Gas-phase diffusion in porous media - Evaluation of an advective-dispersive formulation and the dusty-gas model for binary mixtures

Two models for combined gas-phase diffusion and advection in porous media, the Advective-Dispersive Model (ADM) and the Dusty-Gas Model (DGM), are com monly used. The ADM is based on a simple linear addition of advection calcu lated by Darcy's law and ordinary diffusion using Fick's law with a porosit y-tortuosity-gas saturation multiplier to account for the porous medium. An other approach for gas-phase transport in porous media is the Dusty-Gas Mod el (DGM). This model applies the kinetic theory of gases to the gaseous com ponents and the porous media (or "dust") to develop an approach for combine d transport due to diffusion and advection that includes porous medium effe cts. The objective of this paper is to evaluate these two approaches by com parison to Graham's laws and to isothermal experimental data for He-Ar gas diffusion in a low-permeability graphite. This evaluation shows that the AD M while intuitively appealing, violates Graham's laws and does not compare well to the experimental data. In contrast, the DGM is based on Graham's la ws and shows excellent agreement with the data. The two models are further compared for a hypothetical linear air-water porous media heat pipe, involv ing heat, advection, capillary transport, and diffusion under nonisothermal unsaturated conditions. If diffusion is an important transport mechanism, the DGM is preferable to the ADM because the ADM may give incorrect trends and/or misleading results.