Question 14: Do you have experience with CCR heel catalyst contaminating the circulating inventory during operation? How can this contamination be prevented?
RUSS WILTSE (Honeywell UOP)
The most common situation that results in the contamination of a catalyst load with heel catalyst occurs during a turnaround as a result of accidentally reloading drums of used catalyst that contain heel catalyst back into the reactors. This type of contamination occurs in one or two units every year. However, it is rare for a CCR Platforming™ catalyst inventory to be contaminated with heel catalyst during normal operation. Honeywell UOP is only aware of a small number of instances (about five) where contamination of the circulating catalyst inventory by heel catalyst occurred during normal operation. In these cases, no definitive cause of the contamination was identified.
UOP has identified several scenarios that could lead to heel catalyst contamination during normal operation. These include for the formation of metal catalyzed coke (MCC), reactor internals damage, and abnormal catalyst collector operation.
If MCC masses begin to grow at the bottom of a reactor, they may displace heel catalyst in the bottom head of that reactor and push it into the circulating catalyst. This could cause a slow trickle of heel catalyst to mix into the flowing catalyst contaminating the entire inventory. An event such as this is likely to end with the catalyst flow path in the reactor being completely blocked by the coke mass and the unit having to be shutdown. The risk of MCC formation can be significantly reduced with injection of small amounts of sulfur into the reformer feed and proper fired-heater operation.
Reactor internals damage–in particular, scallop or expander ring damage–could lead to heel catalyst contamination. If one of these components is knocked loose, it could sink to the bottom of the reactor and disturb the heel catalyst in the bottom head of the reactor. There may also be some non-flowing catalyst between the scallops near the reactor wall which could be disturbed if the scallop is dislodged, bent, or crushed flat against the reactor wall. This type of internals damage is best prevented by following prescribed heat-up and cool-down rates, proper use of the UOP Cooldown Mode System (if available), and good turnaround inspection and repair practices.
Abnormal catalyst collector operation typically occurs in response to a catalyst flow issue. The first abnormal operation that can pose a risk of disturbing heel catalyst is “jet loading” of the spent catalyst lock hopper in an atmospheric CCR Platforming™ regenerator. Jet loading is a procedure that is sometimes used to reestablish catalyst flow through plugged external catalyst transfer lines in older CCR units. During the jet loading operation, the heel catalyst at the bottom of the last reactor could be disturbed as the catalyst and gas rush into the external catalyst transfer pipes.
The other abnormal catalyst collector operation that could pose a risk for heel catalyst disturbance is reverse nitrogen purging to unplug a bridged catalyst collector outlet nozzle. Occasionally, if a refiner believes there may be bridging or an obstruction at the catalyst collector outlet nozzle, a reverse flow of nitrogen is directed up the catalyst withdrawal line to try to break up the obstruction. This reverse nitrogen flow might disturb the heel catalyst at the bottom of the catalyst bed in the reactor above.
The keys to preventing the need for the abnormal catalyst collector operation examples above are proper catalyst collector operation (temperature and flow) and monitoring, good insulation with proper weather shielding, and maintaining a low catalyst-fines make.
There are also means of contaminating the circulating catalyst bed with highly coked catalyst without actually disturbing the heel catalyst. Examples of this are pinning, reactor internals damage, and temporary catalyst flow path obstructions. These examples do not actually disturb the heel catalyst. Instead, sections of the circulating catalyst bed are slowed down or stopped. This longer residence time leads to higher coke levels on this portion of the catalyst, thereby creating a heterogeneous coke profile in the spent catalyst. This circumstance may initially be perceived as heel catalyst, but the peak carbon levels are expected to be much lower unless the obstruction in flow persisted for a long time.
Question 15: How do you remove the CCR heel catalyst from the unit during an outage and underwhat atmospheric conditions?
LEWIS KENYON (Honeywell UOP)
Catalyst unloading from a CCR Platforming™ reactor stack is done under nitrogen atmosphere using the normal catalyst transfer line under the catalyst collector. Typically, the last 10 to 15% of the catalyst unloaded will be contaminated with heel catalyst. This percentage may be higher if there has been damage to internals or blockage of catalyst transfer lines.
The catalyst disposal nozzles (off-center on the bottom head) must not be used if the catalyst will be reloaded into the unit, because any portion of the catalyst unloaded from these nozzles is likely to be contaminated with heel catalyst. As the name suggests, the catalyst disposal nozzles should only be used if the catalyst will not be reused and will only be sent for metals recovery.
