Energy & Fuels 2007, 21, 3573–3582
3573
A Laboratory Study of a Pretreatment Approach To Accommodate High-Sulfur FCC Decant Oils as Feedstocks for Commercial Needle Coke Semih Eser* and Guohua Wang Department of Energy and Geo-EnVironmental Engineering, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed October 31, 2006. ReVised Manuscript ReceiVed August 16, 2007
In the delayed coking process, the actual feed to the coking drum consists of a high-boiling fraction of the feedstock, fluid catalytic cracking decant oil (FCC-DO), and the high-boiling end of the liquid products produced by delayed coking (recycle). As distinct from the parent decant oil feed, the molecular composition and coking behavior of actual feed samples to the coking drum (CF) are reported for the first time in this paper. This study examines a commercial pretreatment approach where the feedstock to the fractionator column includes a hydrotreated fraction (HYD) and a vacuum tower bottom (VTB) fraction of a decant oil. Samples of two sets of decant oils including the corresponding HYD, VTB, and CF derivatives were analyzed and carbonized in laboratory reactors to monitor mesophase development from these materials. Decant oil samples and their derivatives, HYD, VTB, and CF, have substantially different molecular composition. The CF samples are characterized by lower degrees of methyl substitution on polyaromatic hydrocarbons (PAH) with higher proportions of thermally stable methyl-PAH isomers. A higher proportion of hydroaromatics and lower concentrations sulfur-containing aromatics characterize the HYD samples, while VTB samples consist exclusively of aromatic ring systems with greater than three condensed rings. Significant differences were also found in the mesophase development from the decant oils and their derivatives. The CF, HYD, and VTB samples produced higher degrees of mesophase development than that obtained from the parent DO.
1. Introduction Delayed coking uses fluid catalytic cracking decant oil (FCCDO) to produce a premium carbon precursor, needle coke.1,2 In this thermal conversion process, the actual feed to the coking drum consists of a high-boiling fraction of FCC-DO and the high-boiling end of the liquid products produced by delayed coking (recycle).3–5 In some cases, the feedstock contains pretreated fractions of FCC-DO in order to reduce the sulfur content of the needle coke product to prevent puffing.6–10 Needle coke is formed via a high degree of mesophase development during the early stages of carbonization.11–15 The overall reaction is determined by the feedstock’s molecular composition.16–19 This study examines a commercial pretreatment approach where the feedstock to the fractionator column includes a hydrotreated * Corresponding author: e-mail
[email protected]. (1) Marsh, H. The Chemistry of Mesophase Formation. In Petroleum DeriVed Carbons; American Chemical Society: Washington, DC, 1986; ACS Symp. Ser. 303, pp 1–28. (2) Mochida, I.; Fujimoto, K.; Oyama, T. Chemistry in the Production and Utilization of Needle Coke. Chem. Phys. Carbon 1994, 24, 111–212. (3) Ellis, P. J.; Paul, C. A. In Tutorial: Delayed Coking Fundamentals; AIChE 1998 Spring National Meeting, New Orleans, 1998. (4) Gary, J. H.; Handwerk, G. E. Petroleum Refining: Technology and Economics, 3rd ed.; Marcel Dekker: New York, 1994; pp xii, 465. (5) Ellis, P. J.; Hardin, E. E. Light Metals 1993, 509–515. (6) Mochida, I.; Nakamo, S.; Oyama, T.; Nesumi, Y.; Todo, Y. Carbon 1988, 26, 751–754. (7) Fujimoto, K.; Sato, M.; Yamada, M.; Yamashita, R.; Shibata, K. Carbon 1986, 24, 397–401. (8) Kawano, Y.; Fukuda, T.; Kawarada, T.; Mochida, I.; Korai, Y. Carbon 2000, 38, 759–765. (9) Kawano, Y.; Fukuda, T.; Kawarada, T.; Mochida, I.; Korai, Y. Carbon 1999, 37, 1725–1730. (10) Machnikowski, J.; Wajzer, L. Fuel 1994, 73, 957–961.
