Refiner outlines approach to FCC design and operations

Richard J. Higgins, Prabhakar R. Satbhai, Ron A. van Wijk
Shell International Oil Products, B.V.
Amsterdam

Royal Dutch/Shell Group's fluid-catalytic cracking (FCC) best practices are based on its design and operational experience, coupled with many years of research and development (R&D) in Amsterdam and Houston.

Shell uses its operational best practices to monitor and troubleshoot fluid-catalytic cracking units (FCCUs), select FCC catalyst, optimize the unit by advanced process control, and plan for FCC turnarounds.

Although FCC gasoline production is decreasing in some markets, FCCUs continue to be a cornerstone for refinery upgrading of heavy hydrocarbons. The flexibility of the FCC process allows the product slate to shift towards petrochemical feeds and diesel in response to demand.

FCC residue-feedstock operations have surged within the past 10 years in response to declining heavy fuel-oil demand.

Shell has designed and licensed over 30 grassroots FCCUs, including 7 for residue feed. Its contributions to FCC technology include riser cracking, stripping cyclones, swirl-tube separators, power-recovery expanders, advanced process control, and on-line optimization.

FCC reactor and regenerator

Shell FCC designs are based on extensive R&D programs and more than 1,000 years of combined FCC operational experience. Design features for reliability have benefited from the experience gained with residue FCC operation. FCC designs include proprietary feed nozzles, riser internals, riser end devices, prestripping cyclones, strippers, third stage separators, air distributors, high efficiency regenerators, and propylene recovery.

There are currently two FCC design configurations: the "Shell 2 vessel" design and the "Shell external reactor" design.

The "Shell 2 vessel" (Fig. 1a [78,135 bytes]) is recommended for feeds with mild coking tendencies, which include certain residue feeds. The reactor cyclones and regenerator swirl-tube separator are inside the vessels; thus, the capital expenditure is low. The latest application of the design is a grassroots residue FCCU at Port Dickson, Malaysia, scheduled to start up in 1999.

The "Shell external reactor" design (Fig. 1b) is preferred for heavy feeds with high coking tendencies. Although it involves more capital expenditure, it delivers improved robustness with difficult feeds.

The "Shell external reactor" design was proven in a revamp at Deer Park, Tex., and then used for Shell's first grassroots residue FCCU at Stanlow, England, which started up in 1988. It has an external riser terminating in a single rough-cut cyclone. The bottom part of this cyclone forms a prestripper bed; the cyclone is thus called a "prestripping cyclone." The rapid stripping in the prestripping cyclone reduces the post-riser coke make.

The catalyst in the prestripping cyclone-vapor outlet is separated in external secondary cyclones. The external reactor-cyclone concept has no reactor dome, which minimizes stagnant areas for potential coke growth.

The external cyclone concept also allows on-line dipleg-clearing procedures.

A multidisciplinary task-force approach has been used to improve the reliability and maintainability of the FCC design. Design features to handle the coke deposition tendency of residue feeds are especially important.

First, Shell seeks to minimize coke-laydown potential via design. Second, Shell includes counter measures so any coke formed will not cause a shutdown. The best performers achieve onstream factors (including scheduled turnarounds) of about 98%.

Recent industry benchmark data shows that Shell units, even though running heavier feeds, have an onstream factor better than the rest of the industry. Shell FCCUs run 6 days/year more than the industry average.

The inherent reliability of the design is demonstrated by the Norco, La., unit, which ran over 3 years without any feed outages until it was shut down recently as a result of a hurricane threat.

Design details

The unit design features low elevations with easy access for inspection and maintenance. The pressure balance and catalyst-piping design provide a smooth and robust catalyst circulation.

Feed injection. The Shell high-performance feed nozzle, shown in Fig. 2 [54,756 bytes], uniformly distributes the feed with a narrow droplet-size distribution. The design provides good mixing of the feed with the catalyst, allows fast vaporization and reaction, and minimizes back-mixing. Six FCCUs are now in operation with these feed nozzles and four more are in the design stage.

The feed nozzles are installed radially. The nozzles are designed for pressure drops of 2-5 bar (30-75 psi) and atomization steam rates of 1-3 wt % on feed.

Higher atomization steam rates may be used for very heavy feeds. The low atomization steam rate and pressure drop allow easy retrofitting into existing FCCUs. Excellent atomization is achieved without small passages in either the steam or oil sections, which avoids any plugging concerns.

Shell has a bottom-entry feed nozzle design with atomization comparable to the side-entry nozzles (verified by full-scale laboratory testing). This design can be applied in special situations when a revamp to side-entry nozzles is not possible. This nozzle is in operation at Durban, South Africa.

