Crude Oil: Fractional Distillation & Cracking (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about the fractional distillation of crude oil and the modification of alkanes by cracking.

Crude oil is separated into fractions by boiling point, but the long-chain fractions this produces exceed demand, so cracking breaks them into shorter, more valuable molecules.

Crude oil is currently the world's main source of organic chemicals, but as a mixture it is not very useful. Fractional distillation separates it into fractions, each containing hydrocarbons of similar chain length and therefore similar boiling point. That separation is not the end of the process: the supply of long-chain fractions exceeds demand, while shorter alkanes and alkenes are more valuable, as fuels and as feedstock for polymers. Cracking converts that low-value surplus into high-demand products.

We'll take the separation first, and why boiling point is what the column sorts on. Then the economic reason for cracking, and what happens to the Carbon carbon bonds when a large alkane is cracked. Finally the two types of cracking, thermal and catalytic, and the different conditions and products of each.

Crude oil is a complex mixture of hydrocarbons, mainly alkanes, which may be straight-chain or branched. It can be separated into fractions, each containing hydrocarbons with similar chain lengths and therefore similar boiling points. Those two properties travel together because of intermolecular forces: hydrocarbons with similar numbers of carbon atoms have similar intermolecular forces, and so similar boiling points.

The separation is carried out in a fractionating column that is hot at the bottom and cooler at the top. Crude oil is heated so that most of it vaporises, and the vapours rise up the column. Hydrocarbons with high boiling points condense lower down, where the temperature is higher, and are drawn off. Those with lower boiling points rise further before condensing and are collected higher up. What comes out is a set of fractions sorted by chain length, and it is that sorting which leaves some fractions in surplus and others in short supply.

Cracking involves breaking carbon carbon bonds in alkanes. The large, less useful hydrocarbon molecules from crude oil are fed into a steel chamber, heated to a high temperature and passed over a zeolite or aluminosilicate catalyst; the chamber contains no oxygen, to prevent the hydrocarbon combusting to water and carbon dioxide. What comes out is a smaller alkane and one or more alkene molecules; decane, for example, cracks to octane and ethene.

The reason for doing this is economic. The supply of long-chain fractions exceeds demand, while shorter alkanes and alkenes are more valuable, as fuels and as feedstock for polymers, so cracking converts low-value surplus into high-demand products, and the low-molecular-mass alkanes it forms make good fuels. The heavier fractions obtained from fractional distillation are exactly the ones that get cracked.

There are two types of cracking. Thermal cracking requires high temperatures, up to 1000 degrees Celsius, and high pressure, up to 70 atmospheres, and produces alkanes and a lot of alkenes. Catalytic cracking uses a lower temperature, around 450 degrees Celsius, and only slight pressure, in the presence of a catalyst such as a zeolite or aluminosilicate, and produces motor fuels and mainly aromatic hydrocarbons.

The conditions decide the products: the high-temperature, high-pressure route is the one that gives a lot of alkenes, and the catalysed route is the one used mainly for motor fuels. Both are doing the same job to the same surplus fractions, breaking C–C bonds to make shorter molecules, but the choice of conditions decides which high-demand product you get.

Fractional distillation is a physical process, not a chemical reaction, so no covalent bonds are broken during the separation. What happens instead is that intermolecular forces between the hydrocarbon molecules are overcome as the crude oil is heated and vaporised. Cracking is the opposite case: it breaks C–C covalent bonds and is a chemical change, so it is not to be confused with boiling, which only overcomes the intermolecular forces between molecules. Remember also that cracking is an endothermic reaction.

And you do not need to memorise every fraction: it is more important to understand the principle of separation based on differences in boiling points, and why fractional distillation is used to obtain useful hydrocarbon fractions from crude oil.

Fractional distillation separates crude oil into fractions of similar chain length and similar boiling point, in a column that is hot at the bottom and cool at the top. Cracking breaks carbon carbon bonds in alkanes to give a smaller alkane and one or more alkenes. Thermal cracking, at up to 1000 degrees celcius and 70 atmospheres, gives a lot of alkenes; catalytic cracking, at around 450 degrees celsius over a zeolite, gives motor fuels and mainly aromatic hydrocarbons.

The two sit together because the fractions distillation produces do not match demand: the long chains are in surplus and the short ones are valuable, so cracking converts one into the other.