fcc study of canadian oil-sands derived vacuum gas oils. 2. effects of feedstocks and catalysts on...

FCC Study of Canadian Oil-Sands Derived Vacuum GasOils. 2. Effects of Feedstocks and Catalysts on

Distributions of Sulfur and Nitrogen in Liquid Products

Siauw Ng,*,† Yuxia Zhu,† Adrian Humphries,‡ Ligang Zheng,§ Fuchen Ding,|Liying Yang,⊥ 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 Petrochemical Technology, Daxing,

Beijing, China 102600, Centre for Chemical Engineering, Beijing Institute of ClothingTechnology, Beijing, China 100029, Syncrude Research Centre, 9421-17 Avenue, Edmonton,

Alberta, Canada T6N 1H4

Received February 15, 2002

This study was conducted in an effort to learn more about the roles of feedstock and catalystin the distributions of sulfur and nitrogen in liquid products after catalytic cracking. Ahydrotreated coker gas oil, a hydrotreated deasphalted oil, a virgin gas oil, all derived fromCanadian oil-sands bitumen, were catalytically cracked in a fluid bed microactivity test (MAT)unit with two commercial catalysts. These were the equilibrium catalysts, bottoms-cracking andoctane-barrel catalysts, respectively, both containing active matrixes. The cracked liquid productswere analyzed for sulfur and nitrogen distributions with boiling point, from which the sulfurand nitrogen contents of gasoline, light cycle oil (LCO), and heavy cycle oil (HCO) fractions weredetermined. Variations of concentrations in each product cut with conversion were discussed,along with the hydrogen transfer indices of catalysts and the hydrogen yields of feeds. The overalldistribution of feed sulfur in cracked products, and the relationship of the sulfur content of feedwith that of LCO, confirmed the findings reported in the literature. The same three feeds werealso cracked with the octane-barrel catalyst in a riser pilot plant. The sulfur and nitrogenconcentrations in each product fraction from MAT were found to be in reasonably good agreementwith those obtained from the riser pilot plant operation. Results from this study will provide thescientific community with a better understanding about the cracking chemistry of sulfur- andnitrogen-containing species in gas oils under different test environments.

1. Introduction

A worldwide trend continues toward new gasoline anddiesel specifications aimed at reducing automobile emis-sions. Sulfur is of particular concern to society, partlydue to its toxic and corrosive nature and its abundancein transportation fuels, and partly due to its adverseeffect on the performance and life of the automobilecatalytic converters, which reduce hydrocarbon and COemissions. In contrast, nitrogen receives no environ-mental attention due to the fact that the NOx producedfrom fuels is negligible compared with those formed athigh temperatures from nitrogen in the air. However,the deleterious effect by nitrogen compounds on theperformance of refining catalysts may cause seriousproduction loss and poor product quality. Thus, both

sulfur and nitrogen levels in feeds and products areclosely watched in fluid catalytic cracking (FCC) opera-tion. The behaviors of these elements during catalyticcracking have been the key research subjects on whichseveral papers were published in the past 2 decades.

On nitrogen poisoning, Fu and Schaffer1 investigatedthe effects of about 30 different nitrogen compounds onthe activity and selectivity of two commercial FCCcatalysts. They found a good agreement between thegas-phase basicity (proton affinity) of a nitrogen com-pound and its poisoning effect on cracking catalysts.Young2 established correlations between compositionsof catalysts and their abilities to crack feedstocksblended with different amounts of quinoline. Scherzerand McArthur3,4 evaluated 6-8 catalysts for crackinghigh-nitrogen feedstocks. They found that high-zeolitecontent and matrix type played a role in enhancing the* 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.⊥ Centre for Chemical Engineering.# Syncrude Research Centre.

(1) Fu, C. M.; Schaffer, A. M. Ind. Eng. Chem. Prod. Res. Dev. 1985,24, 68-75.

(2) Young, G. W. J. Phys. Chem. 1986, 90, 4894.(3) Scherzer, J.; McArthur, D. P. Oil Gas J. 1986, 84 (43), 76-82.(4) Scherzer, J.; McArthur, D. P. Ind. Eng. Chem. Res. 1988, 27,

1571-1576.

1209Energy & Fuels 2002, 16, 1209-1221

10.1021/ef0200370 CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 07/31/2002

nitrogen resistance of FCC catalysts. Silverman et al.5showed that FCC catalysts with high surface areamatrixes could tolerate more nitrogen poisoning. Cormaet al.6 investigated the effect of basic nitrogen oncracking n-heptane over high-silica zeolites.

