Petrochemicals - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 49

radicals can react further and from heptane, eventually several smaller
molecules may be obtained. With a petroleum distillate, the chemistry is even
more complex. The cracking feedstock might have several dozen different
hydrocarbons. However, the chemistry is generally the same. Enough energy
is supplied to cause homolytic bond cleavage. The resultant radicals react
further. The cracked product consists essentially of smaller compounds
and an increase in unsaturation. The cracked product is then separated by
distillation. It is a source of a low boiling fraction containing hydrogen and
methane gas. Subsequent fractions, in order of increasing boiling point are:
ethylene; propylene; a C4 fraction; and then higher boiling fractions.
The C4 fraction will have 1,3-butadiene and other C4 hydrocarbons.
Cracking conditions will have a major effect on the composition. The
C4 components have similar boiling points so the 1,3-butadiene cannot
be separated by a normal distillation process. Rather a technique known
as extractive distillation is used. A solvent such as N-methylpyrrolidone
(NMP) is added to decrease the volatility of the 1,3-butadiene. The high
boiling fraction exits the bottom of the column and is the 1,3-butadiene and
extracting solvent. This high boiling fraction is then separated by boiling
and distillation to give 1,3-butadiene. The 1,3- butadiene is used to prepare
many polymers including synthetic rubbers for a variety of uses including
automotive tires. The raffinate, or material that remains after the extractive
distillation, is the low boiling material that exits the top of the column.

N O

CH 3

1,3- butadiene NMP

The raffinate includes isobutene, also known as isobutylene, as the largest
component followed by 1-butene and then 2- butene, both cis and trans
isomers. Saturated butanes, n-butane and isobutane, are also present.

isobutene 1-butene trans-2-butene cis-2-butene n-butane isobutane

Another process is reforming. Reforming is done with petroleum fractions
that have an octane rating too low to be suitable for gasoline. Gasoline octane
rating is an indication of the propensity of the gasoline/air mixture to prema-
turely self-ignite in the combustion chamber of the engine. This premature
ignition exhibits itself as engine knocking. Isooctane, 2,2,4-trimethylpentane,
has an octane rating of 100 and n-heptane has an octane rating of 0. Gasoline