Obtaining Products from Crude Oil
Specification Point 8.16:
Explain how cracking involves the breaking down of larger, saturated hydrocarbon molecules (alkanes) into smaller, more useful ones, some of which are unsaturated (alkenes)
- Saturated molecules contain single bonds only whereas unsaturated molecules contain double bonds between their carbon atoms.
- Alkanes are saturated compounds and alkenes are unsaturated compounds.
- Alkenes are generally more desirable than alkanes as they are more reactive due to the presence of the carbon carbon double bond, so they can take part in reactions in which alkanes cannot.
- Long chain alkane molecules are further processed to produce other products consisting of smaller chain molecules.
- A process called catalytic cracking is used to convert them into short chain molecules which are more useful.
- Small alkenes and hydrogen are produced using this process.
- Kerosene and diesel oil are often cracked to produce petrol, other alkenes and hydrogen.
Picture showing where industrial catalytic cracking takes place
- Cracking breaks large hydrocarbon molecules down into smaller, more useful molecules.
- Fractions containing large hydrocarbon molecules are heated at 600 – 700°C to vaporise them.
- Vapours will then pass over a hot catalyst of aluminium oxide.
- This process breaks covalent bonds in the molecules, causing thermal decomposition reactions.
- As a result, cracking produces smaller alkanes and alkenes. The molecules are broken up in a random way which produces a mixture of alkanes and alkenes.
- Hydrogen and a higher proportion of alkenes are formed at temperatures of above 700ºC and higher pressure.
Decane is catalytically cracked to produce octane for petrol and ethene for ethanol
Use of Small Chain Hydrocarbons
Specification Point 8.17:
Explain why cracking is necessary
- Although there is a use for each fraction obtained from the fractional distillation of crude oil, the amount of longer chain hydrocarbons separated out from crude oil far outweighs global demands.
- Smaller chain molecules are much more versatile and tend to ignite easier, making them much more useful as fuels.
- Small alkene molecules are used in the production of ethanol, which is the raw material for the manufacture of alcoholic beverages.
- They are also the precursor in the production of many types of plastics and resins via polymerisation reactions.
- Therefore if cracking wasn’t performed there would be a discrepancy between global supply and demand for fuels and other substances produced from small chain hydrocarbon molecules.
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