Assa Lifshitz

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.

Studies with a Single‐Pulse Shock Tube. I. The Cis—Trans Isomerization of Butene‐2

The cis—trans isomerization of butene‐2 was investigated behind reflected shocks in a single‐pulse shock tube of a novel design. The temperature range covered was 1000°—1250°K, and concentrations of 1% and 6% of the butene in argon were used. Analyses were made by vapor‐phase chromatography. The first‐order rate constants obtained in this work fall slightly above the extrapolated Arrhenius curves of two recent low‐temperature studies. An activation energy of 65, rather than 62.8 kcal/mole, is obtained when a straight line is drawn between the high and the low temperature data. |The rate constant obtained is kcis∞=3.5×1014exp(−65×103/RT). Possible sources of errors in evaluating reaction times in the single‐pulse shock tube are discussed.

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.

Kinetics of the Homogeneous Exchange Reaction: 14–14N2+15–15N2⇋214— 15N2. Single‐Pulse Shock‐Tube Studies

The kinetics of the homogeneous exchange reaction in molecular nitrogen highly diluted in argon was studied in a single‐pulse shock tube over a wide range of temperatures, initial pressures, and compositions. The reaction order with respect to the total nitrogen, argon, and the over‐all reaction order were determined. A rate law: Rate=kb[Ar][N214]+kb[Ar ][N215], which is a sum of two parallel reactions, was found to be compatible with the experimental results kb=1013.82±0.31 exp[−(116±5)×103/RT) cc mole−1·sec−1. It was shown that the exchange reaction does not proceed via a radical chain. A mechanism based on vibrational excitation of the nitrogen molecule to a critical vibrational level as a rate determining step is proposed. A rate constant, computed from available vibrational relaxation data in nitrogen agrees very well with the observed rate constant.It was found that slight amounts of oxygen, up to O2/N2∼0.01, had very little effect on the reaction rate. In the presence of higher oxygen concentration...

The reaction H2+D2⇄2HD. A long history of erroneous interpretation of shock tube results

An ultraclean 2 in. i.d. single pulse shock tube coupled to an atomic resonance absorption system was constructed in order to determine hydrogen atom concentration by Lyman‐α absorption. The tube was baked to 300 °C and pumped down to ∼10−7 Torr. Ultrapure argon could be shock heated to ∼2500 K with no spurious H atom absorption. The system was constructed in order to study the kinetics of chemical reactions which are strongly catalyzed by H atoms, under the conditions where no such atoms are present. Specifically, the role of H atoms in the H2+D2→2HD exchange reaction was studied. Mixtures of hydrogen and deuterium diluted in argon were shock heated to 1375–1760 K; samples were then taken from the tube and analyzed mass spectrometrically for the ratio [HD]/[D2]. 1400 K was the highest temperature at which no spurious H atom absorption was observed in a shocked mixture of 1% H2–1% D2. At 1400 K, under the conditions of no absorption, no, or ≤1%HD conversion was obtained. At higher temperatures Lyman‐α abs...

The unusual effect of reagent vibrational excitation on the rates of endothermic and exothermic elementary combustion reactions

Abstract Surprisal analyses are carried out for the reactions H + O 2 ( v ) → OH + O, O + H 2 ( v ) → OH + H, and OH + H 2 ( v ) → H 2 O + H. The rate constants are taken from published quasiclassical trajectory calculations covering the temperature range 300–4000 K. The dichotomy expected when reagent vibrational energy converts an endoergic reaction into an exothermic one is observed. However, contrary to the usual trend observed for the X + HY( v ) → Y + HX reaction (X, Y = halogen), here vibrational energy detracts from the rate of the endothermic reaction, and enhances the rate of the exothermic reaction when compared to statistical prior expectations. There are indications that at sufficiently high temperatures there is a role reversal for reagent vibrational energy. Figures are also provided which relate the slopes of surprisal plots for the general reaction A + BC( v ) → AB + C to the surprisal parameter, λ, for the state-to-state reaction, A + BC( v ) → AB( v ′) + C. The procedure as applied to the H + O 2 reaction reveals large discrepancies with experimental measurements of state-to-state rate constants. All published data are reconciled, however, by means of a non-linear surprisal analysis.

