Phase equilibrium for ozone-containing hydrates formed from an (ozone + oxygen) gas mixture coexisting with gaseous carbon dioxide and liquid water

The paper reports the three-phase (gas + aqueous liquid + hydrate) equilibrium pressure (p) versus temperature (T) data for a (O3 + O2 + CO2 + H2O) system and, for comparison, corresponding data for a (O2 + CO2 + H2O) system for the first time. These data cover the temperature range from (272 to 279) K, corresponding to pressures up to 4 MPa, for each of the three different (O3 + O2)-to-CO2 or O2-to-CO2 mole ratios in the gas phase, which are approximately 1:9, 2:8, and 3:7, respectively. The mole fraction of ozone in the gas phase of the (O3 + O2 + CO2 + H2O) system was from ∼0.004 to ∼0.02. The modified pressure-search method, developed in our previous study [S. Muromachi, T. Nakajima, R. Ohmura, Y.H. Mori, Fluid Phase Equilib. 305 (2011) 145–151] for p–T measurements in the presence of chemically unstable ozone, was applied, having been further modified for dealing with highly water-soluble CO2, for the (O3 + O2 + CO2 + H2O) system, while the conventional temperature-search method was used for the (O2 + CO2 + H2O) system. The measurement uncertainties (with 95% coverage) were ±0.11 K for T, ±6.0 kPa for p, and ±0.0015 for the mole fraction of each species in the gas phase. It was confirmed that, at a given CO2 fraction in the gas phase, p for the (O3 + O2 + CO2 + H2O) system was consistently lower than that for the (O2 + CO2 + H2O) system over the entire T range of the present measurements, indicating a preference of O3 to O2 in the uptake of guest-gas molecules into the cages of a structure I hydrate.

► Aims to present phase-equilibrium data for practical use of ozone-storing hydrates. ► Describes PE measurements for O3 + O2 + CO2 and O2 + CO2 hydrates. ► Describes a measuring technique for unstable (O3) and water-soluble (CO2) guests. ► Presents PE data for 272 K ⩽ T ⩽ 279 K and O3 gas-phase mole fractions up to 0.02. ► Shows the equilibrium pressures being lowered by partially replacing O2 with O3.