Energy and Resource Efficient Production of Fluoroalkenes in High

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Article Energy and Resource Eﬃcient Production of Fluoroalkenes in High Temperature Microreactors

Konstantin Mierdel 1,2,*, Andreas Jess 3,4, Thorsten Gerdes 2,5, Achim Schmidt 2 and Klaus Hintzer 6

1 Chair of Materials Processing, University Bayreuth, 95447 Bayreuth, Germany 2 Institut für innovative Verfahrenstechnik, InVerTec e. V., 95447 Bayreuth, Germany 3 Chair of Chemical Engineering, University Bayreuth, 95447 Bayreuth, Germany 4 Center of Energy Technology, University Bayreuth, 95447 Bayreuth, Germany 5 Chair of Ceramic Materials Engineering, Keylab Glasstechnology, 95447 Bayreuth, Germany 6 3M Dyneon, 84508 Burgkirchen, Germany * Correspondence: [email protected]; Tel.: +49-921-55-7211

Received: 2 August 2019; Accepted: 17 September 2019; Published: 24 September 2019

Abstract: Tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) are the most common monomers for the synthesis of fluoropolymers at industrial scale. Currently, TFE is produced via multistep pyrolysis of chlorodifluoromethane (R22), resulting in a high energy demand and high amounts of waste acids, mainly HCl and HF. In this study, a new chlorine-free process for producing TFE and HFP in a microreactor is presented, starting from partially fluorinated alkanes obtained from electrochemical fluorination (ECF). In the microreactor, high conversion rates of CHF3, which is used as a surrogate of partly fluorinated ECF streams, and high yields of fluoromonomers could be achieved. The energy saving and the environmental impact are shown by a life cycle assessment (LCA). The LCA confirms that the developed process has economical as well as ecological benefits, and is thus an interesting option for future industrial production of fluoroalkenes.

Keywords: microreactor; fluoropolymers; tetrafluoroethylene (TFE); hexafluoropropylene (HFP); life cycle assessment (LCA)

1. Introduction Fluoropolymers are high-tech materials with extraordinary properties (non-flammable, chemically stable, high dielectric strength, and operating temperatures up to 280 ◦C), and thus are not substitutable in numerous technical applications, for example, in the semiconductor/electronic industry, seal- and corrosion-resistant applications, or in future technologies for energy conversion like fuel cells [1]. The worldwide consumption of fluoropolymers was about 270,000 t per year in 2015 [2]. The most common 1 polymer is polytetrafluoroethylene (PTFE) with 140,000 t a− . The complete fluoropolymer production requires about 162,000 t of tetrafluoroethylene (TFE) and 41,000 t of hexafluoropropylene (HFP) per 1 year [2]. In addition, partly fluorinated monomers like vinylidene fluoride (VDF) (43,000 t a− ) and vinyl fluoride (VF) are necessary for polyvinylidene fluoride (PVDF) production, but also for different applications, such as fluorinated membranes or new binding systems in electrodes for lithium ion batteries [3,4]. Industrial production: In contrast to “simple” monomers such as ethylene or propylene, which are produced in a one-step process (steamcracking), the production of fluoromonomers follows an expensive, multistage chemical chlorine-synthesis, which is called the R22 route [5,6]. The R22 route was first reported by Downing and Benning in 1945 [7]. This synthesis, as depicted in Table1 for TFE, starts with the partial chlorination of methane to trichloromethane (Table1, Equation (1)). In

ChemEngineering 2019, 3, 77; doi:10.3390/chemengineering3040077 www.mdpi.com/journal/chemengineering ChemEngineering 2019, 3, 77 2 of 19 this reaction step, not only trichloromethane, but also mono- and dichloromethane are produced, which can only finally be converted into trichloromethane by a respective recycle to the reactor. The other synthesized by-product, tetrachloromethane, cannot be used in further reactions and must be incinerated and disposed of. Further, large amounts of HCl (three mol HCl per mol methane) are formed in this reaction step. In the second synthesis step (Table1, Equation (2)), trichloromethane and hydrogen fluoride, which is produced by the reaction of calcium fluoride and sulphuric acid, are converted to difluorochloromethane (R22) through the use of an antimony chloride catalyst. The third reaction step (Table1, Equation (3)) consists of the pyrolysis of R22 to difluorocarbene and HCl at temperatures between 800 ◦C and 900 ◦C[8]. The gained difluorocarbene reacts immediately to tetrafluoroethylene (TFE) by dimerization (Table1, Equation (4)).

Table 1. Reaction equations of the industrial R22 route.

Reaction ∆rH298 [kJ/mol] Equation 450 C CH + 3 Cl ◦ CHCl + 3 HCl 305 Equation (1) 4 2 3 − SbCl→ CHCl + 2 HF 5 CHClF + 2 HCl 168 Equation (2) 3 → 2 800◦C CHClF2 : CF2 + HCl 120 Equation (3) →800 C 2 : CF ◦ C F 295 Equation (4) 2 → 2 4 −

A closer look at the reaction steps in Table1 illustrates that current TFE production leads to high amounts of unwanted by-products and waste acids, especially hydrochloric acid (HCl) and hydrofluoric acid (HF). A drawback is also the contamination of HCl with low amounts of HF (around 1500 ppm), which makes further use or recycling of the acid impossible. In addition, current TFE production has a high energy demand for the separation, purification, recycling, or incineration of by-products. It is thus highly desirable to develop a chlorine-free and more energy efficient process to produce fluoromonomers. Alternative reaction routes: The first invention of a chlorine-free synthesis of TFE was made in 1957 and involved an electric arc or a plasma process for the reaction of CF4 with coal particles to TFE. However, both processes need huge amounts of energy and achieve only low yields of TFE, and thus could not compete with the conventional R22 route [9,10]. Another approach for the production of fluoromonomers is the pyrolysis of fluoroform (R23), which is a by-product through over fluorination of trichloromethane (Equation (2)) in the R22 route. The annual amount of this waste stream is about several millions of kg per year [11]. Pyrolysis of CHF3 is conducted by Hauptschein and Fainberg [12] at temperatures of 700 to 960 ◦C, residence times about 0.6 s to about 0.08 s, and atmospheric pressure. They reached high conversion rates of about 80% and yields of C2F4 and C3F6 of about 58.1%. Additionally, Gelblum et al. [11] tries to utilize CHF3 waste streams by co-pyrolysing R23/R22 mixtures in the range of 690 to 775 ◦C and contact times less than 2 s. In the sole feed pyrolysis of R23 at 775 ◦C, Gelblum et al. measured conversion rates of CHF3 below 1% and detected TFE as only product gas. In contrast to R22, which was economically favorable earlier because of its availability as a general-purpose refrigerant, a new approach for a complete chlorine-free process is based on electro-chemical fluorination (ECF), where perfluorinated hydrocarbons are formed by reaction of anhydrous hydrofluoric acid (AHF) with the corresponding hydrocarbons. State-of-the-art ECF processes only provide partly- and perfluorinated alkanes, but no alkenes like TFE [13]. Therefore, an additional process step is necessary. Aschauer et al. [14] showed that high yields of TFE could be obtained in a microwave plasma, when the feed consists of perfluorinated alkanes like C2F6 and C4F10, which originated from ECF, as described by Schmeiser et al. or Nagase et al. (Table2)[15,16]. The problem of utilizing feed gases from ECF is the low selectivity to perfluorinated products obtained in this process. Depending on the hydrocarbon used as feed in the ECF, up to 70% of the product spectrum consists of only partly fluorinated alkanes (Table2)[ 15]. Production of 1000 ChemEngineering 2019, 3, 77 3 of 19 t perfluorinated compounds results in waste streams of about 400 t of partly fluorinated residues. Further, hydrofluorocarbon by-products, for example, CHF3,C2HF5, and C2H2F4, are produced, which are not allowed to be released into the atmosphere, because of their high global warming potential (GWP) (Table2)[17].

Table 2. Product distribution of electro-chemical ﬂuorination (ECF) [16,18] and global warming potential (GWP) of ﬂuorocarbons [17].

Total Current 100 yr GWP Feed Reaction a Product Comp. [mol%] Eﬀ. [%] [kg CO2/kg]

CF4 24.1 6500 CHF3 11.0 11,700 CH4 CH4 + 4 HF CF4 + 4 H2 48.2 → CF2H2 8.3 650 CFH3 56.6 150

C2F6 56.2 9200 C2F5H 14.2 2800 C H C H + 6 HF C F + 6 H 65.0 C F H 12.1 1000 2 6 2 6 → 2 6 2 2 4 2 CF3CFH2 10.3 1300 C2F3H3 7.2 300 a The calculation of current efficiency was based on the amount of current assumed to be necessary for forming fluorine with a discharging fluoride ion, which would react with the sample [18].

Currently, all partly ﬂuorinated compounds must be incinerated (Table3) under very high expenditure of energy. Another drawback of incineration is HF formation, which requires neutralization by bases like NaOH or Ca(OH)2 to receive depositable salts (NaF, CaF2) (Table3).

Table 3. Reaction equation of incineration and pyrolysis of CHF3.

