Pulsed field gradient nuclear magnetic resonance study of long-range diffusion in beds of NaX zeolite;evidence for different apparent tortuosity factors in the knudsen and bulk regimes

摘要

The pulsed filed gradient nuclear magnetic resonance method is applied to study self-diffusion of ethane in beds of zeolite NaX for displacements which are orders of magnitude larger than the size of individual crystals.Comparson of the measured diffusivities with those calculated using simple gas kinetic theory indicates that for the samebed of NaX crystals the apparent tortuosity factor in the Knudsen regime is significantly larger than that in the bulk regime.This finding is tentatively attributed to the more pronounced geometrical trapping by surface imperfections in the Knudsen than in the bulk regime.Tortuosity factors,which are much larger in the Knudsen regime than in the bulk regime,were also recently obtained by dyamic Monte Carlo simulation of gas diffusion in various porous systems.