This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
Crude oil contains a wide range of hydrocarbons, organometallics and other
compounds containing sulfur, nitrogen etc. The HCs contain between 1 and 60
carbon atoms. Gasoline contains hydrocarbons with carbon atoms between 3 and
12, arranged in specific ways to provide the desirable properties. Obviously,
a refinery has to either sell the remainder as marketable products, or
convert the larger molecules into smaller gasoline molecules.
A refinery will distill crude oil into various fractions and, depending on
the desired final products, will further process and blend those fractions.
Typical final products could be:- gases for chemical synthesis and fuel
(CNG), liquified gases (LPG), butane, aviation and automotive gasolines,
aviation and lighting kerosines, diesels, distillate and residual fuel oils,
lubricating oil base grades, paraffin oils and waxes. Many of the common
processes are intended to increase the yield of blending feedstocks for
Typical modern refinery processes for gasoline components include
* Catalytic cracking - breaks larger, higher-boiling, hydrocarbons into
gasoline range product that contains 30% aromatics and 20-30% olefins.
* Hydrocracking - cracks and adds hydrogen to molecules, producing a
more saturated, stable, gasoline fraction.
* Isomerisation - raises gasoline fraction octane by converting straight
chain hydrocarbons into branched isomers.
* Reforming - converts saturated, low octane, hydrocarbons into higher
octane product containing about 60% aromatics.
* Alkylation - reacts gaseous olefin streams with isobutane to produce
liquid high octane iso-alkanes.
The changes to the US Clean Air Act and other legislation ensures that the
refineries will continue to modify their processes to produce a less
volatile gasoline with fewer toxins and toxic emissions. Options include:-
* Reducing the "severity" of reforming to reduce aromatic production.
* Distilling the C5/C6 fraction ( containing benzene and benzene precusers )
from reformer feeds and treating that stream to produce non-aromatic high
* Distilling the higher boiling fraction ( which contains 80-100% of
aromatics that can be hydrocracked ) from catalytic cracker product .
* Convert butane to isobutane or isobutylene for alkylation or MTBE feed.
Some other countries are removing the alkyl lead compounds for health
reasons, and replacing them with aromatics and oxygenates. If the vehicle
fleet does not have exhaust catalysts, the emissions of some toxic
aromatic hydrocarbons can increase. If maximum environmental and health
gains are to be achieved, the removal of lead from gasoline should be
accompanied by the immediate introduction of exhaust catalysts and
sophisticated engine management systems,