46
Energy & Fuels 1997, 11, 46-60
Relating Feedstock Composition to Product Slate and Composition in Catalytic Cracking. 3. Feedstocks Derived from Maya, a Mexican Crude J. B. Green,* E. J. Zagula, J. W. Reynolds, L. L. Young, T. B. McWilliams, and J. A. Green National Institute for Petroleum and Energy Research, P.O. Box 2565, Bartlesville, Oklahoma 74005 Received April 22, 1996X
The fluid catalytic cracking (FCC) behavior of compound types present in the >650 °F resid from Maya crude was investigated. Distillation and liquid chromatography 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, nitrogen, nickel, and vanadium partitioning; and, in selected cases, gasoline composition. Results obtained from Maya fractions were compared to those obtained from earlier FCC studies of compound types from Wilmington, CA, and Brass River, Nigeria, >650 °F resids. A conceptual model was proposed that adequately predicted FCC product slates obtained from >650 °F neutral fractions from Maya and the other crudes. An important premise of the model was primary production of gasoline and C3/C4 gases from alkyl side chains and acyclic paraffins in feeds with concurrent conversion of aromatic and naphthenic cores to cycle oils. The product slates calculated from the model agreed well with those determined experimentally. Highly aromatic feedstock constituents presumably formed only coke and light gases. A calculation of hydrogen transfer resulting from cracking indicated no significant hydrogen exchange between aliphatic gasoline or C3/C4 gas precursors and naphthenic/aromatic cores. Implications of the model toward improving FCC feed pretreatment and performance evaluation are discussed.
Introduction This paper is the third 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 lowquality streams to make them into suitable FCC feedstocks. The initial paper presented data obtained from benchscale experiments with liquid chromatographic (LC) fractions derived from Wilmington, CA, crude oil.1 The second dealt with corresponding results from Brass River, a high-quality Nigerian crude.2 Points raised in these two papers include (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.1,2 A brief overview of the relevant literature is also provided in these two papers. X Abstract published in Advance ACS Abstracts, December 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) Green, J. B.; Zagula, E. J.; Reynolds, J. W.; Young, L. L.; Chew, H.; McWilliams, T. B.; Grigsby, R. D. Energy Fuels 1996, 10, 450462.
S0887-0624(96)00059-X CCC: $14.00
Correlation of gasoline yield and composition with feedstock properties was also addressed in the second paper. Specifically, gasoline yield (Gcalcd) was estimated from the proportion (fN) of neutral species in a given feedstock
Gcalcd ) GNfN
(1)
where GN is the yield of gasoline from cracking pure neutrals. The latter may be calculated via
GN ) 10.25[H/(C + S) + log(fcT)] + 28.8
(2)
where H/(C + S) is the atomic ratio of those elements and fcT is the fraction of neutrals boiling below the cracking temperature (970 °F). Gasoline yield from nonneutral types (i.e., acidic or basic) is compared to that of the neutrals by means of the neutral equivalent gasoline yield (NEGY) parameter defined in
NEGY ) (Gmeasd - Gcalcd)/GNfA/B
(3)
C∑ ) MCR + C0(1 + NχN C)
(4)
NEGY is the neutral equivalent gasoline yield, Gmeasd is the actual gasoline yield, and fA/B is the weight fraction of acids/bases in the feed. These same parameters will be used in this paper for the evaluation of results from Maya feedstocks. The prior correlation for total coke yield C∑ will also be tested using data from Maya-based feeds:
In eq 4 MCR is the microcarbon residue (ASTM D 4530), C0 is the catalytic coke formed from a nitrogen-free © 1997 American Chemical Society
Relation of Feedstock Composition to Product Slate Table 1. Compound Type Distribution in Maya as a Function of Boiling Range (Weight Percent)a sample 1740 boiling range, °F acids strong weak total bases strong weak total neutrals polar-neutral sulfide nonsulfide total total a
1741
1740/41
650-930
>930
>650b
4.2 ( 0.1c
17.8 ( 1.2 8.1 ( 0.1 25.9 ( 1.4
13.8d 6.3d 21.1e
1.9 ( 0.3 95.5 ( 1.0 1.6 ( 0.1 6.2 ( 0.5 87.7 ( 0.5 95.5 ( 0.7
10.3 ( 0.1 8.4 ( 0.6 18.7 ( 0.8 54.3 ( 1.2 5.2 ( 1.2 10.0 ( 1.3 38.0 ( 1.2 53.2 ( 2.1
8.0d 6.5d 15.0e 63.5 4.4 9.2 49.1 62.7
101.6
98.9
99.6
b
From LC mass balances. Calculated from data for 650-930 (22.3 wt % of >650 °F) and >930 °F (77.7 wt % of >650 °F) boiling ranges. c Uncertainties given are average deviations from two to three separations. d Includes >930 °F portion only. e Total >650 °F acids or bases.
