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Activity and selectivity of iron fischer-tropsch catalysts in a stirred tank slurry reactor
M. de Pontes, R.L. Espinoza, C.P. Nicolaides, J.H. Scholz and M.S. Scurrell (Editors) Natural Gas Conversion IV
Studies in Surface Science and Catalysis, Vol. 107 9 1997 Elsevier Science B.V. All rights reserved.
163
Activity and selectivity of iron Fischer-Tropsch catalysts in a stirred tank slurry reactor Dragomir B. Bukur, Xiaosu Lang and Lech Nowicki a Department of Chemical Engineering Kinetics, Catalysis and Reaction Engineering Laboratory Texas A & M University College Station, TX 77843-3122, USA Summary Four catalysts prepared by two catalyst manufacturers (UCI and Ruhrchemie) were tested in a stirred tank slurry reactor to determine their performance during Fischer-Tropsch synthesis. Precipitated iron was used as the active metal, and Cu, K and SiO2 were used as promoters. The amounts of promoters (in parts per weight per 100 parts of iron) in these four catalysts were as follows: SiO2 = 4.1-25; K = 1.1-8.7 and Cu = 1.4-7.5. Total BET surface area varied between 136 and 290 m 2/g. The Ruhrchemie and two of the UCI's catalysts (runs SA-3391 and SA-2052) had low deactivation rates, whereas the UCI catalyst with low silica content (run SA-1532) deactivated more rapidly. The intrinsic activity of the low silica catalyst, measured by apparent first order reaction rate constant, was the lowest, whereas the Ruhrchemie catalyst had the highest activity (per gram of iron basis). Activity of the Ruhrchemie catalyst was higher after the CO pretreatment, than after the hydrogen reduction. The low silica catalyst favored production of high molecular weight products. Methane selectivity in run SA-1532 was only 2.2-2.4 wt%, and that of gaseous (C2 - C4) hydrocarbons 10-11 wt%. The extent of secondary reactions (1-olefin hydrogenation, isomerization and/or readsorption) was low on this catalyst. In general, hydrocarbon and olefin selectivities of the Ruhrchemie and the other two UCI catalysts were similar. Methane selectivity in tests with these three catalysts varied between 4.1 and 4.7 wt%, during the first 360 h of testing. 1. I N T R O D U C T I O N Fischer-Tropsch (FT) hydrocarbon synthesis from a coal derived synthesis gas is practiced on commercial scale at SASOL plants in South Africa in fLxed and fluidized bed reactors, utilizing potassium promoted iron catalysts. The fluidized bed reactors have high selectivity togasoline range products, whereas fixed bed reactors produce predominantly diesel fuel and hydrocarbon waxes. This variation in product distribution is achieved through the use of different process conditions and catalysts [ 1]. Several studies directed at iron FT catalyst development and performance evaluation in bench scale slurry reactors were conducted at government, industrial and university laboratories in the United States since 1980's [2-8]. In 1992 the United States Department of a Present address: Institute of Chemical & Process Eng., Lodz Technical University, 90-924 Lodz, POLAND. The work supported by U. S. DOE (Contract DE-AC22-89PC89868) and Texas Engineering Experiment Station.
164
Energy and industrial partners (Air Products, Exxon, Shell, Statoil and UOP) sponsored a FT demonstration run in a 0.57 m in diameter bubble column slurry reactor at LaPorte, Texas [8]. The catalyst for this run, designated L-3950, was manufactured by United Catalysts, Inc. (UCI). Three trial batches of precipitated iron catalysts, prepared by UCI, were evaluated at UOP and Texas A&M University. Here, we report results on catalyst activity, stability and product distribution (hydrocarbon selectivity and olefin selectivities as a function of carbon number) from these tests in a stirred tank slurry reactor. Selected results from our studies [9, 10] with a precipitated iron catalyst synthesized by Ruhrchemie, which was used originally in fixed bed reactors at SASOL, are also presented here for comparison. 2. EXPERIMENTAL
Experiments were conducted in a 1 dm 3 stirred tank slurry reactor (Autoclave Engineers). The feed gas flow rate was adjusted with a mass flow controller and passed through a series of oxygen removal, alumina and activated charcoal traps to remove trace impurities. After leaving the reactor, the exit gas passed through a series of high and low (ambient) pressure traps to condense liquid products. High molecular weight hydrocarbons (wax), withdrawn from a slurry reactor through a porous cylindrical sintered metal filter, and liquid products, collected in the high and low pressure traps, Were analyzed by gas chromatography. The reactants and noncondensible products leaving the ice traps were analyzed on an on-line GC (Carle AGC 400) with multiple columns and both flame ionization and thermal conductivity detectors [8, 10, 12]. Compositions, BET surface areas and pore volumes of catalysts synthesized by UCI, and Ruhrchemie AG are listed in Table 1. In all tests a catalyst was crushed and sieved to either 270/325 mesh (44-53 I.tm in diameter) or less than 270 mesh size, prior to loading to a reactor. A pre-purified normal octacosane was used as the liquid (slurry) medium in tests of the Ruhrchemie catalyst, whereas Ethylflo 164 oil (a hydrogenated 1-decene homopolymer - C 30 obtained from Ethyl Co.) was used in tests of UCI catalysts. Table 1 Physico -Chemical Properties and Test Desil~nations for UCI and Ruhrchemie Catalysts Run number SA-3391 SA-1532 SA-2052 SA-0888/ SB-1370 Catalyst UCI L-3950 UCI 1207UCI L-3950 Ruhrchemie designation 175A (batch 3) LP 33/81 Catalyst composition Fe 100 100 100 100 Cu 4.3 1.4 7.5 5 K 8.7 1.1 2.6 4.2 SiO2 17 4.1 9.2 25 Surface area 251 167 136 290 (m2/g) Pore volume (cm3/g)
0.50
0.18
0.30
0.62
The UCI catalysts were pretreated in situ with synthesis gas (H2/CO molar feed ratio of about 0.7) at 280~ for 12 hours. After the pretreatment the catalysts were tested at: 265~ 2.1 MPa, 2.4 N1/g-Fe/h (where, Nl/h, denotes volumetric gas flow rate at standard temperature
165 and pressure) and H2/CO = 0.7. The Ruhrchemie catalyst was pretreated in situ either with CO at 280~ for 16 h (run SA-0888), or with hydrogen at 220~ for 1 h (run SB-1370). Baseline process conditions in tests of the Ruhrchemie catalyst were: 250~ 1.48 MPa, 3.8 Nl/g-Fe/h, and H 2/CO = 0.67. 3. RESULTS AND DISCUSSION 3.1 Catalyst Activity and Stability Variation of syngas conversion with time for all three tests with UCI catalysts is shown in Figure 1. Results from periods in which catalysts were not tested at the baseline conditions, are not shown here (runs SA-3391 and SA-2052). During the first 300 h of testing the (H2 + CO) conversion was the highest in test SA-3391, whereas the lowest syngas conversion was obtained in test SA-1532. Activities of these three catalysts are compared in Figure 2, in terms of values of apparent first order reaction rate constant evaluated at 260~ The latter value was calculated from data at different temperatures by using the activation energy of 90 kJ/mol, which is a typical value for iron FT catalysts [12]. These results show that the catalyst in run SA-3391 was the most active, whereas the catalyst in run SA-1532 was the least active. Also, both catalysts deactivated with time, whereas in run SA-2052 the catalyst activity increased slightly with time. The most active catalyst (run SA-3391) had the highest total surface area, but the activity in the other two tests did not correlate with the total surface area of the catalyst (Table 1). The apparent rate constants from two tests of the Ruhrchemie catalyst (runs SA-0888 and SB-1370) are also shown in Figure 2 for comparison. Activity of the Ruhrchemie catalyst is markedly higher than that of the UCI catalysts, regardless of the pretreatment procedure employed (CO activation in test SA-0888, and H2 reduction in SB1370), however the catalyst was more active after the CO pretreatment. 500
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Figure 1. Effect of time on stream on (H2+CO) Conversion (a) and methane Selectivity (b) for UCI Catalysts. Values
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. . . .
