K. Brezinsky

The high-temperature oxidation of aromatic hydrocarbons

Abstract Chemical mechanisms of the atmospheric pressure, high-temperature (875–1500K) gas-phase oxidation of benzene, toluene, ethylbenzene and propylbenzene are described and discussed in detail. Major oxidation trends evident from turbulent flow reactor experiments serve as the basis for the overall mechanisms of the oxidation of benzene and alkylated aromatics which are also presented. The potential effects of very high temperatures and pressures on the chemistry of oxidation of aromatics are described. The oxidation of benzene and phenyl radical has been found to proceed in a stepwise C 1 C 5 C 4 sequence. Species profiles obtained from flow-reactor experiments suggest that the oxidation of benzene and phenyl radical follows the generalized route-phenoxy, cyclopentadienyl and butadienyl radical. The oxidation of the C 4 species branches into multiple pathways that yield copious amounts of ethylene and acetylene. The alkylated aromatics are oxidized through a large number of reaction pathways. However, certain major trends are evident: the alkylated aromatics on initial attack either form styrene, benzyl radical or benzene. The styrene reacts further to produce a benzyl radical or benzene.In general, the oxidation of an alkalated aromatic hydrocarbon appears eventually to reduce to the oxidation of either phenyl radical or benzene.

A Flow Reactor Study of the Oxidation of n-Octane and Iso-Octane

Abstract An experimental study has heen conducted in a flow reactor on the oxidation of n-ocltane and iso-octane at 1080 K, one atmosphere pressure and equivalence ratio equal to one. It was found that (despite similar ratios of initial fuel decay) the conversion of the reaction intermediates to CO and COa is much slower during the oxidation of iso-octane than n-octane. Iso-octane produced primarily iso-butylene and propylene as intermediate hydrocarbons whereas n-octane oxidation resulted primarily in the formation or ethne. Both the nature of the intermediates formed from each fuel and the relative rates of oxidation of the intermediates were shown to be related to the number of primary, secondary and tertiary H atoms present in the initial fuel. The basic chemistry revealed by the flow reactor experiments is suggestive of new insights into the chemical phenomena pertinent to knock in spark ignition engines.

Pyrolysis studies of methylcyclohexane and oxidation studies of methylcyclohexane and methylcyclohexane/toluene blends

Abstract High-temperature (1050–1200 K) pyrolysis studies of pure methylcyclohexane (MCH) and oxidation studies of pure MCH and MCH/toluene blends were performed in the Princeton Turbulent Flow Reactor. Since MCH is a proposed endothermic jet fuel, as well as a possibly significant constituent of current commercial automotive and aviation fuel blends, high-temperature studies of MCH provide an understanding of its decay characteristics in both aircraft and automobile engine environments. Ethene, 1,3-butadiene, methane, and propene were found to be the major intermediates produced by MCH pyrolysis. Oxidation of MCH also produced the same intermediates. As expected, the observed oxidation reaction rates were faster than the pyrolysis reaction rates. The MCH/toluene-blend oxidation experiments (1160 K and φ = ∼ 1.3) revealed that, while the two fuels appear to oxidize by independent mechanisms, the rates of oxidation of both fuels are strongly related to the initial concentration of MCH.

High temperature oxidation of aromatic hydrocarbons

High temperature (≈1200 K) oxidationstudies of benzene, toluene and ethyl benzene reveal striking similarities of oxidation pathways in that the overall rate is dominated by the rate of oxidation of the phenyl radical which forms in all three cases. This conclusion is supported by detailed species profiles taken in a turbulent flow reactor. From these profiles a mechanism also has been postulated for phenyl oxidation which follows the route phenoxy, ketocyclohexadienyl, cyclopentaldienyl, butadienyl, vinyl acetylene, butadiene, acetylene and vinyl radicals. CO is expelled from the ring structures to give the next lower order hydrocarbon radical.

Modeling the combustion of toluene-butane blends

A chemical kinetic model has been developed to predict the high-temperature (1200 K) oxidation of neat toluene, neat butane (CH 3 CH 2 CH 2 CH 3 ), and toluene-butane blends in an atmospheric-pressure flow reactor. Because the chemical interactions of toluene and butane would be indicative of those that occur in practical fuel blends, an extensive experimental and kinetic modeling study of their oxidation processes was undertaken. Because the experimental portion of the study indicated that the interactions were limited to radical pool effects, the focus here is on the approach taken to model these effects in the blended fuel experimental results. The basis for the proposed blend model was obtained from neat butane and neat toluene models available in the literature. Prior to its validation against blended fuel experimental data, the new model was compared with neat fuel data to ensure it retained the ability to predict the oxidation of the neat fuels. During this important neat fuel validation step, it was determined that improvements were needed in the existing toluene model. The improvements include addition of iC 4 H 5 reactions, which significantly improve predictions of 1,3-butadiene and acetylene experimental results. Additionally, improvements were made in the modeling of benzaldehyde. Experimentally measured benzaldehyde profiles were obtained with a gas chromatograph better configured to separate polar compounds than in previous experimental toluene studies, and these better profiles led to the adoption of a more appropriate rate constant for the overall reaction that accounts for the formation of benzaldehyde during the oxidation of toluene, C 6 H 5 CH 2 +HO 2 →C 6 H 5 CHO+OH+H The modeling results presented here demonstrate that when the chemical interactions between the various fuel components are limited to radical pool effects, the blended fuel oxidation process is more likely to be predicted when the blend model is properly configured to predict the oxidation processes of the neat fuel components.

