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Ind. Eng. Chem. Res. 2005, 44, 6038-6044
Comparative Study of an Iron Fischer-Tropsch Catalyst Performance in Stirred Tank Slurry and Fixed-Bed Reactors† Dragomir B. Bukur,* Xiaosu Lang,‡ and Lech Nowicki§ Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843
Fischer-Tropsch synthesis (FTS) was studied on a precipitated iron catalyst in a fixed-bed reactor under both “supercritical” (propane or n-hexane as supercritical fluids; P ) 4.1-7.0 MPa) and conventional operating conditions (P ) 1.48 MPa), as well as in a stirred tank slurry reactor (STSR) under conventional conditions. Catalyst activity in the supercritical mode of operation was slightly higher than during conventional FTS in the fixed-bed reactor (FBR). This is attributed to higher diffusivities of reactants in supercritical fluids relative to the conventional mode of operation (pores filled with liquid hydrocarbon wax). Catalyst activity was significantly lower in the STSR than in the FBR, due to incomplete reduction of the catalyst in the STSR. Hydrocarbon selectivities in the FBR were similar in both modes of operation. Methane selectivity in the STSR was lower and C5+ selectivity higher than in the FBR, due to better temperature control in the former and differences in the extent of mixing and 1-olefin readsorption. Total olefin selectivity of high molecular weight olefins was lower and 2-olefin selectivity was higher in the STSR than in the FBR at similar conversions. This indicates that olefin selectivity is not controlled by intraparticle diffusion. It appears that the olefin selectivity in the STSR is largely determined by the residence time of high molecular weight 1-olefins in the liquid phase, 1-olefin solubility dependence on carbon number, and adsorption/desorption phenomena at the catalyst surface. Introduction Fischer-Tropsch synthesis (FTS) has been practiced on a large scale by Sasol in South Africa since the mid 1950s in tubular fixed-bed reactors (TFBRs) and circulating fluidized-bed reactors (CFBRs), using promoted iron catalysts.1 Commercial scale conventional bubbling fluidized-bed reactor and slurry bubble column reactor (SBCR) were placed on stream by Sasol in 1989 and 1993, respectively. These reactors are less expensive to construct, maintain, and operate than the tubular fixedbed or circulating fluidized-bed reactors.2,3 Mike Dudukovic and his research group have made numerous and significant contributions in development of experimental and modeling tools for analysis of complex hydrodynamic behavior of SBCRs that are essential in the SBCR scale-up. Each of these reactor types is employed to produce a certain type of products and has some limitations. TFBRs are used for production of high molecular weight hydrocarbons (waxes) and diesel fuel, whereas CFBRs are used for production of gasoline. These two types of reactors have a rather narrow range of operating conditions with respect to fresh feed composition and † Festschrift in honor of Milorad (Mike) Dudukovic on the occasion of his 60th birthday. * To whom correspondence should be addressed: Professor Dragomir B. Bukur, Department of Chemical Engineering, 3122 Texas A&M University, College Station, TX 77843-3122. Phone: (979) 845-3401. Fax: (979) 845-6446. E-mail: d-bukur@ tamu.edu. ‡ Present address: Dr. X. Lang, Department of Chemical Engineering, University of Saskatchewan, Saskatoon, S7N 0W0 Canada. § Present address: Dr. L. Nowicki, Faculty of Process and Environmental Engineering, Technical University of Lodz, 90924 Lodz, Poland.
the reaction temperature. They are not suitable for direct processing of synthesis gas produced in modern coal gasifiers, where the hydrogen to carbon monoxide molar ratio is 0.5-0.7. Heat-transfer removal is a major problem with TFBRs, and thus they operate at low temperatures, low single pass conversions, and high recycle ratios. Also, waxy products formed during synthesis accumulate in the catalyst pores, decreasing catalyst effectiveness. CFBRs operate at high temperatures (high reaction rates) in order to avoid formation of high molecular weight products, which would wet catalyst particles and have adverse effect on their fluidization properties. Therefore, yields of methane and light gases are high in CFB reactors, and this has a negative impact on the process economics. Slurry processing provides the ability to more readily remove the heat of reaction, minimizing temperature rise across the reactor and eliminating localized hot spots. As a result of the improved temperature control, yield losses to methane are reduced and catalyst deactivation due to coking is decreased. Reaction rates are lower in slurry phase reactors, relative to those in gasphase reactors, due to low reactant concentrations (solubility limitation) and low diffusivities of reactants and products in a liquid medium (mass-transfer limitations). To overcome some of the limitations of conventional FTS, several research groups have conducted studies in supercritical fluids.4-12 Reported advantages of supercritical FTS operation (SFTS) include the following: (a) in situ extraction of high molecular weight products (wax) from the pores, resulting in higher catalyst activity and longer catalyst life; (b) higher olefin selectivities compared to conventional FTS in a fixed-bed reactor; and (c) increased production of high molecular weight hydrocarbons.
