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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.
A 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.
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.
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. Anna Brewer, StudySmarter Originals
Fractional distillation includes the following steps:
Different fractions of crude oil have different uses depending on their properties.
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.
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?
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.
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.
The greenhouse effect. Greenhouse gases trap heat from the sun, thereby warming the Earth. commons.wikimedia.org
See Combustion for more information about burning hydrocarbons.
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.
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.
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