Modelling and Comparative Assessment of Polyamide-6 Manufacturing towards a Sustainable Chemical Industry
Hendrikus Hubertus omas Herps ISBN: 978-94-6416-054-3
DOI: https://doi.org/10.33540/61
Print: Ridderprint | www.ridderprint.nl
Layout: Publiss | www.publiss.nl Modelling and Comparative Assessment of Polyamide-6 Manufacturing towards a Sustainable Chemical Industry
Modellering en comparatieve beoordeling van Polyamide-6 productie op weg naar een duurzame chemische industrie (met een samenvatting in het Nederlands)
Proefschrift
ter verkrijging van de graad van doctor aan de Universiteit Utrecht op gezag van de rector magni cus, prof.dr. H.R.B.M. Kummeling, ingevolge het besluit van het college voor promoties in het openbaar te verdedigen op
vrijdag 6 november 2020 des middags te 4.15 uur
door
Hendrikus Hubertus omas Herps
geboren op 3 juni 1955 te Maastricht Promotor: Prof. dr. E. Worrell
Copromotor: Dr. M. Gazzani veur mien kleinkinder vaan ampa
Table of contents
Table of contents 7 Units and abbreviations 11 1. Introduction 15 1.1. National and international environmental policies 16 1.2. Consequences for the chemical industry 16 1.3. Objectives of the dissertation research 19 Objective 1: methodology 19 Objective 2: polyamide-6 route designs 23 Objective 3: environmental assessment and comparison of PA-6 manufacturing routes 28 1.4. Outline of the thesis 29 2. Methodological characterization of PA-6 manufacturing routes 31 2.1. General outline of pathways to PA-6 32 2.2. System boundaries 33 Use of system boundaries 35 Detailed scoping and assumptions 37 2.3. Material- and energy ows 37 2.4. Comparison of processes 39 2.5. E ciency of molecular process steps (system boundary 3) 41 Carbon consumption 41 Balancing Equation 43 Energy Balance 45 2.6. E ciency of entire process routes (system boundaries 1, 2, 3) 47 Balancing Equation of entire routes 48 2.7. Energy analysis 49 Exergy analysis 49 Primary energy demand 49 2.8. Carbon dioxide emissions 50
7 3. Polyamide-6 production starting from benzene and ammonia 53 3.1. Process design of polyamide-6 production starting from benzene and ammonia 57 Benzene hydrogenation 58 Cyclohexane oxidation 60 Cyclohexanol dehydrogenation 65 Hydroxylamine preparation 66 Cyclohexanone oximation 70 Beckmann rearrangement 71 e-Caprolactam recovery 72 Polymerization of ε-caprolactam 74 3.2. Mass and energy balances 77 Balancing Equation from BBBs to PA-6 77 Raw material consumption and product and waste production 78 Energy balances 81 4. Polyamide-6 production starting from 1,3-butadiene and hydrogen cyanide 85 4.1. Process design of polyamide-6 production starting from 1,3-butadiene and hydrogen cyanide 87 Hydrocyanation of 1,3-butadiene to pentenenitriles 87 Hydrocyanation of pentenenitriles to adiponitrile 91 Hydrogenation of adiponitrile to 6-aminocapronitrile 93 Polymerization of 6-aminocapronitrile 96 4.2. Mass and energy balances 100 Balancing Equation from BBBs to PA-6 100 Raw material consumption and product and waste production 100 Energy balances 102 5. Polyamide-6 production starting from glucose and ammonia 107 5.1. Process design of polyamide-6 production starting from glucose and ammonia 110 Fermentation of glucose (Frost routes and ACA route) 112 Frost route 112 ACA route 113
8 Ion exchange puri cation (Frost routes or ACA route) 116 ε-Caprolactam production from L-lysine (Frost routes) 117 Cyclisation of lysine 117 Deamination of α-amino-ε-caprolactam 120 Polymerization (Frost and ACA route) 124 5.2. Mass and energy balances 127 5.2.1. Frost and Frost PLUS manufacturing route 127 Balancing Equation from BBBs to PA-6 127 Raw material consumption and product and waste production 128 Energy balances 131 5.2.2. ACA manufacturing route 133 Balancing Equation from BBBs to PA-6 133 Raw material consumption and product and waste production 135 Energy balances 137 Sensitivity of biobased manufacturing routes 140 6. Comparative material, energy and GHG assessment of PA-6 manufacturing 143 6.1. Accuracy of the modelling results 145 6.2. Background for the assumptions of the applied energy resources 147 6.3. Material consumption 147 Carbon consumption 147 Practical and environmental constraints and drawbacks 149 6.4. Energy consumption 151 Energy e ciency 151 Primary Energy Demand comparison 155 Practical constraints and drawbacks 157 6.5. Fossil carbon dioxide emission 157 7. Summary, conclusions and recommendations 159 7.1. Background 160 7.2. Objectives of the dissertation research 161 7.3. Main ndings and key results 162 Objective 1 162 Objective 2 164
9 Objective 3 164 7.4. Limitations of the research 169 7.5. Further research 169 8. Samenvatting, conclusies en aanbevelingen 171 8.1. Achtergrond 172 8.2. Doelstellingen van het onderzoek 173 8.3. Belangrijkste bevindingen en resultaten 174 Doelstelling 1 174 Doelstelling 2 176 Doelstelling 3 176 8.4. Beperkingen van het onderzoek 181 8.5. Verder onderzoek 182
References 183 Acknowledgement 191 Curriculum Vitae 193 Appendix A: Primary Energy Demand of Basic Building Blocks 194 Appendix B: Design criteria, assumptions and conditions of the main equipment in the Aspen simulation model for the production of polyamide-6 from benzene and ammonia 197 Appendix C: Design criteria, assumptions and conditions of the main equipment in the Aspen simulation model for the production of polyamide-6 from 1,3-butadiene and hydrogen cyanide 208 Appendix D: Design criteria, assumptions and conditions of the main equipment in the Aspen simulation model for the production of polyamide-6 from glucose and ammonia 214 Appendix E: Method to calculate enthalpy and entropy of polyamide-6 chains 223
10 Units and abbreviations
EJ Exa Joule (1018) Kelvin (k)g (Kilo)gram kJ Kilo Joule (103) kPa Kilo Pascal (103) kton Kiloton (103) kW Kilo Watt (103) L Liter MJ Mega Joule (106) MPa Mega Pascal (106) mton/mt Metric ton (1000 kg)
Cp Heat capacity ΔEx Exergy loss
ΔfH Enthalpy of formation 0 ΔfH Standard enthalpy of formation
ΔrH Enthalpy of reaction 0 ΔrH Standard enthalpy of reaction h Enthalpy of processing stream at P, T [ in kW] h0 Enthalpy of processing stream at P0, T0 [in kW] k E ciency factor in condensate recycling
Mn Molecular weight P Pressure
P0 Atmospheric pressure [0.1 MPa] pH Scale used to specify the acidity or basicity of an aqueous solution pKa Scale used to specify acid dissociation
Pn Polymerization degree S Entropy of processing stream at P, T [in kW]
S0 Entropy of processing stream at P0, T0 [in kW]
Sf Entropy of formation
Sr Entropy of reaction T Temperature
Tm Melting temperature
T0 Temperature at environmental conditions [298.15 K]
α Strength factor (mol/mol ratio) SO3 in oleum β Factor related to the thermodynamically required minimum amount of energy in hydrogen production (methane reforming) 11 6-ACA 6-aminocaproic acid ACN 6-aminocapronitrile ADN Adiponitrile AKA α-ketoadipate AKG α-ketogluterate AKP α-keto pimelate ALCA Attributional Life Cycle Assessment ANOL Cyclohexanol ANON Cyclohexanone AS Ammonium sulphate ATP Adenosine triphosphate BBB Basic Building Block BD 1,3-butadiene
BPh3 Triphenyl borane BTX Benzene Toluene Xylene C3AC Propionic acid C3Salt Potassium propionate CED Cumulative Energy Demand CH3SH Methane thiol CCS Carbon capture and storage CCU Carbon capture and use CFP Carbon footprint CHHP Cyclohexyl hydroperoxide CHX Cyclohexane CLCA Consequential Life Cycle Assessment CPL Caprolactam CW Cooling water DIMERIC Dimeric product in Butadiene route EPD Environmental Product Declaration ESN Ethylsuccinonitrile EU European Union GHG Greenhouse gas GS Generated steam GWP Global Warming Potential HCN Hydrogen cyanide HDDCS Heat drain deep cooling system HMDA Hexamethylene diamine HMF 5-hydroxymethylfurfural
12 HPO Hydroxylamine Phosphate Oximation HSNO Hydroxylamine Sulphate Nitrogen Oxide HSO Hydroxylamine Sulphate Oximation IMINE 6-aminocaproimine IPCC Intergovernmental Panel on Climate Change KA-oil Keton-Alcohol-oil (cyclohexanone/cyclohexanol) KNAW Koninklijke Nederlandse Akademie van Wetenschappen LA Levulinic acid LCA Life Cycle Assessment LCI Life Cycle Inventory LCIA Life Cycle Impact Assessment Metaboli Intermediate product in glucose fermentation (modelled as pyruvic acid) NAD(P)H Nicotinamide adenine dinucleotide phosphate NGO Non-Governmental Organization NRM Natural Raw Material NRTL Non-random-two-liquid equation of state 2M3BN 2-methyl-3-butenenitrile 2MGN 2-methylglutaronitrile PA-6/PA6 Polyamide-6 (Nylon-6) PED Primary Energy Demand PN Pentenenitrile Radfrac Aspen model to simulate a multistage vapour-liquid fractionation RED Renewable Energy Directive (EU) RK Redlich-Kwong equation of state SC Steam condensate THA Tetrahydroazepine THF Tetrahydrofuran VCH Vinyl cyclohexene VK Vereinfacht kontinuierlich (polymerization column) VNCI Vereniging van de Nederlandse Chemische Industrie
13
Introducti on 1 Chapter 1
Environmental issues such as climate change become increasingly important for our society as we face the challenges of limited natural resources and growing human population, with a growing middle class in economically emerging regions aspiring to a higher standard of living. Persisting greenhouse gas emissions seriously aect climate change. e annual global emission of carbon dioxide (as main greenhouse gas) increased to 30–35 gigaton CO2 by 2019. Most of it (80%) originates from the burning of fossil fuels to generate heat and electricity (1)(2). Globally, the contribution of industry is approximately 25% (1). e annual CO2 emission in the Netherlands was 179.3 megatons in 2018. e direct contribution of industry was 36.7 megatons (21%) and energy generation (industry and public) contributes for 58.2 megatons (3).
1.1 National and international environmental policies Following the Paris Agreement (4), national policies such as the Dutch ‘Energieakkoord’ (5) and other national and international agreements based on the 2015 United Nations Climate Change Conference in Paris aim at intensive reduction of greenhouse gas
(GHG) emissions. Carbon dioxide (CO2) emissions will have to be net-zero between 2040 and 2060, according to the IPCC (Intergovernmental Panel on Climate Change) scenarios, lest the average temperature rise will exceed the 1.5–2 °C maximum stipulated in 2015 by the Paris agreement (6). e European Union (EU), on behalf of all Member States, has hard commitments to reduce GHG emissions by at least 40% in 2030 compared to 1990. e recent Dutch government coalition agreement 2017-2021 ‘Trust in the Future’ (5) exceeds the EU commitments: at least 49% GHG reduction in 2030 compared to 1990, which corresponds to an additional annual reduction of 25 million tons CO2 equivalents, while at the European level today even stronger reductions are considered (55%).
In the Netherlands, according to the Dutch government coalition agreement 2017- 2021 (5), relatively large emission reductions (49% GHG reduction in 2030 compared to 1990) have to take place, primarily in the industrial sector, because there is a large technical saving potential. is puts the spotlight on the chemical industry and oil re neries: by far the largest GHG emitters in Dutch industry.
1.2 Consequences for the chemical industry A study by Ecofys and Berenschot for the VNCI (7) reports that, with innovation, it is indeed technically possible for the Dutch chemical industry to achieve the goal of net zero emissions by 2050. However, the speci c question is how the chemical industry
16 Introduction can produce carbon-based products in such a way as to not add greenhouse gases, particularly CO2 to the atmosphere. A major part of the current chemical products contains fossil based carbon that will turn at the end-of-life to gaseous species, such as CO2 (8). Also a few percent of the total fossil carbon produced today is nally embedded in chemical products; the major part of the produced fossil carbon is 1 burned to produce energy for human activities such as the chemical industry (9).
Only a few general strategies can realize a transition to a low carbon economy (5)(10) (11). ese are:
• Reducing the demand for energy: to reduce investments in energy generation capacity, and particularly important as long as renewable energy is still not su ciently available;
• and the net-zero fossil CO2 strategies:
• Power generation without CO2 emissions, i.e. wind, solar, hydropower or nuclear energy in combination with electri cation: a higher proportion of electricity in the use of energy; (12)(13)(14)
• Closing the carbon cycle, by e.g. using biomass, Carbon Capture and Use (CCU), or the extensive recycling of consumer products (e.g. plastic) which contributes to a reduction in the demand for crude oil; (9)(15)(16)(17)(18)
• (Re)activating natural CO2-sinks, e.g. (re)forestation, Carbon Capture and Storage (CCS). (9)(15)(19)
Key hurdles have been identi ed in the net-zero CO2 routes (9): (a) the availability, accessibility, and acceptance of CO2 storage sites (CCS); (b) the very high electricity and energy demand for the CCU route, with the associated strict requirement of very low carbon-intensity of the electricity mix; (c) the availability of land for biomass growth in the case of biomass use, with the associated risks of con icts with other uses (food/feed). Uncertainty about future sustainable (global) supply of biomass is great and the greater the demand for biomass, the greater the risk of indirect land use changes and competition with the food chain (20)(21).
Junginger et al. (22) conclude that it is critical for industry to develop and demonstrate innovative and integrated value chains for biofuels, bioproducts, and biopower. Eventually, biomass products for energy and modern biomaterials may develop into large-scale commodity markets. On the other hand, comprehensive sustainability of large scale biomass production and trading has yet to be secured, while governance of developing biomass markets is a critical issue, with the use of both food crops and woody biomass strongly scrutinized by some policy makers and NGOs.
17 Chapter 1
e Dutch ‘Koninklijke Nederlandse Akademie van Wetenschappen (KNAW)’ concludes that biomass including agricultural waste should not be considered as fuel but as raw material (23). One argument KNAW uses is that it takes 20 to 100 years before the direct CO2 emission of wood burning is absorbed by new planted trees. is argument is based on the Joint Research Centre Annual Report 2013 (24) report which says: ‘Most of the forest feedstock used for bioenergy, as of today, are industrial residues, waste wood, residual wood (e.g. thinning wood, harvest residues, salvage loggings, landscape care wood) for which, in the short to medium term, GHG savings may be achieved. On the other hand, in the case of stem wood harvested for bioenergy purposes only, if all the carbon pools and their development with time are considered in both the bioenergy and the reference fossil scenario, there is an actual increase in CO2 emissions compared to fossil fuels in the short-term (few decades). In the longer term (centuries) also wood may reach the fossil fuel parity points and then generate GHG savings if the productivity of the forest is not reduced because of bioenergy production.’ Another argument KNAW uses is that trees and plants set up at most 0.03% of the incoming sunlight into biomass. e sun radiates per year 4 million EJ energy on the earth from which plants and trees absorb only 1300 EJ. Solar cells already have a return of 16 – 25% and with new concepts it seems returns of 40% are feasible. Land is scarce and therefore the higher returns will eventually win. Additionally, KNAW quotes the competition of industrial biomass vs. food/feed biomass and the related environmental burdens. Bio-energy is therefore mainly a transitional measure, no nal solution.
Hence, the use of renewable energy (power generation without CO2 emissions) and -material feedstock (biomass) will be the most powerful building blocks in the transition to a clean and sustainable chemical industry. However, the actual availability of renewable energy for chemical processes is still very low and the e ciency of closing the carbon loop only modest, in spite of impressive progress during the past years. e switch to solely sustainable energy sources will surely take some decennia to be nalized, as will be the production and use of renewable material building blocks to produce chemical products.
Summarizing, there are still many hurdles to be taken and much research and discussion to be done on the way to a fully sustainable chemical industry. Because delay is no option, the chemical industry should also search for intermediate solutions in the meantime (6). Several approaches are possible, e.g. energy reduction or process intensi cation of commercial fossil based routes (more eective unit operations, reuse of wasted energy). Or using alternative fossil processes implying less, for the time being, fossil processing energy and consequently less environmental impact. On the other hand, in times where renewable energy is still scarce, it is questionable whether such energy should be applied in an intrinsic energy-ine cient process,
18 Introduction
in particular when alternatives with a better CO2 reduction potential are available. In the search for such best sustainable intermediate options, it is useful to include also optional (future) biobased routes to produce the same end product. A comparative assessment of improved fossil based routes and future biobased routes can be useful to 1 de ne the roadmap to sustainable chemical manufacturing.
1.3 Objectives of the dissertation research e dissertation research services three objectives:
1. e development of a method to assess and compare in an unambiguous way dierent manufacturing routes starting with natural raw materials to the same end product. e polyamide-6 (PA-6) manufacturing industry is used as a case study to illustrate such a comparative assessment of dierent manufacturing routes.
2. e reliable design of manufacturing routes to produce polyamide-6 comprising detailed process description and computer aided simulation modelling. We have designed ve manufacturing routes. Two are fossil-based and three are biobased. One of the ve routes is commercially in use, the other routes are still theoretical designs.
3. e environmental assessment and comparison of the selected PA-6 manufacturing routes with the use of the developed evaluation method.
Objective 1: methodology It is imperative that sound comparison methods are available to assess and identify the best intermediate sustainable solution(s). is will require an integral approach, which comprises the comparison and evaluation of the impact of dierent manufacturing routes and unit operations, dierent (combinations of) natural raw materials, and the application of dierent kinds of fossil and renewable energy on chemical manufacturing, in particular the associated CO2 emission. To assess the environmental sustainability of processes and products, many methodologies have been developed to determine environmental consequences of operations. e most generally accepted methodology is Life Cycle Assessment (LCA). LCA is a methodology aimed at evaluating environmental impact, which relates to the entire lifetime of a product, a technological process or an activity. e procedures of LCA are described in the ISO14000 environmental management standards, particularly ISO14040/14044 (25)(26)(27), for which practical guidance is given in various LCA manuals (28). LCA methods can be subdivided in two types: attributional
19 Chapter 1
(ALCA) and consequential (CLCA) life cycle assessments. Several dierent de nitions of both types have been suggested (29)(30), of which the de nitions of Finnveden et al. (31) are the most cited ones in literature:
• ALCA aims to describe the environmentally relevant physical ows to and from a life cycle and its subsystems. is means, attributional assessments give an estimate of what part of the global environmental burdens belongs to the study object or process (32).
• CLCA aims to describe how environmentally relevant ows will change in response to possible decisions. In other words, consequential assessments give an estimate of how the production and use of the study object aect the global environmental burdens (32).
e ALCA practice is more well-established than CLCA, e.g. in environmental product declarations (EPD). In general can be stated that ALCA is more robust and more resistant to abuse in the sense that the results depend less on who is doing the study (32). Another distinction made in LCA application is ex-post and ex- ante LCA. e guidance provided by LCA manuals (28) has been typically applied to modelling and assessing environmental impacts ex-post, meaning aer products have been commercially in use for extended periods of time and information and data are available from empirical experience. Ex-ante is de ned as before a product or technique is commercially deployed at scale and information and insights on the topic under assessment are not (yet) readily available (28). Performing an ex-ante exercise introduces additional challenges, particularly in CLCA. ese include the lack of representative information for the product systems under study, the lack of a clear vision into the future of the technological landscape in which the technology will operate, and the lack of direct access to representative data for lab-scale processes (33) (34)(35)(36).
Many commonly applied forward looking assessment methods have their constraints, either because of the qualitative nature of the results or lack of necessary information and vision into the future. Patel et al. (37)(38) propose a new method, incorporating practical aspects, for quick preliminary assessment of chemical processes at the laboratory stage. e proposed method enables a review of chemical processes within a broader sustainable context and builds upon existing methodologies and combines aspects of techno-economic analysis, LCA and green chemistry. Results from the model give a good preliminary indication regarding the sustainability of a new process compared with a similar conventional process.
In attributional LCA, it appears also that especially choices made in the life cycle impact assessment step (LCIA) can lead to signi cant dierences in the quantitative
20 Introduction outcome of the analysis. Such dierences in interpretation result from e.g. the dierent ways in the selection of impact categories. (31)(39)(40) LCIA results can be expressed as single score endpoint (e.g. carbon footprint (CFP)) or as multi score midpoint/ endpoint results (eco footprint). Multi-score eco-footprint is the ultimate outcome of a LCA study but is not easy to interpret (41). Single score endpoints, like CFP, 1 are easier to interpret and in practise appears to be a good proxy for the overall eco footprint1. e carbon footprint of a manufacturing route is the sum of contributions of greenhouse gases starting from Natural Raw Material (NRM) up to the required end product as it leaves the plant gate (cradle-to-gate approach). In addition, the end- of-life contribution of the end product can signi cantly contribute to the greenhouse gases (cradle-to-grave analysis).
Examples of greenhouse gases are carbon dioxide, methane, nitrous oxide and ozone. e contribution to global warming of greenhouse gases dier signi cantly. E.g., the global warming potential (GWP) of methane is 25 kg CO2 equivalents/kg, the GWP of nitrous oxide (N2O) is 298 CO2 equivalents/kg (GWP is expressed as kg CO2 equivalents (eq)/kg product) (42). Nitrous oxide is a well-known by-product in e.g. fertilizer, nitric acid, adipic acid and caprolactam/nylon manufacturing and should be accounted for in CFP calculations.
e established polyamide-6 manufacturing routes (see description in Objective 2) signi cantly dier in the kind of emitted greenhouse gasses. One of the assessed (fossil based) routes intrinsically emits nitrous oxide (amongst other GHGs), however, the other routes emit nitrous oxide mainly in the production of material feedstock (such as ammonia and hydrogen cyanide). Moreover, end-of-pipe technologies are available to abate nitrous oxide and the use of these technologies is subject to environmental regulations and economic desirability.
We will demonstrate the example of N2O emission in caprolactam manufacturing.
Nitrous oxide can be abated with catalytic technologies by converting N2O into naturally occurring nitrogen and oxygen. e level of abatement strongly depends on the economic competitiveness of the manufacturer and therefore can vary from no abatement up to almost 100% abatement. Comparison of 50 caprolactam production sites (worldwide), see Figure 1-1, shows extreme dierences in N2O emission (and thus CFP) caused by dierences in production technology and abatement technology.
CFP contribution of N2O varies from 0 – 4 kg CO2 eq/kg caprolactam (CPL). Figure 1-1 also reveals separately the CFP contribution of processing energy generation, raw materials and ammonium sulphate co-production.
1 Based on the industrial experience of the author as a LCA practitioner.
21 Chapter 1
Figure 1-1 Comparison of the CFP of 50 caprolactam production sites (worldwide).
e contribution of energy (green bars), raw materials (white bars), N2O (red bars), other GHGs (by-products to CO2, yellow bars) and co-product ammonium sulphate (blue bars) are separately depicted. 2
Although we acknowledge the signi cant contribution of e.g. N2O in environmental assessments, we have decided to focus our analysis primarily on fossil- or biobased carbon consumption (for material and energy supply) and the associated carbon dioxide emission. A clear comparison of the dierent PA-6 routes and exploring sustainable options would become unnecessarily di cult, especially for e.g. policy makers, if we would include all the produced greenhouse gases in the comparative assessment.
Huijbregts et al. (43) conclude in their research that a suitable option to overcome the vulnerabilities and constraints of commonly applied environmental assessment methods is the application of the concept of cumulative energy demand (CED) as a screening factor. CED represents the energy demand during the complete life cycle of a product or process, valued as primary energy. Particularly, from fossil energy demand it is well known that it is dominantly responsible for global warming and consumption of fossil resources (44)(45)(46).
e previous discussed life cycle assessment methods are powerful tools for environmental studies and research applications. Experienced LCA practitioners are familiar with these methods and can, at least in principle, interpret the results in a professional way. However, such a professional interpretation cannot be expected from policymakers and other researchers outside this expertise eld. Such ‘laymen’ oen struggle with the format and the meaning of the LCA results3. Hence, we will keep 2 Based on business intelligence information of the former DSM Fibre Intermediates organization. 3 Based on the industrial experience of the author as LCA-practitioner.
22 Introduction our assessment ‘relatively simple’ and focus solely on, easy to interpret, carbon dioxide emission related to the natural resources which are used to produce the end products and the associated processing energy. Objective 2: polyamide-6 route designs 1 e exploration is done by analysing and comparing a number of dierent, alternative manufacturing routes of one single chemical product, that start from (widely) dierent sets of natural raw materials and include the generation of processing energy. Hence, the impacts of the raw materials, the process routes and steps can be assessed, as well as that of using fossil, biobased, or wind- or solar based energy.
Polyamide-6 as a case study e polyamide-6 manufacturing industry is used as a case study. e polyamide-6 chain is assumed to be representative for the chemical industry in terms of applied process technology and interaction with the ecosystem. And aer decades of continuous improvement it is still oering a wide array of interesting improvement options. e route comparison is based on the shared production of the end product: a kilogram of PA-6. All routes start from NRMs, which we de ne as all resources available in nature such as fossil carbon materials, ores, water and biomass. All of the manufacturing routes start from a zero point: the natural raw materials as they are available in nature.
ere are many existing commercial routes to produce e-caprolactam and PA-6 thereof. All these commercial routes start from fossil feedstocks. We summarize the most applied routes brie y.
• HSO (Hydroxylamine Sulphate Oximation) e HSO/Raschig process (in this study labelled as Benzene-Raschig) is the oldest industrial process to produce e-caprolactam. e route comprises hydrogenation of benzene, oxidation of cyclohexane to cyclohexanone and the production of hydroxylamine via oxidation of ammonia to ammonium nitrite, which is subsequently reduced with sulphur dioxide to hydroxylamine disulphonate. Disulphonate is hydrolysed to hydroxylamine sulphate, and aer oximation with cyclohexanone and neutralization cylohexanone oxime is formed (Raschig process4). Cyclohexanone oxime is rearranged with oleum, and aer neutralization with ammonia, e-caprolactam is formed together with approx. 4.5 to 5 ton ammonium sulphate (AS) per ton caprolactam. e-caprolactam is further polymerized via polyaddition and polycondensation to polyamide-6.
4 e Raschig process is a chemical process for the production of hydroxylamine, developed by the German chemist Friedrich Raschig at the end of the nineteenth century.
23 Chapter 1
• HPO (Hydroxylamine Phosphate Oximation) (47) e hydroxylamine source of this process route is hydroxylamine phosphate. It is produced by hydrogenation of nitric acid. e phosphate salt has the important advantage that it can be used for the oximation of cyclohexanone without the formation of ammonium sulphate. e produced oxime is rearranged with oleum, and aer neutralization with ammonia, e-caprolactam is formed together with approx. 1.5 to 1.8 ton AS per ton caprolactam. e-caprolactam is polymerized via polyaddition and polycondensation to polyamide-6.
• HSNO (Hydroxylamine Sulphate Nitrogen Oxide) In this process hydroxylamine sulphate is produced by catalytic hydrogenation of nitrogen monoxide in dilute sulphuric acid. Hydroxylamine sulphate is used to produce cyclohexanone oxime from cyclohexanone. To achieve full conversion neutralization with ammonia is necessary, and consequently AS is formed, approx. 1 ton per ton caprolactam. Cyclohexanone oxime is rearranged with oleum. e-caprolactam is polymerized via polyaddition and polycondensation to PA-6.
• Ammoximation technology (48)(49)(50) Ammoximation is a ‘new’ technology for preparing cyclohexanone oxime that has been applied by several global caprolactam manufacturers. e technology was developed in the 80-90’s by Sumitomo (Japan). e driver was to have an entirely AS free caprolactam process. ey simultaneously developed the proprietary AS free vapour phase rearrangement process. ey also developed crystallization puri cation and thus introduced an entirely dierent way for making caprolactam in 2000. Sinopec/Baling developed their own ammoximation technology. Baling combined the ammoximation with (traditional) liquid phase Beckmann rearrangement and (traditional) puri cation technology. Ammoximation technology is an important emerging caprolactam/PA-6 route nowadays.
Biobased materials have received interest as one of the available means that should be pursued to curb climate change. In the context of the biobased economy, strategies should include the use of non-fossil carbon feedstock as raw material for the production of chemical products, like PA-6, and for the generation of the required processing energy. Replacing fossil based plastics with biobased material is not straightforward, because many new production systems based on new (biomass) feedstock types would need to be established. Also, not all production routes or biobased materials result in reduction of CO2 emissions compared to fossil based counterparts. (51) Several strategies can be chosen to convert renewable resources into chemicals. De Vries (52) provides two examples of the conversion of sugars into nylon intermediates. 5-Hydroxymethylfurfural (HMF) can be prepared in good yield from fructose. Two hydrogenation steps convert HMF into 1,6-hexanediol. Oxidation converts the last
24 Introduction product into caprolactone, which can be converted into caprolactam (precursor of PA-6) by reaction with ammonia. An even more interesting platform chemical is levulinic acid (LA), which can be obtained directly from lignocellulose in good yield by treatment with dilute sulphuric acid at 200 °C. Hydrogenation converts LA into g-valerolactone, which is ring-opened and esteri ed in a gas-phase process to a mixture 1 of isomeric methyl pentenoates. is mixture is converted in to dimethyl adipate, which is nally hydrolysed to adipic acid (precursor of nylon-66). e conversion of lignin into chemicals is a much more complicated task in view of the complex nature of lignin. For this purpose, De Vries has used three dierent methods: acetalisation, hydrogenation, and decarbonylation.
Moncada Botero (53) assesses the techno-economic performance of the production lines of 1,3-butadiene and e-caprolactam from C6 sugars. Process models are developed to assess the technical performance and derive inputs for the economic analysis. 1,3-butadiene is an important chemical for the production of synthetic rubbers. Butadiene is traditionally produced in oil re neries. Next to the application in rubber production, 1,3-butadiene can also be used as raw material for the production of polyamide-6 as will be explored in Chapter 4. e caprolactam production comprises the production of levulinic acid from C6-sugar prior to the production of g-valerolactone from levulinic acid and subsequent production of e-caprolactam via methyl pentenoates and penteneamides. e production of e-caprolactam from levulinic acid is not commercialized yet and still subject of study. Also other biobased pathways to produce e-caprolactam and/or PA-6, to the authors’ knowledge, are still in the theoretical design phase and not commercially applied.
We have established ve manufacturing routes to produce polyamide-6 from fossil- or biomass based NRMs. Two are fossil- and three are biobased. One of the routes is commercially in production, the other routes are still theoretical designs.
• A commercial e-caprolactam and polyamide-6 route using benzene and ammonia (Benzene-Raschig), and an alternative non-commercial fossil feedstock based route using 1,3-butadiene and hydrogen cyanide (Butadiene). e Benzene- Raschig route comprises the described HSO process. e HSO process is oen used in commercial caprolactam/PA-6 production and has been abundantly described in literature. Hence, we have chosen the commercial HSO route as benchmark manufacturing route.
• ree, still theoretical, biobased routes which start with glucose from sugar beets as carbon feedstock. Two routes ferment glucose to L-lysine and subsequently lysine is chemically transformed to e-caprolactam and PA-6 (Frost and Frost PLUS). e third ferments glucose to 6-aminocaproic acid which is polymerized to PA-6 (ACA).
25 Chapter 1
Figure 1-2 gives an overview of the selected manufacturing routes and the required natural raw materials (NRM) and feedstock, labelled as basic building blocks (BBB), to produce nally PA-6 and co/by-products.
Benzene and 1,3-butadiene, applied as BBBs in the fossil based routes, are products from the naphtha cracking process and consequently originate from crude oil as NRM. Ammonia is produced in the Haber-Bosch process where hydrogen, from the methane (natural gas) reforming process, reacts with atmospheric nitrogen. Hydrogen cyanide is produced in the Andrussow process (54) by the reaction of fossil based ammonia with methane (natural gas) in the presence of atmospheric oxygen. e biobased routes start with glucose and fossil based ammonia (and hydrogen in Frost (PLUS)) as BBBs. Glucose is extracted with hot water from sugar beets and subsequently recovered by distillation. Water is used as activator in the polymerization of caprolactam and 6-aminocapronitrile.
Figure 1-2 Overview of the manufacturing of polyamide-6 via selected routes (listed in the fourth column in the gure). BBBs to produce PA-6 are listed in the second column of the gure and the chemical elements which are the constituents of the nal end products are listed in the third column. PA-6 and other main co/by products are listed in the last column. e main NRMs are depicted at the le side of the gure. Air is used as oxygen supplier for all routes (not shown).
26 Introduction
NRMs, i.e. fossil feedstock or biomass, are used for the production of energy and materials (BBBs). e produced energy, e.g. heat or electricity, is used upstream and downstream. Use upstream enables t for purpose pre-treatment of natural resources and includes mining and transport of fossil resources; sowing, fertilizing, harvesting and transport of glucose resources from the land, or puri cation of nitrogen or water. 1 Downstream energy use enables the production of BBBs from natural resources by chemical reaction and physical processing and, similarly, PA-6 further downstream. Production of BBB comprises e.g. oil re nery, naphtha cracking, BTX plants, water-gas shi- and ammonia plants, bio re neries, sugar production, and production of several auxiliary substances like catalysts. Sulphur is a by-product of energy production. us, the investigated polyamide-6 routes use dierent amounts of dierent raw materials, i.e. dierent BBBs of fossil or renewable origin; dierent natural resources (e.g. water, nitrogen), and dierent amounts of fossil- or renewable energy. Energy can also be generated directly from solar/wind/hydro power sources.
Description of five PA-6 manufacturing routes Table 1-1 summarizes the ve established PA-6 manufacturing routes. Although the product is the same, these routes vary in the used raw materials (renewable and non- renewable (fossil)) as well as in their process design and process ability. Consequently, the environmental impact can vary signi cantly.
Table 1-1 Existing and theoretically possible PA-6 manufacturing routes.
Economical Carbon material feedstock Labelled as Type of processing* phase Non- Benzene Benzene- Chemical Commercial renewable Raschig Non- 1,3-butadiene/hydrogen Butadiene Chemical Non- renewable cyanide commercial Renewable Glucose Frost5 Bio-fermentation and Non- chemical commercial Renewable Glucose Frost PLUS6 Bio-fermentation and Non- chemical commercial Renewable Glucose ACA Bio-fermentation Non- commercial * e polymerization step in all ve routes is chemical and not bio-fermentative.
5 Low deamination yield: 65% of α-amino-e-caprolactam. 6 High deamination yield: 95% of α-amino-e-caprolactam.
27 Chapter 1
Benzene-Raschig manufacturing route e Benzene-Raschig route comprises the reaction of hydroxylamine sulphate with cyclohexanone to produce cyclohexanone oxime. is commercial route is also known as the HSO route. e material feedstock is non-renewable. e Benzene-Raschig route is representative for the state-of-the-art classical and commercial PA-6 manufacturing. e route is considered as benchmark in the present study.
Butadiene manufacturing route is non-commercial process consists of three separate steps. In the rst step 1,3-butadiene is hydrocyanated to form adiponitrile. e hydrogenation of adiponitrile to 6-aminocapronitrile is the second step. e nal step of this process is the polymerization of 6-aminocapronitrile to polyamide-6. e material feedstock is also non-renewable. (55)(56) e Butadiene manufacturing route may be advantageous over the commercial Benzene-Raschig route as a result of reduced energy consumption (56).
Frost, Frost PLUS and ACA manufacturing route e carbon feedstock of these three non-commercial PA-6 production routes is biobased: glucose, which is extracted from e.g. sugar beets. Ammonia and hydrogen are the other feedstocks, these are fossil based. (57) Glucose is fermented to obtain the precursor for polymerization. e precursor for the Frost processes (58)(59) (60) is L-lysine, the precursor for the ACA manufacturing route (61)(62)(63) is 6-aminocaproic acid. L-lysine is consecutively chemically converted to e-caprolactam and PA-6. 6-aminocaproic acid polycondensates directly to polyamide-6. Fossil CO2 mitigation potential for these biobased manufacturing routes should be signi cant. Frost and Frost PLUS routes dier in yield in the deamination section ( nal reaction step to caprolactam).
Objective 3: environmental assessment and comparison of PA-6 manufacturing routes One of the objectives of the dissertation research is to demonstrate the usability of the method by applying such an approach to an important and representative chemical product. Several dierent options to improve its manufacturing are investigated, and mutually compared for the (combined) potential to reduce fossil CO2 emissions caused by manufacturing. Derived questions are e.g. whether preferred options exist which can be the most sustainable with respect to less fossil energy consumption and
CO2 emission, or to what extent (commercial) chemical manufacturing routes can be operated with sustainable energy, and which route will perform best in terms of mitigation of CO2 emission. Mass- and energy balances obtained with Aspen Plus® simulation are used to assess and compare the selected PA-6 manufacturing routes.
28 Introduction
1.4 Outline of the thesis Chapter 2 describes the general outline of industrial chemical pathways to PA-6, particularly the dierences and details. e principle of system boundaries is explained in the context of the evaluation and comparison of PA-6 manufacturing routes. Also dierences between fossil- and renewable based material- and energy ows are 1 described and the possible greening of the energy grid by using primary renewable solar- and wind energy and biomass for energy- and material feedstock generation is investigated. Additionally, we elaborate on ways to characterize PA-6 routes with the use of material- and energy data. e usefulness of balancing equations as a starting point in comparing the theoretical and practical characteristics of manufacturing routes is explained. We elaborate on characterization tools such as carbon atom e ciency, (fossil) CO2 emission, exergy analysis and the primary energy demand concept.
Chapters 3 to 5 describe detailed process designs and related Aspen process simulation of ve possible PA-6 manufacturing routes starting from basic building blocks. Every chapter concludes with the resulting material- and energy balances in accordance with the comparison and characterization tools as de ned in Chapter 2. Chapter 3 analyses the conventional e-caprolactam/PA-6 manufacturing route which starts with fossil based benzene and ammonia. Chapter 4 describes the possible alternative fossil based route using 1,3-butadiene and hydrogen cyanide as feedstock. Chapter 5 elaborates on three possible biobased PA-6 routes: two routes starting with the fermentation of glucose to L-lysine and the consecutive chemical transformation to e-caprolactam and polymerization thereof to PA-6 and another fermentative route to produce 6-aminocaproic acid from glucose and subsequent polycondensation of 6-aminocaproic acid to PA-6.
Chapter 6 analyses the obtained material- and energy balances of the PA-6 manufacturing routes with respect to carbon use and the associated CO2 emission. We have applied the approach to compare and evaluate the impact of dierent manufacturing routes and unit operations, dierent (combinations of) natural raw materials, and the application of dierent kinds of fossil and renewable energy on chemical manufacturing.
Chapter 7 summarizes the preceding chapters and provides conclusions and answers to the objectives of the thesis.
29
Methodological characterizati on of PA-6 manufacturing routes 2 Chapter 2
Assessment methods are indispensable when the aim is to determine or compare the sustainability level of existing chemical processes with options to improve, i.e. alternative feedstocks, energy sources and/or processing routes. To eliminate ambiguity and to identify and clarify uncertainties, chemical production pathways and system boundaries of several polyamide-6 production routes have to be de ned. is chapter describes such pathways from natural resources to polyamide-6 and explains the applied methodology of system boundaries. Additionally, we elaborate on ways to characterize PA-6 routes with the use of material- and energy data.
First it is important to point out that direct comparison of especially multistep processes is not always straightforward. A simple example illustrates this.
Let us compare two commercial fossil processes which operate in two dierent manufacturing routes A and B with dierent sets of BBBs. Suppose that process B would be judged better because of a lower energy consumption, or lower material consumption, or even both. e question is what conclusions could be drawn from such an assessment. Let us start with economics. All other things being equal, the operating expenditures of process B would be lower than that of process A. However, capital expenditure of B could be so much higher that A would still be preferred from an economic viewpoint. In such a case, only increased future costs of waste (e.g.
CO2 tax) or a threat to the “licence to operate” might lead to a change of preference. e environmental conclusion would be similarly poor without deeper analysis. Indeed, process B has the lower environmental burden, but it could operate close to the maximum possible e ciency already while process A could show abundant improvement opportunities potentially surpassing process B. Moreover, A and B may greatly dier regarding improvement options such as the practical use of renewable energy or raw materials. us it is seen that e ciency of entire process routes cannot be judged without an assessment of the e ciency of its process steps.
2.1 General outline of pathways to PA-6 All routes to PA-6 start from Natural Raw Materials, i.e. resources available in nature such as fossil carbon materials, ores, water and biomass. Dierent combinations of NRMs (see Figure 2-1) are processed into dierent BBB chemicals, or used for (fossil) energy generation. Possible dierentiators between the various routes to PA-6 are indicated in blue. Route comparison is based on shared production of a unit mass of PA-6 (1 kg) and the dierent starting points from the natural raw materials as they are available in nature. e comparison focusses on the e ciency of the use of these materials in the various processes and its consequences (environment).
32 Methodological characterization of PA-6 manufacturing routes
2
Figure 2-1 General outline of and main di erences between PA-6 manufacturing routes. e square boxes and black arrows depict the material stream from NRMs via the production of BBBs up to the nal product PA-6. NRMs can be of fossil (non-renewable) or bio (renewable) origin. e produced intermediates (BBBs) are fossil- or biobased chemicals, respectively. Part of the NRMs are used to generate energy (in an energy plant). Energy can also be generated from sustainable sources (e.g. solar, wind, hydropower). e generated energy (blue arrows) is used as processing energy in the di erent manufacturing routes. e Ecosystem services supplies the required (sustainable) energy and NRMs (dotted arrows).
