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Energy & Fuels 1996, 10, 450-462
Relating Feedstock Composition to Product Slate and Composition in Catalytic Cracking. 2. Feedstocks Derived from Brass River, a High-Quality Nigerian Crude J. B. Green,* E. J. Zagula, J. W. Reynolds, L. L. Young, H. Chew, T. B. McWilliams, and R. D. Grigsby National Institute for Petroleum and Energy Research, P.O. Box 2565, Bartlesville, Oklahoma 74005 Received May 4, 1995X
The fluid catalytic cracking (FCC) behavior of compound types present in the >650 °F resid from Brass River (Nigerian) crude was investigated. Liquid chromatography and distillation were employed for separation of selected compound type fractions from the resid; the resulting fractions were then cracked using a bench scale FCC unit. The FCC behavior for each compound type was defined in terms of the resulting product distribution (yields of gas, gasoline, etc.) sulfur and nitrogen partitioning, and in selected cases, gasoline composition. Results obtained from Brass River fractions were compared to those obtained from an earlier FCC study of compound types from Wilmington, CA, >650 °F resid. Correlations were derived for gasoline and coke yields from feedstocks derived from either crude. Brass River is a sweet, paraffinic crude which gives rise to a >650 °F resid with very favorable FCC characteristics. Although the bulk of the FCC gasoline was produced from cracking hydrocarbon types present, significant gasoline production also occurred from heteroatomic compounds (acids/bases) in Brass River. Conversely, negligible gasoline production was observed previously from cracking Wilmington acid/base types. However, feedstocks from both crudes exhibited greater conversion of sulfide sulfur to H2S compared to thiophenic forms of sulfur, and greater carryover of acidic forms of nitrogen (e.g., carbazole) compared to basic forms (e.g., quinoline). Overall gasoline composition depended on hydrocarbon type composition of feedstocks but was also influenced by presence of acids and/or bases in the feed. On the other hand, the detailed distribution of isomers within a given gasoline homolog, e.g., C3-benzenes or C9 isoparaffins, was nearly independent of feed composition. Results obtained for Brass River will serve as benchmarks for future FCC data obtained from low-quality feedstocks.
Introduction This paper is the second resulting from an ongoing program aimed at relating feedstock composition to product slate and composition from fluid catalytic cracking (FCC) processes. This information is being sought in an effort to improve FCC processing of conventional feedstocks and to extend FCC to lower quality feedstocks, such as atmospheric resids from heavy oils. It is anticipated that the results will be used in process models for FCC and to optimize pretreatment of low quality streams to make them into suitable FCC feedstocks. The initial paper presented results obtained from bench scale experiments with liquid chromatographic (LC) fractions derived from Wilmington, CA, crude oil.1 The present one describes analogous work performed on LC fractions from Brass River, a sweet paraffinic crude from Nigeria.2 Brass River was selected as a benchmark high-quality crude to be used as a basis for comparison for the intermediate (e.g., Wilmington) and low-quality crudes which comprise the bulk of this research program. Abstract published in Advance ACS Abstracts, February 1, 1996. (1) Green, J. B.; Zagula, E. J.; Reynolds, J. W.; Wandke, H. H.; Young, L. L.; Chew, H. Energy Fuels 1994, 8, 856-867. (2) Oil and Gas Journal Data Book 1986 Edition; PennWell Co.: Tulsa, OK, 1986: p 40. X
0887-0624/96/2510-0450$12.00/0
Points brought out in the study of Wilmington LC fractions included (1) the significant impact of acidic and basic feed components on FCC product slate and gasoline composition, (2) greater carryover of acidic versus basic forms of nitrogen into liquid products, (3) higher conversion of sulfide forms of sulfur to H2S compared to thiophenic sulfur types, and (4) correlation of FCC coke yield with feedstock microcarbon residue (MCR) data.1 The generality of these findings will be tested here with respect to feedstocks derived solely from Brass River as well as some containing components from both Brass River and Wilmington crudes. Dependence of FCC on feedstock composition has not been a major focus of the open literature. Unit design, operating conditions, catalyst development, and the like have received most of the attention to date. As the result, the present understanding of FCC operating variable-product relationships is significantly better than feedstock-product relationships, as exemplified by a recent paper on benzene levels in FCC gasoline.3 A brief review of applicable papers on feedstock-product correlation was given earlier.1 However, recent papers on hydrogen transfer in FCC4-6 are also pertinent because of the great impact of hydrogen transfer on the conversion process and resulting product slate. For (3) Yatsu, C.; Reid, T. A.; Keyworth, D. A.; Jonker, R.; Torem, M. A. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1993, 38, 576-580.
