Petrochemicals can be defined as a large group of chemicals derived from natural gas and petroleum and further used for a variety of chemical purposes, which are extremely important in modern civilization. From: Advances in Membrane Technologies for Water Treatment, 2015.
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Chemicals from aromatic hydrocarbons
Briefly, aromatic compounds are those containing one or more benzene rings or similar ring structures. The majority are taken from refinery streams which contain them and separated into fractions, of which the most significant fractions are benzene (C6H6), methylbenzene or toluene (C6H5CH3), and the dimethylbenzenes or xylenes (CH3C6H4CH3) with the two-ring condensed aromatic compound naphthalene (C10H8) also being a source of petrochemicals.
In the traditional chemical industry, aromatics such as benzene, toluene, and the xylenes were made from coal during the course of carbonization in the production of coke and town gas. A much larger volume of these chemicals is now made as refinery by-products. A further source of supply is the aromatic-rich liquid fraction produced in the cracking of naphtha or light gas oils during the manufacture of ethylene and other olefins.
Aromatic compounds are valuable starting materials for a variety of chemical products (Chemier, 1992). Reforming processes have made benzene, toluene, xylenes, and ethylbenzene economically available from petroleum sources. They are generally recovered by extractive or azeotropic distillation, by solvent extraction (with water–glycol mixtures or liquid sulfur dioxide), or by adsorption. Naphthalene and methylnaphthalenes are present in catalytically cracked distillates. A substantial part of the benzene consumed is now derived from petroleum, and it has many chemical uses.
Aromatic compounds such as benzene, toluene, and the xylenes are major sources of chemicals (Figure 12.6). For example, benzene is used to make styrene (C6H5CH = CH2), the basic ingredient of polystyrene plastics, as well as paints, epoxy resins, glues, and other adhesives. The process for the manufacture of styrene proceeds through ethylbenzene, which is produced by reaction of benzene and ethylene at 95°C (203°F) in the presence of a catalyst:
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Figure 12.6. Chemicals from benzene, toluene, and the xylenes
In the presence of a catalyst and superheated steam ethylbenzene dehydrogenates to styrene:
Toluene is usually added to the gasoline pool or used as a solvent, but it can be dealkylated to benzene by catalytic treatment with hydrogen:
Similar processes are used for dealkylation of methyl-substituted naphthalene. Toluene is also used to make solvents, gasoline additives, and explosives.
Toluene is usually in demand as a source of trinitrotoluene (TNT) but has fewer chemical uses than benzene. Alkylation with ethylene, followed by dehydrogenation, yields α-methylstyrene [C6H5C(CH3) =CH 2], which can be used for polymerization. Alkylation of toluene with propylene tetramer yields a product suitable for sulfonation to a detergent-grade surface-active compound.
Of the xylenes, o-xylene is used to produce phthalic anhydride and other compounds. Another xylene, p-xylene, is used in the production of polyesters in the form of terephthalic acid or its methyl ester. Terephthalic acid is produced from p-xylene by two reactions in four steps. The first of these is oxidation with oxygen at 190°C (375°F):
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