10.1021/ie0492146 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/15/2005
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In the present study we present for the first time a comparison of catalyst performance (activity and selectivity) in three modes of operation: (1) conventional FTS in the fixed-bed reactor (FBR); (2) SFTS in the FBR, and (3) FTS in a stirred tank slurry reactor (STSR). For this purpose we utilize experimental data from multiple tests in fixed-bed reactors to illustrate reproducibility and reliability of catalyst performance. We include both data reported previously6-7,13-14 as well as the new ones. Reaction studies in these three modes of operation result in significant differences in reactant and product diffusivities (supercritical vs conventional FTS) and catalyst particle sizes employed (FBR vs STSR). Since the intraparticle diffusion resistance depends strongly on the catalyst particle size and reactant diffusivities, the obtained results provide means to assess the effect of intraparticle diffusion on olefin selectivity and product distribution. Experimental Section Reactor System and Operating Procedures. Experiments were conducted in a conventional downflow fixed-bed reactor embedded in an aluminum block with a two-zone heater. Slurry-phase FTS experiments were conducted in a 1 dm3 reactor (Autoclave Engineers). The feed gas (premixed synthesis gas from Iweco Inc.; >99.7% purity) flow rate in both systems was adjusted with a mass flow controller and passed through a series of oxygen removal, alumina, and activated charcoal traps to remove trace impurities. For FBR experiments in a supercritical fluid, either propane or n-hexane (Phillips 66 Co.; 99% purity) was pumped from a liquid dip tube tank using a metering pump (American Lewa, Inc.; Model FMCK-1 or Milton Roy miniPump - Model VS). The feed was premixed and preheated before entering the reactor. After leaving the reactor, the exit gas passed through a series of high and low (ambient) pressure traps to condense liquid products. Noncondensable gases (CO, H2, CO2, and C5- hydrocarbons) were analyzed by on-line gas chromatography (Chandler Engineering Co., Carle AGC 400 series). High molecular weight hydrocarbons (wax), collected in a high-pressure trap (fixed-bed reactor) or withdrawn from the slurry reactor through a porous cylindrical sintered metal filter, and liquid products, collected in a low-pressure ice trap, were analyzed by capillary gas chromatography (Varian 3400 series). Detailed description of our reactor systems, product analysis system, and operating procedures can be found elsewhere.15-17 A precipitated iron catalyst used in this study was synthesized by Ruhrchemie AG (Oberhausen-Holten, Germany),18 and its nominal composition is 100 Fe/5 Cu/ 4.2 K/25 SiO2 (mass basis). Catalyst was calcined in air at 300 °C for 5 h, and then crushed and sieved to about 0.25-0.50 mm (32-60 mesh), for tests in the fixed-bed reactor, or to less than 44 µm (325 mesh) for experiments in the slurry reactor. A purified n-octacosane (∼260 g) was used as the initial liquid medium in the slurry reactor (run SB 1370). The catalyst was reduced in situ in the FBR tests with hydrogen at 220 °C for 1 h, atmospheric pressure, and linear gas velocity of about 150 cm/s (4-5 NL/min; NL ) normal liter at standard temperature, 0 °C, and pressure, 1 bar), as suggested by the catalyst manufacturer. The same reduction temperature and duration were employed in the slurry reactor test SB-1370, at 0.8 MPa and hydrogen flow rate of 7.5 NL/min. Following reduction the catalyst was
Table 1. Comparison of the Ruhrchemie Catalyst Performance in Different Modes of Operation (Process Conditions: 250 °C, H2/CO ) 0.67, P(H2 + CO) ) 1.48 MPa, 2 NL (syngas)/g-cat‚h) conventional FTS
CO conversion (%) (H2 + CO) conversion (%) apparent rate constant (mmol/g-Fe/h/MPa) hydrocarbon selectivity (wt %) CH4 C2-C4 C5-C11 C12+ a
supercritical FTS
fixed-bed reactor
slurry reactor
fixed-bed reactor
56.8 ( 4.2a 57.3 ( 4.2
37.9 ( 2.0 38.9 ( 1.6
56.9 ( 7.3 56.6 ( 6.6
196 ( 17.6
143 ( 7.2
210 ( 21.5
6.0 ( 1.1 17.4 ( 1.5 24.1 ( 3.0 52.5 ( 5.2
4.8 ( 0.5 16.4 ( 1.1 29.1 ( 3.1 49.7 ( 4.1
6.2 ( 0.9 15.8 ( 1.3 23.0 ( 2.5 55.0 ( 3.5
Mean ( standard deviation.
