Page 144 - Fluid Catalytic Cracking Handbook

FCC Catalysts 119

There are three common methods for SO X abatement. These are flue
gas scrubbing, feedstock desulfurization, and SO X additive. The SO X
additive is often the least costly alternative, which is the approach
practiced by many refiners.
The SO X additive, usually a metal oxide, is added directly to the
catalyst inventory. The additive works by adsorbing and chemically
bonding with SO 3 in the regenerator. This stable sulfate species is
carried with the circulating catalyst to the riser, where it is reduced
or "regenerated" by hydrogen or water to yield H 2S and metal oxide.
Table 3-3 shows the postulated chemistry of SO X reduction by a
SO X agent.
To achieve the highest efficiency of SO X additive, it is impor-
tant that:
* Excess oxygen be available; oxygen promotes the SO 2 to SO 3
reaction. SO X additive will only form a metal sulfate from SO V
* The regenerator temperature be lower; lower temperature favors
SO 2 + 1/2 O 2 -> SO 3
* The capturing agent be physically compatible with the FCC
catalyst and easily regenerated in the riser and stripper.
* CO promoter be used, which oxidizes SO 2 to SO 3.
* There be a uniform distribution of air and spent catalyst. Air/

catalyst mixing in the regenerator can significantly affect the SO X
pick-up efficiency.

Table 3-3
Mechanism of Catalytic SO X Reduction

A. In the Regenerator
— »
Sulfur in Coke (S) + O 2 SO 2 + SO 3
l
SO 2 + /2 O 2 —> SO 3
M XO + SO, — > M XSO 4
B. In the Reactor and Stripper
— > M XS + 4H 2O
M XSO 4 + 4H 2
M so + 4H 2 -> M XO + H 2S + 3 H 2O
x
4
M XS + H 2O —> M XO + H 2O
Source: Thiel [9]