The petroleum to be refined is first heated in a special furnace, called pipe still to about 371-426oC. The pressure in the pipe is so adjusted as to avoid evaporation of the crude oil.
The hot petroleum from the pipe still then goes to bottom of all fractionating tower. Here the pressure is released and the liquid vaporizes. The vapors go up the fractionating tower where hydrocarbons that have a high boiling temperature range are easily condensed to liquids in the lower part of the tower.
The fractions like gasoline which have a lower boiling temperature range continue to move up the tower as vapors which are then led o condensers where change to liquid state. Condensed liquid is collected on shelves and then drained off to separate storage tanks. The fractions obtained in this process are not completely separated from one another and yet each fraction contains some impurity of the next lower fraction. To obtain a better separation each of the liquid obtained is redistilled and further treated chemically to remove impurities.