Fractional Distillation

Fractional Distillation

400 million years ago, the world looked totally different. No dinosaurs or ice ages, just oceans filled with small creatures like crustaceans, fish, and phytoplankton. These little guys would die and sink to the ocean floor, where layers of sand and silt would cover them over time. After hundreds of thousands of years, the heat and pressure from all those layers turned them into something we use a lot today - crude oil. We get the different parts of crude oil by separating them through fractional distillation.

A representation of the formation of crude oil
A representation of the formation of crude oil

Crude oil is a mix of different stuff, like hydrocarbons and other organic things. It’s made from plants and animals that broke down under crazy pressures and temperatures over millions of years, way down in the Earth’s crust. Once we get it out, clean it up, and process it, it becomes a super helpful fossil fuel and a big source of organic chemicals. We use it for almost everything, from running our houses to making our plastic bottles and even the soaps and shampoos we use. But, there’s a downside - it’s not good for the environment, and we’ll talk about that soon.

Why do we distill crude oil?

Fractional distillation is a process used to separate a mixture into its component parts, or fractions, by heating them to a temperature at which one or more fractions of the mixture will vaporize. This process is used to separate crude oil into different fractions of hydrocarbons that all have similar sizes and properties. This is done by heating the crude oil to vapourize it and then feeding it into the bottom of a distillation tower. The vapour then rises through the vertical column, and the different fractions are collected at different heights in the tower. These fractions are much more useful to us than the raw mixture, as they are specific hydrocarbons of different lengths, with fewer impurities and contaminants.

How is crude oil distilled?

Fractional distillation of crude oil takes place in a fractionating column. This is a very large chamber typically eight meters in diameter and forty meters high. That’s slightly taller than the statue of Christ the Redeemer in Rio de Janeiro, and just shorter than the distance between the top of the Titanic’s tallest funnels and the water!

The column is filled with rows of trays at different heights. Crude oil enters at the bottom, and the different fractions are piped off from the various trays. A diagram showing the process of fractional distillation, along with examples of some of the fractions produced. Fractional distillation includes the following steps:

What are the uses of crude oil?

Different fractions of crude oil have different uses depending on their properties.

The largest hydrocarbons, with chain lengths of 70 or longer, form bitumen. This is a thick, tar-like substance used for road surfaces and roofing. Medium-length hydrocarbons make up products like diesel oil. The shortest-chain hydrocarbons, which are gases at room temperature, are used as fuel for camping stoves.

For example, butane is useful as a component of petrol because of its low boiling point, meaning we can burn it in internal combustion engines. Naphtha is a fraction containing hydrocarbons with about five or six carbon atoms, and we can crack it industrially to produce alkenes (see Cracking). These are used to make plastics, detergents, and alcohols. Just take a look around you, and you are bound to find a huge range of products made from crude oil.

Disadvantages of crude oil

A substance that powers our vehicles, keeps our electronics ticking over, can be turned into clothes and packaging, and is just lying there under the ocean floors, waiting to be used: why are some people so against extracting and distilling crude oil?

A finite resource

Because crude oil forms so slowly, it is a non-renewable resource.

A renewable resource is one that is replenished naturally at the rate that we use it. Non-renewable resources are therefore resources that we use faster than the rate they are replenished at.

Unless we stop extracting crude oil so quickly, we will soon run out. Our crude oil resources are finite. By relying heavily on crude oil for so many different products, we disadvantage future generations. When our oil reserves run out, they will quickly have to find alternative ways to produce things such as fuel, plastics, and chemical feedstocks, that have become such a fundamental part of everyday life.

Greenhouse gases

Hydrocarbons, which are present in crude oil, are commonly used as fuels for vehicles like cars, boats, and planes because they are a great source of energy. However, burning these hydrocarbons releases carbon dioxide (CO2) and water vapour (H2O) into the atmosphere. These gases are known as greenhouse gases, which trap heat in the atmosphere and contribute to the greenhouse effect. The greenhouse effect is the process by which greenhouse gases in the atmosphere trap radiation from the sun, leading to a rise in global temperatures. This increase in temperature causes a range of problems, including melting glaciers, crop failure, and more extreme weather events such as floods and droughts. Therefore, it is important to find alternative energy sources that do not release greenhouse gases into the atmosphere.

The greenhouse effect. Greenhouse gases trap heat from the sun, thereby warming the Earth

See Combustion for more information about burning hydrocarbons.


Crude oil is an organic mixture that can contain various impurities, such as sulfur, which can cause breathing difficulties, skin irritation, and acid rain when burned. As a result, there are growing concerns about the extraction and use of crude oil. However, there are alternatives to crude oil that are becoming more accessible and affordable, such as natural fibers like linen, cotton, and hemp, as well as renewable energy sources like solar power. Additionally, governments around the world are taking steps to reduce their reliance on fossil fuels and move towards a greener, more sustainable future. Fractional distillation is a key process used to separate crude oil into fractions of alkanes with similar sizes and properties, but it is important to consider the environmental impacts of using these fractions as fuels.

Fractional Distillation

What is fractional distillation?

Fraction distillation is the process of separating a mixture into fractions according to their boiling points.

How does fractional distillation work?

Fractional distillation works by heating a mixture so it evaporates. The vapours rise up a fractionating column with a temperature gradient, so similar length hydrocarbons condense and are collected at different points. Heavier, longer chain hydrocarbons will condense lower down in the column due to their higher boiling points, whilst lighter hydrocarbons continue rising up.

What does fractional distillation separate?

Fractional distillation is mostly used to separate crude oil, a mixture of hydrocarbons, into fractions of alkanes with similar chain lengths.

Why is fractional distillation important?

Fractional distillation is important as a useful separation technique, used to separate mixtures into fractions with similar boiling points.

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