© 1996 American Chemical Society
Feedstock Composition in Catalytic Cracking
example, the reduced aromatic content, coupled with higher olefin plus naphthene contents, of gasoline produced from feedstocks enriched in basic nitrogen compounds was attributed to reduced hydrogen transfer activity resulting from the decrease in effective catalyst acidity.1 The hydrogen transfer properties of a given zeolite are directly related to its acidity and free acid site density.7 Thus, feedstock basic nitrogen content has a significant effect on gasoline yield and composition through its impact on hydrogen transfer activity of the catalyst. Experimental Section The scheme and methodology for crude fractionation and blending of the fractions to obtain FCC feedstocks were the same as used earlier.1 Briefly, the 650-1000 and >1000 °F boiling ranges of the crude were fractionated into nine fractions using LC. These fractions were used singly or in combination as feedstocks to a bench scale fluidized unit. Feedstocks were cracked at 521 °C (970 °F) using a Davison XP-series equilibrium catalyst at a cat/oil ratio of 8.5 ( 0.5/1. The FCC unit, catalyst, and procedures for carrying out cracking experiments were unchanged from the prior work.1 Prior procedures for determining product slate and composition were also generally employed, except for the following refinements. The oven temperature program rate used in the gas chromatographic/mass spectrometric (GC/MS) determination of liquid product composition was decreased from 2 to 1 °C/min over the temperature range corresponding to elution of the bulk of the gasoline range components (25-100 °C). The slower program rate generally improved resolution of marginally separated components. Conversion of the raw GC/MS data (area %) to wt % was performed using average instrument response factors for each hydrocarbon type and carbon number (e.g., C9 isoparaffins). Gasoline range components carrying over into gas sampling bags (C5 and C6 hydrocarbons) were subsequently added to results determined from liquid analyses to obtain total yields for C5 and C6 species. The composition of gas sampling bags was determined in a separate analysis using a Carle Series 400 AGC (Chandler Engineering, Tulsa, OK). Although C5 hydrocarbons were determined individually in gas analyses, C6+ components were determined in aggregate. Thus, for the purpose of calculating net gasoline composition, the C6+ component from gas analyses was factored into the total C6 distribution according to the ratios of individual species determined in the corresponding liquid products. Procedures for calibrating the Carle Series 400 AGC GC used in gas bag analyses were modified after discovering that commercially prepared calibration mixtures for both H2S and olefins degraded significantly with time. Thus, freshly received H2S standards over the concentration range 0.9-3.5 vol % H2S in N2 were used to determine the thermal conductivity detector response to H2S. The prior practice of periodic recalibration with the same standards was discontinued after determining that the variation in instrument response over time generally was negligible compared to the degradation of the standards themselves. Periodically (every 6-12 months), new standards were procured to check instrument calibration, which typically exhibited a relative variation of only 3-5% over a 1 year period. Similarly, degradation of olefins in hydrocarbon standards was noted whenever the standards were (4) Sousa-Aguiar, E. F.; Pinhel da Silva, M.; Murta Valle, M. L.; Forte da Silva, D. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1994, 39, 356-359. (5) Wojciechowski, B. W. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1994, 39, 360-366. (6) Cook, B. R.; Colgrove, S. G. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1994, 39, 372-378. (7) Xie, C.-G.; Pan, R.-N.; Li, Z.-T. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1994, 39, 393-397.
Energy & Fuels, Vol. 10, No. 2, 1996 451 Table 1. Compound-Type Distribution in Brass River as a Function of Boiling Range (wt %)a sample no. 3229 boiling range, °F acids strong weak total bases strong weak total neutrals polar-neutral sulfide non-sulfide total total
3054
3229/3054
650-1000
>1000
>650b
4.4 ( 0.1c
5.2 ( 0.5 5.9 ( 0.7 11.1 ( 0.9
0.36d 0.41d 4.9e
1.9 ( 0.1 94.0 ( 1.0 0.53 ( 0.06 1.6 ( 0.3 89.9 ( 0.8 92.0 ( 0.9 100.3 ( 1.0
5.0 ( 0.3 4.6 ( 0.4 9.6 ( 0.5 81.6 ( 1.8 2.5 ( 0.5 6.7 ( 0.5 71.8 ( 0.3 81.0 ( 0.8 102.3 ( 2.0
0.35d 0.32d 2.4e 93.1 0.67 2.0 88.6 91.3 100.4
a From LC mass balances. b Calculated from data for 650-1000 °F (93.0 wt % of >650 °F) and >1000 °F (7.0 wt %) boiling ranges. c Uncertainties given are average deviations from duplicate separations. d Includes >1000 °F portion only. e Total >650 °F acids or bases.
