fluid catalytic cracking some recent developments in

7/27/2019 Fluid Catalytic Cracking Some Recent Developments In

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Catalysis oday 18 (1993) 509428Elsevier Science Publishers B.V., Amsterdam509

CATTOD 233

Fluid catalytic cracking: some recent developments incatalyst particle design and unit hardwareR. Mann*Department of Chemi cal Engi neeri ng, UM IST, M anchest er M 60 IQD UK)

Abstract

Catalytic cracking is an old process, but is still subject to new pressures for change and improve-ment. In the 199Os, hese pressures arise from the need to crack components drawn from deeper downthe barrel as well as to reformulate fuel products which can meet more stringent environmental re-quirements. Recent efforts at correlating catalyst formulation and cracking performance still rely onlargely empirical understanding of the role of catalyst composition and zeolite/matrix pore architec-ture. However, new tests measuring cracking performance at constant surface area as the zeolite/matrix ratio varies have indicated that the pore architecture should be designed to facilitate a stagedcracking. The whole range of pore sizes from micro-pores ( < 20 A to giant macro-pores ( > 1000Aare now known to be influential in determining cracking yield and activity and these pores should beassembled into an optimal architecture. Unfortunately, the configuration of pores in a typical spraydried FCC catalyst is usually far from such an optimal design. The use of stochastic pore networks tocharacterise macro-pore structures is described in conjunction with some experimental results em-ploying a visualised porosimetry based on low melting point alloy impregnation. Improved designs ofcatalyst will impose new requirements on the fluid&d bed hardware. Some recent improvements inthe chemical engineering of riser reactor/regenerator units are described which centre on atom&dinjection, segregated feeding, closed cyclones, staged stripping and catalyst cooling.

INTRODUCTIONFluid catalytic cracking is now more than fifty years old. Despite this ma-turity, there continues to be significant pressures to upgrade the performanceof existing units.Some of this pressure is quite longstanding and related to the need to dealwith still heavier feedstocks drawn from deeper down the barrel. These heav-ier feeds will often present additional difficulties because of higher sulphur

contents. Improved performance can usually be registered by increases in ole-fins yield.Other pressures for better performance are more recent and are being dri-ven by environmental issues in two main ways. Firstly there is a need to re-*Corresponding author.

0920-586 l/93/$06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.


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510 R. Mann Catal. Today 18 1993) 509-528formulate liquid fuels to give a reduced environmental impact. This refor-mulation requires a reduction in aromatics and an increase in oxygenates.Secondly, emissions from units themselves need to contain less SO,, NO, andparticulates to satisfy tightening legislation.Improvements however continue to be mostly piecemeal and empirical. Thisreflects the lack of any comprehensive understanding of the fundamentals ofthe catalytic cracking event as it takes place within typical fluidisable parti-cles. These particles are configured with a zeolite component dispersed in asilica-alumina support matrix. Unusually for a supported catalyst, both thezeolite and support show catalytic activity. The additional complexities ofmulticomponent cracking and intra-particle diffusion and mass transportpresent a formidable difficulty in uncovering a fundamental understandingsufficient to guide process innovations.CORRELATING CATALYST PROPERTIES AND PERFORMANCE

The empirical basis for the understanding of the performance of an individ-ual unit has, not surprisingly, led to very few general insights into the problemof relating the intricacies of the design of the fluidised particles to the overallperformance of cracking units themselves. It has already been indicated thatthe processes inside the particles are complicated. This is no less true for atypical cracker, which has twin fluid beds connected by a riser around whichthe solid catalyst particles are moved by transported flow with intermittentfluidisation. This intrinsic process complexity is compounded because unitsare invariably feedstock specific and their operational policies are affected bylocal economic factors.Even so, the establishment of some correlation between catalyst character-istics and performance under relevant realistic conditions is of practical im-portance to refineries seeking guidance for catalyst selection. Also, such cor-relations should help to develop more fundamental understanding capable ofleading to improved catalyst particle design. Improved catalyst designs needto optimise the complex and conflicting interactions of multi-species crackingreactions, diffusion and coke deposition.To this end, extensive testing has been reported [ 1 ] using a riser pilot plantdesigned to simulate a commercial FCC unit riser temperature profile andachieve vapour residence times from 2 to 5 seconds. Results from 12 com-mercial catalysts available from 5 reactors, as indicated in Table 1, were ex-amined in great detail. Some measure of the empirically determined varietyof property/performance achievable from the variety of formulations testedcan be gauged from Fig. 1. This shows how a basic measure of conversionvaries with the total specific surface area of the catalyst. This correlation ap-pears to be random. In respect of other property/performance aspects, someunderlying features could be discerned, although no correlation could be found


