FCC Study of Canadian Oil-Sands Derived Vacuum Gas Oils. 1. Feed

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FCC Study of Canadian Oil-Sands Derived Vacuum Gas Oils. 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 Petrochemical Technology, 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 yield structure during fluid catalytic cracking (FCC) of bitumen-derived vacuum gas oils (VGOs). Three nonconventional VGOs, derived from Canadian oil-sands bitumen, were catalytically cracked in a 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 octanebarrel catalyst containing rare earth ultrastable Y zeolite (REUSY) mixed with a small amount of ZSM-5. Both catalysts were embedded in active matrixes. Results indicated that the REY catalyst was more active, producing higher yields of valuable distillates and less coke for the same 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 catalyst properties 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, which could be explained through feed analyses, precursor concentrations determined by GC-MS, and product characterization data from a PIONA analyzer. MAT results were compared with riser pilot plant data at 55 and 65 wt % conversion. In general, at the same conversion, the difference in a given product yield from the two units could be maintained within 15%. Coke yield showed a greater disagreement, however, due to methodological differences in the analysis.

1. Introduction Canada has huge oil-sands bitumen resources that are on a par with Saudi Arabia’s conventional oil reserves.1 Bitumen is a complex hydrocarbon molecule with a relatively low hydrogen-to-carbon atomic ratio and an abundance of chemical impurities. Conversion of bitumen to high-quality transportation fuels is a great challenge, as most refineries are either not equipped to process the bitumen or find it economically unprofitable to process. In Canada, one practical route for overcoming this conversion problem is to upgrade bitumen to a light and bottomless synthetic crude oil (SCO) through adequate processes, typically coking or hydrocracking followed by hydrotreating. Bitumen producers may also sell partially upgraded or raw bitumen to be blended with optimum quantities of conventional crude prior to * To whom correspondence should be addressed. [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.

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being processed in refineries. Both SCO and raw bitumen contain over 35 vol % vacuum gas oil (VGO), which can be used as a feed to the fluid catalytic cracking (FCC) unit. Depending on the degree of upgrading, the derived VGOs carry different amounts of impurities from the bitumen. Among the impurities in an FCC feed, nickel is known as a strong dehydrogenation agent producing large amounts of gases (including hydrogen) and coke; vanadium can destroy zeolite by hydrolysis of the silica/alumina crystal structure, thus reducing catalyst activity; basic nitrogen (about 1/4 to 1/3 of total nitrogen) can neutralize the acidity of the catalyst, rendering it inactive; and Conradson carbon residue (CCR) may block the catalyst pores causing inaccessibility to the molecules that need to be cracked. In addition, the high amounts of diaromatics and larger molecules in VGO produce excessive amounts of lowvalue light cycle oil (LCO), heavy cycle oil (HCO), coke, and dry gas. In an attempt to improve the yield structure during catalytic cracking of bitumen-derived VGOs, a commercial catalyst containing an active matrix with high accessibility to large hydrocarbon molecules was used

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

Canadian Oil-Sands Derived VGOs

to enhance bottoms cracking. For comparison, another equilibrium FCC catalyst (received from a refinery) with distinctly different properties was also included in the test program. Cracked liquid products in various cuts were characterized for sulfur, nitrogen, and hydrocarbon types to determine the performances of these two catalysts with respect to the product quality. The results on the product quality will be presented in subsequent papers that are in preparation. 2. Experimental Section 2.1. Feedstocks. Three feeds were supplied by the Syncrude Research Centre: (1) a laboratory-hydrotreated coker VGO (HTC) with properties close to those of its counterpart from Syncrude’s commercial plant; (2) a laboratory-hydrotreated VGO from a deasphalted oil derived from bitumen (HTDA) (deasphalting was performed using a mixture of n-butane and i-butane as solvent, at typical conditions, in the M. W. Kellog pilot plant in Houston, Texas); and (3) an untreated virgin VGO (VIR) obtained by batch distillation of a whole bitumen to simulate a commercial product from Syncrude’s newly installed vacuum tower. All feeds were distilled to remove 343 °C- and 524 °C+ fractions, except HT-DA, where only the 524 °C- fraction was taken off. Feedstocks were characterized using ASTM and other supplementary methods at the Syncrude Research Centre. 2.2. Catalysts. Two refinery-generated equilibrium catalysts, Akzo Nobel’s HRO 610 (HRO) and catalyst A (CAT-A) from another supplier, were included in this study. HRO, used in a refinery in California that processes Alaskan feeds, was directly received from Akzo Nobel Catalysts LLC., along with the analyses. HRO was developed, on the basis of a special catalyst assembly technology, to give a high Akzo Accessibility Index (AAI), which measures the relative mass transfer rates of hydrocarbons into and out of the catalyst pores (i.e., the adsorption and desorption rates of hydrocarbons). As for CATA, its manufacturer claimed that a new matrix technology was used to lower the Lewis/Bronsted acid site ratio, leading to a reduction in condensation/polymerization reactions that form coke and gas. They also claimed the involvement of a proprietary manufacturing process that controlled the inherent matrix composition and surface area, while increasing the zeolite external surface through reduced crystal size for better bottoms upgrading. The technical information provided suggests that CAT-A contains the ultrastable Y zeolite (USY). Characterization of CAT-A was performed at NCUT using conventional ASTM methods. Both catalysts were calcined at 600 °C for 3 h prior to being loaded into reactors. 2.3. Catalytic Cracking of Feedstocks. Bench-scale cracking experiments were largely conducted at NCUT, with selected runs at an outside laboratory that used a reactor system based on the Advanced Cracking Evaluation (ACE) technology.2 Recently, the ACE unit has gained popularity as a tool for laboratory FCC studies. In this paper, only NCUT results are reported except in a figure which shows a comparison of catalyst activities obtained from the two reactor systems. It should be noted that more runs were performed using NCUT’s unit than from ACE, and the liquid products at NCUT were available for subsequent characterization. At NCUT, bench-scale catalytic cracking was carried out at 510 °C for HTC and VIR, and at 530 °C for HT-DA, using a fluidbed reactor in a MAT unit (Zeton Automat IV). The reactor was loaded with 7 g of catalyst. Catalyst-to-oil ratio was varied to obtain different conversions (the portion of the feed converted to 221 °C- products, including gas and coke). Catalyst contact time was kept constant at 30 s. Details of the (2) Wallenstein, D.; Haas, A.; Harding, R. H. Appl. Catal., A 2000, 203, 23-36.

