Fractional Distillation in the Laboratory

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19/12/2013
Fractional Distillation in the Laboratory

• L.O.
• I can carry out the fractional distillation of
  crude oil safely in the laboratory.
• I can recognise the trends in the properties of
  the fractions relating to their boiling points.

728 PM
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Starter / Settler Activity

• In backs of books put the fractions in the right
  order starting at the top of the fractionating
  column.

Fuel gas
Fraction Boiling Range (oC)
Kerosene 150 - 240
Diesel 220 275
Petrol 40 - 175
Bitumen gt350
Fuel gas Below 40
Lubricating oil 250-350
Petroleum
Kerosene
Diesel

Lub. Oil
Bitumen
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The boiling points of molecules

• We have already said that, in general, the bigger
  the molecule the higher the boiling point.

4

• Here are the boiling ranges of some fractions
  obtained from distillation of petroleum.
• Using the graph on the handout, complete the
  table by estimating the size range of the
  molecules present in each fraction.

Fraction Boiling Range (oC) Number of carbons
Fuel gas Below 40
Petrol 40 - 175
Kerosine 150 - 240
Diesel 220 - 275
1-5
5-10
9-14
13-17
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• When hydrocarbons burn they are reacting with
  oxygen in the air.
• In general, the smaller the molecule the better
  it will mix and then react with the air.
• Flash points are (roughly) the temperature below
  which you cannot ignite the substance.
• Here is some flash points data for oil fractions.
  Link the molecule to the flash point.

Substance Flash Point
Kerosene lt23
Petrol 85 - 150
Fuel Oil 23 - 61
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The viscosity of molecules

• When we pour a liquid the molecules have to pass
  each other. The easier they do this, the runnier
  the liquid is.
• The molecules in fuel oil and bitumen may contain
  up to 400 carbon atoms in long chain structures.
• These chains easily become entangled reducing the
  runniness of the liquid and so those made up of
  big molecules tend to be viscous (treacly) not
  runny.

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The viscosity of molecules

• In contrast, small molecules are more spherical
  and tend not to get tangled.
• They are therefore likely to have a low viscosity
  (be runny).

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Fractional Distillation of Crude Oil in the
Laboratory

• Method
  We heated
• 
  the crude
• 
  oil to
• 
  various
• 
  temperatures
• 
  and collected
• 
  the fractions.

thermometer
crude oil
Fractions collected
heat
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Results
Temp (0C) Appearance Viscosity Ease of lighting Type of flame
Up to 80
80 150
150 250
Over 250
colourless
runny
easy
clean
yellow
thicker
harder
dirty
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• Here are the viscosities of 5 hydrocarbons.
• The higher the value the less runny the
  hydrocarbon is. Draw arrows joining the
  hydrocarbon with the appropriate value.

Hydrocarbon Viscosity (cP)
Pentane - C5H12 0.40
Hexane - C6H12 0.52
Heptane - C7H16 0.24
Octane - C8H18 0.92
Decane - C10H22 0.70
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Industrial Distillation of Crude Oil

• L.O.
• I know how crude oil is fractionally distilled
  industrially

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The colour of molecules

• Small hydrocarbon molecules are gases or
  transparent liquids.
• As the molecules get larger the colour becomes
  increasingly yellow through to the brown/black
  colour of bitumen used on roads and roof repairs.

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• Which is a correct statement about crude oil?
• Formed by the decay of dead sea creatures.
• Consist of a mixture of very large molecules.
• Is purified in an oil rig.

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• Which is a correct statement about fractional
• distillation?
• Oil is separated into fractions with the same
  size molecule.
• Oil is separated into fractions with the same
  density.
• Oil is separated into fractions with similar size
  molecules.

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• Which is a correct order for these fractions
  working down from the top of the column?
• Fuel gas, kerosine, petrol, diesel, bitumen.
• Fuel gas, diesel, kerosine, petrol, bitumen.
• Fuel gas, petrol, kerosine, diesel, bitumen.

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• Which of these is a true statement about the
  changes that occur as hydrocarbon molecules get
  larger?
• Boiling point decreases.
• Viscosity increases.
• Flammability increases.