NMR AND FRACTAL MODELING STUDIES OF TRANSPORT IN POROUS-MEDIA

Magnetic Resonance Imaging (MRI) and PGSE NMR studies have shown that it is necessary to consider heterogeneities in the porous structure ov er different lengthscales in order to be able to understand the relati onship between structure and transport in porous solids. Moreover, the spatial distribution in pore structure is seen to influence strongly mass transfer processes occurring within the porous medium. The MRI st udies described here have suggested that a fractal representation migh t be appropriate in describing heterogeneous porous systems. Numerical simulations have been performed of transport within Cluster-Cluster A ggregate (CCA) structures. A comparison of methods of simulating the d iffusion process is presented. The results of the simulations are comp ared with MRI and PGSE NMR measurements of the tortuosity of commercia l catalyst pellets. In the light of complementary nitrogen desorption and mercury porosimetry data, a multifractal description of porous med ia is also proposed in the form of a Composite CCA structure in which both the macroscopic heterogeneity associated with the pore-size distr ibution and the fractal characteristics of the microscopic pore struct ure itself are represented.