The reaction between H2 and D2 in a shock tube: Study of the atomic vs molecular mechanism by atomic resonance absorption spectrometry

The exchange reaction between hydrogen and deuterium was studied behind reflected shocks in a single pulse shock tube. A vacuum uv monochromator and a Lyman‐α radiation source were attached to the end block of the driven section in order to determine the hydrogen atom profile during the hot phase. These atoms are the result of impurities which are present in the shock tube. Two calibration attempts of the It/I0 vs [H]t around 1250 °K, using the decomposition scheme of propane and the postexplosion conditions in H2/O2/Ar mixtures, were unsuccessful. A calibration method which utilizes an integrated absorption profile is described. A two parameter calibration function (modified Beer–Lambert law) was derived: It/I0=exp(−α[H]βt), where α=2.92×107 and β=0.73 in units of mole‐cm. For each test, a sample was withdrawn from the tube and was analyzed mass spectrometrically for postshock distribution of product and reactants. In addition, the absorption profile at 1215.7 A was recorded and the extent of HD produced...

Thermal decomposition of indene. Experimental results and kinetic modeling

The thermal decomposition of indene was studied behind reflected shock waves in a pressurized driver single-pulse shock tube over the temperature range 1150–1900 K and densities of ≈3×10−5 mol/cm3. GC analyses of post-shock mixtures revealed the presence of the following decomposition products, given in order of increasing molecular weight: CH4, C2H2, CH2=C=CH2, CH3C≡CH, C4H2, C6H6, C6H5−CH3, C6H5−C≡CH, and also naphthalene and its structural isomer, probably 1-methylene-1H-indene. Small or trace quantities of C2H4, C4H4, C5H6, C5H5−C≡CH, and C6H4 were also found in the postshock mixtures. A kinetic scheme based on cyclopentadiene decomposition pathway alone cannot account for the observed product distribution. It can be accounted for if H-atom attachment to the π bond in the five-membered ring followed by consecutive decomposition of the formed indanyl radical is assumed in addition to the indenyl channel. A reaction scheme with the two pathways containing 50 species and 74 elementary reactions reproduces very well the experimental product distribution. In this paper, we show the reaction scheme, the results of computer simulation, and sensitivity analysis. Differences and similarities in the reaction patterns of cyclopentadiene and indence are discussed.

THERMAL DECOMPOSITION OF ACETONITRILE. KINETIC MODELING

The thermal decomposition of acetonitrile in the temperature range 1350–1950 K is modeled with a reaction scheme containing 23 species and 43 elementary reactions. Values of {[product]t/[CH3CN]0}/t, which were reported in a previous investigation are computed with this scheme at 50 K intervals and are compared with the values reported in the literature. Except for acrylonitrile and propyl nitrile at the high-temperature end of the study, very good agreement between the calculation and the experiment is obtained. A sensitivity spectrum of the kinetic scheme is shown and a discussion of the overall mechanism is presented.

Oxidation of cyanogen. I. Ignition behind reflected shocks

The ignition of cyanogen in cyanogen‐oxygen‐argon mixtures, and the distribution of reaction products before and after ignition, were studied behind reflected shocks in a single pulse shock tube. The measurements covered the temperature range 1400–2000°K at pressures P5 varying from roughly 5 to 24 atm. The composition of the reaction mixtures varied from 3.0 to 9.0% C2N2 and 3.0 to 10.5% O2. Under these conditions, the observed induction times ranged between 15 and 450 μsec. A least squares analysis of approximately 150 tests yielded the following empirical relation: τ = 10−13.97 exp(34 700/RT)[C2N2]−1.01 [O2]−0.21[Ar]+0.22, where τ is given in seconds and concentrations are in moles per cubic centimeter. An additional twenty‐three tests were run, in which 1% hydrogen was added to a stoichiometric mixture of 8.6% C2N2 and 8.6% O2. The ignition delay times under these conditions were considerably shorter and the temperature dependence was found to be much weaker, 23.7 compared to 34.7 kcal/mole in the abs...