The Pennsylvania State University
The Graduate School
Department of Energy and Geo-Environmental Engineering
MOLECULAR COMPOSITION OF NEEDLE COKE FEEDSTOCKS
AND MESOPHASE DEVELOPMENT DURING CARBONIZATION
A Thesis in
Materials Science and Engineering
by
Guohua Wang
© 2005 Guohua Wang
Submitted in Partial Fulfillment of the Requirements for the Degree of
Doctor of Philosophy
December 2005
The thesis of Guohua Wang was reviewed and approved* by the following:
Semih Eser Associate Professor of Energy and Geo-Environmental Engineering Thesis Adviser Chair of Committee
Ljubisa R. Radovic Professor of Energy and Geo-Environmental Engineering
Harold H. Schobert Professor of Fuel Science
Alan W. Scaroni Professor of Energy and Geo-Environmental Engineering
Jonathan P. Mathews Assistant Professor of Energy and Geo-Environmental Engineering
A. Daniel Jones Professor of Chemistry, Michigan State University
Gary L. Messing Professor of Ceramic Science and Engineering Head of Dept. of Materials Science and Engineering
*Signatures are on file with the Graduate School. iii
Abstract
This study investigates the molecular composition of fluid catalytic cracking
(FCC) decant oil and its derivatives that are used as feedstocks for delayed coking to
produce needle coke. Needle coke is a premium solid carbon used in manufacturing
graphite electrodes for electric-arc furnace. For the first time in the open literature, this
study reports data on the molecular composition of the feed that is actually introduced
into the coke drum for delayed coking after fractionating the decant oil feedstock together
with the liquid products generated from coking. The coker feed (CF), thus, includes the
high-boiling fraction of the decant oil and the high-boiling fraction of liquid products
(recycle). Hydrotreated fraction of the decant oil samples (HYD) and a vacuum tower
bottom (VTB) obtained from decant oils were also analyzed. More emphasis was placed
on analyzing the high-boiling fraction of the feedstocks that is not amenable to gas
chromatography. Carbonaceous mesophase development from the feedstocks was also
studied to seek relationships between the composition of the feedstocks and the needle coke texture.
Commercial FCC decant oil (DO) samples and their derivatives (CF, HYD, and
VTB) were carbonized in laboratory reactors. DO samples produced semi-cokes that displayed different degrees of mesophase development and CF, HYD and VTB give a higher degree of mesophase development compared to their parent decant oil. Significant differences were observed in the molecular composition of the different decant oil samples and between the decant oils and their derivatives. The principal compounds found in decant oils and their derivatives were found to consist of 3- to 6-ring PAHs iv
(phenanthrene, pyrene, chrysene, benzopyrene, perylene, and benzoperylene). These
PAHs are present in the feedstocks as multi-methyl substituted homologues. Compared to
the parent DO, CF contains higher concentrations of unsubstituted PAHs, a lower degree
of methyl substitution, and a higher proportion of thermally stable alkyl PAH isomers.
HYD, on the other hand, contains a significantly higher proportion of hydroaromatics and
much lower concentrations of sulfur compounds. The VTB fraction consists almost exclusively of PAH with 4 and higher number of condensed rings.
In general, mesophase development from the feedstocks was observed to relate to the overall carbonization reactivity (rate of semi-coke formation) and the relative concentrations of pyrenes and phenanthrenes. This trend confirms a widely accepted notion that the lower the carbonization reactivity, the higher is the degree of mesophase development. Samples of CF and HYD exhibit a relatively low carbonization reactivity
(compared with their parent DO) and exhibit a high degree of mesophase development.
As one exception to the general trend, VTB fractions exhibited the highest carbonization reactivity, but produced a well developed anisotropic coke texture.
The carbonization reactivity of the feedstocks depends on their molecular composition, as exemplified by the comparison of CF with DO. This study shows that the relative distribution of pyrene and phenanthrene compounds plays a significant role in the mesophase development from the needle coke feedstocks. A preliminary quantum chemistry modeling (MNDO level of theory) was conducted using phenanthrenes and pyrenes as models. The results gave a relatively high reaction rate constant for the formation of oligomers with non-planar configuration from phenanthrenes. In contrast, pyrenes produce planar oligomers with lower reaction rate constants. v
TABLE OF CONTENTS
LIST OF TABLES...... viii
LIST OF FIGURES ...... x
ACKNOWLEDGEMENTS...... xiv
Chapter 1 Introduction and Research Objectives...... 1
1.1 Introduction...... 1 1.2 Hypothesis and Research Objectives...... 5 Chapter 2 Literature Review...... 7
2.1 Carbonaceous Mesophase...... 7 2.2 Delayed Coking and Needle Coke Production ...... 8 2.3 Chemistry of Carbonization and Needle Coke Feedstock Composition Characterization ...... 14 2.4 Characterization of Coke Texture...... 26 2.4.1 The use of polarized-light microscope...... 26 2.4.2 Coke texture analysis ...... 29 Chapter 3 Experimental Section ...... 33
3.1 Analysis of the Molecular Composition of Needle Coke Feedstock...... 33 3.1.1 Gas chromatography/mass spectrometry (GC/MS) analysis of decant oil samples...... 33 3.1.2 HPLC/PDA, LD/MS , LC/MS/MS, elemental and NMR analyses of decant oil samples...... 40 3.1.3 Analysis of sulfur compounds ...... 41 3.1.4 NMR analysis...... 41 3.2 Carbonization and Semi-Coke Characterization...... 42 3.2.1 Carbonization experiments ...... 42 3.2.2 Examination of optical texture of semi-cokes ...... 44 3.2.3 Calcination and graphitization of semi-coke samples...... 45 3.3 Experimental Protocol for Preparing Decant Oil Fractions by Vacuum Distillation ...... 46 vi
Chapter 4 Chemical/Physical Properties and Resultant Semi-coke Optical Properties of
Feedstocks...... 48
4.1 Needle Coke Feedstocks...... 48 4.2 Characterization of the Optical Textures of the Semi-Cokes Produced from the Decant Oil Samples and Their Derivatives...... 51 4.3 X-ray Diffraction Analysis of the Heat Treated Semi-cokes...... 61 Chapter 5 Characterization of Needle Coke Feedstocks...... 71
5.1 Gas Chromatography / Mass Spectrometry Analysis of Needle Coke Feedstocks 71 5.1.1 Two-ring aromatic compounds in decant oils ...... 76 5.1.2 Three-ring aromatic compound in decant oils...... 87 5.1.3 Four-ring polycyclic aromatic hydrocarbons in decant oils ...... 100 5.1.4 Normal alkanes in decant oil ...... 107 5.1.5 PAHs in decant oil derivatives...... 110 5.2 High-Performance Liquid Chromatography Analysis of Needle Coke Feedstocks ...... 145 5.3 LC/MS/MS and Laser Desorption Mass Spectrometry Analysis of Needle Coke Feedstocks...... 160 5.3.1 LC/MS/MS analysis of decant oil and its derivatives ...... 160 5.3.2 LD/MS analysis of decant oil and its derivatives...... 176 5.4 1H and 13C Nuclear Magnetic Resonance Spectroscopy Analysis of Needle Coke Feedstocks...... 190 5.5 Summary...... 202 Chapter 6 Mesophase Development and the Molecular Composition of Needle Coke
Feedstocks...... 204
6.1 Vacuum Distillation Product Yields from Decant Oil and Coker Feed ...... 204 6.2 Optical Texture of Semi-Cokes from Vacuum Residuals of Coker Feedstocks... 205 6.3 Changes in Molecular Composition of the Heavier Fractions Obtained by Vacuum Distillation...... 206 vii
6.4 Intermediate Product Yields from Lower Temperature Carbonization of Decant Oil and Decant Oil Derivatives...... 212 6.5 General Trends in the Molecular Composition Change in the Early Stages of Liquid-Phase Carbonization of Decant Oil and Its Derivatives...... 217 6.5.1 Normal alkanes ...... 217 6.5.2 Phenanthrene and pyrene ...... 220 6.6 The Relationship between the Molecular Composition of Feedstocks and Mesophase Development...... 225 Chapter 7 A Semi-empirical Molecular Orbital Study of Thermal Reactivities of Major
PAH in Needle Coke Feedstocks...... 230
7.1 The Overall Reactivity of PAH Molecules...... 232 7.2 Position of Carbon-centered Free Radicals on PAH Ring Structures ...... 236 7.3 Oligomerization of PAH into Mesogens ...... 248 7.4 Summary ...... 264 Chapter 8 Summary, Conclusions and Suggestions for Future Work ...... 265
Bibliography ...... 275
viii
LIST OF TABLES
Table 2.1 Typical delayed coking operation time schedule (Gary and Handwerk, 1994) 10 Table 2.2 Typical needle coke specifications ...... 13 Table 2.3 Nomenclature and optical texture index for coke microtextural description.... 31 Table 2.4 Nomenclature and optical texture index for needle coke microtextural description...... 32 Table 3.1 Standards and their GC/MS characteristics ...... 37 Table 3.2 Standards and their GC/MS characteristics (continued)...... 38 Table 4.1. Elemental composition of feedstock samples, wt %...... 50 Table 4.2 OTIs for semi-cokes from DO samples by carbonization at 500˚C for 3 h...... 53 Table 4.3 OTIs for semi-cokes from DO derivatives by carbonization at 500˚C for 3 h (5 h for HYD)...... 57 Table 4.4 Peak position and half-high width of SRM 640c using Cu Kα radiation...... 64 Table 4.5 Semi-coke crystalline parameters from XRD analysis...... 65 Table 4.6 XRD parameters of calcined cokes...... 67 Table 4.7 XRD parameters of carbons heated to graphitization temperature...... 68 Table 5.1 Needle coke feedstock used in GC/MS analysis ...... 71 Table 5.2 Concentrations of naphthalene and its alkylated substituents in decant oils .... 79 Table 5.3 Relative naphthalene homologues distribution (wt%)...... 81 Table 5.4 Isomer distribution in naphthalene and alkylated derivatives concentrations in decant oils ...... 83 Table 5.5 Concentrations of biphenyl and diphenylmethane and their alkylated derivatives in decant oils...... 85 Table 5.6 Relative distribution of BP homologues in decant oil samples ...... 85 Table 5.7 Phenanthrene and its alkylated substitutes in decant oils (compound identifications are shown in Figure 5.10) ...... 92 Table 5.8 Relative distribution of phenanthrene homologues in decant oils...... 93 Table 5.9 Alkylphenanthrene distribution in decant oils...... 94 Table 5.10 Polycyclic aromatic sulfur compounds concentrations in decant oils (compound identifications are shown in Figure 5.12 and Figure 5.13)...... 98 Table 5.11 Relative distribution of DBT and BNT isomers in decant oils...... 99 Table 5.12. Distribution of pyrene and chrysene and their alkylated derivatives in decant oils (compound identifications are shown in Figure 5.16 and Figure 5.17) ...... 105 Table 5.13 Pyrene and chrysene and their alkylates distribution in decant oils ...... 106 Table 5.14 Methylpyrene and methylchrysene isomer relative distribution in decant oil ...... 106 Table 5.15. Normal alkane distribution in decant oils...... 109 Table 5.16 Naphthalene composition in decant oil and coker feed ...... 112 Table 5.17 Naphthalene concentration and distribution in HYD ...... 113 Table 5.18 Biphenyls distribution in decant oil derivatives (unit: ppm in feed) ...... 115 Table 5.19 Concentrations of naphthalene and alkylated naphthalenes in decant oil and its derivatives, ppm in feeds ...... 116 Table 5.20 Concentrations of biphenyl and alkylated biphenyls in decant oil and its derivatives, ppm in feeds ...... 117 ix
Table 5.21 Two-ring PAH concentrations in decant oil derivatives, ppm in feeds...... 118 Table 5.22 Isomeric distribution in alkylnaphthalenes in decant oil derivatives, wt% .. 120 Table 5.23 Isomeric distribution of biphenyl compounds in decant oil derivatives, wt% ...... 120 Table 5.24 Relative phenanthrene and alkylphenanthrene distribution in decant oil derivatives...... 122 Table 5.25 Phenanthrene isomer distribution in DO and CF, wt% ...... 123 Table 5.26 Comparison of relative distribution of phenanthrene homologues in DO and HYD...... 124 Table 5.27. Phenanthrene isomer distribution in HYD and DO (unit: wt%)...... 124 Table 5.28 Hydrophenanthrene and phenanthrene concentration in HYD derivatives (unit: ppm in feed) ...... 126 Table 5.29. Phenanthrene concentrations in decant oil derivatives (unit: ppm in feed). 127 Table 5.30. Polycyclic aromatic sulfur compounds distribution in decant oil derivatives (unit: ppm in feeds)...... 129 Table 5.31 Four-ring PAHs in decant oil derivatives (unit: ppm in feed) ...... 132 Table 5.32 Relative distribution of pyrene compounds in decant oil derivatives (ppm in feed) ...... 133 Table 5.33 Total chrysene concentration (ppm in feed) and relative distribution (wt%) in decant oil derivatives ...... 134 Table 5.34 Five and more ring PAH distribution in decant oil stream (ppm in feed) .... 144 Table 5.35 Response factors (relative to that of pyrene) of PAH standard compounds 146 Table 5.36 1H NMR downfield shift assignment for functional groups...... 191 Table 5.37 13C NMR downfield shift assignment for functional groups in petroleum.. 196 Table 5.38. Distribution of hydrogen atoms in decant oil and its derivatives ...... 197 Table 5.39. Distribution of carbon atoms in decant oil and its derivatives ...... 197 Table 6.1 Residua yields of feedstocks under vacuum distillation, wt%...... 204 Table 6.2 Relative OTI increment in the semi-cokes from vacuum residua ...... 206 Table 6.3 Distribution of phenanthrene homologs in vacuum residua, wt%...... 208 Table 6.4 Distribution of pyrene homologs in vacuum residua, wt% ...... 208 Table 6.5 Isomer distribution of phenanthrene and pyrene in vac. residua, wt%...... 209 Table 6.6 Isomer distribution of major PAHs in the liquid products of short time carbonization (wt,%)...... 222 Table 7.1 HOMO and LUMO of PAH and alkylated substitutes using MNDOd level of theory ...... 235 Table 7.2 Heat of formation of PAH and their alkylated analogues...... 239 Table 7.3 BDE of PAHs to form aryl radical...... 241 Table 7.4 BDE of forming aryl radicals at various sites on multi-alkylated PAHs...... 242 Table 7.5 BDE of forming aryl-alkyl radical from methyl-PAHs...... 243 Table 7.6 Radical frontier density of edge carbon atoms on unsubstituted PAH...... 246 Table 7.7 Effect of methylation on the radical frontier density of edge carbon in PAH246 Table 7.8 The geometric parameters of inter-molecular addition of PAH ...... 256 Table 7.9 Transition state properties and the kinetics parameters of PAH radical addition reaction...... 259 Table 7.10 Kinetics data for intra-molecular addition of PAH compounds ...... 260
x
LIST OF FIGURES
Figure 2.1 Brooks and Taylor’s mesophase sphere model (Brooks and Taylor, 1965)...... 8 Figure 2.2 A schematic diagram of delayed coking process (Gary and Handwerk, 1994) 9 Figure 2.3 Coke formation in a delayed coker (Ellis and Hardin, 1993)...... 11 Figure 2.4 General carbonization reactions to produce coke and graphite product (Lewis, 1982) ...... 15 Figure 2.5 The structure of carbonaceous mesophase (Mochida, Fujimoto et al., 1994). 16 Figure 2.6 A schematic transition of graphitizable coke to graphite upon heat treatment (Marsh, 1989)...... 16 Figure 2.7 Free valence indexes for different positions in anthracene and phenanthrene19 Figure 2.8 Polarization color and crystal orientation on the coke surface (Marsh, 1989) 28 Figure 3.1 Total ion chromatogram of standard (20μg/ml in DCM) on GC/MS (peak labels are shown in Table 3.1), peak 16 * is the internal standard ...... 39 Figure 3.2 Experimental setup for feedstock vacuum distillation ...... 46 Figure 4.1 A pretreatment scheme used to reduce the sulfur content of the feed to the heater of a delayed coking unit to produce needle coke ...... 49 Figure 4.2 Polarized-light micrographs of sections at the top, middle and bottom of the semi-coke bar obtained from carbonization of DO15...... 52 Figure 4.3 Polarized-light micrographs of the semi-coke sample from DO91...... 54 Figure 4.4 Polarized-light micrographs of the semi-coke sample from DO93...... 54 Figure 4.5 Polarized-light micrographs of the semi-coke sample from DO15...... 54 Figure 4.6 Polarized-light micrographs of the semi-coke sample from DO24...... 55 Figure 4.7 Polarized-light micrographs of the semi-coke sample from DOPSU ...... 55 Figure 4.8 Polarized-light micrographs of the semi-coke sample from DOUP...... 55 Figure 4.9 Polarized-light micrographs of the semi-coke sample from CF15...... 58 Figure 4.10 Polarized-light micrographs of the semi-coke sample from HYD15...... 58 Figure 4.11 Polarized-light micrographs of the semi-coke sample from VTB15...... 58 Figure 4.12 Polarized-light micrographs of the semi-coke sample from CF24...... 59 Figure 4.13 Polarized-light micrographs of the semi-coke sample from HYD24...... 59 Figure 4.14 Polarized-light micrographs of the semi-coke sample from VTB24...... 59 Figure 4.15 X-ray diffractogram for semi-cokes ...... 65 Figure 4.16 X-ray diffractogram for calcined cokes...... 66 Figure 4.17 X-ray diffractogram for coke after graphitization temperature treatment.... 67 Figure 4.18 The d002-spacing of semi-cokes and calcined cokes versus OTI of semi-cokes ...... 69 Figure 4.19 The d002-spacing of graphitized coke vs. OTI of semi-coke ...... 70 Figure 4.20 Crystallite diameter La of graphitized coke vs. OTI of green coke ...... 70 Figure 5.1 GC/MS total ion chromatogram of DO 91, DO93 and DO15...... 73 Figure 5.2 GC/MS total ion chromatogram of DOPSU and DOUP...... 74 Figure 5.3 GC/MS total ion chromatogram of HYD15, CF15 and VTB15 ...... 75 Figure 5.4 TIC of DO91 in the retention time window from 8 to 20 minutes...... 77 Figure 5.5 Multi-ion chromatograms (MIC) of DO91 representing alkylated naphthalenes ...... 78 Figure 5.6 Total concentrations of naphthalene (NAPH), methylnaphthalenes (MN), dimethylnaphthalenes (DMN) and trimethylnapthalenes (TMN) in decant oils ...... 80 xi
Figure 5.7 MIC of DO91 representing alkylated biphenyls and diphenylmethanes...... 84 Figure 5.8 Total ion chromatogram of DO91 (retention time from 20 min to 33 min).... 87 Figure 5.9 Basic three-ring aromatic compounds present in decant oil samples...... 88 Figure 5.10 Multi-ion chromatograms of DO91 representing multi-methyl phenanthrenes ...... 90 Figure 5.11 Concentration of phenanthrene and alkylated phenanthrenes in decant oils. 91 Figure 5.12 Multi-ion chromatograms representing alkylated dibenzothiophenes (DBTs) in decant oil samples...... 96 Figure 5.13 Multi-ion chromatograms representing alkylated benzonaphthothiophenes (BNT) in decant oil samples ...... 97 Figure 5.14 Total ion chromatogram corresponding to four-ring aromatics in DO91 ... 100 Figure 5.15 Structures of four-ring aromatics detected in decant oil ...... 101 Figure 5.16 Multi-ion chromatograms representing alkylated pyrenes in DO91 ...... 103 Figure 5.17 Multi-ion chromatograms representing alkylated benzo(a)anthracene and chrysenes in DO91...... 104 Figure 5.18 Selected ion chromatograms (m/e=47) representing normal alkanes in DO 91 ...... 107 Figure 5.19 Distribution of normal alkanes in decant oils...... 109 Figure 5.20 Naphthalene distribution in decant oil derivatives ...... 111 Figure 5.21 Distribution of tetralin and naphthalene in HYD derivatives...... 114 Figure 5.22 Total concentration of phenanthrenes in decant oil derivatives ...... 121 Figure 5.23. TIC of hydrophenanthrene in HYD stream (Top TIC shows DO in the same window for comparison)...... 125 Figure 5.24 Sulfur-containing compounds in decant oil derivatives ...... 128 Figure 5.25 Comparison of MICs of PASH compounds in decant oil derivatives...... 131 Figure 5.26 Concentration of pyrene homologues in decant oil derivatives ...... 133 Figure 5.27 Multi-ion chromatograms of m/e 216, 230 and 244 representing MPY, DMPY and TMPY in HYD, CF and DO derivatives (intensity is comparable within same stream only)...... 135 Figure 5.28 Multi-ion chromatograms of m/e 242, 256 and 270 representing MCHRY, DMCHRY and TMCHRY in HYD, CF and DO derivatives (intensity is comparable within same stream only)...... 137 Figure 5.29. TIC of decant oil derivatives corresponding to five and higher number ring PAHs...... 139 Figure 5.30. TIC of decant oil derivatives corresponding to five and higher number ring PAHs...... 140 Figure 5.31. The structure of some heavy PAHs in decant oil derivatives...... 141 Figure 5.32 Multi-ion chromatograms of five-ring PAHs in decant oil derivatives...... 143 Figure 5.33. Distribution of heavy PAHs in decant oil derivatives...... 144 Figure 5.34 . HPLC/PDA chromatogram of standard PAH compounds at 254nm ...... 145 Figure 5.35 HPLC/PDA chromatograms of DO 91 and DO93 at 254nm ...... 149 Figure 5.36 HPLC/PDA chromatograms of DO 15 and DO24 at 254nm ...... 150 Figure 5.37 HPLC/PDA chromatograms of DOPSU and DOUP at 254nm...... 151 Figure 5.38 Comparison of HPLC/PDA chromatograms of DO UP and DO15 at 254nm ...... 152 Figure 5.39 HPLC/PDA chromatograms of CF15 and CF24 at 254nm...... 155 xii
Figure 5.40 Comparison of HPLC/PDA chromatograms of DO15 and CF15 at 254nm ...... 156 Figure 5.41 Comparison of HPLC/PDA chromatograms of HYD15 and DO15 at 254nm ...... 157 Figure 5.42 Comparison of HPLC/PDA chromatograms of VTB15 and DO15 at 254nm ...... 158 Figure 5.43 Comparison of HPLC/PDA chromatograms of VTB15 and VTB245 at 254nm ...... 159 Figure 5.44 LC/MS/MS spectra of DO15 (top), CF15( middle) and HYD15 (bottom). 161 Figure 5.45 Daughter ion spectrum of m/e 231 (LC/MS/MS)...... 163 Figure 5.46 Daughter ion spectra of m/e 2451 and 259 (LC/MS/MS) ...... 164 Figure 5.47 Daughter ion spectra of m/e 285 and 295(LC/MS/MS) ...... 165 Figure 5.48 HPLC/MS/MS chromatograms of DO91 and DO93...... 167 Figure 5.49. HPLC/MS/MS chromatograms of DO15 and DO24...... 168 Figure 5.50. HPLC/MS/MS chromatograms of DOPSU and DOUP ...... 169 Figure 5.51. HPLC/MS/MS chromatograms of CF15 and CF24 ...... 172 Figure 5.52. HPLC/MS/MS chromatograms of HYD15 and HYD24...... 173 Figure 5.53 . HPLC/MS/MS chromatograms of VTB15...... 174 Figure 5.54 HPLC/MS/MS chromatograms of VTB24...... 175 Figure 5.55. Laser desorption mass spectra of DO15, CF15 and HYD15...... 177 Figure 5.56. Laser desorption mass spectra of DO91 and DO93 ...... 180 Figure 5.57. Laser desorption mass spectra of DO15 and DO24 ...... 181 Figure 5.58. Laser desorption mass spectra of DOPSU and DOUP...... 182 Figure 5.59. Laser desorption mass spectra of DO24 and HYD24 ...... 184 Figure 5.60. Laser desorption mass spectra of DO15 and HDY15 ...... 185 Figure 5.61. Laser desorption mass spectra of DO24 and CF24 ...... 186 Figure 5.62. Laser desorption mass spectra of DO15 and CF15 ...... 187 Figure 5.63. Laser desorption mass spectra of DO24 and VTB24 ...... 188 Figure 5.64 Laser desorption mass spectra of DO24 and VTB24 (m/e 200~400) ...... 189 Figure 5.65. 1H NMR spectra of DO91, DO93 and DO15...... 192 Figure 5.66. 1H NMR spectra of DO15, HYD15 and CF15...... 194 Figure 5.67. 1H NMR spectra of DO15 and VTB15...... 195 Figure 5.68. 13C NMR spectrum of DO91, DO93 and DO15...... 200 Figure 5.69. 13C NMR spectrum of DO15, HYD15, CF15 and VTB15...... 201 Figure 6.1 Semi-coke OTI from vacuum distillation heavy fractions ...... 205 Figure 6.2 GC/MS TICs of the vacuum residua from DO15...... 207 Figure 6.3 Total PHEN and PY concentration change in DO91 by vacuum distillation209 Figure 6.4 Total PHEN and PY concentration change in DO93 by vacuum distillation210 Figure 6.5 OTI of semi-coke from residua (DO93 and DO91) vs. pyrenes/phenanthrenes concentration ratio ...... 211 Figure 6.6 Correlation between the OTI of semi-coke with pyrene to phenanthrene ratio in the feedstock ...... 212 Figure 6.7 Semi-coke and asphaltene yields during carbonization of DO91 and DO93 at 450˚C...... 214 Figure 6.8 Semi-coke yields from carbonization of DO24 derivatives at 450˚C ...... 215 Figure 6.9 Asphaltenes yields from carbonization of DO24 derivatives at 450˚C...... 215 xiii
Figure 6.10 Alkane concentration in the product from DO24, CF24 and HYD24 within 60 minutes of carbonization at 450˚C...... 218 Figure 6.11 Alkane distribution in the products from DO (top), CF (middle) and HYD (bottom) at 15 and 30 minutes reaction time for carbonization at 450˚C...... 219 Figure 6.12 MICs of PHEN and PY in the product at various reaction time during carbonization of DO at 450˚C...... 220 Figure 6.13 Ratios of PAHs to their methyl-substituted homologs in the products during carbonization of DO15 and CF15 at 450C...... 224 Figure 6.14 The ratio of unsubstituted pyrene to phenanthrene vs. reaction time during carbonization of DO15 and CF15 at 450C...... 224 Figure 7.1 Structures and numbering notations of the predominant PAHs in needle coke feedstocks...... 233 Figure 7.2 Frontier electron density map for radical attack (molecular structures refer to Figure 7.1)...... 247 Figure 7.3 Proposed oligomerization and condensation pathways for phenanthrene and pyrene...... 249 Figure 7.4 Proposed oligomerization and condensation pathways for chrysene, benzopyrene and benz[g,h,i]pyrene ...... 250 Figure 7.5 The conformation of aryl-alkyl radicals of PAHs (From left to right, top to bottom: 9-methylphenanthrenyl, 4-methylpyrenyl, 6-methylchrysenyl, 3- methylbenz[e]pyrenyl, and 5-methylbenz[g,h,i]perylenyl) ...... 253 Figure 7.6 Proposed PAH free radical addition scheme leading to carbonaceous mesophase...... 254 Figure 7.7 IR spectrum and motion vector of 9MPhen+Phen TS...... 255 Figure 7.8 Definition of the geometric parameters of the TS...... 257 Figure 7.9 Conformations of TS of 4MPY-1PY, 6MCHRY-6CHRY, 3MBeP-3BeP and 4MBenzPery-4BenzPery (from left to right, top to bottom)...... 258 Figure 7.10 Energy diagram of reactant, TS and product of 9-MPhen and 4-MPy alkyl- radical addition and condensation reaction...... 262 Figure 7.11 Spatial configurations of the products from intra-molecular addition reactions of phenanthrene (four at top) and pyrene (four at the bottom) ...... 263 xiv
ACKNOWLEDGEMENTS
I am grateful and indebted to my thesis advisor, Dr. Semih Eser, for his kindly
support, encouragement, and his patience to my research progress in this thesis study. I
am thankful to him giving me a chance to do this work. Through my long time of
working with him, I learned a lot from him not only academically, but also personally. I
would like to thank all my committee members, Dr. Arthur Jones, Dr. Jonathan Mathews,
Dr. Ljubisa Radovic, Dr. Alan Scaroni and Dr. Harold Schobert for their advice and
constructive discussions on my research work.