Reaction Equation CHF + 1 O 3 HF + CO Equation (5) 3 2 2 → 2 CHF3 incineration CH + 2 O 2 H O + CO Equation (6) 4 2 → 2 2 Ca(OH) + 2 HF 2 H O + CaF Equation (7) 2 → 2 2 CHF HF+ : CF Equation (8) 3 → 2 CHF3 pyrolysis 2 : CF C F Equation (9) 2 → 2 4 C F + : CF C F Equation (10) 2 4 2 → 3 6

To counteract the disadvantages of thermal utilization and to utilize the complete product spectrum of the ECF, in this study, partly fluorinated alkanes were proven regarding their suitability as starting material for fluoromonomer production. The respective process route of a utilization of partly fluorinated compounds for the production of TFE and HFP is illustrated in Figure1. The synthesis starts with the supply of feed gas as by-products of ECF followed by pyrolysis for generation of monomers like TFE and HFP. The main advantages of this process are the utilization of cheap starting materials (currently waste streams) and the prevention of HCl waste acids. ChemEngineering 2019, 3, 77 4 of 19 ChemEngineering 2018, 2, x FOR PEER REVIEW 4 of 20

Figure 1. DesignDesign of of a a two-step two-step chlorine-free pr processocess for fluoromono ﬂuoromonomermer production.

In the field of fluorine chemistry and fluorine materials, the handling and processing of different In the field of fluorine chemistry and fluorine materials, the handling and processing of different reactions in microreactors are of growing relevance. The reasons for this trend are safer handling of reactions in microreactors are of growing relevance. The reasons for this trend are safer handling of hazardous reactants, a reduction in the (potential) exposure to toxic or hazardous chemical species, hazardous reactants, a reduction in the (potential) exposure to toxic or hazardous chemical species, and higher process safety in terms of thermal runaway by efficient cooling. Moreover, the application and higher process safety in terms of thermal runaway by efficient cooling. Moreover, the application of microreactors in fluorine containing reactions is beneficial in terms of reduced reaction volume, of microreactors in fluorine containing reactions is beneficial in terms of reduced reaction volume, optimal process conditions, and thus higher yields of target products [19,20]. Scale-up of a microreactor optimal process conditions, and thus higher yields of target products [19,20]. Scale-up of a process can be easily done just by numbering up [21]. Microreactors are also advantageous with regard microreactor process can be easily done just by numbering up [21]. Microreactors are also to kinetic studies of highly exothermic or endothermic reactions, for example, here for the highly advantageous with regard to kinetic studies of highly exothermic or endothermic reactions, for endothermic decomposition of fluorinated hydrocarbons. Compared with lab-scale fixed bed reactors, example, here for the highly endothermic decomposition of fluorinated hydrocarbons. Compared microreactors have a much higher surface-to-volume ratio, which enables very efficient heating or with lab-scale fixed bed reactors, microreactors have a much higher surface-to-volume ratio, which cooling, and thus almost isothermal operation. Hence, the determination of kinetic parameters is not enables very efficient heating or cooling, and thus almost isothermal operation. Hence, the hindered by limitations of heat and mass transfer [22]. determination of kinetic parameters is not hindered by limitations of heat and mass transfer [22]. In this work, the experimental results of the pyrolysis of partly fluorinated alkane CHF3 in a In this work, the experimental results of the pyrolysis of partly fluorinated alkane CHF3 in a high high temperature microreactor are presented. On the basis of the rate of CHF3 conversion, the kinetic temperature microreactor are presented. On the basis of the rate of CHF3 conversion, the kinetic data data of this initial reaction step of pyrolysis (dehydrofluorination of CHF3 to HF and difluorocarbene) of this initial reaction step of pyrolysis (dehydrofluorination of CHF3 to HF and difluorocarbene) were determined. In consecutive reactions, fluorinated products such as tetrafluoroethylene (TFE), were determined. In consecutive reactions, fluorinated products such as tetrafluoroethylene (TFE), hexafluoroproplene (HFP), and perfluoroisobutene (PFiB) are formed. The measured product hexafluoroproplene (HFP), and perfluoroisobutene (PFiB) are formed. The measured product distribution was compared with a simulation of the pyrolysis using kinetic data from literature for distribution was compared with a simulation of the pyrolysis using kinetic data from literature for these consecutive steps. The The optimum optimum reaction reaction cond conditionsitions for for monomer monomer formation formation were were investigated. investigated. Finally, thethe ecological ecological and and economical economical potential potentia of thel of process the process was evaluated was evaluated by a life cycleby a assessment life cycle (LCA).assessment (LCA). 2. Materials and Methods 2. Materials and Methods

2.1. Design of the Microreactor for the Pyrolsis of CHF3 2.1. Design of the Microreactor for the Pyrolsis of CHF3 Pyrolysis of partly fluorinated alkanes for the production of fluoromonomers and determination of kineticPyrolysis parameters of partly was fluorinated carried out alkanes in a high for temperaturethe production siliconcarbide of fluoromonomers (SiC) microreactor and determination provided byof 3Mkinetic technical parameters ceramics was (Table carried4). out in a high temperature siliconcarbide (SiC) microreactor provided by 3M technical ceramics (Table 4). Table 4. Technical data of SiC-microreactor. Table 4. Technical data of SiC-microreactor. Material Properties Unit Value Material Properties Unit Value Density g/cm3 >3.15 3 PorosityDensity % g/cm >3.15<2.0 Grain size Porosity µm% <2.0 2–10 Thermal conductivity λGrainSiC at 25size◦C W/(m K) µm 2–10 130 SpecificThermal heat c SiCconductivityat 25 ◦CJ λSiC at 25 /°C(g K) W/(m K) 130 0.69 Specific heat cSiC at 25 °C J/(g K) 0.69 ChemEngineering 2019, 3, 77 5 of 19 ChemEngineering 2018, 2, x FOR PEER REVIEW 5 of 20

TheThe ceramicceramic microreactormicroreactor isis depicteddepicted inin FigureFigure2 2and and has has a a channel channel cross cross section section of of 1 1 mm mm22and and aa reactionreaction volume volume of of 1.2 1.2 mL.mL. TheThe microreactormicroreactor design design consists consists of of four four sections. sections. AnAn in-in- andand outletoutlet forfor thethe suppliedsupplied andand producedproduced gases,gases, aa meandermeander reactionreaction zone,zone, andand aa quenchingquenching zone.zone. TheThe microreactormicroreactor hashas beneficialbeneficial materialmaterial propertiesproperties like like highhigh thermalthermal conductivityconductivity andand specificspecific heat,heat, whichwhich providesprovides almostalmost isothermalisothermal operation,operation, and and thus thus an an accurateaccurate measurementmeasurement of of the the kinetics kinetics of of CHF CHF33 pyrolysispyrolysis andand ofof thethe subsequentsubsequent fluoromonomerfluoromonomer formation, formation, respectively. respectively.

FigureFigure 2.2.Microreactor Microreactor used used for for the the experiments: experiments: (1) gas(1) inlet,gas inlet, (2) reaction (2) reaction zone, zone, (3) thermocouple (3) thermocouple guide tube,guide (4) tube, quenching (4) quenching system, system, and (5) and product (5) product gas outlet. gas outlet.

2.2.2.2. MeasurementMeasurement SetupSetup forfor PyrolsisPyrolsis ofof CHFCHF33 TheThe setupsetup forfor thethe determinationdetermination ofof conversionconversion ofof partlypartly fluorinatedfluorinated substancessubstances isis schematicallyschematically depicteddepicted inin FigureFigure3 3.. It It consisted consisted of of thethe gasgas supply,supply, the the high high temperature temperature micro micro reactor, reactor, the the quenching quenching withChemEngineeringwith aa followingfollowing 2018 exhaust,exhaust 2, x FOR gas PEERgas treatment, treatment, REVIEW and and the the online online gas gas chromatography chromatography (GC) (GC) analysis. analysis. 6 of 20 CHF3 was supplied by Air Liquide with purities of 99.95% and N2 by Sauerstoffwerke Friedrichshafen GmbH with purities of 99.9%. Mass flow controllers from Bronkhorst (EL-FLOW F- 201CV) delivered the gases. The volumetric flow rate of fluoroform was varied from 0.25 to 5 mL min−1. N2 was used as inert carrier gas to adjust a volumetric content of 10% fluoroform in the feed stream. The microreactor was placed in a tubular electric furnace and heated to the desired constant temperature. The reaction temperature was monitored by one thermocouple placed in the meander shaped reaction zone (see Figure 2). After passing through the microreactor, the product gas was quickly quenched below 100 °C to avoid consecutive reactions of TFE to di- or other oligomers. Additionally, hydrogen fluoride (HF) formed through the pyrolysis was neutralized with KOH (40 g/L) during passage through a washing bottle. The HF-concentrations were determined by pH probe, Knick SE555X/1-NMSN, placed in the washing bottles. The moisture behind the washing column was removed via molecular sieve of 0.3 nm (Metrohm Ion analysis). For the detection of the product gas distribution, an online gas chromatography (GC) HP 6890 equipped with a thermal conductivity detector (TCD) and a Carbopack C33 column (6 m, 5% picric acid) was used. The GC was operated in the temperature range from 40 to 110 °C, with a temperature ramp of 10 K/min and a dwell time Figure 3. FlowFlow sheet sheet of of the measurement setup for the pyrolysis of partly fluorinatedfluorinated alkanes. of 5 min at 110 °C. The detector ran at a temperature of 200 °C and the injection temperature was 150 °C. 2.3. PyrolysisCHF3 was Reaction, supplied Kinetic by Modeling, Air Liquide and Life with Cycle purities Assessment of 99.95% (LCA) for and the N Pyrolsis2 by Sauersto of CHF3 ffwerke Friedrichshafen GmbH with purities of 99.9%. Mass flow controllers from Bronkhorst (EL-FLOW Experimental procedure: Before each pyrolysis experiment, the microreactor was purged with F-201CV) delivered the gases. The volumetric flow rate of fluoroform was varied from 0.25 to 2 2 N . The microreactor1 was then heated to the desired reaction temperature under N atmosphere. After 5 mL min− .N2 was used as inert carrier gas to adjust a volumetric content of 10% fluoroform in reaching the target temperature, CHF3 was supplied. The pyrolysis experiments were conducted at the feed stream. The microreactor was placed in a tubular electric furnace and heated to the desired temperatures of 775–875 °C, residence times of 0.4–2.5 s, a molar ratio of CHF3 to N2 of 0.1, and at constant temperature. The reaction temperature was monitored by one thermocouple placed in the almost atmospheric pressure (Table 5).

