Fractional Distillation

Refining of Petroleum

Meaning

Separation of petroleum into simpler fractions after the removal of unwanted materials.

Method Used

Fractional distillation in a fractionating column.

Principle

Difference in the boiling points of the various fractions of petroleum.

Construction of the Fractionating Column

Fractional Distillation of Oil

The fractionating column is a tall steel tower attached to an electrically heated furnace. Throughout the length of the tower, there are trays with raised holes covered with loose fitting caps called the bubble caps. These caps allow the vapors to rise up and not descend down. The tower has outlet pipes at various levels along it's length.

Process

• Crude petroleum is pumped into the furnace where it is heated to a temperature of about 400^oC. Petroleum is converted to vapors in the furnace.

• The vapors of petroleum are fed into the fractionating column from the bottom.

• As the vapors enter the column, they rise up and gradually get cooled.

• At the bottom of the furnace, the temperature is about 400^oC. Here, the component that has a boiling point close to 400^oC condenses (liquifies) first and collects on the tray. From the tray it is let off through the outlet pipe.

• The remaining vapors rise up through the bubble caps.

• When the vapors reach a particular height in the tower, the fraction that has a boiling point close to the temperature at that height condenses on the tray. The remaining vapors continue to rise upwards.

• Like this, at different levels in the tower, different fractions get condensed, collect on the trays and flow out of the outlet pipes.

The following table shows the boiling ranges and uses

of some fractions of petroleum:

Fractions	Boiling Range	Uses
Petroleum gas	< 40°C	* Domestic fuel (LPG)  * Production of H2  * Production of carbon block
Petrol	40° — 170°C	* Fuel for light motor vehicles  * Solvent for dry cleaning
Kerosene	170° — 250°C	* Domestic fuel stoves  * Illumination — lamps  * Aviation fuel (for aeroplane) (purified)
Diesel	250° — 350°C	* Fuel for HM vehicles (Lorry, B us)  * Generating electricity
Fuel oil	350° — 400°C	* Industrial fuel C Fuel for ships
Lubricating oil (residual oil)	> 400°C	* For lubricating machine parts
Paraffin wax (residual oil)	> 400°C	* Fuel in candles C Shoe polish

 

Note :

* Residual oil is that component of petroleum, which is collected first at the bottom of the tower.  It has boiling range of over 400⁰C. It can be further distilled separately to obtain lubricating oil, paraffin wax,asphalt etc.

* Most fractions of petroleum are fuels.

* Asphalt, lubricating oil and petroleum jelly are not used as fuels.