The crude oil obtained from earth’s crust needs to be refined before using it. It is done by a method called fractional distillation which involves separation of two or more liquids having different boiling points. The refining is done by changing the liquid into its gaseous state and then cooling the gaseous mixture at controlled temperatures such that at a time only one of the vapours having boiling point I liquefies first. This is generally done by a special tower called fractionating column. Fractional distillation of petroleum
Crude petroleum is heated to a temperature of 400°C in a furnace. The vapors of petroleum are fed to a fractionating column near Its bottom. As the vapour rises up in the column, it loses heat and hence its temperature drops. Thus the fractioning column maintains different temperatures which decrease in magnitude in upward direction. Due to this difference in temperature the vapors with higher boiling point rise up and condense In different parts of the fractionating column. The vapours or gases which do not liquefy are taken out from the top of fractioning column. in fractional distillation of petroleum, the various products are liquefied at following temperatures a) Residual oil at 400°C taken out from bottom of the column. b) Fuel oil between 350°C to 400°C.
c) Diesel oil between 250°C to 350°C.
d) Kerosene oil between 170°c to 250°C
e) Gasoline or petrol between 40°C to 170°C
l Petroleum gas which does not liquefy or condense is taken out from top of the fractionating column. All other fractions are removed from their respective out lets. Note : Each fraction obtained during the fractional distillation is not a single hydrocarbon but is a mixture of number of hydrocarbons. These hydrocarbons differ in number of carbon atoms present in their molecules.