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Molecular Composition of the High-Boiling Components of Needle Coke Feedstocks and Mesophase Development Guohua Wang and Semih Eser* Department of Energy and Geo-EnVironmental Engineering, The PennsylVania State UniVersity, 101 Hosler Building, UniVersity Park, PennsylVania 16802 ReceiVed October 31, 2006. ReVised Manuscript ReceiVed July 11, 2007
Mesophase development during the carbonization of needle coke feedstocks in delayed coking affects the microstructural anisotropy (or graphitizability) of the resulting coke. Previous work has employed gas chromatography/mass spectrometry (GC/MS) to study the relationships between the molecular composition of the fluid catalytic cracking decant oil (FCC DO) and mesophase development. In this study, the principal emphasis was placed on analyzing the non-GC amenable fraction of DO samples using a combination of analytical techniques, including high pressure liquid chromatography with photodiode array (PDA) and mass spectrometer detectors in tandem (HPLC/PDA and HPLC/MS/MS), and laser desorption mass spectrometry (LD/MS). The experimental data showed that major molecular species in DO samples consist of 3–6 ring multimethyl-substituted polyaromatic hydrocarbons (PAHs), including phenanthrenes, pyrenes, chrysenes, benzopyrenes, perylenes, and benzo[g,h,i]perylenes. Large differences were observed in the molecular composition of decant oil samples, particularly in the distribution of the methylPAH homologues and the degree of methylation on a given PAH. High-boiling PAHs (identified by LD/MS) and low-boiling PAHs (identified by GC/MS) were found to have similar distribution trends in molecular composition. The degree of mesophase development was found to depend strongly on the molecular composition of the feedstock samples. In particular, the ratio of total concentrations of pyrenes/phenanthrenes correlates with the degree of mesophase development during carbonization.
Decant oil from fluid catalytic cracking (FCC) is used as a feedstock for delayed coking to produce needle coke for manufacturing graphite electrodes .1–3 Carbonaceous mesophase development4 leads to the formation of an anisotropic solid (needle coke) by liquid-phase carbonization. The degree of mesophase development from the feedstock determines the coke texture, hence the principal properties of resultant carbon products, such as reactivity and graphitizability. The molecular growth of mesogens and their alignment into lamellar structures constitute the principal chemical and physical processes that control the mesophase development .3,5 Several researchers have studied the relationships between the chemical composition of the feedstock and mesophase development. Feedstock parameters used in these studies include viscosity ,6,7 aromaticity ,8,9 solubility of feedstocks ,10–12 free-
radical initiation and intermediate product stabilites ,13–17 and heteroatom-containing compound distribution.10 The molecular composition of decant oils and its effects on the properties of the resulting cokes have been studied in our laboratory. We have reported on the distribution of major polyaromatic hydrocarbons (PAHs) and their alkylated substituents in the GCamenable fraction (low-boiling point component in decant oils) and its effect on the mesophase development during carbonization.18 To complement our previous work, the objective of this study is to analyze the molecular composition of particularly the highboiling components of the decant oil samples by a combination of analytical techniques, including high pressure liquid chromatography with photodiode array (HPLC/PDA), mass spectrometer detectors in tandem (HPLC/MS/MS), and laser desorption mass spectrometry (LD/MS) to further examine the relationships between the molecular composition of decant oils
* Corresponding author. E-mail:
[email protected]. (1) White, J. L.; Price, R. J. Carbon 1974, 12 (3), 321. (2) Mochida, I.; Oyama, T.; Korai, Y. Carbon 1988, 26 (1), 49–55. (3) Marsh, H. The Chemistry of Mesophase Formation. In Petroleum DeriVed Carbons;ACS Symposium Series; American Chemical Society: Washington, D.C., 1986; Vol. 303, pp 1–28. (4) Brooks, J. D.; Taylor, G. H. Carbon 1965, 3 (2), 185–186. (5) Rodriguez-Reinoso, F.; Santana, P.; Palazon, E. R.; Diez, M. A.; Marsh, H. Carbon 1998, 36 (1–2), 105–116. (6) 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. (7) Siskin, M.; Kelemen, S. R.; Gorbaty, M. L.; Ferrughelli, D. T.; Brown, L. D.; Eppig, C. P.; Kennedy, R. J. Energy Fuels 2006, 20 (5), 2117–2124. (8) Eser, S.; Jenkins, R. G. Carbon 1989, 27 (6), 877–887. (9) Eser, S.; Jenkins, R. G. Carbon 1989, 27 (6), 889–897.
