Coproduction of Acrylic Acid with a Biodiesel Plant Using CO2 As Reaction Medium: Process Modeling and Production Cost Estimation

energies

Article Coproduction of Acrylic Acid with a Biodiesel Plant Using CO2 as Reaction Medium: Process Modeling and Production Cost Estimation

X. Philip Ye * and Shoujie Ren Department of Biosystems Engineering and Soil Science, The University of Tennessee, Knoxville, TN 37996, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-(865)-974-7129; Fax: +1-(865)-974-4514

Received: 19 October 2020; Accepted: 18 November 2020; Published: 20 November 2020

Abstract: Producing value-added chemicals from glycerol is imperative for the sustainable future of biodiesel. Despite worldwide efforts, the commercial production of acrylic acid from glycerol faces challenges, both technologically and economically. Based on our new technology using CO2 as a reaction medium in a two-step process to catalytically convert glycerol to acrylic acid, we established computer simulation models to analyze the energy efficiency and estimate production costs at different scenarios. The analysis was conducted in conjunction with published data of a typical, intermediate-sized biodiesel facility, aiming at the feasibility of producing acrylic acid on-site in the context of a circular economy. Variable analysis in response to the market value of glycerol, the source and cost of carbon dioxide recycling, and the changes in process scale and conditions are also presented. Results indicates that a cost-effective supply of CO2 to the acrylic acid plant is key to the further research and development.

Keywords: glycerol; catalytic dehydration; acrolein; partial oxidation; value-added chemicals; circular economy

1. Introduction Current biodiesel production, although utilizing renewable resources, fits into a linear economy, which is a take–make–dispose model. For a sustainable future of biofuels, it is imperative to move away from a linear economy toward a circular economy with key elements of prioritizing regenerative resources, using waste as a resource, designing for the future, preserving and extending what is already made, and collaborating to create joint value [1]. The common biodiesel production through transesterification of oils produces ~10% by weight of crude glycerol as coproducts. Because of the massive volume of biodiesel production, a worldwide effort to valorize glycerol was initiated [2,3]. Among the value-added chemicals that can be derived from glycerol, acrylic acid produced via the intermediate of glycerol dehydration to acrolein received much attention because this appears to be one of the most promising ways to valorize glycerol [4,5]. There have been extensive efforts in recent years to improve glycerol dehydration to acrolein, including glycerol conversion in sub- and supercritical water [6–8], as well as selective gas-phase conversions [9–13]. Although these different processes gave adequate glycerol conversion rates and selectivity to acrolein, a continuous process catalyzed by solid acids is desirable for industrial application. However, despite their positive initial performance, few solid acid catalysts are known to maintain their catalytic activity long enough for practical application without the need for regeneration, as the high functionality of glycerol leads to severe coking on the catalysts. This fast catalyst deactivation remains a major technical problem for the commercial production of acrolein and acrylic acid from glycerol [5,14].

Energies 2020, 13, 6089; doi:10.3390/en13226089 www.mdpi.com/journal/energies Energies 2020, 13, 6089 2 of 14

We recently developed a novel technology in which supercritical carbon dioxide (SC-CO2) was used for the first time as a reaction medium for the dehydration of glycerol to acrolein catalyzed by a solid acid, showing potential for its industrial application by achieving unprecedented catalyst stability over 528 h of time-on-stream [15]. Further development enabled an integrated process of glycerol dehydration to acrolein, followed by partial oxidation of acrolein to continuously generate acrylic acid wherein crude glycerol with minimum pretreatment can be used as feedstock [16]. Although the process optimization for our new technology is still needed, technical and economic feasibility of the technology needs to be fully understood. At the early stage of a new biobased technology, process modeling and production cost estimation can help to assess potential economic feasibilities, bottlenecks, operation targets for process improvement and identify further research and development effort requirements. Haas et al. [17] reported a process model to estimate the capital and operating costs of a moderate-sized industrial biodiesel production facility using crude, degummed soybean oil as the feedstock; the major process operations in the plant were modeled as continuous-process vegetable oil transesterification, and ester and glycerol recovery. The flexible model could be modified to calculate the effects on capital and production costs of changes in feedstock cost, changes in the type of feedstock employed, changes in the value of the glycerol coproduct, and changes in process chemistry and technology. Although economies of scale usually indicate the cost per unit of output decreasing with increasing production scale, value-added chemical production from glycerol in the context of a circular economy should be considered in conjunction with a typical biodiesel plant. This would enable integrated and cascading processing, better energy efficiency, and minimized cost associated with glycerol handling and shipment. Therefore, the aim of this study was to establish a process simulation model to analyze the technical and economic feasibility of producing acrylic acid from glycerol in conjunction with a typical intermediate-sized biodiesel facility, based on our new technology of using carbon dioxide as a reaction medium in a two-step process to catalytically convert glycerol to acrylic acid. Variable analysis in response to the market value of glycerol, the source and cost of carbon dioxide recycling, and the changes in process scale and conditions are also presented, demonstrating the usefulness of process modeling at the early stages of a new technology.

2. Materials and Methods The intermediate-sized biodiesel facility is based on the one reported by Haas et al.; results of the modeling analysis are available [17]. Table1 summarizes the key scale data and modeling analysis results. The glycerol output of this biodiesel facility is used as a benchmark to design a cascading chemical plant producing acrylic acid from glycerol.

Table 1. Summary of relevant data modeling an intermediate-sized biodiesel facility (based on [17]).