We compare the types and volumes of needed natural materials and energy to manufacture 1 kg PA-6, and investigate advantages and disadvantages of routes regarding e ciencies of energy- and material use and the associated fossil carbon dioxide emission. We will also investigate the in uence on the environment when (part of) the fossil processing energy supply would be substituted with sustainable energy. We de ne ‘system boundaries’ to enable quantitative evaluation and comparison of the manufacturing routes. Reference states are environmental circumstances of 25 °C and atmospheric pressure.
2.2 System boundaries A ‘system boundary’ de nes the processes to be included in the study, and all matter and energy streams outside-in and inside-out of the system boundary. To compare the dierent PA-6 processes from various perspectives three system boundaries will be used. We subdivide the PA-6 production starting with NRMs in three sections. Section 1 represents the energy production, sections 2 and 3 represent the BBB production (section 2) and PA-6 production (section 3).
33 Chapter 2
System boundary 1 encloses the sections 1, 2 and 3 and separates the human realm from nature.
• All installations and equipment7 to produce the required materials and energy, and all use of these installations, materials and energy upstream and downstream, are inside.
• Natural resources such as land, water, minerals, biomass, fossil resources, and potential energy sources such as wind and photons, and natural processes like the natural material cycles are imported.
• Imported streams comprise all natural resource streams at a reference state of 25 °C and 1 atmosphere. However, the required energy of human eorts to extract and transport them is counted within system boundary 1, including the production, transport and distribution of fertilizer for the dedicated agricultural purposes. e carbon content of imported biomass is accounted for as ‘carbon dioxide outside-in’8. Note that solar energy uptake by plants is not accounted for: it is ‘available for free’.
• Exported streams comprise products and waste of known composition and mass. Products are PA-6 and co-products at a reference state of 25 °C and 1 atmosphere. Waste streams, either solids, liquids (e.g. processed water) or gases, are all brought at a reference state of 25 °C and atmospheric pressure within the boundary before
being emitted. Gaseous waste such as CO2 or NOx is emitted to the environment. Note that no matter or heat can escape the system boundary unaccounted for in this way.
System boundary 1 could be considered a black box in principle, meaning that only the required natural resources and the nal product and waste products are considered. However, sophisticated process comparisons requires further re nement. For this purpose we de ne the system boundaries 2 and 3 within system boundary 1.
7 Basically it also contains the production of all required auxiliary equipment. is has been le outside scope because of its, assumed, negligibly low impact in bulk chemical manufacturing (64). 8 In the balance ‘inside-OUT’ minus ‘outside - IN’, carbon dioxide from biomass osets wasted carbon dioxide of vegetable origin, contrary to CO2 of fossil origin which contributes to global warming. Alternatively, plants could also be grown inside system boundary 1 from water, minerals, and carbon dioxide imported outside-in.
34 Methodological characterization of PA-6 manufacturing routes
System boundary 2 is located within system boundary 1 and comprises the sections 2 and 3. It separates the secondary energy9 generation for the chemical industry in system boundary 1.
• All secondary energy generation is within system boundary 1: the production of electricity, heat and high-pressure steam from fossil or renewable sources, which feeds the ‘energy grid’ of electricity, steam, and liquid or gaseous transport- and heating fuels to which all chemical plants are connected. • e chemical plants are within system boundary 2. Reuse of spent heat sources 2 (e.g. lower pressure steam) and use of heat of exothermic reactions in the same plant is part of system boundary 2 (heat integration) 10 .
• Inside-out streams only dier from those of system boundary 1 in the waste
streams from energy production, which are controlled in boundary 1 (CO2, water, ash).
• e material outside-in streams are those of system boundary 1 except for materials used (burnt) for energy generation. Energy comprises the remaining outside-in streams.
System boundary 3 is located within system boundary 2 and includes the PA-6 processes downstream of BBBs. e production of the BBBs remains in system boundary 2. System boundary 3 serves to study the dierent PA-6 processes in detail and allows using process data for well-known BBBs from open literature. is separation is arbitrary.
Use of system boundaries e routes to produce 1 kg of PA-6 can be characterized by accounting the in and out ows of mass (kg) and energy (MJ) per kg PA-6 at the intersections of the system boundaries by the corresponding detailed materials and energy pathways (assuming steady-state production). Depending on the use of the system boundaries the result is a more or less detailed list of consumed and produced materials and energy data per kg PA-6. Before explaining the methodology of comparison, the usefulness and use of the system boundaries will be clari ed.
System boundary 1: Accounting by an observer outside the box will yield speci ed (composition and mass per kg PA-6) consumed fossil- and renewable raw materials, water, and produced coproducts and waste (including wasted energy): a list of materials 9 Secondary energy is the form of energy which is transformed from primary energy sources through energy conversion processes. Examples are electricity and steam, which are transformed from primary sources (renewable and non-renewable), but also re ned fuels such as gasoline or synthetic fuels such as hydrogen fuels. 10 Alternatively, energy could also be delivered from system boundary 2 back to 1 v.v. (applying an appropriate accounting).
35 Chapter 2 and the unused or wasted part of energy generated from part of the materials. Energy is generated and used inside the black box, and dissipated heat (MJ/kg PA-6) is the only remaining testimony of energy generation and use inside11. As mentioned earlier the energy needed to extract the raw materials from the natural environment is also generated entirely inside system boundary 112. E.g. the associated required quantities of fuel (fossil- or biomass based) for energy generation and the resulting waste appear in the accounting of system boundary 1 on the materials list.
e use of solar-, wind- or hydroelectricity inside system boundary 1 does not require the import of fossil- or biobased raw material13, nor does it produce waste
(CO2 emission). However, this kind of energy has still to be accounted for (as primary energy) in the accounting of system boundary 1. e amount of energy will manifest itself indirectly on the list of materials: it replaces the equivalent use of fossil fuel for electricity production. e implications of this approach will be demonstrated later.
System boundary 2 facilitates the analysis of mass- and energy balances. For that purpose we now think of system boundary 1 as transparent and system boundary 2 as another black box inside system boundary 1. To an observer inside system boundary 1 and outside system boundary 2 the ‘outside-in’ -list remains unchanged, but the ‘inside-out’ -list now concerns the materials and energy ows from system boundary 1 into 2. is enables assessment of a): what fractions of the material ows ‘outside-in’ system boundary 1, fossil or biomass, are used for energy generation (electricity or heat); b): how much associated energy and waste (fossil or biobased) are produced and transported into system boundary 2, including energy of wind, solar or hydropower origin, and c): what remaining material streams enter system boundary 2. Inside the black box of system boundary 2 the production of PA-6 takes place. Its ‘outside- in’ mass and energy ows are speci ed, and the mass and energy ows ‘inside-out’ system boundary 2 equal those of system boundary 1. is approach facilitates the use of literature values for dierent types of energy generation, e.g. coal- or natural gas red power plants or cogeneration, by using key gures of the dierent mass- and energy balances. It enables comparing the impact of the energy mix on dierent PA-6 manufacturing routes regarding global warming potential.
System boundary 3, much in the same way as system boundary 2, separates the domain of the relatively well-known manufacturing of BBBs, such as ammonia, hydrogen, benzene or glucose for the dierent PA-6 production routes. e dierent routes to PA-6, discussed in Chapters 3 to 5, are inside system boundary 3. Accounting of the interface between boundary 2 and 3 yields a list of BBBs and mass used in system 11 ‘Dissipated heat’ per kg PA-6 is an important indicator for route (process) ine ciency. 12 It also includes the energy and materials consumed to produce and apply fertilizer on land to grow the biomass used per kg PA-6. 13 e production of renewable energy installations is le out of scope, but could be included as well in principle.
36 Methodological characterization of PA-6 manufacturing routes boundary 3, energy required to produce PA-6 from BBBs, and waste streams associated with production of BBBs. Literature values for the manufacturing of the required BBBs complete the accounting. Note that waste streams originating in system boundary 1 and 2, outside system boundary 3, pass through system boundary 3 unchanged. Note also that energy reused in PA-6 production is kept inside system boundary 3. Alternatively, energy could also be delivered to the energy grid in system boundary 2, and accounted for as an output stream of system boundary 3.
Detailed scoping and assumptions Natural resources are assumed to be unlimitedly available. e composition of solid 2 and liquid products, as well as waste is known. Solid and liquid waste is virtually stored in containers and gaseous waste is allowed to disperse into the environment. Products are collected in a de ned state. Restoration of damage to the environment as a consequence of human activities (e.g. carbon sequestration) is le out of scope. e contribution of the manufacturing of installations to the results is assumed to be small (64) and is neglected. Processes are operated continuously. e use of small quantities of auxiliary materials, such as lubricating oil and catalyst consumption are assumed to be negligible. e required heat to start up equipment at operating temperature is neglected. Heat dissipation due to imperfect isolation or cooling is accounted for via the material- and energy balances of the system boundaries.
2.3 Material- and energy flows Figure 2-2 schematically presents the main material- and energy ows and the three system boundaries. e main ows will be bookkept: energy ows of electricity or heat (steam), both fossil- or renewable, mass ows of fossil or renewable origin, and water as a carrier of energy.
37 Chapter 2
Figure 2-2 Main fossil- or biobased material ows, fossil or renewable energy (electricity, heat) and water (blue lines) through the numbered system boundaries. Plant clusters are represented by squares (chemical) and ovals (energy supply). Fossil mass ows are depicted as solid black lines, fossil energy ows as black dotted lines. Biomass ows are visible as solid green lines, biomass based energy as green dotted lines. Renewable wind, solar, hydro energy is depicted as red dotted lines. Each PA-6 production process can be represented by a di erent combination of ows (type, amount, chemical composition). e IN and OUT of each system boundary is bookkept, which allows comparison of the di erent PA-6 processes from several points of view, e.g. feedstock eciency, renewability, energy
eciency. NB1: not all ows are shown, e.g. CO2 waste from fossil- and biobased energy production. NB2: renewable generation is also supplied with work, i.e. that of wind owing or that of a photon reaching a PV module.
Energy (electricity, heat, locally used fuels) can be generated from fossil- or biobased feedstock, or directly from wind, solar radiation or hydropower. Dierent sources combined in one ‘energy grid’ provide several kinds of energy to plants, such as electricity, high pressure steam or fuel. Examples of source combination are co- ring of coal and biomass in a power plant, or the mixing of renewable and fossil electricity on the grid. e ‘energy signature’, i.e. the amounts of fossil- and non-fossil carbon dioxide per MJ of power output, is used to keep track on the origin of the energy and related greenhouse gas emission. Energy required to get material streams within the system (work ‘IN’ in Figure 2-2) is accounted for as energy use within boundary 1. Dierent energy signatures can be adopted for comparison purposes (see Chapter 6).
Mass entering the system can be renewable or fossil, and leaves the system as PA-6, coproducts, or solid, liquid or gaseous waste (e.g. carbon dioxide). Mass ows entering the system generally serve two purposes: to provide the material to be processed into BBBs and PA-6, or to provide the fuel to generate energy. Dierent options for dierent PA-6 routes can be compared from economic or ecologic points of view, and can lead
38 Methodological characterization of PA-6 manufacturing routes to dierent conclusions. We will focus on carbon resources, but sulphur, atmospheric oxygen and especially nitrogen used for BBB production are accounted as well. e auxiliary material streams, e.g. catalysts, are usually relatively small in large volume production processes and will not be elaborated in detail.
Water enters (reference state: liquid state, 25 °C) and leaves the system unchanged, condensed, puri ed, and cooled. Heat carried by water (steam produced and used) will be accounted for, including unused heat before it is leaving the system. Coproducts leave the system together with waste and polyamide-6. Co-products can 2 be of economic value and worth the eort of separation and puri cation, e.g. the Butadiene route produces hexamethylenediamine (HMDA) which is a base material for the polyamide-6,6 production. Ammonium sulphate is produced as co-product in the other four PA-6 manufacturing routes and is a valued fertilizer for agriculture.
In such joined production it is not always straightforward to divide and allocate the overall environmental impact between the product and the co-product. However, with the help of allocation or system expansion methods it can be done (65). However, in this study we assume that the purpose of PA-6 manufacturing is solely the production of polyamide-6 and that co-production of other valuable products is inevitable and considered as waste and so allocated to PA-6 production.
2.4 Comparison of processes We start with a typical industrial process improvement viewpoint as oered by system boundary 3. Each of the PA-6 processes inside system boundary 3, either as an existing process or as a process concept, can be the subject of improvement eorts, i.e. to the (economically) attractive reduction of costs of energy, raw materials, or investments. Concepts such as raw material- or atom e ciency and energy/exergy analysis contribute both to environmental and economic process assessment by estimating (the limits of) economically justi able improvement of processes and process steps. is requires the detailing of the consecutive process steps inside system boundary 3, and detailed analysis of e.g. catalyst e ciency, the process design, separation processes, and energy reuse options, which is subject of Chapter 3 to 5. e status-quo of a process and options to improve can be made visible as (changes in) the list of materials and energy pertaining to the ins and outs of system boundary 3.
To compare dierent PA-6 processes the scope of comparison should be widened to include the system boundaries 2 and 1 as well. Data for BBBs in system boundary 2 are taken from literature (see Appendix A). In particular, data of ‘Carbon Footprints’ and ‘Cumulative Energy Demands’ for cradle-to-plant-gate manufacturing of BBBs from
39 Chapter 2
NRMs are used. However, straightforward comparison of biobased- and fossil process variants can still be impeded by interpretation di culties (e.g. fossil or biobased carbon dioxide emission). e use of system boundary 1 and the shared thermodynamic reference state of the environment can circumvent this partly14. System boundary 1 reduces the PA-6 process characteristics to used biobased or fossil materials and produced waste of biobased or fossil origin per kg PA-6. It enables analysis of the best options to improve processes, provided the contents of the system boundaries 2 and 3 are known accurately enough. If biomass and renewable energy are considered scarce, the question is relevant in what kind of process the use of a given amount of biomass and renewable energy will lead to the largest reduction of greenhouse gas emission. is could be a new alternative biobased process but also a fossil process ‘greened’ by renewable energy use. e current approach enables an analysis for the selected PA-6 processes by assuming alternative pathways in Figure 2-3, as will be demonstrated.
Figure 2-3 Alternative energy pathways that can be used to support the production of BBBs from natural resources (primary material). Material ows (black) and energy ows (blue), associated with the production of fossil- and renewable BBBs and using energy from a co-generation power plant using natural gas, connect to di erent stages (black-lined
boxes) of the production process. CO2 emission from the fossil power plant is shown as a material stream. e greening of the energy grid by co-using primary renewable solar-, wind- and hydropower energy and biomass for energy- and material feedstock generation is depicted with dark green lines for renewable material ows and light green lines for renewable energy ows. Primary solar energy enabling the growth of sugar beet on land is not included.
14 e impact of dierent land use associated with fossil- and biobased processes is mentioned but not further analyzed.
40 Methodological characterization of PA-6 manufacturing routes
Currently, mainly secondary non-renewable energy is used to support the production of BBBs from primary materials. e generation of this kind of energy is associated with fossil based CO2 emissions and related global warming. is environmental burden can be diminished by (partly) replacing fossil based energy by renewable energy, which is illustrated with the light green lines. e biobased CO2 emissions originating from the secondary renewable energy generation (dark green line) does not harm the environment. e production of PA-6 from BBBs is not included in Figure 2-3, however, (partial) greening of the related processing energy demand can follow the same alternative pathways. 2
2.5 Efficiency of molecular process steps (system boundary 3) Within the methodology of system boundaries we will de ne ways to characterize the performance of PA-6 manufacturing starting from natural resources. e e ciency of PA-6 manufacturing is the domain of system boundary 3, which will be discussed rst (and for the selected processes in detail in the Chapters 3 to 5). Next we turn to the e ciency of complete manufacturing routes.
e e ciency of process steps can be judged by the material- and energy consumption of these steps in system boundary 3. Hallmark of system boundary 3 is that a molecular description of its processes is always possible, at least in principle. For this reason, substances of unde ned molecular composition like oil are not processed in system boundary 3, but in system boundary 2. All BBBs used in this study are de ned at the molecular level.
At the material level, the concept of atom e ciency is oen used to characterise chemical transformations (see below). We will demonstrate the applicability of ‘Balancing Equations’ and elaborate on the theoretically best possible (path independent) pathway and the practically modelled (path dependent) thermodynamic pathways to produce BBBs and PA-6. e expansion of this concept to entire processes is the subject of Section 2.6. e path-dependent approach involves the mapping and assessment of all energy-consuming steps of dierent routes, which is elaborated in Chapter 3 to 5.
Carbon consumption Carbon consumption assessment is based on the principle of atom e ciency. Analysis of atom e ciency is simple and oen applied to transitions of well-de ned reactants (feedstock) into well-de ned products. e general de nition of atom e ciency is the conversion e ciency of a chemical process to turn reactant atoms into the desired product atoms. It can be a useful tool to determine yields of chemical transitions and to investigate potential improvement directions with respect to reduced feedstock
41 Chapter 2 use, e.g. by optimized process conditions or more sophisticated processing steps involving improved catalyst systems. Examples are the hydrogen atom e ciency for the production of ammonia or cyclohexane, and the nitrogen atom e ciency in the synthesis of cyclohexanone oxime using ammonia as one of the raw materials (see Chapter 3).
Atom e ciency cannot always be applied straightforwardly, especially not to entire, integral manufacturing routes such as the production of polyamide-6 from BBBs or natural resources. For example, the hydrogen atom e ciency of ammonia-fed biobased processes is di cult to interpret, because the origin of the atoms may be from glucose, water, or natural gas (used in ammonia synthesis). Similarly, de ning atom e ciency for naphtha cracker products is not straightforward either. Benzene is for 6–7 w% present in crude oil/naphtha, and 1,3-butadiene for 2–3 w%. Both BBBs are also produced at the high temperatures in the cracker. ese BBB products, together with other valuable products, are separated, puri ed and transported for use. us, the naphtha cracker acts as a black box: it scrambles molecular naphtha constituents and atomic fragments into cracker products. As a consequence it is not possible to assign reaction pathways to one particular BBB.
It is, however, oen possible to reduce hydrogen- and nitrogen atom e ciency to carbon atom e ciency (carbon consumption) of several concerted reaction steps
(carbon atom e ciency is useful to investigate environmental burdens like CO2 emissions). We illustrate this below for ammonia.
In the Haber-Bosch process ammonia is synthesized from hydrogen and nitrogen from air:
Equation 2-1
Hydrogen is obtained from methane and water (n.b. its dual origin):
Equation 2-2
and these reactions combine to: Equation 2-3
In Equation 2-3, carbon atom e ciency can be related to the e ciency of ammonia production: per mole (kg) ammonia, 3/8 mole (0.353 kg) methane is required, which is the minimum amount. If the ratio of the minimum amount of 0.353 kg and the measured amount of actually required methane is less than 1, carbon loss to other products is indicated, e.g. to CO.
42 Methodological characterization of PA-6 manufacturing routes
In practice, however, the depicted reaction in Equation 2-3 requires extra energy, which can be obtained for example by the burning of an extra amount of methane. is energy-providing reaction can also be included in Equation 2-3, yielding an overall reaction for ammonia production from natural gas:
Equation 2-4
Here, the value of is related to the practical required minimum amount of energy, 2 which can be determined15. e ideal process with a carbon atom e ciency of 1 thus requires (3/8 +훽 ) moles C per mole ammonia. A carbon e ciency less than 1 occurs when an extra amount of carbon Δ is required to run the reaction depicted in Equation 2-4 in practice,훽 compared with the thermodynamic minimum value. It is common practice that extra energy is required,훽 which can be obtained from the burning of an extra amount of methane, Δ . In that case the carbon atom e ciency of the depicted reaction in Equation 2-4 equals (3/8 + ) / (3/8 + + Δ ). In Chapter 6, the concept of carbon atom consumption훽 has been applied for the comparative assessment of PA-6 manufacturing routes. 훽 훽 훽
Equation 2-3 is an example of a ‘Balancing Equation’. e concept of a balancing equation can be expanded further, even to entire chemical processes, and related to energy pro les and carbon footprints. is is illustrated in the next sections.
Balancing Equation A balancing equation is the simplest chemical reaction equation which:
• only involves de ned raw materials as the reactants;
• yields the reaction products, including all coproducts;
• assumes perfect chemical transformations (‘zero waste’); and
• balances all atoms and charges of all substances involved.
A balancing equation like Equation 2-3 is hypothetic: the actual pathways of molecules can be enormously complex, involving many steps. However, the balancing equation expresses a molecular relationship between the starting materials and the nal product. Because thermodynamic quantities such as enthalpy and free energy are path independent, a balancing equation can be used to characterize the nature of the net
15 is calculated with the reaction enthalpy of the reaction depicted in Equation 2-3 (+48.70 kJ/mol) and the reaction enthalpy of the combustion of additional methane: CH4 + 2 O2 CO2 + 2 H2O (-890.32 kJ/mol).훽 Both reaction heats have to be numerically equalized by tuning (= 0.055). 훽 훽 훽 훽 훽 43 Chapter 2 thermodynamic pathway between the reactants (raw materials) and product: e.g. energetically downhill (exothermic), or uphill (endothermic). If converted to 1 kg PA-6 the balancing equation also sheds light on the minimum amounts of required raw materials.
e fact that PA-6 is a polymer of unde ned molecular composition is a complication. However, instead of polymer we take 1 kg of its repeating unit as the product, i.e.
-C6H11NO-. Enthalpy and free energy of this polymer moiety can be readily estimated from heats of combustion and free energies of the polymer, and this is legitimated by the well-known additivity of free energies of molecular moieties especially in homologous series (see Appendix E). Moreover, it is noted that the (small) error involved (1/n water molecules per polymer chain of n caprolactam moieties) is equal for all ve manufacturing routes and cancels out by route comparison.
We will use this principle as a basic reference frame to evaluate the dierent routes.
Without mentioning the various molecules and the -C6H11NO- moiety, the balancing equation of system boundary 3 can be represented as:
∑ BBBj PA-6 + co-products Equation 2-5 ∑ steps
e ve PA-6 routes in system boundary 3 can be represented by ve balancing equations in this way, all having the polyamide-6 product in common.
e real molecular pathway of processes is an interplay between heating, cooling and work related to several steps. e free energy of each of the many steps within Equation 2-5 needs to be <0 in order for overall-reaction of Equation 2-5 to proceed thermodynamically spontaneously. However in practice this is not always the case and extra energy, thus extra BBBj is applied. is equally holds for chemical reactions in industrial plants and in-vivo molecular transformations in microbes. e extra required amounts of heat and work re ect the path-dependency of these quantities and gives rise to excess material- and energy consumptions compared with the minimum values in Equation 2-5:
∑ BBBj + ∑ BBBj, excess PA-6 + co-products + waste Equation 2-6 ∑ steps
Note that any excess amount of BBBj is transformed into waste (or coproducts, in very speci c cases). However, thermodynamic properties still apply to the BBBj-part and as a consequence, contribute to Equation 2-6. Modeled values of Equation 2-6, and the contrast between these energy- and mass consumptions and the base cases de ned by
44 Methodological characterization of PA-6 manufacturing routes balancing equations (Equation 2-5) will be used for comparison purposes of the ve PA-6 processes.
Although both microbes and industrial processes can be described by Equation 2-6, man and nature follow dierent strategies16. Human large-volume chemical technology seeks to combine the (economically) most optimal steps in Equation 2-6, which includes the most optimal ways to bring the involved reactants together and to separate the products, while simultaneously providing the required energy to enable chemical reaction and physical processes via work or controlled supply of heat. In microbes, reaction is very small-volume and speci cally steered and catalyzed at the molecular 2 level (‘customized manufacturing’). Energy is provided by energy-carrying molecules like NAD(P)H or ATP, which release their energy in concurrent, stoichiometric, energy-consuming molecular transformations, aer which spent energy carriers are regenerated in biochemical cycles in which fuel (glucose) in the cell is spent. In human technology, energy (electricity, or steam) is provided in a non-concurrent non-stoichiometric way by large-volume reaction of fossil fuel with oxygen in power plants, aer which energy is transported (electricity, steam) to enable reactions and physical processes in a chemical plant. In fermentation plants, man strives to harness the production of chemicals by microbes, more in particular by replacing key steps of classical routes from NRMs to products by microbial processing. e PA-6 -routes in this study have been especially selected to enable quantitative comparison of these dierent “strategies to e ciency” in chemical- and fermentation technology.
Energy Balance e energy balance in system boundary 3 comprises externally provided ‘outside- in’ -energy, the internal energy processes due to chemical reaction, and ‘inside-out’ streams of e.g. lower value heat. e balance has been evaluated as their net sum.
16 Note the fundamental dierence between processes in a chemical plant and in a microbe. In a microbe, molecular transformations are stoichiometric coupled on molecular level: by highly speci c enzymes and energy carries. Reactions in a chemical plant occur in much less controlled and thus more diverse molecular environments of the reactants: in general, less selective catalysts, while energy is supplied by convection and conduction of heat. A weakness of even well-developed chemical technology is wasted materials and heat (entropy increase). Well-developed microbial technology shows growth of microbial mass as a by-product and a challenge to isolate dispersed products.
45 Chapter 2
Outside-In
e input consists (next to BBBs) of cooling water and externally generated energy carriers:
• Cooling water at ambient conditions, 25 °C and 1 atmosphere.
• Electricity, e.g. used for mechanical work (transport, mixing) and heating (Dowtherm oil) 17,18 and deep cooling17.
• Steam (known P,T), used for heating, e.g. to reach reaction temperature or for product separations and puri cations.
Inside-Out
Products, coproducts and waste leave system boundary 3 at known amounts (per kg PA), temperature and pressure. Examples of energy carriers are:
• Steam condensate (before reuse in system boundary 1); degenerated steam.
• Cooling water raised at temperatures higher than 25 °C.
• Steam generated within system boundary 3.
Intern
Chemical reactions within system boundary 3 produce or consume heat. e enthalpy of the overall-reaction between raw materials and products can be calculated from the balanced equation (Equation 2-5) and standard enthalpies of formation of products and raw materials. Side reactions, which are other sources of energy-consumption or liberation in practice, can be accounted for as well, by inclusion in process models. Exothermic reaction heat and the amount of heat in products, co-products and waste aer each reaction step relative to 25 °C if unused (i.e. cooled away) are considered waste heat, unless process designs allow for other purposeful applications at lower temperatures, e.g. preheating. In this study the (potential) use of remaining or wasted heat is included explicitly.
Recycling of Energy carriers Dowtherm oil and cooling media are recycled and regenerated (heated/cooled) within system boundary 1. Steam in- and outlets in system boundary 3 (and also system
17 Electricity used within system boundary 3, e.g. to heat Dowtherm oil or to cool cooling medium, ends up in waste heat nally. 18 ermally stable synthetic organic heat transfer uid (produced by DOW Chemical), designed for high temperature heat transfer applications up to 400°C.
46 Methodological characterization of PA-6 manufacturing routes boundary 2) are considered parts of a closed system with a boiler in system boundary 1 which reuses residual steam and condensed water aer use in system boundary 319.
Excess enthalpy as expression for energy content We express the heat content of imported processing steam and of exported waste heat (steam condensate, generated steam and cooling water at elevated temperature) as excess enthalpy. Excess enthalpy is de ned as the enthalpy dierence relative to the 0 standard enthalpy of formation of water (ΔfH water= -285.68 kJ/mol). e ve studied PA-6 manufacturing routes are described in Chapters 3 to 5. e 2 carbon use of the dierent routes is discussed and compared in Chapter 6.
2.6 Efficiency of entire process routes (system boundaries 1, 2, 3) e entire, dierent PA-6 processes will be compared by dierent standards such as total energy consumption, total material consumption, the carbon footprint, the renewable fraction of consumed energy and materials, and the improvement potentials of the processes. To achieve this, the concepts described in the previous section applicable to system boundary 3 can be expanded to entire processes in system boundaries 2 and 1, now starting from NRMs, but this is not always straightforward. For that purpose we need to expand Equation 2-5. In addition, the concepts of primary energy consumption and carbon use are applied to evaluate the impact of substitution of fossil energy and materials and fossil processes or process steps on the environment. e principles are explained in this section.
Similar to Equation 2-5, the overall-reaction of each of the possible PA-6 routes j in Figure 1-2 can be represented by the following overall scheme:
∑ NRMj PA-6 + ∑ co-productsj + ∑ wastej Equation 2-7 ∑ steps
Or, alternatively, in two steps representing system boundary 2 and 3:
∑ NRMj ∑ BBBj PA-6 + ∑co-productsj + ∑wastej Equation 2-8 ∑ steps ∑ steps
19 Allowing all residual heat of used steam to dissipate to the environment is an uncommon industrial practice and could lead to unrealistic image distortion. e remaining heat content of return streams to system boundary 1 is therefore accounted for by speci ed credits on the enthalpy balances of the system boundary 3 and 1.
47 Chapter 2
e entire routes also show dierent practical and theoretical aspects. Practical aspects are the observed or modeled raw material- and energy consumption from which improvement options can be identi ed. eoretical aspects are, again, the enthalpy- and free energy dierences between NRMs, PA-6 and co-products, the minimum NRM consumption assuming perfect chemical transformations (‘zero waste’), and the path-dependent energy (and materials) consumption going from raw materials to products. e thermodynamics start from the dierent initial states of each route
(de ned by ∑ NRMj ) and end with PA-6 as a shared nal state, the BBBs in between acting as an intermediate platform. e paths between NRMj and PA-6, or alternatively between NRMj, BBBj, and PA-6, de ne an overall ‘thermodynamic pro le’ between raw materials and nal state that also can be net uphill or net downhill depending on the nature of the NRM started from. is is comparable with the similar approach in system boundary 3 regarding BBBs in Section 2.5. However, we now face the problem that not all NRM are of a de ned molecular nature, e.g. coal, oil, naphtha, or sugar beets. Consequently, the corresponding thermodynamic state quantities cannot be evaluated, which hampers the straightforward application of Equation 2.7. However, there are ways to overcome this which will be explained below.
Balancing Equation of entire routes First we expand the concept of balancing equations in Equation 2-8 to also include the synthesis of BBBs from NRMs in system boundary 2 as follows:
∑NRMj + ∑NRMj, excess ∑ BBBj +∑ BBBj, excess PA-6 + ∑co-productsj + ∑wastej
Equation 2-9
Here, ‘excess’ pertains to the additional amounts of NRM to produce the additional amounts of BBB, required to produce 1 kg PA-6. If the excess-quantities are zero, Equation 2-9 would represent the balancing equation of the entire PA-6 route. In that case, Equation 2-9 can be used similarly to Equation 2-8 to establish the practically observed or modeled energy- and material balance from available industrial data.
NRMs do not necessarily represent discrete molecular species, e.g. oil or biomass.
Fortunately there are ways to overcome this. Crude oil can be expressed in –CH2– moieties. is means that e.g. benzene equals six –CH2– moieties (maximum error will be ((6x14) – 78)/78= 0.08 and 1,3-butadiene equals four –CH2– moieties (maximum error will be ((4x14) – 54)/54= 0.04. Additionally, we have chosen sugar beet (see Chapter 5) as bio-NRM source (57). Sugar beet consists of (mainly) water and of approx. 25% renewable carbon-containing compounds such as glucose, fructose, lactose. We have de ned molecular glucose as the biobased feedstock. Estimated values of the integral processing energy to prepare glucose from sugar beets (6.55 MJ per kg
48 Methodological characterization of PA-6 manufacturing routes glucose) are known from literature (66). We allocate molecular glucose C-atoms on a 1-to-1 basis to bio-NRM and additionally provide the required (fossil) processing energy of beets in Equation 2-9.
2.7 Energy analysis We apply the principle of exergy to determine the energy e ciency of sub-steps and the total production route from BBBs to PA-6. In addition, we de ne ‘Primary Energy Demand’ to account for the primary energy used in the processing of BBBs from NRMs. 2
Exergy analysis e energy e ciency of manufacturing routes can be expressed as exergy loss (ΔEx) Exergy is equal to the maximum amount of work obtainable when the stream of substance is brought from its initial state to the environmental state, de ned by P0 and
T0, by physical processes involving only thermal interactions with the environment.
Exergy is zero at environmental conditions T0 (298.15 K) and P0 (0.1 MPa). Lost work between two initial states is dependent on the speci c process to accomplish a certain goal and varies with the way the process is performed (irreversibility) (67). e mathematical expression for the loss of exergy (work) between the starting initial state (1) and the nishing initial state (2) of a manufacturing route is:
ΔEx= (h – h ) – (h – h ) – T [(s – s ) – (s – s ) ] 0 1 0 2 0 0 1 0 2 Equation 2-10 where h : enthalpy of processing stream at P, T [ in kW]
h0 : enthalpy of processing stream at P0, T0 [in kW] s : entropy of processing stream at P, T [in kW]
s0 : entropy of processing stream at P0, T0 [in kW] : temperature at environmental conditions [298.15 K] T0 Subscript 1: initial state of the incoming stream
Subscript 2: initial state of the outgoing stream
Primary energy demand Primary Energy Demand (PED) is de ned as the primary energy required to process natural raw materials to BBBs and to generate the required steam and electricity
49 Chapter 2 to produce polyamide-6 from BBBs. Publicly accessible industrial databases oer dierentiated PED information for BBBs and heat (steam) and electricity. e PED concept is further explained in Appendix A, including the database references.
We start the exploration and comparison of the polyamide-6 manufacturing routes with the assumption of natural gas as the only primary energy carrier. We also explore the impact of reuse of waste heat and investigate the impact of (partly) replacing fossil based energy by renewable alternatives. In particular, the eect of the use of sustainable steam and electricity (generated as solar/wind/hydropower) on fossil carbon dioxide emission has been analyzed. e impact of such replacement for the dierent processes, including opportunities and practical constraints, are demonstrated.
2.8 Carbon dioxide emissions Fossil based energy from primary energy carriers like coal, oil, or natural gas emit fossil CO2 which contributes to climate change. e quantity of CO2 emission of these energy carriers strongly depends on the kind of fossil material used. Biobased CO2 emission (energy) does not contribute to global warming (except the relatively small part of fossil CO2 emission cause by e.g. transport of biomass to the energy generation plant). e production of sustainable energy (e.g. solar, wind) does not emit CO2 in principle.
Carbon Footprint of a product or process is de ned as the sum of greenhouse gas emissions and removals in a system, expressed as CO2 equivalents (ISO14067 (68)).
is implies that not only CO2 is counted, but also other process own emissions which contribute to the global warming potential, e.g. nitrous oxide. Furthermore, it is assumed that carbon containing process waste streams are incinerated yielding CO2, unless stated otherwise e.g. in case of recovery of valuable coproducts. e energy may be reused in manufacturing, reducing total energy consumption, if not, reduced energy is considered lost and the CO2 nevertheless emitted. In this thesis we will solely consider CO2 emissions. CO2 emission represents an easy to understand product parameter, which nds its origin in its entire manufacturing process ‘cradle to PA-6 plant gate’. It can be evaluated straightforwardly if the origins of carbon for fuel and material feedstock are known and monitored properly.
Sources of fossil CO2 emission are the generation of energy from primary fossil resources and CO2 emissions due to incineration of fossil carbon containing process waste. If a mix of fossil and renewable energy and feedstock is used, the evaluation of CO2 emission requires detailed carbon accounting. CO2 emission in PA-6 manufacturing is dominated by the contribution of the energy generation plant where
50 Methodological characterization of PA-6 manufacturing routes steam and electricity are produced from (partly) fossil primary energy carriers and subsequently applied in the manufacturing (see Chapter 6). e contribution of the generation of these secondary energy carriers to CO2 emission strongly depends on the kind of fossil material used.
If two entirely fossil-based but dierent processes are compared, e.g. the fossil Benzene-Raschig route and the fossil Butadiene route, their fossil carbon dioxide emission obviously may dier. is can be caused by dierent carbon atom e ciencies of the two processes. e two processes may have dierent (intrinsically) minimum carbon loads, e.g. because of more e cient pathways between NRMs and PA-6 in one 2 process compared with the other. It is interesting to assess these type of dierences in processes. e concept of extended carbon atom e ciency, as previously explained, facilitates such a comparison.
Carbon atom e ciencies of PA-6 routes can be evaluated from practical performance in manufacturing, where imperfect reactions and energy-consuming separations occur. e results can be analysed in terms of energy- and material e ciency from a theoretic perspective. e corresponding carbon loads of these processes represent the lowest possible values of the processes. Any upward deviation is caused by inevitable imperfections in processing. e dierence between this lowest value and the actual carbon load represents the maximum possible theoretical improvement potential.
By monitoring the e ciency of the use of carbon in the material balance and as an energy carrier, and exploring the degrees of freedom to replace fossil carbon by non-fossil carbon, and fossil energy by renewable energy, insights can be obtained on strategies to maximize the energy e ciency and minimize CO2 emissions (see Chapter 6).
51
Polyamide-6 producti on starti ng from benzene and ammonia 3 Chapter 3
e Benzene-Raschig route to produce polyamide-6 is subject of this chapter. e-Caprolactam is produced from benzene and ammonia and subsequently PA-6 is formed by ring-opening polymerization of e-caprolactam. e process is widely used and representative for a classical chemical process: fossil energy and feedstock, large production volume, long development history, optimized operating e ciency, internal recycling loops, energy demanding separations, massive use of cooling water and heating media, as well as emissions to air and surface water. e objective is to obtain a process description and a simulation model for this route and a derived mass and energy balance. e mass and energy balance results are summarized in Section 3.2 and are obtained with Aspen 7.3® simulation of the discussed models in Section 3.1.
Computer aided simulation models of the complete polyamide-6 manufacturing route and the distinct production steps to produce PA-6 from benzene and ammonia (Figure 3-2) are rarely described in literature, particularly for commercial scale production sizes (>200 kton/annum). Zhao et al. (69) describe the simulation and control of a two- step nylon-6 continuous polymerization process with an annual production capacity of 20 kton. A model was set up with Aspen Polymer Plus® and applied to evaluate the sensibility of operational parameters, such as the water and acetic acid content in feed, temperature distribution and pressure of each VK tube20 in the polymerization process. Funai et al. (70)(71) describe the estimation of kinetic parameters and mathematical model validation for the PA-6 process. is work presents the simulation of the hydrolytic polymerization process of PA-6 in a lab-scale semi-batch reactor, using e-caprolactam as monomer and acetic acid as mono-functional acid chain terminator. A computer simulation model was set up with Aspen Polymer Plus® to validate experimental kinetic data.
e descriptions of the various production steps of the current design are based on publicly available literature and challenged with DSM21 experts in the eld of e-caprolactam and PA-6 process design and -production. e conceptual process design and subsequent Aspen 7.3® modelling are discussed in Section 3.1. e carbon containing process stream includes hydrogenation of benzene to cyclohexane, oxidation of cyclohexane to cyclohexanone and cyclohexanol, and dehydrogenation of cyclohexanol to cyclohexanone. e nitrogen containing stream starts with ammonia and ends with hydroxylamine sulphate. e carbon and nitrogen streams are brought together in the cyclohexanone oximation step to produce cyclohexanone oxime, which is subsequently molecularly rearranged to e-caprolactam. Ammonium sulphate is produced as co-product. e-Caprolactam is isolated from ammonium sulphate in the recovery/puri cation section by means of extraction with benzene and subsequently polymerized to polyamide-6. Figure 3-1 illustrates the overall reaction scheme starting from benzene and ammonia. 20 Vereinfacht Kontinuierlich (VK): vertical continuous tube reactor column. 21 Royal DSM is currently a global purpose-led, science-based company in Nutrition, Health and Sustainable Living. DSM also produced chemicals like e-caprolactam until outsourcing this business in 2015.
54 Polyamide-6 production starting from benzene and ammonia
e thermodynamic equation of state, applied in the Aspen models, is RK-NRTL. ese equations of state are commonly used in thermodynamic modelling/simulation of industrial processes.
Redlich–Kwong (RK) equation of state is an empirical, algebraic equation that relates temperature, pressure, and gas volume. It is generally more accurate than the van der Waals equation and the ideal gas equation at temperatures above critical temperatures. e non-random two-liquid model (NRTL) is an activity coe cient model that correlates the activity coe cients of a compound with its mole fractions in the liquid phase concerned. It is frequently applied to calculate phase equilibria. e concept of NRTL is based on the hypothesis of Wilson that the local concentration around a molecule is dierent from the bulk concentration. is dierence is due to a dierence between the interaction energy of the central molecule with the molecules of its own kind and that with the molecules of the other kind. e energy dierence also introduces non-randomness at the local molecular level. e NRTL model belongs to 3 the so-called local-composition models. (72)
55 Chapter 3
Figure 3-1 Overall reaction scheme of polyamide-6 formation from benzene and ammonia.
56 Polyamide-6 production starting from benzene and ammonia
3.1 Process design of polyamide-6 production starting from benzene and ammonia Figure 3-2 depicts the overall process scheme of PA-6 manufacturing starting from benzene and ammonia. Detailed process design descriptions of the distinguished processing blocks will be presented below.
3
Figure 3-2 Overall process scheme of polyamide-6 manufacturing from benzene and ammonia.
