Pyrolysis of Trifluoromethane to Produce Hexafluoropropylene

Figure 1 Schematic diagram of the mini-pilot-plant-scale system for the pyrolysis ... off-line gas chromatograph (HP-5890 Series II, Hewlett-Packard I...
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Ind. Eng. Chem. Res. 2002, 41, 2895-2902

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Pyrolysis of Trifluoromethane to Produce Hexafluoropropylene Dong Ju Moon,* Moon Jo Chung, Honggon Kim, Young Soo Kwon, and Byoung Sung Ahn CFC Alternatives Research Center, Korea Institute of Science and Technology, Seoul 130-650, Korea

Pyrolysis of a mixture of trifluoromethane (CHF3, R23) and tetrafluoroethylene (CF2dCF2, TFE) to produce hexafluoropropylene (CF3CFdCF2, HFP) was investigated by kinetic modeling and pyrolysis experiments. Product distributions from the pyrolysis of R23/TFE mixtures were estimated by a computer simulation using kinetic rate constants of individual reaction steps proposed in the literature and were compared with those obtained from pyrolysis experiments in a bench-scale tubular reactor. In addition, pyrolysis of the R23/TFE mixture in a mini-pilotplant-scale reactor was investigated for the purpose of industrial application under the conditions of reaction temperatures of 700-1000 °C, contact times of 0.01-14 s, and R23/TFE molar ratios of 0.1-5. HFP was mainly produced, along with a small amount of byproducts such as perfluoroisobutylene [PFiB, (CF3)2CdCF2 ], CF3CdCCF3, C2F3H, CF3CHCF2, and CF3CF2CFd CF2. By mixing R23 and TFE in the feed or by recycling the unreacted R23 and TFE to the feed of fresh R23, the reaction temperature could be controlled by carefully utilizing the heat balance between the endothermic pyrolysis of R23 and the exothermic dimerization of TFE. It was found that the simultaneous pyrolysis of a mixture of R23 and TFE with an appropriate molar ratio gave a higher yield of HFP and a lower selectivity of PFiB, a very harmful byproduct, than the pyrolysis of R23 alone. In addition, the pyrolysis of the mixture of R23 and TFE produced less carbon powder and also required less energy to be externally supplied than the pyrolysis of R23. Introduction Hexafluoropropylene (C3F6, HFP) is an important monomeric material for the preparation of copolymers of fluorinated resins. Recently, the demand for HFP, as well as that for tetrafluoroethylene (C2F4, TFE), has been increasing. Both TFE and HFP can be prepared by two-step reactions;1 first, chloroform (CHCl3) is hydrofluorinated to chlorodifluoromethane (CHClF2, R22) over an antimony chlorofluoride catalyst in a liquid-phase reaction, and then, the R22 is pyrolyzed to TFE, HFP, and byproducts at high temperature. However, the yield of HFP is normally far lower than that of TFE. For example, when R22 was pyrolyzed at a reaction temperature of 687 °C and a contact time of 1.8 s in a silver tubular reactor, the conversion of R22 was 38%, and the selectivities of TFE and HFP were 93 and 1%, respectively.1 Even though various reaction conditions for R22 pyrolysis were tested, the selectivity of HFP was still low because it required more steps for the association of C- and CdC than did TFE formation.2-4 Furthermore, it is very difficult to separate pure HFP from an azeotropic mixture of R22 and HFP. Other methods such as the pyrolysis of TFE and octafluorocyclobutane (C4F8, RC318),5 the pyrolysis of poly(tetrafluoroethylene) (PTFE),6 and the pyrolysis of R237-10 have also been proposed for the preparation of HFP, but each of these approaches has certain deficiencies to be overcome. The pyrolysis of TFE mainly produces RC318 with a small amount of HFP. The pyrolysis of PTFE requires a complicated process wherein TFE is first prepared by the pyrolysis of R22, the purified TFE is polymerized to PTFE, and then the PTFE is pyrolyzed to HFP. Even though the selectivity to HFP from the * To whom correspondence should be addressed. Phone: 8202-958-5867. Fax: 82-02-958-5809. E-mail: [email protected]

