Equilibria and Isosteric Heat of Adsorption of Methane on Activated Carbons Derived from South African Coal Discards

Isosteric heat of adsorption (Hst) is critical for evaluating the thermal effects of adsorption-based storage systems. Poor management of the thermal effects of an adsorptive storage system often alters the overall performance of the storage system. In this study, methane equilibrium uptake on activated carbons derived from coal discards and isosteric heat of adsorption were evaluated. The methane adsorption capacity of the produced activated carbons was measured using a high-pressure volumetric analyzer. The isotherm results in temperature ranges of 0-50 °C and pressure of up to 40 bar are analyzed using the Langmuir, Tóth, and Dubinin-Astakhov (DA) isotherm models. The results showed that, for the two activated carbons, the DA model was the best fit. In addition, we evaluated the isosteric heat of adsorption using two theoretical frameworks, Maxwell's thermodynamic relations and the modified Polanyi potential function. The Tóth potential function and Clausius-Clapeyron equations were applied to the Dubinin-Astakhov adsorption model to obtain an analytical expression of Hst. Both methods were compared, and the result showed an overall error margin between 6 and 12%. The values of Hst obtained are over a range of 10-17 kJ/mol. It was observed that Hst decreases with an increase in methane fractional load. The Hst values obtained are useful in designing an efficient thermodynamic scheme for the ANG storage system.