57 Chapter 3
Benzene hydrogenation Benzene hydrogenation to cyclohexane is accomplished by reaction of hydrogen and benzene at elevated temperature and pressure (468 K; 3.3 MPa). e reaction is strongly exothermic (approx. 210 kJ/mol benzene) and the conversion and selectivity reaches almost 100%. It is critical to control the reaction temperature, because of the formation of undesired by-products such as methylcyclopentane that occurs at higher temperatures. Hydrogenation is carried out at the lowest possible temperature. Nickel on silica is most commonly used as catalyst. e hydrogenation is a vapour phase process based on patent GB1214958 (73) and journal publications (74) (75).
e applied Aspen 7.3® simulation model of benzene hydrogenation is shown in Figure 3-3. e de ned design criteria, assumptions and conditions of the main equipment of the present simulation scheme are given in Appendix B.
Figure 3-3 Aspen 7.3® simulation model of benzene hydrogenation.
58 Polyamide-6 production starting from benzene and ammonia
Benzene at ambient conditions (25 mton/h) is pumped (with P1 in Figure 3-3) into the process at 3.38 MPa and 300 K. e stream is mixed with the recycle hydrogen feed (containing 28 mol/mol% hydrogen) and the mixture is subsequently heated to 393 K by means of heat transfer in counter-current ow with the hot reaction mixture of the second hydrogenation reactor (in HEx1). e evaporated reaction mixture ows to the main hydrogenation reactor (R1), where 90 mol/mol% of the benzene is hydrogenated to cyclohexane. e tubular reactor is lled with nickel catalyst and operates at 3.31 MPa and 468 K. e remaining heat duty in the reactor section is eliminated with cooling water.
A small part of the reaction mixture in R1 (2.5 w/w%) is directed to the purge gas chiller (Chil1) where the majority of methane (99 w/w%; 1.5 mton/h) is purged into the environment together with very small amounts of benzene, hydrogen and cyclohexane (40 kg/h). e feed of the purge gas chiller is boosted with gas blower CP2 (DP= 35 3 kPa) and subsequently cooled with cooling water in economizer HEx10 to 323 K. e purge gas chiller (Chil1) operates at 258 K and 3.27 MPa. e cooling medium is deep cooled methanol.
e main stream of R1 is mixed with fresh hydrogen and with the liquid phase of the purge gas chiller (Chil1) and fed to the nishing hydrogenation reactor (R2) via a pre-heater (HEx2). e industrial make-up hydrogen (1292 m3/h) contains 90 mol/ mol% hydrogen and 10 mol/mol% methane and is compressed (with CP1) from 2.1 to 3.1 MPa. e outlet temperature of pre-heater HEx2 is 468 K which is achieved by condensing superheated steam of 2.8 MPa. e reaction conditions of R2 are 3.1 MPa and 488 K. R2 contains also nickel catalyst. Benzene hydrogenation is completed in this second reactor. e generated heat duty in the reactor section is eliminated with cooling water.
e reactor mixture is cooled in counter current ow with the feed stream of the main hydrogenation reactor (in HEx1). e reaction mixture is further cooled with cooling water to 311 K (in HEx3) before entering the ash vessel (Flash1). Flash conditions are 2.17 MPa and 311 K. e gas stream is recycled as hydrogen feed to the main reactor (via recycle compressor CP3, DP= 1.20 MPa) and the liquid phase ows to the puri cation section. e required heat duty for ashing is supplied by condensing 0.3 MPa steam.
e crude cyclohexane – containing dissolved hydrogen and methane - is puri ed in the next ash column (Flash2). e feed of this column is preheated in counter current ow with the nal product cyclohexane to 393 K in heat exchanger HEx4. 10% of the cyclohexane product stream is recycled in the lower part of the ash column (Flash2) via pump P2 (DP= 0.69 MPa) and the reboiler of the column (HEx6). e outlet temperature of the reboiler is 408 K, which is achieved by condensing superheated
59 Chapter 3 steam of 2.8 MPa. e required heat duty for the puri cation column is supplied by condensing 0.3 MPa steam.
e top of the ash column is equipped with a re ux section, containing two condensers (HEx7 and HEx8) and re ux vessel Flash3. Both condensers are cooled with cooling water. Gaseous hydrogen, methane and some cyclohexane leave the re ux vessel and hydrogen and methane are vented to the atmosphere via a vent gas chiller (Chil2). Prior to chilling, the gas stream is compressed (with CP4) to 0.65 MPa (DP= 0.3 MPa) and cooled to 323 K with cooling water (in HEx9). e vent gas chiller operates at 258 K and 0.65 MPa. e cooling liquid is methanol. e liquid phase stream consists mainly of cyclohexane (160 kg/h) which is returned to the top of ash column Flash2. One per cent of this stream is continuously drained as fuel feed.
e main product cyclohexane is cooled in counter current ow with the feed of the puri cation column in heat exchanger HEx4 and subsequently in product cooler HEx5 to 311 K and pumped to the next process section: the cyclohexane oxidation. e remaining heat of the last cooler is eliminated with cooling water.
Cyclohexane oxidation Air oxidation of cyclohexane (CHX) to KA-oil (Keton-Alcohol-oil) was developed by Dupont in the 1940s. It is carried out in three mixed reactors in series using cobalt naphtenate as catalyst. Catalyst concentration in the reaction mixture is typically 1 – 5 ppm (76).
CHX is oxidized in liquid phase by oxygen in air to produce a dilute mixture of oxidation products in unconverted cyclohexane. e intermediate product is cyclohexyl hydroperoxide (CHHP). It is assumed that the conversion of CHX in the oxidation section is 1.5 w/w% per reactor with 80% selectivity towards CHHP. Representative manufacturing results range from 90% selectivity at one percent conversion per pass to about 75% selectivity at ve to six percent conversion per pass (77)(78). CHHP reacts in a subsequent step to form cyclohexanol (ANOL) and/or cyclohexanone (ANON). Part of the CHHP converts to by-products, like esters of ANOL and various mono- and dibasic acids. Organic acids with less than 6 C-atoms are the main by-product. In this study, to avoid modelling of many species of (very) low-concentrations, an average composition of these by-products is assumed: propionic acid (C3AC)22.
e applied Aspen 7.3® simulation model of cyclohexane oxidation is shown in Figure 3-4. e de ned design criteria, assumptions and conditions of the main equipment, depicted in Figure 3-4, are given in Appendix B.
22 Based on the practical experience of the author and the assumption is discussed and veri ed with various DSM experts in the eld of cyclohexane oxidation.
60 Polyamide-6 production starting from benzene and ammonia
3
Figure 3-4 Aspen 7.3® simulation model of cyclohexane oxidation.
61 Chapter 3
Cyclohexane enters the rst cyclohexane oxidation reactor (R1) at 311 K and 1.03 MPa. Air is compressed (with compressor CP1) at the same pressure and 398 K and equally distributed to the three oxidation reactors. e generated air compression heat is eliminated with cooling water in HEx1. e reaction conditions of all oxidation reactors (R1, R2, R3) are 418 K and 1.03 MPa. e reaction equations are depicted in Figure 3-5 and the corresponding fractional conversions in Table 3-1.
Figure 3-5 Reactions of cyclohexane oxidation.
Table 3-1 Fractional conversions of cyclohexane oxidation(79) .
Component Fractional conversion [%]* CHX + 2 O2 2C3AC 0.3 (CHX) CHX + O2 CHHP 1.2 (CHX) CHHP + CHX 2 ANOL 22.5 (CHHP) CHHP ANON + H2O 30.0 (CHHP) * Fractional conversion of reactant as mentioned between brackets.
62 Polyamide-6 production starting from benzene and ammonia
e liquid phase product stream of each reactor is the feed for the next reactor. e vapour outlet of each oxidation reactor consists mainly of cyclohexane, oxygen and inert nitrogen and small amounts of other reaction products. e required heat for operating at reaction conditions is supplied by condensing 2.8 MPa steam. e reaction mixture enters the subsequent peroxide conversion reactor (R4), where rest CHHP is similarly converted to ANOL and ANON. Unreacted cyclohexane is recovered and reused. e gaseous reactor streams are separated in Sep1, Sep2 and Sep3 and condensed in HEx8. e condensation heat is eliminated with cooling water. Subsequently, water is separated from organic material (mainly cyclohexane, approx. 295 ton/h) in Sep7. e organic phase is recycled to R2. Water (215 kg/h) is drained. e separation heat of Sep7 is supplied by cooling water. e remaining gases of this separation are further deep cooled (in HEx9) and ashed (in Sep8). Nitrogen, oxygen, water and part of the cyclohexane (approx. 780 kg/h) is vented via an incinerator in the atmosphere and the deep cooled cyclohexane stream (approx. 12 mton/h) is fed 3 back to R1. Deep cooling of HEx9 is performed with methanol. Flashing heat (Sep8) is supplied by water of 293 K.
e peroxide converter (R4) converts rest CHHP to ANOL and ANON. e generated heat duty is eliminated with cooling water. e reaction equations are depicted in Figure 3-6 and Table 3-2 lists the corresponding fractional conversions.
Figure 3-6 Reactions of peroxide conversion.
Table 3-2 Fractional conversions of peroxide conversion(79) .
Component Fractional conversion [%]*
CHHP + CHX 2 ANOL 43.0 (CHHP) CHHP ANON + H2O 57.0 (CHHP) * Fractional conversion of reactant as mentioned between brackets.
63 Chapter 3
e reaction mixture is diluted with water (P1, 15.4 mton water/h) and the diluted mixture is cooled at 333 K (in HEx2) and degassed (in Sep4). Cooling is performed with cooling water. Degassing heat is supplied by condensing 0.3 MPa steam. e remaining liquid phase is separated in an organic and water phase in a separator tank (Sep5). e separation heat duty is eliminated with cooling water.
e organic phase is puri ed in the cyclohexane recovery column (Dist2). e water phase stream is stripped in steam stripper (Dist1) with 6.85 mton/h superheated 0.3 MPa steam (423 K). e bottom stream (5.6 w/w% propionic acid solution in water) is cooled in counter current ow with the feed stream of Dist1 at 347 K (in HEx4) and transported to an acid recovery unit (16.9 mton/h). e gaseous/aqueous top stream of Dist1 (17.2 mton/h) contains dissolved CHX/ANOL/ANON/C3AC (8.6 w/w%). e stream is condensed in HEx3 at 323 K and supplied to the cyclohexane recovery column (Dist2). e condensation heat is eliminated with cooling water. Dist2 is designed as a model to simulate a multistage vapour-liquid fractionation (Radfrac). e condenser of Dist2 is cooled with cooling water and the reboiler of the column is heated with 2.8 MPa condensing steam.
e top gas stream of Dist2 is cyclohexane with approx. 2.6 w/w% water and approx. 125 ppm ANOL/ANON. e gaseous stream is condensed with cooling water at 318 K (in HEx7) and water is subsequently drained in separator drum (Sep6; 8 mton/h). e remaining cyclohexane (approx. 300 mton/h, contaminated with approx. 550 ppm ANOL/ ANON/water) is recycled to R1. e bottom liquid ow of Dist2 is cooled at 323 K with cooling water (in HEx5) and pumped (with P4) to cyclohexane stripper Dist3. e remaining cyclohexane is stripped in Dist3 with 4.85 mton/h superheated 0.3 MPa steam (423 K). e cyclohexane stream is condensed at 323 K (in HEx6) and recycled to Dist2. e condensation heat is eliminated with cooling water. e product stream is pumped (via P5) to the saponi cation section.
e applied Aspen 7.3® simulation model of saponi cation and ANOL/ANON recovery is shown in Figure 3-7. e de ned design criteria, assumptions and conditions of the main equipment, depicted in Figure 3-7, are given in Appendix B.
Figure 3-7 Aspen 7.3® simulation model of saponi cation and ANOL/ANON recovery.
64 Polyamide-6 production starting from benzene and ammonia
Saponi cation takes place at 366 K and 0.1 MPa in a compartmented reactor with agitators (R5) and with both reactants in counter current ow. Potassium hydroxide (50 w/w% aqueous solution) is added (via P6) in a stoichiometric ratio with the modelled C3AC by-products and heated to 366 K in HEx11 with condensing 0.3 MPa steam. e product stream (with C3AC by-products) is cooled to 366 K with cooling water (in HEx10). e organic ANON/ANOL stream and the aqueous salt stream are separated in Sep9. e required separation energy is supplied by condensing 0.3 MPa steam. e aqueous salt stream is drained. e ANON/ANOL containing stream (with 16 w/w% water) is puri ed in Dist4. Dist4 is a vacuum distillation column (5.3 kPa) designed as Radfrac (P7 is the vacuum system of Dist4). e condenser of the distillation column is cooled with cooling water and the reboiler is heated with 0.3 MPa condensing steam. e ANOL rich stream is dehydrogenated in the ANOL dehydrogenation section. e ANON-stream is dehydrated (in Sep10) and further processed in the oximation section. e dehydration heat duty is eliminated with cooling water. 3 Cyclohexanol dehydrogenation e vapour phase design of J.W. Bruce (80) is used to design and simulate the cyclohexanol dehydrogenation. Dehydrogenation is performed in three mixed reactors in series using a copper based catalyst (CuO-content of approx. 20 w/w%). e selectivity to cyclohexanone is >99.5 w/w% and typical conversions of 50% are achieved (BASF copper based catalyst H3-1123).
e applied Aspen 7.3® simulation model of ANOL dehydrogenation is shown in Figure 3-8. e de ned design criteria, assumptions and conditions of the main equipment, depicted in Figure 3-8, are given in Appendix B.
Figure 3-8 Aspen 7.3® simulation model of cyclohexanol dehydrogenation.
ANOL/ANON produced in the cyclohexane oxidation is pressurized and heated to achieve the required reaction conditions. e overall conversion amounts 87.5 mol/ mol%. e incoming feed stream is mixed with the ANOL recycling stream from the ANON recovery column (Dist2). e mixture is heated and evaporated in heat exchanger HEx1 (453 K) in counter current ow with the hot reaction mixture from 23 Brochure BASF Catalyst H3-11.
65 Chapter 3 the third dehydrogenation reactor R3. e gaseous reaction mixture is compressed to the required reaction pressure with compressor CP1 and the compression heat is eliminated with cooling water in the inter-stage cooler of the compressor (HEx2).
In each reactor (R1, R2, R3) cyclohexanol dehydrogenates at 508 K and 0.13 MPa to cyclohexanone with a conversion of 50%. e endothermic reaction heat is supplied by condensing 4.2 MPa steam. e reaction mixture is cooled at 323 K (in counter current ow in HEx1 and in HEx3 with cooling water) before ashing hydrogen. Hydrogen is separated from the liquid in a deep-cooled knock-out-drum (Flash1) at 268 K. e low temperature condition is supported with deep cooled methanol. Hydrogen is ashed and can be reused for other industrial purposes or ared.
e nal puri cation of the cyclohexanone is performed with two distillation columns that are designed as Radfrac. e rst atmospheric distillation column (Dist1) removes residual hydrogen, which is ared. e bottom product of the column is fractionated in a second vacuum distillation column (Dist2, 5.3 kPa) to obtain pure cyclohexanone (which is used for the oximation with hydroxylamine) as top product and a cyclohexanol rich stream as bottom product. P2 is the vacuum system of Dist2. e latter stream is recycled to the dehydrogenation reactor section (feed of HEx1). e condenser of the distillation columns is cooled with cooling water and the reboiler of the columns is heated with 2.8 MPa condensing steam.
Hydroxylamine preparation e production of hydroxylamine sulphate starts with the oxidation of ammonia, producing nitrogen monoxide (NO) and -dioxide (NO2) and nitrogen as by-product.
Subsequently, NO2 partially reacts further to nitric acid (HNO3) and nitrous acid
(HNO2). Nitrous acid is unstable and decomposes into NO and NO2 (79). e applied Aspen 7.3® simulation model of ammonia oxidation and hydroxylamine preparation is shown in Figure 3-9. e de ned design criteria, assumptions and conditions of the main equipment, depicted in Figure 3-9, are given in Appendix B.
66 Polyamide-6 production starting from benzene and ammonia
Figure 3-9 Aspen 7.3® simulation model of ammonia oxidation and hydroxylamine preparation.
Liquid ammonia feed (1.4 MPa) is vaporized (via P1) at 503 K (in HEx3). Heat is supplied by condensation of 4.2 MPa steam. e vaporized ammonia is mixed with 3 preheated, compressed air to establish an (approx. 10 vol%) ammonia mixture, which is burned over a platinum-rhodium gauze at 1189 K and 0.79 MPa. Compressed make-up air (via CP1, 0.79 MPa) is heated to 503 K (in HEx2) by counter current heat exchange with the gaseous reaction product of R1. e compressor inter-cooler HEx1 is cooled with cooling water. e oxidation reaction of ammonia is exothermic and
NO and NO2 are equally produced (mole based). e gaseous reaction product of R1 is rst cooled in counter current cooling with vent gas of the ammonium nitrite preparation section (in HEx4) and subsequently with a separate heat exchanger HEx5 to 448 K. e vent gas leaves HEx4 at 473 K. e second heat exchanger HEx5 produces 4.2 MPa steam. e gaseous reactor stream is further cooled in counter current ow with the ammonia make-up stream (in HEx2) and the vent gas (in HEx6). Table 3-3 summarizes the reactions that take place during ammonia combustion in R1 and cooling in HEx2/HEx4/HEx5/HEx6.
Table 3-3 Reactions and fractional conversions in the ammonia oxidation (R1, R2) (79).
Reaction Fractional conversion [%]* 4 NH3 + 5 O2 4 NO + 6 H2O 95.0 (NH3) 4 NH3 + 3 O2 2 N2 + 6 H2O 5.0 (NH3) 2 NO + O2 2 NO2 50.0 (NO) 2 NO2 + H2O HNO3 + HNO2 3.5 (NO2) 2 HNO2 NO + NO2 + H2O 100.0 (HNO2)
* Fractional conversion of reactant as mentioned between brackets.
67 Chapter 3
During cooling the formed NO2 reacts further to nitric acid and nitrous acid. Nitrous acid is unstable and breaks down into NO and NO2. Actually, the latter two reactions in Table 3-3 occur in the cooling down heat exchangers (HEx2/HEx4/HEx5/HEx6) but for practical reasons these reactions are simulated in a separate reactor (R2) at 0.79 MPa and 448 K. e reaction heat is eliminated with cooling water. Part of the water formed during the ammonia oxidation (6.9 w/w% HNO3 aqueous solution, 323 K, 2.3 mton/h) is condensed (in HEx7) and removed (in Sep1) as weak nitric acid by- product. e condensation heat is eliminated with cooling water. e separation of the aqueous nitric acid solution requires heat which is supplied by 0.3 MPa steam. e gaseous part of the oxidation reactions is cooled at 283 K (in HEx8) before entering the hydroxylamine preparation section. e cooling medium is deep cooled methanol.
e subsequent hydroxylamine production is quite challenging to simulate in Aspen Plus®, because electrolyte models need to be used. e Aspen data base for these kind of models is incomplete with respect to scalar properties and concentration dependent properties, e.g. enthalpy–molality relations. Also Born-type parameters for electrostatics, based upon dierences in the dielectric constant and activity coe cients for the considered solutions are not available and mostly not determined or reported in literature. Ideally, missing data should be determined experimentally. However, such experimentation is di cult and expensive.
Instead, the reaction model used for the simulation of the hydroxylamine preparation process step is simpli ed and considered as depicted in the ‘gross reaction COLD and HOT section’ in Figure 3-10 at 283 K (so called ‘black box’). No intermediates are modelled in this approach and the only components used are ammonia, nitrogen monoxide, nitrogen dioxide, water, sulphur dioxide, hydroxyl amine, sulphuric acid and ammonium sulphate. e fractional conversion of the gross reaction COLD and
HOT section is assumed to be 90% NO2-based (79).
68 Polyamide-6 production starting from benzene and ammonia
3
Figure 3-10 Reactions in the hydroxylamine preparation section.
In reality, and contrary to the black box approach (as depicted in Figure 3-10), hydroxylamine is processed as its ammonium salt24(81). When the hydroxylammonium ion reacts with cyclohexanone (see next process step: cyclohexanone oximation), sulphuric acid is eliminated from the salt and subsequently neutralized with ammonia. is neutralization reaction is already simulated. In this approach hydroxylamine is not considered as a reactive ion but as a molecule. is simpli cation is acceptable for thermodynamic modelling (in general, the dissolution enthalpy is <5% of the standard enthalpy of formation). Additionally, the black box approach is also acceptable with respect to enthalpy and entropy balances (state variables). e assumption of a single uniform temperature (283 K) instead of two (283 K, 388 K) introduces an error of approx. 5%25, which is acceptable for thermodynamic calculations. However, excess energy balances can be in uenced seriously by the choices made. In particular, the method and conditions of heat exchange with the environment (supporting cooling
24 Hydroxylamine itself is unstable, and for that reason salts are used in the production of cyclohexanone oxime. Ammonium salts are widely used because ammonium sulphate is a useful co-product (fertilizer). 25 Calculated in a separate case comparing reality and black box approach (cyclohexane oxidation/ cyclohexanol dehydrogenation).
69 Chapter 3 and heating media) is decisive, e.g. if the temperature of the black box is set at 283 K and cooled with deep cooled methanol. e hydrolysis of hydroxyl amine disulfonic acid ammonia (NOH(SO3NH4)2) is commonly performed at 378 K and mostly cooled with cooling water. Consequently, choosing 283 K will imply high inaccuracy in the exergy balance (approx. 25%26).
e gaseous part of the ammonia oxidation enters the hydroxylamine preparation reactor R3. e hydroxylamine production is simulated in reactor R3 at 283 K and 0.79 MPa and is based on the ‘Gross reaction COLD and HOT section’ as mentioned at the bottom of Figure 3-10. e produced reaction heat is eliminated with deep cooled methanol. e reaction product is degassed (in Sep2) and subsequently produced sulphuric acid is neutralized with ammonia at 398 K in R4. e gaseous Sep2 outlet of (mainly NO, N2 and O2) is heated in HEx6 and HEx4 and ared. e R4 fractional conversion of the neutralization in is 100% (H2SO4). e heating is partially supported by the neutralization heat and partially by supplying steam. e aqueous reaction mixture containing mainly ammonium sulphate and hydroxylamine is further processed in the cyclohexanone oximation section.
Cyclohexanone oximation Inventa patented a 2-stage counter current oximation reaction yielding > 99% oxime and 99.8% utilization of hydroxylamine (82). Cyclohexanone, reacts exothermically 0 with hydroxylamine at 338 K and atmospheric pressure (DrH = -42.6 kJ/mol). e original process consists of two stages, an oximation column with an excess of hydroxylamine and a sulphate stripper with a substantial excess of cyclohexanone. e pH of the oximation column is controlled between 3.5 at the top and 6.5 at the bottom by injection of ammonia. e produced cyclohexanone oxime contains approx. 5 w/w% water (79).
e applied Aspen 7.3® simulation model of ammonia oxidation and hydroxylamine preparation is shown in Figure 3-11. e de ned design criteria, assumptions and conditions of the main equipment, depicted in Figure 3-11, are given in Appendix B.
26 We have studied two cases: the heat excess of the black box is withdrawn from the reactor zone with deep cooled methanol or with cooling water. e dierence in enthalpy and exergy balance for both cases is relatively high (~25%), however, the impact of that dierence on the enthalpy and exergy balance of the complete polyamide-6 production route is only 3 – 6%. erefore we have averaged the results of both cases.
70 Polyamide-6 production starting from benzene and ammonia
Figure 3-11 Aspen 7.3® simulation model of cyclohexanone oximation.
e ANON stream of the ‘cyclohexane oxidation’ and the ANON stream of the ‘cyclohexanol dehydrogenation’ sections are cooled at 338 K (in HEx1). e 3 combined stream is pressurized at atmospheric conditions (with P1) to react with the hydroxylamine stream of the hydroxylamine preparation plant. e cooling requires cooling water. e hydroxylamine stream is cooled at 338 K (in HEx2), degassed (in Sep1) and pumped (with P2) to reactor R1. e cooling is performed with cooling water; degassing requires energy which is supplied by 0.3 MPa condensing steam. e operating temperature and pressure of the oximation reactor R1 are 338 K and 0.1 MPa. For simplicity the oximation process is simulated as a single reactor in contrast with the Inventa 2-stage process. e oximation reaction is exothermic. e exothermic heat is eliminated with cooling water. e product stream is heated at 353 K (in HEx3) and subsequently the organic oxime is separated from the aqueous ammonium sulphate solution in decanter Sep2. e heating requires 0.3 MPa condensing steam. e separation heat is eliminated with cooling water. e cyclohexanone oxime stream is further processed in the Beckmann rearrangement plant (via P3). e aqueous ammonium sulphate solution (173 mton/h, 39 w/w% ammonium sulphate) is transported (via P4) to the ammonium sulphate crystallization plant.
Beckmann rearrangement e Beckmann rearrangement, named aer the German chemist Ernst Otto Beckmann (1853–1923), is an acid-catalysed rearrangement of an oxime to an amide. Commercial e-caprolactam processes are mainly based on the Beckmann rearrangement of cyclohexanone oxime in oleum. e reaction is highly exothermic and for reasons of temperature control intensive external recycling is applied to cool the reaction mixture (83).
e Aspen 7.3® simulation model of the Beckmann rearrangement is shown in Figure 3-12. e de ned design criteria, assumptions and conditions of the main equipment, depicted in Figure 3-12, are given in Appendix B.
71 Chapter 3
Figure 3-12 Aspen 7.3® simulation model of Beckmann rearrangement.
Cyclohexanone oxime from the oximation section is mixed with oleum in R1 rearrangement reactor . Oleum (19 w/w% SO3 in sulphuric acid) is supplied at ambient conditions (via P1). e process conditions of R1 are 358 K and 0.1 MPa. Due to the extreme exothermic behaviour of the rearrangement reaction a large recycling stream is applied (approx. 4 x feed via Sep1, P2). e exothermic reaction heat is eliminated with cooling water (modelled in R1). e yield of the rearrangement reaction to e-caprolactam is 98% (79). Consecutively the acid e-caprolactam solution is neutralized with stoichiometric amounts of ammonia in a separate neutralization reactor R2 to produce ammonium sulphate. Ammonia is supplied at 283 K and 1.4 MPa. Water is added to the R2 euent to solubilize the ammonium sulphate. e process conditions of the neutralization reactor are 358 K and 0.1 MPa. e neutralization reaction is extremely exothermic, and to cool down the reaction mixture a large recycling stream (approx. 4 x feed via Sep2, P3) is applied. e exothermic reaction heat is eliminated with cooling water (modelled in R2). e aqueous e-caprolactam/ ammonium sulphate reaction mixture is cooled (in HEx1) to reduce e-caprolactam hydrolysis. Subsequently, e-caprolactam is recovered in the e-caprolactam recovery and puri cation section. e-Caprolactam recovery e type and amount of impurities present in the produced e-caprolactam depend on the process route, the purity of the raw materials and the method of e-caprolactam recovery and puri cation. Puri cation of e-caprolactam is generally required for nylon-6 production. From literature of puri cation of e-caprolactam it appears that a combination of treatments is desired. Ion exchange is used to reduce the undesirable inorganic impurities in e-caprolactam, preferable a combination of both acid and alkaline ion exchange. (Vacuum) distillation is required to remove organic contaminants from e-caprolactam (84)(85).
e level of possible organic and inorganic contaminants is too low (500 - 1000 ppm) to be modelled in detail. erefore we have simpli ed the e-caprolactam recovery and puri cation and modelled this process step by means of a benzene extraction and
72 Polyamide-6 production starting from benzene and ammonia a single vacuum distillation. In this study, the aqueous e-caprolactam/ammonium sulphate process stream from the Beckmann rearrangement section is extracted with benzene at 323 K and 0.12 MPa. e extraction process conditions are based on patent DT2619234 (86). e benzene extract contains the majority of the produced e-caprolactam (97 w/w%) and organic impurities. e aqueous ammonium sulphate extract contains 0.5–1 w/w% e-caprolactam. Solubility of benzene in water and vice versa is estimated from literature references (87)(88). Subsequently, benzene is recovered by means of vacuum evaporation and reused. e-caprolactam is puri ed by vacuum distillation.
e applied Aspen 7.3® simulation model of e-caprolactam recovery is shown in Figure 3-13. e de ned design criteria, assumptions and conditions of the main equipment, depicted in Figure 3-13, are given in Appendix B. 3
Figure 3-13 Aspen 7.3® simulation model ofe -caprolactam recovery.
First, the e-caprolactam rich stream of the Beckmann rearrangement section (via P1) is extracted with benzene (in extractor Sep1). e extraction heat is eliminated with cooling water. Benzene supply to Sep1 consists of fresh make-up benzene (via P2; 46.8 kg/h) and recovered crude benzene (124.1 mt benzene/h, 24.1 mt ‘organic components’/h). Organic components, like e-caprolactam and cyclohexanone oxime, are mainly dissolve in the benzene phase of the extraction uid. Ammonium sulphate dissolves in the aqueous phase of the extraction uid. Aqueous ammonium sulphate is further processed in a separate ammonium sulphate recovery/crystallization plant. Residual benzene in the organic e-caprolactam solute of Sep1 is recovered in vacuum asher Sep2. P3 is the vacuum system of Sep2. e evaporation heat is supplied by condensing 0.3 MPa steam. Recovered benzene of Sep2 together with recovered benzene of the e-caprolactam distillation Dist1 is recycled aer being pressurized (with CP1) and cooled (in HEx2) with cooling water.
73 Chapter 3
e crude e-caprolactam stream of Sep2 is further puri ed in a vacuum distillation column (Dist1) at 27 kPa. P4 is the vacuum system of Dist1. e column is modelled as Radfrac. e condenser of the distillation column is cooled with cooling water. e reboiler of the column is heated with 4.2 MPa condensing steam. e top stream of Dist1 contains approx. 21 w/w% benzene. e top stream (160 kg/h) is partly purged (via CP2) and the rest is recycled to extractor Sep1. e puri ed e-caprolactam is cooled with cooling water at 363 K (in HEx1) and pumped (via P5) to the polymerization section.
Polym erizati on of ε-caprolactam Nylon-6 production is commonly carried out in a single stage continuous reactor operating at moderate pressure and high temperature. e column is fed with e-caprolactam and a small amount of water to start the polymerization reactions. Polymer with the required molecular weight (in this study approximately 20,000 kg/kmol) leaves the reactor as a melt. e basic chemical reaction for the hydrolytic e-caprolactam polymerization is depicted in Figure 3-14.
Figure 3-14 Polyamide-6 polymerization reactions.
e initial step is the ring opening of e-caprolactam molecules by the addition of approximately 1 w/w% of water. Reaction conditions are 538 K and 0.25 MPa. Upon hydrolysis to 6-aminocaproic acid, which has both a – NH2 and – COOH reactive group, a ‘built in’ stoichiometry of end groups is ensured. Only a minor amount of e-caprolactam is opened by the reaction with water. e second step is the addition of 6-aminocaproic acid or linear nylon oligomer27 on the carboxylic group of e-caprolactam. In this polyaddition reaction no water is formed in contrast to polycondensation. e major part of the e-caprolactam rings is consumed during the polyaddition to amine groups. e nal step is polycondensation where low linear molecular weight nylon chains are coupled to long polymer chains (Mn= 15,000 – 50,000 kg/kmol) with splitting o of water. A disadvantage of the process is the 10%
27 Polymer chain comprising a few monomer units.
74 Polyamide-6 production starting from benzene and ammonia residual monomer and cyclic oligomers in the nal polymer resulting from equilibrium reactions. is necessitates hot water extraction of the nal polymer before use. Extracted monomer can be recycled to the polymerization process aer the oligomers have been removed in a separation section. (89)
Aspen soware and databases cannot be used easily to perform a polymerization simulation. Aspen models to simulate the complex polymerization of e-caprolactam to PA-6 (ring opening, polyaddition, polycondensation, re-amidation) are subject of study at several polyamide (industrial) research institutes (69)(70). Generally, these Aspen models are based on kinetic and thermodynamic properties, see e.g. Reimschuessel and Tai (90)(91). Possible nal models will be complex and if available subject to intellectual property and protected business intelligence. However, the properties of interest in this study are the state variables enthalpy and entropy of the produced polyamide-6 and these variables are route independent. erefore, we calculated the polymerization related energy changes separately and without Aspen soware. e 3 method to calculate polymerization reaction enthalpy and entropy (as well as standard enthalpy/entropy of formation of nylon-6 polymers) is explained in Appendix E. e calculated reaction enthalpy/entropy of polymerization is used in the current Aspen simulation. In this way, we could apply Aspen 7.3® to simulate only process steps such as chip cutting, hot water extraction/drying of the polymer chips and the removal of water and (reaction) heat in the polymerization column.
e applied Aspen 7.3® simulation model of e-caprolactam polymerization is shown in Figure 3-15. e de ned design criteria, assumptions and conditions of the main equipment, depicted in Figure 3-15, are given in Appendix B.
Figure 3-15 Aspen 7.3® simulation model ofe -caprolactam polymerization.
75 Chapter 3
Puri ed e-caprolactam is pressurized to 0.25 MPa (with P1) and mixed with recycled e-caprolactam and water. A small amount of water is added to simulate the e-caprolactam ring opening reaction. e stream is heated to 513 K (with HEx1). Also acid should be added as catalyst, however, this is not implemented in the model. e heating is accomplished with Dowtherm® MX heating uid. As previously explained, we have not simulated the polymerization reactions (as depicted in Figure 3-14) and normally performed in a so-called VK-column. We have only used the separately calculated reaction enthalpy/entropy data in the simulation model. An arti ce separator block (Sep1) is used to ‘remove’ reacting e-caprolactam and water. Addition of water to Dist1 simulates ‘produced’ polycondensation water (at reaction conditions), and together with the unreacted e-caprolactam and impurities from Sep1 supplied to distillation column Dist1 (Radfrac model). Water is eliminated in Dist1. Approx. 97% of the present water evaporates via Dist1, in analogy with observed data in commercial polymerization plants. e calculated polymerization reaction heat is partly attributed to the evaporation of water; the remaining part is eliminated with cooling water.
e polymer leaving the polymerization column as strands still contains about 10 w/w% e-caprolactam (monomer) and some residual water. e residual e-caprolactam has to be extracted with hot water in a wash column to achieve the nal purity of the polymer. However, the polymer strands are rst cut in chips (granules). e chopper system requires 12.0 kJ/kg PA6 of electricity. In the simulation we assume that this energy is completely absorbed by the polymer and has been taken into account for energy calculations. A water cooled heat exchanger is modelled to simulate this (not shown in Figure 3-15).
Water is added to the ‘polymer’ stream to model the washing of the polymer chips. e required wash water is 4.94 mton/h and enters wash column Sep2. Additionally we modelled the required water to eliminate the heat capacity of the produced (hot) polymer: 279 kg/h (also input of Sep2 via HEx2). Wash water leaves Sep2 at 368 K. e bottom stream of Sep2 represents the residual e-caprolactam content of polyamide-6 chips aer water extraction (washing) and drying. e polymer chip temperature aer drying is 313 K and to eliminate the ‘overheated’ bottom stream of Sep2 this stream has to be ‘cooled’ to 313 K (in HEx3). Wash water is subsequently recovered in the wash water recovery (modelled as Sep3) to obtain liquid e-caprolactam (which is recycled to HEx1 via P2) and contaminated super-heated steam which can be reused for outside heating purposes. e recovery unit is heated by condensing 2.6 MPa steam. e nal drying and cooling of polyamide chips from 368 to 313 K is modelled with recycled nitrogen (Tin= 308 K, Tout= 360 K, pressure= 0.3 MPa). Heated nitrogen (in HEx4) is boosted (with a blower CP1, DP= 20 kPa) and cooled (in HEx5) with cooling water before it is reused. e heat duty which has to be withdrawn is determined by the dierences in the speci c heat of the polymer at 368 and 313 K. 76 Polyamide-6 production starting from benzene and ammonia
3.2 Mass and energy balances Aspen simulation results particularly in mass- and energy data of incoming, intermediate and outgoing process streams of the total production process or part of the production process. e analysis of mass data reveals production yields and prospects of yield improvement. Prospects of yield improvement are based on theoretical balancing equations. Aspen energy data are applied for the analysis of energy yields and prospects for energy optimization.
Balancing Equation from BBBs to PA-6 e balancing equation of the Benzene-Raschig route starting with sulphur dioxide as a BBB is:
3 Equation 3-1
–C6H11NO– is the building block of the PA-6 polymer chain. e number of building blocks can vary depending on the nal chain length. We have de ned a polymer length of 180 blocks. If we replace –C6H11NO– in Equation 3-1 by the molecular formula of the polymer we have to multiply the coe cients of the other compounds with 180. We have chosen for the simple reaction comparison, however, by doing so we have neglected part of the end groups of the polymer chain (OH and NH2). e impact is negligible (18/180= 0.1 g/mol at a molecular weight of –C6H11NO– = 113.16 g/mol). Consecutive rearrangement of cyclohexanone oxime with oleum and neutralization of e-caprolactam sulphate salt with ammonia can be interpreted as the introduction of oleum and a stoichiometric amount (with respect to sulphur) of ammonia as auxiliary chemicals, all per mol –CPL– unit28.
Equation 3-2
Here, α is related to the strength of the oleum used in the rearrangement. If the strength is 19 mass-% SO3, then a= 0.287 mol SO3/mol H2SO4. In practice, virtually all oleum and ammonia used in the rearrangement and neutralization is recovered as the reaction product ammonium sulphate.
e two balanced Equations 3-1 and 3-2 can be considered concerted reactions. An alternative formulation of the balanced equation of the Benzene-Raschig process
28 -CPL- stands for e-caprolactam unit, -C6H11NO- in the PA-6 polymer chain.
77 Chapter 3 concept is therefore:
C6H6 + 3 H2 + 2 SO2 + 5 NH3 + 5/2 O2 -[C6H11NO]- + 2 (NH4)2SO4
∑ steps
H2SO4 +aSO3 + α H2O +2(1+α) NH3 (1 + α ) (NH4)2SO4
Equation 3-3
e condensed form of Equation 3-3 taking the auxiliary reaction into account emerges by adding its two reactions.
Equation 3-4
In Equation 3-4, compared with Equation 3-1, the auxiliary reaction contributes to enthalpy and free energy.
Raw material consumption and product and waste production Figure 3-16 summarizes the material balance of the Benzene-Raschig manufacturing route which is based on the present process descriptions and the results of the corresponding Aspen models and simulations. e mass balance includes raw materials used to produce polyamide-6 (corresponding the balancing equation) and other (auxiliary) materials that are used in the process. e theoretical required amount of raw materials for the Benzene-Raschig route is calculated with Equation 3-4. e theoretical and modelling results are compared in Table 3-4, in mol/mol – CPL– and kg/kg –CPL–.
78 Polyamide-6 production starting from benzene and ammonia
Table 3-4 Comparison of theoretical and modelling results of the Benzene-Raschig manufacturing route.
BBB C6H6 H2 NH3 O2 SO2 SO3 H2SO4 H2O -CPL- (NH4)2SO4 Waste Molar mass 78.11 2.016 17.03 32.00 64.07 80.07 98.09 18.01 113.16 132.17 Reactants Products Balancing equation, 1 3 7+2a 2.5 2 a 1 a 1 3+a mol/mol –CPL– Modeled manufacturing route, 1.35 3.93 8.37 6.29 2.13 0.32 1.13 0.44 1.00 3.59 mol/mol –CPL–
Yield [mol/mol %]* 74 76 90 40 94 90 88 65
Balancing equation, kg/ 0.69 0.05 1.14 0.71 1.13 0.20 0.87 0.05 1.00 3.84 0 kg –CPL– Modeled manufacturing route, 0.93 0.07 1.26 1.78 1.21 0.23 0.98 0.07 1.00 4.19 1.34 3 kg/kg –CPL– a= 0.287
*: ratio of mol/mol –CPL– balancing equation and mol/mol –CPL– modelled manufacturing route
e amount of waste (solid, liquid, gas) has been calculated as the dierence between the mass input of BBBs and the mass of products and coproducts. e modelled waste, based on balancing equation, amounts 1.34 kg/kg PA6 (see Table 3-4).
e total amount of emissions (all outgoing streams, except PA-6), as depicted in Figure 3-16, is 149 kg/s ( 19.93 kg/kg PA6) and signi cantly higher as predicted in Table 3-4 (ammonium sulphate 4.19 and waste 1.34 kg/kg PA6, in total 5.53 kg/kg PA6). However, the main part of the incoming streams of Figure 3-16 can be considered as inert with respect to balancing Equation 3-4 and therefore not accounted for. E.g. methane (as part of hydrogen feed), nitrogen (as part of air), oxygen in the SO2-feed and most of the water feed (in total 7.51 kg/kg PA6) do not react.
79 Chapter 3
Figure 3-16 Overall mass balance of polyamide-6 manufacturing from benzene and ammonia.
80 Polyamide-6 production starting from benzene and ammonia
Energy balances Figure 3-17 shows the exergy balance of polyamide-6 manufacturing starting with benzene and ammonia. e overall loss of work (exergy) amounts 12.6 MJ/kg PA6 due to irreversible processing. e main contributors to this loss are the cyclohexane oxidation manufacturing step and the cyclohexanone oximation manufacturing step: 7.9 and 1.8 MJ/kg PA6, respectively.
3
Figure 3-17 Overall exergy balance of polyamide-6 manufacturing from benzene and ammonia.
81 Chapter 3
We consider the Benzene-Raschig route as a black box. Benzene, hydrogen, ammonia, sulphur dioxide and oleum and small amounts of auxiliary chemicals (like KOH) enter the process (as BBBs). Polyamide-6 (product) and process emissions like co- products, not reacted BBBs and by-products (waste) leave the process. All material ows implicitly represent an enthalpy content calculated with the Aspen models and simulation. e dierence between the enthalpy content of incoming and outgoing streams equals the overall enthalpy of reaction ∆rH (main reaction and side reactions).
e result is ∆rH= -19.6 MJ/kg PA6. Additionally, chemical manufacturing routes use electricity and steam to support their irreversible production processes. Excess process heat is emitted as waste heat via heated cooling water (CW), generated steam (GS) and condensate (SC).
