(191o) A New Modeling Approach for CO2- Water Mixed Gas Adsorption On Coals

Traditional modeling of gas adsorption on wet coals does not include water as a separate adsorbed component and treats water as a "pacifier" of the coal matrix. In this work, we modeled gas adsorption on wet coals by considering water as an active component in a binary mixture. Specifically, we used the simplified local-density/Peng-Robinson (SLD-PR) model to investigate the effects of water present in coals on gas adsorption under the conditions encountered in coalbed methane and CO2 sequestration applications. To conduct this study, new high-pressure gas adsorption measurements were made for CO2 on wet Argonne coals using a volumetric technique.

First, a new data reduction method was implemented that accounts for the presence and effect of water in as many as three equilibrium phases (gas, liquid, adsorbed). Second, the SLD-PR was modified to account for the unique molecular interactions of water in the adsorbed phase. Third, the modified SLD-PR model was used to describe the adsorption of CO2-water mixtures on five well-characterized coals. Finally, a phase-check analysis was performed using the SLD model to investigate the possible formation of a third (aqueous) phase in these systems.

Results indicate that the SLD-PR model is capable of representing the adsorption of this highly asymmetric mixture within the experimental uncertainties, on average. The model parameterization used and the molecular interactions considered for describing water adsorption on coals illustrates a viable method to obtain precise representations of the adsorbed CO2-water mixtures. The phase-check analysis of the same mixture indicated potential formation of a water-rich liquid phase in these systems for coals that contained large amounts of moisture (with the exception of Beulah Zap lignite coal).