The crude oil obtained from earth's crust is refined by following steps.

Step 1: separation of water

Cottrell's process is used for separation of water. Crude oil is an emulsion of oil and salt water. The crude is allowed to flow between two highly charged electrodes. The colloidal water droplets combine to form bigger drops and separates out from the oil.

Step II:    removal of sulphur compounds

Sulphur compounds can be removed by treating it  with copper oxide. Sulphur compounds react to form copper sulphide (solid) which can be removed by filtration.

Step III:   fractional distillation

The crude oil after the removal of water and sulphur compounds is heated to about 400⁰C. except the residue i.e. asphalt and coke, all the constituents volatilizes. These vapours are passed in a fractionating column. Fractionating columns is a tall cylindrical tower containing a number of horizontal stainless steel trays. The tray is provided with a small chimney, covered with lose cap.

As the vapours rises up in tower, they becomes cool and condense. The compounds having higher boiling points are condensed at the lower part of column. Whereas the compounds having lower boiling fractions will condense at top of the tower.