On the sulfur issue, Wormsbecher et al.7 studied thecatalytic effects on the sulfur distribution in FCCgasolines. They found that the mercaptan and thiopheneyields were dependent on conversion but not catalysttype. The tetrahydrothiophene yield was independentof both conversion and catalyst type. The low sitedensity catalysts produced more alkylthiophenes thanthose of high site density. Keyworth et al.8 discusseddifferent approaches to reduce sulfur in FCC gasolineand the distribution of feed sulfur in FCC products.They concluded that FCC feed pretreatment allowedhigher feed rates, and increased conversion and yieldsfor gasoline, LCO, isobutane, and isobutene. FCC octaneperformance was also improved through feed pretreat-ment. Hydrofinishing FCC gasoline degraded octaneperformance but reduced sulfur content and improvedother product properties in the 343 °C- fractions.Myrstad et al.9,10 reported the sulfur reduction of FCCnaphtha by a Zn-containing additive, and the effect ofNi and V on sulfur reduction of FCC naphtha. McClungand Dodwell11 showed that the rare earth level of theFCC catalyst could lower naphtha sulfur content. Thesuggested explanations for the effects of Ni, V, and rareearth were related to the hydrogen transfer activitiesof catalysts. Corma et al.12 studied the mechanism ofsulfur removal during catalytic cracking, by spiking alow sulfur vacuum gas oil (VGO) with sulfur-containingcompounds belonging to the gasoline and light dieselrange. They concluded that the long chain alkylth-iophenes were easier to remove from the gasoline rangethan the short chain counterparts. Benzothiophene wasnot very reactive under FCC conditions and, thus,difficult to crack and remove. In addition to cokeformation, it could undergo alkylation, giving heaviersulfur compounds that did not belong to the gasoline.Hernandez-Beltran et al.13 studied the sulfur reductionin cracked naphtha by a commercial additive. Theyreported that the additive reduced the amount of C1-C4 alkylthiophenes by preventing their formation andpromoted the dehydrocyclization of alkylthiophenes toform alkylbenzothiophenes. The activity of the additiveseemed to be affected by the adsorbed H2S whichpoisoned the active sites.

In part 1 of the present study,14 we evaluated threeCanadian oil-sands derived feedstocks cracked over twocommercial equilibrium FCC catalysts (a bottoms-cracking catalyst HRO 610 and an octane-barrel catalystCAT-A). Results indicated that HRO 610 was moreactive, producing higher yields of valuable distillates,and less coke for the same feed, whereas CAT-A gavemore gases and less gasoline, although the quality ofthis gasoline might be better. Cracking performancecould be related to catalyst properties including zeolitetype, rare earth content, matrix pore structure, zolite-to-matrix ratio, and surface characteristics. The threefeeds were ranked based on their cracking character-istics, which could be explained from feed analyses,gasoline and LCO precursor concentrations determinedby GC-MS, and product characterization data fromPIONA analyzer.

The present work (part 2) covers product quality, withemphasis on the distributions of sulfur and nitrogen inliquid products classified as gasoline (IBP-221 °C), lightcycle oil (LCO, 221-343 °C), and heavy cycle oil (HCO,343 °C+).

2. Experimental Section

2.1. Catalytic Cracking. The same three feeds in part 1(supplied by the Syncrude Research Centre) were investi-gated: (1) a laboratory-hydrotreated coker VGO (HTC), (2) alaboratory-hydrotreated VGO derived from a deasphalted oilfrom bitumen (HT-DA), and (3) an untreated virgin VGO(VIR). Two equilibrium catalysts were employed for cracking:Akzo Nobel HRO 610 (HRO) containing rare earth exchangedY zeolite (REY) and CAT-A (from another supplier) containingrare earth ultrastable Y zeolite (REUSY) mixed with smallamount of ZSM-5. Both catalysts have active matrixes. Tables1 and 2 summarize the properties of feedstocks and catalysts,respectively. Batch cracking experiments were performed atthe National Centre for Upgrading Technology (NCUT) usinga fixed fluid bed reactor in a MAT unit (Zeton Automat IV) at510 °C for HTC and VIR, but 530 °C for HT-DA, with 30 sfeed injection time. A specially designed liquid receiver withextra large volume (300 mL) was used to collect over 99 wt %of liquid products that were free of contamination by washing

(5) Silverman, L. D.; Winkler, W. S.; Tiethof, J. A.; Witoshkin, A.1986 NPRA Annual Meeting, AM-86-62. National Petrochemical &Refiners Association: Washington, DC, 1986.

(6) Corma, A.; Fornes, V.; Monton, J. B.; Orchilles, A. V. Ind. Eng.Chem. Res. 1987, 26, 882.

(7) Wormsbecher, R. F.; Chin, D. S.; Gatte, R. R.; Albro, T. G.;Harding; R. H. 1992 NPRA Annual Meeting, AM-92-15. NationalPetrochemical & Refiners Association: Washington, DC, 1992

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

(10) Myrstad, T.; Seljestokken, B.; Engan, H.; Rytter, E. Appl. Catal.,A 2000, 192, 299-305.

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

(12) Corma, A.; Martinez, C.; Ketley, G.; Blair, G. Appl. Catal., A2001, 208, 135-152.

(13) Hernandez-Beltran, F.; Morenco-Mayorga, J. C.; Quintana-Solorzano, R.; Sanchez-Valente, J.; Pedraza-Archila, F.; Perez-Luna,M. Appl. Catal., B 2001, 34, 137-148.

