Structural Transitions Range of Methane + Ethane Gas Hydrates during the Decomposition Process below the Ice Point

Abstract

The structural transitions in the forming process of methane and ethane gas hydrate were reported previously, this phenomenon has also been observed during the dissociation process of methane and ethane gas hydrate. In our earlier work, the self-preservation and structural transition of 68 mol.% CH4+32 mol.% C2H6 hydrates during dissociation below the ice point have been studied. In this work, the dissociation process of a series of methane + ethane hydrate samples at atmospheric pressure and temperatures below the ice point (272.15 K-269.15 K) was analyzed using in suit Raman spectroscopic, suggesting the hydrate structures transfer from structure I to structure II over a methane vapor composition (yCH4) range of 50 mol.%-68 mol.%. Further investigation shows that the time of structure transition is reduced with the increasing yCH4 at the same decomposition temperature. Meanwhile, hydrate dissociation is retarded with decreasing temperature in this temperature region (272.15 K-269.15 K). The result of this work can be applied to hydrate exploitation in permafrost zone and seabed.