Figure 3-18 summarizes the overall secondary energy balance. e incoming and outgoing energy streams are expressed as excess enthalpy ows (in MJ/kg PA6.) e enthalpy content of ‘Steam’ (incoming), ‘CW’, ‘SC’ and ‘GS’ are de ned as the enthalpy dierence relative to the standard heat of formation of liquid water (- 285.68 kJ/mol). Another excess waste heat ow is ‘Heat drain via deep cooling system (HDDCS)’: excess heat is removed from the chemical process at low temperatures with deep cooled methanol and emitted via an external cooling aggregate (‘refrigerator model’).
Figure 3-18 Modelled secondary energy balance of polyamide-6 manufacturing from benzene and ammonia.
Ideally, the secondary energy balance should be zero. However, the absolute discrepancy of 0.4 MJ/kg can be considered as negligible (<1%).
Part of the excess heat can be reused, which has not been taken into account yet. Steam condensate can be reused for steam generation in system boundary 1 (closed circuit supposed as the base case). A credit can be granted that reduces the total excess energy input. e maximum value of the credit would be 15.1 MJ/kg PA6 if this heat could be reused with 100% e ciency. However, there is an e ciency factor 0 < k < 1 related to condensate recycling. Literature values of increased e ciencies of boiler
82 Polyamide-6 production starting from benzene and ammonia houses typically show k-values of 0.1 – 0.329. Here we take arbitrary k= 0.3. Generated steam is used via system boundary 1 (or recycled within system boundary 3, which is almost equivalent). A credit of 14.1 MJ/kg PA6 can be granted for GS. Depending on e ciencies of heat reuse the Net Excess Enthalpy Input ranges from 38.8 (reuse) to 57.4 MJ/kg PA6 (no reuse).
e reported energy balance (Figure 3-18) is expressed as secondary energy. However, primary energy demand has to be used for the environmental impact analysis in Chapter 6. Primary energy is energy embodied in raw fuels as well as other forms of energy received as input to a system. e method to determine the related primary energy is explained in Section 2.7. and Appendix A. Primary energy demand is labelled as PED. e results are summarized in Table 3-5.
Table 3-5 Primary Energy Demand of polyamide-6 manufacturing from benzene and ammonia. 3 Material/energy feedstock PED [MJ/kg PA6] Benzene 27.4 Hydrogen 6.5 Ammonia 45.2 Sulphur dioxide 22.6 Oleum 12.5 Steam 75.2 (47.9)* Electricity 12.5 Total 201.9 (174.6)* *Steam PED-value between brackets refers to the net value aer reuse of waste energy of the PA-6 production starting from BBBs (generated steam and upgraded condensate).
e environmental impact of the Benzene-Raschig route will be discussed and compared with the other PA-6 manufacturing routes in Chapter 6.
29 Condensate contains a signi cant amount of sensible heat that can account for about 10% to 30% (k-factor, heat recovery e ciency) of the initial heat energy contained in the steam. TLV, Introduction to Condensate Recovery, 2017, www.tlv.com.
83
Polyamide-6 producti on starti ng from 1,3-butadiene and hydrogen cyanide 4 Chapter 4
e co-production of ammonium sulphate in commercial caprolactam production and the energy demand for the recovery of unreacted caprolactam in polymerization trigger the search for alternative production routes without the use of e-caprolactam and with a higher polymerization yield. An example of a possible alternative route is the production of polyamide-6 using 1,3-butadiene and hydrogen cyanide. e initial process description was patented by BASF and further examined by Koning and Meuldijk (56)(92). e process consists of three consecutive steps: hydrocyanation of 1,3-butadiene to adiponitrile, hydrogenation of adiponitrile to 6-aminocapronitrile, and polymerization of 6-aminocapronitrile to polyamide-6, see Figure 4-1.
Figure 4-1 Process steps of polyamide-6 formation starting from 1,3-butadiene.
is Butadiene route is the subject of this chapter (93). e objective is to obtain a process description for the route and a derived mass and energy balance. e route is expected to be more energy e cient than the Benzene-Raschig process (55)(56) (92). e material and energy feedstock is still fossil based and the process involves reactions, internal recycling, energy demanding separations, use of cooling water and heating media, and emissions to air and surface water. e complete route is not yet commercialized.
Computer aided simulation models of polyamide-6 manufacturing routes starting from 1,3-butadiene and hydrogen cyanide (Figure 4-2) are rarely described in literature, particularly for commercial scale production sizes (>200 kton/annum). Adiponitrile, the intermediate reactant of the hydrocyanation of 1,3-butadiene (Figure 4-1), is primarily used in the synthesis of 1,6- hexamethylenediamine, a component of nylon-6,6. Nylon-6,6 is another polyamide and can be considered as a competitor of PA-6. BASF, Invista, Asha and Solutia are the most important global producers of nylon-6,6. Zhu et al. (94) present an analysis of the speci c technical aspects, reaction mechanisms and atom e ciencies of di erent processes to produce adiponitrile with respect to nylon-6,6 production.
86 Polyamide-6 production starting from 1,3-butadiene and hydrogen cyanide
Figure 4-1 depicts the process steps used in the formation of PA-6 starting from 1,3-butadiene. Route variants are discussed in Section 4.1. e mass and energy balance results are summarized in Section 4.2 and are obtained with Aspen 7.3® simulation of the discussed models in Section 4.1. e applied thermodynamic equation of state is RK-NRTL.
4.1. Process design of polyamide-6 production starting from 1,3-butadiene and hydrogen cyanide Figure 4-2 depicts the overall process scheme of PA-6 manufacturing starting from 1,3-butadiene and hydrogen cyanide. Detailed process design descriptions of the distinguished processing blocks will be presented below.
4
Figure 4-2 Overall process scheme of polyamide-6 manufacturing from 1,3-butadiene and hydrogen cyanide.
Hydrocyanation of 1,3-butadiene to pentenenitriles Adiponitrile (ADN) is obtained by hydrocyanation of 1,3-butadiene. e DuPont ADN process is the basis of the hydrocyanation of 1,3-butadiene, which consists of two addition reactions. (95)(96) For the rst reaction a nickel (0) phosphite catalyst is used, and for the second substitution a Lewis acid is added as promoter. Lewis acids provide higher reaction rates, improved distributions of linear products, and longer catalyst life time. e molar ratio of Ni(0) and 1,3-butadiene is 1:1000; the molar ratio of phosphite and Ni(0) is 6:1. (97) e promoter ‘triphenyl borane (BPh3)’ (98) is used for the hydrocyanation of 3-pentenenitrile and 4-pentenenitrile. BPh3 gives a higher yield towards ADN compared with other Lewis acids such as ZnCl2 and FeCl2 (97). Also the isomerization of 3-pentenenitrile towards 2-pentenenitrile is slowed
87 Chapter 4
down (2-pentenenitrile poisons the catalyst). BPh3 is added in a range of 0.005 to 50 moles per mole of nickel (99). In the rst addition reaction, 3-pentenenitrile and 2-metyl-3-butenenitrile are formed and subsequently 2-methyl-3-butenenitrile is isomerized to 3-pentenenitrile, see Figure 4-3 (100)(101). During this isomerisation also 4-pentenenitrile is formed as precursor of ADN.
Figure 4-3 Direct hydrocyanation of 1,3-butadiene to adiponitrile.
e applied Aspen 7.3® simulation model of hydrocyanation of 1,3-butadiene via pentenenitriles to adiponitrile is shown in Figure 4-4. e de ned design criteria, assumptions and conditions of the main equipment, depicted in Figure 4-4, are given in Appendix C.
88 Polyamide-6 production starting from 1,3-butadiene and hydrogen cyanide
4
Figure 4-4 Aspen 7.3® simulation model of hydrocyanation of 1,3-butadiene via pentenenitriles to adiponitrile.
89 Chapter 4
Liquid 1,3-butadiene (BD, 17.9 mt/h, 0.5 MPa, 298 K) and hydrogen cyanide (HCN, 8.95 mt/h, 0.25 MPa, 298 K) are pressurized with P1 and P2 at reactor pressure: 1.62 MPa. e streams are mixed with the BD/HCN recycle stream (via CP1) before entering reactor R1. A separate stream contains the make-up nickel catalyst which is also fed to R1; pressure is 0.1 MPa, temperature is 298 K. e third stream to R1 is a recycle stream (via P6/Sep2) containing mainly catalyst and 2-methylglutaronitrile (2MGN). e liquid phase reaction to 3-pentenenitrile (3PN) is performed at 383 K and 1.62 MPa. BD is converted to pentene nitriles and several by-products: vinyl cyclohexene (VCH), 2MGN, ethylsuccinonitrile (ESN) and 2-pentenenitrile (2PN). e molar ratio of HCN and BD is preferably 95:100 to avoid unnecessary recycling of BD and the formation of by-products, such as VCH. Conversion and selectivity of the various reactions of the rst hydrocyanation step are presented in Table 4-1.
e reaction section is simulated with two serial stoichiometric reactor models: R1 simulates the reaction to 3PN, VCH, 2MGN, 2-methyl-3-butenenitrile (2M3BN) and 4-pentenenitrile (4PN), R2 simulates the isomerization of 2M3BN to 3PN. e generated heat duty in the reactors R1 and R2 is eliminated with cooling water. VCH and 2MGN have to be separated from the reaction mixture immediately aer R1/ R2. One of the key operational speci cations for this separation is temperature. e temperature has to be less than 393 K to prevent catalyst degradation (102). e catalyst is reused. e reaction product of R2 (and R1) is ashed in Flash1 at reaction temperature and low vacuum pressure (75 kPa) to separate in light and heavy component streams. P3 is the vacuum system of Flash1.
Table 4-1 Conversion and selectivity details of hydrocyanation of 1,3-butadiene to pentenenitriles (97) (99).
Component Conversion [%] Selectivity30 [%] Yield [%] Hydrocyanation of 1,3-butadiene 1,3-butadiene 88.0 - Hydrogen cyanide 92.6 - 3-pentenenitrile - 41.4 36.7 4-pentenenitrile - 41.4 36.7 2-methyl-3-butenenitrile - 14.5 13.0 Vinyl cyclohexene - 0.9 0.8 2-methylglutaronitrile - 0.9 0.8 Heavies - 0.9 -31 Isomerization of 2-methyl-3-butenenitrile 2-methyl-3-butenenitrile 95.0 - 3-pentenenitrile - 100.0 95.0
e required heat duty for the separation is supplied by condensing 2.6 MPa steam. e light component stream of Flash1 contains most of the BD and HCN as also 30 Selectivity based on the conversion of 1,3-butadiene (hydrocyanation of 1,3-butadiene) and 2M3BN (isomerization of 2M3BN), respectively. 31 Heavies are not modelled in Aspen and therefore the selectivity is equally divided between the other components.
90 Polyamide-6 production starting from 1,3-butadiene and hydrogen cyanide pentenenitriles. e light component stream– together with another gaseous BD/ HCN containing stream (from Dist1) – is recycled via Sep1 and compressor CP1 to R1. A very small amount of gas is drained (via Sep1) before compressing to avoid component built-up in the recycle stream.
e heavy component stream of Flash1 contains most of the pentenenitriles and is fed to two serial vacuum distillation columns (Dist1/Dist2). Dist1 and Dist2 are operated under vacuum (respectively 10 and 1 kPa) to prevent the dissolved catalyst for degradation. e distillation columns are designed as Radfrac. e condensers of Dist1 and Dist2 are cooled with cooling water. e reboiler of Dist1 is heated with 4.2 MPa pressure condensing steam; Dist2 is heated with 2.6 MPa pressure condensing steam. P4 and P5 are the vacuum systems of Dist1 and Dist2, respectively. Dist1 separates the lights from pentenenitriles, catalyst and 2MGN. e lights are recycled (via CP1) to R1. Dist2 separates pentenenitriles from the catalyst and heavies. e catalyst containing bottom stream of Dist2 is pressurized (in P6) and recycled (via Sep2) to R1. Half of the catalyst recycle stream is drained (with Sep2) to avoid built-up of heavies. Hydrocyanation of pentenenitriles to adiponitrile 4 e reaction of 3PN to ADN takes place in reactor R3. No addition of BD is needed. In the second hydrocyanation step most of 3PN is converted into ADN. Conversion and selectivity of the various reactions of the second hydrocyanation step are presented in Table 4-2.
Table 4-2 Conversion and selectivity details of hydrocyanation of pentenenitriles to adiponitrile (97) (99).
Component Conversion [%] Selectivity32 [%] Yield [%] 3-pentenenitrile 84.15 4-pentenenitrile 84.15 Hydrogen cyanide 93.9 2-pentenenitrile - 2.0 1.7 2-methylglutaronitrile - 3.5 3.0 Ethylsuccinonitrile - 3.5 3.0 Adiponitrile - 90.0 76.5 Heavies - 1.0 -33
However, also 2PN, 2MGN and ESN are formed. ADN has to be separated from this mixture before it can be transformed into 6-aminocapronitrile (ACN). Distillation is
32 Selectivity based on the conversion of 3-pentenenitrile and 4-pentenenitrile. 33 See footnote 31.
91 Chapter 4 not feasible because of the high boiling point of ADN (568 K) in combination with catalyst protection (102)(103). Extraction with n-heptane is used to separate the catalyst from the ADN mixture without harming the catalyst. e molar ratio between 3PN and ADN has to be below 0.65:1 to achieve an optimum phase separation. e amount of n-heptane in the top layer of the extraction is between 80 and 95 w%. BPh3 remains mainly in the high density ADN rich phase. e promoter concentration in n-heptane phase is approx. 0.05 w%. e extraction conditions are atmospheric pressure and a temperature between 305 and 325 K. (104)
e pentenenitriles top stream is pressurized to 0.1 MPa (with P7) before entering R3 for the hydrocyanation to adiponitrile. e reactor conditions are 0.1 MPa and 333 K. e second stream which enters the reactor contains make-up catalyst and hydrogen cyanide, as well as the pentenenitrile rich recycle stream from the extraction section. e catalyst feed stream has a temperature of 298 K and a pressure is 0.1 MPa and contains 12 kg/h of catalyst. e temperature and pressure of the make-up HCN feed are 298 K and 0.25 MPa and the stream contains 8.37 mton/h of HCN. e generated heat duty in the reactor section is eliminated with cooling water. e outlet stream of R3 enters the extraction section and is brought in contact with n-heptane rich extraction solvent (temperature 308.5 K, pressure 0.1 MPa). e extraction section is simulated with two serial separation models with xed split fraction speci cation (Extr1 and Extr2). e solute streams are distilled in Dist3 to remove the catalyst from n-heptane. A second pentenenitrile rich stream is fed to the heptane distillation column Dist3 (via Sep3/P11) to recycle the catalyst back to R3. e condenser of the distillation column (Radfrac model) is cooled with cooling water. e reboiler is heated with 1.8 MPa condensing steam. e extractant (n-heptane) is reused in the extraction section (via P9). e make-up n-heptane demand is 201 kg/h at ambient conditions. e pentene nitrile/catalyst mixture of the bottom of Dist3 is recycled to R3 (via P8).
e nitrile rich bottom layer of the extraction (Extr2) is further puri ed via three serial vacuum distillation columns (Radfrac model, 5 kPa). Dist4 separates unconverted pentenenitriles from adiponitrile, Dist5 separates low boiling components (lights) from adiponitrile, and Dist6 separates high boiling components (heavies) from the required product adiponitrile. e condensers of the vacuum distillation columns are cooled with cooling water. e reboiler of Dist4 is heated with 2.6 MPa condensing steam; Dist5 and Dist6 are heated with 1.8 MPa condensing steam. P10 is the vacuum system of Dist4/5/6.
e main part of pentenenitriles and n-heptane in the top stream of Dis4 is separated in Sep3 and recycled to Dist3 via P11. Lights boiling organic components (lights) and adiponitrile/high boiling components (heavies) are separated in Sep4; lights are purged and adiponitrile/heavies is further puri ed in Sep5. ADN in the bottom stream of Sep4
92 Polyamide-6 production starting from 1,3-butadiene and hydrogen cyanide is separated from the heavies in Sep5 and recycled as feed to Dist4. e required heat duty for the separations is supplied by condensing 0.3 MPa steam.
Hydrogenati on of adiponitrile to 6-aminocapronitrile Adiponitrile hydrogenation proceeds via 6-aminocaproimine (IMINE) to 6-aminocapronitrile, see Figure 4-5 (105). e hydrogenation reaction lasts about 2.5 hours to convert over 90% ADN with a selectivity of 82% towards 6-aminocapronitrile. By-products are HMDA, tetrahydroazepine (THA), and a dimeric product (DIMERIC) in a molar ratio of 5:1:1.
Figure 4-5 Hydrogenation of adiponitrile to 6-aminocapronitrile. 4 e reaction conditions are 7 MPa and 343.1 K. Raney Nickel is preferably used as catalyst (55)(106)(107). Liquid ammonia is used as reaction solvent to suppress the formation of by-products like secondary and tertiary amines. Ammonia is added in a molar ratio of 12:1. e molar ratio of ADN and ammonia impacts the selectivity towards ACN. Higher molar ratios increase the selectivity towards ACN (106). Methanol can also be used as solvent34 (106)(108)(109)(110).
e applied Aspen 7.3® simulation model of hydrogenation of adiponitrile is shown in Figure 4-6. e de ned design criteria, assumptions and conditions of the main equipment, depicted in Figure 4-6, are given in Appendix C.
ADN stream of the ‘Hydrocyanation’ section is pressurized with P1 and cooled with cooling water with HEx1 to the reaction conditions of hydrogenation reactor R1. Conversion and selectivity of various reactions in the hydrogenation reactor are presented in Table 4-3.
34 Ammonia is used as solvent in the hydrogenation section. e recycling and recompression of ammonia requires large amounts of energy (primary energy: 14.6 MJ/kg PA6; secondary energy (electricity): 6.5 MJ/ kg PA6). It could be advisable to use other solvents, e.g. methanol, in order to suppress the undesired energy destruction. However, the use of methanol can result in the detrimental formation of by-products like secondary and tertiary amines.
93 Chapter 4
Figure 4-6 Aspen 7.3® simulation model of hydrogenation of adiponitrile.
94 Polyamide-6 production starting from 1,3-butadiene and hydrogen cyanide
Table 4-3 Conversion and selectivity details of the hydrogenation of adiponitrile to 6-aminocapronitrile (55)(106)(107)(108)(109)(110).
Component Conversion [%] Selectivity [%] Yield [%] Hydrogenation of adiponitrile Adiponitrile 96.0 - Hydrogen (overall) 99.6 - 6-aminocaproimine - 100.035 96.0 Formation of 6-aminocapronitrile 6-aminocaproimine 100.0 - 6-aminocapronitrile - 86.036 86.0 Tetrahydroazepine - 14.036 14.0 Formation of HMDA and dimeric product 6-aminocapronitrile 14.26 - Hexamethylenediamine - 81.637 11.6 Dimeric product - 18.437 2.6
R1 is also fed with fresh ammonia and hydrogen and several recycle streams: a THA rich stream (via P6), an IMINE rich stream (via P8) and an ammonia rich recycle stream (via CP2/Sep3). Make-up ammonia supply (via P2, 25 w% in water) is 12.1 4 mt/h at 298 K and 1.4 MPa. Make-up hydrogen supply (via CP1) is 3.07 mt/h at 298 K and 2.0 MPa. e purity of the applied industrial hydrogen is 90 vol% (7 vol% N2, 0.9 vol% Ar, 1.7 vol% He, 0.4 vol% CH4). e compression heat of the hydrogen ow is eliminated with cooling water (in HEx2). e generated heat duty in R1 is eliminated with cooling water.
e hydrogenation product stream is degassed in Sep1 and Sep2 with dierent ash pressures, respectively 0.1 MPa and 20 kPa; temperature is 343 K. e gas phase of both ashers is compressed with CP2 and recycle to R1. CP2 is cooled with cooling water (in HEx3). Part of the compressed gas (20%) is purged (via Sep3) to avoid build- up of inert gasses. e atmospheric asher is heated with 2.6 MPa condensing steam; the vacuum asher is heated with 0.3 MPa condensing steam.
e remaining product is distilled with three serial vacuum distillation columns Dist1/ Dist2/Dist3 (Radfrac model) to separate THA, HMDA and IMINE from 6-aminocapronitrile. e low boiling components are isolated in Dist1 (20 kPa). e top stream of Dist1 contains mainly HMDA (approx. 40 w%), which is further puri ed in a separate distillation section (Dist4/Dist5). ACN is separated from other components in Dist2 (5 kPa) and subsequently pressurized (with P5) to the pressure of the pre-polymerization reactor. e distillation pressure is maximal 20 kPa to 35 Selectivity is based on the conversion of adiponitrile. 36 Selectivity is based on the conversion of 6-aminocaproimine. 37 Selectivity is based on the conversion of 6-aminocapronitrile (losses).
95 Chapter 4 limit the bottom temperature to 473 K. Above this temperature ‘heavies’ are formed in the reboilers. Dist3 (5 kPa) separates the unreacted ADN from the high boiling components; ADN (and mainly THA) is recycled (via P6) to R1. e bottom stream of Dist3 consists of high boiling components which are drained and can be incinerated. e condensers of Dist1/Dist2/Dist3 are cooled with cooling water. e reboilers of Dist1 and Dist2 are heated with 1.8 MPa condensing steam; Dist3 is heated with 2.6 MPa condensing steam. P4 is the vacuum system of Dist2/3.
e top stream of Dist1 is processed in a separate recovery section. First ammonia is ashed (in Sep4) before entering the vacuum distillation section Dist4 and Dist5. e generated heat duty in Sep4 is eliminated with cooling water. ‘Low boiling’ components are separated from IMINE and HMDA in Dist4 (20 kPa). Dist5 is operated at atmospheric pressure and separates IMINE and HMDA. e IMINE rich top stream of Dist5 is recycled (via P8) to the hydrogenation reactor. e HMDA rich bottom stream of Dist5 can be applied as feedstock for nylon-6,6 production. e condensers of Dist4/Dist5 are cooled with cooling water. e reboilers are heated with 1.8 MPa/2.6 MPa condensing steam. P7 is the pressure system of Dist5.
Polymerization of 6-aminocapronitrile Polymerization of 6-aminocapronitrile consists of hydrolysis, deammoniation (pre- polymerization) and polycondensation (56) (see Figure 4-7). ZrO2 is used as catalyst (55)(92). Process step I is the hydrolysis of 6-aminocapronitrile to 6-aminocaproamide and subsequent pre-polymerization to polyamide-6 with an average chain length of 20 repeating chains (Pn = 20). Ammonia, produced with each coupling of two 6-aminocaproamide molecules in the pre-polymerization, has to be removed. e reaction time of step I is 4 hours. e reaction conditions are 503 K and 3 MPa. At these conditions a conversion of 95 % is reached (55)(111). Process step II involves the polycondensation of prepolymer to commercial grade PA-6 (Mn= 15,000 – 40,000 kg/kmol).
96 Polyamide-6 production starting from 1,3-butadiene and hydrogen cyanide
Figure 4-7 Polymerization of 6-aminocapronitrile (56).
ermodynamic properties of 6-aminocaproamide are not available in literature. Also commonly applied group contribution methods, e.g. Gani, Domalski and Hearing (112)(113), fail. Such methods can only be applied for compounds with one functional 4 group, since the databases of experimental values contain mainly monofunctional compounds. erefore, the applied reaction model for the simulation of step I has been simpli ed. e pre-polymerization is considered as a ‘black box’. e reaction scheme for this ‘black box’ is depicted in Equation 4-1. No intermediates are modelled in this setup and the only components used are 6-aminocapronitrile, water, ammonia and nylon-6 (chain length = 20 caprolactam units, Mn= 2281 kg/kmol). is simple approach will not in uence the nal results, because, rst, only enthalpy and entropy of reactants and products are decisive in the thermodynamic evaluation of step I (state variables, independent of reaction path). Second, also the exergy results will not be in uenced. Indeed, exergy balances are in uenced by the way of heat exchange with the environment (e.g. cooling water, steam). However, all reactions of step I (including the formation of 6-aminocaproamide) take place at the same conditions and therefore we can assume that the exergy results will not be in uenced.
Equation 4-1
97 Chapter 4
e polycondensation is performed in a VK column which separates the produced water from the polymer. e operational conditions of the VK column are 0.25 MPa and approx. 513 K. e conversion of the polycondensation reaction is 99.2 %. e end product of this process is polyamide-6 with a molecular weight of Mn= 20387 kg/ kmol (chain length = 180 caprolactam units). Water formed during polycondensation is removed by means of a condenser at the top of the VK column. e end product is cut to small chips and cooled with dry nitrogen to 313 K.
As already described in Chapter 3, Aspen soware and databases cannot be used to perform a polymerization simulation easily. We calculated the (pre)polymerization related energy changes separately. e method to calculate polymerization reaction enthalpy and entropy (as well as standard enthalpy/entropy of formation of nylon-6 polymers) is explained in Appendix E. e calculated reaction enthalpy/entropy of (pre) polymerization is used in the present Aspen simulation. In this way, we have applied Aspen 7.3® to simulate only mechanical and physical process steps such as chips cutting, hot water extraction/drying of the polymer chips and the removal of water and (reaction) heat exchange in the pre-polymerization vessel and polycondensation column.
e applied Aspen 7.3® simulation model of polymerization of 6-aminocapronitrile (111)(114)(115) is shown in Figure 4-8. e de ned design criteria, assumptions and conditions of the main equipment, depicted in Figure 4-8, are given in Appendix C.
Figure 4-8 Aspen 7.3® simulation model of polymerization of 6-aminocapronitrile.
6-aminocapronitrile is the precursor of the pre-polymerization. e 6-aminocapronitrile stream enters the polymerization section at 3.0 MPa and 412 K. e required water supply for the pre-polymerization reaction amounts 8.44 mt/h (T= 293 K, p= 0.3 MPa) and is pressurized (with P1) at reactor pressure. e pre-polymerization reactor conditions are 503 K and 3 MPa.
98 Polyamide-6 production starting from 1,3-butadiene and hydrogen cyanide
e 6-aminocapronitrile/water mixture is heated in HEx1 with the use of Dowtherm™ heating uid. As explained above we did not simulate the polymerization reactions but only used the separately calculated reaction enthalpy/entropy data in the simulation model.
An arti ce separator block (Sep1) is used to ‘remove’ reacting 6-aminocapronitrile and water from the stream. e residual components of the 6-aminocapronitrile/ water mixture and the produced reaction products water and ammonia are normally separated from the pre-polymer via a condenser on top of the pre-polymerization reactor. is separation is modelled with two Aspen separation blocks (Sep2 and Sep3). Sep2 simulates the separation of ammonia and residual components, Sep3 simulates the evaporation of reaction water.
e (stoichiometric) amounts of reaction products (ammonia and water) are arti cially simulated – at reactor conditions – as incoming streams of these separators. e pre-polymerization reaction is exothermic. However, the evaporation of the reaction products water and ammonia and non-reacted substances require energy. e remaining heat duty is eliminated with cooling water. e produced water in the polycondensation reactor is also separated from the polymer via a condenser on top of the reactor. is separation is modelled with an Aspen separation block (Sep4). 4 e (stoichiometric) amounts of water is arti cially simulated – at reactor conditions – as incoming streams of this separator. e polycondensation reaction heat and the evaporation of polymerization water requires energy which has been eliminated with Dowtherm™ heating uid.
e produced polymer is cut in polymer chips (granules). e chopper system requires 12.9 kJ /kg PA-6 of electricity. In the current simulation set-up we assume that this energy is completely absorbed by the polymer and has been taken into account for energy calculations. A water cooled heat exchanger is modelled for this (not shown in Figure 4-8). e nal drying and cooling of polyamide chips from 368 to 313 K is modelled with recycled nitrogen (Tin= 308 K, Tout= 360 K, pressure= 0.3 MPa). Heated nitrogen (in HEx2) is boosted (with a blower CP1, DP= 20 kPa) and cooled (in HEx3) with cooling water before reused. e heat duty which has to be withdrawn is determined by the dierences in the speci c heat the polymer at 368 and 313 K.
99 Chapter 4
4.2 Mass and energy balances Aspen simulation results particularly in mass- and energy data of incoming, intermediate and outgoing process streams of the total production process or part of the production process. e analysis of mass data reveals production yields and prospects of yield improvement. Prospects of yield improvement are based on theoretical balancing equations. Aspen energy data are applied for the analysis of energy yields and prospects for energy optimization.
Balancing Equation from BBBs to PA-6 e balancing equation of the Butadiene route may start from HCN as a BBB, or further upstream, considering methane as the BBB to start from and including the synthesis of HCN from methane, ammonia and oxygen (the so-called Andrussow process (54)).
Starting from HCN as a BBB:
38 Equation 4-2
Ammonia is a co-product of this route.
Starting from methane as a BBB, Equation 4-3 precedes Equation 4-2:
Equation 4-3
Combining Equation 4-2 and 4-3 yields:
Equation 4-4
Both water and ammonia act as a BBB at the le side of Equation 4-4 and as products at the right side. In Equation 4-5 this has been cancelled out, which in case of ammonia can be interpreted as the perfect recycling of downstream- produced ammonia in Equation 4-2 as a BBB upstream in Equation 4-3. e same reasoning holds for water. Note that Equation 4-5 contains no other co-products than water.
Equation 4-5
Raw material consumption and product and waste production Figure 4-9 summarizes the material balance of the Butadiene manufacturing route
38 -C6H11NO- : see remark in Section 3.2. 100 Polyamide-6 production starting from 1,3-butadiene and hydrogen cyanide which is based on the process descriptions and the results of the corresponding Aspen models and simulations. e mass balance includes raw materials used to produce polyamide-6 (corresponding the balancing equation) and other (auxiliary) materials that are used in the process. e theoretical required amount of raw materials of the Butadiene route is calculated with Equation 4-2. e theoretical and modelling results are compared in Table 4-4, in mol/mol –CPL– and kg/kg –CPL–.
4
Figure 4-9 Overall mass balance of polyamide-6 manufacturing from 1,3-butadiene and hydrogen cyanide.
101 Chapter 4
Table 4-4 Comparison of theoretical and modelling results of the Butadiene manufacturing route.
BBB C4H6 HCN H2 H2O -CPL- NH3 Waste Molar mass 54.09 27.04 2.016 18.01 113.16 17.03 Reactants Products Balancing equation, mol/ 1 2 211 1 mol –CPL– Modeled manufacturing 1.49 2.89* 2.81 1.00 1.00 1.00 route, mol/mol –CPL– Yield [mol/mol %]** 67 69 71 100 Balancing equation, kg/kg 0.48 0.48 0.04 0.16 1.00 0.15 0 –CPL– Modeled manufacturing 0.71 0.69 0.05 0.16 1.00 0.16 0.45 route, kg/kg –CPL–
* e production of HCN requires 4.81 mol CH4/mol -CPL-, 2.61 mol NH3/mol –CPL– and 4.91 mol O2/ mol –CPL–. **: ratio of mol/mol –CPL– balancing equation and mol/mol –CPL– modelled manufacturing route.
e amount of waste (solid, liquid, gas) has been calculated as the dierence between the mass input of BBB and the mass of products and coproducts. e modelled waste amounts, based on balancing equation, 0.45 kg/kg PA6 (see Table 4-4).
e total volume of emissions (all outgoing streams, except PA-6), as revealed in Figure 4-9, is 9.279 kg/s ( 1.33 kg/kg PA6) and signi cantly higher as predicted in Table 4-4
(NH3 0.16 and waste 0.45 kg/kg PA6, in total 0.61 kg/kg PA6). However, part of the incoming streams of Figure 4-9 can be considered as inert with respect to balancing
Equation 4-2 and therefore not accounted for, e.g. N2, Ar, CH4 and He (as part of the hydrogen feed) and n-heptane (as extractant).
Energy balances Figure 4-10 shows the exergy balance of polyamide-6 manufacturing starting with 1,3-butadiene and hydrogen cyanide. e overall loss of work (exergy) amounts 17.8 MJ/kg PA6 due to irreversible processing. e main contributor to this loss is the hydrogenation manufacturing step: i.e. 14.6 MJ/kg PA6.
102 Polyamide-6 production starting from 1,3-butadiene and hydrogen cyanide
4
Figure 4-10 Overall exergy balance of polyamide-6 manufacturing from 1,3-butadiene and hydrogen cyanide.
We consider the Butadiene route as a black box. BD, HCN, H2 and water and small amounts of auxiliary chemicals (e.g. NH3) enter the process (as BBBs). Polyamide-6 (product) and process emissions like co-products, unreacted BBBs and by-products (waste) leave the process. All material ows implicitly represent an enthalpy content calculated with the Aspen models and simulation. e dierence between the enthalpy
103 Chapter 4
content of incoming and outgoing streams equals the overall enthalpy of reaction ∆rH
(main reaction and side reactions). e result is ∆rH= -4.4 MJ/kg PA6. Additionally, chemical manufacturing routes use electricity and steam to support the irreversible production processes. Excess process heat is emitted as waste heat via heated cooling water (CW), generated steam (GS) and condensate (SC).
Figure 4-11 summarizes the overall secondary energy balance. e incoming and outgoing energy streams are expressed as excess enthalpy ows (in MJ/kg PA6.) e enthalpy content of ‘Steam’ (incoming), ‘CW’, ‘SC’ and ‘GS’ are de ned as the enthalpy dierence relative to the standard heat of formation of liquid water (- 285.68 kJ/mol).
Figure 4-11 Modelled secondary energy balance of polyamide-6 manufacturing from 1,3-butadiene and hydrogen cyanide.
Ideally, the secondary energy balance should be zero. However, the absolute discrepancy of 0.5 MJ/kg can be considered as negligible (<1%).
Part of the excess heat can be reused, which has not been taken into account yet. Steam condensate can be reused for steam generation in system boundary 1 (closed circuit supposed as the base case). A credit can be granted, that reduces the total excess energy input. e maximum value of the credit would be 15.8 MJ/kg PA6 if this heat could be reused with 100% e ciency. However, there is an e ciency factor 0 < k < 1 related to condensate recycling. Here we take k= 0.3. Generated steam is used via system boundary 1 (or recycled within system boundary 3, which is almost equivalent). A credit of 9.5 MJ/kg PA6 for GS can be granted. Depending on e ciencies of heat reuse the Net Excess Enthalpy Input ranges from 42.2 (reuse) to 56.4 MJ/kg PA6 (no reuse).
e reported energy balance (Figure 4-11) is expressed as secondary energy. Primary energy demand has to be used for the environmental impact analysis in Chapter 6, as already explained in Section 3.2. e results are summarized in Table 4-5.
104 Polyamide-6 production starting from 1,3-butadiene and hydrogen cyanide
Table 4-5 Primary Energy Demand of polyamide-6 manufacturing from 1,3-butadiene and hydrogen cyanide.
Material/energy feedstock PED [MJ/kg PA6] 1,3-butadiene 24.8 Hydrogen 4.6 Hydrogen cyanide 51.6 Ammonia 0.8 Steam 68.9 (48.5)* Electricity 18.7 Total 169.4 (149.0)* *Steam PED-value between brackets refers to the net value aer reuse of waste energy of the PA-6 production starting from BBBs (generated steam and upgraded condensate).
e environmental impact of the Butadiene route will be discussed and compared with the other PA-6 manufacturing routes in Chapter 6.
4
105
Polyamide-6 producti on starti ng from glucose and ammonia 5 Chapter 5
For commercial manufacturing of polyamide-6 non-renewable energy resources are consumed and fossil fuel based resources are depleted. e use of such resources contributes signi cantly to environmental issues, particularly fossil CO2 and related global warming. It is expected that such non-renewable resources will be replaced by renewable alternatives in the (near) future (116). Biomass is a renewable resource, which is replenished naturally by photosynthesis processes.
Glucose is readily known as renewable feedstock for biobased processes (53). Glucose can be extracted from various biomass sources such as sugar cane, sugar beet, corn and wheat. ese sources also contain starch and lignocellulose which can be transformed into additional glucose (by hydrolysis and decomposition, respectively). However, possible by-products of hydrolysis and decomposition likely inhibit subsequent glucose fermentation. e same disadvantage has been noticed for the by-products molasses and bagasse in sugar production. erefore, only the naturally available glucose in biomass sources is considered in this study as material feedstock for biobased PA-6 manufacturing. Two possible biobased PA-6 routes are the subject of this chapter. e production routes dier in the kind of polymer precursor: production of e-caprolactam from glucose via L-lysine (Frost process) (58)(59)(60) and production of 6-aminocaproic acid from glucose (“AKP” route) (61)(62)(63). Currently there are no such biobased routes in commercial operation. e objective is to obtain a process description for these routes and a derived mass and energy balance.
Computer aided simulation models of the complete polyamide-6 manufacturing route and the distinguished production steps to produce PA-6 from glucose and ammonia are rarely described in literature, particularly for commercial scale production sizes (>200 kton/annum). Moncada Botero (53) assessed the techno-economic performance of the production lines of 1,3-butadiene and e-caprolactam from C6 sugars. Process models were developed to assess the technical performance and derived inputs. e process line for e-caprolactam production involves the production of levulinic acid, g-valerolactone and nally e-caprolactam and is designed for an annual production of 55 kton.
We studied the production of e-caprolactam (via L-lysine) and 6-aminocaproic acid from glucose. Both products can be polymerized to polyamide-6. e consecutive steps for the production of e-caprolactam are the fermentative production of L-lysine from glucose, the chemical conversion of L-lysine into e-caprolactam, and nally the hydrolytic polymerization of e-caprolactam to polyamide-6. Figure 5-1 depicts the overview of the e-caprolactam route. e L-lysine route has been designed for two dierent reaction yields in the deamination section (last reaction step in Figure 5-1): i.e. 65% and 95%. e processes are labelled as Frost and Frost PLUS, respectively (58) (59).
108 Polyamide-6 production starting from glucose and ammonia
Figure 5-1 ε-caprolactam production from glucose (Brochure of Melon Acres, www.melonacres.com/melon-acres-sweet-corn, 2015). e company Royal DSM studied a potential fermentative route to produce 5 6-aminocaproic acid (6-ACA) from glucose (61) (62) (63). is bio-catalysed route (‘AKP-route’) is shown in Figure 5-2.
Figure 5-2 AKP-route to produce 6-aminocaproic acid from glucose.
e AKP-route and the subsequent polymerization of 6-aminocaproic acid is labelled as ACA.
In general, the required energy for biobased processes is partly generated by the combustion of waste and side products from the process itself. However, in this study we have also chosen for fossil based processing energy to make the comparison with fossil processes unambiguous with respect to energy related CO2 emission. e Frost processes have low fermentation yield, which requires internal recycling loops,
109 Chapter 5 and thus energy demanding separations, use of cooling water and heating media, and emissions to air and surface water. e ACA process has a low fermentation yield and uses cooling water and heating media, and emits substances to air and surface water as well. However, the recovery conditions to obtain 6-aminocaproic acid are milder.
Route variants are discussed in Section 5.1. e mass and energy balance results are summarized in Section 5.2 and are obtained with Aspen 7.3® simulation of the discussed models in Section 5.1. e applied thermodynamic equation of state is RK-NRTL.
5.1 Process design of polyamide-6 production starting from glucose and ammonia Figure 5-3 and Figure 5-4 summarize the overall process scheme of PA-6 manufacturing starting from glucose and ammonia. Figure 5-3 shows the Frost manufacturing routes and Figure 5-4 depicts the ACA manufacturing route. Detailed process design descriptions of the distinguished processing blocks will be presented below.
110 Polyamide-6 production starting from glucose and ammonia
5
Figure 5-3 Frost-routes to produce polyamide-6 from glucose.
111 Chapter 5
Figure 5-4 ACA-route to produce polyamide-6 from glucose.
Fermentation of glucose (Frost routes and ACA route) Frost route Vorselen (57) concluded in his study that sugar cane or beet are the most favourable glucose source for the production of ε-caprolactam (or other polymer precursors). Both sources have a relatively high productivity and require a low utility input (per ton sugar). Glucose obtained from sugar beets has been chosen as feedstock for the processes described in this chapter. e reason for this choice is that sugar beets and not sugar cane are grown in Europe (57)(117). Import of sugar cane is possible, however, not favoured because of logistics and associated environmental impact (118)(119).
Commercial fermentation of glucose to L-lysine (further labelled as ‘lysine’) is typically done as a fed-batch39 process (120)(121)(122)(123). First the starting medium is inoculated with bacteria, e.g. ‘Corynebacterium glutamicum’ (121)(124)(125) (126). e actual production of lysine is performed with glucose, a nitrogen source and air. e glucose level is kept low (approx. 5 g/L) to increase the selectivity and productivity of the reaction and to keep the loss of glucose (aer the fermentation is stopped) as low as possible. Ammonia can be used as nitrogen source, supplemented with protein acid hydrolysate (and sometimes urea). Also salts and vitamins are added to complete the bacteria feed. e speci c salts and vitamins vary slightly amongst recipes using dierent bacteria strains (120)(121). Lysine is normally produced as a stable salt: L-lysine.HCl. Technical process conversion yields of 0.45–0.50 kg lysine.
39 Fed-batch culture is de ned as an operational technique in biotechnological processes where one or more nutrients (substrates) are fed (supplied) to the bioreactor during cultivation and in which the product(s) remain in the bioreactor until the end of the run.
