Chemistry 8.1 - 8.4 - Crude oil and fractional distillation

Hydrocarbons and crude oil

The word only is decisive. $\ce{C3H8}$ is a hydrocarbon because its formula contains carbon and hydrogen and no other element. $\ce{CO2}$ is not a hydrocarbon because it contains oxygen and no hydrogen; $\ce{C2H5OH}$ is not a hydrocarbon because it also contains oxygen. A hydrocarbon is a compound, so its carbon and hydrogen atoms are chemically bonded within each molecule.

Crude oil is not one giant molecule and it is not a pure substance. It is a complex mixture of hydrocarbons: many different hydrocarbon molecules are present together and are not chemically bonded to one another. Their carbon atoms can be arranged in chains or rings. The names, formulae and structures of particular ring molecules are not needed here.

Crude oil is important for two broad reasons:

• some of its fractions are used as fuels;
• it is a feedstock, meaning a raw material from which the petrochemical industry makes other useful substances.

Crude oil is also a finite resource. Accessible reserves are limited and natural processes replace crude oil far more slowly than people extract and use it. Finite does not mean that every last drop will vanish on a known date; it means the resource cannot be replenished on a human timescale at the rate it is consumed.

Separating crude oil by fractional distillation

Crude oil is more useful after it has been separated into fractions. A fraction is not one pure compound: it is a simpler mixture of hydrocarbons with a similar range of boiling points.

Fractional distillation works because the hydrocarbons have different boiling points and the fractionating column has a temperature gradient: hot at the bottom and cooler towards the top.

1. Crude oil is heated strongly so that most of it vaporises. The very highest-boiling material may remain liquid.
2. The hot mixture of vapours enters near the bottom of the column and rises.
3. As a vapour rises, the surrounding temperature falls.
4. When the local temperature falls below a hydrocarbon's boiling point, that hydrocarbon condenses. Hydrocarbons with similar boiling points condense in the same region and are drawn off together as a fraction.
5. High-boiling hydrocarbons condense low in the hot column. Lower-boiling hydrocarbons rise farther before condensing; the lowest-boiling gases leave at the top.