Key scale data Annual consumption of 33,635 metric tons degummed soybean oil as feedstock Modeled as continuous process of vegetable oil transesteriﬁcation, biodiesel reﬁning, and ester and glycerol recovery Annual revenues are from 38 million liters (10 million gallons) biodiesel plus 3810 metric tons crude glycerol (80 wt.%) Key modeling analysis results (based on 2003 price) Capital costs $11.3 million to build the biodiesel facility Annual operating cost $21 million Soybean oil feedstock accounts for 88% of total production cost, which is linearly dependent on feedstock cost Unit biodiesel production cost = $0.53/L ($2.00/gal) vs. selling price of biodiesel = $0.61/L ($2.32/gal) Crude glycerol revenue = $0.33/kg, which reduces production costs by ~6%. $0.022/kg ($0.01/lb) reduction in glycerol value results in $0.0022/L ($0.0085/gal) increase of production cost Energies 2020, 13, 6089 3 of 14

2.1. Main Steps of Converting Glycerol to Acrylic Acid Technical data input to our modeling is based on our experimental laboratory results [15,16]. First, glycerol diluted with water goes through dehydration in the medium of supercritical carbon dioxide catalyzed by a solid acid (ZSM-5 is used in the modeling) to generate acrolein, according to the following equation.

H C(OH)CH(OH)CH OH CH = CH CHO + 2H O (1) 2 2 → 2 − 2 Generation of side products of acetol, acetaldehyde, and propionaldehyde is also considered in this first step. Carbon dioxide at supercritical conditions, having liquid-like density and gas-like diffusivity, possesses special solvent and transport properties [18]. The major reaction product of acrolein, which is also considered as a coke precursor [5], is soluble in the highly diffusive SC-CO2 [19] to be quickly moved out of the catalyst bed, preventing it from further secondary reactions that lead to decomposition and coking. Other coke precursors can be dissolved through the combination of water polarity and SC-CO2 non-polarity, washing out both the hydrophilic and hydrophobic coke precursors and thus enabling long catalyst life [15]. Down the stream, the process is similar to that in the industrial production of acrylic acid via oxidation of propylene to acrolein. The use of CO2 here also provides a safe reaction medium for performing oxidation reactions within. After pressure release and a condensation step where all unconverted glycerol and acetol (both have a relatively high boiling point) are separated for further refining, the remaining gaseous stream containing acrolein, acetaldehyde, propionaldehyde, and CO2 is passed through the second reaction of partial oxidation with air catalyzed by a Mo-V based catalyst. Here, acrolein is converted to acrylic acid according to Equation (2).

1 CH = CH CHO + O CH = CH COOH (2) 2 − 2 2 → 2 − Acetic acid and a small amount of propionic acid are generated as side products from the oxidation of acetaldehyde and propionaldehyde, respectively. Acrylic acid is the main revenue product, and acetol and acetic acid are also considered as revenue products. The yield of acrylic acid, acetol, and acetic acid from glycerol is 65 mol%, 12 mol%, and 16 mol%, respectively.

2.2. Process Modeling Unlike the biodiesel model [17] that was based on well-established processes with years of commercial operations, modeling in this work was based on our experimental data at lab scale that was then projected to larger scales of contemporary engineering practices. SuperPro Designer software (Version 8.5, Intelligen Inc., Scotch Plains, NJ, USA) was used to conduct the modeling work, providing modeling of manufacturing and end-of-pipe treatment processes under a single umbrella, as well as project economic evaluation. Because this process modeling was motivated to help assess the potential economic feasibilities, bottlenecks, operation targets for process improvement and identify further research and development effort requirements at the early stage of new biobased technology, the model is intended to capture key unit operations for the conversion of glycerol to acrylic acid and the product refining, representative of contemporary industry practices. However, this modeling is not meant to represent the actual design offered by any single technology provider. Although the model is in its relatively preliminary stages in terms of the level of its detail, it is not intended to replace the thorough engineering analysis that is required in the final design and construction of such a plant; we designed a plant that can recycle most of its process water and carbon dioxide in light of a future circular economy, possibly adding extra cost to the establishment. Energies 2020, 13, 6089 4 of 14

The plant is designed to continuously operate three shifts per day, 350 days per year, converting Energies3810 metric 2020, 13 tons, x FOR of PEER glycerol REVIEW (80 wt.%) from an intermediate-sized biodiesel facility to acrylic4 acidof 14 (1700 metric tons) with acetol (314 metric tons) and acetic acid (350 metric tons) as coproducts. noteworthyIt is noteworthy that this that is thisrather is small rather scale small comparing scale comparing to 80–160 tometric 80–160 tons metric annually tons for annually a commercial for a acryliccommercial acid plant acrylic using acid propylene plant using as propylene feedstock. asThe feedstock. SuperPro The Designer SuperPro flowchart Designer is presented ﬂowchart in is Figurepresented 1, with in Figure four1 color, with-coded four color-coded process sections process modeling sections modelingglycerol dehyd glycerolration dehydration to acrolein, to acrolein,acrolein oxidation to acrylic acid, product refining, and CO2/energy recycling. acrolein oxidation to acrylic acid, product reﬁning, and CO2/energy recycling.

Glycerol dehydration to acrolein

S-129 Acrolein CO /Energy P-16 /CSP-101 oxidation to 2 recycling acrylic acid

Products refining

Figure 1. FlowchartFlowchart for for modeling modeling acrylic acrylic acid production from glycerol.

