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Understanding the phase behaviour of (CO2 + water + permanent gas) systems is critical for implementing carbon capture and storage (CCS) processes, a key technology in reducing CO2 emissions. In this paper, phase behaviour data for (H2O + N2) and (CO2 + H2O + N2) systems are reported at temperatures from 323 to 473 K and pressures up to 20 MPa. In the ternary system, the mole ratio between CO2 and N2 was 1. Experiments were conducted in a newly designed analytical apparatus that includes two syringe pumps for fluid injection, a high-pressure equilibrium vessel, heater aluminium jacket, Rolsi sampling valves and an online gas chromatograph (GC) for composition determination. A high-sensitivity pulsed discharge detector installed in the GC was used to measure the low levels of dissolved nitrogen in the aqueous phase and low water levels in the vapour phase. The experimental data were compared with the calculation based on the γ-φ and SAFT-γ Mie approaches. In the SAFT-γ Mie model, the like parameters for N2 had to be determined. We also obtained the unlike dispersion energy for the (H2O + N2) system and the unlike repulsive exponent and dispersion energy for the (CO2 + N2) system. This was done to improve the prediction of SAFT-γ Mie model. For the (H2O + N2) binary system, the results show that the solubility of nitrogen in the aqueous phase was calculated better by the γ-φ approach rather than the SAFT-γ Mie model, whereas SAFT-γ Mie performed better for the prediction of the vapour phase. For the (CO2 + H2O + N2) ternary systems, both models predicted the experimental data for each phase with good agreement.

An apparatus consisting of an equilibrium cell connected to two vibrating tube densimeters and two syringe pumps was used to measure the saturated phase densities of (CO2 + H2O) at temperatures from (293 to 450) K and pressures up to 64 MPa, with estimated average standard uncertainties of 1.5 kg · m−3 for the CO2-rich phase and 1.0 kg · m−3 for the aqueous phase. The densimeters were housed in the same thermostat as the equilibrium cell and were calibrated in situ using pure water, CO2 and helium. Following mixing, samples of each saturated phase were displaced sequentially at constant pressure from the equilibrium cell into the vibrating tube densimeters connected to the top (CO2-rich phase) and bottom (aqueous phase) of the cell. The aqueous phase densities are predicted to within 3 kg · m−3 using empirical models for the phase compositions and partial molar volumes of each component. However, a recently developed multi-parameter equation of state (EOS) for this binary mixture, Gernert and Span [32] , was found to under predict the measured aqueous phase density by up to 13 kg · m−3. The density of the CO2-rich phase was always within about 8 kg · m−3 of the density for pure CO2 at the same pressure and temperature; the differences were most positive near the critical density, and became negative at temperatures above about 373 K and pressures below about 10 MPa. For this phase, the multi-parameter EOS of Gernert and Span describes the measured densities to within 5 kg · m−3, whereas the computationally-efficient cubic EOS model of Spycher and Pruess (2010), commonly used in simulations of subsurface CO2 sequestration, deviates from the experimental data by a maximum of about 8 kg · m−3.