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Making crude oil useful. Before crude oil can be used, it must first be separated into fractions . This process is called fractional distillation. Each fraction contains a mixture of different hydrocarbon molecules of a similar size and with similar properties.
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Making crude oil useful Before crude oil can be used, it must first be separated into fractions. This process is called fractional distillation. Each fraction contains a mixture of different hydrocarbon molecules of a similar sizeand with similar properties. Very large molecules can be split into several smaller molecules, which are more useful. This process iscalled catalytic cracking.
Fraction Boiling point (°C) LPG gasoline naphtha kerosene diesel lubricating oil fuel oil residue Boiling point of fractions Each fraction of crude oil contains a mixture of different compounds. This means that the boiling point of the fraction is not a fixed temperature but a range. < 0 20-200 20-180 180-260 260-340 370-600 >330 n/a
Generally, the smaller the molecules in a fraction, the more volatile and flammable the fraction. Volatility and flammability Fractions that have a low boiling point evaporate easily. The easier a fraction evaporates, the more volatile it is. When fractions burn, they react with oxygen in the air. The more volatile a fraction is, the easier it mixes with air. This means the fraction ignites and burns easily. Fractions that ignite and burn easily are flammable.
LPG fuel oil gasoline kerosene diesel Composition of crude oil The exact composition of crude oil depends on where in the world the oil comes from. Crude oil from the North Sea is a ‘light’ oil because it contains a large amount of small molecules and so a larger proportion of lighter fractions. Crude oil from Saudi Arabia is a‘heavy’ oil because it contains a large amount of large molecules and so a larger proportion of heavier fractions.
The longer the hydrocarbon chains in a fraction, the more viscous the fraction will be. What is viscosity? Some fractions of crude oil are thin and runny. Other fractions are thick and sticky. The runniness of a liquid is called viscosity. For example, the residue from fractional distillation has a very high viscosity (it is viscous) and cannot be easily poured. Gasoline has a low viscosity and pours easily. What is the relationship between the length of a hydrocarbon chain and the viscosity of a fraction?
Molecule size and viscosity Why are fractions with large hydrocarbon molecules more viscous than fractions with small hydrocarbon molecules? The longer chains of large hydrocarbon molecules are easily entangled. Smaller molecules have shorter chains and are less likely to become entangled.
decrease in size of molecules Colour of fractions The colour of a fraction depends on the size of the molecules it contains. As the molecules get smaller, the colour of the fraction becomes lighter, from dark brown to light brown, orange/yellow and transparent.
Properties of fractions How do the properties of a fraction affect its uses?
Hydrocarbons as fuels Over 85% of crude oil is used as a fuel for heating and transport. Only 8% is used to make plastics and other chemicals. A large amount of oil is used as fuel because the hydrocarbons in the fractions burn easily and produce a large amount of useful energy. A fuel is a substance that reacts with oxygen to produce useful energy.
What is LPG used for? Liquid petroleum gas (LPG) contains propane and butane. They are stored under pressure in metal cylinders. LPG is used for heating and cooking, especially where piped gas cannot be used, such as camp sites and boats.
What is gasoline used for? Gasoline is one of the most important fractions because it is used as a fuel for cars. The gasoline fraction must be refined before it can be used in cars as petrol. Impurities are removed, and chemicals are added to ensure the engine runs smoothly. In the past, these chemicals were lead compounds but because of lead’s harmful effect on health and the environment, they have been replaced by safer compounds.
What is naphtha used for? Naphtha is a very important fraction that has many uses. For example, it is: • a raw material (feedstock) in the production of chemicals and plastics • used as a cleaning fluid and industrial solvent(a substance that can dissolve another substance) • added to petrol and other fuels.
What are kerosene and diesel used for? Kerosene, also called paraffin, is mainly used as aviation fuel for jet aircraft. It is also used in central heating systems, lamps and stoves. Diesel is mainly used as a fuel for lorries, buses and cars, and in some boats.
What are lubricating and fuel oil used for? Lubricating oil, as its name suggests, is used as a lubricant in products with moving parts, such as engines. Fuel oil is a heavy fraction used as a fuel for large industrial boilers, oil-fired power stations and ships.
What is the residue used for? Residue contains paraffin waxes and bitumen, the heaviest product to come from an oil refinery. Paraffin waxes (not the same as paraffin/kerosene) are used in candles and waterproofing. Bitumen is used in road construction and waterproof roof material.
Glossary • crude oil – A naturally-occurring mixture of different-sized hydrocarbon molecules. • flammable – A substance that easily catches fire. • fraction – A mixture of hydrocarbon molecules with similar boiling points. • hydrocarbon – A molecule containing only hydrogen and carbon. • viscosity – A measure of how easily a liquid flows. • volatile – A compound that evaporates easily.
