Fe/KClO4 İkili Karışımlarının Isıl Bozunma Kinetiğinin Analitik Ve Nümerik Yöntemlerle Tayin Edilmesi
Özet
The proposed thesis aims to investigate the kinetic dynamics of heat pellets used in thermal batteries which is a type of a primary battery. For this purpose, heat (Q) – temperature (T) curves for Fe/KClO4 mixtures with different weight compositions were obtained using non-isothermal thermal analysis. Heat (Q) – temperature (T) curves were then converted to conversion (α) – temperature (T) curves utilizing a data processing script that was developed with MATLAB™ (R2014A). For thermal decomposition of each heat pellet with different weight composition, the apparent activation enery (Ea) was determined using Starink’s isoconversional method. Malek’s kinetic procedure was followed to determine the reaction model, (f(α)) with the obtained activation energy. The kinetic parameters that were analytically was used as the initial values for the program that was developed with the COMSOL Multiphysics that aims to minimize the difference between experimental and calculated values in a iterative fashion. Lastly, the optimized kinetic parameters that were calculated using numerical methods were used in the heat pellet burning simulation which was defined with COMSOL Multiphysics to calculate the burn rate of heat pellets. These burn rate values were compared with the literature values to validate the obtained kinetic parameters.
In the first part of the study, for heat pellets with different weight compositions (w/w) (82/18, 84/16, 86/14, 88/12) apparent activation energies were determined as 208, 210, 218, 222 respectively using the Starink isoconversional method. The activation energy decreases with the increasing % content of KClO4. The reason for this increasing trend is the increased probability of oxidation reaction with increasing % content of KClO4. Additionally, activation energy values with respect to conversion revealed that multi-step decomposition (KClO4 decomposition and Fe oxidation) takes place until the overall conversion reaches 0.4.
The reaction model for the thermal decomposition of heat pellets was found to follow Śesták-Berggren’s reaction model that was found by using the apparent activation energies obtained for different heat pellets together with the Malék’s kinetic procedure. The master plots, y(α) and z(α) revealed that the thermal decomposition of heat pellets takes place according to the nucleation model.
The obtained kinetic parameters were used to calculate the conversion (α) vs. time (t) to compare with the experimental values. Comparison showed that there was no overlapping between calculated and experimental values. Therefore, for the optimization of kinetic parameters the iterative approach that was designed with COMSOL Multiphysics to minimize the difference between model and experimental values was employed. The optimized kinetic parameters obtained from optimization tool was used to calculate the time (t) – conversion (α) curve which was compared with the experimental time (t) – conversion (α) curve. Correlation factor (r) between these curves were calculated as 0.98 – 0.99.
The optimized kinetic parameters were used in the simulation of burning process of heat pellets. For the heat pellets with weight compositions of 82/18, 84/16, 86/14 and 88/12 the burn rates were calculated as 26.0, 15.7, 11.5 and 10.7 cm/s, respectively. With increasing % content of KClO4, the burn rate increases exponentially. This behavior is supported with the obtained kinetic parameters for heat pellets. Additionally, the calculated burn rate values were found to be close to the literature values. This correlation verifies that the obtained kinetic parameters are accurate.
In order to accurately model the activation and operation processes of thermal batteries, accurate kinetic parameters of the thermal decomposition of heat pellets should be determined. Consequently, the kinetic dataset of the thermal decomposition of heat pellets which is crucial for the design processes relying on modelling are obtained within the scope of this study.