Figure 1. Pretreatment scheme used to reduce the sulfur content of the feed to the delayed coking unit to produce needle coke.
fraction (HYD) and a vacuum tower bottom (VTB) fraction of a decant oil, rather than feeding a whole decant oil feedstock to the fractionator for delayed coking. The bottom from the fractionator contains these two components as well as the recycled liquid product from coking to constitute the actual feed (11) Brooks, J. D.; Taylor, G. H. Carbon 1965, 3, 185–186. (12) Greinke, R. A. Early Stages of Petroleum Pitch Carbonization Kinetics and Mechanisms. In Chemistry and Physics of Carbon; Walker, J. P. L., Ed. 1994; Vol. 24, p 1. (13) Marsh, H.; Walker, J. P. L. The Formation of Graphitizable Carbons via Mesophase: Chemical and Kinetic Considerations. In Chemistry and Physics of Carbon; Walker, J. P. L., Ed.; New York, 1979; Vol. 15, p 229. (14) White, J. L. Mesophase Mechanism in the Formation of the Microstructure of Petroleum Coke. In Petroleum DeriVed Carbons; Deviney, M. L., O’Grady, T. M., Eds.; American Chemical Society: Washington, DC, 1976; ACS Symp. Ser. 21, p 282. (15) White, J. L.; Price, R. J. Carbon 1974, 12, 321.
10.1021/ef060541v CCC: $37.00 2007 American Chemical Society Published on Web 09/28/2007
3574 Energy & Fuels, Vol. 21, No. 6, 2007
Eser and Wang
Figure 2. Naphthalenes and phenanthrenes in DO, VTB, CF, and HYD samples.
Figure 3. Total concentration of pyrenes in decant oils and their derivatives. Table 1. Description of Optical Units and Index Assignments type
size/shape
index
mosaic small domain domain flow domain
60 µm >60 µm long, >10 µm wide
1 5 50 100
to the coking drum (CF). The objective of this study is to compare the molecular composition of the parent decant oils with the corresponding coker feeds (CF) and the significance of these differences on the mesophase development and, therefore, on the properties of the resulting cokes. Most importantly, the composition and coking behavior of the actual feed to the coke drums (CF) are presented in comparison to
Table 2. Elemental Composition of Feedstock Samples (in wt %) DO15 HYD15 CF15 VTB15 DO24 HYD24 CF24 VTB24 C 89.8 H 9.1 N 0.3 S 1.5 C/H 0.82 (atomic ratio)
90.2 9.4 0.2 0.7 0.79
90.6 90.2 8.3 7.5 0.3 0.2 0.9 1.3 0.91 1.00
89.5 9.3 0.2 0.5 0.80
90.1 9.9 0.2 0.1 0.76
90.2 90.7 9.7 8.9 0.3 0.1 0.3 0.4 0.77 0.85
those of the parent decant oils. A companion paper presents data on the molecular composition of high-boiling components of decant oils.20 2. Experimental Section
(16) Liu, Y.; Eser, S. In GC/MS Characterization and Carbonization of FCC Decant Oils, Extended Abstracts,21st Biennial Conference on Carbon, 1993; 1993; pp 288–289. (17) Eser, S.; Wang, G. In Effects of Molecular Composition and Carbonization ReactiVity of FCC Decant Oil and Its DeriVatiVes on Mesophase DeVelopment;ACS Meeting, Anaheim, CA, 2004; American Chemical Society: Washington, DC, 2004.