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R. Mann / Catd. T&y 18 (1993) 509428TABLE 1Catalysts ested n minatm riser unit (Hsing and Pratt [ 1 )

511

Vendor CatalystAlUO Vision 49AAh0 Advance 407BAkZO Octaboost 627-3Davison Super Nova-D LREDavison Super Octacat RN+Davison Astra-278Davison XP-750+Engelhard Dynasiv 850+Engelhard Precision-RFiltrol HRO-600Filtrol FX-500Katalistiks Alpha-550

75 *74 .

&f 73.r 72.

- 71.G 70 .I? 69 .s

l

68.v 67 .66 .

Conversion vs total fresh surface areaat cat/oil = 6.0

* .l

w

ll.

..

65 -!200 215 230 245 260 275 290 305 320 335 350

Surface areo, m2/gFig. 1. Conversion vs. surface area for 12 commercial catalysts.between overall working activity and unit (zeolite) cell size. A key factor isthe so-called zeolite/ (support) matrix ratio and features of performance couldbe more clearly identified with yield rather than conversion performance. Thusdry gas yield decreased with increasing z~lite/mat~x ratio as shown in Fig.2. Other clear correlations were established between: coke yield - increasedwith increasing unit cell size; bottoms cracking - increased when the zeolite/matrix ratio decreased, and CJC, olefms - increased with increasing zeo-litefmatrix ratio. However, it is obvious from Fig 2 that even a clear corre-lation between empirical performance measures leaves a degree of uncer-tainty in the quality of the correlation. A more fundamental approach requiresa rational framework which incorporates the architecture or physical geome-


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512 R. Mann / Cat al . Today 18 1993) 509-528

Dry gas vs Zeolite/Matrix ratioot 69 wt% conversion

4.5 1,4. 3 - .K .

r. 4. 1.

:cl, x 39n

3. 7 . ..

35 _I05 0.6 1 1 1.4 1.7 2.0 2 3 2 6 2. 9 3. 2 3. 5

Fig. 2. Dry gas yield vs. zeolite/matrix ratio.4

- kiko

r ko = Cracking rate01 hexadecane0 0

16

1 2 3 4 5Naphlhenic rungs

12 13 14 16Carbon number

Fig. 3. Catalytic cracking with molecular sieving.try of the zeolite/matrix assembly. New developments in this direction prom-ise a degree of understanding that is for the first time giving a genuine impetusto radical improvements in catalytic cracking performance.FCC CATALYST PORE ARCHITECTURE AND PERFORMANCE

The importance of pore architecture in determining cracking performancewas initially limited to the internal pore structure of zeolite crystallites. Theresults for several model compounds compared to the cracking rate for hex-adecane are shown in Fig. 3 [ 2 1, from which it is clear that there is an acces-sibility limitation due to the zeolite portal areas which is effectively molecular