(14) Ng, S. H.; Zhu, Y.; Humphries, A.; Zheng, L.; Ding, F.; Gentzis,T.; Charland, J. P.; Yui, S. Energy Fuels 2002, 16, 1196.

Table 1. Summary of Feedstock Properties

feed HTC HT-DA VIR

density at 15.6 °C, g/mL 0.9511 0.9430 0.9712sulfur, wt % 0.43 0.70 3.25total nitrogen, wppm 2150 2450 1930basic nitrogen, wppm 439 613 610Conradson carbon residue, wt % 0.50 2.00 0.33aromatic carbon, % 24.7 20.9 25.4524 °C+ by simdist, wt % 8.3 36.9 2.0

Hydrocarbon Type 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

benzothiophenes 0.7 1.2 5.0dibenzothiophenes 3.0 2.0 4.6benzonaphthothiophenes 0.5 0.2 1.1

polar compounds 3.8 7.1 5.7

1210 Energy & Fuels, Vol. 16, No. 5, 2002 Ng et al.

solvents (e.g., CS2). The same three feeds were also crackedwith CAT-A in a modified ARCO-type riser pilot plant at 490-520 °C. Experimental details about analyses of feedstocks andcatalysts, and cracking operations, can be found elsewhere.14

2.2. Characterization of Cracked Liquid Products.Total liquid products (TLPs, 0.5-1.7 mL depending on thecatalyst-to-oil ratio), without prior separation, were character-ized for simulated distillation (ASTM 2887), and distributionsof sulfur and nitrogen by boiling point using a gas chromato-graph (GC) with Sievers sulfur chemiluminescence detector(GC-SCD) and Antek nitrogen chemiluminescence detector(GC-NCD). Sulfur and nitrogen standard solutions, and amixture of normal paraffins, were used to establish S and Ncalibration factors and the retention time-boiling point rela-tionship. Statistics showed that for two solutions, each con-taining 31.45 µg S/mL and 21.16 µg N/mL, the standarddeviations were 0.777 µg S/mL and 0.240 µg N/mL, corre-sponding to 2.47 and 1.13% relative standard deviation (RSD)for sulfur and nitrogen, respectively. Boiling point distributionwas calculated by SimDist Expert software developed bySeparation Systems Inc. The result was presented in the formof wppm off, or wt % off, versus boiling point. Distributions ofsulfur and nitrogen in TLP byproduct type, and concentrationsof sulfur and nitrogen in each product (i.e., gasoline, LCO, andHCO) were then calculated. Details of these calculations weredescribed previously.15

3. Results and Discussion

3.1. Concentrations of Sulfur and Nitrogen inTLPs. Upon cracking with HRO or CAT-A, both sulfurand nitrogen concentrations in TLPs decreased linearlywith conversion (Figures 1 and 2). Note that thetrendlines in each plot were somewhat parallel betweenfeeds and between catalysts, indicating that the removalof sulfur or nitrogen species in these cases followed moreor less similar mechanisms. For the three feeds at agiven conversion, the order of sulfur or nitrogen con-centrations in TLPs corresponded to the order of sulfuror nitrogen concentrations in the feed. Between the twocatalysts, CAT-A showed better sulfur selectivity butpoorer nitrogen selectivity for the same hydrotreatedfeed (HTC or HT-DA) but very little difference in sulfuror nitrogen selectivity for the same virgin feed (VIR).Table 3 shows that at 65 wt % conversion, the muchhigher C/O ratios used for CAT-A (relative to those forHRO), containing ZSM-5 that favored the production ofgaseous products,14 resulted in lower TLP yields for allthree feeds. This contributed greatly to the lowerretentions of both sulfur and nitrogen, in general, inTLPs from the three feeds cracked with CAT-A, despite

the fact that some of these TLPs had higher sulfur ornitrogen concentrations than their respective counter-parts. Table 3 indicates that at 65 wt % conversion,about 45 wt % feed sulfur or nitrogen was retained inTLPs, except for HTC which retained 51-65 wt % feedsulfur and VIR, which retained about 37 wt % feednitrogen. It seemed that the sulfur species in HTC weremore difficult to remove after more severe hydrotreat-ment, as compared with those in HT-DA.

3.2. Sulfur and Nitrogen Distributions in TLPsby Product Type. Figures 3 and 4 depict the correla-tions of sulfur and nitrogen distribution yields (theamount of sulfur or nitrogen in a product fraction overthe total amount of sulfur or nitrogen in TLP), respec-tively, of different products with conversion for feedscracked with CAT-A. Similar correlations were alsonoticed for the same feeds cracked with HRO.15 Figures3 and 4 show the following,

•In the gasoline fraction, sulfur distribution yieldincreased slightly whereas that of nitrogen decreasednoticeably with conversion. Sulfur compounds in lightnaphtha have been identified as thiols, thiophenes,mercaptans, and aliphatic sulfides, including cyclic andlinear sulfides. Those in heavy naphtha and middledistillate (bp 200-350 °C) are mainly benzothiophenesand dibenzothiophenes series.8,16 These sulfur mol-ecules, except benzothiophenes and dibenzothiophenes,are produced from the cracking of sulfur compounds inthe FCC feed.8

•In the LCO fraction, both sulfur and nitrogen dis-tribution yields showed linear and parallel increaseswith conversion, at the expense of those in the HCOfraction. However, the sulfur distribution yield in-creased at a higher rate.