Energy & Fuels, Vol. 16, No. 5, 2002 1197 experiments were reported elsewhere.3 A specially designed liquid receiver with extra large volume (300 mL) was used to collect over 99 wt % of liquid products that were free of contamination by washing with solvents (e.g., CS2). This permitted the total liquid products (TLPs) to be further characterized, without prior separation, for simulated distillation (ASTM 2887) and other determinations. The coke yield was determined by in situ combustion of the spent catalyst, followed by measuring the CO2 concentration of the flue gas that was passing through the catalytic reactor to convert CO to CO2. This technique measured the total coke, including the noncatalytic “feed residue coke”,4 which might form on the catalyst surface and reactor inner wall. The same three feeds were also cracked with CAT-A at 490520 °C in a modified ARCO-type riser pilot plant.5 The feed was charged to the bottom of the reactor at a rate of 600 g/h. Nitrogen was used to disperse the feed in the reactor. The cracked products and the catalyst were separated in the disengager at the top of the reactor. The catalyst was trapped while the cracked products passed through the cooler, where TLP was recovered. The spent catalyst moved to the stripper where the entrained oil with the catalyst was stripped off using heated nitrogen. The catalyst then went to the regenerator, for burning off the coke, prior to being recirculated to the bottom of the reactor. Total weight of the catalyst loaded was 2 kg with a makeup rate of 50-100 g daily. The flue gas collected in a gas bag was analyzed by GC. The collected TLPs were distilled into liquefied petroleum gas (LPG), gasoline, light cycle oil (LCO), and heavy cycle oil (HCO) in two distillation units: the True Boiling Point (ASTM D2892) and vacuum (ASTM D1160) units. The coke yield was determined from the concentrations of CO and CO2 produced during burning the spent catalyst.

3. Results and Discussion 3.1. Feedstock Properties. The quality of feedstock charged to an FCC unit is the biggest single factor affecting product yields and quality. Table 1 summarizes the feedstock properties. Among the three feeds, HTDA, which contains 36.9 wt % 524 °C+, has the highest concentrations of total nitrogen (2450 wppm), CCR (2.00 wt %), metals (10.9 wppm Ni+V), and polars (7.1 wt %) that usually contain aromatics and heteroatoms. However, probably due to deasphalting and hydrotreatment, HT-DA has the lowest aromatics content as shown by the lowest aromatic carbon (20.9%), the highest aniline point (81.0 °C), H/C atomic ratio, and GC-MS aromatics. It has the highest API gravity, the highest gas and gasoline precursors (64.9 wt %) in terms of saturates and 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 aromatic sulfur. Thus, as far as catalytic cracking is concerned, HT-DA is considered the most reactive feed. The second feed, VIR, is the only nonhydrotreated feed and is very high in sulfur (3.25 wt %) and aromatics. It has the lowest aniline point, H/C atomic ratio and saturates, but has the highest aromatic carbon and GC-MS aromatics. It also has the lowest API gravity, the lowest gas and gasoline precursors, and the highest LCO precursors. (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 Technology; Magee, J. S., Mitchell, M. M., Jr., Eds.; Studies in Surface Science and Catalysis 76; Elsevier Science Publishers B. V.: Amsterdam, 1993; pp 145-182. (5) Wachtel, S. J.; Baillie, L. A.; Foster, R. L.; Jacobs, H. E. Oil Gas J. 1972 (Apr. 10), 104-107.

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Table 1. Feedstock Properties

Table 2. Equilibrium Catalyst Properties

feed

HTC

HT-DA

VIR

catalyst

density at 15.6 °C, g/mL API gravity, degree refractive index at 20 °C sulfur, wt % total nitrogen, wppm basic nitrogen, wppm hydrogen, wt % carbon, wt % H/C atomic ratio Conradson carbon residue, wt % Ni, wppm V, wppm aniline point, °C aromatic carbon, % 343 °C- by simdist, wt % 524 °C+ by simdist, wt %

0.9511 17.3 1.5323 0.43 2150 439 11.5 87.8 1.562 0.50

0.9430 18.6 1.5269 0.70 2450 613 11.8 87.1 1.619 2.00 4.1 6.8 81.0 20.9 1.5 36.9

0.9712 14.2 1.5397 3.25 1930 610 11.1 85.1 1.549 0.33 0.0 0.1 50.8 25.4 6.0 2.0

35.4 5.0 30.4 57.5 29.5 12.7 5.4 6.5

28.7 1.8 26.9 65.6 22.5 14.4 7.2 10.8

1.2 2.0 0.2 7.1 64.9 14.9

5.0 4.6 1.1 5.7 51.2 21.7

X-ray diffraction unit cell size (fresh), Å unit cell size (equilibrium), Å zeolite content, wt % nitrogen adsorption-desorption total surface area, m2/g zeolite surface area, m2/g matrix surface area, m2/g zeolite/matrix (Z/M) micropore volume (mL/g) zeolite content, wt % Hg porosimetry pore volume, mL/g pore area, m2/g av pore diameter, Å water absorption pore volume, mL/g average particle size, microns SiO2 wt % Al2O3, wt % Re2O3, wt % (on catalyst) Re2O3, wt % (on zeolite) Na2O, wt % TiO2, wt % Fe2O3, wt % CaO, wt % MgO, wt % P2O5, wt % SO4, wt % Ni, wppm V, wppm Cu, wppm

63.6 24.7 4.5 8.3

GC-MS Analysis, wt % saturates 34.4 paraffins 4.7 cycloparaffins 29.7 aromatics 61.8 mono29.5 di13.6 tri6.3 tetra- and up 8.2 aromatic sulfur 2-ring compounds 0.7 3-ring compounds 3.0 4-ring compounds 0.5 polar compounds 3.8 gas and gasoline precursorsa 63.9 light cycle oil (LCO) precursorsb 15.8

