Thermal cracking reaction mechanisms

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... 

Thermal cracking tends to deposit carbon on the catalyst surface which can be removed by steaming. Carbon deposition by this mechanism tends to occur near the entrance of the catalyst tubes before sufficient hydrogen has been produced by the reforming reactions to suppress the right hand side of the reaction. Promoters, such as potash, are used to help suppress cracking in natural gas feedstocks containing heavier hydrocarbons. Carbon may also be formed by both the disproportionation and the reduction of carbon monoxide... 

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. 

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). 

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

Thermal cracking of organic substances is an important reaction in the petroleum industry and has been extensively studied for over seventy years. At least for simple alkanes, the decay is first order in good approximation and therefore was long believed to occur in a single, unimolecular step [21]. However, in the 1930s, Rice and coworkers [22-24] established the presence of free radicals under the conditions of the reaction by means of the Paneth mirror technique [25,26], This observation led Rice and Herzfeld to propose a chain mechanism [22,27,28], Extensive later studies proved the essential features of their mechanism to be correct not only for hydrocarbons, but also for many other types of organic substances. 

Rates of thermal cracking are first-order in good approximation for propane, butane and still higher hydrocarbons [21], This is remarkable because chain mechanisms with initiation by break-up of a reactant normally result in reaction orders of one half or one-and-a-half, depending on which radical is consumed by termination. First-order behavior can result from "mixed" termination, which, however, can in most cases be ruled out as dominant mechanism (see Section 9.3). A more probable explanation is a combination of effects that key hydrocarbon radicals participate in several steps of different molecularities, that some steps are reversible, and that some unimolecular ones require collision partners. 

The mechanism of thermal cracking and pyrolysis is discussed by Buekens et al. [7], They proposed four types of pyrolysis reaction. In the case of PE, PP and PS cracking only two type of the mechanisms were stated ... 

Mechanism of Hydropyrolysis of -Paraffins. The hydrogen carbon ratio in the total hydropyrolysis product from 1 is higher (H C > 2.2) than that in 1 itself (H C = 2.12) in all experiments performed (Tables II and III). This clearly indicates participation of hydrogen in the process. To account for the observed differences between hydropyrolysis and conventional thermal cracking (Table I), and to rationalize the variations in product composition as a function of reaction conditions,... 

Fluid catalytic cracking (FCC) (Fig. 13.5) was first introduced in 1942 and uses a fluidized bed of catalyst with continuous feedstock flow. The catalyst is usually a synthetic alumina or zeolite used as a catalyst. Compared to thermal cracking, the catalytic cracking process (1) uses a lower temperature, (2) uses a lower pressure, (3) is more flexible, (4) and the reaction mechanism is controlled by the catalysts. Feedstocks for catalytic cracking include straight-run gas oil, vacuum gas oil, atmospheric residuum, deasphalted oil, and vacuum residuum. Coke inevitably builds up on the catalyst over time and the issue can be circumvented by continuous replacement of the catalyst or the feedstock pretreated before it is used by deasphalting (removes coke precursors), demetallation (removes nickel and vanadium and prevents catalyst deactivation), or by feedstock hydrotreating (that also prevents excessive coke formation). 

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