•In both the gasoline and LCO fractions and atconstant conversion, VIR had the highest sulfur distri-bution yield, followed by HT-DA and HTC. HT-DA hadthe highest nitrogen distribution yield, followed by HTCand VIR. These were in the same order as theirrespective sulfur and nitrogen concentrations in feeds.In the HCO fraction, the above orders were reversed asa result of the 100 wt % sulfur or nitrogen mass balancein TLP.

(15) Zhu, Y.; Ng, S. H.; Ding, F.; Humphries, A.; Yui, S. Prepr.-Am. Chem. Soc., Div. Pet. Chem. 2001, 46, 374-377.

(16) Poirier, M. A.; Smiley, G. T. J. Chromatogr. Sci. 1984, 22, 304-309.

Table 2. Summary of Equilibrium Catalyst Properties

catalyst HRO 610 CAT-A

unit cell size, Å 24.35 24.28nitrogen adsorption-desorption

total surface area, m2/g 148 150zeolite surface area, m2/g 65 100matrix surface area, m2/g 83 50zeolite/matrix (Z/M) 0.78 2.00zeolite content, wt % 10.6 15.6

water absorptionpore volume, mL/g 0.45 0.31

Al2O3, wt % 49.7 39.7RE2O3, wt % (on zeolite) 17.5 7.8Ni, wppm 242 291V, wppm 434 314

Table 3. Feed Sulfur and Nitrogen Retained in TLP at 65wt % Conversion

feedfeed sulfur

concentration,wt %

feed nitrogenconcentration,

wppm

HTC0.432150

HT-DA0.702450

VIR3.251930

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

C/O ratio 8.7 11.1 4.5 6.6 8.2 ∼13TLP yield, wt % 75.3 70.0 76.0 68.7 74.6 70.0S concentration in

TLP, wppm3705 3135 4200 3995 20100 20400

N concentration inTLP, wppm

1295 1400 1475 1565 985 985

feed S retained inTLP, wt %

64.9 51.0 45.6 39.2 46.1 43.9

feed N retained inTLP, wt %

45.4 45.6 45.8 43.9 38.1 35.7

Canadian Oil-Sands Derived VGOs Energy & Fuels, Vol. 16, No. 5, 2002 1211

It can be seen from Figure 3 that, at low conversion,most of the sulfur existed in HCO, with less in LCO andmuch less in the gasoline fraction. As conversionincreased, large sulfur-containing molecules in HCO and

LCO were cracked to smaller species, with or withoutsulfur removal. The net result was that the sulfurdistribution yield of HCO decreased, whereas that ofLCO increased with conversion. At high conversion, the

Figure 1. Sulfur concentrations of TLP versus conversion for all feeds and catalysts.

Figure 2. Nitrogen concentrations of TLP versus conversion for all feeds and catalysts.

Figure 3. Sulfur distribution yields in product cuts versus conversion for all feeds and CAT-A.


former might be exceeded by the latter. Figure 4 showssimilar trends as those in Figure 3, except that thenitrogen distribution yields of HCO or LCO decreasedor increased at a slower pace, such that at highconversion the yields of HCO were still much higherthan those of LCO. This implied that the nitrogenspecies, compared with those of sulfur, were moredifficult to remove at FCC conditions. These nitrogenspecies were originated from the feeds containing avariety of nitrogen compounds commonly found in thecrudes: pyridine, quinoline, acridine (basic), hydroxy-pyridine, hydroxyquinoline (weekly basic), indole, andcarbazole (neutral).17 As well, the nitrogen distributionyields of gasoline, although small in magnitude, de-creased noticeably at higher conversion. The removednitrogen species are believed to be hydrocarbons con-taining single-bonded nitrogen such as amines.

3.3. Sulfur and Nitrogen Distribution Yields ofEach Product Cut. Figures 5-7 and Figures 8-10show the relationships, for sulfur and nitrogen, respec-tively, between conversion and distribution yields ofgasoline, LCO, and HCO, for all feeds and catalysts. In

addition to the observations described previously, onecan also differentiate the catalyst performance in crack-ing the sulfur or nitrogen species. In general, betweenthe two catalysts and at a given conversion, CAT-A gavelower sulfur and nitrogen distribution yields in gasolineand LCO cuts, but higher yields in the HCO cut for thesame feed.

3.4. Simulated Distillation of TLPs. In a MATstudy, the small amount of liquid product can only besubjected to simulated distillation (simdist) by gaschromatograph to determine the gasoline, LCO, andHCO yields relative to TLP. The relationships betweenconversion and the simdist yields of three product cutsfor all feeds and catalysts are illustrated in Figures 11-13 with the following remarks:

•As expected, at low conversion, the majority of TLPwas HCO (of which the simdist yield decreased mono-tonically with conversion), followed by gasoline (of whichthe simdist yield increased linearly with conversion) andLCO (of which the simdist yield showed somewhatirregular patterns, with some increases and decreaseswith conversion). The LCO simdist yield varied in arelatively narrow range (max. 5 wt % variation for a(17) Raith, J.; Lanik, A. Erdoel-Erdgas Zeitschrift 1982, 98 Jg, 169.

