The production of fuels from crude oil is the most widely used, and profitable, industrial application of fractional distillation. In its natural form, crude oil is a thick, viscous mixture of saturated hydrocarbons called alkanes. These alkanes range from light compounds such as methane, ethane and propane through medium-weight gasolines and kerosenes up to heavy compounds like heating oil, asphalt and bitumen.
Alkanes are generally nonreactive, so it is impossible to extract a distinct alkane from a mixture using a chemical reactant or catalyst. However, they do have differing boiling points, with the heavier compounds boiling at higher temperatures than the lighter compounds.
Methane and ethane boil at less than 30 degrees Fahrenheit, which is why they are gaseous at room temperature. Gasolines boil between 185 and 400 F, kerosenes between about 400 and 570 F and asphalt boils at over 750 F.
Oil refineries feature large-scale distillation columns for the separation of alkanes. In these columns, the oil mixture is heated very slowly. At certain temperatures, specific fractions of the mixture will boil off. The evaporated fraction is then removed from the column and cooled to give a liquid, which can be further purified by heating in another distillation column.
Fractional distillation can also be used to extract gases from a mixture. Instead of heating, a gaseous mixture is cooled to cause individual gases to condense. This technique is used to extract constituent gases from air. Air comprises 78 per cent nitrogen, yet nitrogen gas is very unreactive, making it impossible to extract using chemicals. However, cooling air to -320 F will cause nitrogen to condense into a liquid. Helium is separated in a similar fashion, however this requires air to be cooled to -457 F, just a couple of degrees above absolute zero.
Most chemistry students are acquainted with laboratory fractional distillation methods. A simple distillation apparatus comprises a vertical column that is connected to a condenser to cool any vapor that has been produced. A mixture of products generated in a chemical experiment can be separated by heating. For example, bromobenzene reacts with nitric acid to produce ortho-bromonitrobenzene and para-bromonitrobenzene. These compounds have different boiling points, so they can be separated using fractional distillation.