Samples of two sets of decant oils (designated as 15 and 24) and their derivatives were used in this study, including the corresponding HYD, VTB, and CF derivatives. The processes from which these streams are derived are shown in Figure 1. The original decant oil (DO) is separated by vacuum distillation into a gas oil fraction (GO) and a bottom fraction (VTB). The higher
High-Sulfur FCC Decant Oils
Energy & Fuels, Vol. 21, No. 6, 2007 3575
Table 3. Distribution (%) of Homologues and Isomers of Phenanthrenes and Pyrenes in DO and CF homologues
DO15
CF15
DO24
CF24
PHEN MPHEN DMPHEN TMPHEN TEMPHEN PY MPY DMPY TMPY
4 21 37 28 10 4 19 39 38
12 35 31 16 6 31 29 33 6
4 21 35 33 6 6 22 39 33
13 37 32 16 2 26 34 27 13
isomers
do15
cf15
do24
cf24
2-MN 1-MN 3-MPHEN 2-MPHEN 9-MPHEN 1-MPHNE 2-MPY 4-MPY 1-MPY
66 34 28 37 20 14 32 40 28
78 22 41 46 6 6 68 18 14
66 34 31 35 19 15 40 30 30
80 20 39 47 7 7 69 18 13
Table 4. Concentrations (ppm) Aromatic and Hydroaromatic Compounds in HYD Streams naphthalenes tetralins phenanthrenes H-phenanthrenes
HYD15
DO15
HYD24
DO24
1634 3449 24872 7906
12096
510 1895 18675 9035
6049
54058
49277
boiling fraction of GO is hydrotreated and fed to the coker fractionator along with VTB. Thus, the coker feed (CF) stream consists of hydrotrated gas oil (HYD), VTB, and the recycled product from the coke drums. 2.1. Carbonization of Feedstocks and Examination of Coke Texture. Carbonization experiments were performed at 500 °C for 3 h under autogenous pressure in batch reactors.20 The optical texture was characterized according to the shape and size of the isochromatic areas observed on the surface of the semicoke under a polarized-light microscope (Nikon Microphot-FXAII). A systematic scanning of the whole surface of the specimen was used for texture classification. An overall optical texture index (OTI) was calculated by numerically weighted addition of the contribution from each optical unit.18 Briefly, the population of the texture or optical unit of semicoke samples under a polarized-light microscope were determined by the size and shape of respective unit classified as shown Table 1. The overall OTI is calculated by the following equation: OTI )
∑ f (OTI) i
(1)
i
where fi is the numerical fraction of individual texture types from microscopic analysis and (OTI)i is the index assigned to each texture type. It should be noted that the relative magnitudes of optical texture index assigned to different textures are different from index
values used by previous researchers21 to emphasize in OTI the importance of flow domain and domain textures in needle cokes in terms of graphitizability. 2.2. Gas Chromatography/Mass Spectrometry (GC/MS) Analysis of Decant Oil Samples. The GC/MS analyses were performed on a Shimadzu GC-17A gas chromatograph and a QP5000 mass spectrometer. For each analysis, a prepared sample in dichloromethane solution was analyzed with reference to EPA method 8270c.22 2.3. HPLC/PDA, LD/MS, and LC/MS/MS Analysis of Decant Oil Samples. High-pressure/performance liquid chromatography (HPLC) with photodiode array (PDA) detector was used to analyze the heavier PAH species that would not resolve from the GC column. The HPLC separation was performed on a Waters SE600 pump and a Waters 996 PDA detector. A normal phase HPLC column (25 cm × 46 mm) packed with 5 µm silica with pore size of 60 Å (RingSep, ES Industries, West Berlin, NJ) was used for separating aromatic compounds by the difference in the numbers of aromatic rings. Laser desorption/mass spectrometry (LD/MS) analysis was performed on a Voyager DE-STR matrix-assisted laser desorption time-of-flight mass spectrometer (MALDI-TOF-MS). The feedstock samples were first dissolved in toluene and tipped on the matrix holder before subjecting to the laser desorption. Scanning was operated in a linear mode that provided a higher sensitivity for high molecular weight molecules. HPLC/MS/MS analysis was carried out by a tandem mass spectrometry Finnigan MAT (San Jose, CA) TSQ 7000 triple-stage quadruple instrument equipped with an atmospheric pressure chemical ionization source (APCI). MS/MS measurements in the daughter ion scan mode were performed using argon as collision gas in the second quadruple mass detector. The column and solvent elution conditions in HPLC/MS/MS were the same as those used in the HPLC/PDA experiment.
