Formation and growth of structure-H hydrate crystals on a water droplet in contact with methane gas and a large-molecule guest substance liquid

This paper reports on our visual observations of the formation and growth of structure-H hydrate crystals on a water drop partially exposed to methane gas and partially immersed in a pool of a liquid large-molecule guest substance (LMGS) for a structure-H hydrate. In each experiment, 2,2-dimethyl butane (neohexane), methylcyclohexane, or tert-butyl methyl ether was used as an LMGS with methane as a small-molecule guest substance. The temperature and pressure of the test section were set at 273.5 ± 0.2 K and 2.5 ± 0.06 MPa, respectively, to avoid possible structure-I methane hydrate formation, which may occur at a pressure above 2.7 MPa at this temperature. Hydrate crystals first formed on the water drop surface and then floated up to the apex of the drop. The hydrate crystals that were thus accumulated on the apex of the drop grew to form a cap or shell that partially covered the upper area of the drop surface. This hydrate crystal shell exhibited a coarse, apparently polycrystalline, surface texture. The polycrystalline hydrate crystals continued to grow while maintaining the form of a shell intervening between the liquid water and the methane gas. These crystals eventually covered the entire upper area of the water drop surface exposed to methane gas. This hydrate crystal growth process was commonly observed with all three of the LMGSs tested in this study. No preferential growth of the hydrate crystals on the methane-water-LMGS three-phase interfacial line, where the three substances necessary for structure-H hydrate formation are in mutual contact, was observed in any experimental run.