feedstock (6.81 ( 0.72 from prior work), N is the feedstock nitrogen content (wt %), and χN C is the fraction of feed nitrogen incorporated into coke. Maya is a common export crude blend from Mexico.3 Since it is a blend, its composition is more variable than that of crudes from a single field. The distillate and resids employed in this work are derived from a sample of the crude containing 3.34 wt % S and 49 and 251 ppm of Ni and V, respectively. These data compare favorably with published values except for V, which was specified at 333 ppm.3 Due to its widespread availability and low quality, Maya is commonly used in process studies and development.4-7 Some of its detailed structural characteristics are known.8 Its main value for the authors’ purpose was its significantly different characteristics from those of either Wilmington or Brass River crudes. 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 650-930 °F and >930 °F boiling ranges of the crude were fractionated into nine fractions using liquid chromatography (LC). These fractions were used singly or in combination as feedstocks to a bench-scale (approximately 4 g of oil and 35 g of catalyst per charge) fluidized unit. Feedstocks were cracked at 521 °C (970 °F) using a Davison XP series equilibrium catalyst at a catalyst/oil ratio of 8.5 ( 0.5/1. The feedstocks were charged to the unit over a 30 s time period. The sweep gas (N2) velocity was set such that the catalyst volume was displaced approximately every 20 s. The FCC unit, catalyst, and procedures for carrying out cracking experiments were unchanged from the prior work,1 except for refinements added later.2 One of these refinements was conversion of GC/MS gasoline compositional data, which (3) Aaland, L. R. Oil Gas J. 1981, Feb 9, 132-135. (4) Pearson, C. D.; Green, J. B. Fuel 1989, 68, 456-464. (5) Pearson, C. D.; Green, J. B. Fuel 1989, 68, 465-474. (6) Wells, J. W.; Zagula, E. J.; Brinkman, D. W.; Anderson, R. P. Catalytic Cracking of Mayan Gas Oil and Selected Hydrotreated Products; Topical Report NIPER-280, NTIS DE88001212; NTIS: Springfield, VA, 1989; 38 pp. (7) Heck, R. H.; DiGuiseppi, F. T. Energy Fuels 1994, 8, 557-560. (8) Grandy, D. W.; Danner, D. A.; Youngless, T. L.; Feulmer, G. P.; Young, D. C.; Petrakis, L. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1985, 30 (1), 29-36.
Energy & Fuels, Vol. 11, No. 1, 1997 47 were reported in area percent in the initial paper, to weight percent (wt %) in the second paper. For the interested reader, the revised Wilmington gasoline compositions, converted to wt % basis, have been submitted with this paper as Supporting Information. An additional feature of work with Maya feedstocks and products was analysis of each for Ni and V. These analyses were carried out by ashing, dissolving the ashes in aqueous acid, and analyzing the solutions by atomic absorption spectrophotometry using aqueous standards. In the ashing step, 1 mL of high-purity sulfuric acid was added to 1 g of sample; this mixture was heated overnight in a Vycor evaporating dish on a hot plate. The resulting chars were ashed in a muffle furnace at 550 °C for 2 h. The ashes were dissolved in 0.5 mL nitric acid, 0.25 mL each of hydrochloric and sulfuric acids (all of high purity), and 3-5 mL of water and transferred out of the Vycor dishes to plastic 25 mL volumetric flasks prior to analysis. The standards used to calibrate the spectrophotometer were prepared with an aqueous acid matrix which matched that of the samples. Depending on the concentration of Ni and V in the samples, either flame or graphite furnace atomic absorption mode was employed in the analysis.
