Exploration of chloride ion impurities on solid-liquid-vapor phase diagram and thermal performance of nitrate molten salts

Nitrate molten salts for heat storage play a crucial role in modern energy technologies such as concentrated solar power (CSP), but the influence of impurity chloride ions on nitrate molten salts requires further investigation. This article reports the solid-liquid-vapor phase diagram, thermophysical properties, and thermal stability of sodium/potassium nitrate system with impact of chloride ion impurity. Based on Common Ion Solvent (CIS) theory, a modified thermodynamic model and computational method for solid-liquid-vapor phase equilibrium in a binary system containing impurity ions are developed and validated. The results indicate that with the increase of chloride content, there is little change in the composition of the lowest melting and highest boiling points, but the temperature significantly decreases. Adding 3.0 mol% KCl reduced the lowest liquid phase temperature by 4.57 °C and the highest boiling temperature by 13.26 °C. Density increases with impurities, while specific heat and thermal conductivity decrease; correlations with temperature and ion concentration are established. Thermal conductivity drops by 5.59 % with 3.0 mol% KCl, because the short-range ordered vibrations of ions are disrupted by impurity ions. Experimental analysis of short-term and long-term thermal stability of systems containing impurity indicates that chloride ion impurity can increase the evaporation rate of salt due to higher vapor pressure but reduce thermal decomposition of nitrate.