Avrami-Erofeev equation dehydration process

The dehydration and rehydration reactions of calcium sulfate dihydrate (gypsiun) are of considerable technological importance and have been the subject of many studies. On heating, CaS04.2H20 may yield the hemihydrate or the anhydrous salt and both the product formed and the kinetics of the reaction are markedly dependent upon the temperature and the water vapour pressure. At low temperatures (i.e. < 383 K) the process fits the Avrami-Erofeev equation (n = 2) [75]. The apparent activation energy for nucleation varies between 250 and 140 kJ mol in 4.6 and 17.0 Torr water v our pressure, respectively. Reactions yielding the anhydrous salt (< 10 Torr) and the hemihydrate ( (HjO) >17 Torr) proceeded by an interface mechanism, for which the values of E, were 80 to 90 kJ mol. At temperatures > 383 K the reaction was controlled by diffusion with E, = 40 to 50 kJ mol. ... 

The final stages of the above reaction overlapped with the onset of the nucleation and growth process that continued to complete the dehydration. Growth of three dimensional nuclei was confirmed microscopically. This second rate process was well described by the Avrami-Erofeev equation with = 2 and E, for crystals was 175 30 kJ mol (with a considerable scatter of data) below 460 K and a more reproducible reaction rate, with E, = 153 10 kJ mol, for powder. Above about 450 K there were some indications of intracrystalline melting of single crystals and the value of , increased markedly to 350 50 kJ mol (again with significant scatter of data). 

The r-time curves for the decomposition of anhydrous cobalt oxalate (570 to 590 K) were [59] sigmoid, following an initial deceleratory process to a about 0.02. The kinetic behaviour was, however, influenced by the temperature of dehydration. For salt pretreated at 420 K, the exponential acceleratory process extended to flr= 0.5 and was followed by an approximately constant reaction rate to a = 0.92, the slope of which was almost independent of temperature. In contrast, the decomposition of salt previously dehydrated at 470 K was best described by the Prout-Tompkins equation (0.24 < a< 0.97) with 7 = 165 kJ mol . This difference in behaviour was attributed to differences in reactant texture. Decomposition of the highly porous material obtained from low temperature dehydration was believed to proceed outwards from internal pores, and inwards from external surfaces in a region of highly strained lattice. This geometry results in zero-order kinetic behaviour. Dehydration at 470 K, however, yielded non-porous material in which the strain had been relieved and the decomposition behaviour was broadly comparable with that of the nickel salt. Kadlec and Danes [55] also obtained sigmoid ar-time curves which fitted the Avrami-Erofeev equation with n = 2.4 and = 184 kJ mol" . The kinetic behaviour of cobalt oxalate [60] may be influenced by the disposition of the sample in the reaction vessel. 

The isothermal dehydrations of more complex drug hydrates have also been studied. Zhu and Grant (38) examined the dehydration of nedocromil magnesium pentahydrate, which shows the two distinct dehydration processes discussed previously (23). They found that the lower-temperature dehydration process (about 60-90°C) fitted the Avrami-Erofeev equation (A2) whereas the higher-temperature process (about 180-200°C) fitted the Prout-Tompkins and the Avrami-Erofeev... 

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