Predictions of the adsorption equilibrium of methane/carbon dioxide binary gas on coals using Langmuir and ideal adsorbed solution theory under feed gas conditions

Presently many research projects focus on the adsorption and predictions of methane, nitrogen, carbon dioxide and their mixtures on coals under equilibrium gas conditions. It is highly important to develop a method for prediction of CO2/CH4 adsorption on coal under feed (source) gas conditions in the fields of enhanced coal bed methane recovery and CO2 sequestration in coal bed with CO2 injection. The objective is to develop reliable models to predict the adsorption behavior of mixtures on coals under feed gas conditions. In this context, the adsorption and predictions of methane, carbon dioxide and three binary mixtures of these gases on two China coals under feed gas conditions have been performed on the experimental data and derived equation.Firstly, the experimental measurements of CH4, CO2 and their binary gas adsorption isotherms to investigate the adsorption behavior and use for prediction of adsorption equilibrium were conducted at 301 K at pressures up to 10 MPa. Secondly, a calculation formula of adsorbed amount of total gas was derived from relation between adsorbed amount and feed gas composition at standard state on condition of unknown equilibrium gas concentration during adsorption. Thirdly, the extended Langmuir, ideal adsorbed solution (IAS) theory in conjunction with Langmuir were used to predict the experimental data of binary gas adsorption based on an iterative algorithm. Finally, the prediction accuracy for experimental data of total and adsorbed amount of component gas and free gas compositions were analyzed.Experimental data indicate adsorbed amounts of the total gas and single-component CO2 increase, but adsorbed amounts of CH4 increase at low equilibrium pressure and decrease at high pressure with increase of CO2 concentration in feed gas. Model predictions based on single-component isotherms and the calculation formula show that the quantitative formula obtained can be used to predict adsorption equilibrium of CH4/CO2 binary gas under feed gas conditions. The binary gas prediction data are quite variable and depend upon the feed gas composition. The IAS–Langmuir provides, on average, a better fit to total gas adsorption but is variable for the single-component adsorption with the feed gas composition. The predictions of total and component gas adsorption and equilibrium gas compositions illustrate similar trends with their experimental data.