A Fluid Catalytic Cracking Unit (FCCU) has been an integral part of oil refineries since 1942, when it was introduced in the United States by Exxon Corporation in response to a growing wartime need for hydrocarbon based fuels. An FCCU accepts chains of hydrocarbons and breaks them into smaller ones in a chemical process called cracking. This allows refineries to utilize their crude oil resources more efficiently, making more products such as gasoline for which there is a high demand.
Crude oil contains a wide variety of hydrocarbons of various lengths. Depending on the length of the hydrocarbon, it can be used in a variety of ways. For example, cooking gas usually has four carbons, while gasoline for cars is a longer chain, containing eight carbons. Lubricating oils are even longer, with 36 carbons in the hydrocarbon chain. When oil is refined, these hydrocarbons are separated out for use.
However, a barrel of crude oil will not always yield the desired ratio of hydrocarbons. For example, the market may be heavy for gasoline, but light for lubricating oil. Instead of discarding the lubricating oil, it is chemically cracked in an FCCU so that it can be turned into gasoline and other hydrocarbons with shorter changers. Hydrocarbons can be cracked in other ways, but chemical cracking in an FCCU is the most common and efficient.
The FCCU uses an extremely hot catalyst to crack the hydrocarbons into shorter chains. Zeolite, bauxite, silica-alumina, and aluminum hydrosilicate are all catalysts commonly used in an FCCU unit. Both the oil and catalyst in the FCCU are usually extremely hot, and the oil is often in a vapor form. The catalyst splits the long hydrocarbon chains into shorter units, and the mixture travels from the FCCU to another distillation column so that the cracked hydrocarbons can be extracted.
Catalysts can be reused for additional cracking after the carbon which coats them after the process has been removed. In the 1930s, when the concept of an FCCU first began to be developed, a team of scientists designed an FCCU which would work in a continuous cycling mode, capable of processing 13,000 barrels of oil a day. A continuous FCCU has a primary reactor, a distillation column for separating out the cracked hydrocarbons, and a regeneration unit for cleaning the catalysts and preparing them for reuse.