subjected to mild heating over long periods. Conversion of 2-methyl-1-butene to 2-methyl-2-butene was particularly evident. To minimize errors from olefin degradation, fresh working hydrocarbon calibration standards were periodically withdrawn from the main calibration gas cylinder, which was stored at room temperature. Continuous mild (60 °C) heating of the working standard was necessary to ensure that representative proportions of C6+ components contained in the blend were injected into the GC. Hydrocarbon types in 650-1000 °F neutral fractions from feeds were determined by high-resolution MS using published procedures.8
Results Feed Composition. Table 1 lists the distributions of compound types determined from LC separation of the 650-1000 and >1000 °F boiling ranges of Brass River crude. Those data were used in turn to calculate the distribution of compound types for the >650 °F Brass River resid indicated in the table. As noted in the table, the >650 °F resid consisted of 93% 650-1000 °F components and only 7% >1000 °F material. Table 2 shows elemental analyses and microcarbon residues (MCR, ASTM D 4530) for fractions listed in Table 1. Relative to data reported earlier for Wilmington crude, Brass River contains substantially lower levels of acidic, basic, polar-neutral, and sulfide types. Also, the heteroatom contents of Brass River acid and base fractions are generally lower than that of the corresponding Wilmington fraction. The generally decreased heteroatom levels for Brass River 650-1000 °F acids and bases, in conjunction with their higher MCR values, indicate that they exhibit higher average molecular weights than the corresponding Wilmington fractions. On the other hand, the higher heteroatom content and generally higher MCR for Wilmington >1000 °F acid/base types probably results from their greater multifunctionality and higher aromaticity compared to the corresponding Brass River fractions. Table 3 shows compound type distributions in the 650-1000 °F neutral fraction from each crude determined by high-resolution MS. The highly paraffinic (8) Teeter, R. M. Mass Spectrom. Rev. 1985, 4, 123-143.
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Table 2. MCR and Elemental Data for Brass River Fractions (wt %) boiling range, °F 650-1000
>1000
fraction
C
H
N
S
total
MCR
whole distillate acids bases neutrals polar/sulfide-free neutrals whole resid strong acids weak acids strong bases weak bases neutrals polar/sulfide-free neutrals
87.4 80.9 85.1 86.7 86.2 86.3 82.1 81.1 86.2 82.7 86.9 86.7
12.72 9.48 10.21 12.77 12.79 11.89 9.82 9.58 9.78 10.67 12.67 12.33
0.146 1.49 2.99 0.022 0.018 0.36 0.54 1.12 2.53 1.66 0.108 0.064
0.23 0.28 0.78 0.15 0.11 0.58 0.61 0.75 0.85 1.61 0.53 0.22
100.5 92.2 99.1 99.7 99.1 99.1 93.1 92.6 99.4 96.7 100.2 99.3
0.05 10.4 4.2 0.16 0.02 5.9 20.5 18.4 28.1 17.6 3.01 1.38
Table 3. Feed (650-1000 °F Neutrals) Hydrocarbon Type Distributions Determined by High-Resolution MS wt % compound type
formula
Brass River
Wilmington
22.4 13.7 10.6 10.6 6.0 1.4
650 °F resid >650 °F neutrals >650 °F polar/ sulfide-free neutrals N + SA >650 °F neutrals N + WA >650 °F neutrals N + DA >650 °F neutrals BRN/Wil DA >650 °F neutrals
8 9
N + SB BRN/Wil SB
10 11 12
WR N N - P/S
wt % in feed
component B
elemental compositon, wt % feed C H N S
MCRa (wt %)
87.29 12.67 0.16 0.24 86.86 12.76 0.029 0.19 86.16 12.76 0.021 0.12
0.97 0.30 0.10
15.0 15.0 15.0 13.8
86.15 85.99 85.96 86.17
0.11 0.19 0.25 0.32
0.25 0.27 0.20 0.33
3.3 3.0 1.8 0.92
15.0 15.0
86.76 12.31 0.40 85.72 12.20 0.47
0.29 0.62
4.5 5.8
15.0 14.9 15.0
86.23 12.44 0.27 86.60 12.38 0.47 86.40 12.33 0.61
0.40 0.28 0.34
2.9 0.88 0.53
wt % in feed
100 100 100 85.0 85.0 85.0 86.2
>650 °F neutrals >650 °F neutrals
85.0 85.0
N + WB >650 °F neutrals N + DB >650 °F neutrals BRN/Wil DB >650 °F neutrals
85.0 85.1 85.0
>1000 ° F strong acids >1000 °F weak acids 650-1000 °F acids 650-1000 °F acids from Wilmington crude oilb >1000 °F strong bases >1000 °F strong bases from Wilmington crude oilb >1000 °F weak bases 650-1000 °F bases 650-1000 °F bases from Wilmington crude oilb
12.31 12.28 12.27 12.29
a Microcarbon residue (ASTM D 4530). b These fractions are derived from Wilmington, CA, crude oil. All other feedstock components are derived from Brass River, Nigeria, crude.