3. Results and Discussion 3.1. Elemental Analysis of DO and DO Derivatives. The feedstock samples in this study include two sets of the different fractions obtained from two decant oil samples using the pretreatment scheme shown in Figure 1. The elemental composition of the feedstock samples is given in Table 2. As Table 2 shows, the decant oil samples (DO15 and DO24) have similar carbon and hydrogen contents. The nitrogen contents of the samples are generally low. However, the sulfur content differs significantly between the two decant oil samples. The DO15 has about 3 times higher sulfur content than DO24. This wide variation in the elemental composition among the decant oil samples can be related to the differences in the crude oil composition and to the differences in the operating conditions used in the refining processes. Decant oil derivatives CF, HYD, and VTB generally have slightly higher carbon contents than the parent DO, but the hydrogen content varies substantially
Table 5. Polycyclic Aromatic Sulfur Compounds (PASH) Distribution in Decant Oil Derivatives (Units: ppm in Feeds) compound
DO15
VTB15
CF15
HYD15
DO24
VTB24
CF24
HYD24
DBT MDBT DMDBT TMDBT total DBT BNT[1,2] BNT[2,1] BNT[2,3] MBNT DMBNT total BNT
248 3942 10810 6527 21527 219 1152 345 5363 8165 15244
66 403 890 983 2341 89 426 139 432 213 1299
0 1282 1697 2531 5511 0 1678 0 183 391 2253
0 0 479 1421 1900 0 78 0 0 201 279
350 1787 4968 6013 13118 0 767 166 1656 7738 10328
27 244 750 728 1748 67 333 120 309 145 974
157 674 1197 871 2899 0 1197 0 0 459 1656
0 0 278 1016 1294 0 32 0 0 98 130
3576 Energy & Fuels, Vol. 21, No. 6, 2007
Eser and Wang
Figure 4. Comparison of MICs of PASH compounds in decant oil DO 15 and its derivatives.
among these derivatives. The sulfur contents in HYD and CF are significantly lower than those of the original decant oil, as intended. 3.2. GC/MS Analysis of DO Streams. Figures 2 and 3 give the concentrations of the methylated homologues of major pah compounds in the two sets (15 and 24 series) of DO and its derivatives, i.e., naphthalenes (naphthalene, NAPH, methylnaphthalene, MN, dimethylnaphthalenes, DMN, and trimethylnaphthalenes, TMN), phenathrenes (phenanthrene, PHEN, methylphenanthrenes,MPHEN,dimethylphenanthrenesDMPHEN, trimethylphenanthrenes, TMPHEN, and tetramethylphenanthrenes, TEMPHEN), and pyrenes (pyrene, PY, methylpyrenes, MPY, dimethylpyrenes, DMPY, and trimethylpyrenes, TMPY). Compared with DO, CF streams contain a higher concentration of phenanthrenes and pyrenes than their parent decants oils. Lower concentrations of pahs are present in HYD, and VTB samples have the least amount of GC-amenable PAHS. The difference between CF and DO is seen not only in the increased concentrations of three- and four-ring aromatic compounds but also in the increased proportion of unsubstituted and methylsubstituted PAH homologues. There are differences also in the distribution of pah homologues and methyl-PAH isomers in DO and CF (Table 3). The overall difference of CF from DO is the shift in PAH distribution to less alkyl-PAH and more thermally stable isomer distribution for a given alkyl-PAH in CF. It should be noted that the relative distributions of PAH homologues are more similar, in general, across the two different DO and two (18) Eser, S. Carbonaceous Mesophase Formation and Molecular Composition of Petroleum Feedstocks. In Supercarbon: Synthesis, Properties and Applications; Yoshimura, S., Chang, R. P. H., Eds.; Springer-Verlag: Berlin, 1998; pp 147–155. (19) Filley, R. M. Molecular composition and early carbonization chemistry of FCC decant oils. Master Thesis, Pennsylvania State University, 1997. (20) Wang, G.; Eser, S. Energy Fuels 2006, in press.
