Thermodynamic properties of KCaCl(NO3)2 compound and influence of kinetic effects

Abstract

The thermodynamic properties of the intermediate compound KCaCl(NO3)2 are important for the thermochemical description of the KCl–CaCl2–KNO3–Ca(NO3)2 reciprocal system, for its utilization as a phase change material or heat transfer fluid in thermal energy storage. The formation of KCaCl(NO3)2 as intermediate compound in this reciprocal system was confirmed by X-Ray diffraction analysis. Aside from the main phase of KCaCl(NO3)2, other crystalline phases were also identified. Kinetic effects were observed during the study of this compound. For example, the enthalpy of fusion of the 50KCl–50Ca(NO3)2 mixture depends on the cooling rate of the previous cycle. The samples, which crystallized with 10 K/min, show higher enthalpy of fusion (22.4 kJ/mol) compared to samples cooled with 0.5 K/min - 16.3 kJ/mol. This effect is related to the different ratio of crystallized phases (KCaCl(NO3)2, KCaCl3, Ca(NO3)2 and various Potassium Calcium nitrates) in this mixture. The heat capacity and enthalpy increment of the KCaCl(NO3)2 compound in solid and liquid phases are determined with two different DSC instruments. The selected value of the fusion enthalpy for KCaCl(NO3)2 compound is 22.4 ± 1.2 kJ/mol, which was obtained after fast (10 K/min) cooling. The comparison of the experimental results with calculated values based on Neumann-Kopp rule confirms the reliability of the obtained data.