Phase equilibria measurements and modelling of CO2–rich fluids/brine systems

Abstract

An attractive option for reducing the emissions of CO2 to the atmosphere is CCS1 processes which is capturing and transporting the captured CO2 to large storage sites, e.g., saline aquifers. However, to achieve this goal there are several economic barriers and challenging engineering problems that cannot be taken care of without having a precise knowledge of thermo-physical properties of such systems. Robust models validated with precise experimental data are probably the most important part of the above-mentioned knowledge. In this study the PC-SAFT equation of state was applied to calculate thermo-physical properties and phase equilibria of CCS and flow assurance systems of interest. The model was tuned for numerous binary systems containing common gases and hydrocarbons by adjusting the BIPs2 to experimental VLE or LLE3 data. The PC-SAFT pure compound parameters of water, glycols and alcohol were also optimised using saturation data and their BIPs with non-polar substances were adjusted to experimental data. Second order derivatives of the PC-SAFT model contributions to Helmholtz free energy were also derived analytically to calculate second order thermodynamics properties of a few pure compounds. To experimentally study the phase equilibria of CO2-brine systems, solubilities of CO2 in different concentrations of NaCl, KCl, CaCl2 and MgCl2 aqueous solutions were measured over a wide range of temperatures and pressures. The water content of CO2-rich phases in equilibrium with sodium chloride aqueous solutions and pure water were also measured over a broad range of temperature and pressures up to 550 bar. To validate the experimental data, the CPA4 and the PC-SAFT equation of states were tuned to literature data and the developed model calculations were compared with the experimental data measured in this work.