I want to give my thanks here to all the staff and my colleagues at the department
of Geo-Environmental Engineering and at the Energy Institute for their help and
assistance. I also want to thank Dr. David Clifford for his advices in the GC/MS analysis
and Dr. Xiaoliang Ma for his help and discussion on the Molecular Orbital calculations.
Mr. Robert Miller and Dr. John Bassett, at the Chicago Carbon Company,
Lemont, IL, who provided financial support and many helpful discussions, are gratefully acknowledged.
This thesis is a dedication to my beloved parents. Without their love and encouragements, nothing in my life could be possible.
To my wife Weidong and our daughter Amy, they are my delight and I thank them for adding pleasure and happiness to my life. 1
Chapter 1
Introduction and Research Objectives
1.1 Introduction
Tighter environmental controls on transportation fuels and the deteriorating
quality of crude oil feedstocks result in increased output of heavy petroleum residua at
refineries. Broadly speaking, two processes are used to increase the liquid fuel yield from
petroleum residua: coking and hydrocracking. Both processes affect the hydrogen
distribution in the product by rejecting carbon (coking) and by adding external hydrogen
(hydrocracking).
Three major commercial coking processes are employed in modern refineries:
delayed coking, fluid coking and flexicoking (Gary and Handwerk, 1994). Currently in
the USA, there are approximately fifty-five cokers in operation including forty-nine units are delayed cokers; the others are fluid cokers and flexicokers (Ellis and Paul, 1998).
Delayed coking is a low-temperature carbonization process that has been used in
refineries since the early 1930s. It requires a lower capital investment and operating costs than hydrocracking. Delayed coking also offers a great versatility of handling a wide range of feedstocks. The most common practice of employing delayed coking is to treat vacuum residua to produce distillates (coker gas oil) for downstream catalytic cracking.
The coke produced is typically considered as a by-product and is mainly used as inexpensive solid fuel for power plant. However, by selecting suitable feedstocks and modifying operating conditions, a premium coke, called needle coke, can be produced in 2 a delayed coker (Debiase, Elliott et al., 1986). Needle coke consists of elongated carbon crystallites that can produce a graphitic structure upon further heat treatment. The highly graphitizable needle coke is mainly used as fillers in the production of high-power graphite electrode for the electric-arc furnace to recycle scrap steel (Mochida, Fujimoto et al., 1994).
The conversion of an isotropic liquid aromatic feedstock into anisotropic carbon lies fundamentally in the emergence and development of a carbonaceous mesophase during low-temperature carbonization. The stacking of large planar polyaromatic hydrocarbons produced in liquid-phase carbonization bears a rudimentary resemblance to the crystalline structure of graphite. Although a high degree of alignment of such crystalline structure can only be observed in high-temperature treated carbons by crystallography, it has been shown that the rudimentary stacking of graphene layers in the coke structure determines the final crystal properties of the graphitized carbon.
The most important property that determines the market value of needle coke is the coefficient of thermal expansion (CTE) value of the heat-treated products. Graphite electrodes are produced from two raw materials, needle coke (the filler) and coal tar pitch
(the binder). During the preparation of graphite electrodes, the processing conditions at different stages (such as calcination and graphitization) may affect the final product’s
CTE value to various extents. The most important factor that determines the CTE value is, however, the degree of mesophase development from which the needle coke is produced during delayed coking. The mechanical strength, electrical conductivity, and thermal shock resistance are closely related with the thermal expansion properties of graphite electrodes. 3
It is well documented that some aromatic hydrocarbons produce carbonaceous
mesophase during liquid phase carbonization(White, 1976). Carbonaceous mesophase is
an anisotropic, nematic liquid crystal that consists of disc-like (aromatic) molecules. The
driving forces for the formation of an anisotropic liquid crystal include the thermal
stability of large polyaromatic hydrocarbons and the minimization of the free energy by
parallel stacking of such molecules. Mesophase development during carbonization depends on chemical/ molecular composition of the starting materials. Because of the complex molecular composition of needle coke feedstocks (e.g., the bottom product from catalytic cracking, or decant oil), averaged or bulk parameters (such as aromaticity or
condensation index) are used to evaluate the behavior of these feedstocks during
carbonization. A detailed investigation into the molecular composition of coker feedstock
and the mesophase formation poses a challenging task. This fact arises from two aspects:
the complex molecular composition of the starting materials and the complexity of the
chemical reaction pathways during delayed coking.
Fluid catalytic cracking (FCC) decant oil is the primary petroleum-derived feedstock to the delayed coker for producing needle coke. The feedstock (such gas oil) to the FCC unit is catalytically converted into hydrogen-rich light distillates and hydrogen lean bottom fraction. The heaviest fraction of the bottom product is decanted to remove catalyst fines to make up the FCC decant oil, a highly aromatic feedstock suitable for needle coke production in a delayed coker.
Coking of FCC decant oil and mesophase development have been studied by several investigators (Mochida, Nakamura et al., 1976; White, 1976; Marsh, 1986;
Mochida, Shimizu et al., 1988; Eser and Jenkins, 1989b). These investigators all report 4 that intermediate carbonization reactivity and the prolonged presence of a viscous liquid phase during carbonization favors the mesophase development. These conditions facilitate the formation of large planar molecules and their alignment to produce elongated anisotropic domains.
A more recent investigation into the chemical composition of FCC decant oil related to the mesophase development was carried out by Filley and Eser (Filley and
Eser, 1997). A detailed analysis of the molecular composition of FCC decant oil samples was achieved on the GC-amenable fraction. Their work shows that a balance of thermally reactive compounds (such as alkanes) and thermally stable PAH plays a significant role in mesophase formation from decant oils.
Industrial needle coke production often requires a pretreatment of FCC decant oil before it is charged into a coking drum. The pretreatment serves two purposes: reducing the sulfur content in the needle coke and increasing the coke yield. High sulfur content is detrimental to the physical properties of the graphite product because of puffing, an irreversible volume expansion of final graphite product, caused by the rapid sulfur evolution during high temperature treatment. Some of the organic sulfur compounds in coker feedstocks produce highly refractory sulfur species during carbonization that are retained in the coke (Whittaker and Grindstaff, 1969).
Therefore, the actual feedstock to the coke drum in the delayed coking process may have a different molecular composition than that of the original decant oil. A thorough literature review revealed no report of a study on such treated FCC decant oil derivative with respect to the mesophase formation and needle coke production. 5
Understanding the effect of decant oil pretreatment on the molecular composition of the
actual feedstock to the coke drum is critically important for the study of mesophase
development in delayed coking.
1.2 Hypothesis and Research Objectives
It is possible to achieve a better understanding of the relationships between
feedstock composition and mesophase development through a detailed molecular
composition analysis of decant oil samples and their derivatives coupled with laboratory
coking experiments and microscopic characterization of the resulting cokes. A special
emphasis should be placed on the analysis of the heavy ends of the coker feedstocks.
The overall objective of this thesis work is to investigate the relationships between the molecular composition of needle coke feedstocks and the optical texture of cokes produced in the laboratory.
The specific objectives are:
1. To analyze the molecular components, especially, the higher end constituents in
decant oils and their derivatives;
2. To characterize the optical texture of cokes;
3. To seek relationship between the distribution of the major molecular components
of the feedstocks and mesophase development.
These objectives serve to carry out an in-depth study on the molecular
composition of needle coke and the chemistry of mesophase formation during liquid-
phase carbonization. Analyzing the composition and coking behavior of the various 6
streams combined to coker feed to the coke drum will contribute to understanding the
coke texture development in delayed coking. 7
Chapter 2
Literature Review
2.1 Carbonaceous Mesophase
Mesophase development was first observed in the carbonization of coal seams by
Brooks and Taylor (Brooks and Taylor, 1965). Studies after Brooks and Taylor’s
discovery showed that the mesophase development takes place in the liquid-phase
carbonization of almost all carbonaceous feedstocks. This new phase, having fluid
properties (of a liquid) but optically anisotropic nature (of a solid crystal), is called
mesophase or carbonaceous mesophase to distinguish it from the conventional liquid
crystals.
The mesophase is produced by thermal cracking and thermal polymerization
reactions during the liquid-phase carbonization. At the initial stages of carbonization, the
constituents of the starting material thermally crack to produce free radical intermediates.
These intermediates polymerize to form large polynuclear aromatic hydrocarbon (PAH)
molecules. Such lamellar molecules (called mesogens) stack parallel to each other and
precipitate as spherules dictated by the minimum surface energy (Marsh and Cornford,
1976). The emergence of mesophase spherules in the isotropic carbonizing media is thought to result from homogenous nucleation, and the microstructure of mesophase sphere takes up the Brooks and Taylor configuration in which the PAH lamella stack parallel to the equatorial planar but curve toward the surface to meet the isotropic liquid at right angle (Brooks and Taylor, 1968). 8
Figure 2.1 Brooks and Taylor’s mesophase sphere model (Brooks and Taylor, 1965)
The nascent mesophase spheres grow in size through either polymerization
(condensation) of the PAH lamella from the parent isotropic matrix or through
coalescence with other mesophase spheres. At higher temperatures or longer reaction
durations, the large mesophase spheres form large domains that are deformed by volatiles
evolution and eventually solidify into solid product, or semi-coke. Lamella of the
condensed PAHs assume the rudimentary carbon hexagonal network. Thus, the size and
orientation of this hexagonal carbon network evolved during the mesophase development
determine the microstructure of the resultant carbon and its graphitizability, as revealed
by Franklin (Franklin, 1951).
2.2 Delayed Coking and Needle Coke Production
Delayed coking is a severe thermal cracking process used in petroleum refineries
to upgrade and convert petroleum residuum (bottoms from atmospheric and vacuum distillation of crude oil) into liquid and gas product streams, leaving behind a solid 9
concentrated carbon material, the petroleum coke. A fired heater with horizontal tubes is
used in the process to reach thermal cracking temperatures of 485 to 505˚C (905 to
941˚F). The residence time in the furnace is very short (about three minutes) and coking is delayed until the feed reaches the large coking drums downstream of the heater. This is a semi-continuous process in which the flow of the feed stream is continuous, but the removal of coke is intermittent. The feed stream is switched between a pair (or more) of drums. One drum is online filling with the feedstock while the other drum is being cooled, hydraulic-decoked and warmed up. A schematic diagram of delayed coking process is shown in Figure 2.2 (Gary and Handwerk, 1994).
Figure 2.2 A schematic diagram of delayed coking process (Gary and Handwerk, 1994)
The process starts with the introduction of fresh feed to the fractionator, which is 10
essentially a distillation column. Liquid and vapor products from the coke drum are also
introduced into the fractionator. Therefore, the feed to the heater (and to the coke drum)
includes the heavy end of fresh feed and the recycled heavy end of the coker products.
A sequence of events in a typical delayed coking operation cycle is shown in Table 2.1
(Gary and Handwerk, 1994) .
Table 2.1 Typical delayed coking operation time schedule (Gary and Handwerk, 1994)
Operation Hours Fill drum with coke 24 Switch and steam out 3 Cool 3 Drain 2 Unhead and decoke 5 Head up and test 2 Warm up 7 Spare time 3
Total 48
The feedstock in the heater tubes undergoes thermally induced chemical
reactions including the cleavage of weak bonds, dehydrogenation, oligomerization
and condensation to form solid coke and liquid and gaseous products. Figure 2.3
gives a graphical description of the coke formation in the coke drum in a delayed
coker (Ellis and Hardin, 1993). 11
Figure 2.3 Coke formation in a delayed coker (Ellis and Hardin, 1993)
12
When the feedstock is switched to a drum, the processes of pyrolysis, mesophase formation and growth, and deformation into coke start progressively upward from the bottom of the coke-drum. As the height of the coke layer increases, a main channel develops randomly at the center of the coke drum for the evolution of the volatiles. With further filling, the main channel branches off into smaller diameter channels. The feedstock enters the drum through these channels, and the majority of coke is formed in a quiescent or non-turbulent environment because of a larger number of smaller diameter channels in the upper section of coking drum. Gases and vapors from either cracking or distillation exert additional forces to align mesophase domains to anisotropic structure.
The volatile product, cooled by heavy coker gas oil to avoid the plugging in the volatile product line, is removed from the top of the drum to the fractionator. The volatiles are separated into gases, gasoline, diesel, heavy coker gas oil, and recycle streams. The recycle stream is mixed with the fresh coker feedstock before being introduced to the heater and coker drum. Recycling the heavy coker products helps prevent coke deposition in heater tubes and increase the conversion of feedstock into desired products. For vacuum residue feedstock, the recycle ratio (the ratio of recycle stream to fresh feed) is in the range of 0.1 to 0.5 (Rose, 1971), while for more refractory feedstocks such as highly aromatic feeds, the ratio is usually about 0.8 to 1.0 (Janssen,
1984; Gary and Handwerk, 1994).
Two major kinds of solid products can be produced in a delayed coking process depending upon feedstock properties and operating conditions, sponge coke and needle coke. 13
Sponge coke or regular coke is the solid product from delayed coking of vacuum
distillation residua. It is porous, and produced in irregularly shaped lumps. Most sponge
coke is used as fuel. Some sponge coke with low sulfur and low metal content can be
used to make anodes used in aluminum production.
Needle coke is a premium coke from delayed coking of carefully selected highly
aromatic feedstocks. It has the needle appearance and has an elongated crystalline microstructure. Because of its very low CTE and electrical resistance, needle coke is used
to make graphite electrodes used for recycling steel in electric arc furnace. The coke from
the delayed coker, is called green coke, and must be calcined at 1000˚C to 1400˚C to
expel all volatile matter and increase its density before it can be used as filler in the
production of synthetic graphite electrodes. The major specification of needle cokes
(green, calcined and graphitized) is listed in Table 2.2 (Elliot, 1995).
Table 2.2 Typical needle coke specifications
Green Calcined Graphite Artifact Volatile Chemical Matter 5-7 (VCM), wt%,Dry Basis Sulfur, wt% 0.5 Max. 0.5 Max. Ash, wt% 0.1 Max. 0.1 Max. Real Density, g/cm3 2.10-2.14 CTE, x10-7/°C 2.5 (30-125°C) Electrical resistance 320 Ohm-in, x10-6 Flexural Strength, psi 2500 Granulometry @ -1.0 mm 25% Max.
14
The production of needle coke by delayed coking is slightly different from
producing sponge coke, which is considered as a by-product in process design to
maximize the yield of distillate liquids. The objective of delayed coking for needle coke
production is, however, to obtain a high yield and high quality of solid coke, the liquid
products become by-products. Compared with the regular (sponge coke) delayed coker
operating conditions, needle coke production requires higher temperature (505-520˚C),
higher pressure (50-90 psi) and higher recycle ratio (0.8 –1.0). Selecting suitable feedstocks is the most important prerequisite to producing high quality needle coke
(Swain, 1991).
2.3 Chemistry of Carbonization and Needle Coke Feedstock Composition
Characterization
The main interest in carbonization chemistry is to relate the chemical structure of the starting feedstock with the properties (such as the microstructures) of resulting cokes.
Mesophase formation and development during carbonization plays the most important role in the conversion of liquid feedstocks into the solid coke product.
The chemical and physical changes that take place during the carbonization of the organic compounds include an increase in C/H ratio, molecular weight and molecular size, and a decrease in solubility of the products (Greinke, 1994). The general reaction pathways in mesophase formation follow the free radical induced thermal polymerization and condensation. The general scheme in carbonization of aromatic compounds (using anthracene as an example) leading to the mesophase, coke, and graphite is shown in
Figure 2.4 (Lewis, 1982). The molecular arrangement in the mesophase and the transition 15
of the mesophase into coke and synthetic graphite during heat treatment are shown in
Figure 2.5 (Mochida, Fujimoto et al., 1994) and Figure 2.6 (Marsh, 1989), respectively.
Figure 2.4 General carbonization reactions to produce coke and graphite product (Lewis, 1982) 16
Figure 2.5 The structure of carbonaceous mesophase (Mochida, Fujimoto et al., 1994)
Figure 2.6 A schematic transition of graphitizable coke to graphite upon heat treatment (Marsh, 1989)
17
Electron spin resonance (ESR) was used for monitoring the very early stages of
the carbonization of aromatic compounds (Lewis and Singer, 1981). The ESR data
showed that with the increasing reaction time, newly formed complex free radicals
appeared in the reaction media. Although the ESR approach does not allow for examining the short-life transient reactive radicals that may lead to the formation of
mesogenic molecules, the chemistry of stable radicals within the mesophase turned out to
be quite revealing. The ‘stable’ free radicals are stabilized by resonance within the
extensive aromatic network of carbon atoms. Monitoring the intermediate products from
simple starting aromatics (like naphthalene and anthracene) with field desorption mass
spectrometry (FDMS) and gel Permeation chromatography (GPC) has established that the
predominant products correspond to the oligomers of parent reactants (Lewis, 1980). The distinct mass spectra for the same molecular weight oligomers indicate that the isomerization reaction occurs among the intermediate carbonization products. In a general sense, Lewis proposed the following major chemical reactions involved in the carbonization of aromatic compounds (Lewis, 1980):
1. C-H, C-C bond cleavage to form reactive free radicals.
2. Intermediate molecular rearrangement
3. Thermal polymerization
4. Polymerized aromatics condensation
5. Elimination of side chains and hydrogen in condensed aromatics to form solid
coke.
The thermally induced formation of larger PAHs from simple aromatics involves two important chemical changes as far as the larger hexagonal network is concerned. One 18
is the thermal polymerization, in which the planar starting aromatics and, more
importantly, the planar intermediate molecules are necessary to form mesogens and
further to produce highly graphitizable carbons (Edstrom and Lewis, 1969; Isaacs, 1970;
Lewis, 1980).
The other major change is the thermal isomerization of intermediate molecules.
One example is that an unstable five-member ring may be transformed to a more stable
six-member ring system (Walker and Weinstein, 1967; White, 1976). This isomerization
reaction may account for the difficulty of relating the starting structure with the
graphitizability of resultant carbons.
As well acknowledged (Lewis and Kovac, 1978; Singer and Lewis, 1978; Lewis,
1982; Yokono, Iyama et al., 1984; Mochida and Korai, 1986), the very first reaction in
the liquid-phase carbonization is the formation of free radicals. Two kinds of free radicals
can be produced from aromatic hydrocarbons: σ-radical (phenyl radical) and π-radical
(benzyl radical). The unpaired electron of a σ-radical is localized; therefore, the σ-radical is very unstable. In contrast, the unpaired electron of a π-radical is delocalized via conjugation in the aromatic ring and, thus the π-radical is considerably more stable.
Reactivity of starting materials is closely related to the ease of free radical
initiation. Some reactivity parameters can be used to express the thermal reactivity of a
hydrocarbon. These parameters include free valence index (Walker and Weinstein, 1967), unpaired spin densities (Yokono, Miyazawa et al., 1979), localization energy (Lewis and
Singer, 1967), thermochemical properties (Lewis, 1980; Stein, 1981) and steric effects
(Lewis and Edstrom, 1963). Kinetic data observed from carbonization experiments can also be used to compare the thermal reactivity of hydrocarbon. Differences in thermal 19
reactivity can have a significant influence on mesophase development. One example is
the carbonization of the 3-ring unsubstituted anthracene and phenanthrene. It is well
established that linear aromatic anthracene produces a highly graphitizable carbon
precursor, while its angular analog phenanthrene, yields a carbon with much lower
graphitizability (Weintraub, 1967). Kinetic data showed that the activation energy for
anthracene pyrolysis is about 2/3 of that for phenanthrene (based on first-order kinetics)
(Peters, Jenkins et al., 1991; Schabron and Speight, 1997).
The reactivity is related to the electronic configuration of the bonds in a PAH
molecules. This can be seen from the difference in π-electron bonding between
anthracene and phenanthrene (Sasaki, Jenkins et al., 1993b). The 9- and 10- positions
(bearing maximum free valence value, see Fig. 2.4) on anthracene have higher reactivity
and they are located diametrically across the molecule. For phenanthrene, the most
reactive (9- and 10- ) positions have relatively low free valence index value and they are
on the same side of the molecule. Because of the difference in reactivity and position of
the active sites on the molecules, anthracene carbonizes through more gradual
oligomerization at relatively low temperature. This provides a longer duration of liquid
stage that promotes mesophase development (Sasaki, Jenkins et al., 1993b).
0.451
0.448 0.52 0.459 9 10 0.408 8 1 2 0.402 8 9 1 7 7 2 6 5 4 3 6 3 5 10 4 0.441 0.408
Figure 2.7 Free valence indexes for different positions in anthracene and phenanthrene
20
Needle coke is produced from well-developed mesophase of complex industrial
feedstocks in the early stages of carbonization. A general requirement for the needle coke
formation is the moderate chemical reactivity of both the starting compounds in the
feedstock and the intermediate species during the carbonization. This chemical
environment in the carbonizing media would allow the mesogens (precursor to
mesophase) to grow in preferred orientation in the liquid media with relatively low viscosity for a prolonged time period (Marsh and Cornford, 1976).
