(271c) Big Impact By Small Radicals: Dominant Role of Termination Reactions With Hydrogen Atoms in Gas Phase Radical Chemistry

Free radical reactions constitute the primary mechanism for numerous industrial processes, such as pyrolysis, combustion, atmospheric chemistry, and polymerization [1]. Pyrolysis of hydrocarbons is the dominant petrochemical process for light olefins and an important source of aromatics. Accurate kinetic models based on elementary reactions, applicable over a wide range of process conditions, are essential to design, understand, and optimize these large-scale processes.

The effective Arrhenius activation energy and the reaction orders are the experimental observables in kinetic studies and they describe the sensitivity of reaction rate to temperature and reactant concentrations, respectively. Based on a kinetic analysis, it can be shown that radical recombination reactions determine these overall kinetic parameters [2] as they compete for radicals with the propagation steps. Indeed, different dominant recombination reactions limit the propagation steps differently, and therefore affect the overall kinetic parameters. It is often assumed that the dominant recombination reaction is determined by the relative radical concentrations [3]. In this study, we evaluate the validity of this assumption using the recently developed ab initio elementary kinetic model for ethane cracking [4]. The simulations show that, at industrial conditions, recombination reactions with hydrogen radicals play a key role, despite the relatively low hydrogen radical concentrations. Moreover, the importance of recombination reactions involving the small hydrogen radical leads to approximately first-order kinetics, in agreement with experimental data [5]. The importance of recombination reactions with the hydrogen radical results from rate coefficients that are at least 10-fold higher than for reactions involving alkyl radicals, and reach values close to the collision frequency. Small hydrogen radicals loose significantly less rotational entropy to reach the recombination transition state than larger alkyl radicals, explaining their higher rate coefficient. The high activity and central role of small radicals in recombination reactions is likely general and is expected to play a key role in radical reaction mechanisms ranging from atmospheric chemistry to combustion.

References

1.         Broadbelt, L.J. and Pfaendtner, J., AIChE Journal, 51, 2112 (2005).

2.         Wright, M.R., An Introduction to Chemical Kinetics, John Wiley & Sons Ltd (Chichester), 221-224 (2004).

3.         Laidler, K.J., Chemical Kinetics, 3/E, Harper & Row Publishers (New York), 311-314 (1987).

4.         Sun, W. and Saeys, M., AIChE Journal, 57, 2458 (2011).

5.         Froment, G.F., Van de Steene, B.O., Van Damme, P.S., Narayanan, S., and Goossens, A.G.,  Ind. Eng. Chem. Process Des. Dev.,15, 495 (1976).