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Chemical analysis and production
Gasoline is produced in oil refineries. Material that is separated from crude
oil via distillation, called virgin or straight-run gasoline, does not meet the
required specifications for modern engines (in particular octane rating; see
below), but will form part of the blend.
The bulk of a typical gasoline consists of hydrocarbons with between 5 and 12
carbon atoms per molecule.
Many of these hydrocarbons are considered hazardous substances and are regulated
in the United States by OSHA. The Material Safety Data Sheet for unleaded
gasoline shows at least fifteen hazardous chemicals occurring in various
amounts. These include benzene (up to 5% by volume), toluene (up to 35% by
volume), naphthalene (up to 1% by volume), trimethylbenzene (up to 7% by
volume), MTBE (up to 18% by volume) and about 10 others.[5]
The various refinery streams blended together to make gasoline all have
different characteristics. Some important streams are:
Reformate, produced in a catalytic reformer with a high octane rating and high
aromatic content, and very low olefins (alkenes).
Cat Cracked Gasoline or Cat Cracked Naphtha, produced from a catalytic cracker,
with a moderate octane rating, high olefins (alkene) content, and moderate
aromatics level. Here, "cat" is short for "catalyst".
Hydrocrackate (Heavy, Mid, and Light), produced from a hydrocracker, with medium
to low octane rating and moderate aromatic levels.
Virgin or Straight-run Naphtha (has many names), directly from crude oil with
low octane rating, low aromatics (depending on the crude oil), some naphthenes (cycloalkanes)
and no olefins (alkenes).
Alkylate, produced in an alkylation unit, with a high octane rating and which is
pure paraffin (alkane), mainly branched chains.
Isomerate (various names) which is obtained by isomerising the pentane and
hexane in light virgin naphthas to yield their higher octane isomers.
(The terms used here are not always the correct chemical terms. They are the
jargon normally used in the oil industry. The exact terminology for these
streams varies by refinery and by country.)
Overall a typical gasoline is predominantly a mixture of paraffins (alkanes),
naphthenes (cycloalkanes), aromatics and olefins (alkenes). The exact ratios can
depend on
the oil refinery that makes the gasoline, as not all refineries have the same
set of processing units.
the crude oil used by the refinery on a particular day.
the grade of gasoline, in particular the octane rating.
Currently many countries set tight limits on gasoline aromatics in general,
benzene in particular, and olefins (alkene) content. This is increasing the
demand for high octane pure paraffin (alkane) components, such as alkylate, and
is forcing refineries to add processing units to reduce the benzene content.
Gasoline can also contain some other organic compounds: such as organic ethers
(deliberately added), plus small levels of contaminants, in particular sulfur
compounds such as disulfides and thiophenes. Some contaminants, in particular
thiols and hydrogen sulfide, must be removed because they cause corrosion in
engines.
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