A Helmholtz free energy equation of state for the NH3–H2O fluid mixture: Correlation of the PVTx and vapor–liquid phase equilibrium properties

• A Helmholtz free energy EOS is developed for the NH3–H2O fluid mixtures.
• A simple generalized departure function is used in the new EOS.
• The EOS can predict both PVTx and VL phase equilibrium properties of NH3–H2O mixture.
• Volume and phase equilibrium composition can be calculated by an iterative algorithm.

An equation of state (EOS) explicit in Helmholtz free energy was developed to calculate the PVTx and vapor–liquid phase equilibrium properties of the NH3–H2O fluid mixture. This EOS, where four mixing parameters are used, is based on highly accurate EOSs for the pure components (H2O and NH3) that NIST recommends and contains a simple generalized departure function presented by Lemmon and Jacobsen (1999) [38]. Comparison with thousands of reliable experimental data available indicates that the EOS can calculate both vapor–liquid phase equilibrium and volumetric properties of this binary fluid system, within or close to experimental uncertainties up to 706 K and 2000 bar over all composition range. The average absolute deviation is 0.68% in molar volume, and the average composition error of vapor phase and that of liquid phase except for those at the near-critical region are in general less than 0.03 and 0.07 in mole fraction, respectively.