The quadruple point in carbon dioxide hydrate forming system, ice + water-rich liquid + hydrate + carbon dioxide-rich gas four-phase equilibrium condition, has been determined based on supercooled water-rich liquid + hydrate + carbon dioxide-rich gas and water-rich liquid + hydrate + carbon dioxide-rich gas three-phase equilibrium conditions measured in this study combined with ice + hydrate + carbon dioxide-rich gas three-phase equilibrium conditions reported in the literature (K. Yasuda, R. Ohmura, J. Chem. Eng. Data 53 (2008) 2182–2188). The intersection of the three types of the three-phase equilibrium conditions was used to determine the quadruple point. The three-phase equilibrium condition experiments were performed at temperatures between 269.45 K and 275.75 K, and at pressures between 0.825 MPa and 1.708 MPa. The determined p–T condition at quadruple point was T = 271.6 ± 0.2 K and p = 1.04 ± 0.02 MPa. It was also observed that the freezing of supercooled water during some experimental runs performed below the quadruple point temperature, which causes sudden increase in both pressure and temperature.
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