Petrol

getting enough petrol

fluid catalytic cracking

the catalyst

mechanism of catalysis

Fluid Catalytic Cracking

The branched molecules help to give petrol with higher octane number, less inclined to knocking.

To find out how a fluid catalytic cracker operates click the numbered buttons on the diagram

Current designs of FCC reactors are processing anywhere from 13,000 to 125,000 barrels of feedstock per day
1 barrel = 159dm³

1
Preheated hydrocarbon feedstock is fed into the bottom of the riser tube where it meets very hot regenerated catalyst. This causes the feed to vapourise and propels the catalyst and hydrocarbon mix up the riser. The mixture is a seething fluidised bed in which the solid particles flow like a liquid.

The feedstock is cracked as it passes up the riser which is between 1 and 2 m in diameter and 60 m high. It takes the mixture only about 2 seconds to flow from the bottom to the top top of the riser and the catalytically selective cracking occurs during this time . The cracking reactions are endothermic.

Most feedstock molecules have between 30 and 40 carbon atoms, but can range from 25 - 100 carbon atoms. 70 -250 tonnes per hour of feedstock pass up the riser.

2
Depending on temperature and residence time some non-selective thermal cracking reactions can occur. At the top of the riser the mixture enters a riser termination device where separation of catalyst and product occurs. The catalyst falls to the bottom of the reactor and is then steam-stripped. 10-20 kg of steam per ton of catalyst is used to force additional product out of the pores before regeneration. The products leave the top of the reactor to the fractionator.

3
One of the products from cat cracking reactions is a carbon or coke that forms on the catalyst surface. This poisons the acid sites on the catalyst so that it eventually becomes inactive. To reactivate the catalyst , which has the texture of fine beach sand, it needs to be regenerated to remove this "coke".

4
The separated catalyst enters a combustion chamber where it is mixed with superheated blown air to create a sand storm effect. It can take up to 200 tonnes per hour of air to burn the coke off the catalyst.

Energy released from the burning coke heats up the catalyst. and this heat is supplied back to the bottom of the riser for cracking of the feedstock by circulation of the regenerated catalyst.

5
At the top of the regenerator cyclones separate any remaining catalyst from the flue gases(left over air, CO₂ and CO)

6
The regenerated catalyst is fluidized with steam and then flows back to the riser where it passes energy to the incoming feedstock so that cracking can occur without additional heating. Depending on the size of a unit anywhere from 20 to 100 tons/minute of catalyst are re-circulating in the system.

A Fluid Catalytic Cracker

In the 1960s synthetic zeolites started to replace silica/alumina catalysts. A big advantage of zeolites is that they are more selective, giving a higher yield of products in the more useful C₅ to C₁₀ range, and fewer in the C₃ to C₄ range. They also give more branched molecules which makes for a higher octane petrol

The oil industry switched to zeolites almost overnight and still uses them. The combination of higher activity and improved selectivity saves vast amounts of energy and money, making cheaper products possible.

The greater effectiveness of catalytic cracking with zeolites rather than silica-alumina saved over 200 million barrels of crude oil in 1977 in the USA alone, about 30 million tonnes.

Over 400 fluid catalytic cracking units are in operation at oil refineries around the world today. The reactions are varied. They include:

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