Energy & Fuels 2006, 20, 7-10
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Process Design and Solvent Recycle for the Supercritical Fischer-Tropsch Synthesis Wensheng Linghu, Xiaohong Li, Kenji Asami,* and Kaoru Fujimoto Department of Chemical Processes and EnVironments, Faculty of EnVironmental Engineering, The UniVersity of Kitakyushu, Hibikino 1-1, Wakamatsu-ku, Kitakyushu, Fukuoka 808-0135, Japan ReceiVed April 18, 2005. ReVised Manuscript ReceiVed October 8, 2005
A recycle reactor system for supercritical Fischer-Tropsch synthesis was successfully designed and tested. The new reactor system has these characteristics: (1) integration of supercritical Fischer-Tropsch reactions, natural separation of produced wax from liquid phase, and recycle of the solvent and (2) natural recycle of solvent driven by self-gravity. A 20% Co/SiO2 catalyst and n-hexane were used as a catalyst and supercritical fluid, respectively. The results show that the average CO conversion at the steady state was 45% with recycle and 58% without recycle. The lumped hydrocarbon products distribution did not have any obvious difference between with and without recycle operation; however, R-olefin content of products with recycle was lower than that without recycle. The XRD result indicates that most of the reduced cobalt remains in the metallic state during the Fischer-Tropsch reactions for both cases.
Introduction During the last two decades, Fischer-Tropsch synthesis (FTS) has received widespread interest as an economically viable route to convert synthesis gas to environmentally friendly transportation fuels.1-3 As a new reaction phase introduced into the conventional FTS process, supercritical phase FTS (SFTS) has shown some advantages due to their unique characteristic in molecular diffusion and solubility power, such as: (1) gaslike diffusivities of reactants as well as products, (2) liquidlike heat transfer capacities, and (3) in situ extraction of the heavy products from the catalyst surface and pore. The above characteristics would be useful in overcoming some of the disadvantages of gas-phase and slurry-phase FTS.4-11 Bukur and co-workers have investigated the SFTS on a precipitated iron catalyst in a fixed bed reactor.8 Propane was used as the SCF. The lumped hydrocarbon product distribution under supercritical conditions was similar to those obtained during reaction at the baseline conditions (gas-phase conditions); however, higher selectivity of 1-olefin was obtained during SFTS. The authors considered that the increased 1-olefin selectivity during the SFTS is due to higher diffusivities and desorption rates of 1-olefin relative to normal FTS. Subramaniam and co-workers7,12,13 reported SFTS results obtained from * To whom correspondence should be addressed. Phone: +81-093-6953284. Fax: +81-093-695-3376. E-mail address:
[email protected]. (1) Soled, S. L.; Iglesia, E.; Fiato, R. A.; Baumgartner, J. E.; Vroman, H.; Miseo, S. Top. Catal. 2003, 26, 101. (2) Dry, M. E. Catal. Today 2002, 71, 227. (3) Martinez, A.; Lopez, C.; Marquez, F.; Diaz, I. J. Catal. 2003, 220, 486. (4) Fan, L.; Fujimoto, K. Appl. Catal., A 1999, 186, 343. (5) Fan, L.; Yokota, K.; Fujimoto, K. Top. Catal. 1995, 2, 267. (6) Fan, L.; Yokota, K.; Fujimoto, K. AIChE J. 1992, 38, 1639. (7) Subramaniam, B. Appl. Catal., A 2001, 212, 199. (8) Lang, X.; Akgerman, A.; Bukur, D. B. Ind. Eng. Chem. Res. 1995, 34, 72. (9) Huang, X.; Roberts, C. B. Fuel Process. Technol. 2003, 83, 81. (10) Snavely, K.; Subramaniam, B. Ind. Eng. Chem. Res. 1997, 36, 4413. (11) Jacobs, G.; Chaudhari, K.; Sparks, D.; Zhang, Y.; Shi, B.; Spicer, R.; Das, T. K.; Li, J.; Davis, B. H. Fuel 2003, 82, 1251.