(10) Menendez, R.; Granda, M.; Bermejo, J.; Marsh, H. Fuel 1994, 73 (1), 25–34. (11) Marsh, H.; Cornford, C. Mesophase: The Precursor to Graphitizable Carbon. In Petroleum DeriVed Carbons;ACS Symposium Series; Deviney, M. L., O’Grady, T. M., Eds.; American Chemical Soceity: Washington, D.C., 1976; Vol. 21, pp 266–281. (12) Siskin, M.; Kelemen, S. R.; Eppig, C. P.; Brown, L. D.; Afeworki, M. Energy Fuels 2006, 20 (3), 1227–1234. (13) Lewis, L. C.; Singer, L. S. Carbon 1967, 5 (4), 373. (14) Lewis, I. C.; Singer, L. S. Electron Spin Resonance and the Mechanism of Carbonization. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1981; Vol. 17, p 1. (15) Lewis, I. C. Carbon 1980, 18 (3), 191. (16) Walker, J. P. L.; Weinstein, A. Carbon 1967, 5 (1), 13–17. (17) Yokono, T.; Miyazawa, K.; Sanada, Y.; Marsh, H. Fuel 1979, 58 (9), 692. (18) Filley, R. M.; Eser, S. Energy Fuels 1997, 11 (3), 623–630.
1. Introduction
10.1021/ef0605403 CCC: $37.00 2007 American Chemical Society Published on Web 08/23/2007
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Figure 1. Polarized-light micrographs of the semicoke samples from DO-A1, DO-A2, DO-B1, and DO-B2 (from top to bottom). Table 1. OTIs for Semicokes from DO Samples by Carbonization at 500 °C for 3 h optical texture unit counts decant oil DO-A1 (I) DO-A1 (II) average DO-A2 (I) DO-A2(II) average DO-B1 (I) DO-B1(II) average DO-B2 (I) DO-B2 (II) average
flow domains
domains
small domains
mosaics
isotropic pitch
113 136
70 74
13 9
0 0
1 3
48 53
97 105
31 59
18 9
2 1
84 70
35 36
8 6
0 0
1 1
103 130
17 20
15 18
0 0
2 3
OTI 76 79 78 51 48 50 80 79 80 83 84 84
and the degree of mesophase development upon carbonization. A companion paper presents a laboratory study of a pretreatment approach to use high-sulphur decant oil as feedstock for producing needle coke.19 2. Experimental Section 2.1. Needle Coke Feedstocks. The feedstocks used in this study are the fluid catalytic cracking (FCC) decant oils (DOs) that were used in two commercial needle coke plants. They are designated as decant oils DO-A1, DO-A2, DO-B1, and DO-B2. 2.2. Carbonization of Feedstocks and Coke Optical Texture Examination. Carbonization experiments were performed at 500 °C for 3 h under autogeneous pressure. About 4 g of feedstock was placed in a aluminium foil cylinder in a 15 mL vertical tubing (19) Eser, S.; Wang, G. Energy Fuels 2006, submitted for publication.
bomb reactor, and the loaded reactor was purged by nitrogen for 3 times to expel the oxygen inside the reactor before it was plunged into a preheated fluidized-sand bath heater. The optical texture of the resulting semicoke (the whole quantity obtained as a cylindrical bar) was characterized according to the shape and size of the isochromatic areas observed on the polished surface under a polarized-light microscope (Nikon Microphot-FXAII). A systematic scanning of the whole surface of the specimen was achieved on an automatic stage control system. A 0.9 mm × 0.8 mm mask and 5× object lens were used to acquire surface images from the semicoke specimen. At least 250 images were examined for each specimen. The coke texture was characterized by the distribution of various optical units on the specimen surface .20–22 This was done by measuring the occurrence of the size and shape of anisotropic domains under a polarized-light microscope. A factor (index) is assigned to each descriptive unit depending on its contribution to the anisotropy and the overall optical texture index (OTI) was calculated by the following equation:23 OTI ) ∑fiOTIi where fi is the numerical fraction of individual texture types from microscopic analysis and OTIi is the index assigned to each texture type. 2.3. Gas Chromatography/Mass Spectrometry (GC/MS) Analysis of Decant Oil Samples. The GC/MS experiments were performed on a Shimadzu GC-17A gas chromatograph and QP5000 mass spectrometer. For each analysis, approximately 60 mg of decant oil sample was dissolved in 20 mL of DCM with 50 ng/mL of perdeuterated pyrene (pyrene-D10, Cambridge Isotope Laboratories, Inc., Andover, MA) added as an internal standard. (20) Marsh, H.; Neavel, R. C. Fuel 1980, 59 (7), 511. (21) Oya, A.; Qian, Z.; Marsh, H. Fuel 1983, 62 (3), 274–278. (22) Hole, M.; Foosnæs, T.; Øye, H. A. Light Met. 1991, 575–579. (23) 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 and New York, 1998; pp 147–155.
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Figure 2. Distribution of naphthalene (NAPH), phenanthrene (PHEN), and their methyl-substituted homologues in DO samples.
Figure 3. Distribution of pyrene (PY), chrysene (CHRY), and their methyl-substituted homologues in DO samples.
