FCC Study of Canadian Oil-Sands Derived Vacuum Gas Oils. 1. Feed and Catalyst Effects on Yield Structure

FCC Study of Canadian Oil-Sands Derived Vacuum GasOils. 1. Feed and Catalyst Effects on Yield Structure

Siauw Ng,*,† Yuxia Zhu,† Adrian Humphries,‡ Ligang Zheng,§ Fuchen Ding,|Thomas Gentzis,⊥ Jean-Pierre Charland,§ and Sok Yui#

National Centre for Upgrading Technology, 1 Oil Patch Drive, Suite A202, Devon,Alberta, Canada T9G 1A8, Akzo Nobel Catalysts LLC., 2625 Bay Area Boulevard,Suite 250, Houston, Texas 77058, CANMET Energy Technology CentresOttawa,

1 Haanel Drive, Ottawa, Ontario, Canada K1A 1M1, Beijing Institute of PetrochemicalTechnology, Daxing, Beijing, China 102600, CDX Canada, Inc., 1210, 606-4 Street SW,Calgary, AB, Canada T2P 1T1, Syncrude Research Centre, 9421-17 Avenue, Edmonton,

Alberta, Canada T6N 1H4

Received February 15, 2002

This paper demonstrates the important roles of feedstock and catalyst in determining the yieldstructure during fluid catalytic cracking (FCC) of bitumen-derived vacuum gas oils (VGOs). Threenonconventional VGOs, derived from Canadian oil-sands bitumen, were catalytically cracked ina fluid-bed microactivity test (MAT) reactor. Two commercial equilibrium catalysts were used:a bottoms-cracking catalyst containing rare earth exchanged Y zeolite (REY), and an octane-barrel catalyst containing rare earth ultrastable Y zeolite (REUSY) mixed with a small amountof ZSM-5. Both catalysts were embedded in active matrixes. Results indicated that the REYcatalyst was more active, producing higher yields of valuable distillates and less coke for thesame feed, whereas the catalyst containing REUSY/ZSM-5 gave more light gases and less gasoline(although the quality of this gasoline might be better). These results could be related to catalystproperties including zeolite type, rare earth content, matrix pore structure, zeolite-to-matrix ratio,and surface characteristics. The three feeds were ranked based on their yield structures, whichcould be explained through feed analyses, precursor concentrations determined by GC-MS, andproduct characterization data from a PIONA analyzer. MAT results were compared with riserpilot plant data at 55 and 65 wt % conversion. In general, at the same conversion, the differencein a given product yield from the two units could be maintained within 15%. Coke yield showeda greater disagreement, however, due to methodological differences in the analysis.

1. Introduction

Canada has huge oil-sands bitumen resources thatare on a par with Saudi Arabia’s conventional oilreserves.1 Bitumen is a complex hydrocarbon moleculewith a relatively low hydrogen-to-carbon atomic ratioand an abundance of chemical impurities. Conversionof bitumen to high-quality transportation fuels is a greatchallenge, as most refineries are either not equipped toprocess the bitumen or find it economically unprofitableto process. In Canada, one practical route for overcomingthis conversion problem is to upgrade bitumen to a lightand bottomless synthetic crude oil (SCO) throughadequate processes, typically coking or hydrocrackingfollowed by hydrotreating. Bitumen producers may alsosell partially upgraded or raw bitumen to be blendedwith optimum quantities of conventional crude prior to

being processed in refineries. Both SCO and raw bitu-men contain over 35 vol % vacuum gas oil (VGO), whichcan be used as a feed to the fluid catalytic cracking(FCC) unit. Depending on the degree of upgrading, thederived VGOs carry different amounts of impuritiesfrom the bitumen. Among the impurities in an FCCfeed, nickel is known as a strong dehydrogenation agentproducing large amounts of gases (including hydrogen)and coke; vanadium can destroy zeolite by hydrolysisof the silica/alumina crystal structure, thus reducingcatalyst activity; basic nitrogen (about 1/4 to 1/3 of totalnitrogen) can neutralize the acidity of the catalyst,rendering it inactive; and Conradson carbon residue(CCR) may block the catalyst pores causing inacces-sibility to the molecules that need to be cracked. Inaddition, the high amounts of diaromatics and largermolecules in VGO produce excessive amounts of low-value light cycle oil (LCO), heavy cycle oil (HCO), coke,and dry gas.

In an attempt to improve the yield structure duringcatalytic cracking of bitumen-derived VGOs, a com-mercial catalyst containing an active matrix with highaccessibility to large hydrocarbon molecules was used

* To whom correspondence should be addressed. E-mail:[email protected]. Fax: 1-780-987-5349.

† National Centre for Upgrading Technology.‡ Akzo Nobel Catalysts LLC.§ CANMET Energy Technology CentresOttawa.| Beijing Institute of Petrochemical Technology.⊥ CDX Canada, Inc.# Syncrude Research Centre.(1) Newell, E. P. Oil Gas J. 1999, 97(26), 44-53.

1196 Energy & Fuels 2002, 16, 1196-1208

10.1021/ef0200368 CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 07/31/2002

to enhance bottoms cracking. For comparison, anotherequilibrium FCC catalyst (received from a refinery) withdistinctly different properties was also included in thetest program. Cracked liquid products in various cutswere characterized for sulfur, nitrogen, and hydrocarbontypes to determine the performances of these twocatalysts with respect to the product quality. The resultson the product quality will be presented in subsequentpapers that are in preparation.

2. Experimental Section

2.1. Feedstocks. Three feeds were supplied by the Syn-crude Research Centre: (1) a laboratory-hydrotreated cokerVGO (HTC) with properties close to those of its counterpartfrom Syncrude’s commercial plant; (2) a laboratory-hydrotreat-ed VGO from a deasphalted oil derived from bitumen (HT-DA) (deasphalting was performed using a mixture of n-butaneand i-butane as solvent, at typical conditions, in the M. W.Kellog pilot plant in Houston, Texas); and (3) an untreatedvirgin VGO (VIR) obtained by batch distillation of a wholebitumen to simulate a commercial product from Syncrude’snewly installed vacuum tower. All feeds were distilled toremove 343 °C- and 524 °C+ fractions, except HT-DA, whereonly the 524 °C- fraction was taken off. Feedstocks werecharacterized using ASTM and other supplementary methodsat the Syncrude Research Centre.

2.2. Catalysts. Two refinery-generated equilibrium cata-lysts, Akzo Nobel’s HRO 610 (HRO) and catalyst A (CAT-A)from another supplier, were included in this study. HRO, usedin a refinery in California that processes Alaskan feeds, wasdirectly received from Akzo Nobel Catalysts LLC., along withthe analyses. HRO was developed, on the basis of a specialcatalyst assembly technology, to give a high Akzo AccessibilityIndex (AAI), which measures the relative mass transfer ratesof hydrocarbons into and out of the catalyst pores (i.e., theadsorption and desorption rates of hydrocarbons). As for CAT-A, its manufacturer claimed that a new matrix technology wasused to lower the Lewis/Bronsted acid site ratio, leading to areduction in condensation/polymerization reactions that formcoke and gas. They also claimed the involvement of a propri-etary manufacturing process that controlled the inherentmatrix composition and surface area, while increasing thezeolite external surface through reduced crystal size for betterbottoms upgrading. The technical information provided sug-gests that CAT-A contains the ultrastable Y zeolite (USY).Characterization of CAT-A was performed at NCUT usingconventional ASTM methods. Both catalysts were calcined at600 °C for 3 h prior to being loaded into reactors.

