Progress in the Study on the Phase Equilibria of the CO 2-H 2O and CO 2-H 2O-NaCl Systems

Abstract

To study the feasibility of CO 2 geological sequestration, it is needed to understand the complicated multiple-phase equilibrium and the densities of aqueous solution with CO 2 and multi-ions under wide geological conditions (273.15—473.15K, 0—60MPa), which are also essential for designing separation equipments in chemical or oil-related industries. For this purpose, studies on the relevant phase equilibria and densities are reviewed and analyzed and the method to improve or modify the existing model is suggested in order to obtain more reliable predictions in a wide temperature and pressure range. Besides, three different models (the electrolyte non random two-liquid (ELECNRTL), the electrolyte NRTL combining with Helgeson model (ENRTL-HG), Pitzer activity coefficient model combining with Helgeson model (PITZ-HG)) are used to calculate the vapor-liquid phase equilibrium of CO 2H 2O and CO 2H 2O-NaCl systems. For CO 2-H 2O system, the calculation results agree with the experimental data very well at low and medium pressure (0—20MPa), but there are great discrepancies above 20MPa. For the water content at 473.15K, the calculated results agree with the experimental data quite well. For the CO 2-H 2O-NaCl system, the PITZ-HG model show better results than ELECNRTL and ENRTL-HG models at the NaCl concentration of 0.52mol·L −1. Bur for the NaCl concentration of 3.997mol·L −1, using the ELECNRTL and ENRTL-HG models gives better results than using the PITZ-HG model. It is shown that available experimental data and the thermodynamic calculations can satisfy the needs of the calculation of the sequestration capacity in the temperature and pressure range for disposal of CO 2 in deep saline aquifers. More experimental data and more accurate thermodynamic calculations are needed in high temperature and pressure ranges (above 398.15K and 31.5MPa).