Absorption and adsorption of methane and carbon dioxide in hard coal and active carbon

The paper shows what can be deduced on sorption mechanisms in hard coals and active carbon by using a theoretical model of sorption of small molecules in elastic submicroporous materials. This model (referred to as the multiple sorption model-MSM) describes both adsorption and absorption phenomena. Basic assumptions and formulas of the MSM are presented. The computations were performed for isotherms of CO2 and CH4 at elevated pressures on three coal samples of different rank and on an active carbon. Nonideality of the sorbates is handled by an original state equation providing consistent information on fugacity and cohesion energy corresponding to a given molar volume of sorbate molecules in the sorption system. Surface structure of the studied coals and energetic parameters of the systems determined with MSM are compared to those obtained by using BET and Dubinin-Radushkievitch equations. It; was stated that MSM provides more information on surface structure (including submicropores) and clearly explains sorption properties of hard coals (together with expansion phenomena). It is also applicable to examination and prediction of adsorption on microporous materials (like active carbon).