Riser/reactor. The FCC risers have short contact time and are equipped with proprietary internals for lower pressure drop and reduced back-mixing. Shell recently developed, tested, and patented a new generation of riser internals. The company expects yield improvements to result in 1-2% higher conversion or payouts within 1 month.

The riser-exit design provides rapid separation of hydrocarbons and catalyst. The post-riser residence time is kept low via close-coupled cyclones (minimize post-riser thermal cracking for low dry gas and diolefin yields, e.g., butadiene).

Stagnant areas that potentially can form coke are avoided, most notably for the Shell external reactor design.

Stripper. It is important to keep the hydrogen content in coke low (5-6 wt %) for product yields and catalyst activity maintenance. Low hydrogen content can be easily achieved with a long residence time stripper, but the tradeoff is excessive gas and nonbeneficial coke production.

The key to a good stripper design is the proper sizing of the counter-current stripping stages and appropriate use of internals. The hydrodynamics of various stripper internals have been tested in the research facilities to decide which deliver the best performance.

Regenerator. The regenerator design differs from some FCC licensors. The design is a single-stage, counter-current partial-burn regenerator. Full burn, however, has been applied to meet site-specific requirements.

The regenerator design retains catalyst activity by avoiding oxidizing conditions and maintaining moderate temperatures, which minimizes catalyst destruction by vanadium and sodium.

The Shell designed counter-current burn regenerator achieves carbon on regenerated catalyst (CRC) levels of 0.05-0.15 wt % for deep partial burns, such as 9 vol % CO in the dry flue gas. Thus, the benefit of low CRC normally associated with full burn is also achieved by the partial burn regenerator.

The operation is reliable with no afterburn problems, as shown by cooler temperatures in the dilute phase than in the bed.

The design of the regenerator includes spent catalyst-inlet devices to minimize afterburn and catalyst deactivation. The mechanical design of the regenerators allows high-temperature operation of the cyclone system, the air distributor, and other internals. Catalyst coolers can be used to add feedstock flexibility.

Flue gas cleanup and power recovery. Shell's third-stage separator (TSS) is an industry standard. It uses patented swirl tubes to remove practically all 10 m and larger catalyst from the hot flue gas.

The TSS can compete with an electrostatic precipitator (Fig. 3 [55,082 bytes]). In certain cases, it can be installed inside the regenerator vessel to simplify the flue gas line-up (Fig. 1a). Power recovery is applied when justified. Add-on packages are used as needed for compliance with environmental emissions (e.g., NO_x, SO_x).

Main fractionator (MF) and gas plant. Shell designs the MF overhead, wet-gas compressor, and absorber to achieve 95+% propylene recovery. The single column absorber/de-ethanizer design uses low-pressure distillate and chilling to optimize the C₃+ recovery.

Computational Fluid Dynamic modeling has been done for the MF-inlet piping design to avoid coking. Coking and fouling of the MF bottoms is controlled via the design temperature, residence time, and exchanger velocities. MF-overhead fouling is controlled with the design overhead temperature, top circulating reflux temperature, and exchanger washing.

Instrumentation. The FCCUs are provided with sophisticated instrumentation and trip systems for reliability and protection. Easy-to-understand diagrams of the FCC trip systems are prepared for operators to prevent mistakes during use, maintenance, and testing. The trip systems are designed with appropriate voting and time delays to avoid lost profits due to "nuisance" trips.

Alarm philosophies are to allow awareness of malfunctions in key instrumentation and easy prioritization of alarms in upset conditions.

Revamp designs

Revamps allow many opportunities to increase profitability. Shell examines the overall performance with respect to reliability, refinery goals, feed/product economics, and catalyst options to decide what technology to apply.

There are many unit-specific revamp examples. Installation of the proprietary feed nozzles at a Shell FCCU in the U.S. improved the product value by $3 million/year by reducing dry gas and increasing gasoline. A unit in Australia increased throughput by over 20% by relieving a gas constraint with the installation of close-coupled cyclones. Installation of a customized spent catalyst distributor at an FCCU in Europe enabled a reduction of regenerator flue-gas oxygen by 0.5%, thus relieving a coke-burn constraint.

Best operations practices

Proper unit operation is required to get the full benefit of state-of-the-art hardware.

Best practices for operations apply to many aspects: unit interfaces (feedstock selection, upstream and downstream interactions, hydrocarbon inventories), operational details (constraints, key variable targets, yields, product properties, on-line and off-line analyses, catalyst inventories, environmental compliance), technical services (process monitoring, catalyst-selection methods), process control (instrumentation, alarm system, trip system, control strategy, optimization methods), and management decisions (staffing, training, turnaround premises).

