Multiphase Analysis for High-Pressure Adsorption of CO2/Water Mixtures on Wet Coals

Traditional modeling of gas adsorption on wet coals does not consider water as a separate adsorbed component and treats the adsorbed water as a “pacifier” of the coal matrix. In a recent work, we presented a new modeling approach for investigating the competitive adsorption of gas/water mixtures in high-pressure systems. We used the simplified local-density (SLD) model to investigate the effect of including the competitive adsorption of water present in coals on gas adsorption under the conditions encountered in coalbed methane (CBM) and CO₂ sequestration applications. In continuation of that work, we present here a new multiphase algorithm to investigate the three-phase adsorption equilibrium of CO₂/water mixtures on wet coals. When water is treated as one of the adsorbed components in a high-pressure gas adsorption system, as many as three fluid phases may coexist at equilibrium (gas, adsorbed, and liquid phases). A new algorithm is presented in this work to facilitate a Gibbs energy-driven multiphase analysis of the system. The algorithm employs a phase-insertion technique, which involves formally inserting a water-rich, bulk liquid phase and solving a three-phase flash problem, wherein the three phases are the adsorbed, bulk gas, and liquid phases. At equilibrium, the 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 minimum in the Gibbs energy at the given temperature and pressure, subject to material balance constraints, provides the equilibrium phase distribution in these systems. Multiphase analysis was performed for high-pressure CO₂/water mixture adsorption on four wet coals utilizing this algorithm. Analysis indicates that a water-rich liquid phase is present in coals that contain large amounts of inherent moisture. For these coals, the water-rich phase appeared at the higher pressures in the isotherm and the fraction of this phase increased with bulk pressure, reaching a maximum near the CO₂ critical pressure. In contrast, the low-moisture coals did not appear to contain a third-phase at equilibrium.