tested at baseline process conditions of 1.48 MPa, 250 °C, 2 NL/g-cat‚h, and H2/CO ) 0.67 (conventional FTS in the FBR and STSR). In several FBR runs after a period of testing at the baseline conditions, the total pressure was increased to 5.5 or 7 MPa in runs with propane (FA-0844, FA-1724, and FA-1075) or to 4.1 MPa in run FA-2984 with n-hexane, while keeping the partial pressure and the flow rate of the syngas at the baseline conditions. Hexane flow rate, as a liquid, was 1 mL/min. In all these four tests the total reaction pressure and reaction temperature (250 °C) were above critical properties of pure propane (369.8 K, 4.25 MPa) or n-hexane (507.5 K, 3.01 MPa), and this is referred to as “supercritical” FTS (SFTS). However, by this we are not implying that the reaction mixture was a single-phase supercritical fluid. Results Fixed-Bed Reactor TestssConventional FTS. Results from four FBR tests (runs FA-3143, FA-0844, FA-2984, and FA-1075) at the baseline conditions (i.e., without propane or n-hexane in the feed) are summarized in Table 1, and olefin selectivities from these tests are shown in Figure 1. Experimental data were obtained at the following times on stream: 40-70 h (runs FA-3143, FA-2984, and FA-1075) and 240-260 h (run FA-0844). Mean (average) values from the four tests and the corresponding standard deviations for each of the measured quantities (CO and syngas conversions, apparent reaction rate constant, and lumped hydrocarbon selectivities: CH4, C2-C4, C5-C11, and C12+) are listed in Table 1. Mean values from all four tests are representative of catalyst performance in the FBR at the baseline conditions. Standard deviations as a percentage of the mean value range from 7.3% (for conversions) to 15.9% (methane selectivity) and are indicative of very good reproducibility of experimental data. Differences in total olefin content (defined as 100 × linear olefins/(linear olefins + n-paraffins)) and 2-olefin content (100 × 2-olefin/(1-olefin + 2-olefin)) among different tests are small (except for ethylene selectivityFigure 1a) and the mean values are representative of the catalyst performance in the FBR at the baseline process conditions. Fixed-Bed Reactor TestssFTS in “Supercritical” Fluids. Four tests were conducted with a feed consisting of synthesis gas (H2/CO ) 0.67) and either propane (runs FA-0844, FA-1724, and FA-1075) or n-hexane (FA-
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Figure 1. Total olefin (a) and 2-olefin selectivity (b) in the fixedbed reactor tests at 250 °C, 1.48 MPa, 2 NL/g-cat‚h, and H2/CO ) 2/3. Table 2. Process Conditions for Fixed-Bed Reactor Tests with Supercritical Fluids (Other Process Conditions: 250 °C, 2 NL (syngas)/g-cat‚h, H2/CO ) 2/3) test ID/SCF
FA-0844 propane
FA-1724 propane
FA-2984 hexane
FA-1705 propane
total pressure (MPa) syngas (MPa) SCF (MPa) densitya (kg/m3) TOS (h)
7.0 1.48 5.52 67 92-215
5.5 1.48 4.02 49 70-140
4.1 1.48 2.62 68 70-190
5.5 1.48 4.02 49 220-300
a Density of feed calculated at reaction conditions using SoaveRedlich-Kwong EOS.
2984). Process conditions and estimated values of feed (syngas plus propane or n-hexane) densities at reaction conditions are shown in Table 2. Average values and the corresponding standard deviations of activity parameters and lumped hydrocarbon product distribution from these four tests are shown in Table 1. Variations of experimental data among different tests were generally small, and the highest variability was obtained for methane selectivity (standard deviation: 14.5% of the mean value). Total olefin content (selectivity) and 2-olefin content as a function of carbon number for these four tests are shown in Figure 2. A solid line was drawn through average values from these four tests. Differences in olefin selectivities between different tests were generally small, and the corresponding mean values are representative of the catalyst performance in the fixed-bed reactor under “supercritical” conditions. Additional
Figure 2. Total olefin (a) and 2-olefin selectivity (b) during supercritical FTS in the fixed-bed reactor tests at 250 °C, 4.1-7 MPa, 2 NL (syngas)/g-cat‚h, and H2/CO ) 2/3.
