Kinetic Modeling of Pure and Multicomponent Gas Permeation Through Microporous Membranes: Diffusion Mechanisms and Influence of Isotherm Type

The main transport mechanisms involved in pure and multicomponent gas permeation through real microporous membranes are reviewed in this article. They include viscous flow, Knudsen diffusion, bulk diffusion (in mixtures), surface diffusion, and activated gaseous diffusion. The individual contribution of each mechanism may be discriminated from permeation experiments, and can be used to detect the occurrence of defects in the membrane structure. In the case of multicomponent mixtures, the milestone theory of Maxwell-Stefan can be advantageously applied to model the transfer mechanisms embodied. The separation of mixtures can be predicted from data measured for pure gases; here, computer simulations may provide relevant information concerning the loading influence upon diffusivities. With respect to surface diffusion, equilibrium plays a major role in the process, which requires accurate isotherms to compute the corresponding Maxwell-Stefan thermodynamic factors. New single/multicomponent factors are derived here for the first time for Freundlich, Dual-site Langmuir, and Dual-site Langmuir-Freundlich isotherms. The influence of loading upon the surface diffusivities is also addressed, and the most significant theories and approaches adopted to model the phenomenon are discussed.