FCC (fluid catalytic cracking) decant oil is the main feedstock to delayed coker for needle coke production. Extensive work has been done on the carbonization of decant oils (Mochida, Fujimoto et al., 1994). Generally, the formation of the mesophase from decant oil feedstock involves not only the chemical reaction during carbonization, but also the physical process in which the external forces from the evolution of product gases through the reacting media help to orient the mesophase into the preferred needle
structure in the resulting coke. During carbonization, the timing of the chemical and physical events such as the mesophase growth, the viscosity increase and the gas or
volatiles release are the key factors affecting the needle coke properties. These factors
depend on the chemical properties of the feedstocks and the structure and reactivity of
intermediate products during carbonization, as well as on the carbonization conditions,
such as temperature and pressure.
The overall reactivity during carbonization of the coker feedstocks may be
affected by not only that of the individual components, but also by the interactions among the chemical species in the feedstock due to the complex chemical and molecular composition. A typical coker feedstock can be considered as being made up of three 21
groups of components with regard to their roles in carbonization (Marsh, 1986): (1). A
low-molecular-weight fraction that acts as a solvent during carbonization and either
vaporizes from liquid phase or becomes incorporated into the mesophase. (2). A higher-
molecular-weight fraction that is central to the formation of mesophase. (3). A heaviest fraction, present in petroleum pitches or coal tar pitches, that is insoluble in nearly all solvents, and is detrimental to mesophase development.
Pertaining to the needle coke feedstocks, solubility fractionation was widely used to describe the complex composition. The asphaltenes fractions (pentane-insoluble and toluene-soluble fraction) were found to have a dominant effect on the mesophase development of feedstocks (Eser and Jenkins, 1989a; Eser and Jenkins, 1989b). These fractions have a higher C/H ratio and a higher propensity for carbonization to initiate the mesophase development. Larger optical texture size in the solid coke was obtained from the feedstocks with higher hydrogen aromaticity of the asphaltenes fraction. The importance of the hydrogen aromaticity of feedstock was attributed to chemical reactivity of starting material (the rate of carbonization), the molecular planarity of starting compounds and intermediates, and the fluidity of carbonizing medium.
Chromatography fractionation of pitch-like materials has been used to correlate
the carbonization behavior with chemical or group composition (Menendez, Granda et
al., 1994). It was found that a higher amount of aliphatic fractions and/ or polar
compounds are associated with the high reactivity of the feedstock. Heavier aromatic
fractions will also tend to exhibit higher reactivity.
The in-depth molecular composition analysis on needle coke feedstock (the
decant oil) and the effect on the mesophase development was reported by Liu and Eser 22
(Liu and Eser, 1993; Liu and Eser, 1995). Aromatic compounds up to five rings and aliphatic compounds with up to 30 carbon atoms in decant oil were identified by GC/MS.
The results showed that predominant PAHs in decant oil are in the form of polymethylated compounds. The configuration of aromatic ring systems and the degree of alkyl substitution appear to influence the mesophase development. A higher proportion of alkylated three-ring compounds (such as phenanthrene) would hinder a selective coupling reaction, while alkylated four-ring compounds (such as pyrene) would serve as hydrogen shuttler that would promote the mesophase growth. High concentration of normal alkanes would increase the reactivity of feedstock and tend to remove hydrogen from the liquid phase for stabilization of small alkane radicals. In terms of optical texture of the resultant coke, a proportionally high pyrene and chrysene concentration in a feedstock will produce a needle coke of high quality (Liu and Eser, 1995).
GC/MS analysis of coker feedstock has its inherent limitation. Significant proportions of heavy hydrocarbon liquids escape detection by this analytical method. The problem arises from the relatively low molecular mass detection limits of this technique.
GC columns are normally limited to a little over 300 amu for aromatic compounds.
Columns able to reach 400 °C might extend these ranges by some 100 amu; however, a high-temperature GC may result in thermal cracking or coking of reactive molecules in the GC column. Therefore, the information from high-temperature GC/MS may not reflect the intact large molecules. In a previous work by Filley (Filley, 1997), up to five- ring aromatics were well resolved on GC/MS, the total concentration of compounds identified by GC still remains a small fraction of the virgin decant oil (10-20%). The 23
heavier compounds could play a more important role during the initial stage of
carbonization.
Several alternative mass spectrometric analytical techniques are available for
determining the molecular weight distribution (MWD) in heavy feedstocks. These mass
spectrometers utilize a “soft ionization” method to produce relative high abundances of
molecular ions without significantly fragmentizing the sample molecules (Lattimer,
1989). Field desorption mass spectrometry (FD/MS) ionizes the analyte (either liquid or
solid) in a high strength electro-magnetic field through desorption and ionization of the
molecules from the probe. This technique has been used in determining molecular weight
distribution of the products from synthesized mesophase pitches (Sakanishi, 1992;
Evans, 1995). Laser desorption mass spectrometry (LD/MS) is a new soft ionization mass spectrometry for non-destructive analysis. The main advantage of LD/MS over
FD/MS is the simplicity of mass spectrum which shows mainly single-charged molecular ions with hardly any fragmentation (Nielen, 1999). Matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) allows rapid ionization of analytes by absorbing laser radiation to evaporate the sample into the gas phase (Cotter, 1997). MALDI-MS has been widely used in the analysis of biopolymer and synthetic polymers since its first application in 1988 (Tanaka, Waki et al., 1988); however, the application to heavy petroleum or coal tar pitch product is limited. Today, most of the MALDI work performed on heavy hydrocarbon feedstock has been done by
Herod and co-workers, who analyzed the molecular weight distribution of a standard coal tar pitch and its solvent fractions (Begon, Megaritis et al., 1998; Herod, Lazaro et al.,
2000; Herod, Lazaro et al., 2000). They reported that, for narrow polydispersity (narrow 24 solvent elution fraction), the obtained MWD information from MALDI-TOF-MS shows a good agreement with the MWD from size exclusion chromatography (SEC). Their research result indicates that MWD in the heavy fraction in the feedstock (which is insoluble in any solvent such as THF or pyridine) can be accurately determined by
LD/MS or MALDI-MS.
Through a literature search, no study on molecular weight distribution or composition of heavy fraction in the decant oil has been reported. The only work published so far is the LD/MS molecular weight distribution analysis on the decant oil derived mesophase pitch by Thies and co-workers (Edwards, Jin et al., 2003; Edwards,
2004). They used 7,7,8,8-tetracyanoquinodimethane (TCNQ) as the matrix and dense gas extraction to analyze the MWD in a series of fractions from decant oil mesophase pitch.
The LD/MS spectra of the sample show that a series of monomer, dimer, trimer, and tetramer moieties are present in the mesophase pitch.
Soft ionization mass spectrometry gives the whole picture of molecular distribution in the sample, which can be used to screen or profile the complex sample.
However, this MS technique alone lacks structural information. Complementary (or hyphenated) analytical techniques are necessary in order to obtain the structural information of the samples.
High-pressure liquid chromatography (HPLC) is the most commonly used hyphenated molecular characterization method. Thermally liable PAH compounds are more easily analyzed since they are not exposed to excessive heat. PAHs with high molecular weights can also be analyzed because volatility is not an issue for optical detection (such as UV detector). However, the high chromatographic efficiency, which is 25
available from capillary GC, is not currently available from conventional LC. Thus, LC
chromatographic peaks are more likely to contain unresolved components. Therefore,
manipulation of the composition of both stationary and mobile phases (analytical column and solvent), and more selective detection methods such as mass spectrometry are needed
(Anacleto, Ramaley et al., 1995). One difficulty of applying LC/MS technique to PAH
molecules is the insufficient ionization of unsubstituted PAH compounds because of their
high thermal stability and low polarity (Moriwaki, Imaeda et al., 1999). Co-elution of
standard PAH compounds with unknown sample may be used to identify unknown PAHs
compounds to some extent; however, when distinguishing isomers of large PAH,
fragment ions are needed to provide valuable information. Tandem mass spectrometry
(MS/MS) serves better for this purpose: this technique uses two MS detectors in series:
the first one selects stable molecular ions and the second one ionizes the same molecular
ions into informative fragments. Thus by combining the information from two sources of
mass spectra, it becomes relatively easy to identify the PAHs in the sample. Mansoori
(Mansoori, 1998) has performed a LC/MS/MS research work on a set of 16 PAH
standard compounds (from two-ring naphthalene to 7-ring coronene) using atmospheric
pressure chemical ionization (APCI). His work demonstrated that ionization of stable
PAHs was affected by the collision energy as well as the molecule itself. Peak ratios of
stable molecular ion to that of its fragments can be a reliable indicator for isomeric
identification. Linear response ratio was observed in those 16 PAH standard sets, and the
quantification of known PAHs in a reference coal tar confirms the linearity of MS/MS
response.
26
2.4 Characterization of Coke Texture
2.4.1 The use of polarized-light microscope
Optical microscopy is the principal experimental approach to the examination of solid products from carbonization (Mash and Smith, 1978). The wave-mode theory of light describes light waves vibrating at right angles to the direction of light propagation with all vibration directions being equally probable. Polarizing filters can restrict only one vibration direction to produce a linearly or plane-polarized-light. Two polarizing
filters are used in a polarized-light microscope, one between the light source and the
specimen, the polarizer, and the other one between the specimen and the eyepiece, the
analyzer. Plane polarized-light from polarizer with the permitted direction fixed at
“horizontal “ or “east-west” direction reaches the specimen. The light reflected, or
transmitted, by the specimen goes through the second polar (the analyzer), which is
usually oriented with its crystal direction at right angles to the polarizer, permitting light
to vibrate at “ vertical” or “north-south” direction. Such a configuration is called cross-
polarization. In this configuration, no light will reach the observer (extinction), if the
specimen does not change the direction of polarized light (Bradbury, 1989).
Based on the variation in the speed of light traveling in the materials with regard
to its direction, materials can be classified into optically isotropic and optically
anisotropic materials. The velocity of light in optically isotropic materials is independent
of the direction; while the light speed in optically anisotropic materials depends on the 27
direction in which it propagates. In the latter case, the light is imaginarily divided into
two components that are plane-polarized in mutually perpendicular planes.
In a uniaxial crystal (e.g., the hexagonal crystal in the graphite), the speed of one
component (ordinary ray, or o-ray) is constant while the speed of the other component
(extraordinary ray, or e-ray) varies with its direction. Crystals showing this characteristic
are birefringent. A birefringent crystal specimen inserted in the light path between
polarizer and analyzer causes the optical path difference (OPD). The OPD is expressed in
the fraction of a particular wave length (λ). Usually the green light (wavelength 550nm)
is used as reference. When white light passes through 1λ OPD crystal (placed between
polarizer and analyzer), the green light is extinguished by the analyzer (since it has been
plane polarized east-west, it has no north-south component), and other light (with
wavelengths other than 550nm) results in a mixture of all color components passing
through the analyzer. Such polarization color is deep violet-red which can be regarded as
the white light minus those colors (green light) that interfere destructively (Robinson and
Bradbury, 1992).
The optical path difference, hence the polarization color observed in the
specimen, depends on birefringence (the numerical difference of refractive indices of two polarized components) and the thickness of the specimen. A birefringent crystal may be
inserted intentionally between the two polars to enhance the polarization colors by adding
or extracting extra optical path difference. The birefringent crystal for this purpose is
called a retardation plate or compensator. A 1λ or whole wavelength retardation plate is
usually used because the resultant color (deep red-violet) by retardation is most sensitive
to small change in OPD and to the human’s visual perception. 28
Polarized-light microscopy can be used both with reflected and transmitted light.
Reflected polarized-light microscope is useful for opaque materials such as cokes. When examined under crossed polarizer/analyzer, polarized-light microscope with whole wavelength retardation plate inserted between polarizer and the specimen, the coke specimen creates a mixture of polarization colors as a result of the interference of two light components split by the direction of layers of graphite crystallites in the specimen.
These colors are usually yellow, blue and purple.
Figure 2.8 Polarization color and crystal orientation on the coke surface (Marsh, 1989)
Purple or magenta color is produced when the two light components are traveling at the same speed reaching the analyzer (i.e. OPD is zero). In this case, the light must be incident on the surface of specimen parallel to the optical axis or on optically isotropic material. Therefore, the purple color corresponds to the basal plane of carbon crystallite lying parallel to the polished surface, since the optical axis of a hexagonal crystal is perpendicular to its basal surface. Yellows and blues are from the prismatic edges exposed in stacked lamellar planes; they are interchangeable when the specimen stage is 29 rotated by 180˚. Purples do not change color when the specimen is rotated because the incident polarized-light is always parallel to the crystal’s optical axis. Isotropic carbon also shows purplish color and remains purple on rotated stage, but the color is much darker in shade (Forrest and Marsh, 1977).
2.4.2 Coke texture analysis
Two aspects of analysis are of importance for optical microscopic examination of solid carbon or coke: The textural component analysis and the structural analysis. In this section, only texture analysis is reviewed here; the structural analysis (the size, shape, and distribution of crack and pore) of anisotropic cokes can be found elsewhere (Qiao, 2000).
As described in the above section, under plane polarized-light, the polished carbon/coke surface displays regions with a mixture of polarization colors resulting from different orientation of the lamellae in the carbon body. The overall appearance of these colors, i.e., the shape and size of isochromatic areas, is defined as the optical texture of the specimen (Marsh and Smith, 1978).
The low-temperature carbonization coke shows a polycrystalline structure. Its texture varies greatly depending on the anisotropy. Non-graphitizable coke (isotropic) shows much less order of orientation of its crystallites (<0.5µm in diameter in isochromatic unit) under the polarized-light microscope and appears “homogeneous”. For the anisotropic coke, e.g., needle coke, on the other hand, the texture is more heterogeneous and the size of the microcrystal varies in a broader range (isochromatic unit ranging from 1µm up to 500µm). 30
Nomenclatures have been proposed for describing the optical texture of coke or
carbon. The classification of optical units has evolved over the years to better describe
the specific man-made carbons (Sanada, Furuta et al., 1973; Oya, Qian et al., 1983; Hole,
Foosnæs et al., 1991; Eser, 1998).
Coke texture is usually characterized by the statistical analysis of various optical units among the surface of polished coke sample. This is done by measuring (by visual perception or computerized automatic system) the size and shape of anisotropic domains under a polarized-light microscope. Typically, a factor (index) is assigned to each
descriptive unit depending on its contribution to the anisotropy. An overall optical
texture index (OTI) is calculated by following equation:
OTI = f ∗OTI ∑ i i
where fi is the numerical fraction of individual texture types from microscopic
analysis, and OTIi is the index assigned to each texture type.
Table 2.3 lists the classification and OTI of optical units for the metallurgical
coke (Oya, Qian et al., 1983). A very fine distinction among mosaic domains was made
in this nomenclature and OTI assignment because of the predominant presence of mosaic
domain in the metallurgical coke.
31
Table 2.3 Nomenclature and optical texture index for coke microtextural description. Unit Description Size (µm) and shape OTI
I Isotropic Optical inactive 0
MF Fine-grained mosaics <1.5 1
MM Medium-grained mosaic 1.5-5 3
MC Coarse-grained mosaics 5-10 7
<30 length MFA Medium-flow anisotropy 7 <5 width
MS Supra-mosaics Aligned 10
30-60 length CFA Coarse-flow anisotropy 20 5-10 width
SD Small domains 10-60 20
>60 length FD Flow domain anisotropy 30 >10 width
D Domain >60 30
For the textural characterization of the needle coke, the above nomenclature may not work well because the optical units present in needle coke consist almost exclusively of domain and flow domain structure. Furthermore, in terms of needle coke’s anisotropic properties, the contribution of flow domains should be larger than that of domains. An alternative classification was proposed to differentiate the OTI of the highly anisotropic 32 needle coke and listed in Table 2.4 (Eser, 1998). This method of OTI characterization combines all optical units less than 10µm into the mosaic, and higher OTI values are assigned to flow domain and domain units. This revised evaluation method of overall
OTI is found to be more sensitive to the optical texture difference in needle cokes (Qiao,
2000).
Table 2.4 Nomenclature and optical texture index for needle coke microtextural description. Type Size/Shape Index (OTI)
Mosaic <10 um 1
Small Domain 10-60 um 5
Domain >60 um 50
Flow Domain >60 um long,>10 um wide 100
33
Chapter 3
Experimental Section
This chapter describes the experimental and analytical procedures used in this
thesis research. Six FCC decant oil samples (DO91, DO93, DO15, DO24, DOPSU and
DOUP) and derivatives from two of the decant oil samples (DO15 and DO24) that
produced various extent of mesophase development were selected to conduct molecular
composition characterization and carbonization experiments. These feedstocks came
from two commercial needle coke plants. The following sections describe the experimental setup and procedures used for analyzing molecular composition of the feedstock samples.
3.1 Analysis of the Molecular Composition of Needle Coke Feedstock
3.1.1 Gas chromatography/mass spectrometry (GC/MS) analysis of decant oil samples
GC/MS has been explored to characterize the complex molecular composition of needle coke feedstock, such as decant oil (Filley, 1997). GC/MS offers the advantage of combining good chromatographic separation on GC with qualitative identification of species by mass spectra. The GC/MS experiments were performed on a Shimadzu GC-
17A gas chromatograph and QP-5000 mass spectrometer.
For each analysis, approximately 60 mg of decant oil sample was dissolved in 20
ml of dichloromethane with 50 ng/ml of per-deuterated pyrene (pyrene-D10, purchased 34
from Cambridge Isotope Laboratories, Inc., Andover, MA) added as an internal standard.
The prepared sample in dichloromethane (DCM ) solution was analyzed with reference to
EPA method 8270c (EPA, 1996). The injection volume of each sample is fixed at 3
microliters in splitless mode. Each sample was injected at least two times to ensure
reproducible GC/MS results. Electron ionization (EI) at 70 eV was used for the
fragmentation of GC-separated molecular species. The mass range detected in MS is
from 50 to 550 m/e. Gas chromatographic separation was performed on a fused silica
capillary column (30 m long, 0.25 mm ID, Restek XTI-5, Restek Technology, PA). GC oven was temperature programmed from 25 to 290˚C: the temperature was increased from 25˚C to 140˚C at a rate of 10˚C/min; from 140˚C to 290˚C, the rate of temperature increment was set at 5˚C/min. A holding time of 10 minutes was maintained at final temperature to prevent the heavy residue in the sample from being trapped in the GC column. The temperature of both GC injector and MS detector were set at 310˚C.
A mixture of 30 compounds including polyaromatic hydrocarbons and normal alkanes (obtained from Supelco, Inc., Bellefonte, PA) as external standard for calibration was injected to the same GC/MS system. This serves a two-fold purpose: determination of the retention time of individual compounds in the mixture, and determination of the response factor for each standard compound for quantitative molecular composition analysis.
The concentrations of PAH and normal alkanes in decant oil and related samples were determined by an internal-standard method. Six different concentrations (10 mg/ml,
20 mg/ml, 40 mg/ml, 60 mg/ml, 80 mg/ml and 100 mg/ml) of standard compounds in the internal standard-spiked DCM solution were injected into the GC/MS system at 35
aforementioned conditions. The ratio of the peak area of a standard to that of internal standard is plotted against the ratio of concentration of that standard to the internal standard. The obtain curve (the calibration curve) was found to be linear in the tested sample concentrations range, and the slope of the straight line is called a response factor of that known compound. When an unknown compound was injected with internal standard, the ratio of peak areas was given by the peak integration software in the GC/MS system. The concentration of an unknown compound is determined by the following linear equation:
Cu=Au*Cis/RF*Ais
where Cu and Cis are the concentration of unknown compound and internal standard, Au and Ais are the peak areas of unknown compounds and internal standard.
The total ion chromatogram (TIC) was used for compound identification. The
assignment of peaks was done by matching the retention time and mass spectra with that
of standard compounds that were injected into the GC/MS at the same conditions. Some
heavier polyaromatic standards are not available. In such cases, peaks were identified by
comparing the mass spectrum with the standard mass spectrum in a computerized
database library that collects the mass spectra of about 62000 compounds (NIST62,
purchased along with GC/MS software).
Selected ion chromatogram or base peak was chosen for each standard
compound’s quantification. The calibration of the instrument was carried out based on
these selected ion chromatograms. The base peak of an aromatic compound corresponds
to the molecular ion, while the base peak for a normal alkane corresponds to
fragmentation ion of m/e 43. Because TIC represents the accumulation of the response
signals from all fragments at specific retention time on a mass spectrometer, using 36 selected ion chromatogram instead of TIC minimizes the co-elution or overlap of TICs of compounds with similar fragmentation patterns.
Table 3.1 gives the response factors and base or molecular ions for 30 standard compounds. As can be seen in Table 3.1, the response factors vary among the compound groups. Larger-ring aromatics show a lower response factor than smaller-ring aromatics.
Figure 3.1 shows the TIC of standard compounds used in the GC/MS study. All the standards have the same concentration; therefore, caution should be taken when comparing the intensity of peaks for different compounds on the GC/MS chromatograms.
The much lower intensity of heavy compound does not necessarily indicate the low concentrations of these compounds present, because the responses of heavy compounds on GC/MS are low. In the same class of an aromatic compound, the response factors decrease with the increasing number of alkyl-substituents. The similar trend in response factors is also true for normal alkanes. Based on this observation, the response factors for the compounds without available standards are extrapolated from the response factors in the homologous series.
By the convention in geo-environmental sample analysis, the reported concentration of compound is expressed in nanogram of compound in one milligram of sample; this is equivalent to one part per million (ppm). 37
Table 3.1 Standards and their GC/MS characteristics Peak Name Structure Formula Mol. Primary Retention Response No. Wt. Ion, Time, Factor m/e min
1 Tetralin C10H12 132 104 9.261 2.322 2 Naphthalene C10H8 128 128 9.612 3.610 3 1-Methylnaphthalene
C11H10 142 142 11.578 2.013 4 Biphenyl
C12H10 154 154 12.704 2.155 5 2,6 Dimethylnaphthalene
C12H12 156 156 13.179 1.759 6 1,6 Dimenthylnaphthalene
C12H12 156 156 13.544 1.614 7 Fluorene
C13H10 166 166 16.853 1.828 8 Dibenzothiophene
C12H8S 184 184 20.683 2.078 9 Phenanthrene
C14H10 178 178 21.306 2.209 10 Octadecane C18H38 254 43 21.427 2.060 11 Anthracene C14H10 178 178 21.533 2.021 12 2-Methylphenanthrene
C15H12 192 192 24.047 1.385 13 Eicosane C20H42 282 43 26.031 1.916 14 3,6 Dimethylphenanthrene
C16H14 206 206 26.44 1.202 15 Fluoranthene
C16H10 202 202 27.652 1.566 16 Pyrene-d10 (IS*) C16H20 212 212 28.746 17 Pyrene
C16H10 202 202 28.833 1.446 38
Table 3.2 Standards and their GC/MS characteristics (continued) Peak Name Structure Formula Mol. Primary Retention Response No. Wt. Ion, Time, Factor m/e min 18 Docosane C22H46 310 43 30.396 1.572
19 Tetracosane C24H50 338 43 34.486 1.225 20 Benzo(a)anthracene
C18H12 228 228 35.733 0.843 21 Chrysene C H 228 228 35.95 0.758 18 12 22 Hexacosane C26H54 366 43 38.288 0.897 23 Benzo[b]fluorancene
C20H12 252 252 41.572 0.514 24 Benzo[k]fluorancene
C20H12 252 252 41.699 0.528 25 Octacosane C28H58 394 43 41.83 0.617 26 Benzo[a]pyrene
C20H12 252 252 43.107 0.415
27 Triacontane C30H62 422 43 45.147 0.442 28 Indeno[1,2,3-cd]pyrene
C22H12 276 276 48.244 0.200 29 Dibenzo[a,h]anthracene
C22H14 278 278 48.45 0.227 30 Benzo[g,h,i]perylene
C22H12 276 276 49.405 0.236
39
Figure 3.1 Total ion chromatogram of standard (20μg/ml in DCM) on GC/MS (peak labels are shown in Table 3.1), peak 16 * is the internal standard
40
3.1.2 HPLC/PDA, LD/MS , LC/MS/MS, elemental and NMR analyses of decant oil
samples
High-pressure/performance liquid chromatography (HPLC) with photo-diode
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
Water 996 PDA detector. Data collecting and processing was carried out on Millennium
32 HPLC software (Waters). A normal phase HPLC column (25 cm x 46 mm, packed
with 5 μ silica with pore size of 60 Å, RingSep, ES Industries, NJ) was used for separating aromatic compounds by the difference in the numbers of aromatic rings.
Binary solvents (dichloromethane and normal hexane) were used as mobile phase.
