https://www.engr.colostate.edu/~marchese/combustion10/4.pdf

A Comprehensive Modeling Study of n-Heptane Oxidation

Rate coefficients for alkyl and alkoxy radical decomposition are important in combustion, biological, and atmospheric processes. In this paper, rate constant expressions for C1C4 alkyl and alkoxy radicals decomposition via β-scission are recommended based on the reverse, exothermic reaction, the addition of a hydrogen atom or an alkyl radical to an olefin or carbonyl species with the decomposition reaction calculated using microscopic reversibility. The rate expressions have been estimated based on a wide-range study of available experimental data. Rate coefficients for hydrogen atom and alkyl radical addition to an olefin show a strong temperature curvature. In addition, it is found that there is a correlation between the activation energy for addition and (i) the type of atom undergoing addition and (ii) whether this radical adds to the internal or terminal carbon atom of the olefin. Rate coefficients for alkoxy radical decomposition show a strong correlation to the ionization potential of the alkyl radical leaving group and on the enthalpy of reaction. It is shown that the activation energy for alkyl radical addition to a carbonyl species can be estimated as a function of the alkyl radical ionization potential and enthalpy of reaction. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 250–275, 2006

Rate constant estimation for C1 to C4 alkyl and alkoxyl radical decomposition

https://digital.library.unt.edu/ark:/67531/metadc902029/m2/1/high_res_d/928525.pdf

Oxidation and combustion of the n-hexene isomers: A wide range kinetic modeling study

Recent shock tube measurements [1,2] of the reactions of OH radicals with various hydrocarbons provide the incentive for revising an earlier model [3] used to carry out thermochemical transition state theory calculations for the reaction rate coefficients of OH radicals with alkanes. In this article, details of the revised model are presented and calculations are described and compared with experiments. Of particular interest is the question of the reliability of an additive formulation for H atom abstraction rate coefficients; i.e., can we evaluate the total rate coefficient as the sum of site-specific rate coefficients, ktotal = Σinki, where we sum either over primary, secondary, and tertiary H atoms or over some larger number of classes of atoms? It is argued that there are two major reasons for expecting different primary, secondary, or tertiary H atoms to have different rate parameters: (1) activation energies may depend on next-nearest neighbors and (2) activation entropies can be mass-dependent. In principle the first factor invalidates the common procedure of treating the total rate coefficient for OH + RH H abstraction processes as the sum of invariant primary, secondary, and tertiary rates multiplied by the respective number of such H atoms in the molecule. The second factor places limits on the accuracy of any additive formulation. A separate question is whether there is sufficient experimental evidence to justify these distinctions, or whether, given experimental and theoretical uncertainties, it is adequate to treat them all as equivalent. It is concluded that (1) mass-dependent variations are just barely discernible; and (2) there are measurable differences among various primary H atom abstractions, and possibly among secondary atoms, but the data base does not justify distinguishing among tertiary H atoms.

Are reaction rate coefficients additive? Revised transition state theory calculations for OH + alkane reactions

A detailed chemical kinetic model for oxidation of C 2 H 4 in the intermediate temperature range and high pressure has been developed and validated experimentally. New ab initio calculations and RRKM analysis of the important C 2 H 3  + O 2 reaction was used to obtain rate coefficients over a wide range of conditions (0.003–100 bar, 200–3000 K). The results indicate that at 60 bar and medium temperatures vinyl peroxide, rather than CH 2 O and HCO, is the dominant product. The experiments, involving C 2 H 4 /O 2 mixtures diluted in N 2 , were carried out in a high pressure flow reactor at 600–900 K and 60 bar, varying the reaction stoichiometry from very lean to fuel-rich conditions. Model predictions are generally satisfactory. The governing reaction mechanisms are outlined based on calculations with the kinetic model. Under the investigated conditions the oxidation pathways for C 2 H 4 are more complex than those prevailing at higher temperatures and lower pressures. The major differences are the importance of the hydroxyethyl (CH 2 CH 2 OH) and 2-hydroperoxyethyl (CH 2 CH 2 OOH) radicals, formed from addition of OH and HO 2 to C 2 H 4 , and vinyl peroxide, formed from C 2 H 3  + O 2 . Hydroxyethyl is oxidized through the peroxide HOCH 2 CH 2 OO (lean conditions) or through ethenol (low O 2 concentration), while 2-hydroperoxyethyl is converted through oxirane.

