Membrane Reactors for the Water-Gas Shift Reaction

E. Kikuchi, S. Uemiya, N. Sato, H. Inoue, K. Ando, and T. Matsuda, Membrane reactor for the water-gas shift reaction, Chem. Lett, No. 2, 489 (1989). 

Criscuoli, A., Basile, A., and Drioli, E. An analysis of the performance of membrane reactors for the water-gas shift reaction using gas feed mixtures. Catalysis Today, 2000, 56, 53. 

Giessler, S., Jordan, L., Diniz da Costa, J.C., and Lu, G.Q. Performance of hydrophobic and hydrophilic silica membrane reactors for the water gas shift reaction. Separation and Purification Technology, 2003, 32, 255. 

Both theoretical and experimental studies have been performed on palladium-based membrane reactors for the water-gas shift reaction. Ma and Lund simulated the performance achievable in a high temperature water-gas shift membrane reactor using both ideal membranes and catalysts [18]. By comparing the results obtained with those related to the existing palladium membrane reactors, they concluded that better membrane materials are not needed, and that higher performances mainly depend on the development of a water-gas shift catalyst not inhibited by CO2. Marigliano et al. pointed out how the equilibrium shift conversion in membrane reactors is an increasing function of the sweep factor (defined as the ratio between the flow rate of the sweep at the permeate side and the flow rate of CO at the reaction side) [19]. The ratio is an index of the extractive capacity of the system. 

Barbieri G, Brunetti A, Granato T, Bernardo P, Drioli E (2005), Engineering evaluations of a catalytic membrane reactor for the water gas shift reaction , Ind. Eng. Chem. Res., 44,7676-7683. 

A silica membrane reactor will be efficient and effective therefore, when the membrane, working at full capacity, is able to process all the H2 produced by the reaction. This situation arises when the DaPe = 1 and simulations for silica membrane reactors for the water gas shift (WGS) reaction have indeed demonstrated that maximum CO conversion was achieved at DaPe close to 1 (Battersby et al., 2006 Ikuhara et al, 2007). Thus the DaPe number is a valuable metric to evaluate the potential performance of a membrane reactor and a valuable, yet simple, design tool to ensure that both the reactor and membrane components work together for maximum efficacy. However, the DaPe number does not take into account the selectivity of the membrane which obviously does affect the membrane reactor performance. Both experimental and simulation studies have shown that higher permeation results in higher conversion and product yield enhancements (Battersby et al., 2006 Boutikos and Nikolakis, 2010 Lim et al., 2010).That is not to say that a membrane with a low selectivity cannot be successfully utilized in a membrane reactor set-up. Provided the membrane has nominal selectivity for the desired products over reactants, the conversion of equilibrium-limited reactions will be enhanced in a membrane reactor system. However, the product purity will remain dilute and thus additional operational and capital expenditure will be required for further downstream processing. If the membrane is unable to separate gases then the system behaves as a packed bed reactor. 

Silver membranes are permeable to oxygen. Metal membranes have been extensively studied in the countries of the former Soviet Union (Gryaznov and co-workers are world pioneers in the field of dense-membrane reactors), the United States, and Japan, but, except in the former Soviet countries, they have not been widely used in industry (although fine chemistry processes were reported). This is due to their low permeability, as compared to microporous metal or ceramic membranes, and their easy clogging. Bend Research, Inc. reported the use of Pd-composite membranes for the water-gas shift reaction. Those membranes are resistant to H2S poisoning. The properties and performance characteristics of metal membranes are presented in Chapter 16 of this book. 

A schematic of a PBMR, in this case for the water-gas shift reaction, is given in Fig. 14.2. Of course the catalytic reactor and the membrane unit can also be separated from each other, but can still be used to enhance the yield of a catalytic process, as will be shown in Section 14.3 (see also Fig. 14.5). 

As a particular example, an economic study carried out by Criscuoli et al. [29] for the water-gas shift reaction in a Pd-based membrane reactor is discussed below. 

S. Battersby, M. C. Duke, S. Liu, V. Rudolph, J. C. Diniz da Costa, Metal doped sihca membrane reactor operational effects of reaction and permeation for the water gas shift reaction, J. Membr. Sci. 316 (2008) 46-52. 

V. Violante, A. Basile, E. Drioli, Composite catalytic membrane reactor analysis for the water gas shift reaction in the tritium fusion fuel cycle. Fusion Eng. Des. 30 (1995) 217-223. 

Comaglia, C. A., Adrover, M. E., Miinera, J. F., Pedemera, M. N., Borio, D. O., Lombardo, E. A. (2013). Production of ultrapure hydrogen in a Pd—Ag membrane reactor using noble metals supported on La—Si oxides. Heterogeneous modeling for the water gas shift reaction. International Journal qf Hydrogen Energy, 38, 10485—10493. 

Barhieri et al. developed a membrane reactor for water-gas shift 544]. A palladium/ silver film containing 23 wt.% silver, which was between 1- and 1.5-pm thick was produced hy sputtering. This film was coated onto a porous stainless steel support. This patented production method allowed a much higher ratio of pore size to film thickness compared with conventional methods. Tubular membranes of 13-mm outer diameter, 10-20-mm length and 1.1-1.5-pm thickness, respectively, were prepared. A commercial Cu based catalyst supplied by Haldor-Topsoe was used for the water-gas shift reaction. At 210 °C a permeating flux of 4.5 L (m s) was determined for pure hydrogen at 0.2-bar pressure drop. At a reaction temperature of 260-300 °C, and 2085 h gas hourly space velocity, the thermodynamic equilibrium conversion could be exceeded by 5-10% with this new technology. 

Battersby, S., Duke, M.C., Liu, S., Rudolph, V. and da Costa, J.C.D. (2008) Metal doped silica membrane reactor Operational effects of reaction and permeation for the water gas shift reaction. Journal of Membrane Science, 316,46-52. 

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