Universal kinetic description for the hydration reaction of sodium carbonate anhydride across different temperatures and water vapor pressures

The objective of this study was to establish a unified kinetic theory applicable to both reversible thermal decomposition of solids and solid-gas reactions. To this end, the reversible thermal dehydration of sodium carbonate monohydrate (SC-MH) and the hydration of its anhydride (SC-AH) were selected as a model system. The kinetics of both reactions can be described as a function of temperature (<i>T</i>), degree of reaction (<i>α</i>), and partial pressure of water vapor (<i>p</i>(H₂O)). A universal kinetic description of the thermal dehydration of SC-MH has been previously achieved over a range of <i>T</i> and <i>p</i>(H₂O) values, thereby yielding the intrinsic Arrhenius parameters at various <i>α</i> and for each physico-geometrical reaction step. This was based on an extended kinetic equation accompanied by an accommodation function of <i>p</i>(H₂O) with reference to the equilibrium pressure (<i>P</i>_eq(<i>T</i>)) of the reaction. In this article, the applicability of the extended kinetic equation to the hydration of SC-AH is demonstrated by highlighting the common and differential points of the universal kinetic description between the forward and reverse reactions. The universal kinetic description of the hydration reaction over different <i>T</i> and <i>p</i>(H₂O) values was achieved based on the extended kinetic equation in the schemes of the formal kinetic characterization and the physico-geometrical kinetic modeling. However, the extended kinetic analysis of the hydration reaction yielded negative Arrhenius parameters. This outcome was attributed to the contribution of the thermodynamic terms associated with the equilibrium conditions to the apparent Arrhenius parameters. Consequently, the intrinsic Arrhenius parameters were obtained by considering the contributions of the thermodynamic terms.