Glycerol feed is first first diluted with recycled water to 30 wt.% (V (V-101),-101), and pressurized to 7.8 MPa (DP-101). This feedstock of 30 wt.% glycerol is merged with recycled CO at the same pressure (DP-101). This feedstock of 30 wt.% glycerol is merged with recycled CO2 at the2 same pressure (MX- (MX-101)101) and flows and flowsinto the into dehydration the dehydration reactor reactor(RX-101) (RX-101) operated operated at 320 °C at and 320 7.8◦C MPa and with 7.8 MPaZSM with-5 as aZSM-5 solid asacid a solid catalyst, acid catalyst,where the where glyc theerol glycerol is converted is converted to acrolein to acrolein with acetol, with acetol, acetaldehyde, acetaldehyde, and propionaldehydeand propionaldehyde as byproducts. as byproducts. After After being being depressurized depressurized (GTV (GTV-10)-10) and and cooled cooled with with a a line line of recycled CO (HX-105), the reaction effluent passes through a condensation step (HX-101), where part recycled CO2 (HX-105), the reaction effluent passes through a condensation step (HX-101), where part of water and all the acetol and unconvertedunconverted glycerol are condensed.condensed. The condensates go through a distillation (C (C-101)-101) medium to generate 99.8% acetol as a revenue product. The uncondensed stream (S(S-114)-114) is mixed with air (MX-102)(MX-102) and flowsflows into the partial oxidationoxidation reactor (RX-102),(RX-102), which is operated at 300 °C◦C at near near-atmospheric-atmospheric pressure, where the acrolein is converted to acrylic acid with acetic acid and propionic acid as side products; a Mo-VMo-V based catalystcatalyst isis usedused inin thisthis reactorreactor [[16]16].. A condensationcondensation step step (HX-102) (HX-102) separates separates the the second second reaction reaction effluent effluent into two into streams. two streams. The gaseous The stream (S-124) contains CO ,N , and O , and the CO is recycled. The condensate stream (S-104), gaseous stream (S-124) contains2 2CO2, N2,2 and O2, and the2 CO2 is recycled. The condensate stream (S- 104),consisting consisting of acrylic of acrylic acid, acid, acetic acetic acid, acid, propionic propionic acid, acid, and and water, water, passes passes through through aa liquid-liquidliquid-liquid extraction (DX (DX-101)-101) with ethyl acetate as the extractant, which can be recycled (via stream S S-122-122 and unit V V-102).-102). The bottom partition partition of of the the extractor extractor mainly mainly consists consists of of water water (S (S-123),-123), which which is is recycled. recycled. The top partition, comprised of extracted acrylic acid, acetic acid, acid, and and propionic propionic acid acid in in ethyl ethyl acetate, acetate, is first first added with hydroquinone hydroquinone as as the the inhibitor inhibitor (MX (MX-104)-104) to to prevent prevent polymerization polymerization of of acrylic acrylic acid, acid, before passing through two stages of distillation. The first first stage of distill distillationation (C (C-104)-104) recovers ethyl acetate (via stream S S-122);-122); the second distillation generates acetic acid (99.5%) at the top and acrylic acid (99.1% plus 0.23% hydroquinone as anti-polymerizationanti-polymerization additive) at thethe bottom.bottom. To eefficientlyfficiently utilize utilize process process heat heat and an COd 2CO, the2, the gaseous gaseous stream stream after theafter condensation the condensation of the secondof the secondreaction reaction effluent (S-124)effluent is (S first-124) cooled is first with cooled a recycled with linea recycled of CO2 line(HX-106), of CO followed2 (HX-106), by compressionfollowed by compression to 2.2 MPa (G-101) and chilling to −20 °C (HX-107), so the CO2 in the liquid phase is separated from gaseous N2 and O2 (CSP-101) and recycled. The recycled CO2 line (S-118), going through two stages of compression (DP-102 and G-102) to reach 7.8 MPa, is heated with the effluent

Energies 2020, 13, 6089 5 of 14 to 2.2 MPa (G-101) and chilling to 20 C (HX-107), so the CO in the liquid phase is separated from − ◦ 2 gaseous N2 and O2 (CSP-101) and recycled. The recycled CO2 line (S-118), going through two stages of compression (DP-102 and G-102) to reach 7.8 MPa, is heated with the eﬄuent line of the dehydration reaction (S-121), before ﬂowing back to the dehydration reactor (RX-101) via the mixer (MX-101).

2.3. Estimation of Production Costs A depreciable life of 10 years was assumed for the project. Estimation of production costs was based on the ﬂowchart above (Figure1) that forms the “Base Model” of this study. The following categories that could be speciﬁed in SuperPro Designer were considered in the estimation of production costs.

1. Material-related information. This includes the different unit costs (e.g., purchase price, selling price, waste treatment/disposal cost) of pure components, and stock mixtures. 2. Stream-related information. This includes the classification of input and output streams into different categories (e.g., raw materials, revenues, wastes, etc.). 3. Operation-related information. This includes the costs of labor and utilities (heat transfer agents and power) for each operation. 4. Equipment-related information. This includes the capital and operating costs for each equipment (e.g., purchase cost, installation, maintenance, consumables, etc.).

