Pressure and temperature conditions for the four-phase equilibrium in systems that include liquid water, structure H hydrate, methane gas, and liquid hydrocarbon substances (cycloheptane, cycloheptanone, or oxacycloheptane) were measured over the temperature range (273.5 to 284.1) K. The measurements indicate that, at 274 K, the equilibrium pressures of the systems with cycloheptane, cycloheptanone, and oxacycloheptane were lower by (1.6, 1.3, and 1.5) MPa, respectively, than those of the structure I methane hydrates in equilibrium with liquid water and methane vapor. Powder X-ray diffraction measurements of the formed hydrate samples confirmed structure H hydrates.

This article reports the pressure−temperature conditions for the four-phase methane vapor + aqueous liquid + hydrocarbon liquid + hydrate equilibrium in the methane + water + 2,2-dimethylpentane system. The pressure and temperature ranges are from (2.430 to 4.823) MPa and from (273.7 to 279.3) K, respectively. The four-phase equilibrium pressures of this system at temperatures below 278.5 K are lower than the three-phase methane vapor + aqueous liquid + hydrate equilibrium pressures in the 2,2-dimethylpentane free system, whereas the four-phase and three-phase equilibrium pressures are equal at temperatures above 278.5 K. This difference in the equilibrium pressures due to the addition of 2,2-dimethylpentane indicates the formation of a structure-H hydrate at temperatures below 278.5 K. The crystallographic structures of the hydrates formed in this system have been identified by powder X-ray diffraction measurements.