The crude petroleum oil is a complex mixture of several hydrocarbons with water, salt and earth particles. So, before it can be used, it has to be purified. The process of separating crude petroleum oil into more useful fractions of hydrocarbons is called refining. The refining of petroleum is done by process of fractional distillation.
The crude petroleum oil is heated to about 400 degree in a furnace and fed into the fractionating column from near its base. At this temperature, all the components of petroleum except asphalt vaporize. As the mixture of hot vapors of hydrocarbons rises in the column, it cools. The hydrocarbon fraction with the highest boiling point liquefies first and is collected. Then a little higher in the column, the hydrocarbon fraction having a little lower boiling point liquefies and so on. The uncondensed gases, which do not get liquefies in the fractionating column, are taken out from the upper part of the column of petroleum gas. The various fractions thus separated are easily collected.
The various fractions obtained by the fractional distillation column upward are: Asphalt, Paraffin Wax, Lubricating Oil, fuel Oil, Diesel, kerosene, petrol or Gasoline and petroleum Gas. Asphalt, paraffin Wax and lubricants are not used as a fuel. Diesel is used to drive heavy vehicles.