2.3. Catalytic Cracking of Feedstocks. Bench-scalecracking experiments were largely conducted at NCUT, withselected runs at an outside laboratory that used a reactorsystem based on the Advanced Cracking Evaluation (ACE)technology.2 Recently, the ACE unit has gained popularity asa tool for laboratory FCC studies. In this paper, only NCUTresults are reported except in a figure which shows a com-parison of catalyst activities obtained from the two reactorsystems. It should be noted that more runs were performedusing NCUT’s unit than from ACE, and the liquid productsat NCUT were available for subsequent characterization. AtNCUT, bench-scale catalytic cracking was carried out at 510°C for HTC and VIR, and at 530 °C for HT-DA, using a fluid-bed reactor in a MAT unit (Zeton Automat IV). The reactorwas loaded with 7 g of catalyst. Catalyst-to-oil ratio was variedto obtain different conversions (the portion of the feed con-verted to 221 °C- products, including gas and coke). Catalystcontact time was kept constant at 30 s. Details of the

experiments were reported elsewhere.3 A specially designedliquid receiver with extra large volume (300 mL) was used tocollect over 99 wt % of liquid products that were free ofcontamination by washing with solvents (e.g., CS2). Thispermitted the total liquid products (TLPs) to be furthercharacterized, without prior separation, for simulated distil-lation (ASTM 2887) and other determinations. The coke yieldwas determined by in situ combustion of the spent catalyst,followed by measuring the CO2 concentration of the flue gasthat was passing through the catalytic reactor to convert COto CO2. This technique measured the total coke, including thenoncatalytic “feed residue coke”,4 which might form on thecatalyst surface and reactor inner wall.

The same three feeds were also cracked with CAT-A at 490-520 °C in a modified ARCO-type riser pilot plant.5 The feedwas charged to the bottom of the reactor at a rate of 600 g/h.Nitrogen was used to disperse the feed in the reactor. Thecracked products and the catalyst were separated in thedisengager at the top of the reactor. The catalyst was trappedwhile the cracked products passed through the cooler, whereTLP was recovered. The spent catalyst moved to the stripperwhere the entrained oil with the catalyst was stripped off usingheated nitrogen. The catalyst then went to the regenerator,for burning off the coke, prior to being recirculated to thebottom of the reactor. Total weight of the catalyst loaded was2 kg with a makeup rate of 50-100 g daily. The flue gascollected in a gas bag was analyzed by GC. The collected TLPswere distilled into liquefied petroleum gas (LPG), gasoline,light cycle oil (LCO), and heavy cycle oil (HCO) in twodistillation units: the True Boiling Point (ASTM D2892) andvacuum (ASTM D1160) units. The coke yield was determinedfrom the concentrations of CO and CO2 produced duringburning the spent catalyst.

3. Results and Discussion

3.1. Feedstock Properties. The quality of feedstockcharged to an FCC unit is the biggest single factoraffecting product yields and quality. Table 1 summarizesthe feedstock properties. Among the three feeds, HT-DA, which contains 36.9 wt % 524 °C+, has the highestconcentrations of total nitrogen (2450 wppm), CCR (2.00wt %), metals (10.9 wppm Ni+V), and polars (7.1 wt %)that usually contain aromatics and heteroatoms. How-ever, probably due to deasphalting and hydrotreatment,HT-DA has the lowest aromatics content as shown bythe lowest aromatic carbon (20.9%), the highest anilinepoint (81.0 °C), H/C atomic ratio, and GC-MS aromatics.It has the highest API gravity, the highest gas andgasoline precursors (64.9 wt %) in terms of saturatesand monoaromatics, and the lowest LCO precursors(14.9 wt %), defined as the summation of diaromatics,2-ring aromatic sulfur, and 1/2 of the 3-ring aromaticsulfur. Thus, as far as catalytic cracking is concerned,HT-DA is considered the most reactive feed. The secondfeed, VIR, is the only nonhydrotreated feed and is veryhigh in sulfur (3.25 wt %) and aromatics. It has thelowest aniline point, H/C atomic ratio and saturates, buthas the highest aromatic carbon and GC-MS aromatics.It also has the lowest API gravity, the lowest gas andgasoline precursors, and the highest LCO precursors.

(2) Wallenstein, D.; Haas, A.; Harding, R. H. Appl. Catal., A 2000,203, 23-36.

(3) Ng, S. H. Energy Fuels 1995, 9, 216-224.(4) Scherzer, J. Correlation between Catalyst Formulation and

Catalytic Properties. In Fluid Catalytic Cracking: Science and Tech-nology; Magee, J. S., Mitchell, M. M., Jr., Eds.; Studies in SurfaceScience and Catalysis 76; Elsevier Science Publishers B. V.: Amster-dam, 1993; pp 145-182.

(5) Wachtel, S. J.; Baillie, L. A.; Foster, R. L.; Jacobs, H. E. Oil GasJ. 1972 (Apr. 10), 104-107.

Canadian Oil-Sands Derived VGOs Energy & Fuels, Vol. 16, No. 5, 2002 1197

However, its total nitrogen and CCR (0.33 wt %) arethe lowest. Based on the overall quality, VIR is consid-ered the poorest feed. The final feed, HTC, is a sweetVGO with the lowest sulfur (0.43 wt %). Its quality isslightly inferior to that of HT-DA, but is much superiorto that of VIR.

3.2. Catalyst Properties. Table 2 gives the proper-ties of two equilibrium catalysts, CAT-A and HRO.Compared with the latter, CAT-A is characterized by:

• lower unit cell size (24.28 Å, versus 24.35 Å forHRO);

• lower rare earth content on zeolite (7.8 versus 17.5wt % of HRO). The presence of rare earth in a zeoliteincreases its stability and activity. The improved stabil-ity is due to the formation of polynuclear rare-earth-containing hydroxy complexes in the zeolite sodalitecages, whereas the improved activity results from thehigher number of acid sites through the partial hydroly-sis of hydrated rare earth ions [RE(OH2)3+ f REOH2+

+ H+].4 In association with these, rare earth impedeszeolite dealumination during hydrothermal treatment,rendering a higher unit cell size.

• higher zeolite content and higher zeolite surfacearea, but lower matrix surface area with a much higherzeolite/matrix (Z/M) ratio (2.00 versus 0.78 of HRO),although the total surface areas of the two catalysts arealmost the same.

• lower pore volume (0.31 versus 0.45 mL/g of HRO,by water absorption). Our study (Figure 1) indicates alinear correlation between the pore volume by waterabsorption and that by mercury intrusion, which mea-sures essentially the larger pores in the matrix, typicallyover 200 Å.6 The correlation gives a correspondingmercury intrusion volume of 0.37 mL/g for HRO. Com-

bining this value with the prorated pore area of 101 m2/g(61 × 50/83) yields 147 Å average pore diameter (4 ×0.37 × 108/101)for HRO. This matrix opening is sub-stantially larger than that of CAT-A (109 Å), providinggreater access to heavy molecules that are to be pre-cracked.

• relatively high silica (55.7 wt %) and low alumina(39.7 wt %) contents compared with 49.7 wt % Al2O3for HRO. The bulk silica/alumina ratio in a catalystcannot correlate with the Si/Al ratio in the zeolite, whichis an important parameter that reflects the zeolitequality.

