Describe the separation of petroleum into useful fractions by fractional distillation

Fractional Distillation of Petroleum

Objective (AO1‑AO3)

• Explain how crude oil is separated into useful fractions by fractional distillation.
• Identify the main fractions obtained and state their typical uses (as required by the syllabus).
• Design a simple laboratory set‑up, record temperature data safely and interpret the results.

Syllabus Context (Unit 11 – Fuels)

• 11.3 Fuels – identification of fossil fuels, composition of petroleum and uses of the fractions.
• Links to other fuel topics (required only at a brief level):
  • 11.4 Alkanes – gasoline consists mainly of C₄–C₁₂ alkanes.
  • 11.5 Alkenes – large alkanes are thermally cracked to give smaller alkanes and alkenes before/distillation.
  • 11.6 Alcohols – ethene from cracking can be hydrated to ethanol.
  • 11.7 Carboxylic acids – ethene oxidation gives acetic acid.
  • 11.8 Polymers – ethene polymerises to polyethylene.

1. Fossil Fuels – Quick Overview (Unit 11 focus)

Only petroleum (crude oil) is examined in this unit. The other two fossil fuels are listed for context.

2. Composition of Petroleum

• Hydrocarbons
  • Alkanes (paraffins) – straight‑chain and branched.
  • Cyclo‑alkanes (naphthenes).
  • Alkenes (olefins) – present in small amounts or produced by cracking.
  • Aromatics – benzene, toluene, xylene (minor).
• Non‑hydrocarbon impurities – sulphur, nitrogen, oxygen, metals and water.
• These components differ markedly in boiling point, which is the basis for fractional distillation.

3. Purpose of Pre‑heating

• Crude oil is first heated to about 150 °C:
  • To vaporise and remove water (which boils at 100 °C) and very light gases (methane, ethane, propane, butane).
  • Removing these components prevents foaming in the column and improves the efficiency of the subsequent fractionation.

4. Main Fractions Obtained (ordered as in the syllabus)

Note: The ranges are approximate; actual values vary with the source of crude oil and the operating pressure of the column.

5. Equipment for Fractional Distillation (AO2) – with Justification

• Reboiler (bottom heater) – supplies heat to generate vapour from the crude oil.
• Fractionating column (packed with glass beads or structured packing) – provides many theoretical plates; each plate gives a condensation‑evaporation cycle that sharpens the separation.
• Thermometers / temperature probes
  • Bottom thermometer – monitors the temperature of the vapour leaving the reboiler.
  • Mid‑column thermometer – indicates the temperature at which a particular fraction begins to condense.
  • Top thermometer – shows the temperature of the vapour that will become the next lighter fraction.
• Condenser – cools vapour from the top of the column so that gases can be collected separately and liquid fractions can be condensed.
• Draw‑off trays or valves – positioned at heights that correspond to the boiling‑point ranges of the desired fractions.
• Receiving flasks (labelled) – collect each fraction for later identification.
• Heating mantle / Bunsen burner – provides controllable heat to the reboiler.

6. Safety Checklist (AO2)

• Wear safety goggles, lab coat and heat‑resistant gloves.
• Carry out the experiment in a well‑ventilated area or fume hood – petroleum vapours are highly flammable.
• Keep a Class B fire extinguisher within easy reach.
• Never leave the heating mantle unattended.
• Use tongs or heat‑proof clamps when handling hot glassware.
• Allow the column to cool completely before dismantling.

7. Designing a Simple Laboratory Distillation (AO2)

1. Assemble the apparatus in the order: reboiler → packed column → condenser → labelled receiving flasks.
2. Secure a thermometer at the top of the column (to monitor the temperature of the vapour that will condense into the next fraction).
3. Place ~200 mL of crude oil in the reboiler and heat gently to ≈ 150 °C to drive off water and the lightest gases.
4. Increase the heat slowly. Record the temperature at the top of the column every minute.
5. When the temperature stabilises (plateau) within a known boiling‑point range, open the corresponding draw‑off valve and collect the liquid in the pre‑labelled flask.
6. Continue heating, moving to higher‑temperature plateaus, until only a viscous residue (bitumen) remains.
7. Switch off the heat, allow the set‑up to cool, then dismantle and label each collected sample.

8. Step‑by‑Step Process (Conceptual Explanation – AO1)

1. Pre‑heating – removes water and light gases, preventing column fouling.
2. Boiling in the reboiler – creates a mixture of vapours that rise into the column.
3. Fractionating action – vapour contacts cooler packing, partially condenses, then re‑evaporates on encountering hotter vapour higher up. Each condensation‑evaporation cycle acts as a “theoretical plate”.
4. Condensation on trays – when the vapour reaches a region where its temperature equals its boiling point, it condenses on a tray and is drawn off as a liquid fraction.
5. Progression of fractions – lighter fractions condense lower in the column; heavier fractions travel higher before condensing.
6. Top products – the lightest gases pass through the top, are cooled in the external condenser and collected separately.

9. Sample Temperature Data (AO3) and Worked Example

Worked interpretation:

• From minutes 0–5 the temperature rises steadily – no fraction is being collected yet (pre‑heating stage).
• Minutes 6–9 show a plateau at ≈ 130 °C. This lies within the boiling‑point range for petrol (30 – 200 °C), so the liquid drawn off during this period is labelled “Petrol”.
• Minutes 10–14 a second plateau appears at ≈ 150 °C, matching the range for kerosene / jet fuel (150 – 250 °C). The liquid collected here is labelled “Kerosene”.
• When the temperature rises beyond 250 °C the next plateaus would correspond to diesel, lubricating oil and finally fuel oil/bitumen.

Possible sources of error to discuss: heat loss to the surroundings, imperfect packing (fewer theoretical plates), slight pressure variations, and overlapping boiling ranges.

10. Concise Links to Other Fuel Topics (required by the syllabus)

• Alkanes (11.4) – gasoline is mainly a mixture of C₄–C₁₂ alkanes; their combustion releases the energy used in spark‑ignition engines.
• Alkenes (11.5) – thermal cracking of long‑chain alkanes produces smaller alkanes and alkenes (e.g., ethene) that increase the yield of gasoline and diesel.
• Alcohols (11.6) – ethene obtained from cracking can be hydrated to give ethanol, an alternative fuel.
• Carboxylic acids (11.7) – oxidation of ethene gives acetic acid, a useful industrial chemical.
• Polymers (11.8) – polymerisation of ethene produces polyethylene, illustrating the connection between fuel production and the petro‑chemical industry.

11. Key Points to Remember (AO1)

• Separation relies on differences in boiling points; lower‑boiling components are collected higher in the column.
• A tall, well‑packed column provides many theoretical plates, giving sharper separations.
• Accurate temperature control and recording are essential for identifying each fraction.
• Exact boiling‑point ranges may shift with the source of crude oil and with column pressure.
• Fractional distillation is normally preceded by cracking to maximise the amount of light, useful fractions.

12. Summary (AO1)

Fractional distillation converts crude petroleum into a series of valuable hydrocarbon products. By heating the oil, vaporising it, and allowing the vapour to rise through a packed column, repeated condensation‑evaporation cycles separate the mixture according to boiling point. The main fractions – gases, petrol, kerosene, diesel, lubricating oil and fuel oil/bitumen – supply the energy and raw materials for transport, industry and everyday life. Understanding this process also underpins the related syllabus topics of alkane combustion, thermal cracking, and the petro‑chemical synthesis of alcohols, acids and polymers.