Making crude oil useful
Crude oil is a mixture of hydrocarbons. These are separated into useful products, such as fuels, using a process called fractional distillation.
The demand for short hydrocarbon molecules is greater than their supply in crude oil, so a reaction called cracking is used. Cracking converts long alkane molecules into shorter alkanes and alkenes, which are more useful. The exploitation of oil can damage the environment - for example, through oil spills.
Fossil fuels
Crude oil, coal and gas are fossil fuels. They were formed over millions of years, from the remains of dead organisms,
- coal was formed from dead plant material
- crude oil and gas were formed from dead marine organisms.
Fossil fuels are non-renewable. They took a very long time to form and we are using them up faster than they can be renewed. Fossil fuels are also finite resources. They are no longer being made or are being made extremely slowly. Once they have all been used up, they cannot be replaced.
How crude oil was formed (background information only)
Crude oil is found trapped in some of the sedimentary rocks of the Earth's crust.
Millions of years ago, huge numbers of microscopic animals and plants - plankton - died and fell to the bottom of the sea. Their remains were covered by mud.
As the mud sediment was buried by more sediment, it started to change into rock as the temperature and pressure increased. The plant and animal remains were ‘cooked’ by this process, and slowly changed into crude oil.
Oil is less dense than the water in the rocks and will rise as a result of pressure from below (as can be seen in the animation above). often the oil will escape altogether if the rocks are permeable (liquids can pass through them).
If some of the rocks above the oil are impermeable the oil cannot rise through them, so it gets trapped underneath.
Problems of exploiting oil
Geologists can often tell where oil is trapped by looking at the structure of the rocks. Oil tends to be trapped where rocks are domed upwards, or where permeable rocks are in contact with impermeable rocks at a fault line.
Drilling for oil
Oil companies can drill down through the impermeable rocks to get it out. They are then able to turn the oil into products that we can use.
Crude oil takes millions of years to form, and we are using it up more quickly than it is created. Present estimates suggest world supplies of crude oil will run out in about 30 years, unless we use it more efficiently. There are additional reserves of oil in rocks called oil shale. However, it is expensive to extract oil from oil shale because it needs to be heated to release it.
Environmental problems
Oil is carried from oil fields to refineries using ocean-going tankers. If it is spilled, it causes considerable damage to the environment:
- oil slicks travel across the sea, far from the original spill
- beaches and wildlife are harmed when they are coated with oil.
The oil damages feathers and birds may die. Detergents are often used to help clean up oil slicks, but these in turn may harm wildlife.
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Distillation
Distillation is a process that can be used to separate a pure liquid from a mixture of liquids. It works when the liquids have different boiling points. Distillation is commonly used to separate ethanol (the alcohol in alcoholic drinks) from water.
The mixture is heated in a flask. Ethanol has a lower boiling point than water so it evaporates first. The ethanol vapour is then cooled and condensed inside the condenser to form a pure liquid.
The thermometer shows the boiling point of the pure ethanol liquid. When all the ethanol has evaporated from the solution, the temperature rises and the water evaporates.
This is the sequence of events in distillation:
heating → evaporating → cooling → condensing
Fractional distillation
Hydrocarbons have different boiling points. They can be solid, liquid or gas at room temperature,
- small hydrocarbons with only a few carbon atoms have low boiling points and are gases
- hydrocarbons with between five and 12 carbon atoms are usually liquids
- large hydrocarbons with many carbon atoms have high boiling points and are solids.
Because they have different boiling points, the substances in crude oil can be separated using fractional distillation.
The fractionating column
Fractional distillation is different from distillation in that it separates a mixture into a number of different parts, called fractions. A tall column is fitted above the mixture, with several condensers coming off at different heights. The column is hot at the bottom and cool at the top. Substances with high boiling points condense at the bottom and substances with lower boiling points condense on the way to the top.
The crude oil is evaporated and its vapours condense at different temperatures in the fractionating column. Each fraction contains hydrocarbon molecules with a similar number of carbon atoms.
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