/pdf/experimental-and-kinetic-modeling-study-of-c2h4-oxidation-at-5c96retwyk.pdf

Experimental and Kinetic Modeling Study of C2H4 Oxidation at High Pressure

Elementary reactions involved in the oxidation and decomposition of oxirane have been studied in two ways at 480 °C. First, by adding small amounts of oxirane to slowly reacting mixtures of H2+O2, rate constants have been determined for H and OH attack on the additive: H + C2H4O → C2H3O + H2(22H) OH + C2H4O → C2H3O + H2O. (21H) Combination with independent low-temperature data gives values of log10(A22H/dm3 mol–1 s–1)= 10.9±0.2, E22H= 41±2 kJ mol–1 and log10(A21H/dm3 mol–1 s–1)= 10.25±0.15, E21H= 15.1 ± 1.9 kJ mol–1. The major products are CH4 and CO, and CO, and a value of (7.5 ± 2)× 105 dm3 mol–1 s–1 is obtained for the rate constant of the CH3+H2 reaction. Secondly, decomposition studies have been carried out at total pressures (with N2) of 60 and 500 Torr with oxirane pressures varied between 1 and 20 Torr. The major products are CH4, CO, CH3CHO, C2H6, C2H4 and H2, and a comprehensive mechanism is presented to account for their occurrence. In addition to consumption in free-radical processes through H and CH3 attack, oxirane isomerises to give excited CH3CHO* molecules which undergo three competing reactions: [graphic omitted] Little effect of total pressure is observed on the relative rate of each process. From the relative yields of CH4 and C2H6, a value of k4=(4.0 ± 0.2)× 105 dm3 mol–1 s–1 is obtained, which in combination with low-temperature data gives log10(A4/dm3 mol–1 s–1)= 9.03±0.15 and E4= 49.5±1.3 kJ mol–1. It is suggested that C2H4 is formed in radical–radical reactions of the CH2CHO radical. Evidence is presented to support the view that the C—H bond dissociation energy in oxirane is very similar to that in C2H6, and values of D([graphic omitted])= 420.2±4.0 kJ mol–1 and ΔfHo298(C2H3O)= 149.6 kJ mol–1 are suggested: CH3+ C2H4O → CH4+ C2H3O. (4)

Studies of the decomposition of oxirane and of its addition to slowly reacting mixtures of hydrogen and oxygen at 480 °C

Measurements have been made over a wide range of mixture composition of the yields of cis- and trans-but-2-ene, but-1-ene, isobutene and methane, when propene is added to slowly reacting mixtures of H2 and O2 at 480 °C. From the initial yields of the butenes relative to that of methane, rate constants have been determined for the overall addition of CH3 radicals to propene and for specific terminal and non-terminal addition. Values of kint= 2.18 × 106, klt= 1.60 × 106 and kln= 5.8 × 105 dm3 mol–1 s–1 at 480 °C are obtained. [graphic omitted] Combination with low-temperature data for the overall addition gives A1nt=(2.57 ± 0.83)× 108 dm3 mol–1 s–1 and E1nt= 30.3 ± 3.2 kJ mol–1 over the range 80–480 °C. Combination of the present results with the very limited low-temperature data for specific addition gives A1t= 108.25 ± 0.25 dm3 mol–1 s–1 and E1t= 29.5 ± 3 kJ mol–1, and A1n= 108.30 ± 0.30 dm3 mol–1 s–1 and E1n= 36.5 ± 5 kJ mol–1. The main cause of the selectivity thus appears to arise from activation-energy differences as suggested for H-atom addition to propene. A maximum value of the rate constant for abstraction at the allyl position in propene by CH3 radicals has been determined at 480 °C and combined with low-temperature data to give maximum values of A= 4.4 × 107 dm3 mol–1 s–1 and E= 31.5 kJ mol–1. Arising from the difference in the A factors and the almost identical activation energies for addition and abstraction, then addition will always be the more important primary process, but at combustion temperatures reverse decomposition of the methyl adducts will reduce the importance of addition and abstraction will predominate.

Annette Keen

Papers

Studies of the decomposition of oxirane and of its addition to slowly reacting mixtures of hydrogen and oxygen at 480 °C

Rate constants for the addition of methyl radicals to propene