Fractional Distillation

This lesson covers the process of fractional distillation as required by the AQA GCSE Chemistry specification (5.8.1). Fractional distillation is the key industrial process used to separate crude oil into useful fractions. Understanding how and why this process works is essential for your exam.

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Why Separate Crude Oil?

Crude oil in its raw form is not very useful. It is a mixture of hundreds of different hydrocarbons with different chain lengths and properties. To make use of crude oil, it must be separated into groups of hydrocarbons with similar numbers of carbon atoms. Each group is called a fraction.

Each fraction has different uses depending on its properties. The separation is possible because the different hydrocarbons have different boiling points.

Exam Tip: Never say crude oil is "split" by fractional distillation — it is separated. Splitting suggests a chemical change, but fractional distillation is a physical process.

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The Process of Fractional Distillation

Fractional distillation takes place in a fractionating column at an oil refinery. The process works as follows:

Step-by-Step Process

1. Crude oil is heated in a furnace until it vaporises (turns into 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 up the column, it cools down
5. Different hydrocarbons condense (turn back into liquid) at different heights, depending on their boiling points
6. Hydrocarbons with high boiling points condense near the bottom of the column (where it is hottest)
7. Hydrocarbons with low boiling points rise higher before condensing, and are collected near the top of the column
8. Each fraction is collected at a different level of the column

graph TD
    A["Crude Oil Heated in Furnace"] --> B["Hot Vapour Enters Column"]
    B --> C["Fractionating Column"]
    C --> D["Top: Cool ~25°C"]
    C --> E["Upper: ~70°C"]
    C --> F["Middle: ~180°C"]
    C --> G["Lower: ~270°C"]
    C --> H["Bottom: Hot ~350°C+"]
    D --> I["Refinery Gas (C1-C4)"]
    E --> J["Petrol / Gasoline (C5-C8)"]
    F --> K["Kerosene (C9-C12)"]
    G --> L["Diesel (C13-C16)"]
    H --> M["Fuel Oil, Bitumen (C17+)"]

    style C fill:#34495e,color:#fff
    style D fill:#3498db,color:#fff
    style H fill:#e74c3c,color:#fff

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The Fractions of Crude Oil

The main fractions obtained from fractional distillation are:

Fraction	Carbon Atoms	Boiling Point Range	State at Room Temp	Main Uses
Refinery gases	C1 – C4	Below 25°C	Gas	Heating, cooking (LPG), chemical feedstock
Petrol (gasoline)	C5 – C8	25–75°C	Liquid	Fuel for cars
Naphtha	C6 – C10	70–180°C	Liquid	Chemical feedstock for making plastics
Kerosene	C9 – C12	160–250°C	Liquid	Jet fuel (aviation), heating
Diesel oil	C13 – C16	220–350°C	Liquid	Fuel for buses, lorries, some cars
Fuel oil	C17 – C20	350–400°C	Thick liquid	Fuel for ships and power stations
Bitumen	C20+	Above 400°C	Solid/semi-solid	Road surfacing, roofing

Exam Tip: You do not need to memorise the exact carbon numbers or temperatures for each fraction. However, you do need to know the order of fractions, their relative boiling points, and their main uses.

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Why Does Fractional Distillation Work?

The separation works because the hydrocarbons in crude oil have different chain lengths, and therefore different boiling points.

Chain Length and Boiling Point

As the number of carbon atoms in a hydrocarbon increases:

Property	Short-chain hydrocarbons	Long-chain hydrocarbons
Boiling point	Low	High
Viscosity	Low (runny)	High (thick, sticky)
Flammability	High (ignite easily)	Low (harder to ignite)
Colour	Light / colourless	Dark (brown/black)
Volatility	High (evaporate easily)	Low

The reason for the increase in boiling point is the intermolecular forces between the hydrocarbon molecules. Longer molecules have more electrons and therefore stronger London dispersion forces between them. More energy is needed to overcome these forces, so longer-chain hydrocarbons have higher boiling points.

Exam Tip: The forces broken during boiling are the intermolecular forces (forces between molecules), NOT the covalent bonds within the molecules. This is a very common mistake. Covalent bonds are not broken during fractional distillation.

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Trends in Properties of Fractions

Understanding the trends in the properties of crude oil fractions is critical for exam success.

Boiling Point Trend

As you move down the fractionating column (from top to bottom):

• The chain length of the hydrocarbons increases
• The boiling point increases because longer molecules have stronger intermolecular forces
• More energy is needed to separate the molecules, so they condense at higher temperatures

Viscosity Trend

Viscosity is a measure of how "thick" or resistant to flow a liquid is.

• Short-chain hydrocarbons (top of column) are runny (low viscosity)
• Long-chain hydrocarbons (bottom of column) are thick and sticky (high viscosity)

This is because longer molecules become more entangled and have more intermolecular forces holding them together.

Flammability Trend

Flammability describes how easily a substance catches fire.

• Short-chain hydrocarbons are highly flammable — they ignite easily
• Long-chain hydrocarbons are less flammable — they are harder to ignite

Short-chain hydrocarbons are gases or volatile liquids that mix easily with air, making them better fuels.

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Supply and Demand

Not all fractions are produced in equal quantities by fractional distillation, and not all fractions are in equal demand.

Problem	Detail
Excess supply	There is more long-chain hydrocarbon produced than is needed (e.g., fuel oil, bitumen)
Excess demand	There is more demand for short-chain hydrocarbons (e.g., petrol, diesel) than crude oil can supply

This mismatch between supply and demand is one of the main reasons for cracking — a chemical process that breaks long-chain hydrocarbons into shorter, more useful ones. Cracking is covered in a later lesson.

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Industrial Importance

Fractional distillation is one of the most important industrial processes in the world. Oil refineries operate continuously, processing millions of barrels of crude oil per day.

The petrochemical industry depends on fractional distillation to provide: