2006_PT_Recent Advances in FCC Technology

63 (2006) 2–8www.elsevier.com/locate/powtec

Powder Technology 1

Recent advances in FCC technology

Ye-Mon Chen

Shell Global Solutions (US) Inc., United States

Available online 20 March 2006

Abstract

Although the fluid catalytic cracking (FCC) process has been commercially established for over 60 years, the technology continues to evolve tomeet new challenges. This paper presents examples of recent FCC technology advances through integrated R&D programs that bridgeunderstanding in process science and engineering practice in which Shell Global Solutions 1 has contributed significantly.

On the reactor side, advances in feed injection, riser internals and riser termination have been proven to work synergistically toimprove reactor performance. Earlier generation of modern feed injection technology was introduced in the 1980's, using direct impactmechanisms for atomization. This paper discusses the newest generation of the technology utilizing two-phase choking for atomization,which has been demonstrated to be much more energy efficient, as validated by consistently achieving more uniform temperature profilesacross the riser.

The FCC riser is known for its shortcomings of density and velocity variations. The newest riser internal technology minimizes theseshortcomings and promotes ideal plug flow. The FCC is a sequential reaction process in which many desirable products are the intermediates.Thus, cracking reactions must be terminated after a desirable reaction time; otherwise, desirable products will continue to crack, leading to excessproductions of light gases and coke. Improved riser termination technology sharpens the termination of reactions by the combination of the uniquedesign of primary stripper cyclones and close coupled secondary cyclones.

On the regenerator side, conventional flue gas cleaning requires two stages of cyclone separation followed by electro-static precipitators (ESP)or scrubbers. New developments in the Third Stage Separator (TSS) technology provide enhanced capability for achieving low particulateemissions in flue gas to comply with the requirements more stringent, particulate control environmental regulations.

The majority of FCC units in the US have gone through various stages of de-bottlenecking, and many are limited by catalyst circulation.Catalyst circulation enhancing technology (CCET) has been demonstrated to improve standpipe stability, resulting in significant improvements incatalyst circulation rates.© 2006 Published by Elsevier B.V.

Keywords: Fluid catalytic cracking; FCC; Third Stage Separator; TSS; Catalyst; Catalyst circulation; CCET; Rier; Riser termination

1. Introduction

The fluid catalytic cracker (FCC) is the primary conversionunit in most US refineries. It converts, or cracks, low valueheavy ends of the crude oil into a variety of higher-value, lightproducts. In US, the primary function of FCC units is to producegasoline. About 45% of worldwide gasoline production come

E-mail address: [email protected] Shell Global Solutions is a network of independent technology companies in

the Royal Dutch/Shell Group. In this material the expression ‘Shell GlobalSolutions’ is sometimes used for convenience where reference is made to thesecompanies in general, or where no useful purpose is served by identifying aparticular company.

0032-5910/$ - see front matter © 2006 Published by Elsevier B.V.doi:10.1016/j.powtec.2006.01.001

either directly from FCC units or indirectly from combinationwith downstream units, such as alkylation. Although FCC is amature process commercially deployed for over 60 years, thetechnology continues to evolve to meet new challenges [1–3].Modern FCC units can take a wide variety of feedstocks and canadjust operating conditions to maximize production of gasoline,middle distillate (LCO) or light olefins to meet different marketdemands.

The success of developing new FCC technology requiresthe integration of in-depth understanding of the underlyingprocess science and innovation in engineering practices. Thispaper presents selected examples of recent FCC technologyadvances in which Shell Global Solutions has played a sig-nificant role.

mailto:[email protected]

http://dx.doi.org/10.1016/j.powtec.2006.01.001

3Y.-M. Chen / Powder Technology 163 (2006) 2–8

2. Feed injection system

2.1. Process considerations

The feed injection system is by far the most criticalcomponent of the modern FCC reactor design. Several recentdevelopments in the FCC process have made the feed injectionsystem increasingly important.

Engineering innovation in response to:

1. shorter reaction time;2. higher regenerator temperature; and3. heavier feedstock

First, due to the development of highly active zeolite FCCcatalyst, the reaction time has been shortened significantly to afew seconds in the modern riser reactor. Since catalytic crackingreactions can only occur after the vaporization of the liquidhydrocarbon feedstock, mixing and feed vaporization must takeplace in the riser as quickly as possible. Otherwise, thermalcracking reactions will dominate.

