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, isothermalphase equilibrium (pressure - composition) relations for the methane + ethane mixed-gas hydrate system were measuredaccurately at 279.1 K - 288.1 K. In addition, the equilibrium composition in the mixed-gas hydrate phase was investigatedby mass and volume balances analysis. At all temperature conditions, the equilibrium curves exhibit discontinuity aroundgiven equilibrium compositions (water free), which supports that the structural phase transition occurs. The pressure ofstructural transition point increases as temperature rises, and the mole fractions of both hydrate phases at the structuraltransition point are almost independent of temperature.
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