Crude oil, fractional distillation and cracking

Most hydrocarbons in crude oil fractions belong to the alkane homologous series. Alkanes are saturated hydrocarbons: they contain only single bonds between carbon atoms. An unsaturated hydrocarbon, by contrast, contains a carbon–carbon double bond.

A homologous series is a family of compounds with four connected features:

• They share the same general formula.
• Neighbouring members differ in molecular formula by $\mathrm{CH_2}$: one carbon atom and two hydrogen atoms.
• Their physical properties change gradually through the series.
• They have similar chemical properties.

For chain alkanes, the general formula is $\mathrm{C_nH_{2n+2}}$, where $n$ is the number of carbon atoms. For example, methane is $\mathrm{CH_4}$, ethane is and propane is . Each successive member adds . Longer alkane molecules therefore contain more carbon and hydrogen atoms.

Their chemical behaviour remains similar, but their physical properties do not stay identical. For example, boiling points increase gradually as the molecules become larger.

How chain length affects a fraction’s properties

A crude oil fraction is a mixture of hydrocarbons with similar boiling points and broadly similar molecular sizes. Different fractions have different properties because they contain different-sized molecules.

Viscosity means resistance to flow. A runny liquid has low viscosity; a thick liquid that flows slowly has high viscosity.

Larger hydrocarbon molecules experience stronger intermolecular attractions. More energy is needed to separate the molecules, so they have higher boiling points. These stronger attractions also make it more difficult for molecules to move past one another, increasing viscosity. Boiling overcomes attractions between molecules; it does not break the covalent bonds within them.

Shorter-chain hydrocarbons ignite more easily, making them particularly useful as fuels. These differences in properties help explain why crude oil is separated rather than simply used as one mixture.

Separating crude oil by fractional distillation

Fractional distillation separates hydrocarbons using their different boiling points. It is a physical separation: the molecules are collected in different groups, not changed into new substances.

Crude oil is heated so that much of it vaporises, and the heated mixture enters near the bottom of a fractionating column. The column is hot at the bottom and becomes progressively cooler towards the top.

The vapours rise and cool. A hydrocarbon condenses when it reaches a region cool enough for it to become liquid:

• Larger molecules have higher boiling points, so they condense in the hotter, lower parts of the column.
• Smaller molecules have lower boiling points, so they travel further upwards before condensing in cooler regions.
• The smallest hydrocarbons remain gases and leave at the top.
• The heaviest material remains at the bottom as a residue, including bitumen.

Fractions are drawn off at different heights. Each contains several hydrocarbons with similar boiling points, so fractional distillation produces simpler mixtures, not pure individual hydrocarbons.

Industrial fractional distillation runs continuously: heated crude oil enters while the separated fractions are collected. The fractions can then be used or processed further to provide fuels and petrochemical feedstock.

The main fractions and their uses

The following fractions are listed from the top towards the bottom of the column. Moving down this list, molecules generally become larger, boiling points and viscosity increase, and ignition becomes more difficult.

Not every useful fraction is a fuel. Bitumen, for example, is useful as a road-surfacing and roofing material rather than as an easily ignited fuel.

Cracking changes the molecules

Fractional distillation only separates the molecules already present in crude oil. Cracking goes further: it breaks larger, saturated alkane molecules into smaller, more useful molecules. Some products are smaller alkanes; others are unsaturated hydrocarbons called alkenes, which contain a carbon–carbon double bond.

The large hydrocarbons are heated to vaporise them. Their vapours are then either passed over a hot catalyst or mixed with steam and heated to a very high temperature. Covalent bonds within the large molecules break, and smaller molecules form. This is a chemical change, unlike fractional distillation.

One possible example is the cracking of decane into octane and ethene:

$$\mathrm{C_{10}H_{22} \rightarrow C_8H_{18} + C_2H_4}$$

The atoms are conserved: both sides contain ten carbon atoms and twenty-two hydrogen atoms. Octane is a smaller alkane that can be used in petrol; ethene is an alkene. Actual cracking produces mixtures of products rather than always following this one equation.

Why cracking is necessary

Fractional distillation does not necessarily supply the fractions in the quantities people want. Demand for shorter-chain hydrocarbons is greater than demand for longer-chain hydrocarbons, because shorter chains ignite more easily and are useful as fuels.

Cracking converts less-wanted larger molecules into smaller, higher-demand products. It therefore helps refineries make better use of crude oil: separation groups the existing molecules, while cracking changes their sizes to produce more useful substances.

• Hydrocarbons contain carbon and hydrogen only.
• Crude oil is a complex, finite mixture of hydrocarbons with carbon chains or rings; it supplies fuels and petrochemical feedstock.
• Petrol, kerosene and diesel oil are non-renewable fossil fuels from crude oil. Methane is a non-renewable fossil fuel found in natural gas.
• Most fraction hydrocarbons are alkanes: saturated, with only single carbon–carbon bonds.
• A homologous series has the same general formula, neighbouring members differing by $\mathrm{CH_2}$, gradually changing physical properties and similar chemical properties.
• Chain alkanes: ${\mathrm{C}}_{\mathrm{n}}$.