Troubleshoot FCC Catalyst Losses
Catalyst losses from the FCC unit are of great concern to the refiner due to their impact on the environment, unit operation and catalyst cost. As a consequence, the losses are held down to the minimum practical level. In a well designed and operated unit losses below 0.1 pounds per barrel of fresh feed (0.1 Lbs/Bbl-FF) are achieved. However, problems with excessive catalyst losses (>0.2 Lbs/Bbl - FF) are not uncommon as there are numerous possibilities for creating the trouble. These include changes in operating conditions, mechanical problems (cyclone, air grid, etc. damage) or poor catalyst retention properties.
The following discussion will assist the refiner in trouble-shooting the most frequently encountered loss problems. Additionally, points of concern are illustrated in Figure 1.
Increased Loss Determination
Catalyst loss problems are easily determined by the refiner when they observe any increases in either:
- Ash content of the slurry oil is Basic Sediment & Water (BS&W)
- Stack opacity
- Fines collection from the precipitator or tertiary recovery system
- Catalyst additions in order to hold unit inventory constant.
Loss Patterns Provide Clues to Cause
Observing the catalyst loss patterns provides clues as to the possible cause of the problem. Two typical patterns are:
Decrease In 0-40 Micron Fraction
The first of these patterns occurs when a catalyst loss problem is accompanied by a reduction in the 0-40 micron fraction of the circulating inventory and increasing average particle size (APS). Additionally, the increased losses are limited either to the reactor or regenerator side of the unit.
These patterns indicate that the cyclone system was experiencing problems due to:
- excessive dip leg catalyst backup
- holes in the cyclone (Fig. 2) or internal plenum
- spalled coke or refractory lodged in the dip leg
- flapper valve (Fig. 3) problems (coke build-up on the counter weight, counter weight improperly set or valve lodged open with coke)
- trickle valve (Fig. 4) problems (erosion of the valve seat or the valve immersed in an unfluidized zone)
- false bed or stripper levels which interfere with flapper or trickle valve operation. (generally, flapper valves do not perform well if covered, while trickle valves perform best when submerged).
The catalyst losses will increase disproportionately from the vessel (side) containing the malfunctioning / damaged cyclone. However, losses from the unaffected side might also increase if the fresh catalyst make-up rate is raised to maintain inventory.
No Decrease In 0-40 Micron Fraction
A simultaneous increase in catalyst losses on both the regenerator and reactor sides, without a decrease in the 0-40 micron fraction, indicates an increase in catalyst breakage. The following are possible causes:
- catalyst attrition by high velocity gas jets. These could be from a damaged feed nozzle, aeration nozzle, stripping steam distributor or open blast taps
- air distributor damage that results in impingement of catalyst against the vessel walls or other hardware
- damaged torch oil nozzle that results in poor atomization
- use of fresh catalyst that is "soft" (non-attrition grade), possesses low bulk density or contains excessive amounts of 0-40 micron material.
Particle size distribution and microscopic examination of the catalyst fines are useful in determining the cause of the loss problem. Average particle size of fines from the main fractionator tower bottoms or the fines leaving in the overflow of the secondary regenerator cyclone can serve as a guide to the cause of the problem. High average particle size (APS) and the presence of extremely coarse particles indicate a hole or a malfunctioning cyclone. Low APS of these fines suggests a catalyst related problem or the presence of an "attritor" in the unit.
Operating Conditions Affect Losses
Operating conditions of the FCC unit can contribute to increased catalyst losses regardless of the cyclone problems discussed earlier.
Operating variables that affect catalyst entrainment and inlet velocity to the cyclones as well as fines generation in the FCC unit will effect catalyst losses. Some of the major variables affecting these are discussed below.
Catalyst Circulation Rate
Catalyst attrition in the unit is directly related to the catalyst circulation rate. At a given feed rate, increasing catalyst circulation will increase fines generation. The increased losses would be observed on both sides of the unit.
Superficial Gas Velocity
Increasing superficial gas velocity in a vessel will increase catalyst entrainment to the cyclones and result in increased catalyst losses. In most instances, cyclone efficiency improves with higher gas throughput but not sufficiently to offset the increase in solids loading. Superficial velocity will be increased on the reactor side of the unit by:
- increasing feed rate
- increasing reactor temperature and unit conversion
- reducing operating pressure
- using or increasing dilution water/steam to the riser/ reactor.
The superficial velocity on the regenerator side will be increased by:
- increasing air rate
- increasing regenerator temperature
- reducing operating pressure
- increasing quench steam.
- Increasing the superficial velocity will increase cyclone inlet velocity and increase cyclone pressure drop. The dip leg exit is sealed against the pressure difference by the static head of the catalyst in the dip leg. Increased pressure drop will result in increased catalyst back-up in the cyclone dip legs.
Normally, a margin of several feet is allowed between the top and the maximum operating level in the dip leg to allow for fluctuations in catalyst unit inventory. Excessively high levels of catalyst back-up in the dip leg will result in re-entrainment of catalyst inside the cyclone with subsequent loss.
- Occasionally, a unit will experience increased catalyst losses when the feedrate and consequently the air rate is reduced substantially below the design level. In this case, the catalyst entrainment is reduced but not sufficiently to over come the decrease in the cyclone efficiency.
In order to minimize the problem, the superficial velocities should be maintained artificially high by using additional steam or air on the reactor and regenerator sides of the unit respectively.
If the throughput reduction is for a substantial length of time, some refiners have taken a few cyclones out of service in order to improve operating efficiency of the remaining cyclones. In this instance, the total solids entrainment remains the same but losses are reduced due to improved cyclone efficiency as a result of higher inlet velocities.
Depending on the nature of the loss problem, the refiner should consider the following solution.
Use Harder and Denser Catalyst
A denser, more attrition resistant catalyst will reduce fines production, lower catalyst loading and will be retained better in the unit. As a consequence, the stack opacity and slurry BS&W will be reduced. A catalyst switch should be contemplated if current unit retention is inadequate due to increased capacity or severe attrition conditions.
Shutdown to Repair Mechanical Problems
Mechanical problems generally require a unit shut down to correct. As an interim measure, many refiners increase the amount of purchased equilibrium catalyst in the FCC make up in order to compensate for increased losses while maintaining desired catalyst activity in the unit.
Reduce Cyclone Loading
A reduction in the cyclone loading by reducing the feed rate, dilution steam/water, catalyst circulation, etc. sometimes is beneficial in reducing losses. A switch to a denser more attrition resistant catalyst may also improve unit operation and retention.
Install Redesigned Dip Legs
Dip leg sizing should be checked. If the size is inadequate for the new sustained operating conditions, then the dip legs should be replaced. An undersized dip leg will choke and reentrain the fines back into the gas flow while an oversized dip leg may allow the catalyst to deaerate and plug.
Guidelines for estimating entrainment, pressure drop, dip leg sizing and cyclone efficiency are contained in Chapter 13 of the API Publication 931. Cyclone manufacturers should be consulted when vigorous evaluation of cyclones are required.
There are several things that the refiner can do as preventative measures to ensure proper operation of the cyclones. These should include thorough cyclone inspection for holes, refractory loss or coke build up prior to start up.
During the start up the coke build up in the reactor cyclones can be minimized by using a light feedstock and by ensuring that the reactor is at operating temperature prior to introducing feed.
There are numerous reasons for increased catalyst losses from an operating FCC unit. In determining the cause, the refiner should review in great detail recent operating conditions and the physical properties of equilibrium/fresh catalysts. Additionally, list current mechanical problems in the unit. Compare these data to the period when the losses were normal. The review will help to pin-point the reasons for the problem.