Isothermal Phase Equilibria for Methane + Ethane + Water Ternary System Containing Gas Hydrates

Abstract

It is well known that methane + ethane mixed-gas hydrate exhibits the structural phase transition between structure -I and-II although both of pure guest species form the structure-I hydrate at certain pressures and temperatures (Subramanian, Kini, Dec and Sloan, Chem. Eng. Sci., vol. 55, pp. 1981-1999, June 2000). In the present study, isothermal phase equilibrium (pressure-composition) relations for the methane + ethane mixed-gas hydrate system were measured accurately at 279.1 K-288.1 K. In addition, the equilibrium composition in the mixed-gas hydrate phase was investigated by mass and volume balances analysis. At all temperature conditions, the equilibrium curves exhibit discontinuity around given equilibrium compositions (water free), which supports that the structural phase transition occurs. The pressure of structural transition point increases as temperature rises, and the mole fractions of both hydrate phases at the structural transition point are almost independent of temperature.