We used the built-in cost model and database in SuperPro Designer that estimated each equipment purchase cost based on size, the material of construction, and other characteristics, and the cost was brought up to the year of 2017 with an inflation rate of 2% per year. All the process vessels were specified to meet The American Society of Mechanical Engineers (ASME) standards of vessel construction that would increase the cost. Concerning construction materials, all the unit operation vessels or reactors are specified to be constructed of stainless steel with the exceptions of the two large CO2 compressors (G-101 and G-102), having carbon steel as the specified construction material. We followed the methodology of parametric input of Haas et al. [17] to the SuperPro Designer for capital investment adjustment. Direct fixed capital was estimated based on the total equipment purchase cost plus the installation cost. Unlisted equipment purchase cost was estimated as 20% of the listed equipment purchase cost, and unlisted equipment installation cost is two times the unlisted equipment purchase cost. Working capital was estimated to cover 30 days of utilities, labor, and raw materials, and the startup and validation cost account for 5% of the direct fixed capital. In the same way, operating cost was adjusted considering the facility-dependent cost that included depreciation, maintenance, and miscellaneous costs of insurance, local taxes, and factory expenses. A lumped estimate of 16,800 labor-hours per year was assigned to labor cost. Because environmental pollution regulations vary from location to location, no precise calculation of waste stream treatment costs was attempted. However, in the annual operating budget, $50,000 was allocated for waste stream disposal charges. Because this study focused on process modeling and production cost estimation at the early stage of a developing technology, economic factors of economic life, internal rate of return, corporate tax rate, salvage value, debt fraction, construction interest rate, long term interest rate, and marketing and distribution expenses, were not accounted for. The existence of governmental credits or subsidies was not considered. Based on the input of material throughputs and operation conditions, SuperPro Designer would first solve the mass and energy balances, and then perform economic calculations. Importantly, the establishment of the Base Model allowed adjustment of input parameters for variable analysis in response to the market value of glycerol, the source and cost of carbon dioxide recycling, and the changes in process scale and conditions. All the following tables and figures are based on our modeling and economic calculations using SuperPro Designer. Energies 2020, 13, 6089 6 of 14

3. Results and Discussion

3.1. Overview of Base Model A summary of the production cost estimation is presented in Table2. Detailed information on the data and terminology is provided in the annotated economic evaluation report generated by SuperPro Designer (Supplemental Information). Approximately $7.5 million total capital investment is needed to establish the designed facility converting glycerol to acrylic acid. Also listed in Table2 are major operating costs and revenue values of our input into the modeling. The purchasing price of crude glycerol is taken from the selling price reported in Haas et al. [17]. This price and those for revenue products represent a medium value of their ﬂuctuating prices in the past ten years. The total labor cost is estimated as $462,000 annually, slightly less than that ($490,600) of an intermediate-sized biodiesel facility reported in Haas et al. [17], since this project has fewer number of unit operations.

Table 2. Results of production cost estimation for Base Model (2017 price).

Summary Total capital investment $7,468,000 Annual operating cost $3,987,000 Main product revenue $3,882,797 Co-products revenue $573,117 Total revenue $4,455,914 Unit production cost $2.33/kg acrylic acid Unit production revenue $2.60/kg acrylic acid Gross proﬁt margin 10.53% Capital investment itimized Equipment purchase cost $2,782,000 Direct ﬁxed capital $6,866,000 Working capital $258,605 Startup & validation $343,318 Material throughputs Annual consumption of glycerol (80%) 3,810,000 kg Annual production of acrylic acid 1,712,437 kg Annual production of acetic acid 351,194 kg Annual production of acetol 314,778 kg Major operating costs and revenue values Glycerol (feedstock) $0.33/kg Acrylic acid (main revenue) $2.27/kg Acetic acid (revenue) $0.74/kg Acetol (revenue) $1.00/kg Total labor cost $462,000/year

Generally, this “Base Model” acrylic acid plant is profitable, although the profit margin is smaller, comparing to the intermediate-sized biodiesel facility [17]. Because the model is relatively preliminary in the level of its detail, and it is not meant to replace the thorough engineering analysis that is required in the final design and construction of such a plant, we will focus on the difference between the unit production cost and unit production revenue (profit margin) relative to the Base Model in the ensuing analysis and discussion. This will help to assess the potential economic feasibilities, bottlenecks, operation targets for process improvement and identify further research and development effort requirements at the early stage of a new biobased technology. Breaking down annual operating costs reveals more information (Figure2). Unlike the intermediate-sized biodiesel facility where the soybean oil feedstock accounts for 88% of total operating cost, the largest portion of annual operating cost is the utilities, followed in order by raw materials, facility-dependent costs, and labor (Figure2A). Dividing the operating cost by the four designed Energies 2020, 13, x FOR PEER REVIEW 7 of 14

Model in the ensuing analysis and discussion. This will help to assess the potential economic feasibilities, bottlenecks, operation targets for process improvement and identify further research and development effort requirements at the early stage of a new biobased technology. Breaking down annual operating costs reveals more information (Figure 2). Unlike the Energiesintermediate2020, 13,-sized 6089 biodiesel facility where the soybean oil feedstock accounts for 88% of 7total of 14 operating cost, the largest portion of annual operating cost is the utilities, followed in order by raw materials, facility-dependent costs, and labor (Figure 2A). Dividing the operating cost by the four processdesigned sections process shows sections that theshows dehydration that the sectiondehydration occupies section the highest occupies percentage, the highest followed percentage, by the COfollowed2/Energy by the recycle CO2 section/Energy (Figure recycle2 B).section After (Figure analysis 2B). of After the utilities analysis category of the utilities (Figure category2C), we found(Figure that2C), thewe topfound contributors that the top to contributors utility costs areto utility standard costs power are standard and CaCl power2 brine and used CaCl for2 brine deep used cooling. for Itdeep is noteworthy cooling. It is to noteworthy point out that to point the most out that expensive the most piece expensive of equipment piece of (~$1equipment million) (~ is$1 themillion) CO2 compressoris the CO2 compressor (G-101) that (G initially-101) that pressurizes initially pressurizes a large volume a large of volume CO2 from of CO near2 from atmospheric near atmospheric pressure topressure 2.2 MPa to before 2.2 MPa deep before cooling deep to separatecooling andto separate recycle theand CO recycle2. The the standard CO2. The power standard consumption power ofconsumption this CO2 compressor of this CO ranks2 compressor third-highest ranks among third all-highest unit operations, among all after unit the operations, dehydration after reactor the (RX-101)dehydration and reactor the partial (RX-101) oxidation and the reactor partial (RX-102) oxidation that reactor handle (RX reactions-102) that athandle high temperaturesreactions at high of ~300temperatures◦C. This analysis of ~300 indicates°C. This analysis that the indicates use of SC-CO that2 theas ause reaction of SC- mediumCO2 as a andreactio its recyclingn medium entails and its a highrecycling operating entails cost, a high especially operating in terms cost, especially of utilities. in terms of utilities.