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R. Mann / Catal . Today 18 (I 993) 509-528 513

sieving with/without reaction. As the number of naphthenic rings increases(beyond 1 ), the crackability increases, but the cracking rate (relative to thepotential activity of the zeolite cages) falls off due to the increasingly limitedaccessibility of the intra-zeolite acid sites.This shape-selecting concept has now been recognised as being importantfor the larger pores, exterior to the zeolite, which arise from the amorphoussupport matrix and possibly also from the exterior surfaces of zeolite crystal-lites. As an idealised simplification, three possible stages of the cracking pro-cess can be envisaged [ 3 1. Firstly, large asphaltenes crack and metals are de-posited. Secondly, products of the primary cracking pre-crack further in meso-pores. Thirdly, final cracking of the smaller molecules takes place inside zeo-lite cages. This concept of an architecture for the catalyst which promotesstaged cracking is clearly important for the design of FCC resid catalyst thatwill be simultaneously efficient/effective/selective and resistant to deactiva-tion. Moreover, as Fig. 4 shows schematically, size, shape, acidity and acces-sibility have to be designed for and particles have to be fabricated and assem-bled accordingly.In practise, the pore size distribution formation ought to be manipulatedinto a continuous form similar to that shown in Fig. 5. The micro-pores ( 1000 ) serve as liquid catching pores andgive preliminary cracking of asphaltenes as well as accommodating metalsdeposition.Extensive experimental results relating to this theoretical concept have ex-plored the cracking performance of a set of test catalysts for which the ratioof silica matrix material relative to an active selective alumina was varied[ 41. This study used catalysts with low rare earth (RE) content comprisingdealuminated Y zeolites. These have reduced unit cell size with fewer but

PORE ACIDITY DISTRIBUTIONLOW ACIDITY MEDIUM ACIDITY HIGH ACIDITY

I

NICKEL-ORGANICMOLECULE LCOIHCO

,,, RE?; \\\, ,, ,, ;..-4;.LpG

-----, BULK

Fig. 4. Schematics of staged cracking.


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514 R. Mann / Cat al . Today 18 1993) 509-528

SMALL PORESSURFACE - c 20 AAREA _

f 11

EFFECT OF ACTIVE MATRIXAT CONSTANT Z/M

MESO PORES LIQUID CATCHINGRELATIVE 30. I OOOA LARGE PORESACTIVITY -

._I > r oooA. - - -

PORE DIAMETER cTHE PORE ARCHITECTURE DESIGN CONCEPT

Fig. 5. Pore size distributions for catalytic cracking.

EFFECT OF ACTIVE MATRIX AT CONSTANT Z/MFRESH !?.AVS ZEOJTE/Am MATRIX350 ,

; ;_.3,- --- -ys 280 - FA/S - 0.84 + 0w02 270ccii 260

250

2101 2.0 4.0 6.0 6.0 10.0 12.0 14.0 16.0

Fig. 6. Overall surface area vs. alumina( FS) /silica( S ) ratio.

more active sites, typically applied in fluid catalytic cracking units (FCCUs)for producing higher octane gasolines. The results reported explore the effectof the active matrix at a fixed zeolite/matrix or z/m ratio. Fig. 6 shows howthe overall surface area of the fresh catalysts varied with the selective alumina


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R. M ann / Cat al . Today 18 (1993) 509428 515EFFECT OF ACTIVE MATRIX AT CONSTANT Z/M

NIT. PORE SIZE DISTRIBUTIONS BY ADS.0.2

0 180.160.140 12

0.10 080.060.040.02

0

ORE VOLUME:CC/G_ .__~--.__---~~- -----A FA/S=2.67

PORE DIAMETEFkAFig. 7. Pore volume distributions and zeolite/matrix ratios.

EFFECT OF ACTIVE MATRIX AT CONSTANT Z/M4.8 -4.6 -4.4 -4.2 -

4-3.8 -3.6 -

f 3.4 -3.2 -

S-2.1 -2.6 -2.4 -2.2 -

HwHH+\NL288 92 96 too 104 108 112 116WlRIX AERACE PORE DWElER(OESORPllON)A- H:fA/S-2.67 - u:FA/S-0.84 - LFA/S-0.23

Fig. 8. Coke yield vs. matrix average pore diameter.(FA) /silica (S ) ratio. In effect the total area is held constant whilst the zeo-lite/active matrix surface area is varied.Results for a micro-activity test (MAT) unit were reported for a hydro-treated, aromatic west coast VGO. The lower hydrogen transfer activity ofthe reactive matrix was evident from the higher olefinicity gained at the ex-