A gradient elution mode was programmed as follows: Pure n-hexane flows at 1.0 ml/min
for 5 minutes, then, DCM solvent was gradually added into the mobile phase, and the
composition of solvent (n-hexane and DCM) was changed linearly from 100% n-hexane
to 100% DCM in 30 minutes. During this gradient elution, the total flow rate of solvents
was kept constant (1ml/min). At 35 minutes, a higher flow rate (2.0 ml/min) of pure
DCM was used to flush off any resides that might be trapped inside the HPLC column.
The absorption of separated species detected by PDA was scanned and recorded
from 200 nm to 400 nm UV light wavelength continuously. The identification of heavier
PAH molecules in the sample was done by comparing the UV spectra from decant oil
sample with the library (which was created by injecting available standard PAH
compounds) and with those reported in the literature (Friedel and Orchin, 1951; Sadtler
Research Laboratories, 1964). 41
Laser desorption/ mass spectrometry (LD/MS) analysis of coker feedstocks was performed on a Voyager DE-STR matrix-assisted laser desorption time-of-flight mass spectrometer (MALDI-TOFMS). It operated in a linear mode which provided higher sensitivity of high molecular weight molecules.
HPLC/MS/MS analysis was carried out by a tandem mass spectrometry Finnigan
MAT (San Jose, CA, USA) TSQ 7000 triple-stage quadrupole 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 quadrupole mass detector. The column and solvent elution conditions in HPLC/MS/MS were the same as those used in the HPLC/PDA experiment.
3.1.3 Analysis of sulfur compounds
A HP5890 GC with a sulfur-selective detector, a single flame photometric detector (FPD), was used to identify the distribution of GC-amenable sulfur compounds in the feedstock samples. The GC column and separation condition in GC/FPD is the same as that used in the GC/MS analyses. The quantification of sulfur compounds was performed on GC/MS, because the response ratio on FPD detector is non-linear and highly dependent upon the concentration range (Ma, Sakanishi et al., 1997).
3.1.4 NMR analysis
The NMR data for decant oil and coke feeds were collected on a Bruker 360 pulsed FT NMR spectrometer with a scanning width 12 ppm. Tetramethylsilane (TMS) 42 was used at as internal standard for both 1H and 13C NMR spectra. The NMR spectra peak assignments are described in Chapter 5.
3.2 Carbonization and Semi-Coke Characterization
3.2.1 Carbonization experiments
The experimental approach to this study required a large number of carbonization experiments in order to produce semi-coke samples from decant oil and decant oil derivatives and vacuum fractions of decant oil derivatives with selected decant oil samples. A tubing reactor was used in this study. The capacity of the reactor is about 15 ml, which enables rapid heating and quenching the reactant for isothermal operation.
About 4 grams of decant oil sample was loaded into an aluminum foil cylinder inside the reactor. The charged reactor was sealed by threaded weld connectors. To prevent the threads of connectors from sintering under high temperature and high pressure, a graphite-based lubricant was coated onto the threads before sealing. Nitrogen was used to purge the reactor for three times to minimize the amount of oxygen inside the reactor.
The prepared reactor was inserted into the preheated sand bath and kept there until prescribed reaction time elapsed. Usually, four reactors were inserted into the same preheated sand bath. The temperature dropped by about 2˚C after the reactors were inserted and recovered in less than two minutes. Agitation was not used during carbonization experiments.
After the desired reaction time period had passed, the reactors were rapidly removed from the sand bath and quenched in cold water. When the reactor was fully 43
cooled, the gases in the reactor were vented under a fume hood. The solid product (semi-
coke) formed inside the aluminum foil cylinder was extruded from the reactor as a whole
semi-coke bar.
For the purpose of semi-coke texture determination, the feedstocks were
carbonized at 500˚C for 3-5 hours. The solid product was extruded from the cylindrical
reactor as a whole coke bar wrapped by aluminum foil. The solid semi-coke bar
(measured about 1” long and 0.3” in diameter) was washed with 20 ml of dichloromethane to remove liquid products produced during carbonization. The solvent- washed coke bar was left to dry under a fume hood overnight in order to remove residual volatiles before being molded and polished into a coke pellet for optical microscopy characterization.
In some low-temperature (450˚C) carbonization experiments where the carbonization reactivity was monitored, the products (liquid and solid coke) were completely removed from the reactor. Semi-coke and asphaltene products were separated by solvent extraction as follows: the carbonization product was dissolved in 100 ml
DCM. After thorough stirring, the solution was filtered in vacuum. The DCM-insoluble is defined as semi-coke. The DCM was first removed from filtrate under a vacuum rotary evaporator, and the DCM-soluble was dissolved in 50ml pentane. After filtration, the pentane insoluble (but DCM soluble) fraction was dried and weighed. This fraction is defined as asphaltenes. The DCM soluble fraction from low-temperature carbonization experiment was also subjected to GC/MS molecular composition analysis.
44
3.2.2 Examination of optical texture of semi-cokes
A standard technique, ASTM D3997, used for preparing coke samples for optical microscopic examination, requires a fine powder (particles <75 micrometer). Since the
majority of the anisotropic domains in needle coke samples are greater than 60
micrometers, grinding these samples to a fine powder will result in the loss of the most
significant piece of textural information: the flow domain content. Therefore, in this
study, no grinding was made of the semi-coke samples.
The dried cylindrical coke bar is placed longitudinally in a pellet mold cup (1 inch
x φ1.25 inch, internal dimension) in which the internal wall of the mold cup has been
coated with a thin layer of releasing agent (Miller-Stephenson, Inc.). A mixture of epoxy
resin and hardener (6 parts of Epofix rein and 1 part of Epofix hardener, Struers. Inc.)
was added into the cup to the point where the coke bar was completely immersed in the
resin. The mold cup was placed in a vacuum oven for 30 minutes under 20 mmHg
vacuum where the cracks and pores are impregnated with resin. The impregnated coke
bar was allowed to cure at room temperature for 12 hours before ejecting the briquette for
polishing.
The cured pellets were mounted onto a pellet holder in a polishing apparatus
(Leco VP-160, Leco Inc.) and were ground on a series of 240, 400 and 600 grit sand disk
papers (Leco Inc.). The disk rotated at about 300 rpm for 2 minutes for each polishing
stage. Water was used as lubricant and coolant during grinding. Finally, two grades of
aluminum oxide powder slurry (0.3 and 0.05 mm particle size) on cloth were used for
fine polishing. The surface of pellet was checked to be free of scratching from sand and
polishing powders and have a maximum exposure of coke. 45
The optical texture was characterized according to the shape and size of the isochromatic areas observed on the surface of the semi-coke under microscope. A systematic scanning of the whole surface of the specimen was achieved on an automatic stage control system (Nikon Microphot-FXAII). A 0.9 mm X 0.8 mm mask and 5X object lens were used to acquire surface images. At least 250 images were examined for each specimen. The occurrence of different optical textures (see Table 2.4) was counted and the overall optical texture index (OTI) was calculated according to the following equation:
OTI = ∑ f i∗OTI i where fi is the numerical fraction of individual texture types from microscopic analysis and OTIi is the index assigned to each texture type.
3.2.3 Calcination and graphitization of semi-coke samples
Calcination is performed in a tubular furnace (Thermolyne 21100) with automatic temperature controller. The heat-treatment used a 3-stage temperature programming: sample was heated from room temperature to 450˚C at 25˚C/min and slowed to 10˚C/min to 1000˚C. One hour holding time at final heating temperature was maintained.
Throughout the calcination experiments, a stream of nitrogen was continuously introduced into the furnace (80 ml/min).
Graphitization used a Series 10 graphite resistance furnace (Centorr Associates,
NH). The carbon/coke samples were placed in a graphite crucible which was loaded into a heating core in the furnace and was heated to 2270˚C under 14 kPa of ultra pure argon. 46
The holding time at final temperature was fixed for 1 hour for all the coke samples before
the furnace was cooled down slowly by natural convection in the presence of argon.
3.3 Experimental Protocol for Preparing Decant Oil Fractions by Vacuum
Distillation
Selected feedstocks were fractionated in a simple vacuum distillation. The experimental setup is shown in Figure 3.2.
Figure 3.2 Experimental setup for feedstock vacuum distillation
For distillation, about 90 grams of decant oil was placed in a 150 ml three-neck distilling flask (sample level was at about 1/2 capacity of the flask). A fine capillary glass tube connected to a nitrogen tank was introduced into the decant oil liquid through one 47
neck on the distilling flask. The loaded flask was seated in a heating mantle. A three-way adapter connected the distilling flask and a water-cooled condenser. The temperature of
distillation was measured by inserting a glass thermometer at the top of that adapter. The
distillates were collected in a series of receivers attached to a vacuum adapter that could
rotate for collecting a certain temperature range of products. The open end of the vacuum
adapter was connected to a vacuum pump where the pressure was recorded on a vacuum gauge. Prior to vacuum operation, all the glassware joints were sealed by applying a thin coat of vacuum grease. To prevent rapid splashing (bumping) of liquid under vacuum, careful reduction of pressure is extremely important for the successful vacuum distillation operation.
Samples of decant oil and coker feed were separated into fractions with reference to ASTM D-1160. The vacuum was set at 2 mm Hg throughout distillation. To prevent decant oil samples from undergoing possible thermal cracking, the maximum temperature was kept under 245˚C.
Four vacuum residua were collected by distilling off light fractions at vapor temperatures of 125.5˚C, 150˚C, 165˚C and 182˚C. These heavy fractions correspond to the atmospheric equivalent temperatures (AET) of 300˚C, 330˚C, 350˚C and 370˚C, according to the temperature-pressure conversion table in ASTM D-1169. Different heavy cuts from such vacuum distillation were analyzed by GC/MS, and the mesophase development from these fractions was monitored by the carbonization and optical texture examination. 48
Chapter 4
Chemical/Physical Properties and Resultant Semi-coke Optical Properties of
Feedstocks
4.1 Needle Coke Feedstocks
As described in Section 2.2, for a delayed coking process aimed at producing a premium needle coke, the liquid streams coming out of the fractionator tower are considered by-products. Compared to delayed coking used for maximizing the yield of liquid products, the coking process for producing needle coke is carried out at slightly higher coking temperature and pressure, and higher recycle ratios to increase the solid product (needle coke) yield. For producing high-quality needle coke with a low CTE value (<2x10-7/°C) the decant oil feedstock is subjected to different pretreatments to reduce the sulfur content and to alter the chemical composition of the coker feed. Figure
4.1 illustrates a pretreatment scheme that consists of a pre-fractionation and selective hydrotreatment of the light fraction of the decant oil feed. This scheme is used to reduce the sulfur content of the heater charge without hydrotreating the bottom end of the decant oil feed. Samples used in this study included some of the commercially produced streams shown in Figure 4.1 to investigate the relationships between the molecular composition of the carbonized samples and mesophase development during carbonization. 49
Figure 4.1 A pretreatment scheme used to reduce the sulfur content of the feed to the heater of a delayed coking unit to produce needle coke
The abbreviations used in Figure 4. 1 are defined as follows:
DO: virgin decant oil; it is the FCC bottoms product without catalyst fines;
HYD: the heavy fraction of the hydrotreated gas oil from decant oil;
VTB: the vacuum tower bottom fraction of the decant oil;
CF: coker feed, the ultimate feed to the heater and to the coking drum(s). It consists of HYD, VTB and recycled bottom of the liquid products from coking.
The feedstock samples in this study include six decant oil samples and two sets of the different fractions obtained from two decant oil samples using the pretreatment scheme shown in Figure 4.1. The elemental composition of the feedstock samples is given in Table 4.1. 50
Table 4.1. Elemental composition of feedstock samples, wt %
DO91 DO93 DOPSU DOUP DO15 HYD15 CF15 VTB15 DO24 HYD24 CF24 VTB24
C 89.4 88 90.7 89 89.8 90.2 90.6 90.2 89.5 90.1 90.2 90.7 H 8 7.5 8.7 10.7 9.1 9.5 8.3 7.5 9.3 9.9 9.7 8.9 N 0.5 0.4 0.3 0.3 0.3 0.2 0.3 0.2 0.2 0.2 0.3 0.1 S 2.7 4.9 0.3 0.4 1.5 0.9 0.9 1.3 0.5 0.1 0.3 0.4 C/H 0.93 0.98 0.87 0.69 0.82 0.79 0.91 1.00 0.80 0.76 0.77 0.85 (atomic ratio)
As Table 4.1 shows, the decant oil samples have similar carbon contents in the range of 88 to 90.7 wt% , but the hydrogen content varies from 7.5 to 11 wt%. The atomic carbon-to-hydrogen ratio is also listed in this table. The decant oil DO93 has the highest C/H ratio (0.98), followed by DO91, DOPSU, DO15, DO24, and DOUP (0.69), respectively. The nitrogen contents of the samples are generally low and vary between
0.2 and 0.5 wt% However, the sulfur content changes significantly among the decant oil samples. The DO93 has the highest sulfur content of 4.9 wt% while the DOUP and
DOPSU samples have the lowest sulfur content (0.3 wt%). 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, but the hydrogen content varies among these derivatives. The sulfur contents in HYD and CF are significantly lower than those of the original decant oil, as intended. 51
The visual appearances of the decant oil samples and their derivatives are also
different. For example, the DO91 and DO93 samples are darker in color and more
viscous than the other decant oil samples. The samples CF have light brown color and
lower viscosity. Two extreme differences in appearance are found in the HYD and VTB
samples, as expected: HYD samples appear light greenish and the least viscous, while the
VTB samples have the appearance of pitch, being brittle solids at room temperature.
4.2 Characterization of the Optical Textures of the Semi-Cokes Produced from the Decant Oil Samples and Their Derivatives All the feedstock samples except HYD were carbonized isothermally at 500˚C for
3 hours. When HYD samples were carbonized under these conditions, the product
consisted of a viscous liquid after 3 hours of reaction. To produce semi-coke for texture
analysis, HYD samples were carbonized for 5 hours at 500˚C.
Under a polarized-light microscope, the optical texture of a semi-coke appears to
be heterogeneous because of the variation in texture along the height of the semi-coke bar sample (along the height of the vertical reactor), and even on a single “point” on the coke surface. On any given point where pores are present, the texture on or near the pore wall always take an elongated flow domain structure. Such a structure is associated with external force orientation (Mochida, Fujimoto et al., 1994) that results in shearing the
hardening mesophase domains. There is a general pattern of change in the optical unit
size distribution along the height of the cylindrical semi-coke bar. The smallest size
optical units tend to concentrate at the bottom of the semi-coke bar (the bottom of the
reactor). The thickness of this section is usually very small, about 1-2 mm; at the very 52
top of the semi-coke bar, usually mesophase spheres or isotropic pitch regions are found.
In between these two sections, the optical unit size grows larger in moving from the
bottom to the top and the texture becomes more uniform. This mid-portion of the sample
bar takes about 90% of the semi-coke volume and constitutes the most appropriate
section to characterize the texture of the semi-coke samples. Figure 4.2 shows the change
in the size of the optical units on the semi-coke bar depending on the location in the
tubing reactor. A similar pattern of changing optical texture size is also observed in
samples obtained from the pilot coker (Hardin and Ellis, 1992) and commercial coke
drums (from private conversation with Mr. Bob Miller at Chicago Carbon Company,
Lemont, IL).
Figure 4.2 Polarized-light micrographs of sections at the top, middle and bottom of the semi-coke bar obtained from carbonization of DO15 53
The polarized-light micrograph images of the semi-cokes from the carbonization of studied decant oil samples are shown in Figure 4.3 to Figure 4.8. Two images are shown for each feedstock, and each image consists of three micrographs combined edge- to-edge to display a larger portion of a given area on the semi-coke surface. The bottom image on each figure shows an area from the bottom portion of the semi-coke and the top image shows a representative area from the middle section of the respective semi-coke sample bars.
Table 4.2 gives the calculated optical texture index (OTI) values for the semi- cokes from carbonization of decant oil samples (see Section 2.4.2 for the definition and calculation of OTI).
Table 4.2 OTIs for semi-cokes from DO samples by carbonization at 500˚C for 3 h. Optical Texture Unit Counts Decant Oil Flow Domains Domains Small Domains Mosaics Isotropic Pitch OTI DO91 (I) 113 70 13 0 1 76 DO91 (II) 136 74 9 0 3 79 Average 78 DO93 (I) 48 97 31 18 2 51 DO93 (II) 53 105 59 9 1 48 Average 50 DO15(I) 74 77 15 0 2 68 DO15(II) 79 65 16 0 1 70 Average 69 DO24 (I) 77 38 0 0 0 83 DO24 (II) 110 58 0 0 0 83 Average 83 DOPSU (I) 84 35 8 0 1 80 DOPSU(II) 70 36 6 0 1 79 Average 80 DOUP (I) 103 17 15 0 2 83 DOUP (II) 130 20 18 0 3 84 Average 84
54
Figure 4.3 Polarized-light micrographs of the semi-coke sample from DO91
Figure 4.4 Polarized-light micrographs of the semi-coke sample from DO93
Figure 4.5 Polarized-light micrographs of the semi-coke sample from DO15
55
Figure 4.6 Polarized-light micrographs of the semi-coke sample from DO24
Figure 4.7 Polarized-light micrographs of the semi-coke sample from DOPSU
Figure 4.8 Polarized-light micrographs of the semi-coke sample from DOUP
56
The overall OTI value for each semi-coke is the averaged results from two parallel carbonization and optical texture examination experiments for each sample, with a reproducibility of ±2 units for each OTI examination. Also listed in the table are the optical texture distributions on each semi-coke.
The OTI represents the degree of mesophase development during carbonization
(Eser, 1998). The higher the OTI value, the higher the degree of mesophase development toward the textural anisotropy of the needle coke texture. As can be seen from Table 4.2, the decant oil samples produced semi-cokes with significantly different optical textures resulting from different extents of mesophase development during carbonization. The DOUP and DO24 have the highest OTI values (84 and 83), followed by DOPSU, DO91 and DO15. The semi-coke from DO93 gave the lowest OTI value of
50.
The appearance of the optical units in the poorest semi-coke from DO93 is very different from those of the semi-cokes from the other decant oil samples. The most significant distinction is the long and wide cracks seen throughout the surface of the semi-coke from DO93. This is an evidence for a significant shrinkage of the semi-coke after 500˚C reaction due to the hardening of mesophase. In addition to the severe
shrinkage, there exists a large amount of small-size pores in the bulk of this semi-coke.
The edges of pores appear to be “eroded” compared with the smooth edges of the pores in
DO15 and DO91, for example. Most of the pores in the semi-coke from DO93 are not
filled or partially filled by the mounting resin during pellet preparation, indicating that
most of these pores are closed. This physical appearance of semi-coke from DO93
suggests that mesophase was formed and hardened into anisotropic coke more quickly 57 compared to the carbonization of the other decant oil samples. This explains the low OTI of semi-coke DO93 as well (Filley and Eser, 1997).
The optical texture analysis results for the semi-cokes of the derivatives from
DO15 and DO24 are listed in Table 4.3 and the micrographs of the semi-cokes produced from these derivatives are presented in Figure 4.9 to Figure 4.14.
Table 4.3 OTIs for semi-cokes from DO derivatives by carbonization at 500˚C for 3 h (5 h for HYD) Optical Texture Unit Counts Decant Oil Flow Domains Domains Small Domains Mosaics Isotropic Pitch OTI DO15(I) 74 77 15 0 2 68 DO15(II) 79 65 16 0 1 70 Average 69 CF15(I) 75 61 7 0 15 74 CF15(II) 77 49 16 0 12 72 Average 73 HYD15 (I) 79 88 0 0 24 74 HYD15 (II) 98 95 0 0 22 75 Average 75 VTB15 (I) 136 27 14 0 0 85 VTB15 (II) 145 19 20 0 0 85 Average 85
DO24 (I) 77 38 0 0 0 83 DO24 (II) 110 58 0 0 0 83 Average 83 CF24 (I) 74 31 0 0 6 85 CF24 (II) 98 38 0 0 3 86 Average 85 HDY24 (I) 53 8 0 0 10 93 HDY24 (II) 65 12 0 0 20 92 Average 92 VTB24 (I) 105 35 0 0 0 88 VTB24 (II) 113 29 0 0 0 90 Average 89
58
Figure 4.9 Polarized-light micrographs of the semi-coke sample from CF15
Figure 4.10 Polarized-light micrographs of the semi-coke sample from HYD15
Figure 4.11 Polarized-light micrographs of the semi-coke sample from VTB15
59
Figure 4.12 Polarized-light micrographs of the semi-coke sample from CF24
Figure 4.13 Polarized-light micrographs of the semi-coke sample from HYD24
Figure 4.14 Polarized-light micrographs of the semi-coke sample from VTB24
60
Decant oil derivatives HYD and VTB and the coker feed (CF) produced semi- cokes that display much improved mesophase development compared to that produced by the parent decant oils. The OTI values of CF and VTB semi-cokes 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 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 semi-cokes given in Table 4.3 were determined on the semi-cokes produced by 5-hour carbonization experiments.
One of the most distinctive features observed on the semi-cokes obtained from
HYD and CF is the relatively high proportion of isotropic pitch units. Generally, there is a small number of undeformed mesophase spheres in the semi-coke from the decant oils as illustrated in Figure 4.2. (This texture unit was called isotropic pitch in this study.) As the data in Table 4.3 show, both CF and HYD produced semi-cokes that show 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 semi-coke from the VTB fraction where there is no undeformed mesophase sphere at all, which indicates a higher carbonization reactivity from this heavy fraction of decant oil. Another prominent feature in the HYD-derived semi-cokes is the almost exclusive domain and flow domain textures in these semi-cokes. The sizes of these optical texture units are much larger than those seen in the semi-cokes produced from either decant oil or CF samples. 61
4.3 X-ray Diffraction Analysis of the Heat Treated Semi-cokes
In an effort to relate the optical texture index of the semi-cokes with the
crystalline structures of carbons in the needle coke products (calcined and graphitized
carbons), selected semi-cokes with various OTI values (obtained from DO91, DO93,
DO15 and DOUP) were heat treated. The procedures for calcinations and graphitization
heat treatment are described in Section 3.2. The resulting carbons were subjected to the
analysis of x-ray diffraction (XRD).
ASTM method D 5187 suggests scanning calcined cokes between 14 and 34° 2θ
(for copper radiation tube) at a rate of 1°/minute. This 2θ range corresponds to the peak
reflected from the (002) set of planes for graphite. This peak is the most intense and is the only resolvable peak from non-graphitic carbons. The amorphous carbon masks all the other less intense peaks generated from other sets of planes. However, since the coke samples in this study are graphitizable, a preliminary full scan of 14 to 90° 2θ was performed on the calcined coke to confirm that there is no peak with a statistically significant intensity beyond 47° 2θ. Therefore, semi-coke and calcined coke samples were scanned from 14 to 47° 2θ. The graphitized samples were scanned in the full range
from 14 to 90° 2θ.
The x-ray diffractograms of semi-coke, calcined coke and graphitized cokes are
shown in Figure 4.15, Figure 4.16, and Figure 4.17, respectively. The x-ray
diffractograms were drawn on one graph with an arbitrary offset in intensity for each
sample to aid visual comparison of different samples. For graphitized samples, the
diffractograms were presented in a lower intensity to emphasize the difference in
graphite’s characteristic peaks at higher angles. 62
The acquired raw XRD data were processed by a computer program DMSNT
(Diffraction Management System for NT, version 1.37). Background smoothing and elimination was necessary to determine the peaks and full width at half maximum peak
(FWHM). This was done by using “Box Car filter” set at 1.5° width and the Kα2 component artifacts removal method. The peaks of most interest are those correspond to planes (002), (100), (101) and (110). They are identified by first using a routine, “digital
filter”, in which two parameters can be set by the user: estimated standard deviation
(ESD) multiplier and ripple multiplier. Increasing either or both of these multipliers will
cause the filter to find fewer peaks. In this study, the ripple multiplier was fixed at 2.5
while the ESD multiplier, depending on the sample scanned, was changed from 12 to 20
to isolate the peaks of interest. These isolated peaks were then curve-fitted using Pearson
VII algorithm. For the green and calcined coke samples, because the pregraphitic (002) peaks are often skewed and the profile for the low-angle side of the peak is quite different from that of the high-angle side of the peak, a modified Pearson VII (Split-Pearson VII) algorithm was employed. This technique, as the name implies, divides each diffraction peak into two halves at the peak summit. Each side of the peak is fitted independently using the Pearson VII algorithm. The final FWHM is the average of two half-widths at left and right (FWHML and FWHMR).
The d-spacing parameter, which is given by the DMSNT program, is actually calculated according to Bragg’s Equation:
nλ=2dhkl sinθ
63
where n is the number of layers; λ is the wavelength of bean of radiation source, for Cu Kα, λ=1.54056Å; d is the interplane (hkl) spacing, d(002) is of most interest; θ is the diffraction angle.
