Cracking

It is a process of decomposing higher of the petroleum fraction into lower hydrocarbons with low boiling points by strong heating. The process involves the cleavage of carbon–carbon and carbon-hydrogen bonds and results in the formation of molecules of lower hydrocarbons. By this process, the higher boiling fractions of petroleum, like fuel oil, are converted into gasoline. Thus cracking can be employed to improve the yield of gasoline. The types of hydrocarbons formed depend upon the conditions employed for cracking. In thermal cracking the higher hydrocarbons are heated to a high temperature, whereby lower hydrocarbons are formed. For example,

CH3 (CH2)8CH3 ——— C8H18 +CH2= CH2

Decane Octane Ethylene

Cracking is generally employed for the production of good quality gasoline.

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Cracking can also be carried out at lower temperature by the use of suitable catalysts like silica and alumina. This type of cracking is referred to as catalytic cracking.

Polymerisation

This process involves the combination of small molecules with each other to from bigger molecules. The lower unsaturated hydrocarbon such as C2H4 and C4H8 (which are either present as such in petroleum or produced during cracking) are polymerized into liquid gasoline by heating with suitable catalysts like phosphoric acid.