Nonuniform catalyst distribution for inorganic membrane reactors: Theoretical considerations and preparation techniques

Abstract

The performance characteristics of an inert membrane reactor with catalyst on the feed side (IMRCF), a catalytic membrane reactor (CMR), and a fixed-bed reactor (FBR) are compared for uniform and Dirac-type catalyst distributions for the case of a first-order reversible reaction under isothermal conditions. It is demonstrated that for all three reactor types, a Dirac delta distribution of catalyst, located at the external surface (feed side) in either the membrane (CMR) or the pellet (IMRCF and FBR) gives superior performance as compared to the uniformly distributed case. In addition, for the IMRCF, the case of an arbitrary number of reactions following arbitrary kinetics under steady-state nonisothermal conditions, with finite external mass and heat transfer resistances for both the pellet and the inert membrane, is considered. It is shown analytically that in the realistic case where the local catalyst loading is bounded, the optimal distribution of catalyst is in general a multiple step function. As the bound increases, the step function approaches a Dirac delta distribution. Novel preparation techniques for supported thin metal composite membranes for IMRCF applications and inorganic membranes with nonuniform distribution of catalyst for CMRs are also described. The first technique employs electroless plating along with osmotic pressure to yield supported thin membranes of controlled microstructure, porosity and thickness. The preparation of silver films deposited on commercial alumina membrane is presented as an example. Heat treatment in oxygen demonstrates that Ag/alumina membranes thus prepared are thermally more stable than similar membranes prepared using standard electroless plating procedures. In the second technique, using sequential slip-casting of inert and platinum impregnated alumina sols, precise control of the catalyst layer thickness and location within the alumina membrane is obtained.