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 ia 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.