C09.04: Cracking Hydocarbons

• Cracking is a reaction in which larger saturated hydrocarbon molecules are broken down into smaller, more useful hydrocarbon molecules, some of which are unsaturated, by vapourising them and passing over a hot catalyst.
• The original starting hydrocarbons present in crude oil are alkanes. The products of cracking include alkanes and alkenes, members of a different homologous series.
• Catalytic cracking uses a temperature of approximately 550 degrees Celsius and a catalyst known as zeolite which contains aluminium oxide and silicon oxide.
• Steam cracking uses a high temperature of over 800 degrees Celsius and no catalyst.
• The size of hydrocarbon molecules decreases during cracking.
• The temperature range used in catalytic cracking is typically between 450 to 550 degrees Celsius.
• Cracking is significant in the production of fuels because it breaks down larger hydrocarbon molecules into smaller ones, which are more useful for making gasoline and other high-demand fuels.
• The purpose of using a catalyst in catalytic cracking is to increase the rate of the cracking reaction and improve the efficiency of the process.
• Cracking increases the resulting fuels' volatility and octane rating (stability - based on the pressure at which a fuel will spontaneously combust in a testing engine).
• Reasons for cracking:

  • It helps to match the supply of fractions with the demand for them
  • It produces alkenes, which are useful as feedstock for the petrochemical industry
• The supply is how much of a fraction an oil refinery produces. The demand is how much of a fraction customers want to buy. Very often, fractional distillation of crude oil produces more of the larger hydrocarbons than can be sold and less of the smaller hydrocarbons that customers want.

  Smaller hydrocarbons are more useful as fuels than larger hydrocarbons. Since cracking converts larger hydrocarbons into smaller hydrocarbons, the supply of fuels is improved. This helps to match supply with demand.
• Cracking in the lab: