(193t) A New Algorithm for Multiphase Analysis of Gas/Water Mixture Adsorption On Coals
AIChE Annual Meeting
Monday, October 17, 2011 - 6:00pm to 8:00pm
Coalbeds typically contain large amounts of water. Since a large proportion of (natural) gas in such reservoirs is in an adsorbed state, an adsorption equilibrium model is required to describe the adsorption/desorption behavior on coals. Traditionally, these models do not account explicitly for water as a separate adsorbed component in a gas mixture and only treat water as a “pacifier” of the coal adsorbent surface. Further, the conventional adsorption models (thus far) account for the presence of only two equilibrium phases - gas and adsorbed phases. This simplified modeling approach may lead to significant errors in simulations of enhanced coalbed methane recovery and CO2 sequestration potential of coalbed reservoirs.
When water is one of the adsorbed components in a high-pressure gas adsorption system, as many as three phases may coexist at equilibrium. In this work, a new multiphase (three-phase) algorithm is presented and used to investigate the high-pressure adsorption behavior of CO2/water gas mixtures on wet coals. The algorithm uses a phase-insertion technique, which involves formally inserting a third (liquid) phase and solving a series of three-phase flash problems, wherein the three phases are the adsorbed, bulk gas, and liquid phases. A Gibbs-energy-driven algorithm was used to conduct both fixed and variable feed multiphase analyses for CO2/water mixture adsorption on wet coals. The simplified local density/Peng-Robinson (SLD-PR) adsorption model was used in the multiphase flash algorithm.
Results indicated that high-moisture containing coals form a third (liquid) phase at nearly all pressures in the range of measurements. Further, the predictions obtained utilizing a fixed, experimentally measured feed were qualitatively consistent with estimates obtained through variable-feed calculations. The analysis also indicates that low-moisture coals contain only two phases at equilibrium and do not form a free-water phase.