Crude Oil and the Hydrocarbon Industry

Crude Oil — A Vital Finite Resource

Crude oil is one of the most important natural resources on Earth. It is used both as a source of fuels and as a feedstock for making an enormous range of chemicals, including polymers, medicines, dyes and fertilisers. Almost every consumer product we use involves crude oil somewhere in its manufacture or transport. This makes modern life crucially dependent on it.

Crude oil is finite — it took millions of years to form from the compressed remains of ancient marine organisms, so it is being consumed far faster than it can be replaced. Decisions about how we use crude oil must balance short-term economic benefits with the need to conserve this resource for future generations. This is an important example of weighing risk, benefit and long-term consequence in a real-world context.

Hydrocarbons and the Alkane Homologous Series

Crude oil is a complex mixture of hydrocarbons. The most common are alkanes, which form a homologous series — a family of related compounds with the same general formula (\(C_nH_{2n+2}\)) and similar chemical properties. Each member differs from the next by one \(CH_2\) unit.

The first four alkanes are: methane (\(CH_4\)), ethane (\(C_2H_6\)), propane (\(C_3H_8\)) and butane (\(C_4H_{10}\)). As the chain length increases, so does the boiling point and viscosity, because longer molecules have stronger intermolecular forces between them. Shorter alkanes are more flammable and volatile — valuable properties for fuels.

Fractional Distillation

Crude oil is separated into useful fractions by fractional distillation. The oil is heated and the vapour rises up a tall fractionating column that is cooler at the top than at the bottom. As vapours rise, they cool and condense at different heights depending on their boiling points. Fractions with shorter molecule chains (lower boiling points) are collected near the top; fractions with longer chains (higher boiling points) condense lower down or remain as residue at the base.

Cracking

After distillation, there is too much of the heavier, longer-chain fractions (such as fuel oil) and not enough of the lighter, shorter-chain fractions (such as petrol and diesel). Cracking is used to convert long-chain hydrocarbons into shorter, more commercially valuable molecules. It also produces alkenes, which are unsaturated and highly reactive — making them valuable starting materials for the manufacture of polymers.

There are two main methods of cracking. In catalytic cracking, hydrocarbon vapour is passed over a hot catalyst (usually a zeolite, a porous aluminium silicate material). In steam cracking, hydrocarbon vapour is mixed with steam and heated to very high temperatures. Both methods break C—C bonds. A typical cracking reaction splits a long-chain alkane into a shorter alkane and an alkene:

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

Here, a ten-carbon alkane is cracked into octane (a useful petrol component) and ethene (an alkene used to make polyethene). The ethene can be identified by testing with bromine water — alkenes decolourise bromine water because they react across the double bond; alkanes do not.