Adsorption/Desorption of Multicomponent Gas Mixtures at In-Seam Conditions

Adsorption isotherms were measured for binary and ternary mixtures of CH₄, CO₂ and N₂ on coal at 30°C and pressures to 5200 kPa. The data shows that equilibrium gas and adsorbate phase compositions differ considerably and that the total amount of gas mixture adsorbed is strongly dependent on composition and system pressure. The experimental measurements were used to test the applicability of models based on adsorbate solution theory to the prediction of multicomponent adsorption data from pure component isotherms at pressures of interest in coalbed gas applications. It is concluded that IAS theory, which assumes ideality of the adsorbed phase, provides predictions which would be of acceptable accuracy in many coalbed gas applications. Attempts to apply RAS theory to the data show that a clear need exists for the development of models to correctly predict adsorbate activity coefficients for gas mixtures on coal at elevated pressures. The experimental adsorption data reported in the paper will be useful in the development and testing of such models.