Rate studies arc reported of the effect of rehydrationdehydration cycling on the vapor hydration behavior of solid KzCO3. Isothermal rate data were obtained a t different temperatures and water vapor pressures for the reaction of narrowlysized anhydrous particles. Effects of different particle preparation histories on the rehydration rate were investigated and correlations of rate with particle pore structure explored. Rehydration rates of dehydrated K2C03*3/2IIzO were found to depend on the conditions of the prior dehydration. Rehydration is comparatively very slow at relative pressures below PIP,., N 1.5; rates increase linearly with pressure above PIP, -3. Hydration rates of KzCO3 particles obtained as anhydrous are substantianyslower than those of identically-sized crystals produced by prior dehydration of KpC03*3/2II20; after one rehydrationdehydration cycle, rehydration rates are increased by as much as two orders of magnitude and this distinction between sources virtually disappears. Diffusional resistances based on calculated water vapor diffusivities are qualitatively consistent with the observed effects of cycling but d o not by themselves account fully for the observations.
M.
SCOPEThe hydration reaction of KzCO:, with water vapor is of interest from two different perspectives. First, this system serves as a convenient prototype for the study of kinetics in reversible gas-solid reactions of inorganic salts, a topic of recognized industrial importance. Second, water and KzCO3 were shown (Stanish and Perlmutter, 1981) to hold significant promise as a refrigerant-absorbent pair in a low-temperature heat pump cycle. Since little information is available concerning the kinetics of hydration and dehydration in the water-KzC03 system, a n experimental study of these phenomena is needed to more accurately determine the feasibility and potential of a real KzC03-based heat pump device.The objectives of this study were to identify the variables that are most important in determining the rehydration rate of KzCO3 particles and to quantify their effects. This investigation includes evaluation of the importance of the source of the solid and the method of preparation in determining the rate of rehydration. In addition, the effect of water vapor pressure on reaction rate is quantified. Finally, changes in rehydration rate with rehydrationdehydration cycling are explored, and correlations are sought with the differences in particle pore structure revealed by mercury intrusion porosimctry data. The behavior of this system is compared to that reported for thc rcvcrsiblc decomposition of AglC03.
CONCLUSIONS AND SIGNIFICANCEThe rate of rehydration of dehydrated K&O3*3/2HzO crystals is affected by the prior dehydration conditions; samples that were dehydrated more rapidly (at higher temperature and/or lower pressure) were subsequently found to rehydrate more rapidly as well. Slow rehydration and the appearance of induction and acceleration in rehydration conversion time curves were characteristics of KzC03*3/2HzO dehydrat...