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

However, its total nitrogen and CCR (0.33 wt %) are the lowest. Based on the overall quality, VIR is considered the poorest feed. The final feed, HTC, is a sweet VGO with the lowest sulfur (0.43 wt %). Its quality is slightly inferior to that of HT-DA, but is much superior to that of VIR. 3.2. Catalyst Properties. Table 2 gives the properties 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 for HRO); • lower rare earth content on zeolite (7.8 versus 17.5 wt % of HRO). The presence of rare earth in a zeolite increases its stability and activity. The improved stability is due to the formation of polynuclear rare-earthcontaining hydroxy complexes in the zeolite sodalite cages, whereas the improved activity results from the higher number of acid sites through the partial hydrolysis of hydrated rare earth ions [RE(OH2)3+ f REOH2+ + H+].4 In association with these, rare earth impedes zeolite dealumination during hydrothermal treatment, rendering a higher unit cell size. • higher zeolite content and higher zeolite surface area, but lower matrix surface area with a much higher zeolite/matrix (Z/M) ratio (2.00 versus 0.78 of HRO), although the total surface areas of the two catalysts are almost the same. • lower pore volume (0.31 versus 0.45 mL/g of HRO, by water absorption). Our study (Figure 1) indicates a linear correlation between the pore volume by water absorption and that by mercury intrusion, which measures essentially the larger pores in the matrix, typically over 200 Å.6 The correlation gives a corresponding mercury intrusion volume of 0.37 mL/g for HRO. Com-

a

HRO 610

CAT-A

24.66 24.35 n/aa

n/aa 24.28 13

148 65 83 0.78 n/aa 10.6

150 100 50 2.00 0.05 15.6

0.37b 101b 147b

0.22 61 109

0.45 67.9 n/aa 49.7 1.85 17.5 0.31 n/aa 0.66 n/aa n/aa n/aa n/aa 242 434 20

0.31 79.5 55.7 39.7 1.21 7.8 0.19 1.62 0.67 0.09 0.17 0.40 0.01 291 314 n/aa

n/a ) not available. b Estimated value.

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 substantially larger than that of CAT-A (109 Å), providing greater access to heavy molecules that are to be precracked. • relatively high silica (55.7 wt %) and low alumina (39.7 wt %) contents compared with 49.7 wt % Al2O3 for HRO. The bulk silica/alumina ratio in a catalyst cannot correlate with the Si/Al ratio in the zeolite, which is an important parameter that reflects the zeolite quality. The Z/M ratios, unit cell sizes, and rare earth contents of the two catalysts suggest that CAT-A and HRO belong to groups E and B, respectively, according to the classification system by Scherzer4 for FCC catalysts. According to this classification, catalysts in group E are designed to maximize octane-barrels. They consist of zeolites with medium unit cell sizes and matrixes of medium activities. These catalysts have a moderate ability to crack bottoms and give moderate yields of C3+C4 hydrocarbons. Group B catalysts, compared with those in group E, consist of zeolites with a larger unit cell size, higher rare earth contents, and an active matrix. These catalysts are good for cracking bottoms or resids. In general, they also have good gasoline selectivity, generate less C3+C4 hydrocarbons and more coke. Other than the quantity and quality of zeolite and active matrix, both of which determine the activity and selectivity of a catalyst, the additives and metals in the (6) Peters, A. W. Instrumental Methods of FCC Catalyst Characterization. In Fluid Catalytic Cracking: Science and Technology; Magee, J. S., Mitchell, M. M., Jr., Eds.; Studies in Surface Science and Catalysis 76; Elsevier Science Publishers B. V.: Amsterdam, 1993; p 193.


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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 and three catalysts.

catalyst can also affect the cracking performance. X-ray analysis suggests that CAT-A, as received, may contain some ZSM-5, a shape-selective zeolite. Conventionally, ZSM-5 is added to increase octanes through preferential cracking of low-octane, straight-chain olefins and paraffins (mostly C7+) in gasoline to smaller compounds, preferentially olefins, and by isomerization of the lowoctane linear olefins to branched and high-octane olefins. As a result, gasoline has higher octane numbers but its yield is lower, whereas yields of C6- increase with higher iso-to-normal ratios for aliphatics of the same carbon number. Although HRO underwent considerable hydrothermal deactivation, as reflected by a drop of unit cell size from 24.66 to 24.35 Å, the two catalysts were not seriously contaminated in the refinery FCC operation. The dehydrogenation activity of nickel, vanadium, copper, and iron, expressed as equivalent nickel (Ni + V/5 + Cu + Fe/10) is 809 wppm for HRO and 824 wppm (excluding Cu) for CAT-A. It is not clear if the relatively high

concentrations of TiO2 and P2O5 in CAT-A are indications of the presence of a metal trap or passivator. 3.3. Conversion and Yield Structure. In this study, since the reaction temperature, the catalyst weight, and the catalyst contact time are all fixed for a given 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 Figures 2-9. Key observations are given below. 3.3.1. Comparison of the Performance Between ACE and Fluid-Bed MAT. Figure 2 compares the coversions of HT-DA obtained from two reaction systems, NCUT’s conventional and ACE proprietary, both using fixed fluid-bed reactors but with different operating protocols. Typical ACE testing conditions have been reported elsewhere.2 Figure 2 shows that for HT-DA cracked with three catalysts, the two systems gave essentially the same conversion profiles, though the absolute conver-

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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.

sion levels tended to be different at a given C/O ratio. Yield profiles (product yields versus conversion) from the 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 yield relative to NCUTs results for the two catalysts. We are



Figure 6. Relationship between gasoline yield and conversion.

Figure 7. Relationship between LCO yield and conversion.