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Figure 2. Apparent first order reaction constant of Ruhrchemie and UCI catalysts as a function of time on stream. ratio,
UR,
and
partial
pressure
quotient,
Kp = Pco2P,2/PcoP,2o, provide information on relative rates of water-gas-shift (WGS)
166
reaction (lower usage ratios or higher values of Kp correspond to higher WGS activity). Catalyst used in run SA-3391 had the highest WGSactivity (UR = 0.56-0.62, Kp = 14-22), whereas the catalyst used in run SA-2052 had the lowest WGS activity (UR = 0.61-0.70, Kp = 3.6-9). The WGS activity of the Ruhrchemie catalyst during testing at 1.48 MPa, 265~ 3.8 N1/g-Fe/h, H2/CO = 0.67 was similar to that of the UCI catalysts (UR = 0.66, Kp = 4.4-11).
3.2 Hydrocarbon Product Distribution Variation of methane selectivity (mol % C basis) with time, for all three UCI catalysts, is shown in Figure 1b, whereas average values of hydrocarbon selectivities (wt%) for the UCI and the Ruhrchemie catalysts are listed in Table 2. UCI catalyst with low silica and potassium contents (run SA-1532), had the lowest methane (2-2.3 mol%) and gaseous hydrocarbons selectivities. It favored the production of high molecular weight hydrocarbons, and its selectivity was fairly stable with time on stream. The other two UCI catalysts, used in runs SA-3391 and SA-2052, had similar selectivities, but both produced more methane and less high molecular weight products than the catalyst used in run SA-1532. Methane selectivities of these two catalysts were 3.5-4 mol% (4.1-4.4 wt%), whereas C12+ selectivities varied between 52 and 56 wt%. Average values of hydrocarbon selectivities obtained during initial 360 h of testing of the Ruhrchemie catalyst at the baseline conditions were similar to those obtained in runs SA-3391 and SA-2052. The Ruhrchemie catalyst was also tested at 265~ 1.48 MPa, 3.8 Nl/g-Fe/h and H2/CO=0.67. In both tests there was a significant shift toward lower molecular weight hydrocarbons. Methane selectivity increased to: 6.0 wt% (SB-1370) and 7.0 wt% (SA-0888), whereas C12+ selectivity decreased to: 42.9 wt% (SB1370) and 23.0 wt% (SA-0888). This shift toward low molecular weight products is not entirely due to higher reaction temperature, but also due to aging of the catalyst, since data at 265~ were obtained at 570- 590 h on stream [ 10]. Table 2 Comparison of Slurry Reactor Test Results with UCI and Ruhrchemie Catalysts Run numder SA-3391 SA-1532 SA-2052 S A - 0 8 8 8 SB-1370 Catalyst UCI L-3950 UCI 1207-175A UCI L-3950 Ruhrchemie Ruhrchemie (batch 3) Reduction H 2 / C O = 0 . 7 , H2/CO=0.7, H2/CO=0.7, CO, 16 h H2, 1 h Conditions 12 h, 280~ 12 h, 280~ 12 h, 280~ 280~ 220~ TOS (h) 227-322 35-301 140-276, 0-343 0-360 401-4.44 %CO Conv. 73-80 35-52 68-75 40-43 34-39 %(H2+CO) Conv. 71-75 33-49 65-72 43-46 38-41 Hydrocarbon Selectivities (wt-%): (1-I4 4.4 2.3 4.1 4.7 4.5 C2-C4 16.5 10.7 18.5 20.6 15.5 C5-C 11 23.6 16.2 22.4 23.2 27.7 C12+ 55.5 70.8 55.0 51.5 52.3 Test conditions for UCI Catalysts: 265~ 2.1 MPa, 2.4 Nl/g-Fe/h, feed H2/CO=0.7 Test conditions for Ruhrchemie Catalyst: 250~ 1.48 MPa, 3.8 Nl/g-Fe/h, feed H2/CO--0.67
3 . 3 0 i e f i n Selectivities Olefin selectivities of the UCI and Ruhrchemie catalysts are shown in Figure 3. Selectivities of catalysts used in runs SA-3391 and SA-2052 were nearly the same, and followed the usual trend of declining selectivity with carbon number, and relatively low
167
