Thermodynamic equilibrium and kinetic study of lanthanum chloride heptahydrate dehydration for thermal energy storage

Abstract

In the past decade, thermochemical materials have gained attention as promising options for heat storage. Compared to sensible heat storage and phase change materials, thermochemical materials offer certain important advantages such as high energy density and low heat loss during the storage process. Lanthanum chloride hydrates are interesting and promising materials owing to their high energy density and application at low temperature and water-vapor pressure. The thermodynamic properties of the LaCl3–H2O system and dehydration kinetic of LaCl3⋅7H2O are investigated using coupled thermogravimetry and differential scanning calorimetry under a controlled humidity atmosphere and X-ray diffraction. As the literature on the LaCl3 thermodynamic equilibrium is inconsistent, the phase diagram is experimentally determined. The findings of the first part of the study are presented in the form of a reliable phase diagram. From a kinetic point of view, it has been determined that the dehydration of lanthanum sulfate heptahydrate can be analyzed using the shrinking core model, where the growth of a new phase alone is the rate-determining step. The areic reactivity of growth decreases with the increase in water-vapor pressure and increases with the increase in temperature. The mechanism of the dehydration reaction is presented, in which the desorption of water molecules is the rate-determining step.