PTFE pyrolysis is higher than that from the pyrolysis of R22 or TFE, it is an undesirable method because it is expensive. Hauptschein and Fainberg7 reported that HFP can be prepared by the pyrolysis of R23 in a temperature range of 700-1500 °C. They performed the pyrolysis reaction in the said broad temperature range because of the difficulty in controlling the reaction temperature. In addition, when the pyrolysis of R23 was carried out at a temperature above 1000 °C, a tremendous amount of carbon was formed with little HFP because of the uncontrollable high reaction temperature induced locally by the highly exothermic dimerization of TFE, an intermediate. Nevertheless, the overall reaction required a continuous external supply of heat. Politanskii et al.11 reported that R23 decompose into carbene (:CF2) and HF. Edward et al.12 suggested that the dimerization of the :CF2 produced in the pyrolysis of R23 proceeds to form TFE. Butler et al.13 reported that TFE is dimerized to form RC318. Artkinson et al.14 suggested that the reaction of TFE with :CF2 proceeds to produce cyclo-C3F6, and Politanskii et al.15 reported that cyclo-C3F6 is further converted to C3F6, HFP. However, no adequate description of the full mechanism of formation of HFP from R23 has been reported in the literature. In this work, we suggest a plausible reaction mechanism for the formation of HFP from the pyrolysis of R23 by comparing the results of kinetic modeling using the kinetic data for individual reaction steps reported in the literature and the experimental results of R23 pyrolysis. The optimum reaction conditions for the pyrolysis of R23/TFE mixtures were also investigated using computer simulations for the cases of both adiabatic and isothermal reactions. Pyrolysis experiments were carried out in both a bench-scale reactor and a mini-pilot-plant-scale reactor.

10.1021/ie010065q CCC: $22.00 © 2002 American Chemical Society Published on Web 05/17/2002

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Figure 1. Schematic diagram of the mini-pilot-plant-scale system for the pyrolysis of mixtures of R23 and TFE.

Experimental Section Pyrolysis System. The pyrolysis of R23 was carried out in a tubular reactor with an outer diameter of 3/8 in. The bench-scale reaction system consisted of four sections: the feed supplying section, the reactor, the quenching column, and the GC analysis section. The pyrolysis of R23/TFE mixtures was also carried out in the mini-pilot scale reactor system depicted in Figure 1. It consisted of five sections: the feed supplying section, the reaction section, the separation and recycling section, the purifying section, and the GC analysis section. R22 and R23 were supplied by Ulsan Chemical Co. TFE was synthesized from the pyrolysis of R22 and purified by two-step distillation. The purities of R23 and TFE were 99.5 and 99.9%, respectively. The gases were delivered by mass flow controllers, and process water was fed by high-pressure liquid delivery pumps. The preheater and the pyrolysis reactor were made of Inconel 600 tubes with an outer diameter of 3/4 in. The pyrolysis temperature was monitored by four thermocouples inside the reactor, which were placed along the direction of the gas flow, and heating was controlled by a PID controller that measured the temperature in the center of the reactor. The product gas was quenched by chilled water at the exit of the reactor to prevent the formation of solid polymers at a high temperature. The hydrogen fluoride (HF) produced from the reaction was washed out in the water when the product gas passed through the quenching column. The trace amount of HF remaining in the acid-removed gas was neutralized with alkali solution, and the moisture was removed in dryers charged with molecular sieve 5A. The compressed products were delivered to the first distillation column, where the unreacted R23 and TFE were separated from

