Sodium Chloride Influence on Dissociation Behavior of CO₂ Hydrate Below the Melting Point of Ice

Understanding the influence of raw water quality on the stability of the CO2 hydrate during transportation to sequestration sites is crucially important for CO2 capture and storage involving CO2 hydrate transportation. Although raw water purification is a key consideration, the effects of using low-quality waters that are readily available for industrial use remain unclear. To address this issue, this study provides knowledge for the discussion of water quality targets for raw water used to transport CO2 using the self-preservation of CO2 hydrate. This study evaluated the dissociation behavior of CO2 hydrate formed from pure water and 0.0058 and 0.59 mass % NaCl aqueous solutions to elucidate impurity effects on hydrate dissociation behavior. Hydrate dissociation was induced by depressurization and assessed at constant temperatures of 253–272 K and during temperature ramping to ascertain the upper-temperature limit of the self-preservation effect. Hydrate dissociation in the pure water system was restricted at 253–270 K but not at 271 and 272 K, indicating that self-preservation of CO2 hydrate appears at temperatures of 253–270 K. The hydrate dissociation was restricted at 253–270 K for the 0.0058 mass % aqueous solution system and at 253 K for the 0.59 mass % aqueous solution system. In these cases, at 269–270 K for the 0.0058 mass % aqueous solution and at 253 K for the 0.59 mass % aqueous solution, the restriction effect of hydrate dissociation tended to be weak: The self-preservation of the CO2 hydrate does not appear or is weakened even at temperatures where the self-preservation is fundamentally apparent. Temperature ramping measurements of the hydrate dissociation behavior elucidated that the temperature at which the self-preservation phenomenon of CO2 hydrate disappears was almost constant at approximately 271 K, irrespective of the temperature in the pure water system. At 253–267 K, the addition of NaCl lowered the temperature at which the self-preserving effect of the CO2 hydrate disappeared. Moreover, the decrease in the temperature was greater with increased NaCl concentration. In the NaCl aqueous solution system at 268–270 K, where the initial hydrate dissociation amount was much greater and the restriction effect of hydrate dissociation was weak, the temperature at which the self-preservation phenomenon of the CO2 hydrate disappears was as high as that in the pure water system. This fact suggests that the large amounts of ice around the hydrate grain shield the remaining hydrate particles from erosion by NaCl. Findings show that the CO2 hydrate dissociation is controlled by competition between the formation and growth of ice related to the self-preservation phenomenon and the inhibition of ice formation by erosion of NaCl from the surroundings. These findings suggest that, for CO2 hydrate storage using self-preservation at lower temperatures, complete raw water purification is not necessary; partial raw water purification offers a promising approach to enhance the stability of CO2 storage over a wider temperature range.