C1.4 Crude oil and fuels

What is crude oil?
Crude oil is a mixture of a large number of compunds, mostly hydrocarbons (molecules of hydrogen and carbon atoms only) Most of the hydrocarbons in crude oil are alkanes. It is a fossil fuel (formed from tiny animals and plants in the sea) which gives us petrol and is found between porous and non-porous rock.

What is a mixture?
Two or more elements and/or compounds not chemically combined together. The chemical properties of each substance are unchanged.

What are alkanes?
Saturated molecules with the general formula CnH2n+2
For example, butane has 4 carbon atoms so the chemical formula would be C4H10.

What does the term saturated mean?
The carbon atoms in the molecule are joined together by only single covalent bonds (not double)

methane, CH₄   ethane, C₂H₆  propane, C₃H8  butane, C₄H₁₀

Remember that carbon atoms each form 4 covalent bonds and hydrogen atoms each form 1. Covalent bonds form when atoms share electrons. 

Properties of different fractions from crude oil:

The higher the number of carbon atoms in an alkane molecule:

• Less volatile/higher boiling point (condenses lower in the fractionating column)
• More viscous (substances with many carbon atoms are usually solids)
• Less flammable

Fractional distillation

Used to separate crude oil into several fractions in a continuous process. The fractions are mixtures of hydrocarbon molecules with similar properties.

1. Oil is heated to about 400 degrees c to vaporise/evaporate it

2. Vapour from the oil enter the fractioning tower, which is at a higher temperature at the bottom

3. Each fraction condenses at a different level and temperature because it has a different range of boiling points 

Most fuels contain carbon, hydrogen and possibly sulfur. During combustion, the fuels may release the following gases to the atmosphere: carbon dioxide, water (vapour), carbon monoxide, sulfur dioxide, oxides of nitrogen, and particulates. 

Complete combustion

Combustion of hydrocarbon fuels releases energy to the surroundings (exothermic reaction): hydrogen reacts with oxygen and is oxidised to produce water vapour:
2H2 + O2 -> 2H2O
And carbon reacts with oxygen and is oxidised to produce carbon dioxide:
C + O2 -> CO2 

Complete combustion occurs when there is plenty of oxygen.

Incomplete/partial combustion

Where there is insufficient oxygen for complete combustion. Hydrogen is still oxidised to water vapour, but the carbon is not fully oxidised, and produces carbon monoxide instead. Particulates/solid particles are also released. 

Possible products from hydrocarbon/fossil fuel combustion:

Carbon dioxide
Contributes to global warming/the greenhouse effect, hence climate change and rising sea levels. Can be reduced by capturing and storing carbon.

Carbon monoxide
Toxic gas- reduces oxygen to organs and red blood cells. Hard to detect and is produced by incomplete combustion. Can be reduced by catalytic converters which convert CO to CO2.

Sulfur dioxide
When burnt, sulfur oxidises to sulfur dioxide gas (S + O2 -> SO2) and leads to acid rain, which damages rocks, buildings, trees and aquatic life. Can be reduced by reducing sulfur from the fuel e.g. scrubbing, or removing sulfur dioxide from waste gases after burning e.g. in power stations.

Nitrogen oxides
Form at high temperatures e.g car engines. Produced from nitrogen and oxygen in the air to produce NOx. Can contribute to acid rain and can be reduced using catalytic converters.

Particulates
Contain soot/carbon and unburned fuel. Absorbs sunlight so contributes to global dimming and can cause lung and heart disease. 

Fossil fuels are in limited supply and are associated with many pollution problems.
Biofuels are produced from plant material. Burning biofuels releases less carbon dioxide overall than that of fossil fuels, almost carbon neutral: carbon dioxide is absorbed during photosyntesis when plants for biofuels are grown.

Advantages:

• Biodiesel can be used in existing diesel engines
• Almost carbon neutral
• No sulfur dioxide released
• Cheaper
• Bioethanol can be mixed with petrol

Disadvantages:

• Farmland could have been used for food instead of biofuel production
• Destruction of habitats to grow fuel
• Non renewable resources used indirectly (making fertilisers, providing energy during manufacture and transport)

Hydrogen
Can be made through electrolysis (passing electricity through water). Water vapour is the only product made when hydrogn burns: 2H2 + O2 -> 2H2O. 

However, unsafe- vioent reaction with oxygen, difficult to obtain and store, mostly produced from fossil fuels today. Electricity for electrolysis may be generated by fossil fuels.