Kinetics of hydration‐dehydration reactions considered as solid transformations

Abstract

A mechanism is proposed for the dehydration-rehydration process in solid inorganic salts, and model rate equations are derived and applied to the observed behavior of potassium carbonate. Quantitative expressions for the effect of pressure on the reaction rates are derived using basic principles from nucleation and heterogeneous phase transformation theory. Model equation predictions agree with experimental dehydration and rehydration rate data at all but extreme pressures. The basic rate equation is also used to interpret the data of Eckhardt and Flanagan (1964) for the effect of pressure on the dehydration of manganous formate dihydrate. The mechanism on which the model equations are based is also consistent with the observed effects of cycling and of high temperature pretreatment on the KzC03 rehydration rate. SCOPEA new mechanistic interpretation is offered for the analysis of the kinetics of gas-solid reactions and applied in detail to the reversible dehydration and rehydration of solid inorganic salts. These processes are considered as solid-solid transformations, and an expression is derived for the intrinsic rate of reaction in terms of the velocity of the movement of the boundary separating the reactant and product phases. The rate dependence on pressure and temperature predicted by the model equations is compared to those observed for both dehydration and rehydration in experiments with potassium carbonate hydrate and to that reported in the literature for the dehydration of manganous formate dihydrate. In addition, the effects of cycling and of high-temperature pretreatment on the reaction rates of KzCO3 are presented, and the ability of the proposed mechanism to account for these effects is discussed.A new mechanistic rate model has been formulated for reversible gas-solid reactions and applied to interpret the dehydration and rehydration kinetics of salt hydrates. Quantitative rate expressions predicting the effects of pressure and temperature were derived using basic principles from nucleation and heterogeneous phase transformation theories. The dehydration model rate equation provides accurate quantitative predictions for the velocity of movement of the KzCOs*3/2HzO dehydration interface at all three temperatures studied. The dehydration equation also provided a close correlation with data from the literature for the dehydration of manganous formate dihydrate.A comparison of the rehydration model equation with experimental KzC03 particle rehydration rates revealed good agreement of the model with the experimental data for reduced pressures within the range 1 < (Pes/P) < 2. At higher rehydration pressure, the reaction rates are not controlled by the transformation process at the reaction interface and the model equations are not applicable. Within the range of moderate pressures both above and below the equilibrium pressure, the model effectively describes the rapid decline of both dehydration and rehydration rates as the equilibrium pressure is approached.The model equations imply that reaction ra...

Understanding the Hydration Process of Salts: The Impact of a Nucleation Barrier

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“…Among the investigated compounds, K 2 CO 3 hydration was reported earlier (1984) as kinetically hindered, without further experimental elaboration on the low vapor pressure region. 19 The height h is taken here as the unit cell axis which is parallel to the layer of water molecules in the hydrated structure. 26,27 Figure 3.…”

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A DFT based equilibrium study on the hydrolysis and the dehydration reactions of MgCl2 hydrates

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A Scaling Rule for Power Output of Salt Hydrate Tablets for Thermochemical Energy Storage

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