in the process of having these reverse trends confirmed by another ACE reactor. Note that in Figure 2 an equilibrium catalyst Cobra 54 from Akzo Nobel was included for comparison purposes. 3.3.2. Conversion. Figure 3 illustrates the increases in conversion with C/O ratio for all feeds and catalysts, based on the fluid-bed MAT results. At a given C/O ratio, and for the same catalyst, HT-DA gave the highest conversion, followed by HTC (about 8 wt % less), which gave slightly higher or similar conversion than VIR. This trend agrees with the feed quality in general. At a given C/O ratio, and for the same feed, HRO gave 3-8 wt % higher conversion than CAT-A, depending on the C/O ratio and the feed type. This is to be expected, based on the differences in their matrix characteristics, rare earth contents, and unit cell sizes.4 For the two hydrotreated feeds, the paired conversion profiles from the two catalysts tended to converge at high C/O ratio, but for VIR the paired profiles remained far apart throughout. This suggests that the lower activity of CAT-A could be compensated for by higher C/O ratios (more catalyst per 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 less pronounced for VIR due to its lower nitrogen content and its more refractory nature in terms of having more large aromatic molecules (two and higher ring aromatics), as indicated in Table 1. 3.3.3. Dry Gas. Dry gas (H2, H2S, and C1-C2 hydrocarbons) is a low value product that should be kept to a minimum. Excessive dry gas production may cause limitations in the plant operation in terms of gas compression. Figure 4 shows that yields of dry gas increased with conversion for all feeds and catalysts. Components in the dry gas are secondary products from thermal cracking and catalytic cracking of gasoline and butenes.7 These cracking reactions are mostly nonselective, resulting in yield profiles parallel to each other. Being an end product, similar to coke, dry gas should exhibit an exponential increase in yield at higher conversions. Figure 4 shows that for the same catalyst and at a given conversion, VIR produced 0.7 to 1 wt % (7) John, T. M.; Wojciechowski, B. W. J. Catal. 1975, 37, 348-357.

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Figure 8. Relationship between HCO yield and conversion.

Figure 9. Relationship between coke yield and conversion. Table 3. Dry Gas, Hydrogen, and Hydrogen Sulfide Yields at 65 wt % Conversion HRO 610

CAT-A

HTC HT-DA VIR HTC HT-DA VIR hydrogen, wt % 0.13 hydrogen sulfide, wt % 0.02 dry gas, wt % 1.89

0.16 0.09 1.82

0.13 0.16 1.00 0.02 2.84 2.22

0.19 0.06 2.45

0.16 0.77 3.08

more dry gas than HT-DA, which gave a higher (by 0.3 wt %) or similar yield than HTC, depending on the catalyst used. This was not surprising since VIR contained much higher sulfur (3.25 wt %) than HT-DA (0.7 wt % S) and HTC (0.43 wt % S), while about 40 wt % feed sulfur appeared in the reactor gas as H2S.8 Table 3 shows the yields of dry gas, hydrogen, and hydrogen sulfide at 65 wt % conversion. Evidently, H2S was predominant in dry gas for VIR but not for the other two feeds, which were hydrotreated leaving mostly the hard-to-remove aromatic sulfur species, the majority of which were retained in cycle oils and coke after crack(8) Keyworth, D. A.; Reid, T. A.; Asim, M. Y.; Gilman, R. H. 1992 NPRA Annual Meeting, AM-92-17. National Petrochemical & Refiners Association: Washington, DC, 1992.

ing. For HT-DA and HTC, their dry gas yields, relative to each other, should be determined by reaction temperature (20 °C higher for HT-DA) and feed quality including sulfur content and the tendency of coke formation, which could affect the nonselective cracking through pore blockage of catalysts. As a result, HT-DA gave 0.3 wt % higher dry gas yield than HTC when CAT-A was used. The effects were not pronounced in the case of HRO. This was probably related to CAT-A’s higher gas-forming tendency, which will be discussed later. Table 3 also indicates that for the same feed, HRO produced 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-A could be attributed partly to its more isolated acid sites, resulting from a smaller zeolite cell constant, which enhanced gas make.9 Higher hydrogen yield also could be attributed to a higher level of metal contaminants, such as nickel, which is known as a strong dehydroge(9) Pine, L. A.; Maher, P. J.; Wachter, W. A. J. Catal. 1984, 85, 466476.



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

yield, wt % propane propylene n-butane i-butane n-butenes i-butene LPG olefinicity,a g/g C3 C4 LPG hydrogen transfer indexb iso-to-normal ratio, g/g i-butane/ n-butane i-butene/ n-butenes increase in C3d/ increase in C4d (relative to HRO), g/g a

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/HTC

HT-DA/ HTC

VIR/ HTC

0.8 2.49 0.59 2.93 1.96 0.58 9.44

0.69 2.61 0.50 1.98 2.39 1.00 9.17

0.91 2.58 0.54 2.75 1.92 0.73 9.43

1.58 3.00 0.93 4.05 2.09 0.78 12.43

1.42 3.62 0.82 3.33 2.59 1.25 13.03

1.64 3.00 0.86 3.82 1.95 0.88 12.15

1.78 1.21 1.58 1.38 1.07 1.33 1.32

2.08 1.38 1.63 1.69 1.08 1.25 1.42

1.80 1.16 1.60 1.39 1.02 1.20 1.29

1.00 1.00 1.00 1.00 1.00 1.00 1.00

1.17 1.15 1.03 1.22 1.01 0.93 1.08

1.01 0.97 1.01 1.01 0.95 0.90 0.98

0.74 0.42 0.53 0.20

0.79 0.58 0.65 0.51

0.74 0.45 0.55 0.27

0.66 0.37 0.47 0.19

0.72 0.48 0.57 0.37

0.65 0.38 0.48 0.23

0.89 0.87 0.89 0.96

0.91 0.83 0.87 0.74

0.88 0.84 0.87 0.87

4.96

3.94

5.07

4.34

4.09

4.42

0.87

1.04

0.87

0.30

0.42

0.38

0.37

0.48

0.45

1.25

1.15

1.18

1.00

1.00

1.00

1.54

2.24

2.31

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

nation catalyst. This was aggravated by the fact that CAT-A, with lower activity, required much higher C/O ratio to achieve the same conversion as obtained by HRO (Figure 2). However, since both catalysts contain relatively low Ni and V, the dehydrogenation activities involved may not be very strong. As indicated in Table 3, CAT-A produced more H2, which could have saturated some of the aromatic sulfur species before they were cracked to form H2S.10 However, the resulting H2S yield from CAT-A was less. It is possible that the higher H2S yield 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 hydrocarbons) is considered a valuable product since it consists of components that can be used as alkylation and petrochemical feedstocks. For example, propylene is widely used in the polymer industry; isobutylene is the building block for methyltertiarybutyl ether (MTBE) and ethyltertiarybutyl ether (ETBE), the octane boosters; and isobutane can be alkylated with C3 to C5 olefins to form high-octane branched compounds that makeup 11% of the gasoline pool in US refineries.11 The slightly concave yield profiles of LPG in Figure 5 indicate that, as conversion increased, the combined cracking rate of gas oil feed and gasoline to form LPG was marginally higher than its conversion rate to form dry gas and coke. Among the three feeds, HT-DA (the lightest in terms of density), being cracked at a higher temperature, gave about 1 wt % more LPG than the other two feeds when CAT-A was used, but showed little difference in the case of HRO. The higher discrimination on LPG yields by CAT-A was probably related to its tendency to produce more gas. Between the two catalysts, CAT-A gave a higher LPG yield than HRO by 3-4 wt % for the same feed. As LPG is an important 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. Hydrocarbon Process. 1985, (Sept.), 67-71.

