Thermodynamic Modeling of Aqueous Na+-K+-Ca2+-Cl?-HCO3?-CO32?-CO2 Solution

The enormous amount of highly saline water known as the “produced water” generated during oil and gas exploration can have salinity up to seven times that of seawater, raising a great concern for disposing it in the environment along with the scale deposition in the process equipment. To desalinate the water for further use, a comprehensive thermodynamic model is required. We are developing a comprehensive thermodynamic model to accurately calculate thermodynamic and phase equilibria properties, including salt precipitation, for high salinity produced waters [1]. As a part of the larger effort, the present study focuses on the development of a thermodynamic model for the aqueous electrolyte solutions involving sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), chloride (Cl^–), carbonate (CO₃^2–), bicarbonate (HCO₃^–) ions, and molecular CO₂.

Utilizing the electrolyte nonrandom two-liquid (eNRTL) theory [2], this model requires parameterization of eNRTL binary interaction parameters and physical properties parameters of pure components, i.e., water, ions, molecular solute, and precipitating salts. With the fully parameterized eNRTL model, thermodynamic properties including CO₂ solubility and salt precipitation of the aqueous Na⁺-K⁺-Ca²⁺-Cl^−-HCO₃^−-CO₃^2−-CO₂ solution and its subsystems can be accurately predicted. The model is validated against data at a temperature range of 273.15 – 473.15 K and electrolyte concentrations from infinitely dilution to salt saturation. The thermodynamic model developed is an indispensable tool to predict calcium carbonate scale deposition and support mass and energy balance calculations in desalination and water treatment processes. The model should also be an excellent tool for modeling CO₂ capture with brine solutions.

References:

[1] S. Tanveer and C.-C. Chen, "A comprehensive thermodynamic model for high salinity produced waters," AIChE Journal, vol. 66, no. 1, pp. 1-10, 2020.

[2] Y. Song and C.-C. Chen, "Symmetric electrolyte nonrandom two-liquid activity coefficient model," Industrial & Engineering Chemistry Research, vol. 48, no. 16, pp. 7788-7797, 2009.