Hydrogen from benzene pyrolysis

Pyrolysis. Benzene undergoes thermal dehydrocondensation at high temperatures to produce small amounts of biphenyls and terphenyls (see Biphenyl AND terphenyls). Before the 1970s most commercial biphenyl was produced from benzene pyrolysis. In a typical procedure benzene vapors are passed through a reactor, usually at temperatures above 650°C. The decomposition of benzene iato carbon and hydrogen is a competing reaction at temperatures of about 750°C. Biphenyls are also formed when benzene and ethylene are heated to 130—160°C ia the presence of alkaH metals on activated AI2O3 (33). 

By-products from EDC pyrolysis typically include acetjiene, ethylene, methyl chloride, ethyl chloride, 1,3-butadiene, vinylacetylene, benzene, chloroprene, vinyUdene chloride, 1,1-dichloroethane, chloroform, carbon tetrachloride, 1,1,1-trichloroethane [71-55-6] and other chlorinated hydrocarbons (78). Most of these impurities remain with the unconverted EDC, and are subsequendy removed in EDC purification as light and heavy ends. The lightest compounds, ethylene and acetylene, are taken off with the HCl and end up in the oxychlorination reactor feed. The acetylene can be selectively hydrogenated to ethylene. The compounds that have boiling points near that of vinyl chloride, ie, methyl chloride and 1,3-butadiene, will codistiU with the vinyl chloride product. Chlorine or carbon tetrachloride addition to the pyrolysis reactor feed has been used to suppress methyl chloride formation, whereas 1,3-butadiene, which interferes with PVC polymerization, can be removed by treatment with chlorine or HCl, or by selective hydrogenation. 

Free radical substitutions are also known to occur in pyrolytic reactions. An example of this type is the formation of biphenyl from benzene at 700° C (this reaction can be viewed as an oxidation because of the hydrogen elimination). It is likely that similar reactions take place in the pyrolysis of coal and kerogen. 

Hydrogen sulfide is the main decomposition product seen from the pyrolysis of poly(thiophene-2,5-diyl). Some 2,2 -bithiophene (17.4% of pyrolysate) and only a small proportion of thiophene are generated (less than 5% of pyrolysate). However, the pyrolysis in He also forms char, which is not volatile and cannot be seen in the pyrogram. The bonds that appear to cleave more easily are the S-C bonds and the bonds between the thiophene units (C-C type). Since the hydrogen content of the polymer is low, the formation of SH2 is associated with the formation of char. The elimination of some carbon and sulfur as CS2 or S explains the formation of benzene, thiophene, etc. 

One of the oldest pyrochemical reactions is the formation of biphenyl from benzene. The older prescriptions require that benzene vapor be passed through a red-hot iron tube, whereby biphenyl is accompanied by hydrogen, carbon, and terphenyls as pyrolysis products. Zanetti and Egloff255 give 750° as the optimum temperature for production of biphenyl, larger proportions of by-... 

Aromatic compounds such as benzene, toluene, or the different xylenes are mainly produced by the hydrogenated C5+-stream (pyrolysis gasoline) of a steam cracker. Besides the aromatics, this stream contains the different aliphatics and naphthenes. There is the question if all the aromatics (C6-C12) can be separated from the other C -Cu compounds by extractive distillation using for example sulfolane as entrainer. Simplifying, it is assumed that the aliphatics only consist of n-alkanes (n-hexane-M-dodecane). A temperature of 80 °C is chosen. The separation problem and the column configuration is shown in Figure 11.20. 

Thienyldihydrothiophene is the thiophene analogue of the phenyl-cyclohexadiene intermediate deduced by Fields and Meyerson (1966c) from scrambling of protium and deuterium in pyrolysis of deuteriated benzene. Both intermediates account for the formation of dimeric species unaccompanied by highly energetic free hydrogen atoms. 

The feedstock consists of a mixture of C8 aromatics typically derived from catalytically reformed naphtha, hydrotreated pyrolysis gasoline oran LPG aromatization unit. The feed may contain up to 40% ethylbenzene, which is converted either to xylenes or benzene by the Isomar reactor at a high-conversion rate per pass. Feedstocks may be pure solvent extracts or fractional heartcuts containing up to 25% nonaromatics. Hydrogen may be supplied from a catalytic reforming unit or any suitable source. Chemical hydrogen consumption is minimal. 

Application Increase the value of steam cracker pyrolysis gasoline (py-gas) using conversion, distillation and selective hydrogenation processes. Pygas, the C5-C9 fraction issuing from steam crackers, is a potential source of products such as dicyclopentadiene (DCPD), isoprene, cyclopentane, benzene, toluene and xylenes. 

HDA [HydroDeAlkylation] A proprietary dealkylation process for making benzene from toluene, xylenes, pyrolysis naphtha, and other petroleum refinery intermediates. The catalyst, typically chromium oxide or molybdenum oxide, together with hydrogen gas, removes the methyl groups from the aromatic hydrocarbons, converting them to methane. The process also converts cresols to phenol. Developed by Hydrocarbon Research with Atlantic Richfield Corporation and widely licensed worldwide. 

In this configuration, the hydrogen and the methane from the demethanizer column are spUt into their component streams. The hydrogen is for use in various downstream processes and the methane is used as a fuel-gas stream. Bottoms from the de-ethaniser are further split into C3 and C4. stream. The C3 is treated similarly to the C2 to produce polymer grade propylene. After removing the C4 fraction, which is passed to downstream separation units, the heavy components form pyrolysis-gasoUne. The latter may be further separated to produce benzene, toluene and xylene. 

The pyrolysis of PVC produces a highly aromatic oil in addition to hydrogen chloride yields of more than 50 wt% [7-9, 14, 19]. The oil contains mainly aromatic compounds. Benzene has been identified as the main aromatic compound at 22.1 wt% in the oil from the fluidized-bed pyrolysis of PVC [19]. Benzene has also been identified as the... 

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