The Z/M ratios, unit cell sizes, and rare earth contentsof the two catalysts suggest that CAT-A and HRObelong to groups E and B, respectively, according to theclassification system by Scherzer4 for FCC catalysts.According to this classification, catalysts in group E aredesigned to maximize octane-barrels. They consist ofzeolites with medium unit cell sizes and matrixes ofmedium activities. These catalysts have a moderateability to crack bottoms and give moderate yields ofC3+C4 hydrocarbons. Group B catalysts, compared withthose in group E, consist of zeolites with a larger unitcell size, higher rare earth contents, and an activematrix. These catalysts are good for cracking bottomsor resids. In general, they also have good gasolineselectivity, generate less C3+C4 hydrocarbons and morecoke.

Other than the quantity and quality of zeolite andactive matrix, both of which determine the activity andselectivity of a catalyst, the additives and metals in the

(6) Peters, A. W. Instrumental Methods of FCC Catalyst Charac-terization. In Fluid Catalytic Cracking: Science and Technology;Magee, J. S., Mitchell, M. M., Jr., Eds.; Studies in Surface Science andCatalysis 76; Elsevier Science Publishers B. V.: Amsterdam, 1993; p193.

Table 1. Feedstock Properties

feed HTC HT-DA VIR

density at 15.6 °C, g/mL 0.9511 0.9430 0.9712API gravity, degree 17.3 18.6 14.2refractive index at 20 °C 1.5323 1.5269 1.5397sulfur, wt % 0.43 0.70 3.25total nitrogen, wppm 2150 2450 1930basic nitrogen, wppm 439 613 610hydrogen, wt % 11.5 11.8 11.1carbon, wt % 87.8 87.1 85.1H/C atomic ratio 1.562 1.619 1.549Conradson carbon residue, wt % 0.50 2.00 0.33Ni, wppm 4.1 0.0V, wppm 6.8 0.1aniline point, °C 63.6 81.0 50.8aromatic carbon, % 24.7 20.9 25.4343 °C- by simdist, wt % 4.5 1.5 6.0524 °C+ by simdist, wt % 8.3 36.9 2.0

GC-MS Analysis, wt %saturates 34.4 35.4 28.7

paraffins 4.7 5.0 1.8cycloparaffins 29.7 30.4 26.9

aromatics 61.8 57.5 65.6mono- 29.5 29.5 22.5di- 13.6 12.7 14.4tri- 6.3 5.4 7.2tetra- and up 8.2 6.5 10.8aromatic sulfur

2-ring compounds 0.7 1.2 5.03-ring compounds 3.0 2.0 4.64-ring compounds 0.5 0.2 1.1

polar compounds 3.8 7.1 5.7gas and gasoline precursorsa 63.9 64.9 51.2light cycle oil (LCO) precursorsb 15.8 14.9 21.7

a Saturates + monoaromatics. b Diaromatics + 2-ring aromaticsulfur + [(1/2)(3-ring aromatic sulfur)].

Table 2. Equilibrium Catalyst Properties

catalyst HRO 610 CAT-A

X-ray diffractionunit cell size (fresh), Å 24.66 n/aa

unit cell size (equilibrium), Å 24.35 24.28zeolite content, wt % n/aa 13

nitrogen adsorption-desorptiontotal surface area, m2/g 148 150zeolite surface area, m2/g 65 100matrix surface area, m2/g 83 50zeolite/matrix (Z/M) 0.78 2.00micropore volume (mL/g) n/aa 0.05zeolite content, wt % 10.6 15.6

Hg porosimetrypore volume, mL/g 0.37b 0.22pore area, m2/g 101b 61av pore diameter, Å 147b 109

water absorptionpore volume, mL/g 0.45 0.31

average particle size, microns 67.9 79.5SiO2 wt % n/aa 55.7Al2O3, wt % 49.7 39.7Re2O3, wt % (on catalyst) 1.85 1.21Re2O3, wt % (on zeolite) 17.5 7.8Na2O, wt % 0.31 0.19TiO2, wt % n/aa 1.62Fe2O3, wt % 0.66 0.67CaO, wt % n/aa 0.09MgO, wt % n/aa 0.17P2O5, wt % n/aa 0.40SO4, wt % n/aa 0.01Ni, wppm 242 291V, wppm 434 314Cu, wppm 20 n/aa

a n/a ) not available. b Estimated value.

1198 Energy & Fuels, Vol. 16, No. 5, 2002 Ng et al.

catalyst can also affect the cracking performance. X-rayanalysis suggests that CAT-A, as received, may containsome ZSM-5, a shape-selective zeolite. Conventionally,ZSM-5 is added to increase octanes through preferentialcracking of low-octane, straight-chain olefins and paraf-fins (mostly C7+) in gasoline to smaller compounds,preferentially olefins, and by isomerization of the low-octane linear olefins to branched and high-octane ole-fins. As a result, gasoline has higher octane numbersbut its yield is lower, whereas yields of C6- increasewith higher iso-to-normal ratios for aliphatics of thesame carbon number.

Although HRO underwent considerable hydrothermaldeactivation, as reflected by a drop of unit cell size from24.66 to 24.35 Å, the two catalysts were not seriouslycontaminated in the refinery FCC operation. The de-hydrogenation activity of nickel, vanadium, copper, andiron, expressed as equivalent nickel (Ni + V/5 + Cu +Fe/10) is 809 wppm for HRO and 824 wppm (excludingCu) for CAT-A. It is not clear if the relatively high

concentrations of TiO2 and P2O5 in CAT-A are indica-tions of the presence of a metal trap or passivator.

3.3. Conversion and Yield Structure. In thisstudy, since the reaction temperature, the catalystweight, and the catalyst contact time are all fixed for agiven feed, the only variable is the catalyst-to-oil ratio(C/O), which affects the weight hourly space velocity(WHSV) through the relationship WHSV ) 3600/[(C/O)t], where t is the catalyst contact time in seconds.Cracking results are graphically presented in Figures2-9. Key observations are given below.

3.3.1. Comparison of the Performance Between ACEand Fluid-Bed MAT. Figure 2 compares the coversionsof HT-DA obtained from two reaction systems, NCUT’sconventional and ACE proprietary, both using fixedfluid-bed reactors but with different operating protocols.Typical ACE testing conditions have been reportedelsewhere.2 Figure 2 shows that for HT-DA cracked withthree catalysts, the two systems gave essentially thesame conversion profiles, though the absolute conver-

Figure 1. Linear correlation of mercury intrusion volume with water absorption volume for catalysts.

Figure 2. Comparison of catalyst activities between two reactor systems (conventional fluid-bed and ACE) for HT-DA feed andthree catalysts.


sion levels tended to be different at a given C/O ratio.Yield profiles (product yields versus conversion) fromthe two systems for all feeds were also similar between

HRO and CAT-A, except for coke yields. In this case,the ACE reactor gave reverse trends in coke yieldrelative to NCUTs results for the two catalysts. We are

Figure 3. Relationship between conversion and catalyst-to-oil ratio.

Figure 4. Relationship between dry gas yield and conversion.

Figure 5. Relationship between LPG yield and conversion.


in the process of having these reverse trends confirmedby another ACE reactor. Note that in Figure 2 anequilibrium catalyst Cobra 54 from Akzo Nobel wasincluded for comparison purposes.

