What is fractional distillation of petrol?
Petroleum is mixture of hydrocarbon molecules. The molecules are of different sizes and have a different number of carbon atoms. The smaller molecules have fewer number of carbon atoms and have lower boiling points, on the contrary the larger molecules have more number of carbon atoms and have higher boiling points. In its raw state petroleum is difficult ignite and is of very little use. Only if the crude petroleum is 'refined' appropriate fuels and chemicals can be derived.
The refining is done by separating all the different fuels and chemicals by fractional distillation in an oil refinery. Fractional distillation involves heating the crude petroleum, so that at different temperatures the different fuels and chemicals will vaporise and be collected in a pure state. The crude petroleum is heated in a furnace and the oil vaporizes and passes up a column and at different heights the different fuels and chemicals are collected. For instance, petroleum gas comes out at the top of the column as its molecules have the lowest boiling point and lubricating oils come out last because their molecules have higher boiling points. The residue left behind after the process is over is bitumen.
Different engines demand different types of oil with unique properties.
Therefore crude oil is destilled according to its fractions. For more details see answer from lit24.
The composition of oil can vary and depends on its origin. Generally speaking it is a composition of alcanes and aromatic compounds.
The specific fractions are:
C1-C4 (Propane, liquid gas)
C4-C12 (gasoline, petrolether)
C12-C15 (kerosin, heating oil)
C15-C25 (dieseloil, gasoil)
>C25 (wax, asphalt)
Benzol, Toluol, Ethylbenzol, Xylole
Note that the chain lengths increases from C1 to C25 and with it decreases the volatility and water-solubility.
Fractional distillation is a process used for separating more than one liquids from a solution. This process makes use of the difference in boiling points of liquids to be separated. When a mixture of liquids with different boiling points is heated the liquid with lower boiling point starts to boil first and vaporizes at rapid rate. During this process of vaporization the liquid uses the heat applied to the solution for the latent heat of vaporization. This prevents the temperature of the solution to rise above the boiling point of the liquid that is boiling. The vapour thus produced is then condensed to convert it again in liquid, which is called the distillate.
For example, common alcohol boils at 78 degrees C, while water oils at 100 degrees C. Therefore when a mixture of two is heated the alcohol evaporates first, and the distillate thus obtained contains lager proportion of alcohol then water. This distillate can then again be subject to the process further increasing the percentage of alcohol in the distillate. Thus the process is carried out in parts or fraction. This accounts for the name fractional distillation.
As the alcoholic beverages when brewed originally contain substantial percentage of water the process of fractional distillation is used for making distilled alcoholic beverages. Fractional distillation is also used for making petrol. The petrol crude is a mixture of many different types of hydrocarbons mixed together. The process of fractional distillation is used to separate these different ingredients of petroleum crude during the manufacture of petrol and other fuels derived from petroleum crude.