FCC Catalysts for Today's FCC
Part One: Process Model Studies
by J.B. McLean and E.B. Bovo
Dramatic yield improvements have been demonstrated commercially using the latest FCC process technology advances. These technologies include improvements in feed dispersion, stripper and regenerator design, and riser termination for short contact time cracking. Riser termination designs in particular have received much attention, for both new units as well as revamps. Effective systems have been developed to rapidly separate catalyst from hydrocarbon vapors, reducing non-selective post-riser cracking. Improved selectivities result, reducing coke and dry gas formation and allowing unit operation at higher severity.
Proper catalyst selection is essential to realizing the maximum potential benefit from these hardware improvements. Both fresh catalyst properties and catalyst management policy are important variables. In this Catalyst Report, process model studies are presented for several hypothetical cases, which have incorporated an advanced termination device. These studies include the use of 1) high activity catalyst for maximum conversion or throughput, 2) high matrix catalyst for reducing slurry yield, and 3) a metals tolerant catalyst for processing resid feed. In all cases improved yields are achieved when incorporating catalyst change effects, which may not have been possible without the advanced hardware, due to unit constraints. Commercial data supports the above catalytic changes with the implementation of these advanced technologies. The documentation of this commercial performance will be covered in a subsequent Catalyst Report.
It has been recognized for several years that significant post riser cracking occurs in commercial FCC units resulting in substantial production of dry gas and other low valued products (Ref. 1). There are two mechanisms by which this post-riser cracking occurs, thermal and dilute phase catalytic cracking.
Thermal cracking results from extended residence times of hydrocarbon vapors in the reactor disengaging area, and leads to high dry gas yields via non-selective free radical cracking mechanisms.
Dilute phase catalytic cracking results from extended contact between catalyst and hydrocarbon vapors downstream of the riser. While much of this undesirable cracking was eliminated in the transition from bed to riser cracking, there is still a substantial amount of non-selective cracking occurring in the dilute phase due to the significant catalyst holdup which occurs without an advanced termination system design.
Dilute phase cracking produces both additional dry gas as well as additional coke resulting in higher "delta coke". High hydrogen transfer selectivities are also characteristic of this secondary reaction process, resulting in gasoline octane loss and reduced LPG olefinicity through formation of C3/C4 saturates , especially isobutane. Reaction mix sampling studies for various units have shown that up to 50% of the total dry gas and C3/C4 saturates can be formed from post-riser cracking, in most cases with little increase in more valuable yields of gasoline or C3/C4 olefins (Ref. 2). Additional conversion, which occurs in the dilute phase, varies among units, but in general is no more than a few percent. Because most units operate against constraints, which are related to the total coke and gas production, unit operating severity is typically restricted as a result of these non-selective reaction mechanisms.
Advanced Termination Devices
The major FCC process licensors have developed advanced riser termination systems to minimize post-riser cracking (Refs. 3-7), and many units have implemented these in both new unit and revamp applications. In addition, some refiners have implemented their own "in-house" designs for the same purpose. Due to the complexity and diversity of existing FCC units as well as new unit design differences, there are many variations of these systems such as "closed" cyclones, "close-coupled" cyclones, "direct connected or coupled" cyclones, "high containment systems", "short contact time", etc. There are differences in the specific designs, and some may be more appropriate for specific unit configurations than others, but all serve the same fundamental purpose of reducing the undesirable post-riser reactions.
There are many options for taking advantage of reduced post-riser cracking to improve yields. Some combination of higher reactor temperature, higher cat/oil ratio, higher feed rate, and/or poorer quality feed is typically employed. It is interesting that very little notice in the references cited is given to what types of modifications to the catalyst should be made to complement these designs, particularly for revamp applications. FCC catalyst formulations today are highly specialized and customized to each refiner's specific needs. Therefore it is not surprising that significant catalyst modifications should be appropriate when a change of this magnitude in a unit's yield pattern occurs due to hardware effects. In fact, a change of catalyst has been necessary in most cases where revamps have been performed incorporating advanced riser termination systems. Typical catalyst selection objectives such as low coke and dry gas selectivity are reduced in importance due to the process changes, while other features such as activity stability and bottoms cracking selectivity become more important for the new unit constraints. Changes in catalyst management policy, balancing addition rates with fresh catalyst properties, may also be appropriate.
