Apparent diffusion coefficient for adsorption-controlled gas transport in nanoporous media

Abstract

We present an apparent diffusion coefficient derived from a diffusion-based mass conservation equation for single-component gas transport in nanoporous media. Multiple transport and storage mechanisms have been considered, including Knudsen diffusion and viscous flow for the free phase and surface diffusion and multilayer adsorption for the sorbed phase. The real gas effect, confinement effect, and dynamic adsorption layer thickness are also considered. A pore-scale simplified local density method is utilized to obtain sorbed phase properties, such as volume fraction, concentration, and adsorption isotherm. The developed apparent diffusion coefficient is applied to investigate the effects of fluid–solid attraction, fluid–solid species, and pore sizes on the gas diffusion and adsorption in nanoporous media. The results reveal that the sorbed phase occupies a significant volume of nanopores, and the entire pore space becomes sorbed-phase volume in pores with diameters ≤ 2 nm. The total mass transport in these pores is dominated by surface diffusion, and free-phase diffusion contribution increases as the pore size increases. Furthermore, in strong adsorption systems such as CO2-coal, the apparent diffusion coefficient is up to two orders of magnitude smaller than in weak adsorption systems (e.g., N2-coal and CH4-coal). This is attributed to the reduced free-phase volume fraction and increasing adsorption capacity. The proposed apparent diffusion coefficient can be used in the diffusion-based models for gas transport in nanoporous media.