Crystal Growth of Clathrate Hydrate in Liquid Water Saturated with a Simulated Natural Gas

Abstract

This paper reports the visual observation of the formation and growth of clathrate hydrate crystals in liquid water presaturated with a simulated natural gas (methane + ethane + propane mixture). The compositions of the methane + ethane + propane gas mixtures are (i) 90:7:3, (ii) 94.1:5.8:0.1, and (iii) 99.47:0.51:0.02 in molar ratio. A hydrate film first formed to intervene between the mixed gas and liquid water, and then hydrate crystals grew in the liquid water phase. The morphology of hydrate crystals grown in liquid water distinctly varied depending on the system subcooling ΔTsub. When ΔTsub is smaller than ∼7 K, hydrate crystal growth in liquid water was not observed. At ∼7 K < ΔTsub < ∼12 K, hydrate crystals with polygonal morphology were observed. At ΔTsub > ∼12 K, polygonal crystals were replaced by dendritic crystals. These changes in morphology were observed with all three gas mixtures. It was found that the morphology in the system with mixed gas of the molar ratio 99.47:0.51:0.02 was different from those of the other two systems. We also observed the hydrate crystals floating to the hydrate film from liquid water phase. Floating crystals formed in the bulk of liquid water, attached the hydrate film, and then continued to grow in liquid water. The morphology of floating crystals varied with ΔTsub and the gas composition.