Thermal stability, hydrolysis and thermodynamic properties of molten KCl-CuCl

Abstract

KCl-CuCl is known as an inorganic salt mixture with a particularly low melting point, below 200 °C. There are very few studies published investigating this binary system. In this study KCl-CuCl is considered as a candidate for liquifying biomass as a pre-step before a high pressure and temperature (40–50 bar, 450–500 °C) hydro-pyrolysis process. Its low melting point makes it a good candidate to liquify biomass at mild conditions (low temperature and pressure) and avoid producing char and ash. Thermal stability and stability against hydrolysis have been measured for compositions close to the eutectic composition up to 500 °C. The results show that KCl-CuCl is thermally stable, and no mass loss was observed up to 500 °C. Moreover, it is chemically stable in contact with water and no HCl was detected in hydrolysis experiments. In addition, the modelling of the system was studied. Although there is a phase diagram for this system in the literature based on the experimental data, no thermodynamic parameters have been calculated for this system and no database was found for KCl-CuCl solution. Therefore, Calphad modelling of the binary KCl-CuCl molten salt is performed in this study and FactSage is employed to assess the thermodynamic parameters and generate the phase diagram. For this purpose, a series of experiments have been carried out to investigate transition points and thermodynamic properties of mixtures between 40 and 80 mol% of CuCl by cooling curve and differential scanning calorimetry. The intermediate compound K 2 CuCl 3 is considered stoichiometric, and its Gibbs energy modelling relies on ab initio calculated enthalpy of reaction from the base salts and optimization of the standard entropy. The liquid solution is modelled with a subregular solution model using Redlich-Kister polynomials. The phase diagram of the system is generated based on thermodynamic data and experimental results. The results show that the predicted eutectic point of the binary system is located at T = 145.9 °C and 64.9 CuCl mol%. The calculated results are in excellent agreement with the measured values. The high thermal stability and stability against hydrolysis qualify eutectic KCl-CuCl mixtures as promising molten salt for biomass liquefaction. However, corrosion limits the choice of possible reactor materials.