Second, the regenerator temperature is getting higher toachieve more complete catalyst regeneration. Because of thehigher regenerator temperature, control of thermal cracking inthe riser has become more critical. The typical modern riser toptemperature is controlled in the range of 950 to 1050 °F, buttypical regenerated catalyst temperature is much higher, in therange of 1250 to 1350 °F. The feed injection system plays thekey role in reducing thermal cracking reactions by cooling offthe lower riser quickly with fast mixing and vaporization of thefeed.

Third, the FCC feedstock is getting heavier. As the feed getsheavier, the boiling point increases, which makes feedvaporization more difficult. At the same time, the viscosity ofthe feed also increases, which makes feed atomization moredifficult.

2.2. Engineering practice

The success of a feed injection design depends on bridgingthe underlying science to the engineering design aspects inorder to achieve the process objectives. The three key aspects ofa feed injection nozzle design are: Feed atomization, feeddistribution and mixing with catalyst.

• Feed atomization — This aspect of the feed nozzle designis the most obvious. The objective of feed atomization isto generate fine droplets for fast vaporization. Thechallenge of a successful feed nozzle design is to producethe finest feed atomization using the least amount ofenergy. Many different FCC feed nozzle designs have beenpatented [4–8].The first generation of modern FCC feed nozzles [4] wasintroduced in the 1980's. The atomization mechanism of this

design was based on generating a high velocity liquid jet toimpinge on a target, shattering the liquid jet into dropletsupon hitting the target, and conveying the droplets withsteam to the nozzle exit. Although this design belongs toLefebvre's [9] definition of twin-fluid atomizers, whichutilize steam to assist atomization of liquid feed, the primaryatomization mechanism is still a single fluid atomizerutilizing the energy of liquid alone for atomization. As aresult, this nozzle requires high feed pressure and high steamusage to achieve desirable feed atomization.The second generation of modern FCC feed nozzles [5] wereintroduced in the early 1990's which all belong to Lefebvre's[9] definition of internal mixing, twin-fluid atomizers. Theatomization mechanism of these designs was based oninternally mixing the steam and liquid feed upstream of thenozzle exit, using some forms of proprietary mixing devices,and conveying the two-phase mixture to the nozzle exit.These designs were an improvement over the first generationof feed nozzles, particularly in reducing the requirement ofhigh feed pressure. However, all internally mixed twin-fluidatomizers have a common drawback: the two-phase mixturewill stratify during conveying to the nozzle exit, regardlesshow well the two phases are mixed initially, leading to lessefficient atomization.The newest generation of FCC feed nozzles [7,8] wasintroduced in the late 1990's which are twin-fluid atomizersusing two-phase chocking as the atomization mechanism.These nozzles have the atomization steam mixed with thefeed just before the final outlet of the nozzle, preventing thestratification of two-phase flow. As the homogeneous two-phase flow passes through the final outlet, the suddenexpansion of the two-phase flow shatters the liquid into fineatomization.

• Feed distribution — Feed distribution in the FCC riser,sometimes called riser coverage, is as critical as feedatomization in achieving FCC processing objectives. Dueto space limitation in the riser area, only limited number ofnozzles can be used. The industry trend [2] is to use multiplenozzles at a single riser elevation with each nozzlegenerating a wide angle spray. The more uniform theindividual nozzle generates in a wider angle, the less numberof nozzles is required to achieve desirable feed distribution inthe riser.It is important to recognize that a feed nozzle generates aspray with some form of liquid flux and droplet sizedistributions across the entire spray pattern. These distribu-tions vary both in time and space. The challenge of asuccessful feed nozzle design is not only to produce thefinest feed atomization, but also to produce the most stableand uniform spray. The newest generation of FCC feednozzles [7,8] tends to have more uniform feed distribution asa result of better control of homogeneity of two-phase flow atthe nozzle exit.

• Mixing with catalyst — The objective is to achieve auniform radial riser temperature profile as quickly aspossible. In doing so, the regenerated catalyst is uniformlycooled down by vaporization of the hydrocarbon feedstock,

Fig. 1. Feed injection system with adjustable onjection angle (US Patent 5, 979,799, 1999).

