High-pressure phase equilibrium and Raman microprobe spectroscopic studies on the CO2 hydrate system

The three-phase coexistence curve of CO2 hydrate + saturated water + saturated liquid CO2 was investigated in the temperature range from 289 to 294 K and pressure up to 500 MPa. The temperature maximum point on the equilibrium curve was confirmed at 294 K and 328 MPa. The sign of(dp/dT) of the three-phase coexistence curve changes from positive to negative at this boundary point. The equilibrium curve continues up to the four-phase coexistence point, which lies at a slightly higher pressure than that of melting curve of CO2. The laser Raman microprobe spectroscopic analysis reveals that the stretching and bending vibration energies of CO2 entrapped into the hydrate cage are lower than that of pure CO2 at the same temperature and pressure. The Raman spectrum for the intermolecular vibration mode (O-O stretching) of water molecules was detected around 200 cm(-1) in the hydrate phase. The pressure dependence of the Raman shift reveals that the hydrate lattice constructed of water molecules is shrunk by a pressure increase. The Raman spectroscopy suggests that the hydrate-cage-like structure is provided previously in the aqueous solution in equilibrium.