Yield model. A key to getting the most from an FCC design is an accurate yield model. Optimization of operating conditions can be done faster and more accurately by using the process model.

Feed analysis. Shell applies near-infra-red (NIR) combined with chemometrics to obtain fast and accurate values of various feedstock properties. This NIR technique does not require any sample pretreatment. The analytical data are used directly for the on-line optimizer.

Three locations are using this type of analysis now; several more are in the development stage.

Feed injection. Engineering studies using measuring techniques such as riser temperature profiles, riser velocity profiles, riser conversion profiles, and tracing can be used to improve feed/catalyst contacting. Important parameters such as feed atomization, feed distribution, and catalyst fluidization are evaluated.

Fresh catalyst makeup. One of the most important points of FCC operation is proper fresh catalyst-addition rate. It is sometimes difficult to arrive at the right rate because of the conflict between the highly visible costs of fresh catalyst and the "hidden" yield-improvement profits. Shell has a method to optimize the fresh catalyst addition rate.

Reactor and stripper. The optimum feed temperature and reactor temperature combination is determined by the FCC process model.

To optimize the performance of strippers, test runs are conducted at the operating unit to set the best steam/catalyst ratios and bed levels. Steam rates are only varied within design limits to avoid catalyst attrition or catalyst ingress into the steam distributors. The MF loading, dew point, and overhead condenser capacity are other factors involved in optimizing the steam rates.

Regenerator. Regenerator operation involves optimizing the air distribution between alternate locations (main grid, fluffing distributors, burp tubes, catalyst risers, catalyst coolers), maintaining optimum promotion, and selecting the best bed height and regenerator pressure.

The regenerator-temperature effect on yields and catalyst activity is studied for each unit. Many units have some flexibility on the regenerator temperature.

For regenerators with the ability to operate in either combustion mode, proper procedures have been developed for smooth transitions between partial and complete CO combustion (used when economics such as fuel-oil sales price or other conversion-unit availabilities change).

MF/gas plant. The MF heat removal is adjusted among the various choices to optimize separation performance. The gas-plant column pressures, feed preheats, and reflux rates are adjusted for best economic performance. On-line analyses or inferred properties are applied to allow closer approach to product specifications.

For amine-treating units, variables such as heat-stable salt content, lean amine sulfide content, absorber acid-gas loading, amine-regenerator bottoms temperature, and overhead water ammonia content are controlled to ensure reliability.

Monitoring

Regular and detailed unit monitoring is required for early detection of nonoptimum unit performance and optimization of the catalyst-addition rate. It is especially important for evaluating hardware changes or catalyst changes.

Process-performance monitoring consists of analyzing process and laboratory data for the FCCU. Using the process model, the product yields and properties are corrected for variations in feedstock quality and operating conditions. The availability of electronic data transfer allows expert review at a different location. This results in recommended process changes with proven benefits of several million dollars per year.

Fig. 4 [81,941 bytes] shows data from a European FCCU which improved the bottoms cracking (even with higher throughput) as a result of a proper analysis of which constraints to push, catalyst-addition changes, and stripping improvements.

Unit monitoring by periodic test runs range from once per week to once per month. These test runs can use plant-rundown data or reactor-overhead samples. The reactor overhead test-runs carried out by the Shell Research & Technology Centre in Amsterdam are called Precision Test Runs (PTRs), routine activities in Shell FCCUs.

PTRs provide accurate yield data and minimize unit disturbances while gathering data based on different operating conditions. PTRs eliminate potential data errors as a result of flow meters and external-stream corrections, while avoiding the need to wait for the separation section to stabilize.

PTRs help calibrate the FCC process model, evaluate best operating conditions, assess the impact of a catalyst or hardware change, and troubleshoot. A PTR at an FCCU in Europe showed the effect of changing the stripper operating conditions, which led to improved operations valued at $2.6 million/year.

FCC troubleshooting

Shell analyzes the fundamental causes behind problems to determine the best long-term solution to each problem. During the years of operating experience, a wide variety of FCC problems have been encountered and solved; so proven solutions are available.

For example, at a residue unit in the Far East, monitoring showed lower-than-expected air flow for the blower based on the speed, ambient temperature, and discharge pressure. An on-line cleaning technique increased the blower air flow by 3%.

Shell has a team that is highly specialized in diagnostic techniques for hardware-related problems in an FCCU. The techniques consist of radiological methods, helium tracing, and specialized sampling methods for catalyst, flue gases, and hydrocarbon vapors.

Catalyst selection

A review of catalyst alternatives for each FCCU is done about every 2 years.

