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The petroleum is refined by subjecting the crude oil to fractional distillation .Crude petroleum is heated to a temperature of 400⁰ c or slightly above in a furnace .The vapours of petroleum are fed into a tall fractionating column near to its bottom .As the vapours rise up in the column ,they lose heat and hence their temperature drops .Thus ,fractionating column maintains itself  at different temperatures which decrease in magnitude in the upward direction .Due to this difference in temperature ,the vapours with higher boiling point condense first ,but the vapours with the lower boiling point  condense in different parts of fractionating column .The vapours (gases) which do not liquefy are taken out from the top of fractionating column .

In the fractional distillation of petroleum ,the residual oil at liquefies at 400 ⁰C ,fuel oil at

350^oC -400 ⁰C ,diesel oil at 250 ⁰ C -350 ⁰C ,kerosene oil at 170 ⁰C -250⁰C  and gasoline at 40 ⁰ C -170 ⁰ C .

Petroleum gas which do not condense is  taken out from the top of the fractionating column .All other fractions are removed from their respective outlets .