an Fe catalyst in a fixed bed reactor with near-critical n-hexane as SCF. They claimed that the catalyst effectiveness increased with pressure of the SCF due to the alleviation of pore-diffusion limitations. The results suggested to them that the pressure tuning of the density and transport properties of SCF offers a powerful tool to optimize catalyst activity and product selectivity during FTS. Huang and Roberts9 utilized a Co catalyst (15%Co0.5%Pt/Al2O3) and a fixed bed reactor and reported that SFTS has a marked effect on the hydrocarbon product distribution with a shift to higher carbon number products owing to enhanced heat and mass transfer from catalyst surface. In previous studies by our group,4,6,14-17 we investigated the reaction performances of SFTS from the viewpoints of extraction capability and mass diffusion efficiency. We also compared the results obtained from three different reaction phases: gas, liquid, and supercritical phase. n-Hexane was employed as SCF, and three different catalysts, Co, Fe, and Ru, were used, respectively. The results suggested that although the rate of the reaction and the diffusion of the reactants in the SFTS were slightly lower than those in the gas-phase reaction, the removal of reaction heat and waxy products from the catalyst surface was much more effective than in the gas-phase reaction. The olefin content in the hydrocarbon products was much higher in the SFTS than in the liquid- and gas-phase reactions. We all know that high partial pressure of solvent, which means a large amount of solvent, is required during the SFTS to conduct the reaction under the supercritical conditions. Therefore, if we can design a kind of reactor system in which supercritical media can be utilized in recycle, the amount of supercritical solvent will be reduced remarkably. How to recycle supercritical solvent and whether solvent recycle will affect the reaction behavior of FTS reaction during the SFTS, however, (12) Bochniak, D. J.; Subramaniam, B. AIChE J. 1998, 44, 1889. (13) Snavely, K.; Subramaniam, B. Ind. Eng. Chem. Res. 1997, 36, 4413. (14) Tsubaki, N.; Sun, S.; Fujimoto K. J. Catal. 2001, 199, 236. (15) Yokota, K.; Fujimoto, K. Ind. Eng. Chem. Res. 1991, 30, 95. (16) Tsubaki, N.; Fujimoto, K. Fuel Process. Technol. 2000, 62, 173. (17) Yokota, K.; Hanakata, Y.; Fujimoto, K. Nat. Gas ConVers. 1991, 61, 289.
10.1021/ef050109y CCC: $33.50 © 2006 American Chemical Society Published on Web 11/04/2005
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Table 1. Average r-Olefin Content of Products and n-Hexane Content in the Solvent Stream During Different Time Periodsa R-olefin content of products, %
n-hexane content in the solvent stream, %
time on stream, h
with recycle
without recycle
with recycle
without recycle
0-4 5-8 9-12 13-16 17-20 21-24 average, %
11.9 17.1 15.8 18.5 16.8 18.9 16.5
13.1 17.5 19.9 21.6 23.0 22.7 19.6
98.5 97.6 97.4 97.3 97.6 97.6 97.7
98.8 98.9 98.9 98.8 98.9 98.9 98.9
a Reaction conditions: 250 °C, P total ) 4.5 MPa, P(CO+H2) ) 1.0 MPa, Psolvent ) 3.5 MPa, W/F ) 5 g-cat.h/mol, and recycle ratio ) 6.
