In this paper, we present experimental arguments for the guest replacement of methane hydrate with carbon dioxide using in situ Raman spectroscopy at high pressures. The surface shielding of carbon dioxide hydrate formed in the outer layer plays a key function of retarded dissociation of methane hydrate in the core. The water phase produced during the replacement reaction allows the reaction to proceed rapidly by enhancing diffusion of both methane and carbon dioxide, resulting in complete and relatively fast recovery of methane gas from methane hydrate. These results may strongly encourage further efforts in the study of guest stability of clathrate hydrate and may provide useful applications for future technologies.
An important practical feature of gas hydrate is that vast quantities of methane in the form of the gas hydrate exist in the permafrost zone and the subsea sediment (Kvenvolden, 1999). To develop the method for commercially producing natural gases from methane hydrate layers, the environmental and safety issues, which are of particular importance, should be first clarified. In the case of carbon dioxide hydrate, the disposal of global warming gases, mainly carbon dioxide, on the ocean floor using the hydrate formation process has been carefully studied (Saito et al., 2000). Several field experiments were conducted to test and prove ideas for carbon dioxide ocean disposal by in-situ hydrate formation (Brewer et al., 1999).
It is now possible to closely monitor the exchange process of guest molecules of gas hydrate with other hydrate-formers. Neutron diffraction experiments have enabled recent success in studying guest replacement, especially accompanying structural transition, and provide information for equilibrium exchange between guest molecules in the hydrate and free molecules in the surrounding phase (Halpern et al., 2001).