Most of the energy used worldwide comes from coal, crude oil and natural gas. Crude oil is a mixture of mostly hydrocarbons - mostly alkanes. It is converted into a small number of fractions by fractional distillation.Crude oil vapour is passed into the bottom of a fractionating column which has a temperature gradient and is cooler at the top than at the bottom. The vapour rises up the column through a series of bubble caps and condenses at different heights where the temperature matches the temperature range of the molecules in the fraction. Smaller molecules condense at lower temperatures.