Study of ultramicroporous carbons by high-pressure sorption. Part 5.—CH4 isotherm and diffusion kinetics

Abstract

Sorption–desorption kinetics and the isotherm for CH₄ in as-received TCM 128 ultramicroporous carbon fibre are reported at 35°C up to 15 atm. The CH₄ isotherm exhibits a similar hysteresis to that seen for N₂ and CO₂ on the carbon but at a much lower pressure. The low-pressure hysteresis might reflect adsorption in restricted regions composed of constrictions analogous to the tiny constrictions which were the cause of the hystereses seen with N₂ and CO₂. For the larger CH₄, however, some of the regions that were easily accessbile to N₂ may be behaving as ‘restricted’ to CH₄ owing to a molecular-sieving phenomenon. Analysis of the diffusion kinetics indicates that the large CH₄ molecules cause a slow barrier build-up over a few constrictions in series at the outer pores of the carbon. The desorption kinetic response, which is much faster than the adsorption, follows the Fickian model and thus reveals diffusion coefficients of the order 10^–13 cm² s^–1.