Hydrate Phase Equilibria of the Carbon Dioxide, Methane, and Water System

Abstract

Three-phase equilibria for the carbon dioxide + methane + water system were obtained by employing the isobaric temperature search method. Based on these isobaric hydrate equilibrium studies, the ternary hydrate, water-rich liquid, and vapor equilibrium lines generated at different compositions of carbon dioxide and methane were all located between two three-phase equilibrium lines of simple hydrates formed by a single guest component. The upper quadruple points where the four phases hydrate, water-rich liquid, CO2-rich liquid, and vapor coexist were measured for the composition range of 100−82.50 mol % carbon dioxide. Below 82.50 mol % carbon dioxide, the upper quadruple points do not exist because none of the components in the vapor phase, neither methane nor carbon dioxide, is able to liquefy at these conditions. In addition, two-phase equilibria of vapor and hydrate were also determined at the three different pressures 20, 26, and 35 bar. Judging from the resulting T−x diagram, the concentration of carbon dioxide in the hydrate phase was found to be higher than 90 mol % when the corresponding equilibrium vapor-phase composition was more than 40 mol % carbon dioxide. The carbon dioxide concentration and relative selectivity over methane in the hydrate phase appeared to increase with decreasing pressure.