Hydrocarbon recombination reactions

The two major parallel reaction pathways competing for the feedstock are the (desirable) hydrocarbon recombination reactions, which result in lighter products of interest, and the (undesirable) coking reactions, which yield carbon from light hydrocarbon feedstocks and highly naphthenic structures from heavier fe stocks. 

The radicals and other reaction components are related by various equiUbria, and hence their decay by recombination reactions occurs in essence as one process on which the complete conversion of CO to CO2 depends. Therefore, the hot products of combustion of any lean hydrocarbon flame typically have a higher CO content than the equiUbrium value, slowly decreasing toward the equiUbrium concentration (CO afterburning) along with the radicals, so that the oxidation of CO is actually a radical recombination process. 

In view of the observations of the ionic dissociation of nitro-cyano compounds, it is hardly surprising that even a hydrocarbon could dissociate ionically into a stable carbocation and carbanion, provided that the medium is polar enough to prevent the recombination reaction and to ensure equilibration. 

The simplest systems where electron-transfer chemiluminescence occurs on interaction of radical ions are radical-anion and radical-cation recombination reactions in which the radical ions are produced from the same aromatic hydrocarbon (see D, p. 128) by electrolysis this type of chemiluminescence is also called electro-chemiluminescence. The systems consisting of e.g. a radical anion of an aromatic hydrocarbon and some other electron acceptor such as Wurster s red are more complicated. Recent investigations have concentrated mainly on the energetic requirements for light production and on the primary excited species. 

It is seen that the rate constant ks is lower for compounds with electron-accepting substituents than with electron-donating substituents, which implies a dependence of the rate of Ar20 and R02 recombination on the electron density at the para- and ort/zo-positions of the benzene ring of the phenoxyl radical. The activation energies of this reaction vary from -33 to 10 kJ mol-1 however, the concurrent variation in the pre-exponential factor from 103 to 1010 L mol-1 s-1 causes a strong compensatory effect. It can also be seen that phenoxyl radicals readily react with peroxyl radicals k= 10s—109 L mol-1 s-1), whereas the disproportionation of peroxyl radicals is sufficiently slower (see Chapter 2). Hence, during the oxidation of hydrocarbons in the presence of phenols when k7[ArOH] > /c2[RH], the recombination reaction of ArO with R02 is always faster than the reaction of disproportionation of peroxyl radicals. 

The recombination zone falls into the burned gas or post-flame zone. Although recombination reactions are very exothermic, the radicals recombining have such low concentrations that the temperature profile does not reflect this phase of the overall flame system. Specific descriptions of hydrocarbon-air flames are shown later in this chapter. 

The second stage involves pyrolysis of the primary products CH3CI and CH2CI2. In this stage, higher hydrocarbons are formed through recombination reactions such as... 

Following the conclusions of Bowman [1], then, from the definition of prompt NO, these sources of prompt NO in hydrocarbon fuel combustion can be identified (1) nonequilibrium O and OH concentrations in the reaction zone and burned gas, which accelerate the rate of the thermal NO mechanism (2) a reaction sequence, shown in Fig. 7, that is initiated by reactions of hydrocarbon radicals, present in and near the reaction zone, with molecular nitrogen (the Fenimore prompt-NO mechanism) and (3) reaction of O atoms with N2 to form N2O via the three-body recombination reaction,... 

At low temperatures the hydrocarbon radicals are removed by reaction (47) (or for some studies by reaction (47) and the recombination reaction requiring the use of a mixed order analysis programme [96]) but as the... 

Methyl and hydroxyl radicals are formed at an early stage in hydrocarbon combustion processes. The kinetics of the recombination reaction... 

The oxidative dehydrogenation reactions over these catalysts are similar to the gas phase result of shock tube experiments determined by Skinner et al. (ref. 6). This observation supports the fact that the recombination reactions of methyl radicals in the gaseous phase are an important source of ethane and that the ethene is a secondary product derived from ethane. This secondary reaction proceeds in the gaseous phase as well as the catalyst surface. The major role of the MgO surface is to produce the methyl radical efficiently. The active sites for cleaving the H-CH3 bond should be moderated by Li to enforce C2 selectivity. In addition to gas phase oxidation, the direct surface oxidation of the hydrocarbon adsorbate is very significant especially for acidic materials. 

Neutralization processes of ions in the radiolysis of ethane or ethylene with SFg have been shown to lead to the formation of the SF5 radical. Compounds of the type RSF5 are formed as a result of the recombination reactions with hydrocarbon radicals. A kinetic e.s.r. study of the self-reaction of SF5, and a spectroscopic and kinetic e.s.r. study of its reaction with 1,1-di-t-butylethylene, have been reported. The radical undergoes self-reaction by a second-order process and adds to 1,1-di-t-butylethylene to give BU2CCH2SF5, which decomposes by a first-order process. 

The products of the photochemical reactions of aliphatic hydrocarbons in the presence of nitrogen oxides are reasonably well understood. For example, -butane is converted to CH2O, CH3CHO, and methyl ethyl ketone, together with the nitrogen-containing species methyl, ethyl and isopropyl nitrate (14-16) and perox-yacetyl nitrate ( PAN, 17 Altshuller et al., 1969). Radical (RO-, RC = 0 and ROO-) recombination reactions with NO2 probably account for the formation of... 

L. N. Wilson and E. W. Evans, Electron recombination in hydrocarbon-oxygen reactions behind shock waves, J. Chem. Phys. 46, 859-863 (1967). 

Termination of hydrocarbon radicals is not the only recombination reaction. Other possibilities (depending on the structure of the radicals) are (i) the termination by disproportionation, where hydrogen is eliminated, which yields an olefin (R-CHj-CHj R-CH=CH2-I-H ), (ii) termination by transfer (e.g. in reaction with antioxidants), (ill) termination by recombination with a hydroxyl radical, which leads to an alcohol (R-CHj-CHj -I- HO R-CH -GHj-OH) and (iv) further oxidation of the terminal hydroperoxide (R-CHj-CHj -I- Oj -> R-CHj-CHj-O-O -> R-CHj-CHj-O-OH). In the terminal hydroperoxide, the link between oxygen atoms is then cleaved, which gives an alkanal (R-CHj-CHj-O-OH R-CH2-CH=0- -H20). Alternatively, the link between carbon atoms can be cleaved, which results in a shorter alkyl radical (R-CHj ). Some of these reactions are discussed in relation to the secondary reactions of hydroperoxides. 

The thermal pyrolysis of hydrocarbons in general can be characterized as a set of fragmentation and recombination reactions which occur in series and parallel. Any given reaction, in accordance wife fee generally accepted free-radical chain mechanism produces either final product or feedstock for further reaction. 

As shown in Table 4, hydrocarbon compounds react with HO significantly faster than the self-quenching reaction H0 +H202 —> H O + HO. Although the recombination reaction of HO is also very fast, it should not be considered as a competitive reaction owing to the extremely low concentration of the radicals. Thus, Fenton s test can be considered as a stabihty test against HO attack. 

Polymerization and condensation are considered as side reactions during visbreaking process. They occur when part of the formed hydrocarbons recombines to produce relatively stable high molecular weight products, which include poly-aromatics with condensed aromatic structure that are coke precursors. 

---