Fuels From Crude Oil
Crude oil contains many different compounds that boil at different temperatures. These burn under different conditions so need to be seperated to make useful fuels.
We can seperate mixtures of liquids by distillation, simple distillation of crude oil can produce liquids that boil within different temperature ranges.
Most of the compounds in crude oil are hydrocarbons, this means they contain only carbon and hydrogen. Many of these hydrocarbons are alkanes, with a general formula Cn H2n+2. Alkanes contain as many hydrocarbon atoms as possible in each molecule and so we call them saturated hydrocarbons.
Molecules can be represented by:
- a molecular formula that shows the number of each type of atom.
- a structural formula that shows how the atoms are bonded together.
Crude oil is seperated at refineries by fractional distillation. The crude oil is vaporised and fed into a fractionating column. This is a tall tower that is hot at the bottom and cooler at the top.
Oil is vapourised before it enters the column and enters the column at 350 degrees, any residue (a thick sticky mixture of long chain hydrocarbons used to make roads and flat roofs) then drops out the the column. Diesel and gas oils used as fuels for diesel engines and as bolier are the next to come out the column. Next Keronsene, used for aircraft fuel, condenses and leaves the column. Petrol, used for car engines, condenses next. Lastly at around 50 degrees refinery/petroleum gases (short chain hydrocarbons and low boiling point alkanes, used as fuel) condenses.
Inside the column there are many trays with holes to allow gases through, as the vapours move up the column and condense on the tray when they reach their boiling points. There are outlets are different levels to collect the liquid fractions. Hydrocarbons with the lowest boiling point and smallest molecules are collected at the top of the tower, the fractions collected at the bottom have the highest boiling points. Frations with low boiling points burn better and so are more useful.
When pure hydrocarbons burn competely they produce carbon dioxide and water.
Propane + oxygen = carbon dioxide + water
C3H8 + 5O2 = 3CO2 + 4H2O
However, fuels we use are not always burnt completely and they many contain other substances.
In a limited air supply, such as an enginem carbon monoxide and carbon may also be produced, and some of the hydrocarbons may not burn. Carbon and unburnt hydrocarbons form tiny particles of air.
Most fossil fuels contain sulphere compounds, when the fuel burns these sulfur compounds produce sulfur dioxide.
The products from burning fossil fuels are released into the atmosphere in exhaust gases.
- Carbon dioxide is a greenhouse gas. It reduces the amount of heat lost from the Earth and causes global warming,.
- Particulates cause health problems and cause global dimming by reflecting sunlight away from the Earth.
- Sulfur dioxide dissolves in water and forms acid rain. The amount of sulfur dioxide released can be reduced by removing sulfur dioxide from waste gases after burning.
Fossil fuels resourses are being used up and will run out in the future. Plants can provide sugars to make ethanol, or can produce oils that can be used as biodiesel.