A model for hindered diffusion in pores

Abstract

Hindered diffusion is a longstanding interest to chemical engineers working in such fields as heterogeneous catalysis, adsorptive and biochemical separations, and environmental engineering. In this work, a novel and universal model unifying the pore diffusion and surface diffusion was deduced theoretically to account for the pore diffusion control or surface diffusion control in pores. Based on the model, a versatile restrictive factor expression was proposed to describe the correlation between the effective diffusivity and the ratio of molecular diameter to pore diameter. Various variations of restrictive factor versus the ratio of molecular diameter to pore diameter from reported literatures were interpreted better than previous restrictive factor equations. Especially, the restrictive factor with non-monotonic variations could not be explained with the reported restrictive factor equations, but interpreted well by equation in this work. Furthermore, the diffusion tortuosity was first proposed herein to account for the effect of molecule diffusion path on diffusion behavior in pores.