Figure 4. Nitrogen distribution yields in product cuts versus conversion for all feeds and CAT-A.

Figure 5. Sulfur distribution yields of gasoline versus conversion for all feeds and catalysts.


yield profile). At high converion, gasoline predominated,followed by the two unconverted products LCO andHCO.

•Among the three feeds and at a given conversion, ingeneral, HD-DA showed the highest simdist yields,followed by HTC and VIR, in both gasoline and HCO

Figure 6. Sulfur distribution yields of LCO versus conversion for all feeds and catalysts.

Figure 7. Sulfur distribution yields of HCO versus conversion for all feeds and catalysts.

Figure 8. Nitrogen distribution yields of gasoline versus conversion for all feeds and catalysts.


cuts, regardless of catalysts used. In the LCO cut, theorder was the reverse, i.e., VIR showed the highestsimdist yield, followed by HTC and HT-DA, regardless

of the catalysts used. Note that the orders of simdistyields (relative to TLP), in a cut for the three feeds,corresponded to the orders of their respective product

Figure 9. Nitrogen distribution yields of LCO versus conversion for all feeds and catalysts.

Figure 10. Nitrogen distribution yields of HCO versus conversion for all feeds and catalysts.

Figure 11. Gasoline simulated distillation yields (relative to TLP) versus conversion for all feeds and catalysts.


yields (relative to feed) in the same cut, which wererelated to the precursor concentrations.14 A productyield (e.g., gasoline yield) equals the simdist yield of thesame product multiplied by the yield of TLP (relativeto feed). However, for gasoline yield, the C5+ compo-nents in the vapor phase have to be taken into consid-eration as well.

• Between the two catalysts, at a given conversion,HRO generally gave higher simdist yields in bothgasoline and LCO cuts, but lower yield in the HCO cut,for the same feed.

3.5. Sulfur Concentrations in Product Cuts. Thetheme of this study is to investigate the effects offeedstock and catalyst properties on sulfur and nitrogenconcentrations in each product fraction. A concentrationwas calculated using the sulfur or nitrogen concentra-tion in the TLP (Figure 1 or 2), multiplied by the sulfuror nitrogen distribution yield in a product cut (Figures5-7 or Figures 8-10), and divided by the yield of eachcut in the TLP based on the simulated distillation(simdist yield of each cut, Figures 11-13). Among thethree parameters, i.e., the concentration, the distribu-tion yield, and the simdist yield in each cut, somecarried more weight than others in calculation, depend-ing on the magnitudes of changes with conversion.

Figures 14-16 illustrate the correlations of sulfurconcentrations in gasoline, LCO, and HCO, with conver-sion for all feeds and catalysts. It can be observed thefollowing.

•Gasoline had the lowest sulfur concentration, fol-lowed by LCO and HCO.

•Sulfur concentrations in the gasoline fraction de-creased linearly whereas those in both LCO and HCOfractions increased linearly with conversion. Note thatin gasoline and HCO fractions the trendlines represent-ing sulfur concentration-conversion relationships (Fig-ures 14 and 16) were in opposite directions comparedwith their respective trendlines representing sulfurdistribution yield-conversion relationships (Figures 5and 7). In this case, the simdist yields of gasoline (Figure11) and HCO (Figure 13) showed the greatest impacts.

•Between the two catalysts, CAT-A gave better sulfurselectivities in the same product cut for all feeds exceptVIR. For this feed, both catalysts showed proximatesulfur selectivities in gasoline and LCO fractions.

•In general, the order of sulfur concentrations in eachproduct cut corresponded to the order of the sulfurconcentrations in the feeds.

The rises or declines of sulfur concentrations withconversion were essentially the net effect of four major

Figure 12. LCO simulated distillation yields (relative to TLP) versus conversion for all feeds and catalysts.

Figure 13. HCO simulated distillation yields (relative to TLP) versus conversion for all feeds and catalysts.


reactions: (1) the decomposition of light and crackablesulfur species (e.g., mercaptans and sulfides) giving upH2S; (2) the reduction in molecular size of compoundswithout disturbing the sulfur atom; (3) the enrichment

of uncrackable compounds containing sulfur in thearomatic ring; and (4) the formation of coke, on thecatalyst surface, from refractory sulfur-containing aro-matics. In general, in the gasoline fraction, the sulfur

Figure 14. Sulfur concentrations of gasoline versus conversion for all feeds and catalysts.

Figure 15. Sulfur concentrations of LCO versus conversion for all feeds and catalysts.

Figure 16. Sulfur concentrations of HCO versus conversion for all feeds and catalysts.


content decreases with conversion because reaction 1is prevailing relative to reactions 2 and 3. Similarly, inthe HCO fraction, the increase in sulfur concentrationwith conversion is attributed to the predominance ofreaction 3. In the LCO fraction, the concentration cango either way depending on which reaction is dominant.Reaction 4 can affect the magnitude of the increasedconcentration with conversion resulting from reaction3.

