An investigation into the thermodynamic characteristics of methane adsorption on different clay minerals

• The improved D-A model was deducted by combining the D-A equation and the adsorption characteristic curve.
• The calculation models of isosteric heat of adsorption and variation value of surface free energy were established, respectively.
• The variation value of surface free energy increased rapidly with increasing the pressure, whereas decreased with increasing the temperature.
• The isosteric heat of adsorption decreased with increasing the methane capacity.

In this work, we investigate the methane adsorption characteristics of common clay minerals using experimental and theoretical analyses. Models for the isosteric heat of adsorption and variation of the surface free energy were established using thermodynamic theory and an improved D-A model, respectively. The models were used to investigate changes of the isosteric heat of adsorption and variation of the surface free energy of the different clay minerals. Then, the adsorption thermodynamics and adsorption capacity characteristics of the different clay minerals were investigated. An improved D-A model was developed by combining the D-A equation and adsorption characteristic curve and was used to fit the methane adsorption isotherms of the clay minerals. The model fitting results replicated the experimental results. The variation of the surface free energy is a function of pressure and temperature, which increased rapidly with the pressure increase at low pressures, while the free energy increased more slowly at higher pressure and decreased with increasing temperature. The order of the variation of the surface free energy from largest to smallest is illite, chlorite, kaolinite and montmorillonite, which is in disagreement with the order of the methane adsorption capacity, indicating that the specific surface area is a major factor that affects the methane adsorption capacity of the clay minerals. The isosteric heat of adsorption decreased with increasing methane capacity. The initial isosteric heats of adsorption of the clay minerals were 26.088 kJ/mol, 25.543 kJ/mol, 20.503 kJ/mol and 24.229 kJ/mol for montmorillonite, kaolinite, illite and chlorite, respectively. This indicates that the intermolecular forces between the methane molecules and the clay minerals decreases in the following order: montmorillonite, kaolinite, chlorite and illite.