1. Cracking and related refinery processes
We depend largely on crude, the gases associated with it and natural gas (mainly methane) as the source of liquid fuels (petrol, diesel) and the feedstock for the chemical industry.
Oil, and the gases associated with it, consists of a mixture of hundreds of different hydrocarbons, containing any number of carbon atoms from one to over a hundred.
Most of these are straight chain, saturated hydrocarbons which are used for burning, and have little direct use in the chemical industry or as fuel for cars.

2. Thus the various fractions obtained from the distillation of crude oil and the associated gases have to be treated further in oil refineries to make them useful.
The most valuable fractions for the chemical industry, and for producing petrol, are liquefied petroleum gas (LPG), naphtha (mixture of flammable liquid hydrocarbons), kerosine and gas oil. These are treated in several ways including cracking, isomerisation and reforming.

3. A view of the steam crackers at Ludwigshafen in Germany.
The main products are ethene and propene, used to make polymers.

4. The refinery
Petrol (gasoline) contains a mixture of hydrocarbons, with 5 to 10 carbon atoms. The mixture of C5-C10 hydrocarbons obtained directly from the distillation of crude oil contains a high proprtion of straight-chain alkanes.
However, if this mixture is used as petrol, it does serious damage to a car's engine.
Petrol containing a high proportion of straight chain alkanes tends to ignite in the cylinder of the car engine as the piston increases the pressure and before the cylinder reaches the optimum position.
The term knock is used as pre-ignition can be heard. Severe knock can cause serious engine damage.

5. However, branched-chain alkanes, cycloalkanes and aromatic hydrocarbons are much more resistant to knock and straight-chain alkanes are converted into them in a series of processes in the refinery.
The resistance of petrol to knock is measured in terms of an octane rating (octane number). The higher the number, the less likely is a fuel to pre-ignite.
The octane rating is on a scale where heptane is given an arbitary score of 0 and 2,2,4-trimethylpentane (iso-octane) one of 100.

6. These processes are also used to convert staight-chain hydrocarbons to hydrocarbons which are much more useful to make chemicals which are then used to make a huge range of compounds from polymers to pharmaceuticals.
Cracking
Cracking, as the name suggests, is a process in which large hydrocarbon molecules are broken down into smaller and more useful ones, for example:
High temperature

7. Cracking is conducted at high temperatures, by two processes
Steam cracking which produces high yields of alkenes.
Catalytic cracking in which a catalyst is employed and which produces high yields of branched and cyclic alkanes.
Steam cracking
Modern steam cracking plants are very large, usually producing 1-3 million tonnes of products annually and cost about 1 billion dollars to build.
The reactant gases (ethane, propane or butane) or the liquids (naphtha or gas-oil) are preheated and vaporised, are mixed with steam and heated to K in a tubular reactor . They are converted to low relative molecular mass alkenes (plus by-products).

8. A steam cracker is one of the most technically complex and energy intensive plants in the chemical industry.
The products from steam cracking include a mixture of C1 - C4 hydrocarbons and are separated by fractional distillation. Some of the columns are: 1- Separates the C4 hydrocarbons from the C1 - C3 hydrocarbons 2- Separates out the C3 hydrocarbons 3- Separates out the C2 hydrocarbons 4- Separates out the methane 5- Separates propene from propane 6- Separates ethene from ethane.
A view of the steam cracking unit at Wilton in the north-east of England.

9. Catalytic Cracking
A catalyst allows lower reaction temperatures to be used. In fluidised catalytic cracking, the feedstock is gas oil which is vaporised and passed through a zeolite, produced as a fine powder, heated to about K in the reactor.
It is so fine that it behaves like a fluid and continuously flows out of the furnace with the cracking products. The temperature, residence time and the catalyst determine the product proportions. After cracking, the catalyst is separated from the products, regenerated and recycled.

10. A gas of which ethene and propene are the main constituents.
The products are:
A gas of which ethene and propene are the main constituents.
A liquid which is used for petrol and contains branched- chain alkanes, cycloalkanes and aromatic hydrocarbons
A high boiling residue used as a fuel oil.
A catalytic cracker as used to produce alkenes from gas oil.

11. Isomerisation
Isomerisation is the process in which hydrocarbon molecules are rearranged into a more useful isomer. The process is particularly useful in enhancing the octane rating of petrol, as branched alkanes burn more efficiently in a car engine than straight-chain alkanes.
An important example is the isomerisation of butane (from LPG) to 2-methylpropane (isobutane):
Butane vapour is passed over a solid catalyst, aluminium chloride, on an inert solid at ca 300 K. The two alkanes are then separated either by distillation or by passing them through aluminosilicate.
An other example is;

12. Reforming
Reforming is another process in which hydrocarbon molecules are rearranged into other molecules, usually with the loss of a small molecule such as hydrogen. An example is the conversion of an alkane molecule into a cycloalkane or an aromatic hydrocarbon for example:
This is a very important process for the petroleum and chemical industries. It enables straight chain alkanes to be converted into branched-chain alkanes, cyclohexanes and aromatic hydrocarbons which are used to enhance the octane number of petrol.

13. Alkylation
Alkylation is the transfer of an alkyl group from one molecule to another. In a refinery, alkylation refers to the alkylation of alkanes.
For example, 2-methylpropane (isobutane) with alkenes, in the presence of a strong acid catalyst such as hydrofluoric acid or sulfuric acid. The reaction is carried out at mild temperatures (between 273 and 303K). Cooling is needed as the reaction is exothermic.
The product from 2-methylpropane and 2-methylpropene (isobutene) is a mixture of, branched-chain alkanes, mainly 2,2,4-trimethylpentane (isooctane):

14. Dealkylation
The opposite of alkylation occurs when methylbenzene is heated with hydrogen over a catalyst. The dealkylation of methylbenzene, produces benzene, a more valuable product in the chemical industry. Methylbenzene vapour and hydrogen are passed over a catalyst of chromium, platinum or molybdenum, supported on silica or aluminium oxide at K at atm pressure: