Porous membrane reactors

Porous membranes are characterized by high permeability, but low selectivity. The transport mechanisms in porous membranes can be viscous flow, Knudsen diffusion, surface diffusion, capillary condensation, and/ or molecular sieving, depending on the membrane pore size and its surface characteristics. The performance of porous MRs is very much dependent on the membrane structures. Close adherence to a rigid protocol is necessary to obtain membranes of consistent quality. 

This chapter starts with a general description of porous membranes and separation mechanisms related to the pore structure. A variety of porous membranes - including glass, ceramic, carbon, and silica - and their MRs will be illustrated individually. Considerable attention will be focused on 

Inorganic Membrane Reactors Fundamentals and Applications First Edition. Xiaoyao Tan and Kang Li. 

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Gas/vapor phase hydrogen-consuming reactions using porous membrane reactors... 

Tsotsis et al. [1992] considered a case where two reaction zones exist in a porous membrane reactor one inside the membrane matrix and the other in the tubular region which is packed with catalyst particles. They presented a packed-bed catalytic membrane tubular reactor model under isothermal and co-current flow conditions. Thus, Equations (10-37), (10-6) and (10-45) ail reduce to the condition... 

Mohan and Govind [1988c] applied their isothermal packed-bed porous membrane reactor model to the same equilibrium-limited reaction and found that the reactor conversion easily exceeds the equilibrium value. The HI conversion ratio (reactor conversion to equilibrium conversion) exhibits a maximum as a function of the ratio of the permeation rate to the reaction rate. This trend, which also occurs with other reactions such as cyclohexane dehydrogenation and propylene disproportionation, is the result of significant loss of reactant due to increased permeation rate. This loss of reactant eventually negates the equilibrium displacement and consequently the conversion enhancement effects. 

Figure 10.9 Effect of permeation to reaction rate ratio on reaction conversion in a porous membrane reactor [Mohan and Govind 1988c]...

Figure 11.7 Comparison of co-current and counter-current porous membrane reactors for decomposition of ammonia with a hydrogen permselectivity determined by Knudsen diffusion at 627 C [Collins ct al., 19931...

Maximum conversion ratio of reaction A B in a porous membrane reactor at total (bottom product) recycle... 

Figure 11.28 Effect of hydrogen selectivity on conversion of ammonia decomposition in a counter-current porous membrane reactor (Collins et al.. 1993]...

Based on the above considerations, the types of reactions that are amenable to inorganic membrane reactors in the first wave of industrial implementation will probably be as follows (1) The reactions are heterogeneous catalytic reactions, particularly dehydrogenation processes (2) The reaction temperature exceeds approximately 200°C (3) When the reactions call for high-purity reactant(s) or produces) and the volume demand is relatively small, dense membrane reactors (e.g., Pd-based) can be used. On the other hand, if high productivity is critical for the process involved, porous membrane reactors are necessary to make the process economically viable. 

Based on matenal considerations, membrane reactors can be classified into (1) organic-membrane reactors, and (2) inorgamc-membrane reactors, with the latter class subdivided into dense (metals) membrane reactors and porous-membrane reactors Based on membrane type and mode of operation, Tsotsis et al. [15] classified membrane reactors as shown in Table 3. A CMR is a reactor whose permselective membrane is the catalytic type or has a catalyst deposited in or on it. A CNMR contains a catalytic membrane that reactants penetrate from both sides. PBMR and FBMR contain a permselective membrane that is not catalytic the catalyst is present in the form of a packed or a fluidized bed PBCMR and FBCMR differ from the foregoing reactors in that membranes are catalytic. 

A comparison of DGM and the extended Fick model for the transition region has been made by Veldsink et al. [46] and is illustrated by many transport data and applied to describe transport in a macro-porous membrane reactor. Their main conclusion is that for ternary mixtures the use of the DGM model is necessary and predicts the transport of a gas mixture within a few percent (5%). For binary gases usually the extended Fick model is sufficient, but with an overall pressure over the membrane the accuracy is less than that obtained by use of the DGM. A further discussion will be given in Section 9.7. 

Milne, A.D., 2008. The Apphcation of the Attainable Region Concept to the Oxidative Dehydrogenation of N-butanes in Inert Porous Membrane Reactors. University of the Witwatersrand, Johannesburg. 

Milne, D., Glasser, D., HUdebrandt, D., Hausherger, B., 2004. Application of the attainable region concept to the oxidative dehydrogenation of 1-butane in inert porous membrane reactors. Ind. Eng. Chem. Res. 43, 7208. 

Oklany, J. S., Hou, K., Hughes, R. (1998). A simulative comparison of dense and micro-porous membrane reactors for the steam reforming of methane. Applied Catalysis A General, 170, 13—22. 

Temperature (°C) Traditional reactor (g) Porous membrane reactor (g) Dense membrane reactor (g)... 

Finally, most of catalytic tests have been carried out using fixed-bed reactors. However, other reactor types should be considered. Kao et present an analysis of methane oxidative coupling on a Li/MgO packed porous-membrane reactor (PMR) and by a fixed-bed reactor (FBR). They conclude a maximal 30% yield at 53% selectivity in PMR, while the maximal yield achieved in the FBR of identical dimensions and temperature was 20.7%, with 52.5% selectivity. 

A model-based performance analysis of fixed-bed, fluidized-bed, and porous-membrane reactors has been reported, concluding that fluidized-bed reactors can improve the yield up to 26% (which is still below the industrial requirements), while fixed-bed reactors cannot be used industrially. However, membrane reactors offer the possibility of increasing the yield by fine oxygen distribution through the membrane. In this way, it has been suggested that it is possible to optimize the reaction conditions in all investigated reactor concepts. This alternative operation mode using a PBMR, is proposed in order to increase the efficiency of the OCM with a performance of 23.2% yield, 53.9% selectivity, and 42.7% methane conversion. 

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