Non – Isothermal
Table of contents
The paper describes the methodology of study of chemical kinetics under non-isothermal conditions and different theories available. A short critical review is given of the available approaches to determine kinetic parameters from thermal analysis data. We provided with a case study of thermal degradation of Chitosan. The assessment of the kinetic models of chitosan pyrolysis and to estimation the kinetics triplet (E, A and the shape of the most appropriate f(α)-function) by means of thermogravimetric data, using different calculation procedures has been discussed in detail as a case study. The comparison of the obtained results showed that they strongly depend on the selection of proper mechanism function for the process. Therefore, it is very important to determine the most probable mechanism function. In this respect the iso-conversional calculation procedure turned out to be the most appropriate.
Usually the kinetics of chemical reactions are studied under isothermal conditions, i.e, with the temperature of the reacting system constant over space and time. For this the process is conducted in such a way that effects of self-heating of the material as a result of reaction heat are negligible while the temperature of external heat sources (ovens) does not change with time. Restricting the process in this way, during the course of the reaction one measures either the concentrations of the reacting substances or of the end products (which is generally a difficult matter) or some quantity which gives an overall characterization of the process (as a rule, for gases this is the pressure in the reaction vessel; for condensed substances it is the remaining mass of the reacting substance, the quantity of evolved gaseous products, or the quantity of heat evolved)
Two trends in chemical kinetics
To study of the mechanism of a reaction, the establishment of one chemical event or another and the influences on it, the detection and investigation of intermediate products, etc.
A process as a whole is described in the form of kinetic equations and the determination of the parameters entering into them, a detailed understanding of the chemical mechanism of the process being relegated to second place. This approach to the study of a chemical reaction is sometimes called the formal-kinetic treatment.
Investigations in the domain of formal kinetics are just as important as the study of the mechanism of a reaction as they enable one to carry out calculations of the rates of a chemical process under various realistic conditions, Advances in the domain of the mathematical modelling of chemotechnical processes and calculations of chemical reactions intensified the urgency of this trend as it was found that for many reactions of practical importance kinetic equations were not obtained, despite the fact that progress had been made in understanding the mechanism.
Classification of non-isothermal methods
As the kinetics of a chemical reaction, in most cases, is studied at high temperatures, the mechanism of the heating of the substance is very essential. From this point of view all the mechanisms can be separated into two groups:
1) Methods with surface heating (the substance is heated from an external heat source by heat supply through the surface)
2) Methods with volumetric heating (the substance is heated by...
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