DETERMINATION OF EFFECTIVE MACROPORE DIFFUSION-COEFFICIENTS BY DIGITAL IMAGE-PROCESSING

In this contribution, a new method is proposed for determining effecti ve macropore diffusion coefficients in porous media by digitized micro photographs of porous support materials and random walk simulations. T he method introduced allows calculation of the effective diffusion coe fficient as a function of the mean free path length over a wide variet y of values. A versatile method for the preparation of porous substanc es for light microscopy is described. The frequently applied model of an active shell, which is used to model particle/solid collisions, was found to give incorrect results in conjunction with the application o f the first passage time algorithm, which was applied to save computin g time in the simulation of gas diffusion. It was possible to show tha t more realistic results are obtained if a Knudsen layer is used to mo del particle/solid collisions. Furthermore, in investigating diffusion in two- and three-dimensional representations of capillary tubes, it was found that results of simulations performed to calculate transport properties of fluids in porous media, based on two-dimensional model systems, cannot in any case be transferred to the corresponding three- dimensional systems.