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?
-If you overfeed the Sb passivator, you could potentially cause fouling in the MF bottoms. As long as you control the dosage this shouldn’t be a problem.
-Fouling in the bottoms circuit is usually only seen when excess antimony is injected. The correct dosage of antimony is specific to each unit. Antimony is only stable if it reacts with nickel or vanadium on the catalyst. Excess antimony will not stick to the catalyst, but instead will concentrate in the bottoms product. The amount of antimony that will remain stable ranges from 20% to 80% of the nickel on catalyst and varies with catalyst type, regenerator conditions, Ni/V ratio, metals deposition rate and presence of metals traps. Generally, refiners should control the antimony injection rate to be no more than 20% above the stable amount. In other words, if your deposition efficiency is less than 80% you should reduce additions. You mention adding fines that already contain antimony. Presumably, this antimony is on the fines because it has been stabilized by the nickel and vanadium on the fines and should therefore not give any problems in the bottoms circuit.
– According to a former colleague of mine well versed on the subject, the laydown efficiency for various forms of antimony are not equal. Much better laydown efficiency is obtained with hydrocarbon-based antimony solutions, compared to the more common water-based colloidal suspensions. It is important to accurately monitor how much passivator is being injected, and calculate how much is actually staying on the catalyst, how much is lost with fines and catalyst withdrawals, and how much is unaccounted for. Laydown efficiency in the region of 50-75% is not uncommon.
I am assuming that there is a reason to suspect Sb as the cause for fouling (a chemical analysis of a coke deposit for example), but some amount of Sb in the bottoms circuit is to be expected and one should be cautious not to exclude other likely causes of slurry circuit fouling/coking.
–I have also similar experience in the past though it could be different with this case. With the security policy, I cannot share the story in detail so I want to ask the understanding on vague description.
To reduce the amounts of off-gas, excess Sb was introduced for several months in RFCC. After that, the problem of catalyst carryover was happened caused by the deposit in dipleg. So we should have to check the Sb effect as one of the main reasons of catalyst carryover.
With analysis of deposits, we could find that the average Sb contents in deposit surprisingly was about 10 vol%(over 30 wt%) and Sb was existed with the form of metal particle or layer in mixing with lump coke and catalyst. We couldn’t find any clue about filament coke or linkage with adjacent coke on surface of Sb metal.
So we concluded and reported that excess Sb seems to be that it doesn’t act as the catalyst for directly making catalytic coke by dehydration/olefin oligomerization reaction but it affects fouling in RFCC units by making deposit that could interrupt smooth flow of catalyst.
This problem has no longer been occurring after using the proper dosage of Sb and cleaning the deposit in unit at regular maintenance.
See this post and the names of the commenters, as well as other posts in the Refining Community/ CatCracking Forum
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