EQUATION OF STATE DESCRIPTION OF THERMODYNAMIC PROPERTIES OF NEAR-CRITICAL AND SUPERCRITICAL WATER

The pressure-volume-temperature (PVT) behavior of pure water is calculated with the associated perturbed anisotropic chain theory (APACT). The results are compared with existing experimental data. The pressure range of interest is from 25 to 250 000 bar and the temperature range is from 473.15 to 1273.15 K. Special attention is paid to the near- and supercritical region. One set of pure-component parameters, obtained by fitting experimental subcritical liquid-density and vapor-pressure data, is used to evaluate the PVT behavior of pure water over the entire pressure and temperature range. It is shown that, close to the critical point, the phase behavior of this compound cannot be predicted with a cubic equation of state within reasonable accuracy limits. The agreement between the experimental data and the calculated results with APACT is very good for pressures up to 50 000 bar, including the near-critical region. The difference between the APACT two-site and the APACT three-site model appears to be very small. In order to understand the molecular structure of supercritical water, the extent of hydrogen bonding is calculated as a function of pressure and temperature, using APACT (two-site and three-site), the Sanchez-Lacombe-Panayiotou (LP) equation of state and the statistical associating fluid theory (SAFT). The three models investigated show qualitative agreement in the prediction of hydrogen bonding in supercritical water, but APACT predicts a larger degree of association than SAFT and the SLP equations of state for all temperatures.