Diffusion in microporous membranes: Measurements and modeling

Permeation of gases and vapors through microporous membranes, principally zeolite membranes, is reviewed, focusing on macroscopic diffusion models, measurement techniques, and applications of models to experimental data. Brief reference is made to the choice of adsorption isotherms for single components or mixtures. Concerning diffusion, the Maxwell-Stefan (M-S) model is reviewed in some detail, as it is the one universally adopted in recent literature. Emphasis is placed on the coverage dependence of the diffusion coefficients and on the proper handling of the cross terms. In the experimental technique section, the key distinctions are between steady state and transient measurements, and between the use or nonuse of a sweep gas. A few special techniques are also briefly reviewed. Interpretation of transient measurements using the time lag method is reviewed in some detail, especially for coverage-dependent diffusion coefficients. Several of the studies reviewed focus on the ability of the M-S model to match the measurements, and specifically on the suitability of various simplifying approximations. Two common approximations are (i) to treat the M-S diffusion coefficients as coverage-independent and (ii) to neglect the cross terms.