Supercritical water desalination (SCWD) of multi-component brines

The phase behaviour of two model multi-component salt-water mixtures (NaCl-KCl-H2O and NaCl-Na2SO4-H2O) was investigated for the application of supercritical water desalination (SCWD). The experimental investigation was a two-step procedure starting with qualitative analysis followed by quantitative measurements. The qualitative experiments served as a screening step to visually inspect phase behaviour. The quantitative results for NaCl-KCl-H2O, showed similarities in phase behaviour between the salts, which led to the assumption that the ternary mixture behaved as an ideal mixture between two binary solutions. The ternary equilibrium concentrations in the SCW phase could therefore be predicted using the binary salt-water models of the respective salts and accounting for the feed concentrations. Based on the quantitative results for NaCl-Na2SO4-H2O, the phase behaviour of the mixture was divided into three regions. The third region was the suitable region for SCWD, as the precipitated salts (Na2SO4) dissolved in the hydrothermal brine (NaCl), forming a vapour-liquid equilibrium.