How does fractional distillation differ from simple distillation?

Distillation is a process used to purify a liquid that is
contaminated in some manner. In simple distillation, you are dealing with one volatile
material tht you want to have by itself.  Distillation involves heating the liquid to
its boiling point, then condensing the vapors, and collecting the purified
liquid.

For example, you may have some water that also
contains salt or other substances but you want just pure water. By using the
distillation process you can end up with pure water.  You would put the salt water in a
container, heat the container to boil the water, and then pass the steam through a
condenser (a cooling coil) where the steam is converted back to liquid water. The cooled
water is then recovered as a pure substance.

In fractional
distillation you generally have a mixture of volatile substances with different boiling
points. A good example is crude oil which contains a wide variety of different
substances with a wide range of boiling points. The crude oil is introduced into the
bottom of the fractionation tower and heated.  As the various materials in the crude oil
reach their respective boiling points they evaporate and rise up into the tower. As they
rise, the substances with the highest boiling points cool back to liquids first, while
those with the lowest boiling points rise the highest before condensing. By removing the
various condensed liquids at different points along the height of the fractionation
tower, the original crude oil can be fractionated into useable
materials.

Depending on the purity of the materials
required, a number of fractionations may be done to isolate the specific substances
desired.