product, it is worthwhile to examine the detailed distribution in LPG produced. Table 4 gives a breakdown of LPG for all feeds and catalysts at 65 wt % conversion. The following comments are offered. •Propylene and isobutane are the two most prominent single compounds. Propylene is a product from primary cracking of a gas oil feed. It can also be formed from secondary cracking of gasoline and butene.7 The abundance of isobutane, a secondary product, is a result of hydrogen transfer after the prevailing olefin cracking and isomerization. •All individual yields increased when the catalyst was switched from HRO to CAT-A, with higher increases in paraffins than in olefins for the same feed, possibly due to a richer hydrogen environment. As a result, olefinicities of C3, C4, and LPG, and hydrogen transfer indices (defined as isobutene-to-isobutane ratio12) were lower when CAT-A was in use. Relative to HTC, HT-DA and VIR showed higher and equivalent selectivities, respectively, for each LPG component, except for i-butene. •When CAT-A was used, the iso-to-normal ratios of butenes were higher and the increase in propylene was approximately twice as much as the increase in butenes, as compared with the case of HRO. These observations supported the X-ray analysis that CAT-A might contain the shape selective catalyst ZSM-5,13 which was partially responsible for the increases in LPG and dry gas, including H2. It is not known at this stage why higher iso-to-normal ratios of butanes from the same feed, as compared with the case of HRO, were not observed when CAT-A was used. 3.3.5. Gasoline. Gasoline (C5-221 °C boiling point) is the major and the most desirable product in FCC (12) McClung, R. G.; Dodwell, G. In Proceedings of the Engelhard FCC Seminar; Venice, Italy, June 11-13, 1997. (13) Dwyer, F. G.; Degnan, T. F. Shape Selectivity in Catalytic Cracking. In Fluid Catalytic Cracking: Science and Technology; Magee, J. S., Mitchell, M. M., Jr., Eds.; Studies in Surface Science and Catalysis 76; ; Elsevier Science Publishers B. V.: Amsterdam, 1993; pp 499-530.

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

HT-DA

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

VIR

HTC

HRO

CAT-A

HRO

CAT-A

HRO

CAT-A

i-C5 n-C5 i-C5/n-C5

2.63 0.21 12.59

3.63 0.29 12.35

2.06 0.25 8.25

2.94 0.30 9.72

2.60 0.24 10.97

3.11 0.25 12.35

i-C6 n-C6 i-C6/n-C6

0.85 0.08 10.39

1.16 0.09 12.99

1.24 0.15 8.46

1.29 0.13 9.56

0.81 0.08 10.58

0.69 0.06 11.01

i-C7 n-C7 i-C7/n-C7

1.78 0.11 16.64

1.05 0.07 14.42

2.70 0.30 8.90

1.11 0.12 8.92

1.59 0.10 15.23

0.74 0.06 12.52

i-C8 n-C8 i-C8/n-C8

1.30 0.09 13.91

0.85 0.06 14.14

1.58 0.15 10.32

0.87 0.10 9.12

1.11 0.10 11.55

0.64 0.06 10.80

i-C9 n-C9 i-C9/n-C9

1.04 0.08 12.33

0.57 0.05 11.97

1.11 0.13 8.85

0.70 0.08 8.79

0.83 0.07 11.12

0.51 0.05 9.50

C10 i-C10 n-C10 i-C10/n-C10

0.81 0.07 11.88

0.45 0.05 9.86

0.87 0.11 8.29

0.51 0.07 7.29

0.61 0.06 10.20

0.28 0.06 4.40

C5

C6

C7

C8

C9

operation. Figure 6 depicts the gasoline yield profiles for all feeds and catalysts. Overcracking was observed for HT-DA at about 70 wt % conversion, and was expected for HTC and VIR at lower conversions since they contained more aromatics. HTC had 1.5-2 wt % higher gasoline yield than VIR over the entire range of conversion in this study. HT-DA showed lower gasoline selectivity than HTC below 58-61 wt % conversion, but higher, up to 3 wt % yield or equivalent magnitude, thereafter, depending on the catalyst used. This crossover was related to the higher nitrogen and CCR contents of HT-DA, which seriously poisoned the catalyst at low conversions. As C/O ratio increased, resulting in higher conversion and less catalyst poisoning, gasoline yield of HT-DA increased sharply and eventually exceeded that of HTC. HT-DA could have had even higher gasoline yield if it were cracked at a lower temperature. The striking feature of Figure 6 is that HRO outperformed CAT-A by 4-6 wt % in gasoline for the same feed at a given conversion. This could be attributed to the following factors. •Compared with CAT-A, HRO was 35% higher in rare earth, which reduced aluminum loss from the framework of zeolite under hydrothermal conditions. This enabled HRO to maintain more acid sites, resulting in increased activity, and a higher hydrogen transfer rate;4 the net result: more gasoline produced, higher conversion, but also higher coke yield. • The active matrix in HRO, with large pores, played a significant role in precracking the big molecules into smaller fragments, which then gained access to acid sites in zeolite pores for further cracking. When CAT-A was used, the gasoline selectivity of HT-DA was lower probably due to the added ZSM-5, which selectively cracked the linear or singly branched C7+ paraffins and olefins in gasoline into smaller olefins and isoparaffins, some of which fell into LPG range.13 This is to be discussed later in more detail. Tables 5 and 6 summarize yield distributions determined from PIONA analysis for normal- and isoalkanes

HT-DA

VIR

HRO CAT-A HRO CAT-A HRO CAT-A C5d i-C5d n-C5d i-C5d/n-C5d C6d i-C6d n-C6d i-C6d/n-C6d C7d i-C7d n-C7d i-C7d/n-C7d C8d i-C8d n-C8d i-C8d/n-C8d C9d i-C9d n-C9d i-C9d/n-C9d C10d i-C10d n-C10d i-C10d/n-C10d