nature of Brass River, versus the predominantly naphthenic/aromatic character of Wilmington, is obvious from the table. Table 4 lists the feedstocks prepared for the catalytic cracking experiments. As noted in the table, three of the feedstocks were comprised of the Brass River >650 °F neutrals blended with an acid or base fraction derived from Wilmington crude oil. The Brass River >650 °F neutral fraction exhibited a low MCR
and low concentrations of N and S; yet, it was interesting to note the still lower levels for each parameter in the case of the polar/sulfide-free neutrals. Based on the observed difference in their sulfur contents, it was estimated that approximately 40% of the sulfur present in the >650 °F neutrals was of sulfide type. For this calculation, it was assumed that the polar/ sulfide-free neutrals contained only thiophenic-type sulfur.
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Energy & Fuels, Vol. 10, No. 2, 1996 453
Table 5. Overall Product Distributions from Brass River Feedstocks Obtained at 521 ( 1 °C (970 °F) and a Cat/Oil Ratio of 8.5 ( 0.5a A nitrogen partitioningi
wt % feed feedb
gasc
gasolined
LCOe
HCOf
coke
total
convg
1. WR 2. N 3. N - P/S 4. N + SA 5. N + WA 6. N + DA 7. BRN/Wil DA 8. N + SB 9. BRN/Wil SB 10. N + WB 11. N + DB 12. BRN/Wil DB
16.9 15.5 18.1 15.3 15.3 13.5 14.3 15.1 17.5 15.5 11.8 13.3
46.5 46.0 47.6 44.0 44.0 43.8 43.5 42.8 39.0 44.0 39.4 37.7
13.1 12.2 12.0 11.8 11.8 13.1 12.6 12.3 10.3 11.9 14.3 13.2
14.1 17.3 13.0 17.8 19.0 18.7 18.0 15.8 16.2 18.0 21.1 21.3
8.5 7.2 7.9 10.6 10.4 10.1 9.3 13.5 15.4 10.0 11.0 12.7
99.1 98.2 98.6 99.5 100.6 99.1 97.7 99.5 98.4 99.4 97.6 98.2
71.9 68.9 73.6 69.9 69.7 67.3 67.1 71.3 71.9 69.4 62.2 63.7
gas.
effh
sulfur partitioningj
liquid
coke
gas
liquid
coke
total
12.0 12.9 10.7 6.9 15.0 23.2 25.8 4.6 4.7 6.7 11.5 11.7
88.0 87.1 89.3 93.1 85.0 76.8 74.2 95.4 95.3 93.3 88.5 88.3
70.6 59.7 50.2 71.1 66.9 56.3 63.7 64.9 54.7 71.7 59.5 54.7
24.4 35.2 42.9 27.6 29.0 39.8 31.9 22.9 14.3 19.8 29.4 27.0
3.4 2.6 5.7 5.6 2.5 1.7 1.9 7.5 26.4 3.3 8.7 18.3
98.4 97.5 98.8 104.3 98.4 97.8 97.5 95.3 95.4 94.8 97.6 100.0
64.7 67.0 64.7 63.0 63.1 65.1 64.9 60.0 54.3 63.3 63.4 59.2
B feedb 1. WR 2. N 3. N - P/S 4. N + SA 5. N + WA 6. N + DA 7. BRN/Wil DA 8. N + SB 9. BRN/Wil SB 10. N + WB 11. N + DB 12. BRN/Wil DB
liquid product analysis (wt % of liquid) C H N S 86.8 87.2 87.2 86.1 86.4 87.1 87.5 86.9 87.9 86.7 86.3 86.9
12.79 12.99 12.86 12.87 12.87 12.66 12.25 12.62 12.00 12.63 12.55 12.32
0.026 0.0047 0.0030 0.0099 0.037 0.075 0.110 0.025 0.034 0.024 0.072 0.098
0.076 0.086 0.069 0.092 0.102 0.103 0.140 0.092 0.133 0.105 0.108 0.126
H2
C1
0.37 0.29 0.33 0.24 0.28 0.27 0.31 0.38 0.40 0.30 0.48 0.52
0.69 0.56 0.69 0.63 0.70 0.65 0.71 0.76 0.93 0.71 0.76 0.81
gas distribution (wt % feed) C2′s C3 C3) i-Bu n-Bu 0.94 0.79 0.96 0.91 1.03 0.85 0.97 0.98 1.24 1.00 0.88 1.01
1.08 0.99 1.26 1.00 1.18 0.82 0.98 0.96 1.39 1.05 0.70 0.82
3.46 3.03 3.69 3.03 3.24 2.86 3.01 3.07 3.36 3.15 2.57 2.80
3.76 3.32 4.40 3.38 3.81 2.86 3.22 3.13 4.11 3.35 2.09 2.53
C4)T
Gcalc (wt %)k
3.39 3.15 3.71 3.16 3.38 3.06 2.83 3.30 3.22 3.29 2.98 3.07
42.8 46.0 46.0 39.1 39.1 39.1 39.6 39.1 39.1 39.1 39.2 39.1
0.84 0.70 0.99 0.74 0.85 0.61 0.68 0.73 0.97 0.75 0.49 0.59
NEGY1 1.17 (0.31)m 0.71 0.71 0.68 0.61 0.54 -0.01 0.71 0.03 -0.20
a Absolute standard deviations for product yields are (1-2 wt %, conversion and efficiency (1-3 wt %, N and S partitioning (2-4%, liquid C and H contents (0.5 and (0.10, respectively; relative standard deviations for liquid N and S contents and gas distributions are approximately (10% of the stated value. b See Table 4. c Includes C1-C4 hydrocarbons and H2S. d g C5 (82 °F)-430 °F. e 430-650 °F. f >650 °F (approx. 650-1000 °F). g Conversion ) gas + gasoline + coke. h Gasoline efficiency ) (gasoline/conversion) × 100%. i Relative proportion (%) of feed nitrogen found in liquid versus coke (formation of NH3 was negligible). Coke nitrogen calculated by difference. j Relative proportion (%) of feed sulfur found in gas versus liquid versus coke. k Calculated gasoline yield. l Neutral-equivalent gasoline yield of acid/base fraction. m Estimated NEGY for combined sulfide + polar-neutral fractions.
