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An attempt has been made to form continuously either a structure-I or a structure-H hydrate using methane as the common guest substance and methylcyclohexane as the second guest for the structure-H hydrate. The experimental technique we tested was to spray water into a high-pressure chamber charged with methane gas. In the experiments to form the structure-I hydrate, water droplets sprayed from a single nozzle at the top of the chamber coalesced into a water pool underlying the methane gas phase. In the experiments to form the structure-H hydrate, the water droplets fell onto a methylcyclohexane layer superposed on a pool of water then merged with the pool. Water was continuously drained from the bottom of the chamber and circulated back through the spray nozzle. During this circulation, the water passed through a heat exchanger outside the chamber to release the heat generated by hydrate formation. The pressure in the chamber was maintained at a prescribed level (typically 2.5−3.7 MPa) by supplying methane gas through a port at the top of the chamber. The rate of methane flowing into the chamber was continuously measured to determine the instantaneous rate of hydrate formation in the chamber. The results of the measurements indicate that comparable methane-storing rate may be obtained for structure-H and structure-I hydrates, but with the structure-H hydrate formed at lower pressures. The process of hydrate formation was observed visually through a pair of glass windows in the chamber wall, alternatively using a digital video camera and a microscope connected to a high-speed video-recording system. This paper discusses the mechanisms of hydrate formation revealed by these observations and by the measurements of the rate of hydrate formation.

This paper describes an experimental study on the statistical nature of clathrate-hydrate nucleation in a quiescent hydrochlorofluorocarbon-in-water system in which a hydrate was once formed and then dissociated. The primary objective of the study is to investigate how hydrate nucleation in the system depends on its thermal history (i.e., the time evolution of system temperature), through which the preceding hydrate dissociation was carried out, and thereby better characterize the nature of the “memory effect” for hydrate formation. Isolated drops of HCFC-141b (CH3CCl2F) immersed in water were employed as test samples for use in detecting hydrate formation on the drop surfaces by video-monitoring. The thermal history of these samples was characterized by the temperature at which the hydrate covering the drops dissociated and also by the length of time for which the samples were held at that temperature before they were cooled again to a prescribed level to induce hydrate nucleation. Thirty to fifty samples were used in each thermal history program, to collect a sufficient amount of induction-time data for statistical data processing. The processed data indicated that hydrate nucleation in a system with a “memory” of prior hydrate formation/dissociation is an intrinsically stochastic event and that the rate of nucleation strongly depends on the thermal history of the system. A qualitative discrepancy is found between these data and predictions based on assumptions that the rate of nucleation is essentially the same for all samples subjected to the same thermal-history program and that the rate is held constant with time under constant hydrate-formable thermodynamic conditions. A hypothetical explanation is provided for this point, assuming that the “memory” of prior hydrate formation/dissociation that remains in individual samples could differ despite the samples having the same thermal history.