ethylene selectivity (60 wt%). Low silica catalyst, used in run SA-1532, had high ethylene selectivity (85%) and higher olefin content in high molecular weight hydrocarbons (C 11+ products) than the other two catalysts. Low hydrogenation activity of this catalyst is consistent with its low methane selectivity. Comparison of 2-olefin selectivities (which is a measure of the extent of 1-olefin isomerization) of the four catalysts at 95-187 h on stream is made in Figure 4. The 2-olefin selectivity of all three UCI catalysts was rather low. Catalysts used in runs SA-3391 and SA2052 had similar 2-olefin selectivities, and the 1-olefin isomerization activity increased with carbon number (C9+ products). The low silica catalyst (SA-1532) had the lowest 2-olefin content, and selectivity of C 10+ 2-olef'ms was fairly constant (--10%). Total olefin selectivity (C10+ hydrocarbons) of the Ruhrchemie catalyst was lower, whereas the 2-olefin selectivity was higher than that of the UCI catalysts used in runs SA3391 and SA-2052 (Figures 3 and 4). 90
,
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,
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,
4
!
i
I
6
i
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Carbon Number
Figure 3. Comparison of olefin selectivides of UCI and Ruhrchemie catalysts. 40
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o 9 9 9
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SA-0888, 95 h (Catalyst Ruhrchemir LP 33/81) SA-3391, 187 h (Catalyst: UC[ I.,-3950) SA-1532, 164 h (Catalyst: tiC[ 1270-175A) SA-2052, 168 h (Catalyst: tiC[ L-3950, batch 3)
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Figure 4. Comparison of 2-olefin selectivities of UCI and Ruhrchemie catalysts.
168 As can be seen in Figure 3, the olefin selectivity dependence on carbon number of hydrocarbon products passes through a maximum. This is believed to be due to secondary reactions of l-olefins [ 13, 14]. Ethylene is more reactive than other low molecular weight olefins, whereas the decrease in olefin content with increase in molecular weight has been attributed to their greater adsorptivity [ 14, 15], higher solubility resulting in longer residence time in a slurry reactor [2, 16], lower diffusivities [ 17], and/or 1-olefin readsorption [ 18-20]. The increase in 2-olefin selectivity with increase in carbon number is due to the same factors which affect the total olefin selectivity. The longer residence time of high molecular weight 1-alkenes either in the catalyst pores or in the reactor itself increases probability for secondary reactions (1-olefin hydrogenation, isomerization and readsorption). REFERENCES
2 3.
.
8. 9.
10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
M. E. Dry, in J. R. Anderson and M. Boudart (Editors), Catalysis Science and Technology 1, Springer, New York, 1981, p. 159. T. J. Donnelly and C. N. Satterfield, Appl. Catal., 52 (1989) 93. J. C. W., Kuo, Final Report on US. DOE Contract No. DE-AC22-83PC60019, Mobil Res. Dev. Corp., Paulsboro, New Jersey, 1985. H. W. Pennline, M. F. Zarochak, J. M. Stencel and J. R. Diehl, Ind. Eng. Chem. Res. 26 (1987) 595. H. Abrevaya, R. R. Frame and W. M. Targos, in G. J. Stiegel and R. D. Srivastava (Editors), Liquefaction Contractors' Review Meeting Proc., US. Department of Energy, Pittsburgh Energy Technology Center, Pittsburgh, 1991, p. 219. B. H. Davis and F. L. Tungate, in G. J. Stiegel and R. D. Srivastava (Editors), Liquefaction Contractors' Review Meeting Proc. , US. Department of Energy, Pittsburgh Energy Technology Center, Pittsburgh, 1991, p. 275. S. Soled, E. Iglesia and R. A. Fiato, Catal. Lett., 7 (1990) 271. D. B. Bukur, L. Nowicki and X. Lang, Chem. Eng. Sci. 49 (1994) 4615. B. L. Bhatt, E. S. Schaub, E. C. Heydorn, D. M. Herron, D. W. Studer and D. M. Brown, in G. J. Stiegel and R. D. Srivastava (Editors), Liquefaction Contractors' Review Meeting Proc. , US. Department of Energy, Pittsburgh Energy Technology Center, Pittsburgh, 1992, p. 403. D. B. Bukur, S. A. Patel and X. Lang, Appl. Catal., 61 (1990) 329. D. B. Bukur, L. Nowicki and S. A. Patel, submitted to Can. J. Chem. Eng., 1995. W. H. Zimmerman and D. B. Bukur, Can. J. Chem. Eng., 68 (1990) 292. H. Arakawa and A. T. Bell, Ind. Eng. Chem. Proc. Des. Dev., 22 (1983) 97. R. B. Anderson, in P. H. Emmett (Editor), Catalysis Vol. 4, Van Nostrand-Reinhold, New York, (1956) p. 29. H. Schulz and H. Gokcebay, in J. R. Kosak (Editor), Catalysis of Organic Reactions, Marcel Dekker, New York, 1984, p. 153. R. A. Dictor and A. T. Bell, J. Catal. 