4 Y.-M. Chen / Powder Technology 163 (2006) 2–8

thus minimizing thermal cracking reactions in the lower risersection. The newest generation of FCC feed nozzles [7,8]tends to have faster mixing with catalyst because of suddenexpansion of two-phase chock flow at the nozzle exit,creating a strong suction to draw in catalyst. Commercialexperience has shown that, in addition to feed atomizationand feed distribution, the feed injection angle also plays asignificant role in mixing with catalyst and hence thetemperature profile in the riser. Although most of modernFCC units have the feed nozzles installed through risershrouds at a fixed angle, a new feed nozzle design [8], shownin Fig. 1, enables an FCC unit to adjust the feed injection

Measured Riser Ra

erutare

pme

T

Radia

New Shell NozzleConventional Nozzle

+100

-10

Fig. 2. Comparison of temperature p

angle while using the existing riser shrouds. This enables anFCC unit to optimize mixing of feed and catalyst byadjusting the injection angle to achieve the best performanceof the unit.Fig. 2 shows radial temperature profiles of a commercialFCC riser right above feed injection before and after the feednozzle change, including adjusting the injection angle. Asshown prior to the change, the riser temperature profile wasrather non-uniform, with lower temperature at the center andhigher temperature near the wall. The temperature differen-tial was close to 100 °C. After replacing the feed injectionnozzles of the latest design and adjusting the injection anglefor optimum mixing, the temperature profile became muchmore uniform, with temperature difference reduced to about20 °C. The remaining non-uniformity of the temperatureprofile was due mainly to the asymmetrically catalyst inletfrom the regenerator standpipe, which can only be addressedby changing the lower riser design.In addition to the temperature profile change, commercialexperience has confirmed that using the newest generationfeed nozzles, such as the one shown in Fig. 1, cansubstantially reduce dry gas and increase gasoline yield.These results are in line with the expectation that better feedinjection design reduces thermal cracking reactions, whichare the primary source for dry gas. As a result, catalyticcracking reactions are maximized and more desirableproducts are produced.

3. Riser internals


Chemical reactions taking place in the FCC riser arenumerous and complex. Thermal and catalytic crackingreactions as well as many side reactions, such as hydrogentransfer, are progressing simultaneously. In addition, FCCfeedstock is a cocktail of hydrocarbon mixture with differentchemical species vaporizing and reacting at different rates.

dial Temperature Profile

l Distance

rofile across a FCC reactor riser.

Critical Process Considerations

1. uniform catalyst distribution desirable;2. catalyst back-mixing undesirable;3. uniform temperature desirable; and3. uniform gas distribution desirable


Although it is difficult to cover all aspects of FCC reactionkinetics in the riser, several process considerations arecritical:

• First, catalytic reactions require the presence of catalyst.More even catalyst distribution across the riser is importantbecause it leads to more uniform local catalyst-to-oil ratioand more even reaction rates.

• Second, catalytic reactions result in coke deposition on thecatalyst, which reduces catalyst activity and selectivity.Spent catalyst back-mixing down the riser is highlyundesirable.

• Third, thermal cracking reactions are more sensitive toreaction temperature. More even temperature distributionacross the riser is desirable because it leads to overallreduction in thermal cracking reactions and more evencatalytic reaction rates.

• Fourth, gas radial mixing in the riser is relatively slow. Moreeven gas velocity distribution across the riser is desirablebecause it leads to more even reaction residence timedistribution.


Modern FCC units use vertical risers for cracking reactionswith very few design variables downstream of feed injection.Still, the critical process considerations can be partiallyachieved by applying engineering innovations.

Successful design of a FCC riser must consider the followinghydrodynamic trends of a co-current gas–solids two-phaseupflow:

• Solids concentration is higher near the wall than the center.

Fig. 3. Comparison of riser veloci

• Solids always move upward in the center zone, but can beeither upward or downward near the wall.

• Gas velocity is higher near the center and lower near thewall.

Furthermore, due to the progress of the cracking reactions,the molar flow rate, and hence the volumetric flow rate of thehydrocarbon vapor, increases as it moves up the riser. In atypical FCC unit, the volume expansion is in the range of 3 to 4times of the vapor of the original feedstock. Thus, the riserdiameter may be increased once or twice after feed injection tokeep the vapor velocity within the desirable range. These stepchanges in riser diameter promote catalyst reflux near the wall.Modern riser design also includes a sharp 90° turn at the top,which further promotes catalyst reflux down the riser wall.

