Diffusion of binary mixtures in microporous materials: Overshoot and roll-up phenomena

Diffusion of mixtures in microporous materials such as zeolites is complicated by two important factors: (1) the diffusion phenomena is strongly dependent on the adsorbed species loadings, and (2) the diffusion of one component has a direct influence on the diffusion of the other component(s). In order to describe the diffusion process we adopt the Maxwell-Stefan formulation; this allows the prediction of mixture behaviour on the basis of information of the diffusion characteristics of the pure components in the mixture. Calculations are presented for several examples to illustrate the unusual characteristics of microporous diffusion. For uptake of a binary mixture into a spherical particle, for example, it is shown that the species with the higher mobility can “overshoot” its equilibrium loading during the transient uptake process. The slower-diffusing species experiences the “roll-up” phenomenon in its breakthrough characteristics through a packed bed adsorber filled with microporous particles.