Ind. Eng. Chem. Res. 2005, 44, 9653-9658
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Membrane Application in Fischer-Tropsch Synthesis to Enhance CO2 Hydrogenation Martin P. Rohde,* Dominik Unruh, and Georg Schaub Engler-Bunte-Institut, Bereich Gas, Erdo¨ l und Kohle, Universita¨ t Karlsruhe (TH), Engler-Bunte-Ring 1, D-76131 Karlsruhe, Germany
The present work experimentally demonstrates that the conversion of CO2 to long-chain hydrocarbons via CO2 shift and Fischer-Tropsch reaction can be enhanced by in situ H2O removal. The experiments are carried out in a packed-bed reactor with an integrated silica membrane. Though the membrane exhibits low permselectivities regarding H2O under FischerTropsch conditions, the deficiencies in permselectivity can be overcome by the choice of H2 and H2/CO2 as the sweep-gas. The increase in conversion and yield is attributed to the removal of H2O, which is shown by experimental data analysis based on a reaction engineering type model of the inert membrane packed-bed reactor. 1. Introduction Water vapor is the main by-product of the FischerTropsch (FT) synthesis, and at high synthesis gas conversions, high H2O partial pressures appear in the vapor phase. The effects of water vapor on the FT catalysts and kinetics have been studied by many authors and can be summarized as follows: In the case of cobalt-based (Co-based) catalysts, kinetic studies showed significant deactivation by reoxidation at high H2O/H2 partial pressure ratios and high H2O absolute pressures.1,2 Dry3 reported that iron-based (Fe-based) catalysts also deactivate due to oxidation by CO2 and H2O, but additionally, water vapor strongly inhibits the FT reaction rate, e.g., see refs 4-6. Thus, the selective removal of H2O during the FT synthesis could be helpful in reducing catalyst inhibition and deactivation, and furthermore, it could result in increased per pass conversions through a higher partial pressure of the reactants and higher residence times. The in situ removal of H2O during the FT synthesis by means of hydrophilic membranes was proposed by Espinoza et al.7,8 Espinoza et al. carried out permeation experiments with several types of hydrophilic zeolite membranes (mordenite/ZSM-5 on an R-Al2O3 support) under conditions typical for FT (200-300 °C and 2 MPa). Fischer-Tropsch experiments with integrated membranes were not performed. In a recent review paper, Coronas and Santamaria9 classified the case of “in situ removal of H2O during the FT synthesis” as a conversion enhancement by the removal of catalyst poisons and inhibitors, but for Febased catalysts, this case should also be classified as “conversion (and yield) enhancement by equilibrium displacement”. Fe-based catalysts in general and potassium-promoted (K-promoted) ones in particular are active for the CO/CO2 shift reaction. In the presence of H2O, CO is converted to CO2 under typical FT conditions and the hydrocarbon yield declines (see Scheme 1). However, CO2 can be converted to CO by the reverse shift reaction, and in a subsequent step, the intermedi* To whom correspondence should be addressed. Tel.: +49-(0)721 6083172. Fax: +49-(0)721 606172. E-mail:
[email protected].
Scheme 1. Reaction Network of CO2 Hydrogenation Proposed by Riedel et al.10 for K-promoted, Fe-based Catalysts: (1) FT Reaction, (2) CO2/CO Shift Equilibrium, and (3) Direct CO2 Hydrogenation
ate CO reacts via the FT reaction to hydrocarbons. Earlier investigations by Riedel et al.10 regarding CO2 hydrogenation under FT conditions on a K-promoted, Fe-based catalyst showed that conversion of CO2 to longchain hydrocarbons is possible and that the main reaction path of CO2 hydrogenation is a two-step mechanism with CO as the intermediate, as shown in Scheme 1. The direct CO2 hydrogenation is not significant. Unruh et al.11 showed in a modeling study that the selective removal of H2O during the FT reaction could displace the CO/CO2 shift equilibrium in favor of CO and enhance the hydrocarbon yield and the conversion of CO2 to long-chained hydrocarbons. Practical interest in CO2 conversion during the FT synthesis results from CO2 being a potential constituent of synthesis gases from either coal, biomass, or natural gas.12 On the basis of the earlier work on CO2 hydrogenation by Riedel10,13 and on modeling studies performed by the authors,11,14 the present work focuses on the experimental demonstration of membrane integration for H2O removal in a FT lab-scale fixed bed reactor. For this purpose, synthesis gas with H2/CO2 (3/1) is chosen, because the largest measurable effects are expected for this composition. The experimental setup is shown in Figure 1. The tubular membranes used (provided by ACA, Berlin) consist of commercial ultrafiltration supports (γ-Al2O3 on R-Al2O3, dpore < 5 nm) coated with two (γ-138) or three (γ-143) layers of tetraethyl orthosilicate (TEOS). Polymeric SiOx structures in the pores reduce the pore diameters below 1 nm. Because of steric reasons, some Si-OH groups do not crosslink, and the functional layer becomes hydrophilic. Estimated from
10.1021/ie050289z CCC: $30.25 © 2005 American Chemical Society Published on Web 06/15/2005
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molecular diffusion, Knudsen diffusion, and/or viscous flow. The permeation experiments at elevated temperatures reveal that the functional silica layer exhibits irreversible changes of the pore structure. Instead of a detailed description of the individual transport mechanisms, a simple phenomenological approach was chosen. According to eq 1, the molar flow rate of component
N˙ i,tmb ) AMQi(PFyiF - PSyiS)
Figure 1. Photograph and schematic drawing of packed-bed inert membrane reactor (IMPBR) for Fischer-Tropsch synthesis.