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516 R. Mann / Cat al . Today 18 1993) 509-528pense of aromaticity. The zeolite hydrogen transfer activity was much lowerthan for a corresponding high rare earth (RE) formulation, although still muchgreater than for the active matrix component. Fig. 7 shows how the pore vol-ume distribution was modified for two values of the zeolite/matrix ratio. Ata high value of 2.67, the catalyst has a larger proportion of the smaller inter-mediate cracking matrix pores. Fig. 8 then shows one of the very many resultsreported [ 41 which indicates how the coke yield falls as the matrix averagepore diameter increases. All the results reported by Yanik et al. [ 41, compris-ing some 40 figures, ranging through many measures of activity, selectivityand yield, confirmed the validity of the staged cracking concept in Fig. 4.However, it is clear that whilst the concept of staged cracking is importantin understanding general issues relating to quantitative design of cracking cat-alyst particles, a more precise quantification of the complex multi-speciescracking and diffusional processes is needed. This in turn requires better def-inition of the pore architectures themselves.MODELLING FCC CATALYST PORE STRUCTURES

The simplest approach to modelling the pore spaces inside FCC particles isto envisage that the zeolite pores are uniformly distributed amongst supportpores represented by a parallel bundle. This is a mathematically highly con-venient formulation, which is depicted in Fig. 9. This model has already beendeveloped to analyse the relative activities of the zeolite and matrix in theface of coke laydown [ 5,6 1.

Idealized catalyst pore structure of an FCC catalystFig. 9. Simple parallel bundle model for a supported zeolite catalyst.


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R. M ann / Cat al . To& y 18 (1993) 509-528 517

However, scanning electron micropscopy (SEM) studies of typical com-mercial FCC particles, show, in common with most porous materials, that thepore spaces are an evident tangled mass of widely varying sizes. Pores are thusexpected to be randomly jumbled together, but to be interconnected thor-oughly amongst one another. Fig. 10 shows how an FCC particle of about 70m in diameter appears when viewed by an SEM. So called stochastic porenetworks are a suitable model concept for representing randomness and in-terconnectively in porous catalysts. A simple 2-D example of size 30x 30 isdepicted in Fig. 11. Large and small pores can be randomly allocated withinsuch a simple square network, providing a reasonably realistic and yet stilltractable description of the pore structure. Such simple pore networks can beanalysed for diffusion, reaction and coke laydown. Some results for the crack-ing of cumene over a commercial catalyst (SUPER-D) showed how coke de-position within the zeolite could be separated from coke laydown on the sup-port. Moreover, the model clearly indicated how interior accessibility couldbe affected as the support pores became plugged with coke [ 71. In the longer

Fig. 10. SEM view of a FCC particle.


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518 R. Mann / Catal. To&y 18 (1993) 509-528

Fig. 11. A simple 30 x 30 2-D stochastic pore network.

Fig. 12. A 10 x 10 x 10 3-D stochastic pore network.term, it will eventually be desirable to apply 3-D pore networks to representthe support/matrix pore structure. An example of a stochastic pore networkin 3-D is visualised in Fig. 12.


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The use of 3-D networks is obviously more realistic for calculating multi-species cracking, diffusion and coke laydown. Furthermore, the linking of atheoretical 3-D network structure to the matrix/support pores in typicalcracking catalyst particles can now be linked to techniques which can visual-ise the pore spaces exposed by a plane section through a particle. One experi-mental method for achieving this uses the pressurised penetration of a fluidlow melting point alloy (LMPA) into the pore spaces. On lowering the tem-perature, the alloy is solidified within the pores and thereby comprises a mea-sure of the accessible pore volume at the penetrating pressure. This is a kindof visualised porosimetry. Assemblies of particles subjected to this techniquecan then be sectioned and polished to provide an image of the pore spaces.