The oxidation of ethylbenzene near 1060K

Abstract Flow reactor data from the oxidation of ethylbenzene are analyzed to deduce the major reactions involved in removing the ethyl group. Three major routes are found: (i) direct cleavage of the sidechain followed by the oxidation of the benzyl radical, (ii) displacement of the ethyl by a radical species, and (iii) abstraction of a hydrogen from the ethyl group that leads to the formation of styrene from which the removal of the vinyl group occurs through displacement or oxidative attack. Because of the importance of styrene in the ethylbenzene mechanism, results from a styrene oxidation are also reported. The experimental results are used to derive quantitative information on the relative importance of the three major paths through linear regression of equations derived with a steady state analysis. Finally the reaction sequence beginning with abstraction shows a strong analogy to results for the oxidation of ethane, and suggests that some results for the alkanes can serve as guides in understanding the results for higher normal alkylated aromatics.

Design of a high-pressure single pulse shock tube for chemical kinetic investigations

A single pulse shock tube has been designed and constructed in order to achieve extremely high pressures and temperatures to facilitate gas-phase chemical kinetic experiments. Postshock pressures of greater than 1000 atmospheres have been obtained. Temperatures greater than 1400 K have been achieved and, in principle, temperatures greater than 2000 K are easily attainable. These high temperatures and pressures permit the investigation of hydrocarbon species pyrolysis and oxidation reactions. Since these reactions occur on the time scale of 0.5–2 ms the shock tube has been constructed with an adjustable length driven section that permits variation of reaction viewing times. For any given reaction viewing time, samples can be withdrawn through a specially constructed automated sampling apparatus for subsequent species analysis with gas chromatography and mass spectrometry. The details of the design and construction that have permitted the successful generation of very high-pressure shocks in this unique app...

Supercritical Pyrolysis of Decalin, Tetralin,and N-Decane at 700—800K. Product Distribution and Reaction Mechanism

Abstract Supercritical pyrolysis mechanisms of potential endothermic fuels and hydrogen donor additives decahydronaphthalene (decalin) and tetrahydronaphthalene (tetralin) were examined in a specially constructed silica–lined flow reactor in which pressure and temperature were varied independently. Pressure and temperature were varied over ranges of 700—810 K and 0.2 to 10 MPa. Under these conditions, major products of supercritical decalin pyrolysis included: light alkanes and alkenes, methylhexahydroindane, indene, methylcyclohexenes, and indane. Major products of supercritical tetralin pyrolysis included: naphthalene, methylindane, ethane, methane, ethene and phenyl butane. The major products found in these experiments are contrasted with those found in gas phase pyrolysis studies. Quantification of the major products indicated that C6 to C5 ring contraction was found to occur preferentially with increasing pressure. The formation of these more compact products is qualitatively consistent with a reacti...

Chemistry of Polycyclic Aromatic Hydrocarbons Formation from Phenyl Radical Pyrolysis and Reaction of Phenyl and Acetylene

An experimental investigation of phenyl radical pyrolysis and the phenyl radical + acetylene reaction has been performed to clarify the role of different reaction mechanisms involved in the formation and growth of polycyclic aromatic hydrocarbons (PAHs) serving as precursors for soot formation. Experiments were conducted using GC/GC-MS diagnostics coupled to the high-pressure single-pulse shock tube present at the University of Illinois at Chicago. For the first time, comprehensive speciation of the major stable products, including small hydrocarbons and large PAH intermediates, was obtained over a wide range of pressures (25-60 atm) and temperatures (900-1800 K) which encompass the typical conditions in modern combustion devices. The experimental results were used to validate a comprehensive chemical kinetic model which provides relevant information on the chemistry associated with the formation of PAH compounds. In particular, the modeling results indicate that the o-benzyne chemistry is a key factor in the formation of multi-ring intermediates in phenyl radical pyrolysis. On the other hand, the PAHs from the phenyl + acetylene reaction are formed mainly through recombination between single-ring aromatics and through the hydrogen abstraction/acetylene addition mechanism. Polymerization is the common dominant process at high temperature conditions.

Vibrational spectra of liquid crystals. XI. Correlation functions obtained from 4‐octyloxy, 4′‐cyanobiphenyl CN stretching in smectic, nematic, isotropic, and solution phases

Correlation functions from the Raman scattering associated with the C=N stretching mode of the mesogen 4‐octyloxy, 4′‐cyanobiphenyl (80 CB) in aligned smectic, aligned nematic, and isotropic phases have been calculated. These functions, calculated from IVH spectra, show that the band contour is dominated by vibrational relaxation processes. Correlation functions calculated from spectra of the C=N band of the mesogen dissolved in benzene, carbon tetrachloride, chloroform, and methyl thiocyanate suggest that ’’vibrational dephasing’’ is primarily responsible for the bandwidth and shape in liquid crystal phases, in the isotropic phase, and in solutions. The solution studies indicate the presence of a wide distribution of local environments in the mesomorphic and isotropic phases of the mesogen as well as in CHCl3 and CH3SCN. These environments average out to a more narrow distribution, i.e., there is ’’motional narrowing,’’ in C6H6 and CCl4. The results lead to an understanding of why motional narrowing occu...