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Avoiding FCCU Outages & Mechanical Failures

INDUSTRY Q&A: FCC Operations

 FCC Consultants Bob Ludolph and Ken A. Peccatiello

Question #1:  In troubleshooting catalyst losses, can you describe any cases where a unit shutdown was imminent (e.g., severe cyclone failure) if the losses could not be stopped quickly? What is your recommendation for avoiding shutdowns due to excessive catalyst losses?

Bob Ludolph

Bob Ludolph

 Bob Ludolph: There are a number of scenarios that can lead to a unit shutdown due to excessive catalyst losses. Generally, inability to maintain regenerator bed level leads to an imminent unit shutdown. Since the stripper bed level is controlled with the spent catalyst valve any catalyst loss from the reactor or the regenerator is reflected in a loss of regenerator level. Refiners will face the decision to shut down whenever the catalyst loss is so high that:

There may be opportunities to mitigate the catalyst loss and delaying the need for shutdown. The delay would allow the refiner to troubleshoot and develop corrective action plans for staying on-line and keeping operating costs down. The refiner could consider:

Ken A Pecatielo

Ken A Pecatielo

Ken A. Peccatiello: There have been circumstances where catalyst losses have been very severe and a shutdown was imminent. However, it is very unusual for the cause of an immediate/emergency shutdown to be cyclone damage due to normal “wear-and-tear.” The normal “wear-and-tear” damage can usually be observed, monitored, and anticipated well in advance by operations and technical personnel, thus avoiding an emergency outage; a good monitoring program will allow operations and technical personnel to work with planning and scheduling to plan an outage as the damage and losses become unsustainable.

The rapid and imminent causes are usually the result of a significant event or a catastrophic failure.  The usual causes are refractory failure/loss following an upset or a thermal cycle which plugs or restricts one or more cyclone diplegs leading to massive catalyst carry-over.  A second example is the use of “smear-coating” or “butter-coating” the refractory in the cyclones.  “Smear-coats” or “butter-coats” do not adhere to the base refractory and spall off almost immediately upon start-up. Since the secondary cyclones are relatively small in diameter to most units (usually 8-10 inches), a major spall of this type can easily plug (and has) the dipleg.   An example unique for the reactor side, would be coke spalling from reactor cyclone gas outlet tubes following a thermal cycle and again plugging or restricting the cyclone dipleg.  These situations can be observed in several manners:

The failure mentioned previously may not be easily observed upon the restart / dry-circulation period when catalyst circulation rates are generally very low and the catalyst loadings to the cyclones are extremely low (generally less than 10% of operating catalyst circulation rates).  The situation will manifest itself once feed has been introduced into the unit and catalyst circulation rates, and thus catalyst loadings to the cyclones, are increased.

A more unusual, but still possible (possible because it has occurred) circumstance is the loss of a cyclone.  A loss in this case means the cyclone failed or dropped from its supports.  This can happen following an extreme thermal excursion (usually in the regenerator) or following some seismic activity for either or both the reactor and/or the regenerator sides (usually discovered upon a re-start as the seismic activity most likely took the unit off-line). Again, the same five previously mentioned items will be key indicators of a problem or issue. An FCCU/RFCCU process engineer should know the following critical pieces of information for their unit:

  1. At what rate would catalyst be lost from the unit if a regenerator primary cyclone were to plug?
  2. At normal catalyst circulation rates.
  3. At minimum feed rate catalyst circulation rates (minimum feed rate is usually the point required for main fractionator operational and product yield stability).
  4. At what rate would catalyst be lost from the unit if a regenerator secondary cyclone were to plug?
  5. At normal catalyst circulation rates.
  6. At minimum feed rate catalyst circulation rates.
  7. At what rate would catalyst be lost from the unit if a reactor primary/rough-cut cyclone were to plug?
  8. At normal catalyst circulation rates.
  9. At minimum feed rate catalyst circulation rates (minimum feed rate is usually the point required for main fractionator operational and product yield stability).
  10. At what rate would catalyst be lost from the unit if a reactor secondary cyclone were to plug?
  11. At normal catalyst circulation rates?
  12. At minimum feed rate catalyst circulation rates.

The ability of process personnel to quickly analyze and respond to any of these is the difference between a short five-to-eight day disruption or two-to-three week outage.

Question #2: What defines FCC under-utilization and where do you look for profit improvement in such situations?

 Bob Ludolph: In many respects, what defines under-utilization applies to when FCC feedrates are down or when operations are normal; the difference is a matter of degree.  In general, under-utilization can be defined whenever:

Where to look for profit improvement?  Three places should be evaluated:

  1. The sources and types of feed components that are cracked
  2. The technology, formulation and blend of catalyst and additives cracking the feed
  3. The operation of the cracking, combustion, flue gas and product recovery equipment.

Let’s discuss the opportunities that come with each of these as well as potential obstacles hindering your success in pursuing them.

