Quantitative approaches for the description of solubilities of inorganic compounds in near-critical and supercritical water

The decreased solubility of salts in supercritical water is of great interest for industrial applications and scientific work. Several methods to quantify this decreased solubility are described and reviewed by applying them on experimental solubility data. The salts used for comparison are NaCl, NaNO3, Na2CO3, Na2SO4, PbO and CuO. The approaches used for comparison are of semi-empirical and empirical origin. Resulting from this comparison, one approach based upon the description of the phase equilibrium between the solid salt phase and the supercritical water phase is chosen as the most appropriate one. Additionally, parameters for the description of the solubilities of NaCl, NaNO3, Na2CO3, Na2SO4, PbO and CuO with this approach at supercritical conditions are presented. To extend the known solubility data of salts an experimental setup was designed which uses a continuous feed stream with a known concentration. This stream is pressurized and heated to the temperature and pressure of interest. Due to the elevated temperatures and pressures, a supersaturation occurs and precipitation takes place. The solubility of NaCl in the range of 380–410 °C and 170–235 bar is determinated with this method. The experimental results agree well with other studies which investigated the solubility of NaCl and extend the known solubility data to higher densities.