The degree of variation of sulfur concentration in aproduct cut with conversion is feedstock and catalystdependent. With respect to the feedstocks, the types ofsulfur compounds and their distributions with boilingpoint play an important role. Myrstad et al.9,10 reportedthat saturated sulfur species such as mercaptan andtetrahydrothiophene in gasoline were very reactive andeasily cracked to release H2S. The more aromatic thespecies, the lower its reactivity, e.g., thiophene beingless reactive than either mercaptan or tetrahydro-thiophene, and benzothiophene being less reactive thanthiophene. Also, for alkylthiophenes, the reactivitydecreased with an increasing degree of alkylation (e.g.,C3-thiophene was less reactive than C2-thiophene) prob-ably because of the steric hindrance. These results werein line with those reported by other researchers.12,18

Before investigating the catalyst effect on productsulfur concentration, it is advantageous to review somebasic mechanisms reported in the literature for thesulfur removal reactions. In the simplest case, saturatedspecies can be cracked to yield H2S.9,10,12 The conversionof the aromatic sulfur species requires a hydrogensource from the feed hydrocarbon.18 Gatte and co-workers19 reported that catalysts with high acid sitedensity and, therefore, high hydrogen transfer activity,preferentially hydrogenated thiophene to tetrahy-drothiophene, which then could be cracked to releaseH2S. In contrast, catalysts with low acid site densityand low hydrogen transfer activity, favored alkylation

of thiophene to heavier sulfur species. In this study,HRO containing rare earth exchanged Y zeolite (REY)with larger unit cell size and higher acid site densitythan CAT-A, did produce more H2S for the three feedsat 65 wt % conversion (0.02, 0.09, and 1.00 wt % H2Sfor HTC, HT-DA, and VIR, respectively, versus 0.02,0.06, and 0.77 wt % H2S for the same feeds when CAT-Awas used).14 However, the higher amount of H2S pro-duced by HRO did not reflect its superiority in removingsulfur species in TLP and each product cut. Rather,CAT-A seemed to give better sulfur selectivities in TLPand the same product cut for all feeds except VIR.

For better understanding, we investigated the indi-vidual catalyst’s hydrogen transfer activity, usuallyexpressed as the hydrogen transfer index (HTI), whichis defined as the ratio of isobutene to isobutane yields.11

A higher HTI is an indication of low hydrogen transferactivity, and vice versa. Figure 17 shows the gradualdecline of HTI with conversion. It appeared that HTIof a catalyst was feedstock dependent. For the samecatalyst and at a given conversion, HT-DA gave thehighest HTI, followed by VIR and HTC. Between thetwo catalysts, CAT-A with smaller unit cell size andlower acid site desity exhibited lower HTI, or higherhydrogen transfer activity for the same feed. This is incontradiction to the reported observations in the litera-ture.19,20 Thus, further explanation is necessary. In thisstudy, for the same feedstock, there appear to be fourfactors which can affect the HTI value of a catalyst.They are

1. acid site density of Y zeolite reflected by unit cellsize: the lower the density, the higher the HTI;

2. matrix type: active matrix also contains acid sitesusually associated with aluminum atoms. Active matrixcan enhance C3+C4 yields and olefinicity, and thusfavors higher HTI;20

3. zeolite-to-matrix ratio: at constant catalyst activity,an increase in zeolite/matrix ratio results in a decreasein olefin/paraffin ratio in LPG, giving lower HTI;20

(18) Wormsbecher, R. F.; Weatherbee, G. D.; Kim, G.; Dougan, T.J. 1993 NPRA Paper AM-93-55. National Petrochemical & RefinersAssociation: Washington, DC, 1993.

(19) Gatte, R. R.; Harding, R. H.; Albro, T. G.; Chin, D. S.;Wormsbecher, R. F. Prepr.-Am. Chem. Soc., Fuel Div. 1992, 37, 33-40.

(20) Scherzer, J. Correlation between Catalyst Formulation andCatalytic 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.

Figure 17. Hydrogen Transfer Indices versus conversion for all feeds and catalysts.


4. absence or presence of ZSM-5: the addition ofZSM-5 entails the loss of some gasoline and the forma-tion of LPG olefins with enhanced C3/C4 olefin selectiv-ity.20

Thus, the expected higher HTI of CAT-A, due to factor1 plus possibly factor 4, was apparently offset by theprevailing effects from facors 2 and 3, since CAT-A hada higher zeolite/matrix ratio (2.00 versus 0.78 for HRO)and its matrix was likely less active than that in HRO(as reflected by the lower matrix surface area andaluminum content). The lower ultimate HTI values ofCAT-A seemed to support partially its better sulfurselectivity. However, closer examination of HTI revealedthat at high conversion the HTIs for the two catalystsapproached the same value for the same feed (Figure17) while the corresponding sulfur concentrations weresignificantly different in most cases (Figures 14-16).Thus, HTI alone could not explain the observation athigh conversion.

