Influence of Sodium Chloride on the Formation and Dissociation Behavior of CO₂ Gas Hydrates

We present an experimental study on the formation and dissociation characteristics of carbon dioxide (CO₂) gas hydrates using Raman spectroscopy. The CO₂ hydrates were formed from sodium chloride/water solutions with salinities of 0–10 wt %, which were pressurized with liquid CO₂ in a stirred vessel at 6 MPa and a subcooling of 9.5 K. The formation of the CO₂ hydrate resulted in a hydrate gel where the solid hydrate can be considered as the continuous phase that includes small amounts of a dispersed liquid water-rich phase that has not been converted to hydrate. During the hydrate formation process we quantified the fraction of solid hydrate, x_H, and the fraction of the dispersed liquid water-rich phase, x_L, from the signature of the hydroxyl (OH)-stretching vibration of the hydrate gel. We found that the fraction of hydrate x_H contained in the hydrate gel linearly depends on the salinity of the initial liquid water-rich phase. In addition, the ratio of CO₂ and water was analyzed in the liquid water-rich phase before hydrate formation, in the hydrate gel during growth and dissociation, and after its complete dissociation again in the liquid water-rich phase. We observed a supersaturation of CO₂ in the water-rich phase after complete dissociation of the hydrate gel and were able to show that the excess CO₂ exists as dispersed micro- or nanoscale liquid droplets in the liquid water-rich phase. These residual nano- and microdroplets could be a possible explanation for the so-called memory effect.