The prepared sample in dichloromethane solution was analyzed with reference to EPA method 8270c.24 2.4. 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 (24) EPA Semivolatile Organic Compounds by Gas Chromatography/ Mass Spectrometry. http://www.epa.gov/epaoswer/hazwaste/test/pdfs/ 8270c.pdf (accessed July 2007)
the GC column. The HPLC separation was performed on a Waters SE600 pump and Waters 996 PDA detector. A normal phase HPLC column (25 cm × 46 mm, packed with 5 µ silica with pore size of 60 Å (RingSep, ES Industries, NJ) was used for separating aromatic compounds with respect to the number 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). It was oper-
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Figure 4. TIC of DO-A1 corresponding to five-ring PAHs.
Figure 5. HPLC/PDA chromatograms of DO-A1 (top) and DO-A2 (bottom) at 254 nm.
ated in a linear mode which provided higher sensitivity to highmolecular-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 a 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. The analytical procedures can be found elsewhere.25
3. Results and Discussion 3.1. Mesophase Development from Decant Oil Feedstocks. The polarized-light micrograph images of the semicokes from the carbonization of studied decant oil samples are shown in Figure 1. Each image consists of three micrographs
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Figure 6. HPLC/PDA chromatograms of DO-B1 (top) and DO-B2 (bottom) at 254 nm.
combined edge-to-edge to display a larger portion of a given area on the semicoke surface. The images were taken from the middle section of the coke bar that was believed to show a representative area of the respective semicoke sample bars. Table 1 gives the overall optical texture index (OTI) values for the semicokes from the carbonization of decant oil samples. The value of OTI (mean of two duplicate experiments) represents the degree of mesophase development during carbonization.23 The higher the OTI, the higher the degree of mesophase development toward the anisotropic texture of needle coke. As shown in Table 1, DO-B2 has the highest OTI value (84), followed by DO-B1 and DO-A1. The semicoke from DOA2 gives the lowest OTI value of 50. The appearance of the optical units in the poorest semicoke from DO-A2 is very different from those of the semicokes from the other decant oil samples as shown in Figure 1. The most significant distinction is the long and wide cracks seen throughout the surface of this semicoke. This is an evidence for a significant shrinkage of the semicoke after 500 °C reaction due to the hardening of the mesophase. In addition to the severe shrinkage, there exists a large amount of small(25) Wang, G. Molecular Composition of Needle Coke Feedstocks and Mesophase Development During Carbonization. Ph.D Thesis, The Pennsylvania State University, University Park, PA, 2005.
size pores in the bulk of this semicoke. The edges of pores appear to be “eroded” compared with the smooth edges of the pores in DO-A1, for example. Most of the pores in the semicoke from DO-A2 are not filled or only partially filled by the mounting resin (dark purple areas seen on pellet surfaces) during pellet preparation, indicating that most of these pores are closed pores. Along with the small anisotropic domain size, the appearance of semicoke from DO-A2 suggests that the mesophase was formed and hardened into anisotropic coke more quickly compared to the carbonization of the other decant oil samples.18 3.2. GC/MS Analysis of Hydrocarbons in Decant Oil Feedstocks. The quantitative GC/MS analysis results for major GC-amenable PAHs in decant oils are presented in Figures 2 and 3 for the concentrations of naphthalene and phenanthrene and pyrene and chrysene, respectively. These major PAH compounds in this GC-amenable fraction comprise 2-ring to 4-ring aromatics with a high degree of alkyl substitution. The total concentration of naphthalenes (naphthalene and alkynaphthalene) in decant oil is much lower than that of the higher PAHs. DO-A1, DO-A2, and DO-B2 contain about the same concentration of total naphthalenes, while DO-B1 has only one half of the naphthalenes present in the other samples. Phenanthrenes and pyrenes constitute the majority of PAHs in the GC-
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Figure 7. HPLC/MS/MS chromatograms of DO-A1 (top) and DO-A2 (bottom).
amenable fraction. More phenanthrenes are present in DO samples in set A than in set B. Particularly, DO-2A has about four times higher phenanthrenes than DO-2B. Larger variations in pyrene content were observed between the decant oil sets and between the samples within the same set. The DO-A2 and DO-B1 have lower pyrene contents than in DO-A1 and DOB2. Compared with phenanthrene and pyrene concentrations, chrysenes appeared to be in lower quantity. However, this seemingly low concentration of chrysenes (compared with phenanthrenes and pyrenes) can be attributed to the inability of the GC temperature column to evaporate such high-boiling PAHs into the mass spectrometer to be detected, leading to low overall concentrations.
The relative distributions of major PAH homologues among decant oil samples fall into two sets. The PAHs in set B, DOB1 and DO-B2, have lower extents of alkyl substitution (15% nonsubstituted naphthalene) than those in set A samples (DOA1 and DO-A2) that contain