the acid-removed products and recycled to the reactor after being stored in the R23/TFE mixture reservoir. High-boiling-point compounds containing HFP were delivered to the second distillation column where highpurity HFP (over 99.9%) could be collected from the middle of the column. The mixture of unreacted R23 and TFE was recycled and mixed with fresh R23, a makeup feed, to keep an appropriate molar ratio of R23 to TFE in the feed. Pyrolysis Reaction. The pyrolysis system was purged with N2 gas to remove oxygen before the pyrolysis experiment. All reactions were conducted at temperatures of 750-1200 °C, residence times of 0.1-5 s, and R23/TFE feed molar ratios of 0.1-10 under atmospheric pressure. The effluent gas was analyzed by an on-line or off-line gas chromatograph (HP-5890 Series II, Hewlett-Packard Inc.) equipped with a TCD and a Poraplot Q capillary column (0.000 32-m o.d. and 25-m lengh). The GC was isothermally operated under the conditions of a detector temperature of 200 °C, an injection temperature of 150 °C, a column temperature of 90 °C, and a helium head pressure of 5 psig. Each component in the product stream was identified by GC/ MS (HP-5890/5971, Hewllet Packard Inc.) using the same column. Kinetic Modeling. A plausible reaction mechanism for HFP formation from the pyrolysis of R23 was suggested. Using the kinetic constants of individual reaction steps reported in the literature, the formation of HFP was calculated and considered for various pyrolysis conditions using computer simulations. The conversion of R23 and the concentrations of products were calculated by using the Runge-Kutta method.16,17

Ind. Eng. Chem. Res., Vol. 41, No. 12, 2002 2897 Table 1. Individual Reaction Constants in Kinetic Models rate equation

rate constant

ref

2 3 4 5 6 7 8 9

k2 ) 1013.44e-69 000 cal/RT, [L/s] k3 ) 109.94e0 cal/RT, [L/(mol s)] k4 ) 1016.66e-70 400 cal/RT, [L/s] k5 ) 107.40e-24 000 cal/RT, [L/(mol s)] k6 ) 1016.00e-74 300 cal/RT, [L/s] k7 ) 106.06e-8500 cal/RT, [L/(mol s)] k8 ) 1013.25e-38 600 cal/RT, [L/s] k9 ) 1015.04e-50 400 cal/RT, [L/s]

10 11 11 12 12 13 13 14

Reaction Mechanism On the basis of the kinetic data of the individual reaction related to the formation of HFP reported by various researchers,11-15 it was assumed that HFP could be produced from R23 through the following consecutive reaction

CHF3 f CF2dCF2 f C4F8 f CF3CFdCF2

(1)

According to the kinetic data reported, a reaction mechanism for the pyrolysis of CHF3 to CF2CFCF3 was possibly expressed by the individual reactions in eqs 2-9. k2

CHF3 98 [:CF2] + HF

(2)

k3

2[:CF2] 98 C2F4

(3)

k4

2[:CF2] 79 C2F4

(4)

k5

2C2F4 98 C4F8 k6

k7

C2F4 + [:CF2] 98 c-C3F6 k8

C2F4 + [:CF2] 79 c-C3F6 k9

[:CF2] )

[

(

)

]

x(R2 + 8k3β) - R 4k3

(10)

(11)

where

R)

(

)

k7k9[C2F4] k8 + k9

(14)

d[C4F8] ) (k5[C2F4]2 - k6[C4F8]) dt

(15)

Results and Discussion

(9)

k7[:CF2][C2F4] k8 + k9

d[C3F6] ) k9[c-C3F6] ) R[CF2] dt

(7)

The reaction constant for each reaction (i) is represented by ki. Table 1 displays the reaction constants for individual reactions. Using the steady-state approximation for the intermediates, the concentrations of these species can be represented by

[c-C3F6] )

d[C2F4] ) (1/2k2[R23] - 3/2r[HFP] - 2r[C4F8]) (13) dt

(6)

(8)

c-C3F6 98 C3F6

The production rates for TFE, HFP, and RC318 were determined from the carbon balance and could be represented by

The conversions and concentrations of the products from the pyrolysis of R23 were simulated with these kinetic models using the Runge-Kutta method under adiabatic and isothermal reaction conditions with various reaction temperatures and retention times.

