Benzene pyrolysis conditions

Pyrolysis of CH4 and NH3 gives very low yields of amino acids. The pyrolysis conditions are from 800° to 1200°C with contact times of a second or less. However, the pyrolysis of CH4 and other hydrocarbons gives good yields of benzene, phenylacetylene, and many other hydrocarbons. It can be shown that phenylacetylene would be converted to phenylalanine and tyrosine in the primitive ocean.17 Pyrolysis of the hydrocarbons in the presence of NH3 gives substantial yields of indole, which can be converted to tryptophan in the primitive ocean. 

Figure 15.3 Thermal cracking mechanism for polyethylene terephthalate. Note under mild conditions terephthalic acid predominates, but under more severe pyrolysis conditions the terephthalic acid decomposes to benzoic acid and benzene...

Since biomass pyrolysis product mixtures are very complex and selectivities are low for specific products, considerable effort has been devoted to improving selectivities. Selectivities can sometimes be increased by addition of coreactants or catalysts, or by changing the pyrolysis conditions (cf. Nikitin et al, 1962). For example, the pyrolysis of maplewood impregnated with phosphoric acid increased the yield of methanol to 2.2 wt % of the wood as compared to 1.3 wt % obtained on dry distillation of the untreated wood. Addition of sodium carbonate to oak and maple increased the yield of methanol by 100 and 60%, respectively, compared to pyrolysis yields without sodium carbonate. Other weakly alkaline reagents exhibited a similar effect. Pyrolysis of wood in a stream of benzene, xylene, or kerosine increased the yields of acetic acid, aldehydes, and phenols and reduced the yield of tars. Optimization of pyrolysis conditions will be shown later to have large effects on product distributions and yields. 

The use and importance of aromatic compounds in fuels sharply contrasts the limited kinetic data available in the literature, regarding their combustion kinetics and reaction pathways. A number of experimental and modelling studies on benzene [153, 154, 155, 156, 157, 158], toluene [159, 160] and phenol [161] oxidation exist in the literature, but it would still be helpful to have more data on initial product and species concentration profiles to understand or evaluate important reaction paths and to validate detailed mechanisms. The above studies show that phenyl and phenoxy radicals are key intermediates in the gas phase thermal oxidation of aromatics. The formation of the phenyl radical usually involves abstraction of a strong (111 to 114 kcal mof ) aromatic—H bond by the radical pool. These abstraction reactions are often endothermic and usually involve a 6 - 8 kcal mol barrier above the endothermicity but they still occur readily under moderate or high temperature combustion or pyrolysis conditions. The phenoxy radical in aromatic oxidation can result from an exothermic process involving several steps, (i) formation of phenol by OH addition to the aromatic ring with subsequent H or R elimination from the addition site [162] (ii) the phenoxy radical is then easily formed via abstraction of the weak (ca. 86 kcal moT ) phenolic hydrogen atom. 

Cyclization proceeded in nearly 100% selectivity in the case of thermal reaction of butadiene (1 ), yielding 4-vinylcyclohexene (VCH) for the first step and ethylene, cyclohexene, cyclohexa-diene, and benzene in the secondary steps. Similar highly selective cyclizations were observed for the reactions between butadiene and ethylene, propylene, 1-butene, cis-2-butene, trans-2-butene or isobutylene (1), yielding cyclohexene (HCH), 4-methyl-cyclohexene (MCH), 4-ethylcyclohexene, cis-4,5-dimethylcyclo-hexene, trans-4,5-dimethyIcyclohexene or 4,4-dimethylcyclohexene, respectively. Based on the above information, it can be said that butadiene plays an important role in the formation of cyclic compounds in pyrolysis conditions. 

The unique phenomenon in the pyrolysis of vinylidene chloride/vinyl chloride copolymer is the trimer formation. Under pyrolysis conditions, the polymer will directly undergo the thermal dehydrochlorination to form a conjugated polyene [205]. The polymer will then unzip, followed by a radical cyclisation to form benzene, chlorobenzene, dichlorobenzene, and trichlorobenzene. The mechanism can be expressed as ... 