different CF samples than those in the corresponding pairs of DO and CF samples. For example, as seen in Table 3, DO15 and DO24 have similar distributions of phenanthrene (4%, 4%), methylphenanthrene (21%, 21%), and dimethylphenanthrene (37%, 35%) concentration, respectively. In contrast, CF15 and CF24 have the following distributions: phenanthrene (12%, 13%), methylphenanthrene (35%, 37%), dimethylphenanthrene (31%, 32%), respectively. These distributions may represent equilibrium distributions resulting from the severe reaction conditions in both fluid catalytic cracking and delayed coking processes. The similarities in isomer distributions between the DO and CF samples can also be seen in the concentrations of specific methyl-PAH isomers. For example, methylnaphthalene isomers 1-methylnaphthalene and 2-methylnaphthalene are distributed (34%, 66%) respectively in both DO15 and DO24, compared to (22%, 78%) in CF15 and (20%, 80%) in CF24, respectively, as shown in Table 3. These results also indicate the attainment of, or close approach to, equilibrium concentrations in FCC and delayed coking operations. The HYD feeds contain much less PAH compounds than their parent DOS (Table 4). The relative homologue distribution is different from either DO or CF. Compared to DO, there is an increase in the proportion of methyl- and dimethyl-PAHS. Tetralin and hydrophenanthrene (H-PHEN) are present in significant concentrations in the HYD streams. Similar to CF samples, HYD samples contain relatively low concentrations of more highly substituted PAH (three or more methyl groups on the ring systems). The GC-amenable polyaromatic sulfur hydrocarbon (PASH) compounds distributions in decant oil derivatives are listed in (21) Oya, A; Qian, Z.; Marsh, H. Fuel 1983, 62, 274–278. (22) EPA Semivolatile Organic Compounds by Gas Chromatography/ Mass Spectrometry. http://www.epa.gov/epaoswer/hazwaste/test/pdfs/ 8270c.pdf.
High-Sulfur FCC Decant Oils
Energy & Fuels, Vol. 21, No. 6, 2007 3577
Figure 5. Comparison of HPLC/PDA chromatograms of DO15 and CF15 at 254 nm.
Figure 6. Comparison of HPLC/PDA chromatograms of HYD15 and DO15 at 254 nm.
Table 5. Total concentration of PASH compounds in VTB feeds is ∼10% of that in its parent decant oil. C2- and C3-DBTS are the primary sulfur aromatics. For heavy PASHs, unsubstituted
Figure 7. Comparison of HPLC/PDA chromatograms of VTB15 and DO15 at 254 nm.
BNT[2,1 d] was found to be the dominant compound. In HYD streams, the DBT, MDBT, and BNT were almost completely removed. The primary PASH compounds were found to be 4,6dimethyl-DBT and three TMDBT isomers (2,4,6-, 3,4,6-, and
3578 Energy & Fuels, Vol. 21, No. 6, 2007
Eser and Wang
Figure 8. LC/MS/MS spectra of DO15 (top), CF15 (middle), and HYD15 (bottom). Table 6. OTIS for Semicokes from DO Derivatives by Carbonization at 500 °C for 3 h (5 h for HYD) optical texture unit counts decant oil DO15(I) DO15(II) aVerage CF15(I) CF15(II) aVerage HYD15 (I) HYD15 (II) average VTB15 (I) VTB15 (II) aVerage DO24 (I) DO24 (II) aVerage CF24 (I) CF24 (II) aVerage HDY24 (I) HDY24 (II) aVerage VTB24 (I) VTB24 (II) aVerage
flow domains
domains
small domains
mosaics
isotropic pitch
74 79
77 65
15 16
0 0
2 1
75 77
61 49
7 16
0 0
15 12
79 98
88 95
0 0
0 0
24 22
136 145
27 19
14 20
0 0
0 0
77 110
38 58
0 0
0 0
0 0
74 98
31 38
0 0
0 0
6 3
53 65
8 12
0 0
0 0
10 20
105 113
35 29
0 0
0 0
0 0
OTI 68 70 69 74 72 73 74 75 75 85 85 85 83 83 83 85 86 85 93 92 92 88 90 89
1,4,6-TMDBT). These DBT compounds are proven to be the refractory compounds that could survive the hydrodesulfurization process.23,24 Figure 4 compares the multi-ion chromatogram of MDBT, DMDBT, and TMDBT compounds distribution in decant oil derivatives. The CF feed has a simpler composition of sulfur compounds than DO. Two methyl-DBTS (4-methylDBT and 2-MDBT), three DMDBTs (4,6-DMDBT, 2,6-DMDBT, and 3,6-DMDBT), and three TMDBTS (4-E,6-MDBT, 2,4,6-TMDBT, and 1,4,6-TMDBT) are the predominant PASHs in the CF stream. Of these, 4-MDBT and 4,6-DMDBT account for about 50% of C1- and C2-DBT isomers, and 2,4,6-TMDBT accounts for over 80% of C3-DBTS. The high concentration of MDBT and 2,6- and 3,6-DMDBT in CF (those compounds (23) Ma, X.; Sakanishi, K.; Isoda, T.; Mochida, I. Fuel 1997, 76, 329– 339. (24) Gates, B. C.; Topsoe, H. Polyhedron 1997, 16, 3213.