This has been done for a sample of an (unused) industrial cracking catalyst(SUPER-D) and the results are shown in Fig. 13 [ 8 1. This section is of size500 p x 600 e, which exposed cross-sections of some 200 (or so) crackingcatalyst particles. Several features of this result are worth noting. Firstly, al-though the exposed sections appear to show particles of variable diameter,this is because the sections pass randomly through approximately sphericalparticles of roughly equal size. In this case the catalyst particles are some 70pm in diameter. Secondly, and perhaps most significantly, there is a startlingnon-uniformity of penetrated internal matrix/support pore structure as in-dicated by the extent of invasion of the low melting point alloy. The individ-ual particles internal pore spaces are much more widely variable than theyare similar. This variability ranges from the total non-penetration of some 50particles (about 25%) to the largest exposed section which contains giant sizedpore voids. In between these two extremes, there appears to be a continuousvariation in the indicated porosity and associated pore space dimensions.Thirdly, all the particles show a significant skin effect, so that interior accesscan be expected to be restricted by smaller exterior pores. This outer skin isalso probably the explanation for the large fraction of totally unpenetratedparticles which appear completely black in Fig. 13.The major conclusion from Fig. 13 is that whatever the optimal pore struc-ture of the matrix/support should be in terms of meso-pores linked to pre-cracking pores as staged pathways to the zeolite micro-pores, the assembly ofparticles in Fig. 13 could hardly be expected to begin to approximate it. Be-cause of the intrinsic wide variation from particle to particle, each one wouldhave different flux/permeation/diffusivity characteristics. Moreover, eachone would behave quite differently with respect to coke laydown and build-up. Finally, each one would then have quite different burn-off behaviour atthe regeneration stage. The subsidiary conclusion from Fig. 13 is then that theprocedures used to fabricate cracking catalyst particles need to be examinedand revised so as to be able to control individual pore structures much moreprecisely. Only in this way will it become possible to fabricate particles whichhave optimally tailored pore structural characteristics.


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520 R. Mann / Cat al . Today 18 1993) 509428

Fig. 13. Low melting point alloy (LMPA) impregnation of cracking catalyst.

As a post-script to Fig. 13, it ought to be possible to draw some inferencesas to the average particle structure. This should serve as some guide to im-provements, since it is reasonable to assume that the disparate set of particlesin Fig. 13 on average delivers reasonable performance. Thus, Fig. 14 showsthe particle which is subjectively judged to represent a pore structure average.This detailed view of an individual particle clearly shows an outer skin, whichhas been penetrated by alloy through three closely adjacent large peripheralpores (upper right particle periphery).Fig. 15 then shows a computer generated random section through a 3-Dstochastic pore network by low melting point alloy (LMPA) impregnation.The computer image has been generated from a so-called regular 3-D networkextended to accommodate 5 degrees of randomness. These 5 degrees of ran-


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Fig. 14. Alloy penetration of an average FCC particle.

domness are: pore diameter, pore length, pore direction, pore cross-sectionand wall roughness. The computer generated regeneration in Fig. 15 is consis-tent with mercury porosimetry results. The random 3-D network of Fig. 15contains pore elements up to 3~ in diameter. The computer image howevershows pore features up to 9p in (equivalent ) diameter. The imaged repre-sentation exhibits these large pore features due to overlapping of pores as theplane section passes close by pore junctions (nodes). A feature may thus con-tain elements of several pores and thereby appear enlarged in the plane of thesection. This aggregation of pore sections is readily discernible in Fig. 15.The foregoing assessment of the pore structures of actual cracking catalystparticles has indicated the need for catalyst manufacturing and preparationtechniques to recognise the importance of dovetailed pore structures that couldoptimise architectures so as to deliver staged cracking of the heavier residfeedstocks. Techniques developed for imaging stochastic pore networks [ 91can be readily adapted to construct pore assemblies with any geometricalproperties. In this respect, Fig. 16 shows a concept network in which poresizes are controlled from the interior (smallest) to the exterior (largest). Thediffusional and reaction performance of such networks can be readily calcu-lated, so that candidate pore structures can be screened by a modelling stage.The spiralled network of Fig. 16 would appear to have a desirable geometry


