On the phase behaviour of the carbon dioxide-water systems at low temperatures

CO2 coming from capture processes is generally not pure and can contain impurities such as water. In a typical gas sweetening unit, the acid gas stream is the overhead product of a solvent regeneration tower and is thus saturated with water. The presence of water in the acid gas stream can result in ice and/or gas hydrate formation and cause blockage. In spite of the obvious importance of this, limited data are available on the phase behaviour of CO2 in presence of water and there is a lack of reliable predictive model for providing qualitative description of such systems over a wide range of thermodynamic conditions.

In this work, water contents for pure carbon dioxide in equilibrium with hydrates at 137.9 bar were generated by a reliable water content measurement set-up. The accuracy and reliability of the experimental measurements are demonstrated by comparing measurements with reliable literature data. Three different thermodynamic approaches have been employed to investigate the phase behaviour of the carbon dioxide – water system: the Valderama-Patel-Teja (VPT) equation of state combined with the Non Density Dependent (NDD) mixing rules, the Cubic-Plus-Association (CPA) equation of state and the Soave-Redlich-Kwong (SRK) equation of state with the Huron-Vidal mixing rules combined with a modified NRTL local composition model. The hydrate-forming conditions are modelled by the solid solution theory of van der Waals and Platteeuw. The results show that the VPT EoS with NDD mixing rules is better able to predict the phase behaviour of CO2 rich systems at low temperature conditions compared to the other EoS.