Comparisons of adsorption models for methane adsorption equilibrium on activated carbon

Comparisons of accuracies of adsorption models in predicting adsorption data of supercritical methane on activated carbon were carried out for practical application of adsorption of natural gas (ANG). Ajax activated carbon was selected as an adsorbent, six isotherms of excess adsorption amount of methane were measured at temperature from 268.15 K to 338.15 K and pressure up to 12.5 MPa. Parameters of Langmuir, Langmuir-Freundlich and Toth equations were firstly set by the linear fit of adsorption data, absolute amounts and densities of the adsorbed phase of supercritical methane were then determined by the modified models. Isosteric heats of methane adsorption on Ajax activated carbon were determined by adsorption isosteres on the absolute amounts. Comparisons were made between experimental data and those predicted by models. Results showed that the adsorbed phase densities of supercritical methane on the activated carbon varied with equilibrium temperatures and pressures, the mean value of isosteric heat of adsorption set by absolute amounts was 15.72 kJ/mol, which was smaller than that plotted from the excess amounts. The relative errors between the experimental data and those predicted by Langmuir, Langmuir-Freundlich and Toth equations varied with the pressure, The accumulated relative errors were respectively 6.449 8%, 7.918 4% and 0.910 0% at lower pressure range from 0 to 0.025 MPa, but will respectively be 0.491 1%, 0.161 3% and 0.369 4% while pressure was in range of 1～10 MPa. Toth equation performed well in predicting the equilibrium data in the whole pressure range, but the results from Langmuir-Freundlich equation had higher prediction accuracies while the pressure got higher.