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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.

Kinetic characteristics of thermal energy storage (TES) using tetrabutylammonium acrylate (TBAAc) hydrate were experimentally evaluated for practical use as PCMs. Mechanical agitation or ultrasonic vibration was added to detach the hydrate adhesion on the heat exchanger, which could be a thermal resistance. The effect of the external forces also was evaluated by changing their rotation rate and frequency. When the agitation rate was 600 rpm, the system achieved TES density of 140 MJ/m3 in 2.9 hours. This value is comparable to the ideal performance of ice TES when its solid phase fraction is 45%. UA/V (U: thermal transfer coefficient, A: surface area of the heat exchange coil, V: volume of the TES medium) is known as an index of the ease of heat transfer in a heat exchanger. UA/V obtained in this study was comparable to that of other common heat exchangers, which means the equivalent performance would be available by setting the similar UA/V. In this study, we succeeded in obtaining practical data for heat storage by TBAAc hydrate. The data obtained in this study will be a great help for the practical application of hydrate heat storage in the future.