Table 5. Experimental parameters of CHF3 pyrolysis.

Parameters Unit Range Temperature °C 775–875 Pressure mbar 1013–1263 (absolute) Flow rate (standard temperature and pressure (STP)) mL/min 7.5–50 Residence time s 0.4–2.5

Modelling of CHF3 pyrolysis: Several researchers have already investigated the thermal decomposition of fluoroform (CHF3) over a broad temperature and residence time range, as well as with shockwave techniques or turbulent reactors. The main products in the pyrolysis of CHF3 reported in literature are the valuable monomers TFE and HFP, but also undesired partly fluorinated alkanes and alkenes like C2F6, C2HF3, C3F8, and C2F5H, as well as toxic compounds like i-C4F8 and HF [23]. It is generally agreed that the initial step in the synthesis of fluoromonomers is the highly endothermic decomposition of fluoroform yielding difluorocarbene (:CF2) [24]. Hence, an efficient supply of heat is needed to keep the dehydrofluorination going. The decomposition of CHF3 starts up at 700 °C and is intensified by an increase of temperature and residence time. Reported activation energies for the decomposition of CHF3 are in the range of 244–295 kJ/mol [24,25]. By dimerization of the generated difluorocarbene, the main product TFE is formed, but dimerization of difluorocarbene is reversible under reaction conditions, which is experimentally evidenced by Couture et al. [26] in their studies to produce HFP out of TFE. The yield of TFE passes a maximum value at a certain reaction (residence) time in consequence of the consecutive reaction with difluorocarbene to HFP [27]. Thus, the formation of fluoromonomers by pyrolysis of fluoroform depends on the generation and consumption of difluorocarbene. The complete reaction network of CHF3 pyrolysis, which consists of seven reactions, is described in detail in Section 3.3. In this study, the calculation of the CHF3 conversion was done based on the kinetic parameters (pre-exponential factor, activation energy) determined by respective own experiments. The rates of formation of the consecutive products tetrafluoroethylene (TFE), hexafluoropropylene (HFP), and perfluoroisobutene (PFiB) were calculated based on the respective kinetic parameters reported in the ChemEngineering 2019, 3, 77 6 of 19 meander shaped reaction zone (see Figure2). After passing through the microreactor, the product gas was quickly quenched below 100 ◦C to avoid consecutive reactions of TFE to di- or other oligomers. Additionally, hydrogen ﬂuoride (HF) formed through the pyrolysis was neutralized with KOH (40 g/L) during passage through a washing bottle. The HF-concentrations were determined by pH probe, Knick SE555X/1-NMSN, placed in the washing bottles. The moisture behind the washing column was removed via molecular sieve of 0.3 nm (Metrohm Ion analysis). For the detection of the product gas distribution, an online gas chromatography (GC) HP 6890 equipped with a thermal conductivity detector (TCD) and a Carbopack C33 column (6 m, 5% picric acid) was used. The GC was operated in the temperature range from 40 to 110 ◦C, with a temperature ramp of 10 K/min and a dwell time of 5 min at 110 ◦C. The detector ran at a temperature of 200 ◦C and the injection temperature was 150 ◦C.

2.3. Pyrolysis Reaction, Kinetic Modeling, and Life Cycle Assessment (LCA) for the Pyrolsis of CHF3 Experimental procedure: Before each pyrolysis experiment, the microreactor was purged with N2. The microreactor was then heated to the desired reaction temperature under N2 atmosphere. After reaching the target temperature, CHF3 was supplied. The pyrolysis experiments were conducted at temperatures of 775–875 ◦C, residence times of 0.4–2.5 s, a molar ratio of CHF3 to N2 of 0.1, and at almost atmospheric pressure (Table5).

Table 5. Experimental parameters of CHF3 pyrolysis.

Parameters Unit Range

Temperature ◦C 775–875 Pressure mbar 1013–1263 (absolute) Flow rate (standard temperature and pressure (STP)) mL/min 7.5–50 Residence time s 0.4–2.5

Modelling of CHF3 pyrolysis: Several researchers have already investigated the thermal decomposition of fluoroform (CHF3) over a broad temperature and residence time range, as well as with shockwave techniques or turbulent reactors. The main products in the pyrolysis of CHF3 reported in literature are the valuable monomers TFE and HFP, but also undesired partly fluorinated alkanes and alkenes like C2F6,C2HF3,C3F8, and C2F5H, as well as toxic compounds like i-C4F8 and HF [23]. It is generally agreed that the initial step in the synthesis of fluoromonomers is the highly endothermic decomposition of fluoroform yielding difluorocarbene (:CF2)[24]. Hence, an efficient supply of heat is needed to keep the dehydrofluorination going. The decomposition of CHF3 starts up at 700 ◦C and is intensified by an increase of temperature and residence time. Reported activation energies for the decomposition of CHF3 are in the range of 244–295 kJ/mol [24,25]. By dimerization of the generated difluorocarbene, the main product TFE is formed, but dimerization of difluorocarbene is reversible under reaction conditions, which is experimentally evidenced by Couture et al. [26] in their studies to produce HFP out of TFE. The yield of TFE passes a maximum value at a certain reaction (residence) time in consequence of the consecutive reaction with difluorocarbene to HFP [27]. Thus, the formation of fluoromonomers by pyrolysis of fluoroform depends on the generation and consumption of difluorocarbene. The complete reaction network of CHF3 pyrolysis, which consists of seven reactions, is described in detail in Section 3.3. In this study, the calculation of the CHF3 conversion was done based on the kinetic parameters (pre-exponential factor, activation energy) determined by respective own experiments. The rates of formation of the consecutive products tetrafluoroethylene (TFE), hexafluoropropylene (HFP), and perfluoroisobutene (PFiB) were calculated based on the respective kinetic parameters reported in the literature. The differential equations describing both the CHF3 conversion and the product yields at different residence times and temperatures were solved by Matlab R2015b using ode 45 solver. Life cycle analysis (LCA): The evaluation of the ecological and economical potential of the developed process (LCA) was carried out according to ISO 1404X standards with the software Simapro ChemEngineering 2019, 3, 77 7 of 19

8.3.0 and the Database ecoinvent 3.3. The assessment criteria are the cumulative energy demand (CED) in version V1.09 and the 18 impact categories of the ReCiPe method midpoint (H) v1.12/European ReCiPe (H).

3. Results

3.1. Calculation of Conversion and Yield in the Pyrolysis of CHF3

The pyrolysis of CHF3 can be regarded as a ﬁrst order reaction:

dcCHF r = 3 = k c . (11) CHF3 − dτ CHF3 CHF3

3 1 The reaction rate r (mol m− s− ) is deﬁned here based on the residence time (τ) in the tubular 3 reactor and depends on the rate constant kCHF3 (1/s) and the concentration of CHF3 (mol m− ). For an isothermal and ideal plug ﬂow reactor, integration of Equation (11) yields the following conversion:

k τ X = 1 e− CHF3 . (12) CHF3 − The assumption of a plug flow reactor is only valid in good approximation for a highly turbulent flow (Reynolds number Re > 2300). For the microreactor used in this study, Re is defined as

u dhyd Re = , (13) ν where u is the gas velocity (at reaction conditions), dhyd is the characteristic (hydraulic) diameter, and ν is the kinematic viscosity. For a quadratic channel with height and width s (both here 1 mm), dhyd is given by the following (A: cross sectional area; C: circumference):

4 A 4 s2 d = · = = s. (14) hyd C 4 s For the given microreactor, Re is very small, for example, 23 for a typical residence time of 0.4 s and temperature of 875 ◦C. Thus, the ﬂow is strictly laminar under the applied conditions in this study. For laminar ﬂow, the conversion of CHF3 in a tubular reactor is usually calculated as follows [28]:

2 Z∞ x Da Da Da e− X = 1 1 e −2 dx, (15) lam − − 2 · − 4 x Da 2

Da = k (T) τ, (16) CHF3 · EA ( ) RT kCHF3 T = kCHF3,0e− , (17)

Vreactor Lreaction zone τ = . = . (18) u (T) VFeed(T) Feed Da is the so-called Damköhler number. Solutions of the integral in Equation (15) are tabulated [29,30]. In the case of laminar flow, the assumption of plug flow behavior is not valid and usually a lower conversion (as determined by Equation (15)) is achieved compared with an ideal plug flow reactor (PFR) (Equation (12)). However, the advantage of the small dimensions (above all the small diameter) of the used microreactor is a relatively fast diffusion in the radial direction, that is, the characteristic ChemEngineering 2019, 3, 77 8 of 19

time for radial diﬀusion, τD, is much shorter than the average residence time (τ)[28]. In Equation (19), rt is the radius of reaction channel, and Dmol is the diﬀusivity of gas (N2).

