Petrochemicals - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 52

S
O O
Sulfolane
If the aromatic content is low, the aromatics can be extracted from the
hydrocarbon mixture by a liquid - liquid extraction. This is the same principle
as performed in a small scale in the laboratory with a separatory funnel with
two immiscible liquid layers. On the industrial scale, it is done continuously
in a counter-current extraction column.
Crystallization is used to separate p-xylene from its C8 isomers. The
melting point of p-xylene is 13 ∘ C which is significantly higher than the other
xylenes or ethylbenzene. Therefore, p-xylene can be selectively crystallized
by cooling a C8 mixture.
Another method to separate p-xylene relies on the difference in shape of
the isomers. The para isomer can be selectively adsorbed into pores of certain
zeolites. It is then rinsed out of the zeolite bed with a desorbing solvent that
can be easily separated by distillation.
5.2 COAL, NATURAL GAS AND SHALE OIL
In addition to oil, coal and natural gas are other fossil fuels that are used for
energy and conversion to higher value organic chemicals. Coal has been used
as a fuel source since the cave man. Historically, and to some extent even
today, coal has been used to produce chemicals. However, most coal is used
by power plants to produce electricity. In the United States, this represents
over 90% of consumption with the other major use as a fuel for heat and
power. About a billion tons of coal are mined each year in the United States.
Worldwide production is about 8 billion tons.
Natural gas is predominantly methane. The composition varies but it can
also include significant amounts of ethane, propane, butane, and isobutane.
Other gases such as carbon dioxide, nitrogen, hydrogen sulfide, and helium
can also be present. In this state, with the heavier hydrocarbons, it is some-
times referred to as “wet gas.” Natural gas is available from several sources
including some derived from oil and some from coal beds. Tight gas refers
to natural gas located in impermeable hard rock and shale gas is natural gas
entrapped in shale deposits. The U.S. produced about 23 trillion cubic feet of
natural gas in 2011 and this is likely to increase. One reason is the increase
in production from shale deposits. Shales are fine-grained sedimentary rocks
that can hold petroleum or natural gas. Historically they have been drilled by
vertical wells and infusing chemically treated water and sand to break up the