| External mass transfer resistance in feed and permeate channel boundary layers dramatically influences the separation properties of high-flux microporous asymmetric membranes having a permselective layer wherein transport is controlled by Knudsen diffusion. When multicomponent mixtures permeate through these membranes the significance of external resistance may differ widely among species. This can make the boundary layer more selective than the membrane itself.; Boundary layer selectivity predominated with a highly permeable large-pore membrane. This provided the framework for a largely unexplored process for the selective separation of low molecular weight gases (e.g., H{dollar}sb2,{dollar} He) from mixtures that has been called "Membrane Atmolysis".; Hydrodynamic boundary layers may also be important when reaction and separation are combined in a catalytic membrane reactor (CMR), based on a porous inorganic membrane. Theory and experiments were combined to quantitate the advantages of using CMR architectures to overcome equilibrium limitations on dehydrogenation reactions. A theoretical analysis is developed that takes into account external mass transfer effects as well as reaction and diffusion inside pores, to examine "supra-equilibrium conversion" in CMR's. We conclude that partial pressure drops across these structures are the primary source of enhanced conversion, and membrane H{dollar}sb2{dollar} permselectivity is only secondary. Inclusion of mass transfer in external boundary layers in our theoretical analysis, was crucial to successful simulation of experimental results with a laboratory-scale CMR. |