2 rt Lreaction zone τD = τ = . (19) Dmol uFeed(T)

In this study, the residence time τ (0.4–2.5 s) is always higher than the characteristic time of diffusion τD (0.1 s, determined by Equation (19)). In this case, we may use the axially dispersed plug flow model and the so-called axial dispersion coefficient Dax, respectively, to inspect the deviation from an ideal PFR (see, for example, the work of [28]):

D 1 D d ax = = ax t , (20) uLt Bo udt Lt

2 2 u dt Dax = Dmol + . (21) 192Dmol

In the Equations (20) and (21), u is the gas velocity, and dt and Lt are the diameter and length of the reactor, respectively. The Bodenstein number Bo represents the ratio of convective flux to diffusive flux [28]: uL Bo = t . (22) Dax

For Re higher than 10, the ﬁrst term on the right-hand side of Equation (21) (Dmol) can be neglected and the Bo as a measure for the deviation from plug ﬂow behavior reads as follows:

  Lt Dmol  1 τ Bo = 192   = 192τ 50 ( f or Re > 10). (23) u  2  4τ ≈ τ 4rt D D

In general, plug ﬂow is almost reached if Bo > 80 [28]. In the used microreactor, Bo is always >175 (875 ◦C, 0.4 s). Thus, Equation (12) is valid and was used to calculate the conversion of CHF3. The yields and selectivities of TFE and HFP are calculated by the residual molar rates in mol/s (reactor outlet) and the initial molar rate of CHF3 (Equations (24)–(27)). The factors 2 and 3 consider the stoichiometry of the respective reactions.

. nTFE YTFE = 2 . , (24) nCHF3,0

. nHFB YHFP = 3 . , (25) nCHF3,0 . nTFE YTFE STFE = 2 . . = , (26) n n XCHF3 CHF3,0 − CHF3 . nHFB YHFP SHFP = 3 . . = . (27) n n XCHF3 CHF3,0 − CHF3 For the ﬁrst order assumption, the rate constant is given (based on Equation (12)) by the following:

1 X k = ln − CHF3 . (28) CHF3 − τ ChemEngineering 2019, 3, 77 9 of 19

The pre-exponential factor k0 and the activation energy EA were determined by the Arrhenius equation (Equation (29) and the respective plot based on experiments at diﬀerent temperatures (775 to 875 ◦C) and residence times (0.35 to 2.5 s). All experiments were conducted at atmospheric pressure.

E 1 ln(k ) = ln(k ) A . (29) CHF3 CHF3,0 − R T

3.2. Kinetic Parameters, Conversion, and Product Distribution in the Pyrolysis of CHF3 Kinetic parameters: The experimental data are fitted with a least square fitting for the calculation of the kinetic parameters. The data are shown in Figure4a and the straight lines are in good agreement withChemEngineeringChemEngineering the first order 2018 2018, ,2 approach.2, ,x x FOR FOR PEER PEER TheREVIEW REVIEW values of kCHF3,0 and EA are listed in Figure4b. 9 9 of of 20 20

TemperatureTemperature // °C°C 1.41.4 900900 850850 800800 750750 775°C 775°C 11 800°C 800°C 1.21.2 825°C 825°C 850°C 850°C 1.01.0 875°C 875°C

0.80.8 -1 -1

0.60.6 0.10.1 k(T) / s k(T) / s ln(1/(1-X)) / - ln(1/(1-X)) ln(1/(1-X)) / - ln(1/(1-X)) 0.40.4 CHFCHF33 0.20.2 EEAA:: 194194 kJ/molkJ/mol 0.00.0 kk00:: 3.84E+083.84E+08 1/s1/s 0.00.0 0.5 0.5 1.0 1.0 1.5 1.5 2.0 2.0 2.5 2.5 0.010.01 ResidenceResidence timetime ττ // ss 8.48.4 8.6 8.6 8.8 8.8 9.0 9.0 9.2 9.2 9.4 9.4 9.6 9.6 9.8 9.8 ReciprocalReciprocal temperaturetemperature // 1010-4-4 KK-1-1 ((aa)) ((bb))

FigureFigureFigure 4. 4.4.Measurement MeasurementMeasurement ofof kinetickinetic paramete parametersparametersrs inin in thethe the pyrolysispyrolysis pyrolysis ofof of CHFCHF CHF33:: 3((a:(a)) adeterminationdetermination) determination ofof k(T)k(T) of k(T) atat at didifferentdifferentﬀerent temperatures, temperatures,temperatures, ( (b(bb))) Arrhenius ArrheniusArrhenius plot plot fo forforr determination determination ofof of EEEAA A andandand kkCHFCHFkCHF3,03,0.. 3,0.

FirstFirstFirst orderorderorder assumptionassumptionassumption andandand conversion:conversion:conversion: TheTheThe measuredmeasured measured CHFCHF CHF33 conversion3conversionconversion forfor for differentdifferent different concentrationsconcentrationsconcentrations ofofof fluoroformfluoroformfluoroform isis shownshown inin in FigureFigure Figure 555 a(a)(a) for forforthree threethree temperatures. temperatures.temperatures. TheThe The conversionconversion conversion isis is constantconstantconstant for forfor each eacheach temperature, temperature,temperature, whichwhich is isis a a clear clear indicationindication ofof of aa a firstfirst first orderorder order reaction.reaction. reaction.

6060 100100 cCHF3 cCHF3 10% 10% 0.4s 0.4s cCHF3 cCHF3 20% 20% 0.8s 0.8s 5050 cCHF3 50% cCHF3 50% 8080 2.5s 2.5s τ c : 10% τ:: 1.7-2.31.7-2.3 ss cCHF3CHF3: 10% 4040 % % p:p: 30-4030-40 mmbarbar (rel.)(rel.) % % / / 6060 / / / / 3030 CHF3 CHF3 CHF3 CHF3 X X X X 4040 2020

1010 2020 Sim.Sim. Meas.Meas. 00 00 700700 750 750 800 800 850 850 900 900 600600 700 700 800 800 900 900 1000 1000 TemperatureTemperature // °C°C TemTempperatureerature // °°CC ((aa)) ((bb))

FigureFigureFigure 5. 5.5. ( (a(a)a)) Conversion ConversionConversion of of CHF CHF33 forforfor differentdifferent diﬀerent temperaturestemperatures temperatures andand and ((bb ()) b comparisoncomparison) comparison ofof simulated ofsimulated simulated andand and measuredmeasuredmeasured conversion conversionconversion of ofof CHF CHFCHF33 overover temperaturetemperature forfor for differentdifferent diﬀerent residenceresidence residence times.times. times.

InInIn Figure FigureFigure5b, 55 the (b),(b), conversion thethe conversionconversion of CHF ofof 3CHFCHFis shown33 isis shownshown as a function asas aa functionfunction of temperature ofof temperaturetemperature for di ﬀ forforerent differentdifferent residence times.residenceresidence The times.calculatedtimes. TheThe calculated CHFcalculated3 conversion CHFCHF33 conversionconversion (dashed lines) (dashed(dashed matches lines)lines) matches thematches experimental thethe experimentalexperimental data (dots) datadata quite (dots)(dots) well quitequite wellwell (for(for thethe assumptionassumption ofof aa firstfirst orderorder reactireaction).on). MeasuredMeasured conversionconversion ofof aboutabout 25%25% atat 875875 °C°C andand 0.40.4 ss isis inin goodgood agreementagreement withwith CHFCHF33 conversionconversion ofof 27.3%27.3% inin thethe experimentsexperiments ofof HauptscheinHauptschein etet al.al. [12].[12]. ProductProduct distribution:distribution: TheThe productsproducts ofof CHFCHF33 pyrolysispyrolysis areare practicallypractically onlyonly CC22FF44 andand CC33FF66.. CC22FF66 andand i-C i-C44FF88 are are only only formed formed in in traces traces at at the the highest highest meas measuredured temperatures. temperatures. The The yields yields and and selectivities selectivities ofof TFETFE andand HFPHFP (as(as valuablevaluable products)products) atat shortestshortest andand longestlongest adjustedadjusted residenceresidence timetime areare shownshown inin FigureFigure 66 andand 77 inin aa temperaturetemperature rangerange ofof 775775 toto 875875 °C.°C. WithWith increasingincreasing temperaturetemperature andand residenceresidence time,time, thethe yieldyield andand selectivity,selectivity, respectively,respectively, ofof thethe consecutiveconsecutive productproduct HFPHFP (C(C33FF66)) increases.increases. InIn thisthis study,study, aa maximummaximum yieldyield ofof 23%23% TFETFE (0.4(0.4 ss atat 875875 °C)°C) andand ofof 17%17% TFETFE andand 35%35% HFPHFP (2.5(2.5 ss atat 875875 °C)°C) isis achieved.achieved. ComparedCompared withwith yieldsyields ofof HauptscheinHauptschein etet al.al. [12],[12], thethe generatedgenerated yieldsyields ofof TFETFE andand HFPHFP areare inin thethe samesame range.range. Thus,Thus, withwith thethe samesame processprocess paraparametersmeters (880(880 °C°C andand 0.40.4 s),s), thethe calculatedcalculated yieldyield byby ChemEngineering 2019, 3, 77 10 of 19