has never been investigated until now. On the basis of the above discussion, it is very important and necessary to study the effect of solvent recycle during the SFTS process to reduce the cost of solvent for possibly operating SFTS on a large scale. The present work was undertaken with the objective to design and test a novel reactor system for the recycle of solvent in the SFTS and evaluate the performance of SFTS under solvent recycle condition. The experimental result was also compared with that obtained without solvent recycle (solvent one-pass through) operation. Choice of the Supercritical Fluid Choice of the supercritical solvent for FTS was based on the following criteria:4 1. The critical temperature and pressure of the solvent should be slightly lower than the typical reaction temperature and pressure. 2. The solvent should not be poisonous to the catalysts and should be stable under the reaction conditions. 3. The solvent should have a high affinity for aliphatic hydrocarbons to extract the wax from the catalyst surface. Typical reaction conditions used in this study were T ) 250 °C, Ptotal ) 4.5 MPa, P(CO+H2) ) 1.0 MPa, Psolvent ) 3.5 MPa, and W/F(CO+H2) ) 5 g cat‚h/mol. Herein, W/F(CO+H2) was defined as the weight of the catalyst divided by the flow rate of (CO+H2). The recycle ratio of solvent was defined as the total flow rate of solvent passed through the catalyst bed divided by the flow rate of fresh solvent introduced into the system. On the basis of the above criteria, n-hexane, whose critical temperature and critical pressure are 233.7 °C and 2.97 MPa, respectively, was used as the supercritical solvent. Under the solvent recycle, the n-hexane fraction in the recycled solvent was always higher than 97% (see Table 1 for detailed data). The calculated n-hexane partial pressure is about 3.4 MPa. Therefore, even at the solvent recycle conditions, the system should be at supercritical condition. Design of Recycle Reactor System A schematic diagram of the recycle reactor system is shown in Figure 1. Feed gas (H2/CO/Ar of 2:1:0.1, Ar is used as an internal standard gas) and fresh solvent (n-hexane) were introduced into a preheated vaporizer (260 °C) concurrently by a Brooks 5850E mass flow controller and a high-pressure liquid pump (JP-H 2, Japan), respectively. The ratio of n-hexane to feed gas in mole is 3.5:1. Since the boiling point of n-hexane is only 68 °C, n-hexane was vaporized in the vaporizer, and calculated partial pressure of n-hexane was 3.5 MPa when total pressure of the system was maintained at 4.5 MPa. Therefore,
Figure 1. Schematic diagram of the recycle reactor setup for SFTS 1. Syngas mass flow controller; 2. Solvent tank; 3. High-pressure liquid pump; 4. Pressure relief valve; 5. Vaporizer; 6. Fixed bed reactor; 7. Separator; 8. Recycle pipe; 9. Cooler; 10. Glass tank; 11. Micrometering valve; 12. Cold trap; 13. Back pressure regulator; 14A. GC with TCD; 14B. GC with FID.
n-hexane was at the supercritical conditions. Then vaporized solvent along with feed gas enters into a conventional downflow fixed bed reactor, where CO reacts with H2 to form a broad range of hydrocarbons under the supercritical conditions. After leaving the reactor, the exit mixture, containing unreacted syngas, products, and solvent, passes through a wax separator, which is maintained at about 150 °C, to condense heavy products (wax). In fact, the wax separator is a hot trap. In general, FTS product on Co-based catalyst consists mostly of normal paraffins and linear R-olefins. The boiling points of n-nonane (n-C9H20) and n-decane (n-C10H22) are 150.8 °C and 174.0 °C, respectively. The wax herein was defined as the solid fraction condensed in the hot trap (wax separator) and containing mainly C10+. The noncondensable gases pass though a gas line heated with ribbon heater at about 250 °C and then enters a water-cooled heat exchanger, where the solvent, consisting of main n-hexane and some content of lighter fraction of products, was condensed. After leaving the heat exchange, the gas-liquid mixture enters a high-pressure glass tank. A portion of the condensed solvent is recycled to the vaporizer, and the flow rate of recycled solvent (n-hexane fraction in the recycled solvent was always higher than 97%, detailed data will be listed in Table 1) can be adjusted by using a micrometering valve (Whitey SS 22RS4, Swagelok Co., Japan). It should be noted that the driving force for the solvent recycle comes from the gravity of the solvent. Therefore, to keep the solvent recycle stable, the glass tank is placed about 2 m higher than the vaporizer. The rest of the portion of the condensed solvent and noncondensable gas passes through an ice trap, where condensed liquid is collected continuously by a products tank. The noncondensable gas passes through a back pressure regulator measured with a wet test meter before being vented. The reactor is the core of the reaction system. The reactor used in this study was a stainless steel tube with an inner diameter of 10 mm and height of 400 mm. The reactor had an isothermal zone ((2 °C) of 150 mm. The catalyst was loaded in the center section, and the height of the catalyst bed was