An effective engineering solution to improve the riserhydrodynamics is the use of wall baffles [10,11]. Fig. 3 showsthe comparison of velocity and concentration profiles in thesame riser before and after installation of wall baffles. As shownin Fig. 3, both gas and solid velocity profiles are more uniformin the riser with wall baffles, improving the riser hydrodynamicscloser to desirable plug flow reactor. Furthermore, the overallpressure drop through the riser is reduced with the wall baffles,due to the reduction of catalyst back-mixing. Commercialexperience confirms the benefits for the use of the wall bafflesin the FCC riser.

The concept of a downer FCC reactor has been an activeresearch subject of circulating fluidized bed. The concept of thedowner is based on the premise that solids back-mixing wouldno longer be an issue in co-current gas–solids two-phasedownflow. However, the downer has its own challenges anddrawbacks. A few cases of commercial experience with downerFCC reactors by others have not been clear success at all [12].

4. Riser termination and close-coupled cyclones


It is important to recognize that FCC is a sequential reactionprocess in which many desirable products, such as gasoline, areintermediates. Modern FCC risers are designed with theresidence time required for maximizing desirable products.

ty and concentration profiles.

Fig. 4. Internal close-coupled cyclone systems, (WO 02/085527 A2, 2002).


However, once the reactor mixture leaves the riser, crackingreactions must be terminated as sharply as possible; otherwise,desirable products will continue to crack, leading to excessproductions of light gases and coke.


A successfully designed riser termination device (RTD)terminates the cracking reactions as soon as possible. There area number of variations of riser termination devices to achievesuch objective by combination of some or all of following threeprocess tactics:

• Separating catalyst from the product gas, which willterminate catalytic reactions

• Reducing product gas residence time, which will reduce bothcatalytic and thermal cracking, and

• Reducing temperature [13], which will reduce catalyticcracking, but more importantly, will reduce thermal crackingreactions.

A couple of related process considerations should also bekept in the background in the design of riser termination:

• At riser outlet, product gas consists of both interstitial gasphase and the intra-particle gas trapped inside catalyst pores.

• Reducing temperature generally reduces stripping efficiencydownstream.

Older FCC units have the reactor mixture discharged fromthe riser into an open reactor vessel, leading to substantial postriser residence time and over cracking. Modern FCC riserdesign includes an improved riser termination device, whichallows quick separation of catalyst and product gas tominimize post-riser cracking. One such device is called thepre-stripping cyclone [14], which incorporates all threeprocess tactics above. Fig. 4 shows a schematic drawing ofthis cyclone design in which the upper part is a typicalcyclone for fast separation of catalyst and interstitial productgas. The lower part of the stripper cyclone serves as thestripper bed where steam is injected to further removeentrained interstitial product gas as well as part of the intra-particle product gas trapped inside catalyst pores. The upperand lower part of the stripper cyclone is separated by astabilizer, which separates the vigorous spinning motion abovefrom the more quiescent stripper region below. The strippingsteam also serves to reduce the temperature by a few degrees.The advantage of this design is to achieve earliest possibledisengagement of product gas and catalyst, thus minimizingpost-riser cracking.

Modern FCC units' riser termination further includes thearrangement called close-coupled cyclones. The objective of theclose-coupled cyclones is to reduce post-riser cracking by notallowing product vapor to enter the reactor vessel. There areseveral variations of close-coupled designs deployed in theindustry. Fig. 4 shows one example [14] in which the vaporoutlet from the primary cyclone is “closely coupled” to the inlet

of the secondary cyclone, by aligned the two hydrodynamically.There is a gap between the primary cyclone outlet duct and thesecondary inlet duct to allow stripper vapor to enter thesecondary cyclones. In this arrangement, since the secondarycyclones are operating below reactor pressure, the productvapor passes from the riser, to the primary cyclones, secondarycyclones and straight to the fractionation without entering thereactor vessel.

5. Standpipe flow


The FCC process requires continuous catalyst circulationbetween the reactor and the regenerator in order to achievetwo main process objectives. One is to restore catalyst activityby burning off coke deposition on spent catalyst. The other isto keep the FCC unit in heat balance by continuouslyremoving heat from the regenerator and providing heat to thereactor for vaporizing and cracking the liquid hydrocarbonfeedstock.


Catalyst circulation between the reactor and the regeneratoris driven by unit pressure balance. Although the pressurebalance of the FCC unit consists of numerous parameters, mostof these parameters are set by overall unit layout and processconditions and, therefore, seldom vary. In contrast, pressuregains in the standpipes can vary significantly, which ultimatelydetermine the pressure differentials available for slide valvecontrol for achieving several critical functions in the FCCprocess:

• controlling reactor riser temperature and cracking severity,• regulating reactor catalyst level, and• serving as safeguard against flow reversal.


The standpipe is a simple catalyst transfer conduit with veryfew critical design requirements. Pressure gain in a standpipe isknown to depend mainly on two design elements. The firstelement is aeration along the standpipe, which is relativelystraightforward. As catalyst flows downward in the standpipe,the static pressure increases, and the gas phase moving alongwith the catalyst is compressed. In order to maintain properfluidization of the catalyst flow and to continue the pressuregain, aeration gas is added to compensate for the volumetric lossdue to the increase in static pressure.

The second key design element of a standpipe is thestandpipe inlet, which is the focus of this discussion. Theconventional standpipe design adds an inlet hopper at the top ofthe standpipe [15]. The basic design concept is that bubbles areknown to enter the standpipe together with the catalyst flow.The inlet hopper provides residence time for bubbles to coalesceand grow into large bubbles; since large bubbles have a higherriser velocity, they have a better chance of escaping back intothe fluidized bed, thereby reducing gas entrainment in thestandpipe.

The fundamental flaw of the conventional design conceptis that, while the objective of the standpipe inlet design issupposed to reduce gas entrainment into the standpipe, thedesign requires that many bubbles be drawn into the hopperin the first place. The consequences are that

• If the inlet hopper is too small, many bubbles drawn into theinlet hopper do not have enough residence time to coalesceand grow; instead, the bubbles flow directly into thestandpipe, leading to high gas entrainment.

• If the inlet hopper is large enough to allow small bubblesto grow, large bubbles could hang stationary inside thehopper for an extended period of time, which cantemporarily restrict catalyst flow into the standpipe. Whenthe bubbles finally grow large enough to escape, therelease of the large bubbles creates a sudden surge ofcatalyst flow, causing a sudden pressure swing in thestandpipe. Thus, even if the inlet hopper functions asintended, the sequence of growing and releasing of largebubbles leads to a unstable standpipe operation, which itis supposed to prevent.

It is also important to recognize that a FCC unit is designedto operate in a wide range of conditions. Therefore, a standpipeinlet hopper could be too small at the high end of catalystcirculation and too large at the low end.

The new standpipe inlet design, called catalyst circulationenhancement technology (CCET) [16], takes a differentapproach. The basic concept of this design is to remove excessbubbles from the fluidized bed before catalyst entering thestandpipe. As shown in Fig. 5, this standpipe inlet design usesthe disk outside the standpipe to trigger a local, partial de-fluidization and to form a dense bed region outside thestandpipe, as shown conceptually by the circle in Fig. 5. Byre-introducing a small amount of fluidization gas above thedisk, the fluidization condition of the standpipe inlet region isoptimized and controlled independently from the processconditions of the process. This enables standpipe to operate awide range of conditions with high pressure build-up and stableoperation. Commercial experience confirms that this newtechnology can improve catalyst circulation rate by as muchas 50%.

6. Third Stage Separator (TSS)


The FCC process requires continuous regeneration of spentcatalyst by burning off coke with air in the regenerator operatingat relatively high fluidization of ∼3 ft/s superficial velocity. Theregenerator is typically designed with multiple pairs of two-stage cyclones to capture and return entrained catalyst back tothe fluidized bed. However, in order to handle large volume offlue gas, these regenerator cyclones are relatively large in

Fig. 5. New standpipe inlet design.

Fig. 6. Shell Global Solutions' Third Stage Separator (TSS).


diameter and are not very effective in capturing small catalystparticles.