FCC Product Distribution. Table 5 summarizes the overall product distributions obtained from the catalytic cracking experiments. Each result represents the mean from 2-4 replicate experiments. Independent mass balances and product analyses were carried out for each replicate. The typical precision of each type of result is footnoted in the table. Variations in FCC behavior between replicates are the largest source of error for most parameters. However, sampling and analysis errors probably contributed significantly to imprecision of data obtained on spent catalysts (sulfur) and gas composition. The high degree of suitability of Brass River as an FCC feedstock is immediately obvious from the comparable product distributions obtained from the whole >650 °F resid versus the >650 °F neutral fraction. In comparison, the yield of light products from the whole resid was typically only about half that derived from the neutral fraction for Wilmington and other lower quality crudes examined in this work. Although the lower proportion of acidic and basic types present in the Brass River >650 °F resid may partially account for its favorable product distribution, the active participation of these types in cracking processes yielding light products is what largely differentiates it from the low quality crudes. As discussed below, this latter point is best illustrated by the calculated parameters listed in the last two columns of Table 5.
The calculated gasoline yield Gcalc was based solely on the proportion of neutrals in a given feedstock (fN) and the gasoline yield determined from cracking the pure neutrals fraction (GN):
Gcalc ) GNfN
(1)
This simple relationship assumes that gasoline is produced only through cracking of neutral species. To test this assumption, a “neutral equivalent gasoline yield” (NEGY) is subsequently calculated for the acid/ base type(s) in a given feedstock via eq 2, where Gmeas
NEGY )
Gmeas - Gcalc GN fA/B
(2)
is the actual gasoline yield, and fA/B is the weight fraction of acids/bases in feed. The NEGY term was used in the earlier paper1 but was defined differently than above. The present expression directly relates gasoline production from acids/ bases to that of the neutrals from a given feedstock. For example, NEGY ) 1 implies equivalent production to that of the neutrals; NEGY ) 0 denotes no gasoline production; a negative NEGY value indicates inhibition of gasoline production from neutrals. With the aid of the information given above, it may be seen that substantial production of gasoline (NEGY
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Green et al.
Table 6. Gasoline Yield from Wilmington Feedstocks Containing Acids/Bases gasoline yield feed (wt %)a 90.75 N + 9.25 SA 83.31 N + 16.69 SA 82.99 N + 17.01 WA 85.30 N + 14.70 DA 86.72 N + 12.87 SB 85.31 N + 14.69 WB 86.94 N + 13.06 DB W (65.5% N) 84.37 W + 15.63 SA N N-P/S
measd 37.4 35.1 36.1 39.2 33.5 35.6 34.4 26.7 24.1 41.4 41.4
calcd 37.6 34.5 34.4 35.3 35.9 35.3 36.0 27.1 22.9 (41.4) (41.4)
NEGYb -0.05 0.09 0.24 0.64 -0.45 0.05 -0.30 -0.03 0.06 (1.00)c
a SA, WA, SB, WB ) >1000 °F strong acids, weak acids, strong bases and weak bases, respectively. DA and DB ) 650-1000 °F acids and bases. W ) whole resid, N ) neutrals, N-P/S ) polar/ sulfide-free neutrals (>650 °F). b Estimated error for NEGY ) ( 0.1. c Estimated NEGY for combined sulfide + polar-neutral fractions.
