Morphologies, kinetics and structures of methane hydrate in the system containing tetrahydrofuran and cyclopentane

Abstract

The morphologies of methane (CH4) hydrate formation in the system containing different mole ratios of tetrahydrofuran (THF) and cyclopentane (CP) at two different temperatures are systematically studied in this work. The results showed that for all cases conducted at 274.2 K, a hydrate film quickly covers the entire interface between the oil (CP) and the THF solution phase after gas loading, and then a hydrate layer forms, thereby significantly decreasing the following gas-liquids mass transfer efficiency. Raman spectrum shows this hydrate layer is caused mainly by pure THF hydrate. Interestingly, as the temperature increase to 278.2 K, the hydrate layer breaks. This is because, after hydrate layer formation, hydrate still grows at the contact interface of the metal, gas, and liquid phase, and then moves upward. In this process, liquids are gradually transported to the reaction front to form hydrate by capillary force and the hydrate layer’s downward curvature gradually increases as the consumption of liquid until rupture. Besides, typical reaction kinetics shows that there are three stages of hydrate formation in the THF + CP + CH4 system at 278.2 K. (1) hydrate layer formation (pure THF hydrate formation); (2) CP-CH4 hydrate formation; (3) large amount of THF-CH4 hydrate formation (after hydrate layer rupture). Further, this work also confirmed that after the hydrate layer rupture, the CH4 occupancy in 512 cages of sII at the same temperature and promoter’s mole fractions is independent of the type of promoters (CP, THF, and CP + THF) used in the system.