A B C

Utilities CaCl2 brine Std Power CO2/E recycle 35.2% Raw materials 34.9% 39.7% 30.8% 28.9% Dehydration 44.7%

Labor- dependent Products refining 11.6% Steam Facility- 13.7% Heating oil Cooling 11.2% dependent Partial 5.9% Waste 22.9% water oxidation 6.6% treatment 10.7% Chilled 1.3% water Consumables 1.7% 0.2%

FigureFigure 2.2. BreakdownBreakdown of annualannual operatingoperating cost forfor thethe BaseBase Model,Model, ((AA)) byby categories,categories, ((BB)) byby processprocess sections,sections, andand ((CC)) furtherfurther breakdownbreakdown ofof thethe utilityutility category.category.

3.2.3.2. Factor EEffectsffects onon BaseBase ModelModel FactorsFactors andand theirtheir eeffectsffects onon thethe BaseBase ModelModel areare presentedpresented inin FigureFigure3 .3. HereHere we we define define the the unit unit productionproduction revenuerevenue minusminus unitunit productionproduction costcost forfor acrylicacrylic acidacid (AA)(AA) asas “Unit(R-C)”;“Unit(R-C)”; whenwhen thisthis valuevalue becomesbecomes negative,negative, thethe plantplant willwill notnot bebe profitable.profitable. TheThe glycerolglycerol purchasingpurchasing priceprice slightlyslightly aaffectsffects thethe totaltotal investment for for the the facility facility but but has has a more a more pronounced pronounced impact impact on the on annual the annual operating operating cost. cost.This ThisBase BaseModel Model plant plant can be can marginally be marginally profitable profitable if the if feedstock the feedstock price price rises rises to $0.4 to $0.400/kg /(Figurekg (Figure 3A).3A). Historically,Historically, thethe acrylic acrylic acid acid price price greatly greatly fluctuated, fluctuated, heavily heavily dependent dependent on the on petroleum the petroleum market andmarket the and supply the ofsupply propylene. of propylene. Nonetheless, Nonetheless, the global the global acrylic acrylic acid market acid market is expected is expected to reach to reach USD $22.6USD billion$22.6 billion by 2022, by with2022, demandwith demand in superabsorbent in superabsorbent polymer polymer production production as a key as drivinga key driving factor forfactor market for market growth, growth, according according to a study to a study by Grand by Grand View View Research, Research, Inc., SanInc. Francisco,, San Francisco, CA, USACA, USA [20]. It[20] appears. It appears that acrylic that acrylic acid priced acid priced at $2/kg at is$2/kg the break-even is the break point-even for point the Basefor the Model Base (Figure Model3 B).(Figure 3B). With further research and development in reaction catalysis, it is reasonable to achieve higher acrylicWith acid further yield and research lower and reaction development temperatures. in reaction The profit catalysis, margin it is can reasonable be increased to achieve by 74% higher if the acrylicacrylic acidacid yieldyield canand belower increased reaction by temperatures. 15% to reach 80The mol% profit (Figure margin3C). can If be the increased reaction temperatureby 74% if the foracrylic both acid dehydration yield can andbe increased partial oxidation by 15% canto reach be lowered 80 mol% to (Figure 280 ◦C, the3C). total If the capital reaction investment temperature and annualfor both operating dehydration cost and will partial be reduced oxidation by 0.7% can and be lowered 1.8%, respectively; to 280 °C, the utility total costs capital will investment be reduced and by 4.3%,annual while operating the Unit(R-C) cost will will be reduced be increased by 0.7% by 14.8%and 1.8%, (Figure respectively;3D). utility costs will be reduced by 4.3%, while the Unit(R-C) will be increased by 14.8% (Figure 3D).

Energies 2020, 13, 6089 8 of 14 Energies 2020, 13, x FOR PEER REVIEW 8 of 14

A Investment ($ million) Annual operating cost ($ million) Unit(R-C) ($/kg AA) 8 7.47 7.38 7.47 7.63 7 6.14 6 5 3.99 4.18 4 3.17 3 2 0.75 1 0.27 0.16 0 -1 Base 0.33 $/kg 0.09 $/kg 0.4 $/kg 1 $/kg -2 Glycerol purchasing price -0.99 B Total revenue ($ million) Annual operating cost ($ million) Unit(R-C) ($/kg AA) 5.13 5 4.46 3.99 3.96 3.95 3.96 3.96 4 3.28 3 2 1 0.69 0.27 0.00 0 -1 Base 2.27 $/kg 1.6 $/kg-0.39 2.0 $/kg 2.7 $/kg Acrylic acid selling price C Total revenue ($ million) Annual operating cost ($ million) Unit(R-C) ($/kg AA) 4.92 5 4.46 3.99 3.99 4 3 2

