Page 90 - Fluid Catalytic Cracking Handbook

FCC Feed Characterization 67

1. Catalyst Addition Rate
A higher catalyst addition rate dilutes the concentration of metals
and allows less time for the vanadium to get fully oxidized,

Alkaline Earth Metals
Alkaline earth metals in general, and sodium in particular, are
detrimental to the FCC catalyst. Sodium permanently deactivates the
catalyst by neutralizing its acid sites. In the regenerator it causes the
zeolite to collapse, particularly in the presence of vanadium. Sodium
comes from two prime sources:
• Sodium in the fresh catalyst
« Sodium in the feed
Fresh catalyst contains sodium as part of the manufacturing process.
Chapter 3 discusses the drawbacks of sodium that are inherent in the
fresh catalyst.
Sodium in the feed is called added sodium. For all practical purposes,
the adverse effects of sodium are the same regardless of its origin.
Sodium usually appears in the form of sodium chloride. Chlorides
tend to reactivate aged metals on the catalyst and allows them to cause
more damage.
Sodium originates from the following places:
• Caustic that is added downstream of the crude oil desalter. Caustic
is injected downstream of the desalter to control overhead corrosion.
Natural chloride salts in crude decompose to HC1 at typical unit
temperatures. Caustic reacts with these salts to form sodium chloride.
Sodium chloride is thermally stable at the temperature found in the
crude and vacuum unit heaters. This results in sodium chloride being
present in either atmospheric or vacuum resids. Most refiners dis-
continue caustic injection when they process residue to the FCC unit.
It can still be present in purchased feedstocks, however.
• Water soluble salts that are carried over from the desalter. An
effective desalting operation is more important than ever when
processing heavy feedstocks to the cat cracker. Chloride salts are
usually water soluble and are removed from raw crude in the desalter.
However, some of these salts can be carried over with desalted crude.
• Processing of the refinery "slop." A number of refiners process
the refinery slop in their desalter. This can adversely affect the