Mechanisms and kinetics models for hydrocarbon pyrolysis

A quantum chemical investigation is presented for the determination of accurate kinetic and thermodynamic parameters for hydrocarbon radical reactions. First, standard enthalpies of formation are calculated at different levels of theory for a training set of 58 hydrocarbon molecules, ranging from C1 to C10, for which experimental data are available. It is found that the CBS-QB3 method succeeds in predicting standard enthalpies of formation with a mean absolute deviation of 2.5 kJ/mol, after a systematic correction of −1.29 kJ/mol per carbon atom and −0.28 kJ/mol per hydrogen atom. Even after a systematic correction, B3LYP density functional theory calculations are not able to reach this accuracy, with mean absolute deviations of 9.2 (B3LYP/6-31G(d)) and 12.9 kJ/mol (B3LYP/6-311G(d,p)), and with increasing deviations for larger hydrocarbons. Second, high-level transition state geometries are determined for 9 carbon-centered radical additions and 6 hydrogen additions to alkenes and alkynes and 10 hydrogen a...

Ab Initio Calculations for Hydrocarbons: Enthalpy of Formation, Transition State Geometry, and Activation Energy for Radical Reactions

Accurate activation energies for 67 hydrocarbon radical addition and β-scission reactions are calculated with the CBS-QB3 ab initio method. An extension of Benson's group additivity method to activation energies is introduced. The underlying hypotheses, that is, the group concept and the additivity approximation, are validated with ab initio data. Standard activation group additivity values are obtained from the ab initio calculations for reactions involving primary, secondary, tertiary alkylic, allylic, benzylic, and vinylic radicals. The proposed group contribution method yields accurate activation energies for radical addition and for β-scission reactions. The effect of substituents on the carbon atoms of the reactive center on the activation energy can be as large as 95 kJ/mol for the adding radical, and 187 kJ/mol for the product radical of the β-scission. Non-nearest-neighbor effects such as gauche and cis interactions have an influence of less than 3 kJ/mol per interaction on the activation energies. However, for hydrocarbons that are heavily branched near the reactive center, these interactions can become important. © 2004 American Institute of Chemical Engineers AIChE J, 50: 426–444, 2004

Ab initio group contribution method for activation energies for radical additions

Computer generation of a network of elementary steps for coke formation during the thermal cracking of hydrocarbons

Upgrading of residual oil in sub- and supercritical water: An experimental study

Unrestricted Hartree−Fock and density functional theory calculations have been carried out to investigate the detailed kinetics and mechanisms of hydrocarbon thermal cracking. The calculations of the elementary reactions involved in the overall cracking of paraffin molecules agree well with the generally accepted free radical mechanism. The results can be summarized as follows:  (1) Initiation cracking, the calculated bond dissociation energy (BDE) for C−C homolytic scission is ∼95 kcal/mol at the MP2/6-31G* level and ∼89 kcal/mol at the B3LYP/6-31G* level. No transition states are found in the reactions. (2) H-transfer reaction, the calculated energy barriers (activation energy) are 15−17 kcal/mol at the MP2/6-31G* level and 10−12 kcal/mol at the B3LYP level. The reverse reaction has about the same energy barrier. The calculated transition state structures for both reactions lie in the middle between the reactant and product. (3) Radical decomposition−“β scission”, the activation energy for the radical d...

G. B. Hieshima

Papers

Understanding the Kinetics and Mechanisms of Hydrocarbon Thermal Cracking: An Ab Initio Approach†