Thermodynamic analysis of molecular simulations of N2 and O2 adsorption on zeolites under plateau special conditions

Abstract

Adsorption thermodynamics of N2 and O2 on two most commonly used zeolites (5A and Li-LSX) under plateau special conditions of wide ranges of temperature (238–298 K) and pressure (0–300 kPa) for pressure swing adsorption (PSA) oxygen production were studied based on molecular simulations and theoretical model fitting. Adsorption isotherms for each adsorbate-adsorbent pair at different temperatures were fitted by Langmuir, Freundlich and Sips models, obtaining equilibrium parameters including adsorption capacity, strength, heterogeneity and N2/O2 selectivity. Thermodynamic parameters such as Gibb's free energy, enthalpy, and entropy changes were numerically derived, and isosteric adsorption heats and distributions of potential energy were simulated. The results suggest that the adsorption capacity and affinity follow the order of N2-Li-LSX > N2-5A > O2-5A > O2-Li-LSX, leading to higher N2/O2 selectivity on Li-LSX compared to 5A. For Li-LSX, N2/O2 selectivity increases significantly with temperature above 273 K due to the difference in adsorption thermodynamics of N2 and O2 varying with temperature in terms of different adsorption heterogeneities. For 5A, the homogeneous distributions of N2 and O2 as well as the wide scopes of affinities from Na+ and Ca2+ exhibit N2/O2 selectivity with negligible temperature dependence, which is beneficial for stable oxygen production with a fixed PSA process under large day-and-night temperature difference. This study provides theoretical basis for obtaining an optimal balance between efficiency and stability of oxygen production by using a proper mixture of Li-LSX and 5A.