Results Feedstock Composition. Table 1 lists the distributions of compound types determined from LC separation of the 650-930 °F and >930 °F boiling ranges of Maya crude. Those data were used in turn to calculate the distribution of compound types for the >650 °F resid indicated in the table. The 930 °F cut point cited for the two boiling ranges was the crossover point for their GC simulated distillation profiles. In fact, there was considerable overlap in their distillation curves. As noted in the table, the relative proportion of 650-930 °F material in the >650 °F resid was only 22.3 wt %. By comparison, the proportion of 650-1000 °F material in Brass River >650 °F was 93 wt %.2 The distribution of compound types in Maya was similar to that of Wilmington.1 The largest difference in the compound type distributions between the two >650 °F resids was the greater proportion of strong acids in Maya (13.8 wt %) compared to Wilmington (7.4 wt %). However, when comparisons of distributions of relatively crude fractions such as those listed in Table 1 are made, it should be remembered that each classification, e.g., strong acids, actually includes a wide range of potential subtypes and homologs. Thus, Maya versus Wilmington strong acids, for example, may in fact be quite dissimilar in composition. Table 2 shows elemental and MCR data for each LC fraction and whole distillate/resid. As expected, acid/ base fractions were substantially enriched in MCR precursors and all heteroatoms, except sulfur, compared to the respective whole material. The extremely refractory nature of the >930 °F strong acid and strong base fractions in particular is evident from their high heteroatom content, high MCR, and low hydrogen contents. Table 3 compares high-resolution mass spectrometric (MS) hydrocarbon type results for Maya distillate neutrals with the corresponding fractions from Brass River and Wilmington crudes. The MS method used is applicable only to neutral types boiling below approximately 1050 °F.9 The Maya fraction contains an intermediate level of paraffins, but it is lower in (9) Teeter, R. M. Mass Spectrom. Rev. 1985, 4, 123-143.
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Energy & Fuels, Vol. 11, No. 1, 1997
Green et al.
Table 2. MCR and Elemental Data for Maya Fractions wt %
boiling range, ° F 650-930
>930
C
H
N
S
total
MCR
Ni
whole distillate acids bases neutrals polar/sulfide-free neutrals
84.7 80.6 81.4 85.3 85.4
11.87 8.23 9.38 11.94 11.93
0.20 2.79 2.64 0.023 0.017
2.82 1.98 4.96 2.61 2.12
99.6 93.6 98.4 99.8 99.5
0.22 13.9 5.8 650 °F resid 100 N >650 °F neutrals 100 N - P/S >650 °F polar/sulfide- 100 free neutrals N + SA >650 °F neutrals 85.0 N + WA >650 °F neutrals 85.0 N + DA >650 °F neutrals 85.0 N + SB >650 °F neutrals 85.0 N + WB >650 °F neutrals 85.0 N + DB >650 °F neutrals 85.0
wt % in feed
component B
>930 °F strong acids >930 °F weak acids 650-930 °F acids >930 °F strong bases >930 °F weak bases 650-930 °F bases
15.0 15.0 15.0 15.0 15.0 15.0
elemental composition, wt % feed C H N S Oa
MCR,b wt %
83.7 10.30 0.53 4.62 0.83 84.8 11.37 0.113 3.88 85.9 11.62 0.064 2.50
20.5 7.0 5.1
83 16 5.7
441 152 1.8
84.1 84.1 84.2 84.3 84.2 84.4
13.3 10.6 8.0 13.3 10.1 6.8
51 27 14 62 30 14
311 235 130 290 209 129
10.82 10.99 10.90 10.84 11.08 11.07
0.31 0.31 0.51 0.38 0.27 0.49
4.20 4.00 3.60 4.20 4.22 4.04
0.54 0.64 0.82 0.30 0.22
metal content, ppm (w/w) Ni V
By difference. b Microcarbon residue (ASTM D 4530).