112 Polyamide-6 production starting from glucose and ammonia
HCl/kg glucose; productivities of 0.65–0.75 kg lysine.HCl/(kg glucose.h) and biomass speci c product formation rates of 0.08–0.24 kg lysine.HCl/(kg dry biomass.h) have been reported (124)(127)(128). Aer each fermentation batch the reactor (fermenter) and auxiliary equipment is sterilized with steam of 373 K. To produce e.g. 220 kton PA-6/annum, 290 kton lysine/annum is needed provided that no product is lost in the consecutive steps. We have calculated that 92 fermenters of 500 m3 (fermenter lling degree 40 v/v%) are required to produce 0.7 kg lysine.HCl/(kg glucose.h); including bacteria grow vessels, compressors, pumps and gas/liquid/solid separation.
However, the energy destruction in the production of polyamide-6 from L-lysine is signi cant, mainly caused by recovery processes and (de)compression energy. It can be bene cial in this respect to design a completely fermentative process to produce polyamide-6 from renewable feedstock.
ACA route e patented ‘AKP-route’ as depicted in Figure 5-2 could be a possible fermentation route. e fermentation starts with the conversion of a carbon source (e.g. glucose) to a-ketogluterate (AKG). e preparation method includes the conversion of AKG to a-ketoadipate (AKA) and the conversion of AKA to a-keto pimelate (AKP). e biocatalyst can be an enzyme or any organism embedded in fungi, yeast, euglenoids, archaea and bacteria. e preparation of 6-aminocaproic acid from AKP can be a completely chemical route and/or a completely biochemical route. (61)(62) e overall stoichiometric reaction for the aerobic 6-aminocaproic acid formation 5 from glucose is as follows:
Equation 5-1
e reaction includes the oxidation of glucose to carbon dioxide and water necessary for bacteria growth and activity. Patents and reports regarding the aerobic 6-aminocaproic acid formation from glucose indicate production yields and production conditions which are comparable with the aerobic fermentation of glucose to L-lysine. Hence, the stoichiometric reaction for the aerobic fermentation has been adapted in the model.
e applied Aspen 7.3® simulation model of fermentation of glucose and ion exchange puri cation (Frost and ACA routes) is shown in Figure 5-5. e de ned design criteria, assumptions and conditions of the main equipment of the present simulation scheme are given in Appendix D.
113 Chapter 5
Figure 5-5 Aspen 7.3® simulation model of glucose fermentation and ion exchange puri cation.
114 Polyamide-6 production starting from glucose and ammonia
Each bacteria grow vessel (R1) and fermenter (R2) are fed with glucose solution and are aerated. R1 feed consists of 1376 kg/h aqueous glucose solution (35 w/w% via P1) and 1894 Nm3/h air (2.21 mton/h) at ambient conditions (via CP1). R2 feed (via P2) contains 17.82 mton/h aqueous solution (7.3 w/w% glucose, 1.2 w/w% ammonia, 0.2 w% yeast) and 5090 Nm3/h air (5.93 mton/h) at ambient conditions (via CP2). R1 and R2 operate at 304.5 K and 0.1 MPa. Gas and liquid feeds are supplied at 0.5 MPa, using pumps and compressors, to simulate the pressure drop due to the viscosity of the reaction medium and the reactor height. e generated compression heat of each compressor is eliminated with cooling water (with HEx1 and HEx2). In R1 glucose is converted into biomass (modelled as glucose). Half of the glucose feed is oxidized to simulate the energy necessary for bacteria growth. In R2 glucose is converted to lysine (or 6-aminocaproic acid) using ammonia that is dissolved in the feed medium. Also in each fermenter almost half of the glucose feed is oxidized to simulate the energy necessary as bacteria food. Furthermore, some additional biomass is formed from glucose and the yeast extract and some glucose reacts to intermediate molecules, modelled as pyruvic acid (‘metaboli’). Finally some ammonia is oxidized to nitrogen to model the uptake of nitrogen into the biomass. Fractional conversions of the involved reactions for an industrial production are summarized in Table 5-1.
Table 5-1 Fractional conversions of glucose fermentation(120) .
Reaction Mole conversion Frost processes Microbe growth reactor Reaction Glucose biomass 0.5000 5 Glucose + 6O2 6CO2 + 6H2O 0.5000 Fermentation reactor Glucose + 2NH3 Lysine + 2H2O + O2 0.4440 Glucose + 6O2 6CO2 + 6H2O 0.4146 Yeast biomass 0.9870 Glucose biomass 0.0789 Glucose + O2 2 metaboli + 2H2O 0.0490 2NH3 + 1.5 O2 N2 + 3H2O 0.0001 ACA process Microbe growth reactor Glucose biomass 0.5000 Glucose + 6O2 6CO2 + 6H2O 0.5000 Fermentation reactor 2Glucose + 2NH3 2ACA + 2H2O + 3O2 0.4440 Glucose + 6O2 6CO2 + 6H2O 0.4146 Yeast biomass 0.9870 Glucose biomass 0.0789 Glucose + O2 2 metaboli + 2H2O 0.0490 2NH3 + 1.5 O2 N2 + 3H2O 0.0001
115 Chapter 5
Each bacteria grow vessel and each fermenter are cooled with cooling water. Gasses are emitted via Flash1. e liquid broth is ltrated (in Sep1) yielding wet biomass and a clear lysine (or 6-aminocaproic acid) containing broth. Biomass can be dried and combusted. e wet biomass is assumed to contain 50 w/w% water40. Periodical sterilization of the fermentation equipment is performed with 2.6 MPa steam; for each grower/fermenter unit 18.4 kg steam/h is simulated (not shown in Figure 5-5). e Aspen simulation block ‘MULTI’ multiplies the product stream of Sep1 (x92: required number of fermentation set-ups).
Ion exchange purification (Frost routes or ACA route) Aer fermentation, lysine (or 6-aminocaproic acid) is puri ed. First, bacteria are removed by means of centrifugation and ltration. Some CO2 remains dissolved in the solution. is is neglected in the process design. Subsequently, ion exchange is used to purify the solution. To do so the solution is rst acidi ed with sulphuric acid to a pH between 3 and 4 to protonate lysine (or 6-aminocaproic acid) once (Lys- H+, ACA-H+)). is protonated amino group attaches to the alkaline ion resin end- group. ereaer, the broth is acidi ed to a pH between 1 and 2 providing lysine (or 6-aminocaproic acid) with a 2+ charge. is increases the driving force for adsorption at low concentrations, ensuring very low losses of lysine/6-aminocaproic acid. e eluent is treated as acidic waste water. e ion exchanger with the attached lysine/6- aminocaproic acid is subsequently ushed with ammonia solution. It is assumed that lysine/6-aminocaproic acid is loaded onto the resin for 80% of the maximum resin adsorption capacity. Ammonia will exchange with lysine/6-aminocaproic acid yielding a solution with lysine/6-aminocaproic acid and residual ammonia. Generally, 1.5 moles of ammonia are used to remove 1 mole of amino acid. e ion exchanger is then regenerated with additional acid. Aer regeneration, the eluent contains dissociated sulphuric acid and removed ammonia, yielding an aqueous ammonium sulphate solution41. Modern ion exchangers are built as carrousels, with each part of the carrousel going through a full adsorption cycle during one turning cycle of the carrousel. ese carrousels can be regarded as simulated moving beds (129). In the current design the adsorption is modelled as a single step.
Clear lysine (or 6-aminocaproic acid) containing broth is puri ed with an ion exchange carrousel (via P3, Figure 5-5). First the broth is acidi ed in Sep2 with 1.8 molar sulphuric acid (via P4, 298 K, 0.1 MPa). e adsorption takes place and 98.5% of the lysine (or 6-aminocaproic acid) adsorbs onto the resin (modelled in Sep2). e adsorbate is then mixed with an ammonia solution to simulate the elution step (modelled in Sep3). Fresh 25 w/w% ammonia solution (via P5, 298 K, 0.1 MPa) and recycled ammonia solution (from Dist1) is used to achieve that 1.5 mole ammonia is
40 Based on expert judgement at DSM Del (former Gist Brocades). 41 e ammonium sulphate crystallization is not modelled in the current process design. Potential ammonium sulphate production is 0.9 kg/kg PA-6.
116 Polyamide-6 production starting from glucose and ammonia added for each mole of adsorbate. e adsorbed ammonia is desorbed with 1.8 molar sulphuric acid (298 K, 0.1 MPa) owing through the ion exchanger in a molar ratio of 2
NH3 : 1 H2SO4. is yields an 22 w/w% aqueous ammonium sulphate solution that can be crystallized to fertilizer. e concentrated adsorbate/ammonia stream is distilled (in Dist1) at ambient pressure to yield a concentrated lysine (or 6-aminocaproic acid) solution and a concentrated ammonia solution which is recycled to the elution step (Sep3). Dist1 is designed as Radfrac. e condenser of the distillation column is cooled with cooling water. e reboiler of the column is heated with 2.6 MPa condensing steam. Concentrated lysine solution is pressurized (with P6) to 0.3 MPa and processed in the cyclisation section (Frost processes). Concentrated 6-aminocaproic acid solution is directly processed in the polycondensation section.
ε-Caprolactam producti on from L-lysine (Frost routes) Industrial production of ε-caprolactam from lysine is not yet realized. Only lab- scale research has been performed to prove the principle of the process (58)(59). e preparation of ε-caprolactam from lysine is performed via cyclisation of lysine to α-amino-ε-caprolactam and subsequent deamino-hydrogenation (further called ‘deamination’). Cyclisation of lysine is performed by heating lysine in alcohol under re ux. e alcohol acts as catalyst and is not consumed. e examples in Frost’s patent (58) show that 1,2-propanediol gives the best yield and productivity. No additional chemicals are needed, only proper solvent and temperature (60)(83). Frost (58)(59) also developed two methods to deaminohydrogenate α-amino-ε-caprolactam. One method implies the use potassium hydroxide and hydroxylamine-O-sulphonic acid. e other 5 method describes the reaction of H2/H2S (hydrogen/hydrogen sulphide) gas with dissolved α-amino-ε-caprolactam (solvent is tetrahydrofuran (THF)) and the use of Pt- S/C catalyst. e reaction conditions are 523 K and 4.5 MPa. e second method seems to be the most economical option and has been applied in the present design (59).
Cyclisation of lysine e lysine solution is heated and subsequently fed to the cyclisation reactor containing 1,2-propanediol at its boiling point, i.e. 461 K. e selectivity from lysine to α-amino-ε- caprolactam is 96%. It is assumed, that half of the remaining lysine does not react and the other half reacts to heavy oligomers (60). e chemical reaction is depicted in Figure 5-6.
Figure 5-6 Chemical reaction of the cyclisation of lysine.
117 Chapter 5
e applied Aspen 7.3® simulation model of cyclisation of lysine is shown in Figure 5-7. e de ned design criteria, assumptions and conditions of the main equipment, depicted in Figure 5-7, are given in Appendix D.
Figure 5-7 Aspen 7.3® simulation model of cyclisation of lysine.
118 Polyamide-6 production starting from glucose and ammonia
Lysine solution is mixed with make-up alcohol (1,2-propanediol, 298 K, 0.3 MPa), recycled alcohol from the cyclisation section (Dist5), recycled alcohol from the deamination section and recycled lysine (via P3). e mixture is fed to reactor R1, which operates at 461 K and 0.1 MPa. Although the cyclisation reaction is exothermic, the incoming mixed stream has still to be heated to the reaction temperature. Net energy has to be added to the system by means of condensing 2.6 MPa steam. 96% of the lysine reacts to α-amino-ε-caprolactam and water, and 2% reacts to heavy oligomers forming 5 moles of water per mole of heavy oligomers. Heavy oligomers as produced in the cyclisation section are modelled as hexalysine, see Figure 5-8.
Figure 5-8 Model of heavy oligomers produced in cyclisation section (hexalysine).
e separation of reaction gas and liquid is modelled with ash vessel Sep1 at reactor conditions. e gas phase of Sep1 enters an atmospheric distillation column (Radfrac model, Dist1)) to condense α-amino-ε-caprolactam that partly evaporates in the reactor, along with some alcohol/water/light impurities.
Top separation section: e condensed stream of Dist1 is processed in the rst alcohol/α-amino-ε-caprolactam steam distillation column of the ‘bottom separation’ section (Dist3). e remaining gas of Dist1 consists mainly of alcohol and water and is fed to the rst atmospheric 5 water/alcohol distillation column (Radfrac model, Dist5), yielding liquid alcohol that is recycled to R1. Also two water/alcohol streams coming from the ‘bottom separation’ section are led to the water/alcohol distillation Dist5. e gaseous top stream of Dist5 contains mainly water and some alcohol. e stream is puri ed in a second atmospheric distillation column (Radfrac model, Dist6). In Dist6 almost all remaining alcohol is condensed and recycled to P1. e top stream is emitted as gaseous waste water. e condenser of Dist5 and Dist6 are cooled with cooling water that evaporates to steam; the reboiler of the columns are heated with 2.6 MPa condensing steam.
Bottom separation section: e bottom stream of Sep1 is cooled to 400 K (in HEx1) and heated to 431 K (in HEx2 aer decompression via P2) before entering Dist2. e pressure of the vacuum distillation column Dist2 is 20 kPa42 to avoid high temperatures when separating α-amino-ε- caprolactam and 1,2-propanediol from the higher boiling lysine and oligomers. e
42 e vacuum pressure of Dist2 is modelled by decompressing the stream via P2 before entering the column. To prevent cavitation in P2, the stream is rst condensed (cooled) to 400 K and aer (arti cial) decompression heated again to 431 K.
119 Chapter 5 condenser of Dist2 is cooled with cooling water that evaporates to steam; the reboiler is heated with heating oil (Dowtherm MX®). Lysine with heavies is partially drained (via P4) and partially recycled via P3 to R1. Some α-amino-ε-caprolactam has to remain present to keep lysine dissolved and to avoid degradation of lysine. Recovered α-amino-ε-caprolactam and alcohol at the top of Dist2 are pressurized to atmospheric pressure (via CP1) and subsequently fed to steam distillation column Dist3, together with the α-amino-ε-caprolactam and alcohol stream coming from Dist1. e added steam (1.86 mton/h, 2.6 MPa) allows 97% of the α-amino-ε-caprolactam to leave as bottom stream. Alcohol (containing 8.5 w% α-amino-ε-caprolactam and water) leaves the top of Dist3 and is fed to Dist5. e bottom product of Dist3 contains mainly α-amino-ε-caprolactam and some remaining alcohol. e remaining alcohol is removed with high pressure steam (1.63 mton/h, 4.2 MPa) in Dist4. is allows to recover 60% of the alcohol. e bottom stream of Dist4 is puri ed α-amino-ε- caprolactam, which is further processed in the deamination section. e top stream, containing water and alcohol, is fed to Dist5. e condensers of Dist3 and Dist4 are cooled with cooling water.
Deamination of α-amino-ε-caprolactam Puri ed α-amino-ε-caprolactam is mixed with fresh and recycled THF, pressurized and fed to the deamination reactor. Also hydrogen and methane thiol (CH3SH, catalyst promoter) gas is fed to the reactor to improve selectivity and yield. e patent of
Frost (59) prescribe hydrogen sulphide. However, CH3SH gas is easier to recycle and therefore used in this process design. e deamination produces ammonia gas and ε-caprolactam. e selectivity of α-amino-ε-caprolactam towards caprolactam is 65 mol/mol%. It is assumed that 30 mol/mol% is recycled and 5 mol/mol% will form oligomers (Frost process). Deamination reactions are well known in research practise and the achieved yields of this reaction could be improved43. To establish mass- and energy balances at higher yields and to evaluate the thermodynamic consequences thereof, also a production with a selectivity of 95 mol/mol% and 5 mol/mol% oligomer formation is modelled (Frost PLUS process).
e chemical reaction is depicted in Figure 5-9.
Figure 5-9 Chemical reaction of the deamination of α-amino-ε-caprolactam.
43 Based on expert judgement at Eindhoven University of Technology.
120 Polyamide-6 production starting from glucose and ammonia
e applied Aspen 7.3® simulation model of deamination of α-amino-ε-caprolactam is shown in Figure 5-10. e de ned design criteria, assumptions and conditions of the main equipment, depicted in Figure 5-10, are given in Appendix D.
5
Figure 5-10 Aspen 7.3® simulation model of deamination of α-amino-ε-caprolactam.
121 Chapter 5
e puri ed α-amino-ε-caprolactam stream is mixed with make-up tetrahydrofuran (via P1, 298 K, 0.1 MPa) and recycled THF and pressurized to 4.5 MPa (via CP1 and P2). Water is added to simulate the production of heavies. To avoid cavitation errors in the Aspen simulation, rst the stream is ashed (in Sep1), and therea er gas is compressed (via CP1) and liquid pumped (via P2) at 4.5 MPa before entering R1. Heavy oligomers as produced in the deamination section are modelled as hexacaprolactam, see Figure 5-11. e heavy oligomer formation requires 1 mole water per mole oligomer. is is probably not the case in reality, hence this small amount of water (169 kg/h) is simulated by adding to R1.
Figure 5-11 Model of heavy oligomers produced in deamination section (hexacaprolactam).
e combined streams are fed to the deamination reactor R1 (via CP2) along with make-up H2/N2 (90/10 vol%, 298 K, 2.1 MPa), make-up CH3SH-N2 (90/10 vol%,
298 K, 2.0 MPa) and recycled gas (via Sep7) to establish a H2:CH3SH ratio and a H2: α-amino-ε-caprolactam ratio as described in the work of Frost (59). R1 operates at 523 K and 4.5 MPa and yields vapour and liquid and is heated with Dowtherm® MX.
To avoid cavitation errors in the Aspen simulation, rst the reactor output stream is ashed (in Sep2) and therea er gas is compressed (via CP3) and liquid pumped (via P3) at 50 kPa before entering Dist1. e combined stream is distilled in a primary vacuum column (Dist1 at 50 kPa). Here light components, i.e. nitrogen, oxygen, ammonia,
CH3SH, alcohol and water, evaporate and α-amino-ε-caprolactam, ε-caprolactam, lysine and heavy components leave the column as bottom stream. e condenser of Dist1 is cooled with cooling water and the reboiler is heated with Dowtherm® MX. e bottom stream is processed in the ‘bottom separation’ section. e top stream of Dist1 is fed to the ‘top separation’ section.
Bottom separation section: e consecutive distillation column (Dist2, Radfrac model) operates at 20 kPa. P4 is the vacuum system of Dist2. e top stream of Dist2 contains 99.95% pure caprolactam which is further processed in to the polymerization section. e bottom stream of Dist2 is separated in Sep 3 where 99% of the heavy oligomers, 100% of the lysine, 1% of the α-amino-ε-caprolactam and 0.5% of the present ε-caprolactam is purged (via
122 Polyamide-6 production starting from glucose and ammonia
P5). e remainding part is re-pressured to 4.5 MPa, cooled with cooling water to 523 K (in HEx1) and recycled to R1. To avoid cavitation errors in the Aspen simulation, rst the stream is ashed (in Sep4) and thereaer gas compressed (via CP4) and liquid pumped (via P6) at 4.5 MPa before entering HEx1. e condenser of Dist2 is cooled with cooling water; the reboiler is heated with Dowtherm® MX. Sep3 is heated with Dowtherm® MX.
Top separation section: e top stream of Dist1 is re-pressured to 4.5 MPa (with CP5), cooled to 550 K (via HEx2), and fed in a high pressure distillation column (Dist3, Radfrac model) where water, THF and alcohol are condensed. e condenser of Dist3 is cooled with deep cooled methanol; the reboiler is heated with 4.2 MPa condensing steam. e top stream of Dist3 is partially condensed (in Sep7) at 255 K and 4.5 MPa to purge some produced ammonia (95% is recycled to R1, 5% is purged). Sep7 is cooled with deep cooled methanol. e bottom stream of Dist3 is decompressed to 0.1 MPa (with P7) and fed to an atmospheric distillation column (Radfrac model) to separate water. However, such a column cannot be modelled properly with Aspen and an approximation has to be simulated. Water in such a stream forms an azeotrope with THF. Additional alcohol has to be added in order to reach a molar alcohol fraction of 0.10. Additional alcohol (entrainer) breaks the water-THF azeotrope and consequently THF and water can be more easily distilled (130). is is done with make-up 1,2-propanediol (via P8, 0.1 MPa, 298 K) in combination with re uxed alcohol. First the mixture is cooled in HEx3 to 341 K (this is the boiling point of THF with 10% alcohol present). THF evaporates and water does not evaporate at this temperature (130). e mixture is led into Sep5 5 where 99.5% of the water and alcohol leave as ‘bottom’ stream and 99.99% of THF and small quantities of impurities leave as ‘top’ stream, resembling the desired design speci cations of the actual distillation column. In the simulation, the separation block only calculates the heat duty necessary for the evaporation, but not a separate reboiler and condenser duty. e complete heat exchange (cooling rst to 341 K and then separating) is supported by condensing 2.6 MPa steam. Since the separation should be quite easy now the azeotrope is broken, and water and THF have a dierence of 34 K in boiling point, it is expected that only a small re ux ratio will be necessary. is will result in a small condenser duty and added reboiler duty. e THF is recycled to R1 (via Sep1). e alcohol/water mixture contains a small THF impurity. e mixture is distilled in Dist4 to purge water and THF impurity as top stream, and to recycle 1,2-propanediol as bottom stream. is is modelled by removing the THF impurity before the distillation column and mixing it with the top stream aerwards to avoid the eects of the azeotrope (simulation constrain). e necessary heat is supplied by condensing 2.6 MPa steam. e condenser of Dist4 is cooled with cooling water, the reboiler is heated with condensing 2.6 MPa steam. Alcohol is recycled to both the
123 Chapter 5 deamination section (HEx3) and the cyclisation section, in a 98.66:1.34 %ratio. In this way only a small amount of fresh alcohol has to be added in both sections, e.g. a small fraction is recycled to the cyclisation section to replenish the alcohol that leaves the cyclisation section with the produced α-amino-ε-caprolactam.
Polymerization (Frost and ACA route) e polymerization design of the Frost processes (Lysine route) is similar to the polymerization design as has been described for the Benzene-Raschig route (Section 3.1)44. e applied Aspen 7.3® simulation model of the caprolactam polymerization is shown in Figure 5-12. Design criteria, assumptions and conditions of the main equipment used for the simulation of the caprolactam polymerization are listed in Appendix B.
Figure 5-12 Aspen 7.3® simulation model of caprolactam polymerization in the Frost routes.
44 Note: the chopper system requires 12.8 kJ/kg PA6 of electricity in the Frost polymerization and 10.9 kJ/ kg PA6 in the Frost PLUS polymerization.
124 Polyamide-6 production starting from glucose and ammonia
e applied Aspen 7.3® simulation model of 6-aminocaproic acid polymerization is shown in Figure 5-13. e de ned design criteria, assumptions and conditions of the main equipment, depicted in Figure 5-13, are given in Appendix D.
5
Figure 5-13 Aspen 7.3® simulation model of 6-aminocaproic acid polymerization in the ACA route.
125 Chapter 5
Crude 6-aminocaproic acid (6-ACA) is pumped with P1 to water separator Sep1. Sep1 operates at 400 K and atmospheric pressure. e main part of the water is purged. Heating is accomplished with 0.3 MPa steam. Subsequently, ‘puri ed’ 6-ACA is pressurized (with P2) to 0.25 MPa and mixed with recycled 6-ACA. e stream is heated to 513 K (in HEx1) to reach reactor condition. e heating is accomplished with Dowtherm® MX.
As explained in Section 3.1 we did not simulate the polymerization reactions. We only use the separately calculated reaction enthalpy/entropy data in the simulation model. An arti ce separator block (Sep2) is used to ‘remove’ the reacting 6-aminocaproic acid from the stream. Addition of water is simulated as ‘produced’ polycondensation water (at reaction conditions) and together with the unreacted 6-ACA and impurities led to distillation column (Dist1, Radfrac model) to evaporate water. Approx. 97% of the present water evaporates via Dist1, in analogy to commercial polymerization. e exothermic polymerization reaction heat is partly used to evaporate the water and the remaining part is cooled by evaporating cooling water.
e polymer leaving the polymerization column as strands still contains 6-aminocapronic acid and some residual water. e residual 6-ACA has to be extracted with hot water in a wash column to achieve the nal purity of the polymer. However, the polymer strands are rst cut in chips (granules). e chopper system requires 9.8 kJ /kg PA-6 of electricity. In the current simulation set-up we assume that this energy is completely absorbed by the polymer and has been taken into account for energy calculations. A water cooled heat exchanger is modelled for this (not shown in Figure 5-13).
Water is added to the ‘polymer’ stream to model the washing of the chips. e required wash water is 3.09 mt/h and enters wash column Sep3. Additionally we modelled the required water to eliminate the heat capacity of the produced (hot) polymer: 341 kg/h (also input of Sep3 via HEx2). Wash water leaves Sep3 at 368 K. e bottom stream of Sep3 represents the residual 6-ACA content of polyamide-6 chips aer water extraction (washing) and drying. e polymer chip temperature aer drying is 313 K and to eliminate the ‘overheated’ bottom stream of Sep3 this stream has to be ‘cooled’ to 313 K (in HEx3).
Wash water is subsequently recovered in the wash water recovery (modelled as Sep4) to obtain liquid 6-ACA (which is recycled to HEx1 via P3) and contaminated super-heated steam which can be reused for outside heating purposes. e recovery unit is heated by condensing 2.6 MPa steam. e nal drying and cooling of polyamide chips from
368 to 313 K is modelled with recycled nitrogen (Tin= 308 K, Tout= 360 K, pressure= 0.3 MPa). Heated nitrogen (in HEx4) is boosted (with a blower CP1, DP= 20 kPa) and cooled (in HEx5) with cooling water before reuse. e heat duty which has to be withdrawn is determined by the dierences in the speci c heat of the polymer at 368 and 313 K.
126 Polyamide-6 production starting from glucose and ammonia
5.2 Mass and energy balances Aspen simulation results in mass- and energy data of incoming, intermediate and outgoing process streams of the total production process or part of the production process. e analysis of mass data reveals production yields and prospects of yield improvement. Prospects for yield improvement are based on theoretical balancing equations. Aspen energy data are applied for the analysis of energy yields and prospects for energy optimization.
5.2.1 Frost and Frost PLUS manufacturing route Balancing Equation from BBBs to PA-6 e microbial pathway runs from glucose and ammonia to lysine. e simplest balanced equation that links glucose and ammonia to products is:
Equation 5-2
Equation 5-2 de nes a thermodynamic relationship between the used BBBs and the released products irrespective of the real pathway of the molecules within the microorganism. It is not energy-balanced (all balancing equations are not). e required energy for the reaction to proceed is released by energy carriers like NAD(P) H and ATP, which are regenerated at the expense of more glucose. Regeneration of energy carriers in microbes is comparable with energy generation by incineration of 5 fossil fuel in chemical plants. e ATP- and NAD(P)H- governed microbial reaction pathways and convection- and conductivity governed energy supply in chemical plants are subject to the same laws of thermodynamics and physical chemistry.
Lysine is next transformed into a-amino-ε-caprolactam and -CPL- using chemical technology. ese reactions are expressed in Equation 5-3 and Equation 5-4.
Equation 5-3
45 Equation 5-4
e simplest overall balancing equation is found by adding Equation 5-2, Equation 5-3 and Equation 5-4.
45 See footnote 38.
127 Chapter 5
Equation 5-5 Frost and Frost PLUS have identical balancing equations but dier in yield. BBBs are glucose, ammonia, and hydrogen used in the chemical technology step. e cancelling- out of ammonia in Equation 5-5 is similar to that in the Butadiene route in Equation 4-5. In practice, lysine needs puri cation by means of ion exchange. As to account for the alkalinity of lysine (pKa side chain is 10.54) in the broth, Equation 5-2 can be rewritten as:
Equation 5-6
In this reaction, H2CO3 did not react to H2O and CO2 as in Equation 5-2, but one of the protons remains on lysine, and the hydrogen carbonate counter ion remains in solution. Reaction with excess sulphuric acid (the second acidi cation, see Section 5.1) to 1 Equation 5-7 e double- protonated lysine is neutralized via reaction with ammonia: Equation 5-8 e overall balancing equation is: Equation 5-9 is is similar to the auxiliary use of sulphur and ammonia to enable the main reaction in the Benzene-Raschig process (Equation 3-3 and 3-4). Raw material consumption and product and waste production Figure 5-14 summarizes the material balance of the Frost and Frost PLUS manufacturing route which is based on the present process descriptions and the results of the corresponding Aspen models and simulations. e mass balance includes raw 128 Polyamide-6 production starting from glucose and ammonia materials used to produce polyamide-6 (corresponding the balancing equation) and other (auxiliary) materials that are used in the process. e theoretical required amount of raw materials of the Frost and Frost PLUS route is calculated with Equation 5-9. e theoretical and modelling results are compared in Table 5-2, in mol/mol – CPL– and kg/kg –CPL–. 5 Figure 5-14 Overall mass balance of polyamide-6 manufacturing from glucose and ammonia (Frost routes). 129 Chapter 5 Table 5-2 Comparison of theoretical and modelling results of the Frost and Frost PLUS manufacturing route. BBB C6H12O6 NH3 H2 H2SO4 O2 -CPL- H2O CO2 (NH4)2SO4 Waste Molar mass 180.16 17.03 2.016 98.09 32.00 113.16 18.01 44.01 132.17 Reactants Products Balancing 7/6 3 1 1 1 4 1 1 equation, mol/ mol –CPL– Frost Modeled 4.09 5.58 1.68 4.00 1.00 15.14 10.75 0.81 manufacturing route, mol/mol –CPL– Yield [mol/mol 29 54 60 25 %]* Frost PLUS 3.49 4.72 1.68 3.41 1.00 12.94 9.15 0.69 Modeled manufacturing route, mol/mol –CPL– Yield 33 64 60 29 [mol/mol %]* Balancing 1.86 0.45 0.02 0.87 1.00 0.64 0.39 1.17 0 equation, kg/kg –CPL– Frost Modeled 6.51 0.84 0.03 3.47 6.91 1.00 2.41 4.18 0.95 9.22 manufacturing route, kg/kg –CPL– Frost PLUS 5.55 0.71 0.03 2.96 5.89 1.00 2.06 3.56 0.81 7.71 Modeled manufacturing route, kg/kg –CPL– *: ratio of mol/mol –CPL– balancing equation and mol/mol –CPL– modelled manufacturing route. e amount of waste (solid, liquid, gas) has been calculated as the dierence between the mass input of BBB and the mass of products and co-products. e modelled waste, based on balancing equation, for the Frost and Frost PLUS route amounts 9.22 and 7.71 kg/kg PA6 respectively. e total amount of emissions (all outgoing streams, except PA-6), as shown in Figure 5-14, is 843 kg/s ( 120.22/102.43 kg/kg PA646) and signi cantly higher as predicted in Table 5-2 (ammonium sulphate 0.95/0.8146, water 46 46 46 46 2.41/2.06 , CO2 4.18/3.56 and waste 9.22/7.71 kg/kg PA6, in total 16.76/14.14 kg/kg PA6). However, the main part of the incoming streams of Figure 5-14 can be considered as inert with respect to balancing Equation 5-9 and therefore not accounted for, e.g. nitrogen (as part of air) and most of the water feed (80.24/68.1646 kg/kg PA6 used to supply glucose and ammonia). 46 Result split by “/”: results Frost process/results Frost PLUS process. 130 Polyamide-6 production starting from glucose and ammonia Energy balances Figure 5-15 depicts the exergy balance of polyamide-6 manufacturing starting with glucose and ammonia (Frost processes). e overall loss of work (exergy) amounts 38.5/44.346 MJ/kg PA6 due to irreversible processing. e main contributor to this loss is the deamination manufacturing step: 32.2/38.946 MJ/kg PA6. 5 Figure 5-15 Overall exergy balance of polyamide-6 manufacturing from glucose and ammonia (Frost routes). 131 Chapter 5 We consider the PA6 route Frost and Frost PLUS as a black box. Glucose, oxygen, hydrogen, ammonia and sulphuric acid enter the process (as BBBs). PA-6 (product) and process emissions like co-products, not reacted BBBs and by-products (waste)) leave the process. All material ows implicitly represent an enthalpy content calculated with the Aspen models and simulation. e dierence between the enthalpy content of incoming and outgoing streams equals the overall enthalpy of reaction ∆rH (main 47 reaction and side reactions). e result is ∆rH= 0.4/0.1 MJ/kg PA6 . Additionally, chemical manufacturing routes use electricity and steam to support their irreversible production processes. Excess process heat is emitted as waste heat via heated cooling water (CW), generated steam (GS) and condensate (SC). Figure 5-16 summarizes the overall secondary energy balance. e incoming and outgoing energy streams are expressed as excess enthalpy ows (in MJ/kg PA6.) e enthalpy content of ‘Steam’ (incoming), ‘CW’, ‘SC’ and ‘GS’ are de ned as the enthalpy dierence relative to the standard heat of formation of liquid water (- 285.68 kJ/mol). Another excess waste heat ow is ‘Heat drain via deep cooling system (HDDCS)’: excess heat is removed from the chemical process with deep cooled methanol and emitted in an external cooling aggregate (‘refrigerator model’). Figure 5-16 Modelled secondary energy balance of polyamide-6 manufacturing (Frost / Frost PLUS route48). Ideally, the secondary energy balance should be zero. However, the absolute discrepancy of 0.4/0.5 MJ/kg48 can be considered as negligible (1%). Part of the output excess heat can be reused, which has not been taken into account yet. Steam condensate can be reused for steam generation in system boundary 1 (closed circuit supposed as the base case). A credit can be granted, that reduces the total excess energy input. e maximum value of the credit would be 17.3/14.948 MJ/ kg PA6 if this heat could be reused with 100% e ciency. However, there is an e ciency factor 0 < k < 1 related to condensate recycling. Here we take k= 0.3. Generated steam is used via system boundary 1 (or recycled within system boundary 3, which is almost equivalent). A credit of 29.4/25.248 MJ/kg PA6 can be granted for GS. Depending on 47 See footnote 46. 48 See footnote 46. 132 Polyamide-6 production starting from glucose and ammonia e ciencies of heat reuse the Net Excess Enthalpy Input ranges from 47.5/65.348 (reuse) to 82.1/95.048 MJ/kg PA6 (no reuse). e reported energy balance (Figure 5-16) is expressed as secondary energy. Primary energy demand has to be used for the environmental impact analysis in Chapter 6 (see also Section 3.2). e results are summarized in Table 5-3. Table 5-3 Primary Energy Demand of polyamide-6 manufacturing from glucose and ammonia (Frost manufacturing routes). Material/energy feedstock Frost Frost PLUS PED [ MJ/kg PA6] PED [ MJ/kg PA6] Glucose 42.6 36.4 Hydrogen 2.7 2.3 Sulphuric acid 33.7 28.7 Ammonia 29.9 25.5 Steam 69.1 (19.8)* 59.1 (16.4)* Electricity 74.9 119.5 Total 252.9 (203.6)* 271.5 (228.8)* *Steam PED-value between brackets refers to the net value aer reuse of waste energy of the PA-6 production starting from BBBs (generated steam and upgraded condensate). 5.2.2 ACA manufacturing route Balancing Equation from BBBs to PA-6 e microbial pathway runs from glucose and ammonia to 6-aminocaproic acid, C6H13NO2. e simplest relationship between glucose and ammonia, 6-aminocaproic 5 acid and co-products is: Equation 5-10 6-aminocaproic acid is converted into PA-6: Equation 5-11 e overall balancing equation is: 49 Equation 5-12 e microbial pathway to 6-aminocaproic acid (Equation 5-1) is: 49 See footnote 38. 133 Chapter 5 6-aminocaproic acid is converted into PA-6: 49 Equation 5-13 Combining Equation 5-1 and Equation 5-13 gives: Equation 5-14 e dierence between Equation 5-12 and 5-14 is: Equation 5-15 Equation 5-15 can be considered an auxiliary, energy-providing reaction to Equation 5-1. In practice, 6-aminocaproic acid needs puri cation by means of ion exchange. Lysine is very alkaline, and is dominantly positively charged in aqueous environments below pH 9 (pKa of the amino side chain group is 10.54, the iso-electric point is 9.74). However, 6-aminocaproic acid has a much lower isoelectric point of 7.6, and thus exists as a neutral Zwitterion in the broth. Equation 5-10 can be rewritten as: Equation 5-16 Reaction with sulphuric acid yields a solution of the sulphate salt of 6-aminocaproic acid protonated on the amino group and a neutral carboxylic acid group, Equation 5-17. Equation 5-17 e protonated 6-aminocaproic acid, retained by ion exchange, is set free by reacting with ammonia. Equation 5-18 134 Polyamide-6 production starting from glucose and ammonia e overall balancing equation is: Equation 5-19 is is similar to the auxiliary use of sulphur and ammonia to enable the main reaction in the Benzene-Raschig process (Equation 3-3 and 3-4). Raw material consumption and product and waste production Figure 5-17 summarizes the material balance of the ACA manufacturing route that is based on the present process descriptions and the results of the corresponding Aspen models and simulations. e mass balance includes raw materials used to produce polyamide-6 (corresponding the balancing equation) and other (auxiliary) materials that are used in the process. e theoretical required amount of raw materials of the ACA route is calculated with the use of balancing Equation 5-19. e theoretical and modelling results are compared in Table 5-4, in mol/mol –CPL– and kg/kg –CPL–. 5 135 Chapter 5 Table 5-4 Comparison of theoretical and modelling results of the ACA manufacturing route. BBB C6H12O6 NH3 H2SO4 O2 -CPL- H2O CO2 (NH4)2SO4 Waste Molar mass 180.16 17.03 98.09 32.00 113.16 18.01 44.01 132.17 Reactants Products Balancing equation, mol/ 5/4 2 1/2 1 7/2 3/2 1/2 mol –CPL– Modeled manufacturing 3.13 5.18 3.90 18.71 1.00 8.73 8.23 0.63 route, mol/mol –CPL– Yield [mol/mol %]* 40 39 13 Balancing equation, kg/ 1.99 0.30 0.43 1.00 0.56 0.58 0.58 0 kg –CPL– Modeled manufacturing 4.98 0.78 3.38 5.29 1.00 1.39 3.20 0.74 8.10 route, kg/kg –CPL– *: ratio of mol/mol –CPL– balancing equation and mol/mol –CPL– modelled manufacturing route. e amount of waste (solid, liquid, gas) has been calculated as the dierence between the mass input of BBB and the mass of products and co-products. e modelled waste amounts 8.10 kg/kg PA6. e total amount of emissions (all outgoing streams, except PA-6), as shown in Figure 5-17, is 883 kg/s ( 96.40 kg/kg PA6) and signi cantly higher as predicted in Table 5-4 (ammonium sulphate 0.74, water 1.39, CO2 3.20 and waste 8.10 kg/kg PA6, in total 13.43 kg/kg PA6). However, the main part of the incoming streams of Figure 5-17 can be considered as inert with respect to balancing Equation 5-19 and therefore not accounted for, e.g. nitrogen (as part of air) and most of the water feed (64.64 kg/kg PA6 used to supply glucose and ammonia). 136 Polyamide-6 production starting from glucose and ammonia 5 Figure 5-17 Overall mass balance of polyamide-6 manufacturing from glucose and ammonia (ACA route). Energy balances Figure 5-18 depicts the exergy balance of polyamide-6 manufacturing starting with glucose and ammonia (ACA process). e overall loss of work (exergy) amounts 2.3 MJ/kg PA6 due to irreversible processing. e main contributor to this loss is the fermentation step: 1.4 MJ/kg PA6. 137 Chapter 5 Figure 5-18 Overall exergy balance of polyamide-6 manufacturing from glucose and ammonia (ACA routes). 138 Polyamide-6 production starting from glucose and ammonia Similar to the modelling of the Frost processes, we also consider the ACA polyamide-6 route as a black box. ∆rH= 1.0 MJ/kg PA6. Figure 5-19 Modelled secondary energy balance of polyamide-6 manufacturing (ACA route). Ideally, the secondary energy balance should be zero. However, the absolute discrepancy of 0.3 MJ/kg can be considered as negligible (3%). Depending on e ciencies of heat reuse the Net Excess Enthalpy Input ranges from 8.6 (reuse) to 10.0 MJ/kg PA6 (no reuse). e reported energy balance (Figure 5-19) is expressed as secondary energy. Primary energy demand has to be used for the environmental impact analysis in Chapter 6 (see also Section 3.2). e results are summarized in Table 5-5. Table 5-5 Primary Energy Demand of polyamide-6 manufacturing from glucose and ammonia 5 (ACA manufacturing route). Material/energy feedstock PED [ MJ/kg PA6] Glucose 32.6 Sulphuric acid 32.8 Ammonia 27.8 Steam 5.5 (3.5)* Electricity 13.8 Total 112.5 (110.5)* *Steam PED-value between brackets refers to the net value aer reuse of waste energy of the PA-6 production starting from BBBs (generated steam and upgraded condensate). e environmental impact of the mass- and energy balances of the Frost, Frost PLUS and ACA route will be discussed and compared with the other studied PA-6 manufacturing routes in Chapter 6. 139 Chapter 5 Sensitivity of biobased manufacturing routes e processing energy consumption of biobased routes depends strongly on the kind of biobased material feedstock and the yield of the fermentation step. So far we applied sugar beets as a natural resource for glucose. However, in many regions in the world the main ‘sugar/glucose’ resource is sugar cane. e PED value of sugar cane glucose is signi cantly lower than the PED value of sugar beet glucose: 2.51 MJ/kg vs. 6.55 MJ/ kg (66)(131). Hence, the total PED value (with heat integration) decreases from 110.5 to 90.5 MJ/kg PA6 (ACA route). e fermentation feed of the biobased processes is glucose (from sugar beets), ammonia and air. Part of the glucose is used to grow and maintain microbes for the fermentation process. Approx. 