The oxidation of higher-order hydrocarbons

H. The Oxidation of Higher-Order Hydrocarbons TABLE 2 Relative Importance of Intermediates in Hydrocarbon Combustion... 

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. 

Thus methyl radicals are consumed by other methyl radicals to form ethane, which must then be oxidized. The characteristics of the oxidation of ethane and the higher-order aliphatics are substantially different from those of methane (see Section HI). For this reason, methane should not be used to typify hydrocarbon oxidation processes in combustion experiments. Generally, a third body is not written for reaction (3.85) since the ethane molecule s numerous internal degrees of freedom can redistribute the energy created by the formation of the new bond. 

Because hydrocarbon radicals of higher order than ethyl are unstable, the initial radical C H2 +1 usually splits off CH3 and forms the next lower-order olefinic compound, as shown. With hydrocarbons of higher order than C3H8, there is fission into an olefinic compound and a lower-order radical. Alternatively, the radical splits off CH3. The formaldehyde that forms in the oxidation of the fuel and of the radicals is rapidly attacked in flames by O, H, and OH, so that formaldehyde is usually found only as a trace in flames. 

Numerous other possible reactions can be included in a very complete mechanism of any of the oxidation schemes of any of the hydrocarbons discussed. Indeed, the very fact that hydrocarbon radicals form is evidence that higher-order hydrocarbon species can develop during an oxidation process. All these reactions play a very minor, albeit occasionally interesting, role however, their inclusion here would detract from the major steps and important insights necessary for understanding the process. 

Since diffusion rates vary with pressure and the rate of overall combustion reactions varies approximately with the pressure squared, at very low pressures the flame formed will exhibit premixed combustion characteristics even though the fuel and oxidizer may be separate concentric gaseous streams. Figure 6.1 details how the flame structure varies with pressure for such a configuration where the fuel is a simple higher-order hydrocarbon [1], Normally, the concentric fuel-oxidizer configuration is typical of diffusion flame processes. 

Many extensive models of the high-temperature oxidation process of methane have been published [20,20a, 20b, 21]. Such models are quite complex and include hundreds of reactions. The availability of sophisticated computers and computer programs such as those described in Appendix H permits the development of these models, which can be used to predict flow reactor results, flame speeds, emissions, etc., and to compare these predictions with appropriate experimental data. Differences between model and experiment are used to modify the mechanisms and rate constants that are not flrmly established. The purpose here is to point out the dominant reaction steps in these complex models of methane oxidation from a chemical point of view, just as modem sensitivity analysis [20, 20a, 20b] can be used to designate similar steps according to the particular application of the mechanism. The next section will deal with other, higher-order hydrocarbons. 

However, the kinetics of CO and hydrocarbon oxidation over platinum has a negative first order dependence on CO concentration for most of the ranges of temperature and concentration of interest to automotive catalysis. Voltz et ah (I) demonstrated that, under a total pressure of 1 atm. at 400°-800°F, the kinetics depend inversely on CO concentration from 0.2% to 4%. Therefore, when there are diffusion effects, a decline in CO concentration toward the interior of a porous catalytic layer would lead to an increase in reaction rates in the interior. Diffusion then has a beneficial rather than a detrimental effect on overall kinetic rates, and a thick catalytic layer may effect a higher conversion than a thin layer under identical ambient conditions. 

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