Diffusion coefficients in bidisperse porous structures

Abstract

On the basis of theoretical analysis the reason for experimentally observed differences in effective diffusion coefficients in porous media when stationary and nonstationary methods are applied to the measurements has been explained. It has been shown that these differences appear as a function of the porous structure of the media and are especially significant for bidisperse porous pellets. In order to verify the theoretical conclusions effective diffusion coefficients were measured for five different porous pellets, one monodisperse and four bidisperse structures using the dynamic membrane method. The experimental results fully confirm the theoretical conclusion that the diffusion coefficients estimated from the zero and first moments of the response curves should be identical and thus equal to those obtained in the stationary methods. For all bidisperse structures considered values of diffusion coefficients estimated from the second moments on the basis of the formula for the monodisperse structure are lower than the diffusion coefficients estimated from the zero and first moments, thus fully supporting the theoretical conclusions.