FCC IS FAR FROM BEING A MATURE TECHNOLOGY

Amos A. Avidan
Mobil Research & Development Corp.
Paulsboro, N.J.

Fluid catalytic cracking has been evolving for 50 years without becoming a mature technology.

It is not likely to become one soon because, along with the rest of petroleum refining technology, FCC is facing some of its toughest challenges.

FCC has become the major refining upgrading process because it is simple, relatively inexpensive to construct and operate, and surprisingly flexible. FCC meets ever-changing product, environmental, and operational demands quickly and profitably. Its preeminent position is likely to keep expanding, and the process will adapt to tomorrow's refining needs.

Some recent developments in FCC include hardware changes such as efficient feed/catalyst mixing devices, short contact time cracking, elimination of postriser thermal cracking, and resid cracking hardware.

While still relying on the unique Y zeolite commercialized 25 years ago, FCC catalyst systems have become increasingly more sophisticated, blending numerous functional components, custom-tailored for each application.

Process optimization and advanced control are at the forefront of today's applied computer technology. Emissions have been cut by orders of magnitude through hardware, catalyst, and operational changes.

FCC's role as the refinery's main profit center and state-of-the-art technology showpiece is likely to continue.

Light olefins from the FCC are the source feed for premium gasoline blending components such as methyl tertiary butyl ether (MTBE) and alkylate.

The trend to process resid in FCC is likely to accelerate while the product slate changes. Integration of FCCs with petrochemical plants may provide incremental olefins to supplement thermal cracking.

FCC CATALYSTS

Cracking catalyst systems have also been evolving continuously for over 50 years. The catalysts are at the heart of the FCC process and they provide:

Low coke yield and effective carbon rejection. (Thermal crackers, such as cokers, can yield more than 30% coke, whereas catalytic cracking produces less than 10% coke.)
High selectivity to desired products (light olefins and gasoline).
Enhancement of desired properties, such as octane.
In situ control of emissions, such as SOx.

Worldwide FCC catalyst production was over 1,100 tons/day in 1990 with sales of over $500 million/year. In 1990, estimated nameplate capacity was 1,750 tons/day and actual capacity 1,450 tons/day. Overcapacity has kept prices low and competition stiff, and has resulted in several consolidations.

FCC catalysts and additives are specialty chemicals, and their cost is a small fraction of the uplift in FCC.

RECENT DEVELOPMENTS

Cracking catalysts have undergone many evolutionary and revolutionary changes. Milestones are listed in Table 1.

Today's FCC catalyst system is a complex mixture of functional components. The main component is the catalyst itself, containing Y zeolite, which provides the primary cracking function. Other components currently include:

Combustion promoter: Combustion promoters are used to reduce CO emissions and afterburn in FCC regenerators.
ZSM-5 additive: The ZSM-5 increases octane and light olefins yields. There are currently over 50 commercial FCCUs using ZSM-5, and this number is expected to increase considerably as the push to produce more light olefins continues.
Light olefins are used to produce alkylate and ethers such as MTBE-major components of reformulated gasoline. Typical yield shifts with ZSM-5 are shown in Table 2.
A recent development in ZSM-5 technology is the use of high-activity additives, which have cut catalyst makeup by at least a factor of two.
Another development is the highselectivity ZSM-5 additives, which crack less gasoline and produce less liquid petroleum gas (LPG), to achieve the same octane increase. The increase in octane is obtained mainly by improved isomerization; hence, motor octane increases more with high-selectivity ZSM-5 than with high-activity ZSM-5.
Desulfurization additives: Desulfurization additives promote oxidation of SO2 to SO3 in the regenerator, and adsorption of SO3 onto alumina, which is then transferred to the riser. SO3 is reduced in the riser and catalyst stripper to H2S, which is later recovered in the gas plant.
SOx emissions are reduced by up to 70% in complete CO combustion, and up to 50% in partial CO combustion.
The Y zeolite-containing FCC catalyst is a multifunctional mixture of zeolite, active alumina, silica-alumina, clay, rare earth oxides, and other components (Fig. 1).
The catalyst is custom-tailored to each application, which explains why there appear to be as many FCC catalyst "types" as there are FCCUs (over 350). Major adjustments in the catalyst formulation include the zeolite and matrix.
Zeolite content: There has been a steady increase in zeolite content of FCC catalysts-from 10% in the 1960s to over 35% today. Some of today's FCC catalysts contain up to 50% zeolite. The calcined zeolite surface area is the best indication of active zeolite content.
Zeolite type: There are many derivatives of Y zeolite-basically they are Na56[SiO2]136[AlO2]56.250H2O. These are made by changing synthesis conditions, treatment steps, and exchange agents (Fig. 2). 1 The three most important categories used today are presented in Table 3.
The zeolite can be dealuminated thermally, chemically, or by both methods. Crystallization time and conditions can be adjusted to produce more "perfect" crystals with less defects, less nonframework alumina, etc.
While coke selectivity, yields, and octane are typically a simple function of the zeolite silica-to-alumina ratio (or the unit cell size [UCS]) and the rare earth-to-zeolite ratio, there are catalysts that offer better results than the average correlation (Fig. 3). 2
Rare earth. Elements help increase the hydrothermal stability and activity of the catalyst. They are carefully balanced with zeolite type and content.
Active alumina: Alumina type, pore size distribution, and matrix surface area are important. Amorphous alumina was the active ingredient in cracking catalysts prior to the introduction of zeolites. While increasing bottoms conversion, amorphous alumina increases coke and gas yields.

