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Fractional Distillation

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Chemistry

Four hundred million years ago, the world was a different place. This was a time before the dinosaurs, before the glaciers of the ice ages took over the planet. Spiders had only been crawling about for 80 million or so years, the first four-legged tetrapods had just made it on to land, and green plants hadn’t yet evolved into forests. The majority of the Earth was covered in oceans full of crustaceans, fish, and various types of vegetation like phytoplankton - microscopic green algae. When these organisms died, their remains fell to the ocean floors and were gradually covered in layers of silt and sand. As the layers grew higher over hundreds of thousands of years, the conditions intensified. Eventually the heat and pressure pushing down upon the remains were so great that the dead organic matter started turning into a substance of great importance to humans today - crude oil. We separate crude oil into its useable constituents through a process known as fractional distillation.

Fractional Distillation formation of crude oil over millions of years StudySmarterA representation of the formation of crude oil. Anna Brewer, StudySmarter Originals

Crude oil is a complex mixture of hydrocarbons and other organic impurities.

Crude oil is formed through the breakdown of plant and animal matter under high pressures and temperatures over millions of years, deep within the Earth’s crust. When mined, purified, and refined, it is our main source of organic chemicals and an extremely useful fossil fuel. It plays a role in almost all aspects of our lives, from powering our homes to making up our plastic bottles and providing us with the basis of our soaps and shampoos. However, it has a negative environmental impact, as we’ll find out later.

Why do we distill crude oil?

When crude oil is mined, it is a thick, black liquid. It isn’t useful to us in its original state, which is a jumbled mixture of hydrocarbons of different lengths, made murkier with various impurities and contaminants. To separate it into useful products, we use fractional distillation.

Fractional distillation is a process that involves separating a mixture into smaller samples with similar boiling points, known as fractions.

By distilling crude oil, we can obtain different fractions of hydrocarbons that all have similar sizes and properties. These are much more useful to us than the raw mixture.

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.

Fractional Distillation, fractional distillation process diagram, StudySmarterA diagram showing the process of fractional distillation, along with examples of some of the fractions produced. Anna Brewer, StudySmarter Originals

Fractional distillation includes the following steps:

  • The crude oil is heated to high temperatures and evaporates.
  • The evaporated mixture rises up the fractionating column, which has a temperature gradient. This means that the bottom of the column, where the crude oil is heated, is warmer than the top.
  • The vapours rise up the column through the rows of trays. As they move further up the column, the temperature gradually decreases. When hydrocarbons from the crude oil reach a temperature that is cool enough for them to condense, they collect in the trays and are piped off. Similar length hydrocarbons have similar boiling points and so are collected together as a fraction. The remaining molecules keep rising up the column until it is cool enough for them to condense.
  • Heavier, longer-chain hydrocarbons have a higher boiling point and so will condense lower down in the column, whereas lighter, shorter-chain hydrocarbons will continue rising towards the top of the column.

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

As you now know, crude oil is made up of hydrocarbons of all different lengths and sizes. We burn lots of these hydrocarbons as fuels for cars, boats, and planes. Whilst they are great sources of energy, hydrocarbons release carbon dioxide () and water vapour () when burnt.

This is a serious problem, because carbon dioxide and water vapour are both greenhouse gases.

A greenhouse gas is a gas that absorbs radiation from the sun reflected off the Earth, instead of letting it escape back into outer space.

This traps heat in the atmosphere and warms the planet, contributing to something called the greenhouse effect.

The greenhouse effect is a term used to describe how greenhouse gases in the atmosphere absorb radiation from the Earth, which traps heat and warms the Earth up. It works in much the same way as a greenhouse used for growing plants, hence the name.

By burning crude oil fractions, humans are contributing to the steady increase in global temperature that is melting glaciers, causing crop failure, and intensifying freak weather events like floods and droughts.

Fractional distillation, the greenhouse effect, StudySmarterThe greenhouse effect. Greenhouse gases trap heat from the sun, thereby warming the Earth. commons.wikimedia.org

See Combustion for more information about burning hydrocarbons.

Impurities

Crude oil is an organic mixture, and can contain many different impurities, such as sulfur. These impurities come from the bodies of the deep-sea creatures that break down to form crude oil, as explored above. When we burn crude oil as a fuel, we release the impurities back into the environment.

Sulfur, for example, burns to form sulfur dioxide. You may know that this gas causes breathing difficulties, skin irritation and corrosive acid rain. Sounds fun, right?

For all of these reasons, extracting and distilling crude oil remains a controversial topic and many parties are actively protesting against it. But it isn’t all bad news. Alternatives to crude oil are becoming ever cheaper and more accessible. You may have drunk coffee from a fully-compostable cup, worn clothes made from natural linen, cotton or even hemp, or powered your phone with solar energy. The UK government recently announced plans to phase out all sales of new petrol and diesel cars by 2030. Although this may seem like an unachievable goal right now, it is a great step in the right direction towards a greener, more sustainable future.

Fractional Distillation - Key takeaways

  • Crude oil is a mixture of hydrocarbons formed from plant and animal matter compressed under high temperatures and pressures over millions of years.
  • Fractional distillation uses a temperature gradient to separate crude oil into fractions of alkanes with similar sizes and properties.
  • Burning the products of crude oil distillation has negative impacts on the environment, but crude oil is an important part of modern life.

Fractional Distillation

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

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.

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

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

Final Fractional Distillation Quiz

Question

What is crude oil?

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Answer

A mixture of hydrocarbons of different chain lengths and other organic impurities.

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Question

How is crude oil formed?

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Answer

Through the breakdown of plant and animal matter deep within the Earth’s crust under high pressures and temperatures, over millions of years.

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Question

What is fractional distillation?

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Answer

A process that involves separating a mixture into fractions, which are samples containing molecules with similar boiling points.

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Question

How is crude oil distilled?


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Answer

  • Crude oil is heated so it evaporates. 
  • The vapours rise up a fractionating column that has a temperature gradient, until they reach a temperature cool enough for them to condense into trays.
  • The condensed vapours are piped off to be collected.

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Question

Why are longer-chain hydrocarbons collected lower down in the fractionating column?


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Answer

They have a higher boiling point than shorter chain hydrocarbons, and so will condense at a higher temperature.

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Question

Give five uses of fractions obtained from crude oil fractional distillation.


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Answer

  • Fuels. 
  • Plastics.
  • Chemical feedstock.
  • Camping stove gas canisters.
  • Road surfaces.

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Question

Where is the hottest part of the fractionating column?


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Answer

The bottom.

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Question

 Define renewable resource.


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Answer

A resource that is produced naturally at the same rate as it is used.

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Question

Crude oil is formed from the remains of dead organisms. This is a natural process, so why is crude oil not a sustainable resource?


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Answer

Crude oil takes millions of years to form. We are using it at a faster rate than it is being created.

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Question

What are the products of hydrocarbon combustion?


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Answer

Carbon dioxide and water vapour.

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Question

Name an impurity often found in crude oil.


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Answer

​Sulfur.

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Question

Give three arguments against extracting and refining crude oil.


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Answer

  • It is non-renewable.
  • It burns to produce a greenhouse gas.
  • It contains damaging impurities that have negative environmental impacts when burnt.

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