Metal Catalyzed Coking (MCC)
A look back on my career and customers that faced significant MCC formation/damage and insight into how to avoid the same fate.
Metal Catalyzed Coking is a term that is known by most within the Catalytic Naphtha Reforming community. Particularly those that run in Aromatics Mode, but also those that run in Motor Fuels. MCC is unique from amorphous coke in both its structure and how it’s formed. It’s not new to our industry and can only be characterized correctly with high magnification (SEM) Scanning Electron Microscope.
MCC become evident during the late 80’s ~ early 90’s when CCR Naphtha Reforming made a significant step-change in lowering Rx operating pressures. This change resulted in a significant boost in aromatics yield, octane (for motor fuels) and H2 production. At the same time with these significant gains, it was also understood that there was an increase in coke production. UOP (licensor of this technology) made design changes to their Regenerator to increase coke burning capacity without increasing the physical size of the Regenerator. The refining industry loved it, but there was a learning curve to operating at higher severity and understanding the formation of Metal Catalyzed Coke.
MCC has a unique filamentous appearance that differs from amorphous coke that’s typically found in low-flow or stagnate areas within the Rx’s. MCC’s filamentous appearance is not evident to the naked eye, even optical microscopy is not powerful enough to accurately characterize it. One needs to use a Scanning Electron Microscope (SEM) to properly identify MCC. There are articles with images posted on the internet that date back nearly 50 years of MCC vs amorphous coke.
The formation of this filamentous coke (MCC) is typically characterized by carbon ingress into the grain boundaries of steel while operating at very high Reactor Inlet Temperatures (RIT’s) approaching or above 1000°F (538°C) and at a reduced Reactor pressure set by the Products Separator at 35 ~ 50 psi(g) or (2.5 ~ 3.5 Kg/cm2). It’s largely agreed that initial formation of MCC begins in the furnace tubes where carbon ingress displaces Fe, Cr and other metals from the interior of the furnace tubes. These displaced metals, particularly Fe are a catalyst for the hot feed that contacts it and results in coke formation. As more and more of these metals are displaced by carbon they create a chain of active metal sites and carbon that form this spaghetti looking filamentous coke.
So, why is MCC so frequently found in the Rx’s and not in furnaces? It’s widely believed that these filaments eventually break off and are carried into the reactors with the hot feed, and at some point get lodged into a scallop or other area while remaining catalytically active to form more and more MCC. At times MCC growth is so severe it can deform and permanently damage Rx internals and result in a loss of catalyst containment.
Mitigation of MCC!
Most licensors have upgraded their furnace tubes to 9 Cr. This helps and will become evident to why as you read further. The most effective way to mitigate MCC is to maintain the proper amount of S (usually added via DMDS injection) in your naphtha feed. Your licensor will have a set of guidelines to follow, and they should be strictly adhered to. Generally, S addition rates are adjusted to result in 0.25 ~ 1.0 wppm S in the feed, and this amount varies with operating severity. The S in feed reacts with the Cr in the tube metallurgy to create a chromium sulfide (Cr2S3) barrier that passivates the tube metallurgy. Raising the Cr content in the furnace tubes was done to increase the amount of Cr available to react with the S to build a more effective barrier.
Some Don’ts to remember:
Do not depend on your chemical pump setting (dial or otherwise) to provide a reliable injection rate. Many customers have experienced significant damage due to MCC by not having any S injection going while the chemical pump was running, but not pumping. A level glass calibration is easy to do if set up correctly… and is the only way to accurately determine the feed S injection rate.
Never assume some slip of S from the NHT will contribute to your S injection rate. Some customers have insisted that the S slip from the NHT was enough to mitigate MCC, and that the additional S injection specified was having a negative impact on yield structure. One of those customers was down for 3 months replacing scallops and other Rx equipment. That’s a big loss in yield!!
Lastly, watch for flame impingement in your furnaces. You can (and we’re pretty sure this has happened) develop a localized hot spot where MCC will form even with the prescribed S injection rate for your operation. It’s believed that these localized hot spots can become so hot that your normal S injection guidelines will not mitigate MCC formation in that area.
Have a question regarding your MCC formation? Drop us a line:
