3. Examples of Catalysis in the Petrochemical Industry
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This page looks briefly at some of the basic processes in the petrochemical industry (cracking, isomerisation and reforming) as examples of important catalytic reactions.
Cracking is the name given to breaking up large hydrocarbon molecules into smaller and more useful bits. This is achieved by using high pressures and temperatures without a catalyst, or lower temperatures and pressures in the presence of a catalyst.
The source of the large hydrocarbon molecules is often the naphtha fraction or the gas oil fraction from the fractional distillation of crude oil (petroleum). These fractions are obtained from the distillation process as liquids, but are re-vaporised before cracking.
The hydrocarbons are mixed with a very fine catalyst powder. These days the catalysts are zeolites (complex aluminosilicates) - these are more efficient than the older mixtures of aluminium oxide and silicon dioxide.
The whole mixture is blown rather like a liquid through a reaction chamber at a temperature of about 500°C. Because the mixture behaves like a liquid, this is known as fluid catalytic cracking (or fluidised catalytic cracking).
Although the mixture of gas and fine solid behaves as a liquid, this is nevertheless an example of heterogeneous catalysis - the catalyst is in a different phase from the reactants.
The catalyst is recovered afterwards, and the cracked mixture is separated by cooling and further fractional distillation.
There isn't any single unique reaction happening in the cracker. The hydrocarbon molecules are broken up in a fairly random way to produce mixtures of smaller hydrocarbons, some of which have carbon-carbon double bonds. One possible reaction involving the hydrocarbon C15H32 might be:
Or, showing more clearly what happens to the various atoms and bonds:
This is only one way in which this particular molecule might break up. The ethene and propene are important materials for making plastics or producing other organic chemicals. The octane is one of the molecules found in petrol (gasoline).
Hydrocarbons used in petrol (gasoline) are given an octane rating which relates to how effectively they perform in the engine. A hydrocarbon with a high octane rating burns more smoothly than one with a low octane rating.
Molecules with "straight chains" have a tendency to pre-ignition. When the petrol / air mixture is compressed they tend to explode, and then explode a second time when the spark is passed through them. This double explosion produces knocking in the engine.
Octane ratings are based on a scale on which heptane is given a rating of 0, and 2,2,4-trimethylpentane (an isomer of octane) a rating of 100.
In order to raise the octane rating of the molecules found in petrol (gasoline) and so make the petrol burn better in modern engines, the oil industry rearranges straight chain molecules into their isomers with branched chains.
One process uses a platinum catalyst on a zeolite base at a temperature of about 250°C and a pressure of 13 - 30 atmospheres. It is used particularly to change straight chains containing 5 or 6 carbon atoms into their branched isomers.
Reforming is another process used to improve the octane rating of hydrocarbons to be used in petrol, but is also a useful source of aromatic compounds for the chemical industry. Aromatic compounds are ones based on a benzene ring.
Reforming uses a platinum catalyst suspended on aluminium oxide together with various promoters to make the catalyst more efficient. The original molecules are passed as vapours over the solid catalyst at a temperature of about 500°C.
Isomerisation reactions occur (as above) but, in addition, chain molecules get converted into rings with the loss of hydrogen. Hexane, for example, gets converted into benzene, and heptane into methylbenzene.