Matrix activity should be carefully balanced with zeolite activity and usually needs to be kept much lower.

FCC catalyst can contain additives, such as ZSM-5, combustion promoter, vanadium trap, SOx transfer agents, and other ingredients. Alternatively, these agents can be used as separate particle additives. Silica-alumina, clay filler, or natural clay are used to help hold the entire particle together.

Today's spray-dried microspheres exhibit excellent physical retention properties despite much higher zeolite content (Table 4). With properly functioning dust collection systems (cyclones, third-stage separators, electrostatic precipitators), dust emissions from FCCUs can be negligible.

FCC catalysts are supplied in various grades of particle sizes and attrition resistance. At the refiner's choice, calcination can reduce loss-on-ignition and improve attrition resistance.

A coarse grade can be supplied, but it does not flow as well as normal fines content material (about 25% smaller than 40 p,). It can also limit catalyst circulation in poorly designed standpipes. Separate high-density fines, or fluidization additives, are available.

RESID FCC CATALYSTS

The major problems in resid processing are handling high Conradson Carbon Residue (CCR) and metals levels in the feed. CCR contributes to coke yield (up to 75% of CCR goes to coke), hence the need for better catalyst coke selectivity.

Of the feed metals, nickel and vanadium are the most problematic. Nickel catalyzes unwanted side reactions to form coke and dry gas, especially H2. Vanadium causes irreversible destruction of the FCC catalyst.

Higher catalyst makeup rates can offset most of the metals effects. A proper choice of catalyst makeup strategy, for example, can anticipate feed quality changes. This is illustrated in Fig. 4, where the resulting equilibrium catalyst (E-Cat) metals level is plotted against feed metals level at several makeup rates.

There are at least two major schools of thought in the design of commercial resid FCC (RFCC) catalysts, and they are not necessarily contradictory.

The first approach stresses the importance of careful tailoring of zeolite-to-matrix activity ratio. 3 This is illustrated in Fig. 5, where one indicator of selectivity-dry gas make-is plotted against zeolite-to-matrix surface area ratio.

Increasing matrix activity increases bottoms conversion, but at some point, undesirable effects, such as higher coke and dry gas make, are predominant.

The second approach stresses the importance of tailoring matrix alumina tv pe and pore size distribution:

Liquid catching large pores ( 1 00 A in diameter) with lower activity to control coke and gas make
Meso pores (30-100 A) with higher activity
Small pores (

The meso pores are directly responsible for reducing bottoms yield with aromatic or naphthenic feedstocks, whereas the smaller pores and the zeolite are more important for paraffinic feedstocks (Fig. 6). 4

Because many resid molecules are typically larger than 30 A, the RFCC catalyst matrix needs to be more open than conventional catalyst. The shape-selective (zeolitic) portion of the catalyst is less useful for the primary cracking of a resid molecule-particularly for aromatic or naphthenic molecules.

