Quadratic autocatalysis and self-heating in hydrocarbon oxidation

Abstract

The oxidation of normal butane under very fuel-rich conditions leads to isothermal reaction which obeys a quadratic autocatalytic rate law. When self-heating occurs the maximum rate is reached only in the final stages of the ‘slow’ reaction. The accompanying chemiluminescent emission (from CH₂O^*) is identified as the ‘pic d' arrêt’; it results form the enhancement of free-radical concetrations as the maximum of the authocatalytic reaction rate is reached under non-isothermal conditions. Criticality, leading to cool-flame phenomena at sub-atmospheric pressures, takes place as a result of the autocatalysis accompanied by self-heating.

The experimental features are described in this paper from measurements made by mass spectrometry, thermocouples, and photomultiplier in a well stirred, closed vessel. The results are interpreted by use of simple numerical models representing quadratic autocatalysis, and routes to the prediction of criticality in hydrocarbon oxidation are discussed against the background of the formal analytical theory derived Melentiev and Todes. Their critical criterion based on the Semenov parameter ψ_cr, familiar in the context of thermal ignition theory, appears to match the numerical and experimental conditions very satisfactorily. The present results are relevant to the prediction of spontaneous explosion hazards where hydrocarbon gases or vapours may mix with limited amounts of air.