Crystal growth behavior of methane hydrate in the presence of liquid hydrocarbon

This paper reports the visual observation of the formation and growth of methane hydrate crystals in the methane + decane + water three-phase quiescent system. This three-phase system is a simplified model of the condition within an oil-producing pipeline. A hydrate crystal nucleated at a random point on the water droplet surface and then became a hydrate film that covered the whole surface of the water droplet. The morphology of individual hydrate crystals grown at the water droplet surface were triangular or polygonal. The hydrate crystal size and the hydrate-covered water droplet shape distinctly varied depending on the system subcooling (ΔT_sub). The hydrate crystal size got smaller and more slender with the increase in subcooling, and the hydrate-covered water droplet shape collapsed even more at a lower subcooling. To estimate the effect of non-hydrate-forming liquid existence, we compared the observational results obtained in the systems with and without liquid hydrocarbon. Although the liquid hydrocarbon phase offers the resistance of the mass transfer of methane, the observations indicate that the results observed in the systems with liquid hydrocarbon are similar to the results observed in the system without liquid hydrocarbon. It turns out that the controlling process for the methane hydrate crystal growth would exist within the liquid water phase. It would be possible to transfer the observational results obtained from the system without liquid hydrocarbon to the hydrate growth behavior in the system with liquid hydrocarbon, which is relevant to the condition within an oil-producing pipeline.