Process Design and SolVent Recycle for Supercritical FTS
Energy & Fuels, Vol. 20, No. 1, 2006 9
about 100 mm. The reaction temperature was defined with the highest temperature of the catalyst bed, measured by a thermocouple that was placed at the center of the catalyst bed. The design presented above gives the recycle reactor system these characteristics: (1) integration of supercritical FTS reactions, natural separation of produced wax from liquid phase, and recycle of the solvent and (2) natural recycle of solvent driven by self-gravity. Experimental Section The catalyst used was a SiO2-supported cobalt catalyst, which was prepared by impregnating a commercially available Q-15 SiO2 gel (Fuji Silysia Chemical Ltd., Japan) with cobalt nitrate from its aqueous solution by incipient wetness impregnation method. The nominal catalyst composition was Co: 20 and SiO2: 80 by weight. The catalyst preparation procedure has been described previously.18 The catalyst was calcined in air at 200 °C for 2 h and then crushed and sieved to 20-40 mesh size. A quantity of 1 g of catalyst was diluted 1:3 by weight with SiO2 prior to loading into the reactor. The catalyst was reduced in situ in hydrogen flow at 400 °C for 3 h. The characteristics of the catalyst used are as follows: BET surface area 131 m2/g, pore volume 0.56 cm3/g, and Co metal dispersion 3.15%. Each reaction was conducted for 24 h. Gaseous compounds were analyzed online by two-coupled gas chromatographs. CO, CO2, and CH4 were analyzed by using an activated charcoal column with a thermal conductivity detector (TCD). Light hydrocarbons (C1-C6) were analyzed by using an Al2O3-KCl capillary column with a flame ionization detector (FID). A capillary column (DB-2881) with a FID was used for the analysis of the liquid products. The n-hexadecane was used as the internal standard for liquid products. A Rigaku, RINT 2000 X-ray diffractomter equipped with a Cu radiation source was used to analyze fresh and used catalyst in the 20-70° 2θ range at a generator voltage of 40 kV and a generator current of 20 mA. A scan rate of 2° min-1 was used.
Results and Discussion Two tests were conducted to assess the effects of solvent recycle on the performance of SFTS. The first test was carried out under the recycle ratio of 6 (denote as with recycle), and the second one was tested at the solvent one pass through conditions (denote as without recycle), while the total flow rate of solvent passed through the catalyst bed was kept the same as the first test. Figure 2 shows CO conversion as a function of time on stream under conditions with or without solvent recycle. CO conversion decreased remarkably with time on stream at the initial stage (about 0-8 h) and gradually reached the steady state after 8 h on stream for both systems. The average CO conversion for the time period of 9-24 h was 45% with recycle and 58% without recycle. This result may be caused by two factors. The first one, the main reason, is the competitive adsorption of primary products, R-olefins and reactant, CO on same catalytic site. The suggestion that the competitive adsorption of olefins and CO on the same metal site occurs during FTS process has also been reported by other researchers.19,20 We know that linear normal paraffins and R-olefins are the main primary products of the FTS reactions on Co-base catalyst; however, normal paraffins are unreactive and R-olefins can readsorb on the catalyst surface and undergo secondary reactions, such as hydrogenation, isomerization, and double bond shift.4,8,21 Under the condition with recycle ratio of 6, about (18) Linghu, W.; Li, X.; Asami, K.; Fujimoto, K. Fuel Process. Technol. 2004, 85, 1121. (19) Jordan, D. S.; Bell, A. T. J. Phys. Chem. 1986, 90, 4797. (20) Schulz, H.; Steen, V. E.; Claeys, M. Top. Catal. 1995, 2, 223.
Figure 2. Variation in CO conversion with time on stream under with and without recycle. Reaction conditions: 250 °C, Ptotal ) 4.5 MPa, P(CO+H2) ) 1.0 MPa, Psolvent ) 3.5 MPa, W/F ) 5 g cat‚h/mol, and recycle ratio ) 6.
