Fluid Catalytic Cracking Selectivities of Gas Oil Boiling Point and

Jul 1, 1996 - Columbia, Maryland 21044. The product selectivities of the fluid catalytic cracking (FCC) process are strongly dependent on the properti...
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Ind. Eng. Chem. Res. 1996, 35, 2561-2569

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Fluid Catalytic Cracking Selectivities of Gas Oil Boiling Point and Hydrocarbon Fractions Robert H. Harding,*,† Xinjin Zhao,‡ Kuangnan Qian,† Kuppuswamy Rajagopalan,‡ and Wu-Cheng Cheng‡ W. R. Grace & Co.sConn. and Grace Davison, Washington Research Center, 7500 Grace Drive, Columbia, Maryland 21044

The product selectivities of the fluid catalytic cracking (FCC) process are strongly dependent on the properties of the petroleum gas oil reactant. In order to elucidate the complex relationship between gas oil chemical composition and product selectivity, a new technique has been developed which experimentally determines the product distribution of specific gas oil fractions in a realistic chemical environment. This “incremental yield analysis” approach is examined with a representative industrial gas oil. The gas oil is characterized and then divided into boiling point fractions by distillation and into hydrocarbon-type fractions with a chromatographic method. The FCC selectivity of each gas oil fraction is then determined by microactivity testing of blends of each fraction with the original gas oil. Results show that hydrocarbon type is a more significant determinant of the product spectrum than boiling point. Introduction Fluid catalytic cracking (FCC) is an important industrial process which is used to upgrade heavy petroleum gas oils into gasoline, diesel fuel, light olefins, and other valuable products (Venuto and Habib, 1979). Naturallyoccurring gas oils are a complex blend of hydrocarbons, including paraffins, isoparaffins, naphthenes, aromatics, and asphaltenes. A typical gas oil also contains significant quantities of multiringed molecules containing heteroatoms, such as nitrogen and sulfur. Worldwide gas oil reserves are evolving toward higher molecular weight crudes and a greater concentration of heteroatoms with continued use. Therefore, a great deal of current research is oriented toward the development of catalysts and processes for cracking the heavy end of the gas oil (Mitchell et al., 1993). In this paper, we explore the FCC cracking kinetics and selectivities of a gas oil as a function of boiling point range and feed composition. Past research in this area has tended toward correlating the properties of a wide range of gas oils with the products produced by a single catalyst (Fisher, 1990; Pachovsky and Wojciechowski, 1975). Historically, it has been difficult to deconvolute the effects of molecular weight and hydrocarbon type, since these properties tend to correlate in natural gas oils. The kinetic rates and selectivities of gas oil during cracking are difficult to determine due to the convolution of the kinetics with deactivation, diffusion, competitive adsorption, and bimolecular interactions such as hydrogen transfer (Pine et al., 1984). In order to gain a better understanding of the determinants of FCC selectivities, researchers have studied a hierarchy of hydrocarbon mixtures. The simplest and most complete experiments have focused on the catalytic cracking of single model compounds (Townsend and Abbot, 1993; Corma et al., 1992). These experiments have determined the primary kinetic pathways for many of the individual components in the gas oil. These model compound experiments have elucidated the basic carbenium ion pathways of FCC (Venuto and Habib, 1979) and have explored the role of second† ‡