0.68 0.71 0.95

0.82 0.67 1.23

1.29 1.14 1.14

1.29 0.90 1.44

0.93 0.77 1.20

0.82 0.61 1.33

0.41 0.23 1.76

0.46 0.20 2.33

1.05 0.61 1.71

0.89 0.39 2.30

0.22 0.27 0.84

0.42 0.18 2.39

0.16 0.21 0.76

0.39 0.12 3.18

0.15 0.39 0.38

0.70 0.21 3.25

0.22 0.23 0.97

0.42 0.13 3.36

0.46 0.14 3.26

0.30 0.07 4.09

0.84 0.32 2.63

0.52 0.12 4.19

0.50 0.14 3.49

0.34 0.08 4.32

0.37 0.08 4.44

0.19 0.04 4.87

0.73 0.18 3.98

0.34 0.07 4.71

0.36 0.08 4.50

0.22 0.04 5.41

0.26 0.04 6.00

0.11 0.02 4.57

0.48 0.09 5.50

0.20 0.04 4.78

0.22 0.03 6.79

0.13 0.02 5.15

Table 7. Comparison of Actual Product Yields at 70 wt % Conversion with Precursor Concentrations HRO dry gas, wt % LPG, wt % gasoline, wt % total gas-plusgasoline, wt % gas-plus-gasoline precursors, wt % LCO, wt % LCO precursors, wt % polars

CAT-A

HTC HT-DA

VIR

HTC HT-DA

VIR

2.2 11.2 46.8 60.2

2.2 11.2 49.4 62.8

3.2 11.2 45.8 60.2

2.5 14.0 43.2 59.7

2.8 14.9 43.2 60.9

3.5 13.9 42.0 59.4

63.9

64.9

51.2

63.9

64.9

51.2

19.8 15.8

19.4 14.9

21.2 21.7

17.5 15.8

16.7 14.9

18.6 21.7

3.8

7.1

5.7

3.8

7.1

5.7

and alkenes, in gasoline at 65 wt % conversion for all feeds and catalysts. In general, for the same feed and catalyst, the yields gradually decreased at higher carbon number 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 the calculations, whereas the gaseous C5 components were. In comparison with HRO, Table 5 generally indicates that 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 same carbon number and for the same type. Slight differences were noticed for alkenes in Table 6. Compared with HRO, for the same feed and for the same 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 consequence of the shape-selective ZSM-5 in CAT-A. Although



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

CAT-A and HRO are of different bases, which lead to individual product slates, the observed phenomena agree with the reported findings for ZSM-5 in the literature,13 namely, that ZSM-5 selectively cracks the linear and monomethyl C7+ olefins and, less preferably, paraffins. Some of the resulting olefins can undergo facile catalytic isomerization, or form paraffins through hydrogen transfer. The facile isomerization leads to higher iso-to-normal ratios for the cracked olefins and paraffins. As a result, there is a net decrease in the amount of low-octane C7+ olefins and a corresponding increase in the amount of high-octane C5 and C6 compounds. Table 7 compares the actual yields of gas-plusgasoline and LCO (to be discussed later), at 70 wt % conversion, against their corresponding precursor concentrations, based on hydrocarbon types by GC-MS (Table 1). It is assumed that saturates and monoaromatics are the precursors that will predominantly contribute to the production of gasoline and its derived products,14 including dry gas, LPG, and some catalytic coke, whereas diaromatics, 2-ring aromatic sulfur, and one-half of the 3-ring aromatic sulfur, are precursors of LCO. The precursor concentration sets an upper limit for gas-plus-gasoline yield, or a lower limit for LCO yield, as a guide. Beyond this limit, the feed tends to produce excessive gas and coke rather than the valuable liquid products. Although it is a useful concept to determine precursor concentrations based on hydrocarbon types, accurate assignment of the components to the precursor categories can be challenging. This is because the analysis by MS for the hydrocarbon type (CnH2n+z) is based on the hydrogen deficiency in the molecule and the results are reported based on the z numbers. For example, simple naphthenobenzenes such as C10H12 (z ) -8), classified by MS as monoaromatics, can undergo hydrogen transfer to yield LCO rather than gasoline, and thus provide an overestimate of the gasplus-gasoline yield. Likewise, fluorenes such as diphenylenemethane C13H10 (z ) -16), classified as diaromatics, can be cracked into a gasoline fraction under (14) Fisher, I. P. Appl. Catal. 1990, 65, 189-210.

proper conditions, since the two phenyl groups are connected through single carbon-carbon bonds that are crackable. This will provide an overestimate of the LCO yield. Further, polars, mostly aromatics that contain heteroatoms, and part of the coke yield, are not included in the precursor formulas. These will contribute more uncertainties to precursor concentrations. Nevertheless, compositional analysis by MS is generally used as a guide to predict the maximum or minimum yield of a cracked fraction. There may be a special challenge for gas oils derived from heavy oils and bitumens, which contain substantial amounts of naphthenoaromatic compounds. Table 7 shows that at 70 wt % conversion, where maximum gasoline yields occurred for HT-DA, the differences between actual and predicted maximum gas-plus-gasoline yields were within 4.5 wt %, except for those of VIR, which gave abnormally high actual yields. Increasing conversion will further narrow the gaps between the actual and predicted yields in some cases (Figures 4-7). 3.3.6. LCO. LCO (221-343 °C boiling point) is a product whose value depends on the location of the refinery. In some countries where gasoline is not in high demand, the FCC unit is used as a major producer of middle distillate because of its low capital cost (onefourth of that of a hydrocracker). With proper design and 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 initial boiling point (∼182 °C). In Europe and the U.S., many refiners seek seasonal means of increasing LCO yields to meet higher fuel oil demands in the winter. In the western U.S. and Canada, LCO has less market value because of warmer weather and/or the popular use of natural gas for home heating. Thus, LCO is not a highly desirable product, especially when derived from aromatics-rich feeds. The cetane number of LCO from FCC is generally low (less than 30) and its sulfur content is high. The lower the LCO yield, the poorer its quality due to the enrichment of aromatics at high conversion. (15) Ritter, R. E. 1988 NPRA Annual Meeting, AM-88-57. National Petrochemical & Refiners Association: Washington, DC, 1988.

