DF3 - Crude Oil
Crude Oil is a mixture of hundreds of different hydrocarbons - compounds of only carbon and hydrogen. The hydrocarbons are separated into fractions by fractional distillation. A fraction is a mixture of compounds with a specific boiling point range. These are then used as fuels.
- Gasoline - 25 to 75 degrees Celsius - 5 to 7 carbons - petrol
- Naptha - 75 to 190 degrees Celsius - 6 to 10 carbons - medicines
- Kerosene - 190 to 250 degrees Celsius - 10 to 16 carbons - jet fuel
- Gas Oil - 250 to 350 degrees Celsius - 14 to 20 carbons - diesel
Oil from the North Sea is pumped along pipes on the seabed to UK refineries. At the refinery, the crude oil is heated to vaporise it and the vapour passes into a distillation column, where there is a temp. gradient - coolest at the top; hottest at the bottom. The less volatile hydrocarbons condense on the trays across the column.
Crude oil contains a surplus of the high boiling fractions - like gas oil and residue - but not enough of the lower boiling fractions, like gasoline. The demand for petrol and diesel is greater than the supply.
DF3 - Crude Oil
The refinery must convert crude oil into useful components. To do this, the structure of the alkane molecules present must be altered to produce different alkanes, such as cycloalkanes - rings of carbon atoms - and arenes - aromatic hydrocarbons with benzene ring. This involves different processes: 'isomerisation', 'reforming' and/or 'cracking'. The products are blended to produce high-grade petrol.
Alkenes contain C=C double bond. These are needed to make useful compounds, like plastics, medicines and dyes.
Vacuum Distillation is a technique for distilling liquids which decompose when heated to their boiling point. Distillation cannot be carried out at atm pressure, but if external pressure is reduced, then so is the boiling point, and distillation can then be achieved below decomposition temp.
DF3 - Petrol
A mixture of petrol vapour and air is ignited in a cylinder in a car engine. The vapour-air mixture is made in the carburettor, but when it's cold, petrol is difficult to vapourise, so it's difficult to start the car.
Petrol companies make different blends for different times of the year. In winter, they put more volatile components so it vaporises more readily. This means putting in more hydrocarbons with small molecules - eg butane and pentane.
In hot weather, you don't want too many volatile components, because you'd lose petrol from the tank by evaporation - which is costly and polluting. The fuel pump then delivers a mixture of liquid and vapour to the carburettor, and this means that not enough fuel gets through to keep the engine running - vapour lock.
Another important property that blenders must take into accound is the octane number of the petrol. This is the tendency of a petrol-based fuel to auto-ignite. The fuel-air mixture in the engine has to ignite at the right time. Many hydrocarbons auto-ignite under high compression. The mixture catches fire as it's compressed, and two explosions occur - one due to the compression, and another when the spark occurs (when engine is turned on). This produces a problem - 'knocking' sound - in the engine.
DF3 - Octane Number
Auto-ignition is the explosion of a fuel without a spark. The octane number is a measure of the tendency of petrol to auto-ignite. This causes:
- reduced engine performance
- engine damage.
octane number 100 --> octane number 0
low tendency to auto-ignite high tendency to auto-ignite
- Short-chain compounds have higher octane number than long-chain compounds
- Branched-chain compounds have higher octane number
- Cycloalkanes have higher octane numbers
- Arenes have higher octane number than cycloalkanes
- Oxygenates have higher octane numbers and are added to petrol to increase octane number
- Cracking, Reforming, Isomerisation increase octane numbers of petrol components
DF3 - Diesel
Diesel fuel is a mixture of much larger hydrocarbon molecules than those found in petrol.
- Diesel from crude oil contains about 75% saturated hydrocarbons (mostly alkanes and cycloalkanes)
- Contains 25% aromatic hydrocarbons
- avg chemical formula: C12H26, with a range of 10 to 15 carbons
Diesel engine only takes in air. Then, it compresses it and injects fuel into compressed air. The heat of the compressed air ignites the fuel spontaneously. In this way, a diesel engine works by auto-ignition.