Page 134 - Chemical and process design handbook

Speight_Part II_B 11/7/01 3:11 PM Page 2.75

BENZENE

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Benzene (C H , boiling point: 80 C, density: 0.8789, flash point: –11 C,
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ignition temperature: 538 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 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 distillation
because of azeotrope formation with various other hydrocarbons. Recovery
is 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 and
separation, thermal or catalytic dealkylation of toluene, and disproportion-
ation. Benzene is also obtained from pyrolysis gasoline formed in the
steam cracking of olefins.
If benzene is the main product desired, a narrow light naphtha fraction
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boiling over the range 70 to 104 C is fed to the reformer, which contains
a noble metal catalyst consisting of, for example, platinum-rhenium on a
high-surface-area alumina support. The reformer operating conditions and
type of feedstock determine the amount of benzene that can be produced. The
benzene product is most often recovered from the reformate by solvent
extraction techniques.
In the platforming process (Fig. 1), the feedstock is usually a straight-
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run, thermally cracked, catalytically cracked, or hydrocracked C to 200 C
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naphtha. The feed is first hydrotreated to remove sulfur, nitrogen, or oxy-
gen compounds that would foul the catalyst, and also to remove olefins
present in cracked naphthas. The hydrotreated feed is then mixed with
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recycled hydrogen and preheated to 495 to 525 C at pressures of 116 to
725 psi (0.8 to 5 MPa). Typical hydrogen charge ratios of 4000 to 8000
standard cubic feet per barrel (scf/bbl) of feed are necessary.

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