An equation of state for accurate thermodynamic modeling of water and carbon dioxide from triple points to 647 K and 100–200 MPa

Abstract

An equation of state with near-critical corrections is developed for water (H2O) and carbon dioxide (CO2). The equation is constrained by the critical conditions and optimized with highly accurate pressure–volume–temperature (PVT) and phase equilibrium data at temperatures from the triple point to 647.096K and pressures from zero to 80MPa. The resulting equation is systematically checked in the above P–T range. For H2O, the vapor pressures are reproduced within 0.032%, with an average deviation of only 0.005%; the average deviations of the saturated vapor and liquid volumes, and the volumes in the single-phase regions are 0.047%, 0.019% and 0.058%, respectively. The maximum deviations of the above-mentioned volumes are 1.26%, 0.91% and 1.06%, respectively, which are all in the immediate vicinity of the critical point. Below 520K, the deviations of PVT and saturation properties are all within 0.06%. For subcritical CO2, the average and maximum deviations of vapor pressures are 0.011%, 0.077%, respectively; the average deviations of the saturated vapor and liquid volumes, and the volumes in the single-phase regions are 0.099%, 0.058% and 0.126%, respectively. The largest deviations of the above volumes are 0.230%, 0.173% and 0.358%, respectively. For supercritical CO2 below 650K, the average volume deviation is 0.080%. The equation also agrees very well with many high-quality density and phase equilibrium data not used to fit the equation. The equation gives excellent prediction of fugacity coefficients, residual enthalpies and entropies and heats of vaporization, and can be extrapolated to 200MPa for H2O, or to 100MPa for CO2. The equation can serve as a good starting point for the thermodynamic modeling of many industrial and geological processes, including carbon capture and sequestration.