Feed Opportunities

Crudes or purchased feedstocks with lower qualities than what are normally cracked offer the opportunity to improve profitability. Higher sulfur, acid number, carbon, and/or metal feeds can be economically attractive while being processed at levels that minimize your risk. Conversely, some refiners introduce feedstocks with better than normal quality in order to fill out their recovery sections while cracking less feed. Internally to the refinery there are incremental feedstocks, like atmospheric or vacuum resid, that can be cracked to raise overall volume gain for the facility. Also, FCC products can be recycled to obtain more valuable yield distributions, like gasoline re-cracked to generate more olefins or heavy cycle oil re-cracked to more valuable liquid products.

Be mindful that an opportunity feedstock may be less crackable than what its reported bulk properties suggest. Excessive yields of fuel gas and coke could result, negatively impacting the heat balance and reducing the feedstock profitability. If the hydrothermal stability of the catalyst is insufficient, then the catalyst may not be able to tolerance additional metals. Additional or new process chemicals for mitigating fouling or corrosion may be needed when cracking more difficult feedstocks.

Catalyst Opportunities

Optimization of catalyst selectivities, activity or both is worth reviewing with your supplier. Through reformulation or blending ratio changes, the yield structure or metals tolerance can be shifted in a favorable direction.  Introduce cracking additive technology (e.g. ZSM-5, bottoms reducer) to shift the yield structure quicker or respond to feed quality changes better. Consider trialing new catalyst or catalyst additive technologies to pursue further product distribution improvement.  Lowering catalyst additions would reduce your expenses. Raising catalyst additions may improve cracking selectivities. Trial each to determine what fits your operation better. Also consider adding equilibrium catalyst on a regular basis to reduce your expenses.

Adjusting catalyst technology or formulation could result in physical property changes of the circulating inventory. Monitor the fluidization parameters since fines retention or catalyst attrition may be less. If catalyst additions are reduced, monitor the particle size distribution (PSD) of the inventory…you may have to adjust the fresh catalyst size grade. Standpipes may be over- or under-aerated depending on the operating conditions…adjust aeration rates as necessary for proper pressure build.  Closely review the properties of any equilibrium catalyst under consideration for purchase…incompatibility with the fresh catalyst technology could lead to under-performance.

Equipment Operation Opportunities

It is likely that the reduced rate operation will free up combustion air. This opens up the opportunity to operate differently and consume the unused coke burn capacity. Pertaining to the riser operation, lowering or removing feed preheat will increase catalyst circulation, conversion and volume gain. Feed dispersion steam can be conserved to reduce sour water production, or increasing the steam/feed ratio may actually improve feed/catalyst contacting.  Being too aggressive with dispersion steam conservation can lead to poor feedstock atomization, feed nozzle plugging, or “wetted” spent catalyst going to the regenerator. Conduct steam/feed trials to determine what works better. Also consider taking feed nozzles out-of-service to improve overall atomization – review the design and procedures before trying this.

By introducing or increasing riser lift gas you may reduce the net dry gas production by “conditioning” the metals on the regenerated catalyst. You can also introduce or increase cracked naphtha injection to promote light olefin production. Under reduced feed rate conditions there will be more riser contact time, which should promote bottoms cracking. You may realize higher slurry ash content as a result, which may impact product blending or increase the erosion rate of system piping. Also, the riser velocity may get too low, which promotes backmixing, resulting in higher fuel gas and coke yields, gasoline overcracking to LPG and other selectivity shifts.

In the reactor section, with the air available, reactor temperature can be raised for more conversion and volume gain. Lowering the reactor pressure will result in less hydrogen transfer (better octane, olefin yield), better stripping, and reduced rotating equipment cost. Longer reactor residence time could also lead to product mix deterioration and vessel coking. Potentially higher butadiene yield may impact the alkylation plant. Slurry fouling rate may accelerate from higher conversion operations – you may need an antifoulant program. LCO properties will shift and may impact distillate blending for cetane, sulfur, and gravity.

In the stripper section, longer stripper residence time (from lower catalyst circulation) can lead to better stripping efficiency, product recovery, and lower coke hydrogen content, especially if reactor temperature is increased. Conserving stripping steam may help with sour water management but be careful of going too low. Like the feed nozzles, getting too aggressive with stripping steam conservation can result in steam distributor nozzle plugging and lower actual stripper bed level, which may uncovering cyclone diplegs. Operating at lower feed rate can also result in higher density spent catalyst, leading to higher slide/plug valve differentials – this presents the opportunity to shift the pressure balance in a positive way by lowering the reactor pressure.

In the product recovery section, generally the lower feed rate condition opens up wet gas capacity. This means you can handle lower suction pressure or lower molecular weight gas streams – this flexibility may expand your feedstock and catalyst options. Downstream recovery sections are likely to be under-loaded, resulting in better liquid/vapor product separations, product purities and improved treating conditions.

