Thermodynamic properties and microscopic analysis of gas hydrate for sequestrating of carbon dioxide into deep ocean

Abstract

Gas hydrates are crystalline compounds formed by specific configuration of hydrogen-bonded water around other molecules. The water (host) molecules arrange themselves to form cavities that can include other (guest) molecules. Methane hydrates exist widely in nature, and they have potentially great importance not only in relation to environmental issue such as global warming, but also as a possible energy resource. Carbon dioxide hydrates can be formed on deep ocean floor, and they are proposed as a method of carbon sequestration. One of the unique properties of hydrates is a change of the specific volume of hydrate formers during their transition from a free gas or liquid state into a solid hydrate state and vice versa. The specific volume of water increases by 26-32 vol.% during the phase transition into a hydrate state, compared with 9 vol.% during the phase transition into ice state. This property allows us to create methods of storage and transportation for large volumes of gas in a hydrate state. The gas hydration process for the storage of carbon dioxide on the ocean floor has recently concentrated. This technology has the possibility of managing huge amount of carbon dioxide by just spreading in ocean floor. For the fundamental understanding of $CO_2$ hydrate in the deep ocean, basic three-phase equilibria and kinetics of $CO_2$ hydrate were measured. In the case of phase equilibria, liquid water and vapor (H-Lw-Vw) for $CO_2-H_2O-NaCl$ (or clay) system are measured. The salt effect on the formation of carbon dioxide hydrate was confirmed. As expected, the addition of NaCl inhibited hydrate formation. The H-Lw-V equilibrium curve of the 3 wt % NaCl and 10 wt % clay solution almost overlapped with that of the 3 wt % NaCl-only solution, but was slightly shifted to the left. The capillary action in the pore induced the corresponding hydrate equilibrium pressure to rise to a higher pressure than that in the bulk water at a specified temperature. By this ef...