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R. M ann / Cat al . Today 18 (1993) 509-528

real image computer generated image

Fig. 15. Average FCC particle modelled by a slice through a 3-D network.for a staged cracking reaction which requires pre-cracking pores capable oftaking up metals deposits.Other workers have also developed pore network modelling for catalyticreaction applications [ 101, allowing for variable connectivity of 3-D net-works within a spherical envelope and extended their approach to nested net-works, of the type shown in Fig. 17 [ 111. Calculations of the effect of cokelaydown on particle deactivation behaviour have also been carried out in reg-ular 3-D lattice networks [ 121.Attention has also recently been focused in parallel on the impact of cokedeposition in uniform lattice models of the zeolite cage/channel structure[ 13 1, although this problem is itself further complicated by the fact that siteoccupation by molecules in typical configurational diffusion itself causes ac-


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smaest oxw on interior/lqt pores on csteriorFig. 16. A concept 2-D network for optimising staged cracking.

Micro.Pbrc Mscro-PoreI Prriodlc Buuodrrr

a bw c

Fig. 17. Nested stochastic pore networks in 2-D.

cess blocking and reduced activity [ 141. The problem of effectively jointingthe diffision and reaction behaviour of zeolites to the transport and crackingperformance of the support matrix obviously requires fiuther work, which


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will ultimately be necessary for the complete modelling of modem crackingcatalysts.NEW DEVELOPMENTS IN FCCU HARDWARE

Since fluid catalytic cracking is the dominant upgrading process in a refin-ery, the fluid catalytic cracking unit (FCCU) has a key role to play in meetingnew fuel based challenges for the 1990s. These challenges are summarisedschematically in Table 2.The US Clean Air Act amendments of November 1990 require reductionsin vehicle exhaust gas emissions, which could be met by greater use of alky-lates and ethers. These have high octane values with zero aromatics and inaddition the ethers have a beneficially high oxygen content. Light olefins areprecursors to these particular gasoline components, so that increasing the ole-fin potential from FCCUs is highly desirable.Emissions from the regenerator itself are also becoming more tightly con-trolled by US State and Federal regulatory bodies. This requires regeneratorsto reduce SO, and NO, emissions as well as particulates.These environmental requirements for the product balance and the processhave to be achieved in the face of continuing pressures to process heavierhigher sulphur crudes (in the jargon of the industry, to improve the upliftfrom a barrel of crude). As Johnson et al. [ 151 have so succinctly stated ...the FCC (unit) will be called upon to process poorer quality feeds whilemeeting strict environmental laws and increase the yield of lighter olefins forcleaner fuels. New technology is required to upgrade FCCUs which are gen-erally already running at capacity limits.Table 3 then lists a summary of six key innovations in unit hardware, whichprogress the three challenges of Table 2. These hardware changes have all beenimplemented in the recent Orthoflow and Resid FCC design depicted in Fig.18, which resulted from collaborative development between Mobil and Kel-logg [ 151.TABLE 2FCC: Major challenges for 1990s (Johnson et al. [ 171)Producing reformulated fuels Reduce aromatics

Increase oxygenatesMeeting environmental limits SO,NO,ParticulatesUplifting heavier feedstocks Higher S

Increased olefms


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525. M ann / Cat al . Today 18 (1993) 509-528TABLE 3FCC units: new hardwaredevelopments (Johnson et al. [ 17 1)Feed atomisation - improves liquid productsSegregated njection - increases light oleflnsClosed cyclones - reduces thermal crackingStaged stripping-lowers regenerator emperatureCatalyst di~~bution - reduces after burningCatalyst cooling - better conversion for heavier feeds

DISEHGAGER

REGENERATORSPENT c*T*l.vs,

Fig. 18. Mobil/Kellog orthoflow and resid FCC unit (Johnson et al. [ 171).

