Feedback on my last blog “Can Feed Sulfur be Both Good and Bad,” led to our new topic on catalyst pinning. This is a great topic suggested by a LinkedIn connection. With that, I went to the Internet and searched for articles published on “Catalyst Pinning”. I found several, but they were all scholarly articles that didn’t appear to look at root causes beyond the limitations with feed gas density and flow. So, with focus on CCR reforming I wanted to share some additional thoughts on causes, propagation of catalyst pinning and potential outcomes.
I’ll also let you know how to find the pinning curves for your radial flow reactor (UOP only). Lastly, some “Please Don’t Dos” to keep your unit on-stream when catalyst pinning is forcing you to reduce throughput and you’re bottlenecking your entire complex.
What is Catalyst Pinning?
Catalyst pinning occurs when the force of total feed flowrate exceeds the catalysts’ ability to fall by gravity along the centerpipe. This results in catalyst being held up against the centerpipe and either slows or completely stops its flow down through the Rx’s. Note: Total feed flow = the mass of all liquid feed and recycle gas.
Pinning Curves and Pin-Margin
All radial flow reactors are designed to process 100% of the design feed flowrate without pinning, plus some margin. That additional margin is referred to as pin-margin and will vary from one Rx to the next depending on its design. This margin can vary from 15 ~ 50+% and is always based on “clean screen conditions” and this needs to be remembered while contemplating how to use any additional margin that may be available. This is good time to engage your licensor to obtain copies of your pinning curves and leverage your service agreement by discussing them with a technical service representative.
Some licensors, do not provide pinning curves in their Process Design and Engineering packages or Schedule “A”. You’ll need to put in a request to access copies of them. Keep in mind that much of the older unit design information predates wide use of computers. Depending on the age of your unit, this information may be on micro-film or other paper records no longer stored locally. So, don’t expect to get a quick turn-around on your request. Ask before you need them.
Cause and Effect of Catalyst Pinning
A likely source of pinning is damaged catalyst (chips and fines) getting stuck or wedged into the centerpipe profile wire. This doesn’t mean chips and fines are not being elutriated into the fines removal system, but it could be. There are several places along the catalysts path through the regenerator where catalyst fines can be generated, including the lift to the top of the Rx’s. Damaged Rx internals can also be a cause. Your licensor should have a decision tree (or provide other support) to help determine the most likely root cause.
When the open area of the centerpipe starts to become obstructed, the feed flow velocity and dP across the centerpipe will increase, and at some point, you may begin to pin whole catalyst pills to the centerpipe and prevent them from flowing down the Rx’s. When catalyst is pinned to the centerpipe it will see more feed per Rx pass and the coke on pinned catalyst will begin to increase. This can be a problem if that catalyst starts to flow again, as the regenerator may get hit with a shot of high coke catalyst some hours later. This is never welcomed and can result in coke slippage into the chlorination zone and potential damage to the regenerator.
If catalyst pinning gets worse a subsequent concern will be the fluidization of whole catalyst pills between the face of the scallops and OD of the centerpipe. When this happens, more damaged catalyst is created and can move into downstream Rx’s, initiating more widespread pinning. In this case, one should be observing an increasing dP across the effected Rx with downstream Rx’s following shortly thereafter. Moreover, damaged catalyst doesn’t flow well and is more prone to stacking up and bridging in Rx catalyst transfer pipes and standpipes fitted with restriction orifices on the regenerator side.
Some Please Don’t Dos
Widespread catalyst pinning followed by catalyst fluidization is not common but does happen. As previously mentioned, if damaged catalyst begins to stack up in catalyst transfer pipes or standpipes, one may observe catalyst transfer problems. This is more common with MCC formation but can also result from physical catalyst damage. If catalyst transfer lines start to plug (regardless of the cause), please don’t try to unplug them by creating a way to backflow through them. This has been done and any favorable outcome is short-lived. Moreover, if attempted on the Rx side this can disturb the heel catalyst in the bottom head of the last Rx and introduce heel catalyst with 10 ~ 35+ wt% coke into the normal catalyst inventory. This cannot easily be recovered from on the fly.
On the regenerator side I’ve seen similar efforts attempted with non-flowing catalyst. One result that comes to mind was catalyst being lifted into the regen gas blower circuit. Ultimately, the regen gas blower tripped on high motor current which ended further efforts.
Each customer has to look at their own economics of a unit shutdown or how to manage a unit that is feed constrained. There have been some cases where customers have come up with some extraordinary ideas to keep making product, but the risks associated with the method chosen need to be fully understood.
This topic has the potential to go forward into many other discussions i.e. maximum dP across the centerpipe, expansion bellows, etc., but I try to limit each blog to a 5-minute read. We welcome your feedback. Feel free to fire back a question or share an experience on this topic to keep the post alive.
If you have a question or comment on Catalyst Pinning, please contact us. We appreciate your feedback.