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3. Industrial catalysis
3.6 Producing petrol last page page 15 next page
Structures in reaction
Picture 3.14. An example of a reforming reaction.

3.6.5 Reforming
This is another treatment that is used to produce molecules that will raise the octane number of petrol. This process changes the shape of molecules rather than their size (as in cracking which makes smaller molecules).

The starting material is naphtha, another fraction from the distillation of crude oil. In this process straight chain alkanes are converted to cycloalkanes and then to aromatic hydrocarbons.

The naphtha is preheated to about 500oC with hydrogen. It is fed into reactor vessels at about 20 times atmospheric pressure in the presence of the catalyst. Rearrangement reactions take place producing aromatic and other hydrocarbon molecules with high anti-knock vlaues.

Before the mixture is fed into the reactors, it is passed through a Hydrofiner. Sulphur and sulphur compounds, which would damage the catalyst, are removed by reacting with the hydrogen forming hydrogen sulphide.

reforming
reforming hexane arrow cyclohexane arrow benzene
octane no. 25 83 106

catalyst

finely divided platinum on the surface of aluminium oxide. Sometimes the process is referred to as platforming.
Photo of cat converter
Picture 3.15. Catalytic cracker at the Fawley Esso refinery.
3.6.6 Cracking
Cracking is used to convert large hydrocarbon molecules into smaller molecules. Some of these are branched or cyclic hydrocarbons that are used to raise the octane number of petrol while others are alkenes that are used as building blocks to make other chemicals.

Much of the cracking to produce petrol involves heating heavy oils such as gas oil with steam in the presence of a catalyst. It is consequently known as cat cracking for short.

catalyst

a form of zeolite, zeolite Y

The catalyst powder is injected into the stream of steam and vaporised oil before it enters a 60 metre high riser reactor at between 700 and 800 K and at a slight pressure. The mixture forms a fluidised bed in which the catalyst particles flow like a liquid. The mixture gets to the top of the tube in about 2 seconds. The large hydrocarbons are cracked to form a mixture of smaller ones. This product mixture is fed into fractioning columns to separate the different hydrocarbons.

However, during the process, carbon will have formed on the surface of the catalyst.The catalyst is separated from the product and fed into a regenerating compartment. In here, the carbon is burnt off in a stream of air so that the catalyst can be reused.

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Question 13

Why is it important that carbon should be burnt off the surface of the zeolite catalyst before it is reused?