Application of a modified Dubinin-Radushkevich equation to adsorption of gases by coals under supercritical conditions

The classic Dubinin-Radushkevich (DR) equation, which is used to describe the adsorption of gases, requires a "saturation pressure" in its calculation, which is not defined for supercritical conditions. By using gas density instead of gas pressure (and adsorbed phase density rather than saturation pressure) to describe the sorption of the gas onto the surface, the DR equation can be applied in supercritical conditions. A proportionality term (k times the gas density) was added to this modified DR equation to account for possibilities such as Henry's law dissolution by the coal, errors in cell volume and helium density, and differences in the accessibility to coal between helium and the other gases. The sorption characteristics of three dry Australian bituminous coals when exposed to carbon dioxide, methane, and nitrogen were measured over the range 0-20 MPa at 53 degrees C. The data were fitted by the modified DR equation to within 1% of the sorption capacity across the isotherm in nearly all cases, substantially better than any of the alternatives tested. Without the proportionality term, sorption at high pressures was substantially underpredicted.