(for the assumption of a first order reaction). Measured conversion of about 25% at 875 ◦C and 0.4 s is in good agreement with CHF3 conversion of 27.3% in the experiments of Hauptschein et al. [12]. Product distribution: The products of CHF3 pyrolysis are practically only C2F4 and C3F6.C2F6 and i-C4F8 are only formed in traces at the highest measured temperatures. The yields and selectivities of TFE and HFP (as valuable products) at shortest and longest adjusted residence time are shown in Figures6 and7 in a temperature range of 775 to 875 ◦C. With increasing temperature and residence time, the yield and selectivity, respectively, of the consecutive product HFP (C3F6) increases. In this ChemEngineeringChemEngineeringstudy, a maximum 2018 2018, ,2 2, yield,x x FOR FOR ofPEER PEER 23% REVIEW REVIEW TFE (0.4 s at 875 ◦C) and of 17% TFE and 35% HFP (2.5 s at 875 10 10◦ ofC) of 20 20 is achieved. Compared with yields of Hauptschein et al. [12], the generated yields of TFE and HFP are in EquationsEquationsthe same range.(24) (24) and and Thus, (25) (25) of withof Hauptschein Hauptschein the same processet et al. al. is is 17 parameters17.9%.9% for for TFE TFE (880 and and◦C 6.6% 6.6% and for 0.4for HFP. s),HFP. the The The calculated highest highest yield yieldyield of byof 58.1%58.1%Equations is is reached reached (24) andat at 960 960 (25) °C °C of and and Hauptschein 0.3 0.3 s. s. The The maximum maximum et al. is 17.9% yield yield forof of fluoromonomers TFEfluoromonomers and 6.6% for (52%) (52%) HFP. is Theis obtained obtained highest in in yieldthis this studystudyof 58.1% atat lower islower reached temperaturestemperatures at 960 ◦C and (about(about 0.3 s.100100 The °C)°C) maximum andand longerlonger yield residenceresidence of fluoromonomers timestimes (2.5(2.5 s), (52%)s), whichwhich is obtained couldcould bebe in explainedexplainedthis study by by at the lowerthe reversible reversible temperatures dimeri dimeri (aboutzationzation of 100of TFE TFE◦C) to to and difluorocarbene difluorocarbene longer residence and and times the the consecutive consecutive (2.5 s), which reaction reaction could to beto HFPHFPexplained at at the the adjusted byadjusted the reversible residence residence dimerization times times [26]. [26]. of TFE to difluorocarbene and the consecutive reaction to HFP at the adjusted residence times [26].

4040 4040 0.4s 0.4s 0.4s 0.4s 2.5s 2.5s 2.5s 2.5s c : 10% ccCHF3: :10% 10% c CHF3: 10% 3030 CHF3 3030 CHF3 % % % %

/ / 20 / 20

/ / 20 / 20 HFP TFE HFP TFE Y Y Y Y

1010 1010

00 00

775775 800 800 825 825 850 850 875 875 775775 800 800 825 825 850 850 875 875 Temperature / °C TemperatureTemperature / /°C °C Temperature / °C (a(a) ) (b(b) )

Figure 6. Measured product formation in the pyrolysis of CHF3: (a) yield of C2F4 and (b) yield of C3F6 FigureFigure 6. 6. MeasuredMeasured product product formation formation in in the the pyrolysis pyrolysis of CHF 3::( (aa)) yield yield of of C C22FF44 andand ( (bb)) yield yield of of C C33FF6 6 (line:(line: guide guide guide for for for the the the eye, eye, dot: dot: experimental experimental experimental data). data). data).

100 100 100 100 0.4s 0.4s 2.5s 2.5s 8080 8080

60 60 % % 60 60 % % / / / / HFP TFE HFP TFE

S 40 S 40

2020 2020 0.4s 0.4s 2.5s 00 2.5s 00 750750 775 775 800 800 825 825 850 850 875 875 900 900 750750 775 775 800 800 825 825 850 850 875 875 900 900 TemperatureTemperature / /°C °C TemperatureTemperature / /°C °C

(a(a) ) (b(b) )

FigureFigure 7. 7. Selectivity SelectivitySelectivity of of fluoromonomers fluoromonomers ﬂuoromonomers TFE TFE ( a (a) )and and HFP HFP ( b (b) )in inin the thethe pyrolysis pyrolysispyrolysis of ofof CHF CHFCHF33 3(line: (line: guide guide guide for for thethe eye, eye, dot: dot: experimental experimental experimental data). data). data).

Long-termLong-term process process stability stability of of CHF CHF 3-pyrolysis3-pyrolysis-pyrolysis in in in the the the SiC-microreactor: SiC-microreactor: SiC-microreactor: After AfterAfter the the the evaluation evaluation evaluation of of suitablesuitable process process parameters, parameters, the the CHF CHF3 33pyrolysis pyrolysis was was carried carried out out at at residence residence times times of of 0.4 0.4 and and 2.5 2.5 s s for for severalseveral hours hours to to study study the the “long-term” “long-term” microreactor microreactor stability stability and and performance. performance. Figure Figure 8 88 shows showsshows that thatthat isothermalisothermal conditions conditions and and a a stable stable reactor reactor operation operation were were reached reached in in both both cases cases after after about about 1 1 h. h. The The timetime of of 1 1 h h is is necessary necessary for for heating heating the the oven oven to to target target temperatures temperatures and and purging purging all all tubes tubes and and washing washing bottlesbottles with with the the same same concentration concentration of of reaction reaction products. products. ChemEngineering 2019, 3, 77 11 of 19 isothermal conditions and a stable reactor operation were reached in both cases after about 1 h. The time of 1 h is necessary for heating the oven to target temperatures and purging all tubes and washing bottlesChemEngineering with the 2018 same, 2, x concentration FOR PEER REVIEW of reaction products. 11 of 20

1000 100 1000 100

800 80 800 80 °C % °C / % CHF3 600 60 / 600 60 HFP TFE , Y TFE , Y , 400 40 400 HFP 40 , Y , CHF3 CHF Temperature /

3 X CHF3 Temperature /

20 TFE 20 X 200 TFE 200

0 0 0 0 0 100 200 300 400 500 600 700 0 120 240 360 480 600 Time on stream / min Time on stream / min (a) (b)

FigureFigure 8. 8.Time Time on on stream: stream: ( a()a) production production of of CC22FF44 (0.4 s) s) and and ( (bb)) production production of of C C2F24F and4 and C3 CF36 F(2.56 (2.5 s) at s) at 875875◦C. °C.