1 0.27 0.47 0 Base 65 mol% yield 15% yield increase Acrylic acid yield D Investment ($ million) Annual operating cost ($ million) Utilities ($ million) Unit(R-C) ($/kg AA) 8 7.47 7.42 7 6 5 3.99 3.92 4 3 2 1.40 1.34 1 0.27 0.31 0 Base 300-320 280 Reaction temperature ℃ ℃ Figure 3. Factor effects on Base Model, (A) effect of glycerol purchasing price, (B) effect of acrylic acid Figureselling 3. Factor price, effects (C) e ffonect Base of increasing Model, (A acrylic) effect acidof glycerol yield, and purchasing (D) effect price, of lowering (B) effect reaction of acrylic temperature. acid selling price, (C) effect of increasing acrylic acid yield, and (D) effect of lowering reaction temperature. 3.3. Effect of CO2 Input on Base Model 3.3. Effect of CO2 Input on Base Model In our research and development of the SC-CO2 technology to extend the service life of solid acidIn our catalysts, research the and amount development of CO2 ofinput the hasSC-CO not2 beentechnology optimized. to extend The Basethe service Model life requires of solid cycling acid catalysts, the amount of CO2 input has not been optimized. The Base Model requires cycling 9270 kg per hour of CO2, 50 times the molar input of glycerol. Our simulation results shown in Figure 4 demonstrate the critical need to reduce the CO2 input for the improvement of this new technology.

Energies 2020, 13, 6089 9 of 14

9270 kg per hour of CO2, 50 times the molar input of glycerol. Our simulation results shown in Figure4 demonstrateEnergies 2020 the, 13, criticalx FOR PEER need REVIEW to reduce the CO2 input for the improvement of this new technology.9 of 14

A Base CO2 volume Base x 50% Base x 38% 8 7.47 5.97 6 5.48 4.46 4.46 4.46 3.99 4 3.33 3.18

2 0.75 0.27 0.65 0 Investment Total revenue Annual operating cost Unit(R-C) ($ million) ($ million) ($ million) ($/kg AA) Major costs and revenue

B Raw materials Utilities Facility-dependent Labor-dependent Waste treatment Consumables 100% 11.6 13.9 14.6 80% 22.9 21.9 21.0 60% 35.2 27.9 26.4 40%

20% 28.9 34.6 36.3 0% Base CO2 volume Base x 50% Base x 38% Breakdown of operating cost by category

C Std Power Steam Cooling water Chilled water Heating oil CaCl2 brine 100%

32.3 80% 34.4 33.7

7.2 6.0 8.7 60% 1.7 2.7 3.0 6.5 8.4 9.0 11.0 40% 16.5 18.4

20% 39.2 32.7 28.7

0% Base CO2 volume Base x 50% Base x 38%

Further breakdown of utilities category

FigureFigure 4. E ﬀ4.ects Effects of reducing of reducing CO 2COinput2 input on Baseon Base Model, Model, (A) on(A) major on major costs costs and revenue,and revenue, (B) on(B) operating on costoperating by category, cost (byC) category, on itemized (C) on utility itemized costs. utility costs.

If theIf COthe 2COinput2 input can can be cutbe cut down down by by 50%, 50%, the the total total capital capital investment investment willwill bebe reducedreduced by 20% from $7.47from million $7.47 to million $5.97 million;to $5.97 themillion; annual the operatingannual operating cost will cost be will decreased be decreased by 17% by from17% from $3.99 $ million3.99 to $3.33million million, to and$3.33 the million Unit(R-C), and the will Unit(R be increased-C) will be by increased 140% from by 0.27140% to from $0.65 0.27/kg to AA. $0.65/kg Further AA. cutting Further cutting down CO2 input to 38% of that of the Base Model will further reduce capital down CO2 input to 38% of that of the Base Model will further reduce capital investment and operating investment and operating cost, elevating the profit margin (Figure 4A). cost, elevating the profit margin (Figure4A). Breaking down the operating cost by category reveals that the utility costs exhibit a significant Breaking down the operating cost by category reveals that the utility costs exhibit a significant reduction with the decreasing CO2 input, and raw materials rise to the top cost category (Figure 4B). reductionAmong with the utilities the decreasing used, the COreduction2 input, of and standard raw materialspower consumption rise to the is top pronounced cost category (Figure (Figure 4C). 4B). AmongFuture the research utilities should used, theprioritize reduction the reduction of standard of powerCO2 input consumption without sacrificing is pronounced its function (Figure of 4C). Futureextending research catalyst should service prioritize life. the reduction of CO2 input without sacrificing its function of extending catalyst service life. 3.4. Effect of No CO2/Energy Recycling

Changes are made to the Base Model by removing the process section of CO2/Energy recycling shown in Figure 1, while keeping other process conditions the same. In this NR-Model, there is a

