Page 143 - Fluid Catalytic Cracking Handbook

118 Fluid Catalytic Cracking Handbook

ppm, is typically dispersed on a support. The effectiveness of the
promoter largely depends on its activity and stability.
Promoter is frequently added to the regenerator two to three times
a day, normally at a rate of 3 to 5 pounds (1 to 2.3 kg) promoter per
ton of fresh catalyst. The concentration of platinum required in a unit
inventory is about 0.5 to 1.5 ppm. The promoter addition rate may
be increased if antimony solution is being used to passivate the nickel.
The use of CO promoter, particularly during unit start-ups, improves
the stability of the regeneration operation. However, not every cat
cracker can justify combustion-promoted operation. Heat balance,
availability of combustion air, NO X emission metallurgical limits, and
the presence of CO boiler are some of the factors that should be
considered before using combustion promoter. For example, in units
operating with low oxygen levels and partial combustion, a promoted
system could increase carbon on regenerated catalyst (CRC). This is
because CO combustion reaction competes with carbon burning reac-
tion for the available oxygen. The combustion of CO to CO 2 will also
increase NO X emissions. This is largely due to the oxidation of inter-
mediates such as ammonia and cyanide gases into nitrogen oxide (NO).

SOX Additive

The coke on the spent catalyst entering the regenerator contains
sulfur. In the regenerator, the sulfur in the coke is converted to SO 2
and SO 3. The mixture of SO 2 and SO 3 is commonly referred to as SO X,
and approximately 80% to 90% of SO X is SO 2, with the rest being
SO 3, The SO X leaves the regenerator with the flue gas and is eventually
discharged to the atmosphere. Coke yield, thiophenic sulfur content
of the feed, the regenerator operating condition, and the type of FCC
catalyst are the major factors affecting SO X emissions.
The environmental impact of SO X emissions has gained much
attention over the past ten years. The United States Environmental
Protection Agency (EPA) New Source Performance Standards (NSPS)
went into effect in 1989. The ruling covers new, modified, and recon-
structed FCC units since January 1994. It should be noted that the
Southern California Air Quality Management District (SCAQMD)
board has established a limit of 60 kilograms of SO X per 1,000 barrels
of feed for the existing FCC units.