W. R. Grace & Co.sConn. Grace Davison.

ary pathways, which depend on carbonium (Haag and Dessau, 1984) and cyclopropenium (Sie, 1993) intermediates. Although a significant quantity of kinetic information has been derived at this level, studies of single model compounds do not completely address the selectivity of that model compound in the gas oil environment or the interaction of gas oil molecules (Martin et al., 1989). Model compound tests also tend to be reported at low coke levels. The variation of coke levels makes rate comparisons difficult (Lin et al., 1992). In addition, since model compounds representing high molecular weight gas oil species are difficult to obtain, these experiments do not typically address the role of gas oil boiling point or issues such as concarbon coke production. A second tier of experiments have used mixtures of model compounds to explore FCC cracking kinetics and selectivities. Mixtures of typically 2-5 individual model compounds have given much insight into the role of competitive adsorption (Abbot and Wojciechowski, 1987; Santilli and Zones, 1990), initiation (Abbot, 1990), and bimolecular reactions (Krannila et al., 1992) in FCC. Mixtures of model compounds have also been used to deconvolute effective first-order rate constants from competitive adsorption and deactivation (Harding et al., 1993). Although they provide significant insight into the nonlinear kinetic interactions of hydrocarbons, these mixture experiments have the same limitations as the individual model compounds: the smaller size ranges and limited molecular variety are only an idealization of gas oil cracking. Experiments which mix a single model compound into a complete gas oil form a third tier of molecular detail. These experiments are designed either to determine how a specific molecular type affects the global product yields (such as coke production) or to determine the cracking selectivities of specific model compounds in a realistic chemical environment (Cook and Colgrove, 1994). Since the cracking of these model compounds is measured in a large pool of gas oil species and their products, only the most significant product species can be directly measured. Chemical labeling of the model compound before cracking can increase the number of product species that can be experimentally assigned. Although these model compound experiments are per-

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2562 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 Table 1. Feedstock Properties API gravity, @ 289 K aniline point, K sulfur, wt % total nitrogen, wt % basic nitrogen, wt % Conradson carbon, wt % simulated distillation, vol %, K IBP 5 10 20 30 40 50 60 70 80 90 95 FBP K factor n-d-m analysis Cp Cn Ca

22.5 346 2.59 0.086 0.034 0.25 490 580 597 616 635 655 675 696 719 745 773 798 826 11.52 59.5 18 22.4

formed in a more realistic chemical environment, the amount of direct chemical information is significantly more limited than that obtained with the individual model compound testing. There appears to be a tradeoff between the level of product detail obtained in an experiment and the complexity of the reactant mixture. In this paper, we describe a set of experiments which add an intermediate level of complexity between the third tier of model compound experiments and experiments on a full gas oil. Instead of mixing a model compound into a representative gas oil, we blend a gas oil fraction into that representative gas oil. The gas oil fractions are derived by two techniques: (1) the gas oil is distilled into boiling point fractions and (2) the gas oil is separated into paraffinic, aromatic, and polar fractions by a modification of a standard ASTM method. Each fraction is then blended back into the initial gas oil to maintain similar standard chemical environments. By blending the separated fractions back into the gas oil, we create a set of gas oil blends composed of identical molecules, but which have enhanced concentrations of specific molecular subsets. Another advantage of reblending the gas oil fractions is that the catalyst/oil ratios and other physical characteristics of the microactivity testing are kept roughly constant for constant conversion comparisons. We use a modified version of ASTM D2007 to separate the gas oil into paraffinic, aromatic, and polar molecular type fractions. The use of chromatographic methods to separate gas oils has been extensively used to characterize gas oils (Menges et al., 1993). Some researchers have also collected the chromatographically-separated

fractions and tested them for coke-producing tendency (Green et al., 1992). We believe this work represents the first study that examines the catalytic performance of gas oil chromatographic fractions. This study compares the role of gas oil boiling point and hydrocarbon type in determining product selectivity. In the next section, we describe the experimental separation techniques, the gas oil characterization techniques, and the microactivity test procedure. In the following section, we report the product selectivities for each gas oil blend based on an “incremental yield analysis” technique. We examine the effect of enriching a specific part of the gas oil on the selectivity of specific product components. Using an approximate linear combination technique, we estimate the product selectivities of each gas oil fraction. To determine the robustness of our product selectivity estimates, we examine how these estimates change as a function of blending concentration and compare our estimates with the selectivities of an individual fraction. Experimental Methods Properties of the Sour, Imported Heavy Gas Oil (SIHGO) are shown in Table 1. The gas oil was separated by distillation (ASTM D-1160) into four boiling fractions: 900 °F). Modified feedstocks were prepared by blending onethird by weight of each boiling fraction with two-thirds by weight of the original gas oil. The SIHGO feedstock was also separated by chemical type using a modified clay-gel separation method (ASTM D-2007). Fifty grams of the gas oil was dissolved in 500 mL of heptane and poured over a series of two packed columns of 7.6 cm i.d. The first column contained 500 g of attapulgite clay, and the second column contained 250 g of attapulgite on top of 1000 g of silica gel (supplier: Forcoven Products, Humble, TX). The columns were then washed with 2500 mL of heptane. This procedure concentrated the polar components in the first column, the aromatic components in the second column, and the saturates (paraffinic/naphthenic) components in the eluted fraction. The columns were separated and washed with 2500 mL of a 50/50 mixture of acetone and toluene. The gas oil fractions were recovered by distilling off the solvents. A small percentage of the gas oil (97 wt %) paraffinic and naphthenic components. The measurements also showed that the aromatic fraction contained most of the saturated sulfur and some aromatic sulfur

