Dynamics and Morphology of CO₂ Hydrate Crystals Formed in Highly Concentrated Sodium Chloride Aqueous Solution

Clathrate-hydrate-based desalination would surpass reverse osmosis membrane technology in terms of the water recovery ratio and energy consumption on an industrial scale. For engineering practice, comprehension of hydrate crystal growth and morphology is essential because these kinetics affect the efficiency of the hydrate formation and dewatering process. We measured the three-phase equilibrium conditions of CO2 + NaCl aqueous solution with a mass fraction of 0.20. The equilibrium temperature at 2.1 MPa was determined to be 265.9 K. We observed crystal growth and the morphology of the CO2 hydrate in the same system at various subcooling temperatures under 2.1 MPa. Dendritic and polyhedral crystals were observed at large and small subcooling temperatures. The crystal growth rate was higher at large subcooling temperatures in the same system. At a given subcooling temperature, the growth rate in NaCl aqueous solution was obviously lower than in pure water under a given driving force of crystal growth. We discussed this difference in the aspects of viscosity, CO2 solubility, and thermal conductivity. These observations indicate that both hydrate formation rate and dewatering efficiency depend on subcooling temperature and are in a trade-off relation. The results would facilitate the optimization of the operation conditions of hydrate-based desalination plants.