Energies 2020, 13, 6089 10 of 14

3.4. Eﬀect of No CO2/Energy Recycling

Changes are made to the Base Model by removing the process section of CO2/Energy recycling shownEnergies in Figure 2020, 131, x, whileFOR PEER keeping REVIEW other process conditions the same. In this NR-Model, there10 is of a 14 source of CO2 as input to the plant, requiring two gas compressors and an extra heater to bring the input CO2 source of CO2 as input to the plant, requiring two gas compressors and an extra heater to bring the to the required pressure and temperature before entering the dehydration reactor. Five scenarios are input CO2 to the required pressure and temperature before entering the dehydration reactor. Five consideredscenarios for are the considered NR-Model for inthe contrast NR-Model to thein contrast Base Model to the (FigureBase Model5): (1) (Figure the input 5): (1) CO the2 inputhas aCO pressure2 of 1has Bar a with pressure no cost; of 1 (2)Bar thewith input no cost; CO (22)has the ainput pressure CO2 has of 10a pressure Bar with of no 10 cost;Bar with (3) theno cost; input (3) CO the2 has a pressureinput ofCO 202 has Bar a andpressure costs of $15 20 Bar per and metric costs ton; $15 (4)per the metric input ton; CO (4)2 theat 1input Bar CO has2 noat 1 cost Bar has and no the cost volume can beand cut the down volume by can 50%; be andcut down (5) the by input 50%; and CO 2(5)at t 1he Bar input can CO be2 at cut 1 downBar can by be 50%cut down in volume by 50% and in costs $25 pervolume metric and ton.costs $25 per metric ton.

A Base model 1 Bar free CO2 10 Bar free CO2 20 Bar CO2@$15/ton 1 Bar free 50% CO2 1 Bar 50% CO2@$25/ton

8 7.47 7.11 7

6 5.39 5.12 5.30 4.67 5 4.35 4.49 4.47 3.99 4 3.52 3.49 3 2 1 0.54 0.56 0.27 0.06 0 -0.02 -0.01 -1 Investment ($ million) Annual operating cost ($ million) Unit(R-C) ($/kg AA) Major costs and revenue

B Raw materials Utilities Facility-dependent Labor-dependent Waste treatment Consumables 100% 11.6 12.1 15.0 11.8 15.1 11.8 80% 14.4 22.9 18.2 27.2 21.7 23.3 20.6 60% 20.8 35.2 28.7 26.6 40% 32.6 51.8 20% 47.8 28.9 26.5 32.8 33.1 0% Base model 1 Bar 10 Bar 20 Bar CO2 1 Bar 1 Bar 50% CO2 free CO2 free CO2 @$15/ton free 50% CO2 @$25/ton Breakdown of operating cost by category

C Std Power Steam Cooling Water Chilled Water Heating oil CaCl2 Brine 100% 21.7 30.5 24.1 24.1 80% 34.2 33.4 5.8 6.1 6.1 1.6 2.5 2.5 5.8 11.2 10.0 60% 1.7 2.3 11.7 12.7 12.7 6.5 11.5 12.5 2.5 11.0 13.7 17.5 17.5 40% 16.1 17.6 48.2 20% 38.9 37.1 37.1 28.6 21.1 0% Base model 1 Bar 10 Bar 20 Bar CO2 1 Bar 1 Bar 50% CO2 free CO2 free CO2 @$15/ton free 50% CO2 @$25/ton Further breakdown of utilities category

Figure 5. Eﬀects of ﬁve scenarios of no CO2/Energy recycling on NR-Model in contrast to Base Model, Figure 5. Effects of five scenarios of no CO2/Energy recycling on NR-Model in contrast to Base Model, (A) on(A) major on major costs costs and and revenue, revenue, ( B(B)) on on operatingoperating cost cost by by category, category, (C ()C on) onitemized itemized utility utility costs. costs.

Energies 2020, 13, 6089 11 of 14

Although the total capital investment can be lowered by ~5% if we do not recycle CO2/Energy in the first scenario, the annual operating cost would be increased by 10%, resulting in a 78% lower profit margin (Figure5A) compared to the Base Model. This proves the advantage of including CO2/Energy recycling in the design of our Base Model. If the input CO2 is at 10 Bar (scenario 2), significant reductions of capital investment (by 31%) and operating cost (by 12%) can be achieved with an increase of profit margin by 100%, comparing to the Base Model. This analysis indicates that the initial compression of a large volume of CO2 input at near atmospheric pressure is an energy-intensive process, also requiring costly large equipment. Further rise of the input CO2 pressure to 20 Bar (scenario 3) would render a plant close to the break-even point even if the CO2 costs $15 per metric ton. As we have shown in the analysis for the Base Model in Section 3.3, the reduction of CO2 input volume has a pronounced impact on lowering investment and operating costs. The same is true for the NR-Model if the CO2 input is cut down by 50% (scenario 4), and the plant would be close to the break-even point even if the CO2 costs $25 per metric ton (scenario 5). Figure5B shows the relative percentage changes of each operating cost category for all the five scenarios in comparison with the Base Model, reinforcing that (1) initial compression of a large volume of CO2 is costly in terms of both utilities and facility; (2) the top two utilities cost items are the standard power and CaCl2 brine which are mainly used for CO2 compression and cooling (Figure5C); and (3) the reduction of CO2 input would significantly increase profit margin. It is noteworthy to point out that the future development of this bio-based technology should consider cascading processing in conjunction with emerging Carbon Capture and Utilization or Recycling (CCU or CCR) technologies that can efficiently provide CO2.

