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Temperature programmed desorption is a typical non-isothermal method and therefore it needs a careful experimentation and a careful evaluation of the experimental data, in order to obtain meaningful kinetic parameters.

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Abstract  

On the basis of the theory of thermokinetics proposed in the literature, a novel thermokinetic method for determination of the reaction rate, the characteristic parameter method, is proposed in this paper. Mathematical models were established to determine the kinetic parameters and rate constants. In order to test the validity of this method, the saponifications of ethyl benzoate, ethyl acetate and ethyl propionate, and the formation of hexamethylenetetramine were studied with this method. The rate constants calculated with this method are in agreement with those in the literature, and the characteristic parameter method is therefore believed to be correct.In the light of the characteristic parameter method, we have developed further two thermo-kinetic methods, the thermoanalytical single and multi-curve methods, which are convenient for simultaneous determination of the reaction order and the rate constant. The reaction orders and rate constants of the saponifications of ethyl acetate and ethyl butyrate and the ring-opening reaction of epichlorohydrin with hydrobromic acid were determined with these methods, and their validity was verified by the experimental results.

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Kinetic parameters of thermal decomposition of compounds of general formulaM 2 I M II[Ni(NO2)6], whereM I= K+, Rb+ or Cs+ andM II= Ca2+, Sr2+ or Ba2+, were investigated on the basis of the respective thermal curves. Calculations of the reaction order and activation energy carried out by the Coats-Redfern method and by Doyle's method (modified by Zsakó) gave similar results, The reaction order is 2 for all the compounds investigated. In the group of potassium salts the activation energy increases fromM II=Ca2+ toM II=Ba2+. In the groups of rubidium and caesium salts, the lowest activation energy is observed whenM II=Sr2+. Such behaviour of the nitritonickelates is explained in terms of structures and the principle of maximum density.

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The wide variations of calculated activation energies for solid decomposition suggests that there is no discrete activated state. Further, the statistical distribution on which the calculations are based is not a realistic concept. The lowest energy possible — and most frequently occurring — is the energy of the bulk crystal. Within the crystalline solid, vibrational interactions transfer energy so rapidly that a substantial difference from the average energy is not achievable within the crystal. The lack of a statistical distribution rules out the use of the Arrhenius equation unless it is independently verified for the particular system.

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A differential method is proposed which uses local heating rates to evaluate non-isothermal kinetic parameters. The method allows to study the influence of the deviation of the true heating rate with respect to the programmed one on the values of the kinetic parameters. For application, the kinetic parameters of the following solid-gas decomposition reaction were evaluated: [Ni(NH3)6]Br2(s)→[Ni(NH3)2]Br2(s)+4NH3(g). The results obtained revealed significant differences between the values of the non-isothermal kinetic parameters obtained by using local heating rates and those obtained by using the programmed heating rate. It was also demonstrated that the kinetic equation which makes use of the local heating rates permits a better description of the experimental (α, t) data than the kinetic equation which uses the programmed constant heating rate.

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Feedforward neural networks have been used for kinetic parameters determination and signal filtering in differential scanning calorimetry. The proper learning function was chosen and the network topology was optimized, using an empiric procedure. The learning process was achieved using simulated thermoanalytical curves. The resilient-propagation algorithm have led to the best minimization of the error computed over all the patterns. Relative errors on the thermodynamic and kinetic parameters were evaluated and compared to those obtained with the usual thermal analysis methods (single scan methods). The errors are much lower, especially in presence of noisy signals. Then, our program was adapted to simulate thermal effects with known thermodynamic and kinetic parameters, generated electrically, using a PC computer and an electronic interface on the serial port. These thermal effects have been generated by using an inconel thread.

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In this paper, a systematic analysis of the errors involved in the determination of the kinetic parameters (including the activation energy and frequency factor) from five integral methods has been carried out. The integral methods analyzed here are Coats-Redfern, Gorbachev, Wanjun-Yuwen-Hen-Zhiyong-Cunxin, Junmeng-Fusheng-Weiming-Fang, Junmeng-Fang and Junmeng-Fang-Weiming-Fusheng method. The results have shown that the precision of the kinetic parameters calculated by the different integral methods is dependent on u (E/RT), that is, on the activation energy and the average temperature of the process.

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In pseudo bi-component separated-stage model (PBSM), the effect of the TG value at separation points on the kinetic parameters is studied by residual and theoretical analysis. Simultaneously, a new method to determine the point that is the end of 1st reaction or the initial of 2nd reaction is developed. The investigations have improved the calculation procedure of PBSM. We performed thermogravimetry (TG) analysis on oil tea wood with two-step consecutive model and parallel model. Comparison between the results of the two models and improved PBSM shows well agreements. The influence of different separation points on kinetic parameters is presented.

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An improved version of the Coats-Redfern method of evaluating non-isothermal kinetic parameters is presented. The Coats-Redfern approximation of the temperature integral is replaced by a third-degree rational approximation, which is much more accurate. The kinetic parameters are evaluated iteratively by linear regression and, besides the correlation coefficient, the F test is suggested as a supplementary statistical criterion for selecting the most probable mechanism function. For applications, both non-isothermal data obtained by theoretical simulation and experimental data taken from the literature for the non-isothermal dehydration of Mg(OH)2 have been processed.

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Abstract  

A program for the evaluation of non-isothermal kinetic parameters is presented. The program allows evaluation of the kinetic parameters under constant heating rate or constant reaction rate conditions. The simulation of temperature vs. conversion curves is also possible. A regression method is included, which allows a discrimination between various conversion functions and also evaluation of the activation parameters. The program was tested with various simulated decomposition curves and the non-isothermal decomposition curves of calcium oxalate. The program is written in Visual BASIC 4.0 and can be run under Windows 95 .

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