Since saturation of aromatic sulfur species is onepossible reaction route, and the saturation reactionsrequire hydrogen, it is worthwhile to examine thehydrogen yield-conversion relationship which is shownin Figure 18. For the same catalyst and at a given

conversion, HT-DA gave the highest hydrogen yieldwhile the other two feeds showed more or less the sameyield. The high hydrogen yield by HT-DA was partlydue to the higher reaction temperature (530 versus 510°C for the other two feeds) and partly due to the catalystpoisoning by the abundant coke formation at highconversion.14 Between the two catalysts, CAT-A pro-duced more hydrogen for the same feed due predomi-nantly to its more isolated but stronger acid sites whichpromoted cracking, although its higher zeolite/matrixratio and less active matrix favored lower hydrogenyield. Thus, CAT-A provided a more ideal environmentfor saturation of aromatic sulfur species and gave abetter sulfur selectivity, in general.

3.6. Nitrogen Concentrations in Product Cuts.Figures 19-21 show the relationship between conver-sion and nitrogen concentrations in gasoline, LCO, andHCO, for all feeds and catalysts. Similar to the case ofsulfur, one can also observe the following.

•Gasoline had the lowest nitrogen concentration,followed by LCO and HCO.

•Nitrogen concentrations in both gasoline and LCOfractions decreased linearly, whereas those in the HCOfraction increased linearly with conversion. The trend-

Figure 18. Hydrogen yields versus conversion for all feeds and catalysts.

Figure 19. Nitrogen concentrations of gasoline versus conversion for all feeds and catalysts.


lines for LCO and HCO in the concentration-conversionplots (Figures 20 and 21) were in the opposite directionscompared with their respective trendlines in the distri-bution yield-conversion plots (Figures 9 and 10). Thiswas due mostly to the influences of the nitrogenconcentrations in TLPs (Figure 2) in the case of LCO,and the simdist yields (Figure 13) in the case of HCO.

•Between the two catalysts, CAT-A gave better ni-trogen selectivities in the same product cut for all feeds,despite the fact that CAT-A showed poorer nitrogenselectivities in TLPs for the same hydrotreated feed, i.e.,HTC or HT-DA (Figure 2).

•The order of nitrogen concentrations in each productcut corresponded to the order of the nitrogen concentra-tions in the feeds.

The trends that the nitrogen concentrations decreasedor increased with conversion were determined by thenet effect of the major reactions, which were similar tothose for sulfur except that (1) NH3 is released whenthe light and crackable nitrogen species (e.g., aminesand amides) are decomposed; (2) some saturated nitro-gen compounds can undergo dehydrogenation, for ex-ample piperidine can be converted to pyridine;1 (3) thebasic nitrogen species, especially the polyaromatic

bases, may be irreversibly adsorbed onto the catalystsurface forming the additive coke21 and be partiallyremoved from the liquid product; and (4) the aromaticnitrogen species cannot be saturated followed by break-ing the C-N bond, under FCC conditions. Thus, thepossible routes or reactions responsible for the changesin nitrogen concentrations in product cuts are morecomplicated than those for sulfur. The mechanism forthe nitrogen removal in catalytic cracking has seldombeen reported in the literature.

3.7. Comparison with Results in the Literatures.3.7.1. Feed Sulfur Distribution in FCC Products. Key-worth et al.8 reported approximate feed sulfur distribu-tion in four products:

•about 40% in reactor gas as H2S;•3-10% in gasoline (typically 5-10% for non-hy-

drotreated feeds and 3-6% for hydrotreated feeds);•about 50% in cycle oil (LCO+HCO);•the balance of sulfur in the coke (about 5% for regular

feeds, but it could exceed 10% for hydrotreated or resid

(21) Venuto, P. B.; Habib, E. T., Jr. Fluid Catalytic Cracking withZeolite Catalysts; Chemical Industries 1; Marcel Dekker: New York,1979; p 74.

Figure 20. Nitrogen concentrations of LCO versus conversion for all feeds and catalysts.

Figure 21. Nitrogen concentrations of HCO versus conversion for all feeds and catalysts.


feeds which had less sulfur contributions to reactor gasand gasoline but more to cycle oil and coke, since sulfurin these feeds is mainly tied up with polynucleararomatics. High sulfur in coke was also found by Hulinget al.22).

The amounts of sulfur in H2S and coke are usuallydifficult to determine accurately due to (1) the abun-dance of olefins in product gas, which may react withH2S, and (2) the restricted small amount of samples (i.e.,spent catalysts with less than 1 wt % coke) required bythe modern analytical instrument for sulfur analysis.Hernandez-Beltran et al.13 also reported difficulty inaccurate determination of H2S in the dry gas. In thisstudy, uncertainty existed in estimating the feed sulfurdistribution in coke for the two hydrotreated feeds, butit was safe to assume 5% for VIR. The feed sulfur ingasoline, LCO, and HCO could then be readily calcu-lated, while the balance was counted as the sulfur inH2S. Table 4 shows the feed sulfur distributions at 60and 65 wt % conversion, respectively. In general, theresults agreed with the findings reported in the litera-ture.8

3.7.2. LCO Sulfur Concentrations. Letzsch and Ash-ton23 reported that, as a rule of thumb, LCO hadapproximately the same sulfur content as the feed formany FCC units. This was also observed in our work(Table 5).