(5)

2C2F4 79 C4F8

Figure 2. Effect of the addition of steam on the pyrolysis of R23 performed in the small reactor (steam/R23 molar ratio ) 4, reaction temperature ) 900 °C, residence time ) 0.36 s, reaction pressure ) 1 atm).

and β ) (k2[R23] + 2k4[C2F4]) (12)

Pyrolysis Reaction of R23. First, in the preparation of HFP from the pyrolysis of R23, the effects of the addition of steam, the pyrolysis temperature, and the residence time in the reactor were experimentally investigated in a small tubular reactor. Figure 2 shows the effect of adding steam on the pyrolysis of R23. The pyrolysis of a mixture with molar ratio of steam/R23 ) 4 was performed at a temperature of 900 °C, a residence time of 0.36 s, and a reaction pressure of 1 atm. Unlike the pyrolysis of R23 alone, most of the R23 decomposed into CO2, CO, H2, and CF4 when steam was supplied. Even though the conversion of R23 was very high, the selectivity of HFP was very low. Generally, superheated steam is known to play an important role as a diluent gas in the pyrolysis of R22 (CF2HCl) to produce TFE (CF2dCF2). However, this experimental result shows that the steam might act as an additive, enhancing the decomposition and oxidation of R23 possibly to an active oxygen species such as COF2, which could then be converted to CO2 and CO in an alkali scrubber. It was clearly found that feeding superheated steam or water vapor is undesirable in the case of R23 pyrolysis. Figure 3 shows the effect of reaction temperature on the product distribution in the pyrolysis of R23 for a residence time of 0.36 s and a reaction pressure of 1

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Figure 3. Effect of reaction temperature on the product distribution in the pyrolysis of R23 performed in the small reactor (residence time ) 0.73 s, reaction pressure ) 1 atm).

Figure 4. Effect of reaction temperature on the product distribution in the pyrolysis of R23 performed in the small reactor (residence time ) 0.73 s, reaction pressure ) 1 atm).

atm. The conversion of R23 increased with increasing reaction temperature up to 1000 °C. The concentration of TFE increased to a maximum with increasing reaction temperature and then fell. However, the concentrations of HFP; perfluoroisobutylene [(CF3)2CdCF2, PFiB]; and other products such as CF3CCCF3, C2F3H, CF3CHCF2, and CF3CF2CFCF2 increased with increasing temperature. Figure 4 shows another effect of the reaction temperature on the pyrolysis of R23 performed at a residence time (0.73 s) longer than that used for Figure 3. With a longer residence time, we can clearly see that the conversion of R23 and the production of HFP, PFiB, and the other byproducts increased but the production of TFE decreased with increasing temperature. At temperatures below 800 °C, the conversion of R23 to HFP was very low, and Teflon powder was produced. In contrast, at a temperature of around 900 °C, the exothermic dimerization of TFE was activated, and a trace of carbon was formed. Because of the exothermic generation of a huge amount of local heat above 900 °C, it was difficult to control the reaction temperature, and as a result, large amounts of carbon and byproducts were formed. Therefore, the selectivity

Figure 5. Effect of residence time on the product distribution in the pyrolysis of R23 performed in the small reactor (reaction temperature ) 900 °C, reaction pressure ) 1 atm).