compounds. The polar fraction is made up of S- and N-containing compounds, as well as some multiring aromatics. The catalyst used in this study is a low matrix rareearth ultrastable Y catalyst (REUSY). Properties of the catalyst are shown in Table 4. The catalyst was hydrothermally deactivated in a fluidized bed for 4 h at 1088 K under 100% steam at 0 psig. Cracking experiments were performed in a microactivity unit (ASTM D-3907) at a temperature of 798 K and a contact time of 30 s. Conversion was varied by changing the catalyst to oil ratio. Results and Discussion Interpolated, mass-balanced yields for SIHGO and all the paraffinic feed blends are provided in Table 5. As the feed is enriched with paraffinic components, its reactivity is increased, which produces higher conversions at the same C/O ratio. In addition, the selectivity to produce gasoline is increased and the selectivities for

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2564 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 Table 3. Compositions of the SIHGO Feed

undesirable products (coke, C1 + C2) are decreased. An analysis of all the selectivities (Figure 3) indicates that the selectivity changes are roughly linear with increases in paraffinic components in the range investigated. We define “incremental yield” as

Y′ ) [Yblend - (1 - X)Ybase]/X where Yblend is the yield of the blended feedstock, Ybase is the yield of the base feedstock, and X is the weight fraction of the blend component. The incremental yield,

Y', is the change in yield due to the addition of the blend component, normalized by the weight fraction of the blend component. The yields may be interpolated at constant conversion, constant catalyst/oil ratio, or constant coke yield. In this work, we choose to interpolate at constant catalyst/oil ratio, since it is based on the assumption that the blend component and the gas oil are cracking at the same WHSV. Although a constantconversion interpolation permits better product yield comparisons, it is based on the incorrect assumption that each component in the feed cracks at the same rate.

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Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 2565 Table 4. REUSY Catalyst Properties

Table 6. Comparison of Incremental Yields and Yields from Pure Component Cracking

Chemical Analysis SiO2, wt % Al2O3, wt % Re2O3, wt % Na2O, wt % SO4,wt % Fe, wt % TiO2, wt % Ni, ppm V, ppm

66.2 27.4 2.79 0.46 1.51 0.4 0.9 0 40

Physical Analysis ABD (g/cm3) average particle size (µm)

0.79 69

Properties after Thermal Treatment for 3 h @ 811 K total surface area (m2/g) 237 zeolite surface area (m2/g) 200 matrix surface area (m2/g) 37 unit cell size (Å) 24.56 Properties after Hydrothermal Deactivation for 4 h @ 1088 K total surface area (m2/g) 158 zeolite surface area (m2/g) 135 matrix surface area (m2/g) 23 unit cell size (Å) 24.28 Table 5. Effect of Paraffin Enrichment on MAT Yields

% conversion catalyst/oil hydrogen, wt % total C1 and C2, wt % propylene, wt % propane, wt % total C3s, wt % isobutane, wt % total butenes, wt % total C4s gasoline, wt % light cycle oil, wt % heavy cycle oil, wt % coke, wt % gasoline composition paraffins isoparaffins aromatics naphthenes olefins GC RON GC MON