results from tests FA-0844 and FA-1724, during both conventional and SFTS can be found elsewhere.6-7 FTS in the Stirred Tank Slurry Reactor. In STSR test SB-1370 the baseline conditions (1.48 MPa, 250 °C, 2 NL/g-cat‚h, and H2/CO ) 0.67) were maintained during the first 350 h on stream, and five mass balances were performed during this time period. Average values (and the corresponding standard deviations) from these five balances are shown in Table 1, whereas total olefin and 2-olefin selectivity changes with time are shown in Figure 3. Catalyst activity (not shown) and 2-olefin selectivity (Figure 3b) increased slightly with time, whereas hydrocarbon selectivity shifted toward lower molecular weight products.13 However, the magnitude of these changes was rather small (as evidenced by reported values of standard deviations in Table 1) and the mean values from five mass balances will be used for comparison with results obtained in the fixed-bed reactor. Discussion Catalyst Activity and Selectivity Comparisons. Average values of CO and syngas conversions from four FBR tests in conventional mode of operation were nearly the same as the corresponding average values obtained under supercritical conditions. Apparent reaction rate constant (based on the assumption that FTS reaction rate is first order in hydrogen) was slightly higher (∼7%) in the supercritical mode of operation, due to higher diffusivities of reactants in supercritical fluids relative to the conventional mode of operation (pores filled with liquid hydrocarbon wax). The highest values of conver-
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Figure 3. Total olefin (a) and 2-olefin selectivity changes with time in the STSR test SB-1370 at 250 °C, 1.48 MPa, 2 NL/g-cat‚ h, and H2/CO ) 2/3.
sions and apparent rate constant were obtained in run FA-2984 with “supercritical” n-hexane, which may be attributed to partial extraction of hydrocarbon wax from the catalyst pores. This results in higher diffusivities of reactants and thus higher reaction rates. Syngas and CO conversions were significantly lower in the STSR in comparison to those in the FBR, due to lower catalyst activity and differences in flow patterns (perfect mixing vs plug flow behavior). A possible reason for lower catalyst activity in run SB-1370 is that the degree of catalyst reduction was lower in a slurry reactor than in the fixed-bed reactor.13 Catalyst activity in run SB1370 continued to increase with time-on-stream due to creation of active sites during FTS.13 Recently,14 we were able to improve activity of the Ruhrchemie catalyst in the STSR by employing more severe reduction conditions (i.e., H2 reduction at 250 °C for 4 h). From experimental data in run SB-2886 at 260°C, 1.48 MPa, 2.1 NL/g-cat‚h, and H2/CO ) 0.67, the estimated value of the apparent reaction rate constant at 250 °C was 186 mmol/(g-Fe/h/MPa). This is consistent with the hypothesis that low activity in run SB-1370 was due to incomplete reduction of the catalyst. Methane selectivity in the STSR was lower than in the fixed-bed reactor tests, primarily due to better temperature control in the former. This cannot be ascribed to differences in conversion levels in STSR and FBR tests as shown by comparing methane selectivities in STSR test SA-2866 with the average value from FBR tests. Methane selectivity (5.3 wt %) in run SB-2886 was lower than the average value in FBR tests (6.2 wt %) at comparable levels of syngas conversions (61.8% in SA-
2866 vs 56.6% in FBR tests) even though the reaction temperature in the STSR test was 10 °C higher than that in the FBR tests. Gasoline yield (C5-C11 hydrocarbons) was higher in the STSR test SB-1370 (29%) than that in the fixed-bed reactor tests (∼23-24%), whereas C5+ (liquid plus wax products) selectivity was similar in all three modes of operation (∼79% in the STSR vs 76-78% in the fixed bed). Hydrocarbon product distributions in FBR tests were similar in both modes of operation. Several researchers reported more pronounced effects of supercritical operation on the product distribution and catalyst activity under operating conditions that result in liquid-like densities of supercritical fluids.8-12 For example, Bochniak and Subramanian8 tested the Ruhrchemie catalyst at 240 °C, H2/CO ) 1/2, and total pressure between 3.5 and 7 MPa using n-hexane as the supercritical fluid. They reported a significant increase in the catalyst activity and 1-olefin selectivity with increase in pressure and found that carbon number distribution could be described by a single value of the chain growth parameter. These results were interpreted in terms of enhanced diffusivities of reactants and products and enhanced desorption of 1-olefins from the catalyst surface. Huang and Roberts12 reported enhancement in syngas conversion, 1-olefin selectivity, and heavy hydrocarbons production on alumina-supported cobalt F-T catalyst during supercritical FTS (n-hexane as SCF; T ) 250 °C, H2/CO ) 2/1, and P ) 3.5-8 MPa) relative to conventional (gas-phase) FTS. Jacobs et al.11 used a mixture of hexane and pentane (55/45 wt %) as a supercritical solvent in their study of FTS at 220°C, H2/CO ) 2/1, and 8.24 MPa over an alumina-supported cobalt catalyst. They reported enhanced activity and olefin selectivity, longer catalyst life, and lower methane selectivity in comparison to FTS without addition of supercritical fluid. However, they did not observe any increase in the chain growth probability as reported by Huang and Roberts12 and Fujimoto and co-workers.9,10 It should be noted that enhancements in catalyst activity, stability with time, and/or increased selectivity to desired products obtained during supercritical phase operation need to be weighed against increased capital and operating costs due to operation at higher pressure and recycling of a supercritical fluid. As shown in Figures 1-3 total olefin content dependence on carbon number of hydrocarbon products passes through a broad maximum (C3-C5 range). This shape is believed to be due to either secondary hydrogenation of l-olefins19,20 or 1-olefin readsorption and subsequent chain termination to paraffins.21-23 Ethylene is more reactive than other low molecular weight (MW) olefins, whereas the decrease in olefin content with increase in molecular weight (carbon number) has been attributed to one or more of the following factors: greater adsorptivity of higher molecular weight olefins;24-25 increased solubility with increased MW resulting in longer residence time in a slurry reactor;19-20,26 and/or diffusionenhanced 1-olefin readsorption.23,27,28 Higher olefin isomerization activity with increase in carbon number is due to the same factors that affect the 1-olefin hydrogenation and/or readsorption activity. Longer residence time of high molecular weight 1-olefins either in the catalyst pores or in the reactor itself increases probability for secondary reactions (hydrogenation, isomerization, and/or 1-olefin readsorption followed by chain termination to either n-paraffins or 2-olefins).
