Crude oil has many compounds that boil at different temperatures. Useful fuels can be extracted as long as when it is heated, the products of the crude oil is seperated.
Mixtures can we seperated with distillation. Compunds inside the mixture that have different boiling points are named fractions.
Most of the compounds in crude oil are hydrocarbons which only contain hydrogen and carbon. And many of these hydrocarbons are alkanes.
Alkanes have the formula: C(n)H(2n+2)
N = 2 for Ethane
N = 3 for Propane
N = 4 for Butane
We already know that crude oil is seperated into fractions using distillation, but together this is known as "fractional distillation". The reason behind this is that the hydrocarbons in crude oil are different sizes (the bigger the hydrocarbon, the higher the boiling point).
The oil is vaporised before it enters the column. The column then has many fractions at which the energy in the hydrocarbons will be lost and a certain hydrocarbon will fall into a certain fraction due to it's boiling point.