SIHGO

8% added paraffins

16% added paraffins

25% added paraffins

52.8 2 0.05 1.1 2.7 0.5 3.2 2.2 3.7 6.3 40.6 25.1 22.2 1.64

54.9 2 0.04 1.0 2.7 0.5 3.2 2.3 3.7 6.4 42.6 23.7 21.3 1.61

57.1 2 0.04 1.0 2.8 0.5 3.3 2.5 3.9 6.8 44.5 23.0 19.8 1.50

60.5 2 0.04 1.0 3.1 0.4 3.5 2.7 4.0 7.0 47.5 22.9 16.6 1.46

38.3 33.7 29.3 11.2 21.1 88.7 79.2

40.2 34.9 29.0 11.0 19.8 87.9 78.7

41.0 35.3 27.1 11.2 20.6 87.3 78.1

41.4 35.7 27.3 11.6 19.6 86.7 77.8

Physically, the incremental yield of a fraction is an estimate of the product yields produced by that fraction in the environment of the base feedstock. In a sense, the incremental yield is analogous to the concept of partial molar property. If there were no interaction between the blend component and the base feedstock, i.e., an “ideal mixture”, we would expect the incremental yield to be equal to the yield of pure component cracking. However, bimolecular interactions, such as hydrogen transfer, alkyl transfer, and dimerization, between the blend component and other components in the base feedstock would cause the incremental yield to differ from the yield of pure component cracking. Since these feed interactions may be nonlinear, our linear interpolation technique provides only an approximation of the yields from the blend component. This was indeed observed in the case of paraffin cracking. Table 6 shows a comparison between the incremental yields, calculated from cracking of paraffinenriched feed blends, and the yields derived from cracking of the pure paraffinic component. Both the incremental and actual yields for paraffinic components demonstrate very high gasoline selectivity and low coke selectivity relative to the base SIHGO feed (Tables 5

incremental yield of paraffinic fraction

yields from cracking of pure paraffinic fraction

81.1 2 -0.032 0.5 3.6 0.4 4.0 4.0 4.4 9.2 66.5 12.5 6.4 0.9

83.9 2 0.024 0.7 6.7 0.6 7.3 3.1 10.0 17.0 58.1 12.0 4.1 0.7

55.6 45.0 13.1 10.5 20.8 80.3 73.4

45.0 37.3 29.4 10.0 16.5 87.4 77.2

% conversion catalyst/oil hydrogen, wt % total C1 and C2, wt % propylene, wt % propane, wt % total C3s, wt % isobutane, wt % total butenes, wt % total C4s gasoline, wt % light cycle oil, wt % heavy cycle oil, wt % coke, wt % gasoline composition paraffins isoparaffins aromatics naphthenes olefins GC RON GC MON

Table 7. Yield Distribution with Different Boiling Point Fractions

% conversion catalyst/oil hydrogen, wt % total C1 and C2, wt % propylene, wt % propane, wt % total C3s, wt % isobutane, wt % total butenes, wt % total C4s gasoline, wt % light cycle oil, wt % heavy cycle oil, wt % coke, wt % gasoline composition paraffins isoparaffins aromatics naphthenes olefins GC RON GC MON