50 w/w% of the glucose is used for the production of the polyamide-6 precursor lysine or 6-aminocaproic acid. Only 44.4 w/w% of the glucose reacts to the considered precursor, 4.9 w/w% to metabolic by-products and 1.35 w/w% of the glucose feed does not react. Hence, the nal titer of the fermentation broth will be 24 g precursor/L. e maximum titer is 28 g/L broth (at 44.4 + 4.9 + 1.35= 50.65 w/w% glucose conversion). Partial poisoning of microbes in biobased processes is practically possible, causing lower titer values. We have examined the sensibility of the ACA route with respect to ‘total PED’ in the range of 28 to 10 g/L. e results are shown in Figure 5-20. e starting point for the calculation of ‘total PED’ at various titers is the present model result at 24 g a-aminocaproic acid/L broth (110.5 MJ/kg PA6, with heat integration). PED values at other titers are calculated by means of extrapolation relative to this starting point. We included for reasons of comparison the calculated PED value of both fossil based PA-6 manufacturing routes (with heat integration). We can conclude from Figure 5-20 that the fossil based Butadiene route already matches the biobased ACA route at a titer of 18 g/L; the fossil based Benzene-Raschig route matches at 15 g/L. is phenomena can be compared with catalyst deactivation in fossil based processes. 140 Polyamide-6 production starting from glucose and ammonia Figure 5-20 Sensitivity of ACA PA-6 manufacturing process with respect to total PED and the 6-ACA titre in the fermentation broth. 5 141 Comparati ve material, energy and GHG assessment of PA-6 manufacturing 6 Chapter 6 Greenhouse gas emissions seriously aect global warming and climate change. e main contributor of GHG emission is carbon dioxide which originates e.g. from the burning of fossil fuels to generate heat and electricity. Many industrial and human activities emit CO2. One of them is the chemical industry which has been studied in this work with the example of polyamide-6 manufacturing. In any model-based (comparative) assessment, the quality of the outcome is dependent on the quality of the data input. It is imperative to ensure that good quality data are collected where possible (which can be the case for incumbent processes). In cases where exact data are not available (in case of hypothetical routes), these should be substituted with data based on reasonable assumptions that are clearly explained (37). Moreover, the quality of the process design and modelling results depends also on the expertise and skills of the analyst. is may bring subjectivity to the estimation of e.g. the e ciency of the design, and material- and energy balances and may nally aect the quality of the generated data (53). e ve featured PA-6 routes are in very dierent stages of industrial development, ranging from proof of concept to commercial. e actual environmental performance is only known for one process: the commercial Benzene-Raschig route, as has been described in Chapter 3. e performance of other manufacturing routes is hypothetical and based on explicit assumptions as have been described in Chapter 4 and 5. is complicates the interpretability of ‘comparing’ of these routes. In addition, there is no doubt that all ve processes could be signi cantly improved. e main question is therefore not a comparison of the ve processes based on the modelled outcomes, but whether and which of the currently non-commercial PA-6 processes could surpass the environmental performance of the current commercial process. Furthermore what are the required conditions to achieve this goal, and what are the vulnerabilities and opportunities? In practice, improvements of chemical industrial processes can be achieved in dierent ways. For example new steps on an existing manufacturing route or as a whole new process. Improvements can be incremental contributions (e.g. energy integration) as well as a radical innovation such as the fermentation route. However, one speci c improvement can in uence PA-6 manufacturing routes analysed here, such as the improvement of the ammonia synthesis: a raw material for all ve PA-6 processes. Last but not least, the change to renewable processing energy can potentially improve the environmental performance of all processes. In practice, all the possibilities, either separately or combined, can be challenged. It must also be recognised that any process can be improved such, at least in principle, that the performance can meet the highest environmental regulations (with the use of end-of-pipe technology). e only question is: at what price? Apart from investment 144 Comparative material, energy and GHG assessment of PA-6 manufacturing costs, the operational performance of these processes can vary in terms of e.g. energy consumption and agriculture land use. And if renewable energy is scarce, it is questionable whether renewable energy should be applied in an intrinsic energy- ine cient process, in particular if there are alternatives available. We have used a holistic approach for the comparative material, energy and GHG assessment (see Chapter 2), that comprises the comparison and evaluation of the impact of dierent manufacturing routes and unit operations, dierent (combinations of) natural raw materials, and the application of dierent kinds of fossil and renewable energy for chemical manufacturing, in particular the resulting CO2 emission. We have used material- and energy balances of PA-6 manufacturing routes for the assessment. Before we start the comparative assessment, we will rst zoom in (Section 6.1) on the accuracy of the modelling results as have been reported in Chapter 3 to 5. We will also discuss in Section 6.2., the assumptions used in a scenario where the utilities are generated from fossil fuels (also for the biomass-based processes) and one where the utilities are generated by renewables (including the current petrochemical processes). 6.1 Accuracy of the modelling results e uncertainty in conclusions or claims about polyamide-6 manufacturing result from the possible sources of error in the obtained mass- and energy data. A quantitative sensitivity analysis is preferred, however, such evaluation has not been applied in the current study. e reason for this choice is the large number of input variables and the unknown partial uncertainty of certain variables. A precise sensitivity analysis requires a detailed study of all input variables, which could be tackled and assessed in future studies. 6 However, it is possible to provide a qualitative sensitivity consideration based on practical experience with process simulation and life cycle analysis. In depth discussions with DSM experts in these elds made it possible to identify and to estimate the possible sources of error in the mass- and energy data and to approximate the quantitative accuracy of the obtained results. First, uncertainty depends on the type of processes compared. For example the resulting uncertainty in comparing two fossil routes or two biobased routes on the one hand is smaller than in comparing a fossil- and a biobased route on the other hand. e obvious reason is partial elimination of shared error sources in shared fossil data or shared biobased data, which contrasts with the accumulation of errors in fossil- and biobased data when comparing fossil with biobased. 145 Chapter 6 Second, the overall accuracy is largely determined by the accuracy of mass- and energy data of the main process ows. e impact of small(er) side streams to the overall variance is oen only small, if not negligible, so less accurate characterization can be tolerated in such cases. ird, we used literature data of mass- and energy consumptions for processes outside system boundary 3, in boundary 1 and 2. Many of these data are ‘industrial averages’, meaning that they re ect the averaged performance of a varying number of existing commercial plants producing a particular basic building block. e detailed mass- and energy performance of each plant is accurately known, usually within 1%, by only to the plant owner. e reason is that ‘single-plant’ data are considered strictly con dential because they reveal crucial commercial information about cost positions and technology intelligence to the knowledgeable expert. e way around was created under the increasing demand of more openness about the environmental performance of industry, and this resulted in ‘environmental product declarations’, EPDs, of base chemicals (including the relevant basic building blocks) and polymers such as polyamide-6 (Ecoinvent database, Plastics Europe database (132)(133)(134)). ese EPDs are based on Life Cycle Analysis methods and contain averaged mass- and energy consumption data based on detailed material and energy balances of existing basic building block plants like those of polyamide-6 within system boundary 3. Participating companies under embargo disclosed plant data to a neutral party who assessed the average industrial performance e.g. in Europe. Uncertainty levels mentioned in these EPDs are typically approximately 15% and are dominated by the variance caused by averaging the results of participants; the uncertainty of underlying life cycle inventory data and methodology (e.g. Ecoinvent inventory database) is negligible in this respect (135). e advantage of this approach is that the energy- and mass data of basic building blocks obtained in this way can provide required mass- and energy data for the PA6 process variants within system boundary 3 from a generally accepted quantitative reference platform within system boundaries 1 and 2. Much eort has been invested in accurate data for the polyamide-6 process variants within system boundary 3 using the detailed process designs as described in Chapter 3 to 5. In this case, uncertainty can originate from limited practical process knowledge, process design choices, the level of detail of process models and Aspen simulations, and the availability and accuracy of literature data. e accuracy of the modelling results mainly depends on the level of design details. Models of important plant streams are detailed and usually considerably more accurate than small side streams whose maximum possible impact on the results was taken as a measure for the level of detail needed. e accuracy and reliability of (basic or estimated) process data, input variables, thermodynamic data, and approximations related to the applied equation of state are also sources of uncertainty. 146 Comparative material, energy and GHG assessment of PA-6 manufacturing e advice of the consulted DSM experts, based on their experience with comparable processes, designs and Aspen® simulations, is to assume a cumulative accuracy, expressed as relative variance, of maximum 15%. e accuracy of the previous key factors that aect the result are included to this maximum extent. We do not expect that the current absolute results will be aected, if a quantitative sensitivity analysis would be performed that results in a higher overall accuracy. If the accuracy would be higher (and the relative variance lower), the comparative conclusions can slightly change. In other words, if the results are currently comparable within a variance of ±15%, they can be signi cantly dierent at lower relative variance. However, the assessment of possible vulnerabilities of the processes and the evaluation of the opportunities to eliminate such vulnerabilities will most probably not be in uenced by the level of accuracy. 6.2 Background for the assumptions of the applied energy resources We start the comparative assessment with the assumption that the required processing energy for all routes originates from natural gas (valued as methane) as primary energy carrier. In this way we can make an unambiguous comparison between the dierent manufacturing routes as such. In practice, however, part of the energy can be of renewable origin (biomass, solar, wind, hydropower), depending on the energy mix of the grid. To investigate the environmental impact of renewable processing energy, we subsequently assume that steam and electricity used to produce polyamide-6 from basic building blocks is of solar/wind/hydropower origin. By doing this, we can investigate whether preferred options exist which can be the most sustainable with respect to less fossil energy consumption and CO2 emission, or to what extent (commercial) chemical manufacturing routes can be operated with sustainable energy, 6 and which route will perform best in terms of mitigation of CO2 emission. 6.3 Material consumption Although the product is the same, the studied PA-6 manufacturing routes vary in the used raw materials (renewable and non-renewable (fossil)). Carbon consumption Carbon (C) consumption, either of fossil or bio origin, has been used as an indicator for the analysis of the yield of PA-6 manufacturing. Nitrogen and hydrogen exhaustion is also been traced back to fossil carbon consumption as has been explained in Chapter 147 Chapter 6 2. Fossil C-consumption as well as biobased carbon consumption have an impact on the environment, albeit in dierent ways. E.g. fossil carbon consumption in uences environmental burdens like global warming. Bio (renewable) carbon consumption can interfere with food/feed production and (indirect) land use. We will elaborate the consequences in this chapter. Figure 6-1 summarizes the carbon consumption (consumption of natural resources). e results are derived from the material and energy balances as depicted in Chapter 3 to 5. We have distinguished in fossil based (brown bars) and biobased carbon (green bars) material feedstock. C-consumption of the energy part (grey bars) has been calculated by using the heat of combustion of natural gas (-797.5 MJ/kmol)50. We have also assumed that the purpose of all manufacturing routes is solely the production of polyamide-6 and that co-production of other valuable products is inevitable (and considered as waste). Figure 6-1 Total carbon consumption of the ve PA-6 manufacturing routes, expressed in kg C/kg PA6. e material feedstock data are derived from the results in Table 3-4, Table 4-4, Table 5-2, Table 5-4, the energy feedstock data are derived from the results in Table 3-5, Table 4-5, Table 5-3, Table 5-5. e energy feedstock data are with reuse of generated steam and condensate. 50 Glucose is used as feedstock in the biobased PA-6 routes (Frost and ACA). Glucose (sugar) processing can use bagasse, a by-product of the process, as its major processing energy source (157). However, still high amounts of fossil energy are used in the processing stage of sugar beet (158). We have assumed, for an unambiguous comparison, that all PA-6 routes use natural gas as primary energy carrier. 148 Comparative material, energy and GHG assessment of PA-6 manufacturing Notably, with the use of fossil (natural gas) based processing energy, both fossil feedstock based routes and the bio feedstock based ACA route perform comparably within the assumed accuracy. Both Frost routes perform signi cantly worse (higher carbon consumption). In case of replacing fossil based processing energy by renewable energy (only brown and green columns), all bio feedstock based routes perform equally worse compared with the fossil feedstock based routes. Hence, it is questionable whether renewable energy should be applied in bio feedstock based processes, in particular if there are fossil feedstock based alternatives available. e assumed titer of the ACA fermentation broth route is 24 g a-aminocaproic acid/L broth. If the titer would decrease to e.g. 14 g a-aminocaproic acid/L the total C-consumption would increase from 3.893 to 5.317 kg C/kg PA6. In such case, the biobased ACA route even exceeds the total carbon consumption of fossil based routes in both energy scenarios. Practical and environmental constraints and drawbacks e use of biobased feedstock to manufacture polyamide-6 implies also practical and environmental constraints and drawbacks. Interference of biobased PA-6 manufacturing with food/feed production and (indirect) land use can be considerable, as will be explained with the next example. e studied biobased routes use in average 2.27 kg biobased C/kg PA6, which equals 5.614 kg glucose/kg PA6. e glucose content of sugar beets is approximately 25%. Hence, approximately 22.5 kg sugar beets have to be harvested to produce 1 kg PA-6. A commercial production level of 250 kton PA6/ annum51 would require 5614 kton/annum, which needs 623 km2 (136) agricultural land to grow the beets (approximately the surface of South-Limburg (NL)). e annual sugar beet production in the Netherlands in 2013 was 5709 kton/annum (136). Moreover, replacing fossil based feedstock by biobased feedstock in existing plants is not easily done. Chemical plants will need to undergo major changes (read: nancial 6 investments) to be adapted to these new raw materials, since using a dierent starting material will oen require an entirely new process. Building a green eld plant with the current achievable low processing yield is also technically and probably nancially unattractive. With the present technology (Chapter 5) the required fermenter volumes for a commercial production scale are simply immense. e production attractiveness could be enhanced by improving the reaction conversion and selectivity of the fermentation process. is would lead to less renewable feedstock use and use of arable land and also to less fossil processing energy and mitigation of climate change eects. e negative consequences when switching to biobased energy fuel have been discussed by the Dutch ‘Koninklijke Nederlandse Akademie van Wetenschappen (KNAW)’ in 2015 (23) and can be illustrated with the example of wood as primary energy carrier. 51 Global annual PA-6 production capacity is approx. 6000 kton (159). 149 Chapter 6 Table 6-1 gives an overview of the practical and environmental consequences of wood combustion to produce sustainable steam and electricity to produce PA-6 from BBBs. e second column depicts the required primary energy for steam and electricity generation. e combustion heat of wood is 15 MJ/kg wood. e third column shows the corresponding kton of wood to support the production of 250 kton PA-6 per annum (commercial size). Column four gives the required m3 forest felling. e annual Dutch forest felling in 2013 was 1.3 million m3 (137). Clearly, the use of wood as energy supplier for industrial PA-6 production would infringe Dutch forest resources seriously. Table 6-1 Practical and environmental consequences of wood combustion to generate renewable steam and electricity in polyamide-6 manufacturing. Manufacturing route Renewable energy Required wood as Required forest felling (steam and electricity) primary energy fuel for needed to support an required to produce an annual production annual production of PA-6 from BBBs of 250 kton PA-6 250 kton PA-6 [MJ/kg PA6] [kton] [million m3] Benzene-Raschig 60.4 1007 1.34 Butadiene 67.2 1120 1.49 Frost 94.7 1578 2.10 Frost PLUS 135.9 2265 3.01 ACA 17.3 288 0.38 e challenge is how the increasing demands for food and bioenergy/material can be met in the future, particularly when water and land availability will be limited. Growing demand for biomass for energy in Europe and beyond, alongside growing interest in the use of biomass to replace petroleum and other conventional materials in the production of industrial products and chemicals, necessitates consideration of how the limited supplies of biomass can be used most e ciently (138). e EU Renewable Energy Directive (RED) targets, implemented to achieve climate change mitigation, aect the level of agricultural production in the EU and in the rest of the world (139). Mandatory sustainability criteria have been formulated in the RED for biomass feedstocks to be used in biofuels production (140). However, stricter criteria could reduce the cropping potential and change the crop mix signi cantly. Aer all, arable crops for biofuels usually compete with food and feed crops for higher quality land requiring a compensation for indirect land-use change emissions. ese stricter sustainability criteria can only be applied successfully if they are accompanied by a change in demand, in particular for lignocellulose biomass for advanced biofuels and other bioenergy uses (140). According to Böttcher et al. (139), global greenhouse gas emissions from agriculture and land-use change are anticipated to rise signi cantly up to 2030 due to various drivers (e.g. Gross Domestic Product and population, and also bioenergy demand) despite basic sustainability criteria implemented by the RED. 150 Comparative material, energy and GHG assessment of PA-6 manufacturing e production of energy crops implies a risk of polluting water and air, losing soil quality, enhancing erosion, and reducing biodiversity (141). Fernando et al. (141) conclude in their research that woody and lignocellulose crops have an advantage over annual crop systems, namely regarding erodibility and biodiversity. Alternative technologies are already being developed to overcome problems related to land competition as well as to mitigate environmental impacts: 2nd generation biofuels that use non-edible feedstock such as lignocellulose crops or bio waste from dierent sources (e.g. forest, agriculture, industry or municipalities) (142). More research eort is needed to improve yields of biomass crops growth and grow of crops on bad lands. Nitrogen and phosphorus are known to be essential nutrients for plant growth and development. Large quantities of chemical fertilizers are used to replenish soil nitrogen and phosphor, resulting in high costs and severe environmental contamination (e.g. pollution of groundwater and GHG emissions). Consequently, there has been a growing level of interest in sustainable agricultural practices to alleviate detrimental eects of intensive farming currently practiced. E.g. increasing and extending the role of bio-fertilizers would reduce the need for chemical fertilizers and decrease adverse environmental eects. (20)(143) 6.4 Energy consumption Of all factors that are important for our future, energy may be one of the most critical problems that we have to face in the 21st century. Global energy production is still governed by energy from fossil resources. However, fossil based energy generation has to be limited and nally stopped, also in industry (5)(7). By 2050 we need to achieve net zero GHG emissions to meet the Paris Goals. In particular, greenhouse gas emissions (e.g. CO emissions) can in uence and nally limit industrial activities. 2 6 erefore, it will become inevitably important to nd production routes with reduced processing energy demand and to change to more sustainable energy sources. We will assess the energy impact for the dierent polyamide-6 routes with the use of exergy analysis (to assess energy e ciency) and Primary Energy Demand comparison (to investigate the eectiveness of renewable energy implementation). Energy efficiency e principle of exergy loss is useful to compare the energy e ciency of dierent PA-6 production routes starting from BBBs. Figure 6-2 reveals the signi cant dierences in exergy loss between the ve PA-6 manufacturing routes. e overview is a summary of the exergy analysis as presented in Figure 3-17, Figure 4-10, Figure 5-15 and Figure 5-18. e vertical error bars represent ± 15% relative accuracy. 151 Chapter 6 Figure 6-2 Overview of the exergy loss of the PA-6 manufacturing routes. A premature conclusion could be that the bio feedstock based ACA- route is by far the most energy e cient process (lowest exergy loss). However, more detailed exergy analysis reveals constraints and opportunities within each manufacturing route. As will be discussed in the next sections, both fossil based routes have technological potential to surpass the fermentative route with respect to energy e ciency (exergy loss). Benzene-Raschig route Figure 6-3 reveals the exergy loss of the consecutive processing blocks of the Benzene- Raschig route. e cyclohexane oxidation towards cyclohexanone and cyclohexanol appears to be the major contributor to work loss (expressed as exergy loss). Figure 6-3 Exergy losses in polyamide-6 production starting from benzene and ammonia. e graphical presentation subdivides the production into the consecutive processing blocks. 152 Comparative material, energy and GHG assessment of PA-6 manufacturing e catalytic system currently in use for industrial cyclohexane oxidation employs homogeneous cobalt salts, molecular oxygen, and temperatures above 423 K, with conversions around 4% and selectivities of 85% to cyclohexanone and cyclohexanol. Under these conditions, the rate of cleavage of the C–H bond is high, which favours the formation of signi cant amounts of cyclohexyl hydroperoxide that decomposes to cyclohexanol and cyclohexanone at the end of the process and also generates carboxylic acids as by-products. However, as observed in other radical processes, the selectivity decreases with higher conversion rate. (77) erefore, the industrial process requires a rigid control of the conversion to maintain reasonable selectivity values. Many eorts have been made to develop new catalysts to oxidize cyclohexane under mild conditions and with dierent oxidants. e ideal oxidant is certainly molecular oxygen, which is cheap and quite selective if temperatures not higher than 343 K are used. e challenge is to nd active catalysts for these conditions. (78)(144)(145) Butadiene route Figure 6-4 presents the exergy loss of the consecutive processing blocks of the Butadiene route. e hydrogenation of adiponitrile to 6-aminocapronitrile appears to be the major contributor to work loss (expressed as exergy loss). 6 Figure 6-4 Exergy losses in polyamide-6 production starting from 1,3-butadiene and hydrogen cyanide. e graphical presentation subdivides the production into the consecutive processing blocks. Liquid ammonia is used as the preferred reaction solvent in the hydrogenation section to suppress the formation of by-products such as secondary and tertiary amines. e recycling and recompression of the ammonia requires large amounts of primary energy (14.6 MJ/kg PA6; secondary energy (electricity): 6.5 MJ/kg PA6). Methanol, as second best option, can also be used as solvent. e application of methanol is subject 153 Chapter 6 to further research (109). Replacement of pressurized ammonia by liquid methanol would almost completely blur the contribution of the hydrogenation section in the overall exergy loss. Milder conditions in the recovery of methanol (at low pressures) is the reason. In the latter case, this new fossil based manufacturing route would surpass the benchmark route with respect to the e ciency of energy use and even approaches the level of the ACA route. Frost/Frost PLUS route Figure 6-5 shows the exergy loss of the consecutive processing blocks of the Frost routes. e deamination of α-amino-ε-caprolactam towards ε-caprolactam appears to be the major contributor to work loss (expressed as exergy loss). We have to conclude that the Frost manufacturing routes, although promising from a biobased economy prospective, do not have the potential to surpass fossil based polyamide-6 manufacturing routes with respects to the e ciency of energy use. Figure 6-5 Exergy losses in polyamide-6 production starting from glucose and ammonia. e graphical presentation subdivides the production into the consecutive processing blocks. e top gure depicts the Frost route and the bottom gure shows the exergy analysis of the Frost PLUS route. 154 Comparative material, energy and GHG assessment of PA-6 manufacturing ACA route Figure 6-6 presents the exergy loss of the consecutive processing blocks of the ACA route. e fermentation of glucose appears to be the major contributor to work loss (expressed as exergy loss). Figure 6-6 Exergy losses in polyamide-6 production starting from starting from glucose and ammonia (ACA route). e graphical presentation subdivides the production into the consecutive processing blocks. However if partial poisoning of microbes occurs, the fermentative ACA route will surpass the sophisticated fossil based routes w.r.t. exergy losses. In this case, and if renewable energy is still scarce, it would be questionable whether renewable energy should be applied in such a biobased process, in particular if there are better (fossil) alternatives available for the next decades52. Primary Energy Demand comparison 6 Primary (processing) energy (PED) used in the production of BBBs contributes substantially to the total primary energy demand of PA-6 manufacturing starting from NRMs. Figure 6-7 shows that the relative contribution of material feedstock (BBBs) in the total PED demand varies between 40 and 85%. Less (fossil) energy demanding BBB manufacturing would be favourable with respect to fossil CO2 emission, e.g. the production of glucose from sugar cane is less energy demanding than the production of glucose from sugar beets (PED 2.51 vs. 6.55 MJ/kg, see Appendix A). 52 However, the use of fossil feedstock implies that incineration at the end-of-life results in CO2 emissions. Hence, it will depend on the stringency of the CO2 emission limits, if fossil feedstocks can be used without measures to counter the emissions at end-of-life. 155 Chapter 6 Technologies are currently available to produce electricity and steam with biomass energy or with solar, wind and/or hydropower. Replacing other fossil energy carriers is also possible, at least in principle, however not commercially applied yet. E.g. the simultaneous combustion of certain hydrocarbon fractions of naphtha to support the production of benzene and 1,3-butadiene or the co-combustion of methane to support the endothermic methane reforming to produce hydrogen could be replaced by the import of renewable/ sustainable energy via high temperature heat transfer liquids (e.g. liquid potassium). We have limited our assessment to the replacement of fossil based steam and electricity, used in the manufacturing of PA-6 from BBBs, by renewable ones. e PED impact on CO2 emission will be elaborated in this section and in the next section we will extend the CO2 discussion with the impact of the material feedstock. Figure 6-7 depicts the consumed primary energy of the ve simulated routes. e results summarize Table 3-5, Table 4-5, Table 5-3 and Table 5-5 (with reuse of generated steam and condensate). Figure 6-7 Primary energy demand in PA-6 manufacturing. Figure 6-7 reveals the PED, dierentiated in the contribution of steam (brown bars) and electricity (grey bars) as used in the production of PA-6 from BBBs and in the contribution of material feedstock (blue bars) and the total PED to produce PA-6 starting from natural resources (yellow bars). e vertical error bars represent ± 15% relative accuracy. Notably, the material feedstock contribution is dominant and almost comparable (range 81.8 – 114.2 MJ/kg PA6) for all routes. Steam contribution in the fossil feedstock based processes is relatively high compared with the biobased routes. 156 Comparative material, energy and GHG assessment of PA-6 manufacturing Electricity contribution in the Frost routes is extremely high compared with other routes. Comparison of the total PED results (yellow bars) reveals high energy demands for the Frost routes and to a lesser extent for both fossil based routes. e biobased ACA route appears to be the most energy e cient manufacturing route. Primary energy consumption relates to CO2-emission and in case of fossil based energy carriers to global warming. Hence, the ACA route performs as the most favourable route in this respect. However, if we replace fossil based steam and electricity by renewable energy we will notice an almost comparable PED (blue bars) for fossil based and biobased PA-6 routes (within ± 15% inaccuracy). In particular the fossil based Butadiene process could very well compete with the biobased routes. Moreover, if partial poisoning of microbes would occur, the ACA route would even surpass the fossil based routes in PED. And again one could argue, if it is questionable whether renewable energy should be applied in such a biobased process, in particular if there are better (fossil) alternatives available. Practical constraints and drawbacks e practical impact of sustainable energy use is demonstrated in Table 6-2. Table 6-2 Practical impact of using renewable steam and electricity in polyamide-6 manufacturing. Manufacturing Renewable energy Renewable Total Total renewable route to generate steam electricity required renewable energy to support required in the in the production energy an annual production of PA-6 of PA-6 from BBBs required in the production of 250 from BBBs production kton PA-6 of PA-6 from BBBs [MJ/kg PA6] [MJ/kg PA6] [MJ/kg PA6] [MJ/annum] Benzene-Raschig 47.9 5.6 53.5 1.34x1010 Butadiene 48.5 8.4 56.9 1.42x1010 Frost 19.8 33.6 53.4 1.34x1010 6 Frost PLUS 16.4 53.6 70.0 1.75x1010 ACA 3.5 6.2 9.7 0.24x1010 For comparison reasons: annual Dutch solar energy production was approx. 1.3x1010 MJ/annum in 2018 (146) and annual Dutch windmill energy production was approx. 3.6x1010 MJ/annum in 2018 (146). 6.5 Fossil carbon dioxide emission e energy related fossil carbon dioxide emission is the major contributor in the CO2 emission of PA-6 routes. However, also wasted fossil material carbon feedstock 157 Chapter 6 can contribute to climate change. Fossil C-atoms emitted as waste/by-products can be incinerated or be land lled. In both cases these C-atoms end as CO2 in the atmosphere. e GHG emission is immediately notable in case of incineration (short time eect), however, in the case of land lling GHG eects will be noticed over a longer time period. Polyamide-6 has an embedded carbon footprint of 2.3 kg CO2/kg PA6 which will contribute to global warming at the end-of-life stage of the product (if the C-atoms originate from fossil material feedstock). Figure 6-8 gives an overview of the dierent fossil carbon dioxide emission contributions. Figure 6-8 Fossil carbon dioxide emission of PA-6 routes [kg CO2/kg PA6]. The material feedstock data (green and blue bars) are derived from the results in Figure 6-1, the energy feedstock data (yellow and grey bars) are derived from the results in Figure 6-7. We notice comparable total CO2 emissions for all routes, except for the biobased ACA route, if fossil based steam and electricity is used (within ± 15% inaccuracy). e use of sustainable (not biobased) steam and electricity (Figure 6-8 without yellow bars) signi cantly decreases the cradle-to-grave CO2 emission of PA-6 manufacturing routes, particularly those with high electricity demand (both Frost routes). We notice an improvement of 1.3–1.4x for fossil feedstock based routes, 1.2x for the ACA route and 1.7–2.2x for the Frost routes. 158 Summary, conclusions and recommendati ons 7 Chapter 7 7.1 Background Anthropogenic greenhouse gas emissions are the main cause for global warming and climate change. Most of the greenhouse gas emissions (80%) originate from the burning of fossil fuels to generate heat and electricity. National policies such as the Dutch ‘Energieakkoord’ and other (inter)national agreements strive to reduce these emissions. Relatively high emission reductions have to take place in the industry sector because of its large technical saving potential. is focuses the spotlight on the chemical industry and oil re neries: by far the largest greenhouse gas emitters in the Dutch industry. For the chemical industry, the use of renewable energy and -material feedstock could potentially be the most powerful part in the transition to a clean and sustainable economy. However, the switch to such alternative sources will take some decennia to be nalized. In the meantime the industry should search for intermediate solutions. Several approaches are possible, e.g. energy reduction or process intensi cation of commercial fossil based routes (more eective unit operations, reuse of waste energy). Another possibility is using new, more advanced fossil fuel-based processes that require less fossil energy input, and therefore lead to less environmental impact. With the goal of a rigorous benchmark of these possibilities and therefore the identi cation of the best intermediate sustainable solution(s), it is of paramount importance to develop and apply reliable comparison methods. Life Cycle Assessment methodologies are commonly applied to assess the environmental consequences of operations and products. However, it appears that the quantitative outcome of such analysis strongly depends on choices made in the inventory analysis and the applied impact assessment step. e current thesis describes a method to assess the sustainability of chemical manufacturing. e polyamide-6 manufacturing industry is used as a case study. We have established ve manufacturing routes to produce polyamide-6 from fossil- or renewable raw materials such as fossil carbon materials, ores, water, air and biomass. Widely dierent options for improved manufacturing processes are investigated, and mutually compared with respect to the (combined) potential to reduce natural raw material consumption and fossil carbon dioxide emissions (see Table 7-1). 160 Summary, conclusions and recommendations Table 7-1 Overview of the studied polyamide-6 manufacturing routes. Type of processing Natural raw material Economical Labelled as (polymerization step is feedstock phase chemical) Crude oil, natural gas, air Benzene-Raschig Chemical Commercial Crude oil, natural gas, air Butadiene Chemical Non-commercial Sugar beets, natural gas, air Frost Bio-fermentation and Non-commercial (low deamination yield) chemical Sugar beets, natural gas, air Frost PLUS Bio-fermentation and Non-commercial (high deamination yield) chemical Sugar beets, natural gas, air ACA Bio-fermentation Non-commercial Although the product is the same, the studied manufacturing routes vary in the used raw materials. Basically we assume that the required processing energy for all routes originates from natural gas (methane) as primary energy carrier. Next the replacement of fossil based energy by renewable energy is explored. 7.2 Objectives of the dissertation research e dissertation research services three objectives: 1. e development of a method to assess and compare in an unambiguous way dierent manufacturing routes starting with natural raw materials to the same end product. e polyamide-6 (PA-6) manufacturing industry is used as a case study to illustrate such a comparative assessment of dierent manufacturing routes. 2. e reliable design of manufacturing routes to produce polyamide-6 comprising detailed process description and computer aided simulation modelling. We have designed ve manufacturing routes. Two are fossil-based and three are biobased. One of the ve routes is commercially in use, the other routes are still theoretical designs. 3. e environmental assessment and comparison of the selected PA-6 manufacturing routes with the use of the developed evaluation method. 7 Table 7-2 provides an overview of the thesis chapters and the objectives they address. 161 Chapter 7 Table 7-2 Overview of chapters and the addressed objectives. Chapter Title Objectives 1 2 3 2 Methodological characterization of PA-6 manufacturing routes x 3 Polyamide-6 production starting from benzene and ammonia x 4 Polyamide-6 production starting from 1,3-butadiene and hydrogen cyanide x 5 Polyamide-6 production starting from glucose and ammonia x 6 Comparative material, energy and GHG assessment of PA-6 manufacturing x 7 Summary of main results and concluding remarks x x x 7.3 Main findings and key results is section presents a summary of the main ndings and key results of this thesis for each objective. Objective 1 e development of a method to assess and compare in an unambiguous way dierent manufacturing routes starting with natural raw materials to the same end product. e polyamide-6 (PA-6) manufacturing industry is used as a case study to illustrate such a comparative assessment of dierent manufacturing routes. Assessment methods are indispensable when the aim is to determine or compare the sustainability level of existing chemical processes with options to improve, i.e. alternative feedstock, energy sources and/or processing routes. To eliminate ambiguity, and to identify and clarify uncertainties, chemical pathways, system boundaries of several production routes have to be de ned. Routes, that are compared by using this framework are based on the production of a unit mass of end product (1 kg) starting from the natural raw materials as available in nature. e comparison focusses on the e ciency of the use of these raw materials in the various processes and possible environmental consequences. Chapter 2 oers an extensive description of a method to assess and compare dierent manufacturing routes starting with natural raw materials to produce the same end product. Several characterization methods are described to analyse material- and energy balances in a straightforward and unambiguous way: (extended) carbon atom e ciency, balancing equations, exergy and primary energy demand analysis and (fossil) carbon dioxide emission. e quantitative outcome of the analysis is easy to interpret, for policymakers and researchers without professional LCA expertise, and does not suer from choices made in the selection of impact categories (see also Chapter 2). e analysis can support economic as well as environmental assessments. We have applied ‘system boundaries’ to enable quantitative evaluation and comparison of the manufacturing routes. Reference states are environmental circumstances of 25 °C and atmospheric pressure. 