The distillation unit yields straight run products contained in the crude oil. However, some of these are not suitable in quantity and quality to meet the present requirements. For example, the quality of gasoline found naturally in crude oil does not satisfy car engine requirements. Also one needs higher yields of middle distillates, which the distillation unit alone cannot provide. These requirements of more middle distillates, better gasoline and more LPG, have resulted in the evolution of the Fluid Catalytic Cracking Units.
In these units, feedstock is charged to a Reactor in which it is contacted with hot catalyst, made of Silica-Alumina, that vaporizes this feedstock and at the same time brings about its chemical decomposition by cracking. The cracked vapors pass over to the Fractionator where they are separated into gas, gasoline, cycle oils and clarified oil.
During the cracking reaction, some carbon gets deposited on the surface of the catalyst, which is continuously removed by "burning" in the Regenerator. A Stripper that entrains and separates hydrocarbons by stripping with steam reduces the load on the Regenerator. Hot regenerated catalyst is then returned to the Reactor to renew the cycle.
The catalyst, in the form of a fine powder moves between the three main vessels as a fluid. Cracking produces higher quality gasoline and other valuable products. Gas is burnt in the refinery furnaces. LPG is sold to domestic and industrial customers. Cycle oils are blended to diesel and Clarified oil is blended with Short Residue from the Feed Preparation Unit to produce furnace oil / LSHS.