SIHGO

644blend

644-700 blend

700-755 blend

755+ blend

71.0 4.6 0.07 2.1 4.3 1.2 5.5 4.5 4.3 9.9 49.7 18.7 10.3 3.8

71.0 4.7 0.08 1.9 4.3 1.2 5.6 4.7 4.3 10.0 49.5 19.8 9.2 3.9

71.0 4.6 0.07 2.0 4.3 1.2 5.5 4.5 4.3 9.9 49.8 18.2 10.8 3.7

71.0 4.7 0.08 2.2 4.3 1.3 5.6 4.4 4.4 9.8 49.2 18.0 11.0 4.1

71.0 4.6 0.08 2.2 4.3 1.2 5.6 4.4 4.3 9.7 49.2 18.1 10.9 4.2

4.7 44.1 33.5 7.8 9.9 88.8 81.1

4.8 42.7 37.1 7.1 8.3 88.7 81.1

4.6 44.7 33 7.6 10.1 88.8 81.3

4.6 43.8 34.1 7.4 10 89.0 81.2

4.5 44.8 33.1 7.4 10.3 88.8 81.0

and 6). However, the calculated incremental yields are, in general, better (higher gasoline, lower light gas yields) than the yields of pure paraffins cracking. The difference between the two yield slates in Table 6 is attributed to interactions between the paraffins and the base feed. For example, the paraffinic fraction appears to show more hydrogen-transfer products when it is cracked in the presence of the gas oil than when it is cracked individually. A likely explanation of this phenomenon is that the products of other species present in the full gas oil donate hydrogen to the products of the paraffinic fraction. Interpolated, mass-balanced yields for SIHGO and the different feed blends (paraffins, polars, aromatics, and different boiling range blends) at a constant 71 wt % conversion are provided in Tables 7 and 8. The activity and selectivity results reported in these tables represent time-averaged results for the 30 s of reaction between the catalyst and feed components. During this period, the catalyst deactivates due to coke formation. When either feeds or catalysts with dramatically different

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2566 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996

Figure 3.

coking tendencies are used in this type of measurement, the deactivation can disguise the activity and selectivity rankings (Weekman, 1969). The feed blending approach and the significantly shorter reaction time compared to the Weekman experiments used in this work resulted in similar deactivation characteristics for all the feeds. For example, coke-on-catalyst at the end of the experiment, which is a measure of the extent of deactivation, varied over a narrow range of 0.8-1.0 wt % for all the feeds tested in Tables 7 and 8. When coke-on-catalyst varies over a wider range (from a factor of 2 to 1 order of magnitude), additional data analysis may be needed to determine deactivation disguise of selectivity. We can conclude from these results that for the

SIHGO feed, with the REUSY catalyst, changes in boiling point distribution did not significantly influence reactivity and selectivity. However, changes in chemical composition resulted in large changes in reactivity and selectivity. Incremental yields for all the feed components in the blend at constant feed conversion were calculated from this data (Tables 9 and 10). The high selectivity (65 wt %) for gasoline and low selectivity for coke observed for paraffinic components are consistent with reported results (Fisher, 1990), indicating that paraffinic components of the feed are gasoline precursors. The polar components have low reactivity, as expected. Incremental yields for this fraction indicate a high selectivity

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Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 2567 Table 8. Yield Distribution with Different Hydrocarbon Fractions

conversion, wt % catalyst/oil ratio hydrogen, wt % total C1 and C2, wt % propylene, wt % propane, wt % total C3s, wt % isobutane, wt % total butenes, wt % total C4s gasoline, wt % light cycle oil, wt % heavy cycle oil, wt % coke gasoline composition paraffins isoparaffins aromatics naphthenes olefins GC RON GC MON

SIHGO

polar blend

paraffin blend

aromatic blend

71.0 4.6 0.07 2.1 4.3 1.2 5.5 4.5 4.3 9.9 49.7 18.7 10.3 3.8

71.0 5.9 0.10 2.7 4.3 1.5 5.9 4.5 4.2 9.9 47.1 18.2 10.8 5.3

71.0 3.3 0.05 1.5 4.1 0.9 5.1 4.2 4.5 9.6 52.1 18.7 10.3 2.6

71.0 6.1 0.10 3.0 4.3 1.8 6.2 4.8 3.7 9.9 46.0 18.8 10.2 5.9

4.7 44.1 33.5 7.8 9.9 88.8 81.1

4.6 44.1 35.6 6.2 9.5 89.1 81.3

6.1 43.9 28.3 9.4 12.3 85.9 79.1

4.4 43.6 41.3 4.5 6.2 90.1 82.0

for coke and dry gas (C1 + C2). Incremental yields for aromatic components indicate significantly poorer (lower) gasoline selectivity than paraffinic components but better gasoline selectivity than the polar components. Multiring aromatic species are reported to be coke and bottoms precursors (Fisher, 1990). Thus, the incremental yield analysis has provided results consistent with those reported by Fisher. In addition to the analysis of calculated incremental yields, the measured yields (at constant 71 wt % conversion) on different blended feeds were related to feed properties like the amount of high boiling material (611 K+), polars, and aromatics in the feed. Results in Figure 4 indicate that reactivity for the SIHGO feed blends (as measured by the C/O ratio required to achieve a constant 71 wt % conversion) is independent of the amount of high boiling material. High boiling and higher molecular weight components have, in general, intrinsically higher reactivity than lower molecular weight components of the same chemical type (e.g., n-paraffins). This is counterbalanced by the increased

