Rate from shock tube data

In contrast to the relatively limited number of experimental approaches utilized to determine electron collisional information for C02 laser species, many different types of experiments have been employed in the determination of heavy particle rates as a function of temperature, for temperatures slightly below room temperature up to several thousand degrees. At room temperature, measurements have been obtained using sound absorption and/or dispersion as well as impact-tube and spectrophone techniques. High temperature rate data have been obtained primarily from shock tube experiments in which electron beam, infrared emission, schlieren, and interferometric diagnostic techniques are employed. For example, as many as 36 separate experiments have been conducted to determine the relaxation rate of the C02 bending mode in pure C02 [59]. The reader is referred to the review by Taylor and Bitterman [59] of heavy-particle processes of importance to laser applications for a detailed description and interpretation of available experimental and theoretical data. 

The rate coefficient for the reaction of H atoms with N2O to give N2 and OH has been extracted by Henrici and Bauer from their shock-tube data on N2O-H2 mixtures in the temperature range 1700-2600 °K. The kinetics was followed by ultraviolet absorption measurements of OH and NO concentrations. The bi-mo lecular rate coefficient is reported as 4 x 10 ° exp (— 12,000/RT ) l.mole . sec . ... 

Extensive efforts have been made to reconcile the high temperature dissociation results with the recombination rate coefficients. The shock tube data span the interval 1200—2300°K and the recombination work has been extended from measurements at room temperature to 1300°K by the method of flash photolysis and a thermostatted cell [29]. 

Following a series of flame computations over an extended range of initial compositions and conditions, the validity of an assumed kinetic mechanism and set of rate parameters may be tested by comparison of computed properties with experiment. In the case of the flame profiles there must be comparison not only for major reactants and products, but also for reactive intermediates. It is necessary to be circumspect as far as minor intermediates are concerned (for example, with mole fractions less than about 10"" ), since there may be large uncertainties in the measurements themselves. When assessing rate parameters it is also necessary to carry out sensitivity analyses (Chapter 7) to determine the relative importance of each elementary reaction and intermediate in the mechanism. The final mechanism and rate parameters must, further, be consistent with results from studies of other combustion phenomena, such as data obtained from shock tube, explosion limit, or fast flow studies. 

The rate constant data selected for final analysis were largely obtained from shock tube experiments, although data from premixed flames and static and flow reactors were also included. The experimental techniques employed were numerous, including various forms of emission and absorption spectroscopy, laser schlieren deflection, molecular beam techniques, and chemical analysis. Those rate constant data that were rejected from consideration in selecting a recommended expression were excluded for several possible reasons, including the use of insensitive or questionable experimental techniques, excessive scatter, or large differences from other results considered to be more reliable. 

FIGURE 5.1 Arrhenius behavior over a large temperature range. (Data from Monat, J. P., Hanson, R. K., and Kruger, C. H., Shock tube determination of the rate coefficient for the reaction N2 + O- NO + N, Seventeenth Symposium (International) on Combustion, Gerard Faeth, Ed., The Combustion Institute, Pittsburgh, 1979, pp. 543-552.)... 

The derived values of k29, k30 and k 30 are in fair agreement with rate coefficients obtained by independent methods. Thus, for example, the derived value of k-30 at 298 °K of 142 l2.mole-1.sec 1 should be compared with 66 l2.mole-2. sec 1 obtained by Ray and Ogg21 s from a study of NO oxidation at high N02/N0 ratios. Similarly, the derived value of k30 at 820 °K of 1.3 x 107 l.mole-1.sec-1 compares well with the k30 value of 1.5 x 107 l.mole-1.sec-1 from the data of Schott and Davidson226 obtained in a shock-tube study of N205 pyrolysis. The derived value of k29 is an order of magnitude smaller than the other estimates listed in Table 15. 

When the commodity chemical propylene oxide is heated to high temperature in the gas phase in a shock tube, unimolecular rearrangement reactions occur that generate the CsHgO isomers allyl alcohol, methyl vinyl edier, propanal, and acetone (Figure 15.9). Dubnikova and Lifshitz carried out a series of calculations to determine the mechanistic pathway(s) for each isomerization, with comparison of activation parameters to those determined from Arrhenius fits to experimental rate data to validate the theoretical protocol. 

Henrici et al. [504] carried out a shock tube study of the CO + F2O reaction in mixtures heavily diluted with argon at higher temperatures. They obtained data on overall CO2, O2 and COF2 production from single pulse experiments, and they also made time-resolved optical measurements of the rate of formation of CO2 and depletion of F2O by studying the emission at 4.3 pm and the absorption at 2200 A, respectively. The major path for the decomposition of F2O was assumed to be by reactions (xcii)—(xciv)... 

The kinetics of the reaction between OFj and CO have been studied in shock tubes in the temperature range of 800-1400 K (at about 133-267 kPa) [943], Significant amounts of COFj are produced, in addition to CO, Oj and F. COF is understood to be formed from [COF] as a result of the following elementary steps, the rate constants of which were estimated from thermodynamic and molecular data ... 

At the present time, the fuels which can be described by this modeling approach include hydrogen, carbon monoxide, methane, methanol, ethane, ethylene, acetylene, propane, and propylene. The reaction mechanism used to describe the oxidation of these fuels has been developed and validated in a series of papers (3-7). The elementary reactions and their rate expressions are summarized in Reference (7) and are not reproduced here due to space limitations. Reverse reaction rates are computed from the forward rates and the appropriate thermodynamic data (8). This mechanism has been shown to describe the oxidation of methane (3,A), methanol (5), ethylene (6), and propane and propylene (7) over wide ranges of experimental conditions. It has also been used to describe the shock tube oxidation of ethane (4,9), and acetylene (10). 

Kriegel et al 1987). Vibrational excitation of molecular ions is more efficient in collisions with buffer gas atoms or molecules heavier tlian helium (e.g. N2, Ar, etc.). This is demonstrated below. However, the occurrence of vibrational excitation is unfortvmate from the viewpoint of astrochemistry, since vibrational excitation of ions in drift tubes can obscure the influence of kinetic excitation on the rate coefficients of molecular ion-neutral reactions. Such information is needed in some astrophysical situations such as in interstellar MHD shocks. In these situations, the mean free times between collisions of the ions with the ambient gas are usually longer than the radiative lifetimes of the vibrational states of the ions and therefore the ions will generally be vibrationally relaxed. Thus data are required on the variation of the rate coefficients with Ej. for the reactions of ions in their ground vibrational state. So, drift tube data on molecular ion reactions must be applied with caution to astrophysical situations except that is for data obtained at low e/N where vibrational excitation of the molecular ions is minimal. 

The following types of unimolecular reactions can be distinguished cleavage of the ordinary bond and formation of two radicals elimination to form stable molecules isomerization reactions. Table 4.1 contains the examples and Arrhenius parameters of the rate constant for these types of reactions. The experimental studies presented in Table 4.1 were carried out in shock tubes except for the decomposition of CCI2HCH2CI when laser heating of the gas mixture was used. It is seen from these data that the highest pre-exponential factors belong to the rate constants of decomposition at the ordinary bond. Recombination reactions, which, as a rule, occur without a barrier, are inverse for these reactions. Unlike recombination reactions, inverse reactions of elimination and isomerization have substantial potential barriers. 

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