Cation exchange modification of clinoptilolite -thermodynamic effects on adsorption separations of carbon dioxide, methane, and nitrogen

This study details an experimental analysis of the thermodynamics of adsorption equilibrium behaviour of pure CO2, CH4, and N-2 gases on raw, Fe3+, Ca2+, and Cs+ cation exchanged clinoptilolite. Equilibrium adsorption isotherms were experimentally determined for CO2, CH4, and N-2 at three different temperatures ranging from 303K up to 363K, at pressures up to 8.0atm using the microgravimetric technique. Kinetic adsorption uptake data was measured at different temperatures for CH4 and N-2 gases, and the results were compared for raw and Ca2+ clinoptilolite. Heat of adsorption values were calculated for all three gases on the studied clinoptilolite samples using the chromatographic method. These results were compared to the isosteric adsorption enthalpies determined from the adsorption isotherms at different temperatures. The isosteric heat of adsorption was the highest for CO2 followed by CH4, and N-2. Temperature dependent Sips, Toth, and Dual-Site Langmuir isotherm models were fitted with experimental data and selectivity factor values were calculated for CO2/CH4 and CH4/N-2 gas separations using the regressed isotherm models. Ca2+ clinoptilolite shows the best potential for CO2/CH(4 )separations due to its high selectivity. However, higher temperatures are needed in order to compensate for the slow kinetic behaviour. For CH4/N-2 separations, Fe3+ clinoptilolite selectivity displays both high values and less variability compared to Cs+ clinoptilolite over a broader range of temperatures and pressures.