TRANSPORT HETEROGENEITY IN POROUS PELLETS .1. PGSE NMR-STUDIES

This paper is the first in a series of reports describing Nuclear Magn etic Resonance (NMR) and numerical modelling studies of the representa tion of the structure of porous media and associated transport phenome na. In this paper the interpretation of Pulsed Gradient Spin Echo (PGS E) NMR data are considered. In extending earlier studies in which the porous medium is assumed to be macroscopically homogeneous, we have co nsidered the effect of spatial variations in both the diffusivity and relaxation properties within the porous medium, occurring over lengths cales of order > 100 mu m. Expressions for the log-attenuation plot an d the diffusivity are derived. The theory gives excellent agreement wi th experimental PGSE data obtained for a variety of porous catalyst su pport pellets, and both the effective diffusivity of the materials and an indication of the degree of heterogeneity of diffusion are obtaine d. Sample tortuosities are found to be in the range 1.3-4.3. The resul ts suggest that macroscopically heterogeneous materials have a greater tortuosity than more homogeneous materials.