This primary cracking, or precracking, must be achieved by the matrix.

Sufficient amorphous matrix activity must be present to convert the resid asphaltenes down to the gas oil size, but too much activity can lead to excess coke and dry gas make.

Metals passivation in resid FCC operation is handled by:

Vanadium traps, such as tin or barium and strontium titanates and MgO; zeolite type and content.
Nickel passivation, such as antimony and bismuth; matrix properties.

FUTURE DEVELOPMENTS

The Y zeolite has been the main ingredient in FCC catalysts for 25 years. Usage of USY-containing catalysts in the U.S. has been increasing steadily since the mid-1980s and currently amounts to about 70% of the total. 3 This trend will probably accelerate internationally as well.

The Y zeolite's superior selectivity and activity over larger pore amorphous alumina or silica-alumina has been fine-tuned over the years. One big question in FCC catalysts for the future is: Will there be another zeolite, molecular sieve, or any other microporous structure superior to the Y zeolite?

To date, this question has not been answered, but there has been considerable research and speculation.

With the increasing importance of resid cracking, larger-pore materials have been proposed as cracking catalysts (Table 5). None of the materials with pore openings larger than Y zeolite have so far shown significant cracking activity or good hydrothermal stability.

The hypothesis that larger zeolite pore openings are most efficient for resid cracking has not been proven. Current resid catalyst design practice is:

Lowest-UCS USY zeolite
High zeolite content
Low rare earth level
Controlled ratio of matrix-to-zeolite activity
Controlled matrix pore size distribution
Metals tolerance, traps, and passivation.

Catalyst zeolite content is likely to keep increasing, particularly with emphasis on resid processing. Today's premium FCC catalysts, with about 40% zeolite, can be extrapolated to about 50% zeolite before significant degradation in zeolite availability and physical properties occurs. Future developments may allow even higher zeolite levels.

One additional factor supporting the need for higher-zeolite catalysts is the forecasted increase in the use of additives which dilute the concentration of Y zeolite in the FCC catalyst inventory. These additives will include further use of SO, transfer agents and better vanadium and nickel traps, as well as potentially new additives.

The use of ZSM-5 is likely to increase dramatically if proposed regulations mandating reformulated fuels (2.7% minimum oxygen in gasoline, maximum aromatics levels) take place.

The FCC complex is currently the source of some of the "dirtiest" fuel components (such as light cycle oil [LCO] and FCC heavy naphtha), as well as some of the "cleanest." These clean components include light olefins, which can be converted to alkylate, MTBE, tertiary amyl methyl ether (TAME), and synthetic diesel.

Matrix technology is likely to keep improving in terms of metals tolerance, more selective bottoms cracking, and physical properties. Catalyst demetallization technology may become more widespread in commercial use.

Equilibrium catalyst metals can be removed by a chemical washing process such as Demet or by a magnetic separation process like MagnaCat. 5-7 Such processes can restore activity and selectivity and save on catalyst makeup in a resid FCC.

FCC HARDWARE

FCC hardware has also been improved continuously, usually driven by product demands, catalyst innovations, process objectives, and most importantly, the need to improve reliability and meet environmental regulations.

Key hardware areas in a modem FCCU are shown in Fig. 7. Feed nozzles have been improved and now may include feed atomizers. Many different designs are used, from low pressure drop atomization to high pressure drop atomization (150 psi).

Some claims have been made as to "shattering" of feed asphaltenes, thereby preventing high-CCR resids from converting 75% of the CCR to coke.

Another major hardware improvement commercialized in recent years is closed cyclones. 8 The use of closed cyclones in FCC reactors has eliminated postriser, nonselective thermal cracking.

The most dramatic effect has been the reduction in dry gas components, such as methane and ethane, by more than 40%. Gasoline and distillate yields and gasoline octane are also increased, while heavy fuel oil yield is decreased. Butadiene concentration in LPG is decreased as well, by up to 50%.

The benefits of closed cyclones increase with the trend toward shorter contact times and higher cracking temperatures. As the temperature is raised, competition from nonselective thermal cracking in the reactor vessel becomes more significant. It then becomes imperative to redesign the product/catalyst separation system to eliminate thermal cracking.

