Clathrate hydrate crystal growth in liquid water saturated with a guest substance: Observations in a methane + water system

This paper reports on a visual study of formation and growth of clathrate hydrate crystals in liquid water saturated (prior to hydrate formation) and in contact with methane gas under the pressure of 6-10 MPa at a temperature of 273.5 K. Irrespective of the pressure set in the experimental system, in most of the experimental runs we observed that a hydrate film first formed to intervene between methane gas and liquid water, and then hydrate crystals grew in liquid water from the hydrate film. Distinct variations in the morphology of hydrate crystals grown in liquid water were observed depending on the pressure. At pressures of 6-8 MPa, hydrate crystals with skeletal, columnar morphology were observed. At the pressure of 10 MPa, the skeletal, columnar crystals were replaced by dendritic crystals. The dependency of the morphology on the degree of driving force for mass-transfer-controlled hydrate-crystal growth is discussed, comparing the present observations with those reported in the literature. Another category of hydrate formation and growth was observed in some experimental runs. The hydrate crystals first formed at the inner surface of the test cell in contact with liquid water instead of the methane-water interface. These crystals floated up to the methane-water interface, where they became a polycrystalline hydrate film, and continued to grow in liquid water.