Modified potential theory for modeling supercritical gas adsorption

Theoretical modeling of adsorption plays a crucial role in providing better understanding of the adsorption phenomena, isotherms and isosteric heats. However, when modeling the adsorption of gas mixtures containing hydrogen, it is necessary to accommodate a wide temperature range because of hydrogen's low critical temperature. In this work, we extend the multicomponent potential theory of adsorption's (MPTA) capability of predicting adsorption isotherms to a wide temperature range by introducing a temperature dependent Dubinin potential parameter and use it to model adsorption isotherms of supercritical hydrogen, nitrogen and methane on various activated carbons. This extended MPTA can accurately predict the adsorption isotherms when used with NIST equation of state (EOS). The resulting isosteric heats of adsorption of hydrogen agree well with the experimental data for similar volume filling scenarios. Hydrogen's low temperature adsorbed-phase pressure inside the activated carbon's micropore volume reaches the melting pressure of solid hydrogen. This causes the transition of adsorbed hydrogen from supercritical gas to solid-like phase which is clearly observed in our model. Our study, thus, provides a better understanding of physisorption of hydrogen inside the micropores. ► Extension of MPTA to cover a wide temperature range of hydrogen adsorption. ► Comparison of two adsorption potentials and two equations of state. ► Low temperature hydrogen density at the adsorbent interface near that of solid phase. ► Calculated isosteric heat for hydrogen agrees with that found in literature. ► NIST EOS and DRA potential is optimal combination for excess adsorption prediction.