Hardware improvements needed to convert an FCC to an RFCC are shown in Fig. 8. They are similar to conventional FCC designs, with the emphasis shifting to better feed nozzles, higher use of dispersion steam (up to 10 wt %), and the use of bed coils or catalyst coolers to remove heat from the regenerator (Fig. 8).9

Typical catalyst coolers allow for 75150 MMBTU/hr of heat removal. Catalyst coolers can replace, or be used together with, CO boilers, which also remove heat from the FCCU.

FUTURE DEVELOPMENTS

As with catalyst, future developments in FCC hardware are likely to be a continuation of recent trends, particularly with increased processing of resid. Hardware such as feed nozzles and catalyst coolers will be improved. Contact times in FCC risers may be reduced even further than today's practice (Table 6).

Considerable work on ultrashort contact time has been done by researchers in academia, such as the pioneering work of Bergougnou and his coworkers; and in industry, such as the Stone & Webster QC process.

Tied to the concept of ultrashort contact time is the issue of downflow vs. upflow reactor. There is little difference in hydrodynamics of performance of upflow vs. downflow pilot plants (the slip factor for both are close to 1).

However, there is a large difference in slip factors and backmixing between upflow and downflow commercial-size reactors. The slip factor for upflow can be as low as 0.3, whereas it is about I for downflow. Because of scale-up difficulties and poor understanding of the hydrodynamics involved, it is not easy to predict the advantages of downflow, if any.

FCC PROCESS

The FCC process combines catalyst, hardware, and process technology to produce optimal results. The main elements of FCC process technology include:

Operating strategy, steady-state optimization
Process control
Environmental control
Integration with other refinery units and refinery energy balance.

The FCC process is a major factor in refinery profitability, and today it is the main upgrading process. It has been the most profitable and flexible refining process for nearly half a century because of its ability to meet changing demands.

FCC gasoline started out as a component in aviation gasoline, along with alkylate, and FCC naphtha still provides excellent front-end volatility and octane. FCC is very flexible in feed, and accepts a continuously changing feed slate of vacuum distillates, atmospheric resid, vacuum tower bottoms, coker gas oils, lube extracts, and slops.

FCC rejects carbon without producing coke and, in effect, consumes its own worst component as fuel. The process is also tolerant of wide crude source swings, is a net exporter of energy, and produces hydrogen.

FCCUs help desulfurize the feed by 50-70%, and can be designed to operate with extremely low levels of particulates, SOx, NOx, and CO emissions. The catalyst is relatively insensitive, and the system as a whole is flexible and recovers quickly from upsets. FCC is indeed a flexible and unique technology.

RECENT DEVELOPMENTS

The trend to convert FCC to RFCC has been gathering momentum for over 30 years (Fig. 9). The major grassroots projects are listed in Table 7. This table does not include many units which add resid to vacuum gas oil operation or have been revamped to resid operation.

The units that do not have catalyst coolers (Stone & Webster units) rely on two-stage regeneration (two flue gas trains) for heat removal. The economics for resid upgrading have fluctuated considerably, but the long-term trend seems to almost always favor resid upgrading.

The keys to resid upgrading are:

Resid FCC catalyst
Hardware (such as catalyst coolers)
Process optimization and control.

A proper choice of catalyst makeup strategy, for example, can anticipate feed quality changes. The types of resids that can be processed in today's conventional, resid, and pretreater RFCC units are shown in Fig. 10. 10

About 25% of the world's total crude reserves can be processed (as atmospheric resid) in RFCC units. Feed pretreatment and further process improvements can increase this number to almost 50%.

Most new grassroots FCCUs today, particularly in the major growth area of the Pacific Rim, are RFCC units. This trend is likely to accelerate in Europe and North America as well.

FCC modeling and optimization have also advanced considerably to the point of being used routinely in the field. While hydrodynamic models of FCC reactors and regenerators are still far from being perfect, kinetic models and steady-state optimizers are performing well.

Kinetic models can use feed, catalyst, and operating data to predict yields, product properties, catalyst management, and emissions. 11

FCC emissions regulations have been tightening continuously since the 1970s (Table 8). So far, FCCUs have managed to meet each environmental challenge successfully (Table 9).

