The CPA Equation of State and an Activity Coefficient Model for Accurate Molar Enthalpy Calculations of Mixtures with Carbon Dioxide and Water/Brine

TL;DR

This paper evaluates the CPA equation of state and a CO2 activity coefficient model for accurately predicting molar enthalpy in CO2-water/brine mixtures, demonstrating their effectiveness across various conditions.

Contribution

It introduces and validates the CPA equation of state and Duan and Sun's activity coefficient model for molar enthalpy calculations in CO2-water and brine mixtures, expanding applicability and accuracy.

Findings

CPA equation of state accurately predicts molar enthalpy for various mixtures.

Duan and Sun's model yields accurate partial molar enthalpy of CO2 in NaCl solutions.

CPA model can be modified for improved experimental agreement.

Abstract

Thermodynamic property calculations of mixtures containing carbon dioxide (CO$_2$) and water, including brines, are essential in theoretical models of many natural and industrial processes. The properties of greatest practical interest are density, solubility, and enthalpy. Many models for density and solubility calculations have been presented in the literature, but there exists only one study, by Spycher and Pruess, that has compared theoretical molar enthalpy predictions with experimental data. In this report, we recommend two different models for enthalpy calculations: the CPA equation of state by Li and Firoozabadi, and the CO$_2$ activity coefficient model by Duan and Sun. We show that the CPA equation of state, which has been demonstrated to provide good agreement with density and solubility data, also accurately calculates molar enthalpies of pure CO$_2$, pure water, and both CO$_2$-rich and aqueous (H$_2$O-rich) mixtures of the two species. It is applicable to a wider range of conditions than the Spycher and Pruess model. In aqueous sodium chloride (NaCl) mixtures, we show that Duan and Sun's model yields accurate results for the partial molar enthalpy of CO$_2$. It can be combined with another model for the brine enthalpy to calculate the molar enthalpy of H$_2$O-CO$_2$-NaCl mixtures. We conclude by explaining how the CPA equation of state may be modified to further improve agreement with experiments. This generalized CPA is the basis of our future work on this topic.