single-gas experiments at 105 °C, the H2O/H2 permselectivities of these silica membranes are exceptionally high (between 100 and 350)15 due to capillary condensation and surface diffusion. The investigation of the permeability characteristics of the membranes under typical FT conditions is part of the present investigation. Experimental data analysis is based on reaction engineering type models. 2. Mathematical Model The mathematical models of the packed-bed reactor (PBR) and of the packed-bed inert membrane reactor (IMPBR) are based on the following observations and assumptions: Given temperature variations of max. (1-2 K along the axis, the lab-scale reactor systems are considered isothermal. Catalyst particle size is 200 °C, the initial permselectivities decrease until a stable situation is reached. These characteristics are due to chemical changes in the functional layer. With increasing temperature, the concentration of the OH groups on the silica surfaces decreases monotonically. Above 150 °C, hydrophilic silanol groups condense and form hydrophobic siloxane bridges.18 The fact that the permeances in the case with
Figure 3. Curves indicate calculated data: (s), PBR; (- - -), IMPBR. Symbols indicate measured data: (O,b), XCO2; (],[), XC; (4,2), YCO,CO2. See conditions and permselectivities in Tables 4 and 6 (γ-143) (sweep flow ratio Ψ ) 3.3 and sweep gas H2/CO2 (3/1)).
reaction are in general smaller than in the case without reaction suggests that the transport rate through the membrane may be lowered by a film of liquid hydrocarbons on the membrane surface. A gradual plugging of the membrane by wax could not be observed. 4.2. Effect of the Membrane. The effect of H2O removal during the FT synthesis with H2/CO2 (3/1) synthesis gas is plotted descriptively in Figure 3. The solid lines show the development of the conversion and yield along the reactor axis for the packed-bed reactor without membrane (PBR). The conversion and yield are calculated with the kinetics according to the procedure outlined in section 3.1. The CO yield passes through a maximum which confirms the two-step mechanism with CO as an intermediate (Scheme 1). The broken lines represent the evolution of the CO2 conversion and the CO yield in the packed-bed reactor with membrane (IMPBR) with H2/CO2 (3/1) as the sweep gas. By removing H2O via the membrane, the equilibrium is displaced and CO2 is converted to a larger content, leading to 50% higher conversions (closed symbols). This level of conversion at 250 °C with an integrated membrane is comparable to the conversion that Riedel et al.10 achieved with a similar catalyst (Fe/ Al2O3/Cu/K, 100/13/10/10 wt) at the same residence time τmod in a conventional PBR at 300 °C. Under the aspect of the production of long-chain hydrocarbons, lower temperatures are favorable to the distribution of the FT products. A significant difference between the product distributions (e.g., RASF and olefin content) in the IMPBR and the PBR was not observed, which is in accordance to the results of Riedel et al.10 on CO2 hydrogenation on Fe-based catalysts. However, because of the loss of CO across the membrane, the CO yield maintains rather high, resulting in a lower total carbon conversion/hydrocarbon yield. This fact suggests the need for membranes which offer even higher permselectivities regarding CO. 4.3. Effect of Sweep Flow Ratio. With increasing sweep flow rate, the permeating components are removed faster and to an enhanced degree out of the reaction zone due to lower partial pressures on the sweep side, resulting in higher conversions (Figure 4). The improved removal of H2O displaces the equilibrium; however, the loss of the intermediate CO also occurs to a larger extent. Therefore, the CO yield increases more strongly than the total carbon conversion, as the calculated curves show in Figure 4.
