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.