Chemical reactions, kinetics thermal decomposition

The initial decomposition chemistry involves unimolecular reactions. This was the conclusion of the first gas-phase kinetics study [84] and has been repeatedly confirmed by subsequent bulb and shock-tube experiments [85, 86]. That first study used shock heating to induce thermal decomposition [84], The data were interpreted in terms of simple C-N bond fission to give CH2 and N02. A more extensive and definitive shock-tube study was reported by Zhang and Bauer in 1997 [85]. Zhang and Bauer presented a detailed kinetics model based on 99 chemical reactions that reproduced their own data and that of other shock-tube experiments [84, 86]. An interesting conclusion is that about 40% of the nitromethane is lost in secondary reactions. 

It has been of considerable interest to develop a theoretical model for predicting the behavior of fire. Excellent articles by Martin and others reflect the strides made in this direction through a number of investigations. Except for Martin s work, which is briefly reviewed, most of these studies (involving the disciplines of physics and mathematics) are beyond the scope of the present article. However, it should be noted that some of the formulas and correlations developed are based on the chemical kinetics, as well as on physical principles. Thus, the lack of sufiBcient knowledge regarding the nature of the combustion process and the reactions involved has led to serious limitations that have been handled by various forms of approximation. For instance, the pioneering work of Bamford, Crank, and Malan was based on the assumption that thermal decomposition. 

Early kinetic experiments on the thermal decomposition of nitro compounds established that for the simplest derivative, nitromethane, the process was first order, but that the reaction was chemically complex owing to further reactions between the products and nitromethane. Cottrell et re-examined the nitromethane pyrolysis and reported values of = 53.2 kcal.mole" and log A = 13 for the Arrhenius parameters of the homogeneous decomposition a radical mechanism was proposed, initiated by C-N cleavage... 

Analytical pyrolysis is considered somehow apart from the other thermoanalytical techniques such as thermometry, calorimetry, thermogravimetry, differential thermal analysis, etc. In contrast to analytical pyrolysis, thermoanalytical techniques are not usually concerned with the chemical nature of the reaction products during heating. Certainly, some overlap exists between analytical pyrolysis and other thermoanalytical techniques. The study of the kinetics of the pyrolysis process, for example, was found to provide useful information about the samples and it is part of a series of pyrolytic studies (e.g. [6-8]). Also, during thermoanalytical measurements, analysis of the decomposition products can be done. This does not transform that particular thermoanalysis into analytical pyrolysis (e.g. [9]). A typical example is the analysis of the gases evolved during a chemical reaction as a function of temperature, known as EGA (evolved gas analysis). 

Vyazovkin and Liimert [56] argue that kinetic data, A and E, values obtained on the assumption of a one-step reaction may be incorrect because the possibility that thermal decompositions proceed by multistep processes has been ignored. This potential error can be avoided by using isoconversional methods to calculate Arrhenius parameters as a fimction of a. A real isokinetic relationship in a multistep process can be identified fi om the dependence of and its confidence limits, on a. The contribution fi om the second reaction step is negligible at the start of chemical change and thereafter rises as ar increases. 

Much of the interest in the field of thermal decompositions has been in studies of relatively simple solid reactants to minimise the problems of interpretation of behaviour in contributing towards a set of fundamental principles for the subject. In spite of such an approach, chemical correlations are often not readily discemable. Reactants containing chemically similar components do not always give comparable reactions, whereas resemblances in kinetic behaviour are sometimes very clearly apparent on heating materials with very different chemical constituents. One of the major aims of decomposition studies, namely the prediction of thermal behaviour from chemical and other properties, thus still remains unfulfilled, as discussed in the concluding Chapter 18. 

An important effort in this investigation was the thermal decomposition study of the shales. Considerable effort has been made to find a simple kinetic model which will accurately describe the weight loss curves for non-isothermal pyrolysis at various heating rates. In the past, many researchers have proposed and tested theoretical kinetic models for this reaction Q-4), however, most attempts at finding a suitable model have been focused on finding a very accurate fit to experimental data. Successive studies have increasingly emphasized microscopic details (i.e., diffusion models, exact chemical composition, etc.) in an attempt to find a precise model to fit the weight loss curves. In this... 

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