Species-Based Modeling of Binary Gas Mixture Transport in Nanoporous Media with Adsorption

We present a predictive species-based model for binary gas mixture transport through nanoporous media using modified Maxwell–Stefan formulations for the free and sorbed phases. The developed model considers the transport of free and sorbed phases through nanopores and incorporates the extended Langmuir isotherm for multicomponent adsorption. The diffusive mass fluxes for both phases are coupled to obtain the governing equations with binary effective diffusion coefficients and capacity factors that account for adsorption. The governing equations are functions of the free-phase composition and pressure. The model is utilized to conduct a sensitivity analysis of key parameters. Co- and counter-diffusion processes are simulated to investigate CO2/CH4 flux from and CO2 injection into organic-rich shale and coal samples, representing moderate and high adsorption capacity systems, respectively. The results show that the sorbed phase occupies nearly half of the pore volume in the chosen coal sample. Higher diffusion coefficients were obtained for low or moderate adsorption systems. For both co- and counter-diffusion processes, the sorbed phase concentrations are higher in the coal sample. The contribution of the sorbed phase to the total mass depends on both pore size and adsorption affinity for low-pressure systems, while it depends only on adsorption affinity for high-pressure systems.