of HFP became low even though the conversion of R23 increased with the increase in temperature. Figure 5 shows the effect of residence time on the concentrations of products from the pyrolysis of R23. The reaction was carried out in a small reactor that was isothermally controlled at 900 °C. As the residence time increased, both the conversion of R23 and the production of HFP, PFiB, and the other byproducts increased. However, the concentration of TFE, an intermediate, showed a maximum at 0.36 s and then decreased with increasing residence time. It was experimentally shown that controlling both the reaction temperature and the residence time was important in obtaining a high selectivity of HFP and in minimizing the formation of side products such as PFiB, Teflon powder, or solid carbon in the pyrolysis of R23. Among the byproducts, PFiB is the most harmful toxic gas,18 which is usually formed as a byproduct in the production of TFE or the thermal degradation of poly(tetrafluoroethylene) (PTFE) at about 425 °C. Therefore, in the development of a HFP process the reaction mechanism and the optimum reaction conditions for the formation of HFP and PFiB should be considered and predicted to obtain a high yield of HFP and to minimize the formation of PFiB. Computer Simulation for Kinetic Modeling of R23 Pyrolysis. Computer simulations for the kinetic modeling of R23 pyrolysis were carried out for isothermal and adiabatic reaction conditions using the reaction mechanism and the reaction constants for the individual reaction steps reported in the literature.10-15 Figure 6 compares the computer simulation results with the experimental results for the effect of residence time on the isothermal pyrolysis of R23 in the absence of steam at 900 °C. Even though the experimental product distribution did not exactly match the calculated value for each component, the trends showed the general characteristics of concentration-time curves for a consecutive reaction. In the calculations, a trend was predicted that R23 would decrease exponentially, TFE would rise to a maximum (17.4%) and then slowly decrease, and HFP would continuously increase with increasing residence time. RC318 was produced only in a trace amount. The highest production rate of HFP

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Figure 6. Computer simulation results for the effect of residence time on the isothermal pyrolysis of R23 (reaction temperature ) 900 °C).

Figure 7. Computer simulation results for the effect of residence time on the isothermal pyrolysis reaction of R23 with steam (reaction temperature ) 900 °C, steam/R23 molar ratio ) 4).

occurred when the concentration of TFE reached at a maximum. From the computer simulations with the kinetic model, it was predicted that the pyrolysis of R23 would proceed quickly and that all reactions would finish within a few seconds at temperatures above 900 °C. Figure 7 shows the computer simulation results for the effect of residence time on the isothermal pyrolysis of R23 at 900 °C in the presence of steam; the molar ratio of steam to R23 was 4. From a comparison with Figure 6, it was found that the conversion of R23 decreased slowly and almost linearly rather exponentially with increasing residence time. In addition, the product ratio of HFP to TFE was far lower than that in Figure 6 for any residence time. RC318 was produced in a trace amount regardless of steam supply. Although it was difficult to predict the formation of CO2, CO, H2, and CF4 in the presence of steam as shown in Figure 2, the computer simulation indicated that the supply of steam or water vapor in the pyrolysis of R23 is undesir-

Figure 8. Computer simulation results for the effect of residence time on the adiabatic pyrolysis of R23 (inlet temperature ) 900 °C).

able because both the conversion of R23 and the selectivity of HFP decrease in the presence of steam. The adiabatic pyrolysis of R23 was simulated in the same manner. Figure 8 shows the computer simulation results for the effect of residence time on the adiabatic pyrolysis of R23 for an inlet temperature of 900 °C in the absence of steam. As the residence time increased, the conversion of R23 slowly increased and TFE was predominantly produced. At a residence time of 0.4 s, the conversion of R23 was less than 7%, and the selectivities of TFE and HFP were 6 and 0.8%, respectively. However, the conversion of R23 and the selectivities of TFE and HFP did not change thereafter, even though the residence time increased greatly. The low change in conversion with changing residence time was interpreted as resulting from the decrease of the reaction temperature that accompanied the endothermic pyrolysis of R23. That is, the pyrolysis temperature decreased along the flow direction in the reactor under adiabatic conditions. From a comparison of Figure 8 with Figure 6 for the case of isothermal pyrolysis at 900 °C, it can be seen that the conversion of R23 and the selectivities of TFE and HFP significantly depend on the reaction temperature. A simulation of the adiabatic pyrolysis of R23 with steam supply was also conducted in the same manner. For adiabatic pyrolysis with a steam/R23 molar ratio of 4 at an inlet temperature of 900 °C, the conversion of R23 and the selectivities of TFE, HFP, and RC318 showed behaviors similar to those in Figure 8. At a residence time of 0.4 s, the selectivities of TFE and HFP were 6.8 and 0.3%, respectively, and they did not change much with increasing residence time because the pyrolysis temperature decreased as the pyrolysis of R23 proceeded for longer residence times under adiabatic conditions. In most of the pyrolysis reactions conducted, more PFiB was formed with increasing reaction temperature or increasing residence time. On the basis of the individual reactions reported (eqs 2-9) and the experimental results obtained, it was possible to consider that PFiB might be formed through the following reaction mechanism k16

