Flame rate constant measurement

We now understand why some spontaneous reactions do not take place at a measurable rate they have very high activation energies. A mixture of hydrogen and oxygen can survive for years the activation energy for the production of radicals is very high, and no radicals are formed until a spark or flame is brought into contact with the mixture. The dependence of the rate constant on temperature, its... 

Low energy ion-molecule reactions have been studied in flames at temperatures between 1000° and 4000 °K. and pressures of 1 to 760 torr. Reactions of ions derived from hydrocarbons have been most widely investigated, and mechanisms developed account for most of the ions observed mass spectrometrically. Rate constants of many of the reactions can be determined. Emphasis is on the use of flames as media in which reaction rate coefficients can be measured. Flames provide environments in which reactions of such species as metallic and halide additive ions may also be studied many interpretations of these studies, however, are at present speculative. Brief indications of the production, recombination, and diffusion of ions in flames are also provided. 

The results show that at 2 torr, ku = 2.5 X 10 8 and at 760 torr ku = 1.0 X 10 8 cm.3 molecule-1 sec.-1 This is reasonably good agreement in view of the possible errors. Furthermore, the values of ku obtained are consistent with earlier estimates based on comparisons with similar reactions (10, 19). Our purpose in presenting it here is to illustrate the potential use of flames in estimating more accurate rate constants for reactions like Reaction 14. Of course, the influence of diffusion must always be accounted for in such estimations diffusion is particularly important at low pressures and for small ion concentrations. (It is often advantageous to work at low pressures because the spatial resolution is much better than at 1 atm. At low pressures most measurements are made in or close to the reaction zone itself. At high pressures, where the reaction zone is thinner, measurements are made both in the reaction zone and in the burned gases.)... 

Prompt NO mechanisms In dealing with the presentation of prompt NO mechanisms, much can be learned by considering the historical development of the concept of prompt NO. With the development of the Zeldovich mechanism, many investigators followed the concept that in premixed flame systems, NO would form only in the post-flame or burned gas zone. Thus, it was thought possible to experimentally determine thermal NO formation rates and, from these rates, to find the rate constant of Eq. (8.49) by measurement of the NO concentration profiles in the post-flame zone. Such measurements can be performed readily on flat flame burners. Of course, in order to make these determinations, it is necessary to know the O atom concentrations. Since hydrocarbon-air flames were always considered, the nitrogen concentration was always in large excess. As discussed in the preceding subsection, the O atom concentration was taken as the equilibrium concentration at the flame temperature and all other reactions were assumed very fast compared to the Zeldovich mechanism. 

Using laser fluorescence measurements on fuel-rich H2/02/N2 flames seeded with H2S, Muller et al. [43] determined the concentrations of SH, S2, SO, S02, and OH in the post-flame gases. From their results and an evaluation of rate constants, they postulated that the flame chemistry of sulfur under rich conditions could be described by the eight fast bimolecular reactions and the two three-body recombination reactions given in Table 8.4. 

A method was proposed to obtain the kinetic rate constant at a fixed temperature with a one-point measurement. This method is comparable to gas-chromato-graphic concentration measurements and can in principle be executed with a convenient gas-chromatograph equipped with a flame ionization detector (FID) [37]. The background of this method is introduced in the following. 

Possible applications of laser enhanced ionization in flame diagnostics are 1. simultaneous observation of ionization and fluorescence signals from various levels might provide more information on the sequence of processes leading to and from the ionization continuum 2. the measurement of ion mobilities, relating to cross-sections for elastic collisions between ions and flame particles 3. measurement of ionization rate constants relating to cross-sections for inelastic collisions between excited atoms and other flame particles 4. measurement of recombination rate constants, relating to cross-sections for inelastic collisions between ions, electrons and neutrals. 

Method 2. Saturation Method for Sequential Pumping. In this method, atomic fluorescence of the inorganic probe is produced at 3+1 and at 3+2 after excitation at 1+3 and/or 2+3 respectively. However, in this case, it is necessary to "saturate" the excited level, 3, in order to use the methodic In addition, in order for the flame temperature to be evaluated it is necessary for the mixing first order rate constant, k2i, between the metastable, 2, and ground state, 1, to be much greater (> 20X) than the sum of the total deactivation rate constants between levels 3 and 1 and also between 3 and 2. This method also requires calibration of the spectrometric measurement system, saturation of level 3, corrections or minimization of scatter and post filter effects, and beam matching of 2 dye laser beams are needed for the excitation process. 

The present volume is concerned with low-temperature oxidation of hydrocarbons but the data sources cited in this section cover both higher and lower temperature regimes. As indicated earlier most of the direct measurements of rate constants of elementary reactions have been carried out at high temperatures (>1000 K) or temperatures close to ambient. It is only relatively recently that experimental techniques have been modified to produce substantial quantities of data for the intermediate temperatures pertinent to low-temperature oxidation of hydrocarbons, and much of the data for modelling low-temperature oxidation of hydrocarbons must be obtained from extrapolation of low-temperature data, interpolation between high- and low-temperature data, or by estimation methods. Consequently both the evaluations produced for modelling flames and those for atmospheric modelling are relevant. 

Independent data on surface growth have been provided by Are eva e al. (14). At 1700 K they measured a rate constant of 10 g/cm -s-atm for surface growth on pure carbon from which is similar to the rate constant that we calculate for old (high C/H ratio) soot in our flame. The agreement supports the conclusion that surface growth is contributed primarily by acetylene. 

Also included on Fig. 8 is the rate constant determination for reaction (2) in a flame by Fenimore and Jones (9). The line drawn in this region for the temperature dependence of the direct reaction (2) corresponds to an Arrhenius form. It has a frequency factor 2 x 10" cm sec and activation energy of 9 kcal/mole, estimated respectively by the OH + CH A-factor and an estimate of AH for reaction (2) coupled with a 2 kcal/mole barrier. While this forms a reasonable description of the two experimental results, it would be desirable to measure points intermediate in temperature. 

The last reaction, while highly exoerglc, has been found by Tal rose (51) to be of negligible importance in this system the second reaction is known to produce vlbrationally excited HF yielding laser action, but we have not studied this channel. Tal rose has also studied the reaction to produce IF using mass spectrometric probing of fluorine flames and has measured a rate constant k = 2 x 10 cm sec . ... 

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