Oil refineries
Graphic of cracking
A generalised cracking reaction. Each angle represents a carbon so this shows molecules with 20 to 30 carbons being cracked into molecules with 2 to 7 carbons.

The crude oil product with the biggest demand is petrol, which is made from the naphtha fraction. This fraction contains molecules with 5 to 10 carbons in their chains. As such, it is one of the lighter fractions.

Although around 20% of a barrel of crude oil eventually goes to petrol, there is nowhere near this amount of naphtha in the raw crude. So, the demand is much greater than what is available.

However, there is a surplus of heavier fractions like heavy gas oil, which has very few direct uses. Its molecules have more than 20 carbons in their chains.

The solution is to break up these long chain molecules into the shorter molecules that are useful in petrol. We call this process cracking.

The cracking reaction can be induced using high temperatures in the presence of steam or a catalyst.

The steam cracker reacts steam with heavy gas oil. The heavy gas oil is called the feedstock - i.e. it is what we feed into the cracker. The long chain hydrocarbons are split into much shorter chains with 1 to 8 carbon atoms. The main product is ehtene (with 2 carbons). The C5 to C8 hydrocarbons go into the petrol blend.

The catalytic cracker takes a number of feedstocks, including heavy gas oil, treated fuel oil and residue from the lubricant treatment plant.

The feedstock is mixed with a hot catalyst and passes up through the reaction vessel (shown on the right in the picture). The catalyst allows the cracking reaction to take place at a relatively low temperature (about 500°C).

The products pass out of the top of the reactor vessel (you can see the pipe in the picture) to a fractionating column to be separated. About 20% of the product goes into petrol. The rest are lighter hydrocarbons which pass into the refinery processes.

The catalyst flows back into the regenerator on the left where it is reheated to burn of the carbon ready for another round of catalysing.

You can see an animation of the catalytic cracking process (including a ride on a catalyst crystal) by clicking here.

Photo of cat cracker
The catalytic cracker is the grey vessel on the right. This is where long hydrocarbons are split into shorter, more valuable molecules.