Radical reactions hydrocarbon cracking

Propene and 1-butene, respectively, are produced in this free radical reaction. Higher hydrocarbons found in steam cracking products are probably formed through similar reactions. 

When liquid hydrocarbons such as a naphtha fraction or a gas oil are used to produce olefins, many other reactions occur. The main reaction, the cracking reaction, occurs by a free radical and beta scission of the C-C bonds. This could be represented as ... 

Let us discuss in general gas-phase processes of oxidative dehydrogenation of hydrocarbons involving as reagents substances that easily induce free radical transformations of substrates. Many such substances are known that dissociate to free radicals or induce free radical reactions. However, the most widespread in investigations are compounds that are able to shift dehydrogenation and cracking product ratios toward the first process. 

Gas phase free radical reactions are used in industry for pyrolysis, halogenation and combustion reactions. Nowadays, and probably for a long time to come, the thermal cracking of hydrocarbons constitutes the main production route for olefins, which are the basic feedstocks of the chemical industry around the world. Hydrocarbon pyrolysis is thus of considerable economic interest, as is shown by the very large amount of effort dedicated both to fundamental and applied research in this field (see, for example, refs. 35—37). 

Sources of previous work on hydrocarbon cracking can be found in Ref.52. Cracking of CH4 and of C4H10 has also been studied in the presence of C02 and H20 vapor53, s4 The interesting observation is that the rupture of a C—H bond remains the slow step which occurs at rates very close to those measured in mixtures with hydrogen. Further oxidation to CO follows via C2 species. In the presence of 0252>, conversion to CO occurs through self-accelerated radical chain reactions. 

As a result of the interaction between the catalyst and the hydrocarbon molecule, one part of the molecule is bonded with the catalyst via a strong double-electron bond. Another part of the molecule is bonded with the catalyst via a weak one-electron bond. The two surface compounds formed are unstable and are thus very reactive. This explains the high velocity of catalytic reactions. Typical metal catalysts are Fe, Co, Ni, Ru, Rh, Re, Ir and Pt. The activity of the metal catalysts is explained by the non-saturated d-shell (or d-level) orbital that acts as a free radical during thermal cracking. 

The approach to hydrocarbon cracking taken by the Froment school is to model the actual elementary steps of radicals at the various molecular configurations [38]. These are relatively few initiation hydrogen abstraction from a primary, secondary, or tertiary carbon and radical decomposition by scission of a carbon-carbon bond in /3-position to the unpaired electron. Boolean relation matrices are used to reflect the structures of the hydrocarbon reactants by indicating the existence and location of all their carbon-carbon bonds. Computer software generates reaction networks on the basis of known rate coefficients and activation energies at the various positions. Froment states the number of components in naphtha cracking as around 200, that of radicals as 40, and that of elementary radical steps... 

Fundamental work on the kinetics of hydrocarbon cracking is fully justified by strong industrial interest (1 to 5), Up to now kinetic studies of complex radical reactions in the gaseous phase have been conducted mainly in batch reactors. Nevertheless, the use of a continuous flow stirred tank reactor (CFSTR) seems very promising because it gives a direct measurement of reaction rates (6) and also it is well adapted to the study of fast reactions. 

The kinetics of the cracking reactions in the reactor tubes were expressed in terms of a radical reaction scheme, presented in Chapter 1 and already applied in Chapter 9. For the cracking of light hydrocarbons, 1200 elementary reaction steps between 128 species were considered, implying the solution of a stiff set of 128 continuity equations for the reacting species in addition to the total mass, momentum, and energy equations. 

The similarity of oxidation rates of different hydrocarbons in the higher temperature regions is probably related to the predominance of alkyl radical cracking reactions under these conditions (reaction 28). The products of such reactions would be similar for most common hydrocarbons (96). 

Mechanism. The thermal cracking of hydrocarbons proceeds via a free-radical mechanism (20). Siace that discovery, many reaction schemes have been proposed for various hydrocarbon feeds (21—24). Siace radicals are neutral species with a short life, their concentrations under reaction conditions are extremely small. Therefore, the iategration of continuity equations involving radical and molecular species requires special iategration algorithms (25). An approximate method known as pseudo steady-state approximation has been used ia chemical kinetics for many years (26,27). The errors associated with various approximations ia predicting the product distribution have been given (28). 

The first step in cracking is the thermal decomposition of hydrocarbon molecules to two free radical fragments. This initiation step can occur by a homolytic carbon-carbon bond scission at any position along the hydrocarbon chain. The following represents the initiation reaction ... 

The cleavage of carbon-carbon o- bonds is the main reaction in thermal cracking of naphtha, one of the commonest processes in the hydrocarbon conversion industry. Thus, radical dissociation or cleavage of carbon-carbon cr bonds has been quite familiar to synthetic and physical organic chemists. 

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