as reliable and can be used for performing kinetic predictions in
practical applications.
As mentioned above, the dependence of the activation energy
of the dehydration reaction (Eq. (2.4)) on the conversion fraction
suggests that the process is a multi-step process. More specifically,
according to Fig. 4b, the transformation of calcium sulphate dihydrate
into soluble calcium sulphate anhydrite III can be assumed to
be a three step process [12,13], corresponding to different mechanisms:
- Step I: Nucleation (˛ < 0.1);
- Step II: Nuclei growth (0.1 < ˛ < 0.7);
- Step III: Diffusion of water molecules (˛ > 0.7).
In order to define the kinetic parameters of each process,
a model-fitting approach was used, with respect to Eq.
(3.1), and the results are tabulated in Table 3. The calculated
kinetic parameters are quite close to those reported in [13],
where Ea = 137
±
24.5 kJ mol−1 (0 < ˛ < 0.1), Ea = 107.3
±
2.4 kJ mol−1
(0.1 < ˛ < 0.7) and Ea = 106.9
±
1.5 kJ mol−1 (0.7 < ˛ < 1), for pure
calcium sulphate. The main reason for the observed discrepancies
between the kinetic parameters of this study and those
for pure calcium sulphate dihydrate is that the values reported
in [13] were defined at autogenous water vapor partial pressure.
Thus, the discrepancies are more obvious at high values
of conversion fraction, where the water vapor concentration and
partial pressure are very significant. Furthermore, Lou et al. [15]
reported that the activation energy for the dehydration of two
different types of flue gas desulphurization (FGD) gypsums, at
negligible water vapor partial pressure, was Ea = 130
±
11 kJ mol−1
(0.1 < ˛ < 0.4) and Ea = 78
±
6 kJ mol−1 (0.5 < ˛ < 0.9) for gypsum one
and Ea = 136
±
13 kJ mol−1 (0.1 < ˛ < 0.4) and Ea = 79
±
7 kJ mol−1
(0.5 < ˛ < 0.9) for gypsum two, respectively. The differences
between these values and the values found in this paper
could be due to the different origin of the raw material
and/or the impurities, such as fly ash, that accelerate gypsum
dehydration and thereby decreasing the activation energy
[15].
as reliable and can be used for performing kinetic predictions inpractical applications.As mentioned above, the dependence of the activation energyof the dehydration reaction (Eq. (2.4)) on the conversion fractionsuggests that the process is a multi-step process. More specifically,according to Fig. 4b, the transformation of calcium sulphate dihydrateinto soluble calcium sulphate anhydrite III can be assumed tobe a three step process [12,13], corresponding to different mechanisms:- Step I: Nucleation (˛ < 0.1);- Step II: Nuclei growth (0.1 < ˛ < 0.7);- Step III: Diffusion of water molecules (˛ > 0.7).In order to define the kinetic parameters of each process,a model-fitting approach was used, with respect to Eq.(3.1), and the results are tabulated in Table 3. The calculatedkinetic parameters are quite close to those reported in [13],where Ea = 137±24.5 kJ mol−1 (0 < ˛ < 0.1), Ea = 107.3±2.4 kJ mol−1(0.1 < ˛ < 0.7) and Ea = 106.9±1.5 kJ mol−1 (0.7 < ˛ < 1), for purecalcium sulphate. The main reason for the observed discrepanciesbetween the kinetic parameters of this study and thosefor pure calcium sulphate dihydrate is that the values reportedin [13] were defined at autogenous water vapor partial pressure.Thus, the discrepancies are more obvious at high valuesof conversion fraction, where the water vapor concentration andpartial pressure are very significant. Furthermore, Lou et al. [15]reported that the activation energy for the dehydration of twodifferent types of flue gas desulphurization (FGD) gypsums, atnegligible water vapor partial pressure, was Ea = 130±11 kJ mol−1(0.1 < ˛ < 0.4) and Ea = 78±6 kJ mol−1 (0.5 < ˛ < 0.9) for gypsum oneand Ea = 136±13 kJ mol−1 (0.1 < ˛ < 0.4) and Ea = 79±7 kJ mol−1(0.5 < ˛ < 0.9) for gypsum two, respectively. The differencesbetween these values and the values found in this papercould be due to the different origin of the raw materialand/or the impurities, such as fly ash, that accelerate gypsumdehydration and thereby decreasing the activation energy[15].
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