The changes in at omi si ngfeed i nj ect i on mprove the contacting in the zonewhere liquid feed and hot, active (regenerated) catalyst are brought together.Podr Ontario increases coke and dry gas which adversely affects both con-version and selectivity. The new nozzle designs, jointly developed by Kelloggand Mobil, produce a flat, fan shaped spray which gives a tighter droplet sizedistribution. Performance has also been improved by co-injecting steam with


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526 R. M ann Cat al . Today 18 1993) 509-528

the feedstock and using an improved geometrical distribution in the riser.The consequent improvement in mixing reduces thermal cracking at the riserbase.Incorporation of segregatedfeed injection separates the riser into two zones.A high nitrogen feed is introduced at the base and a low nitrogen feed part-way up the riser. This helps to increase the conversion of heavy ends, partic-ularly when the maximum riser top temperature is acting as a constraint.CIosed cyclones reduce any thermal cracking after exit from the top of theriser. Yield losses from earlier designs were attributable to post-riser non-se-lective thermal cracking. Mobil made use of a plexiglass scale model to de-duce a closed cyclone design which reduced the spread of hold-up times ofvapour in the reaction vessel, thereby giving substantial improvements inyields.Steam stripping of spent catalyst is a basic operation in FCC. Any under-flow of hydrogen rich hydrocarbons is detrimental because it gives higher thandesirable regenerator bed temperatures and higher individual catalyst particletemperatures, which acts in turn to decrease the conversion. These effects areshown in Fig. 19. In the Orthoflow design a more efficient st aged catal yst

str ipping s achieved by two stages which produces a lower 6% hydrogen incoke at 2 to 2.5 kg steam per 1000 kg of recirculating catalyst.Spent catalyst, especially when fresh (and tending therefore to be moreheavily coked), when it contacts combustion air close to the inlet, can expe-rience a temperature rise sufficiently high to provoke sintering. This has beenknown for a long time [ 16 ] and causes undue deactivation. Regenerat or per-

fo rmance can be improved by dispersing the spent catalyst well away from theincoming combustion air. This is also an important factor when workingheavier feedstocks which tend to give higher coke on catalyst. A more uni-form catalyst distribution thereby enables heavier feeds to be more effectivelyaccommodated. Kellogg has developed and commercialised a spent catalystdistributor which achieves this more effective uniform dispersal (US PatentNo. 4 150 090). This improved contacting also reduces NO, formation andtherefore NO, emissions. Table 4 shows typical improvements obtained. Theseimprovements are claimed to be due to the initial burning-off taking place inTABLE 4Improved air/catalyst contacting (Johnson et al. [ 17 ] )

Before AfterExcess O2 vol.%AfterbumOcFeed rateFeed N,-tNO,

2.523BaseBase

1.019

+5%-45%


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zones of tower oxygen ~n~en~tion, in turn resulting in a lower parti& tem-perature rise. As Table 4 shows, not only have NO, emissions reduced, butmore feed can be processed without reaching detrimental& high tempera-tures. Reduced emissions are therefore achieved at gre&er ~rou~put.The use of dense phase ~~~~~~~oding estricts the r~e~emtor tempera-ture rise when processing feedstocks that produce heavier coke laydown.Temperatures which are too high reduce the conversion to more desirableproduets and ~~es~n~~y give a high dry gas yield. Early unit designsused tube flow of the dense phase in a shell and tube heat exchanger, whichcaused u~a~ptabIe erosion rates and gave poor availability in service, Laterimprovement used regenerator in-bed coils, which improved pliability butproved insufficiently flexible. New imp~vements now make use of tow veloc-ity flow of the dense phase on the shell-side, with steam on the tube-side [ 17 1,In the new designs, the tubes are freely suspended from the tube sheet as abayonet-TV future. This design gives lower thermaI stresses peskily onstart-up) ) which improves ~~ab~ity by reducing tube f&lwes. This approachalso gives a high heat transfer capability similar to that achieved by regener-ator bed coils, so that smaller shell sizes are needed. Heat removal rates inexcess of 29 mW are being achieved using this approach.