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Figure 4. Comparison of olefin selectivities in different modes of operation.
Comparison of olefin selectivities in different modes of operation is shown in Figure 4. Total olefin content was the highest (Figure 4a) and the 2-olefin content was the lowest (Figure 4b) in the “supercritical” mode of operation. This was particularly evident for higher molecular weight products, and it is related to the ability of “supercritical” fluids (propane or n-hexane) to facilitate desorption of 1-olefins and remove some wax from the catalyst pores. The latter results in higher diffusivities and faster removal of high MW 1-olefins from the pores, thus reducing the extent of secondary reactions. Olefin selectivities during slurry-phase operation were generally between values obtained during conventional and SFTS in the fixed-bed reactor. Syngas conversion in the STSR test SB-1370 (∼39%) was markedly lower than that obtained in the fixed-bed reactor tests (∼57%), and this is expected to favor primary reactions (i.e., more 1-olefins and less 2-olefins). Comparison of olefin selectivities in the fixed-bed reactor and the STSR at comparable conversions is shown in Figure 5. The syngas conversion in the STSR test (SB2886) at 260 °C, 1.48 MPa, 2.1 NL/(g-cat‚h), and H2/CO ) 0.67 was about 62%. The total olefin content was lower and the 2-olefin content was higher than the corresponding values obtained during conventional FTS in the fixed-bed reactor at 57% conversion. Relatively small differences in the reaction temperature (250 °C vs 260 °C) in these two tests are not expected to have a marked effect on olefin selectivities, based on results from our study with the Ruhrchemie catalyst in the fixed-bed reactor.7 However, differences in residence times of high molecular weight products in the STSR
Figure 5. Comparison of olefin selectivities in the fixed-bed reactor tests (250 °C) and the STSR test SB-2886 (260 °C) at 1.48 MPa, 2-2.1 NL/g-cat‚h, H2/CO ) 2/3, and similar conversions (5762%).
and FBR may have had impact on olefin selectivities, as discussed at the end of the following section. In all modes of operation and reactors the H2/CO consumption ratio was nearly the same as the feed ratio, resulting in similar values of the exit H2/CO ratios. Thus, the catalyst was exposed to the same H2/CO ratio inside the reactor and observed hydrocarbon and olefin selectivities were not affected by differences in the reaction environments. Role of Intraparticle Diffusion. Exxon’s researchers have proposed an elegant and quantitative mathematical model for prediction of hydrocarbon product distribution (including olefin selectivity) based on a concept of intraparticle diffusion-enhanced 1-olefin readsorption.21,23,27-28 To check whether our experimental observations of olefin selectivity trends can be explained qualitatively in terms of intraparticle diffusional effects, one needs an estimate of Thiele modulus for 1-olefin readsorption reaction. Dimensionless Thiele modulus, Φ, is a measure of importance of intraparticle diffusional limitations (Φ , 1, negligible diffusional limitations, or kinetic regime; Φ . 1, strong diffusional limitations). According to Iglesia et al.27 and Iglesia,29 the Thiele modulus for 1-olefin readsorption reaction can be approximately expressed as
Φn ) Lcxkr,nθM/RpDn
(1)
where Lc ) pellet radius (m), kr,n ) readsorption rate constant of 1-olefin containing n carbon atoms (m3/
Ind. Eng. Chem. Res., Vol. 44, No. 16, 2005 6043 Table 3. Diffusivities of Selected n-Alkenes in FT Wax and Supercritical Fluids diffusivity, D12 × 109 (m2/s) fluid
1-octene
1-hexadecene
FT waxa propaneb n-hexaneb
5.9 89 124
2.8 51 78
a Diffusivities in FT wax are at 250 °C and 1.4 MPa. b Diffusivities in supercritical fluids are at 250 °C and fluid density of 70 kg/m3.
