Petrochemicals - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 50

with an octane rating of 90 would have similar combustion behavior to a 90:10
ratio of isooctane : n-heptane. Recognize that something can have an octane
rating of 90 or even 100 and contain no octane or isooctane. The rating has to
do with the combustion properties and not how much octane or isooctane is
in the fuel. Octane ratings generally increase with branching. In reforming,
the petroleum distillate is heated in the presence of an isomerization catalyst,
such as halogen-treated alumina or non-acidic zeolite L, and a hydro-
genation/dehydrogenation metal, e.g., a Group 8, 9, or 10 metal such as
platinum. This forms branched isomers and cyclic compounds. Distillation
of the reformate gives a hydrocarbon stream of increased octane level. The
distillation is also a source of BTX, a mixture of benzene, toluene, and xylene
There are several separation techniques used to isolate the various com-
ponents of the petroleum product stream. The most common is distillation.
Distillation depends upon a mixture having a different composition in the
vapor phase from the composition of the liquid phase. Consider a simple
distillation of a 50-50 molar ratio of a two-component mixture. When the
mixture is heated to the boiling point, typically the vapor is enriched in
the lower boiling component. The enrichment will vary based upon the
difference in boiling points. If the vapor has an 80:20 molar ratio of low boiler
to high boiler, condensation of that vapor will result in a fraction that is 80%
(molar basis) of the lower boiling component. If that were distilled again,
the collected vapor may be 95% of the low boiler. Repeated distillation can
give material which is an even purer low boiling fraction. At the same time
that the low boiler is obtained by the collection of the vapor phase, the high
boiler is obtained by collection of the liquid phase. Rather than doing several
of these simple distillations over and over again, a packed column is used.
In the column, the vapor condenses on the packing material and then some
of it is returned as liquid back down the column and some of it revaporizes;
essentially it is redistilled. Each condensation and revaporization is like a
simple distillation. The number of distillations within a column is called the
number of plates. A column is said to have a certain number of theoretical
plates. Depending on the surface area of the packing material, the height
of the column, and the return ratio of the condensed liquid back down the
column, the column can be the same as doing many simple distillations.
Industrial distillation columns can be many stories high and can have over
100 theoretical plates. Because of their size and height, they can often
be seen from a distance. These distillation processes are typically run in
a continuous manner with feedstock constantly entering the column and
fractions continuously being removed from various points along the column.
Fractional distillation by itself is not always suitable for isolation of the
desired component. For example in the separation of BTX, alkanes such as
cyclohexane, n-heptane and others can codistill with benzene and toluene.