Rapid compression studies on spontaneous ignition of isopropyl nitrate Part II: Rapid sampling, intermediate stages and reaction mechanisms

Abstract

Rapid compression studies, which offer a means of studying gas reactions that proceed too fast for conventional techniques but too slowly for accurate shock tube work, require special techniques for quenching and sampling at millisecond or submillisecond intervals. These are developed and used here in a study of the intermediate products of the rapid decomposition of isopropyl nitrate (IPN). Satisfactory element balances are found. Product abundances are consistent with an initial formation of NO 2 plus isoproxyl (iPrO) radicals that is followed by decomposition of iPrO to yield acetaldehyde and CH 3. The principal features of the reaction mechanism for rapid decomposition are established and used as a basis to interpret the behavior during rapid oxidation. Studies on stoichiometrically similar mixtures of NO 2 + CH 3CHO contribute to these aspects. In decomposition, temperatures rise to around 600°K at the end of the compression stroke. Conditions then remain constant for nearly a hundredth of a second and 90% of the IPN has been consumed by the end of 8 ms. Dynamic absorption spectra show that NO 2 is an important intermediate product during this period, although it is wholly consumed. The final products include substantial yields of CH 2O, CH 3CHO, and CH 3NO 2, and smaller amounts of CH 3OH and CH 3ONO. Carbon oxides are only minor products. Each rises smoothly in yield from the start. In oxidation, temperatures rise and IPN concentrations fall along paths at first identical to those of decomposition, and quite similar compositions of stable intermediate products are seen. But within a short interval (∼5 ms) there is a very marked change: temperatures now reach around 900°K due to self-heating, the intermediates are consumed in ignition, and CO 2 becomes the predominant carbon-containing product.