The effective crystallite size L, must be estimated from the amount of broadening of peak, β, using Scherrer equation: L=Kλ/(β cosθ)
L is usually quoted as stack height or effective thickness, Lc, and stack width or
effective diameter, La. For hexagonal carbon/graphite crystallite, Scherrer constant K has
a value of 0.9 for Lc and of 1.84 for La. β is peak broadening and is determined by following relationship: β2=B2-b2 where B is the peak broadening of the carbon sample, i.e., FWHM in radians b is the instrument broadening.
In this study, an external standard silicon powder (SRM 640c from National
Institute of Standards and Technology, 2000) was scanned at the same conditions as in carbon XRD experiment. The purpose of using external standard before running actual coke/graphite sample is to verify the alignment of diffractometer to make position correction if any were needed. The peak positions and parameters of the reference material are listed in Table 4.3.
64
Table 4.4 Peak position and half-high width of SRM 640c using Cu Kα radiation h k l 2θ, degrees FWHW, degree
1 1 1 28.44 0.0976 2 2 0 47.31 0.1003 3 1 1 56.12 0.0981 4 0 0 69.13 0.0974 3 3 1 76.38 0.1012 4 2 2 88.03 0.1006
The table also gives the parameter b in the Scherrer equation. Since silicon crystal shows no diffraction peaks at 26.4° 2θ (location of (002) peak of carbon), the peak breadth at 28.44° 2θ was used in Scherrer equation to calculate Lc (002). In a similar
manner, the FWHM at 47.31 and 76.38° 2θ was used as instrumental broadening for the
calculation of the stack width La (100) and La (110) of carbon/graphite crystallite
respectively.
As the diffractograms in Figure 4.15 shows, semi-cokes exhibit only broad (002)
peaks. The peak position, d002 spacing and crystallite thickness Lc (002) by using Split-
Pearson VII algorithm curve fitting are listed in Table 4.5. All the semi-cokes show a
large spacing (>3.44 Å) and a similar stacking height (about 30 Å), indicating that these
cokes are non-graphitic. Semi-coke DO93, which produced the least mesophase
development during carbonization, also showed the largest d002 spacing (3.609 Å) among
all the semi-coke samples. 65
6000
5000
4000 DO_UP
3000 DO15 Intensity
2000
1000 DO93
DO91
0 14 19 24 29 34 39 44 2θ°
Figure 4.15 X-ray diffractogram for semi-cokes
Table 4.5 Semi-coke crystalline parameters from XRD analysis
d Position FWHM FWHM L Sample 002 L R c (Å) (2θ,°) (°) (°) (Å) DO_91 3.418 26.005 3.98 2.089 26 DO_93 3.609 24.644 2.94 2.961 28 DO15 3.502 25.048 2.577 2.681 31 DOUP 3.458 25.739 4.026 2.303 26
The XRD diffractograms of calcined (1000˚C for 1 hour) cokes showed a sharper and less skewed profile than those of semi-coke samples (see Figure 4.16). The positions of the peak at the (002) basal plane for all coke were shifted to higher angles, indicating a decreasing d002 spacing. A weak peak at high angles (about 43˚) was also developed on 66 the diffractogram, reflecting the increasing diffraction of (100) plane in the hexagonal carbon unit cells in the calcined cokes.
6000
5000
4000 y DO_UP 3000 Intensit
DO15
2000
DO93
1000
DO91
0 14 19 24 29 34 39 44 2 Theta, Degree
Figure 4.16 X-ray diffractogram for calcined cokes
Cokes heated at graphitization temperature show the most intense d002 plane peaks on the XRD diffractgrams. Some characteristic peaks of graphite also emerged in the higher angle region as shown in Figure 4.17. In order to show the details of the refractions from the planes other than (002), the y axis of diffractograms of the
“graphitized” cokes was scaled up to about 1/3 full intensity of the d002 peak. 67
30000
25000
20000
DO15
15000 Intensity
DOUP 10000
DO93 5000
DO91 0 20 30 40 50 60 70 80 2 theta, degree
Figure 4.17 X-ray diffractogram for coke after graphitization temperature treatment
The peak width at (100) plane in the diffractogram from calcined cokes and the peak width at (110) plane in the diffractogram from graphitized cokes were used to calculate the effective diameter of the crystallites. The XRD parameters for calcined and graphitized cokes are listed in Table 4.6 and 4.7 respectively.
Table 4.6 XRD parameters of calcined cokes
d002 Position FWHML FWHMR Lc Position FWHML FWHMR La (100) Sample (Å) (2θ,°) (°) (°) (Å) (2θ,°) (°) (°) (Å)
DO91 3.400 26.189 2.450 2.450 33 44.029 2.610 2.610 67 DO93 3.561 24.986 2.124 2.816 33 43.406 3.020 3.020 56 DO15 3.438 25.845 2.443 2.443 33 43.184 3.359 3.359 52 DOUP 3.393 26.241 2.482 2.482 33 43.860 2.482 2.482 71
68
Table 4.7 XRD parameters of carbons heated to graphitization temperature
d002 Position FWHML FWHMR Lc Position FWHML FWHMR La (100) Sample (Å) (2θ,°) (°) (°) (Å) (2θ,°) (°) (°) (Å)
DO91 3.377 26.370 0.337 0.337 253 77.842 0.197 0.197 1239 DO93 3.380 26.349 0.305 0.305 282 77.788 0.247 0.247 925 DO15 3.379 26.202 0.328 0.328 261 77.589 0.214 0.214 1107 DOUP 3.370 26.429 0.336 0.336 254 77.989 0.187 0.187 1325
From the tables, it can be seen that the cokes are graphitizable, as expected from
these needle coke feedstocks. The d002 spacing of “graphitized” cokes approaches 3.370
Å. These d002 values of the coke samples are still larger than that of graphite (3.354 Å), most probably because of the relatively low graphitization heat-treatment temperature
(2270˚C) used in this study compared to the conventional graphitization temperature
(2800˚C). The effective crystallite thickness for the calcined cokes appears to be the same (33 Å) for all coke samples, while the crystallite diameter is about twice the thickness. The heat treatment at 2270˚C increases both the thickness and the diameter, but the increase in the diameter was more pronounced. For example, Lc increased about 8
times for DO91 graphitized coke, whereas La increased 20 times upon heating to 2270˚C.
It appears that high OTI cokes (DOUP and DO91) have much larger La for the
“graphitized” cokes than the low OTI cokes (DO93 and DO015).
An attempt was made to correlate the OTI value of semi-cokes with the XRD
parameters of the calcined and “graphitized” cokes. The plots of d002 spacing and La versus OTI are shown in Figure 4.18, Figure 4.19 and Figure 4.20, respectively. 69
Reasonably good trends in both d002 and La were found for the samples included
in this analysis. Qiao in her Ph.D thesis (Qiao, 2000) related the OTI of semi-cokes to
the physical properties of both calcined and graphitized cokes. Her results showed that
there exists a good linear relation between the OTI of semi-cokes and the coefficient of
thermal expansion (CTE) values of calcined and graphitized carbons. The relationship
between the OTI and the degree of graphitizability of the semi-cokes in this study further
confirms that OTI is a good measure of semi-coke or needle coke quality.
Relating the OTI of semi-cokes and XRD parameters of calcined and graphitized
cokes to the CTE of synthetic graphites is more complicated because of the complex
contribution from binders and pores (Engle, 1970; Seehra and Pavlovic, 1993). Still, the
OTI values of semi-cokes as predictors of the degree of graphitizability can be related to
the thermal expansion behavior of the graphitized cokes.
3.650 Semi-coke 3.600 Calcined-coke
3.550
3.500 d002 ,Å
3.450
3.400
3.350 40 50 60 70 80 90 OTI of semic-coke
Figure 4.18 The d002-spacing of semi-cokes and calcined cokes versus OTI of semi-cokes 70
3.382
3.380
3.378
3.376
d002 ,Å 3.374
3.372
3.370
3.368 40 50 60 70 80 90 OTI of semi-coke
Figure 4.19 The d002-spacing of graphitized coke vs. OTI of semi-coke
1400
1300
1200
1100 La, Å
1000
900
800 40 50 60 70 80 90 OTI of semi-coke
Figure 4.20 Crystallite diameter La of graphitized coke vs. OTI of green coke
71
Chapter 5
Characterization of Needle Coke Feedstocks
This chapter presents the data on decant oil samples and their derivatives obtained by various instrumental analyses to highlight their general molecular composition. The first part reports the analytical data acquired by GC/MS. The second part presents the
HPLC/PDA, LC/MS/MS, LD/MS and NMR analyses to reveal the molecular composition of the heavier aromatic hydrocarbons in the needle coke feedstock. A summary of the analysis results is given in the last section.
5.1 Gas Chromatography / Mass Spectrometry Analysis of Needle Coke Feedstocks
Feedstock samples (see Table 5.1, detailed explanation on the sources of decant oil derivatives refers to Figure 4.1 in Chapter 4) were analyzed on a GC/MS instrument according to the procedures given in the EPA’s semi-volatile organic compound testing method 8270C (EPA, 1996).
Table 5.1 Needle coke feedstock used in GC/MS analysis Decant oil Decant oil derivative DO91 DO15 DO15 DO24 DOPSU DOUP CF15 CF24 HYD15 HYD24 VTB15 VTB24
The GC/MS total ion chromatograms (TIC) for decant oil (DO), coker feed (CF), hydrotreated decant oil fraction (HYD) and vacuum tower bottom fraction (VTB) are shown in Figure 5.1 to Figure 5.3.
72
Two-ring aromatic compounds in the needle coke feedstocks are well-resolved in
the temperature range of 120˚C to 180˚C with the elution at retention time from 8 min to
20 min. These compounds are mainly naphthalene and the alkyl-substituted naphthalenes.
Three-ring to five-ring aromatics elute from the GC column after the two-ring aromatics.
Since the higher degree of alkylation on these PAHs in the needle coke feedstock, the
elution of the higher alkyl substituents of three-ring and four-ring aromatics overlaps in
the column temperature range from 180˚C to 285˚C (retention times from 20 to 45 min).
Five-ring aromatics usually do not resolve well in decant oil samples, however, good
resolution of less alkylated substituents of these five-ring aromatic compounds were found in the coker feed (CF) streams. Further increase in the column temperature and
duration did not give further evolution of heavier compounds, as indicated by the TIC chromatograms.
In addition to the aromatic compounds, aliphatic species also present in the needle coke feedstocks as revealed by the GC/MS analysis, these aliphatic compounds are mainly in the forms of normal alkanes which are resolved at almost same retention time
(or temperature) interval over the whole retention time (or temperature) range of GC column.
Following sections present and discuss the GC/MS data on the aromatic compounds in groups with respect to the number of aromatic rings in the needle coke feedstock samples. Aliphatic compounds concentration and distribution are also given in the ensuing sections.
73
Figure 5.1 GC/MS total ion chromatogram of DO 91, DO93 and DO15 Column: Restek XTi-5 (5% phenyl, 95% dimethylpolysiloxane), i.d.: 0.32mm; length: 30 meters 74
Figure 5.2 GC/MS total ion chromatogram of DOPSU and DOUP Column: Restek XTi-5 (5% phenyl, 95% dimethylpolysiloxane), i.d.: 0.32mm; length: 30 meters
75
Figure 5.3 GC/MS total ion chromatogram of HYD15, CF15 and VTB15 Column: Restek XTi-5 (5% phenyl, 95% dimethylpolysiloxane), i.d.: 0.32mm; length: 30 meters
76
5.1.1 Two-ring aromatic compounds in decant oils
The two-ring aromatic compounds consist of naphthalene and biphenyl and their
corresponding alkylates. They eluted from 8 to 20 minutes (column temperature: 120-
185˚C) with very good resolution under the experimental conditions used. All decant oil
samples show a similar peak-retention time profile with different intensities. In this
temperature/time window, about 23 peaks are well-resolved on the total ion
chromatogram; Figure 5.4 shows TIC of DO91 in this retention time and column
temperature window. The analysis of mass spectra reveals that most of these peaks
correspond to the unsubstituted naphthalene (peak 1), methylnaphthalenes (peak 3, 4),
C2-substituted (peak 6 to 12) and C3-substituted naphthalenes (peak 14 to peak19).
Alkyl substituents of more than C3 groups were not observed on decant oil samples. Two
peaks (peak 21, peak 23) at later retention times are from methyl biphenyl and
dimethylbiphenyl.
Also eluted in this temperature window are the normal alkanes (peak 2, 5, 13, and
20); the analysis of the normal alkanes will be presented in Section 5.1.5.
The identification of aromatic compounds in this part of the chromatogram can be
readily achieved by matching their mass spectra patterns with those of the standard compounds in the library of computerized database software. However, the
differentiation among isomers can not be uniquely performed from the mass spectrum
alone. By comparing retention times of unknown isomers with those of the standard
compounds (see Table 3.1) or by matching the relative peak elution orders of unknown
isomers with that in published literature (Rowland, Alexander et al., 1984; Radke, 77
Garrigues et al., 1990; Lai and Song, 1995), the identification of these isomers can be
positively confirmed. Figure 5.5 gives the composite multi-ion chromatograms (MIC)
with mass to charge ratios of m/e 142, 156 and 170, representing the base peaks of
methylnaphthalene, C2-naphthalenes, and C3-naphthalenes respectively. These
naphthalene compounds are labeled numerically the same as in the total ion
chromatogram (Figure 5.4), and their molecular structures with conventional numbering
protocol are also shown in this figure.
Figure 5.4 TIC of DO91 in the retention time window from 8 to 20 minutes
The concentrations of naphthalene and alkylated naphthalenes in the decant oil samples, determined by integrating the peaks in MIC and expressed in ppm (one microgram, or 1x10-6g, of individual compound in one gram of decant oil), are listed in
Table 5.2.
Naphthalenes present in decant oil predominantly as alkylated naphthalene homologues. The alkylation is almost exclusively multi-methyl substitution, i.e.,
methylnaphthalenes (MN), dimethylnaphthalenes (DMN), and trimethylnapthalenes 78
(TMN). Some ethyl or methylethyl-substituted naphthalenes are also detected in the
decant oil samples, but these naphthalene compounds are in trace amounts.
Figure 5.5 Multi-ion chromatograms (MIC) of DO91 representing alkylated naphthalenes
79
Table 5.2 Concentrations of naphthalene and its alkylated substituents in decant oils Concentration, ppm. in decant oil Compound DO91 DO93 DO15 DO24 DOPSU DOUP Naphthalene 865 653 693 307 1335 2311
2-MN 2395 2306 1891 847 1133 2085 1-MN 1164 1240 993 444 660 1218 Total MN 3560 3546 2884 1291 1793 3302
2-EN 374 372 280 140 155 315 2,6+2,7-DMN 2099 2172 1409 705 860 1733 1,3-DMN 2017 2154 1382 668 882 1779 1,6-DMN 1123 1172 760 392 475 925 1,4-DMN 707 727 468 255 287 567 1,5-DMN 221 225 163 83 92 132 1,2-DMN 384 413 282 135 167 316 Total DMN 6926 7235 4744 2379 2918 5767
MEN 797 752 438 250 276 523 1,3,7-TMN 1076 1117 628 346 401 753 1,3,6-TMN 1329 1398 761 410 478 831 1,3,5-+1,4,6-TMN 923 1023 533 296 343 639 2,3,6-TMN 1076 1139 577 327 400 691 1,2,7- +1,6,7-TMN 744 998 561 297 380 525 1,2,6-TMN 392 555 277 145 131 277 Total TMN 6337 6982 3775 2072 2409 4237
Total Naphthalenes 17688 18416 12096 6049 8455 15618
80
The total naphthalenes (naphthalene and C1- to C3-naphthalenes) concentration
(Figure 5.6.) varies greatly among decant oil samples. DO91 and DO93 contain the most
naphthalenes, while DO24 and DOPSU contain less than half of the naphthalenes in the other decant oil samples.
20000
18000 TMN DMN 16000 MN NAPH 14000
12000
10000
8000 Conc, ppm in DO ppm in Conc, 6000
4000
2000
0 DO91 DO93 DO15 DO24 DO_PSU DO_UP
Figure 5.6 Total concentrations of naphthalene (NAPH), methylnaphthalenes (MN), dimethylnaphthalenes (DMN) and trimethylnapthalenes (TMN) in decant oils
Table 5.3 lists the relative distributions of naphthalene homologues among decant
oil samples. The decant oil samples fall into two sets according to their multi-methyl
substitution patterns. The first set of decant oil samples, DOPSU and DOUP, is less
methyl substituted, which can be easily seen from the higher proportion (about 15%) of 81
unsubstituted naphthalene. The second set of decant oil samples (DO91, DO93, DO15
and DO24) are more alkaylated (95% in alkylated naphthalenes). Among these samples,
DO93, which produced the lowest degree of mesophase development, has the lowest
naphthalene and highest trimethylnaphthalenes proportions (concentrations), indicating the highest degree of PAH alkylation this decant oil.
Table 5.3 Relative naphthalene homologues distribution (wt%) DO91 DO93 DO15 DO24 DOPSU DOUP NAPH 4.9% 3.5% 5.7% 5.1% 15.8% 14.8% MN 20.1% 19.3% 23.8% 21.3% 21.2% 21.1% DMN 39.2% 39.3% 39.2% 39.3% 34.5% 36.9% TMN 35.8% 37.9% 31.2% 34.3% 28.5% 27.1%
The distribution of isomers of C2-naphthalenes and C3-naphthalenes was found to be similar in all decant oil samples as data in Table 5.3 describe. The only noticeable difference can be seen in the methylnaphthalene isomer distribution between the two sets of samples. The decant oils in the first set (DOUP and DOPSU) have higher
1-MN/2-MN ratios than those in the second set.
The different distribution pattern of homologous naphthalenes and methyl-
naphthalene isomers suggests that these two sets of decant oils are derived from the FCC
processes under different operating conditions and/or from very different crude oils.
There are two methylnaphthalene isomers, 2- MN and 1- MN. Both of them were
found to be present in decant oil samples, with a relative ratio of about 3:1. For multi- methyl-naphthalenes, the number of isomers increases rapidly. For example, there are 10 82 possible dimethylnaphthalene (DMN) isomers and 16 possible trimethylnaphthalene
(TMN) isomers. However, the actual number of multi-methyl naphthalenes present in decant oil samples is much fewer. Only 7 out of 10 DMN isomers and 8 out of 16 TMN isomers were found to exist with significant amount in decant oil samples, and these isomers tend to have low steric hindrance or high thermal stability (the thermal stability of methylPAH will be discussed in detail in Chapter 7). Some of the isomers were not well separated in the GC column, so the total concentrations of those co-elutes are reported in Table 5.4.
The naphthalene isomer distribution, which is the same for the decant oil samples within the same sample set, but are different between sample sets, suggests that the distribution of the naphthalene isomers is controlled by the chemical equilibrium
(thermodynamics) in FCC processes.
Two major peaks (peak 21 and 23 in Figure 5.4) at retention times of 17-20 minutes represent the elution of bridged two-ring aromatic compounds. These peaks showed the molecular ions with m/e 168 and 182 that correspond to the 3-methylbiphenyl
(MBP) and 3,5-dimethylbiphenyl (DMBP) respectively. The MIC of m/e 168 and 182 is presented in Figure 5.7. As the MIC indicates, two more MBP isomers (peak a,b) and nine more DMBP isomers (peaks c to j) also exit in the sample, although they are in lesser abundance than 3-MBP and 3,5-DMBP. Methyldiphenylmethanes (MDPM), the structural isomers to DMBP, were also found in the same GC elution window, but these
MDPMs are in even lesser amount. No measurable amount of biphenyl (m/e 152) was observed in any decant oil. The identification of DMBP isomers was mainly inferred from the work of Trolio et al (Trolio, Grice et al., 1999). 83
Table 5.4 Isomer distribution in naphthalene and alkylated derivatives concentrations in decant oils Concentration, wt %
Compound DO91 DO93 DO15 DO24 DOPSU DOUP
2-MN 67% 65% 66% 66% 63% 63% 1-MN 33% 35% 34% 34% 37% 37% Total MN 100% 100% 100% 100% 100% 100%
2-EN 5% 5% 6% 6% 5% 5% 2,6+2,7-DMN 30% 30% 30% 30% 29% 30% 1,3-DMN 29% 30% 29% 28% 30% 31% 1,6-DMN 16% 16% 16% 16% 16% 16% 1,4-DMN 10% 10% 10% 11% 10% 10% 1,5-DMN 3% 3% 3% 3% 3% 2% 1,2-DMN 6% 6% 6% 6% 6% 5% Total DMN 100% 100% 100% 100% 100% 100%
MEN 13% 11% 12% 12% 11% 12% 1,3,7-TMN 17% 16% 17% 17% 17% 18% 1,3,6-TMN 21% 20% 20% 20% 20% 20% 1,3,5-+1,4,6- TMN 15% 15% 14% 14% 14% 15% 2,3,6-TMN 17% 16% 15% 16% 17% 16% 1,2,7- +1,6,7- TMN 12% 14% 15% 14% 16% 12% 1,2,6-TMN 6% 8% 7% 7% 5% 7% Total TMN 100% 100% 100% 100% 100% 100%
84
Figure 5.7 MIC of DO91 representing alkylated biphenyls and diphenylmethanes
The concentrations of MBP, DMBP and MDPM in decant oil samples are listed in
Table 5.5. and the relative distribution of the BP homologues is shown in Table 5.6. All decant oil samples have a similar relative BP distribution. About 2/3 of biphenyls are in the form of DMBPs for all DO samples except DOUP. The biphenyls in DOUP tend to be evenly distributed. BP isomer distribution pattern could not be drawn, because of the lower concentration of the isomers. However, 3-MBP and 3, 5-DMBP are the predominant forms of the bridged two-ring aromatics.
85
Table 5.5 Concentrations of biphenyl and diphenylmethane and their alkylated derivatives in decant oils Concentration, ppm. in decant oil
Compound DO91 DO93 DO15 DO24 DOPSU DOUP 2-methyl-1,1'-biphenyl 123 85 286 97 96 56 3-methyl-1,1'-biphenyl 404 658 890 312 376 243 4-methyl-1,1'-biphenyl 269 327 340 191 87 180 Total MBP 796 1070 1517 600 559 479 2,3’-dimethylbiphenyl 85 0 0 56 0 0 2,5-dimethylbiphenyl 100 0 223 126 0 0 2,4-dimethylbiphenyl 97 192 271 133 0 0 3-methyldiphenylmethane 80 224 160 82 0 0 2- and 4- methyldiphenylmethane 41 35 43 116 0 0 3,5-dimethylbiphenyl 424 621 839 309 342 233 4-ethylbiphenyl 104 224 0 136 38 46 3,3’-dimethylbiphenyl 92 312 369 155 133 124 3,4’-dimethylbiphenyl 229 228 282 119 112 86 4,4’-dimethylbiphenyl 179 266 339 138 85 63 3,4-dimethylbiphenyl 196 397 525 211 223 0 Total DMBP and MDPM 1626 2498 3052 1579 933 552 Total MBP,DMBP and MDPM 2423 3568 4568 2179 1491 1031
Table 5.6 Relative distribution of BP homologues in decant oil samples Percentage in total BP homologues, % Compound DO91 DO93 DO15 DO24 DOPSU DOUP MBP 35% 32% 35% 30% 37% 46% DMBP 65% 68% 65% 70% 63% 54%
86
The total concentration of biphenyls is much lower than that of naphthalenes and
varies among decant oil samples. The contents of biphenyls increase in the order of
DOUP, DOPSU, DO24, DO91, DO93 and DO15.
Comparing with the OTI values of semi-cokes from the decant oil samples
(reported in Chapter 4), it can be seen that biphenyl-rich decant oil produces less
developed mesophase during carbonization. This should be expected considering the fact that biphenyl produces non-graphitizable carbon because of its non-planar spatial configuration (Walker, 1990).
MDPMs (methyldiphenylmethanes) were observed only in the second set of samples (DO91, DO93, DO15 and DO 24) although their concentrations were very low. 87
5.1.2 Three-ring aromatic compound in decant oils
Shown in Figure 5.8 is the typical total ion chromatogram corresponding to the elution of three-ring aromatics in a decant oil sample.
Figure 5.8 Total ion chromatogram of DO91 (retention time from 20 min to 33 min)
Phenanthrene (PHEN) and alkylphenanthrenes are the most abundant three-ring
PAHs in decant oil samples, although an insignificant amount of anthracene and fluorene
were also detected. Dibenzothiophene (DBT) and alkyl DBTs are the primary
heterocycles, and, a large quantity of those compounds is present in higher sulfur content
decant oil samples such as DO93 and DO91. The structures and conventional numbering
of these basic three-ring aromatic compounds (both PAH and heterocycles) are shown in
Figure 5.9.