When it comes to the regenerator, combustion air will likely be available. You could continue to conserve in the interest of compressor energy savings or apply the excess for higher coke burns. You could also reduce or eliminate the cost of oxygen enrichment if it is part of your base operation. If the refinery needs steam and you have a catalyst cooler increase the coke burn to produce extra steam. If superficial velocities are down and there is less regenerator afterburn then your feedstock and catalyst options grow.  You can also save on combustion promoter which may also result in lower emissions. Too low of a combustion air rate can lead to air distributor nozzle erosion and lower regenerator bed penetration. The burn will become uneven with the radial temperature differences growing. The catalyst regeneration will be less uniform resulting in a “salt & pepper” appearance that could impact the catalyst cracking selectivities. The fluidized density will also vary throughout the bed, potentially impacting the stability of the catalyst circulation returning to the riser. Consider plugging off “extra” air distributor nozzles during the next outage if the operation is expected to last a very long time.

In the flue gas system, turboexpander vibrations from fines deposits could be reduced or eliminated. Less or no walnut shelling would be required.  However, turboexpander power generation may be lost if the regenerator pressure is lower than the normal operation.  Fuel to fired boilers could be reduced or burners modified to reduce emissions. Lower fines accumulation on boiler internals may mean less opacity spiking during sootblowing cycles. Lower chemical cost would be expected for NOx and SOx reduction equipment technologies.

Realize that lower vapor and flue gas rates may result in the loss of cyclone efficiency and less reactor/regenerator catalyst retention. Larger particle sizes may be preferentially lost. Consider sealing off “unnecessary” cyclone pairs during an outage if efficiencies are expected to be low for a very long time. Lowering operating pressure should help in this situation. High slide/plug valve differentials may also lead to high valve erosion rates.  See if the pressure balance could be adjusted to address this. Process control valves may perform with less stability since they may be operating near the low end of their range. Retuning may be necessary.

Additional Opportunities

Consider riser, reactor, stripper, recovery, regenerator and flue gas system modifications. Alter or upgrade equipment to address plant constraints and reliability issues. Add new equipment/technology to expand processing capabilities and maximize profitability under reduced rate conditions.

Lastly, take the opportunity to improve your LP vectors for better representations of feed quality and operating condition changes. Also develop various business scenarios of interest and assemble the model projections for each. Conduct plant trials that simulate the cases and provide the necessary data. Update the LP as necessary.

Ken A. Peccatiello: I would classify an FCCU or an RFCCU as under-utilized with two very different and distinct definitions. The first definition of an under-utilized FCCU/RFCCU is based upon the unit as a stand-alone entity. I do not necessarily consider the unit under-utilized if it is not at maximum feed throughput/rate or at maximum conversion. The true test of the definition for an under-utilized unit is if the unit is constrained?  Is the unit operating or being allowed to operate within parameters within the control of the FCCU/RFCCU operations and/or technical team? Is the unit up against one or more major constraints?  Is the unit limited by: main air blower (MAB); wet gas compressor (WGC); hydraulically on catalyst circulation or slide valve; main fractionator bottoms (MFB) heat removal capacity; main fractionator overhead (MFO) cooling/condensing capacity; hydraulically in the vapor recovery unit (VRU) area?  Is the unit constrained singularly or with multiple constraints?

If the unit is not constrained, then the unit can be considered under-utilized unless it comes into conflict with the second definition. The second definition is if the refinery is constrained in such a manner that the FCCU/RFCCU is not operating at one or more of its constraints.  If the refinery is operating in such a manner that it is uneconomical or unable to push the FCCU/RFCCU to a constraint, then I do not “punish” the FCCU/RFCCU, as this condition is beyond the control on the FCCU/RFCCU operations and/or technical team.  There have been many circumstances where the refinery has cut crude runs either for economical reasons or due to one or more processing unit problems or issues.  Anything that is “beyond the control” of the FCCU/RFCCU team should not count against them.

It is the duty of the FCCU/RFCCU operations and technical team to always look for ideas/solutions in order to maximize the profitability of their unit; which usually means push the unit to multiple constraints.  However, the team also needs to understand the place that their unit has in the over-all scheme of the refinery.  There may be times when the FCCU/RFCCU may need to be under-utilized for the greater economic good of the entire refining complex.


Find more technical articles at the blog and in the FCCU Forum.

Stay current by attending RefComm® CatCracking conferences around the world.

In RefComm® Past Presentations, find these articles.

Troubleshooting Catalyst Losses in the FCC Unit

Twenty Questions Identify Probable Cause of High FCC Catalyst Loss

Monitoring Mitigating and Troubleshooting FCC Catalyst Losses



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Posted by: Rene Gonzalez

Rene G Gonzalez is the Director for and contributing editor for As a chemical engineer (Texas A&M University: 1982), Gonzalez has worked in various engineering capacities throughout the energy industry value chain, primarily in refinery processing and operations.

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