Phase Equilibria of D2O Hydrates for Hydrate-Based Tritium Separation

Clathrate-hydrate-based tritium separation from isotope water is a promising process for removing tritium that is not effectively separated by conventional methods. Clathrate hydrates (hereafter hydrates) are crystalline compounds composed of water and guest molecules. Hydrate-based tritium separation utilizes the property that heavy water (D₂O) forms hydrates under milder temperatures than light water (H₂O). Efficient industrial operation requires a guest compound that forms hydrates at high temperatures and low pressures and has a large difference in phase equilibrium temperature between H₂O and D₂O hydrates (ΔT_DH). In this study, we measured the phase equilibrium conditions of D₂O hydrates formed with HFC-134a, HFC-32, and HFC-23. The formation of D₂O hydrates with these guests can be a route to tritium separation through co-precipitation of T₂O. HFC-134a formed hydrates under the mildest conditions, with ΔT_DH values of 2.8 K, 1.8 K, and 2.4 K for HFC-134a, HFC-32, and HFC-23. In addition to the three investigated guests, the potentials of propane, cyclopentane, and cyclopentane + CO₂ hydrate systems for hydrogen isotope separations were also compared, suggesting that HFC-134a and cyclopentane may be suitable guests for tritium separation. Present and previous studies have also shown a strong positive correlation between the hydration number and ΔT_DH (correlation coefficient = 0.76). This trend may be ascribed to the fact that a higher proportion of water molecules in the hydrate amplifies the effect of replacing H₂O with D₂O. These results indicate that the equilibrium conditions of D₂O hydrates may be approximately predicted to identify suitable guests for tritium separation.