CF3CF)CF2 + [:CF2] 98 (CF3)2CF)CF2

(16)

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CF2)CF2 + CF3CF)CF2 98 (CF3)2CF)CF2 + [:CF2] (17) k18

2CF3CF)CF2 98 (CF3)2CF)CF2 + CF2)CF2 (18) It was assumed that PFiB might be predominantly formed by the reaction mechanism involving HFP and carbene (:CF2) (eq 16) because molecule-molecule reactions (eqs 17 and 18) are generally known to occur more slowly than molecule-radical reactions (eq 16). In addition, it was experimentally found that the concentration of PFiB was high when that of HFP was low. Therefore, the following modified consecutive reaction is plausible in interpreting the reaction mechanism for the formation of PFiB from R23.

CHF3 f CF2dCF2 f C4F8 f CF3CFdCF2 f (CF3)2CdCF2 (19) Optimization of Pyrolysis Reaction Conditions. The pyrolysis reaction of R23 is an endothermic reaction, which requires continuous supply of heat from outside to keep the reaction temperature constant. On the other hand, the dimerization of TFE is an exothermic reaction that requires the continuous removal of heat. Therefore, when the pyrolysis of R23 is performed at a temperature above 1000 °C, TFE, the major intermediate, undergoes dimerization to a significant extent, thus generating a large amount of heat. As a result, the reaction temperature rapidly increases and becomes very difficult to control. If a heat balance between the endothermic pyrolysis of R23 and the exothermic dimerization of TFE can be used, then the reaction temperature can be easily controlled so as to minimize carbon formation. From this point of view, a computer simulation for the pyrolysis of an admixture of R23 and TFE at an appropriate molar ratio was conducted. Figure 9 shows the computer simulation results for the temperature versus residence time in the adiabatic pyrolysis of mixtures with various molar ratios of R23 and TFE. Without TFE, the temperature of R23 pyrolysis slowly decreased as the residence time increased. Without R23, the temperature of TFE pyrolysis rapidly increased as the residence time increased. However, it was found that the pyrolysis temperature

Figure 9. Computer simulation results for the effect of the R23/ TFE molar ratio on the pyrolysis temperature.

Figure 10. Computer simulation results for the adiabatic pyrolysis reaction of mixtures of R23 and TFE (R23/TFE feed molar ratio ) 1, reaction temperature ) 1000 °C).

Figure 11. Computer simulation results for the isothermal pyrolysis reaction of mixtures of R23 and TFE (R23/TFE feed molar ratio ) 1, reaction temperature ) 1000 °C).

could be controlled in a certain stable range when the amount of TFE in the admixture of R23 and TFE was maintained within the range of 50-80 mol %. In pyrolysis experiments within this composition range, coke formation was remarkably reduced, and high yields of HFP were obtained. Figure 10 shows the computer simulation results for the effect of residence time on the adiabatic pyrolysis of a mixture of 50 mol % R23 and 50 mol % TFE at an initial reaction temperature of 1000 °C. For residence times longer than 0.3 s, the reaction temperature was maintained constant at 1026 °C. Most of the reactions seemed to be completed within 0.2 s, and the conversion of R23 and the selectivity of HFP could both be expected to be as high as 99%. Meanwhile, Figure 11 shows the computer simulation results for the isothermal pyrolysis reaction of the R23/TFE mixture with an R23/TFE molar ratio of 1 at 1000 °C. As for the results in the adiabatic pyrolysis reaction, most reactions seemed to finish within 0.2 s, and both the conversion of R23 and the selectivity of HFP were estimated to be as high as 99%. From a comparison of Figures 10 and 11, it was found that the product distributions showed similar

