C9.4 - Cracking hydrocarbons

• Some of the heavier fractions from the fractional distillation of crude oil are not in high demand., however the hydrocarbons in them are made up of large molecules. They are thick liquids/solids with high boiling points making them difficult to vaporise and do not burn easily.
• Oil provides the fuel for most modern transportation, including cars, trucks, trains, and airplanes. Alkenes - The petrochemical industry uses some of the hydrocarbons in oil to make things like polymers, solvents, lubricants and detergents.
• Fortunately, the larger, less useful hydrocarbon molecules can be broken down into smaller, more useful ones in an endothermic process called cracking.
• This process takes place at an oil refinery in steel vessels called crackers. In the cracker, a heavy fraction distilled from crude oil is heated to vaporise the hydrocarbons. The vapour is then either: 1) passed over a hot catalyst 2) mixed with steam and heated to a very high temperature
• Cracking is a thermal decomposition reaction - breaking molecules down by heating them.
• 1. The first step is to heat long-chain hydrocarbons to vaporise them
• 2. Then the vapour is passed over a hot powdered aluminium oxide catalyst
• 3. The long-chain molecules split apart on the surface of the specks of catalyst - this is a cataytic cracking
• 5. You can also crack hydrocarbons if you vaporise them, mix them with steam and then heat them to a very high temperature. This is known as steam cracking.
• Long chain hydrocarbon molecule → shorter alkane molecule + alkene
• Alkenes are useful because they have double bonds which can be used to make polymers such as polyethene (plastic bags).
• Catalytic cracking is endothermic so it requires a lot of energy to run but it's more efficient than steam cracking at producing smaller molecules from longer ones.
• Steam cracking is an exothermic process so it produces lots of energy that can be harnessed to power other processes or generate electricity.
• Cracking produces more gasoline than crude oil does naturally.