Fractional Distillation

This lesson covers fractional distillation of crude oil as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the process, explain how it separates crude oil into useful fractions, and describe the uses and properties of each fraction.

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Why Separate Crude Oil?

Crude oil straight from the ground is not very useful. It is a complex mixture of hundreds of different hydrocarbons. To make it useful, we separate it into groups of hydrocarbons with similar chain lengths and boiling points. These groups are called fractions.

The separation technique used is fractional distillation — it works because the different hydrocarbons have different boiling points.

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The Process of Fractional Distillation

Fractional distillation takes place in a fractionating column at an oil refinery.

Steps

1. Crude oil is heated in a furnace until most of it turns into vapour (gas).
2. The hot vapour enters the bottom of a tall fractionating column.
3. The column is hot at the bottom and cool at the top — there is a temperature gradient.
4. As the vapour rises, it cools down.
5. When a hydrocarbon reaches the level where the temperature equals its boiling point, it condenses back into a liquid.
6. The liquid is collected on trays at that level and piped off as a fraction.
7. Hydrocarbons with the lowest boiling points rise to the top before condensing.
8. Hydrocarbons with the highest boiling points condense near the bottom (or do not vaporise at all and are collected as a thick residue — bitumen).

The Fractionating Column

graph TB
    TOP["🔝 TOP — Coolest"] --> R["Refinery gases<br/>C₁–C₄<br/>Below 25 °C"]
    R --> P["Petrol (gasoline)<br/>C₅–C₁₀<br/>25–60 °C"]
    P --> N["Naphtha<br/>C₅–C₁₂<br/>60–180 °C"]
    N --> K["Kerosene<br/>C₁₀–C₁₆<br/>180–250 °C"]
    K --> D["Diesel<br/>C₁₄–C₂₀<br/>250–350 °C"]
    D --> FO["Fuel oil<br/>C₂₀–C₅₀<br/>350–400 °C"]
    FO --> B["Bitumen<br/>C₅₀+<br/>Above 400 °C"]
    B --> BOT["🔽 BOTTOM — Hottest"]

    style TOP fill:#2196f3,color:#fff
    style R fill:#e1f5fe,color:#000
    style P fill:#b3e5fc,color:#000
    style N fill:#81d4fa,color:#000
    style K fill:#4fc3f7,color:#000
    style D fill:#29b6f6,color:#fff
    style FO fill:#039be5,color:#fff
    style B fill:#01579b,color:#fff
    style BOT fill:#d32f2f,color:#fff

Exam Tip: Fractional distillation is a physical process — no chemical bonds are broken. The hydrocarbons are separated based on differences in their boiling points (which depend on chain length and intermolecular forces).

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Fractions and Their Uses

Fraction	Approximate Carbon Chain Length	Boiling Point Range	Main Uses
Refinery gases	C₁–C₄	Below 25 °C	Heating, cooking (LPG: propane/butane)
Petrol (gasoline)	C₅–C₁₀	25–60 °C	Fuel for cars
Naphtha	C₅–C₁₂	60–180 °C	Feedstock for making chemicals and plastics
Kerosene	C₁₀–C₁₆	180–250 °C	Jet fuel (aircraft)
Diesel	C₁₄–C₂₀	250–350 °C	Fuel for lorries, buses, trains
Fuel oil	C₂₀–C₅₀	350–400 °C	Fuel for ships, power stations, heating
Bitumen	C₅₀+	Above 400 °C	Road surfacing, roofing

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Why Does Fractional Distillation Work?

Fractional distillation works because different hydrocarbons have different boiling points:

• Short-chain hydrocarbons have weak intermolecular forces, so little energy is needed to overcome them → low boiling point → they rise to the top of the column.
• Long-chain hydrocarbons have stronger intermolecular forces (more surface area, more points of contact), so more energy is needed → high boiling point → they condense near the bottom.

The key scientific principle: as chain length increases, intermolecular forces increase, so the boiling point increases.

Exam Tip: The actual covalent bonds within the molecules are NOT broken during fractional distillation. It is the intermolecular forces (forces between molecules) that are overcome.

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Supply and Demand

Fractional distillation produces fixed proportions of each fraction, but the demand for each fraction is not the same as the supply:

Fraction	Supply	Demand
Short-chain fractions (petrol, LPG)	Moderate	High
Long-chain fractions (fuel oil, bitumen)	High	Low

Because there is more demand for short-chain hydrocarbons than can be supplied by fractional distillation alone, oil companies use a process called cracking to convert surplus long-chain hydrocarbons into shorter, more useful ones. (This is covered in a later lesson.)

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Industrial Importance

Fractional distillation is carried out on a massive scale at oil refineries. Key points:

• Refineries operate continuously — 24 hours a day.
• The process is energy-intensive — the furnace requires a large amount of fuel.
• The fractionating column may be 60 metres tall or more.
• Products are transported by pipeline, tanker and road to where they are needed.

Exam Tip: You could be asked to label a fractionating column diagram. Remember: short chains at the top, long chains at the bottom. The column is hottest at the bottom and coolest at the top.

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Summary

• Fractional distillation separates crude oil into fractions based on boiling point.
• The fractionating column has a temperature gradient — hot at the bottom, cool at the top.
• Short-chain hydrocarbons have low boiling points and are collected at the top.
• Long-chain hydrocarbons have high boiling points and are collected at the bottom.
• Each fraction has different uses — from LPG and petrol to bitumen for roads.
• The process works because of differences in intermolecular forces between molecules of different chain lengths.

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Worked Examples

Worked Example 1 — Predicting where a fraction exits

Question: A hydrocarbon with a boiling point of 150 °C enters a fractionating column. Where will it exit?

Step 1: The column is hot at the bottom (around 400 °C) and cool at the top (around 25 °C).

Step 2: A hydrocarbon condenses once it reaches a part of the column cooler than its boiling point.

Step 3: 150 °C is in the kerosene range, so the hydrocarbon will exit in the kerosene/paraffin fraction, roughly midway up the column.

Worked Example 2 — Matching a fraction to its use

Question: A student is told a fraction "is solid at room temperature and has chains above 70 carbons long". Which fraction is it and what is it used for?

Answer: The description matches bitumen. It is used for road surfaces and roofing.

Worked Example 3 — Why condensing occurs

Question: Explain why pentane (boiling point 36 °C) does not exit at the bottom of the column but exits at the top.