3.3.3.3. Simulation Simulation of of Product Product Distribution Distribution

TheThe gas gas phase phase pyrolysispyrolysis of of CHF CHF3 at3 attemperatures temperatures between between 775 and 775 875 and °C 875 was◦ Cexamined was examined in more in moredetail detail using using both bothexperimental experimental data and data modeling and modeling techniques. techniques. The first step The in first the step pyrolysis in the of pyrolysis CHF3 ofis CHF dehydrofluorination3 is dehydrofluorination to hydrogen to hydrogen fluoride fluoride (HF) and (HF) difluorocarbene and difluorocarbene (:CF2); (:CFsee 2Equation); see Equation (8) (8)[24,31,32]. [24,31,32 ].Similar Similar to the to thereaction reaction of R22, of the R22, generated the generated difluorocarbene difluorocarbene dimerizes dimerizes immediately immediately to TFE to[33]. TFE The [33]. formation The formation of HFP—Equation of HFP—Equation (10)—takes (10)—takes place by placeconsecutive by consecutive reaction of reaction TFE with of another TFE with CF2 carbene. The overall reaction network can be described by a series of consecutive reactions listed another CF2 carbene. The overall reaction network can be described by a series of consecutive reactions in Table 6. The kinetic parameters for CHF3 decomposition were experimentally determined in this listed in Table6. The kinetic parameters for CHF 3 decomposition were experimentally determined in study. For the remaining consecutive reactions, literature references were used (Table 6). this study. For the remaining consecutive reactions, literature references were used (Table6). The kinetic parameters of the reaction of C2F4 (TFE) with a difluorocarbene radical (CF2) to C3F6 The kinetic parameters of the reaction of C2F4 (TFE) with a difluorocarbene radical (CF2) to C3F6 (HFP) were calculated by fitting k4(T) for each temperature and by the Arrhenius plot. Compared (HFP) were calculated by fitting k4(T) for each temperature and by the Arrhenius plot. Compared with with the study of Ainagos [34], EA is in the same range, but k0 is lower (values in Table 6). The reaction the study of Ainagos [34], E is in the same range, but k is lower (values in Table6). The reaction network was then finally simulatedA with the kinetic parameters0 listed in Table 6. The comparison of network was then finally simulated with the kinetic parameters listed in Table6. The comparison of experimental and simulated data is shown in Figure 9 for constant pyrolysis temperatures of 850 °C experimentaland 875 °C and and simulated a residence data time is shown up to in 3 Figure s. The9 for measured constant pyrolysisand simulated temperatures (dimensionless) of 850 ◦C and 875 C and a residence time up to 3 s. The measured and simulated (dimensionless) concentrations concentrations◦ of the educt CHF3 and the products TFE, HFP, and PFiB are in good agreement. of the educt CHF3 and the products TFE, HFP, and PFiB are in good agreement. Table 6. Kinetic parameters of the consecutive reaction network. Table 6. Kinetic parameters of the consecutive reaction network. kn 0 −1 −1 −1 A Reaction k 1[s or1 s M1 ] E [kJ/mol] Literature Reaction kn (T)(T) k0 [s− or s− M− ] EA [kJ/mol] Literature 8 8 CHF𝐶𝐻𝐹3 →HF 𝐻𝐹+:𝐶𝐹+ : CF2 k1 k1 4.16 4.1610 × 10 195195 thisthis study study → × 8 2 : 𝐶𝐹 → 𝐶𝐹 (𝑇𝐹𝐸) k2 8.71 ×8 10 0 Poltanskii [35] 2 : CF2 C2F4 (TFE) k2 8.71 10 0 Poltanskii [35] → × 16 𝐶𝐹 → 2 : 𝐶𝐹 k3 4.57 ×16 10 293 Poltanskii [35] C2F4 2 : CF2 k3 4.57 10 293 Poltanskii [35] → × this study (calc. 𝐶 𝐹 + : 𝐶𝐹 → 𝐶 𝐹 (𝐻𝐹𝑃) 4 8 k 4.97 ×8 10 122 this study C2F + : CF2 C3F6 (HFP) k 4.97 10 122 from data) 4 → 4 × (calc. from data) 13 𝐶𝐹 → 𝐶𝐹 + :𝐶𝐹 k5 1.91 × 10 333 Poutsma. [36] + 13 C3F6 C2F4 : CF2 k5 1.91 10 16 333 Poutsma. [36] 𝐶𝐹 + : 𝐶𝐹→ → 𝑖−𝐶𝐹 (𝑃𝐹𝑖𝐵) k6 1.20× × 10 385 Bauer [37] 16 C3F6+ : CF2 i C4F8 (PFiB) k6 1.20 10 385 Bauer [37] → − × ChemEngineering 2019, 3, 77 12 of 19 ChemEngineering 2018, 2, x FOR PEER REVIEW 12 of 20

1.0 1.0 Sim. Sim. Meas. /l)

/l) Meas. 0.8 CHF3 0.8 CHF3 CHF3,0

TFE CHF3,0 TFE HFP HFP 0.6 PFiB 0.6 PFiB (g/l)/(g (g/l)/(g / / / 0.4 0.4 0,CHF3 0,CHF3

0.2 0.2 c(t)/c c(t)/c

0.0 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 residence time / s residence time / s (a) (b)

Figure 9. Simulation of CHF3 pyrolysis: (a) 850 °C and (b) 875 °C. Figure 9. Simulation of CHF3 pyrolysis: (a) 850 ◦C and (b) 875 ◦C.

3.4.3.4. Life Life Cycle Cycle Assessment Assessment (LCA) (LCA) EnvironmentalEnvironmental protection protection andand resourceresource efficiency efficiency gain gain increasing increasing importance importance in inpolitics politics and and economicseconomics as as a a result result of of changing changing legislativelegislative frameworksframeworks or or a a shortage shortage of of ra raww materials. materials. In Inaddition, addition, thethe transition transition to to renewable renewable energy energy resources resources has has far far reaching reaching e effectsffects on on the theavailable available energyenergy mixmix forfor an industrialan industrial process. process. Under Under economic economic and and environmental environmental aspectsaspects like the the Paris Paris international international climate climate agreement,agreement, new new processes processes need need to to bebe moremore cost- an andd resource-efficient. resource-efficient. Thus, Thus, the the energetic energetic evaluation evaluation ofof production production processes processes has has become become an importantan importan partt part in process in process development development in the in last the two last decades. two Onedecades. possibility One possibility to evaluate to a evaluate process regardinga process rega requiredrding required energies, energies, their ecological their ecological damage damage potential, potential, and waste disposal is the life cycle assessment (LCA). Implementation of an LCA enables and waste disposal is the life cycle assessment (LCA). Implementation of an LCA enables the direct the direct comparison of different process routes and the identification of savings potential with comparison of different process routes and the identification of savings potential with regard to energy regard to energy and raw materials [38]. and raw materials [38]. In this work, a comparative LCA of the proposed new chlorine-free pyrolysis process and the In this work, a comparative LCA of the proposed new chlorine-free pyrolysis process and the incineration process of fluoroform, which is the actual state of the art for handling partly fluorinated incinerationcompounds, process was done. of fluoroform, In addition, which the current is the indu actualstrial state R22 of route the artto produce for handling fluoromonomers partly fluorinated was compounds,used for evaluating was done. the In savings addition, pote thential current of this industrialnew process R22 route. route to produce fluoromonomers was used forTo evaluating compare these the savings three processes, potential ofwe this started new processby analyzing route. the single process steps of the differentTo compare process these routes three and processes, set the system we started boun bydaries. analyzing Afterwards, the single the process calculation steps of of mass the di andfferent processmaterial routes flows and and set thethe system thermal boundaries. and electric Afterwards, process theenergies calculation took ofplace mass on and the material basis flowsof andthermodynamic the thermal and and electric reaction process requirements. energies For took a placebetter oncomparability the basis of thermodynamicbetween the industrial and reaction R22 requirements.route and our For lab ascale better process, comparability a scale up betweenfor the handling the industrial of 300 tCHF R223 route/a was and conducted. our lab scaleEvaluation process, a scalecriteria up were for the thehandling cumulative of 300energy tCHF demand3/a was (CED conducted.) and theEvaluation ReCiPe in different criteria wereimpact the categories, cumulative energylike climate demand change, (CED) human and thetoxicity, ReCiPe ozone in didepletion,fferent impact and resource categories, depletion. like climate change, human toxicity, ozone depletion, and resource depletion. 3.4.1. Functional Unit, System Boundaries, Material Flow, and Process Energy 3.4.1. Functional Unit, System Boundaries, Material Flow, and Process Energy Our work compares the incineration of partly fluorinated alkanes (s. Table 1) with the newly developedOur work process compares for recycling the incineration compounds of originating partly fluorinated from ECF alkanes (s. Table (s. 2) Table and 1the) with R22 theroute newly (s. developedTable 2). process for recycling compounds originating from ECF (s. Table2) and the R22 route (s. Table2).The first step contains the comparison of the incineration and pyrolysis of 1 kg of CHF3, which is defined as the functional unit (FU) in this work (Equation 28). The first step contains the comparison of the incineration and pyrolysis of 1 kg of CHF3, which is defined as the functional unit (FU) in thisFU work= 1 kg (Equation CHF3. (30)). (28) Afterwards, the amount of generated fluoromonomers (TFE and HFP), which is obtained by the FU = 1 kg CHF3. (30) pyrolysis of 1 kg CHF3 (CHF3 route), is compared in a second step with the similar amount of fluoromonomerAfterwards, thegenerated amount by ofthe generated industrial fluoromonomersR22 route. (TFE and HFP), which is obtained by In all cases, only the production of the fluoromonomers (TFE, HFP) was considered, in order to the pyrolysis of 1 kg CHF3 (CHF3 route), is compared in a second step with the similar amount of simplify the researched process chains. All subsequent products, for example, fluoropolymers, that fluoromonomer generated by the industrial R22 route. ChemEngineering 2019, 3, 77 13 of 19

ChemEngineeringIn all cases, 2018 only, 2, x FOR the productionPEER REVIEW of the fluoromonomers (TFE, HFP) was considered, in order 13 of to20 simplify the researched process chains. All subsequent products, for example, fluoropolymers, that are generatedare generated by polymerization by polymerization are neglected are neglected because be thecause energy the energy costs for costs their for production their production and disposal and aredisposal similar. are The similar. defined The system defined boundaries system boundaries of the LCA of forthe incinerationLCA for incineration and pyrolysis and pyrolysis of 1 kg of of CHF 1 kg3 areof CHF shown3 are in shown Figure in 10 Figure. The CHF 10. The3 route CHF in3 Figureroute in 10 Figure consists 10 ofconsists the supply of the of supply the CHF of 3thefrom CHF the3 from ECF bythe train, ECF by the train, pyrolysis the pyrolysis step atthe step optimum at the optimum process process parameters parameters (875 ◦C, (875 2.5 °C, s, CHF 2.5 s,3 CHFconversion3 conversion 73%, and73%, yield and yield fluoromonomers fluoromonomers 52%). 52%). This This is followed is followed by the by exhaustthe exhaust gas treatmentgas treatment by quenching by quenching the productthe product gas, neutralizationgas, neutralization of gaseous of gaseous HF, and HF, the followingand the following distillation distillation of the generated of the monomers.generated Themonomers. process ofThe incineration process of of incineration partly fluorinated of partly compounds fluorinated includes compounds the CHF includes3 supply, the the CHF incineration3 supply, step,the incineration and neutralization step, and of neut the generatedralization of HF. the generated HF.

