Transport heterogeneity in porous pellets—II. NMR imaging studies under transient and steady-state conditions

Abstract

In the first paper of this study, PGSE NMR was used to measure liquid diffusion in porous catalyst support pellets. The results were explained in terms of heterogeneity in the structure and transport properties of these materials. In this part of the study, NMR imaging is used to study diffusion within the same samples. By acquiring a series of images during the course of a transient diffusion process, it is possible to estimate the effective diffusivity of water confined within the pore space of the pellets. NMR imaging is also used to produce two-dimensional images which show clearly that porous catalyst support pellets contain significant heterogeneities, as suggested by the spatially non-resolved PGSE NMR experiments presented in Paper I (Hollewand and Gladden, Chem. Eng. Sci. 50, 309–326, 1995). Finally, the PGSE and imaging NMR experiments are combined to produce images of the local diffusivity of water contained within individual pellets. These images allow the degree of diffusional heterogeneity to be quantified and give results in agreement with the PGSE NMR study. The results demonstrate that it is necessary to consider heterogeneities in the porous structure over different lengthscales in order to be able to understand the relationship between structure and transport in porous solids.