Crude oil is a mixture of compounds called hydrocarbons. Many useful materials can be produced from crude oil. It can be separated into different fractions using fractional distillation, and some of these can be used as fuels. Unfortunately, there are environmental consequences when fossil fuels such as crude oil and its products are used.
Hydrocarbons and alkanes
Hydrocarbons
Most of the compounds in crude oil are hydrocarbons. This means that they only contain hydrogen and carbon atoms, joined together by chemical bonds. There are different types of hydrocarbon, but most of the ones in crude oil are alkanes.
Alkanes
The alkanes are a family of hydrocarbons that share the same general formula. This is:
CnH2n+2
The general formula means that the number of hydrogen atoms in an alkane is double the number of carbon atoms, plus two. For example, methane is CH4 and ethane is C2H6. Alkane molecules can be represented by displayed formulae in which each atom is shown as its symbol (C or H) and the chemical bonds between them by a straight line.
Structure of alkanes
| alkane | formula | chemical structure | ball-and-stick model |
|---|---|---|---|
| methane | CH4 | ||
| ethane | C2H6 | ||
| propane | C3H8 | ||
| butane | C4H10 |
Notice that the molecular models on the right show that the bonds are not really at 90º.
Alkanes are saturated hydrocarbons. This means that their carbon atoms are joined to each other by single bonds. This makes them relatively unreactive, apart from their reaction with oxygen in the air, which we call burning or combustion.
Boiling point and state at room temperature
Hydrocarbons have different boiling points, and can be either 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.
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
Fractional distillation differs from distillation only 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 low boiling points condense at the top. Like distillation, fractional distillation works because the different substances in the mixture have different boiling points.
Fractional distillation of crude oil
Because they have different boiling points, the substances in crude oil can be separated using fractional distillation. The crude oil is evaporated and its vapours allowed to condense at different temperatures in the fractionating column. Each fraction contains hydrocarbon molecules with a similar number of carbon atoms.
Oil fractions
The diagram below summarises the main fractions from crude oil and their uses, and the trends in properties. Note that the gases condense at the top of the column, the liquids in the middle and the solids stay at the bottom.
The main fractions include refinery gases, gasoline (petrol), naphtha, kerosene, diesel oil, fuel oil, and a residue that contains bitumen. These fractions are mainly used as fuels, although they do have other uses too.
Hydrocarbons with small molecules make better fuels than hydrocarbons with large molecules because they are volatile, flow easily and are easily ignited.
Combustion of fuels
Complete combustion
Fuels burn when they react with oxygen in the air. The hydrogen in hydrocarbons is oxidised to water (remember that water, H2O, is an oxide of hydrogen). If there is plenty of air, we get complete combustion and the carbon in hydrocarbons is oxidised to carbon dioxide:
hydrocarbon + oxygen → water + carbon dioxide
Incomplete combustion
If there is insufficient air for complete combustion, we get incomplete combustion instead. The hydrogen is still oxidised to water, but instead of carbon dioxide we get carbon monoxide. Particles of carbon, seen as soot or smoke, are also released.
Sulfur
Most hydrocarbon fuels naturally contain some sulfur compounds. When the fuel burns, the sulfur it contains is oxidised to sulfur dioxide.
Summary
The combustion of a fuel may release several gases into the atmosphere, including:
- water vapour
- carbon dioxide
- carbon monoxide
- particles
- sulfur dioxide
These products may be harmful to the environment.
Sulfur dioxide
Sulfur dioxide is produced when fuels that contain sulfur compounds burn. It is a gas with a sharp, choking smell. When sulfur dioxide dissolves in water droplets in clouds, it makes the rain more acidic than normal. This is called acid rain.
Effects of acid rain
Acid rain reacts with metals and rocks such as limestone. Buildings and statues are damaged as a result. Acid rain damages the waxy layer on the leaves of trees and makes it more difficult for trees to absorb the minerals they need for healthy growth. They may die as a result. Acid rain also makes rivers and lakes too acidic for some aquatic life to survive.
Reducing acid rain
Sulfur dioxide can be removed from waste gases after combustion of the fuel. This happens in power stations. The sulfur dioxide is treated with powdered limestone to form calcium sulfate. This can be used to make plasterboard for lining interior walls, so turning a harmful product into a useful one.
Sulfur can be removed from fuels at the oil refinery. This makes the fuel more expensive to produce, but it prevents sulfur dioxide being produced. You may have noticed 'low sulfur' petrol and diesel on sale at filling stations.
Global warming
Carbon dioxide from burning fuels causes global warming, a process capable of changing the world’s climate significantly.
As you can see from the graphs, the amount of carbon dioxide in the atmosphere has increased steadily over the past 150 years, and so has the average global temperature.
Carbon dioxide is a greenhouse gas. It absorbs heat energy and prevents it escaping from the Earth’s surface into space. The greater the amount of carbon dioxide in the atmosphere, the more heat energy is absorbed and the hotter the Earth becomes.
Global dimming
Tiny particles that are released when fuels are burned cause global dimming. Like global warming, this process may change rainfall patterns around the world.
The amount of sunlight reaching the Earth’s surface has decreased by about 2 per cent every ten years, because more sunlight is being reflected back into space. The particles from burning fuels reflect sunlight, and they also cause more water droplets to form in the clouds. This makes the clouds better at reflecting sunlight back into space.
It is likely that global dimming has hidden some of the effects of global warming, by stopping some of the Sun’s energy reaching the Earth’s surface in the first place. Governments around the world are introducing controls on pollution. There is the possibility that as the air becomes less polluted by smoke and soot, global dimming will decrease, causing the effects of global warming to become more obvious.
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