162 Summary, conclusions and recommendations Within the methodology of system boundaries we have de ned ways to characterize the performance of polyamide-6 manufacturing starting from natural resources. (Extended) carbon atom e ciency Atom e ciency analysis is useful to determine yields of chemical transitions and to investigate potential improvement directions with respect to lower feedstock use. However, atom e ciency cannot always be applied in a straightforward manner, especially not to entire, integral manufacturing routes such as the production of polyamide-6 from basic building blocks or natural resources. Hence, we have suggested ways to solve these constraints and have de ned and applied the principle of extended carbon atom e ciency. Balancing equation e balancing equation assumes a perfect chemical transformation (‘zero waste’) and expresses the theoretical molecular relationship between the starting materials and the nal product. e equation also sheds light on the minimum amounts of required raw materials. However, any extra excess amount of starting materials is transformed into waste (or co-products, in very speci c cases). In other words, the dierence between the theoretical yield (balancing equation) and the practical yield shows the maximum yield improvement potential of a chemical conversion. Exergy analysis and primary energy demand Energy e ciency of manufacturing routes can be expressed as exergy loss. Loss of work between two initial states is dependent on the speci c process to accomplish a certain end stage and varies with the way the process is performed (loss of entropy due to irreversible character of processing). Primary Energy Demand is de ned as the primary energy required to process natural raw materials to basic building blocks and to generate the required steam and electricity to produce polyamide-6 from basic building blocks. Carbon dioxide emission In this thesis we solely consider carbon dioxide emissions originating from fossil- or 7 biobased natural raw materials. Sources of carbon dioxide emission are the generation of energy from primary resources and carbon dioxide emissions due to incineration of carbon containing process waste, and emissions at end-of-life. If a mix of fossil and renewable energy and feedstock is used, the evaluation of carbon dioxide emission requires detailed carbon accounting. e adopted methodology has been applied in ve process designs to produce polyamide-6 from natural resources. By monitoring the e ciency of the use of carbon 163 Chapter 7 as material feedstock and as an energy carrier, and by next exploring the degrees of freedom to replace fossil carbon by non-fossil carbon, and fossil energy by renewable energy, insights can be obtained on strategies that maximize the bene cial eect for the environment, i.e. carbon dioxide emissions and energy e ciency of the entire process routes. Objective 2 e reliable design of manufacturing routes to produce polyamide-6 comprising detailed process description and computer aided simulation modelling. We have designed ve manufacturing routes. Two are fossil-based and three are biobased. One of the ve routes is commercially in use, the other routes are still theoretical designs. We have established ve manufacturing routes to produce polyamide-6 from fossil- or biomass based NRMs. e process designs are based on publicly available literature sources and have been extensively discussed with professional experts within Royal DSM. We have simulated and optimized the designs with Aspen Plus® simulation soware. Mass- and energy balances thus obtained are explored with the methodological characterization tools as described in Objective 1 and used to explore the impact of the use of dierent natural raw materials on the (fossil) carbon consumption and associated CO2 emission in polyamide-6 manufacturing. Key results for Objective 1 and 2 e ve process designs and -simulations (see Chapter 3 – 5) can be considered to be representative for polyamide-6 manufacturing, as will be demonstrated with the main ndings of Objective 3. Moreover, we can consider the applied assessment methodology as sound, also based on the argumentation and the outcome of the results of Objective 3. Objective 3 e environmental assessment and comparison of the selected PA-6 manufacturing routes with the use of the developed evaluation method. We have applied the de ned method to the ve manufacturing routes with the aim to assess the reliability of their designs and to illustrate the usability of the developed methodology. Comparative material, energy and GHG assessment of PA-6 manufacturing, as elaborated in Chapter 6, are further evaluated. Carbon consumption e yield of an organic chemical process is generally expressed as the ratio of the theoretically calculated amount of ‘carbon’ feedstock (based on the balancing equation principle) and the practically required amount of ‘carbon’ feedstock (based on the modelling), in mol/mol. 164 Summary, conclusions and recommendations e yield of the two examined fossil feedstock based polyamide-6 processes are 0.74 (benchmark route) and 0.68 [mol/mol], respectively. e yield of the Benzene-Raschig route is an established number (0.74 mol/mol) in polyamide-6 production. is justi es the conclusion that the present process design and -simulation (Chapter 3) can be considered to be representative to benchmark polyamide-6 industry. e yield of the three examined bio feedstock based processes are in the range of 0.29– 0.40 [mol/mol]. A premature conclusion could be that bioprocesses perform worse than fossil based processes. However, (as we have explained in Chapter 6, Figure 6-1) if we apply the principle of extended carbon atom e ciency (starting from NRMs) and take into account the material and energy carbon feedstock, the carbon consumption (yield performance) of the respective processes imply other conclusions. e fossil based processes yield a carbon consumption of 4.138 (benchmark process) and 3.697 [kg carbon/kg PA6], respectively. e biobased routes show carbon consumptions of 5.996–6.040 (Lysine route) and 3.893 (6-aminocaproic acid route). If renewable (not biobased) processing energy is applied, the carbon consumption of all bio feedstock based manufacturing routes even transcend the fossil feedstock based routes in this respect: 2.229–2.974 vs. 1.453–1.509 kg carbon/kg PA-6. Primary energy demand Chapter 3 describes the polyamide-6 production starting from benzene and ammonia. is route can be considered as the benchmark of e-caprolactam and polyamide-6 production. Benzene, ammonia, hydrogen, oxygen and sulphur are the (chemical) basic building blocks for this manufacturing route. ese chemical building blocks are extracted or produced from natural raw materials like crude oil, natural gas and air. e obtained energy balance reveals a primary energy demand of 174.6 MJ/kg PA6: 60.4 MJ on the account of steam and electricity to produce polyamide-6 from basic building blocks and 114.2 MJ related to the production of basic building blocks from natural raw materials. PlasticsEurope reports a primary energy demand of 90.7 MJ and 38.5 MJ respectively; in total 129.2 MJ/kg PA6 (147). However, PlasticsEurope data (147) are anonymous industrial averages of respective manufacturing sites and intrinsically dierent e-caprolactam and polymerization technologies. e PlasticsEurope primary 7 energy data to produce basic building blocks from natural raw materials are subject of Life Cycle Assessment allocation and system expansion methods which are not revealed in detail due to the anonymity of the data sources. e data of our simulation and evaluation are not subject to the above described constraints. All primary energy is assigned exclusively to polyamide-6 and not in proportions to all valuable end products such as the co-product ammonium sulphate. is argumentation, next to yield performance, also justi es the conclusion that the present process design 165 Chapter 7 and -simulation (Chapter 3) can be considered to be representative for benchmark polyamide-6 manufacturing. Chapter 4 describes the polyamide-6 production starting from 1,3-butadiene and hydrogen cyanide, which is currently not in commercial use. Hydrogen cyanide, 1,3-butadiene and hydrogen are the (chemical) basic building blocks for this manufacturing route. ese chemical building blocks are extracted or produced from natural raw materials like crude oil, natural gas and air. e obtained energy balance reveals a primary energy demand of 149.0 MJ/kg PA6: 67.2 MJ on the account of steam and electricity to produce polyamide-6 from basic building blocks and 81.8 MJ related to the production of basic building blocks from natural raw materials. e primary energy demand for the production of steam and electricity is comparable with the energy needed in the previous described benchmark manufacturing route. However, the energy used for BBBs production is substantially lower than in the benchmark route (81.8 vs. 114.2 MJ/kg PA6) which can be traced back to the ammonium sulphate co-production in the benchmark route (sulphur dioxide and oleum production accounts for 35.1 MJ/kg PA6)53. Chapter 5 describes the polyamide-6 production starting from glucose and ammonia. Glucose is fermented to obtain the precursor for polymerization. e precursor is L-lysine (Frost manufacturing routes) or 6-aminocaproic acid (ACA manufacturing route). Frost and Frost PLUS routes54 dier in deamination yield. Glucose, ammonia and hydrogen are the (chemical) basic building blocks for these manufacturing routes. ese building blocks are extracted or produced from natural raw materials like sugar beets, natural gas and air. e obtained energy balance of the Frost and Frost PLUS route reveal total primary energy demands of 203.6 and 228.8 MJ/kg PA6, respectively. e steam and electricity part is 94.7 and 135.9 MJ, respectively. e BBB part is 108.9 and 92.9 MJ, respectively. Striking is the substantially higher primary energy demand for steam and electricity, which is caused by higher electricity needs for compressors and vacuum pumps. e energy balance of the ACA route reveals a primary energy demand of 110.5 MJ: 17.3 MJ on the account of steam and electricity production and 93.2 MJ related to the production of basic building blocks from natural raw materials. We have acknowledged that the current titer in the process design (24 g a-aminocaproic acid/L broth) could 53 Due to allocation method: all to PA-6. 54 John W. Frost developed and patented the synthesis of e-caprolactam from L-lysine in the beginning of this century (58)(59). e Frost routes have been evaluated by commercial caprolactam producers as possible biobased production alternatives. Also fully fermentative routes, like the ACA route, have been subject of industrial/academic studies to nd alternative biobased routes to produce polyamide-6. 166 Summary, conclusions and recommendations be substantially lower in practice which results in lower fermentation yields. Lower titer values are realistic in practical circumstances where (partial) poisoning of microbes is always possible. If such poisoning of microbes would occur, the biobased ACA route could exceed the studied fossil based manufacturing routes in primary energy consumption. Exergy analysis e exergy analysis in Chapter 6 appears to be a useful tool to compare the energy e ciency of dierent PA-6 production routes starting from BBBs. Detailed exergy analysis reveals that both fossil based routes have technological potential to exceed the bio feedstock based ACA route with respect to energy e ciency (exergy loss). e ACA route will certainly perform worse in energy e ciency than the fossil feedstock based routes, if partial poisoning of microbes would occur. We also concluded that the Frost manufacturing routes, although promising from a biobased economy prospective, do not have the potential to exceed fossil based polyamide-6 manufacturing routes with respect to energy e ciency of the process. Carbon dioxide emission e energy related fossil carbon dioxide emission is the major GHG contributor of PA-6 routes. Also wasted fossil material carbon feedstock can contribute signi cantly when incinerated or land lled. Polyamide-6 has an embedded carbon footprint of 2.3 kg CO2/kg (which releases at the end-of-life). We notice signi cant mitigation of fossil CO2 emission when fossil based processing steam and electricity is replaced by renewable ones (Chapter 6). Key results for Objective 3 When looking at the PA-6 manufacturing process, we can conclude: • If fossil based steam and electricity is used in the production of PA-6 from basic building blocks, the cradle-to-grave fossil CO2 emission is comparable for all routes, except for the signi cantly lower emitting fermentative ACA route (fossil routes and biobased Frost routes range from 12.6 – 15.1 vs. 7.0 kg CO2/kg PA6 for the ACA route). 7 • If sustainable steam and electricity is used in the production of PA-6 from basic building blocks, the cradle-to-grave fossil CO2 emission decreases signi cantly. e fossil-fuel based routes have comparable fossil CO2 emissions (9.8 and 11.8 kg CO2/kg PA6), however, higher than in the biobased routes (range: 6.0 – 7.4 kg CO2/kg PA6) 167 Chapter 7 • If fossil based steam and electricity is used in the production of PA-6 from basic building blocks, the cradle-to-plant gate fossil CO2 emission is still comparable for all routes, except for the signi cantly lower emitting fermentative ACA route (fossil routes and biobased Frost routes range from 11.2 – 13.8 vs. 7.0 kg CO2/kg PA6 for the ACA route). • In case of sustainable steam and electricity use in the production of PA-6 from basic building blocks, the cradle-to-plant gate fossil CO2 emission of the fossil- fuel based Butadiene route and the biobased routes are comparable (6.0 – 7.5 kg CO2/kg PA6) and lower than the CO2 emission of the fossil based Benzene- Raschig route (9.5 kg CO2/kg PA6). • e energy e ciency of the fermentative ACA process is signi cantly higher than the e ciency of the fossil-fuel based routes and the biobased Frost routes. e Frost routes have energy demanding puri cation steps (compressing and decompressing) resulting in high exergy losses and low energy e ciency. • e fossil-fuel based PA-6 routes, particularly the improved Butadiene route, have the technological potential to surpass the biobased routes with respect to energy e ciency and primary energy loss. • Replacing fossil based feedstock by biobased feedstock in existing plants is not straightforward, since the use of a dierent starting material will oen require an entirely new process and major changes in the plant layout. Moreover, the feasibility of replacing fossil based feedstock by biobased feedstock is questionable whenever a steady supply of similar biomass is not possible. • From an energy e ciency perspective alone, it is questionable for the studied processes if the biobased process is preferred. When not accounting for end- of-life emissions, e.g. if they are compensated via carbon capture and storage or direct air CO2 capture, a shi to renewable energy sources in the existing process must be prioritized. However, if the total life-cycle GHG emissions are considered this result may change and favour the biobased route, though at the expense of increased energy use. 168 Summary, conclusions and recommendations 7.4 Limitations of the research In this thesis, methodological particularities and limitations were identi ed. With respect to process designing and process simulation: • We have used the standard thermodynamic properties as available in the Aspen® data library. Unavailable thermodynamic properties of certain intermediates and end products have been estimated with group contribution methods, which may introduce higher uncertainties in the simulation results. • We have applied black box approaches in certain process models, which also introduces increased uncertainty in the simulation results. • We have used literature data for mass- and energy consumption in the manufacturing of base chemicals (BBBs) to produce polyamide-6. Many of these data are ‘industrial averages’, meaning that they re ect the averaged performance of a varying number of existing commercial plants. • We have provided a qualitative sensitivity consideration based on practical experience with process design and Aspen simulation. e accuracy of the results, expressed as relative variance, is estimated to be maximal 15%. We do not expect that the current absolute results will be aected by a lower relative variance, however, the comparative conclusions can slightly change. Results that are currently comparable within a variance of ±15% can be signi cantly dierent at lower relative variance. With respect to the (comparative) environmental assessment: We have focussed our comparative assessment on energy (exergy) e ciency, carbon consumption and related carbon dioxide emission. Other greenhouse gas emissions (e.g. N2O) are not included in the comparative assessment, although its impact on climate change can be incremental (depending on the degree of abatement (end-of- pipe technology)). 7 7.5 Further research Because the fossil-fuel based PA-6 routes have the technological potential to be more energy e cient, we conclude that it is advisable to critically assess all processing alternatives to reveal the impact on energy and material feedstock reduction and resulting reduction of the carbon footprint. A quantitative sensitivity analysis is preferred, however, such evaluation has not been 169 Chapter 7 applied in the current thesis study. We do not expect that the current conclusions will be aected if a quantitative sensitivity analysis is performed (due to possible higher overall accuracy). However, such quantitative sensitivity analysis can reveal possible sources of partial (too) high inaccuracy that can lead to adapted (intermediate) results. e current method and comparative assessment has been applied to certain industrial examples of polyamide-6 manufacturing. e method and (comparative) assessment, at least in principle, can also be applied to other industrial processes, either fossil or bio feedstock based. We suggest to apply the methodology to the integrated production of 1,3-butadiene and e-caprolactam from C6 sugars and the consecutive polymerization (the thesis research of Moncada Botero (53)) and compare the results with the results of the current work. 170 Samenvatti ng, conclusies en aanbevelingen 8 Chapter 8 8.1 Achtergrond Antropogene broeikasgasemissies zijn de belangrijkste oorzaak van de opwarming van de aarde en de klimaatverandering. De meeste broeikasgasemissies (80%) zijn aomstig van de verbranding van fossiele brandstoen om warmte en elektriciteit op te wekken. Nationaal beleid zoals het Nederlandse ‘Energieakkoord’ en andere (inter)nationale afspraken streven ernaar deze uitstoot te verminderen. In de industrie moeten relatief hoge emissiereducties plaatsvinden vanwege het grote technische besparingspotentieel. Dit richt de aandacht op de chemische industrie en oliera naderijen: verreweg de grootste broeikasgas uitstoters in de Nederlandse industrie. Voor de chemische industrie kan het gebruik van hernieuwbare energie en biomaterialen mogelijk het krachtigste middel zijn in de overgang naar een schone en duurzame economie. Het zal echter nog enkele decennia duren voordat de overstap naar dergelijke alternatieve bronnen is afgerond. Ondertussen moet de industrie zoeken naar tussenoplossingen. Er zijn verschillende benaderingen mogelijk, b.v. energiereductie of procesintensivering van commerciële op fossiele grondstoen gebaseerde routes (eectievere unit operations, hergebruik van rest energie). Een andere mogelijkheid is het gebruik van nieuwe geavanceerdere op fossiele grondstoen gebaseerde processen, die minder fossiele energie vereisen en daarom tot verminderde milieu-impact leiden. Het is van het grootste belang om betrouwbare vergelijkingsmethoden te ontwikkelen en toe te passen teneinde deze mogelijkheden op rigoureuze wijze te vergelijken en daarmee de beste intermediaire duurzame oplossing(en) te identi ceren. Levenscyclusanalyse methodologieën (LCA) worden gewoonlijk toegepast om de gevolgen voor het milieu door activiteiten en producten te beoordelen. Het blijkt echter dat de kwantitatieve uitkomst van een dergelijke analyse sterk aangt van de gemaakte keuzes in de inventarisanalyse en de toegepaste eectbeoordelingsstap. Het huidige proefschri beschrij een andere methode om de duurzaamheid van chemische productie te beoordelen. De productie van polyamide-6 (nylon) wordt gebruikt als studiecase. We hebben vijf productieroutes opgesteld om polyamide-6 uit fossiele of hernieuwbare grondstoen zoals fossiele koolstofmaterialen, ertsen, water, lucht en biomassa te produceren. Er zijn verschillende opties voor verbeterde productieprocessen onderzocht en onderling vergeleken met betrekking tot het (gecombineerde) potentieel om het verbruik van natuurlijke grondstoen en de uitstoot van fossiele kooldioxide te verminderen (zie tabel 8-1). 172 Samenvatting, conclusies en aanbevelingen Tabel 8-1 Overzicht van de bestudeerde polyamide-6 productie routes. Type verwerking Economische Natuurlijke grondstof Gelabeld als (polymerisatie stap is fase chemisch) Ruwe olie, aardgas, lucht Benzene-Raschig Chemisch Commercieel Ruwe olie, aardgas, lucht Butadiene Chemisch Niet- commercieel Suikerbieten, aardgas, lucht Frost Bio-fermentatie Niet- (lage de-aminatie opbrengst) en chemisch commercieel Suikerbieten, aardgas, lucht Frost PLUS Bio-fermentatie Niet- (hoge de-aminatie opbrengst) en chemisch commercieel Suikerbieten, aardgas, lucht ACA Bio-fermentatie Niet- commercieel Hoewel het product hetzelfde is, verschillen de bestudeerde productieroutes in gebruikte grondstoen. We gaan er in principe van uit dat de benodigde verwerkingsenergie voor alle routes aomstig is van aardgas (methaan) als primaire energiedrager. Vervolgens wordt de vervanging van fossiele energie door hernieuwbare energie verkend. 8.2 Doelstellingen van het onderzoek Het onderzoek hee drie doelstellingen: 1. De ontwikkeling van een methode om op eenduidige wijze verschillende productieroutes te beoordelen en te vergelijken vanaf de gebruikte natuurlijke grondstoen tot eenzelfde eindproduct. De polyamide-6 industrie wordt als casestudy gebruikt om een dergelijke vergelijkende beoordeling van verschillende productieroutes te illustreren. 2. Het maken van betrouwbare ontwerpen van polyamide-6 productieroutes met gedetailleerde procesbeschrijvingen en computerondersteunde simulatie modelleringen. We hebben vijf productieroutes ontworpen. Twee zijn fossiel- en drie zijn biogrondstof gebaseerd. Eén van de routes is al commercieel in productie, de andere routes zijn nog steeds theoretische ontwerpen. 3. De beoordeling en vergelijking van de geselecteerde polyamide-6-productieroutes met behulp van de ontwikkelde evaluatiemethode. 8 173 Chapter 8 8.3 Belangrijkste bevindingen en resultaten Deze sectie gee een samenvatting van de belangrijkste bevindingen en de belangrijkste resultaten voor elke doelstelling van dit proefschri. Doelstelling 1 De ontwikkeling van een methode om op eenduidige wijze verschillende productieroutes te beoordelen en te vergelijken vanaf de gebruikte natuurlijke grondstoen tot eenzelfde eindproduct. De polyamide-6 industrie wordt als casestudy gebruikt om een dergelijke vergelijkende beoordeling van verschillende productieroutes te illustreren. Beoordelingsmethoden zijn onmisbaar wanneer het doel is het duurzaamheidsniveau van bestaande chemische processen te bepalen of te vergelijken, inclusief opties voor verbetering zoals het gebruik van alternatieve grondstoen, -energiebronnen en/ of -verwerkingsroutes. Voor het identi ceren en verduidelijken van onzekerheden moeten zogenaamde ‘systeemgrenzen’ van de verschillende productieroutes worden gede nieerd. Routes, die op deze wijze worden vergeleken, gaan uit van de productie van een eenheidsmassa eindproduct (1 kg) en vanaf de grondstoen zoals die in de natuur beschikbaar zijn. De vergelijking richt zich op de e ciëntie in het gebruik van deze grondstoen in de verschillende processen en de daaruit voortkomende mogelijke milieu gevolgen. Hoofdstuk 2 gee een uitgebreide beschrijving van een methode om verschillende productieroutes vanaf de natuurlijke grondstoen tot eenzelfde eindproduct te beoordelen en te vergelijken. Er worden verschillende karakteriseringsmethoden beschreven om materiaal- en energiebalansen op een ongecompliceerde en ondubbelzinnige manier te analyseren: (uitgebreide) koolstofatoom e ciëntie, reactievergelijkingen, exergie- en primaire energie analyse en (fossiele) kooldioxide emissie. Het kwantitatieve resultaat van de analyse is voor beleidsmakers en onderzoekers zonder professionele LCA-expertise gemakkelijk te interpreteren. De analyse kan zowel voor economische als milieu beschouwingen worden gebruikt. We hebben het begrip ‘systeemgrenzen’ toegepast om een kwantitatieve evaluatie en vergelijking van de productieroutes mogelijk te maken. Referentie parameters zijn als 25 ° C en atmosferische druk gede nieerd. Binnen de ‘systeemgrenzen’ hebben we manieren gede nieerd om de prestaties van polyamide-6-productie van natuurlijke bronnen tot en met eindproduct te karakteriseren. (Uitgebreide) koolstofatoom e ciëntie Atoome ciëntie analyse is nuttig om opbrengsten van chemische conversies te bepalen en om mogelijke verbeteringsrichtingen met betrekking tot lager verbruik van reactanten te onderzoeken. Atoome ciëntie kan echter niet altijd op een eenvoudige 174 Samenvatting, conclusies en aanbevelingen manier worden toegepast, vooral niet op totale, integrale productieroutes zoals de productie van polyamide-6 vanuit reactanten of natuurlijke grondstoen. Daarom hebben we manieren voorgesteld om deze beperkingen op te lossen en hebben we het principe van uitgebreide koolstofatoom e ciëntie gede nieerd en toegepast. Reactievergelijkingen De reactievergelijking gaat uit van perfecte chemische conversie (‘nul afval’) en gee de theoretische moleculaire relatie tussen de uitgangsmaterialen en het eindproduct weer. De reactievergelijking gee ook de minimale hoeveelheden reactanten aan. Maar elke extra benodigde hoeveelheid uitgangsmateriaal wordt in afval (of in zeer speci eke gevallen in co-producten) omgezet. Met andere woorden, het verschil tussen de theoretische opbrengst (reactievergelijking) en de praktische opbrengst gee het maximale verbeterpotentieel van de opbrengst van een chemische conversie aan. Exergie- en primaire energie analyse Energie e ciëntie van productieroutes kan als exergieverlies worden uitgedrukt. Verlies van arbeid (exergie) tussen twee begintoestanden is aankelijk van het speci eke proces om een bepaalde eindfase te bereiken en varieert met de manier waarop het proces wordt uitgevoerd (verlies van entropie als gevolg van het onomkeerbaar karakter van het productie proces). Primaire energie behoee wordt gede nieerd als de primaire energie die nodig is om natuurlijke grondstoen tot reactanten te verwerken en om de benodigde stoom en elektriciteit op te wekken om polyamide-6 uit reactanten te produceren. Kooldioxide emissie In dit proefschri kijken we alleen naar de uitstoot van kooldioxide aomstig van natuurlijke grondstoen (fossiel of biomassa gebaseerd). Bronnen van kooldioxide emissie zijn de opwekking van energie uit primaire grondstoen, kooldioxide emissies als gevolg van verbranding van koolstooudend procesafval en emissies aan het einde van de levensduur van het product. Als een mix van fossiele en hernieuwbare energie en -grondstoen wordt gebruikt, vereist de evaluatie van de kooldioxide emissie een gedetailleerde koolstooekhouding. De gebruikte methodologie is in vijf procesontwerpen om polyamide-6 uit natuurlijke grondstoen te produceren getest. Door de e ciëntie van het gebruik van koolstof als grondstof en als energiedrager te monitoren en door vervolgens de vrijheidsgraden te verkennen om fossiele koolstof te vervangen door niet-fossiele koolstof en fossiele 8 energie door hernieuwbare energie, kunnen inzichten worden verkregen over strategieën die het gunstige eect voor het milieu maximaliseren: d.w.z. kooldioxide emissies en energie e ciëntie van de gehele procesroute. 175 Chapter 8 Doelstelling 2 Het maken van betrouwbare ontwerpen van polyamide-6 productieroutes met gedetailleerde procesbeschrijvingen en computerondersteunde simulatie modelleringen. We hebben vijf productieroutes ontworpen. Twee zijn fossiel- en drie zijn biogrondstof gebaseerd. Eén van de routes is al commercieel in productie, de andere routes zijn nog steeds theoretische ontwerpen. We hebben vijf routes uitgewerkt om polyamide-6 uit fossiele of biomassa grondstoen te produceren. De procesontwerpen zijn op informatie uit openbare literatuurbronnen gebaseerd en zijn uitvoerig met DSM experts besproken. De ontwerpen zijn met Aspen Plus® simulatiesoware gesimuleerd en geoptimaliseerd. De aldus verkregen massa- en energiebalansen zijn met de methodologische manieren van karakteriseren, zoals beschreven in doelstelling 1, verkend en gebruikt om de impact van het gebruik van verschillende natuurlijke grondstoen op de (fossiele) koolstofverbruik en de daaruit voortvloeiende CO2-uitstoot bij de productie van polyamide-6 te onderzoeken. Belangrijkste resultaten voor doelstelling 1 en 2 De vijf procesontwerpen en -simulaties (beschreven in de hoofdstukken 3, 4 en 5) kunnen als representatief voor de productie van polyamide-6 worden beschouwd, zoals zal worden aangetoond met de belangrijkste bevindingen van doelstelling 3. Bovendien kunnen we vaststellen dat de toegepaste beoordelingsmethodologie geschikt is, mede gebaseerd op de argumentatie en de uitkomst van de resultaten van doelstelling 3. Doelstelling 3 De beoordeling en vergelijking van de geselecteerde polyamide-6-productieroutes met behulp van de ontwikkelde evaluatiemethode. We hebben de gede nieerde methode op de vijf productieroutes toegepast met als doel de betrouwbaarheid van de ontwerpen te beoordelen en de bruikbaarheid van de ontwikkelde methodologie te illustreren. De vergelijkende materiaal-, energie- en broeikasgas beoordeling van polyamide-6-productie (zoals uitgewerkt in hoofdstuk 6) wordt verder geëvalueerd. Koolstofverbruik De opbrengst van een organisch chemisch proces wordt normaliter uitgedrukt als de verhouding van de theoretisch berekende hoeveelheid ‘koolstof’ grondstof (gebaseerd op de reactievergelijkingen) en de praktisch vereiste hoeveelheid ‘koolstof’ grondstof (gebaseerd op de modellering), uitgedrukt in mol/mol. De opbrengst van de twee onderzochte op fossiele grondstoen gebaseerde polyamide-6-processen is respectievelijk 0,74 (referentieroute) en 0,68 [mol/mol]. De vastgestelde opbrengst van 176 Samenvatting, conclusies en aanbevelingen de Benzene-Raschig route is realistisch (0,74 mol/mol) voor polyamide-6 productie. Dit rechtvaardigt de conclusie dat het huidige procesontwerp en de simulatie (hoofdstuk 3) voor de ‘benchmark’ productie van polyamide-6 als representatief kan worden beschouwd. De opbrengst van de drie onderzochte op biomassa gebaseerde processen ligt tussen 0,29 en 0,40 [mol/mol]. Een voorbarige conclusie zou kunnen zijn dat bio- processen slechter presteren dan fossiele processen. Echter, (zoals we hebben uitgelegd in hoofdstuk 6, Figuur 6-1) als we het principe van uitgebreide koolstofatoom e ciëntie toepassen (vanaf natuurlijke grondstoen) en rekening houden met materiaal- en energie grondstof, zal het koolstofverbruik van de respectievelijke processen tot andere conclusies leiden. De op fossiele grondstoen gebaseerde processen hebben dan een koolstofverbruik van respectievelijk 4,138 (benchmark proces) en 3,697 [kg koolstof/kg PA6]. De biomassa routes laten een koolstofverbruik zien van 5,996–6,040 (Lysine route) en 3,893 (6-aminocapronzuur route). Als duurzame (niet biomassa gebaseerde) verwerkingsenergie wordt toegepast, overstijgt het koolstofverbruik van alle op biomassa gebaseerde productieroutes het koolstofverbruik van op fossiele grondstoen gebaseerde routes: 2,229-2,974 vs. 1,453-1,509 kg koolstof/kg PA-6. Primaire energie analyse Hoofdstuk 3 beschrij de productie van polyamide-6 uitgaande van benzeen en ammoniak. Deze route kan als de benchmark voor de productie van e--caprolactam en polyamide-6 worden beschouwd. Benzeen, ammoniak, waterstof, zuurstof en zwavel zijn de bouwstenen voor deze productieroute. Deze chemische bouwstenen worden gewonnen of geproduceerd uit natuurlijke grondstoen zoals ruwe olie, aardgas en lucht. De verkregen energiebalans toont een primaire energie verbruik van 174,6 MJ/ kg PA6: 60,4 MJ t.b.v. stoom en elektriciteit om polyamide-6 uit de chemische bouwstenen te produceren en 114,2 MJ om de chemische bouwstenen uit natuurlijke grondstoen te produceren. PlasticsEurope rapporteert een primaire energie behoee van respectievelijk 90,7 MJ en 38,5 MJ; in totaal 129,2 MJ/kg PA6. De gegevens van PlasticsEurope zijn echter anonieme industriële gemiddelden van afzonderlijke productielocaties en intrinsiek verschillende e-caprolactam- en polymerisatie technologieën. De PlasticsEurope primaire energie gegevens om (chemische) grondstoen uit natuurlijke grondstoen te produceren zijn aan LCA allocatie methoden onderworpen die vanwege de anonimiteit van de gegevensbronnen niet in detail worden vrijgegeven. De gegevens van onze simulatie en evaluatie zijn aan de hierboven beschreven beperkingen niet onderworpen. Alle primaire energie wordt uitsluitend aan polyamide-6 toegewezen en niet verhoudingsgewijs aan alle waardevolle eindproducten (zoals b.v. het bijproduct ammoniumsulfaat). Deze 8 argumentatie rechtvaardigt dan ook de conclusie dat het huidige procesontwerp en -simulatie (hoofdstuk 3) als representatief voor ‘benchmark’ polyamide-6 productie kan worden beschouwd. 177 Chapter 8 Hoofdstuk 4 beschrij de niet-commerciële productie van polyamide-6 uitgaande van 1,3-butadieen en waterstofcyanide. Waterstofcyanide, 1,3-butadieen en waterstof zijn de bouwstenen voor deze productieroute. Deze chemische bouwstenen worden uit natuurlijke grondstoen zoals ruwe olie, aardgas en lucht gewonnen of geproduceerd. De verkregen energiebalans toont een primaire energie verbruik van 149,0 MJ/kg PA6: 67,2 MJ voor stoom en elektriciteit om polyamide-6 uit de chemische bouwstenen te produceren en 81,8 MJ om de chemische bouwstenen uit natuurlijke grondstoen te produceren. De primaire energieconsumptie voor de productie van stoom en elektriciteit is vergelijkbaar met de energie die nodig is in de eerder beschreven benchmark productieroute. De energie die voor de productie van chemische bouwstenen wordt gebruikt is echter aanzienlijk lager dan in de benchmark route (81,8 vs. 114,2 MJ/kg PA6) en is terug te voeren op de coproductie van ammoniumsulfaat in de benchmarkroute (de productie van zwaveldioxide en oleum is verantwoordelijk voor 35,1 MJ/kg PA6). Hoofdstuk 5 beschrij de productie van polyamide-6 uitgaande van glucose en ammoniak. Glucose wordt gefermenteerd om het monomeer voor de polymerisatie te verkrijgen. Het monomeer is L-lysine (Frost productie routes) of 6-aminocapronzuur (ACA productie route). Frost en Frost PLUS routes verschillen in de-aminatie opbrengst. Glucose, ammoniak en waterstof zijn de bouwstenen voor deze productieroute. Deze bouwstenen worden uit natuurlijke grondstoen zoals suikerbieten, aardgas en lucht gewonnen of geproduceerd. De verkregen energiebalans van de Frost en Frost PLUS route toont een totale primaire energie verbruik van respectievelijk 203,6 en 228,8 MJ/kg PA6. De stoom- en elektriciteit bijdrage is respectievelijk 94,7 en 135,9 MJ. De bijdrage van de chemische bouwstenen is respectievelijk 108,9 en 92,9 MJ. Opvallend is de substantieel hogere behoee aan primaire energie voor stoom en elektriciteit, hetgeen wordt veroorzaakt door hogere elektriciteit behoeen voor compressoren en vacuümpompen. De energiebalans van de ACA -route laat een primaire energie verbruik zien van 110,5: 17,3 vanwege de productie van stoom en elektriciteit en 93,2 MJ gerelateerd aan de productie van chemische bouwstenen uit natuurlijke grondstoen. We hebben aangenomen dat de huidige titer in het procesontwerp (24 a-aminocapronzuur/liter bio reactiemassa) in de praktijk aanzienlijk lager zou kunnen zijn, wat resulteert in lagere fermentatierendementen. Lagere titerwaarden zijn realistisch in praktische omstandigheden waar (gedeeltelijke) vergiiging van microben altijd mogelijk is. Als een dergelijke vergiiging van microben zou optreden, zou de bio ACA-route de fossiele productieroutes in primair energieverbruik kunnen overschrijden. 178 Samenvatting, conclusies en aanbevelingen Exergie analyse De exergie analyse in hoofdstuk 6 blijkt een nuttig hulpmiddel te zijn om de energie e ciëntie van verschillende polyamide-6 productieroutes te vergelijken, startend vanuit de bouwstenen. Uit een gedetailleerde exergieanalyse blijkt dat beide op fossiele grondstoen gebaseerde routes het technologisch potentieel hebben om de op biomassa gebaseerde ACA route met betrekking tot energie e ciëntie (uitgedrukt in exergieverlies) te overtreen. Als er gedeeltelijke vergiiging van microben zou optreden, zal t.a.v. energie e ciëntie de ACA route zeker slechter presteren dan de op fossiele grondstoen gebaseerde routes. We concludeerden tevens dat de Frost productieroutes, hoewel veelbelovend vanuit een biomaterialen perspectief, niet het potentieel hebben om fossiele polyamide-6 productieroutes met betrekking tot de energie-e ciëntie te overtreen. Kooldioxide emissie De energie gerelateerde uitstoot van fossiele kooldioxide is de belangrijkste broeikasgas bijdrage in polyamide-6 productie. Ook als afval geëmitteerde fossiele koolstof kan aanzienlijk bijdragen tot broeikasgas eecten in geval van verbranden of vuilstorten. Polyamide-6 hee een eigen kooldioxide voetafdruk van 2,3 kg CO2/kg (die vrijkomt bij verbranding aan het einde van de levensduur). We zien een aanzienlijke vermindering van de fossiele CO2-uitstoot wanneer stoom en elektriciteit uit hernieuwbare bronnen wordt gegenereerd (hoofdstuk 6). Belangrijkste resultaten voor doelstelling 3 Als we naar het polyamide-6-productieproces kijken kunnen we concluderen: • Als bij de productie van polyamide-6 uit bouwstenen stoom en elektriciteit, opgewekt uit fossiele brandstoen, worden gebruikt, is de ‘cradle-to-grave’ fossiele CO2-uitstoot vergelijkbaar voor alle routes, behalve voor de aanzienlijk lagere emitterende ACA-route (fossiele routes en bio-Frost routes variëren van 12,6 – 15,1 versus 7,0 kg CO2/kg PA6 voor de ACA-route). • Als bij de productie van PA-6 uit bouwstenen duurzame stoom en elektriciteit worden gebruikt, neemt de ‘cradle-to-grave’ fossiele CO2-uitstoot aanzienlijk af. De routes op basis van fossiele grondstoen hebben dan vergelijkbare fossiele CO2-emissies (9,8 en 11,8 kg CO2/kg PA6), maar wel hoger dan in de bio routes (bereik: 6,0 – 7,4 kg CO2/kg PA6). • Als bij de productie van polyamide-6 uit bouwstenen stoom en elektriciteit, opgewekt uit fossiele brandstoen, worden gebruikt, is de ‘cradle-to-plant gate’ 8 fossiele CO2-uitstoot nog steeds vergelijkbaar voor alle routes, behalve voor de aanzienlijk lager emitterende ACA-route (fossiele routes en bio-Frost routes variëren van 11,2 – 13,8 vs. 7,0 kg CO2/kg PA6 voor de ACA-route). 