Traditional modeling of gas adsorption on wet coals does not include water as a separate adsorbed component; instead, the adsorbed water is viewed 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 effect of the water present in coals on gas adsorption under the conditions encountered in coalbed methane (CBM) and CO2 sequestration applications. To conduct this study, our previously acquired measurements for high-pressure CO2 adsorption on wet coals were utilized.

When water is treated as one of the adsorbed components in a high-pressure gas adsorption system, as many as three phases may coexist at equilibrium. To investigate the phase behavior of CO2/water adsorbed gas mixtures on wet coals, a new algorithm was developed to facilitate a Gibbs energy-driven multiphase analysis of this system. The algorithm uses a phase-insertion technique, which involves formally inserting a third (liquid) phase and solving a three-phase flash problem, wherein the three phases are the adsorbed, bulk gas and liquid phases. At equilibrium, the total Gibbs energy of the system is calculated based on the phase distribution obtained at each step. This calculation is repeated sequentially with incrementally increased amounts of the inserted third phase. A Gibbs energy analysis for CO2/water mixed gas adsorption on wet coals was conducted utilizing this new algorithm.

Results involving CO2/water mixtures on four well-characterized coals indicate that the SLD-PR model is capable of representing the adsorption of these highly asymmetric mixtures within the experimental uncertainties. The Gibbs analysis of these coals indicates that there is a third phase present in systems that contain large amounts of moisture. The third, water-rich phase appeared typically at the higher pressures in the isotherm.

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