97 (1986) 121. K. Yokota and K. Fujimoto, Ind. Eng. Chem. Res., 30 (1991) 95. R. J. Madon, S. C. Reyes and E. Iglesia, J. Phys. Chem. 95 (1991) 7795. R. J. Madon and E. Iglesia, J. Catal. 139 (1993) 576. W. Zimmerman, D. Bukur and S. Ledakowicz, Chem. Eng. Sci. 47 (1992) 2707.
Studies in Surface Science and Catalysis, Vol. 107 9 1997 Elsevier Science B.V. All rights reserved.
163
Activity and selectivity of iron Fischer-Tropsch catalysts in a stirred tank slurry reactor Dragomir B. Bukur, Xiaosu Lang and Lech Nowicki a Department of Chemical Engineering Kinetics, Catalysis and Reaction Engineering Laboratory Texas A & M University College Station, TX 77843-3122, USA Summary Four catalysts prepared by two catalyst manufacturers (UCI and Ruhrchemie) were tested in a stirred tank slurry reactor to determine their performance during Fischer-Tropsch synthesis. Precipitated iron was used as the active metal, and Cu, K and SiO2 were used as promoters. The amounts of promoters (in parts per weight per 100 parts of iron) in these four catalysts were as follows: SiO2 = 4.1-25; K = 1.1-8.7 and Cu = 1.4-7.5. Total BET surface area varied between 136 and 290 m 2/g. The Ruhrchemie and two of the UCI's catalysts (runs SA-3391 and SA-2052) had low deactivation rates, whereas the UCI catalyst with low silica content (run SA-1532) deactivated more rapidly. The intrinsic activity of the low silica catalyst, measured by apparent first order reaction rate constant, was the lowest, whereas the Ruhrchemie catalyst had the highest activity (per gram of iron basis). Activity of the Ruhrchemie catalyst was higher after the CO pretreatment, than after the hydrogen reduction. The low silica catalyst favored production of high molecular weight products. Methane selectivity in run SA-1532 was only 2.2-2.4 wt%, and that of gaseous (C2 - C4) hydrocarbons 10-11 wt%. The extent of secondary reactions (1-olefin hydrogenation, isomerization and/or readsorption) was low on this catalyst. In general, hydrocarbon and olefin selectivities of the Ruhrchemie and the other two UCI catalysts were similar. Methane selectivity in tests with these three catalysts varied between 4.1 and 4.7 wt%, during the first 360 h of testing. 1. I N T R O D U C T I O N Fischer-Tropsch (FT) hydrocarbon synthesis from a coal derived synthesis gas is practiced on commercial scale at SASOL plants in South Africa in fLxed and fluidized bed reactors, utilizing potassium promoted iron catalysts. The fluidized bed reactors have high selectivity togasoline range products, whereas fixed bed reactors produce predominantly diesel fuel and hydrocarbon waxes. This variation in product distribution is achieved through the use of different process conditions and catalysts [ 1]. Several studies directed at iron FT catalyst development and performance evaluation in bench scale slurry reactors were conducted at government, industrial and university laboratories in the United States since 1980's [2-8]. In 1992 the United States Department of a Present address: Institute of Chemical & Process Eng., Lodz Technical University, 90-924 Lodz, POLAND. The work supported by U. S. DOE (Contract DE-AC22-89PC89868) and Texas Engineering Experiment Station.
