Combined isotherm of adsorption and absorption on coal and differentiation of both processes

Sorption of hydrogen and helium on coal and activated charcoal was measured at 298 K within the pressure range 0–1 bar and the obtained isotherms were characterized by a maximum deviation of the desorption branch from the adsorption branch implied with their shape the process of irreversible absorption. At the sorption of carbon dioxide on coal, the simultaneously running adsorption and absorption were registered by a combined isotherm AD/AB, which desorption branch was for individual coals characterized with variously sized hysteresis. After separation of both partial sorption processes, a scheme of reconstruction of the combined isotherm AD/AB has been proposed. This provided a corrected isotherm of separate physical adsorption AD for calculation of parameters of the microporous structure according to Dubinin, as well as a pure absorption isotherm AB, for which the analysis of experimental values confirmed its linearity according to Henry’s equation. Since the isotherm AD/AB including besides filling of micropores also occupation of intramolecular voids within the macromolecular coal matrix complies very well with the interpretation in Dubinin’s coordinates, the designation nanoporous system has been suggested for this set of cavities. From the difference of the spatial parameters calculated from the isotherms AD/AB and AD, the effective size of intramolecular voids could be estimated.