Propyl radical isomerization and heterogeneous effects in the pyrolysis of propane below 500°C

Abstract

A detailed study of product formation in the initial stages of pyrolysis of propane has been carried out with a static system at pressures between 26 and 416 mmHg at temperatures between 441 and 503°C. Under all conditions employed the stoicheiometry 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 ) with α = 10 0.59 ± 0.15 exp – [(2580 ± 540)/ RT ] + 10 -2.39 ± 0.46 exp [(24000 ± 1600)/ RT ] [C 3 H 8 ], where [C 3 H 8 ] (mole ml. -1 ) is the initial propane concentration. At any given temperature and pressure α is almost independent of extent of reaction and is identical in vessels with surfaces of Pyrex, potassium chloride on Pyrex, and quartz with surface to volume ratios in the range 0.9 to 6.3 cm -1 . The individual rates of product formation are also unaffected by the nature of the surface at 503°C. In contrast, at 456°C an increased surface to volume ratio leads to markedly reduced rates. Furthermore, there is no marked self-inhibition at temperatures below 460°C, in contrast to findings at temperatures of 500°C and above. It is shown that the pressure dependence of α results from competition between reactions (13) and (6 s ) CH 3 CHCH 3 + C 3 H 8 → CH 3 CH 2 CH . 2 + C 3 H 8 , (13) CH 3 CHCH 3 → H . + 3 H 6 (6 s ) at the ‘low’ temperatures of this study. The experimental results yield a value for k 13 of 10 12.1 exp( —17000/ RT ) ml. mole -1 s -1 if the value of k 6s is 10 15.0 exp( —42000/ RT ) s -1 , which appears to be the best currently available estimate. Although reaction (13) can provide a partial explanation for the finding of diminished self­ inhibition at low temperatures, it is shown that reaction (14) H → wall (14) is mainly responsible for this effect. Moreover, reaction (14) accounts for the observed dependence of rate on surface to volume ratio at low temperatures, and allows a semi-quantitative understanding of the temperature and pressure dependence of rates of product formation which cannot be explained in terms of purely homogeneous termination reactions.