Crude Oil, Alkanes and Fractional Distillation

Why crude oil matters

Crude oil is a mixture of many different hydrocarbons: compounds containing carbon and hydrogen atoms only. It is extracted from rocks beneath the land and seabed and is a main source of the hydrocarbons used in modern life.

Crude oil has two important roles. It supplies fuels, such as petrol and diesel for vehicles and kerosene for aircraft. It is also a feedstock for the petrochemical industry: a raw material that is processed to make other substances. These include polymers used in plastics, solvents, detergents and many medicines. Our dependence on oil therefore extends beyond transport and heating to the materials used in packaging, clothing and everyday products.

Crude oil formed over millions of years from the remains of living organisms exposed to heat and pressure. We use it much faster than new oil forms, so it is a finite resource. It cannot be replenished on the timescale of human use. Its importance as both a fuel source and a source of materials makes this limited supply significant.

Alkanes: a family of hydrocarbons

The hydrocarbons in crude oil vary greatly in molecular size. Its fractions contain mainly alkanes, hydrocarbons with only single covalent bonds. They are described as saturated because their carbon atoms carry the maximum possible number of hydrogen atoms for that carbon framework.

Alkanes form a homologous series: a family with a common general formula and similar chemical properties. Each successive member differs by one $\mathrm{CH}_2$ unit. Their general formula is:

$$\mathrm{C}_n\mathrm{H}_{2n+2}$$

Here, $n$ is the number of carbon atoms in one molecule. Methane is $\mathrm{CH}_4$, ethane is $\mathrm{C}_2\mathrm{H}_6$ and propane is . For octane, , so the number of hydrogen atoms is : its formula is .

Crude oil is not one alkane, and it does not have one chemical formula. It contains many different compounds mixed together.

Why different hydrocarbons boil at different temperatures

There are two types of attraction to distinguish. Strong covalent bonds hold the carbon and hydrogen atoms together within each molecule. Much weaker intermolecular forces attract separate molecules to one another.

When a liquid boils, energy is transferred to it by heating. Molecules become separated as the intermolecular attractions are overcome, allowing them to move freely as a gas. The covalent bonds inside each molecule remain intact: a hydrocarbon molecule in the vapour is still the same compound as it was in the liquid.

Larger, longer-chain hydrocarbon molecules have stronger intermolecular attractions than smaller molecules. More energy is needed to overcome these attractions, so larger molecules generally have higher boiling points. Smaller molecules have weaker attractions and lower boiling points.

The same distinction between bonds and forces matters in other changes of state. Melting requires energy to overcome enough intermolecular attraction for molecules to move past one another; it does not break their covalent bonds. During condensation or freezing, energy is transferred to the surroundings as molecules become more closely held together. Changes of state are physical changes, not chemical reactions.

Separating crude oil into useful fractions

Different uses need hydrocarbons with different properties. Rather than separating every individual compound, a refinery separates crude oil into fractions. Each fraction is still a mixture, but its molecules have similar sizes and a similar range of boiling points.

Fractional distillation uses these differences in boiling point:

1. Crude oil is heated in a furnace so that most of it vaporises.
2. The hot vapours enter near the bottom of a fractionating column. The column is hottest at the bottom and becomes cooler towards the top.
3. Vapours rise and cool. Higher-boiling hydrocarbons condense in the hotter, lower parts of the column.
4. Lower-boiling hydrocarbons remain as vapour for longer, rise further and condense in cooler regions higher up.
5. Liquid fractions are drawn off at different levels. The smallest hydrocarbons leave the top as gases, while material that did not vaporise remains at the bottom as a residue.

For example, petrol is collected above kerosene and diesel because its hydrocarbons are generally smaller and have lower boiling points. Naphtha provides a chemical feedstock, fuel oil is used by ships, and the heavy bitumen residue is used for road surfaces.

Industrial fractional distillation operates continuously: heated crude oil enters while fractions are collected. It separates existing compounds without changing them chemically. The whole process connects molecular size to intermolecular forces, boiling point and finally the height at which a fraction is collected.

Crude oil and its importance

• Crude oil is a finite mixture of hydrocarbons, mainly alkanes.