164
Energy and industrial partners (Air Products, Exxon, Shell, Statoil and UOP) sponsored a FT demonstration run in a 0.57 m in diameter bubble column slurry reactor at LaPorte, Texas [8]. The catalyst for this run, designated L-3950, was manufactured by United Catalysts, Inc. (UCI). Three trial batches of precipitated iron catalysts, prepared by UCI, were evaluated at UOP and Texas A&M University. Here, we report results on catalyst activity, stability and product distribution (hydrocarbon selectivity and olefin selectivities as a function of carbon number) from these tests in a stirred tank slurry reactor. Selected results from our studies [9, 10] with a precipitated iron catalyst synthesized by Ruhrchemie, which was used originally in fixed bed reactors at SASOL, are also presented here for comparison. 2. EXPERIMENTAL
Experiments were conducted in a 1 dm 3 stirred tank slurry reactor (Autoclave Engineers). The feed gas flow rate was adjusted with a mass flow controller and passed through a series of oxygen removal, alumina and activated charcoal traps to remove trace impurities. After leaving the reactor, the exit gas passed through a series of high and low (ambient) pressure traps to condense liquid products. High molecular weight hydrocarbons (wax), withdrawn from a slurry reactor through a porous cylindrical sintered metal filter, and liquid products, collected in the high and low pressure traps, Were analyzed by gas chromatography. The reactants and noncondensible products leaving the ice traps were analyzed on an on-line GC (Carle AGC 400) with multiple columns and both flame ionization and thermal conductivity detectors [8, 10, 12]. Compositions, BET surface areas and pore volumes of catalysts synthesized by UCI, and Ruhrchemie AG are listed in Table 1. In all tests a catalyst was crushed and sieved to either 270/325 mesh (44-53 I.tm in diameter) or less than 270 mesh size, prior to loading to a reactor. A pre-purified normal octacosane was used as the liquid (slurry) medium in tests of the Ruhrchemie catalyst, whereas Ethylflo 164 oil (a hydrogenated 1-decene homopolymer - C 30 obtained from Ethyl Co.) was used in tests of UCI catalysts. Table 1 Physico -Chemical Properties and Test Desil~nations for UCI and Ruhrchemie Catalysts Run number SA-3391 SA-1532 SA-2052 SA-0888/ SB-1370 Catalyst UCI L-3950 UCI 1207UCI L-3950 Ruhrchemie designation 175A (batch 3) LP 33/81 Catalyst composition Fe 100 100 100 100 Cu 4.3 1.4 7.5 5 K 8.7 1.1 2.6 4.2 SiO2 17 4.1 9.2 25 Surface area 251 167 136 290 (m2/g) Pore volume (cm3/g)
0.50
0.18
0.30
0.62
The UCI catalysts were pretreated in situ with synthesis gas (H2/CO molar feed ratio of about 0.7) at 280~ for 12 hours. After the pretreatment the catalysts were tested at: 265~ 2.1 MPa, 2.4 N1/g-Fe/h (where, Nl/h, denotes volumetric gas flow rate at standard temperature
165 and pressure) and H2/CO = 0.7. The Ruhrchemie catalyst was pretreated in situ either with CO at 280~ for 16 h (run SA-0888), or with hydrogen at 220~ for 1 h (run SB-1370). Baseline process conditions in tests of the Ruhrchemie catalyst were: 250~ 1.48 MPa, 3.8 Nl/g-Fe/h, and H 2/CO = 0.67. 