3.5. Effect of Threefold Scale-Up Although we intended to conduct the process modeling and production cost estimation of a cascading acrylic plant based on a typical intermediate-sized biodiesel facility, larger scales of biodiesel facilities exist. Therefore, we scale up the Base Model to evaluate the scaling impact. Two scenarios are considered here as shown in Figure6 in contrast to the Base Model: (1) a threefold scale-up in term of glycerol input volume (and proportionally other inputs of materials) keeping all process conditions the same as those of the Base Model, and (2) a threefold scale-up in term of glycerol input volume keeping all process conditions the same as those of the Base Model, except that the CO2 input is reduced by 50%. The threefold scale-up (3 scale) plant requires approximately a doubled capital investment and × 2.6 times the annual operating cost, comparing to the Base Model, and the profit margin is doubled as indicated by the Unit(R-C). Reducing CO2 input by 50% significantly lowers the capital investment and operating cost, leading to a more than tripled profit margin (Figure6A). With the threefold scale-up, the relative percentage of utilities cost jumps to nearly a half of the total operating cost, while that of the facility-dependent cost shrinks (Figure6B), although now the most expensive piece of equipment for initial CO2 compression (G-101) costs nearly $2.9 million, indicating an energy-intensive plant. Reducing CO2 input by 50% brings the percentage of the cost of utilities down to the same level as that for the Base Model (Figure6B). Looking further into the itemized utility costs (Figure6C) reveals that the scale-up increases the percentage of standard power cost to 48% of the total utilities cost, again mainly due to the large power consumption for compressing the CO2. It is clear that even for a threefold scale-up, the reduction of CO2 input should be prioritized in further research and development. Although continuous development of better catalysts remains as one way to extend catalyst life [21], the potential of “green” process engineering should be explored. We would like to point out that Carbon Capture and Utilization or Recycling (CCU or CCR) technologies are advancing quickly. In the North American market, ammonia producers in the United States are used to a price range of around US$3–15 per metric ton for bulk gaseous/supercritical CO2, which varies significantly depending on the location; the price for pipelined CO2 has historically been in the range of US$9–26 per metric ton, which includes the cost of the pipeline infrastructure (capital and operational costs) [22]. Energies 2020, 13, 6089 12 of 14

Energies 2020, 13, x FOR PEER REVIEW 12 of 14 Ethanol is already a part of the bioeconomy. A typical ethanol plant producing 50 million gallons ofof ethanol ethanol per per year year also also produces produces ~150,000 ~150,000 metric metric tons tons of of high high-purity-purity CO CO22 annuallyannually [23] [23].. These These are are potentialpotential sources sources that that could could provide provide CO CO22 toto an an acrylic acrylic acid acid plant plant in in a a cost cost-e-effectiveﬀective manner. manner.

Base model 3xScale 3xScale 50% CO2 A 15.61 16 14 12 10.56 10.31 10 7.47 8.12 8 6 3.99 4 2 0.27 0.59 1.02 0 Investment ($ million) Annual operating cost ($ million) Unit(R-C) ($/kg AA) Major costs and revenue

B Raw materials Utilities Facility-dependent Labor-dependent Waste treatment Consumables 100% 4.4 5.7 11.6 17.6 15.6 80% 22.9

60% 35.4 47.7 35.2 40%

20% 42.6 28.9 29.9

0% Base model 3xScale 3xScale 50% CO2 Breakdown of operating cost by category

C Std Power Steam Cooling Water Chilled Water Heating oil CaCl2 Brine 100%

30.0 33.033.0 80% 34.2

6.2 5.8 1.4 1.4 8.28.2 60% 1.7 5.4 2.5 6.51.7 2.5 6.5 9.1 8.28.2 11.0 40% 16.116.1

48.0 20% 38.9 32.032.0

0% Base model 3xScale 3xScale3xScale 50% CO2 Further breakdown of utilities category Figure 6. Eﬀects of two scenarios of scale-up in contrast to Base Model, (A) on major costs and revenue, Figure(B) on operating6. Effects costof two by category,scenarios (Cof) onscale itemized-up in utilitiescontrast costs.to Base Model, (A) on major costs and revenue, (B) on operating cost by category, (C) on itemized utilities costs.

Energies 2020, 13, 6089 13 of 14

4. Conclusions and Future Perspective This study establishes simulation models in the context of a circular economy to analyze the technical and economic feasibility of producing acrylic acid from glycerol in conjunction with a typical intermediate-sized biodiesel facility, based on our new technology of using carbon dioxide as reaction medium in a two-step process to catalytically convert glycerol to acrylic acid. The results of our process modeling and production cost estimation indicate that coproduction of acrylic acid with an intermediate-sized biodiesel plant can be economically feasible, contributing to the bioeconomy and circular economy, and the profit margin can be doubled with a threefold scale-up. Other than market factors such as fluctuating prices of glycerol and acrylic acid, our simulation reveals that from the technical standpoint, the reduction of CO2 input without sacrificing its function of extending catalyst service life has the highest positive impact on the economic benefit, followed by further increase of acrylic acid yield and reduction to milder reaction conditions. The profit margin can be increased by 74% if the acrylic acid yield can be increased by 15% to reach 80 mol%, whereas the profit margin can be increased by 140% if the CO2 input can be cut down by 50%. Our results clearly pointed out directions for further research and development of this new technology in the way that should consider cascading processing in conjunction with emerging Carbon Capture and Utilization or Recycling Technologies that can cost-effectively provide CO2. New biobased technologies are emerging as scattered dots, and it is now the time to connect the dots to develop cascading and integrated technologies towards a circular economy.

Supplementary Materials: SuperPro Designer Economic Evaluation Report is available online at http://www. mdpi.com/1996-1073/13/22/6089/s1. Author Contributions: X.P.Y. was responsible for conceptualization, methodology, investigation, supervision, writing, reviewing and editing. S.R. conducted the data curation, investigation, draft model preparation. All authors have read and agreed to the published version of the manuscript. Funding: This work was partially supported by the Department of Agriculture HATCH project No. TEN00521. Acknowledgments: We thank the support by the U.S. Department of Agriculture HATCH project. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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