Crude oil is a mixture of hydrocarbons. Fractional distillation is used to separate out this mixture based on the different boiling points. To reduce the chances of knocking, straight chain alkanes can be converted to branched or cyclic hydrocarbons via reforming.
Fractional distillation
Crude oil is a mixture of hydrocarbons. However, this mixture is not useful so it is separated out using fractional distillation.
1.
Crude oil
2.
Heater
3.
Gases
4.
Petrol
5.
Naptha
6.
Kerosene
7.
Gas oil
8.
Mineral oil
9.
Residue
10.
Tray
11.
Fractionating column
Procedure
1.
Crude oil is vaporised at around 350°C.
2.
Different hydrocarbons have different boiling points which means they will evaporate off at different temperatures as the column heats up and the liquids boil.
3.
When the boiling point is reached, the liquid will turn into a gas and rise up the column then condense out.
4.
Liquids with the highest boiling point will not reach the top of the column. They run to the lowest pipe, forming residue.
5.
The fractionating column is cooler at the top. The hydrocarbons with the lowest boiling point do not condense and are drawn out as gases at the top.
6.
As the different hydrocarbons have different boiling points they will condense at different levels of the fractionating column. The fractions are collected accordingly.
Heavy fractions
Heavy fractions, such as bitumen, are cracked to give smaller, more useful molecules. Lighter fractions such as petrol have a greater demand.
Example
decane→propene+heptaneC10H22→C3H6+C7H16
There are two types of cracking:
Thermal cracking
Catalytic cracking
Thermal cracking
catalytic cracking
Occurs at high temperature and pressure.
Occurs at high temperature and lower pressure than thermal cracking in the presence of a zeolite catalyst.
Produces many alkenes which are used to produce polymers.
Produces aromatic hydrocarbons and motor fuels.
The use of catalysts reduces costs as the same reaction can be done at a lower temperature and pressure. Catalysts also speed up reactions thus saving time.
Knocking
Petrol contains straight chain alkanes such as pentane. Straight chain alkanes are subject to knocking. This is where the mixture of alkanes is compressed in the engine and erupts suddenly. Using branched alkanes reduces the chances of knocking and ensures that combustion is done efficiently.
Reformation
Straight chain alkanes can be converted to branched and/or cyclic hydrocarbons via reforming in the presence of a catalyst such as platinum.
