Clathrate hydrates have recently received attention as novel storage and transportation materials for natural gases or hydrogen. These hydrates are treated as powders or particles, and moderate storage temperatures (around 253 K) are set for economic reasons. Thus, it is necessary to consider the sintering of hydrate particles for their easy handling because the hydrates have a framework similar to that of ice, even though their sintering would require guest molecules in addition to water molecules. We observed the sintering process of clathrate hydrates to estimate the rate of sintering. Spherical tetrahydrofuran (THF) hydrate particles were used in observations of sintering under a microscope equipped with a CCD camera and a time-lapse video recorder. We found that THF hydrate particles stored at temperatures below the equilibrium condition sintered like ice particles. The sintering part was confirmed to be not ice, but THF hydrate, by increasing the temperature above 273 K after each experiment. The sintering rate was lower than that of ice particles under the normal vapor condition at the same temperature. However, it became of the same order when the atmosphere of the sample was saturated with THF vapor. This indicates that the sintering rate of THF hydrate was controlled by the transportation of guest molecules through the vapor phase accompanied with water molecules.

Clathrate hydrates have recently received attention as novel storage and transportation materials of natural gases. For the economical storage or transportation of the produced natural gas hydrates, the effect of sintering should be considered because the hydrate particles would be stored in relatively moderate conditions. In the present study, microscopic observations were carried out on gas hydrate particles to investigate their sintering properties. The spherical gas hydrates were prepared from spherical ice particles with carbon dioxide (CO2) or ethane (C2H6) pressures higher than the dissociation ones. The initial sintering rate of CO2 hydrates at temperatures between 271.4 K and 278.2 K and pressures between 1.9 MPa and 2.9 MPa was similar to that of THF hydrate under saturated conditions reported previously. Conversely, the C2H6 hydrate particles were hard to sinter at temperatures between 273.5 K and 281.7 K and pressures between 0.5 MPa and 2.0 MPa. We discussed the sintering process of gas hydrates and their rate-determining process which depended on guest molecules under their stable conditions. The sintering of methane hydrate particles under stable and self-preservation conditions was also investigated to demonstrate the storage and transportation conditions of natural gas hydrate particles.

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