Catalytic membrane reactors microporous membranes

Tsuru, T., K. Yamaguchi, T. Yoshioka, and M. Asaeda, Methane steam reforming by microporous catalytic membrane reactors, AIChE., 50(11), 2794-2805, 2004. 

The conditions are substantially more favorable for the microporous catalytic membrane reactor concept. In this case the membrane wall consists of catalyti-cally active, microporous material. If a simple reaction A -> B takes place and no permeate is withdrawn, the concentration profiles are identical to those in a catalyst slab (Fig. 29a). By purging the permeate side with an inert gas or by applying a small total pressure difference, a permeate with a composition similar to that in the center of the catalyst pellet can be obtained (Fig. 29b). In this case almost 100% conversion over a reaction length of only a few millimeters is possible. The advantages are even more pronounced, if a selectivity-limited reaction is considered. This is shown with the simple consecutive reaction A- B- C where B is the desired product. Pore diffusion reduces the yield of B since in a catalyst slab B has to diffuse backwards from the place where it was formed, thereby being partly converted to C (Fig. 29c). This is the reason why in practice rapid consecutive reactions like partial oxidations are often run in pellets composed of a thin shell of active catalyst on an inert support [30],... 

The insertion of catalytically active guests, such as transition metal ions, is an example of the potentialities of zeolite membranes for applications in catalytic membrane reactors. The well-known catalytic properties of supported vanadium oxides for selective oxidations have recently prompted a number of studies on the possibility of inserting vanadium in the framework of crystalline microporous silica and aluminosilicate powders. " ... 

All reports agree that acceptance of catalytic membrane reactors on a commercial scale is at least 10 years away. Here, more experimental performance data for particular processes and process conditions are required to stimulate further development. This needs the commercial availability of a larger assortment of microporous as well as dense membranes with a variety of combinations of good separation factors and good performance values. 

An innovative potential application of membrane technology in catalysis and in catalytic membrane reactors is the possibility to produce catalytic crystals with a weU-defined size, size distribution, and shape by membrane crystallization (Fig. 27.9) (Di Profio et al., 2003, 2005). This innovative technology makes use of the evaporative mass transfer of volatile solvents through microporous hydrophobic membranes in order to concentrate feed solutions above their samration limit, thus attaining a supersaturated environment where crystals may nucleate and grow. In addition, the presence of a polymeric membrane increases the probability of nucleation with respect to other locations in the system (heterogeneous nucleation). 

The dimerization of isobutene carried out in a forced-flow polymeric catalytic membrane reactor was reported by D. Fritsch and co-workers. The authors prepared composite porous membranes consisting of a catalytic layer made of solid add catalysts, such as siUca supported Naflon , Nafion NR50, Amberlyst 15 and silica supported tungstophosphoric add dispersed in polymeric binders such as Teflon AF, Hyflon AD, polytrim-ethylsilylpropyne, or polydimethylsiloxane (PDMS), cast on microporous support membranes made of polyacrylonitrile (PAN) or Torlon . The membranes were assembled in the membrane reactor into which isobutene was fed in the retentate side with a build-up pressure of 4 bar. The liquid product was collected on the permeate side. 

Kurungot, S., Yamaguchi, T., Nakao, S.-L, Rh/y-AljOj catalytic layer integrated with sol-gel synthesized microporous silica membrane for combact membrane reactor applications, Catal. Lett. 2003, 86, 273-278. 

Sorptive reactor concepts where periodic operation is used to temporarily store or remove educts or products in the fixed bed can be considered close to industrial realization, whereas membrane reactor concepts with permselective inert or catalytically active microporous membranes are still at the laboratory stage. They promise the highest potential for a further improvement of catalytic reactor technology and present the biggest challenges [54]. 

Itoh et al. [1984] modeled a 1.5-m long packed-bed membrane reactor for catalytic decomposition of HI. The reactor consists of a large number of microporous hollow fiber... 

When the catalyst is immobilized within the pores of an inert membrane (Figure 25.13b), the catalytic and separation functions are engineered in a very compact fashion. In classical reactors, the reaction conversion is often limited by the diffusion of reactants into the pores of the catalyst or catalyst carrier pellets. If the catalyst is inside the pores of the membrane, the combination of the open pore path and transmembrane pressure provides easier access for the reactants to the catalyst. Two contactor configurations—forced-flow mode or opposing reactant mode—can be used with these catalytic membranes, which do not necessarily need to be permselective. It is estimated that a membrane catalyst could be 10 times more active than in the form of pellets, provided that the membrane thickness and porous texture, as well as the quantity and location of the catalyst in the membrane, are adapted to the kinetics of the reaction. For biphasic applications (gas/catalyst), the porous texture of the membrane must favor gas-wall (catalyst) interactions to ensure a maximum contact of the reactant with the catalyst surface. In the case of catalytic consecutive-parallel reaction systems, such as the selective oxidation of hydrocarbons, the gas-gas molecular interactions must be limited because they are nonselective and lead to a total oxidation of reactants and products. For these reasons, small-pore mesoporous or microporous... 

Itoh N., Shindo Y, Haraya T. and Hakuta T, A membrane reactor using microporous glass for shifting equilibrium of cyclohexane dehydrogenation, J. Chem. Eng. Jpn. 21 399 (1988). Tsotsis T.T., Champagnie A.M., Vasileiadis S.P., Ziaka E.D. and Minet R.G., Packed bed catalytic membrane reactors, Chem. Engng. ScL 47 2903 (1992). 

More specifically in the area of this overview, zeolite materials constitute the main group of microporous membranes with regard to their potential membrane-reactor apphcations. The wide variety of existing zeolite structures, together with the possibility of modifying their adsorption and catalytic properties, provides us with a working material of high flexibihty. As a... 

D. Cazanave, A.G. Fendler, J. Sanchez, R. Loutaty and J.-A. Dalmon, Control of transport properties with a microporous membrane reactor to enhance yields in dehydrogenation reactions. Paper presented at the 1st International Workshop on Catalytic Membranes, September 1994, Lyon-Villeurbanne, France. 

Langhendries et al [5.74] analyzed the liquid phase catalytic oxidation of cyclohexane in a PBMR, using a simple tank-in-series approximate model for the PBMR. In their -reactor the liquid hydrocarbon was fed in the tubeside, where a packed bed of a zeolite supported iron-pthalocyanine catalysts was placed. The oxidant (aqueous butyl-hydroperoxide) was fed in the shellside from were it was extracted continuously to the tubeside by a microporous membrane. The simulation results show that the PBMR is more efficient than a co-feed PBR in terms of conversion but only at low space times (shorter reactors). A significant enhancement of the organic peroxide efficiency, defined as the amount of oxidant used for the conversion of cyclohexane to the total oxidant converted, was also observed for the PBMR. It was explained to be the result of the controlled addition of the peroxide, which gives low and nearly uniform concentration along the reactor length. 

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