Fractional distillation of crude oil

In a nutshell

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. 

Procedure

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

​$$decane \space \rightarrow propene \space + \space heptane \\ \ \\C_{10}H_{22} \space \rightarrow C_3H_6 \space + \space C_7H_{16}$$​​

There are two types of cracking:

1. Thermal cracking
   
2. Catalytic cracking

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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.

Example

​$$octane \xrightarrow{Pt} 2,4-dimethylhexane \\ \ \\CH_3(CH_2)_6CH_3 \xrightarrow{Pt} CH_3CH_2(CH_3)CH_2CH_2(CH_3)CH_2CH_3 \\$$

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Example

​$$hexane \xrightarrow{Pt} cyclohexane \space + \space hydrogen \\ \ \\C_6H_{14} \xrightarrow{Pt} C_6H_{12} \, + \, H_2$$​​