Alexander Burcat

IUPAC Critical Evaluation of Thermochemical Properties of Selected Radicals. Part I

This is the first part of a series of articles reporting critically evaluated thermochemical properties of selected free radicals. The present article contains datasheets for 11 radicals: CH, CH2(triplet), CH2(singlet), CH3, CH2OH, CH3O, CH3CO, C2H5O, C6H5CH2, OH, and NH2. The thermochemical properties discussed are the enthalpy of formation, as well as the heat capacity, integrated heat capacity, and entropy of the radicals. One distinguishing feature of the present evaluation is the systematic utilization of available kinetic, spectroscopic and ion thermochemical data as well as high-level theoretical results.

Kinetics of Methane Oxidation

To supplement previous ignition delay investigations on methane‐oxygen‐argon mixtures, product distributions were determined for methane‐oxygen‐argon and methane‐hydrogen‐oxygen‐argon mixtures that had been heated in a single‐pulse shock tube. On the basis of these combined data and literature values, extensive kinetic calculations, involving numerical integration of 23 reactions, have been made. The calculations have reproduced ignition delay times or product distributions of 14 sets of experimental data for hydrogen‐oxygen‐argon, methane‐oxygen‐argon, and methane‐hydrogen‐oxygen‐argon mixtures, and seem to be able to correlate all available data within the limits of experimental error.

The Effect of Higher Alkanes on the Ignition of Methane-Oxygen-Argon Mixtures in Shock Waves

Abstract Ignition delay times and product distribution were observed in mixtures of methane-oxygen-argon to which different amounts of ethane, propane, butane and pentane were added. All produced appreciable reduction in ignition delay time, which at low additive concentrations could be correlated in terms of the thermal effects. However, chemical analysis of quenched reacting mixtures and kinetic model calculations indicated that the additive influence waschemical andnot thermal. An explanation is advanced for the thermal correlation and its breakdown.

Shock-tube investigation of ignition in propane-oxygen-argon mixtures

A detailed shock-tube study of ignition-delay times in propane-oxygen-argon mixtures is presented. Ignition delays were determined from pressure and heat-flux measurements in the reflected shock region. The induction times measured ranged from 12 to 600 μsec and the temperature range covered was 1250°–1600°K. The pressures varied from 2 to 10 atm, and the equivalence ratios of the various mixtures ranged from 0.125 to 2.0 The influence of each parameter on ignition-delay times was separately determined. The dependence upon the concentrations close to stoichiometric composition, derived from more than 150 shocks is the following: τ = 4.4 × 10 − 14 exp ⁡ [ ( 42.2 × 10 3 ) / RT ] [ Ar ] 0 [ C 3 H 8 ] 0.57 [ O 2 ] − 1.22 sec ⁡ , where the concentrations are in moles/cc. Additional experiments were carried out in order to determine the product distribution before and after ignition. Considerable decomposition of propane takes place before ignition. The rate of decomposition is close to that of pure propane, and is very little affected by the presence of oxygen.

Detailed kinetics of cyclopentadiene decomposition studied in a shock tube

Mixtures of cyclopentadiene diluted with argon were used to investigate its decomposition pattern in a single pulse shock tube. The temperatures ranged from 1080 to 1550 K and pressures behind the shock were between 1.7–9.6 atm. The cyclopentadiene concentrations ranged from 0.5 to 2%. Gas-chromatographic analysis was used to determine the product distribution. The main products in order of abundance were acetylene, ethylene, methane, allene, propyne, butadiene, propylene, and benzene. The decomposition of cyclopentadiene was simulated with a kinetic scheme containing 44 species and 144 elementary reactions. This was later reduced to only 36 reactions. The ring opening process of the cyclopentadienyl radical was found to be the crucial step in the mechanism.

