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Fractional Distillation

IGCSE 0620 · Separation · Crude oil
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Refinery column Running
    Plant consoleContinuous process

    1 · Furnace

    Steady

    The crude is heated to about 400 °C in the furnace and enters near the bottom as a hot mixture of liquid and vapour.

    2 · Pick a fraction

    Drag the plant to turn it. Tap a numbered take-off on the column, or a card beside it, to read that fraction's boiling range, molecule size and use.

    The theory Crude oil
    Quick check
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    What this experiment shows

    Crude oil is a mixture of hydrocarbons with a huge spread of boiling points, and a fractionating column separates it into fractions - groups of molecules with similar chain lengths and similar boiling ranges. The column is hot at the bottom, where the crude enters at about 400 °C, and cool at the top. Each vapour rises until it reaches a tray cold enough for it to condense, and is drawn off there. Short chains have weak forces between them, so they boil low and travel high; long chains condense almost at once near the base.

    refinery gas → petrol → naphtha → kerosene → diesel → fuel oil → lubricating oil → bitumen

    What you need

    • An industrial fractionating column, tens of metres tall, with bubble-cap trays
    • A furnace to heat the crude to about 400 °C
    • Draw-off pipes and receiving tanks at each height
    • In the lab: a fractionating column, thermometer, condenser and receiver

    How it is done

    1. Heat the crude oil in a furnace until most of it is vapour.
    2. Feed it into the column near the bottom.
    3. Let the vapour rise through the trays, condensing and re-boiling at each one.
    4. Draw off each fraction at the height where its boiling range is reached.
    5. Take the residue, bitumen, from the bottom of the column.

    What you should see

    • Refinery gas, below 20 °C, 1 to 4 carbons - bottled gas.
    • Petrol, 20 to 70 °C, 5 to 10 carbons - fuel for cars.
    • Kerosene, 120 to 200 °C, 10 to 16 carbons - fuel for aircraft.
    • Diesel, 200 to 300 °C, 14 to 20 carbons - lorries and diesel engines.
    • Bitumen, above 400 °C, more than 40 carbons - road surfacing.

    Where marks are lost

    • Saying longer molecules are heavier so they sink. It is intermolecular forces, not weight, that fix the boiling point.
    • Calling a fraction a pure compound. It is a group of hydrocarbons with a similar range.
    • Putting the fractions in the wrong order. Shortest chains leave at the top.
    • Saying the column is hot at the top. It is coolest at the top.

    Questions students ask

    Why are long-chain hydrocarbons taken off near the bottom?
    A longer chain touches its neighbours over more of its length, so the intermolecular forces are stronger and more energy is needed to separate the molecules. They condense first, low down where the column is hottest.
    Is a fraction a pure substance?
    No. It is a mixture of hydrocarbons whose boiling points fall in a narrow range, which is why each one is quoted as a range rather than a single value.
    What happens to the fractions nobody wants?
    Long-chain fractions are cracked into shorter, more useful molecules such as petrol and alkenes for making plastics.

    Try these next

    Written for Cambridge IGCSE Chemistry 0620 and CBSE Class 9-12 by Ajay Shekhawat, founder of MrChemCoach. Run the simulator above, then check yourself against the questions.