Chemistry SS1: Cracking and Reforming

Cracking

• Cracking is the process of breaking down large hydrocarbon molecules into two or more smaller hydrocarbon molecules.
• It is used to increase the quantity of petrol, as the fraction from which petrol is produced (C6 - C12) is small compared to other fractions with a greater number of carbon atoms.
• There are two types of cracking in use in the petroleum industry:
  • One type is more controllable, allowing for desirable products of certain chain lengths to be obtained.
  • The other type yields a higher grade of petrol than the one obtained directly from the petrol fractions during the distillation of crude oil.

Benefits of Cracking

• It increases the yield of petroleum.
• It provides a petrol mixture rich in branched-chain hydrocarbons with an attendant increase in octane number.
• It yields as a by-product, a large quantity of ethane, propane, butane, etc. used for making plastics, synthetic rubber, detergent, and many important chemicals like ethanol and phenol.

Reforming

• Reforming is the process of converting long-chain hydrocarbons to shorter and branched-chain molecules to improve its anti-knock properties.
• It usually takes place in the presence of catalysts such as oxides of silicon and aluminium at about 6000C and pressure between 8 and 15 atm.
• Reforming is an isomerisation process, changing a compound into its isomers.

Octane Number

• The octane number or octane rating of petrol is a measure of the proportion of branched-chain hydrocarbons to the straight-chain hydrocarbons in a given blend of gasoline (petrol).
• Straight-chain hydrocarbons (e.g. n-heptane) burn too rapidly in the car engine, causing irregular motion of the pistons and resulting in rattling noise (knocking).
• Branched-chain hydrocarbons (e.g. 2,2,4 trimethylpentane) burn very smoothly in engines.
• Octane number is a measure of the performance of the fuel in engines and is rated according to the percentage of branched-chain hydrocarbons to straight-chain hydrocarbons present.