83% of liquid products (paraffins and olefins) was recycled in the system along with recycled solvent. This is similar to that of the addition of R-olefins into the feed gas. Because the adsorption equilibrium constants of hydrocarbons increase exponentially with chain length of hydrocarbons, R-olefins, especially those with longer carbon chains, would readsorb more strongly on the catalytic site and would cover a large active area of the catalyst, thereby inhibiting CO adsorption onto the metal site. The above result is also in good agreement with the result reported by Fan4 that relatively lower CO conversion was observed in the case of the addition of 1-C16H32 into feed gas. Another one may be the inhibition of FTS reaction due to the increase of product concentration under solvent recycle condition. To confirm the above explanation, we measured average R-olefin content of C2-C25 products and n-hexane content in the solvent stream during different time periods under with and without recycle conditions, and the results are shown in Table 1. It can been seen that average R-olefin content of products under with recycle condition was obviously lower than that under without recycle conditions in every reaction time period. In general, under similar reaction conditions, R-olefin content of FTS products at a lower CO conversion level should be higher than that at a higher CO conversion level. That both lower R-olefin content of FTS products and lower CO conversion were observed under solvent recycle indicates that the readsorption and secondary reactions of R-olefins under with recycle are much stronger than that under without recycle. The analysis data in Table 1 also show that although the n-hexane content in the solvent stream under with recycle was slightly lower than that under without recycle, the n-hexane content in the recycled solvent stream was always higher than 97%. In addition, it should be noted that the liquid pump used in the work was a semicontinuous piston pump, and every 2 h was a recycle, which affected the stability of the system to some degree. In our case, the experimental error for CO conversion was about 5%. The changes of carbon species of hydrocarbon products as a function of reaction time under with and without recycle conditions are shown in Figure 3. The hydrocarbon products were divided into C1 (CH4), C2-C4 fractions, C5-C10 fractions, and C10+ fractions. The selectivities of C1 to C10 fractions were calculated from the data analyzed by GC. The selectivity of C10+ is defined as follow: C10+ selectivity ) 100 - selectivity (21) Claeys, M.; Steen, E. V. Catal. Today 2002, 71, 419.
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were shown in Figure 3b. The above results indicate that the recycle operation should not have marked influence on the lumped hydrocarbon products distribution. Furthermore, X-ray diffraction was used to determine the changes of Co crystal structures of the catalyst during different status: after calcination, after reduction, and after reaction under with or without recycle. We found that fine crystals of Co3O4 were formed during the calcinations in air at 200 °C for 2 h. The diffraction peaks of Co3O4 spinel phase disappeared, and metallic Co phase appeared after reduction in hydrogen flow at 200 °C for 3 h. In general, the reduction of Co3O4 under H2 flow rate includes two steps: Co3O4 to CoO and CoO to Co°. The above result suggests that Co3O4 is completely reduced to metallic Co. These results are in agreement with the reports published by other researchers.3,22 After reaction for 24 h the peaks of metallic Co crystals were still predominant, but a very small Co3O4 (311) peak at 36.8° was also detected for both catalysts at with or without recycle conditions. This means that most of the reduced cobalt remains in the metallic state during the FTS reactions for both with and without recycle. Conclusions
Figure 3. Changes of carbon species of hydrocarbon products vs reaction time. Reaction conditions: 250 °C, Ptotal ) 4.5 MPa, P(CO+H2) ) 1.0 MPa, Psolvent ) 3.5 MPa, W/F ) 5 g cat‚h/mol and recycle ratio ) 6.
of C1 to C10 fractions. It can be seen from Figure 3a that, under with recycle conditions, CH4 selectivity was almost constant irrespective of reaction time. The selectivity of C2-C4 fractions and C5-C10 fractions increased from about 6 and 17% to about 10 and 21% when reaction time increased from the initial stage to 24 h. At the same time, C10+ fraction selectivity decreased from about 67 to 59%. This result indicates that with the time on stream, the hydrocarbon product distribution had a shift to light carbon number products. The similar results were also observed under without recycle conditions, the data of which
The recycle reactor system for SFTS designed and tested in this article can be operated successfully under the reaction conditions. This system has these characteristics: (1) integration of supercritical FTS reactions, natural separation of produced wax from liquid phase, and recycle of the solvent and (2) natural recycle of solvent driven by self-gravity. Under the reaction conditions used in this study, average CO conversion at the steady state was 45% with recycle and 58% without recycle. The lumped hydrocarbon product distribution has no obvious difference between with and without recycle operation; however, lower R-olefin content of products was observed during the recycle operation. The XRD result indicates that solvent recycle did not have any effect on the Co crystal structure of catalyst. The lower CO conversion under with recycle should be mainly attributed to the competitive adsorption of recycled R-olefin product and CO on the same catalytic site, which inhibits the reaction of CO. Additional work is planned to optimize the recycle process. Acknowledgment. This work was entrusted from Nippon Oil Corporation as a part of the NEDO-ATL Project (Japan). We are grateful for the use of the X-ray diffractometer at the Instrumentation Center of The University of Kitakyushu (Japan). EF050109Y (22) Adachi, M.; Yoshi, K.; Han, Y.; Fujimo, K. Bull. Chem. Soc. Jpn. 1996, 69, 1509.