3.3.2. Conversion. Figure 3 illustrates the increasesin conversion with C/O ratio for all feeds and catalysts,based on the fluid-bed MAT results. At a given C/Oratio, and for the same catalyst, HT-DA gave the highestconversion, followed by HTC (about 8 wt % less), whichgave slightly higher or similar conversion than VIR.This trend agrees with the feed quality in general. At agiven C/O ratio, and for the same feed, HRO gave 3-8wt % higher conversion than CAT-A, depending on theC/O ratio and the feed type. This is to be expected, basedon the differences in their matrix characteristics, rareearth contents, and unit cell sizes.4 For the two hy-drotreated feeds, the paired conversion profiles from thetwo catalysts tended to converge at high C/O ratio, butfor VIR the paired profiles remained far apart through-out. This suggests that the lower activity of CAT-A couldbe compensated for by higher C/O ratios (more catalystper unit weight of feed) to effect cracking or to overcome

some deleterious effects of feedstock on catalyst (e.g.,nitrogen poisoning). The compensation effect was lesspronounced for VIR due to its lower nitrogen contentand its more refractory nature in terms of having morelarge aromatic molecules (two and higher ring aromat-ics), as indicated in Table 1.

3.3.3. Dry Gas. Dry gas (H2, H2S, and C1-C2 hydro-carbons) is a low value product that should be kept to aminimum. Excessive dry gas production may causelimitations in the plant operation in terms of gascompression. Figure 4 shows that yields of dry gasincreased with conversion for all feeds and catalysts.Components in the dry gas are secondary products fromthermal cracking and catalytic cracking of gasoline andbutenes.7 These cracking reactions are mostly nonselec-tive, resulting in yield profiles parallel to each other.Being an end product, similar to coke, dry gas shouldexhibit an exponential increase in yield at higherconversions. Figure 4 shows that for the same catalystand at a given conversion, VIR produced 0.7 to 1 wt %

(7) John, T. M.; Wojciechowski, B. W. J. Catal. 1975, 37, 348-357.

Figure 6. Relationship between gasoline yield and conversion.

Figure 7. Relationship between LCO yield and conversion.


more dry gas than HT-DA, which gave a higher (by 0.3wt %) or similar yield than HTC, depending on thecatalyst used. This was not surprising since VIR con-tained much higher sulfur (3.25 wt %) than HT-DA (0.7wt % S) and HTC (0.43 wt % S), while about 40 wt %feed sulfur appeared in the reactor gas as H2S.8 Table3 shows the yields of dry gas, hydrogen, and hydrogensulfide at 65 wt % conversion. Evidently, H2S waspredominant in dry gas for VIR but not for the othertwo feeds, which were hydrotreated leaving mostly thehard-to-remove aromatic sulfur species, the majority ofwhich were retained in cycle oils and coke after crack-

ing. For HT-DA and HTC, their dry gas yields, relativeto each other, should be determined by reaction tem-perature (20 °C higher for HT-DA) and feed qualityincluding sulfur content and the tendency of cokeformation, which could affect the nonselective crackingthrough pore blockage of catalysts. As a result, HT-DAgave 0.3 wt % higher dry gas yield than HTC whenCAT-A was used. The effects were not pronounced inthe case of HRO. This was probably related to CAT-A’shigher gas-forming tendency, which will be discussedlater.

Table 3 also indicates that for the same feed, HROproduced more H2S at 65 wt % conversion than CAT-A,although the latter gave higher dry gas and H2 yields.The higher yield of dry gas (including H2) by CAT-Acould be attributed partly to its more isolated acid sites,resulting from a smaller zeolite cell constant, whichenhanced gas make.9 Higher hydrogen yield also couldbe attributed to a higher level of metal contaminants,such as nickel, which is known as a strong dehydroge-

(8) Keyworth, D. A.; Reid, T. A.; Asim, M. Y.; Gilman, R. H. 1992NPRA Annual Meeting, AM-92-17. National Petrochemical & RefinersAssociation: Washington, DC, 1992.

(9) Pine, L. A.; Maher, P. J.; Wachter, W. A. J. Catal. 1984, 85, 466-476.

Figure 8. Relationship between HCO yield and conversion.

Figure 9. Relationship between coke yield and conversion.

Table 3. Dry Gas, Hydrogen, and Hydrogen SulfideYields at 65 wt % Conversion

HRO 610 CAT-A

HTC HT-DA VIR HTC HT-DA VIR

hydrogen, wt % 0.13 0.16 0.13 0.16 0.19 0.16hydrogen sulfide, wt % 0.02 0.09 1.00 0.02 0.06 0.77dry gas, wt % 1.89 1.82 2.84 2.22 2.45 3.08


nation catalyst. This was aggravated by the fact thatCAT-A, with lower activity, required much higher C/Oratio to achieve the same conversion as obtained byHRO (Figure 2). However, since both catalysts containrelatively low Ni and V, the dehydrogenation activitiesinvolved may not be very strong. As indicated in Table3, CAT-A produced more H2, which could have saturatedsome of the aromatic sulfur species before they werecracked to form H2S.10 However, the resulting H2S yieldfrom CAT-A was less. It is possible that the higher H2Syield by HRO was due to its higher congested acid sites,which promote hydrogen transfer8 to remove sulfur.

3.3.4. LPG. In FCC operation, LPG (C3+C4 hydrocar-bons) is considered a valuable product since it consistsof components that can be used as alkylation andpetrochemical feedstocks. For example, propylene iswidely used in the polymer industry; isobutylene is thebuilding block for methyltertiarybutyl ether (MTBE)and ethyltertiarybutyl ether (ETBE), the octane boost-ers; and isobutane can be alkylated with C3 to C5 olefinsto form high-octane branched compounds that makeup11% of the gasoline pool in US refineries.11

The slightly concave yield profiles of LPG in Figure5 indicate that, as conversion increased, the combinedcracking rate of gas oil feed and gasoline to form LPGwas marginally higher than its conversion rate to formdry gas and coke. Among the three feeds, HT-DA (thelightest in terms of density), being cracked at a highertemperature, gave about 1 wt % more LPG than theother two feeds when CAT-A was used, but showed littledifference in the case of HRO. The higher discriminationon LPG yields by CAT-A was probably related to itstendency to produce more gas. Between the two cata-lysts, CAT-A gave a higher LPG yield than HRO by 3-4wt % for the same feed. As LPG is an important FCC

product, it is worthwhile to examine the detaileddistribution in LPG produced. Table 4 gives a break-down of LPG for all feeds and catalysts at 65 wt %conversion. The following comments are offered.

•Propylene and isobutane are the two most prominentsingle compounds. Propylene is a product from primarycracking of a gas oil feed. It can also be formed fromsecondary cracking of gasoline and butene.7 The abun-dance of isobutane, a secondary product, is a result ofhydrogen transfer after the prevailing olefin crackingand isomerization.

•All individual yields increased when the catalyst wasswitched from HRO to CAT-A, with higher increases inparaffins than in olefins for the same feed, possibly dueto a richer hydrogen environment. As a result, olefinici-ties of C3, C4, and LPG, and hydrogen transfer indices(defined as isobutene-to-isobutane ratio12) were lowerwhen CAT-A was in use. Relative to HTC, HT-DA andVIR showed higher and equivalent selectivities, respec-tively, for each LPG component, except for i-butene.