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Figure 4. Effect of sweep-gas ratio in IMPBR. Lines indicate calculated data with fixed permselectivities. Symbols indicate measured data: (sb), XCO2; (- -[), XC; (- - -2), YCO,CO2. See conditions and permselectivities in Tables 4 and 6 (γ-143).
is close to this optimal value. The effect on conversion can be amplified by increasing the sweep flow ratio. However, a reduction of the sweep pressure or the use of an inert sweep gas would lead to a strong decline of reactor performance due to reactant loss. In the case of the ideal membrane (Figure 5, curves 1 and 3), the selective removal of H2O results in significantly higher conversions and hydrocarbon yields compared to the membrane γ-143. Because there is no loss of reactants across the membrane, the conversion curves exhibit no maximum with increasing values of 1/Pe, and the effect of the membrane can be strongly reinforced as well by larger sweep flow rates and by decreased pressure on the sweep side (Figure 5, curves 1s3). The results of the ideal membrane show the potential of in situ H2O removal during the FT synthesis regarding the conversion of CO2 to long-chain hydrocarbons and the need for highly selective and permeable membranes. 6. Summary and Conclusions
Figure 5. Calculated data for an ideal membrane with SH2O,ref ) 0.76 (curves 1 and 3) and real membrane permselectivities (curves 2 and 4): (s), XCO2; (- - -) XC (see Table 6 (γ-143), sweep gas H2/ CO2 ) 3/1, sweep flow ratios for curves 1 and 2 Ψ ) 3.3, sweep flow ratios for curves 3 and 4 Ψ ) 10, T ) 250 °C, lab-scale reactor Pe-1 ) 27).
5. Modeling Study: Real Membrane Versus Ideal Membrane Figure 5 represents the results of a case study carried out for an IMPBR (i) with the membrane γ-143 with permselectivities determined from the experiment (see Table 6) and (ii) with an ideally permselective membrane regarding H2O. H2/CO2 (3/1) is applied as the sweep gas. The Pe´clet number Pe represents the ratio of the characteristic time of transport through the membrane with respect to the characteristic time of the convective axial transport. For large values of Pe, axial flow dominates and the IMPBR changes into a PBR. In the case of the lab-scale reactor in the present study, the value of Pe is small (0.037; 1/Pe ) 27) due to the high permeance of silica membranes and a high specific membrane area. For small values of Pe, the transmembranal flow is large with respect to the axial convection and the membrane can affect the reaction kinetics and, for example, displace the equilibrium. In the case of the nonideal membrane γ-143, the evolution of the conversion vs 1/Pe depends strongly on the choice of sweep gas. With H2/CO2 (3/1) as the sweep gas, the conversion passes a maximum (Figure 5, curves 2 and 4). Above the maximum, the decrease in conversion is caused by the enhanced cofeeding of CO2 across the membrane and by the enhanced loss of the intermediate CO to the sweep side. Therefore, an optimal value of Pe exists under given conditions, and the Pe value of the lab-scale reactor used in the present study
The present work focused on the experimental demonstration of membrane integration for H2O removal in a Fischer-Tropsch (FT) lab-scale fixed bed reactor. The aim was the enhancement of CO2 conversion to longchain hydrocarbons by equilibrium displacement through the selective removal of H2O. The experiments were carried out in an inert membrane packed-bed reactor (IMPBR) with a TEOS coated ceramic membrane and a K-promoted, Fe-based catalyst. The analysis of the experimental results was supported by reaction engineering type models. The following conclusions can be drawn from the results: (i) The experimental results show that CO2 hydrogenation to long-chain hydrocarbons can be enhanced by the removal of H2O via a membrane, if a suitable sweep gas is applied, i.e., H2 or H2/CO2 (3/1). The increase in conversion can be attributed to the removal of H2O during the reaction by numerical analysis of the experimental results with the mathematical model of the IMPBR. (ii) TEOS based membranes do not offer sufficiently high permselectivities regarding H2O under FT conditions. In the case of Ar as the sweep gas, the performance of the IMPBR declines due to severe reactant loss. In the case of H2 and H2/CO2 (3/1) as the sweep gas, the increase in the hydrocarbon yield (total carbon conversion) remains low because of the loss of the intermediate CO. (iii) The applied mathematical model of the IMPBR is able to describe and explain the experimental results very well for various conditions. The results of the case study show the potential of selective in situ H2O removal during the FT synthesis but also describe the need for more selective membranes. Acknowledgment This work was financially supported within the ReFuelNet project by the BMBF (Bundesministerium fu¨r Bildung und Forschung). Part of the modeling work was carried out within EU Project LTCPO-GTL (ENKGCT-2002-00682). The authors thank J. Caro, Universita¨t Hannover, and P. Ko¨lsch, R. Scha¨fer, and M. Noack, ACA Berlin, for the preparation of the membranes and helpful advice. The authors also thank M. Claeys, University of Cape Town, for providing the catalyst.
