Kinetics and stability of CH4-CO2 mixed gas hydrates during formation and long-term storage.

The formation of CH4-CO2 mixed gas hydrates was observed by measuring the change of vapor-phase composition using gas chromatography and Raman spectroscopy. Preferential consumption of carbon dioxide molecules was found during hydrate formation, which agreed well with thermodynamic calculations. Both Raman spectroscopic analysis and the thermodynamic calculation indicated that the kinetics of this mixed gas hydrate system was controlled by the competition of both molecules to be enclathrated into the hydrate cages. However, the methane molecules were preferentially crystallized in the early stages of hydrate formation when the initial methane concentration was much less than that of carbon dioxide. According to the Roman spectra, pure methane hydrates first formed under this condition. This unique phenomenon suggested that methane molecules play important roles in the hydrate formation process. These mixed gas hydrates were stored at atmospheric pressure and 190 K for over two months to examine the stability of the encaged gases. During storage, CO2 was preferentially released. According to our thermodynamic analysis, this CO2 release was due to the instability of CO2 in the hydrate structure under the storage conditions.