Fractional Distillation
of Crude Oil
Most fuels contain carbon and hydrogen. When these
fuels burn, carbon dioxide, water and heat energy are
produced. The equation for the combustion of natural
gas, which contains mainly methane (CH ), can be
4
written as:
methane + oxygen → carbon dioxide + water + heat
energy
Fractional distillation
• Fractional distillation differs from distillation only in that it
separates a mixture into a number of different parts,
called fractions. A tall column is fitted above the mixture,
with several condensers coming off at different heights.
The column is hot at the bottom and cool at the top.
Substances with high boiling points condense at the
bottom and substances with low boiling points condense at
the top. Like distillation, fractional distillation works
because the different substances in the mixture have
different boiling points.
Crude oil/Petroleum
• Petroleum and crude oil is a dark brown, foul smelling liquid. It
is a mixture of different hydrocarbons. Different fractions can
be separated by fractional distillation.
• Process: Crude oil is heated and vaporized sample is passed
through the fractionating column which is cool at the top and
hot at the bottom.
• Fractions can be separated due to their different boiling points.
• The smaller hydrocarbons with lower boiling points condenses
at the top and larger hydrocarbons with higher boiling points
condenses at the bottom of the column.
• In the fractionating column,
• (a) decreasing chain length
• (b) higher volatility
• (c) lower boiling points
• (d) lower viscosity
• As the number of carbon atoms in hydrocarbon molecules increases, the
physical properties of the compounds change. Most of these changes are
the result of increasing attractions between neighbouring molecules.
• As the molecules become bigger, the intermolecular forces of attraction
become stronger and it becomes more difficult to pull one molecule
away from its neighbors.
• As the molecules become bigger, the following changes occur.
• Boiling point increases: the larger the molecule, the higher the boiling point.
This is because large molecules are attracted to each other more strongly
than smaller ones. More energy is needed to break these stronger
intermolecular forces of attraction to produce the widely separated molecules
in the gas.
• The liquids become less volatile: the bigger the hydrocarbon, the more
slowly it evaporates at room temperature. This is again because the bigger
molecules are more strongly attracted to their neighbours and so don't turn
into a gas so easily.
• The liquids become more viscous and flow less easily: liquids containing
small hydrocarbon molecules are runny. Those containing large molecules
flow less easily because of the stronger forces of attraction between their
molecules.
• The liquids become darker in colour.
• Bigger hydrocarbons do not burn as easily as smaller ones. This limits the
use of the bigger ones as fuel.
Uses of fractions
• Note that the gases condense at the top of the column,
the liquids in the middle and the solids stay at the
bottom.
• The main fractions include refinery gases, gasoline
(petrol), naphtha, kerosene, diesel oil, fuel oil, and a
residue that contains bitumen. These fractions are
mainly used as fuels, although they do have other uses
too.
• Hydrocarbons with small molecules make better fuels
than hydrocarbons with large molecules because they
are volatile, flow easily and are easily ignited.
