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Acetylene - Wikipedia
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A Nature Research Journal. THIS book is the latest representative of a series of short lecture-monographs.
The title would seem to be rather grandiloquent for a work of this type; but, in fact, the author has managed to consider, albeit superficially, most of the reactions and transformations of this important class of organic compounds. By Ernst David Bergmann. Reprints and Permissions. By submitting a comment you agree to abide by our Terms and Community Guidelines. Acetylene is one of the major intermediates in any hydrocarbon flame and its oxidation is relevant to important phenomena and chemical processes occurring in the combustion of hydrocarbon fuels.
However, most computational studies on combustion mechanisms, where acetylene itself is the principal fuel, have focused on laminar flames and benzene formation was the main concern. Kinetic modeling results usually describe concentration profiles of stable species and free radicals in the ground electronic states that can be validated by experimental data, which have been generally obtained using molecular-beam mass spectrometry and laser induced fluorescence. Although the chemiluminescent radicals are minor species, they are probes of the combustion processes since they are intermediate species with short lifetimes that characterize the reaction zones, where the reagents are principally consumed.
Thus, they are suitable to use in following the chemistry of small species and other features of the combustion reactions. To validate the proposed mechanism the reactions for excited radical formation and decay are considered and the simulated and experimental chemiluminescence profiles resulting from decay of excited radicals were compared. Based on the analysis of production rates of the kinetic modeling established, the key reaction paths for the production of the excited radicals are also identified.
The combustion reactions were carried out in a closed chamber with constant small volume ca.
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The gaseous mixtures were prepared using a vacuum manifold to introduce acetylene and oxygen directly into the combustion chamber at a fixed initial composition and pressure. The combustible mixture was ignited by a spark plug at the center of the combustion chamber. The light emitted from the reaction inside the whole chamber was analyzed by a monochromator Oriel, , detected simultaneously by a photodiode and a photomultiplier Burle, 1P28A and recorded by an oscilloscope Nicolet, Once the combustion of the fixed mixture amount is initiated, the large number of species produced reacts among themselves and with the original components of the initial mixture, until the reaction rate falls off, while exhibiting a defined chemiluminescence time behavior.
Numerical methods for kinetic modeling have been fully discussed in the literature and thus will only be described briefly. In this work, we have used the KINAL program package, 19 which is a public domain program based on the Runge-Kuttasemi-implicit method. The production rate analysis requires calculation of the P ij matrix elements, which show the contribution of reaction j to the rate of production of species i. The starting mechanism was based on the studies of Eraslan and Brown 9 and Hidaka et al. The initial mechanism also included reverse reactions. They were each inserted, as new reactions, since the KINAL package only works with forward reactions.
The Arrhenius parameters for these formation reactions of excited species were estimated from similar reactions, which lead to the same species in their ground states, as well as those used for our earlier simulation of ethanol combustion.
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In the simulated combustion processes, soot particle formation is negligible as our previous study of soot temporal evolution by laser extinction measurements reported. The proposed reaction mechanism displayed in Table 1 is very different from the initial one described above.
To obtain it, the initial model was optimized through the reduction procedure based on ROPA results and later by applying kinetic parameters from other studies, those that better fitted to the experimental chemiluminescence curves of the excited radicals. The temperature of the combustion processes, in this study, was a parameter adjusted by running the kinetic modeling at different temperatures until to reproduce the experimental chemiluminescence temporal scales.
Simulations with 50 K higher or lower than K resulted in chemiluminescence temporal scales very different from those experimentally observed. The proposed reaction mechanism Table 1 was fitted to K to reproduce the observed experimental chemiluminescence time of about 2. It is well known that the principal path of acetylene oxidation is the reaction of acetylene with atomic oxygen, producing CHCO and CH 2 radicals as primary products: 1,11, This value is within the range of the reported determinations and it is that which best fit our experimental chemiluminescence profiles.
Frank et al. Although this reaction is usually reported for the singlet CH 2 radical in kinetic modeling, 11,25 there are some studies that consider the same reaction for the triplet CH 2 radical. This rate coefficient is very similar to those used recently by Frenklach and co-workers. Results and Discussion.
Acetylene Chemistry: Chemistry, Biology, and Material Science
The experimental data show a short time interval to reach maximum chemiluminescence intensity around 0. Based on this temporal behavior, an approximation to the combustion process may be proposed which occurs as two different events. Flame propagation throughout the chamber is the dominant process in the first part of the chemiluminescence process.
