Diffusional limitations on the kinetics of dehydration reactions of hydrated barium chloride

Abstract

The dehydration kinetics of BaCl 2·2H 2O powders were examined as a function of water vapor pressure, sample bed depth, particle diameter and temperature. The conversion behavior was modeled in both the kinetic and the combined diffusion and kinetic limited regimes with modified forms of the Avrami-Erofe'ev equation. Kinetic data were obtained by thermogravimetric techniques and by calorimetry. Structural data for intermediates and products were obtained by X-ray diffraction and optical microscopy. Dehydrations were conducted isothermally between 317 and 335 K either under vacuum or in controlled pure water vapor atmospheres ranging from 40 to 6.67 × 10 2 Pa. The dehydration rate decreased as the water vapor pressure, sample bed depth and particle diameter increased. The estimated effective bed diffusivity is 1.0×10 −6 cm 2 s −1. After a critical pressure was exceeded, severe reductions in rate were observed. The dehydration of BaCl 2·2H 2O proceeded stepwise via the crystalline monohydrate above about 80 Pa at 317 K; below this pressure the crystalline monohydrate was not observed. Transient increases in water vapor pressure in the sample bed caused crystallization of the monohydrate under conditions where it did not otherwise form.