Heel catalysts are pills that do not move with the normal circulation of catalyst; they remain in the reactors (at the bottom of each reactor and between the scallops) and develop a very high level of coke (up to 50 wt%). Average heel catalyst volume for downflow reactors is about 9% but about 7% for up flow reactors due to improved flow paths. Because the coke level is so high, heel catalyst cannot be loaded back into the unit as the regeneration tower is not able to safely burn that level of coke. Drums or bins of normal, unloaded catalyst that is contaminated with heel catalyst are detectable by visual inspection and can be confirmed by lab testing of average coke content must be segregated and not loaded back into the unit.
Not all heel catalyst will be removed from the reactor stack during the first phase of the unloading. When the circulating catalyst is unloaded by gravity through the normal flow path, some heel catalyst will be found at the bottom of each reactor. In addition to being very high in coke, it may also contain hydrocarbons or trace pyrophoric iron sulphide scale. Therefore, this catalyst must be handled carefully and removed from the reactors before they can be made safe for inspection and maintenance.
The remaining heel catalyst is generally removed by vacuum. Care should be taken during this operation due to the risk of self-ignition. Honeywell UOP recommends that the reactors be kept under inert conditions until the heel catalyst is removed, even though this requires inert personnel entry. Only once all reactors are free of catalyst and the reactor section has been completely isolated from the other sections of the unit should air be admitted to the reactors following the refiner’s standard safety procedures.
Some refiners chose to introduce air prior to entry for removal of residual heel catalyst because of the inherent risks entailed with inert vessel entry. They consider that the risk of self-ignition is lower than the risk associated with inert entry, since instances of self-ignition are rare. UOP advises against this as consequences of ignition can be significant; but if this choice is made, then we suggest that an ample draft of air be established through the reactor stack in order to facilitate the removal of residual hydrogen and hydrocarbon vapour and to oxidize any pyrophoric material before any personnel enter the reactors.
Question 12: What operating strategies do you employ to successfully regenerate catalyst in a continuous catalyst regeneration (CCR) unit with a carbon content in excess of 10 wt%?
MICHAEL CROCKER and STEVEN PHILOON (Honeywell UOP)
The burn zone in a Honeywell UOP Platforming™ CCR Regenerator is designed for operation at 5 wt% carbon on catalyst or about 5.25 wt.% coke at the design catalyst circulation rate. We find that most units can operate normally at coke levels 40% above the design (about 7.4 wt.% coke) and some at even higher levels of coke.
Note: The design coke-burning capacity is determined with a clean regenerator inner screen and 100% of design regenerator burn-zone recirculating gas flow. If the inner screen is fouled with catalyst chips and fines, and if the regeneration gas flow is less than the clean screen flow, then the coke-burning capacity of the regeneration tower will be reduced.
For most units and most circumstances, the recommended approach to deal with a high level of coke will be to operate the regeneration tower in black-burn mode at a reduced catalyst circulation rate. During this period of operation, there will be only upper air introduced into the burn zone; the rate of combustion air will be controlled to ensure that the peak burn-zone temperatures are kept below 1100°F (595°C). The rate of catalyst circulation is adjusted to keep the peak burn-zone temperature below the top TIpoint in the catalyst bed. The burn-zone profile should generally retain its normal shape. It is important that the observed peak temperature in the burn zone is NOT at the TI point in the catalyst bed. If this is the case, it is possible the real peak temperature is actually at a location higher in the bed and at a higher temperature. This can happen when the catalyst circulation rate is too low for the current regeneration gas oxygen level. Note that organic chloride should be injected into the feed of the platforming unit to help maintain the level of chloride on the catalyst while the regenerator is operating in black-burn mode.
In some circumstances, it may be necessary to allow the operation of the regeneration tower to shift to partial carbon burn. This will happen when the coke level on the catalyst exceeds the amount that can be burned, given the constraints of the maximum burn-zone peak temperature and the location of the peak temperature below the top TI. The lower portion of the burn-zone profile will not have its normal shape; the lower temperatures will rise as coke burning is continuing in that section of the catalyst bed. It may be that you are only reducing the coke on catalyst by 5 to 7 wt.%. If the catalyst entering the burn zone has 11 wt% coke, it may have 4 to 6 wt% as it leaves the regeneration tower.
