Refining of petroleum:

Petroleum (crude) extracted from earth's crust is an intimate mixture of many organic compounds. Crude petroleum needs to be purified and its components are to be separated. This process of purification of crude petroleum and separation of its constituents into useful products is called refining of petroleum. The process of petroleum refining involves three steps.

1) Separation of water: Crude petroleum consists of salt water mixed with components of petroleum. Water is separated by allowing crude oil to flow between two highly charged electrodes. Water gets separated and settles on electrodes which is later removed.

2) Removal of sulphur compounds: Crude oil free from water is treated with copper oxide sulfur present in crude oil reacts with copper oxide and precipitates copper sulphate which is removed by filtration.

3) Fractionation: The crude oil which is free from water and sulphur is heated to 400°C in an iron tank. All components of crude oil get evaporated leaving behind a residue called 'asphalt' The vapours are passed through a tall, cylindrical fractionating column. Fractionating column contains large number of horizontal stainless steel trays. Each tray consists of a chimney which is covered with a loose cap. The crude oil vapours go up in the fractionating column and the components are cooled and condensed at different heights depending on boiling points of the components. The components with height boiling points are condensed at the lower trays and the components with lower boiling points are condensed at higher trays.
The products obtained after fractionation are
(a) uncondensed gas - used as fuel
(b) petroleum ether - used as solvent
(c) naptha - used as solvent and fuel
(d) kerosene - used as motor fuel
(e) diesel - fuel for diesel engines
(f) heavy oil - as lubricant
(g) residue - as road tar.