3. RESULTS AND DISCUSSION 3.1 Catalyst Activity and Stability Variation of syngas conversion with time for all three tests with UCI catalysts is shown in Figure 1. Results from periods in which catalysts were not tested at the baseline conditions, are not shown here (runs SA-3391 and SA-2052). During the first 300 h of testing the (H2 + CO) conversion was the highest in test SA-3391, whereas the lowest syngas conversion was obtained in test SA-1532. Activities of these three catalysts are compared in Figure 2, in terms of values of apparent first order reaction rate constant evaluated at 260~ The latter value was calculated from data at different temperatures by using the activation energy of 90 kJ/mol, which is a typical value for iron FT catalysts [12]. These results show that the catalyst in run SA-3391 was the most active, whereas the catalyst in run SA-1532 was the least active. Also, both catalysts deactivated with time, whereas in run SA-2052 the catalyst activity increased slightly with time. The most active catalyst (run SA-3391) had the highest total surface area, but the activity in the other two tests did not correlate with the total surface area of the catalyst (Table 1). The apparent rate constants from two tests of the Ruhrchemie catalyst (runs SA-0888 and SB-1370) are also shown in Figure 2 for comparison. Activity of the Ruhrchemie catalyst is markedly higher than that of the UCI catalysts, regardless of the pretreatment procedure employed (CO activation in test SA-0888, and H2 reduction in SB1370), however the catalyst was more active after the CO pretreatment. 500
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Figure 1. Effect of time on stream on (H2+CO) Conversion (a) and methane Selectivity (b) for UCI Catalysts. Values
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Figure 2. Apparent first order reaction constant of Ruhrchemie and UCI catalysts as a function of time on stream. ratio,
UR,
and
partial
pressure
quotient,
Kp = Pco2P,2/PcoP,2o, provide information on relative rates of water-gas-shift (WGS)
166
reaction (lower usage ratios or higher values of Kp correspond to higher WGS activity). Catalyst used in run SA-3391 had the highest WGSactivity (UR = 0.56-0.62, Kp = 14-22), whereas the catalyst used in run SA-2052 had the lowest WGS activity (UR = 0.61-0.70, Kp = 3.6-9). The WGS activity of the Ruhrchemie catalyst during testing at 1.48 MPa, 265~ 3.8 N1/g-Fe/h, H2/CO = 0.67 was similar to that of the UCI catalysts (UR = 0.66, Kp = 4.4-11).
3.2 Hydrocarbon Product Distribution Variation of methane selectivity (mol % C basis) with time, for all three UCI catalysts, is shown in Figure 1b, whereas average values of hydrocarbon selectivities (wt%) for the UCI and the Ruhrchemie catalysts are listed in Table 2. UCI catalyst with low silica and potassium contents (run SA-1532), had the lowest methane (2-2.3 mol%) and gaseous hydrocarbons selectivities. It favored the production of high molecular weight hydrocarbons, and its selectivity was fairly stable with time on stream. The other two UCI catalysts, used in runs SA-3391 and SA-2052, had similar selectivities, but both produced more methane and less high molecular weight products than the catalyst used in run SA-1532. Methane selectivities of these two catalysts were 3.5-4 mol% (4.1-4.4 wt%), whereas C12+ selectivities varied between 52 and 56 wt%. Average values of hydrocarbon selectivities obtained during initial 360 h of testing of the Ruhrchemie catalyst at the baseline conditions were similar to those obtained in runs SA-3391 and SA-2052. The Ruhrchemie catalyst was also tested at 265~ 1.48 MPa, 3.8 Nl/g-Fe/h and H2/CO=0.67. In both tests there was a significant shift toward lower molecular weight hydrocarbons. Methane selectivity increased to: 6.0 wt% (SB-1370) and 7.0 wt% (SA-0888), whereas C12+ selectivity decreased to: 42.9 wt% (SB1370) and 23.0 wt% (SA-0888). This shift toward low molecular weight products is not entirely due to higher reaction temperature, but also due to aging of the catalyst, since data at 265~ were obtained at 570- 590 h on stream [ 10]. Table 2 Comparison of Slurry Reactor Test Results with UCI and Ruhrchemie Catalysts Run numder SA-3391 SA-1532 SA-2052 S A - 0 8 8 8 SB-1370 Catalyst UCI L-3950 UCI 1207-175A UCI L-3950 Ruhrchemie Ruhrchemie (batch 3) Reduction H 2 / C O = 0 . 