In developing a systematic scheme of polymer identification, pyrolysis conditions must be such that all polymers degrade rapidly. However, at temperatures above 1000 °C the pyrograms will also be less suitable for identification, since secondary reactions become predominant, leading to increasing amounts of simple molecules such as carbon dioxide, acetylene, ethylene or benzene which gives less characteristic patterns than those observed for monomers or primary degradation products. Pyrolysis temperatures between 500 and 800 C (optimum 610 °C) for 10 seconds are recommended. 

Ethylbenzene (C6H5CH2CH3) is one of the Cg aromatic constituents in reformates and pyrolysis gasolines. It can be obtained by intensive fractionation of the aromatic extract, but only a small quantity of the demanded ethylbenzene is produced by this route. Most ethylbenzene is obtained by the alkylation of benzene with ethylene. Chapter 10 discusses conditions for producing ethylbenzene with benzene chemicals. The U.S. production of ethylbenzene was approximately 12.7 billion pounds in 1997. Essentially, all of it was directed for the production of styrene. 

The use of pyrolysis for the recycling of mixed plastics is discussed and it is shown that fluidised bed pyrolysis is particularly advantageous. It is demonstrated that 25 to 45% of product gas with a high heating value and 30 to 50% of an oil rich in aromatics can be recovered. The oil is found to be comparable with that of a mixture of light benzene and bituminous coal tar. Up to 60% of ethylene and propylene can be produced by using mixed polyolefins as feedstock. It is suggested that, under appropriate conditions, the pyrolysis process could be successful commercially. 23 refs. 

The formation of the parent system p-phenylenebismethylene (8 Scheme 1) was first attempted in the gas phase from the pyrolysis of C-labeled l,4-bis(5-tetrazo-lyl)benzene. Under such conditions, it was not possible to detect the intermediate directly and specify it in detail, but its formation was deduced from the product analysis [72]. In 1998, though, irradiation of the bisdiazo precursor 8-D2 made possible the characterization of 8 by IR and UV/vis spectroscopy [73]. The identification was based on trapping experiments with HCl (to form 9) and oxygen (Scheme 1) and by simulating the IR spectrum of 8 [UB3LYP/6-31G(d,p)] [73]. 

It was found that the majority of the bromine was concentrated in the carbon residue and while majority of the nitrogen accumulates in the liquid products irrespective of degradation conditions (134). Besides a large amount of styrene and benzene derivatives the pyrolysis oils contained around 1000 ppm of nitrogen, 1000-4000 ppm bromine, 5000-5200 ppm chlorine and 800-1300 ppm oxygen (135). 

The outline procedure involves the initial reaction of the 2,4,6-triphenyl-pyrylium halide with the primary amine to yield the corresponding 2,4,6-triphenylpyridinium halide (see Section 8.4.1, and also Section 5.15.3, p. 768) this reaction proceeds either at room temperature in a suitable solvent, or more efficiently under reflux in benzene with azeotropic removal of water. Pyrolysis of the pyridinium halide under controlled conditions then yields the alkyl (or aralkyl) halide in good yield. The mechanism of the reaction in this case is probably of the Sn2 type. 

Thermal cracking investigations date back more than 100 years, and pyrolysis has been practiced commercially with coal (for coke production) even longer. Ethylene and propylene are obtained primarily by pyrolysis of ethane and heavier hydrocarbons. Significant amounts of butadiene and BTXs (benzene, toluene, and xylenes) are also produced in this manner. In addition, the following are produced and can be recovered if economic conditions permit acetylene, isoprene, styrene, and hydrogen. 

The pronounced differences among the product distributions from the three dichlorobenzenes rule out any extensive scrambling of the chlorine atoms, such as was found for deuterium in deuteriated benzene at high temperatures. That such scrambling of chlorine atoms does not occur is indicated further by our failure to detect mono- and trichlorobenzenes among the pyrolysis products of the dichlorobenzenes alone under the same conditions. 

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