Fractional Distillation

Fractional distillation definition

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.

Fig. 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.

Fig. 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.

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.

Greenhouse gases trap heat in the atmosphere and warm the planet, contributing to something called the greenhouse effect.

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.

Fig. 3 - The greenhouse effect. Greenhouse gases trap heat from the sun, thereby warming the Earth

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.