Kinetic factors in pyrolytic reactions

For pyrolytic reactions, the variation of the molar concentration [A] of a substance during the pyrolysis is not always the most appropriate parameter to be monitored. The calculation of [A] can be a problem for many types of samples, and very frequently during pyrolysis not only one decomposition process takes place. In this case, the overall reaction kinetics must be considered. A more convenient parameter for monitoring pyrolytic reactions is, for example, the sample weight. For a reaction of the first order, by multiplying rel. (2.3.2) with the volume V and the molecular weight M of the substance A, since W = [A] V M, the following expression is obtained  

In some cases, the pyrolysis leaves a residue of undecomposed sample of the weight Wf (final weight). In this case rel. (2.3.35a) will be 

This type of relation can be applied to pyrolytic processes even if the reaction is not of the first order. In this case the reaction rate is described by the relation  

When the pyrolytic process does not occur in gas phase, different problems appear. Although equations of the type (2.3.35a) can be used in certain cases, these may lead to incorrect results. Various empirical models were developed for describing the reaction kinetics during the pyrolysis of solid samples [6]. Most of these models attempt to establish equations that will globally describe the kinetics of the process and fit the pyrolysis data. 

The temperature dependence of the rate of reactions is particularly important for the pyrolytic processes. Rel. (2.3.5) and (2.3.35a) can be used for the understanding of the common choices for the pyrolysis parameters, their combination leading to  

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In order to understand better the problems related to kinetic factors in pyrolytic reactions, a few basic concepts will be reviewed. The reaction rate of a chemical process where R is a reactant and P is a product is defined as the variation of the concentration of R or of P versus time. The reaction rate can be expressed by the relation ... 

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