Figure 4. Activity and coke yield vs feed 611+.

intracrystalline diffusion limitation of the higher boiling point components for the active sites in the zeolitic crystals. Thus, the observed reactivity was independent of the amount of high boiling material. This reactivity trend may be influenced by the mass-transfer properties of the system and is therefore probably catalyst dependent. The feed blend with the highest percentage of components with boiling points above 611 K exhibited significantly higher coke selectivity than the other feeds. Coke selectivity also increased with increasing concentration of aromatics and polars in the feed (Figure 5). Aromatic and polar components have a greater propensity to form coke than the paraffinic component (Gates et al., 1979). The high coke selectivity of the feed blend with the most high boiling material (Figure 4) is likely to be due to a higher concentration of aromatics and polars in that feed blend. The increase in coke selectivity with increasing concentration of polars and aromatics results in a corresponding decrease in gasoline selectivity (Figure 6). Increasing the percentage of olefins in wet gas (C3 and C4 hydrocarbons) is sometimes desirable, since C3 and C4 olefins can be alkylated to produce high-octane gasoline components. The percentage of olefins of C3 and C4 hydrocarbons was relatively independent of the concentration of polars in the feed blends. The percentage of olefins in wet gas decreased with increasing aromatics (Figure 7). As multiring aromatics become

Table 9. Incremental Yields of Various Boiling Point Fractions

catalyst/oil % conversion hydrogen, wt % methane, wt % total C1 and C2, wt % propylene, wt % total C3s, wt % isobutylene, wt % total butenes, wt % isobutane, wt % total C4s gasoline, wt % light cycle oil, wt % heavy cycle oil, wt % coke, wt % gasoline composition paraffins isoparaffins olefins naphthenes aromatics GC RON GC MON

SIHGO observed

644incremental

644-700 incremental

700-755 incremental

755+ incremental

SIHGO calculated

5 71.6 0.08 0.7 2.1 4.4 5.7 0.7 4.4 4.6 10.0 49.4 18.5 9.9 4.2

5 72.1 0.09 0.5 1.8 4.4 5.5 0.4 3.8 5.3 10.7 50.2 21.0 6.9 3.8

5 75.0 0.09 0.7 2.3 4.6 6.0 0.7 4.1 5.3 11.2 51.6 15.3 9.6 3.9

5 72.8 0.10 0.9 2.7 4.5 5.9 0.8 4.3 4.5 10.2 49.4 15.8 11.4 4.5

5 74.1 0.09 0.9 2.7 4.7 6.2 0.7 4.3 4.7 10.5 49.4 15.3 10.5 5.3

5 73.3 0.09 0.7 2.3 4.5 5.8 0.6 4.1 5.0 10.7 50.2 17.4 9.3 4.2

48.7 44.1 9.1 7.3 34.9 88.8 81.1

53.0 48.7 9.4 6.3 31.3 88.4 81.1

53.6 49.6 8.5 6.0 32.0 88.9 81.7

47.8 43.6 10.1 6.6 35.4 89.5 81.4

49.1 44.7 10.4 6.7 33.6 88.8 80.9

51.3 47.0 9.5 6.4 32.8 88.9 81.3

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Figure 5. Coke yields vs polars and aromatics.

Figure 7. LPG olefinicity vs aromatics or polars in feeds.

Figure 6. Gasoline yield vs polars or aromatics.

Figure 8. Dry gas yields vs aromatics or polars.