An elementary reaction-kinetic model for the gas-phase formation of 1,3,6,8- and 1,3,7,9-tetrachlorinated dibenzo-p-dioxins from 2,4,6–trichlorophenol

A 71-step reaction-kinetic model for the formation of 1,3,6,8- and 1,3,7,9-tetrachlorodibenzo-p-dioxins (TCDDs) from the oxidation of 2,4,6-trichlorophenol in the presence of hexane is developed based on experimental data and a simpler model that was previously published in the literature. The rate of reaction of phenoxyl-radicals with molecular oxygen has recently been experimentally demonstrated to be at least five orders of magnitude slower than the rate used in the previous model. With this correction, inclusion of radical-radical recombination reactions of 2,4,6-trichlorophenoxyl-radicals, and other minor modifications, the revised model yields satisfactory agreement between experimental and predicted concentrations of TCDDs above 900 K. The results of this study indicate that high-temperature, gas-phase reactions of chlorinated phenols that contain an ortho-chlorine will form poly-chlorinated dibenzo-p-dioxins (PCDDs) in yields approximately four to five orders of magnitude greater than believed, based on the previous modeling results.

High temperature oxidation of propene

Abstract Shock tube measurements of ignition delay times of propeneoxygenargon mixtures are presented. The concentrations are varied from 0.6 to 1.6% propene and from 2.7 to 14.4% oxygen diluted in argon. Temperatures in the range of 1274–1840K are achieved with averaged initial pressures ranging from 50 to 133 Torr. The product distribution is analyzed using a gas chromatograph. The overall ignition delay correlation equation, deduced from over 100 experiments, is given by τ= 5.3 x 10 -14 exp ( 137,500 RT ) [C 3 H 6 ] 0.23 [O 2 ] -1.12 [Ar] 0 s The reaction mechanism for propeneoxygenargon mixtures is investigated numerically and the results compared to the experimental data. The numerical calculations are performed with two integration techniques, LSODE and CHEMEQ, converging on a kinetic scheme that includes 59 reactions and 29 species. The agreement between the experimental data and the numerical predictions is partial, suggesting that the reaction mechanism is not complete.

Calculation of the ignition delay times for methaneoxygennitrogen dioxideargon mixtures

The experimental ignition delay values of methane-oxygen-argon-nitrogen dioxide mixtures published recently are confirmed and explained with the help of calculated ignition delay data. Calculation was done using a computer program with 38 reactions whose values were taken from the literature. Only two of these reactions were iteratively fitted and their rates were found to be 2.4 x 10/sup 11/ cc/mole sec for CH/sub 3/ + NO/sub 2/ ..-->.. NO + CH/sub 3/O and 1.0 x 10/sup 11/ cc/mole sec for the CH/sub 3/O + O/sub 2/ ..-->.. CH/sub 2/O + HO/sub 2/ reaction.

Cracking of propylene in a shock tube

Abstract The high-temperature decomposition of propylene has been investigated in a single-pulse shock tube in the temperature range 1160–1700 K. The stable products observed were hydrogen, methane, ethylene, acetylene, ethane, allene and propyne. The rate of evolution of the different products was found to have a similar concentration-dependence except for ethane, which was found to be dependent only on the third-body concentration. A computer model of the kinetics is presented, according to which the first step of the propylene decomposition has a rate constant (calorie units) of: k 1 [ Ar ] ( mol/cm 3 s ) = (10 13 ± 0.5 ) exp [(−74 ± 1) /sx 10 3 RT ]

Ab‐Initio Calculations and Ideal Gas Thermodynamic Functions of Cyclopentadiene and Cyclopentadiene Derivatives

Structures, frequencies and energies, ideal gas thermodynamic properties and values, have been calculated for cyclopentadiene, cyclopentadienols, and a number of radicals derived from them. The necessary molecular information for these calculaions was found by ab‐initio molecular orbital calculations. The gometries, vibrational frequencies and moments of inertia of 8 species are reported. In order to estimate the accuracy of the computations the molecular parameters were compared with known values reported in the literature whenever those were available.