Page 26 - Curriculum Visions Dynamic Book

Processing crude oil
Processing crude oil to obtain petrochemicals provides us with some of the most valuable products in use today. This is carbon-chemistry in action. Processing is vital because petroleum is a complicated mixture of chemicals that cannot be used directly. The chemical sorting process is called refining.
In general the most usable fractions are light liquids, such as gasoline, and gases, such as butane. The least desirable are thick liquids. Thus, a petrochemical plant has a second task: to break up, or “crack”, the large molecules of the heavy, thick liquid products to make more of those with
higher demand.
Cracking is possible because hydrocarbons can be converted from one type to another quite easily. Cracking also has another benefit: if a petrochemical plant did not perform this valuable piece of chemistry, then the amount of waste liquid to be disposed of would
be colossal.
Eventually the raw materials for a vast array
of complicated products such as plastics, solvents, synthetic fibres, synthetic rubber as well as fuels and solvents can be produced.
Heated crude oil enters some way from the bottom.
 The tall, slim, towers are the fractionating towers of
a petrochemical plant. The squat tower on the left is the cooling tower of a power station.
 This is a fractionating tower from a refinery and is designed to distil a range of fractions. The fractions with the lowest boiling points condense at the top, whereas those with higher boiling points distil lower down.
At each level small bell caps allow vapours to rise and at the same time trap condensate so that it can be withdrawn from the column.
The lightest fractions remain as gases and are collected for use, perhaps as fuels, or as the building blocks for plastics.
The liquid fraction can be drawn off.
Bell caps
The fractionation column produces a range of fractions such as aircraft fuel (kerosene), diesel oil and gasoline. Some of the heavier oils will be cracked at a later stage.
The fractions with very high molecular weight settle out at the bottom. These are used as boiler fuel and asphalt.
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