Increasing molecular O3 storage capacity in a clathrate hydrate

Abstract

This paper reports an experimental study to further increase the ozone storage capacity in a clathrate hydrate and to better understand the relationship between the gas phase O₃ concentration and the O₃ storage capacity in the hydrate. We performed experiments with the O₃ + O₂ + CO₂ feed gas with an increased O₃ fraction in the gas phase exceeding that covered by a preceding study. To accurately specify the thermodynamic conditions to form the hydrate, we first measured the three-phase (gas + liquid + hydrate) equilibrium conditions for the (O₃ + O₂ + CO₂ + H₂O) and (O₂ + CO₂ + H₂O) systems. The phase equilibrium data cover the temperature range from 272 to 277 K, corresponding to pressures from 1.6 to 3.1 MPa, for each of the two different (O₃ + O₂)-to-CO₂ or O₂-to-CO₂ molar ratios in the feed gas, which are approximately 4 : 6 and 5 : 5, respectively. The O₃ fraction in the gas phase was ∼0.025. Based on the equilibrium data, we prepared crystal samples of the O₃ + O₂ + CO₂ hydrates at a system pressure of 3.0 MPa and a temperature of 272 K. The highest O₃ storage capacity in the hydrates was measured to be 2.15 mass% which is 2.36 times higher than the highest past record of 0.91 mass%. The results also show that the dominant factor to control the O₃ storage capacity in the hydrates is the O₃ mole fraction in the gas phase in contact with the hydrates.