179 Chapter 8 • Als bij de productie van PA-6 uit bouwstenen duurzame stoom en elektriciteit worden gebruikt, zijn de ‘cradle-to-plant gate’ fossiele CO2-uitstoot van de fossiele Butadiene route en de bioroutes vergelijkbaar (6,0 – 7,5 kg CO2 /kg PA6) en lager dan de CO2-uitstoot van de fossiele Benzene-Raschig route (9,5 kg CO2/kg PA6). • De energie-e ciëntie van het ACA-proces is aanzienlijk hoger dan de e ciëntie van de op fossiele grondstoen gebaseerde routes en de bio-Frost routes. De Frost routes hebben veeleisende zuiveringsstappen (comprimeren en decomprimeren), wat resulteert in hoge exergieverliezen en een lage energie-e ciëntie. • De op fossiele brandstoen gebaseerde polyamide-6-routes, met name de verbeterde Butadiene route, hebben het technologische potentieel om de bio routes met betrekking tot energie-e ciëntie en primair energieverlies te overtreen. • Het vervangen van fossiele grondstoen door bio grondstoen in bestaande fabrieken is niet eenvoudig, omdat het gebruik van een ander uitgangsmateriaal vaak een geheel nieuw proces en grote veranderingen in de fabriek lay-out vereist. Bovendien is de haalbaarheid van vervanging van fossiele grondstoen door bio grondstoen twijfelachtig wanneer een constante aanvoer van vergelijkbare biomassa niet mogelijk is. • Alleen al vanuit het oogpunt van energie-e ciëntie is het voor de bestudeerde processen twijfelachtig of bio processen de voorkeur hebben. Wanneer geen rekening wordt gehouden met emissies aan het einde van de levensduur, b.v. als ze worden gecompenseerd via koolstof afvang en -opslag of directe CO2-afvang in de lucht, moet prioriteit aan het toepassen van hernieuwbare energie in het bestaande proces worden gegeven. Als echter naar de totale broeikasgasemissies gedurende de levenscyclus wordt gekeken, kan dit resultaat veranderen en de bio route begunstigen, hoewel dit ten koste van het toegenomen energieverbruik zal gaan. 180 Samenvatting, conclusies en aanbevelingen 8.4 Beperkingen van het onderzoek In dit proefschri werden methodologische bijzonderheden en beperkingen geïdenti ceerd. Met betrekking tot procesontwerp en processimulatie: • We hebben de standaard thermodynamische eigenschappen gebruikt die beschikbaar zijn in de Aspen®-database. Onbekende thermodynamische eigenschappen van bepaalde tussenproducten en eindproducten zijn met groepsbijdrage methoden geschat, wat tot grotere onzekerheden in de simulatieresultaten kan leiden. • We hebben black box-benaderingen in bepaalde procesmodellen toegepast, wat ook tot meer onzekerheid in de simulatieresultaten leidt. • We hebben literatuurgegevens van massa- en energieverbruiken voor de productie van bouwstenen, die in de productie van polyamide-6 worden toegepast, gebruikt. Veel van deze gegevens zijn ‘industriële gemiddelden’, wat betekent dat ze de gemiddelden van verschillende bestaande commerciële fabrieken weerspiegelen. • We hebben op basis van praktische ervaringen met procesontwerpen en Aspen- simulaties een kwalitatieve gevoeligheidsanalyse voorgesteld. De nauwkeurigheid van de resultaten, uitgedrukt als relatieve variantie, wordt op maximaal 15% geschat. We verwachten niet dat de huidige absolute resultaten door een lagere relatieve variantie zullen worden beïnvloed, maar de vergelijkende conclusies kunnen enigszins veranderen. Resultaten die momenteel binnen een variantie van ± 15% vergelijkbaar zijn kunnen bij een lagere relatieve variantie signi cant verschillen. Met betrekking tot de (vergelijkende) milieubeoordeling: We hebben onze vergelijkende beoordeling op energie (exergie) e ciëntie, koolstofverbruik en gerelateerde kooldioxide-uitstoot gericht. Andere broeikasgasemissies (bijv. N2O) worden in de vergelijkende beoordeling niet meegenomen, hoewel het eect op de klimaatverandering kan toenemen (aankelijk van de mate van bestrijding (end-of-pipe-technologie)). 8 181 Chapter 8 8.5 Verder onderzoek Omdat de op fossiele grondstoen gebaseerde polyamide-6-routes het technologische potentieel hebben om energie-e ciënter te zijn, concluderen we dat het raadzaam is om alle verwerkingsalternatieven kritisch te beoordelen om zo de impact op de vermindering van het energie- en materiaalgrondstoen verbruik en de resulterende vermindering van de koolstof dioxide voetafdruk te bepalen. Een kwantitatieve gevoeligheidsanalyse hee de voorkeur, maar deze evaluatie is niet in de huidige thesisstudie toegepast. We verwachten niet dat de huidige conclusies zullen worden beïnvloed als een kwantitatieve gevoeligheidsanalyse wordt uitgevoerd. Een dergelijke kwantitatieve gevoeligheidsanalyse kan echter mogelijke bronnen van gedeeltelijke (te) hoge onnauwkeurigheid aan het licht brengen die tot aangepaste (tussen) resultaten kunnen leiden. De huidige methode en vergelijkende beoordeling is op bepaalde industriële voorbeelden van polyamide-6 productie toegepast. De methode en (vergelijkende) beoordeling, althans in principe, kan ook op andere industriële processen worden toegepast, hetzij op fossiele hetzij op biologische grondstoen gebaseerd. We stellen voor om de methodologie op de geïntegreerde productie van 1,3-butadieen en e-caprolactam uit C6-suikers en de daarop volgende polymerisatie toe te passen en de resultaten te vergelijken (het proefschrionderzoek van Moncada Botero). 182 References References 1. International Energy Agency. World Energy Outlook 2019; Global Energy Trends. 2019. 2. International Energy Agency. World Energy Statistics 2019. 2019. 3. CBS Statline. 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Global Nylon Market - Supply Demand Analytics Service 2018 Report. 2019. 190 Acknowledgement Acknowledgement “I Have a Dream” the American civil rights activist Martin Luther King, Jr. said on August 28th, 1963. My personal believe is that everyone has a dream, at least should have a dream. I had a dream at the age of 12. My dream depicted a scienti c entourage, research people in white lab coats and myself. e message of that dream, in my opinion, was that I should become a doctor. Due to social circumstances, the dream stopped at the age of 20 when I passed my BSc degree. I started a successful carrier at Royal DSM and founded a family, which I never regretted. However, the dream was still there….and should once be ful lled. I started the ful lling of my dream at the age of 51. First, I nalized the MSc study ‘Polymers and Compounds’ at the Eindhoven University of Technology (TU/e) successfully and my mission will be completed aer my PhD defence in 2020: dr. ir. e PhD project started at the TU/e in 2011 under the supervision of prof. dr. Jan Meuldijk. e support and coaching of Jan Meuldijk was very helpful in the rst part of the study. Jan, my special thanks! I have greatly enjoyed working with you. Your feedback was always valuable. I am also very grateful for the support of dr. Paul Brandts of DSM. Paul, it was a heaven’s gi to have you as a coach in structuring my research and formulating the results. It felt not always as heaven (), however, without your help this thesis would never have reached the present quality. I would also like to thank dr. Gerard Krooshof of DSM for the thermodynamic discussions and support. Gerard, your knowledge and experience of thermodynamics are beyond praise. I learned a lot from you about thermodynamic relations, validity of methods, and the use of simulation modelling tools. Also many thanks to Dave Morris for the review of the thesis with respect to the English grammar and his advices as a LCA expert. I am also very grateful for the support of ir. Daan Berends and ir. Olaf Vorselen. Daan and Olaf, thanks for your assistance in the process design and Aspen simulation of the Butadiene route and Biomass routes, respectively. It was a pleasure to be your coach during your MSc graduation projects. In 2017, the cooperation with Jan Meuldijk and the TU/e had to be stopped due to multiple circumstances. e scaold of the thesis was ready, however, the nalization could not be accomplished in Eindhoven. Fortunately, prof. dr. Ernst Worrell of the Utrecht University, Copernicus Institute of Sustainable Development, was willing to 191 Acknowledgement take over the project and to be my promotor. Dr. Matteo Gazzani was appointed as co-promotor. Ernst and Matteo, I’m in nitely grateful for your willingness to take over and your input and enthusiasm during the nalization of my dream. I would also like to thank the members of the assessment committee for their time and eorts spent in reading and assessing this thesis. And last but not least, also many thanks to my lovely wife José who always supported me in doing this study and gave me all the (family) time needed. Especially her down- to-earth philosophy and realism were very useful in di cult moments. “I Had a Dream” and I succeeded in ful lling this dream. It was a great experience and gave me a lot of joy and wisdom and hopefully I have contributed in a certain extent to the discussions towards a sustainable future. My ultimate wish is that everyone and especially the generation of my children and grandchildren and all generations to come will have personal dreams and challenges. Discover your opportunities and do your utmost best to ful l your realistic dreams. You owe it to yourself and the community you are living in. 192 Curriculum Vitae Curriculum Vitae Henny Herps was born on June 3rd, 1955 in Maastricht, e Netherlands. He studied analytical chemistry at the ‘Zuidlimburgse Laboratorium School – Sittard’ and achieved his BSc degree in 1975. Subsequently, he started a 40 years lasting career at Royal DSM. Commencing July 2015, he continued as an independent consultant. He completed the MSc-study ‘Polymers and Compounds’ at the Eindhoven University of Technology in 2009. Two years later, he started a PhD project at the Eindhoven University of Technology on the topic of “Modelling and Comparative Assessment of Polyamide-6 Manufacturing towards a Sustainable Chemical Industry”. He completed his PhD project at the Utrecht University/Copernicus Institute of Sustainable Development and achieved his PhD degree in 2020. 193 Appendix A Appendix A: Primary Energy Demand of Basic Building Blocks Processing energy (provided as heat/steam and electricity) is necessary to run a process. To judge the complete energy impact of an industrial chemical activity all involved manufacturing processes from natural resources up to the nal product and including the manufacturing of energy, have to be considered. In this study we assume NRMs to be ‘free of charge’ immediately before extracting them for (bio)chemical processing. In other words, energy changes are counted starting with the extraction from the earth and the total energy consumed from that point on, re ects the primary energy needed to process NRMs to BBBs, to auxiliary chemicals and to secondary energy carriers such as steam and electricity. In this thesis this consumed energy is labeled as Primary Energy Demand (PED). ere are several ways to determine the PED of BBBs and energy carriers. Undoubtedly, direct energy measurement in existing production plants will give the real processing energy demand of that particular plant. Another way, as a matter of fact the only one when the considered manufacturing process is not (yet) commercial, is estimation by means of process design and model simulation (e.g. via Aspen simulation techniques). However, both ways experience the same di culties. e mass- and energy balances of existing processes are mostly subject to commercial interest and therefore con dential and not publicly available. e process design and modelling imply extensive literature studies and still a lot of assumptions have to be made in the design parameters (mostly, useful (pilot)plant results are not published in literature). A third possibility to obtain PEDs of BBBs is via publicly accessible databases. e European industry uses e.g. the Eco-pro le database of commercial plastics and their precursors (published by PlasticsEurope55) and Life Cycle Assessment inventory databases of Ecoinvent56 57 (132) (133). Eco-pro les (PlasticsEurope) are reports on product-speci c environmental impacts (134). Based on European industry averages of the respective polymer production technologies, they include detailed environmental datasets – the so- called Life Cycle Inventory (LCI) – and environmental key performance indicators. e Ecoinvent database oers LCI and LCIA results. As such both databases are comparable. Both databases are compiled from long-time industrial production averages (anonymous information) regarding mass- and energy balances. We consider these databases as representative for the concerned industry and therefore we will use the databases to determine the PED of the BBBs and energy carriers used in the 55 PlasticsEurope, the association of European plastics manufacturers, was the rst industry organization to assemble detailed environmental data on the processes operated by its member companies with the rm intention of making this information available for public use. ey have more than 100 member companies, producing over 90% of all polymers across Europe. 56 e international Ecoinvent database is a.o. one of the leading life cycle inventory database and managed by the Swiss Ecoinvent Center, a not-for-pro t association which started o as a joint initiative of the ETH Domain and Swiss Federal O ces. e database accommodates more than 2500 background processes oen required in LCA case studies. 57 RIVM, Life Cycle Assessment (LCA), 2016, www.rivm.nl. 194 Appendix A dierent PA-6 manufacturing routes. In this way we will avoid bias and arbitrariness in the calculation of PED. e PED value of glucose from sugar beets is taken from the study of W.J. Corré and J.W.A. Langeveld of the Plant Research International B.V. of the Wageningen University, e Netherlands (66). e PED value of glucose from sugar cane is taken from the study of P.W. Rein of the Louisiana State University (131). e PED values of sulphur dioxide, sulphuric acid and oleum are based on estimations. e basis of the estimation is the assumed consumption of sulphur (secondary sulphur as produced in a naphtha cracker/re nery, partial PED by allocation is 28.9 MJ/kg), required electricity and heat. Assumptions are based on best practice and discussed with experts in the sulphuric acid industry (Table A-1). e contribution of sulphur in the PED is dominant. Notably, the chemical reaction to synthesize sulphuric acid and oleum from elementary sulphur is strongly exothermic. e exothermic reaction heat is even higher than the PED value as mentioned in Table A-1. is surplus of energy is converted into secondary energy (steam) and normally reused on the production site. However, it is not obvious to discount this surplus energy in the PED of sulphuric acid and oleum. Aer all, the production is most probably not on the PA-6 production site. erefore, this surplus energy has not been taken into account. e surplus energy amounts 5.09 MJ/kg oleum and 4.81 MJ/kg H2SO4. e impact on the C-consumption, if allocated to the PA-6 routes, would be: Benzene-Raschig route additionally 0.086 kg C/kg PA6, Frost route additionally 0.251 kg C/kg PA6, Frost PLUS route additionally 0.213 kg C/kg PA6, ACA route additionally 0.245 kg C/kg PA6. We can conclude that the impact is marginal (2 – 6%). 195 Appendix A e summary of PED’s for several BBBs and energy carries are summarized in Table A-1. Table A-1 Primary Energy Demand of several basic building blocks and energy carries in PA-6 manufacturing. BBB, energy carrier Primary energy demand (PED) Reference 1,3-butadiene 34.8 [MJ/kg] Plastics Europe Eco-pro le 2012 (148) Ammonia 35.76 [MJ/kg] Plastics Europe Eco-pro le, 2005 (149) Benzene 29.4 [MJ/kg] Plastics Europe Eco-pro le 2013 (150) Plastics Europe Eco-pro le 2005 Electricity (on site) 2.23 [MJ/MJ] (151) Plant Research International 2008 Glucose (sugar beet) 6.55 [MJ/kg] (66) Louisiana State University 2010 Glucose (sugar cane) 2.51 [MJ/kg] (131) Plastics Europe Eco-pro le 2005 Hydrogen (reformer) 89.22 [MJ/kg] (152) Plastics Europe Eco-pro le 2005 Hydrogen cyanide 74.94 [MJ/kg] (153) Estimation based on best practise Oleum 10.3 [MJ/kg] in industry and on Ecoinvent PED (19 w% SO in H SO ) 3 2 4 value of sulphur Plastics Europe Eco-pro le 2005 Steam (on site) 3.95 [MJ/kg] (154) Estimation based on best practise Sulphur dioxide 18.7 [MJ/kg] in industry and on Ecoinvent PED value of sulphur Estimation based on best practise Sulphuric acid 9.7 [MJ/kg] in industry and on Ecoinvent PED value of sulphur 196 Appendix B Appendix B: Design criteria, assumptions and conditions of the main equipment in the Aspen simulation model for the production of polyamide-6 from benzene and ammonia Note: V= vapour, L= liquid Flash3 Flash2 323 341 V/L 160 HEx10 HeatX Counter current Counter Design Hot: 323 Hot: 0 Flash2 Flash2 398 341 V/L 160 HEx9 HeatX Counter Counter current Design Hot: 323 Hot: 0 Flash1 Flash2 311 2168 V/L 500 HEx8 HeatX Counter Counter current Design Hot: 323 Hot: 0 Flash2 258 652 V/L Chil2 850 HEx7 HeatX Counter Counter current Design Hot 348 Hot 0 Flash2 258 3272 V/L Chil1 12 750 HEx6 HeatX Counter Counter current Design Cold: 408 0 H 6 C 2 ) 6 H 6 500 HEx5 HeatX Counter Counter current Design Hot: 311 Hot: 0 + 3H 6 H 6 RStoic 488 3099 V/L R2 C 1.0 (C 12 200 HEx4 HeatX Counter Counter current Design Cold: 393 0 H 6 C 2 ) 6 H 160 HEx3 HeatX Counter Counter current Design Hot: 311 Hot: 0 6 + 3H 6 H 6 468 3306 RStoic V/L C R1 0.9 (C 165 HEx2 HeatX Counter Counter current Design Cold: 468 0 [K] [kPa] 30 HEx1 HeatX Counter Counter current Design Cold: 393 0 .K] 2 [K] [Pa] [W/ m Aspen code Aspen Temperature Pressure phases Valid stoichiometryReaction Fractional conversion Fractional Aspen code Aspen Shortcut ow direction Calculation Calculation mode Cold/hot stream outlet temperature Pressure Pressure hot drop side Constant U Constant value Benzene hydrogenation Benzene 197 Appendix B 198 CP1 CP2 CP3 CP4 P1 P2 Aspen code Compressor/ Compressor/ Compressor/ Compressor/ Pump Pump isentropic isentropic isentropic isentropic Outlet discharge [kPa] 3099 3341 3375 652 3380 1031 pressure Efficiency Isentropic 0.72 0.72 0.72 0.72 Mechanical 1 1 1 1 Pump 0.75 0.75 Driver 1 1 Cyclohexane oxidation HEx1 HEx2 HEx3 HEx4 HEx5 HEx6 HEx7 HEx8 HEx9 HEx10 HEx11 Aspen code HeatX HeatX HeatX HeatX HeatX HeatX HeatX HeatX HeatX HeatX Heater Shortcut ow Counter Counter Counter Counter Counter Counter Counter Counter Counter Counter direction current current current current current current current current current current Calculation mode Design Design Design Design Design Design Design Design Design Design Cold/hot stream [K] Hot: 398 Hot: 323 Hot: 323 Cold: 369 Hot: 323 Hot 323 Hot: 318 Hot: 323 Hot: 258 Hot: 366 outlet temperature Pressure drop hot [Pa]0000000000 side Constant U value [W/ 160 500 1250 1150 500 1250 1250 160 850 500 m2.K] Temperature [K] 365 Pressure [kPa] 100 R1/R2/R3 R4 R5 Sep1/S Sep4 Sep5 Sep6 Sep7 Sep8 Sep9 Sep10 Aspen code RStoic RStoic RStoic Flash2 Sep Decanter Decanter Flash3 Flash2 Sep Sep Temperature [K] 418 488 366 418 333 323 323 263 Pressure [kPa] 1032 3099 100 1032 100 136 1032 1032 Valid phases V/L V/L V/L V/L L/L L/L V/L Reaction C6H12 + 2O2 CHHP + C6H12 C3Ac +KOH stoichiometry 2C3Ac (1) 2ANOL (1) C3SALT + C6H12 + O2 CHHP ANON H2O CHHP (2) + H O (2) 2 CHHP + C6H12 2ANOL (3) CHHP ANON + H2O (4) Fractional (1): 0.003 (C6H12) (1): 0.43 (CHHP) 1 conversion (2): 0.012 (C6H12) (2): 0.57 (CHHP) (3): 0.225 (CHHP) (4): 0.3 (CHHP) Outlet stream O2= 1 ANON= ANON= conditions split H2=1 0.99 0.074 CH =1 ANOL= H O= 1 fraction 4 2 N2= 1 0.99 H2O= 0.53 Key components H2O and C3Ac H2O and to identify 2nd C3Ac liquid phase Key component 0.9 0.99 threshold for 2nd liquid phase Determine phase Equating component Equating split by fugacity’s of two component liquids fugacity’s of two liquids Appendix B Calculate L-L Property method Property coecient from method 199 Appendix B Dist1 Dist2 Dist3 Dist4 Aspen code RadFrac RadFrac RadFrac RadFrac Calculation type Equilibrium Equilibrium Equilibrium Equilibrium Number of stages 12 16 12 70 Condenser None Partial vapour None Partial vapour Reboiler None Kettle None Kettle Valid phases V/L V/L V/L V/L Convergence Strongly non- Strongly non- Strongly non-ideal Standard ideal liquid ideal liquid liquid Reux ratio Mass 2 Mass 4 Distillate to feed Mass 0.88 Mass 0.327 ratio Feed stream stage 1 and 12 3 (above stage) 1 (above stage) 35 (above (on stage) and 12 (on stage) stage) (on stage) Product stream 1 (V) 1 (V) 1 (V) 1 (V) stage 12 (L) 16 (L) 12 (L) 70 (L) Pressure top stage [kPa] 136 136 136 5.333 CP1 P1 P2 P3 P4 P5 P6 P7 Aspen code Compressor/ Pump Pump Pump Pump Pump Pump Pump isentropic Outlet [kPa] 1032 1032 136 136 200 336 100 5.333 discharge pressure Eciency Isentropic 0.72 Mechanical 1 Pump 0.75 0.75 0.75 0.75 0.75 0.7 0.75 Driver 1 1 1 1 1 1 1 Cyclohexanol dehydrogenation HEx1 HEx2 HEx3 R1/2/3 Flash1 Aspen code HeatX HeatX HeatX RStoic Flash2 Shortcut ow direction Counter Counter Counter current current current Calculation mode Design Design Design ANOL ANON + H2 Cold/hot stream outlet [K] Cold: 453.15 Hot: 508.15 Hot: 0.5 (ANOL) temperature 323.15 Pressure drop hot side [Pa] 0 0 0 Constant U value [W/m2.K] 100 160 600 Temperature [K] 508 268 Pressure [kPa] 130 100 Valid phases V/L V/L Reaction stoichiometry ANOL ANON + H2 Fractional conversion 0.5 (ANOL) 200 Appendix B Dist1 Dist2 CP1 P1 P2 Aspen code RadFrac RadFrac Compressor/ Pump Pump isentropic Calculation type Equilibrium Equilibrium Number of stages 8 70 Condenser Partial vapour Partial vapour Reboiler Kettle Kettle Valid phases V/L V/L Convergence Strongly non-ideal Standard liquid Reux ratio Mass 2 Mass 4 Distillate to feed ratio= Mass 0.02 Mass 0.875 Feed stream stage 4 (above stage) 35 (above stage) Product stream stage 1 (V) 1 (V) 8 (L) 70 (L) Pressure top stage [kPa] 134 5.33 Outlet discharge [kPa] 130 134 5.33 pressure Eciency Isentropic 0.72 Mechanical 1 Pump 0.75 0.75 Driver 1 1 Hydroxylamine preparation HEx1 HEx2 HEx3 HEx4 HEx5 HEx6 HEx7 HEx8 Aspen code HeatX HeatX Heater HeatX HeatX HeatX Heater HeatX Shortcut ow Counter Counter Counter Counter Counter Counter direction current current current current current current Calculation Design Design Design Design Design Design mode Cold/hot [K] Hot: 433 Cold: 503 Cold: Hot: Cold: Hot stream outlet 473 448 473 283 temperature Pressure [Pa] 0 0 0 0 0 0 drop hot side Constant U [W/ 160 30 30 30 160 160 value m2.K] Temperature [K] 503 323 Pressure [kPa] 790 790 201 Appendix B R1 R2 R3 R4 Sep1 Sep2 Aspen code RStoic RStoic RStoic RStoic Sep Flash2 Temperature [K] 1189 448 283 398 Pressure [kPa] 790 790 790 100 283 790 Valid phases V/L V/L V/L V/L V/L Reaction 4NH3 + 2NO2 4NH3 + NO + NO2 2NH3 + stoichiometry 5O2 4NO + H2O + 7H2O + 4SO2 H2SO4 + 6H2O (1) HNO3 + 2HYAM + 2H2SO4 (NH4)2SO4 4NH3 + HNO2 (1) + 2(NH4)2SO4 3O2 2N2 2HNO2 + 6H2O (2) H2O + NO 2NO + O2 + NO2 (2) 2NO2 (3) Fractional (1): 0.95 (1): 0.035 NO2: 0.9 H2SO4: 1 conversion (NH3) (NO2) (2): 0.05 (2): 1 (NH3) (HNO2) (3): 0.5 (NO) Outlet stream H2O= conditions 0.275 split fraction HNO3=1 CP1 CP2 P1 P2 Aspen code Compressor/ Compressor/ Pump Pump Polytropic Isentropic using ASME method Outlet discharge pressure [kPa] 790 790 1400 790 Eciency 0.72 Isentropic 1 Mechanical Pump 0.75 0.75 Driver 1 1 202 Appendix B Cyclohexanone oximation HEx1 HEx2 HEx3 P1 P2 P3 P4 Aspen code HeatX Heater Heater Pump Pump Pump Pump Shortcut ow direction Counter current Calculation mode Design Cold/hot stream outlet [K] Hot: 338 temperature Pressure drop hot side [Pa] 0 Constant U value [W/m2.K] 500 Temperature [K] 338 353 Pressure [kPa] 100 100 Outlet discharge [kPa] 120 120 150 150 pressure Pump 0.75 0.75 0.75 0.75 Driver 1 1 1 1 R1 Sep1 Sep2 Type RStoic Sep Sep Temperature [K] 338 Pressure [kPa] 100 Valid phases V/L Reaction stoichiometry ANON + HYAM OXIME + H2O Fractional conversion 1 (HYAM) Outlet stream conditions split ANON= 1 ANON= 1 fraction ANOL= 1 ANOL= 1 H2O= 1 H2O= 0.013 (NH4)2SO4= 1 OXIME= 1 HYAM= 1 C6H12= 1 OXIME= 1 H2SO4= 1 C6H12= 1 203 Appendix B Beckmann rearrangement HEx1 R1 R2 Sep1 Sep2 Aspen code Heater RStoic RStoic Sep Sep Temperature [K] 348 358 358 Pressure [kPa] 100 100 100 Valid phases V/L V/L Reaction OXIME 2NH3 + H2SO4 stoichiometry C6H11NO (1) (NH4)2SO4 SO3 + H2O H2SO4 (2) Fractional (1): 0.98 1 conversion (OXIME) (2): 1 (H2O) Outlet stream ANON= 0.2 ANON= 0.2 conditions split H2O= 0.2 H2O= 0.2 fraction (NH4)2SO4= 0.2 (NH4)2SO4= OXIME= 0.2 0.2 H2SO4= 0.2 OXIME= C6H12= 0.2 0.2 C6H11NO= 0.2 H2SO4= 0.2 NH3= 0.2 C6H12= 0.2 SO3= 0.2 C6H11NO= 0.2 NH3= 0.2 SO3= 0.2 P1 P2 P3 Aspen code Pump Pump Pump Outlet discharge pressure [kPa] 120 120 120 Pump 0.75 0.75 0.75 Driver 1 1 1 204 Appendix B Caprolactam recovery HEx1 HEx2 Sep1 Sep2 Dist1 Aspen code Heater Heater Sep Flash2 RadFrac Temperature [K] 363 323 358 Pressure [kPa] 27 120 2 Valid phases V/L Outlet stream C6H11NO= 0.025 conditions split (NH4)2SO4= 1 fraction H2O= 0.9954 C6H6=1 Calculation type Equilibrium Number of stages 32 Condenser Partial vapour Reboiler Kettle Valid phases V/L Convergence Standard Reux ratio Mass 2 Bottom to feed ratio Mass 0.975 Feed stream stage 16 (above stage) Product stream stage 1 (V) 32 (L) Pressure top stage [kPa] 27 CP1 CP2 P1 P2 P3 P4 P5 Aspen code Compressor/ Compressor/ Pump Pump Pump Pump Pump isentropic isentropic Outlet [kPa] 120 100 120 120 2 27 100 discharge pressure Eciency Isentropic 0.72 0.72 Mechanical 1 1 Pump 0.75 0.75 0.75 0.75 0.75 Driver 1 1 1 1 1 205 Appendix B 206 Polymerization ofe -caprolactam HEx1 HEx2 HEx3 HEx4 HEx5 Sep1 Sep2 Sep3 Dist1 Aspen code Heater Heater Heater Heater HeatX Sep Sep Flash2 RadFrac Shortcut ow direction Counter current Calculation mode Design Cold/hot stream outlet [K] Hot: 308 temperature Pressure drop hot side [Pa] 0 Constant U value [W/m2.K] 850 Duty [kW] 13.554 Temperature [K] 513 368 313 Pressure [kPa] 250 300 250 300 Temperature [K] 323.15 Pressure [kPa] 100 Valid phases V/L V/L V/L V/L V/L Outlet stream C6H11NO= 0.1 C6H11NO= 0.026 conditions split fraction N2= 1 H2O= 0.0005 Calculation type Equilibrium Number of stages 10 Condenser Partial vapour Reboiler Kettle Valid phases V/L Convergence Standard Reux ratio Mass 2 Bottom to feed ratio Mass 0.8 Feed stream stage 5 (above stage) Product stream stage 1 (V)10 (L) Pressure top stage [kPa] 250 Appendix B CP1 P1 P2 Aspen code Compressor/ Pump Pump isentropic Outlet discharge pressure [kPa] 120 250 250 Eciency Isentropic 0.72 Mechanical 1 Pump 0.72 0.72 Driver 1 1 207 Appendix C Appendix C: Design criteria, assumptions and conditions of the main equipment in the Aspen simulation model for the production of polyamide-6 from 1,3-butadiene and hydrogen cyanide Note: V= vapour, L= liquid Hydrocyanation section R1 R2 R3 Flash1 Sep1 Sep2 Sep3 Sep4 Sep5 Extr1 Extr2 Aspen code RStoic RStoic RStoic Flash2 Sep Sep Sep Sep Sep Sep Sep Temperature [K] 383 383 333 383 Pressure [kPa] 1617 1617 101 75 Valid phases V/L V/L V/L V/L Reaction HCN + BD 3PN (1) 2M3BN 3PN + HCN ADN (1) stoichiometry 2BD VCH (2) 3PN 4PN + HCN ADN (2) 2HCN + BD 2MGN (3) 3PN 2PN (3) HCN + BD 2M3BN (4) 4PN 2PN (4) HCN + BD 4PN (5) 3PN + HCN 2MGN (5) 4PN + HCN 2MGN (6) 3PN + HCN ESN (7) 4PN + HCN ESN (8) Fractional (1): 0.367174 (BD) 0.95 (1): 0.765 (PN) conversion (2): 0.007983 (BD) (2): 0.765 (PN) (3): 0.007983 (BD) (3): 0.017 (PN) (4): 0.129591 (BD) (4): 0.017 (PN) (5): 0.367174 (BD) (5): 0.02975 (PN) (6): 0.02975 (PN) (7): 0.02975 (PN) (8): 0.02975 (PN) Outlet stream BD/ BD/HCN/3PN/ BD/HCN/3PN/ BD/HCN/3PN/ ADN= CAT= 0.45 CAT= 0.85 conditions split HCN/3PN/ 2M3BN/4PN/ 2M3BN/4PN/ 2M3BN/4PN/ 0.9998 n-heptane= n-heptane= fraction 2M3BN/4PN/ 2MGN/VCH/ 2MGN/VCH/ 2MGN/VCH/ 0.95 0.95 2MGN/ ADN/2PN/ ADN/2PN/ 2PN/ESN/CAT/ -10 VCH= 1x10 ESN/CAT/ ESN/CAT/ H2O/n-heptane= 1 n-heptane= 0.5 n-heptane/ H2O= 0.15 H2O= 0.15 208 Appendix C Hydrocyanation section R1 R2 R3 Flash1 Sep1 Sep2 Sep3 Sep4 Sep5 Extr1 Extr2 Aspen code RStoic RStoic RStoic Flash2 Sep Sep Sep Sep Sep Sep Sep Temperature [K] 383 383 333 383 Pressure [kPa] 1617 1617 101 75 Valid phases V/L V/L V/L V/L Reaction HCN + BD 3PN (1) 2M3BN 3PN + HCN ADN (1) stoichiometry 2BD VCH (2) 3PN 4PN + HCN ADN (2) 2HCN + BD 2MGN (3) 3PN 2PN (3) HCN + BD 2M3BN (4) 4PN 2PN (4) HCN + BD 4PN (5) 3PN + HCN 2MGN (5) 4PN + HCN 2MGN (6) 3PN + HCN ESN (7) 4PN + HCN ESN (8) Fractional (1): 0.367174 (BD) 0.95 (1): 0.765 (PN) conversion (2): 0.007983 (BD) (2): 0.765 (PN) (3): 0.007983 (BD) (3): 0.017 (PN) (4): 0.129591 (BD) (4): 0.017 (PN) (5): 0.367174 (BD) (5): 0.02975 (PN) (6): 0.02975 (PN) (7): 0.02975 (PN) (8): 0.02975 (PN) Outlet stream BD/ BD/HCN/3PN/ BD/HCN/3PN/ BD/HCN/3PN/ ADN= CAT= 0.45 CAT= 0.85 conditions split HCN/3PN/ 2M3BN/4PN/ 2M3BN/4PN/ 2M3BN/4PN/ 0.9998 n-heptane= n-heptane= fraction 2M3BN/4PN/ 2MGN/VCH/ 2MGN/VCH/ 2MGN/VCH/ 0.95 0.95 2MGN/ ADN/2PN/ ADN/2PN/ 2PN/ESN/CAT/ -10 VCH= 1x10 ESN/CAT/ ESN/CAT/ H2O/n-heptane= 1 n-heptane= 0.5 n-heptane/ H2O= 0.15 H2O= 0.15 209 Appendix C 210 Dist1 Dist2 Dist3 Dist4 Dist5 Dist6 Aspen code RadFrac RadFrac RadFrac RadFrac RadFrac RadFrac Calculation type Equilibrium Equilibrium Equilibrium Equilibrium Equilibrium Equilibrium Number of stages 36 25 30 20 50 10 Condenser Partial vapour Total Total Total Total Total Reboiler Kettle Kettle Kettle Kettle Kettle Kettle Valid phases V/L V/L V/L V/L V/L V/L Convergence Standard Standard Standard Standard Standard Standard Reflux ratio Mole 20 Mass 0.05 Mass 0.1126 Mass 0.5 Mass 1.5 Mass 0.01 Distillate to feed ratio Mass 0.035 Mass 0.9143 Mass 0.88 Mass 0.172161 Mass 0.2 Mass 0.95 Feed stream stage 13 (above stage) 12 (above stage) 9 and 15 (above 15 (above stage) 15 (above stage) 5 (above stage) stage) Product stream stage 1 (V) 1 (L) 1 (L) 1 (L) 1 (L) 1 (L) 36 (L) 25 (L) 30 (L) 20 (L) 50 (L) 10 (L) Pressure top stage [kPa] 10 1 10 5 5 5 CP1 P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11 Aspen code Compressor/ Pump Pump Pump Pump Pump Pump Pump Pump Pump Pump Pump isentropic Outlet [kPa] 1617 1617 1617 75 10 1 1617 101 101 101 5 10 discharge pressure Efficiency Isentropic 0.72 Mechanical 1 Pump 0.7 0.7 0.72 0.7 0.7 0.7 0.72 0.72 0.72 0.72 0.72 Driver 1 1 1 1 1 1 1 1 1 1 1 Hydrogenation section HEx1 HEx2 HEx3 Aspen code Heater Heater Heater Temperature [K] 343 343 343 Pressure [kPa] 7000 7000 7000 Valid phases V/L V/L V/L R1 Sep1 Sep2 Sep3 Sep4 Aspen code RStoic Flash2 Flash2 Sep Sep Temperature [K] 343 343 343 Pressure [kPa] 7000 7000 20 Valid phases V/L V/L V/L Reaction ADN + H2 IMINE (1) stoichiometry IMINE + H2 ACN (2) IMINE + 2H2 THA + NH3 (3) ACN + 2H2 HMDA (4) ACN + HMDA + 2H2 NH3 + DIMERIC (5) Fractional conversion (1): 0.96 (ADN) (2): 0.86 (IMINE) (3): 0.14 (IMINE) (4): 0.116279 (ACN) (5): 0.026316 (ACN) Outlet stream BD/HCN/3PN/2M3BN/4PN/2MGN/VCH/ NH3= 1 Appendix C conditions split ADN/2PN/ESN/CAT/H2O/n-heptane/HMDA/ fraction THA/IMINE/DIMERIC/ACN/H2/NH3/N2/ Ar= 0.2 211 Appendix C 212 Dist1 Dist2 Dist3 Dist4 Dist5 Aspen code RadFrac RadFrac RadFrac RadFrac RadFrac Calculation type Equilibrium Equilibrium Equilibrium Equilibrium Equilibrium Number of stages 20 50 20 20 60 Condenser Partial vapor Total Total Total Total Reboiler Kettle Kettle Kettle Kettle Kettle Valid phases V/L V/L V/L V/L V/L Convergence Standard Standard Standard Standard Standard Reux ratio Mass 2 Mass 1 Mass 2 Mass 5 Mass 21.5 Distillate to feed ratio Mass 0.2 Mass 0.903 Mass 0.53 Mass 0.15 Mass 0.548 Feed stream stage 10 (above stage) 7 (above stage) 6 (above stage) 10 (above stage) 30 (above stage) Product stream stage 1 (V) 1 (L) 1 (L) 1 (L) 1 (L) 20 (L) 50 (L) 20 (L) 20 (L) 60 (L) Pressure top stage [kPa] 20 5 5 20 100 CP1 CP2 P1 P2 P3 Aspen code Compressor/ Compressor/ Pump Pump Pump isentropic isentropic Outlet discharge pressure [kPa] 7000 7000 7000 7000 20 Efficiency Isentropic 0.72 0.72 Mechanical 1 1 Pump 0.72 0.72 0.72 Driver 1 1 1 Appendix C Polymerization section HEx1 HEx2 HEx3 Aspen code HeatX Heater HeatX Shortcut ow direction Counter current Counter current Calculation mode Design Design Cold/hot stream outlet [K] Cold: 503.15 Hot: 308.15 temperature Pressure drop hot side [Pa] 0 0 Constant U value [W/m2.K] 175 165 Temperature [K] Pressure [kPa] 300 Duty [kW] 42.749 Valid phases V/L Sep1 Sep2 Sep3 Sep4 Aspen code Sep Flash2 Flash2 Flash2 Temperature [K] 503 503 503 Pressure [kPa] 250 100 250 Valid phases V/L V/L V/L Outlet stream conditions split H2O= 0.9481 fraction ACN= 0.9481 CP1 P1 Aspen code Compressor/ Pump isentropic Outlet discharge pressure [kPa] 300 3000 Eciency Isentropic 0.72 Mechanical 1 Pump 0.72 Driver 1 213 Appendix D Appendix D: Design criteria, assumptions and conditions of the main equipment in the Aspen simulation model for the production of polyamide-6 from glucose and ammonia Note: V= vapour, L= liquid Fermentation and ion exchange section HEx1 HEx2 Aspen code HeatX HeatX Shortcut ow direction Counter current Counter current Calculation mode Design Design Cold/hot stream outlet temperature [K] Hot: 304.5 Hot: 304.5 Pressure drop hot side [Pa] 0 0 Constant U value [W/m2.K] 160 160 214 R1 R2 Flash1 Sep1 Sep2 Sep3 Aspen code RStoic RStoic Flash2 Sep Sep Sep Temperature [K] 304.5 304.5 304.5 Pressure [kPa] 100 100 100 Valid phases V/L V/L V/L Reaction C6H12O6 C6H12O6 + 2NH3 6C6H14N2O2 + 2H2O + O2 (1) stoichiometry BIOMASS (1) 2C6H12O6 + 2NH3 2C6H13NO2 + 2H2O + 3O2 (1) C6H12O6 + 6O2 C6H12O6 + 6O2 6H2O + 6CO2 (2) 6H2O + 6CO2 (2) YEAST BIOMASS (3) C6H12O6 BIOMASS (4) C6H12O6 +O2 2METABOLI + 2H2O (5) 4NH3 + 3O2 2N2 + 6H20 (6) Fractional (1): 0.5000 (1): 0.4440 (C6H12O6) conversion (C6H12O6) (2): 0.4146 (C6H12O6) (2): 0.5000 (3): 0.9870 (C6H12O6) (YEAST) (4): 0.0789 (C6H12O6) (5): 0.0490 (C6H12O6) (6): 0.0001 (NH3) Outlet stream H2O= 0.02 C6H14N2O2= 0.985 NH3= conditions split BIOMASS= 1 C6H13NO2= 1 0.833333 fraction N2= 0.0006 Information in red diers for ACA process with respect to the Frost processes Appendix D 215 Appendix D Dist1 Aspen code RadFrac Calculation type Equilibrium Number of stages 10 Condenser Partial vapor Reboiler Kettle Valid phases V/L Convergence Standard Reux ratio Mole 0.1 Distillate to feed ratio Mole 0.35 Feed stream stage 5 (above stage) Product stream stage 1 (V) 10 (L) Pressure top stage [kPa] 100 CP1 CP2 P1 P2 P3 P4 P5 P6 Aspen code Compressor/ Compressor/ Pump Pump Pump Pump Pump Pump isentropic isentropic Outlet [kPa] 500 500 500 500 300 300 300 300 discharge pressure Eciency Isentropic 0.72 0.72 Mechanical 1 1 Pump 0.72 0.72 0.72 0.72 0.72 0.72 Driver 1 1 1 1 1 1 Cyclisation section HEx1 HEx2 Aspen code Heater Heater Temperature [K] 400 431 Pressure [kPa] 101 20 Valid phases V/L V/L R1 Sep1 Aspen code RStoic Flash2 Temperature [K] 461 461 Pressure [kPa] 100 100 Valid phases V/L V/L Reaction stoichiometry C6H14N2O2 AAEC + H2O (1) 6C6H14N2O2 HEXALYS + 5H2O (2) Fractional conversion (1): 0.96 (C6H14N2O2) (2): 0.02 (C6H14N2O2) 216 Dist1 Dist2 Dist3 Dist4 Dist5 Dist6 Aspen code RadFrac RadFrac RadFrac RadFrac RadFrac RadFrac Calculation type Equilibrium Equilibrium Equilibrium Equilibrium Equilibrium Equilibrium Number of stages 16 15 15 10 20 10 Condenser Partial vapour Partial vapour Partial vapour Partial vapour Partial vapour Partial vapour Reboiler Kettle Kettle None None Kettle Kettle Valid phases V/L V/L V/L V/L V/L V/L Convergence Standard Standard Standard Standard Standard Standard Reux ratio Mole 2 Mole 1 Mole 0.5 Mole 0.278405 Mole 1.5 Mass 0.5 Distillate to feed ratio Mole 0.9 Mole 0.95 Mole 0.5015 Mole 0.9 Bottom rate [kg/s] 0.5 Feed stream stage 8 (above stage) 8 (above stage) 1 (above stage) and 15 (on stage) 10 (on stage) 3 (above stage) 5 (above stage) Product stream stage 1 (V) 1 (V) 1 (V) 1 (V) 1 (V) 1 (V) 16 (L) 15 (L) 15 (L) 10 (L) 20 (L) 10 (L) Pressure top stage [kPa] 100 20 100 100 100 100 CP1 P1 P2 P3 P4 Aspen code Compressor/ Pump Pump Pump Pump isentropic Outlet discharge pressure [kPa] 100 120 20 100 100 Eciency Isentropic 0.72 Mechanical 1 Pump 0.72 0.72 0.72 0.72 Driver 1 1 1 1 Appendix D 217 Appendix D 218 Diamination section HEx1 HEx2 HEx3 Aspen code Heater Heater Heater Temperature [K] 523 550 341 Pressure [kPa] 4500 4500 100 Valid phases V/L V/L V/L R1 Sep1 Sep2 Sep3 Sep4 Sep5 Sep6 Sep7 Aspen code RStoic Flash2 Flash2 Sep Flash2 Sep Sep Flash2 Temperature [K] 523 523 255 Pressure [kPa] 4500 100 4500 20 4500 Duty [kW] 0.001 0.001 Valid phases V/L V/L V/L V/L V/L Reaction AAEC + H2 NH3 + C6H11NO (1) stoichiometry 6AAEC + 6H2 + H2O 6NH3 + HEXACAP (2) Fractional (1): 0.65/0.95 (AAEC) conversion (2): 0.15/0.05 (AAEC) Outlet stream C6H14N2O2= 1 C6H14N2O2= 1 THF= 1 conditions split AAEC= 0.01 H2O= 0.995 fraction HEXALYS= 0.99 AAEC= 1 HEXACAP= 0.99 C6H11NO= 0.005 PR1DIOL= 0.995 HEXALYS= 1 THF= 0.0001 HEXACAP= 1 C6H11NO= 1 C6H12O6= 1 BIOMASS= 1 YEAST= 1 METABOLI= 1 (NH4)2SO4= 1 H2SO4=1 Information in red diers for Frost PLUs process with respect to the Frost process Appendix D Dist1 Dist2 Dist3 Dist4 Aspen code RadFrac RadFrac RadFrac RadFrac Calculation type Equilibrium Equilibrium Equilibrium Equilibrium Number of stages 15 15 30 15 Condenser Partial vapour Total Partial vapour Partial vapour Reboiler Kettle Kettle Kettle Kettle Valid phases V/L V/L V/L V/L Convergence Standard Standard Standard Standard Reux ratio Mole 0.1/2 Mole 6 Mole 2.3 Mole 1 Distillate to feed Mole 0.65 Mole 0.53 Mole 0.01 ratio Bottom rate [kmol/s] 0.0862 [kg/s] 8.98 11.1037 Feed stream stage 3 (above stage) 9/8 (above stage) 5 (above stage) 7 (above stage) Product stream stage 1 (V) 1 (V/L) 1 (V) 1 (V) 15 (L) 15 (L) 30 (L) 15 (L) Pressure top stage [kPa] 50 20 4500 100 Information in red diers for Frost PLUs process with respect to the Frost process CP1/CP2/ CP3 P1/P8 P2/P6 P3 P4 P5/P7 CP4/CP5 Aspen code Compressor/ Compressor/ Pump Pump Pump Pump Pump isentropic isentropic Outlet discharge [kPa] 4500 50 300 4500 50 20 100 pressure Eciency Isentropic 0.72 0.72 Mechanical 1 1 Pump 0.72 0.72 0.72 0.72 0.72 Driver 1 1 1 1 1 219 Appendix D Polymerization Frost section HEx1 HEx2 HEx3 HEx4 HEx5 Aspen code Heater Heater Heater Heater HeatX Shortcut ow direction Counter current Calculation mode Design Cold/hot stream outlet [K] Hot: 308 temperature Pressure drop hot side [Pa] 0 Constant U value [W/ 850 m2.K] Type Flash Flash Flash Flash Duty [kW] 12.722/14.616 Temperature [K] 513 368 313 Pressure [kPa] 250 300 250 300 Valid phases V/L V/L V/L V/L V= vapour L= liquid Information in red diers for Frost PLUs process with respect to the Frost process Sep1 Sep2 Sep3 Aspen code Sep Sep Flash2 Temperature [K] 323 Pressure [kPa] 100 Valid phases V/L Outlet stream conditions split fraction C6H14N2O2= 1 C6H14N2O2= 1 NH3= 1 H2O= 0.0005 AAEC= 1 NH3= 1 PR12DIOL= 1 AAEC= 1 HEXALYS=1 PR12DIOL= 1 THF= 1 HEXALYS=1 H2= 1 THF= 1 CH3SH= 1 H2= 1 HEXACAP= CH3SH= 1 1 HEXACAP= C6H11NO= 0.1 1 C6H12O6= 1 C6H11NO= 0.026 BIOMASS= 1 C6H12O6= 1 Yeast= 1 BIOMASS= 1 N2= 1 Yeast= 1 O2= 1 N2= 1 CO2= 1 O2= 1 METABOLI= 1 CO2= 1 H2SO4=1 METABOLI= 1 (NH4)2SO4= 1 H2SO4=1 (NH4)2SO4= 1 220 Appendix D Dist1 Type RadFrac Calculation type Equilibrium Number of stages 10 Condenser Partial vapour Reboiler Kettle Valid phases V/L Convergence Standard Reux ratio Mass 2 Bottom to feed ratio Mass 0.8 Feed stream stage 5 (above stage) Product stream stage 1 (V) 10 (L) Pressure top stage [kPa] 250 CP1 P1 P2 Aspen code Compressor/ Pump Pump isentropic Outlet discharge pressure [kPa] 120 250 250 Eciency Isentropic 0.72 Mechanical 1 Pump 0.72 0.72 Driver 1 1 Polymerization ACA section HEx1 HEx2 HEx3 HEx4 HEx5 Aspen code Heater Heater Heater Heater HeatX Shortcut ow Counter direction current Calculation mode Design Cold/hot stream [K] Hot: 308 outlet temperature Pressure drop hot side [Pa] 0 Constant U value [W/m2.K] 850 Duty [kW] 16.826 Temperature [K] 513 368 313 300 Pressure [kPa] 250 300 250 Valid phases V/L V/L V/L V/L 221 Appendix D Sep1 Sep2 Sep3 Sep4 Aspen code Flash2 Sep Sep Flash2 Temperature [K] 400 323 Pressure [kPa] 100 100 Valid phases V/L V/L Outlet stream conditions split H2O= 1 H2O= 0.0005 fraction NH3= 1 NH3= 1 C6H12O6= 1 C6H12O6= 1 BIOMASS= 1 BIOMASS= 1 Yeast= 1 Yeast= 1 N2= 1 METABOLI= 1 O2= 1 H2SO4=1 CO2= 1 C6H13NO2= METABOLI= 1 0.025 H2SO4=1 C6H13NO2= 0.1 Dist1 Aspen code RadFrac Calculation type Equilibrium Number of stages 10 Condenser Partial vapour Reboiler Kettle Valid phases V/L Convergence Standard Reux ratio Mass 2 Bottom to feed ratio Mass 0.8 Feed stream stage 5 (above stage) Product stream stage 1 (V) 10 (L) Pressure top stage [kPa] 250 CP1 P1 P2 P3 Aspen code Compressor/ Pump Pump Pump isentropic Outlet discharge pressure [kPa] 120 100 250 250 Eciency Isentropic 0.72 Mechanical 1 Pump 0.72 0.72 0.72 Driver 1 1 1 222 Appendix E Appendix E: Method to calculate enthalpy and entropy of polyamide-6 chains ermodynamic properties of polyamide-6 with dierent polymer chain length are not available in Aspen databases and are not published in literature, in particular enthalpy/entropy data. However, these data are essential for the calculation of energy balances of polymerization steps as described in Chapter 3, 4 and 5. We have calculated the standard enthalpy and –entropy of formation of PA-6 polymer chains by following the path of caprolactam ring opening and -addition at high temperatures and subsequent cooling and crystallisation to standard conditions (T0, P0). Aer all, enthalpy and entropy are state variables and as such path independent regarding the calculation. We assume also that the results are independent of the polymer chain length and that high molecular weight polymer chains have extremely low vapour pressures and can be considered to be non-volatile. We have used the thermodynamic results as been reported in the studies of Reimschüssel (90) and Tai (91). e reported reaction enthalpy and entropy at 538 K are respectively ΔrH = -3010 [kJ/mol], Sr= -5.2 [kJ/mol-K] (90)(91). With the use of the enthalpy- and entropy of formation of caprolactam and water at 538 K it is possible to calculate the enthalpy and entropy of formation of polyamide chains at 538 K. ΔfH and Sf of liquid caprolactam and liquid water at 538 K are respectively: ΔfH(CPL) = -252.94 kJ/mol, ΔfH(water) = - 264.73 kJ/mol, Sf (CPL) = -0.583 kJ/mol-K and Sf (water) = - 0.113 kJ/mol-K. ese values are extracted from the Aspen 7.3® database. Subsequently we have corrected for the eect of crystallization and cooling to standard conditions (as is required 0 0 for ΔfH and Sf ). Van Krevelen’s Handbook ‘Properties of Polymers’ (155) describes experimental values for heat capacities (Cp) and enthalpy and entropy changes of crystallization. e enthalpy of melting (Tm= 533 K) is listed as 26.0 kJ/mol -CPL-; entropy of melting is 0.0488 kJ/mol -CPL-K. Experimental heat capacities of PA-6 are listed as 0.164 kJ/mol-K for solid and as 0.242 kJ/mol-K for liquid PA-6 (156). 0 0 We have calculated ΔfH and Sf for a PA-6 chain with 20 monomer units (prepolymer in the Butadiene process) and with 180 monomer units ( nal PA-6 product of all routes). e results are given in Table E-1. 0 0 Table E-1 ΔfH and Sf of PA-6 with di erent polymer chain length. Polymer length Molecular weight (# –CPL– unit) [kg/kmol] [kJ/mol] [kJ/mol.K] 20 2281 -5751 7 180 20387 -48920 61 223