Modelling phase equilibria in the CO2/CH4-H2O-NaCl system using the association equation of state

A new association model, CPA-MHV1, is developed by combining the Soave-Redlich-Kwong (SRK) equation of state (EoS) with the Cubic-Plus-Association (CPA) EoS based on Michelsen's improved Huron-Vidal mixing rule. The model is utilized to investigate the vapor-liquid equilibrium (VLE) of binary mixtures involving methane (CH4), carbon dioxide (CO2), and water (H2O) binary mixtures, as well as ternary mixtures containing sodium chloride (NaCl). CO2 is modeled as a non-associating, non-self-associating (solvation in water) component and as a self-associating component with two, three, or four association sites; furthermore, CH4 is modeled as a nonassociating and pseudo-associating component. In the regression of the adjustable parameters, dependence on temperature is considered and the parameters are fitted as functions of temperature. The results demonstrate that the best performance is achieved when solvation between CO2 and H2O is considered, with the average absolute relative deviation (AARD) of 4.50 % in the H2O-rich phase, 5.48 % in the CO2-rich phase, and 3.28 % in NaCl solution. CH4 with pseudo-association scheme has the highest comprehensive prediction performance, and AARD values in H2O-rich phase, CH4-rich phase and NaCl solution are 5.26 %, 5.90 % and 9.19 %, respectively.

Automated summary

A new association model, CPA-MHV1, combining the SRK equation of state with the CPA equation based on Michelsen's improved Huron-Vidal mixing rule, is developed. The model is used to investigate the vapor-liquid equilibrium of binary mixtures involving CH4, CO2, and H2O, as well as ternary mixtures containing NaCl. The results show that considering the solvation between CO2 and H2O yields the best performance, while CH4 with a pseudo-association scheme has the highest comprehensive prediction performance.