The solubilities of water in liquid CO2 coexisting with water or hydrate

We investigate the solubilities of water in liquid CO2 in the presence and absence of coexisting clathrate hydrate from theoretical calculations of the chemical potentials of water and CO2 in the aqueous, hydrate, and CO2 fluid phases across a wide range of temperatures and pressures. One of the advantages of the present method is that it is applicable to deeply cooled and heavily compressed states where a CO2 hydrate is formed. The experimental solubility curve against temperature is successfully recovered with an appropriate correction for the self-polarization energy implicitly embedded in the pairwise additive interaction model for water. The solubility of water in liquid CO2 coexisting with an aqueous solution decreases upon cooling. It is found that the intervening hydrate steadily reduces the solubility of water compared to that assumed to be in direct contact with the aqueous phase. The significant decrease results from the decrease in the chemical potential of water in the hydrate, relocating the boundary from the water/hydrate to the hydrate/fluid. Liquid CO2 loses its capacity to retain water upon cooling more seriously than that anticipated from the water/fluid boundary, and an excessive amount of water is precipitated into hydrate at low temperatures. The thermodynamic properties thus calculated provide valuable information on problems in the massive transport of CO2, specifically the blockage of pipelines and corrosion of vessels.