Figure 10. System boundaries for (a) incineration of CHF3 and (b) chlorine-free synthesis Figure 10. System boundaries for (a) incineration of CHF3 and (b) chlorine-free synthesis of TFE and of TFE and HFP by pyrolysis of CHF3. ECF, electrochemical fluorination. HFP by pyrolysis of CHF3. ECF, electrochemical ﬂuorination.

Additionally, thethe lifelife cyclecycle assessmentassessment isis basedbased onon thethefollowing followingassumptions: assumptions:

• 3 NoNo energeticenergetic oror ecologicalecological allocationsallocations forfor CHFCHF3 (by-product(by-product ofof ECF);ECF); •• NoNo considerationconsideration of of energetic energetic and and infrastructure infrastructure processes processes for building for building the chemical the chemical plant, because plant, • ofbecause the theoretical of the theoretical scale up andscale its up similarity and its similarity to R22 pyrolysis; to R22 pyrolysis; • TheThe suppliedsupplied energyenergy consistsconsists ofof thethe usualusual energyenergy mixmix inin GermanyGermany (2014);(2014); •• The energy demand for the CHF3 route is subjected to a conservative estimation, whereby a The energy demand for the CHF3 route is subjected to a conservative estimation, whereby a higher • higher saving potential of the overall energy demand is possible. saving potential of the overall energy demand is possible. The generated data for energy and mass flows in the CHF3 route and CHF3 incineration are obtained in this study or estimated in a scale up of the CHF3 pyrolysis process. The data for the balancing of incineration of partly fluorinated compounds originate from an exhaust gas incineration plant. In contrast to the newly developed process to produce fluoromonomers, the standard process of TFE production, R22 pyrolysis, is already stored in the ecoInvent-database. Table 7 shows the heat and material flows for each of the compared processes.

ChemEngineering 2019, 3, 77 14 of 19

The generated data for energy and mass flows in the CHF3 route and CHF3 incineration are obtained in this study or estimated in a scale up of the CHF3 pyrolysis process. The data for the balancing of incineration of partly fluorinated compounds originate from an exhaust gas incineration plant. In contrast to the newly developed process to produce fluoromonomers, the standard process of TFE production, R22 pyrolysis, is already stored in the ecoInvent-database. Table7 shows the heat and material flows for each of the compared processes.

Table 7. Mass balances and process energies for the incineration and pyrolysis of 1 kg of CHF3.

CHF3 Route CHF3 Incineration ECF Transport Pyrolysis Neutralization Distillation HF ECF Freight train tkm/kg 0.9 natural gas kg/kg 0 2.60 Electricity kWh/kg 1.5 0.32 0.21 0.69 1.22

CHF3 kg/kg 1 0.3 1 1 3 CH4 m /kg 0.33 air kg/kg 0 50.65

H2O kg/kg 0 5.60

CO2 kg/kg 0 7.79 HF kg/kg 0.2 0.86 1 0.85 NaOH (18%) kg/kg 3.17 9.49 Salt sewage kg/kg 0.6 1.80 (NaF) Waste water kg/kg 3.4 43.2

N2 kg/kg 3.69 0 TFE/HFP kg/kg 0.54 0

CaF2 kg/kg 2

H2SO4 kg/kg 2.5

3.4.2. ReCiPe Impact Categories

The environmental influences of the evaluated processes, incineration, and pyrolysis of CHF3, are shown in Figure 11 for the selected impact category. Under environmental aspects, the incineration of the CHF3 is the best process for handling partly fluorinated ECF waste streams in all investigated impact categories. However, the incineration of CHF3 generates no fluoromonomers; a closer look must be taken at the two remaining processes for producing TFE and HFP. Comparison of the monomer production processes in Figure 12 clarifies that the standard R22 route causes the highest environmental effects in every impact category. The R22 route performs the worst in the impact categories of climate change, ozone, and fossil depletion, as well as human toxicity. In contrast to the actual industrial process, the CHF3 route gets significantly better environmental results in every single impact category. The saving potential in the categories of climate change and fossil depletion is about a factor of two, a factor of 10 for human toxicity, and a factor of 10,000 for ozone depletion. The advantages of the CHF3 pyrolysis in terms of ozone depletion and climate change result mainly from the high global warming potential (GWP) of CHF3 [17]. Additionally, the developed process avoids undesirable by-products, for example, HF contaminated HCl, which improves the environmental performance even more. ChemEngineering 2019, 3, 77 15 of 19 ChemEngineering 2018, 2, x FOR PEER REVIEW 15 of 20

FigureFigure 11. 11.Selected Selected categoriescategories withwith highest impact impact fact factorsors in in the the comparison comparison of ofincineration incineration and and ChemEngineering 2018, 2, x FOR PEER REVIEW 16 of 20 pyrolysispyrolysis of of CHF CHF3.3.

Comparison of the monomer production processes in Figure 12 clarifies that the standard R22 route causes the highest environmental effects in every impact category. The R22 route performs the worst in the impact categories of climate change, ozone, and fossil depletion, as well as human toxicity. In contrast to the actual industrial process, the CHF3 route gets significantly better environmental results in every single impact category. The saving potential in the categories of climate change and fossil depletion is about a factor of two, a factor of 10 for human toxicity, and a factor of 10,000 for ozone depletion. The advantages of the CHF3 pyrolysis in terms of ozone depletion and climate change result mainly from the high global warming potential (GWP) of CHF3 [17]. Additionally, the developed process avoids undesirable by-products, for example, HF contaminated HCl, which improves the environmental performance even more.

FigureFigure 12. 12.Selected Selected categories categories with with the highestthe highest impact impact factors factors in the comparisonin the comparison of monomer of monomer production

processesproduction R22 processes and CHF R223 pyrolysis. and CHF3 pyrolysis.

3.4.3. Cumulative Energy Demand (CED)

The calculated CEDs of the compared processes are illustrated in Figure 13. Incineration of CHF3 has an energy demand of 28.3 MJ, while pyrolysis of CHF3 and the industrial R22 route consumes 45.1 MJ and 79.5 MJ, respectively. Under an energetic aspect, incineration of CHF3 is the best process to handle ECF waste stream, but at the end of the incineration, only CO2, high amounts of waste acids (HF), and no valuable product are generated. Compared with incineration, the CED of the CHF3 route in Figure 13 (a) show that by increasing the amount of energy by about 40%, valuable monomers like TFE and HFP can be obtained out of a currently unused residue, which actually generates costs for its incineration and disposal.

100 100 79.5 80 80 MJ 60 MJ 60 45.1 45.1 40 40 CED / 28.3 CED / 20 20

0 0 CHF3 CHF3 CHF3 R222 Incineration Route Route Route (a) (b)

Figure 13. (a) Cumulative energy demand (CED) of the compared processes and (b) comparison of new developed chlorine-free against standard monomer production. ChemEngineering 2018, 2, x FOR PEER REVIEW 16 of 20

Figure 12. Selected categories with the highest impact factors in the comparison of monomer ChemEngineering 2019, 3, 77 16 of 19 production processes R22 and CHF3 pyrolysis.

3.4.3.3.4.3. Cumulative Cumulative Energy Energy Demand Demand (CED) (CED)

TheThe calculated calculated CEDs CEDs of of the the compared compared processes processes are are illustrated illustrated in Figure Figure 13.13. Incineration Incineration of of CHF CHF33 hashas an an energy energy demand demand of of 28.3 28.3 MJ, MJ, while while pyrolysis pyrolysis of of CHF CHF33 andand the the industrial industrial R22 R22 route route consumes consumes 45.145.1 MJ MJ and and 79.5 79.5 MJ, MJ, respectively. respectively. Under Under an an energetic energetic aspect, aspect, incineration incineration of of CHF CHF33 isis the the best best process process toto handle handle ECF ECF waste waste stream, stream, but but at at the the end end of of the the incineration, incineration, only only CO CO22, ,high high amounts amounts of of waste waste acids acids (HF),(HF), and and no no valuable valuable product product are are generated. generated. Co Comparedmpared with with incineration, incineration, the CED CED of of the the CHF CHF33 routeroute inin Figure Figure 13 13 (a)a show show that that by by increasing increasingthe the amountamount ofof energyenergy byby about 40%, valuable monomers monomers like like TFETFE and and HFP can be obtainedobtained outout ofof aa currentlycurrently unused unused residue, residue, which which actually actually generates generates costs costs for for its itsincineration incineration and and disposal. disposal.

