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. 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.

Abstract:The three-phase coexisting curve of Ar hydrate + water + gas has been investigated in a pressure range up to 485 MPa and a temperature range of (279.57 to 305.32) K. The Raman spectrum of intermolecular O-O stretching vibration mode has been measured for Ar hydrate crystal along the stability boundary curve. Both the discontinuity of slope of the three-phase coexisting curve and the pressure dependence of the Raman shift reveal that two structural phase transition points exist at (281±1) MPa and (302.7±0.1) K, (456±1) MPa and (304.6±0.1) K in a pressure range up to 485 MPa for the Ar hydrate system.