Coupled gravimetric, manometric and calorimetric study of CO2, N2 and CH4 adsorption on zeolites for the assessment of classical equilibrium models

Abstract

N2, CO2 and CH4 pure gas adsorption equilibria on five zeolites with different structural properties (Si/Al ratio, type of cations contained inside their structure, pore size and pore volume) have been measured over a wide range of pressures (from 10–5 to 80 bar) and temperatures (from 253 to 363 K) by combining high pressure gravimetric technique and high resolution low pressure manometry. These experimental data, coupled with the measurement of the differential heat of adsorption and with some literature information obtained with microscopic studies, have allowed to identify and to analyze the different adsorption mechanisms. The results show that CO2 adsorption mechanism is controlled by molecule–cation interactions at low pressures and by the pore volume filling at intermediate and high pressures. On the contrary, N2 and CH4 adsorption mechanism is controlled by the pore volume filling in the whole range of pressures studied in this work. It is shown that the most popular models used in gas separation modeling such as Toth, Sips and bi-Langmuir do not describe the successive physico-chemical phenomena observed for the adsorption of CO2 on zeolites. Moreover, they are not able both to fit the whole range of experimental data and to predict the isosteric heat of adsorption accurately.