The severity of operations on the reactor side should be moderated to reduce the rate of coke formation. Depending upon the level of coke and the severity of reactor side conditions, it may take several cycles in black-burn mode to reduce the coke level on the catalyst leaving the last reactor to a level that is low enough that the regeneration tower can be safely switched to white-burn mode.
It is important to limit the number of cycles that the regeneration tower operates in black-burn mode as the platinum on the catalyst is not redistributed when the chlorination zone is not operating normally with the regenerator in white-burn mode. At some point, the agglomeration of the platinum will begin to affect catalyst performance (activity and selectivity).It may take several cycles in white-burn mode to return the catalyst to its previous condition.
If you find yourself in a situation where the coke level on the spent catalyst is above 7.5 wt.%, UOP recommends that you contact your UOP Regional Service Manager to discuss the appropriate path forward. In special circumstances, other techniques such as dual zone burning have been used to deal with catalyst that has a high level of coke. The details of these alternatives should be discussed with UOP.
Question 13: When the regenerator in a CCR unit is shut down for an extended periodof time, how do you predict coke on catalyst with no catalyst circulation?
VIVEK GHOSH and STEVENPHILOON (Honeywell UOP)
There are two broad scenarios which might result in the catalyst circulation of a CCR Platforming™ unit being stopped for an extended period of time. The first is that there is maintenance being performed that requires the regenerator to be taken off-line for several days. The maintenance may be scheduled, such as screen cleaning or unscheduled due to a mechanical or other operating problem. In either of these cases, it is not possible to circulate catalyst. The other scenario represents an operational choice. The most common instance of this case occurs when the severity of the reactor operating conditions is so mild that there is insufficient coke deposited on the catalyst to maintain normal operation of the burn zone. For units operating in “low-coke mode", catalyst circulation can be stopped to allow additional coke to lay down on the catalyst. For those units with atmospheric regenerators that routinely run in low-coke mode, Honeywell UOP has a revamp offering that extends the operating envelope significantly. Implementing the revamp has allowed a number of units to operate at lower coke levels and achieve more stable reactor-side operations.
Assuming that no other operating changes can be made to increase the rate of coke formation, for those units operating in “low-coke mode”, UOP recommends that the burn zone be shut down while catalyst circulation continues. This allows for the catalyst to develop a uniform level of coke while avoiding issues associated with restarting catalyst flow. An alternative is to use “batch catalyst movement”. With this approach, catalyst is circulated for a short period, every day or two, which is long enough to collect representative sample of spent catalyst. With either of these approaches, the coke on catalyst is determined by laboratory analysis. The third approach is to have no catalyst circulation. UOP does not recommend this option because of the uncertainty when estimating the coke on the catalyst and the rare, but real, risk of problems restarting catalyst circulation. If the level of coke on the spent catalyst cannot be determined by lab analysis, the following discussion will be helpful for estimating the rate of coke formation.
When the regenerator section is shut down for maintenance while the reactor side continues to operate, it is important to track and control the rate of coke formation so that the regenerator can be brought back onstream before the coke level gets too high. The rate of coke laydown is a function of many factors that may be either easy to control, difficult to control, or essentially uncontrolled at the unit level. The feedrate, H2/HC (hydrogen gas/hydrocarbon) ratio and weighted average inlet temperature (WAIT) are all directly controlled at the unit. The feed distillation and product quality requirements are not directly controlled at the unit, and any change must be negotiated ata multi-unit level. The current condition of the catalyst, the quantity of the coke precursors in the feed, and the risk of an upset are essentially uncontrolled.
If the reactor operating conditions will remain the same –after the regenerator shutdown –as they were prior to the shutdown, the refinery can make an assumption that the coke laydown rate in the reactors during the CCR shutdown period will also not change significantly.
Based on operating experience with CCR and semi-regen units and data from pilot plant studies, it is UOP’s understanding that the rate of coke formation in the last reactor is higher than that in the lead reactors. This result is true because the average bed temperature is higher in the last reactor and coke precursors that are formed in the lead reactors will become coke in the last reactor. For units with about 50%of the total catalyst volume in the last reactor, the rate of coke formation in that reactor can be estimated to be about two-times that of the previous reactors. Using this rule of thumb, the rate of coke laydown in the last reactor can be estimated on a daily basis. This rate can then be used to predict the level of coke on the catalyst in the last reactor.
