The hydrate phase equilibrium behaviors of tetrahydrofuran (THF) + CH 4 , THF + CO 2 , CH 4 + CO 2 , and THF + CO 2 + CH 4 were investigated over wide ranges of temperature, pressure, and concentration. The dissociation conditions of THF + CH 4 and THF + CO 2 hydrates were shifted to lower pressures and higher temperatures from the dissociation boundaries of pure CH 4 and pure CO 2 hydrates. X-ray diffraction results revealed that the CH 4 + CO 2 and THF + CO 2 + CH 4 hydrates prepared from a CH 4 /CO 2 (50:50) gas mixture formed structure I and II clathrate hydrates, respectively. Raman measurements provided detailed information regarding the cage occupancy of CH 4 and CO 2 molecules encaged in the hydrate frameworks. For the CH 4 + CO 2 hydrates, the concentrations of CO 2 in the hydrate phase were higher than those in the vapor phase. In contrast, for the THF + CO 2 + CH 4 hydrates, the concentrations of CO 2 in the hydrate phase were lower than those in the vapor phase.

An in situ Raman spectroscopy study on the guest replacement of methane hydrate with carbon dioxide at high pressures is presented. The surface shielding of carbon dioxide hydrate formed in the outer layer plays a key function, retarding dissociation of methane hydrate in the core. A thorough investigation of Raman spectra of the vibration modes of water molecules enables us to understand the phenomena of the coexistence of methane and carbon dioxide hydrates and water phase for guest replacement reactions. These results should aid further efforts in the study of guest stability of clathrate hydrate as well as future technological applications.

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Microbubble technology is now gathering much interest in industrial fields for its excellent gas-dissolving ability. This study was the first trial in using microbubbles in hydrate formation, and it has been demonstrated that a microbubble system is a promising method for hydrate formation for two reasons:  (1) its excellent gas-dissolution ability and (2) its ability to make the conditions of hydrate nucleation more mild due to the microbubbles' property of increasing interior gas pressure while decreasing in size under water.