Inorganic membrane reactors applications

The inorganic membrane reactor technology is in a state characterized by very few in practice but many of promise. Since the potential payoff of this technology is enormous, it deserves a close-up look. This and the following three chapters are, therefore, devoted to the review and summary of the various aspects of inorganic membrane reactors applications, material, catalytic and engineering issues. 

Armor, ). N., Applications of catalytic inorganic membrane reactors to refinery products, J. Membr. Sci. 1998, 347 (2), 217-233. 

V. T. Zaspalis, A. J. Burggraaf, Inorganic membrane reactors to enhance the productivity of chemical processes, in Inorganic membranes synthesis, characteristics and applications,... 

Ilias and Govind(lO) have reviewed the development of high temperature membranes lor membrane reactor application. Hsieh(4) has summarized the technology in the area of important inorganic membranes, the thermal and mechanical stabilities of these membranes, selective permeabilities, catalyst impregnation, membrane/reaction considerations, reactor configuration, and reaction coupling. 

Typically liquid-phase reactions do not require high temperatures, and as such organic membranes may be suitable for the membrane reactor applications. Justification of using inorganic membranes for these applications comes from such factors as better chemical stability and beuer control and containment of the catalysts. 

While the aforementioned and other novel membrane reactors hold great promises, many material, catalysis and engineering issues need to be fully addressed before the inorganic membrane reactor technology can be implemented in an industrial scale. This is particularly true for many bulk-processing applications at high temperatures and often harsh chemical environments. Those issues will be treated in the subsequent chapters. 

When using commercial inorganic membranes for separation or membrane reactor applications at relatively high temperatures, say, greater than 400°C or so, care should be taken to assess their thermal or hydrothermal stability under the application conditions. This is particularly relevant for small pore membranes because they are often made at a temperature not far from 400 C. Even if such an exposure does not yield any phase changes, there may be particle coarsening (and consequently pore growth) involved. 

Despite the aforementioned efforts, membrane flux decline due to fouling continues to be a major operational issue. Attempts have been made to modify inorganic membranes, mostly their surfaces, to render them less prone to foulant adsorption. One of the frequently encountered fouling problems in biotechnology and food applications is protein adsorption. In membrane reactor applications which are largely associated with hydrocarbons, carbonaceous deposits pose as one of the operational problems. 

Finally, the current status of the inorganic membrane technology is summarized for an overall perspective. The future is speculated based on that perspective to provide a framework for future developments in the synthesis, fabrication and assembly of inorganic membranes and their uses for traditional liquid-phase separation, high-temperature gas separation and membrane reactor applications. 

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. 

Despite the unique properties of inorganic membranes vs. the rather well-established polymeric ones (see Table 1 for a comparison), issues such as membrane instability, insufficient permeability or permselectivity, or simply the unbearable costs implied still hamper the application of inorganic-membrane reactors in the process industry. 

When one of the components can condense within the pores, the capillary condensation mechanism is enabled (Fig. 9). The condensate fills the pores and then evaporates at the permeate side, where a low pressure is imposed [54]. Moreover, the transfer of rather big molecules is generally favored by this mechanism over rather small ones. Provided the pore dimension is small and homogeneous enough, and the pores themselves uniformly dispersed over the membrane, this mechanism allows for very high selectivities (separation factors between 80 and 10(X), as reported in [55]) limited only by the solubility of noncondensable molecules in the condensate. However, capillary forces are strong enough to promote this mechanism only with small pore sizes at relatively low temperatures. Hence, as for surface or multilayer diffusion, the practical chances of application appear poor in inorganic-membrane reactors. 

Some of the most interesting application opportunities that have been tested on separative inorganic-membrane reactors are listed in Table 3, where recent literature references are cited as well. For more information concerning the huge number of reactions ever tested on such reactors, see Ref. 11. Some representative cases are discussed next. 

Table 4 lists some of the most interesting reactions tested until now on nonseparative inorganic-membrane reactors. As for separative applications, we refer the reader to a recent review of ours [11] for a more complete list. A few case studies are discussed next in some detail. 

The major features of and application opportunities for inorganic-membrane reactors have been described in some detail. We can conclude that inorganic-membrane reactors actually show promise for improving either conversion of equilibrium-limited reactions (e.g., dehydrogenations) or selectivity toward some intermediates of consecutive reaction pathways (e.g., partial oxidations). 

However, at least for separative applications, most hopes to find consistent application of inorganic-membrane reactors lie in the development of inorganic membranes having pores of molecular dimensions (<10 A, e.g., zeolitic membranes). Such membranes should moreover be thin enough to allow reasonable permeability, defect-free, resilient, and stable from the thermal, mechanical, and chemical standpoints. Such results should not be achieved only at a lab scale (a lot of promising literature has recently appeared in this context), but should also be reproducible at a large, industrial scale. Last, but not least, such membranes should not be unacceptably expensive, in both their initial and their replacement costs. 

The increasing interest in inorganic membranes for gas applications is undoubtedly due to their excellent high temperamre resistance. Inorganic membrane reactors (including carbon membranes) may thus have a very nice potential for industrial... 

FIGURE 10.21 (See color insert following page 588.) Traditional applications of inorganic membrane reactors for (a) conversion enhancement by product removal, (b) permeation of products and reaction coupling, and (c) selectivity enhancement by reactant distribution. 

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