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What do I need to know about fractional distillation?

Fractional distillation is covered in almost every GCSE chemistry course, although what you need to know varies slightly by syllabus. The main points to take away are explained below.

Why do we do fractional distillation?

Crude oil is a mixture of many different substances. The useful products found in crude oil are called alkanes, they are a homologous group within the hydrocarbon family, made of long chains of carbon atoms with hydrogen atoms bonded around the edges. However, alkanes with different length chains have different uses, so we need to separate the chains by length, which is what fractional distillation does.

How does fractional distillation work?

A common pattern in chemistry is that larger molecules have higher boiling points than smaller ones. Alkanes are no exception: the longer the chain, the higher the boiling point. Fractional distillation uses this fact to separate the chains. Raw crude oil is pumped into afractionation chamber, which is hot at the bottom (about 350oC) and cool at the top (about 25oC), and has special condensation slats built in all the way up the sides.

The small hydrocarbons evaporate to become gases at the bottom and rise up the chamber to condense at the top as they have a low boiling/condensing temperature. The larger hydrocarbons also evaporate, but they condense nearer the bottom of the chamber, as these molecules have a higher boiling/condensing temperature. The final result is that the crude oil is separated into many fractions, each containing alkanes of a similar length.

What are the products used for?

The main use for the alkanes separated by fractional distillation is as fuels. Shorter chain alkanes, such as refinery gases or petrol, are used to power smaller machines like gas lamps or cars. Longer chain alkanes, such as kerosene or fuel oil, are used to power heavy-duty vehicles including aeroplanes and ships.

What happens after fractionation?

The story doesn’t stop there for alkanes. One problem of fractional distillation is that we get many long chain alkanes, but shorter chains are much more useful. Commonly, after fractionation, long chain alkanes are passed over a hot catalyst in a process called cracking. During this process, the long alkanes are broken up into short alkanes and alkenes. Alkenes have a double bond, which means they can be turned into polymers to make plastics, or alcohols and other derivatives.

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