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Sources of Petroleum

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Students learn to identify the industrial source of ethylene from the catalytic cracking of some of the fractions from the refining of petroleum

Crude oil is more like tar than the petrol you get at the service station; some crude oils are even solid at room temperature [see New Scientist 18th Dec, 1999, 32-35]. This is because of the relatively high molecular weight of most molecules in the complex mixture that is crude oil.

Crude oil does not consist simply of straight chain hydrocarbons like hexane and octane, but is a mixture of linear hydrocarbons, branched hydrocarbons, cyclic hydrocarbons, aromatic carbon compounds, and other species containing sulphur, oxygen, and nitrogen in addition to carbon and hydrogen.

As well as petrol, avgas, and other products that are simply burnt, crude oil is the source for the building blocks from which we get plastics and synthetic fibres, products that are everywhere around us in the industrialised world. One of these building blocks (which we will be talking about a lot) is the molecule which is properly named ethene, but is commonly known as ethylene.

Refining Crude Oil

How do we get from crude oil to all of these useful things?

Simple distillation to separate fractions of different boiling points was originally used, but this gives a distribution of products that may not be economic. For example, the crude oil might contain lots of tarry goo and sticky stuff that might be good for lubricating cart axles, but little light hydrocarbons that can be easily burnt as fuel.

Today two linked processes are used to transform amorphous black goo to useful materials: Cracking and Fractionation.