•When CAT-A was used, the iso-to-normal ratios ofbutenes were higher and the increase in propylene wasapproximately twice as much as the increase in butenes,as compared with the case of HRO. These observationssupported the X-ray analysis that CAT-A might containthe shape selective catalyst ZSM-5,13 which was par-tially responsible for the increases in LPG and dry gas,including H2. It is not known at this stage why higheriso-to-normal ratios of butanes from the same feed, ascompared with the case of HRO, were not observedwhen CAT-A was used.

3.3.5. Gasoline. Gasoline (C5-221 °C boiling point) isthe major and the most desirable product in FCC

(10) Myrstad, T.; Engan, H.; Seljestokken, B.; Rytter, E. Appl. Catal.,A 1999, 187, 207-212.

(11) Chapin, L. E.; Liolios, G. C.; Robertson, T. M. HydrocarbonProcess. 1985, (Sept.), 67-71.

(12) McClung, R. G.; Dodwell, G. In Proceedings of the EngelhardFCC Seminar; Venice, Italy, June 11-13, 1997.

(13) Dwyer, F. G.; Degnan, T. F. Shape Selectivity in CatalyticCracking. In Fluid Catalytic Cracking: Science and Technology; Magee,J. S., Mitchell, M. M., Jr., Eds.; Studies in Surface Science andCatalysis 76; ; Elsevier Science Publishers B. V.: Amsterdam, 1993;pp 499-530.

Table 4. LPG Composition and Key Parameters in LPG at 65 wt % Conversion

HRO CAT-A CAT-A/HRO (same feed) CAT-A/HRO (relative to HTC)

HTC HT-DA VIR HTC HT-DA VIR HTC HT-DA VIR HTC/HTCHT-DA/

HTCVIR/HTC

yield, wt %propane 0.8 0.69 0.91 1.58 1.42 1.64 1.78 2.08 1.80 1.00 1.17 1.01propylene 2.49 2.61 2.58 3.00 3.62 3.00 1.21 1.38 1.16 1.00 1.15 0.97n-butane 0.59 0.50 0.54 0.93 0.82 0.86 1.58 1.63 1.60 1.00 1.03 1.01i-butane 2.93 1.98 2.75 4.05 3.33 3.82 1.38 1.69 1.39 1.00 1.22 1.01n-butenes 1.96 2.39 1.92 2.09 2.59 1.95 1.07 1.08 1.02 1.00 1.01 0.95i-butene 0.58 1.00 0.73 0.78 1.25 0.88 1.33 1.25 1.20 1.00 0.93 0.90LPG 9.44 9.17 9.43 12.43 13.03 12.15 1.32 1.42 1.29 1.00 1.08 0.98

olefinicity,a g/gC3 0.74 0.79 0.74 0.66 0.72 0.65 0.89 0.91 0.88C4 0.42 0.58 0.45 0.37 0.48 0.38 0.87 0.83 0.84LPG 0.53 0.65 0.55 0.47 0.57 0.48 0.89 0.87 0.87

hydrogen transferindexb

0.20 0.51 0.27 0.19 0.37 0.23 0.96 0.74 0.87

iso-to-normalratio, g/gi-butane/n-butane

4.96 3.94 5.07 4.34 4.09 4.42 0.87 1.04 0.87

i-butene/n-butenes

0.30 0.42 0.38 0.37 0.48 0.45 1.25 1.15 1.18

increase in C3d/increase in C4d(relative to HRO), g/g

1.00 1.00 1.00 1.54 2.24 2.31

a (olefin in a fraction)/(total compounds in the same fraction). b i-C4)/i-C4.


operation. Figure 6 depicts the gasoline yield profilesfor all feeds and catalysts. Overcracking was observedfor HT-DA at about 70 wt % conversion, and wasexpected for HTC and VIR at lower conversions sincethey contained more aromatics. HTC had 1.5-2 wt %higher gasoline yield than VIR over the entire range ofconversion in this study. HT-DA showed lower gasolineselectivity than HTC below 58-61 wt % conversion, buthigher, up to 3 wt % yield or equivalent magnitude,thereafter, depending on the catalyst used. This cross-over was related to the higher nitrogen and CCRcontents of HT-DA, which seriously poisoned the cata-lyst at low conversions. As C/O ratio increased, resultingin higher conversion and less catalyst poisoning, gaso-line yield of HT-DA increased sharply and eventuallyexceeded that of HTC. HT-DA could have had evenhigher gasoline yield if it were cracked at a lowertemperature. The striking feature of Figure 6 is thatHRO outperformed CAT-A by 4-6 wt % in gasoline forthe same feed at a given conversion. This could beattributed to the following factors.

•Compared with CAT-A, HRO was 35% higher in rareearth, which reduced aluminum loss from the frame-work of zeolite under hydrothermal conditions. Thisenabled HRO to maintain more acid sites, resulting inincreased activity, and a higher hydrogen transfer rate;4the net result: more gasoline produced, higher conver-sion, but also higher coke yield.

• The active matrix in HRO, with large pores, playeda significant role in precracking the big molecules intosmaller fragments, which then gained access to acidsites in zeolite pores for further cracking. When CAT-Awas used, the gasoline selectivity of HT-DA was lowerprobably due to the added ZSM-5, which selectivelycracked the linear or singly branched C7+ paraffins andolefins in gasoline into smaller olefins and isoparaffins,some of which fell into LPG range.13 This is to bediscussed later in more detail.

Tables 5 and 6 summarize yield distributions deter-mined from PIONA analysis for normal- and isoalkanes

and alkenes, in gasoline at 65 wt % conversion for allfeeds and catalysts. In general, for the same feed andcatalyst, the yields gradually decreased at higher carbonnumber due to cracking. However, there were, at times,abrupt drops in C6 yields as the unidentified C6+components in the vapor phase were not included in thecalculations, whereas the gaseous C5 components were.In comparison with HRO, Table 5 generally indicatesthat for the same feed, CAT-A produced:

•more i-C5, n-C5, and i-C6, but probably not n-C6;•less C7 to C10 alkanes for the same carbon number

and for the same type (n- or isoalkane);• higher iso-to-normal ratios for C5 to C6 alkanes, but

lower ratios for C7 to C10 compounds, for the samecarbon number and for the same type.

Slight differences were noticed for alkenes in Table6. Compared with HRO, for the same feed and for thesame carbon number, CAT-A produced:

•more i-C5d to i-C7d iso-alkenes;•less i-C8d to i-C10d iso-alkenes;•less n-C5d to n-C10d n-alkenes;•higher iso-to-normal ratios for C5 to C9 alkenes, but

lower ratios for C10d.The above observations are believed to be a conse-

quence of the shape-selective ZSM-5 in CAT-A. Although

Table 5. Yields of n- and iso-Alkanes in Gasoline at 65 wt% Conversion

HTC HT-DA VIR

HRO CAT-A HRO CAT-A HRO CAT-A

C5i-C5 2.63 3.63 2.06 2.94 2.60 3.11n-C5 0.21 0.29 0.25 0.30 0.24 0.25i-C5/n-C5 12.59 12.35 8.25 9.72 10.97 12.35

C6i-C6 0.85 1.16 1.24 1.29 0.81 0.69n-C6 0.08 0.09 0.15 0.13 0.08 0.06i-C6/n-C6 10.39 12.99 8.46 9.56 10.58 11.01

