Oil products
Making polymers

A polymer is a molecule that is made by joining together smaller molecules, called monomers. Often, the polymer molecule is a long repeating chain made from joining lots of similar molecules. An example is polythene.

Graphic of polymerisation
Unsaturated ethen monomers are joined to make polythene.

We make polythene by joining together molecules of ethene. Like all alkenes, ethene is an unsaturated hydrocarbon. This means it hasn't made as many bonds as it could - it has the capacity to make more bonds.

In the case of ethene, the spare capacity comes from the double bond between its two carbon atoms. If we break this double bond into a single bon, each of the carbons has the ability to bond onto something else. By mixing lots of ethen molecules together, we can make them join onto each other - like links in a chain.

In this way, a chain of ethenes links up to make 'poly-ethene', or polythene.

Polythene is not made on the refinery. However, the refinery produces the ethene (and other unsaturated monomers) and supplies them to the chemical plant next door.

On the other hand, halobutyl rubbers are made on the refinery site. These synthetic rubbers are impermeable to gases and water and are inert (they don't react with anything). This makes them useful for the lining of tubeless car tyres and also for bottle stoppers in the chemical and pharmaceutical industries.

Butyl rubber is another polymer. It is made by joining two different monomers to make the long polymer chain. This is called co–polymerisation.

The monomers are fed into a reaction vessel at -100°C in the presence of an aluminium chlorid catalyst. The monomers react to form a slurry of tiny polymer particles.

These are dissolved in a solvent and reacted with chlorine or bromine (the halogen part of the name) to make halobutyl rubber. The solvent is boiled off and the rubber is dried in large bales which are ready for sale and export.

Polymerisation plant
The refinery also has a polymerisation plant. This joins together short chain alkenes (with one or two carbons) to make longer chain alkenes (with up to 10 or 11 carbons). This is because the cat cracker produces a surplus of the short chain alkenes. The longer chains are sent to the chemical plant or blended into petrol.

The polymerisation reaction is carried out at a high pressure (7 MPa) and at 200°C.

HBR plant
The halobutyl rubber plant. The big tower in the middle is for recovering the solvent, which is reused.