Chemistry Reference
In-Depth Information
XYLENES (C
6
H
4
(CH
3
)
2
)
Properties
There are three xylene isomers, commonly known
as
ortho
-xylene,
meta
-xylene, and
para
-xylene.
They are all colorless liquids. The
ortho
-isomer
boils at 144°C, the
meta
- at 139.1°C, and the
par
a-
at 138.5°C.
Commercial grades
Xylenes are available as an isomer mixture (about 10%
ortho
-, 72%
meta
-, and 18%
para
-) and as the pure
isomers. The mixed xylenes are priced by the gallon
while the pure isomers are priced per pound.
Uses
The mixed xylenes are used as solvents and as octane
enhancers in gasoline. The largest use for each of
the pure isomers is oxidation to the corresponding
dicarboxylic acid.
Manufacture
The xylenes are obtained with benzene (and toluene)
from the catalytic reforming of naphtha and
separated from the aromatic mixture by
distillation. From the mixed isomers, the
ortho
- can
be obtained by distillation because its boiling point
is sufficiently different. The
meta
- and
para
- are
separated by either selective adsorption or by
crystallization.
Producers
ExxonMobil, Chevron Phillips, BP Amoco, Citgo,
Koch, Hess,
Of the over 4 billion pounds of xylene made each year in the U.S., about
40% is recovered as high-purity
para
-xylene, about 20% as
ortho
-xylene,
and the rest used as mixed xylene in gasoline and for solvent purposes. The
reason for the large use in gasoline is because xylene (like benzene and
toluene) has very high fuel
anti-knock
characteristics (high octane rating).
Thus, high-test gasoline has a higher percentage of aromatic hydrocarbons
like xylene than regular-grade.
Because
ortho
-xylene is more readily isolated and purified (by distilla-
tion), it costs less than
para
-xylene. Like all petrochemicals, prices depend
on the price of crude oil; but in early 2001, mixed-xylene was about 17
cents/lb while
para
-xylene was only about 15 cents due to high manufac-
turing capacity and low demand for use for making terephthalic acid. In the
extremely high volumes in which such chemicals are sold, fractions of a
penny difference in price can be very important.