C7i-C7 1.78 1.05 2.70 1.11 1.59 0.74n-C7 0.11 0.07 0.30 0.12 0.10 0.06i-C7/n-C7 16.64 14.42 8.90 8.92 15.23 12.52

C8i-C8 1.30 0.85 1.58 0.87 1.11 0.64n-C8 0.09 0.06 0.15 0.10 0.10 0.06i-C8/n-C8 13.91 14.14 10.32 9.12 11.55 10.80

C9i-C9 1.04 0.57 1.11 0.70 0.83 0.51n-C9 0.08 0.05 0.13 0.08 0.07 0.05i-C9/n-C9 12.33 11.97 8.85 8.79 11.12 9.50

C10i-C10 0.81 0.45 0.87 0.51 0.61 0.28n-C10 0.07 0.05 0.11 0.07 0.06 0.06i-C10/n-C10 11.88 9.86 8.29 7.29 10.20 4.40

Table 6. Yields of n- and iso-Alkenes in Gasoline at 65 wt% Conversion

HTC HT-DA VIR

HRO CAT-A HRO CAT-A HRO CAT-A

C5di-C5d 0.68 0.82 1.29 1.29 0.93 0.82n-C5d 0.71 0.67 1.14 0.90 0.77 0.61i-C5d/n-C5d 0.95 1.23 1.14 1.44 1.20 1.33

C6di-C6d 0.41 0.46 1.05 0.89 0.22 0.42n-C6d 0.23 0.20 0.61 0.39 0.27 0.18i-C6d/n-C6d 1.76 2.33 1.71 2.30 0.84 2.39

C7di-C7d 0.16 0.39 0.15 0.70 0.22 0.42n-C7d 0.21 0.12 0.39 0.21 0.23 0.13i-C7d/n-C7d 0.76 3.18 0.38 3.25 0.97 3.36

C8di-C8d 0.46 0.30 0.84 0.52 0.50 0.34n-C8d 0.14 0.07 0.32 0.12 0.14 0.08i-C8d/n-C8d 3.26 4.09 2.63 4.19 3.49 4.32

C9di-C9d 0.37 0.19 0.73 0.34 0.36 0.22n-C9d 0.08 0.04 0.18 0.07 0.08 0.04i-C9d/n-C9d 4.44 4.87 3.98 4.71 4.50 5.41

C10di-C10d 0.26 0.11 0.48 0.20 0.22 0.13n-C10d 0.04 0.02 0.09 0.04 0.03 0.02i-C10d/n-C10d 6.00 4.57 5.50 4.78 6.79 5.15

Table 7. Comparison of Actual Product Yields at 70 wt %Conversion with Precursor Concentrations

HRO CAT-A

HTC HT-DA VIR HTC HT-DA VIR

dry gas, wt % 2.2 2.2 3.2 2.5 2.8 3.5LPG, wt % 11.2 11.2 11.2 14.0 14.9 13.9gasoline, wt % 46.8 49.4 45.8 43.2 43.2 42.0total gas-plus-

gasoline, wt %60.2 62.8 60.2 59.7 60.9 59.4

gas-plus-gasolineprecursors, wt %

63.9 64.9 51.2 63.9 64.9 51.2

LCO, wt % 19.8 19.4 21.2 17.5 16.7 18.6LCO precursors,

wt %15.8 14.9 21.7 15.8 14.9 21.7

polars 3.8 7.1 5.7 3.8 7.1 5.7


CAT-A and HRO are of different bases, which lead toindividual product slates, the observed phenomenaagree with the reported findings for ZSM-5 in theliterature,13 namely, that ZSM-5 selectively cracks thelinear and monomethyl C7+ olefins and, less preferably,paraffins. Some of the resulting olefins can undergofacile catalytic isomerization, or form paraffins throughhydrogen transfer. The facile isomerization leads tohigher iso-to-normal ratios for the cracked olefins andparaffins. As a result, there is a net decrease in theamount of low-octane C7+ olefins and a correspondingincrease in the amount of high-octane C5 and C6compounds.

Table 7 compares the actual yields of gas-plus-gasoline and LCO (to be discussed later), at 70 wt %conversion, against their corresponding precursor con-centrations, based on hydrocarbon types by GC-MS(Table 1). It is assumed that saturates and monoaro-matics are the precursors that will predominantlycontribute to the production of gasoline and its derivedproducts,14 including dry gas, LPG, and some catalyticcoke, whereas diaromatics, 2-ring aromatic sulfur, andone-half of the 3-ring aromatic sulfur, are precursors ofLCO. The precursor concentration sets an upper limitfor gas-plus-gasoline yield, or a lower limit for LCOyield, as a guide. Beyond this limit, the feed tends toproduce excessive gas and coke rather than the valuableliquid products. Although it is a useful concept todetermine precursor concentrations based on hydrocar-bon types, accurate assignment of the components tothe precursor categories can be challenging. This isbecause the analysis by MS for the hydrocarbon type(CnH2n+z) is based on the hydrogen deficiency in themolecule and the results are reported based on the znumbers. For example, simple naphthenobenzenes suchas C10H12 (z ) -8), classified by MS as monoaromatics,can undergo hydrogen transfer to yield LCO rather thangasoline, and thus provide an overestimate of the gas-plus-gasoline yield. Likewise, fluorenes such as diphe-nylenemethane C13H10 (z ) -16), classified as diaro-matics, can be cracked into a gasoline fraction under

proper conditions, since the two phenyl groups areconnected through single carbon-carbon bonds that arecrackable. This will provide an overestimate of the LCOyield. Further, polars, mostly aromatics that containheteroatoms, and part of the coke yield, are not includedin the precursor formulas. These will contribute moreuncertainties to precursor concentrations. Nevertheless,compositional analysis by MS is generally used as aguide to predict the maximum or minimum yield of acracked fraction. There may be a special challenge forgas oils derived from heavy oils and bitumens, whichcontain substantial amounts of naphthenoaromaticcompounds. Table 7 shows that at 70 wt % conversion,where maximum gasoline yields occurred for HT-DA,the differences between actual and predicted maximumgas-plus-gasoline yields were within 4.5 wt %, exceptfor those of VIR, which gave abnormally high actualyields. Increasing conversion will further narrow thegaps between the actual and predicted yields in somecases (Figures 4-7).

3.3.6. LCO. LCO (221-343 °C boiling point) is aproduct whose value depends on the location of therefinery. In some countries where gasoline is not in highdemand, the FCC unit is used as a major producer ofmiddle distillate because of its low capital cost (one-fourth of that of a hydrocracker). With proper designand operation, an FCC unit can produce 40-45 wt %LCO,15 which can be upgraded to no. 1 (diesel quality)and no. 2 fuel oils that generally have a lower initialboiling point (∼182 °C). In Europe and the U.S., manyrefiners seek seasonal means of increasing LCO yieldsto meet higher fuel oil demands in the winter. In thewestern U.S. and Canada, LCO has less market valuebecause of warmer weather and/or the popular use ofnatural gas for home heating. Thus, LCO is not a highlydesirable product, especially when derived from aromat-ics-rich feeds. The cetane number of LCO from FCC isgenerally low (less than 30) and its sulfur content ishigh. The lower the LCO yield, the poorer its qualitydue to the enrichment of aromatics at high conversion.

(14) Fisher, I. P. Appl. Catal. 1990, 65, 189-210.(15) Ritter, R. E. 1988 NPRA Annual Meeting, AM-88-57. National

Petrochemical & Refiners Association: Washington, DC, 1988.