NOx emissions are affected by regenerator design. 12 For example, a more optimal distribution of air in a dense bed regenerator shows 50% reduction in NOx emissions (Table 10).

FUTURE DEVELOPMENTS

One current trend that is likely to accelerate, and possibly even eventually dominate, catalytic cracking is FCC integration with petrochemical production, Some FCCUs are already integrated with petrochemical plants in various ways, as illustrated in Fig. 11.

Ethylene is extracted from FCC fuel gas, propylene is sold as a petrochemical (typically at a much higher price than if it were converted to gasoline), and H2 is supplied back to the refinery.

The future refinery is likely to be more of a specialty chemical producer, centered around the FCC complex. Through innovations in hardware and catalysts, FCC may also replace thermal cracking as the major route to producing light olefins, particularly from heavy feedstocks.

Another area of great interest is advanced FCC complex control. The current steady-state control scheme is constantly being improved with safety and reliability features and better steady-state optimization. This is done off-line in most cases.

The future may bring more sophisticated on-line steady state optimization and predictive advanced control, which will anticipate FCC feedstock changes. FCCUs that may benefit from such control are those where significant objectives, rates, or feed quality changes take place more than once per week.

Many FCCUs already benefit from the use of state-of-the-art computer technology-they are likely to keep up with rapid advances in computer-related technologies.

Despite its long and colorful history, FCC is far from being a mature "low-tech" technology." FCC technology, catalysts, hardware, and processes will continue to lead petroleum refining in innovation, safety and reliability, environmental impact, and last but not least, profitability.

REFERENCES

1. "Zeolite Y," published by Crossfield Catalysts, Cheshire, England, 1990.

2. Pine, L.A., Maker, P,J., and Wachter, W.A., "Prediction of Cracking Catalyst Behavior by a Zeolite Unit Cell Size Model," J. of Catalysis, Vol. 85, 1984, pp. 466-76.

3. Stokes, G.M., and Mott, R.W., "FCC Resid Processing: An overview," American Institute of Chemical Engineers symposium Series 273, Vol. 85, 1989, pp. 58-77.

4. O'Connor, P., Gerritsen, L.A., Pearce, J.R., Desai, P.H., Humphries, A., and Yanik, S., "Catalyst Development in Resid FCC," Akzo Catalysts Symposium, May 1991, Scheveningen, The Netherlands.

5. Elvin, F.J., "Catalyst Demetallized for Reuse," Hydrocarbon Processing, October 1989.

6. Elvin, F.J., and Pavel, S.K., "Metal removal of FCC catalyst operating in refinery," OGJ, July 22, 1991, P. 94.

7. Kowalczyk, K., Campagna, R.J., Hettinger, W.P. Jr., and Takase, S., "Magnetic Separation Enhances FCC Unit Profitability," National Petroleum Refiners Association Annual Meeting, Paper No. AM-91-51, March 1991, San Antonio.

8. Avidan, A.A., Krambeck, F.J., Owen, H., and Schipper, P.H., "FCC closed-cyclone system eliminates post-riser cracking," OGJ, Mar. 26, 1990, P. 56.

9. Johnson, T.E., "Improve Regenerator Heat Removal," Hydrocarbon Processing, November 1991, pp. 55-57.

10. Khow, F.H.H., Tonks, G.V., Szetch, K.W., van Els, A.C.C., and Van Hatten, A., "The Shell Residue Fluid Catalytic Cracking Process," Akzo Catalysts Symposium, May 1991, Scheveningen, The Netherlands.

11. Krambeck, F.J., "Continuous Mixtures in FCC and Extensions," Mobil Workshop on Kinetics and Thermodynamics of Chemical Reactions in Complex Mixtures, Mar. 15-16, 1990, Bridgeport, N.J.

12. Schipper, P.H., Sapre, A.V., Owen, H., and Knickerbocker, B.M., "FCC Hardware Technology for the 90's," W.R. Grace & Co. Seminar, September 1990, Berlin.

13. Avidan, A.A., Edwards, M., and Owen, H., "Innovative improvements highlight FCC's past and future," OGJ, Jan. 8, 1990, P. 33.