Cracking radical chains

Ethylene Dichloride Pyrolysis to Vinyl Chloride. Thermal pyrolysis or cracking of EDC to vinyl chloride and HCl occurs as a homogenous, first-order, free-radical chain reaction. The accepted general mechanism involves the four steps shown in equations 10—13 ... 

Physical properties of hexachloroethane are Hsted in Table 11. Hexachloroethane is thermally cracked in the gaseous phase at 400—500°C to give tetrachloroethylene, carbon tetrachloride, and chlorine (140). The thermal decomposition may occur by means of radical-chain mechanism involving -C,C1 -C1, or CCl radicals. The decomposition is inhibited by traces of nitric oxide. Powdered 2inc reacts violentiy with hexachloroethane in alcohoHc solutions to give the metal chloride and tetrachloroethylene aluminum gives a less violent reaction (141). Hexachloroethane is unreactive with aqueous alkali and acid at moderate temperatures. However, when heated with soHd caustic above 200°C or with alcohoHc alkaHs at 100°C, decomposition to oxaHc acid takes place. 

Thermal cracking is a free radical chain reaction. The mechanism is given in Fig. 7.8. 

Another key point to note is that chain transfer and termination by radical combination leads to radicals and molecules with more carbon atoms than the feed (propane). Subsequent involvement of these moieties in the radical chain propagation leads to larger molecules. In practice this maimer of radical cracking of ethane and propane cracking leads to some C4, C5 and Ce+ products forming pyrolysis gasoline. 

It is generally accepted that thermal cracking is a free radical chain reaction (a free radical is an atom or group of atoms with an unpaired electron). Free radicals react with hydrocarbons and produce new hydrocarbons and new free radicals ... 

Specihcally with regard to the pyrolysis of plastics, new patents have been filed recently containing variable degrees of process description and equipment detail. For example, a process is described for the microwave pyrolysis of polymers to their constituent monomers with particular emphasis on the decomposition of poly (methylmethacrylate) (PMMA). A comprehensive list is presented of possible microwave-absorbents, including carbon black, silicon carbide, ferrites, barium titanate and sodium oxide. Furthermore, detailed descriptions of apparatus to perform the process at different scales are presented [120]. Similarly, Patent US 6,184,427 presents a process for the microwave cracking of plastics with detailed descriptions of equipment. However, as with some earlier patents, this document claims that the process is initiated by the direct action of microwaves initiating free-radical reactions on the surface of catalysts or sensitizers (i.e. microwave-absorbents) [121]. Even though the catalytic pyrolysis of plastics does involve free-radical chain reaction on the surface of catalysts, it is unlikely that the microwaves on their own are responsible for their initiation. 

It has been assumed that the remaining products are formed by some sort of free radical chain mechanism, but no generalized mechanism like that of Rice s for paraffin pyrolysis has been proposed. Tanaka et al. have been able to simulate product distributions for shorter olefins—up to hexene (10). We shall describe a model for higher alpha-olefin pyrolysis and use it to account for the products from the cracking of several olefins. 

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. 

Thermal cracking is a radical chain process. The chain process contains three main stages chain start, chain growth and chain termination. 

Coke formation on the catalyst proceeds in same way as was shown in section 6.1 for thermal cracking. However, the presence of the catalyst changes the mechanism of the polycondesation reaction. Coke formation in all catalytic processes proceeds by the ion mechanism and not by the radical chain mechanism applicable for thermal processes. One example of a possible pathway for coke formation is shown in reaction (6.52). 

Thermal cracking of organic material has a radical chain mechanism. 

Finally, most cracking processes do not use pure compounds. Instead, they use complex hydrocarbon mixtures containing appreciable amounts of sulfur compounds which can act as both initiators and inhibitors of radical chain reactions. This can become a serious problem since our thermochemical and kinetic data on sulfur compounds are limited. In addition, the inclusion of too many starting reactants can impose intolerable borders on even large computers. Under the circumstances, the scheme must be simplified by a combination of methods which include careful adjustment of "averaged kinetic parameters with available data. Many of these can be done reasonably well by analyzing product ratios. Others cannot be done by any simply specifiable procedures but only by trial and error in terms of unique systems. 

Because of their commercial importance, we still need to do more work on thermal cracking reactions, since their scope and complexity extend considerably beyond the world of Rice-Herzfeld mechanisms. For example, consider the pyrolysis of butane (K.J. Laidler, Chemical Kinetics, McGraw-Hill, New York, 1965). This molecule affords the formation of a number of radical chain-carrier species, and the number of elementary steps increases accordingly... 

RTC mechanism is similar to that of the conventional TC with the essential difference that radical chain carriers are generated not only thermally but also by ionizing radiation. There are two main conditions necessary for a chain cracking reaction ... 

The mechanism of thermal degradation of plastics proceeds through a radical chain reaction pathway with hydrogen transfer steps. In secondary reactions, branched products were only formed as a result of the interaction between two radicals without any rearrangement reactions [48]. As a consequence, thermal cracking of polyolefins leads toward a broad distribution of hydrocarbons up to waxy products. More than 500 °C temperatures are needed to receive more oily products. In contrast, catalytic cracking takes place at lower temperatures and leads to the formation of smaller branched hydrocarbons. This catalytic cracking can potentially lower the costs and increase the yields of valuable products. 

Steam cracking Naphtha b.p. 30-190°C steam - 1 1 Ethane None 750-900°C Free radical chain, C-C cleavage. Ethene, Propene Butenes, Butadiene... 

A radical reaction or radical chain propagation (such as in alkene polymerization) is terminated by either the reaction of two radicals or by disproportionation of the radical into alkane and alkene (Scheme 2.2.6). The latter reaction plays the dominant role in petrochemical cracking processes. Alternatively, a radical reaction can be stopped by adding to the reaction mixture substances that react very easily with radicals by forming very stable radicals themselves so that the propagation reaction is terminated. Examples of such radical scavenger molecules are phenols, quinones, and diphenylamines. 

In this paper, we describe a computational study of the cracking reactions of propane and the production of olefins, especially propene, as a preUminary step in our research of gas-phase production of propylene oxide. The purpose of the smdy is twofold. On the one hand, we aim to compare different computational schemes applied to a subset of reactions for which experimental data exist On the other hand, we want to obtain precise estimates of the thermochemistry and kinetics of the radical chain initiation, propagation and termination reactions involved in the mechanism. Previous computational and experimental studies on this area of research have been performed by several authors, which results we will use to compare to our own. 

The reaction occurs via a first order free radical chain mechanism. The crack reaction temperature is 500-550 °C, the pressure is mostly kept at 2.0-3.0 MPa. The crack furnace is of plug-flow design with tubes being placed in the convection zone of the furnace, which is equipped with a burner. 

Two explanations have been given for the formation of these submicrocracks Zakrewskii [20] proposes a radical chain reaction and Peterlin [58] suggests that the ends of microfibrils are the crack nuclei. 

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