Aromatization during pyrolysis

NMR measurements have been combined with elemental and mass balance data to determine the extent of aromatization during pyrolysis. Hershkowitz et al7 were the first to quantify the increase in the aromatic carbon formed during pyrolysis of Colorado oil shale. Their experiments were conducted at a slow heating rate, under high pressure (2600 kPa) N2 or H2 atmospheres, at temperatures up to 600°C, followed by a 10 min soak period at this temperature. In an N2 atmosphere, the total aromatic carbon in the products increased by 83% over that in the raw shale. In H2 the increase was only 17%. In addition, 87% of the raw shale carbon was recovered in the oil when heated under H2, compared with 67% under N2. An increase in aromatic carbon of about 83% has been observed in pyrolysis studies of Green River oil shale at heating rates of l-720 Ch to 500°C. ... 

Muller, J. and Dongmann, G., Formation of Aromatics during pyrolysis of PVC in the presence of metal chlorides, /. Anal. Appl. Pyrolysis, 45, 59,1998. 

Sakai, T., et al. A Kinetic Study on the Formation of Aromatics During Pyrolysis of Petroleum Hydrocarbons, in Albright, L. F. und Crynes, B. L. Industrial and Laboratory Pyrolyses. ACS Symposion Series 32, Washington 1976, p. 152-177. 

Since conjugated unsaturated macromolecules produce aromatics during pyrolysis, polymers decomposing into such structures evolve plenty of smoke. A characteristic example is PVC which gives polyenic chains after dehydrochlorination then they are unzipping , through segmental cyclisation into benzene and some alkylbenzenes which are known to burn with a sooty flame (cf. Section 4.2.1.1, Table 4.6, Fig. 4.20). 

A Kinetic Study on the Formation of Aromatics During Pyrolysis of Petroleum Hydrocarbons... 

The fact that most alkylated benzenes show the same tendency to soot is also consistent with a mechanism that requires the presence of phenyl radicals, concentrations of acetylene that arise from the pyrolysis of the ring, and the formation of a fused-ring structure. As mentioned, acetylene is a major pyrolysis product of benzene and all alkylated aromatics. The observation that 1-methylnaphthalene is one of the most prolific sooting compounds is likely explained by the immediate presence of the naphthalene radical during pyrolysis (see Fig. 8.23). 

It is also almost certain that the methyl groups are completely eliminated during pyrolysis at 600°C. Infrared (7) and chemical studies (24) on semicoke indicate that semicoke is about 1009 aromatic in character and hence contains no methyl carbon. 

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. 

Unsubstituted Aromatics. The analyses indicated that the mixture was simplified during pyrolysis until it consisted primarily of unsubstituted aromatics such as benzene, naphthalene, phenanthrene, and pyrene. Of course, aromatization is not the only mechanism occurring since liquid products with both higher and lower molecular weights than the starting liquids, as well as gases and carbon, were formed. 

According to Eq. (4.3), the slope of the straight line obtained from a plot of the LOI versus the coke yield during pyrolysis or combustion of a polymer may be a measure of the ability of coke to improve the flame retardancy of polymers. Fig. 15 demonstrates that, when siloxane monomers are introduced into aromatic polycarbonates, the flame retardant effect of the coke increases considerably as compared with polymers not containing such monomers. It has been noted, however... 

The theoretical yield of pyrolysis products calculated on the basis of the additivity of individual plastic pyrolysis at 550°C is in good agreement with the products yields for the mixed plastics pyrolysis. However, with increase of the pyrolysis temperature, the char and gas yield are higher (with a decrease of oil and wax yields), suggesting that there are some interaction of the plastics in the mixture during pyrolysis [8], Increasing the pyrolysis temperature leads to an increase of the aromatics, and a decrease of alkanes, aUcenes and alkadienes in the oils. 

A lthough coke formation is always of importance during pyrolysis processes that are used for production of ethylene and other valuable olefins, diolefins, aromatics, etc., relatively little is known about the factors affecting such coke formation. It has been found that operating conditions, feedstock, pyrolysis equipment, and materials of construction and pretreatments of the inner walls of the pyrolysis tubes all affect the production of coke. General rules that have been devised empirically at one plant for minimizing coke formation are sometimes different than those for another plant. It can be concluded that there is relatively little understanding of, or at least little application of, fundamentals to commercial units. 

Besides solid residues carbon-, hydrogen-, and oxygen containing gases as well as condensable aromatic hydrocarbons, which are designated as tars, are formed during pyrolysis. [4]... 

It is interesting to note that the decarboxylation mechanism of aromatic acids is probably an electrophilic substitution. This reaction is not uncommon during pyrolysis. For example, the decarboxylation of benzoic acid takes place as an aromatic electrophilic substitution ... 

The addition reactions take place at a carbon-carbon multiple bond, or carbon-hetero atom multiple bond. Because of this peculiarity, the addition reactions are not common as the first step in pyrolysis. The generation of double bonds during pyrolysis can, however, continue with addition reactions. The additions can be electrophilic, nucleophilic, involving free radicals, with a cyclic mechanism, or additions to conjugated systems such as Diels-Alder reaction. This type of reaction may explain, for example, the formation of benzene (or other aromatic hydrocarbons) following the radicalic elimination during the pyrolysis of alkanes. In these reactions, after the first step with the formation of unsaturated hydrocarbons, a Diels-Alder reaction may occur, followed by further hydrogen elimination ... 

Side group reactions are common during pyrolysis and they may take place before chain scission. The presence of water and carbon dioxide as main pyrolysis products in numerous pyrolytic processes can be explained by this type of reaction. The reaction can have either an elimination mechanism or, as indicated in Section 2.5 for the decarboxylation of aromatic acids, it can have a substitution mechanism. Two other examples of side group reactions were given previously in Section 2.2, namely the water elimination during the pyrolysis of cellulose and ethanol elimination during the pyrolysis of ethyl cellulose. The elimination of water from the side chain of a peptide (as shown in Section 2.5) also falls in this type of reaction. Side eliminations are common for many linear polymers. However, because these reactions generate smaller molecules but do not affect the chain of the polymeric materials, they are usually continued with chain scission reactions. 

One problem related to the interpretation of pyrolysis data on kerogens is related to the influence of the matrix on pyrolysis products. The clay, diatomaceous materials, and calcareous substrates may have catalytic effects on pyrolysis. For example, it is difficult to determine if some aromatic components in pyrolysates were initially present in the sample or were generated from saturated precursors by dehydrogenation under the influence of heat and the presence of a specific matrix of the kerogen. Terpenoid or steroid related hydrocarbons are better preserved during pyrolysis and may be used as evidence of a certain kerogen origin. 

Some 2,6-bis(1,1-dimethylethyl)-4-methyl phenol (BHT) which is added as an inhibitor, can be seen in the pyrogram (at 72.20 min.). The difference between the pyrolysis results for the poly(vinyl acetate) homopolymer and that of the copolymer with polyethylene indicates that the acetate groups are sparsely distributed in the copolymer. The differences in the generation of aromatic compounds are indicated schematically below. The poly(vinyl acetate) homopolymer will undergo during pyrolysis reactions of the following type ... 

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