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In this study, all feeds contain less than 6 wt % heavy fraction, which boils in the LCO range (Table 1). One can imagine that as conversion increases, the LCO yield should increase initially and then decrease when the rate of its formation (from cracking of the 343 °C+ fraction) is exceeded by that of its decomposition into lighter fractions. Figure 7 shows the general parabolic decreases in LCO yields with increased conversion. In theory, all yield curves should converge and reach zero at 100 wt % conversion. In general, VIR gave relatively higher LCO selectivity than HTC, which in turn was slightly higher in LCO yield than HT-DA. This is in line with LCO precursor concentrations of 21.7, 15.8, and 14.9 wt %, for VIR, HTC, and HT-DA, respectively. Table 7 shows that at 70 wt % conversion, the differences between actual and predicted minimum LCO yields are within 4.5 wt %. Between the two catalysts, HRO gave higher LCO selectivity than its counterpart for the same feed. This is related to HRO’s large pore martrix, which also contains acid sites usually associated with aluminum atoms. The matrix in HRO is believed to have higher activity than that in CAT-A, to facilitate deeper bottoms cracking that leads to lower HCO but higher LCO and gasoline yields. 3.3.7. HCO. HCO (343-525 °C boiling point) is an unwanted, unconverted product with comparatively high aromatics and sulfur. Its yield should be reduced to the minimum. Figure 8 shows the monotonic decreases in HCO yield as conversion increased. The yield magnitude of HCO for all feeds and catalysts at a given conversion should be in the reverse order to that of LCO, based on the simple relationship HCO ) 100 - conversion - LCO. Figure 8 indicates that HT-DA gave slightly higher HCO selectivity than HTC, which in turn was higher in HCO yield than VIR. Between the two catalysts, HRO gave lower HCO selectivity than its counterpart for the same feed. Similar to LCO, all HCO yield curves should converge and reach zero at 100 wt % conversion. 3.3.8. Coke. In FCC operation, coke is necessary in order to supply heat for feed preheat and cracking in a much more economical and efficient fashion than using alternative liquid fuels such as torch oil or refinery fuel.16 However, too much coke can overload the air blower during catalyst regeneration, cause excessively high temperature in the regenerator, and seriously poison the catalyst. Figure 9 shows the exponential increase in coke yield with conversion. In general, for the same catalyst, VIR had the highest coke selectivity, followed by HTC and HT-DA, for which the coke yield could be higher if it were cracked at a lower temperature. Between the two catalysts, CAT-A gave 0.5-1.7 wt % more coke than HRO for the same feed, depending on the feed type and conversion. In this respect, pairs of yield curves for the same feed showed an irregular shape with smaller differences at both ends. This implies that HRO, for which the lines have greater slopes at high conversions, is more sensitive to conversion in coke formation than CAT-A. Under the current test conditions, coke yield can be represented in an alternative format that gives a linear relationship between coke yield and C/O ratio (Figure 10), as shown below. (16) Blazek, J. Catalagram 1987, 75, 1-3.

Ng et al.

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 where catalytic coke is generated from the cracking that occurs at the acid sites of the zeolite and matrix; feed residue coke4 is contributed by carbon residue in the feed; contaminant coke4 derives from catalytic dehydrogenation of metal poisons such as Ni, V, Fe, and Cu; A and n are empirical constants which are catalyst-andfeedstock dependent in Voorhie’s equation;17 and t is the catalyst contact time, which is 30 s in this study. Figure 10 shows that, for the same catalyst and at a given C/O ratio, HT-DA, a more reactive feed with higher CCR, produced more coke, followed by HTC and VIR. Between the two catalysts, HRO with higher activity gave a higher coke yield than CAT-A. Moreover, pairs of straight lines representing the two catalysts for the 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 % coke for HTC, HT-DA, and VIR, respectively. These were 0.8-1.0 wt % higher than their corresponding CCR values of 0.50, 2.00, and 0.33 wt %. Thus, it is believed that the coke yield in this study could be about 1 wt % higher than the expected value. It is possible that some heavy product (HCO) was not recovered at the “cold” spot near the exit of the reactor, but rather was picked up later as coke at a higher reactor temperature (600 °C) during catalyst in situ regeneration. Again, in Figure 10 HRO showed a greater slope than CAT-A for the same feed. The lower coke-making tendency of CAT-A (containing USY zeolite) observed in Figure 10 can be related to its smaller unit cell size as reported in the literature18 (also observed in our ealier work19) and the improved matrix technology, as claimed by the manufacturer. However, the lower coke-making tendency of CAT-A is offset by its much lower conversion compared with that by HRO (at the same C/O ratio), of which the wide pore matrix facilitates effective cracking of large molecules of heavy gas oils. It is thus believed that the coke-making tendency of a Y zeolite is better judged from a “coke yeild-C/O ratio” plot rather than from a “coke yield-conversiobtn” plot, where the conversion 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 the shapes of the yield curves in Figure 9 are a reflection of the results in Figure 3 and Figure 10. In FCC, delta coke (the amount of coke per unit weight of spent catalyst) is an important parameter, the magnitude of which affects the heat balance. Delta coke (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.



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 conversion, wt % dry gas, wt % LPG, wt % gasoline, wt % LCO, wt % HCO, wt % coke, wt % delta coke, wt %

8.92 65.00 1.77 9.36 46.46 21.53 13.47 7.28 0.82

10.84 65.00 2.11 12.44 42.48 18.45 16.55 7.97 0.71

1.92 0.00 0.34 3.07 -3.98 -3.08 3.08 0.69 -0.11

3.82 65.00 1.82 9.28 48.24 21.25 13.75 5.82 1.45

6.35 65.00 2.44 13.14 42.40 17.64 17.36 7.10 1.13

2.53 0.00 0.63 3.86 -5.85 -3.62 3.62 1.28 -0.32

9.07 65.00 2.84 9.44 45.27 23.05 11.95 7.43 0.83

12.66 65.00 3.14 12.07 41.18 20.04 14.95 8.61 0.68

3.59 0.00 0.30 2.63 -4.09 -3.01 3.00 1.18 -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 conversion, wt % dry gas, wt % LPG, wt % gasoline, wt % LCO, wt % HCO, wt % coke, wt % delta coke, wt %