Basis for Process Model Studies
The studies conducted here considered a hypothetical unit, which incorporates an advanced riser termination system in a revamp. Heat balanced process model studies were conducted using a combination of the Profimatics FCC model and BASF's catalyst model to predict process operating condition and catalyst change effects.
Parameters and yields for the base case, chosen for evaluation, are listed in Table 1. The conditions were chosen to be typical for a unit feeding VGO and operating in full combustion. Since none of the base case conditions were considered severe, sufficient latitude for varying operational parameters within commercially practical limits in the post-revamp cases was allowed. The base case catalyst was considered to be an octane barrel oriented RE-USY catalyst, with properties estimated to be typical of the average catalyst based on BASF's equilibrium catalyst data base. This choice of base case catalyst also left considerable flexibility for modifying catalyst properties for alternate cases studied.
Before examining the results from the cases where full advantage is taken of the advanced termination device installation, two post-revamp cases are shown alongside the base case in Table 1. These cases are distinguished from subsequent cases in that no changes were made in catalyst type, catalyst addition or reactor temperature.
Note in the first "After Revamp" case, catalyst circulation was allowed to rise to 31 TPM (a 12% increase) and the preheat was allowed to rise to maintain reactor temperature. Note in the second "After Revamp" case, catalyst circulation was unconstrained, but the preheat was kept the same as in the "base case".
Under such restraints, loss of conversion due to minimizing thermal and dilute phase cracking is clearly illustrated. Realistically, a refiner is not going to operate at lower conversion. Instead a refiner will seek to maximize conversion (or profitability) up to unit constraints.
Different refiners will choose to respond with different methods of maximizing profitability.
Three Possible methods were considered in the studies in this publication:
Each scenario was evaluated first by considering process changes only, and then expanded to include the effects of catalyst changes as well.
Model Study Assumptions
The assumptions regarding process variable or catalyst changes allowable to meet the above methods are described below:
2. Riser temperature, feed preheat and catalyst make-up (for the base case catalyst) were the primary independent variables considered.
The following options were included in the cases where catalyst reformulation was allowed:
2. Increased fresh catalyst activity.
3. Increased matrix activity featuring increased bottoms upgrading.
Performance for cases involving catalyst changes is predicted by integrating relative catalyst activity and selectivity responses into the heat-balanced model and varying conditions subject to the unit constraints. The relative properties for the alternative catalysts are shown in Table 2.
Note that the coke and dry gas selectivities for the high activity and high matrix catalysts are higher than for the base catalyst, so that they work best in conjunction with the delta coke and gas reductions achieved via the hardware modifications. The high activity catalyst was considered for higher conversion and higher feedrate cases. The high matrix catalyst was also considered for the higher feedrate case, with a secondary objective to minimize slurry yield. The resid catalyst differs from the others in that it features lower coke and dry gas selectivities than the base, as well as a more favorable metals response factor, which becomes important for resid feed applications.
Study Results for Max. Conversion Scenario
In a situation where feed availability is fixed, it is desirable to maximize conversion subject to the wet gas and air constraints. Figure 1 shows computer simulation results for this scenario where conversion has been varied by changing reactor temperature or equilibrium catalyst activity. Observations are as follows:
- If reactor temperature is used, as indicated by the line designated Rxn T (1), the reaction temperature can be increased to 1020°F while maintaining catalyst additions at 5 TPD. The maximum achievable conversion in this case is much lower than all other cases due to the stronger wet gas response to temperature.
- Using reactor temperature with no circulation constraint, as shown by the line designated Rxn T (2), will achieve conversion beyond the base case value.
- If the high activity catalyst (Cat Act line) is used, a much higher conversion can be reached at the base reactor temperature without increasing addition rates. The higher activity catalyst would be more expensive on a $/ton basis than the base catalyst, but overall catalyst cost would be lower since lower addition rates could be maintained.
In summary, the highest conversion is obtained for the high catalyst addition case, with the response for the high activity catalyst close behind. However, increasing addition rates to 12 TPD may neither be physically possible nor be economically justifiable so the higher activity catalyst may be more promising. Raising reactor temperature is the least effective way to maximize conversion. While the results improve for the high reactor temperature case if there is no circulation constraint, the results still show less improvement than if the catalyst is changed.
