2.75 BENZENE Benzene (C 6 H 6 , boiling point: 80 o C, density: 0.8789, flash point: –11 o C, ignition temperature: 538 o C), is a volatile, colorless, and flammable liquid aromatic hydrocarbon possessing a distinct, characteristic odor. Benzene is practically insoluble in water (0.07 part in 100 parts at 22°C); and fully miscible with alcohol, ether, and numerous organic liquids. For many years benzene (benzol) was made from coal tar, but new processes that consist of ca
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  2.75 BENZENE Benzene (C 6 H 6 , boiling point: 80 o C, density: 0.8789, flash point: –11 o C,ignition temperature: 538 o C), is a volatile, colorless, and flammable liquidaromatic hydrocarbon possessing a distinct, characteristic odor. Benzene ispractically insoluble in water (0.07 part in 100 parts at 22°C); and fullymiscible with alcohol, ether, and numerous organic liquids.For many years benzene (benzol) was made from coal tar, but newprocesses that consist of catalytic reforming of naphtha and hydrodealkyla-tion of toluene are more appropriate. Benzene is a natural component of petroleum, but it cannot be separated from crude oil by simple distillationbecause of azeotrope formation with various other hydrocarbons. Recoveryis more economical if the petroleum fraction is subjected to a thermal or cat-alytic process that increases the concentration of benzene.Petroleum-derived benzene is commercially produced by reforming andseparation, thermal or catalytic dealkylation of toluene, and disproportion-ation. Benzene is also obtained from pyrolysis gasoline formed in thesteam cracking of olefins.If benzene is the main product desired, a narrow light naphtha fractionboiling over the range 70 to 104 o C is fed to the reformer, which containsa noble metal catalyst consisting of, for example, platinum-rhenium on ahigh-surface-area alumina support. The reformer operating conditions andtype of feedstock determine the amount of benzene that can be produced. Thebenzene product is most often recovered from the reformate by solventextraction techniques.In the platforming process (Fig. 1), the feedstock is usually a straight-run, thermally cracked, catalytically cracked, or hydrocracked C 6 to 200 o Cnaphtha. The feed is first hydrotreated to remove sulfur, nitrogen, or oxy-gen compounds that would foul the catalyst, and also to remove olefinspresent in cracked naphthas. The hydrotreated feed is then mixed withrecycled hydrogen and preheated to 495 to 525 o C at pressures of 116 to725 psi (0.8 to 5 MPa). Typical hydrogen charge ratios of 4000 to 8000standard cubic feet per barrel (scf/bbl) of feed are necessary. PDF Created with deskPDF PDF Writer - Trial ::  The feed is then passed through a stacked series of three or four reactorscontaining the catalyst (platinum chloride or rhenium chloride supportedon silica or silica-alumina). The catalyst pellets are generally supported ona bed of ceramic spheres.The product coming out of the reactor consists of excess hydrogen anda reformate rich in aromatics. The liquid product from the separator goesto a stabilizer where light hydrocarbons are removed and sent to a debu-tanizer. The debutanized platformate is then sent to a splitter where C 8 andC 9 aromatics are removed. The platformate splitter overhead, consisting of benzene, toluene, and nonaromatics, is then solvent extracted.Solvents used to extract the benzene include tetramethylene sulfone (Fig. 2), diethylene glycol,  N  -methylpyrrolidinone process, dimethylfor-mamide, liquid sulfur dioxide, and tetraethylene glycol.Benzene is also produced by the hydrodemethylation of toluene undercatalytic or thermal conditions.In the catalytic hydrodealkylation of toluene (Fig. 3):C 6 H 5 CH 3 + H 2  → C 6 H 6 + CH 4 toluene is mixed with a hydrogen stream and passed through a vesselpacked with a catalyst, usually supported chromium or molybdenum oxides,platinum or platinum oxides, on silica or alumina. The operating tem-peratures range from 500 to 595 o C and pressures are usually 580 to 870 psi 2.76 MANUFACTURE OF CHEMICALS Feedstock        R    e     g      e     n    e     r    a      t     o     r    R  e  a  c   t  o  r    S  e  p  a  r  a   t  o  r  o  r   d   i  s   t   i   l   l  a   t   i  o  n SeparatorSeparatorRecycle hydrogenHydrogenPlatformateLight endsSpent catalystRegenerated catalyst FIGURE 1 Benzene manufacture by the platforming process. PDF Created with deskPDF PDF Writer - Trial ::  (4 to 6 MPa). The reaction is exothermic and temperature control (by injec-tion of quench hydrogen) is necessary at several places along the reactionsequence. Conversions per pass typically reach 90 percent and selectivity tobenzene is often greater than 95 percent. The catalytic process occurs atlower temperatures and offers higher selectivity but requires frequentregeneration of the catalyst. Products leaving the reactor pass through a sep-arator where unreacted hydrogen is removed and recycled to the feed.Further fractionation separates methane from the benzene product. BENZENE  2.77 Feedstock     E  x   t  r  a  c   t  o  r   S   t  r   i  p  p  e  r   E  x   t  r  a  c   t  r  e  c  o  v  e  r  y Raffinate wash waterRaffinateExtract        S      e      p        a      r     a       t      o      r Separator FIGURE 2 Benzene manufacture by sulfolane extraction. Fuel gasBenzeneXylenesHydrogen recycleTolueneMakeuphydrogen    R  e  a  c   t  o  r Separator    S  e  p  a  r  a   t  o  r Recycle toluene and C9 aromatics FIGURE 3 Benzene manufacture by toluene hydrodealkylation. PDF Created with deskPDF PDF Writer - Trial ::  Benzene is also produced by the transalkylation of toluene in whichtwo molecules of toluene are converted into one molecule of benzene andone molecule of mixed xylene isomers.In the process (Fig. 4), toluene and C 9 aromatics are mixed with liquidrecycle and recycle hydrogen, heated to 350 to 530 o C at 150 to 737 psi(1 to 5 MPa), and charged to a reactor containing a fixed bed of noblemetal or rare earth catalyst with hydrogen-to-feedstock mole ratios of 5:1to 12:1. Following removal of gases, the separator liquid is freed of lightends and the bottoms are then clay treated and fractionated to producehigh-purity benzene and xylenes. The yield of benzene and xyleneobtained from this procedure is about 92 percent of the theoretical.Other sources of benzene include processes for steam cracking heavynaphtha or light hydrocarbons such as propane or butane to produce a liq-uid product (pyrolysis gasoline) rich in aromatics that contains up to about65 percent aromatics, about 50 percent of which is benzene. Benzene canbe recovered by solvent extraction and subsequent distillation.Benzene can also be recovered from coal tar. The lowest-boiling frac-tion of the tar is extracted with caustic soda to remove tar acids, and thebase oil is then distilled and further purified by hydrodealkylation.Benzene is used as a chemical intermediate for the production of manyimportant industrial compounds, such as styrene (polystyrene and syn-thetic rubber), phenol (phenolic resins), cyclohexane (nylon), aniline(dyes), alkylbenzenes (detergents), and chlorobenzenes. These intermedi- 2.78 MANUFACTURE OF CHEMICALS Fuel gasBenzeneXylenesHydrogen recycleTolueneMake-up hydrogen    R  e  a  c   t  o  r Recycle streamsTolueneHigh boilersC9 aromatics    R  e  a  c   t  o  r   D   i  s   t   i   l   l  a   t   i  o  n FIGURE 4 Benzene manufacture by the transalkylation of toluene. PDF Created with deskPDF PDF Writer - Trial ::
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