are not present in HYD) indicates these thermally stable pashs were introduced with the recycle stream from the coker. 3.3. HPLC/PDA and HPLC/MS/MS Analyses of Decant Oil Derivatives. Figure 5 compares the HPLC/PDA chromatograms of DO15 and CF15, indicating significant differences in PAH composition of the two materials. On the lighter PAH end, phenanthrene and methylphenanthrene show increased peak heights resulting from the shift to less-substituted phenanthrenes distribution in the CF15. Pyrenes and chrysenes distributions also give the same trend as in phenanthrenes. On the heavier end, large increases in the concentration of benzopyrenes and benzoperylenes are seen in the CF 15. The indistinctive peaks around 25 min show similar UV spectra to that of benzoperyelene, but the peak wavelengths shifted to higher values, suggesting the presence of alkyl-substituted benzoperylene compounds. HYD derivatives gave less-featured HPLC/PDA chromatograms than either DO or CF (Figure 6). The broadening peaks of most PAH compounds in HYD stream suggest the composition of most PAHs in this stream is more close to its parent decant oil. The reduced phenanthrene peak intensity and the disappearance of chrysene (unsubstituted) in HYD feed are also noticeable. A series of intensified peaks at the longer retention times may represent partially hydrogenated PAH with higher molecular weights, but this cannot be verified by the PDA detector. Figure 7 compares HPLC/PDA chromatograms of DO15 and VTB15 samples. Mainly two clusters of peaks eluted from the HPLC column. The first peak cluster in VTB15 shows similar features to those the parent decant oils. The second peak cluster consists almost exclusively of five- and six-ring PAHs. Positively determined PAHs in the second hump are alkylated benzopyrene and benzo[g,h,i]perylenes. These two types of PAH molecules account for about 1/3 of the heavier PAHs in the second peak cluster. PAHs eluted after benzoperylene showed very poor PDA spectra that did not allow any identification of these heavier compounds. Paraffins and naphthenic compounds (cycloalkanes) in needle coke feedstocks cannot be analyzed on the PDA detector because of the absence of conjugated π electrons in these molecules. HPLC analysis gives a broad view of the constitution of the high-boiling-point molecular species. The larger portions of pyrene, benzopyrene, and benzoperylene and “simpler” PAH composition in CF derivatives were found to be the key differences between CF and DO samples. Figure 8 gives the LC/MS/MS spectra of DO15, HYD15, and CF15 that were obtained from direct injection of samples to a dual MS–MS system. The major peaks are a series of compounds with m/e difference of 14 (for example, m/e of 231, 245, 259, and 281, 295); this is indicating that these most abundant compounds are the alkylated homologues with different length side chains (or methyl substitution). From the fragmentation pattern, it can be determined that the m/e 231, 245, 259 are C2-, C3-, and C4-pyrene and m/e 281, 295, 319 are C2-, C3- and C4-benzopyrene. In general, the relative intensity of peaks of PAH homologue represents the relative abundance of PAHs species in the sample. By comparing the pyrene homologues intensity in the mass spectra (peaks with m/e of 231, 245, 259 in Figure 8) of DO15 and CF15, the less alkylated PAH distribution of pyrenes in CF is clearly seen. In addition, the mass ranges in the spectra suggest that decant oil and coker feed consist of mainly four- to six-ring aromatic compounds. The hyd feed mass spectrum differs from DO and CF; the sudden drop in molecular ion intensities above m/e of 350 results from the removal of the heavy end of the decant oil
High-Sulfur FCC Decant Oils
Energy & Fuels, Vol. 21, No. 6, 2007 3579
Figure 9. HPLC/MS/MS chromatograms of DO24 (top) and CF24 (bottom).