Increased Feedrate Scenarios
Another common scenario is where more feed of similar quality may be available, and it is desirable to increase feedrate up to the unit constraints but without losing conversion relative to the original base case. Figure 2 shows the results for increasing feedrate by 10%. The high activity (Hi Act Cat line) catalyst is again compared with a combination of increased temperature and catalyst makeup for the base catalyst (Add Cat line). For the base case catalyst, the make-up rate is increased from 5 TPD to 7.5 TPD and the reactor temperature allowed to rise to 992°F in order to reach the wet gas compression limit. For a given conversion target, a higher incremental feedrate can be achieved with the high activity catalyst.
Another common objective would be to increase feedrate while at the same time minimizing production of low value slurry. The slurry production rate is a function of feedrate, conversion, and bottoms upgrading selectivity. The high activity catalyst maximizes conversion as discussed above, and thus reduces slurry yield relative to the base catalyst.
Alternatively, a high matrix activity catalyst featuring enhanced bottoms upgrading selectivity can be used. Figure 3 shows for a given feedrate and conversion target, the high matrix catalyst achieves the lowest overall slurry yield. The dry gas and regenerator temperature limitations, which would have normally been reached from using this catalyst in the base case, have been removed by the hardware changes associated with advanced riser termination.
Resid Feed Scenario
Another common situation is for a refiner to take advantage of the revamp by running heavier and lower cost resid feed to the unit. Figure 4 shows the results when 10% of the VGO feed has been replaced by a vacuum resid. The base catalyst is compared to a more selective resid catalyst. With both catalysts, a combination of higher reactor temperature (1000°F) and higher catalyst addition rate (7.5 TPD) has been used to reach circulation and air limits while attempting to maximize conversion. The resid catalyst achieves a higher conversion with the difference accentuated due to the higher coke producing tendency of the feed and the higher metals loading on the catalyst relative to the VGO operation. Figure 4 shows a higher quantity of resid feed can be added at a given conversion target when the resid catalyst is used.
Dramatic yield improvements have been demonstrated by implementing the latest FCC hardware technology, in particular advanced riser termination designs to minimize non-selective post-riser cracking. The potential yield benefits are magnified when combined with the proper catalyst choice of the new operation. In nearly all cases, the optimum catalyst choice will change when advanced riser termination designs are implemented. Depending on how an individual refiner chooses to optimize the new operation, beneficial catalyst changes may include higher activity for increased conversion and throughput, higher matrix activity for enhanced bottoms cracking while minimizing coke and gas penalties, or improved resid tolerance for operation with poorer quality feeds. Potential catalyst formulation changes and the combined impact of process and catalyst changes on performance should be addressed by any refiner considering the implementation of these new technologies.
Acknowledgement: The contents of this Catalyst Report were taken from the NPRA Paper AM-94-44, "Improved FCC Yields Through A Combination of Catalyst And Process Technology Advances", by the same authors.
1. A.A. Avidan and F.J. Krambeck " FCC Closed Cyclone System Eliminates Post Riser Cracking" NPRA 1990 Annual Meeting
2. W. Letzsch, R.J. Campagna, and D.C. Kowalczyk "New Optimization Tools and Technologies for the FCCU" NPRA 1993 Annual Meeting Paper AM-93-66
3. R.E. Wrench and P.E. Glascow "FCC Hardware Options for the Modern Cat Cracker" AlChE Symposium Series #29, 1992
4. L.L. Upson and D.E. Wegerer "Rapid Disengager Techniques in Riser Design" ACS 3rd International Symposium on Advances in Fluid Catalytic Cracking August 1993
5. S.L. Long, A.R. Johnson, and D. Dharia "Advances in Residual Oil FCC" NPRA 1993 Annual Meeting Paper AM-93-50
6. D.C. Draemel "Flexicracking IIIR - ER&E's Latest Cat Cracking Design" JPI Petroleum Refining Conference 1992
7. F.H.H. Khouw "Shell Residue FCC Technology: Challenges and Opportunities in a Changing Environment" JPI Petroleum Refining Conference 1992