Comprehensive Thermodynamic Study of the Calcium Sulfate–Water Vapor System. Part 2: Physical Modeling of Adsorption Phenomena

We employ a rigorous thermodynamic modeling approach to investigate the water adsorption phenomena on two calcium sulfate compounds, AIII-CaSO4 and CaSO4·0.5H2O. In part 1 of this work ( Ind. Eng. Chem. Res., 2019, DOI: 10.1021/acs.iecr.9b00856 ), we prepared these two products by the dehydration of synthetic CaSO4·2H2O and obtained quantitative adsorption data as a function of the temperature and water vapor partial pressure. In this part, we develop macroscopic solution models (ideal and nonideal) to model monolayer adsorption on AIII-CaSO4. This allowed the calculation of the energies of adsorption for this phenomenon, evidencing a physisorption mechanism. For the CaSO4·0.5H2O, we interpreted the water adsorption using a multilayer adsorption model (BET model). For both materials, we showed that nitrogen adsorption data was not sufficient to represent their entire surface areas and porosity profiles compared to their water vapor sorption capacity.