NETWORK MODELING OF DIFFUSION-COEFFICIENTS FOR POROUS-MEDIA .2. SIMULATIONS

Gas diffusion often dominates constituent transport in porous media (P M) and is dependent on pore geometry, water content, and water distrib ution in PM, Network models of PM offer the ability to investigate the influence and interaction of pore-scale PM properties and fluid prope rties on macroscopic properties of the system. This study was conducte d to investigate the macroscopic relative gas diffusion coefficient vs . air-filled porosity relationship or diffusion characteristic (DC) of PM using a network model. The network model was used to simulate DCs in wetting and drying PM containing air and water. A network size of n ine by nine by nine spheres was used; increasing the network size to 1 9 by 19 by 19 produced essentially no change in the DC. The DC was ind ependent of Henry's law gas-liquid partition coefficient (H) for H val ues of 0.1, 1.0, and 5.0. The product HRw, where Rw is the ratio betwe en the hulk gas- and liquid-phase diffusion coefficients, strongly inf luences the DC when H values of 1000 to 10 000 are considered; this in dicates that certain organic compounds have DCs independent of air-fil led porosity. Hysteresis in DCs was found in selected network cases, w ith the wetting DCs being greater than the drying DCs for most air-fil led porosities, in accord with some experimental results reported in t he literature. Spatial correlation of network pore space was shown to bring the simulated DCs into better agreement with some experimental D Cs.