If the reactor-side operating conditions during the CCR shutdown period change, then the Relative Coking Factor charts in the UOP CCR Platforming General Operating Manual can be used to estimate the new coke laydown rate. As is always the case with estimations, the greater the magnitude of the changes in the operating conditions, the greater uncertainty in the new estimated coke formation rate.
When operation of the regeneration tower and catalyst circulation is first restarted, it is expected that the coke on the spent catalyst will be high. If the catalyst can be circulated at the design rate, the coke level will decrease during the first catalyst cycle as the unit returns to regular operation and normal catalyst coke levels. If, for any reason, the catalyst circulation rate is significantly below design, the amount of coke on the catalyst may continue to rise.
At the end of the day, past experience with the unit will provide the best basis for coke prediction. If you need to know the maximum number of days you can operate without the regenerator in order to plan for a CCR section shutdown, it would be beneficial to conduct operational tests to quantify the rate of coke formation with your unit, feed, operating conditions, and catalyst.
Question 16: What is your Best Practice for inspecting and preventing erosion in CCR lift lines?
KOLAPO ALADE-LAMBO (Honeywell UOP)
In CCR Platforming™ units, the movement of catalyst through the lift pipe results in contact between the catalyst pills and the inner surface of the lift pipe. In turn, this results in catalyst attrition and erosion of the pipe surface. A rupture in the catalyst lift line can lead to a hazardous situation, especially in a lift line that uses a hydrogen-rich lift gas. Severe lift line erosion is most likely to occur at an elbow or bend in the lift line due to the angle of contact between the catalyst and the elbow during catalyst lift. To prevent or minimize erosion in the catalyst lift lines, it is important to monitor and control the lift gas velocity and catalyst mass flux in the lift lines according to the guidelines of the licensor of the unit.
Honeywell UOP recommends periodic ultrasonic testing of the lift lines to determine the pipe thickness, with the inspection concentrated at the lift line bends as more erosion is usually seen there. For new units or new inspection programs, the frequency of testing should be at least annually to establish the rate of erosion after which the frequency can be adjusted as appropriate.
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Question 17: What are your strategies for managing feed sulfur to reforming units? What are the pros and cons of the different approaches?
STEVENPHILOON (Honeywell UOP)
It is desirable to have a small amount of sulfur in the feed for CCR reforming units in order to reduce the risk of metal catalyzed coke (MCC) formation and heater-tube carburization and dusting. The sulfur interacts with the chromium and the iron to form a protective layer that reduces the penetration of carbon into the metal. However, sulfur is also a poison to the platinum metal function of reforming catalyst; so, the amount in the feed must be kept below the level where it will impact the performance of the catalyst.
The recommended level of sulfur in the feed to a CCR Platforming™ unit varies depending upon the severity of unit operations. The risk of MCC formation increases with decreasing reactor pressure and increasing product octane.
Honeywell UOP’s recommended approach is to operate the naphtha hydrotreating (NHT) unit to remove essentially all of the sulfur in the feed. Thisapproachwill ensure that other contaminants (nitrogen, metals, oxygenates, etc.) are also removed from the feed to the extent achievable by the NHT.Organic sulfur is then added to the Platforming™unit feed with a chemical injection system pumping in a specific and controlled amount of organic sulfur compound to achieve the target recommended by the licensor. This injection of sulfur provides the refiner with independent control of the sulfur in the feed to the unit that can be changed,as needed,if feed rate or operating conditions change
Question 18: The increased production of light straight-run (LSR) from crude units is likely to have an impact on refiners’plans for Tier 3 compliance. What strategies do you employ in order to manage this issue?
JEFF BRAY (Honeywell UOP)
Tier 3 drives hydrotreating of essentially all light naphtha streams. Since most United States refineries have FCCs, it is usually desirable to hydrotreat other gasoline streams more completely to minimize the FCC naphtha olefin saturation and the associated octane loss. Even streams such as alkylate, and butanes can contain sufficient sulfur to impact the pool. Complete hydrotreating of these streams will often require additional hydrotreating capacity. With the increase of light straight-run naphtha yields from crude and the availability of cheap natural gasoline, many sites have become limited in hydrotreating capacity for the gasoline range streams. The regulatory requirement then drives an expansion of hydrotreating, which is very hard to avoid without significant impact on site economics. To make the project add to the site profitability, a key aspect is to try to extend the project not only to just meeting regulatory needs, but also to debottleneck the site so that more material, such as natural gasoline or other condensates, can be upgraded or value added in other ways.