Table 5. Overall Product Distributions from Maya Feedstocks Obtained at 521 ( 1 °C (970 °F) and a Catalyst/Oil Ratio of 8.5 ( 0.5a
feedb 1. WR 2. N 3. N - P/S 4. N + SA 5. N + WA 6. N + DA 7. N + SB 8. N + WB 9. N + DB
sulfur partitioningj
nitrogen partitioningi
wt % feed
gasc gasolined LCOe HCOf coke total convrng G effh liquid
coke
gas
15.8 16.2 18.5 15.3 16.1 14.0 17.1 17.6 15.5
93.4 90.2 89.6 93.9 87.2 70.2 95.6 94.3 80.4
39.5 68.2 65.0 54.3 64.9 63.9 60.2 66.8 63.8
feedb
C
1. WR 2. N 3. N - P/S 4. N + SA 5. N + WA 6. N + DA 7. N + SB 8. N + WB 9. N + DB
85.8 86.1 86.4 86.1 85.3 85.4 85.9 86.0 85.4
30.0 40.4 42.2 34.0 34.9 32.7 33.4 37.6 32.3
11.7 14.4 14.2 13.9 14.4 14.1 11.9 13.3 13.7
9.7 11.8 11.2 12.8 14.9 20.8 11.7 11.2 19.7
31.2 98.4 15.6 98.4 13.4 99.5 23.3 99.2 20.7 101.0 17.7 99.3 25.8 99.8 20.3 99.9 19.0 100.3
liquid product analysis (wt % of liquid) H N S total 11.72 12.17 12.08 11.95 11.68 11.48 11.95 11.87 11.48
0.067 0.016 0.010 0.030 0.061 0.223 0.029 0.024 0.144
1.46 1.37 1.14 1.34 1.56 1.59 1.37 1.38 1.76
99.1 99.7 99.6 99.4 98.6 98.7 99.3 99.3 98.8
77.0 71.4 74.1 72.5 70.3 64.3 76.2 75.5 66.9
39.0 56.4 56.4 46.9 49.7 50.9 43.9 49.8 48.4
6.6 9.8 10.4 6.1 12.8 29.8 4.4 5.7 19.6
HCO metal content, ppm (w/w)
liquid coke total 16.5 24.1 31.4 19.7 25.3 30.2 18.7 20.7 28.8
46.0 102.0 5.9 98.2 3.9 100.3 24.6 98.6 10.3 100.5 5.1 99.2 17.0 95.9 9.3 96.8 6.6 99.2
Ni
V
1.2 2.0 930 °F weak acids and weak bases5). Furthermore, the significant contribution of V to product liquids from the neutral fraction itself (feed 2) may result from its having the highest proportion of V in vanadyl porphyrin forms.5 It was interesting to note the high carryover of Ni from the neutral plus weak base feedstock (no. 8), particularly since no porphyrinic forms of Ni were detected in any Maya fraction.5 Part of the Ni in that fraction may be present in relatively low molecular weight forms with volatility characteristics similar to that of porphyrins. Alternately, the Ni carryover from this feed may in fact be attributable to porphyrinic Ni complexes which are somehow invisible to the conventional visible spectrophotometric porphyrin analysis method. (12) Cimbalo, R. N.; Foster, R. L.; Wachtel, S. J. Oil Gas J. 1972, May 15, 112-122.
50
Energy & Fuels, Vol. 11, No. 1, 1997
Comparison of calculated versus measured coke yields reveals that eq 4 generally underestimates actual coke yield by 1-3 wt % for Maya feeds. Thus, for best fit to the Maya data, the constant C0 should be somewhat greater in magnitude. However, rather than recalculate C0 for each set of feeds, an overall value will be calculated via regression of all data at the end of this project. Gasoline Composition. Table 6 compiles detailed compositions of gasoline obtained from selected feedstocks. Included in the table are separate analyses of nominally duplicate liquids from independent FCC runs using the whole >650 °F resid as the feed (no. 1). As footnoted in the table, one liquid product (A) was actually a composite from three independent FCC runs, while the other (B) was from a single run. The relative agreement between these two sets of data was used to partly assess whether gasoline compositions from different feeds were significantly different or equivalent within normal experimental error. It is recognized that for a true statistical evaluation of data precision, many more replicate analyses would be required. However, given the large magnitude of effort required per analysis, a statistical approach was not practical in this case. Figure 1 provides a summary of overall gasoline composition as a function of feed. It should be noted that the apparent increases in production of cyclohexane and C6 isoparaffins from the whole and polar/sulfide-free neutrals, respectively (feeds 2 and 3), simply reflect carryover of residual solvents used in the LC separation of these two fractions (1:3 benzene/cyclohexane for neutrals; hexane for polar/ sulfide-free neutrals). Similarly, the enhanced level of benzene in gasolines from feeds containing 650-930 °F acids or bases (feeds 6 and 9) reflects carryover of benzene used in their recovery from ion exchange resins used in LC. The residual solvents distill over in the FCC unit and are collected as liquid product. Solvents used in the LC separations are typically removed down to 1-3 wt % of the recovered fraction, since further evaporation to remove residual solvent usually results in significant losses of the fraction itself. Complete solvent removal was carried out only in cases of high boiling, refractory materials such as >930 °F acid or base fractions. With the preceding information in mind, it may be seen that the compositions of gasolines from whole neutrals (feed 2) and polar/sulfide-free neutrals (feed 3) were the same within experimental error. Addition of any acid or base fraction to the neutrals resulted in gasolines with clearly lower concentrations of aromatics and increased levels of normal paraffins and olefins. Addition of distillate (650-930 °F) acids or bases suppressed isoparaffin yields and enhanced cycloolefin yields relative to those in gasoline from neutrals or sulfide/polar-free neutrals. However, the effect of adding >930 °F acid or base types on gasoline range isoparaffin and cycloolefin concentrations was mixed. The isoparaffin content of gasoline from the whole resid (35 wt % >930 °F acids plus bases, Table 1) was equivalent to that of the neutrals, while the cycloolefin content was significantly higher. However, addition of either >930 °F strong acids (feed 4) or strong bases (feed
Green et al.
