According the UOP Process Technology Manual for Fluid Catalytic Cracking Process, the steam to riser can be divided into four categories:
In the first case, steam is injected at the base of the riser. It is used on startup to establish catalyst circulation, at low charge rates, or on shutdown. Steam is sometimes injected with the feed in an attempt to decrease coke make by decreasing hydrocarbon partial pressure. Most plants are designed with steam injection through the feed nozzle, although some plants use three blast points around the base of the riser. The hot, regenerated catalyst is subject to breakage and deactivation if contacted by large amounts of steam over an extended period. The breakage is partially the result of microscopic damage caused by the water vapor, and partially a physical phenomenon. The deactivation is not completely understood, but probably involves the action of the water molecules on the acid sites at high temperature. Steam should be used if necessary, but its use should be kept to a minimum if there is not a large quantity of oil with it. Circulating on steam only for long periods of time will lead to catalyst damage and should be avoided.
The blast points on the riser are used to clear plugs and on some units to help initial catalyst circulation. These should also be kept to minimum usage. The nozzles are angled to give a primarily upward thrust, but a high velocity steam jet from the nozzle will push catalyst against the wall opposite the blast point. This can damage both the riser lining and the catalyst.
Stripping steam contributes to catalyst deactivation but the effects here are less severe. The catalyst is at a lower temperature and the velocity and concentration of the steam is lower than the riser steam. Economically, it is the best material available for removing hydrocarbon from the catalyst.
Purge steam is used to keep catalyst out of slide valve bonnets, expansion joints, and torch oil or cooling spray nozzles. The flow is normally controlled with restriction orifices. The steam pressure upstream of the orifice should be about 5 psi above the vessel or line pressure. If the velocity of steam injected is too high, attrition effects can damage catalyst. Too little steam, as with a plugged orifice, can result in catalyst entering an area with resultant erosion or plugging problems.
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