Figure 10. Linear relationship between coke yield and C/O ratio.


In this study, all feeds contain less than 6 wt % heavyfraction, which boils in the LCO range (Table 1). Onecan imagine that as conversion increases, the LCO yieldshould increase initially and then decrease when therate of its formation (from cracking of the 343 °C+fraction) is exceeded by that of its decomposition intolighter fractions. Figure 7 shows the general parabolicdecreases in LCO yields with increased conversion. Intheory, all yield curves should converge and reach zeroat 100 wt % conversion. In general, VIR gave relativelyhigher LCO selectivity than HTC, which in turn wasslightly higher in LCO yield than HT-DA. This is in linewith LCO precursor concentrations of 21.7, 15.8, and14.9 wt %, for VIR, HTC, and HT-DA, respectively.Table 7 shows that at 70 wt % conversion, the differ-ences between actual and predicted minimum LCOyields are within 4.5 wt %. Between the two catalysts,HRO gave higher LCO selectivity than its counterpartfor the same feed. This is related to HRO’s large poremartrix, which also contains acid sites usually associ-ated with aluminum atoms. The matrix in HRO isbelieved to have higher activity than that in CAT-A, tofacilitate deeper bottoms cracking that leads to lowerHCO but higher LCO and gasoline yields.

3.3.7. HCO. HCO (343-525 °C boiling point) is anunwanted, unconverted product with comparativelyhigh aromatics and sulfur. Its yield should be reducedto the minimum. Figure 8 shows the monotonic de-creases in HCO yield as conversion increased. The yieldmagnitude of HCO for all feeds and catalysts at a givenconversion should be in the reverse order to that of LCO,based on the simple relationship HCO ) 100 - conver-sion - LCO. Figure 8 indicates that HT-DA gaveslightly higher HCO selectivity than HTC, which in turnwas higher in HCO yield than VIR. Between the twocatalysts, HRO gave lower HCO selectivity than itscounterpart for the same feed. Similar to LCO, all HCOyield curves should converge and reach zero at 100 wt% conversion.

3.3.8. Coke. In FCC operation, coke is necessary inorder to supply heat for feed preheat and cracking in amuch more economical and efficient fashion than usingalternative liquid fuels such as torch oil or refineryfuel.16 However, too much coke can overload the airblower during catalyst regeneration, cause excessivelyhigh temperature in the regenerator, and seriouslypoison the catalyst. Figure 9 shows the exponentialincrease in coke yield with conversion. In general, forthe same catalyst, VIR had the highest coke selectivity,followed by HTC and HT-DA, for which the coke yieldcould be higher if it were cracked at a lower tempera-ture. Between the two catalysts, CAT-A gave 0.5-1.7wt % more coke than HRO for the same feed, dependingon the feed type and conversion. In this respect, pairsof yield curves for the same feed showed an irregularshape with smaller differences at both ends. Thisimplies that HRO, for which the lines have greaterslopes at high conversions, is more sensitive to conver-sion in coke formation than CAT-A. Under the currenttest conditions, coke yield can be represented in analternative format that gives a linear relationshipbetween coke yield and C/O ratio (Figure 10), as shownbelow.

where catalytic coke is generated from the cracking thatoccurs at the acid sites of the zeolite and matrix; feedresidue coke4 is contributed by carbon residue in thefeed; contaminant coke4 derives from catalytic dehydro-genation of metal poisons such as Ni, V, Fe, and Cu; Aand n are empirical constants which are catalyst-and-feedstock dependent in Voorhie’s equation;17 and t is thecatalyst contact time, which is 30 s in this study.

Figure 10 shows that, for the same catalyst and at agiven C/O ratio, HT-DA, a more reactive feed withhigher CCR, produced more coke, followed by HTC andVIR. Between the two catalysts, HRO with higheractivity gave a higher coke yield than CAT-A. Moreover,pairs of straight lines representing the two catalysts forthe same feed tended to intersect the y-axis (C/O ) 0)at the same point, i.e., 1.25, 3.02, and 1.27 wt % cokefor HTC, HT-DA, and VIR, respectively. These were0.8-1.0 wt % higher than their corresponding CCRvalues of 0.50, 2.00, and 0.33 wt %. Thus, it is believedthat the coke yield in this study could be about 1 wt %higher than the expected value. It is possible that someheavy product (HCO) was not recovered at the “cold”spot near the exit of the reactor, but rather was pickedup later as coke at a higher reactor temperature (600°C) during catalyst in situ regeneration. Again, inFigure 10 HRO showed a greater slope than CAT-A forthe same feed. The lower coke-making tendency ofCAT-A (containing USY zeolite) observed in Figure 10can be related to its smaller unit cell size as reportedin the literature18 (also observed in our ealier work19)and the improved matrix technology, as claimed by themanufacturer. However, the lower coke-making ten-dency of CAT-A is offset by its much lower conversioncompared with that by HRO (at the same C/O ratio), ofwhich the wide pore matrix facilitates effective crackingof large molecules of heavy gas oils. It is thus believedthat the coke-making tendency of a Y zeolite is betterjudged from a “coke yeild-C/O ratio” plot rather thanfrom a “coke yield-conversiobtn” plot, where the conver-sion can be influenced by other catalyst properties, e.g.,matrix type, matrix pore size, zeolite/matrix (Z/M) ratio,and rare earth content. It should be noted that theshapes of the yield curves in Figure 9 are a reflectionof the results in Figure 3 and Figure 10.

In FCC, delta coke (the amount of coke per unitweight of spent catalyst) is an important parameter, themagnitude of which affects the heat balance. Delta coke

(16) Blazek, J. Catalagram 1987, 75, 1-3.

(17) Voorhies, A. Ind. Eng. Chem. 1945, 37, 318-322.(18) Rajagopalan, K.; Peters, A. W. J. Catal. 1987, 106, 410-416.(19) Ng, S. H.; Rahimi, P. M. Energy Fuels, 1991, 5, 595-601.

coke (wt %) ) catalytic coke +(feed residue coke + contaminant coke)

catalytic coke ) (g coke formed/g feed)100

) {[(coke in catalyst (wt %)) ×(0.01)(g catalyst)]/(g feed)}100

) {Atn}(g catalyst/g feed)

) (constant)(C/O ratio)

coke (wt %) ) [(constant) ×(C/O ratio)] + constant


is calculated from coke yield divided by the C/O ratio.Figure 11 shows the linear decline of delta coke withconversion, for all feeds and catalysts. This is under-standable since higher C/O ratio or less feed per unitweight of catalyst is necessary to achieve higher conver-sion. Figure 11, similar to Figure 10, indicates that HT-DA gave the highest delta coke, followed by HTC andVIR, for the same catalyst and at a given conversion.Between the two catalysts, HRO produced more deltacoke than CAT-A for the same feed at a given conver-sion.