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Nomenclature aj,H2O ) inhibition coefficient for H2O in reaction j AM ) inner surface area of membrane tube, m2 RASF ) hydrocarbon chain growth probability bj,CO2 ) inhibition coefficient for CO2 in reaction j d ) outer diameter of membrane tube, mm dP ) particle diameter, m DaFT ) mcatkFTPF/N˙ F,in ) Damko¨hler number of the FT reaction Φ ) PS/PF ) ratio of pressures on the sweep side and on the feed side k′j ) reaction rate constant of reaction j, mol/(s kg Pa) KP,CO-SH ) equilibrium constant of the CO shift reaction κ ) k′CO2-SH/k′FT ) ratio of the reaction rate constants of the CO2 shift reaction and the FT reaction L ) length of reactor, m mcat ) mass of catalyst, kg NC,i ) number of carbon atoms in component i N˙ iF ) molar flow rate of component i on the feed side, mol/s N˙ iF,in ) molar flow rate of component i at the inlet (z+) 0) of the feed side, mol/s N˙ iS ) molar flow rate of component i on the sweep side, mol/s N˙ iS,in ) molar flow rate of component i at the inlet (z+) 0) of the sweep side, mol/s N˙ i,tmb ) molar flow of component i across the membrane (transmembranal), mol/s N˙ iF+ ) N˙ iF/N˙ F,in ) dimensionless molar flow rate of component i on the feed side N˙ iS+ ) N˙ iS/N˙ S,in ) dimensionless molar flow rate of component i on the sweep side ∆N˙ i,tmb ) (N˙ iS,in - N˙ iS) ) net molar flow of component i across the membrane (transmembranal), mol/s / ∆N˙ i,tmb ) variable in Table 5 νi,j ) stoichiometric coefficient of component i in reaction j pi ) partial pressure of component i, Pa P ) absolute pressure, Pa Pe ) N˙ F,in/(AMQrefPF) ) Pe´clet number Qi ) permeance of component i (eq 1), mol/(s m2 Pa) Qref ) reference permeance, 1‚10-7 mol/(s m2 Pa) r′j ) rate of reaction j, mol/(s kg) R ) ideal gas constant ) 8.134 J/(mol K) F˜ n ) p/(RT) ) molar density, mol/m3 s ) thickness of membrane tube, mm ∆s ) size of the annular gap of the sweep-gas channel, mm Si.ref ) Qi/Qref ) ratio of permeances of component i and reference permeance, permselectivity T ) temperature, K τmod ) mcat/V˙ F,in ) modified residence time, (kg s)/m3 V˙ F,in ) volumetric flow rate at standard conditions STP (273 K, 0.1 MPa), m3/s XCO2 ) conversion of CO2 (Table 5) XC ) conversion of total carbon, i.e., of CO2 and CO, equivalent to YHC,CO2 (Table 5) yiF+ ) N˙ iF/N˙ F ) molar fraction component i on the feed side yiS+ ) N˙ iS/N˙ S ) molar fraction component i on the sweep side YCO,CO2 ) yield of CO (Table 5) YHC,CO2 ) yield of hydrocarbons (Table 5) Ψ ) N˙ S,in/N˙ F,in ) sweep flow ratio
z ) reactor length axial coordinate, m z+ ) z/L ) dimensionless reactor length
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Received for review March 1, 2005 Revised manuscript received May 3, 2005 Accepted May 12, 2005 IE050289Z
