Crude Oil Refining

What do these have in Common

Crude Oil

What is crude oil?

Crude oil is a fossil fuel and one of the most important substances in the world. It is a mixture of hundreds of different compounds.

Crude oil is used to • Make fuels for transport• Heating • Generating electricity.• Making Plastics• Different Chemicals

Every day, the world uses over 70 million barrels of oil.

Crude oil: a brief history

The modern oil industry began in the mid-19th century when kerosene was distilled from crude oil and used as cheap and clean-burning fuel for lamps.

It wasn’t until the development of the internal combustion engine and gasoline-powered cars in the late 19th century that gasoline became highly valuable.

Gasoline was a by-product inkerosene production, and was initially used as a solvent.

This led to a great increase in the demand for crude oil, which has continued to this day.

How was crude oil made?

Crude oil is thought to have been made from the remains of marine plants and animals that died millions of years ago.

These remains sank to thebottom of the sea, where theywere buried in layers of sand andmud, preventing them from rotting. These layers gradually became sedimentary rock.

Over millions of years the layers of rock built up, increasing the heat and pressure. This caused the remains to be broken down into the molecules that form crude oil and natural gas.

Hydrocarbons in crude oil

Many compounds in crude oil only contain the elements carbon and hydrogen. They are called hydrocarbons.

Most hydrocarbons in crude oil are compounds called alkanes.

Alkanes contain a single chain of carbon atoms with hydrogen atoms bonded along the side.

What are alkanes?

Alkanes are a family of hydrocarbon compounds with the general formula CnH2n+2.

The simplest alkane is methane. It has the formula CH4.

The second simplest alkane is ethane. It has the formula C2H6.

The third simplest alkane is propane. It has the formula C3H8.

Where in the world is crude oil?

Finding crude oil at sea

Crude oil is often found trapped in rocks of the sea bed.

Oil rigs or drilling platforms are used to drill through the sea bed to obtain the oil.

Once the oil has been removed from the sea bed, it is pumped in long pipelines to an oil tanker terminal or an oil refinery on land.

Crude oil itself has no uses – it must first be processed or refined. This is done in an oil refinery.

The first step is to separate compounds in the oil into groups called fractions.

Each fraction contains a mix of compounds with a similar number of carbon atoms.

How can crude oil be made useful?

Molecule size and boiling point

The more carbon atoms in a hydrocarbon molecule, the larger the molecule. How does this affect its boiling point?

Generally, the larger a hydrocarbon, the higher its boiling point.

This is because the intermolecular forces between large molecules are stronger than the intermolecular forces between small molecules.

More energy is needed to break the forces between large molecules, and so the boiling point is higher.

Molecules in crude oil can contain anything from just 1 carbon atom to well over 50.

Boiling point of alkanes

What is fractional distillation?

Fractional distillation is a process used to separate a mixture of liquids that have different boiling points.

Fractional distillation is used to separate crude oil into fractions with different

boiling points.

When the mixture is heated, liquids with a low boiling point evaporate and turn to vapour.

Liquids with a higher boiling point remain as liquid. The vapour can then be separated from the liquid.

Fractional distillation of crude oil

Crude oil is separated into fractions by fractional distillation.

1. Oil is heated to about 450 °C and pumped into the bottom of a tall tower called a fractionating column, where it vaporizes.

2. The column is very hot at the bottom but much cooler at the top. As the vaporized oil rises, it cools and condenses.

3. Heavy fractions (containing large molecules) have a high boiling point and condense near the bottom of the column.

4. Lighter fractions (containing small molecules) have a lower boiling point and condense further up the column.

How does fractional distillation work?

Supply and demand

The amount of each type of fraction obtained by fractional distillation does not usually match the amount of each fraction that is needed.

Crude oil often contains more heavier fractions than lighter fractions, which are more useful and therefore more desirable.

The large hydrocarbon moleculesin the heavier fractions can bebroken down into smaller, more useful, molecules to meet demandfor raw materials for fuels and plastics.

Catalytic cracking

Large hydrocarbon molecules can be broken down into smaller molecules using a catalyst. This is called catalytic cracking.

The smaller molecules produced by catalytic cracking are a mixture of alkanes and alkenes.

The hydrocarbon molecules are heated until they turn into vapour, and then mixed with a catalyst. The type of reaction in catalytic cracking is called thermal decomposition.

Alkenes are reactive molecules that are used to make plastics and other chemicals.

What are alkenes?

Alkenes are a family of hydrocarbon compounds with the general formula CnH2n.

Alkenes are very similar to alkanes, but they have one important difference: they contain at least one double covalent bond between carbon atoms.

The simplest alkene is ethene. It has the formula C2H4.

The second simplest alkene is propene. It has the formula C3H6.

Cracking decane

Decane from the naphtha fraction can be cracked to form pentane (for use in petrol), propene and ethene.

decane (C10H22)

pentane (C5H12)

propene (C3H6)

ethene (C2H4)

+

+

Saturated vs. unsaturated

Alkanes are examples of saturated compounds.

Alkenes are examples of unsaturated compounds.

A saturated compound only contains single covalent bonds between carbon atoms.

An unsaturated compound contains at least one double covalent bond between carbon atoms.

A test to distinguish between saturated and unsaturated compounds is to add red bromine water. In the presence of unsaturated compounds, the red colour disappears.

How does catalytic cracking work?

Removing impurities

An important step in refining is removing impurities from fractions. These can damage equipment that uses the fraction, and cause pollution.

One of the most important impurities to remove is Sulphur.

When burnt, this forms the gas Sulphur dioxide, which causes acid rain.

Other impurities that need to be removed include nitrogen, oxygen, water and dissolved metals.

alkanes – A family of hydrocarbon molecules with the general formula CnH2n+2.

alkenes – A family of hydrocarbon molecules with the general formula CnH2n.

catalytic cracking – A reaction where a large molecule is broken down into smaller molecules in the presence of a catalyst.

crude oil – A naturally-occurring mixture of different-sized hydrocarbon molecules.

fraction – A mixture of hydrocarbon molecules of a similar size.

Glossary (1/2)

fractional distillation – The process used to separate crude oil into different fractions.

hydrocarbon – A molecule containing only hydrogen and carbon.

saturated – A compound that only contains single covalent bonds between carbon atoms.

unsaturated – A compound that has at least one double covalent bond between carbon atoms.

Glossary (2/2)

Problems with crude oil

Crude oil is extremely useful but it has several drawbacks:

Burning the products of crude oil as fuel produces gases and particles that contribute to global warming and air pollution.

Removing oil from the ground, as well as oil spillages and slicks, can be very damaging to wildlife and the environment.

The high value of oil means it is sometimes the cause of military conflict, especially because a large amount of oil comes from countries that are politically unstable.

When will crude oil run out?

It is difficult to say when crude oil will run out because no-one knows exactly how much oil there is left in the world.

Some scientists believe there is a lot more oil still to find, but others think that most oil has already been discovered.

There are over 1 trillion (1 million million) barrels of crude oil in proven oil reserves

At current rates, this will last about 44 years, but the amount of oil used in the world increases each year.

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