The physical and chemical processes that are used in an oil refinery

The fractions from this distillation process can still be used in many areas:

* Refinery gas is used to make butane and propane; blend into petrol; and make many organic chemicals

* Naphtha is also used to make other organic chemicals as well as high performance petrol

* Kerosene is used for making jet engine fuel and to make heating fuels such as paraffin.

* Gas oil is also used for central heating fuel as well as making petrol

The residue is somewhat useless in comparison. There is a high quantity of it and only a small demand for it. However, there is another process that can be carried out to obtain the ingredients for roads (bitumen and asphalt); lubricating oils and waxes; and fuel oil (used in large ships and power stations).

Vacuum Distillation

Residue has a very large specific boiling point range and as such it is near impossible to reach this temperature range at atmospheric pressure making further fractional distillation pointless. Even if you did get the residue hot enough to distil the hydrocarbons may crack and cause the residue to decompose. However, if the external pressure is reduced the boiling point range can be brought down to a more practical level and therefore further fractions can be obtained from the residue. This is done using a highly pressurized distillation column.

The most important use of the crude oil refining process is for making petrol.

Making better Petrol

As I mentioned earlier, straight-run gasoline was initially used as a fuel for the internal combustion engine, but with mans need for speed and efficiency something had to be done. The most common problem with straight-run gasoline is 'knocking'. Knocking is what happens when the fuel ignites to quickly. In a petrol engine, the fuel must be made to ignite just before the piston reaches the top of the cylinder. If the fuel ignites at the wrong time then the cycle will be far less effective and less power will be produced. This is because the fuel igniting must move the piston the greatest possible amount. If the fuel were to ignite when the piston is only half way up the cylinder then the power output would not be that great. A hydrocarbon or mixture of hydrocarbons tendency to auto-ignite is measure by its octane number.

Making the right Octane Number

There are many ways for petrol blenders to alter the octane number of the fuel they wish to produce. The first of which was discovered in the 1920s and is by adding an 'anti-knock additive' to the fuel.

Anti-knock additives

The first additive that was used was lead. Exactly how they work is still not fully understood but it did solve the problem. However the effect 'leaded' petrol had on the environment soon became a concern. Leaded petrol made the exhaust fumes even more toxic and poisoned the metal catalysts in the catalytic converters; preventing them from reducing the levels of other pollutants in the exhaust fumes. These reasons soon led to the demise of leaded petrol and it was made illegal to use it in cars.

For those who already had engines designed for leaded petrol the refineries developed lead replacement petrol but turned their main concern to finding another way to alter the octane number. Blenders now use isomerisation, reforming, cracking and adding oxygenates to do this.

Isomerisation

This is performed by taking a straight-chain alkane and heating it in the presence of a catalyst to break and reform the chains into a branched arrangement. Branched chain alkanes have a much higher octane number. An example of this is shown below:

As you can see this isomer of pentane has a much higher octane number than the original pentane. In most refineries today, the isomerisation takes place in the presence of aluminium oxide because it is a catalyst. The products then pass over a form of zeolite, which will sort the branched from the straight chain alkanes, so they can be used in the mixing of fuels.

Catalytic Cracking (or Cat Cracking)

This is the heating of the heavy oils (gas oil) in the presence of a catalyst. Cat cracking reactions can be quite varied (see below).

You may wish to use the alkanes to make:

* Branched alkanes + branched alkenes

* Smaller alkanes + cycloalkanes

* Simply smaller alkanes

Cracking will also produce varied products and therefore zeolites are again used to sort the straight, branched and cyclo chain alkanes/alkenes. A diagram of a cat cracker can be seen below:

The vaporised hydrocarbons are fed into the bottom of the cat cracker and mixed with the zeolite catalyst. They both then enter the separator and are separated. The catalyst will have become covered in carbon and will therefore be useless. It has to pass into the 'regenerator' where the carbon is 'burnt' off using the oxygen in air. The products are smaller, lighter hydrocarbons which can be removed from the separator in a gaseous state.

Oxygenates

The two most common types of oxygenate used in petrol manufacture are alcohols and ethers. Oxygenates are fuels which contain oxygen molecules and they have high octane number. Straight-run gasoline has an octane number of 70 but methanol and ethanol have octane numbers of 114 and 111 respectively.

2-methoxy-2-methylpropane is the most commonly used oxygenate and is used in all performance grades of unleaded:

Unleaded regular 93 octane 0.6% MTBE added

Unleaded premium 95 octane 2.2% MTBE added

Unleaded super plus 98 octane 7.0% MTBE added

Blending to make the right fuel

The ideal fuel can only be obtained from thorough blending and accurate mixing of the following: straight run gasoline; product from isomerisation; product from catalytic cracking; butane; MTBE; Ethanol; and Methanol.

To get the 'right' blend of petrol the blenders must consider the price and performance (octane number) of the petrol. When they have met a balance of these two variables then the petrol will be blended in batches of about 20 000 000 litres and then shipped off to the petrol stations.

Thus, our story ends, we have followed the hydrocarbons from their birth as crude oil to their destiny as petrol for the cars that help sustain the lifestyles we live today.