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Abstract

Much research has been carried out on the combustion of hydrocarbons containing up to about eight carbon atoms. In contrast the gas-phase oxidation of higher molecular weight “hydrocarbons has received almost no attention. This is ascribable partly to the experimental problems associated with the gas-phase handling of compounds of low volatility and partly to the difficulties of obtaining and interpreting the extremely complex analytical data for the product distributions formed from large fuel molecules. Nevertheless, from the technological point of view, the combustion of larger hydrocarbons, such as decane, is of great significance as these compounds are important constituents of numerous practical liquid fuels. Decane is also of special interest as it is the lowest member of the alkane series whose uncatalysed oxidation can be studied in both the gas phase and the liquid phase at similar temperatures.

The Introduction (Section 1) outlines the general features of hydrocarbon combustion. Currently postulated mechanisms are reviewed and compared with those which are generally accepted to account for the observed kinetic and analytical features of the liquid-phase oxidation of hydrocarbons,

The Experimental Section (Section 2) describes the different apparatuses involved in this study. Initially a standard "premix" vacuum system was constructed and used but this was found to give unreliable results on account of the rapid oxidation of decane in the "premix" vessel. A "metal-free" apparatus was therefore built in order to obtain information about the peroxidic intermediates formed during the slow combustion of the hydrocarbon, at lower temperatures, while a "liquid injection" apparatus was used at higher temperatures in order to ensure rapid mixing without the use of a "premix" vessel, The analytical techniques for identifying the complex mixtures of reaction products are also outlined. High-pressure liquid chromatography, with a post-column reactor, was used to detect and analyse peroxides, while the numerous other products were identified and determined using gas chromatography/mass spectroscopy.

The Results Section (Section 3) gives the temperature/ pressure ignition profiles obtained for decane/air mixtures and describes the effects of several high molecular weight fuel additives on the slow combustion/cool flame boundary. The variation with time of the concentrations of the individual products is also presented. At low temperatures, decyl monohydroperoxides are formed in significant amounts but, as the temperature is raised through the slow combustion region, dihydroperoxides replace them as the main initial products. Iii the cool-flame region the yields of peroxides and carbonyl compounds containing less than ten carbon atoms are somewhat lower, while there is a concurrent increase in the yields of O-heterocyclic compounds and decenes. It is shown that the addition of hydrogen bromide to the reaction mixture decreases sharply the net amounts of hydroperoxides formed but at the same time increases the yields of decanones. An increase in the initial oxygen concentration in the reaction mixture is shown to increase the yields of dihydroperoxides and of carbonyl compounds with less than ten carbon atoms but to decrease the amounts of monohydroperoxides and O-heterocyclic compounds, Changes in the initial concentration of decane are shown to have little effect on the product distribution,

Finally, in the Discussion Section (Section 4), the ignition profiles of decane-air mixtures obtained in the "liquid injection" apparatus are shown to be in good agreement with previous results obtained in a "premix" apparatus. The mechanism of decane combustion at reasonably low temperatures is shown to involve the formation of decylperoxy radicals. Some of these abstract a hydrogen atom to form decyl monohydroperoxides which, in the liquid phase, is their almost exclusive fate. In contrast to the behaviour observed in the liquid phase, however, decylperoxy radicals in the gas phase may also isomerise and subsequently form dihydroperoxides and O-heterocycles. During slow combustion, carbonyl compounds with less than ten carbon atoms are shown to be formed mainly by the decomposition of the dihydroperoxides, the measured yields being in excellent agreement with those predicted, In the cool flame region, the rate of isomerisation of decylperoxy radicals and the subsequent formation of O-heterocycles increases rapidly, while at the temperatures involved, radical-radical reactions also become important. Decenes are formed in the cool flame region by the abstractive reaction of oxygen with decyl radicals, but above 623 K the main reaction of decyl radicals is their oxygen-catalysed cracking to form lower alkenes. The random initial attack on decane which occurs even at low temperatures suggests that, in contrast to the behaviour observed during the oxidation of some lower molecular weight alkanes, the main chain-propagating species are hydroxyl radicals rather than alkylperoxy radicals.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science Q Science > QC Physics Q Science > QD Chemistry T Technology > TP Chemical technology

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