8.49 64.61 1.74 9.27 46.42 21.65 13.73 7.00 0.83

9.34 62.20 1.88 11.59 41.70 18.76 19.04 7.00 0.75

0.85 -2.41 0.14 2.32 -4.72 -2.89 5.30 0.00 -0.08

5.50 67.91 2.02 10.31 49.16 20.19 11.90 7.00 1.31

6.18 63.97 2.37 12.83 42.03 17.74 18.29 7.00 1.16

0.67 -3.94 0.35 2.53 -7.13 -2.46 6.40 0.00 -0.15

8.49 65.40 2.87 9.56 45.34 22.92 11.68 7.00 0.82

9.51 60.66 2.86 10.97 39.61 20.78 18.56 7.00 0.73

1.03 -4.74 -0.01 1.41 -5.73 -2.14 6.88 0.00 -0.09

is calculated from coke yield divided by the C/O ratio. Figure 11 shows the linear decline of delta coke with conversion, for all feeds and catalysts. This is understandable since higher C/O ratio or less feed per unit weight of catalyst is necessary to achieve higher conversion. Figure 11, similar to Figure 10, indicates that HTDA gave the highest delta coke, followed by HTC and VIR, for the same catalyst and at a given conversion. Between the two catalysts, HRO produced more delta coke than CAT-A for the same feed at a given conversion. 3.4. Comparison of Catalyst Performance on Common Bases. To assess catalyst cracking performance, MAT results are compared at both constant conversion and constant coke yield. Table 8 compares product yields at 65 wt % conversion where gasoline yields are close to maximum values. In comparison with HRO and for the same feed, CAT-A produced more dry gas, LPG, HCO, and coke, but much less gasoline and LCO, and slightly less delta coke. Further, to achieve this conversion, much higher severities in terms of C/O

ratios were necessary for CAT-A. Thus, for producing more premium products, it may be preferable to use HRO over CAT-A, although the latter may result in a better quality gasoline. The increased LPG yield by CAT-A cannot compensate quantitatively for the decreased 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 process resid-containing feeds, CAT-A yielded lower conversion with much less premium distillates, more LPG and undesirable products (dry gas and HCO). The decreases in delta coke were negligible. CAT-A also required slightly higher C/O ratio to maintain the same coke yield. Again, HRO is the preferred choice to crack these feedstocks for higher yields of valuable products. 3.5. Comparison of Product Yields between MAT and Riser. Table 10 shows MAT and pilot plant product distributions for all three feeds, using CAT-A at 55 and 65 wt % conversion, respectively. The table indicates that, except for coke yield, the bias in MAT yields relative to those of the riser pilot plant can be main-

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Ng et al.

Figure 12. Relationship between predicted and actual riser yields. Table 10. Comparison of Product Yields (wt %) at 55 and 65 wt % Conversion between MAT and Risera conversion feed dry gas MAT riser biasb LPG MAT riser biasb gasoline MAT riser biasb LCO MAT riser biasb HCO MAT riser biasb coke MAT riser biasb

55 HTC

HT-DA

65 VIR

HTC

HT-DA

VIR

1.5 1.7 -11.8

1.9 1.8 5.6

2.6 3.0 -13.3

2.1 2.4 -12.5

2.4 2.5 -4.0

3.1 4.0 -22.5

9.5 9.2 3.3

10.6 9.9 7.1

9.6 10.7 -10.3

12.4 11.6 6.9

13.1 12.1 8.3

12.1 12.0 0.8

39.1 40.9 -4.4

36.8 39.4 -6.6

37.3 38.2 -2.4

42.5 46.3 -8.2

42.4 44.9 -5.6

41.2 44.3 -7.0

19.4 20.0 -3.0

17.6 20.4 -13.7

21.2 20.6 2.9

18.4 16.1 14.3

17.6 17.9 -1.7

20.0 17.4 14.9

25.6 25.0 2.4

27.4 24.6 11.4

23.8 24.4 -2.5

16.6 18.9 -12.2

17.4 17.1 1.8

15.0 17.6 -14.8

5.0 3.2 56.3

5.6 3.9 43.6

5.6 3.1 80.6

7.9 4.7 68.1

7.1 5.5 29.1

8.6 4.7 83.0

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

tained within 15%, in general. This is considered acceptable based on the great differences in reactor design and operation between the two systems. Compared with the riser, the MAT unit tends to give lower dry gas and gasoline, higher LPG, and much higher coke yields, due to the in situ combustion method for total coke yield determination. The deviation in coke yield obtained from the MAT is further aggravated by the consistently higher error by ∼1 wt % (absolute) and much longer catalyst contact time (30 s in the MAT versus just a few seconds in the riser) resulting in higher coke formation on the catalyst in the MAT according to Voorhie’s equation.17 Assuming linear correlations between riser yields and MAT yields, the predicted riser yields based on MAT data (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 the coefficients of determination (R2) for every single set of correlations. R2 values for dry gas, gasoline, and HCO are acceptably good (over 0.91) but for the rest of products, especially LCO, are poor. This is because both the riser and the MAT data are confined to a rather narrow range (Table 10). Thus, from a statistical point of view, it is more difficult to obtain a good correlation in this case, as compared with the case where the data (containing the same error as their counterparts) are spread over a wider range. As well, one or two bad outliers can easily upset the statistical balance if too few data points (only 6 per product in this case) are used for correlation. On the whole, the data points representing the predicted and actual riser yields lie reasonably close to the 1:1 line. 4. Conclusions To achieve higher yields of valuable distillates when cracking oil-sands-derived VGOs, HRO, a bottomscracking catalyst containing REY zolite and a large-pore active matrix, is more suitable than CAT-A, an octanebarrel catalyst containing REUSY/ZSM-5 zeolites and an active matrix. With respect to conversion and product slate, 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 cracking phenomena can be linked to feedstock and catalyst properties. MAT yields can be correlated with riser pilot plant results, although their absolute values can be very much different. Acknowledgment. The authors wish to thank the Analytical Laboratory of the National Centre for Upgrading Technology (NCUT) for their technical support. Partial funding for NCUT has been provided by the Canadian Program for Energy Research and Development (PERD), the Alberta Research Council, and the Alberta Energy Research Institute. EF0200368

FCC Study of Canadian Oil-Sands Derived Vacuum Gas Oils. 1. Feed

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