Understanding the chain mechanism of radical reactions in n-hexane pyrolysis

Abstract

Pyrolysis of n-hexane is interpreted in terms of a chain mechanism. The first-order kinetic equation fits experimental data adequately. The rate constant for feed conversion increases from 1.75 × 10−2 to 3.79 × 10−1 s−1 with increasing temperature from 923 to 1,023 K, giving an apparent activation energy of 208.5 ± 1.0 kJ mol−1. Initial primary products consist of hydrogen, C1–C4 paraffins, C2–C5 olefins, iso-hexanes, and coke. Quantitative interpretation of the chain mechanism shows that different products are formed from different elementary reactions with different selectivity. Increasing temperature promotes homolysis of the feed molecule and β-cracking of radicals and reduces the proportion of H abstraction by radicals, resulting in an increase in production of hydrogen and formation of a more dehydrogenated coke. The average reaction chain length decreases from 7.33 to 5.73 as the temperature is increased from 923 to 1,023 K.