g 0.5) was achieved from most Brass River acid/base types. Interestingly, the NEGY calculated for the whole resid was greater than one. This suggests that the mixture of acids/bases naturally present in Brass River combined to produce an enhanced yield of gasoline. Only the 650-1000 °F Brass River bases and >650 °F polar neutral/sulfide types yielded low NEGY values. Base fractions derived from Wilmington crude resulted in slightly negative NEGY values (feedstocks 9 and 12). For comparison, Table 6 lists calculated gasoline yields and NEGY values for the previously reported Wilmington feedstocks.1 Most Wilmington acid/base fractions yielded NEGY values near zero. Significant gasoline production occurred only from the Wilmington 650-1000 °F acids, which yielded comparable NEGY values when diluted in either Wilmington (0.64, Table 6) or Brass River (0.61, Table 5) neutrals. NEGY values for Wilmington 650-1000 °F bases were also quite comparable for the two neutral diluents (-0.20 (Table 5) versus -0.30 (Table 6)). The greater negativity for the Wilmington >1000 °F strong base NEGY in Wilmington (-0.45) versus Brass River neutrals (-0.01) may relate to its superior dispersion in the Wilmingtonderived diluent. Owing to its naphthenic/aromatic character (Table 3), Wilmington should act as the more effective solvent/dispersent for refractory materials such as its >1000 °F base fraction. Coke yields in Table 5 could be expressed as the sum of coke produced from thermal plus catalytic reactions.
CΣ ) CTh + Ccat
(3)
Thermally produced coke can be closely approximated as feed MCR. Catalytic coke is a function of cat/oil ratio, temperature, catalyst, and selected feedstock properties.9 An acceptable correlation was achieved by approximating Ccat as C0(1 + NχN C ), where C0 is the catalytic coke formed from a nitrogen-free feedstock; N is the feedstock nitrogen content, and NχN C is the fraction of nitrogen incorporated into coke (Table 5). Thus
CΣ ) MCR + C0(1 + NχN C)
(4)
For the catalyst and conditions used here, the mean value determined for C0 was 6.81 ( 0.72. Appropriate C0 values could be determined for other FCC conditions
Table 7. Correlation of Coke Yield with Feedstock MCR and Nitrogen Contenta feed no. 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14
code
coke yield (C∑, wt % feed) calcdd measd SD
A. Brass River >650 °Fb WR 8.6 8.5 N 7.3 7.2 N - P/S 7.0 7.9 N + SA 10.8 10.6 N + WA 10.9 10.4 N + DA 9.9 10.1 BRN/Wil DA 9.3 9.3 N + SB 13.9 13.5 BRN/Wil SB 15.7 15.4 N + WB 11.4 10.0 N + DB 10.5 11.0 BRN/Wil DB 11.0 12.7
0.6 0.5 0.7 0.1 0.6 0.8 0.4 1.1 1.5 0.5 0.1 0.9
B. 2034 650-1000 °F Gasoilc 2034 GO 8.0 8.5 0.8 2034 + DA 9.3 7.8 0.6 2034 + DB 10.3 9.8 0.5 W W + SA N N-P/S N + SA1 N + SA2 N + WA N + DA N + SB N + WB N + DB
C. Wilmington >650 °Fc 25.8 25.7 27.3 25.9 15.0 13.9 12.1 13.7 17.4 19.9 19.0 19.8 21.0 21.5 15.7 16.1 21.2 20.7 19.0 17.6 16.7 16.6
1.0 0.7 0.6 1.3 2.3 0.6 0.6 1.6 1.1 1.1 0.1
NχN C
C0
0.14 0.025 0.019 0.10 0.16 0.19 0.24 0.38 0.45 0.25 0.42 0.54
6.76 6.73 7.65 6.62 6.37 6.96 6.77 6.51 6.63 5.67 7.14 7.90
0.14 0.27 0.46
7.26 5.60 6.45
0.82 0.77 0.35 0.23 0.43 0.45 0.53 0.48 0.65 0.58 0.67
6.76 5.99 5.98 8.09 8.57 7.37 7.12 7.05 6.48 5.92 6.77
a See eq 4. b This work (Table 5). c See ref 1 (Table 5). culated using the average C0 value (6.81 ( 0.72).
