Porous Inorganic Membrane Reactors

11 — CURRENT DEVELOPMENTS AND FUTURE RESEARCH DM CATALYTIC MEMBRANE REACTORS 

Mesoporous alumina membranes have been studied extensively mostly for 

Van Swaaij and coworkers [5,6] proposed the use of nonpermselective macroporous membranes for gas phase reactions, which by their nature require strict stoichiometric conditions. Such reactions include selective catalytic reduction of NO by NH3, and the Clauss reaction. The principle for the CNMR operation is shown in Fig. 11.7. By creating a sharp reaction front within the membrane one avoids the slip by either reactant (NH3 or NO, SO2 or H2S) on either side of the membrane. Small perturbations in the feed could be accommodated in principle by a shift in the reaction front within the membrane. The success of the CNMR concept depends, as one would expect, on the sharpness of the reaction front created within the membrane. For a non-instantaneous reaction the front created is rather diffuse (see Fig. 11.7) and there is, as a result, a reactant slip. 

A number of research groups [83-86] have used a rather related concept for carrying out various selectivity limited reactions in a membrane reactor. The concept is illustrated schematically in the top part of Fig. 11.8 which is from a study by Harold and coworkers [84]. It applies to partial oxidation reactions where the desired reaction product can further react with oxygen to produce an imdesirable total oxidation product. In some instances it makes better sense (in terms of maximizing the reactor yield) to feed the two reactants separately on either side of the membrane rather that to co-feed them on either side. This is shown in the bottom part of Fig. 11.8 which shows the yield to the desired product as a fimction of the Thiele modulus as the degree of feed segregation 

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The advent of reliable quality ceramic membranes entering the industrial market has heightened the interest for porous inorganic membrane reactors at high temperatures,... 

Given in Table 8.8 is a summary of experimental data available on dehydrogenation reactions using porous inorganic membrane reactors with a variety of catalysts. Some of those reactions are industrially important and will be discussed separately as follows. 

Dehydrogenation of cyclohexane to benzene. Another well studied reaction using porous inorganic membrane reactors is dehydrogenation of cyclohexane to make benzene. The operable temperature range is from about 170 to 3 X using a precious metal (e.g., Pt or Pd) as the catalyst either impregnated in the membrane pores or on a carrier such as alumina or silica. 

Dehydrogenation of aliphatic hydrocarbons. A number of aliphatic hydrocarbons experience enhanced dehydrogenation conversions by carrying out the reactions in porous inorganic membranes. Most of the studies use porous alumina membrane tubes as the reactors. 

As a building block for simulating more complex and practical membrane reactors, various membrane reactor models with simple geometries available from the literature have been reviewed. Four types of shell-and-tube membrane reactor models are presented packed-bed catalytic membrane reactors (a special case of which is catalytic membrane reactors), fluidized-bed catalytic membrane reactors, catalytic non-permselecdve membrane reactors with an opposing reactants geometry and catalytic non-permselective membrane multiphase reactors. Both dense and porous inorganic membranes have been considered. 

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. 

In many situations the conversion of a membrane reactor increases as the total permeate rate increases. This is to be expected if the membrane has a perfect or very high pcimselcctivity. In many commercially available porous inorganic membranes, however, the permselectivity is moderate and some reactants as well as products other than the most selective species "leak through the membrane. This leakage rate often increases with the total permeate rate, for example, as the feed side pressure increases. This has an important consequence on the reactor performance. 

The most general case of catalyst-membrane systems are systems containing a conventional granulated catalyst and a membrane catalyst. Two varieties of such systems are possible (1) a pellet catalyst with a monolithic membrane or (2) a pellet catalyst with a porous (sometimes composite) membrane. The inorganic membrane reactors with or without selective permeability are discussed in Chapter 17 of this book. Examples of applications of systems of selective metal-containing membrane and granulated catalyst are presented in Table 5. 

The aim of this chapter is to show that a multidisciplinary approach, focusing on materials, processes and modelling as depicted in Fig. 14.1, is needed to judge the techno-economic feasibility of inorgaiuc membranes in large-scale processes. This will be done by discussing examples of the potential use of porous inorganic membranes in three different membrane reactor applications. 

Much of the impetus for the awakened interest and utilization of inorganic membranes recently came hom a history of about forty or fifty years of some large scale successes of porous ceramic membranes for gaseous diffusion to enrich uranium in the military weapons and nuclear power reactor applications. In the gaseous diffusion literature, the porous membranes are referred to as the porous barriers. For nuclear power generation, uranium enrichment can account for approximately 10% of the operating costs (Charpin and Rigny, 1989]. 

For convenience of discussion, modeling studies of packed-bed inert membrane tubular reactors will be divided into two categories depending on the type of inorganic membranes dense or porous. 

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