Crystal Growth of Clathrate Hydrate Formed with Carbon Dioxide and Deuterium Oxide: Implications for Hydrate-based Tritium Separation

Tritium separation technology is a key method for treating contaminated water from nuclear power plants. Recently, a novel tritium separation method using deuterium oxide (D₂O)-based clathrate hydrate has attracted attention because it can reduce the tritium concentration in contaminated water from 4.77 × 10⁵ Bq/kg to 1.55 × 10³ Bq/kg. In this hydrate-based tritium separation method, tritium is concentrated on the surface of a D₂O-based hydrate substrate. The mechanistic characteristics and formation kinetics of D₂O-based hydrates need to be clarified to develop an efficient process for substrate formation. This study contributes to the process design by revealing the crystal growth of the CO₂ + D₂O hydrate. The crystal growth behavior of the hydrate formed in the CO₂ + D₂O system was experimentally observed under different subcooling temperatures (ΔT_sub) at 3.0 MPa. At all ΔT_sub, hydrate crystals were formed at the gas-liquid interface and extended along the interface. After the interface was covered, hydrate crystals grew in the liquid phase. The variation in the morphology of CO₂ + D₂O hydrate depended on ΔT_sub. At ΔT_sub = 1.6 K, polygonal crystals with side lengths of 0.2-0.6 mm were formed. At ΔT_sub = 2.7 K, columnar crystals with a length of 1-2 mm were formed. At ΔT_sub ≥ 3.3 K, dendritic crystals were formed. With an increase in ΔT_sub, the size of the dendritic branches increased. The crystal growth dynamics and morphological tendency of the CO₂ + D₂O hydrate were comparable to those of the CO₂ + H₂O hydrate at fixed ΔT_sub. Detailed theoretical discussions on the obtained results of surface kinetics, mass transfer, and heat transfer during hydrate formation from H₂O and D₂O were provided, considering the chemical potential, viscosity, and heat transfer of the liquids.