7 , H2/CO=0.7, H2/CO=0.7, CO, 16 h H2, 1 h Conditions 12 h, 280~ 12 h, 280~ 12 h, 280~ 280~ 220~ TOS (h) 227-322 35-301 140-276, 0-343 0-360 401-4.44 %CO Conv. 73-80 35-52 68-75 40-43 34-39 %(H2+CO) Conv. 71-75 33-49 65-72 43-46 38-41 Hydrocarbon Selectivities (wt-%): (1-I4 4.4 2.3 4.1 4.7 4.5 C2-C4 16.5 10.7 18.5 20.6 15.5 C5-C 11 23.6 16.2 22.4 23.2 27.7 C12+ 55.5 70.8 55.0 51.5 52.3 Test conditions for UCI Catalysts: 265~ 2.1 MPa, 2.4 Nl/g-Fe/h, feed H2/CO=0.7 Test conditions for Ruhrchemie Catalyst: 250~ 1.48 MPa, 3.8 Nl/g-Fe/h, feed H2/CO--0.67
3 . 3 0 i e f i n Selectivities Olefin selectivities of the UCI and Ruhrchemie catalysts are shown in Figure 3. Selectivities of catalysts used in runs SA-3391 and SA-2052 were nearly the same, and followed the usual trend of declining selectivity with carbon number, and relatively low
167
ethylene selectivity (60 wt%). Low silica catalyst, used in run SA-1532, had high ethylene selectivity (85%) and higher olefin content in high molecular weight hydrocarbons (C 11+ products) than the other two catalysts. Low hydrogenation activity of this catalyst is consistent with its low methane selectivity. Comparison of 2-olefin selectivities (which is a measure of the extent of 1-olefin isomerization) of the four catalysts at 95-187 h on stream is made in Figure 4. The 2-olefin selectivity of all three UCI catalysts was rather low. Catalysts used in runs SA-3391 and SA2052 had similar 2-olefin selectivities, and the 1-olefin isomerization activity increased with carbon number (C9+ products). The low silica catalyst (SA-1532) had the lowest 2-olefin content, and selectivity of C 10+ 2-olef'ms was fairly constant (--10%). Total olefin selectivity (C10+ hydrocarbons) of the Ruhrchemie catalyst was lower, whereas the 2-olefin selectivity was higher than that of the UCI catalysts used in runs SA3391 and SA-2052 (Figures 3 and 4). 90
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SA-0888, 95 h (Catalyst Ruhrchemir LP 33/81) SA-3391, 187 h (Catalyst: UC[ I.,-3950) SA-1532, 164 h (Catalyst: tiC[ 1270-175A) SA-2052, 168 h (Catalyst: tiC[ L-3950, batch 3)
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Figure 4. Comparison of 2-olefin selectivities of UCI and Ruhrchemie catalysts.
168 As can be seen in Figure 3, the olefin selectivity dependence on carbon number of hydrocarbon products passes through a maximum. This is believed to be due to secondary reactions of l-olefins [ 13, 14]. Ethylene is more reactive than other low molecular weight olefins, whereas the decrease in olefin content with increase in molecular weight has been attributed to their greater adsorptivity [ 14, 15], higher solubility resulting in longer residence time in a slurry reactor [2, 16], lower diffusivities [ 17], and/or 1-olefin readsorption [ 18-20]. The increase in 2-olefin selectivity with increase in carbon number is due to the same factors which affect the total olefin selectivity. The longer residence time of high molecular weight 1-alkenes either in the catalyst pores or in the reactor itself increases probability for secondary reactions (1-olefin hydrogenation, isomerization and readsorption). REFERENCES
2 3.
.
8. 9.
10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
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