Observations of gas hydrate dissociation below the ice point with microfocus X-ray computed tomography

Because of their wide occurrence in the nature and involvement in various natural events, a considerable number of studies have been made on gas hydrates over the past few decades, ant it has greatly enriched our knowledge of time-independent hydrate properties, such as the structures and equilibrium thermodynamics of hydrates. On the other hand, many major challenges remain in understanding time-dependent hydrate processes, including the hydrate formation and decomposition. One remaining puzzle is kinetics of gas hydrate dissociation at temperatures below 273K. Although it is known that gas hydrate dissociation is considerably suppressed in the presence of ice, the detailed mechanism is still well not understood. Using microfocus X-ray computed tomography, distributions of gas hydrate and ice inside hydrate particles during dissociation at 246K were observed. Hydrate samples investigated in this work were synthetic argon and krypton hydrates of diameter several hundred microns. For Ar samples, hydrate dissociation started from the surface, and the ice formation went into the inside of the particles. It took approximately 10 hours to complete Ar hydrate decomposition. On the other hand, in the case of Kr samples, rapid dissociation in which the great part of hydrates decomposed in 30 minutes was followed by considerably slow reaction. In the second regime, the remaining Kr hydrates dispersed in the ice matrix were preserved for a long period of time (days). These observations suggest that the mode of gas hydrate dissociation to ice is dependent on types of guest gases probably due to difference in interaction between the guest and host water molecules.