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Xylene (from Greek
ξύλο,
xylo, "wood"), xylol or dimethylbenzene is an
aromatic hydrocarbon mixture consisting of a benzene ring with two
methyl groups at various substituted positions. The three isomers of
xylene have the molecular formula C8H10,
also representated by the semi-structural formula C6H4(CH3)2. Xylene is a major petrochemical produced by catalytic reforming and
also by coal carbonisation in the manufacture of coke fuel. It
represents about 0.5–1% of crude oil (depending on the source), and
is found in small quantities in gasoline and aeroplane fuels.
Xylenes are mainly produced as part of the BTX aromatics (benzene,
toluene and xylenes) extracted from the product of catalytic
reforming known as "reformate". The mixture is a slightly greasy,
colorless liquid commonly encountered as a solvent. Xylene was named
in 1851, having been discovered as a constituent of wood tar.
Several million tons are produced annually.
In 2011, a global consortium began construction of one of the
world’s largest xylene plants in Singapore.
Isomers
Xylene exists in three isomeric forms. The
isomers can be distinguished by the designations ortho- (o-),
meta- (m-), and para- (p-), which
specify to which carbon atoms (of the benzene ring), the two methyl
groups are attached. By counting the carbon atoms around the ring
starting from one of the ring carbons bonded to a methyl group, and
counting towards the second methyl group, the o-isomer has
the IUPAC name of 1,2-dimethylbenzene, the m-isomer is
1,3-dimethylbenzene, and the p-isomer is 1,4-dimethylbenzene.
Of the three isomers, the p-isomer is the most industrially
sought after after since it can be oxidized to terephthalic acid.
Industrial production
Xylenes are produced by the methylation>
of toluene and benzene. Commercial or laboratory grade xylene
produced usually contains about 40-65% of m-xylene and up to
20% each of o-xylene, p-xylene and ethylbenzene.
The ratio of isomers can be shifted to favor the highly valued p-xylene
via the patented UOP-Isomar process0ff">[7]
or by transalkylationof xylene with itself or trimethylbenzene. These conversions are
catalyzed by zealots.
Properties
The chemical and physical properties of xylene differ according to
the respective isomers. The melting point ranges from −47.87 °C
(−54.17 °F) (m-xylene) to 13.26 °C (55.87 °F) (p-xylene).
The boiling point for each isomer is around 140 °C (284 °F). The
density of each isomer is around 0.87 g/mL (7.26 lb/U.S. gallon or
8.72 lb/imp gallon) and thus is less dense than water. Xylene in air
can be smelled at concentrations as low as 0.08 to 3.7 ppm (parts of
xylene per million parts of air) and can be tasted in water at 0.53
to 1.8 ppm
ATEX Gas detectors are available for 2
explosion classes as follows:
Zone 1 is available with or without display
Gas detectors for ATEX Zone 1 are located in a flameproof enclosure,
EEx d
The shell is such that ignition and
combustion within the shell of a potential explosive gas mixture is
not propagated to the explosive mixture outside the enclosure and
the enclosure can thus endure the resulting explosion pressure
without damage.
In Zone 1
one can expect an explosive atmosphere to occur during normal
operation but the presence of it is relatively
short.
The equipment shall not be a source of
ignition in normal operation or when an error occurs.
Gas detectors for ATEX Zone 2 are of the
design Non-sparking EEx nA
EEx nA is a non-sparking piece of equipment.
In
Zone 2, you need not count with an
explosive atmosphere to occur during normal operation, and if it
should occur, which is probably rare of duration.
The equipment shall not be a source of
ignition during normal operation.
Information on LEL och
explosion hazard
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