Abnormal catalyst losses, coking/fouling at RFCCU main fractionator bottoms.

We are facing abnormal catalyst loss from Regs to flue gas. Therefore, silo fines recycle to Regs is being applied with approximately 1 ton/day (Sb in silo fines is around 1000ppmwt). Can you explain in detail the mechanism of coking/fouling at RFCCU MF’s bottom which relates to Sb coming from Nickel passivator injecting to feedstock?