Free radicals combustion chain reaction

This is a chain reaction with many similarities to the chain reactions that occur in free-radical combustion processes in the previous chapter. For the chain reaction A B + C we represented the process as a kinetic chain involving the chain-propagating intermediate R, which was fed and terminated by initiation and termination reactions (Figure 1 1-2). In the preceding reaction sequence AMj is involved in a similar chain, but now each time the chain goes around the molecule is increased in size by one monomer unit We can represent this process in Figure 11-3. This reaction system is a series reaction in AMj,... 

AUTOXIDATION. A word used to describe those spontaneous oxidations, which take place with molecular oxygen or air at moderate temperatures (usually below 150°C) without visible combustion. Autoxidation may proceed through an ionic mechanism, although in most cases the reaction follows a free radical-induced chain mechanism. The reaction is usually autocatalytic and may be initiated thermally, photoehemically, or by addition of either free radical generators or metallic catalysts. Being a chain reaction, the rate of autoxidation may be greatly increased of decreased by traces of foreign material. 

The degradation and combustion behavior of polycarbonate/POSS hybrid system has been reported recently.48 Different loading contents of trisilanolphenyl-POSS (TPOSS) were melt blended with polycarbonate matrix (PC). The data shown in Table 8.4 indicate that no improvement in thermal stability parameters (i.e., onset decomposition temperature and peak decomposition temperature) was observed compared to the neat polycarbonate. The thermo-oxidative degradation process of the hybrid system proved to be a complicated process, which includes hydrolysis/alcoholysis of the carbonate linkage, free radical oxidative chain degradation, reformation, and branching and cross-linking reactions. 

The alkanes have low reactivities as compared to other hydrocarbons. Much alkane chemistry involves free-radical chain reactions that occur under vigorous conditions, eg, combustion and pyrolysis. Isobutane exhibits a different chemical behavior than / -butane, owing in part to the presence of a tertiary carbon atom and to the stability of the associated free radical. 

Our treatment of chain reactions has been confined to relatively simple situations where the number of participating species and their possible reactions have been sharply bounded. Most free-radical reactions of industrial importance involve many more species. The set of possible reactions is unbounded in polymerizations, and it is perhaps bounded but very large in processes such as naptha cracking and combustion. Perhaps the elementary reactions can be postulated, but the rate constants are generally unknown. The quasi-steady hypothesis provides a functional form for the rate equations that can be used to fit experimental data. 

Combustion is a chain reaction that can be initiated and propagated by free radicals such as the hydroxyl free radical. The hydroxyl radical may be produced by reaction of oxygen with macroalkyl radicals as shown below ... 

This is an extremely important reaction to which we wiU refer throughout this book. It is responsible for all NO, formation in the atmosphere (the brown color of the air over large cities) as well as nitric acid and acid rain. This reaction only occurs in high-temperature combustion processes and in lightning bolts, and it occurs in automobile engines by free-radical chain reaction steps, which will be the subject of Chapter 10. It is removed from the automobile exhaust in the automotive catalytic converter, which wiU be considered in Chapter 7. 

Combustion processes are fast and exothermic reactions that proceed by free-radical chain reactions. Combustion processes release large amounts of energy, and they have many applications in the production of power and heat and in incineration. These processes combine many of the complexities of the previous chapters complex kinetics, mass transfer control, and large temperature variations. They also frequently involve multiple phases because the oxidant is usually air while fuels are frequently liquids or solids such as coal, wood, and oil drops. 

Thermal reactions of light alkanes with oxygen in the combustion process have been studied extensively (6, 7). These studies were typically conducted at high temperatures—flame temperatures. The elementary reactions of the hydrocarbon species often involve reactions with atomic (H, O) or free radical species (OH, alkyl, etc.). The initiation step is the homolytic cleavage of C—C single bonds to form alkyl radicals. The C—C bonds are the weakest bonds in an alkane molecule (Table I). The chain-propagation step... 

Alkanes are fuels they burn in air if ignited. Complete combustion gives carbon dioxide and water less complete combustion gives carbon monoxide or other less oxidized forms of carbon. Alkanes react with halogens (chlorine or bromine) in a reaction initiated by heat or light. One or more hydrogens can be replaced by halogens. This substitution reaction occurs by a free-radical chain mechanism. 

Know the meaning of substitution reaction, halogenation, chlorination, bromination, free-radical chain reaction, chain initiation, propagation, termination, combustion. 

Emission spectroscopy and, to a lesser degree, absorption spectroscopy have provided considerable information on and insight into the chemistry occurring during the process of combustion. In particular, many of the transient free-radical molecules important in the chain reactions were identified and characterized through their emission spectra in flames. Now, new laser spectroscopic techniques offer the promise of obtaining more detailed and precise information, especially for the ground electronic states of many of the molecules involved in combustion. 

Spectroscopic methods are required for free radical intermediates. Laser induced fluorescence of hydroxyl radicals has been used successfully to determine elementary rate parameters associated with the isomerization reaction RO2 QOOH [113]. Laser perturbation of hydroxyl radical concentrations in stabilized cool-flames has been used to obtain global kinetic data for chain-branching rates at temperatures of importance to the low-temperature region [79]. These methods appear to be most suited at present to combustion studies in flow systems. There are also several studies of the relative intensity from OH radical fluorescence during oscillatory cool-flames [58,114]. 

Prior to 1930, attention of fundamental combustion scientists was focused mainly on the morphology of the cool-flame and ignition regions. The acceptance of free radicals, followed by the masterly and elegant Semenov theory (outlined in Chapter 5), which established the principle of branched chain reactions, provided the foundation for modern interpretations of hydrocarbon oxidation. The significant processes are ... 

The chain reaction sustains itself until it is terminated by direct combination of H and Cl, probably at the walls of the containing vessel. Such a reaction therefore tends to propagate itself without further encouragement until the reactants are exhausted. There are many examples of chain reactions in organic chemistry, where the active intermediate is often a free radical such as CH 3. Sometimes these are undesirable e.g. in the premature oxidation of hydrocarbons under pressure, which causes knocking in internal combustion engines) and it is necessary to inhibit them by suitable additives which operate by terminating the chains. 

Free Radical Trap Theories. Combustion vapor-phase reactions have been studied using premixed gas flames such as methane. Considerable information concerning the mechanism of flame propagation has resulted from this work 40, 49, 50). Basically the process occurs predominantly by branching chain reactions among free radicals. The major chain branching reactions are... 

Although ozone is quite reactive, most photochemical oxidations in the atmosphere (as in surface waters) involve still more reactive free radicals. Free radicals are species that contain an unpaired electron therefore, the reaction of any free radical with a chemical other than another free radical still leaves an electron unpaired. The newly formed free radical may readily react with another chemical, forming yet another free radical, and so on. An example of such a free-radical chain reaction is a combustion flame. A free radical is destroyed only when reaction with another free radical causes the two unpaired electrons to pair with each other. 

The combustion process is initiated by an ignition source converting some number of methane molecules into free radicals. Free radicals are in turn converted to OH free radical. Possible oxygenated compounds include aldehyde, alcohol, carboxylic acid, and oxide. The hydroxyl free radical then reacts with methane and is regenerated. The successive (chain type) combustion reaction is impeded by destruction of the OH radicals. Solid surfaces often destroy the OH radicals before they can react with hydrocarbons. The same effect is exploited in a porous-bed flame arrestor. In general, the combustion rates are very fast and nearly measurable with a few exceptional situations where time scales can be expanded to microseconds (KT6 s). The... 

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