Theoretically Based Model for Competitive Adsorption of Subcritical Mixtures

Numerous works have been achieved on the prediction of subcritical fluids, but most of them are about gas adsorption at low pressures or low concentrations, where adsorption consists in the formation of one adsorption layer. However, at higher pressures or concentrations, these mixtures can form an important number of adsorbed fluid layers and may exhibit a gas/liquid transition when the capillary condensation pressure is reached. In this work, we derive from the statistical mechanics a model predicting subcritical mixtures in such conditions. In the proposed model, the adsorbed molecules are supposed to be distributed in two distinct adsorption layers. Two parameters per adsorbed species characterize the interactions of the species in the adsorption layers. One additional parameter characterizes the adsorption capacity of the porous structures. To check the consistency of the model, Monte Carlo simulations on pure compound and mixtures adsorption are used as reference data. Our model shows improvements compared to the Ideal Adsorbed Solution Theory, whose adsorption isotherm are modeled with the BET model developed by Gritti et al. [J. Chromatogr. A 2002, 978, 81107]. Especially, the predictions of the adsorbed phases compositions are in good agreement with molecular simulation results, in both gas and liquid states. The adsorbed amount in the gas phase are also correctly predicted. By introducing an additional empirical parameter which characterizes the liquid adsorption capacity, it is also possible to predict the adsorbed amounts for the liquid phase, providing a consistent approach to model the adsorption of liquid and gas mixtures.