Table 10. Incremental Yields of Different Hydrocarbon Fractions

involves dehydrogenation. Thus, an increase in polars can lead to an increase in selectivity for molecular H2. As the reactivity of the feedstock is reduced, lower space velocities are needed to achieve constant conversion. The resulting longer residence time can lead to a increased thermal cracking. C1 and C2 hydrocarbons are products of thermal cracking, thus, increasing the concentration of polars both reduced feed reactivity and increased the selectivity for C1 and C2 hydrocarbons. A small amount of the gas oil appeared to be irreversibly adsorbed onto the chromatography columns and could not be removed by solvent washing. These materials are presumably heavy asphaltene species found in most gas oils. In a control experiment, the paraffinic, aromatic, and polar fractions were blended to match their concentrations in the original gas oil. The product selectivities of this mixture were very similar to those observed for the original gas oil, except that coke production was slightly lower. The experiment described above is therefore an incomplete picture of coke formation on FCC catalysts and does not address the role of asphaltenes in determining coke selectivity. In some of the cases described above, the incremental yield analysis predicts selectivities that are physically unrealistic. The source of these unrealistic selectivities is most often due to nonlinear interactions between molecular types in the reaction system. For example, the incremental yield analysis predicts a negative gasoline yield for the polar fraction in Table 10. This reduction is probably due to the effect of the high coke yield of the polar fraction on the cracking of paraffinic and aromatic gasoline products. It does not imply that the polar fraction, if cracked separately, would have zero or negative gasoline yield. The strength of the incremental yield approach, that it describes the selectivities of each fraction as it cracks within the rest of the gas

SIHGO aromatic polar SIHGO observed incremental incremental calculated catalyst/oil % conversion hydrogen, wt % methane, wt % total C1 and C2, wt % propylene, wt % total C3s, wt % isobutylene, wt % total butenes, wt % isobutane, wt % total C4s gasoline, wt % light cycle oil, wt % heavy cycle oil, wt % coke, wt % gasoline composition paraffins isoparaffins olefins naphthenes aromatics GC RON GC MON

5 71.6 0.08 0.7 2.1 4.4 5.7 0.7 4.4 4.6 10.0 49.4 18.5 9.9 4.2

5 53.6 0.08 0.9 2.8 2.9 4.4 0.1 1.9 2.7 5.9 34.9 27.7 18.7 5.5

5 8.8 0.25 2.5 6.3 1.4 1.4 0.9 -5.5 -6.9 -5.7 -2.4 37.4 53.8 9.0

5 71.9 0.07 0.9 2.8 4.4 5.9 0.7 3.7 4.4 10.4 48.0 17.1 11.0 4.7

48.7 44.1 9.1 7.3 34.9 88.8 81.1

42.4 39.2 9.0 3.7 45.0 94.1 84.9

32.5 31.3 18.5 7.9 41.1 95.7 84.6

55.7 48.6 8.8 6.8 28.7 85.3 78.6

hydrogen deficient, they tend to be converted to coke (Gates et al., 1979). The reactions involved can include donation of hydrogen from these aromatic compounds to olefins, hydrogenating them to paraffins. Paraffinic components are a source of formation of olefins. Thus, increasing aromatics or a reduction in paraffinic components resulted in a reduction of the percentage of olefins of C3 and C4 hydrocarbons. In addition, the selectivity for light hydrocarbons (C1 + C2) and molecular H2 increased for feed blends with a higher concentration of polars (Figure 8). The conversion of polar molecules into coke precursors and ultimately coke

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oil, also produces nonlinear interactions that are not adequately described by the interpolation. Conclusion A new method of characterizing the reactivity of FCC feed components has been described. This method involves separating a base feedstock into fractions of varying boiling range and chemical type, preparing modified feedstocks by blending the fractions individually back into the base feedstock, cracking the modified feedstocks, and quantifying the changes in reactivity and product yields as a function of feed modification. Using the concept of incremental yields, the reactivity and selectivity of each feedstock fraction in the environment of the base feedstock can be determined. Incremental yields are only slightly affected by changes in boiling point range. However, incremental yields are strongly dependent on chemical type. Paraffinic components exhibit high reactivity, high gasoline yield, high LPG percentage of olefins, and low yields of coke and dry gas. Aromatic components exhibit low reactivity, low gasoline yield, and high yields of coke and dry gas. Polar components are very difficult to crack and make few valuable products. Incremental yields for this fraction are predominately coke, C4 minus and bottoms. Acknowledgment The authors thank Ted Peders and Steve Olsavsky for their testing help. The authors also thank W. R. Grace & Co.sConn. for permission to publish this work. Literature Cited Abbot, J. The Influence of Olefins on Cracking Reactions of Saturated Hydrocarbons. J. Catal. 1990, 126, 684-688. Abbot, J.; Wojciechowski, B. W. Kinetics of Catalytic Cracking of n-Paraffins on HY Zeolite. J. Catal. 1987, 104, 80. Cook, B. R.; Colgrove, S. G. Anthracene Isomerization over Amorphous Silica-Alumina: A Novel Hydrogen Transfer Reaction. Book of Abstracts; Symposium on Hydrogen Transfer in Hydrocarbon Processing (ACS National Meeting, Washington, DC); American Chemical Society: Washington, DC, 1994; pp 372-378. Corma, A.; Miguel, P. J.; Orchilles, A. V.; Koermer, G. S. Cracking of Long Chain Alkyl Aromatics on USY Zeolite Catalysts. J. Catal. 1992, 135, 45-59. Fisher, I. P. Effect of Feedstock Variability on Catalytic Cracking Yields. Appl. Catal. 1990, 65, 189-210.

Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill Book Co.: New York, 1979. Green, J. B.; Shay, J. Y.; Reynolds, J. W.; Green, J. A.; Young, L. L.; White, M. E. Microcarbon Residue Yield and Heteroatom Partitioning between Volatiles and Solids for Whole Vacuum Resids and their Liquid Chromatographic Fractions. Energy Fuels 1992, 6, 836-844. Haag, W. O.; Dessau, R. M. Proc. 8th Int. Congr. Catal. Berlin 1984, 2, 205. Harding, R. H.; Gatte, R. R.; Pereira, C. J. Pseudocomponent Test of the Relative Utilization of Feed Components in Fluid Catalytic Cracking. J. Catal. 1993, 140, 41-52. Krannila, H.; Haag, W. O.; Gates, B. C. Monomolecular and Bimolecular Mechanisms of Paraffin Cracking: n-Butane Cracking Catalyzed by H-ZSM-5. J. Catal. 1992, 135, 115-124. Lin, L.; Gnep, N. S.; Guisnet, M. A Procedure for Comparing the Reactivity of Different Hydrocarbons in Cracking over Zeolites. React. Kinet. Catal. Lett. 1992, 42, 133- 138. Martin, A. M.; Chen, J.-K.; John, V. T.; Dadyburjor, D. B. Coreactant-Induced Modifications of Catalytic Behavior in Zeolitic Systems. Ind. Eng. Chem. Res. 1989, 28, 1613-1618. Menges, R. A.; Spino, L. A.; Armstrong, D. W. Separation and Characterization of Components of Catalytic Cracker Feed using Centrifugal Partition Chromatography. Anal. Chem. 1993, 65, 2873-2877. Mitchell, M. M., Jr.; Hoffman, J. F.; Moor, H. F. Stud. Surf. Sci. Catal. 1993, 76, 293-338. Pachovsky, R. A.; Wojciechowski, B. W. Effects of Charge Stock Composition on Selectivity in the Cracking of Neutral Distillates. Can. J. Chem. Eng. 1975, 53, 659-665. Pine, L. A.; Maher, P. J.; Wachter, W. A. Prediction of Cracking Catalyst Behavior by a Zeolite Unit Cell Size Model. J. Catal. 1984, 85, 466. Santilli, D. S.; Zones, S. I. Secondary Shape Selectivity in Zeolite Catalysis. Catal. Lett. 1990, 7, 383-388. Sie, S. T. Acid-Catalyzed Cracking of Paraffinic Hydrocarbons. 3. Evidence for the Protonated Cyclopropane Mechanism from Hydrocracking/hydroisomerization Experiments. Ind. Eng. Chem. Res. 1993, 32, 403-408. Townsend, A. T.; Abbot, J. Catalytic Reactions of Tetralin on HZSM-5 Zeolite. Appl. Catal. A. 1993, 95, 221-236. Venuto, P. B.; Habib, E. T., Jr. Fluid Catalytic Cracking with Zeolite Catalysts; Marcel Dekker, Inc.: New York, 1979. Weekman, V. W. Ind. Eng. Chem. Process Des. Dev. 1969, 8, 385.

Received for review July 19, 1995 Accepted May 17, 1996X IE950449E

X Abstract published in Advance ACS Abstracts, July 1, 1996.