Phase behaviour of liquid CO₂ with an impurity of water: influence of CO₂ hydrate

The solubility of water in liquid CO₂ coexisting with CO₂ hydrate or liquid water is evaluated in order to investigate the thermodynamic conditions to avoid the formation of CO₂ hydrate in the transportation processes of liquid CO₂. To this end, theoretical calculations have been carried out to obtain the chemical potentials of water and CO₂ in all the phases involved in their coexistence. The solubility of water in liquid CO₂ coexisting with liquid water decreases with decreasing temperature over a wide range of temperature and pressure, except for in the vicinity of the critical point of CO₂. The decrease in the solubility is further enhanced by the formation of hydrate. We estimate the Gibbs energy of hydrate formation, which is an important property for sequestration of CO₂, for cases where the temperature or pressure of water-saturated liquid CO₂ decreases. We also estimate the amount of water precipitated as hydrate during these processes, which has a direct bearing on flow assurance in CO₂ transportation. The present study will contribute to the development of a low-energy, safe CO₂ transport network aiming at achieving large-scale carbon neutrality.