Fractional distillation allows us to separate different lengths hydrocarbons, mainly alkanes, from a mixture, as they are found in crude oil. Different hydrocarbons have different uses:

Fraction

Length of atom chains

Uses

Gas

1-4

Bottled gases for camping etc.

Petrol

5-12

Petrol for combustion engines

Naphtha 

7-14

Petrochemicals

Kerosene

11-15

Plane fuels

Diesel

15-19

Combustion engines

Mineral Oils

20-30

Lubricating oils

Fuel Oils

30-40

Ship fuel and Power stations

Wax

40-50

Candles and lubrication

Bitumen

50+

Road surfaces

Cracking in Industry

  • Crude oil is heated to 350°c and pumped into the bottom of the distillation column
  • The crude oil vaporises, the smaller hydrocarbon molecules with their lower boiling points, rise and condense higher up the column
  • Going down the column the different hydrocarbon chains grow larger, going from gases to more viscous substances such as bitumen. The largest hydrocarbons do not vaporise as their boiling points are higher than 350°c and therefore collect at the bottom forming a thick residue.
  • As the crude oil rises it reaches its fractions, and being far away from the heat source and falls below its boiling point and condenses on to it. This follows a temperature gradient
  • Some gases do not do condense (e.g. propane, ethane) and are just collected at the top.

Cracking in a lab

The difference between industry and on a smaller scale in a lab is minimal however necessary due to the limitation of height and temperature.

  • Put the hydrocarbon mixture in the bottom of the distillation column.
  • Using an electric mantle, as they will heat the mixture to a constant set temperature which a Bunsen burner cannot achieve easily, heat the mixture
  • The mixture that you want to obtain from the set temperature will vaporise and rise up the column.
  • At the top of the column a condenser is used to lower the temperature of the vaporised liquid down, so it can therefore condense back into a liquid. The condenser has a cold circulation of water running through it.
  • A collection beaker is used to collect the required fraction.
  • After no more of the fraction is being collected, raise the temperature and use a different beaker to collect the next fraction.