100 100 79.5 80 80 MJ 60 MJ 60 45.1 45.1 40 40 CED / 28.3 CED / 20 20

0 0 CHF3 CHF3 CHF3 R222 Incineration Route Route Route (a) (b)

FigureFigure 13. 13. ((aa)) Cumulative Cumulative energy energy demand demand (CED) (CED) of of the the compared compared processes processes and and ( (bb)) comparison comparison of of newnew developed developed chlorine-free chlorine-free agains againstt standard standard monomer monomer production. production.

High savings potential of the developed pyrolysis process is evidenced by comparison with the industrial R22 route in Figure 13b. Originating from the residual CHF3 from ECF, 40% energy savings are obtained to produce the same amount of fluoromonomers. The energy demand for the single pyrolysis step of CHF3 is higher than that in R22 pyrolysis because of the higher reaction temperature, but the previous process steps of chlorination from methane to R22 and the required auxiliaries like Cl2, Sb-catalysts, and HF can be replaced completely. Besides the material utilization of partly fluorinated residues, a chlorine-free synthesis of TFE and HFP requires the targeted production of CHF3 via the ECF and the following pyrolysis of the generated CHF3. The calculated energy demand of this combined process compared to the industrial R22 route is shown in Figure 14. The synthesis of CHF3 and the following pyrolysis of a mixture of CHF3 (10%) and nitrogen (90%) has a nearly equal CED compared with the R22 route. Considering the assumption proved earlier of a first order reaction and the consequence of an independence of concentration in Figure5a, the concentration of CHF 3 in the educt stream is able to increase to 50% (Figure5a) and an energy saving of more than 10% is achieved. Additionally, chlorine chemistry is fully substituted. Furthermore, the entire energy consumption of the process can be reduced by implementing gas burners instead of electrical heating of the reactors, with the disadvantage of a higher GWP, and additionally increasing the amount of fluorocarbon against nitrogen. Nevertheless, the advantage of the presented process, the reduction of four process steps in the R22 route to a one-step synthesis using partly fluorinated residues or a maximum two-step process with a target production of CHF3 and a following pyrolysis, is obvious. ChemEngineering 2018, 2, x FOR PEER REVIEW 17 of 20

High savings potential of the developed pyrolysis process is evidenced by comparison with the industrial R22 route in Figure 13 (b). Originating from the residual CHF3 from ECF, 40% energy savings are obtained to produce the same amount of fluoromonomers. The energy demand for the single pyrolysis step of CHF3 is higher than that in R22 pyrolysis because of the higher reaction temperature, but the previous process steps of chlorination from methane to R22 and the required auxiliaries like Cl2, Sb-catalysts, and HF can be replaced completely. Besides the material utilization of partly fluorinated residues, a chlorine-free synthesis of TFE and HFP requires the targeted production of CHF3 via the ECF and the following pyrolysis of the generated CHF3. The calculated energy demand of this combined process compared to the industrial R22 route is shown in Figure 14. The synthesis of CHF3 and the following pyrolysis of a mixture of CHF3 (10%) and nitrogen (90%) has a nearly equal CED compared with the R22 route. Considering the assumption proved earlier of a first order reaction and the consequence of an independence of concentration in Figure 5 (a), the concentration of CHF3 in the educt stream is able to increase to 50% ChemEngineering 2019, 3, 77 17 of 19 (Figure 5(a)) and an energy saving of more than 10% is achieved.

100

79.5 81.5 80 71.3

MJ 60 Pyrolysis 40 Pyrolysis CED / CED

20 ECF ECF

0 CHF R222 CHF3 3 Route Route Route (10%) (50%) FigureFigure 14. ComparisonComparison of of the the cumulative cumulative energy energy demand demand of new developed chlorine-free against standardstandard monomer production.

4. Discussion Additionally, chlorine chemistry is fully substituted. Furthermore, the entire energy consumptionPartly fluorinated of the process compounds can be reduced (such as by CHF implem3) originateenting from gas burners electrochemical instead of fluorination. electrical heating In this ofstudy, the reactors, the chlorine-free with the synthesis disadvantage of tetrafluoroethylene of a higher GWP, and and hexafluoropropylene additionally increasing by pyrolysis the amount of CHF of3 fluorocarbonwas studied. Theagainst pyrolysis nitrogen. was Nevertheless, conducted in athe microreactor advantage atof atmospheric the presented pressure process, in athe temperature reduction ofrange four of process 775 to steps 875 ◦ C.in Conversionthe R22 route of to CHF a one-step3 turned synthesis out to be using a first partly order fluorinated reaction. The residues initial or step a maximumof the gas phasetwo-step pyrolysis process of with CHF a 3targetis formation production of HF of andCHF CF3 and2 (difluorocarbene), a following pyrolysis, which is dimerizesobvious. immediately to C2F4. With prolonged residence time, C3F6 is obtained by the reaction of TFE and CF2. 4.The Discussion activation energy (EA) and the pre-exponential factor (k0) of the decomposition of CHF3 to HF and

CF2 werePartly determined. fluorinated compounds (such as CHF3) originate from electrochemical fluorination. In this study,The the formation chlorine-free of C 2synthesisF4,C3F6, andof tetrafluoroethylene i-C4F8 can be described and byhexafluoropropylene a reaction network consistingby pyrolysis of sixof CHFreactions.3 was Thestudied. simulation The pyrolysis is in good was agreement conducted with in the a experimentalmicroreactor at data. atmospheric The conversion pressure of CHF in a3 temperaturereaches 73% atrange a temperature of 775 to 875 of 875°C. ◦ConversionC, and a yield of CHF of about3 turned 52% out fluoromonomers to be a first order is obtained. reaction. Short The initialresidence step times of the and gas/or phase high pyrolysis temperatures, of CHF isothermal3 is formation conditions, of HF and quenchingCF2 (difluorocarbene), of the product which gas dimerizesto avoid consecutive immediately reactions to C2F4. maximizesWith prolonged the yield residence of fluoromonomers. time, C3F6 is obtained These conditions by the reaction could beof TFErealized and inCF the2. The ceramic activation microreactor energy used(EA) and in this the study.pre-exponential factor (k0) of the decomposition of CHF3The to HF life and cycle CF assessment2 were determined. for the developed CHF3 route demonstrates the ecological and economic benefitThe of formation this process. of C It2F is4, shown C3F6, and that i-C the4F material8 can be utilizationdescribed by of CHFa reaction3 is preferable network to consisting incineration. of six In reactions.contrast to The the simulation generation ofis in high good amounts agreement of waste with acids the atexperimental CHF3 incineration, data. The the conversion material utilization of CHF3 reachesof CHF3 73%requires at a temperature a higher energy of 875 expenditure °C, and a (40%), yield of but about generates 52% fluoromonomers high yields of the is valuable obtained. products Short residenceTFE and HFP. times In and/or comparison high temperatures, with the industrial isotherm R22 route,al conditions, the developed and quenching CHF3 pyrolysis of the requiresproduct 40%gas less energy for the production of the same amount of fluoromonomers. Additionally, a reduction in terms of climate change and ozone depletion of 50% is obtained. The developed process provides a sustainable synthesis of fluoromonomers with the material utilization of partly fluorinated compounds instead of their incineration. In this study, it is shown that the CHF3 pyrolysis process is stable with a time on stream of about 8 h, shows no significant corrosion on the developed microreactor, and is thus ready for an industrial application. A scale up of the process is possible by employing multiple micro reactors in parallel or by a conventional scale up inspired by the R22 route with a tube bundle reactor, which is adjusted to the higher temperatures. Beneficial for the scale up is the proven first order reaction, which simplifies the reaction engineering. With regard to the tremendous saving potential in energy consumption, undesirable by-products, disposing waste materials, and the complete substitution of chlorine chemistry, the developed process is suitable to be transferred into the industrial scale. Especially, the developed reactor enables the pyrolysis of partly and perfluorinated gases and, therefore, becomes a big step in a complete closing of the fluorine cycle. ChemEngineering 2019, 3, 77 18 of 19

Author Contributions: K.M. designed and performed the study and wrote the paper. A.S. helped in the design of the reactor sealing and in analyzing experimental as well as LCA data. T.G. was responsible for funding acquisition and A.J. supported the kinetic measurement and data evaluation. K.H. contributed the used gases and crosschecked the gas samples in the laboratories of 3 M. Funding: Deutsche Bundesstiftung Umwelt (DBU) is gratefully acknowledged for financial support (reference number: 31819/01-31). Acknowledgments: This publication was funded by the German Research Foundation (DFG) and the University of Bayreuth in the funding programme Open Access Publishing. Conflicts of Interest: The authors declare no conflict of interest.

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