d
Cal-
as well. Table 7 summarizes results from application of this correlation to Brass River and Wilmington >650 °F resids as well as a conventional gas oil (sample no. 2034). Individual standard deviations (SD) for experimental coke yields are provided in the table. The pooled standard deviation (σ), a better measure of the overall precision of coke yield data, was calculated to be 1.0. Ninety-six percent of the calculated coke yields agree with the experimental result within 2σ; 73% agree within 1σ. Thus, the model proposed in eq 4 fits the experimental data within the normal limits of statistical error. Preliminary efforts at correlating gasoline yield with feed properties have yielded the following relationship for neutral fractions:
GN ) 10.25[H/(C + S) + log(fcT)] + 28.8
(5)
where GN is the gasoline yield (wt %) from cracking neutral fraction, H/(C + S) is the atomic ratio of those elements, and fcT ) fraction of neutrals boiling below the cracking temperature (970 °F). Table 8 lists parameters for >650 °F neutrals from five crudes investigated to date, including Brass River. Agreement between calculated and measured gasoline yields was well within one SD in all cases. The calculated gasoline yields for neutral fractions were subsequently substituted into eq 1 for estimation of the gasoline yield from whole resids attributable to cracking of neutrals. As shown in Table 9, the proportion of whole resid FCC gasoline attributable to cracking of neutrals ranged from essentially 100%, as in the case (9) Fisher, I. P. Fuel 1986, 65, 473-479.
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Energy & Fuels, Vol. 10, No. 2, 1996 455
Table 8. Correlation of Gasoline Yield with Properties of Neutral Fractionsa gasoline yield (GN, wt %) crudeb
calcd
measd
SD
H/(C + S)
fcTc
Brass River Lagomedio Wilmington Maya Merey
46.0 42.9 41.3 40.2 42.4
46.0 43.4 41.0 40.4 42.0
1.7 1.6 1.5 1.2 1.2
1.75 1.71 1.55 1.57 1.61
0.86 0.46 0.47 0.35 0.52
a See eq 5. b Origin of neutral fraction. c Determined by hightemperature GC simulated distillation.
Table 9. Comparison of Calculated versus Measured Gasoline Yields from Whole >650 °F Resids gasoline yield (wt %) crudea
calcdb
measd
SD
fN
Brass River Lagomedio Wilmington Maya Merey
42.8 35.0 27.1 25.5 29.1
46.5 40.2 26.3 30.0 31.4
0.3 0.8 0.7 0.6 1.2
0.931 0.815 0.655 0.635 0.687
a
Origin of resid. b See eq 1.
of Wilmington, to only about 85%, as observed for Lagomedio or Maya resids. Gasoline production in excess of that predicted from eq 1 must have come from cracking of acid/base types, as discussed earlier in conjunction with the NEGY parameter (eq 2). Further efforts in correlation of gasoline production with feedstock properties will focus on prediction of NEGY for acid/base fractions. Gasoline Composition. Table 10 shows the compositions of gasolines from selected feedstocks. Figure 1 depicts a summary of compound types from each feed based on the table. It should be noted that benzene and cyclohexane used in LC fractionations were incompletely removed from selected feedstocks and thus carried over into the resulting gasolines. From Figure 1, it may be seen that the addition of distillate (650-1000 °F) acids or bases to neutrals generally resulted in gasolines with increased levels of olefins, olefinic naphthenes and saturated naphthenes with corresponding decreases in isoparaffin contents. Each of these effects was much larger for 650-1000 °F bases than acids. Addition of >1000 °F bases to feeds produced no significant changes in compound-type distributions. Figure 2 shows carbon number distributions for aromatic and isoparaffin hydrocarbon classes. In order to make results for each of the gasolines more directly comparable, data from Table 10 were converted to a relative basis (percent total aromatics or isoparaffins) for the figure. Distributions for gasolines from feeds containing distillate bases (feeds 11 and 12) are shifted to higher carbon numbers, in keeping with the lower overall conversion obtained with those feedstocks. Interestingly, the gasolines from feeds containing >1000 °F bases (nos. 8 and 9) are enriched in lower carbon number species, particularly so for the feed containing Wilmington >1000 °F strong bases. Table 11 lists relative isomeric distributions for C7 isoparaffins and C3-alkylbenzenes. As with Figure 2, data in Table 11 have been converted to a relative basis to facilitate comparison between gasolines from different feeds as well as with literature data for the respective isomeric distributions.10,11 Dependence of isomeric dis-
