Fractions

• Crude oil is made up of a complex mix of hydrocarbons both large and small, coming from the remains of plants and animals over millions of years. This mix is of limited use in its raw state, so it’s separated into parts, known as fractions, using a process called fractional distillation.

• Each fraction is essentially a mix of hydrocarbon chains of different lengths. These are often segregated by the number of carbon atoms.

• Fractional distillation works based on the principle that the different sizes of the hydrocarbon molecules mean they evaporate at different temperatures.

• At the bottom of the distillation column, the temperatures are highest and this is where the heaviest fractions with long chain hydrocarbon molecules are collected.

• As you move up the column, the temperature decreases, allowing lighter fractions to be collected. These contain smaller hydrocarbon molecules.

• Examples of fractions (from heaviest to lightest) include fuel oil, diesel oil, kerosene (paraffin), naphtha, petrol and refinery gases. Each fraction has different uses depending on the properties of the hydrocarbons it contains.

• Heavier fractions like fuel oil are used in large ships and power stations. These fractions are less valuable as they are less versatile.

• Lighter fractions such as petrol are highly valued for their use in vehicles because they ignite easily (have low ignition temperatures).

• Organic chemistry is the study of the properties, composition, reactions, and structure of organic compounds, which contain carbon atoms. Many of these organic compounds are produced from the fractions obtained from crude oil.

• Some fractions are not useful as a fuel and so they are chemically processed or ‘cracked’ to produce shorter, more useful molecules. This process is called ‘cracking’.

• It is also possible to use fractions as feedstocks to create a range of different chemicals, which can be used to produce everyday items such as plastic, solvents, detergents and even explosives.

• Alkanes and Alkenes are examples of hydrocarbons that can be made from these fractions. Alkanes are saturated (single bonds only) and Alkenes are unsaturated (contain a carbon-carbon double bond).

• It’s essential to remember that while these fractions and their uses are crucial for modern living, they are non-renewable and their combustion results in greenhouse gases, so the search for alternative, renewable energy sources is an ongoing global concern.