On the TIC in Figure 5.8, four clusters of peaks can be readily distinguished.
These clusters are the responses from methylphenanthrenes (MPHEN),
dimethylphenanthrenes (DMPHEN), trimethylphenanthrenes (TMPHEN) and 88
tetramethylphenanthrenes (TEMPHEN) in retention time windows of 23.5 to 24.6 , 26.4 to 27.8, 29.5 to 30.8 and 31.5 to 32.8 minutes, respectively. DBT and its alkylated substituents elute from the GC column prior to phenanthrenes. At later retention times,
both pyrene and methylpyrenes elute between DMPHEN and TEMPHEN. The
identification and quantification of these four-ring aromatics will be discussed in next
section.
10 9 891 8 1 7 2
7 2 6 3
5 10 4 6 5 4 3
C14H10 C14H10 phenanthrene anthracene 9 1 1 S 8 2 8 2
7 3 7 3 4 4 6 5 6 5
C13H10 C12H8S
fluorene dibenzothiophene
Figure 5.9 Basic three-ring aromatic compounds present in decant oil samples
The multi-ion chromatograms for C1-, C2- , C3- and C4- substituted
phenanthrene are shown in Figure 5.10 (corresponding to m/e 192, 206, 220 and 234). 89
These phenanthrenes are almost exclusively multi-methyl substituted. Because of the
overwhelming numbers of isomers of multi-methyl phenanthrenes and limited
availability of standard compounds, the following procedures were used for the
identification of isomers of phenanthrene homologues. Two phenanthrene standard compounds (2-methyl and 3,6-dimethylphenanthrene) were co-injected with the decant oil sample into GC/MS system; the retention times of these two standard compounds were located by comparing the TICs from standard spiked and original decant oil sample.
Once the positions (elution times) of the standard compounds were obtained, the positions (retention times) of the other isomers was determined by comparing the peak eluting order with those in the published literature (Budzinski, Garrigues et al., 1995;
Kruge, 2000). Differentiation of tetramethylphenanthrene (TEMPHN) isomers could not be achieved, as standard compounds and the literature data on elution times are not available.
The quantitative GC/MS analysis results for phenanthrene homologues and their isomers are given in Table 5.7. The distribution of phenanthrene compounds in the decant oil samples is presented in Figure 5.11. Multi-methyl substituted phenanthrenes constitute over 95% of all phenanthrenes as shown in Table 5.8.
The total phenanthrenes concentration increases in the order of DOUP
DOs are about two to three times higher than the rest of the decant oil samples. Comparing the orders of the concentration between phenanthrene and naphthalene in 90 Figure 5.10 Multi-ion chromatograms of DO91 representing multi-methyl phenanthrenes 91 decant oil samples, there seems to be no clear correlation of the PAH abundance with respect to the size of the aromatic compounds. It is interesting to compare the composition of phenanthrenes in DO91 and DO93. These two have comparable amount of total phenanthrenes, however, the relative distribution of homologous phenanthrenes is very different. Unsubstituted phenanthrene in DO91 (4.5%) is more than twice as high as that in DO93 (2.1%). 120000 PHEN MPHEN 100000 DMPHEN TMPHEN TEMPHEN 80000 Total PHENs 60000 40000 Concentration, ng/mg 20000 0 DO91 DO93 DO15 DO24 DO_PSU DO_UP Figure 5.11 Concentration of phenanthrene and alkylated phenanthrenes in decant oils 92 Table 5.7 Phenanthrene and its alkylated substitutes in decant oils (compound identifications are shown in Figure 5.10) Concentration, ppm. in decant oil Compound DO91 DO93 DO15 DO24 DOPSU DOUP PHEN 3817 2145 1958 2225 941 1080 3-MPHEN 5017 4898 3245 3117 1776 2203 2-MPHEN 6853 7101 4219 3535 2189 2545 9-MPHEN 3355 4182 2309 1859 1009 1304 1-MPHNE 2545 3333 1619 1517 939 1192 Total MPHEN 17771 19514 11393 10027 5913 7244 3-EPHEN 298 657 408 348 229 156 3,6-DMPHEN 2685 3639 1862 1768 1416 1068 2,6-DMPHEN 4894 6466 2969 2762 2574 1885 2,7-DMPHEN 2961 4129 1852 1662 1591 1127 1-EPHEN 1427 1953 1034 657 671 429 1,6-+2,9- DIMPHEN 8337 11238 5189 4344 4565 2851 1,7-DMPHEN 5167 7289 3210 2627 3077 1912 2,3-DMPHEN 2431 3507 1501 1404 1489 990 1,9-DMPHEN 2518 3534 1553 1398 1571 1011 1,8-DMPHEN 609 1195 627 545 540 329 Total DMPHEN 31327 43608 20204 17515 17723 11757 EMP 352 548 195 287 167 157 1,3,10-TMPHEN 790 1087 392 467 335 295 1,3,6-TMPHEN 399 631 232 471 188 8 1,3,9-TMPHEN 1133 1756 422 831 412 255 2,6,10-TMP 473 452 128 346 109 82 2,3,6- +2,6,9- TMPHEN 5442 6871 3041 3052 2475 1741 2,6,10- + 1,2,9- TMPHEN 7109 8994 3846 3706 3291 2399 2,7,9- + 1,3,8- TMPHEN 2617 3348 1607 1559 1324 769 1,3,7-TMPHEN 3829 4919 2177 2164 1863 1451 1,7,9-TMP 811 1050 374 508 320 333 2,7,9-TMPHEN 1026 1327 491 591 472 450 2,3,7-TMPHEN 1303 1240 591 781 506 232 2,3,10-TMP 1367 1099 856 865 732 594 1,2,8-TMPHEN 421 468 512 243 361 150 1,2,7-TMPHEN 953 1074 190 562 162 367 Total TMPHEN 28025 34863 15053 16432 12718 9283 Total TEMPHEN 4661 2610 5449 3077 2471 1974 Total PHENs 85601 102741 54058 49277 39409 31696 93 Table 5.8 Relative distribution of phenanthrene homologues in decant oils DO91 DO93 DO15 DO24 DOPSU DOUP PHEN 4.5% 2.1% 3.6% 4.5% 2.7% 3.0% MPHEN 20.8% 19.0% 21.1% 20.3% 15.0% 22.9% DMPHEN 36.6% 42.4% 37.4% 35.5% 45.0% 37.1% TMPHEN 32.7% 33.9% 27.8% 33.3% 32.3% 29.3% TEMPHEN 5.4% 2.5% 10.1% 6.2% 5.0% 7.8% Total PHENs 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% The isomer distribution in alkylphenanthrenes (MPHEN, DMPHEN and TMPHEN) calculated from the data in Table 5.7 is listed in Table 5.9. Almost identical distribution pattern of DMPHEN and TMPHEN isomers was observed among all the decant oil samples. Four DMPHEN isomers ( 2,6-, 1,6-, 2,9-, and 1,7-DMPHEN) and five TMPHEN isomers ( 1,3,7-,2,3,6- ,2,6,9-, 2,6,10- and 1,2,9- TMPHEN) account for about 80% and 60% of DMPHEN and TMPHEN respectively. As observed for naphthalenes, the distribution of the methylated phenanthrenes appears to be controlled by chemical equilibrium. 3- and 2-MPHEN are the predominant MPHENs; they take up over 2/3 of MPHENs in decant oil samples. However, a noticeable variation was found in the MPHEN isomer distributions, particularly between DO93 and DO91. The combined percentage of 9- and 1-MPHEN in DO93 is 15% more than in DO91. 94 Table 5.9 Alkylphenanthrene distribution in decant oils Weight percent, wt% Compound DO91 DO93 DO15 DO24 DOPSU DOUP 3-MPHEN 28% 25% 28% 31% 30% 30% 2-MPHEN 39% 36% 37% 35% 37% 35% 9-MPHEN 19% 21% 20% 19% 17% 18% 1-MPHNE 14% 17% 14% 15% 16% 16% Total MPHEN 100% 100% 100% 100% 100% 100% 3,6-DMPHEN 9% 9% 10% 11% 8% 10% 2,6-DMPHEN 17% 16% 16% 17% 15% 17% 2,7-DMPHEN 10% 10% 10% 10% 9% 10% 1,6-+2,9-DIMPHEN 28% 27% 28% 26% 27% 26% 1,7-DMPHEN 17% 18% 17% 16% 18% 17% 2,3-DMPHEN 8% 9% 8% 9% 9% 9% 1,9-DMPHEN 9% 9% 8% 8% 9% 9% 1,8-DMPHEN 2% 3% 3% 3% 3% 3% Total DMPHEN 100% 100% 100% 100% 100% 100% 1,3,10-TMPHEN 3% 3% 3% 3% 3% 3% 1,3,6-TMPHEN 1% 2% 2% 3% 1% 0% 1,3,9-TMPHEN 4% 5% 3% 5% 3% 3% 2,6,10-TMP 2% 1% 1% 2% 1% 1% 2,3,6- +2,6,9- TMPHEN 20% 20% 20% 19% 20% 19% 2,6,10- + 1,2,9- TMPHEN 26% 26% 26% 23% 26% 26% 2,7,9- + 1,3,8- TMPHEN 9% 10% 11% 10% 11% 8% 1,3,7-TMPHEN 14% 14% 15% 13% 15% 16% 1,7,9-TMP 3% 3% 3% 3% 3% 4% 2,7,9-TMPHEN 4% 4% 3% 4% 4% 5% 2,3,7-TMPHEN 5% 4% 4% 5% 4% 3% 2,3,10-TMP 5% 3% 6% 5% 6% 7% 1,2,8-TMPHEN 2% 1% 3% 2% 3% 2% 1,2,7-TMPHEN 3% 3% 1% 3% 1% 4% Total TMPHEN 100% 100% 100% 100% 100% 100% Three-ring sulfur-containing aromatic compounds (alkylated DBTs) elute before phenanthrenes on the GC column. Four-ring sulfur compounds benzonaphthothiophenes (BNTs) elute at a little longer retention times. For simplicity, the composition analyses of both DBT and BNT in decant oil samples are presented in this section. 95 These polyaromatic sulfur-containing hydrocarbons (PASH) were identified by either co-injecting with authentic compounds or by comparing with reference data (Kabe, Ishiharam et al., 1992; Ma, Sakanishi et al., 1997; Budzinski, Raymond et al., 1998). The parent PASH structure and retention time of alkyl-substituted homologues are shown in Figure 5.12 and Figure 5.13. The concentrations of GC-measurable PASHs are given in Table 5.10. As the data show, a high PASH content was found in DO93, followed by DO91, DO15 and DO24; trace amounts of the same PASH presents in DOPSU and DOUP. This order of PASH contents, in general, agreed with the results of total elemental sulfur analysis (S: 4.9%, 2.7%, 1.5%, 0.5%, 0.4% and 0.3% in above decant oil samples). In low-sulfur decant oils such as DO91, DO15 and DO24, the most abundant PASHs are multi-methyl PASH homologues with a similar relative distribution, while in high-sulfur DO93, heavy PASHs (BNTs) are the primary sulfur compounds. The PASH’s isomer distribution in decant oil samples is shown in Table 5.11. 4- MDBT was found to be the primary isomer of methyl DBT. The isomers of DMDBT and TMDBT appear to be evenly distributed. There are three benzonaphthothiophene (BNT) isomers (see Figure 5.13). BNT- 2,1d was found to be most dominant isomer (over 60% of BNTs), the structural configuration of BNT-2,1-d tends to partially shield the sulfur atom and make this isomer more stable. Because of the lack of pure alkylated BNT standards and the lack of information from literature, it is not possible to uniquely identify the isomers of methyl- and dimethyl-BNTs. However, considering the special structural stability of BNT-2,1-d, C1- and C2- BNT-2,1-d should be expected to be the most abundant isomers. 96 The PASHs quantified by GC/MS represent about 40%, 54%, 35% and 37% of total elemental sulfurs in DO91, DO93, DO15 and DO24 respectively. In very low sulfur decant oils (DOPSU and DOUP), about 15% of total sulfur was detected by GC/MS. Figure 5.12 Multi-ion chromatograms representing alkylated dibenzothiophenes (DBTs) in decant oil samples 97 Figure 5.13 Multi-ion chromatograms representing alkylated benzonaphthothiophenes (BNT) in decant oil samples 98 Table 5.10 Polycyclic aromatic sulfur compounds concentrations in decant oils (compound identifications are shown in Figure 5.12 and Figure 5.13) Concentration, ppm. in decant oil Compound DO91 DO93 DO15 DO24 DOPSU DOUP DBT 1048 1979 248 350 0 0 MDBT 4913 9725 3942 1787 0 0 4-MDBT 1969 4159 1569 776 0 0 2- and 3-MDBT 2261 5190 1914 879 0 0 1-MDBT 683 376 459 133 0 0 MDBT 4913 9725 3942 1787 0 0 1EDBT 264 796 288 91 0 4,6-DMDBT 1734 4159 1254 676 621 622 2,6-DMDBT 2021 5065 1246 674 398 588 3,6-DMDBT 2145 5552 1478 760 288 0 3,7- +2,8-DMDBT 2384 6219 1497 802 0 0 3,8-DMDBT 674 2261 624 137 0 0 1,4- +1,6-DMDBT 887 2022 556 239 0 0 1,3- +3,4-DMDBT 2317 5280 1401 704 0 0 1,9- +2,4-DMDBT 2946 5320 1703 639 0 0 1,2- + 2,3-DMDBT 1722 2188 763 244 0 0 DMDBT 17093 38862 10810 4968 1308 1210 2,3,7-TMDBT 3096 6384 1158 1390 231 488 2,3,5-TMDBT 1953 4804 879 613 0 0 2,3,6-DMDBT 2591 5451 1166 812 0 0 TMDBT, unknown 1 1252 2200 605 432 0 0 TMDBT unknown 2 1273 1631 405 390 0 0 2,4,6- TMDBT 2314 5183 832 614 0 0 TMDBT, unknown 3 1765 2216 200 477 0 0 1,4,6-TMDBT 3549 6769 1381 1284 0 96 TMDBT 17793 34637 6527 6013 291 583 Total DBT 40848 85204 21527 13118 1538 1794 benzo[b]naphtho[1,2-d]thiophene 1197 1287 219 104 30 benzo[b]naphtho[2,1-d]thiophene 5751 8326 1152 767 604 136 benzo[b]naphtho[2,3-d]thiophene 2481 2952 345 166 107 80 MBNT 11896 30829 5363 1656 1296 1190 DMBNT 14367 67130 8165 7738 0 0 Total BNT 35691 110524 15244 10328 2112 1354 Total PASH 76539 195728 36771 23446 3650 3148 99 Table 5.11 Relative distribution of DBT and BNT isomers in decant oils Percentage of isomers DOPSU Compound DO91 DO93 DO15 DO24 DOUP MDBT 100% 100% 100% 100% 4-MDBT 40% 43% 40% 43% NA 2- and 3-MDBT 46% 53% 49% 49% NA 1-MDBT 14% 4% 12% 7% NA DMDBT 100% 100% 100% 100% EDBT 2% 2% 3% 2% NA 4,6-DMDBT 10% 11% 12% 14% NA 2,6-DMDBT 12% 13% 12% 14% NA 3,6-DMDBT 13% 14% 14% 15% NA 3,7- +2,8-DMDBT 14% 16% 14% 16% NA 3,8-DMDBT 4% 6% 6% 3% NA 1,4- +1,6-DMDBT 5% 5% 5% 5% NA 1,3- +3,4-DMDBT 14% 14% 13% 14% NA 1,9- +2,4-DMDBT 17% 14% 16% 13% NA 1,2- + 2,3-DMDBT 10% 6% 7% 5% NA TMDBT 100% 100% 100% 100% 2,3,7-TMDBT 17% 18% 18% 23% NA 2,3,5-TMDBT 11% 14% 13% 10% NA 2,3,6-DMDBT 15% 16% 18% 13% NA TMDBT, unknown 7% 6% 9% 7% NA TMDBT unknown 7% 5% 6% 6% NA 2,4,6- TMDBT 13% 15% 13% 10% NA TMDBT, unknown 10% 6% 3% 8% NA 1,4,6-TMDBT 20% 20% 21% 21% NA BNT 100% 100% 100% 100% BNT-1,2d 13% 10% 13% 0% NA BNT-2,1d 61% 66% 67% 82% NA BNT-2,3d 26% 23% 20% 18% NA 100 5.1.3 Four-ring polycyclic aromatic hydrocarbons in decant oils Figure 5.14 illustrates the TIC that represents the elution of four-ring aromatic compounds on GC/MS. Figure 5.14 Total ion chromatogram corresponding to four-ring aromatics in DO91 The major four-ring PAHs with measurable quantity in decant oil consist of one peri-condensed, pyrene (PY), two cata-condensed, chrysene (CHRY) and benzoanthracene, and three isomers of benzofluorene. The structures of these PAHs are shown in Figure 5.15. 101 10 1 2 3 9 2 1 4 12 3 5 8 11 6 7 4 10 7 6 5 9 8 Pyrene chrysene C16H10 C18H12 Mol. Wt.: 202 Mol. Wt.: 228 11 10 1 9 2 8 3 7 6 5 4 benzoanthracene 11H-Benzo[b]fluorene C18H12 C17H12 Mol. Wt.: 228 Mol. Wt.: 216 7 8 6 9 1 2 5 11 10 3 10 9 11 4 4 1 8 7 5 6 2 3 11H-Benzo[a]fluorene 7H-Benzo[c]fluorene C17H12 C17H12 Mol. Wt.: 216 Mol. Wt.: 216 Figure 5.15 Structures of four-ring aromatics detected in decant oil 102 The multi-ion chromatograms of above four-ring PAH homologues are shown in Figure 5.16 and Figure 5.17 respectively. As can be seen on the MIC of m/e 216, three methylpyrene (MPY) isomers were very well resolved as three high-intensity peaks. They are identified as 2-MPY, 4-MPY, and 1-MPY respectively. Eight well-defined peaks on the MIC of m/e 230 represent the isomers of dimethylpyrene (DMPY), but the position of methyl substitution could not be determined. Six relatively intense peaks on MIC of m/e 244 are the isomers of trimethylpyrene (TMPY), again, with unknown substitution. Also eluted are several much less intense and poorly defined peaks prior to that of MPY, DMPY and TMPY. The three peaks (ahead of MPY) are identified as the isomers of benzofluorene (7H-benzo(c)fluorene, 11H-benzo(a)fluorene and 11H-benzo(b)fluorene respectively). The rest of the peaks represent the methyl- and dimethyl-benzo(a)fluorene isomers with unknown substitution positions. Chrysene and benzo(a)anthracene were well separated and positively identified on MIC of m/e 228 (Figure 5.17). Out of the eight peaks in the MIC of m/e 242, six peaks are the isomers of MCHRY and the remaining two peaks (ahead of MCHRY) are the isomers of methylbenzo(a)anthracene. Four of the six methylchrysenes (MCHRY) peaks were positively identified as 3-,2-,6- and 1- MCHRY. About 13 poorly resolved DMCHRY isomers appeared in MIC of m/e 256, and the position of the methyl group in these isomers could not be determined, either. The quantitative GC/MS analysis results for four-ring PAHs in decant oil samples are presented in Table 5.12. Since the presence of fluorenes and benzoanthrenes in decant oil samples (as shown in Figure 5.16 and Figure 5.17) is not significant, only pyrenes and chrysenes are listed in this table. 103 Figure 5.16 Multi-ion chromatograms representing alkylated pyrenes in DO91 104 Figure 5.17 Multi-ion chromatograms representing alkylated benzo(a)anthracene and chrysenes in DO91 105 Table 5.12. Distribution of pyrene and chrysene and their alkylated derivatives in decant oils (compound identifications are shown in Figure 5.16 and Figure 5.17) Concentration, ppm. in decant oil Compound DO91 DO93 DO15 DO24 DOPSU DOUP PY 4603 1893 1451 4505 3070 2927 2-MPY 6289 2371 2121 6249 3445 4791 4-MPY 7059 2542 2643 4581 3173 5057 1-MPY 5237 2179 1840 4708 2885 4316 Total MPY 18564 7092 6604 15538 9504 14164 DMPY 42879 12780 13870 27210 14016 29647 TMPY 39696 11538 13281 23035 10416 22143 Total PY 105742 33303 35205 70288 37005 68880 CHRY 3815 2822 1244 2941 835 955 3-MCHRY 8135 5601 2735 5274 1481 2874 2-MCHRY 3971 2042 1481 2608 654 1352 6-MCHRY 1536 730 300 1213 152 518 1-MCHRY 757 561 317 1361 263 195 MCHRY 14399 8934 4831 10457 2549 4940 DMCHRY 5307 15458 5902 4831 3677 7312 Total CHRY 23521 27214 11977 18021 7060 13207 The total pyrene concentration varies substantially among the decant oil samples, and chrysenes were less abundant than pyrenes in all decant oil samples. There seems to be no direct relationship between the semi-coke’s OTI and the individual concentrations of pyrene and chrysene compounds in decant oil samples. For example, DO93 and DOPSU have almost same content of total pyrenes, but they produced semi-coke with very different degrees of anisotropy. But the relative concentration of chrysene (to pyrene), seems to connect with the OTIs of the semi-cokes. For example, DOUP and DOPSU that produced high OTI semi-cokes contain least amount of chrysenes (less than 1/5 of pyrenes) and DO93 (lowest OTI semi-coke) has the highest chrysenes content (more than 4/5 of pyrenes). 106 The distribution of pyrene, chrysene and their homologues in decant oils is shown in Table 5.13. Pyrenes exhibit a consistent distribution profile: Unsubstituted pyrene constitutes less than 8% of total pyrenes; DMPY and TMPY are the most abundant homologues of pyrenes. Table 5.13 Pyrene and chrysene and their alkylates distribution in decant oils DO91 DO93 DO15 DO24 DOPSU DO-UP PY 4% 6% 4% 6% 8% 4% MPY 18% 21% 19% 22% 26% 21% DMPY 41% 38% 39% 39% 38% 43% TMPY 38% 35% 38% 33% 28% 32% CHRY 16% 10% 10% 16% 12% 7% MCHRY 61% 33% 40% 58% 36% 37% DMCHRY 23% 57% 49% 27% 52% 55% The relative methyl isomer distribution of pyrene and chrysene are given in Table 5.14. All three MPYs isomers were found to be nearly evenly distributed; while 3- and 2- MCHRYs were the dominant isomers which accounted for over 80% of MCHRYs in decant oil samples. Table 5.14 Methylpyrene and methylchrysene isomer relative distribution in decant oil DO91 DO93 DO15 DO24 DOPSU DO-UP 2-MPY 33.8% 33.0% 32.1% 40.2% 36.3% 33.8% 4-MPY 38.0% 36.0% 40.0% 29.5% 33.4% 35.7% 1-MPY 28.2% 31.0% 27.9% 30.3% 30.4% 30.5% 3-MCHRY 56.5% 62.7% 56.6% 63.0% 58.1% 58.2% 2-MCHRY 27.6% 22.9% 30.6% 23.0% 25.6% 27.4% 6-MCHRY 10.7% 8.2% 6.2% 8.0% 6.0% 10.5% 1-MCHRY 5.3% 6.3% 6.6% 6.0% 10.3% 4.0% 107 5.1.4 Normal alkanes in decant oil In GC/MS analysis, normal alkanes show characteristic fragmentation ions of m/e 43 and 57. A multi-ion chromatogram representing normal alkanes in DO91 sample is shown in Figure 5.18. Detectable normal alkanes in decant oils range from C12 to C30. Table 5.15 gives the concentration of individual n-alkanes and total alkane distribution among the decant oils. Figure 5.18 Selected ion chromatograms (m/e=47) representing normal alkanes in DO 91 108 Normal alkane distribution by carbon numbers is shown in Figure 5.19. The most abundant normal alkanes are those with carbon number of C24 and C28 for most decant oils. The distribution of normal alkanes in DO24 showed a larger shift to higher normal alkanes, which suggests that, in this decant oil sample, there exists more abundant longer- chain normal alkanes that are not volatile for GC detection. About 11% normal alkanes are present in DOUP, followed by DOPSU and DO91 (7%), DO93 and DO24 (2%) and DO15 (1%). The total normal alkanes quantified may account for a portion of total aliphatic compounds because of the presence of an unresolved complex mixture (UCM) as can be seen from the MIC in Figure 5.18. In crude oils this UCM usually takes up 84% of total aliphatic fraction (Gough and Rowland, 1990). Partial oxidative degradation of the UCM showed that a large variety of branched and alicyclic compounds are present in this portion. Decant oils, which are derived from distillation and catalytic cracking process, contain much less UCM than in crude oil. The area ratio of UCM to total identified normal alkane on selected ion chromatograms is about 1:0.3 to 1:0.4 for the decant oil samples studied. In general, n-alkanes can be readily cracked in carbonization environment to form free radicals. These radicals could serve as a hydrogen source in early stage of carbonization. Moderately high content of alkanes in decant oils was found to produce highly anisotropic textures semi-coke in a previous study (Filley, 1997). 