A thermodynamic model for predicting mineral reactivity in supercritical carbon dioxide: I. Phase behavior of carbon dioxide-water-chloride salt systems across the H 2O-rich to the CO 2-rich regions

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Abstract

Phase equilibria in mixtures containing carbon dioxide, water, and chloride salts have been investigated using a combination of solubility measurements and thermodynamic modeling. The solubility of water in the CO 2-rich phase of ternary mixtures of CO 2, H 2O and NaCl or CaCl 2 was determined, using near infrared spectroscopy, at 90atm and 40 to 100°C. These measurements fill a gap in the experimental database for CO 2-water-salt systems, for which phase composition data have been available only for the H 2O-rich phases. A thermodynamic model for CO 2-water-salt systems has been constructed on the basis of the previously developed Mixed-Solvent Electrolyte (MSE) framework, which is capable of modeling aqueous solutions over broad ranges of temperature and pressure, is valid to high electrolyte concentrations, treats mixed-phase systems (with both scCO 2 and water present) and can predict the thermodynamic properties of dry and partially water-saturated supercritical CO 2 over broad ranges of temperature and pressure. Within the MSE framework the standard-state properties are calculated from the Helgeson-Kirkham-Flowers equation of state whereas the excess Gibbs energy includes a long-range electrostatic interaction term expressed by a Pitzer-Debye-Hückel equation, a virial coefficient-type term for interactions between ions and a short-range term for interactions involving neutral molecules. The parameters of the MSE model have been evaluated using literature data for both the H 2O-rich and CO 2-rich phases in the CO 2-H 2O binary and for the H 2O-rich phase in the CO 2-H 2O-NaCl/KCl/CaCl 2/MgCl 2 ternary and multicompontent systems. The model accurately represents the properties of these systems at temperatures from 0°C to 300°C and pressures up to ~4000atm. Further, the solubilities of H 2O in CO 2-rich phases that are predicted by the model are in agreement with the new measurements for the CO 2-H 2O-NaCl and CO 2-H 2O-CaCl 2 systems even though the new data were not used in the parameterization of the model. Thus, the model can be used to predict the effect of various salts on the water content and water activity in CO 2-rich phases on the basis of parameters determined from the properties of aqueous systems. Given the importance of water activity in CO 2-rich phases for mineral reactivity, the model can be used as a foundation for predicting mineral transformations across the entire CO 2/H 2O composition range from aqueous solution to anhydrous scCO 2. An example application using the model is presented which involves the transformation of forsterite to nesquehonite as a function of temperature and water content in the CO 2-rich phase. © 2012 Elsevier B.V.

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Springer, R. D., Wang, Z., Anderko, A., Wang, P., & Felmy, A. R. (2012). A thermodynamic model for predicting mineral reactivity in supercritical carbon dioxide: I. Phase behavior of carbon dioxide-water-chloride salt systems across the H 2O-rich to the CO 2-rich regions. Chemical Geology, 322323, 151–171. https://doi.org/10.1016/j.chemgeo.2012.07.008

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