Benzene Benzol

For many years benzene was made from coal tar even as late as 1949, when all of it was made by this old process. New processes began to take over in the 1950s, which were used for 50% of the benzene in 1959, 94% in 1972, 96% in 1980, and near 100% in the 1990s. These new processes consist of catalytic reforming of naphtha and hydrodealkylation of toluene in a 70 30 capacity ratio. 

These solvents have high solubility for aromatics but not for nonaromatics. They also have high boiling points for later separation from the aromatics. Fractionation separates the benzene from the solvent and other aromatics. A typical Udex extraction starting with a reformed feed of 51.3% aromatic content gives 7.6% benzene, 21.5% toluene, 21%, xylenes, and 1.2% C9 aromatics. The recovery rate is 99.5% of the benzene, 98% of the toluene, 95% of the xylene, and 80% of the C9 aromatics. 

More toluene is formed than is needed in the catalytic reforming of naphtha. Benzene is always in tight supply. Table 8.7 shows the catalytic reformate production percentages of benzene, toluene, and xylene vs. the U.S. chemical demand. When the price is right it is economical to hydrodealkylate (add hydrogen, lose the methyl) toluene to benzene. This is best done on pure toluene, where the yield can be as high as 98.5%. The reaction can be promoted thermally or catalytically. As much as 30-50% of all benzene is made this way. 

Benzene is a clear, colorless, flammable liquid with a pleasant characteristic odor, bp 80.1°C, flash point -11.1°C, and ignition temperature 538°C. Benzene has been found to be very toxic and is on the list of Known to Be Human Carcinogens published every two years by the Department of Health and Human Services under the National Toxicology Program (NTP). There are four top 50 chemicals on this worst carcinogen list benzene 1,3-butadiene ethylene oxide and vinyl chloride. There are also four chemicals in the top 50 on the Reasonably Anticipated to Be Human Carcinogens list acrylonitrile, ethylene dichloride, formaldehyde, and propylene oxide. 

Benzene has a low threshold limit value or TLV. The time weighted average TLV (TWA) is the allowable exposure for an average 8 hr day or a 40 hr week. The short-term exposure limit TLV (STEL) is the maximum allowable exposure for any 15-min period. For benzene the TWA = 0.5 ppm and the STEL is 2.5 ppm, as given by the American Conference of Governmental Industrial Hygienists (ACGIH). This allowable exposure is much lower than those for toluene and xylene, probably because these latter two compounds have benzyl ic positions that are easily oxidized in vivo to compounds that can be eliminated from the body. 

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Solvents used for dewaxing are naphtha, propane, sulfur dioxide, acetone—benzene, trichloroethylene, ethylenedichloride—benzene (Barisol), methyl ethyl ketone—benzene (benzol), methyl -butyl ketone, and methyl / -propyl ketone. Other solvents in commercial use for dewaxing include /V-methylpyrrolidinone, MEK—MIBK (methyl isobutyl ketone), dichloroethane—methylene dichloride, and propfyene—acetone. 

Roh-alkohol, m. crude (or raw) alcohoL -analyse, /. rough or approximate analysis, -antimon, n. crude antimony, -arbeit, /, (Metal.) ore smelting, -asbest, m. crude asbestos. -aufbereitung, /. preliminary preparation, -benzol, n. crude benzene, benzol (or benzole), -blei, n. crude lead, -blende, /. (Mining) crude blende, -block,... 

Steinkohlenteer, m. coal tar. -benzin, n. benzene benzol(e), -blase,/. coal-tar still, -essenz, /. first light oil. -farbe, /. coal-tar color, -kampher, m. naphthalene, -dl, n. coal-tar oil (leichtes, light oil schweres, heavy oil), -pech, n. coal-tar pitch, -prl-parat, n. coal-tar preparation, coal-tar prod uct-... 

Benzol 90, see Benzene Benzole, see Benzene Benzolene, see Benzene... 

Benzene, Benzole, Pbenylhydtide, Cyclobexatrienq Phene (called Benzol in Ger) CH CH-CH ... 

For many years benzene (benzol) was made from coal tar, but new processes that consist of catalytic reforming of naphtha and hydrodealkylation of toluene are more appropriate. Benzene is a natural component of petroleum, but it cannot be separated from crude oil by simple distillation because of azeotrope formation with various other hydrocarbons. Recovery is more economical if the petroleum fraction is subjected to a thermal or catalytic process that increases the concentration of benzene. 

Greenburg L, Mayers MR, Goldwater L, et al. 1939. Benzene (benzol) poisoning in the rotogravure printing industry in New York City. J Ind Hyg Toxicol 21 395-420. 

HamiltonA. 1922. The growing menace of benzene (benzol) poisoning in American industry. JAm Med Assoc 78 627-630. 

Benzene benzol, phenyl hydride, cyclohexatriene CeHe 78,11... 

Benzene—Benzol—Phenyl hydrid—C H,—78—(not to be confounded with the commercial benzine, a mixture of hydrocarbons of the series CmHjn+, obtained from petroleum) does not exist in nature, but is produced in a number of reactions. It is obtained by one of two methods, according as it is required chemically pure or mixed with other substances. 

The difference in toxicity between benzene (benzol) and benzine is very signihcant and great care should be taken not to confuse them. 

Liebig" mentions that benzene is present in coal-tar Hofmann states that he found no published source for this. About 1834 work was in progress in Manchester to separate naphtha from coal-tar by distillation and John Dale about 1838 tried to convert this into wood naphtha. John Leigh in 1842 showed specimens of benzene, nitrobenzene, and dinitrobenzene to the British Association in Manchester. Mansfield, under Hofmann s direction, separated by fractional distillation of coal-tar not only large amounts of benzene (benzole) but also toluene (b.p. c, 113" ) and what he called cumol (b.p. I43° i45°) and cymol (b.p. lyo -iyz ). Hugo Miiller showed that Mansfield s cumol is xylene, discovered by Cahours in 1850. ... 

Isocyclic compounds — Isocyclische Verbindungen 2.3.2.S Benzene and hydrocarbon substituted benzenes — Benzol- und Eohlenwasserstoff-substituierte Benzole ... 

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