Separating Crude Oil (GCSE Chemistry)

Separating Crude Oil

Separating Crude Oil

Fractions and Their Uses

• Lots of compounds make up crude oil. As previously mentioned, crude oil is made up of various hydrocarbons. These can be separated by a process called fractional distillation, into fractions.
• Within a fraction, molecules contain a similar numbers of carbon atoms. When crude oil gets separated into fractions, similar molecules group together. Usually, molecules group together by the number of carbon atoms that they contain. Therefore, each fraction consists of molecules with a similar number of carbon atoms.
• Fractions can be processed. Once the fractions are separated out of the crude oil mixture, they can be processed. The processed fractions can then be used to produce fuels and feedstock for the petrochemical industry.

The Process of Fractional Distillation

For exams, you are expected to be able to describe the process of fractional distillation.

1. Crude oil is heated. The liquid crude oil is heated within a chamber, to about 350 – 400 °C, until most of it evaporates and forms a gas. Any liquid is drained off as the crude oil gas enters the fractionating column.
2. A temperature gradient is set up in the fractionating column. The bottom of the fractionating column has the highest temperature and the top of the column has the lowest temperature.
3. Long hydrocarbons condense first. The longest hydrocarbons have higher boiling points. This is because there are more weak intermolecular forces between the molecules, as they are larger. More energy is needed to separate the molecules. Larger molecules will cool and condense and turn into liquids at higher temperatures, at the bottom of the column.
4. Short hydrocarbons condense last. The shorter hydrocarbons have  lower boiling points. They have fewer weak intermolecular forces so less energy is needed to separate the molecules. Shorter hydrocarbons remain as gases until they reach the top of the column where it is cooler; they cool and condense here. 

• Different boiling points are utilised. Throughout the process of fractional distillation, the different boiling points of hydrocarbons are utilised. Depending on their boiling points, different fractions are formed at different levels of the column.
• Evaporation and condensation are key processes in fractional distillation. At the beginning of the fractional distillation process, crude oil is heated and most of it evaporates. It enters the fractionating column as a gas. As the gas rises up the column, the crude oil fractions cool and condense out at different levels,  depending on their boiling points.
• Fractions can be used as fuels. Each fraction has different properties due to the length of the carbon chain. Therefore, we can use each fraction for different purposes. The shortest chain hydrocarbons are used for petroleum gas and petrol. Those with the longest chains can be used for heavy fuel oil and as bitumen for road making.

Crude Oil Products

The Petrochemical Industry

• Hydrocarbons can form new compounds. Hydrocarbons from crude oil are used in the petrochemical industry as feedstock in order to  make new compounds. These new compounds can form polymers, solvents, lubricants and detergents.

Natural and Synthetic Carbon Compounds

• Carbon can form different compounds. Due to its structure and bonding, carbon can form various compounds which have different properties.  Compounds with similar properties can form a homologous series. For example, alkanes are a homologous series.

Uses of Fractions

You may be asked about the different names of fractions and their uses, seen in the table below. As you move down the table, the molecules increase in size and therefore have a higher boiling point and are not very volatile. This means they will not flow or ignite as easily. You can relate these properties to the uses of each fraction.

Using Hydrogen in Cars

There are several advantages and disadvantages to using hydrogen as a fuel in cars, rather than petrol. This can be summarised in a table as seen below: