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

We present an experimental study on the formation and dissociation characteristics of carbon dioxide (CO) gas hydrates using Raman spectroscopy. The CO hydrates were formed from sodium chloride/water solutions with salinities of 0-10 wt %, which were pressurized with liquid CO in a stirred vessel at 6 MPa and a subcooling of 9.5 K. The formation of the CO 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, and the fraction of the dispersed liquid water-rich phase, x, from the signature of the hydroxyl (OH)-stretching vibration of the hydrate gel. We found that the fraction of hydrate x contained in the hydrate gel linearly depends on the salinity of the initial liquid water-rich phase. In addition, the ratio of CO 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 CO in the water-rich phase after complete dissociation of the hydrate gel and were able to show that the excess CO 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.