Thermodynamic analysis of high-pressure methane adsorption on coal-based activated carbon

In this work, three different activated carbons were prepared from low-rank bituminous coal by KOH activation method. High-pressure methane adsorption tests were conducted to these three different activated carbons from 273.15 to 373.15 K. Experimental results show that 313.15 K seems to be the critical temperature point for methane adsorption. When the adsorption temperature is lower than 313.15 K, methane adsorption amount would be rapidly decreased with the increase of temperature. The maximum-monolayer adsorption capacity q(m), which is fitted by adsorption experimental result of activated carbon, would be gradually decreased with the increase of temperature. The adsorption equilibrium constant b also presents the similar law as the temperature changes. Thermodynamic parameters show that methane adsorption on activated carbons are nonspontaneous (Delta G degrees > 0) and exothermic (Delta H degrees < 0) physisorption (the magnitude of Delta H degrees < 20 kJ/mol) under high-pressure conditions. And, with increase of temperature, the adsorption feasibility would be decreased. The isosteric heat of adsorption (q(st)) of methane on the activated carbons are 15.78-16.72 kJ/mol, 15.94-18.26 kJ/mol and 15.83-16.97 kJ/mol, respectively. With increase of the adsorption amount, isosteric heat of adsorption present a first decreasing and then increasing rules under the influence of temperature and pressure. The minimum value of qst appears near the adsorption amount of 3 mmol/g. The higher temperature would increase the thermal motion of methane molecules and the higher pressure would increase the density of methane molecules in the fixed space, which would result in the intermolecular interactions of methane that have been ignored are gradually appeared.