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This paper report analyses of thermodynamic stability of structure-H clathrate hydrates formed with methane and large guest molecules in terms of their gas phase molecular sizes and molar masses for the selection of a large guest molecule providing better hydrate stability. We investigated the correlation among the gas phase molecular sizes, the molar masses of large molecule guest substances, and the equilibrium pressures. The results suggest that there exists a molecular-size value for the best stability. Also, at a given molecule size, better stability may be available when the large molecule guest substance has a larger molar mass.

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.

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