The self-inhibited pyrolysis of propane at small extents of reaction

Abstract

A detailed study of product formation in the pyrolysis of propane at small extents of reaction (from 0·05 to 2 mmHg propane consumed) has been carried out in a static system at pressures between 25 and 260 mmHg in the temperature range 510 to 560 °C. Under all conditions employed the stoichiometry of the reaction may be represented as (1+α) C 3 H 8 =H 2 +C 3 H 6 +α(CH 4 +C 2 H 4 ), where α increases slightly with increased temperature but is almost independent of extent of reaction and shows no variation with propane pressure. The product-time plots, however, show a very marked falling rate at propane consumptions of only a few tenths of a mmHg. A simple free radical long chain scheme, of the Rice-Herzfeld type (with no interconversion of isomeric propyl radicals) gives an excellent account of the simple product distribution, but cannot quantitatively account for the observed pressure and temperature dependences of the early, apparently linear, portions of the product-time plots, even when multiple terminations are invoked. It is concluded that the reaction cannot be studied free from self-inhibition, even at the earliest accessible moments of reaction. It is proposed that, under these experimental conditions, at the very onset of reaction, chain termination is due to methyl-methyl combination, but that as reaction progresses the process H+C 3 H 6 →CH 3 ĊHCH 3 becomes important, with a sufficient rise in the [CH 3 ĊHCH 3 ]/[CH 3 ] ratio that terminations involving secondary propyl radicals become important. This theory explains the observed steady fall in the rate of reaction, with the rate of formation of each product given by an equation of the form: rate=const.x[C 3 H 8 ] 3/2 /{1+const.x[C 3 H 6 ]}, where the constant in the denominator is the same for all products.