Explosion limits mechanism

Many of the early contributions to the understanding of hydrogen-oxygen oxidation mechanisms developed from the study of explosion limits. Many extensive treatises were written on the subject of the hydrogen-oxygen reaction and, in particular, much attention was given to the effect of walls on radical destruction (a chain termination step) [2], Such effects are not important in the combustion processes of most interest here however, Appendix C details a complex modem mechanism based on earlier thorough reviews [3,4],... 

Indeed, in developing complete mechanisms for the oxidation of CO and hydrocarbons applicable to practical systems over a wide range of temperatures and high pressures, it is important to examine the effect of the H02 reactions when the ratio is as high as 10 or as low as 0.1. Considering that for air combustion the total concentration (M) can be that of nitrogen, the boundaries of this ratio are depicted in Fig. 3.3, as derived from the data in Appendix C. These modem rate data indicate that the second explosion limit, as determined... 

It is informative, however, to consider the possible mechanisms for dry CO oxidation. Again the approach is to consider the explosion limits of a stoichiometric, dry CO—02 mixture. However, neither the explosion limits nor the reproducibility of these limits is well defined, principally because the extent of dryness in the various experiments determining the limits may not be the same. Thus, typical results for explosion limits for dry CO would be as depicted in Fig. 3.5. 

The higher-order hydrocarbons, particularly propane and above, oxidize much more slowly than hydrogen and are known to form metastable molecules that are important in explaining the explosion limits of hydrogen and carbon monoxide. The existence of these metastable molecules makes it possible to explain qualitatively the unique explosion limits of the complex hydrocarbons and to gain some insights into what the oxidation mechanisms are likely to be. 

The cool-flame phenomenon [15] is generally a result of the type of experiment performed to determine the explosion limits and the negative temperature coefficient feature of the explosion limits. The chemical mechanisms used to explain these phenomena are now usually referred to as cool-flame chemistry. 

Even though there have been appreciably more studies of CS2, COS is known to exist as an intermediate in CS2 flames. Thus it appears logical to analyze the COS oxidation mechanism first. Both substances show explosion limit curves that indicate that branched-chain mechanisms exist. Most of the reaction studies used flash photolysis hence very little information exists on what the chain-initiating mechanism for thermal conditions would be. 

Property parameters. The physical property parameters include state of matter, phase equilibrium, thermal, mechanical, optical, and electromagnetic properties. The chemical property parameters include preparation, reactivity, reactants and products, kinetics, flash point, and explosion limit. The biological property parameters include toxicity, physiological and pharmaceutical effects, nutrition value, odor, and taste. 

Such reactions have been used to explain the three limits found in some oxidation reactions, such as those of hydrogen or of carbon monoxide with oxygen, with an "explosion peninsula between the lower and the second limit. However, the phenomenon of the explosion limit itself is not a criterion for a choice between the critical reaction rate of the thermal theory and the critical chain-branching coefficient of the isothermal-chain-reaction theory (See Ref). For exothermic reactions, the temperature rise of the reacting system due to the heat evolved accelerates the reaction rate. In view of the subsequent modification of the Arrhenius factor during the development of the reaction, the evolution of the system is quite similar to that of the branched-chain reactions, even if the system obeys a simple kinetic law. It is necessary in each individual case to determine the reaction mechanism from the whole... 

Gray fit Yang (Ref 1), a mathematical model was proposed to unity the chain and thermal mechanisms of explosion. It was shown that the trajectories in the phase plane of the coupled energy and radical concentration equations of an explosive system will oive the time-dependent behavior of the system when the initial temperature and radical concentration are given. In the 2nd paper of the same investigators (Ref 2), a general equation for explosion limits (P—T relation) is derived from a unified thermal and chain theory and from chis equation, the criteria of explosion limits for either the pure chain or pure thermal theory can be deduced. For detailed discussion see Refs... 

Use the reaction mechanism for the I2-O2-H2 system developed in the previous problem. Evaluate whether addition of I2 to a stoichiometric hydrogen-oxygen mixture at atmospheric pressure can shift the explosion limit to temperatures above 900 K. 

Devices to dilute the exhaust gas below the lower explosion limit (about 3% H.C.) are regaining popularity for offshore use. The old design of dilution fans for low rates suffered from being mechanical and consuming relatively large amounts of power from the essential power supplies. Entrained oil droplets settling on equipment also created an unpopular environment for dilution fans. 

Explosion Limit Phenomena and Their Use in Elucidation of Reaction Mechanisms... 

It would seem worth while, therefore to restudy the explosion limits of methane-oxygen and ethane-oxygen and also to study the effects of these hydrocarbons on the carbon monoxide-oxygen limits, with a view toward establishing whether these systems are connected in any way. In any case, valuable clues to the mechanisms of combustion of hydrocarbons can probably be obtained. 

From the observation that the first explosion limit is lowered by inert gases such as N2 and He it is necessary to conclude that, at pressures lower than this limit, termination is predominantly at the surfaces and by a diffusion-controlled mechanism. From the further observation that this limit usually lies in the range 1 to 10 mm Hg for spherical flasks of 5 to 10 cm diameter it is further possible to show, by using our approximate analysis [Eq. (XIV.G.2)], that the efficiency of capture of the wall-terminating radicals is of order of 10 or larger (i.e., the probability of capture per collision 6 10 ). 

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