Ind. Eng. Chem. Res., Vol. 41, No. 12, 2002 2901 Table 2. Results of the Pyrolysisa of R23 and Admixtures of R23 and TFE in the Mini-Pilot-Plant-Scale Pyrolysis System expt no. 1 2 3 4 5

feed concentration (%) R23

TFE

100 80.5 61.0 49.5 36.6

0 19.5 39.0 50.5 64.4

product distribution (%) reaction temp (°C) R23 TFE HFP PFiB othersb 880 875 872 878 873

46.6 39.5 33.7 28 24.4

26.1 31.4 34.7 42.9 49.3

19.3 25.5 28.5 25.8 23.7

4.5 1.1 1.0 1.1 0.8

3.5 2.5 2.1 2.2 1.8

a Pyrolysis conditions: R23/TFE supply rate ) 10.13 L/min, residence time ) 2 s. b Others: CF4, CF3CF3, CF3CCCF3, C2F3H, CF3CHCF2, and CF3CF2CFCF2

Figure 12. Effect of recycling the unreacted R23 and TFE on the product distribution in the pyrolysis of R23 performed in the minipilot-plant-scale reactor (average reaction temperature ) 880 °C, residence time ) 2 s, feed supply rate ) 10.13 L/min, reaction pressure ) 1.13 kg/cm2).

behaviors regardless of the different reaction conditions such as adiabaticity or isothermality when the mixture of R23 and TFE was pyrolyzed. This phenomenon was due to the heat balance in the simultaneous pyrolysis of R23 and TFE, and we interpreted this to mean that the heat generated by the dimerization of TFE might be used for the endothermic pyrolysis of R23 under both adiabatic and isothermal conditions. In the isothermal reaction, only a minimum amount of extra external heat was required to maintain the temperature constant. In the experiment of R23 pyrolysis, PFiB and other byproducts, which were not considered in the computer simulation, were formed in significant amounts when the reaction temperature was high or the residence time was long. Because of its high toxcity, the production of PFiB is extremely undesirable. A large-scale pyrolysis reaction of mixtures of R23 and TFE was performed in the mini-pilot plant shown in Figure 1 to confirm the possibility of controlling the reaction temperature using a heat balance between R23 pyrolysis and TFE dimerization and of minimizing the formation of PFiB, other byproducts, and carbon. The reaction conditions were determined from the experimental results of R23 pyrolysis in the small tubular reactor system and the computer simulation results. The product mixture discharged from the reactor was transferred to a distillation column, where the light mixture of R23 and TFE was separated from the heavy compounds involving HFP and recycled to the pyrolysis reactor. Figure 12 shows the effect of recycling the unreacted R23 and TFE to the reactor on the product distribution of R23 pyrolysis. The pyrolysis of R23 was performed at an initial average reaction temperature of 880 °C, a residence time of 2 s, a feed supply rate of 10.13 L/min, and a reaction pressure of 1.13 kg/cm2. Without recycling the unreacted R23 and TFE, the reaction temperature continuously increased with fluctuations. The average concentration of HFP was 19.3%, and the contents of R23, TFE, PFiB, and other byproducts were 46.6, 26.1, 4.5, and 3.5%, respectively. When the unreacted R23 and TFE were recycled and mixed with the fresh R23 to keep an appropriate molar ratio of R23 to TFE such as 50.5% R23 and 49.5% TFE, the pyrolysis of the mixture could be performed under stable conditions of an average reaction temperature of 875 °C, a residence