This part of the combustion process is strongly affected by dynamic factors that are related to the flame propagation phenomenon. After the chemiluminescence reaches its maximum emission intensity, the reagent consumption is the predominant process in the reaction throughout the chamber. After the initial step, reagent consumption increases and the reaction goes to completion with the formation of stable species.
worlleglinor.cf Since the chemiluminescence temporal behavior is faster in the first process than in the second, the observed chemiluminescence in the second process, where turbulence makes the reaction fairly homogeneous, is considered to be emitted from the entire reacting chamber volume. In the second, slow process, the turbulence results in stirring within the reaction zone, which increases heat transfer and radical diffusion. As some studies 50,51 have reported that the reagents are only partially consumed in the initial step and the KINAL is a software package for the analysis of homogeneous gas-phase chemical kinetics it does not include fluid dynamic factors related to the flame propagation, as most of the software package is for combustion kinetic simulations , a better approach is to use the slower second process to describe the combustion reaction as a guideline to validate the model in simulation studies of acetylene combustion in a closed chamber.
This work proposes a mechanism for acetylene combustion including formation reactions for electronically excited species. The simulated temporal behavior of the chemiluminescence originating from the radiative decay of the excited species is compared with the second step of the experimental chemiluminescence, which is used as a criterion for mechanism optimization. The computer simulation results in growth curves of "photon concentration" as a function of time and the differentials of these curves produce the photon production rates as a function of time, which can be associated with the experimental data.
To compare the experimental chemiluminescent profiles with the simulated chemiluminescent profiles, the first part of all curves which represents the fast flame propagation through the whole chamber was removed and the curves were normalized to make the first point coincide with zero on the abscissa and with unity on the ordinate. This procedure does not produce any deformation in the curves and allows comparison between the different groups of results. Similar approaches were done to simulate the formation of the chemiluminescent species in ethanol combustion produced in the same combustion system.
The formation reactions used to reproduce experimental data were:. Reaction r61 , in spite of being slightly endothermic 16 kcal mol -1 , 54 was proposed by Shuler 55 for hydrogen flames. However, none of these authors supplied the reaction kinetic parameters. For both reactions r61 and r62 , kinetic parameters were obtained from similar reactions that produce OH in the ground electronic state.
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In the second, the recombination reaction r62 would need a third body M to transfer the excess energy to produce OH in the ground state, so the same rate constant without M in the reaction is suitable. For this reason the rate constant used by Berman et al. In this way, the rate constant for reaction r60 is about hundred times higher than that for reaction r61 at the temperatures used. The simulated chemiluminescence profiles without r61 in the reaction mechanism did not fit the experimental chemiluminescence profiles very well.
C 2 H radicals could be found in vibrationally excited states 37 increasing ca. Kinetic parameters for reactions r68 and r70 were taken from Eraslan and Brown 9 and the kinetic parameters for the latter reaction are the same as those adopted by Grebe and Homann. The other two reactions r69 and r70 were discarded by ROPA analysis. Grebe and Homann 37 also discard reaction r70 , due to the low concentration of C 2 radical in the ground state and our results support their conclusion. Mechanism for C 2 A 3 P g radical formation. Reactions r73 and r76 were originally proposed by Gaydon. Kinetic parameters used to test reactions r73 , r74 and r75 were applied by Williams and Pasternack 27 for similar reactions of C 2 in the ground electronic state.
For reaction r76 the kinetic parameters employed were also taken from similar reactions of the Eraslan and Brown 9 study. Important reaction paths. The influence of reactions and intermediate species on the simulated chemiluminescence profiles was tested before the optimization of the proposed reaction mechanism, by changing the rate coefficient values within their error limits.
It was verified that these changes in the kinetic parameters of the formation reactions of excited radicals have almost no influence on the simulated chemiluminescence profiles. However, the simulated chemiluminescence profiles are strongly affected by changes in the rate coefficients of reactions that lead to the main precursors CH 2 and CHCO of most of the excited radicals.
The most important reactions for production and consumption of each species inserted in the reaction mechanism were determined through production rate analysis by ROPA. As there is practically no change in a reaction's importance as a function of reaction time, the scheme in Figure 6 can properly represent the reaction paths of acetylene oxidation over the whole reaction period.
The proposed reaction mechanism shows three main routes to acetylene oxidation Figure 6. CH radical has a meaningful role in the formation of excited radicals, despite neither always being directly involved in the production of these species. The present paper and our earlier ethanol study 42 show the possibility of using experimental chemiluminescence data as a guideline to validate kinetic modeling and to understand the chemistry of small hydrocarbon radicals in similar combustion systems closed chamber with small volume , since flame propagation is very fast and the greatest part of the combustion processes can be represented by a fairly homogeneous reaction and, thus, properly simulated by KINAL and other software packages for combustion kinetic simulations.