109 20000 18000 DO91 DO93 DO15 DO24 16000 DO_PSU DO_UP 14000 12000 10000 Con, ppm 8000 6000 4000 2000 0 C14 C15 C16 C17 C18 C19 C20 C21 C22 C23 C24 C25 C26 C27 C28 C29 C30 Figure 5.19 Distribution of normal alkanes in decant oils Table 5.15. Normal alkane distribution in decant oils Concentration, ppm. in decant oil Compound DO91 DO93 DO15 DO24 DOPSU DOUP C12H26 409 0 91 98 264 482 C13H28 527 0 133 120 309 776 C14H30 847 196 200 148 419 204 C15H32 969 275 242 177 568 1500 C16H34 1099 346 271 229 624 575 C17H36 1385 444 332 260 811 2154 C18H38 1665 455 381 311 985 2353 C19H40 2139 566 478 399 1392 2612 C20H42 2768 777 613 467 2000 2753 C21H44 4017 1324 828 557 3255 3295 C22H46 6121 3044 1105 772 5087 4287 C23H48 8146 5117 1204 1088 7184 6789 C24H50 9102 5465 1131 1245 9168 10210 C25H52 8558 4480 1062 1499 9723 15201 C26H54 7286 2955 864 1675 8389 17358 C27H56 5188 1377 673 2098 6922 15679 C28H58 4117 1068 0 2693 4909 12753 C29H60 0 109 0 2815 3262 8964 C30H62 0 0 0 2414 264 6908 Total n-alkanes 64344 27997 9607 22009 65537 114852 110 5.1.5 PAHs in decant oil derivatives The derivatives (CF, HYD, and VTB) from two decant oil samples (DO15 and DO24) that produced semi-coke with different degrees of anisotropy were analyzed using the same procedures described in previous sections. The processes from which these streams are derived, shown in Figure 4.1, are reproduced here for reference. The molecular composition data of these derivatives were summarized together with those of the corresponding decant oils for comparison purposes. Figure 4.1 A pretreatment scheme used to reduce the sulfur content of the feed to the heater of a delayed coking unit to produce needle coke 111 5.1.5.1 Two-ring aromatics in decant oil derivatives Figure 5.20 shows the concentrations of naphthalene homologues in decant oil derivatives. Coker feeds (CF) contain less naphthalene compounds than their virgin decant oils. Total naphthalene concentrations in CF15 and CF24 are about 1/3 and 1/5 of their parent DOs respectively. 5000 4500 4000 3500 NAPH 3000 MN DMN 2500 TMN 2000 Concentration, ppm of feed 1500 1000 500 0 DO15 VTB15 CF15 HYD15 DO24 VTB24 CF24 HYD24 Figure 5.20 Naphthalene distribution in decant oil derivatives The difference between DO and CF is not only seen in the absolute concentrations, but also in the composition of naphthalenes. The most obvious change is the increased unsubstituted naphthalene and the decreased trimethylnapthalenes concentrations in CF derivatives. The extent of naphthalene composition change in CF24 is more drastic than in CF15 (Table 5.16). 112 Table 5.16 Naphthalene composition in decant oil and coker feed DO15 CF15 DO24 CF24 NAPH 6% 9% 5% 12% MN 24% 25% 21% 32% DMN 39% 39% 39% 39% TMN 31% 25% 34% 18% The vacuum tower bottom or VTB, contains the least amount of naphthalenes as shown in Figure 5.20. This should be expected from the operation of vacuum distillation, where the low molecular weight aromatic compounds are stripped off by the difference in the boiling point of PAHs. In hydrotreated decant oil (HYD), naphthalenes were substantially reduced. Alkylated naphthalene homologues are the only detectable compounds (Table 5.17). The higher proportions of alkylnaphthalenes indicates that severe hydrogenation (of naphthalene) and hydrocracking (of large aromatics) occurred in this stream. Comparing this feature of naphthalene concentration and composition in HYD with that in coker feeds, it can be inferred that the lower concentration of naphthalene in CF is due to blending the lower naphthalene content HYD and the increased proportion of naphthalenes with fewer side-chains in CF is attributed to the recycling of the delayed coker liquid products into the coker feed. 113 Table 5.17 Naphthalene concentration and distribution in HYD Concentration, ppm in feed Distribution, wt% DO15 HYD15 DO24 HYD24 DO15 HYD15 DO24 HYD24 NAPH 693 trace 307trace 6% 0% 5% 0% MN 2884 209 129192 24% 13% 21% 18% DMN 4744 490 2379205 39% 30% 39% 40% TMN 3775 935 2072214 31% 57% 34% 42% Total NAPH 12096 1634 6049 510 100% 100% 100% 100% The products of the hydrogenation of naphthalene, tetralin (1,2,3,4- tetrahydonaphthalene) and decalin (decahydronaphthalene) are detected in the HYD. Decalin was only in trace amount, while tetralin and its alkylated derivatives were found in great quantities. C1- to C3 alkyltetralins are the most abundant forms of tetralin in HYDs. The identification of substitution position on tetralin was not performed. Therefore, only the total concentrations of these hydronaphthalenes are reported here. Figure 5.21 compares the distribution of tetralin and naphthalenes in HYD15 and HYD24. As the figure shows, the concentration of tetralin and alkyltetralin exceeds their corresponding naphthalene homologues. The relative abundance of tetralins to naphthalenes is different between the two HYD derivatives. The higher proportion of alkyl tetralins (TRLN) in HYD24 indicates that this feedstock was derived from a more severe hydrogenation. 114 1800 1600 Tetralin Naphthalene 1400 1200 HYD15 1000 800 600 concentration, ng/mg concentration, 400 200 0 TRLN (NAPH) C1-TRLN (MN) C2-TRLN (TMN) C3-TRLN (DMN) 700 600 Tetralin Naphthalene 500 400 HYD24 300 200 100 concentration, ppm in feedstock concentration, 0 TRLN (NAPH) C1-TRLN (MN) C2-TRLN (DMN) C3-TRLN (TMN) Figure 5.21 Distribution of tetralin and naphthalene in HYD derivatives 115 Biphenyl and alkylbiphenyl concentrations in CF, HYD and VTB are tabulated in Table 5.18. All derivatives except HYD contain fewer biphenyls than their parent decant oils. Lowest biphenyls were found in VTB feeds, and biphenyls were proportionally reduced. Only 10% of such compounds retained in VTB 15 and almost no biphenyls existed in VTB24. CF stream contains about 30-50% biphenyls in DOs, and the unsubstituted biphenyl accounts for about 10% in total biphenyl aromatics in the CF feed. Table 5.18 Biphenyls distribution in decant oil derivatives (unit: ppm in feed) DO15 VTB15 CF15 HYD15 DO24 VTB24 CF24 HYD24 BP trace trace 112 235 trace trace 143 406 MBP 1070 110 207 2097 600 39 294 811 DMBP 2498 218 974 4877 1579 34 975 2029 Total BP 3568 327 1293 7209 2179 73 1412 3246 HYD stream has nearly two-fold higher biphenyl content than its parent DO. The increase in biphenyl concentration is due to the hydrodesulfurization of DBT compounds with main products as biphenyls, and from severe hydrogenation/hydrocracking of phenanthrenes (Qian, Yoda et al., 1999). More detailed analysis results are listed in Table 5.18 and Table 5.19 for naphthalene and biphenyl compounds respectively. 116 Table 5.19 Concentrations of naphthalene and alkylated naphthalenes in decant oil and its derivatives, ppm in feeds Compound DO15 VTB15 CF15 HYD15 DO24 VTB24 CF24 HYD24 NAPH 693 18 339 0 307 0 158 0 2-MN 1891 88 730 143 847 59 336 67 1-MN 993 45 209 66 444 29 84 25 MN 2884 133 939 209 1291 88 421 92 2-EN 280 13 141 0 140 0 38 0 2,6+2,7-DMN 1409 92 568 202 705 63 242 84 1,3-DMN 1382 89 332 182 668 68 122 76 1,6-DMN 760 52 194 106 392 33 0 45 1,4-DMN 468 31 169 0 255 0 0 0 1,5-DMN 163 9 23 0 83 0 68 0 1,2-DMN 282 16 59 0 135 0 51 0 DMN 4744 302 1486 490 2379 164 520 205 MEN 438 38 197 124 250 0 0 0 1,37-TMN 628 51 127 156 346 26 78 54 1,3,6-TMN 761 66 183 179 410 39 8 71 1,3,5-+1,4,6-TMN 533 45 74 145 296 24 72 45 2,3,6-TMN 577 56 202 124 327 26 76 44 1,2,7- +1,6,7-TMN 561 50 118 148 297 0 0 0 1,2,6-TMN 277 21 35 58 145 0 0 TMN 3775 327 936 935 2072 115 234 214 Total NAPH 12096 780 3699 1634 6049 366 1333 510 117 Table 5.20 Concentrations of biphenyl and alkylated biphenyls in decant oil and its derivatives, ppm in feeds Compound DO15 VTB15 CF15 HYD15 DO24 VTB24 CF24 HYD24 BP 112 235 142 406 2-methyl-1,1'-biphenyl 85 18 0 167 97 0 0 45 3-methyl-1,1'-biphenyl 658 60 153 1349 312 39 216 542 4-methyl-1,1'-biphenyl 327 32 54 581 191 0 78 224 MBP 1070 110 207 2097 600 39 294 811 2,3’-dimethylbiphenyl 0 0 118 794 56 0 124 338 2,5-dimethylbiphenyl 0 19 283 1327 126 0 260 589 2,4-dimethylbiphenyl 192 20 314 1521 133 0 278 630 3-methyldiphenylmethane 224 0 57 314 82 0 100 0 2- and 4-methyldiphenylmethane 35 0 138 594 116 0 212 268 3,5-dimethylphenyl 621 67 64 327 309 34 0 205 4-ethylphenyl 224 16 0 0 136 0 0 0 3,3’-dimethylphenyl 312 28 0 0 155 0 0 0 3,4’-dimethylphenyl 228 11 0 0 119 0 0 0 4,4’-dimethylphenyl 266 26 0 0 138 0 0 0 3,4-dimethylphenyl 397 30 0 0 211 0 0 0 DMBP 2498 218 974 4877 1579 34 975 406 Total Biphenyls 3568 327 1293 7209 2179 73 1412 3246 Listed in Table 5.21 are the overall two-ring aromatic compound concentrations in decant oil derivatives. VTB is excluded from the table because of the insignificant amount of two-ring aromatics in this sample. Compared with their source decant oils, the total two-ring PAHs are greatly reduced in CF (1/3 of DO) and HYD feeds (4/5 of DO). 118 Table 5.21 Two-ring PAH concentrations in decant oil derivatives, ppm in feeds DO15 CF15 HYD15 DO24 CF24 HYD24 Total Naphthalene 12096 3761 1634 6049 1333 510 Total BP 3568 1293 7209 2179 1412 3246 Total Tetralin trace trace 3449 trace trace 1895 Total Two-ring PAH 15664 5054 12292 8229 2745 5650 It is interesting to notice that although HYD contains 30% tetralins in the two-ring aromatic compounds, CF does not show any significant amount of such hydrogenated compounds. As can be seen in Figure 4.1, when the blend of HYD and VTB enter into the lower part of combination tower, the light-end components are fractionated into light streams without entering into coker feed line. The absence of tetralin in CF indicates that tetralin is distilled into light product streams in the combination tower. Listed below are the normal boiling points of these compounds (Source data: ChemOffice 2002, CambridgeSoft Corp.): Compound Normal boiling point, ˚C Naphthalene 218 Methylnaphthalene 246 Tetralin 207 Methyltetralin 229 Biphenyl 255 Methylbiphenyl 282 The distribution of two-ring PAH isomers among decant oil derivatives is listed in Table 5.22 and Table 5.23. 119 VTBs show an identical multi-methyl-naphthalene isomer distribution pattern to that of their parent decant oils. However, the CFs are significantly different from decant oils; there is an increase in thermally stable isomers. For example, 2-MN increased from 67% to about 78-80%, 2,6 and 2,7-DMN increased from 30% to 38%. Such a composition shift is also observed in HYDs, but the shift is different between HYD15 and HYD24. This variation in CF’s isomer distribution further suggests that the hydrotreatment conditions of DO15 and DO24 were different. CF and HYD bear closer distribution patterns in MBP: Over 92% of methylbiphenyls are in 3- and 4- MBP compared with 80% of the same MBP in DO. Their DMBP isomers distribution is even more different from that of decant oil samples, only 6 DMBPs were detected in measurable amounts in CF and HDY compared to 10 DMBPs in decant oil. Furthermore, the positions of dimethyl substituent groups on the most abundant biphenyl isomers in both CF and HYD correspond to those of stable dimethyldibenzothiophenes (DBT) in DO. This similar distribution of biphenyls in CF and HYD further shows that the biphenyl in CF comes mainly from HYD stream. To a lesser extent, the disappearance of a certain dimethylbiphenyl isomer in CF stream suggests that most of the biphenyls react readily in the carbonizing matrix to form condensed PAH molecules. 120 Table 5.22 Isomeric distribution in alkylnaphthalenes in decant oil derivatives, wt% DO15 VTB15 CF15 HYD15 DO24 VTB24 CF24 HYD24 2-MN 66% 66%78% 68% 66% 67% 80% 73% 1-MN 34% 34%22% 32% 34% 33% 20% 27% 2-EN 6% 4%9% 0% 6% 0% 7% 0% 2,6+2,7-DMN 30% 30%38% 41% 30% 38% 47% 41% 1,3-DMN 29% 30%22% 37% 28% 41% 23% 37% 1,6-DMN 16% 17%13% 22% 16% 20% 0% 22% 1,4-DMN 10% 10%11% 0% 11% 0% 0% 0% 1,5-DMN 3% 3%2% 0% 3% 0% 13% 0% 1,2-DMN 6% 5%4% 0% 6% 0% 10% 0% MEN 12% 12%21% 13% 12% 0% 0% 0% 1,37-TMN 17% 16%14% 17% 17% 23% 33% 25% 1,3,6-TMN 20% 20%20% 19% 20% 34% 4% 33% 1,3,5-+1,4,6-TMN 14% 14% 8% 16% 14% 21% 31% 21% 2,3,6-TMN 15% 17%22% 13% 16% 23% 32% 21% 1,2,7- +1,6,7-TMN 15% 15% 13% 16% 14% 0% 0% 0% 1,2,6-TMN 7% 7%4% 6% 7% 0% 0% 0% Table 5.23 Isomeric distribution of biphenyl compounds in decant oil derivatives, wt% DO15 CF15 HYD15 DO24 CF24 HYD24 2-methyl-1,1'-biphenyl 8% 0% 8% 16% 0% 6% 3-methyl-1,1'-biphenyl 62% 74% 64% 52% 74% 67% 4-methyl-1,1'-biphenyl 31% 26% 28% 32% 26% 28% MBP 100% 100% 100% 100% 100% 100% 2,3’-dimethylbiphenyl 0% 12% 16% 4% 13% 17% 2,5-dimethylbiphenyl 0% 29% 27% 8% 27% 29% 2,4-dimethylbiphenyl 8% 32% 31% 8% 29% 31% 3-methyldiphenylmethane 9% 6% 6% 5% 10% 0% 2- and 4-methyldiphenylmethane 1% 14% 12% 7% 22% 13% 3,5-dimethylphenyl 25% 7% 7% 20% 0% 10% 4-ethylphenyl 9% 0% 0% 9% 0% 0% 3,3’-dimethylphenyl 12% 0% 0% 10% 0% 0% 3,4’-dimethylphenyl 9% 0% 0% 8% 0% 0% 4,4’-dimethylphenyl 11% 0% 0% 9% 0% 0% 3,4-dimethylphenyl 16% 0% 0% 13% 0% 0% DMBP 100% 100% 100% 100% 100% 100% 121 5.1.5.2 Three-ring aromatics in decant oil derivatives Figure 5.22 compares the concentrations of phenanthrene and alkylphenanthrenes in the derivatives of DO15 and DO24. 35000 PHEN 30000 MPHEN DMPHEN 25000 TMPHEN TEMPHEN 20000 15000 10000 concentration, ppm in feed ppm concentration, 5000 0 DO15 VTB15 CF15 HYD15 DO24 VTB24 CF24 HYD24 Figure 5.22 Total concentration of phenanthrenes in decant oil derivatives Contrary to the change observed in naphthalene content, phenanthrenes contents increase in the both CF feeds. The relative increases in phenanthrenes to that of the decant oils are about 60% and 20% in CF15 and CF24 respectively. The relative distribution of phenanthrene homologues is listed in Table 5.24. Similar to the change in the naphthalene distribution, the constitution of phenanthrenes in CF shifts to that with less alkyl substitution. CF24 shows an even further shift than CF15, as can been seen 122 from the higher percentage of PHEN and MPHENs and lower percentage of TMPHEN and TMPHEN. Table 5.24 Relative phenanthrene and alkylphenanthrene distribution in decant oil derivatives DO15 CF15 DO24 CF24 PHEN 4% 12% 4% 13% MPHEN 21% 35% 21% 37% DMPHEN 37% 31% 35% 32% TMPHEN 28% 16% 33% 16% TEMPHEN 10% 6% 6% 2% Phenanthrene isomer distribution in CF also showed a significant change compared with the decant oil as showed in Table 5.25. Two methyl isomers (3- and 2- MPHEN) constitute 85% of total MPHENs in CF samples, while the same isomers account for about 65% in DO; the percentage of three DMPHEN isomers (3,6-, 2,6 and 2,7-DMPHEN) in CF is about twice as high as in DO. The TMPHEN isomeric distribution pattern also differs in CF and DO, but the change of TMPHEN isomer distribution seems to be less drastic than the MPHEN and DMTHEN. A proportionally lower concentration of phenanthrenes than their parent decant oils was found in VTBs, and the distribution of phenanthrenes isomers in VTBs is about the same as that of their parent decant oils. 123 Table 5.25 Phenanthrene isomer distribution in DO and CF, wt% DO15 CF15 DO24 CF24 3-MPHEN 28% 41% 31% 39% 2-MPHEN 37% 46% 35% 47% 9-MPHEN 20% 6% 19% 7% 1-MPHNE 14% 6% 15% 7% MPHEN 100% 100% 100% 100% 3,6-DMPHEN 9% 16% 10% 18% 2,6-DMPHEN 15% 28% 16% 30% 2,7-DMPHEN 9% 16% 9% 17% 1,6-+2,9-DIMPHEN 26% 15% 25% 14% 1,7-DMPHEN 16% 9% 15% 8% 2,3-DMPHEN 7% 6% 8% 5% 1,9-DMPHEN 8% 8% 8% 8% 1,8-DMPHEN 3% 1% 3% 1% DMPHEN 100% 100% 100% 100% 1,3,10-TMPHEN 3% 3% 3% 4% 1,3,6-TMPHEN 2% 4% 3% 4% 1,3,9-TMPHEN 3% 6% 5% 7% 2,6,10-TMP 1% 2% 2% 2% 2,3,6- +2,6,9-TMPHEN 20% 19% 19% 19% 2,6,10- + 1,2,9-TMPHEN 26% 22% 23% 23% 2,7,9- + 1,3,8-TMPHEN 11% 12% 9% 12% 1,3,7-TMPHEN 14% 15% 13% 15% 1,7,9-TMP 2% 3% 3% 3% 2,7,9-TMPHEN 3% 3% 4% 2% 2,3,7-TMPHEN 4% 3% 5% 1% 2,3,10-TMP 6% 3% 5% 3% 1,2,8-TMPHEN 3% 0% 1% 0% 1,2,7-TMPHEN 1% 2% 3% 3% TMPHEN 100% 100% 100% 100% The HYD feeds contain much less (50% of DO) phenanthrene compounds. The relative homologue distribution in HYD samples is given in Table 5.26. Both MPHEN and DMPHEN concentrations in HYD were lower, while TMPHEN was higher than those in DO. This distribution pattern seen in HYD feeds is opposite to that seen in CF feeds (relative to DO). 124 Table 5.26 Comparison of relative distribution of phenanthrene homologues in DO and HYD DO15 HYD15 DO24 HYD24 PHEN 4% 2% 4% 2% MPHEN 21% 16% 21% 14% DMPHEN 37% 32% 35% 31% TMPHEN 28% 41% 33% 45% TEMPHEN 10% 9% 6% 7% Phenanthrene isomer distribution in decant oil and HYD stream is shown in Table 5.27. MPHEN isomers in two HYD derivatives exhibit a similar trend to that in CF, the enrichment of 3- and 2-MPHEN but with lesser degree of distribution shift. DMPHEN isomer distribution, however, shows a different pattern from the same compounds in CF. For example, 1,6- +2,9-DMPHEN, 1,7-DMPHEN and 2,3-DMPHEN give opposite distribution trends in HYD and CF streams. Table 5.27. Phenanthrene isomer distribution in HYD and DO (unit: wt%) DO15 HYD15 DO24 HYD24 3-MPHEN 28% 33% 31% 32% 2-MPHEN 37% 40% 35% 38% 9-MPHEN 20% 14% 19% 15% 1-MPHEN 14% 14% 15% 15% Total MPHEN 100% 100% 100% 100% 3,6-DMPHEN 9% 12% 10% 11% 2,6-DMPHEN 15% 19% 16% 19% 2,7-DMPHEN 9% 12% 9% 12% 1,6-+2,9-DIMPHEN 26% 27% 25% 25% 1,7-DMPHEN 16% 14% 15% 15% 2,3-DMPHEN 7% 9% 8% 10% 1,9-DMPHEN 8% 7% 8% 8% 1,8-DMPHEN 3% 0% 3% 0% Total DMPHEN 100% 100% 100% 100% 125 The effect of hydrotreatment on the composition of phenanthrenes is also seen in the distribution of hydrophenanthrene. Figure 5.23 shows the GC/MS TIC in the retention time between 17 and 26 minutes when hydrophenanthrenes and phenanthrene elute from GC column. Two types of hydrophenanthrenes were identified from the GC/MS analysis, tetrahydrophenanthrene (H4PHEN) and octahydrophenanthrene (H8PHEN). Most of these hydrophenanthrenes are in forms of C1- and C2 substituted compounds. C3- and C4- hydrophenanthrenes are also detected in this stream but they are not present in high enough concentration for quantification. No effort was made to identify the isomers of these hydroaromatic compounds. The total concentration of hydrophenanthrenes in Table 5.28 is reported as a cumulative concentration of the possible isomers. (x1,000,000) 2.5 TIC HYD15 2.0 H4PHEN+ H8PHEN 1.5 1.0 0.5 0.0 17.5 20.0 22.5 25.0 (x1,000,000) TIC 2.0 DO15 1.5 1.0 0.5 0.0 17.5 20.0 22.5 25.0 Figure 5.23. TIC of hydrophenanthrene in HYD stream (Top TIC shows DO in the same window for comparison) 126 Table 5.28 Hydrophenanthrene and phenanthrene concentration in HYD derivatives (unit: ppm in feed) HYD15 HYD24 H4PHEN 538 551 H8PHEN 369 176 MH4PHEN 1829 1535 MH8PHEN 1369 1458 DMH4PHEN 1717 2910 DMH8PHEN 2084 2956 Total H-PHEN 7906 9035 PHEN 450 353 MPHEN 3920 2393 DMPHEN 8068 5959 TMPHEN 10279 8552 TEMPHEN 2155 1417 Total PHEN 24872 18675 The total concentrations of hydrophenanthrenes in HYD15 and HYD24 are about 1/3 and 1/2 of total unsaturated phenanthrenes, respectively. In parallel to the higher tetralin-to-naphthalene ratio in HYD24 than in HYD15 (in Table 5.20), the high hydrophenanthrene concentration offered further evidence that the HYD24 was derived from a more severe hydrotreating process than HYD15. Detailed GC/MS analysis results for phenanthrene and alkylphenanthrenes in decant oil derivatives are reported in Table 5. 29. 127 Table 5.29. Phenanthrene concentrations in decant oil derivatives (unit: ppm in feed) DO15 VTB15 CF15 HYD15 DO24 VTB24 CF24 HYD24 PHEN 1958 539 10352 450 2225 362 7203 353 3-MPHEN 3245 806 11731 1299 3117 581 8008 769 2-MPHEN 4219 974 13141 1559 3535 684 9526 899 9-MPHEN 2309 400 1645 529 1859 365 1403 357 1-MPHNE 1619 321 1842 532 1517 290 1526 368 Total MPHEN 11393 2501 28358 3920 10027 1919 20463 2393 3-EPHEN 408 0 0 0 348 0 0 0 3,6-DMPHEN 1862 425 4250 984 1768 365 3195 669 2,6-DMPHEN 2969 706 7468 1572 2762 621 5386 1146 2,7-DMPHEN 1852 388 4186 946 1662 357 3084 701 1-EPHEN 1034 59 579 0 657 98 40 0 1,6-+2,9-DIMPHEN 5189 939 3967 2165 4344 1066 2462 1499 1,7-DMPHEN 3210 585 2321 1129 2627 656 1448 865 2,3-DMPHEN 1501 269 1479 703 1404 301 993 591 1,9-DMPHEN 1553 304 2000 569 1398 320 1432 487 1,8-DMPHEN 627 114 223 0 545 113 105 0 Total DMPHEN 20204 3790 26471 8068 17515 3934 18145 5959 EMP 195 113 447 224 287 0 282 225 1,3,10-TMPHEN 392 112 437 262 467 63 335 173 1,3,6-TMPHEN 232 177 564 378 471 23 315 164 1,3,9-TMPHEN 422 78 768 0 831 172 592 47 2,6,10-TMP 128 227 216 538 346 319 143 461 2,3,6- +2,6,9-TMPHEN 3041 444 2491 2101 3052 469 1686 1695 2,6,10- + 1,2,9-TMPHEN 3846 849 2908 2313 3706 963 2046 1935 2,7,9- + 1,3,8-TMPHEN 1607 297 1645 950 1559 384 1117 834 1,3,7-TMPHEN 2177 465 1968 1350 2164 537 1363 1133 1,7,9-TMP 374 108 424 491 508 99 288 486 2,7,9-TMPHEN 491 131 354 295 591 133 218 240 2,3,7-TMPHEN 591 156 377 577 781 144 127 242 2,3,10-TMP 856 131 440 354 865 223 229 302 1,2,8-TMPHEN 512 10 0 182 243 0 0 455 1,2,7-TMPHEN 190 88 276 263 562 93 294 160 Total TMPHEN 15053 3386 13315 10279 16432 3624 8946 8552 TEMPHEN 5449 624 4921 2155 3077 640 1152 1417 Total PHEN 54058 10841 83418 24872 49277 10479 55908 18675 128 The GC-amenable PASH compounds distributions in decant oil derivatives are listed in Table 5.30, and graphically presented in Figure 5.24. 