kmol)n-1 (s-1), Dn ) effective diffusivity of reacting species (m2/s), Rp ) pore radius (m), and θM ) metal site density (m-2). We do not have sufficient information to estimate numerical values of all parameters appearing in the expression for the Thiele modulus. However, an estimate of the ratio of Thiele moduli in different modes of operation can be obtained if one assumes that the pore radius and catalyst activity and site density (kr,n and θM) are the same in all modes of operation. For the fixedbed reactor tests operated under conventional and “supercritical” conditions, we have
ΦSFTS/ΦFBR ≈ x(Dn)wax/(Dn)SCF ≈ 0.2-0.3
(2)
where ΦSFTS ) Thiele modulus for the “supercritical” FTS, ΦFBR ) Thiele modulus for conventional FTS in the FBR, (Dn)wax ) effective diffusivity of 1-olefin in a liquid wax, and (Dn)SCF ) effective diffusivity of 1-olefin in a “supercritical” fluid. The characteristic length (particle size) was the same in both modes of operation, whereas the ratio of effective diffusivities is the same as the ratio of the corresponding molecular diffusivities. Molecular diffusivities of selected 1-olefins in a liquid wax and supercritical fluids (ethane, propane, and/or n-hexane) were experimentally determined at the Texas A&M University,30-32 and the reported range of values in eq 2 represents the ratio of molecular diffusivities of 1-olefins between C8 and C16 at the reaction temperature of 250 °C (Table 3). Similarly, the ratio of Thiele moduli for conventional FTS in the STSR and the FBR is approximately
As stated earlier, additional factors that may affect olefin selectivity and carbon number distribution are carbon number dependencies of adsorption/desorption of 1-olefins, physisorption of 1-olefins, and solubility of 1-olefins. All these factors result in preferential accumulation of high molecular weight hydrocarbons in the liquid phase that is determined by vapor-liquid equilibrium, and therefore in a longer residence time compared to lower molecular products that leave the reactor with the gas phase. In both reactors (STSR and FBR) the residence time of the liquid phase is much longer than that of the gas phase, and this increases probability for secondary reactions of high molecular weight olefins 1-olefins. Our results from FBR tests under conventional and “supercritical” conditions may be explained in terms of the diffusion-enhanced 1-olefin readsorption model of Exxon’s researchers27,28 and enhanced desorption of adsorbed primary products in a SCF.4,5 On the other hand olefin selectivity trends in STSR and FBR tests under conventional conditions (Figures 4 and 5) may be explained in terms of differences in residence times of liquid products in these two types of reactors. First, it should be noted that olefin selectivity of C6- products, which leave reactor preferentially with the gas phase, shows the trend that is consistent with the concept that 1-olefins are the primary products; i.e., olefin selectivity is somewhat higher in the STSR than in the FBR when the conversion is smaller (as in Figure 4) and selectivities are similar at comparable levels of conversion (Figure 5). However the olefin selectivity of C7+ products is lower in the STSR than in the FBR (at similar conversions). This may be attributed to a longer residence time of the liquid products in the STSR compared to the FBR. In the STSR the liquid products accumulate in the reactor and are periodically withdrawn from it every 48-100 hours, whereas in the downflow FBR they leave the reactor continuously by gravity. Also, the differences in mixing patterns (perfect mixing in the STSR vs plug flow in the FBR) may have had some effect on olefin selectivities. One would expect that the FBR would favor selectivity of the primary products. Conclusions
ΦSTSR/ΦFBR ≈ (dp)STSR/(dp)FBR ≈ 0.04-0.07
(3)
This estimate was obtained by assuming that the effective diffusivities, pore radius, and intrinsic catalyst activity are the same in both modes of operation. The average particle size for the fixed-bed tests was 375 µm, whereas for the STSR test it was approximately 15-25 µm (less than 44 µm). On the basis of these order of magnitude estimates, it follows that ΦSTSR < ΦSFTS < ΦFBR; i.e., the intraparticle diffusional limitations are expected to be the least significant for the slurry-phase operation. Therefore, if the diffusion-enhanced 1-olefin readsorption were the dominant factor in determining olefin selectivity, one would expect to obtain the highest total olefin selectivity and the lowest 2-olefin selectivity during the slurry-phase operation. However, this was not experimentally observed, and at similar syngas conversions (57-62%) the total olefin selectivity in the STSR was lower, and 2-olefin selectivity higher, than the corresponding selectivities in the FBR during conventional FTS (the highest value of the Thiele modulus).