tributions on feedstock was quite small in each case. Also, FCC and benchmark distributions agreed surprisingly well, especially considering FCC's reputation as a strictly kinetically-controlled process. The benchmark distribution for isoparaffins shown in Table 11 was calculated from a chain growth model developed for the Fischer-Tropsch process.10 This model has been shown to adequately predict natural isoparaffin distributions in crude oil, which should in turn reflect thermodynamic stability. The major difference between calculated versus measured C7 isoparaffin distributions was in the relative abundance of 2versus 3-methylhexane. The enriched abundance of 2-methylhexane in FCC gasoline results in part from the greater stability of tertiary carbonium ions, such as I in eq 6, formed as intermediates in the cracking +
CH3(CH2)3
C
CH2R
CH3(CH2)3
CH3
C
CH2 + R+
hydrogen transfer
CH3
I CH3(CH2)3CH(CH3)2
(6)
process.12 Beta cleavage of such species may ultimately result in products analogous to 2-methylhexane, as indicated in the reaction scheme. On the other hand, FCC and thermodynamic distributions for C3-alkylbenzene isomers agreed essentially within experimental error. Thus, there was no need for any special rationalization of isomeric abundances in that case. Figure 3 compares overall product distributions in gasoline produced from Wilmington versus Brass River neutrals, as well as each neutral faction spiked with Wilmington 650-1000 °F acid or base fractions. Earlier reported1 data for gasolines derived from Wilmington neutrals were converted from area % to wt % for the purpose of this comparison. Differences in gasoline composition from the two plain neutral fractions relate back to the inherent differences in hydrocarbon types present in each feed (Table 3). However, addition of acids and particularly bases to the neutrals before cracking significantly alters the ratio of most compound types in the resulting gasoline. For example, more n-paraffins were produced from the Wilmington neutrals + bases feed than the Brass River neutrals + bases, in spite of the complete absence of n-paraffins in the Wilmington neutrals (Table 3). Also, clear differences in olefin and aromatic yields from plain neutrals were effectively minimized in the gasolines from the corresponding neutrals+bases feeds. Conversely, apparent differences in gasoline range saturated and olefinic naphthene yields were amplified through addition of acids or bases to the feed. Clearly, the impact of acids/bases in the feed on gasoline composition is both significant and neutrals (i.e., hydrocarbon type) dependent. Discussion Correlations of Coke and Gasoline Yield with Feedstock Parameters. The convention adopted in (10) Friedel, R. A.; Sharkey, A. G. Jr. U.S. Bureau of Mines Report; Investigations No. 7122, 1968; 10 pp. (11) Pitzer, K. S.; Rossini, F. D. J. Res. Natl. Bur. Stand. 1946, 37, 95-122. (12) Corma, A.; Wojciechowski, B. W. Catal. Rev. Sci. Eng. 1985, 27(1), 29-150.
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Green et al.
Table 10. Composition of Brass River Gasolines Determined by GC/MS (Wt %)
feed
whole resid
neutrals
neutrals + 650-1000 °F acids BR Wilmington
neutrals + 650-1000 °F bases BR Wilmington
neutrals + >1000 °F strong bases BR Wilmington
Aromatics alkylbenzenes benzene toluene total C2-benzenes ethylbenzene o-xylene m- + p-xylenes total C3-benzenes isopropylbenzene n-propylbenzene 1-ethyl-2-methylbenzene 1-ethyl-3-methylbenzene 1-ethyl-4-methylbenzene 1,2,3-trimethylbenzene 1,2,4-trimethylbenzene 1,3,5-trimethylbenzene total C4-benzenes isobutylbenzene sec-butylbenzene n-butylbenzene 3-isopropyl-1-methylbenzene 4-isopropyl-1-methylbenzene 1-methyl-2-propylbenzene 1-methyl-3-propylbenzene 1-methyl-4-propylbenzene 1,2-diethylbenzene 1,3-diethylbenzene 1,4-diethylbenzene 1,2-dimethyl-3-ethylbenzene 1,2-dimethyl-4-ethylbenzene 1,3-dimethyl-2-ethylbenzene 1,3-dimethyl-4-ethylbenzene 1,4-dimethyl-2-ethylbenzene 1,3-dimethyl-5-ethylbenzene 1,2,3,4-tetramethylbenzene 1,2,3,5-tetramethylbenzene 1,2,4,5-tetramethylbenzene total C5-benzenes (no. isomers) total C6-benzenes (no. isomers) total alkylbenzenes alkylindans indan total C1-indans 1-methylindan 2-methylindan 4-methylindan 5-methylindan total C2-indans (no. isomers) total C3-indans (no. isomers) total alkylindans alkylindenes indene total C1-indenes (no. isomers) total alkylindenes naphthalene total aromatics
0.73 3.0 10.0 1.27 2.30 6.4 9.2 0.03 0.43 0.55 2.0 0.77 0.65 3.4 1.40 4.6