7) alone to neutrals gave rise to gasoline with slightly enriched isoparaffin content and equivalent cycloolefin levels. For Maya, the effects of 650-930 °F acids (feed 6) on both gasoline yield, as noted above, and gasoline composition were much greater than for Wilmington or Brass River. The effects of 650-930 °F bases (feed 9) on gasoline yield and composition were similar for each crude, except that addition of bases to Brass River neutrals did not suppress gasoline aromatic content like it did for Wilmington and Maya. The somewhat greater impact of >930 °F Maya strong bases (feed 7), compared to Wilmington or Brass River >1000 °F strong bases, on both gasoline composition and yield may be attributed to its lower initial boiling point (i.e., greater volatility at the 970 °F cracking temperature). As discussed elsewhere, the observed effects of acid and base fractions in feeds on the resulting gasoline compositions may be rationalized from their relative inhibition of cyclization and hydrogen transfer processes.11 Figure 2 shows carbon number distributions for aromatic and isoparaffin types in gasolines listed in Table 6. The data in the table have been converted to a relative basis (weight percent total aromatics or isoparaffins) to facilitate comparison of gasolines from the different feeds indicated in the figure. Feeds containing >930 °F strong bases (including the whole resid) produced gasolines with somewhat lower carbon number distributions for both aromatics and isoparaffins. This same effect occurred for Brass River neutrals spiked with either Brass River or Wilmington >1000 °F strong bases.2 The Maya neutral plus strong acid feed (no. 4) resulted in gasoline with slightly higher carbon number distributions. Explaining subtle effects of feedstock composition such as these will require a much better understanding of FCC chemistry. Table 7 lists relative isomeric distributions for C7 isoparaffins and C3-alkylbenzenes in gasolines. As with Figure 2, data in Table 7 have been converted to a relative basis for the purpose of intercomparison of gasolines from different feedstocks as well as for comparison with literature data used in benchmark distributions.13,14 Average distributions calculated for Brass River feedstocks2 are also included in the table. The overall conclusion drawn from the table is that each isomeric distribution is largely independent of feedstock composition. The distribution of C3-alkylbenzenes closely approximates the thermodynamic distribution in all cases. Thus, an equilibrium among isomeric C3-alkylbenzenes is effectively achieved, regardless of detailed feedstock composition. Formation of 2-methylhexane is favored in FCC gasoline, relative to the benchmark distribution for C7 isoparaffins, owing to the prevalence of β-cleavage from tertiary carbonium ions in FCC.2 Discussion Conceptual Model for FCC. A proposed conceptual model, suitable for relating feedstock composition to FCC product slate and product composition, is based on the following principles and assumptions: (13) Friedel, R. A.; Sharkey, A. G., Jr. U.S. Bur. Mines Rep. Invest. No. 7122; U.S. GPO: Washington, DC, 1968; 10 pp. (14) Pitzer, K. S.; Rossini, F. D. J. Res. Natl. Bur. Stand. 1946, 37, 95-122.
Relation of Feedstock Composition to Product Slate
Energy & Fuels, Vol. 11, No. 1, 1997 51
Table 6. Composition of Maya Gasolines Determined by GC/MS feeda neutrals whole resid (1)b A B
polar/ whole (2) sulfide-free (3)
neutrals + >930 °F neutrals + 650-930 °F acids (6) bases (9)
strong acids (4)
strong bases (7)
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. of isomers) total C6-benzenes (no. of isomers)
1.9 1.7 4.6 4.4 10.5 11.2 1.64 1.50 2.9 2.6 6.0 7.1 9.6 10.1 0.02 0.01 0.52 0.52 0.54 0.74 2.0 1.9 1.11 1.09 0.71 0.68 3.8 3.7 0.93 1.43 4.6 4.4 0.01
