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Fractional Distillation

Fractional Distillation

The power of hydrocarbons has been known for centuries. The walls and towers of Babylon were built from asphalt; ancient Persians used petroleum for lighting and medical treatments. But the vast majority of hydrocarbons need separating, refining, and processing before they can be used efficiently. To accomplish this, we utilise the same process used to distil alcohol, filter air, and extract plant essences: Fractional distillation.

  • This article is about fractional distillation in chemistry.
  • We'll start by defining fractional distillation.
  • We'll then look more closely at the fractional distillation of crude oil.
  • This will involve not only walking through the process, but also finding out about the products and their uses.
  • Finally, we'll consider the disadvantages of crude oil fractional distillation.

Fractional distillation definition

Fractional distillation is a separating technique that involves dividing a mixture into smaller samples according to their boiling points. These smaller samples are known as fractions.

Fractional distillation has been used for hundreds of years to purify and separate various liquids. However, its biggest use today is in petroleum refineries and petrochemical plants. The easiest way to understand fractional distillation is by looking at how it is used within these industries to refine a substance known as crude oil.

Fractional distillation of crude oil

Four hundred million years ago, the world was a different place. This was a time before the dinosaurs and 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 onto 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.

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

Fractional Distillation crude oil formation diagram StudySmarterFig. 1 - A representation of the formation of crude oil

When crude oil is mined, it is a thick, black liquid full of hydrocarbons (such as alkanes), impurities, and contaminants. It isn’t very useful to us in its original state The hydrocarbons all have different lengths and properties, which makes them hard to use when they are muddled together in such a way. However, if we distil crude oil using fractional distillation, 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.

Fractional distillation: Process, column, and diagram

Let's now look at the process of fractional distillation. Here's how it is carried out:

  1. Fractional distillation of crude oil takes place in a fractionating column. This is a huge chamber, typically eight meters wide and forty meters high. For context, that’s slightly taller than the statue of Christ the Redeemer in Rio de Janeiro, and only just shorter than the distance between the top of the Titanic’s tallest funnels and the water!
  2. Crude oil enters at the bottom of the fractionating column. It is heated to high temperatures and so it evaporates.
  3. The evaporated vapours rise up the fractionating column.
  4. The column has a temperature gradient - the bottom of the column is warmer than the top. This means that as the vapours rise, they gradually cool down.
  5. When the temperature of the column gets cool enough, the vapours condense. They are then collected in trays and piped off for further refinement.

The most important feature of a fractionating column is its temperature gradient. It means that hydrocarbons with different boiling points condense at different heights. This all depends on their size:

  • Heavier, longer-chain hydrocarbons have higher boiling points. They condense at relatively high temperatures, and so their fractions are collected lower down in the column.
  • Lighter, shorter-chain hydrocarbons have lower boiling points. They condense at relatively low temperatures, and so are collected higher up in the column.

Fractional distillation process diagram fractions StudySmarterFig. 2 - A diagram showing the process of fractional distillation. The fractions produced are named.

Uses of fractional distillation

So, we've learned that fractional distillation's primary function is to separate crude oil, which by itself doesn't have many uses, into different fractions. Hydrocarbons within a fraction all share similar boiling points thanks to their similar sizes. A hydrocarbon's size dictates many of its other properties, too, and so molecules in the same fraction also have multiple other properties in common. This gives different fractions different uses - they have a whole range of applications and are much more valuable. By separating crude oil, we've managed to make a relatively worthless substance infinitely more useful! For example:

  • 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. Other long-chain hydrocarbons are good fuels for ships and power stations.
  • Medium-length hydrocarbons make up fuel for vehicles. 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. These hydrocarbons also form diesel.
  • The shortest-chain hydrocarbons, which are gases at room temperature, are used as fuel for camping stoves. You might also use bottled gas to heat your home.

But crude oil fractions have many uses beyond construction and fuel. For example:

  • Naphtha is a fraction containing hydrocarbons with about five or six carbon atoms. When we crack naphtha, we end up with alkenes which are then used to make plastics, detergents, and alcohols.
  • In fact, hydrocarbons make great chemical feedstocks - they're easily transformed industrially into other types of molecules.
  • In addition, many of your clothes are based on hydrocarbons.
  • Hydrocarbons are also found in various paints, solvents, and lubricants.
  • Even the coolant in your refrigerator is derived from a particular short-chain hydrocarbon!

Just take a look around you: You are bound to find a huge range of products that, one way or another, originate from crude oil.

Disadvantages of crude oil fractional distillation

Crude oil is our main source of organic chemicals and an extremely useful fossil fuel when mined, purified, and refined.

It is 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. In fact, you could argue that it plays an essential role in almost all areas of our lives. If so, then why are some people so against using it? Let's now consider the disadvantages of extracting and distilling crude oil. These include:

  • Its renewability.
  • The greenhouse gases associated with its use.
  • Its various impurities.

Renewability

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 such a plethora of products, we disadvantage future generations. When our oil reserves run out, they will quickly have to find alternative ways to produce items 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 (CO2) and water vapour (H2O) 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.

Greenhouse gases trap heat in the atmosphere and warm 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, StudySmarterFig. 3 - The greenhouse effect. Greenhouse gases trap heat from the sun, thereby warming the Earth

See Combustion for more information about burning hydrocarbons and its negative effects.

Impurities

Crude oil is an organic mixture that can contain many impurities, such as sulphur. 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.

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

Crude oil alternatives

For all the reasons explored above, 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 perhaps 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

  • Fractional distillation is a process that involves separating a mixture into smaller samples with similar boiling points, known as fractions. It is primarily used for separating crude oil.
  • Crude oil is a mixture of hydrocarbons formed from plant and animal matter that has been compressed under high temperatures and pressures over millions of years.
  • Fractional distillation uses a temperature gradient to separate crude oil into more useful fractions of hydrocarbons (primarily alkanes) with similar sizes, boiling points, and properties.
    • Heavier, longer-chain hydrocarbons have a higher boiling point. They condense at relatively high temperatures and so are collected as one fraction lower down in the column.
    • Lighter, shorter-chain hydrocarbons have a lower boiling point. They condense at relatively low temperatures and so continue rising towards the top of the column. They are only collected when the temperature in the column drops below their boiling point, making it cool enough for them to condense.
  • Crude oil fractions have various uses depending on their properties. They are used as fuels, construction materials, and chemical feedstock.

  • Burning crude oil fractions as fuel has negative impacts on the environment, but crude oil is widely regarded as an important part of modern life.

Frequently Asked Questions about 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.

Show question

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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