Fractional distillation of petroleum

Fractional distillation of petroleum:

The crude petroleum obtained from the well is a dark colored viscous liquid which contains many impurities such as water, solid particles and gases like CH4 and C2H6. To make it useful for various purposes, it must be separated into various components. The process of separating petroleum into useful fractions and removal of undesirable impurities is called refining. The refining of petroleum is done by process known as fractional distillation.

The first process in the refining process is neutralization of crude oil by washing with acidic or basic solution as needed.


Then the oil is heated in a furnace to about 675K and the vapors obtained are introduced to a fractionating tower. The fractionating tower is divided into various compartments by means of shelves having holes. These holes have bubble caps which allow lighter (more volatile) components to escape while the heavier (less volatile) components condense and flow into trays below. Each shelf is provided with an overflow pipe which keeps the liquid to a certain level and allows the rest to flow down to the lower shelf. Therefore fractions with lower boiling points rise up the tower and condense at different levels as per their boiling points. The components with the highest boiling point liquefies first and collected, then in higher column the component having slightly lower boiling point liquefies and so on.

The various fractions of petroleum are given below.

S. No.


Boiling range (K)




113 to 303

C1 – C6


Petroleum ether

303 to 363

C6 – C7


Petrol or Gasoline

343 to 473

C7 – C12



448 to 548

C12 – C15


Gas oil, Fuel oil and diesel oil

523 to 673

C15 – C18


Lubricating oils, greases and petroleum jelly

623 and higher

C16 and higher


Paraffin wax

Melts between 325 to 330

C20 and higher


Petroleum coke


C30 and higher