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.

Four-phase (ice + structure-H hydrate + large-molecule guest substance liquid + methane-rich vapor) equilibrium pressure−temperature conditions were measured at temperatures from 252 K to 272 K in systems of water + methane + LMGS, where LMGS is a large-molecule guest substance for a structure-H hydrate. The tested LMGSs were 2,2-dimethylbutane (neohexane), methylcyclohexane, and tert-butyl methyl ether. The results obtained in the 2,2-dimethylbutane system agree with the corresponding data reported by Makogon et al., which were the only data set on the structure-H hydrate phase equilibrium at temperatures below the freezing point of water. Among the three systems studied, the lowest equilibrium pressure at a given temperature was observed in the 2,2-dimethylbutane system, and the highest, in the tert-butyl methyl ether system.

An experimental study has been performed involving water spraying onto a cooled metal‐block surface exposed to a hydrate‐forming gas as a means of high‐rate hydrate formation for the purpose of, for example, natural‐gas storage. Special attention has been paid to the effectiveness of conductive cooling through the metal block for directly removing the heat released by the hydrate formation from its site, that is, the surface of the metal block, and, thereby, increasing the rate of hydrate formation. HFC‐32 (CH2F2) that forms a structure‐I hydrate at moderate pressures was used as a model gas to enable visual observations of the hydrate formation inside a large‐windowed spray chamber. The experiments revealed that the cooling by the metal block effectively increases the rate of hydrate formation while water is sprayed at a given volumetric rate and at a given degree of subcooling from the guest‐gas/water/hydrate equilibrium temperature. © 2006 American Institute of Chemical Engineers AIChE J, 2006

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.

Clathrate hydrate formation in a (methane + either 3-methyltetrahydropyran or 2-methyltetrahydrofuran + water) system is demonstrated. The first data of the quadruple (water + structure-H hydrate + either 3-methyltetrahydropyran or 2-methyltetrahydrofuran + methane) equilibrium pressure-temperature conditions are measured over temperatures from 273 to 286 K. In the 3-methyltetrahydropyran system, the equilibrium pressures are lower by 1.6-2 MPa than those of the structure-I methane hydrate formed in the methane + water system at given temperatures. In the 2-methyltetrahydrofuran system, equilibrium pressures at temperatures below 278 K are lower than those for the structure-I methane hydrate, and at temperatures above 278 K, they are higher. These phase equilibria suggest the formation of hydrates other than structure-I methane hydrates in the two systems. The crystallographic structures of the hydrates are determined to be structure H by means of X-ray diffraction as expected from considerations of sizes and shapes of the 3-methyltetrahydropyran and 2-methyltetrahydrofuran molecules.