CCR Naphtha Reforming: Radial Flow Reactors Catalyst Bed Pressure Explained – Avoiding Rx Internals Damage
Introduction:
Before continuing with CCR Naphtha Reforming Turnarounds it seemed appropriate to explain the cause and effects of unchecked thermal cycles and high catalyst bed pressure. This information is vital to understanding how to mitigate Rx internals damage and improves the ability of operators and engineers to evaluate unit equipment and catalyst performance more effectively. Moreover, should prove useful while completing Rx internals inspection to better associate observed damage to previous unit operation and performance.
Radial flow reactor designs continue to evolve and improve; although, even with these improvements their internals remain vulnerable to thermal cycles and high catalyst bed pressure. Most thermal cycles result from recycle gas compressor and subsequent heater trips. Over the course of a run (years) these trips are seemingly unavoidable, but one’s response to it can significantly reduce damage and potential downtime.
Rx Diameter and Catalyst Bed Pressure
As with most process technologies the demand for higher throughput with new construction is frequently requested, and as a result equipment sizing increases, including Rx’s.
Scaling up a technology always presents challenges, and while increasing Rx size – even with the same materials of construction – a larger Rx will expand and contract more in overall length and diameter than a smaller Rx.
Example: If Rx “A” is 16’ in diameter and Rx “B” is 10’ in diameter both will expand and contract the same on a percentage basis. But the measured dimensional changes of Rx “A” will be greater than Rx “B”. During cooldown, Rx “A” will contract in length and diameter more than Rx “B” and therefore more likely to develop high catalyst bed pressure during a thermal cycle.
Some licensors provide upgraded (more robust) Rx internals and improved scallop designs. Moreover, design changes to catalyst transfer systems to alleviate high catalyst bed pressure and improved control over recycle gas flow with alternative heater firing systems. But for this article, it is assumed that these systems are not available or routinely used.
High Catalyst Bed Pressure – Scenario 1
The RG compressor trips on high vibration, which should also trip the naphtha feed pump, the charge heater, and all inter-heaters. If this is not a momentary trip i.e., a few hours or more, the heaters, Rx shells and internals will all begin to cool off. As each Rx cools they begin to slowly contract and squeeze the static catalyst between the face of the scallops and OD of the centerpipe. This results in increased catalyst bed pressure, and if unchecked, this alone may crush the scallop face to relieve the additional pressure. Moreover, the overall scallop length will be reduced from axial contraction and may pull the scallop risers below the coverdeck (center photo).
Note: Newer scallop designs add additional riser height to help with the issue. See flattened “D” shaped scallops shown in the following photos.
Some licensors have integrated systems and procedures to alleviate high catalyst bed pressure. These systems “if present” are initiated by an operator that uses a decision tree to guide them on how and when to engage these systems during an outage or equipment failure. Once initiated, this system will transfer catalyst from the bottom Rx with an alternative catalyst lift line and gas to the top of the Rx stack. UOP’s design is referred to as Cooldown Modeä (CDM), and if you have this system or similar, I highly recommend that it’s used to avoid the damage shown in the above photos. Note: A less effective alternative to relieve catalyst bed pressure is to simply use the spent catalyst lift system w/ nitrogen to move some catalyst to the Disengaging Hopper (DH). Although, it is noted that this option to move catalyst to the DH may not be an option with some unit designs, but if it is, it will directionally help with reducing catalyst bed pressure.
Catalyst Bed Pressure – Scenario 2
In this 2nd scenario the equipment problem that initiated Scenario 1 has been resolved and now the RG compressor can be restarted, and heaters relit. This is where the stated damage in Scenario 1 can be compounded significantly if catalyst bed pressure remains high before moving forward. Note: The scallop material and centerpipe profile wire are only 1 ~ 2 millimeters thick, while the Rx shell is a much thicker steel plate. Thus, the Rx shell and internals do not expand and contract at the same rate.
Once started, the recycle gas compressor will push residual hydrocarbons through the entire Rx circuit. Using a 4 Rx design, the Rx circuit includes a charge heater, 3 inter-heaters, and 4 Rx’s that will all see a rapid transition in temperature in a very short period of time. This includes the hot side of the CFE which (if a plate exchanger) likely has some tight constraints on temperature and rate of change.
Over several hours, the hydrocarbons in each heater will have cooled off much more than in the 4 Rx’s. Once the cool material in the heaters flows to the Rx’s (observed at the Rx inlet TI) it travels down the scallops, across the catalyst bed and out the centerpipe. These relatively thin Rx internals will cool off rapidly while the thick Rx shell lags behind. In this scenario the contraction of the scallops and centerpipe opens up additional volume in the annular catalyst bed allowing more catalyst to flow into it. Note: This rapid contraction may also contribute to scallop risers dropping beneath the coverdeck. This contraction also puts a strain on the centerpipe that directionally could lift the base out of its socket.
Simultaneously, as this cooler material from the heaters is transferred to the Rx’s, the hotter material sitting in the Rx’s is being displaced and flowing downstream to the next piece of equipment. Each Rx that goes through this thermal cycle can expect transient increases in catalyst bed volume to be shortly followed by additional catalyst to fill that volume, then a sharp increase in temperature that reduces the volume of annular catalyst bed forcing the catalyst against the scallops and centerpipe. This roller coaster of temperature variation will sweep through the entire Rx circuit, and likely to happen before you relight any burners on your heaters. This is extremely hard on all Rx internals, including old weld repairs that are rarely as robust as the original welds.
Some commonly seen damage from these thermal cycles includes damaged scallop bases, support rings and possibly support lugs. The photo (bottom right) is of damage to the centerpipe profile wire and referred to as a “fisheye”. This can result in a loss of catalyst containment.
Lastly, the effluent side of the CFE will see a large increase in temperature from the material from the last Rx. This upward swing in temperature may be transient i.e. 15 or 20 minutes, but may exceed 250°F in that short period of time. That could easily damage a plate exchanger, and Packinox generally has a very long lead time for a replacement.
Conclusion: If a systematic approach to restarting the RG compressor and bringing your unit back online is not taken, you are certainly postured to experience the damage noted above. Particularly if you have larger diameter Rx’s.
Recommendation: If you feel vulnerable to what is written here or have seen this before on your unit, reach out to your licensor or Becht experts and begin looking into upgrading your internals and/or adding systems like UOP Cooldown Modeä and Low Flow Low Firingä.
Best Practices:
If you already have a UOP design with Low Flow Low Firing (LFLF) and/or Cooldown Mode (CDM), use it. The primary reason most customers don’t use one or both of these systems is that it’s not built into their normal work processes. This is somewhat understandable as these scenarios don’t happen with regularity in most facilities. Nevertheless, using this equipment is key to mitigating significant equipment damage. Please use it.
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