Kinetics of Gaseous Sorbate Accumulation and Emission Processes from a Porous Medium – is the Diffusion Coefficient a Material Constant?

Abstract

The kinetics of sorption and diffusion processes in porous materials significantly influence applications such as greenhouse gas capture, toxic gas removal, and geological storage. This study investigates whether the diffusion coefficient, a critical parameter in describing these processes, can be treated as a material constant. Experimental analyses were conducted using hard coal samples subjected to methane sorption under controlled conditions. Numerical and analytical models were developed, incorporating linear and Langmuir sorption isotherms, to simulate the diffusion and sorption processes. Results indicate that assuming a linear isotherm allows for satisfactory modelling in low-density sorbate regions but introduces artefacts at higher densities, where the Langmuir isotherm provides superior accuracy. The numerical model, validated against analytical solutions, demonstrates high precision in extracting diffusion coefficients and isotherm parameters, highlighting the interdependence of sorption kinetics and equilibrium conditions. These findings emphasise the need for advanced modelling techniques to accurately characterise diffusion in complex porous systems and establish the diffusion coefficient as a robust material property.