Crude oil is a subtance that contains hundreds of different hydrocarbons of different lengths.
Properties of Hydrocarbons found in Crude Oil:
As the hydrocarbon molecule gets bigger, the intermolecular forces increase, meaning that...
- The boiling point increases because the bigger molecules have a stronger force holding them together, so more heat is needed to break the bonds.
- The liquids become less volatile; the bigger it is, the slower it evaporates so it doesn't turn to gas as easiy as the smaller molecules.
- The liquids become more viscous (flow less easily).
- Bigger hydrocarbons burn less easily so they are less usful as fuels.
Separating Crude Oil - Fractional Distillation
Crude oil is heated and passed into a fractionating column, which is cooler at the top (around 40 degrees) and hotter at the bottom (around 400 degrees). The difference in heat separates the oil into different lengths of hydrocarbons.
- The smaller hydrocarbons condense nearer to the top of the column because their boiling point is much lower. The larger ones condense at the bottom, separating them into different lengths.
- The largest hydrocarbons are not always condensed so they remain at the bottom to be removed. Bitumen is a residue of this, used to tarmac roads - it is very thick.
Uses of the Fractions:
All hydrocarbons release a huge amount of heat when they burn, so they can be used as fuels. E.g. Methane - CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)
- Refinery gases - Used as LGP (liquefied petroleum gas) for domestic heating and cooking.
- Gasoline (petrol) - Used to power transport and other machinery.
- Kerosine - Used as fuel for jet engines and domestic heating oil.
- Diesel oil (gas oil) - Used to power buses, lorries, trains and some cars.
- Fuel oil - Used for ships boilers and industrial heating.
Problems with fractional distillation:
- We have no control over how much of each hydrocarbon there is in the oil so some may have very little petrol when that is the thing we need the most of.
- Apart from burning, the hydrocarbons are fairly unreactive which is not very useful to us.
Cracking is a way of splitting the hydrocarbons to get the most useful ones.
How it works:
- The fraction is heated then passed over a catalyst (silicon dioxide mixed with aluminium oxide) at about 600-700 degrees. It can be done at higher temperatures without a catalyst.
- The molecules are broken up in a random way and there can be many different combinations of molecules from the same hydrocarbon.
- E.g. C13H28(l) -> C2H4(g) + C3H6(g) + C8H18(l)
- Notice that both alkanes and alkenes are formed in the process of cracking.