Influence cinétique et chimique d'alcènes sur la pyrolyse d'alcanes: mise au point et nouveaux résultats

Abstract

On the basis of previously reported, as well as new, experimental results, a review is presented of the kinetic and chemical influences of alkenes on the pyrolysis of alkanes. The results confirm that the addition of free H• atoms to the double bonds of alkenes is not sufficient to explain their inhibiting influence on the pyrolysis of alkanes. On the contrary, our results show that the processes of addition can account for the chemical effects of added alkenes on the pyrolysis of alkanes, namely, the modifications brought to the nature and the ratio of the products formed during the reaction. The results presented here confirm that the addition of alkyl free radicals to the double bond of alkenes does not lead to important chemical effects. The previously reported experimental results (pyrolysis of ethane in the presence of ethylene or propene, the pyrolysis of 2,2-dimethylbutane in the presence of propene, isobutene, or two isopentenes), completed by our new results (pyrolysis of ethane in the presence of isobutene, pyrolysis of isobutane in the presence of ethylene, of propene, of trans-2-butene, or of 2-methyl-2-butene), can lead to very simple general rules, on the basis of which it is possible to explain the results and to predict the modifications (relating to the nature of the products and to their ratios) that will result from the addition of an alkene during the pyrolysis of an alkane at a temperature of 500 °C (773 K). Aside from their fundamental aspects, these observations and their mechanistic consequences could be useful for the preparation of models for these thermal reactions, especially for the simplification of complex models. These observations also explain certain facts observed in industry and can help in solving some of the problems associated with the thermal cracking of heavy oils.