We studied FTS reaction on the precipitated iron catalyst (LP 33/81) prepared by Ruhrchemie AG under conditions representative of industrial practice in two types of reactors (FBR and STSR). In FBR tests we studied the effect of the addition of inert hydrocarbons (propane or n-hexane), at total pressure and temperature that were above their corresponding critical values, on the catalyst performance. We found that the catalyst activity in the STSR (run SB-1370) was lower than that in the fixed-bed reactor when the catalyst was activated in H2 at 220 °C for 1 h. The use of more severe reduction conditions (250 °C for 4 h) in the STSR (run SB-2886) resulted in the catalyst activity similar to that obtained in the FBR with the catalyst reduced under less severe conditions. Methane selectivity was lower and C5+ yield higher in the STSR than in the FBR. This is attributed to better temperature control in the STSR and differences in the extent of mixing and 1-olefin readsorption in these two types of reactors. The total olefin content of high molecular hydrocarbons (C7+) was lower and 2-olefin content higher in the STSR than in the FBR at similar
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conversions. This is inconsistent with hypothesis that the intraparticle diffusion (i.e., diffusion-enhanced 1-olefin readsorption) controls the olefin selectivity. The observed trends in olefin selectivities in the STSR and FBR under conventional operating conditions can be explained in terms of increase in solubility and adsorptivity/physisorption with increase in molecular weight coupled with longer residence in the STSR.19,20,22,33-35 Introduction of “supercritical” fluids during the fixedbed reactor operation had the most pronounced effect on olefin selectivity, whereas the effects on the catalyst activity (slight increase) and carbon number distribution were relatively small. High total olefin content and low 2-olefin content, observed during “supercritical” FTS in the FBR, may be attributed to higher diffusivities of the primary reaction products (1-olefins) and their rapid removal (desorption) from the catalyst surface.4-9 Acknowledgment Financial support for this work by the U. S. DOE was provided through several grants and/or contracts. Literature Cited (1) Dry, M. E. The Fischer-Tropsch Synthesis. In CatalysisScience and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: New York, 1981; Vol. 1, pp 160-255. (2) Jager, B.; Espinoza, R. Advances in low-temperature Fischer-Tropsch synthesis. Catal. Today 1995, 23, 17. (3) Jager, B. Developments in Fischer-Tropsch Technology. Stud. Surf. Sci. Catal. 1997, 107, 219. (4) Yokota, K.; Fujimoto, K. Supercritical phase FischerTropsch synthesis reaction. 2. The effective diffusion of reactant and products in the supercritical phase reaction. Ind. Eng. Chem. Res. 1991, 30, 95. (5) Fan, L.; Yokota, K.; Fujimoto, K. Supercritical phase Fischer-Tropsch synthesis: catalyst pore size effect. AIChE J. 1992, 38, 1639. (6) Lang, X.; Akgerman, A.; Bukur, D. B. Steady-state FischerTropsch synthesis in supercritical propane. Ind. Eng. Chem. Res. 1995, 34, 72. (7) Bukur, D. B.; Lang, X.; Akgerman, A.; Feng, Z. Effect of process conditions on olefin selectivity during conventional and supercritical Fischer-Tropsch synthesis. Ind. Eng. Chem. Res. 1997, 36, 2580. (8) Bochniak, D. J.; Subramaniam, B. Fischer-Tropsch synthesis in near-critical n-hexane: Pressure-tuning effects. AIChE J. 1998, 44, 1888. (9) Fan L.; Fujimoto, K. Fischer-Tropsch synthesis in supercritical fluid: characteristics and application. Appl. Catal., A 1999, 186, 343. (10) Tsubaki, N.; Fujimoto, K. Product control in FischerTropsch synthesis. Fuel Process. Technol. 2000, 62, 173. (11) Jacobs, G.; Chaudhari, K.; Sparks, D.; Zhang, Y.; Shi, B.; Spicer, R.; Das, T. K.; Li, J.; Davis, B. H. Fischer-Tropsch synthesis: supercritical conversion using a Co/Al2O3 catalyst in a fixed bed reactor. Fuel 2003, 82, 1251. (12) Huang, X.; Roberts, C. B. Selective Fischer-Tropsch synthesis over an Al2O3 supported cobalt catalyst in supercritical hexane. Fuel Process. Technol. 2003, 83, 81. (13) Bukur, D. B.; Nowicki, L.; Patel, S. A. Activation studies with an iron Fischer-Tropsch catalyst in fixed bed and stirred tank slurry reactors. Can. J. Chem. Eng. 1996, 74, 399. (14) Ma, W.; Ding, Y.; Carreto-Vazquez, V.; Bukur, D. B. Study on catalytic performance and attrition strength of the Ruhrchemie catalyst for the Fischer-Tropsch synthesis in a stirred tank slurry reactor. Appl. Catal., A 2004, 268, 99. (15) Bukur, D. B.; Patel, S. A.; Lang, X. Fixed bed and slurry reactor studies of Fischer-Tropsch synthesis on precipitated iron catalyst. Appl. Catal. 1990, 61, 329.