3.4. Comparison of Catalyst Performance onCommon Bases. To assess catalyst cracking perfor-mance, MAT results are compared at both constantconversion and constant coke yield. Table 8 comparesproduct yields at 65 wt % conversion where gasolineyields are close to maximum values. In comparison withHRO and for the same feed, CAT-A produced more drygas, LPG, HCO, and coke, but much less gasoline andLCO, and slightly less delta coke. Further, to achievethis conversion, much higher severities in terms of C/O

ratios were necessary for CAT-A. Thus, for producingmore premium products, it may be preferable to useHRO over CAT-A, although the latter may result in abetter quality gasoline. The increased LPG yield byCAT-A cannot compensate quantitatively for the de-creased yield of valuable distillates, especially gasoline.Table 9 shows that for the same feed and at 7 wt % coke,which is a yield acceptable to refineries that processresid-containing feeds, CAT-A yielded lower conversionwith much less premium distillates, more LPG andundesirable products (dry gas and HCO). The decreasesin delta coke were negligible. CAT-A also requiredslightly higher C/O ratio to maintain the same cokeyield. Again, HRO is the preferred choice to crack thesefeedstocks for higher yields of valuable products.

3.5. Comparison of Product Yields between MATand Riser. Table 10 shows MAT and pilot plant productdistributions for all three feeds, using CAT-A at 55 and65 wt % conversion, respectively. The table indicatesthat, except for coke yield, the bias in MAT yieldsrelative to those of the riser pilot plant can be main-

Figure 11. Relationship between delta coke yield and conversion.

Table 8. Comparison of Product Yields at 65 wt % Conversion

feed HTC HT-DA VIR

catalyst HRO CAT-A difference HRO CAT-A difference HRO CAT-A difference

C/O ratio 8.92 10.84 1.92 3.82 6.35 2.53 9.07 12.66 3.59conversion, wt % 65.00 65.00 0.00 65.00 65.00 0.00 65.00 65.00 0.00dry gas, wt % 1.77 2.11 0.34 1.82 2.44 0.63 2.84 3.14 0.30LPG, wt % 9.36 12.44 3.07 9.28 13.14 3.86 9.44 12.07 2.63gasoline, wt % 46.46 42.48 -3.98 48.24 42.40 -5.85 45.27 41.18 -4.09LCO, wt % 21.53 18.45 -3.08 21.25 17.64 -3.62 23.05 20.04 -3.01HCO, wt % 13.47 16.55 3.08 13.75 17.36 3.62 11.95 14.95 3.00coke, wt % 7.28 7.97 0.69 5.82 7.10 1.28 7.43 8.61 1.18delta coke, wt % 0.82 0.71 -0.11 1.45 1.13 -0.32 0.83 0.68 -0.15

Table 9. Comparison of Product Yields at 7 wt % Coke

feed HTC HT-DA VIR

catalyst HRO CAT-A difference HRO CAT-A difference HRO CAT-A difference

C/O ratio 8.49 9.34 0.85 5.50 6.18 0.67 8.49 9.51 1.03conversion, wt % 64.61 62.20 -2.41 67.91 63.97 -3.94 65.40 60.66 -4.74dry gas, wt % 1.74 1.88 0.14 2.02 2.37 0.35 2.87 2.86 -0.01LPG, wt % 9.27 11.59 2.32 10.31 12.83 2.53 9.56 10.97 1.41gasoline, wt % 46.42 41.70 -4.72 49.16 42.03 -7.13 45.34 39.61 -5.73LCO, wt % 21.65 18.76 -2.89 20.19 17.74 -2.46 22.92 20.78 -2.14HCO, wt % 13.73 19.04 5.30 11.90 18.29 6.40 11.68 18.56 6.88coke, wt % 7.00 7.00 0.00 7.00 7.00 0.00 7.00 7.00 0.00delta coke, wt % 0.83 0.75 -0.08 1.31 1.16 -0.15 0.82 0.73 -0.09


tained within 15%, in general. This is consideredacceptable based on the great differences in reactordesign and operation between the two systems. Com-pared with the riser, the MAT unit tends to give lowerdry gas and gasoline, higher LPG, and much higher cokeyields, due to the in situ combustion method for totalcoke yield determination. The deviation in coke yieldobtained from the MAT is further aggravated by theconsistently higher error by ∼1 wt % (absolute) andmuch longer catalyst contact time (30 s in the MATversus just a few seconds in the riser) resulting in highercoke formation on the catalyst in the MAT according toVoorhie’s equation.17

Assuming linear correlations between riser yields andMAT yields, the predicted riser yields based on MATdata (predicted riser yield ) a(MAT yield) + b) are

plotted against the actual riser yields in Figure 12,which shows also the parameters a and b, and thecoefficients of determination (R2) for every single set ofcorrelations. R2 values for dry gas, gasoline, and HCOare acceptably good (over 0.91) but for the rest ofproducts, especially LCO, are poor. This is because boththe riser and the MAT data are confined to a rathernarrow range (Table 10). Thus, from a statistical pointof view, it is more difficult to obtain a good correlationin this case, as compared with the case where the data(containing the same error as their counterparts) arespread over a wider range. As well, one or two badoutliers can easily upset the statistical balance if toofew data points (only 6 per product in this case) are usedfor correlation. On the whole, the data points represent-ing the predicted and actual riser yields lie reasonablyclose to the 1:1 line.

4. Conclusions

To achieve higher yields of valuable distillates whencracking oil-sands-derived VGOs, HRO, a bottoms-cracking catalyst containing REY zolite and a large-poreactive matrix, is more suitable than CAT-A, an octane-barrel catalyst containing REUSY/ZSM-5 zeolites andan active matrix. With respect to conversion and productslate, HT-DA is the best feed, followed by HTC and VIR,although the first feed contains the highest CCR,metals, and nitrogen. Most of the observed crackingphenomena can be linked to feedstock and catalystproperties. MAT yields can be correlated with riser pilotplant results, although their absolute values can be verymuch different.

Acknowledgment. The authors wish to thank theAnalytical Laboratory of the National Centre for Up-grading Technology (NCUT) for their technical support.Partial funding for NCUT has been provided by theCanadian Program for Energy Research and Develop-ment (PERD), the Alberta Research Council, and theAlberta Energy Research Institute.

EF0200368

Figure 12. Relationship between predicted and actual riser yields.

Table 10. Comparison of Product Yields (wt %) at 55 and65 wt % Conversion between MAT and Risera

conversion 55 65

feed HTC HT-DA VIR HTC HT-DA VIR

dry gasMAT 1.5 1.9 2.6 2.1 2.4 3.1riser 1.7 1.8 3.0 2.4 2.5 4.0biasb -11.8 5.6 -13.3 -12.5 -4.0 -22.5

LPGMAT 9.5 10.6 9.6 12.4 13.1 12.1riser 9.2 9.9 10.7 11.6 12.1 12.0biasb 3.3 7.1 -10.3 6.9 8.3 0.8

gasolineMAT 39.1 36.8 37.3 42.5 42.4 41.2riser 40.9 39.4 38.2 46.3 44.9 44.3biasb -4.4 -6.6 -2.4 -8.2 -5.6 -7.0

LCOMAT 19.4 17.6 21.2 18.4 17.6 20.0riser 20.0 20.4 20.6 16.1 17.9 17.4biasb -3.0 -13.7 2.9 14.3 -1.7 14.9

HCOMAT 25.6 27.4 23.8 16.6 17.4 15.0riser 25.0 24.6 24.4 18.9 17.1 17.6biasb 2.4 11.4 -2.5 -12.2 1.8 -14.8

cokeMAT 5.0 5.6 5.6 7.9 7.1 8.6riser 3.2 3.9 3.1 4.7 5.5 4.7biasb 56.3 43.6 80.6 68.1 29.1 83.0

a Using CAT-A. b bias (%) ) [(MAT yield - riser yield)/(riseryield)]100.


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