3.8. Comparison of Sulfur and Nitrogen Resultsbetween MAT and Riser. Table 6 shows a comparisonof sulfur and nitrogen concentrations in different liquidproducts obtained from both a MAT and a riser pilotplant for all feeds cracked with CAT-A at 55 and 65 wt% conversion, respectively. The table indicates that,except for high sulfur (S > 0.7 wt %) in HCO, thedifferences in values from the two units are within 0.002absolute for S between 0.005 and 0.007 wt %, within0.02 absolute for S between 0.04 and 0.07 wt %, andwithin 0.15 absolute when S or N > 0.08 wt %. In thecase of the riser reactor, the sulfur concentration of LCOfrom VIR at 65 wt % conversion (4.10 wt %) was

obviously too high, since it was almost the same inmagnitude as that of HCO (4.15 wt %). The comparisonin Table 6 was acceptable, considering the great differ-ences in reactor configuration and operating principlesbetween the two systems. Compared with the riser, theMAT unit tended to give higher values in nitrogenconcentrations. For HCO, MAT showed consistentlylower sulfur by 0.2-0.4 wt %.

4. Conclusions

As conversion increased, sulfur and nitrogen concen-trations in both TLP and gasoline decreased linearly forall feeds and catalysts. In the LCO fraction, sulfurconcentration increased but that of nitrogen decreasedat higher conversion. In HCO, both sulfur and nitrogenconcentrations increased linearly with conversion. Theorder of sulfur or nitrogen concentrations in eachproduct cut, at a given conversion, corresponded to theorder of the sulfur or nitrogen concentrations in thefeeds. Between the two catalysts, CAT-A gave bettersulfur and nitogen selectivities, in general. These couldbe related to its higher hydrogen transfer ability andits tendancy to produce more hydrogen for a given feed.The feed sulfur distributions in cracked products for VIRagreed well with those reported in the literature. TheLCO sulfur concentrations were similar to those in theircorresponding feeds. In general, the sulfur and nitrogenconcentrations in different liquid products from MATcompared reasonably well with those from the riser pilotplant for all feeds cracked with CAT-A at 55 and 65 wt% conversion, respectively.

Acknowledgment. The authors thank the Analyti-cal Laboratory of the National Centre for UpgradingTechnology (NCUT) for their technical support. Partialfunding for NCUT has been provided by the CanadianProgram for Energy Research and Development (PERD),the Alberta Research Council and the Alberta EnergyResearch Institute.

EF0200370

(22) Huling, G. P.; McKinney, J. D.; Readal, T. C. Oil Gas J. 1975(May 19), 73-79.

(23) Letzsch, W. S.; Ashton, A. G. The Effect of Feedstock on Yieldsand Product Quality. In Fluid Catalytic Cracking: Science andTechnology; Magee, J. S., Mitchell, M. M., Jr., Eds.; Studies in SurfaceScience and Catalysis 76; Elsevier Science Publishers B. V.: Amster-dam, 1993; p 493.

Table 4. Feed Sulfur Distributions in FCC Products forVirgin VGO, wt %

catalyst conversion coke H2S gasoline LCO HCO

HRO 610 60.0 5.0 45.3 5.0 23.7 21.0CAT-A 60.0 5.0 47.7 4.4 20.9 22.0HRO 610 65.0 5.0 48.9 4.9 24.0 17.2CAT-A 65.0 5.0 51.5 4.3 21.3 17.9

Table 5. LCO Sulfur Concentrations (wt %) at 65 wt %Conversion

feedfeed sulfur, wt %

HTC0.43

HT-DA0.70

VIR3.25

HRO 610 0.57 0.73 3.46CAT-A 0.44 0.66 3.43average 0.51 0.70 3.45

Table 6. Comparison of Sulfur and NitrogenConcentrations between MAT and Riser Pilot Planta

gasoline LCO HCO

HTC HT-DA VIR HTC HT-DA VIR HTC HT-DA VIR

sulfur, wt %at 55 wt %conversion

MAT 0.007 0.071 0.46 0.42 0.62 3.28 0.76 0.71 3.57riser 0.005 0.057 0.55 0.41 0.63 3.37 1.09 0.96 3.92

at 65 wt %conversion

MAT 0.005 0.061 0.39 0.44 0.65 3.43 0.81 0.85 3.92riser 0.006 0.041 0.49 0.49 0.76 4.10 1.22 1.14 4.15

nitrogen, wt %at 55 wt %conversion

MAT 0.14 0.18 0.09 0.42 0.43 0.34riser 0.11 0.10 0.08 0.34 0.30 0.25

at 65 wt %conversion

MAT 0.13 0.16 0.08 0.44 0.46 0.35riser 0.10 0.11 0.08 0.31 0.35 0.20

a CAT-A was employed.


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