Fractional Distillation and Cracking

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  • Fractional Distillation and Cracking
    • Crude oil is a mixture of mainly different sized alkane hydrocarbons
      • Different fractions condense at different temperatures
        • This can be used to separate the different chains
          • In a fractional distillation unit
            • Oil is preheated and passed into a column
            • Fractions will condense at different heights
              • Temperature of the column decreases upwards
              • Separation depends on boiling point
                • Small molecules condense at the top
                  • Lower temperatures
                • Large molecules condense at the bottom
                  • Higher temperatures
        • Boiling point depends on the size of the molecules
    • Vacuum distillation
      • Lowers the pressure to lower the boiling point
        • Means the heavy substances from the fractionating column can be distilled further
        • Done without higher temperatures
    • Thermal cracking
      • High pressure
        • 7000 kPa
      • High temperatures
        • 400-900
      • Bonds  broken anywhere in the molecule
        • Happens by C-C and C-H fission
      • Produces mostly alkenes
      • Produces Hydrogen
        • Used in the Habour process
    • Catalytic Cracking
      • Slight pressure
        • Cheaper than thermal cracking
      • High temperature 450
        • Cheaper than thermal cracking
      • Zeolite catalyst
        • Crystalline aluminosilicates, clay like substances
      • Produces branched and cyclic alkanes and aromatic hydrocarbons
        • Used for producing motor fuels
    • Cracking
      • Breaking up large alkane molecules
        • Into smaller alkanes and alkenes
          • C-C and C=C
        • C-C
      • Done because the demand for smaller alkanes is higher than the supply
      • Larger alkanes have a big supply
      • Saves from having to extract crude oil to preserves the stock


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