from vacuum distillation of decant oil. The partially hydrogenated PAHs can be found from peaks m/e of 211 (H4-C2PHEN), 225 (H4-C3-PHEN), and 237 (H4-C4-PHEN). The HPLC/MS/MS chromatograms obtained from DO and CF samples are shown in Figure 9. Compared with their parent decant oils, both CF streams show increased intensities of peaks after 13 min of HPLC elution, indicating the higher concentrations of five- and six-ring pahs present in these CF feeds. The high-boiling-point PAHs in CF24 were very well resolved on LC/MS compared to those in CF15, as can be seen from the well-defined peaks (better baseline under these heavy PAHs on the LC/MS/MS chromatograms). The relative distributions of five-ring aromatics BEP, MBEP, DMBEP, and TMBEP (peaks 13, 14, 15, and 16 in Figure 9) and six-ring PAH benzoperylene,
methylbenzoperylene, and dimethylbenzoperylene (peaks 17, 18, and 19) show similar patterns in these two CF streams. The most abundant five- and six-ring PAH homologues were found to be the isomers of methylbenzopyrene and dimethylbenzoperylenes. Between CF15 and CF24, the better resolution of peaks in this heavy PAH elution time window may suggest the composition of heavy PAHs in CF24 is simpler than that of CF15. The LD/MS spectrum from the VTB stream is shown in Figure 10. The heaviest ends with reasonable abundance in decant oil are the C1–4 alkylated benzoperylene (m/e 276) and dibenzopyrenes (m/e 302). Coronene (m/e 300) appears to be in very low concentration in the decant oils. 3.4. Mesophase Development from DO and Its Derivatives. The optical texture analysis results for the semi-
3580 Energy & Fuels, Vol. 21, No. 6, 2007
Eser and Wang
Figure 10. Laser desorption mass spectra of DO24 and VTB24 (m/e 200–400).
cokes of the derivatives from DO15 and DO24 are listed in Table 6. The reported OTI values show the results from duplicate experiments and the mean OTI from duplicate experiments. Figure 11 contains the micrographs of the semicokes from DO15 and its derivatives. The two decant oil samples produced semicokes with significantly different optical textures resulting from different extents of mesophase development during carbonization. DO24 has the OTI value of 83, while the OTI of DO15 gives a lower value of 69. Decant oil derivatives CF, HYD, and VTB produced semicokes that display much improved mesophase development compared to that produced by the parent decant oils. The OTI values of CF and VTB semicokes show significant increases over those of their parent decant oils. Under the same carbonization reaction conditions (500 °C, 3 h), HYD sample gave a very low yield of the solid product. Under the polarized-
light microscope, the majority of the optical units of the HYD solids appeared to be large mesophase spheres and isolated domains in an isotropic matrix. The OTI values for HYD semicokes given in Table 6 were determined on the semicokes produced by 5-h carbonization experiments. One of the most distinctive features observed on the semicokes obtained from HYD and CF is the relatively high proportion of isotropic pitch units, containing mesophase spheres in most cases. Both CF and HYD produced semicokes that show a relatively large amount of such isotropic units. This occurrence clearly indicates that the CF and HYD samples have lower coking reactivity than their parent decant oils. The opposite trend in the distribution of isotropic pitch is found on the semicoke from the VTB fraction where there is no nondeformed mesophase spheres at all, which indicates a higher carbonization reactivity from this heavy fraction of
High-Sulfur FCC Decant Oils
Energy & Fuels, Vol. 21, No. 6, 2007 3581
Figure 11. Polarized-light micrographs of the semicoke sample from DO15, CF15, HYD15, and VTB15 (top to bottom).
Figure 12. Correlation between the OTI of semicoke with pyrene to phenanthrene ratio in the feedstocks.
decant oil. Another prominent feature in the HYD semicokes is the almost exclusive domain and flow domain textures in these semicokes. The sizes of these optical texture units are much larger than those seen in the semicokes produced from either decant oil or CF samples. Figure 12 shows a plot of OTIS of semicokes from the original DO and their derivatives versus the total pyrenes to phenanthrenes molar ratios. The plot indicates a correlation between the mesophase development (or semicoke texture) and the total pyrenes/phenanthrenes concentration ratios of the DO
samples and their derivatives. A good correlation between pyrenes/phenathrenes ratio in feedstocks and optical texture development in cokes also appeared in related studies,18,20 with discussion of possible reasons for this correlation.18 It has been suggested that the effects of pyrenes to phenanthrenes ratio in needle coke feedstocks on the OTI of the resultant semicoke may be related to the differences in the rate of molecular growth process and the planarity of the resulting oligomers from catacondensed versus peri-condensed PAH. In addition, pyrenes can also serve as possible hydrogen shuttlers that provide a
3582 Energy & Fuels, Vol. 21, No. 6, 2007
prolonged fluidity period during the carbonization. These conditions lead to higher degrees of mesophase development.18 4. Summary and Conclusions FCC-DO and its derivatives involved in a pretreatment scheme have different molecular compositions. The molecular constitution of needle coke feedstocks consists of three- to sixring polyaromatic compounds as dominant components and alkanes as minor constituents. Depending on the crude oil source and operating conditions under which decant oils are produced, about 20–40% of total molecular species, which represent the lower to middle boiling range of the molecular components in the feedstocks, can be quantitatively characterized by GC/MS. HPLC/PDA, HPLC/MS/MS, and LD/MS analyses revealed very important features of heavy PAHs composition that cannot be seen from GC/MS. Although the quantitative characterization of the heavy PAH compounds in the feedstock was not possible, a qualitative and semiquantitative examination of results on the heavy end PAHs showed a continuum in the composition of the structurally closely related PAH compounds such as phenanthrenes and chrysenes, pyrenes, and benzopyrenes. Higher proportions of normal alkanes and lower contents of sulfur-containing aromatics were found in the DO24 decant oil sample, which produced better mesophase development during carbonization than the other decant oil samples. The PAH composition in the better feedstock has the characteristics of lower degree of methyl substitution and higher percentage of heavy PAHs compared with the other decant oil samples. Although the concentrations of major PAH homologues vary significantly among the decant oil samples, the methyl-PAH isomers showed a uniform distribution pattern in decant oil samples, most probably resulting from the thermodynamic equilibrium composition, which depends on the FCC processing operating conditions. Coker feeds, the actual feedstocks to the delayed coker, showed significantly different molecular composition from that of their parent decant oil samples. Coker feeds have low concentrations of normal alkanes and low sulfur contents. Compared to the parent decant oils, coker feeds contain higher proportions of higher alkyl-PAHs and higher portions of unsubstituted and less multi-methylated PAHs, resulting in a less complex PAH composition profile. Further, there is a shift of the composition of alkylated PAH isomers, particularly, the
Eser and Wang
methyl-PAH isomers to the dominant presence of the thermally stable isomers in the coker feeds. Hydrotreated decant oil (HYD), the lightest feedstock, contains a wide range of hydroaromatic compounds resulting from hydrogenation of PAH. The difficulty in resolving and identifying these hydroaromatics with the analytical protocol used in this study limited the proportion of the identified compounds to less than 10% of HYD feedstock. The VTB samples provided an opportunity to study the carbonization behavior of and the mesophase development from the heaviest PAH present in the decant oil. Peri-condensed sixring aromatics such as benzo[g,h,i]perylene and dibenzopyrene and their methylated homologues constitute the dominating PAH species in this feedstock. Mesophase development, hence the anisotropy of coke from FCC-DO and its derivatives, can be related to the differences in the molecular composition in the feedstocks. The higher degree of mesophase development from CF streams can be attributed to the overall reactivity of the hydrocarbons during carbonization reactions. The relatively higher proportion of less alkylated PAH and the low concentration of normal alkanes can explain the gradual conversion of asphaltenes into semicoke, producing highly developed mesophase structures. A correlation was observed between the OTI of the semicokes and the pyrenes to phenanthrenes molar ratio in the starting feeds (DO and CF). These results suggest that the effects of pyrenes to phenanthrenes ratio in needle coke feedstocks on the OTI of the resultant semicoke may be related to the differences in the rate of molecular growth process and the planarity of the resulting oligomers from cata-condensed versus peri-condensed PAH. In addition, pyrenes can also serve as possible hydrogen shuttlers that provide a prolonged fluidity period during the carbonization,leadingtohigherdegreesofmesophasedevelopment. Acknowledgment. This study was supported by the Carbon Research Center at Penn State University and by Chicago Carbon Company, IL. The authors thank Mr. Robert Miller and Dr. John Bassett at Chicago Carbon Company for their support and helpful discussions. Professor A. Daniel Jones of Michigan State University has graciously helped with the acquisition and interpretation of HPLC/MS and LD/MS data. EF060541V