(16) Bukur, D. B.; Lang, X.; Rossin, J. A.; Zimmerman, W. H.; Rosynek, M. P.; Yeh, E. B.; Li, C. Activation studies with a promoted precipitated iron Fischer-Tropsch catalyst. Ind. Eng. Chem. Res. 1989, 28, 1130. (17) Zimmerman, W. H.; Bukur, D. B. Reaction kinetics over iron catalyst used for Fischer-Tropsch synthesis. Can. J. Chem. Eng. 1990, 68, 292. (18) Frohning, C. D.; Rottig, W.; Schnur, F. The fixed bed synthesis. In Chemicals from Coal; Falbe, J., Ed.; Thieme: Stuttgart, 1977; pp 234-257. (19) Schulz, H.; Gokcebay, H. Fischer-Tropsch CO-hydrogenation as a means for linear olefins production. In Catalysis of organic reactions; Kosak, J. R., Ed.; Marcel Dekker: New York, 1984; pp 153-169. (20) Dictor, R.; Bell, A. T. Fischer-Tropsch synthesis over reduced and unreduced iron oxide catalysts. J. Catal. 1986, 97, 121. (21) Madon, R. J.; Reyes, S. C.; Iglesia, E. Primary and secondary reaction pathways in ruthenium-catalyzed hydrocarbon synthesis. J. Phys. Chem. 1991, 95, 7795. (22) Zimmerman, W.; Bukur, D.; Ledakowicz, S. Kinetic model of Fischer-Tropsch synthesis selectivity in the slurry phase. Chem. Eng. Sci. 1992, 47, 2707. (23) Madon, R. J.; Iglesia, E. The importance of olefin readsorption and H2/CO reactant ratio for hydrocarbon chain growth on ruthenium catalysts, J. Catal. 1993, 139, 576. (24) Anderson, R. B. Catalysts for the Fischer-Tropsch Synthesis. In Catalysis; Emmett, P. H. Ed.; Van Nostrand-Reinhold: New York, 1956; Vol. IV, pp 29-255. (25) Schulz, H.; Rosch, S.; Gokcebay, H. Selectivity of the Fischer-Tropsch CO-hydrogenation. In Coal: Phoenix of ’80s, Proc. 64th CIC. Coal Symp.; Al-Taweel, A. M., Ed.; Canadian Society for Chemical Engineering: Ottawa, Canada, 1982; Vol. 2, pp 486-493. (26) Donnelly, T. J.; Satterfield, C. N. Product distributions of the Fischer-Tropsch synthesis on precipitated iron catalysts. Appl. Catal. 1989, 52, 93. (27) Iglesia, E.; Reyes, S. C.; Madon, R. J.; Soled, S. L. Selectivity control and catalyst design in the Fischer-Tropsch synthesis: Sites, pellets and reactors. J. Catal. 1991, 129, 238. (28) Iglesia, E.; Reyes, S. C.; Madon, R. J. Transport-enhanced R-olefin readsorption pathways in Ru-catalyzed hydrocarbon synthesis. Adv. Catal. 1993, 39, 221. (29) Iglesia, E. Fischer-Tropsch synthesis on cobalt catalysts: Structural requirements and reaction pathways. In Natural Gas Conversion IV; de Pontes, M., Espinoza, R. L., Nicolaides, C. P., Scholz, J. H., Scurrell, M., Eds.; Elsevier Science: Amsterdam, The Netherlands. Stud. Surf. Sci. Catal. 1997, 107, 153. (30) Erkey, C.; Rodden, J. B.; Akgerman, A. Diffusivities of synthesis gas and n-alkanes in Fischer-Tropsch wax. Energy Fuels 1990, 4, 275. (31) Noel, J. M.; Erkey, C.; Bukur, D. B.; Akgerman, A. Limiting mutual diffusion coefficients of 1-octene and 1-tetradecene in nearcritical ethane and propane. J. Chem. Eng. Data 1994, 39, 920. (32) Eaton, A. P. Infinite dilution diffusion coefficients in supercritical fluids. Ph.D. Thesis, Texas A&M University, College Station, TX, 1996. (33) Kuipers, E. W.; Vinkenburg, I. H.; Oosterbeek, H. Chain length dependence of R-olefin readsorption in Fischer-Tropsch synthesis. J. Catal. 1995, 152, 137. (34) van der Laan, G. P.: Beenackers, A. C. M. Hydrocarbon selectivity model for the gas-solid Fischer-Tropsch synthesis on precipitated iron catalysts. Ind. Eng. Chem. Res. 1999, 38, 1277. (35) Schulz, H.; Claeys, M. Appl. Catal., A Kinetic modeling of Fischer-Tropsch product distributions. 1999, 186, 91.
Received for review August 26, 2004 Revised manuscript received December 20, 2004 Accepted December 29, 2004 IE0492146
