Dissociation Behavior of Clathrate Hydrates to Ice and Dependence on Guest Molecules

Abstract

Self‐preservation: The existence of gas hydrates (see picture) outside their stability zone for prolonged periods is shown to depend on the interaction strength between guest molecules (red) and water (blue), as reflected by the dissociation pressures at 273 K.

Anomalous Preservation of CH4 Hydrate and its Dependence on the Morphology of Hexagonal Ice

“…This anomalous preservation has been observed in this temperature region only for CH 4 hydrate upon dissociation by rapid pressure release from the high pressures at which it is stable, to an ambient pressure of CH 4 gas. On the other hand, we recently found that dissociation of gas hydrates by temperatureramping depends on the interaction strength between guest molecules and H 2 O molecules, as reflected by the dissociation pressures of the various hydrates at 273 K. [6] That work suggests that neither the thermodynamic stability nor the crystal structure, but the nature of the guest molecules determines whether self-preservation phenomena should be expected. Herein, we report the relationship between the morphology of ice grown from hydrate dissociation and hydrate stability in the temperature region of anomalous preservation by the pressure-release method.…”

“…In our previous study, dissociation of C 2 H 6 and C 3 H 8 hydrate under atmospheric pressure of N 2 gas proceeded in a single step up to 220 K whereas the dissociation rate for CH 4 hydrate decreased and as the sample self-preserved up to 268 K during temperature ramping. [6] Additionally, it has been reported that CH 4 + C 2 H 6 hydrate dissociates as a single entity without preferential release of either CH 4 or C 2 H 6 . [13] Therefore, the preservation of gas hydrates by the rapid pressure-release method also depends on the nature of the guest molecules, but not on either the thermodynamic stability or the crystal structure.…”

“…Here, the initial dissociation rates may depend on the type of guest molecules; C 2 H 6 and C 3 H 8 hydrates dissociate readily but CH 4 hydrate is self-preserved during temperature ramping as reported earlier. [6] It is also reported that morphologies of ice change depending on the type of atmospheric gas present during growth, for example, air, N 2 ,O 2 ,C O 2 and H 2 , even though the mechanism is poorly understood.…”

“…Then, a mobile water layer may also support the contact between the dissociating hydrate and the plate-like ice Ih. Additionally, it was suggested that the nature of the ice crystals and the interaction between guest molecules and H 2 O molecules [6] both affect anomalous preservation phenomena for gas hydrates. We think that the gas hydrates which show self-preservation phenomena by the temperature-ramping method in earlier studies, [6,23] such as N 2 ,O 2 , Ar, and H 2 hydrate, will also show anomalous preservation by the rapid pressure-release method.…”

Review of Fundamental Properties of Gas Hydrates: Breakout Sessions of the International Workshop on Methane Hydrate Research and Development

“…S. Takeya (AIST) presented a brief overview of the self-preservation effect [27,32] and the common understanding about the effect at the time were summarized as follows:…”