New environmental regulation mandates reduction in fluegas particulate emission. In addition, the new regulation alsorequires major reduction in flue gas NOx emission, which istypically achieved by Selective Catalytic Reduction (SCR)process with a catalyst bed prone to plugging. Conventionaltechnology for flue gas particulate removal includes electro-static precipitators (ESP) and wet scrubbers. New cyclonictechnology provides an alternative as a viable solution.


The cyclonic design of the catalyst fine removal downstreamof the regenerator cyclones must achieve two objectives:

• a high separation efficiency for catalyst fines, which requiresa small diameter cyclonic separator

• a capability of handling a large flue gas volume.

The solution is a device called a Third Stage Separator(TSS), shown in Fig. 6 [17]. In order to achieve high separationefficiency of catalyst fines, the TSS system uses a speciallydesigned separation element, called a swirl tube, with arelatively small diameter. A key difference from conventionalcyclone design is that the swirl tube is an axial flow cyclonicseparator using a swirl vane at the inlet to induce fast rotatingmotion. By using the axial flow separator, a large number ofswirl tubes can be installed in a common TSS vessel to handlethe large volume of flue gas.

Recent advances in the swirl tube design enable thetechnology to reach a cut point of 2 microns. This has beenproven commercially to meet the most stringent particulateemission requirements in the world, including MACT IIrequirement in the US. Furthermore, by reducing the catalystfine loading to below 50 mg/m3, downstream flue gas treatmentequipment, such as SCR, is protected.

7. Concluding remarks

The FCC process has been the most important refiningconversion process in the past 60 years. Through a fewexamples, this paper highlights some recent advances of thetechnology to meet new demands and challenges. Although theFCC process continues to evolve, there is no doubt that it willcontinue to serve a central role in the future of the refiningbusiness.

References

[1] A.D. Reichle, Fluid Catalytic Cracking hits 50 year mark on the run, OGJ,May 18 1992, p. 41.

[2] J.R. Murphy, Evolutionary design changes mark FCC process, OGJ, May18 1992, p. 49.

[3] A.A. Avidan, FCC is far from being a mature technology, OGJ, May 181992, p. 59.

[4] R.R. Dean et al.,“Residual oil feed process for fluid catalytic cracking”, USpatent 4,434,049 (1984).

[5] R.E. Bedaw et al.,“Atomizing spray nozzle for mixing a liquid with a gas”,U.S. patent 5,240,183 (1993).

[6] J. Haruch, “Nozzle for use in fluidized catalytic cracking”, European patentEP 0-717-095-A2 (1995).

[7] Y. Chen et al., “Feed nozzle”, U.S. Patent 5,794,857 (1998).[8] Y. Chen et al., “Feed nozzle”, U.S. Patent 5,979,799 (1999).[9] A.H. Lefebvre, Atomization and Sprays, Hemisphere Publishing Corp.,

1989.[10] J.E. Gwyn, Entrance, exit and wall effects on gas/particulate solids flow

regimes, 4th International Conf. On Circulating Fluid Beds, 1993, p. 679.[11] H.W.A. Dries. “Reactor riser for a fluidized-bed catalytic cracking plant”

— US patent 6,596,242.[12] Innovative Technology Meets Processing and Environmental Goals;

Flying J Commissions New MSCC and TSS — V.J. Memmott and B.Dodds, AM 03–13, 2003 NPRA meeting.

[13] J.G. Schwartz et al., Catalytic cracking unit with external cyclone and oilquench system — US Patent 5,089,235, 1992.

[14] Y. Chen, T. Dewitz, H. Dirkes, H. Dries, R. Samson, Cyclone Separator,2002, WO 02/085527 A2.

[15] Fluid catalytic cracking, in: Y. Chen, Wen-Ching Yang (Eds.), Handbookfor Fluidization and Fluid-particle Systems, 2003.

[16] Y. Chen, D.J. Brostem, Standpipe inlet enhancing particulate solidscirculation for petrochemical and other processes, US patent 6228328,2001.

[17] FCCU Particulate Emissions Control with a Shell Third Stage Separator –a case study – Weaver, E.H. (of Belco Technologies Corporation),contributions from Geiger, F., presented at NPRA, New Orleans, Louisiana(USA), September 2002.

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2006_PT_Recent Advances in FCC Technology