SS1 Chemistry Lesson Note on Cracking and Reforming of Petroleum Fractions; Knocking and Octane Rating

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CRACKING OF PETROLEUM FRACTIONS, REFORMING, OCTANE NUMBER AND KNOCKING

CRACKING AND REFORMING

The quality of petrol obtained from the Fractional distillation of crude oil is about 20% by volume.

Cracking is therefore employed so as to provide more petrol, since its demand is high and as a source of alkenes.

CRACKING: cracking is the process whereby large molecules of heavy oils are broken into smaller molecules (light oils) at high temperature, pressure and in the presence of a catalyst. Usually, petroleum fractions with more than eleven carbon atoms in their molecules can be cracked.

During cracking, ethene, C₂H₄ is produced; it is the major raw material in the petrochemical industry.

CRACKING TECHNIQUES

The two main cracking processes are: thermal cracking and catalyst cracking.

There is also a less common cracking known as hydro cracking.

1. THERMAL CRACKING: Thermal cracking involves heating heavy oils, such as lubricating oil, diesel oil and kerosene to about 600⁰C under pressure; it leads to the decomposition and formation of a mixture of more volatile alkanes, alkenes and hydrogen. The mixture is then separated by fractional distillation. e.g.

 

2. CATALYTIC CRACKING: in catalytic cracking, the heavy oil is heated to about 500⁰C under reduced pressure and in the presence of natural clay (a mixture of silicon (iv) oxide and aluminium oxide) as the catalyst. Under these conditions, a mixture of alkanes and alkenes, which can be separated by fractional distillation, is obtained e.g.

C₁₈H₃₈      →         C₆H₁₄ + C₈H₁₆ +2C₂H₄

Note: note that the products of cracking depend on the starting material and the reaction conditions, such as temperature, pressure and the nature of the catalyst used.

In hydro cracking, hydrogen is passed into the reaction mixture during the cracking process. Under this condition, only the lower saturated alkanes are obtained.

REFORMING: In reforming the molecules do not crack but are reformed or reshaped. In reforming straight chain alkanes are converted catalytically into branched chain isomers, cycloalkanes and benzene, the purpose is to convert low grade petro to higher grades, i.e. improve the quality. Reforming techniques includes: ISOMERISATION, CYCLIZATION AND AROMATIZATION

DIFFERENCE BETWEEN CRACKING AND REFORMING

1. Cracking involves breaking large molecules of petrol fractions into smaller molecule, while reforming involves rearrangement of atoms in the molecules of petroleum fraction to obtain branched and cylic hydrocarbons.
2. Cracking can be achieved thermally or catalytically, while reforming is in the presence of a catalyst.
3. Cracking occurs at a temperature higher than that of reforming.
4. Cracking is employed to increase the quantity of petrol, while reforming is to improve its quality.

OCTANE NUMBER OR RATING

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