Hydration of salts as a two-step process: Water adsorption and hydrate formation

Abstract

K2CO3 is a promising salt for thermochemical heat storage. For a high performance, the thermochemical reaction must take place as close as possible to the equilibrium, while ensuring sufficient reaction rates. In this work, we studied the near-equilibrium hydration kinetics of K2CO3 and other salts (CuCl2, MgCl2 and LiCl). We proposed a generic two-step mechanism for the hydration of salts, consisting of (1) adsorption of water vapour and dissolution of ions from the initial phase (a wetting film) and (2) formation of the hydrate crystal (crystallisation from the wetting film). The two steps are assumed to be in momentarious balance during the hydration process. As a result, nucleation is rate limiting at low supersaturations of water vapour (inside the metastable zone), and water diffusion to the wetting film is rate limiting at high supersaturations (outside the metastable zone). We have seen that the vapour pressure of the wetting film stabilises at the metastable zone boundary p*. The driving force for hydration outside the metastable zone (MZ) is therefore the pressure difference between the atmospheric vapour pressure and the vapour pressure of the wetting film, p − p*. Non-Parametric Kinetic analysis of the hydration of K2CO3 indicates that nucleation plays a central role inside the metastable zone (at low supersaturations) as expected. Outside the MZ, the analysis suggests a steady conversion rate, in agreement with a water vapour diffusion limitation. The diffusion limited process at high supersaturations hardly depends on the temperature, but mainly on the pressure difference, as expected. It is further shown that the diffusion limited process can be characterised with an apparent activation energy. However, this apparent activation energy is in fact the hydration enthalpy and does not refer to a real energy barrier.