Application of the Unified Kinetic Theory for the Reversible Thermal Dehydration and Hydration Reactions in the Calcium Oxalate Monohydrate–Anhydride–Water Vapor System

Abstract

This article provides a demonstration of the universal kinetic descriptions of the reversible thermal dehydration and hydration processes of inorganic salts across varying temperatures (T) and partial pressures of water vapor (p(H2O)). Thermal dehydration of calcium oxalate monohydrate (CaOX-MH) and the hydration of its anhydride (CaOX), which exhibit a perfect reversible nature, were selected as a model system. In each reaction process, the reaction rate exhibited variations influenced by a combined effect of chemical equilibrium and reaction kinetics. An extended kinetic equation was formulated by incorporating an accommodation function (AF), which expresses the dependence of the reaction rate on p(H2O) in individual reaction processes. The extended kinetic equation was employed to elucidate the kinetic behaviors of both the thermal dehydration and hydration processes as a function of T, degree of reaction (α), and p(H2O) with reference to the equilibrium pressure of the reactions (Peq(T)). The AF functioned to unify the kinetic curves into a kinetic surface in the three-dimensional kinetic coordinate of T–1, α, and ln[(dα/dt)/h(p(H2O), Peq(T))]. The universal kinetic description across varying T and p(H2O) values was achieved in the individual reaction processes by revealing the isoconversional and isothermal kinetic relationships of the kinetic surfaces in the three-dimensional kinetic coordinate. Furthermore, the intrinsic Arrhenius parameters for each reaction process were derived through a correlation analysis of the apparent Arrhenius parameters determined based on the extended kinetic equation. The demonstration of the practical kinetic analysis as applied to both the thermal dehydration and hydration processes validates the kinetic theory for the reversible thermal dehydration and hydration processes, providing novel insight into the previously unrevealed characteristics of the reversible processes in the CaOX-MH–CaOX–water vapor system.