time of 2 s, a feed supply rate of 10.13 L/min, and a reaction pressure of 1.13 kg/cm2. The average concentration of HFP was 25.5%, and the contents of R23, TFE, PFiB, and byproducts were 39.5, 31.4, 1.1, and 2.5%, respectively. It was found that the production of PFiB and byproducts could be remarkably reduced while the yield of HFP was increased when the unreacted R23 and TFE was mixed with the fresh R23 in the feed. Pyrolysis results for R23/TFE mixtures of various feed molar ratios are listed in Table 2. In most experiments, the selectivities of HFP and PFiB in the pyrolysis of R23 or of the admixture of R23 and TFE increased with increasing residence time, but the selectivity of TFE decreased. The optimum molar ratio of R23 to TFE in the feed was experimentally found to be in the range of 1-4. If the ratio was lower than 0.25, the exothermic heat generated from the dimerization of the large amount of TFE exceeded the endothermic heat required for the pyrolysis of R23 so that the reaction temperature rapidly increased and was hard to control. If the ratio was higher than 4, the endothermic heat required for the pyrolysis of the large amount of R23 far exceeded the exthothermic heat from the dimerization of TFE so that the reaction needed additional external heat supply. Meanwhile, it was found that a low reaction temperature and a short residence time were good for minimizing the production of PFiB, the highly toxic byproduct, as well as the other byproducts. The heavy products separated from the unreacted R23 and TFE in the first distillation column were transferred to the second distillation column where HFP was purified from the other byproducts. The HFP collected from the middle of the second column showed an average purity of over 99.92%. Conclusions An effective method for preparing HFP of high selectivity and high yield from the pyrolysis of R23 was studied by experiment as well as by kinetic modeling. From the pyrolysis experiments in a small tubular reactor, it was revealed that HFP and TFE were produced as the major products and that perfluoroisobutylene, CF3CCCF3, C2F3H, CF3CHCF2, and CF3CF2CFCF2 were produced as byproducts. Based on the experimental results, a consecutive reaction mechanism was proposed, where HFP was presumably produced through the pyrolysis of R23 and dimerization of TFE, an intermediate formed from R23 pyrolysis. It was found that the reaction temperature and the residence time were important factors for obtaining a high selectivity of HFP and for minimizing the formation of side products such as toxic PFiB, polymer powders, or solid carbon in the pyrolysis of R23. The addition of super-

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heated steam or water vapor to the R23 pyrolysis was also shown to be undesirable for the formation of HFP because it enhanced the decomposition of R23 to CO2, CO, H2, and CF4 rather than playing a role in controlling the reaction temperature as a diluent gas. Computer simulations of the pyrolysis of R23 and TFE were conducted by using the presumed reaction mechanism and the kinetic data for the individual reaction steps reported in the literature. It was found that the simultaneous pyrolysis of R23 and TFE in an appropriate molar ratio could improve the stability of the reaction temperature because it could balance the heat required for the endothermic pyrolysis of R23 with that generated by the exothermic dimerization of TFE. A large-scale experiment in a mini-pilot plant showed that the pyrolysis of R23/TFE mixtures enables the reaction temperature to be controlled at a stable level so that the selectivity of HFP can be improved, the formation of PFiB and carbon can be minimized, and the amount of externally supplied energy can also be minimized. The optimum conditions for the pyrolysis of R23/TFE mixtures determined experimentally are R23/ TFE molar ratios of 1-4, reaction temperatures of 850900 °C, and residence times of 0.5-2 s. PFiB, the most harmful side product, was proposed to be produced through the formation of HFP, which possibly reacted with carbene (:CF2). It was also found that the selectivity of PFiB increased with increasing reaction temperature and residence time. However, it could be markedly reduced while the yield of HFP remained high when mixtures of R23 and TFE were pyrolyzed simultaneously.

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Received for review January 22, 2001 Revised manuscript received March 6, 2002 Accepted March 13, 2002 IE010065Q