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 x 10(5) Bq/kg to 1.55 x 10(3) 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 (Delta T-sub) at 3.0 MPa. At all Delta T-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 Delta T-sub. At Delta T-sub = 1.6 K, polygonal crystals with side lengths of 0.2-0.6 mm were formed. At Delta T-sub = 2.7 K, columnar crystals with a length of 1-2 mm were formed. At Delta T-sub >= 3.3 K, dendritic crystals were formed. With an increase in Delta T-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 Delta T-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.