Each FCCU is considered unique in terms of hardware, performance and constraints, feed quality, and processing objectives. The catalyst-selection process includes in-depth discussions with the FCC-catalyst suppliers about preferred candidate catalysts.

Dedicated laboratory testing is required to judge the performance of potential new catalysts. Experience shows that appropriate FCC catalyst-evaluation programs have a very short payout time, with cost being negligible compared to the impact of not choosing the best catalyst.

In the beginning of the laboratory evaluation, artificially aged catalysts are tested in a riser pilot plant using the actual unit feedstock and a close match of the actual process conditions. The riser pilot plant yields then can be used together with the FCC process model to determine the expected performance in the commercial FCCU.

Table 1 [30,254 bytes] shows the importance of choosing the best catalyst on the first try.

During the changeover to a new catalyst, careful monitoring is done to establish the benefits. Riser-pilot plant testing of the actual unit equilibrium catalyst (Ecat) at an early stage of the changeout provides accurate data on the impact of the catalyst change, thus allowing for fine tuning of the catalyst formulation or of the catalyst-addition rate.

Riser-pilot plant facilities are available in Amsterdam and Houston. Riser-pilot plant testing is also used for troubleshooting purposes to determine if a unit performance problem is catalyst or hardware related.

Advanced process control and on-line optimization

Advanced process control (APC) improves the reliability and profitability of the FCCUs. APC allows the best operating strategy and constraint handling to be in effect 24 hr/day.

Shell has applied APC in over 25 FCCUs since the initial application over 20 years ago. The control applications consist of standardized packages for easy implementation and maintenance. The applications run in distributed control systems or supervisory systems.

Shell uses proven control algorithms for the complex multivariable interactions experienced in an FCC reactor/regenerator and MF. The Shell Multi-variable Optimizing Controller (SMOC) at a residue FCCU in the Far East led to a steadier operation, a closer approach to constraints, and increased throughput giving a value added of $2.3 million/year.

The advanced control is typically integrated with an on-line closed loop optimizer for maximum profitability. Many of the best practices for FCC operating variables are handled by the optimizer. Although decisions can be done off-line, some benefits are missed by not having continuous on-line evaluation of unit conditions and constraints.

Shell's Plant Optimization Technology (a proprietary technology known as SPOT) integrates an FCC yield model and a distillation simulation package. Application of on-line optimization results in a step change in operator understanding of how to improve the unit operation, which provides a bonus to the value added from ongoing fine tuning. An FCC optimization system can be on-line and closed loop within 9 months of project initiation.

Turnaround planning

Prior to scheduled FCC turnarounds, workshops are held to scrutinize the turnaround premise and the work planning to produce a more profitable strategy. Important topics addressed are turnaround timing and hydrocarbon utilization for the refinery during the turnaround.

The lessons learned from other FCCUs are applied to minimize shutdown durations. Workshops dedicated to review the shutdown methods, catalyst unloading, decontamination steps, work practices, and start-up procedures have led to turnaround-duration reductions of at least 2 days/site. Typical times for some key activities are shown in Table 2 [46,311 bytes].

Bibliography

1. Nieskens, M.J.P.C., Borley, M.J.H., Roebschlaeger, K.H.W., and Khouw, F.H.H., "The Shell residue fluid catalytic cracking process commercial experience and new developments," NPRA Annual Meeting, March 1990.
2. Chockalingam, A., and Khouw, F.H.H., "Shell Residue FCC Technology, Challenges and Opportunities in a Changing Environment," JPI, Petroleum Refining Conference, Japan, 1992.
3. Satbhai, P.R., Dirkx, J.M.H., Higgins, R.J., and van Dijk, A., "Update on Shell Residue FCC Process and Operation," AIChE 1998 Spring Meeting, New Orleans, March 1998.
4. Mercera, P.D.L., Dirkx, J.M.H., and Hadjigeorge, G., "Catalyst Selection, Testing, and Monitoring at Shell," Akzo Nobel FCC Seminar, Noordwijk, The Netherlands, June 1998.
5. Van der Pas-Toornstra, H.M., and van Arkel, J.A., "SPOT : Shell Plant Optimisation Technology," European Refining Technical Conference-Computing, London, June 1998.
6. Satbhai, P.R., Dirkx, J.M.H., Higgins, R.J., and Mercera, P.D.L., "Best Practices in Shell FCC Units," Akzo Nobel FCC Seminar, Mumbai, India, October 1998.
7. Dries, H.W.A., and van der Werf, R.P., "Meeting New Dust Emission Targets: from FCCU diagnosis towards a TSS upgrade," Grace Davison Technology Conference, Lisbon, Portugal, September 1998.

The Authors