REFERENCEI L.H. Hsing and R.E. Pratt, carrelations between FCC Catalyst Properties and Perform-ance in a Riser Pilot Plant, Paper 126f, AIChE Annual Meeting, Los Angeles,CA ( 1991).2 D.M. Nate, Catalytic Cracking over Crystalline ~~nosi~~te~ Ind. Eng. Chem. Prod,Res. Develop_.,9 (2) ( 1970) 203-209.3 P. Hettinger, ~velopment of a Reduced Crude Cracking Crttalyst, ACS S~rn~si~~ Se-ries, 375 ( 1989) 308.4 S.J. Yanik, P. OConner, M.F. Rrady, D.K. Abner and MC. Friedrich, FCC Catalyst Pore

Architecture and Performance, AIChE Symposium Series No. 291,88 ( 1992) 9-20,S R, Mann and I.R. Moore, ~~~~tion of a Supine Zeolite Cracking ~t~yst: A PoreStructural Approach, I. Chem. E. Symposium Series, 87 (1984) 25-33.6 R. Mann and G. Thomson, variation of a Supported Zeolite Catalyst: Simulation ofDiffusion, Reaction and Coke Laydown in a Parallei Bundle, Chem. Eng. Sci., 42 ( 1987)555-563.7 R. Mann, G. Thomson and P.N. Sharratt, Deactivation of a Supported Zeolite Catalyst:Diffusion, Reaction and Coke Deposition in Stochastic Pore Networks, Chem. Eng. Sci,,41(1986)711-719.8 K Khalaf, Appli~~on of Stochastic Pore Network Models to Craekiug Catalyst, Ph.D,Thesis, UMIST ( 1987).9 R. Mann and H.N.S. Yousef, Q~ti~cati~n of Random Pore Structures of Porous Adsor-bents, Adsorption Science and Technology, 8 (4) ( 1991) 196-2 16.10 R.L. Portsmouth and L.F. Gladden, Determination of Pore Connectivitiy by Mercury Po-rosimetry, Chem. Eng. f&i., 46 ( 1991) 3023-3036.11 M.P. Hollewand and L.F. Gladden, Repres~~~ou of Porous Catalysts using Random PoreNetworks, Chem. Eng. Sci., 47 ( 1992) 2757-2762.


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528 R. Mann / Cat al . Today 18 1993) 509-52812 S. Arabi and M. Sahimi, Computer Simulations of Catalyst Deactivation. II: Effect of Mor-phological, Transport and Kinetic Parameters on the Performance of a Catalyst, Chem.Eng. Sci., 46 (1991) 1749-1755.13 A.O.E. Beyne and G.F. Froment, A Percolation Approach for the Modelling of Deactiva-tion of Zeolite Catalysts by Coke Formation: Diffusional Limitations and Finite Rate ofCoke Growth, Chem. Eng. Sci., 48( 3) (1993) 503-511.14 J.G. Tsilcoyiannis and J. Wei, Diffusion and Reaction in High-Occupancy Zeolite Cata-lysts. I: A Stochastic Theory Chem. Eng Sci., 46 ( 1991) 233-253.15 T.E. Johnson, Catalyst Coolers for Fluid Catalytic Cracking: Past and Present, Int. Conf.on Circulating Fluid Bed 4, Nagoya, Japan ( 1990).16 A. Bondi and R.S. Miller, Rapid Deactivation of Fresh Cracking Catalyst, I and EC Proc.Des. and Develop., 1 (3) ( 1962).17 T.E. Johnson, P.R. Niccum, M.F. Raterman and P.H. Schipper, FCC for the 90s: NewHardware Developments, AIChE Symposium Series No. 29 1,88 ( 1992) 9-20.

fluid catalytic cracking some recent developments in

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