The Oxidation of Propane at Low and Transition Temperatures

Abstract

An experimental study of the oxidation of propane in the temperature range 563–743 K. was carried out using a static reactor. The oxidation mechanism was found to undergo a transition from a low temperature reaction regime (T<600K) to that of an intermediate temperature regime ( T> 650 K) separated by a region of negative temperature coefficient (600-650 K). In the lower temperature regime alkylperoxy radicals are formed and become the dominant radical species. These can react in several ways, the most important of which leads to the formation of hydroperoxides. Hydroperoxides were determined to be the main chain branching intermediates responsible for the acceleration of the reaction and for the formation of cool flames. At the intermediate temperatures hydroperoxyl radicals are dominant and lead to hydrogen peroxide, which is the main branching intermediate in this regime. These conclusions are based in large measure upon the hydrocarbon products formed which consist mainly of oxygenated species at the lower temperatures and lower alkanes and alkenes at the intermediate temperatures. The negative temperature coefficient and the change in mechanism are due to the competition between reactions involving the addition of Oa to the alkyl radical forming the alkylperoxy radical and reactions involving the abstraction of hydrogen from the alkyl radical by Oa to form the conjugate alkene and the hydroperoxyl radical. The effect of increasing the reaction vessel surface-to-volume ratio was to enhance heterogeneous termination, resulting in longer reaction times. Comparison of the reaction products from vessels with different surface-to-volume ratios showed there to be no significant effect of surface on the main reaction paths of the mechanism. Some aspects of the mechanism at these temperatures are compared to those in the high temperature regime (T>1000K).