Hydrocarbon pyrolysis, molecular reaction

Plot the selectivity to C4H8 as a function of ethane conversion. Does it behave like a secondary or primary product Consult the paper by Dean (1990), and describe additional reactions which lead to molecular weight growth in hydrocarbon pyrolysis systems. While some higher molecular weight products are valuable, the heavier... 

Molecular reaction schemes have a long history of use in the design of pyrolysis coils. Since the pioneer works of Myers and Watson [46] and Schutt [47] on propane pyrolysis, improved by Snow and Schutt [48], molecular reaction schemes have been applied to the modelling of the pyrolysis of light and heavy hydrocarbons [38, 49—56]. Froment and co-workers have extensively promoted molecular reaction schemes in a series of papers [57—61] a brief account can be found in a book by Froment and Bischoff [25]. 

Staveley and Hobbs and Hinshelwood studied the inhibition of the reaction by nitric oxide, and found under certain conditions that some 13 % was uninhibitable. Several investigations have shown that this residual reaction is not a molecular reaction. Stevenson et al studied the pyrolysis of propane containing radioactive carbon, and concluded that isotopic mixing took place at the same rate, relative to the pyrolysis rate, when the reaction was completely inhibited as when it was uninhibited Hinshelwood et al obtained a similar result. Poltorak and Voievodsky showed that when propane is pyrolyzed in the presence of D2, D atoms appear in the hydrocarbon fraction at a rate, relative to the rate of decomposition, that is independent of the amount of nitric oxide present. All of these results show that free radicals are still important in the reaction occurring in the presence of nitric oxide, and provide no support for the view that a molecular mechanism plays a significant role in the propane pyrolysis. Evidence reading to the same conclusion is provided by the experiments of Niclause et which show that in certain reaction vessels the propane pyrolysis is completely... 

Anthracene dimers as well as dihydroanthracene have been identified as initial reaction products in all pyrolysis studies of anthracene. As shown in Chart I, 11 dimers from anthracene are possible. Because the 9-position is the most reactive, one might expect a predominance of the 9,9 -dimer. However the 2,9-dimer was reported as the major product in one study (18). Many of the other possible dimers were also obtained, depending on the reaction conditions employed. Both steric effects and reactivity factors must, therefore, be taken into account for considering the possible reaction products in aromatic hydrocarbon pyrolysis. The results for anthracene show how the lack of a functional group and the nonspecificity for molecular recombination lead to complex product mixtures in aromatic pyrolysis. 

An excess of crotonaldehyde or aUphatic, ahcyhc, and aromatic hydrocarbons and their derivatives is used as a solvent to produce compounds of molecular weights of 1000—5000 (25—28). After removal of unreacted components and solvent, the adduct referred to as polyester is decomposed in acidic media or by pyrolysis (29—36). Proper operation of acidic decomposition can give high yields of pure /n j ,/n7 j -2,4-hexadienoic acid, whereas the pyrolysis gives a mixture of isomers that must be converted to the pure trans,trans form. The thermal decomposition is carried out in the presence of alkaU or amine catalysts. A simultaneous codistillation of the sorbic acid as it forms and the component used as the solvent can simplify the process scheme. The catalyst remains in the reaction batch. Suitable solvents and entraining agents include most inert Hquids that bod at 200—300°C, eg, aUphatic hydrocarbons. When the polyester is spHt thermally at 170—180°C and the sorbic acid is distilled direcdy with the solvent, production and purification can be combined in a single step. The solvent can be reused after removal of the sorbic acid (34). The isomeric mixture can be converted to the thermodynamically more stable trans,trans form in the presence of iodine, alkaU, or sulfuric or hydrochloric acid (37,38). 

Nowadays silenes are well-known intermediates. A number of studies have been carried out to obtain more complex molecules having Si=C double bonds. Thus, an attempt has been made to generate and stabilize in a matrix 1,1-dimethyl-l-silabuta-l,3-diene [125], which can be formed as a primary product of pyrolysis of diallyldimethylsilane [126] (Korolev et al., 1985). However, when thermolysis was carried out at 750-800°C the absorptions of only two stable molecules, propene and 1,1-dimethylsilacyclobut-2-ene [127], were observed in the matrix IR spectra of the reaction products. At temperatures above 800°C both silane [126] and silacyclobutene [127] gave low-molecular hydrocarbons, methane, acetylene, ethylene and methylacetylene. A comparison of relative intensities of the IR... 

Mos of the solid carbonaceous material available to industry is derived from the pyrolysis of petroleum residues, coal, and coal tar residues. Understanding the reactions occurring during pyrolysis would be beneficial in conducting materials research on the manufacture of carbonaceous products. The pyrolysis of aromatic hydrocarbons has been reported to involve condensation and polymerization reactions that produce complex carbonaceous materials (I). Interest in the mechanism of pyrolysis of aromatic compounds is evidenced in a recent study by Edstrom and Lewis (2) on the differential thermal analysis of 84 model aromatic hydrocarbons. The study demonstrated that carbon formation was related to the molecular size of the compound and to energetic factors that could be estimated from ionization potentials. 

Pyrolysis reactions in fuel-rich flame zones may lead, however, to emissions of polycyclic aromatic compounds (PACs) and soot. The close correlation between the concentrations of PACs and the bioactivity of flame samples is indicative of some of the health hazards involved in the emissions of PACs (Fig. 3). Fluxes of PAC species determined in fuel-rich, natural gas turbulent diffusion flames show the build up of hydrocarbons of increasing molecular weight along flames (Fig. 4). It is postulated that PACs are formed by the successive addition of C2 through C5 hydrocarbons to aromatic compounds (Fig. 5). 

The occurrence of molecular eliminations in photolysis is something that does not occur to a significant extent in the pyrolysis of hydrocarbons. Most frequently the molecular decomposition is accompanied by free-radical decomposition, with the fraction of each often dependent on the wavelength of the light used to initiate reaction. Such a case is the photolysis of methane where the following reactions occur ... 

Besides these primary reactions, the successive decompositions of decyl radicals must also be taken into account as must the pyrolysis of the resulting alkenes. It is quite evident that a manual compilation of the whole set of reactions would be unmanageable, particularly when the molecular weight of the hydrocarbon components is increased. 

As already observed above, the pyrolysis of naphthas and gasoils to produce light alkenes is a process whose chemical complexity is dictated both by the characterization of the hydrocarbon mixture and by the complete definition of the kinetic mechanism. Due to the large number of species involved and the need to take into account all the relevant interactions between the different species, the number of elementary reactions becomes very large. For this reason, it is useful to classify the reactions on the one hand and very convenient to apply automatic procedures in order to generate the kinetic scheme, on the other. Likewise, as the molecular weight of the molecules rises, it is not only useful but sometimes necessary to adopt carefully evaluated simplifying rules. 

The use of computer generation systems in modelling the pyrolysis of large hydrocarbons is no longer considered simply an alternative to manual mechanism construction. It has become a necessity. The quantity of species and reactions becomes enormous, increasing molecular weight. This is particularly true if the focus is not merely on linear alkanes but also on other typical components of naphthas and gasoils, such as Bo-alkancs or cyc/o-alkanes, where the number of possible isomers increases exponentially with the number of carbon atoms in the molecule. 

Carbon black is formed by pyrolysis of hydrocarbons. The process of carbon formation is complex, involving cracking, dehydrogenation and subsequent accretion into large molecular species. It is probable that there is a stage in the reaction sequence at which liquid droplets are formed, which then undergo partial graphitization with additional... 

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