12000 DBT 10000 MDBT DMDBT TMDBT 8000 BNT MBNT 6000 DMBNT 4000 2000 Concentration, in feeds ppm 0 DO15 VTB15 CF15 HYD15 DO24 VTB24 CF24 HYD24 Figure 5.24 Sulfur-containing compounds in decant oil derivatives Total concentration of PASH compounds in VTB feeds is approximately 10% of that in its parent decant oil. C2 and C3-DBTs are the primary sulfur aromatics. For heavy PASHs, unsubstituted BNT[2,1 d] was found to be the dominant compound. 129 Table 5.30. Polycyclic aromatic sulfur compounds distribution in decant oil derivatives (unit: ppm in feeds) Compound DO15 VTB15 CF15 HYD15 DO24 VTB24 CF24 HYD24 DBT 248 66 0 0 350 27 157 0 MDBT 3942 403 1282 0 1787 244 674 0 4-MDBT 1569 166 622 0 776 118 375 0 2- and 3-MDBT 1914 194 399 0 879 126 299 0 1-MDBT 459 43 261 0 133 0 0 0 Total DMDBT 10810 890 1697 479 4968 750 1197 278 EDBT 288 0 0 0 91 0 0 0 4,6-DMDBT 1254 133 971 479 676 108 508 278 2,6-DMDBT 1246 118 368 0 674 100 229 0 3,6-DMDBT 1478 136 132 0 760 132 308 0 3,7- +2,8-DMDBT 1497 128 3 0 802 132 151 0 3,8-DMDBT 624 0 0 0 137 0 0 0 1,4- +1,6-DMDBT 556 42 82 0 239 0 0 0 1,3- +3,4-DMDBT 1401 142 142 0 704 105 0 0 1,9- +2,4-DMDBT 1703 115 0 0 639 113 0 0 1,2- + 2,3-DMDBT 763 76 0 0 244 59 0 0 0 TMDBT 6527 983 2531 1421 6013 728 871 1016 2,4,6-TMDBT 1458 233 2078 767 1390 163 749 509 TMDBT, unknown (peak 61) 879 118 0 0 613 100 0 0 TMDBT, unknown (peak 62) 1166 128 0 0 812 60 0 0 TMDBT, unknown (peak 63) 905 49 0 0 432 18 0 0 TMDBT unknown (peak 64) 405 82 0 0 390 97 0 0 1,4,6- TMDBT 632 118 112 309 614 190 0 228 TMDBT, unknown (peak 67) 0 78 0 0 477 34 0 0 3,4,6-TMDBT 1081 177 342 345 1284 67 122 280 Total DBT 21527 2341 5511 1900 13118 1748 2899 1294 Benzo[b]naphtho[1,2-d]thiophene 219 89 0 0 0 67 0 0 Benzo[b]naphtho[2,1-d]thiophene 1152 426 1678 78 767 333 1197 32 Benzo[b]naphtho[2,3-d]thiophene 345 139 0 0 166 120 0 0 MBNT 5363 432 183 0 1656 309 0 0 DMBNT 8165 213 391 201 7738 145 459 98 Total BNT 15244 1299 2253 279 10328 974 1656 130 Total PASH 36771 3640 7763 2179 23446 2722 4555 1424 130 In HYD streams, the DBT, MDBT and BNT were almost completely removed. The primary PASH compounds were found to be 4,6-dimethylDBT and 3 TMDBTs isomers (2,4,6-, 3,4,6- and 1,4,6-TMDBT). These DBT compounds are proven to be the refractory compounds that could survive the hydrodesulfurization process (Houalla, Nag et al., 1978; Ma, Sakanishi et al., 1994; Gates and Topsoe, 1997). Figure 5.25 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 methylDBTs (4-methyl DBT 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 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 are not present in HYD) indicates these thermally stable PASHs were introduced with the recycle stream from the coker. 131 S S HYD15 2,6-DMDBT 3,4,6-TMDBT S S 1,4,6-TMDBT 4-E,6-MDBT CF15 S S 4,6-DMDBT BNT(2,1d) S 4-MDBT S 2,6-DMDBT S S 3,6-DMDBT 2-MDBT DO15 22 23 24 25 2627 28 3033 35 3537 38 40 Retention Time, minute Figure 5.25 Comparison of MICs of PASH compounds in decant oil derivatives 132 5.1.5.3 Four-ring aromatics in decant oil derivatives Pyrene and chrysene comprise the majority of four-ring aromatics in decant oil derivatives. CF streams, relatively higher (to DO) concentrations of five-member ring PAHs (benzofluorenes) were found to exist, however, the concentrations of benzofluorenes are only 10% of that of total chrysenes. Table 5.31 lists the distribution of pyrene, chrysene and their alkyl substituted analogs in DO and its derivatives. Table 5.31 Four-ring PAHs in decant oil derivatives (unit: ppm in feed) Compound DO15 VTB15 CF15 HYD15 DO24 VTB24 CF24 HYD24 PY 1451 1014 20352 1362 4505 1230 33389 3269 MPY 6604 3332 18934 5667 15538 3846 43156 12138 2-MPY 2121 1326 12947 2097 6249 1439 29896 4611 4-MPY 2643 974 3423 1845 4581 1136 7578 3646 1-MPY 1840 1032 2565 1725 4708 1271 5682 3881 DMPY 13870 3897 21656 8295 27210 5828 34381 18878 TMPY 13281 1879 3728 2180 23035 4121 16600 6382 TOTAL PY 35205 10123 64670 17504 70288 15025 127526 40668 CHRY 1501 276 18821 159 2941 445 16124 478 MCHRY 6242 1147 17062 2384 12047 1807 18192 2862 3-MCHRY 3448 630 10033 937 5274 899 11533 -- 2-MCHRY 1701 311 4433 386 2608 376 3948 -- 6-MCHRY 395 84 971 42 1213 166 593 -- 5-MCHRY 53 11 286 785 1361 159 1544 2355 4-MCHRY 147 13 647 158 1228 175 500 352 1-MCHRY 498 98 692 77 362 32 72 154 DMCHRY 7790 1351 7193 3504 17629 2253 9658 6046 TMCHRY 4790 760 1484 16535 2134 694 Total CHRY 20324 3534 44560 6048 49152 6639 44668 9386 benzoanthracene 476 97 3792 -- 1329 107 5883 -- benzo(c)fluorene -- -- 4395 ------1748 -- benzo(a)fluorene -- -- 4737 ------3924 -- 133 The comparisons of pyrene compounds among decant oil derivatives are shown in Figure 5.26 and Table 5.32. 50000 45000 PY d 40000 MPY 35000 DMPY 30000 TMPY 25000 20000 15000 10000 concentration, ppm in fee 5000 0 DO15 VTB15 CF15 HYD15 DO24 VTB24 CF24 HYD24 Figure 5.26 Concentration of pyrene homologues in decant oil derivatives Table 5.32 Relative distribution of pyrene compounds in decant oil derivatives (ppm in feed) Compound DO15 VTB15 CF15 HYD15 DO24 VTB24 CF24 HYD24 PY 4% 10% 31%8% 6% 8% 26% 8% MPY 19% 33% 29% 32% 22% 26% 34% 30% DMPY 39% 39% 33% 47% 39% 39% 27% 46% TMPY 38% 19% 6% 12% 33% 27% 13% 16% TOTAL PY 100% 100% 100% 100% 100% 100% 100% 100% Compared with their parent decant oils, CF feeds contain almost twice the amount of pyrenes, while VTB and HYD show a decrease in pyrene concentrations. The homologous distribution pattern of pyrene shows a lower proportion of higher alkylated 134 pyrenes in all the decant oil derivatives. The most pronounced pyrene alkylation change was observed in CFs, which is readily seen from Table 5.32. Figure 5.27 compares multi-ion GC/MS chromatograms of alkylated pyrene in HYD24, CF24 and DO24, which illustrates the isomeric alkylpyrene distribution among the derivatives of decant oil sample. The isomeric distribution of pyrene homologues in HYD stream exhibits a similar pattern to DO, however, the constitutional isomers of MPY, benzo(a)- and benzo(c)- fluorenes disappeared from this stream. The content of methyl and dimethyl isomers of benzofluorenes was also greatly reduced. The CF stream has very different alkylpyrene isomer distribution profile from that of its parent DO. As shown in the Figure 5.27, the peak intensity of 2-MPY exhibits a great extent of increase at the expense of those of 4-MPY and 1-MPY. Similar isomeric distribution changes were also observed in DMPY and TMPY relative distribution in this stream. The total concentrations and distribution of chrysene compounds in decant oil derivatives are shown in Table 5.33. Table 5.33 Total chrysene concentration (ppm in feed) and relative distribution (wt%) in decant oil derivatives DO15 VTB15 CF15 HYD15 DO24 VTB24 CF24 HYD24 CHRY 7% 8% 42% 3% 6% 7% 36% 5% MCHRY 31% 32% 38% 39% 25% 27% 41% 30% DMCHRY 38% 38% 16% 58% 36% 34% 22% 64% TMCHRY 24% 22% 3% 0% 34% 32% 2% 0% Total CHRY 100% 100% 100% 100% 100% 100% 100% 100% 135 methylpyrene HYD24 Dimethylpyrene Trimethylpyrene 4-methylpyrene 2-methylpyrene CF24 1-methylpyrene benzo(a)fluorene DO24 benzo(c)fluorene 30 31 32 333233 34 35 35 36 37 38 39 Retention time, min Figure 5.27 Multi-ion chromatograms of m/e 216, 230 and 244 representing MPY, DMPY and TMPY in HYD, CF and DO derivatives (intensity is comparable within same stream only). 136 As in the change of pyrenes, a proportional reduction in chrysene and its homologues was observed in VTB feeds. In HYD streams, chrysene and higher methylchrysene (e.g., trimethyl chrysenes) were reduced to a great extent. The total concentrations of chrysenes in CF feeds either increase (in CF15) or remain unchanged (in CF24). However, common to both CF feeds are the substantial increments in unsubstituted chrysene and a shift to less alkyl substitution. The isomer distributions of chrysenes in DO derivatives are compared in the multi-ion chromatograms in Figure 5.28. Relatively small shift in MCHRY isomers distribution was observed in CF feed (10% increase in 3-MCHRY). The most obvious difference can be seen in the MCHRY isomer distributions of the HYD feed where 1- MCHRY dominates. The much lower intensity of chrysene in HYD, therefore, the lower their concentrations, indicates that chrysene was more easily hydrogenated than four-ring pyrenes. Similar finding was also reported by Korre (Korre, Klein et al., 1995). Benzo(a)anthracene, which is about one-third in concentration of chrysene in virgin decant oil, was also observed to be present in much lower content in HYD. GC/MS analysis did not show any significant amount of dihydrochrysene, or hexahydrochrysene in this HYD derivative. The extensive reduction in chrysene and the insignificant amount of hydrochrysenes detected may indicate a severe hydrocracking of the chrysene hydrogenation products. 137 HYD24 2-methylchrysene 3-methylchrysene CF24 6-methylchrysene 1-methylchrysene DO24 benzo(a)anthracene Dimethylchrysene chrysene Trimethylchrysene Tetramethylchrysene 35 3737 38 39 39 41 43 41 43 4544 45 46 Retention time, min Figure 5.28 Multi-ion chromatograms of m/e 242, 256 and 270 representing MCHRY, DMCHRY and TMCHRY in HYD, CF and DO derivatives (intensity is comparable within same stream only). 138 5.1.5.4 Five plus rings aromatics in decant oil derivatives The resolutions of five+ rings aromatic compounds from GC column are usually poor for decant oil samples. CF feed, on the other hand, showed much improved resolution in this range of heavy ends PAHs. The total ion chromatograms corresponding to the heavy PAH in decant oil derivatives are given in Figure 5.29 and 5.30 for the two sample sets, respectively. The heavy PAHs detected in decant oil derivatives include benzopyrenes and perylene (five-ring PAH), and benzo[g,h,i]perylene (six-ring PAH). Five-member-ring benzo(b)- and benzo(k)fluoranthenes are also present in large quantities. The structures and molecular weights of these heavy PAHs are shown in Figure 5.31. The identification of these PAHs were based on co-injecting standard materials with the decant oil derivates on GC/MS. The identification of benzo(e)pyrene, for which no standard was available, was made by the relative peak elution sequences of m/e 252 with reference to the published literature (Budzinski, Jones et al., 1997). 139 HYD15 41 42 43 44 45 46 47 48 49 benzo(e)pyrene CF15 benzo[g,h,i ]perylene 41 42 43 44 45 46 47 48 49 DO15 41 42 43 44 45 46 47 48 49 Retention time, min Figure 5.29. TIC of decant oil derivatives corresponding to five and higher number ring PAHs 140 HYD24 CF24 benzo(e)pyrene benzo[]peryleneg,h,i DO24 41 42 43 44 45 46 47 48 49 Retention time, min Figure 5.30. TIC of decant oil derivatives corresponding to five and higher number ring PAHs 141 20 19 1 a 18 2 13 15 b 12 14 3 c 17 k 4 11 16 d j 9 h 10 i 8 5 g e 7 f 6 benzo(b)fluoranthene benzo(k)fluoranthene C20H12 C20H12 Mol. Wt.: 252 Mol. Wt.: 252 16 2 15 1 a 16 2 b 15 1 a b 14 3 17 19 20 c 14 3 4 17 19 13 18 20 c 4 d 13 18 9 d 12 5 9 i h 8 12 5 g e 8 7 h 11 10 f 6 i g e 11 10 7 6 f 21 22 perylene benzo[ghi]perylene C20H12 C22H12 Mol. Wt.: 252 Mol. Wt.: 276 20 19 12 13 10 18 11 14 1 9 16 15 2 17 7 4 8 3 6 5 benzo(a)pyrene benzo(e)pyrene C20H12 C20H12 Mol. Wt.: 252 Mol. Wt.: 252 Figure 5.31. The structure of some heavy PAHs in decant oil derivatives 142 Shown in Figure 5.32 are the multi-ion chromatograms of the five-ring PAHs and their C1-and C2-substituted analogs in decant oil derivatives. Four peaks were well resolved in the MIC of m/e 252, and they represent the responses from benzo(b)fluoranthene, benzo(e)pyrene, benzo(a)pyrene and perylene respectively. Benzo(k)fluoranthene eluted as a shoulder on benzo(b)fluoranthene peak, but its intensity is much lower. At least 12 peaks were resolved in the MIC of m/e 266; the less intensive peaks in the front of the cluster are probably of C1-substituted benzo(b)fluoranthenes, and the rest would be methylbenzopyrenes. The C2-substituted five-ring PAH could not be uniquely determined. It can be seen in Figure 5.32 that the distribution of homologous heavy PAHs is similar to those of lighter PAHs such as phenanthrene or pyrene. Unsubstituted benzopyrene, benzofluoranthene and perylene and their methyl-substituted compounds are the primary forms of five-ring PAH in decant oil derivatives. Benzofluoranthene and perylene show different distributions between DO and CF. The significantly high concentration of benzofluoranthene in CF suggests that either this PAH is less reactive and survives the liquid phase carbonization environment or it is a product of the delayed coking. Although it is impossible to uniquely determine the methyl isomers, the most abundant isomers would be expected to be the thermally stable ones. Total concentrations of unsubstituted five- and six-ring aromatic PAHs in decant oil derivatives are shown in Figure 5.33. Both CF feeds are “heavier” in PAH composition than their parent decant oils. Between the coker feeds, CF24 has larger proportions of benzopyrene and benzoperylene. 143 HYD24 4142 43 44 45 46 46 47 48 CF24 benzo(e)pyrene benzo(a)pyrene perylene 4142 43 44 45 46 46 47 48 Methylbenzopyrenes DO24 benzo(k)fluoranthene C2- substituents benzo(b)fluoranthene 4142 43 44 45 46 46 47 48 Retention time, min Figure 5.32 Multi-ion chromatograms of five-ring PAHs in decant oil derivatives 144 16000 14000 Benzofluoranthene benzopyrene 12000 Benzoperylene 10000 8000 6000 concentration, ppm in feed 4000 2000 0 DO15 VTB15 CF15 HYD15 DO24 VTB24 CF24 HYD24 Figure 5.33. Distribution of heavy PAHs in decant oil derivatives HYD and VTB samples have lower concentrations of five-ring PAHs than the corresponding decant oil samples and benzopyrene was the predominant PAHs present in both derivatives. Table 5.34 lists the GC/MS results for these heavy PAHs in the decant oil derivatives. Table 5.34 Five and more ring PAH distribution in decant oil stream (ppm in feed) Compound DO15 VTB15 CF15 HYD15 DO24 VTB24 CF24 HYD24 benzo(b)fluoranthene 87 -- 3269 690 638 -- 4694 237 benzo(k)fluoranthene 14 -- 626 -- 267 -- 926 -- Benzo(e)pyrene 710 274 3906 2634 2968 598 9150 1539 benzo(a)pyrene 722 152 2393 -- 1862 192 4296 -- perylene -- -- 489 -- 780 -- 989 -- C1-benzopyrene+perylene 1731 715 6096 972 14330 671 12377 3037 C2- benzopyrene+perylene 961 442 2928 436 19013 511 3618 752 benzo[g,h,i]perylene -- 513 837 -- 2011 1063 2887 1872 145 5.2 High-Performance Liquid Chromatography Analysis of Needle Coke Feedstocks Because of the limitation of GC/MS in resolving high-boiling-point PAHs, high- performance liquid chromatography (HPLC) analyses were performed to complement the GC/MS analysis of decant oils and their derivatives. Figure 5.34 gives an HPLC/PDA chromatogram of a mixture of reference PAHs at 254 nm wavelength. The relative response factors (to pyrene) are listed in Table 5.35. On the analytical column used, PHEN (unsubstituted and substituted) showed almost twice greater responses than four-ring aromatics (PY and CHRY); the responses of five-ring PAHs are roughly in the same range of four-ring PAHs. Six-ring PAHs (benz[g,h,i]perylene and dibenzo(a,h)anthracene ) show the lowest responses (about 1/5 of four-ring aromatic PAHs). Figure 5.34 . HPLC/PDA chromatogram of standard PAH compounds at 254nm 146 Table 5.35 Response factors (relative to that of pyrene) of PAH standard compounds Peak number Compound Response factor 1 fluorene 0.76 2 dibenzothiophene 0.59 3 anthracene 3.66 4 phenanthrene 2.33 5 2-methylphenanthrene 2.52 6 3,6-dimethylphenanthrene 2.19 7 pyrene 1 8 pyrene-d10 0.32 9 benzo(a)anthracene 1.13 10 chrysene 1.73 11 benzo(b)flouranthene 1.09 12 benzo(k)flouranthene 1.3 13 benzo(a)pyrene 1.19 14 dibenzo(a,h)anthracene 0.28 15 indeno(1,2, 3-cd)pyrene 1.48 16 benzo(g,h,i)perylene 0.52 Chromatograms at various wavelengths can be extracted from the PDA detector; however, it was found that the most convenient way of comparing PAH distribution among feedstocks is to use the chromatogram at wavelength of 254 nm, which covers almost all peaks, especially for those of five and more ring PAHs. Shown in Figures 5.35 to 5.38 are the HPLC chromatograms of decant oil samples. As expected, HPLC reveals the important information on the composition of the high-boiling-point PAHs that can not be achieved by GC/MS. By comparing the retention time and UV spectrum of major peaks with standard materials, the heavier PAHs in decant oils were found to be, primarily, in the forms of benzopyrene and benzoperylene. 147 As in low-molecular weight PAHs, C1- to C4 benzopyrene and benzoperylene substituents are present in higher quantity than the base (unsubstituted) PAH compounds. The tailing at longer retention time (after 25 minutes) on HPLC/PDA suggested that even larger PAHs present in decant oil were not well resolved on the HPLC column. Although these larger PAHs (if there are) had shown much lower intensities, it does not necessarily mean that these compounds are in lower quantity since these larger PAHs tend to respond poorly (lower response factors) in HPLC as the data on reference compounds showed in Figure 5.34. Peak purity examination on well-resolved four- to six-ring PAHs showed that most of the peaks on HPLC chromatograms consisted of a mixture of isomers of alkylated PAH compounds. The poor baseline in the extracted chromatogram did not permit statistically meaningful quantitative results. Therefore, a semi-qualitative comparison in PAH distribution, as visualized by peak height and area from the chromatograms, is discussed in this section. DO91, DO93, DO15 and DO24 show similar HPLC profile with the observable difference being in the intensity of peaks at certain retention times, indicating that the concentrations of PAHs in those decant oil samples are different. As Figure 5.31 shows, the relative high intensity peaks for pyrenes in DO91 agree with the GC/MS quantitative results, that the concentration of pyrenes in DO91 is much higher than that in DO93. The broadening peaks in phenanthrenes in DO93 reflect the elution of dibenzothiophene compounds in this time window. Similar observation can also be found in low-sulfur decant oil DO15 and DO24, while the steep ramp in phenanthrene peaks indicates less 148 abundance of DBT compounds which results in simplified peak constitution (this can be seen more clearly in the chromatogram for DO24). DOPSU and DOUP show a similar PAH distribution pattern but very different from those in the previous decant oil sample set (DO91, DO93, DO15 and DO24). The differences in HPLC/PDA profiles between these two sets of samples also confirm the difference as indicated in GC/MS results discussion. The PAHs in DOPSU and DOUP were much better-resolved on HPLC/PDA than the other set of samples. Figure 5.34 compares the peaks of PAHs in DOUP with those in DO15. The sharper and higher peaks of pyrenes in DOUP than in DO15 clearly showed that the pyrenes are present in higher concentrations in DOUP. This is consistent with the GC/MS results. The very well-resolved peaks in the range of phenanthrenes in DOUP indicated a simpler composition of C2-phenanthrene isomers in DOUP. The high-molecular weight PAH distribution in DOPSU and DOUP also drastically differs from the other set of samples. The enhanced peaks in the longer retention time on HPLC/PDA chromatograms indicated that the major heavy PAHs in DOPSU and DOUP are less complex in composition. 149 Figure 5.35 HPLC/PDA chromatograms of DO 91 and DO93 at 254nm 150 Figure 5.36 HPLC/PDA chromatograms of DO 15 and DO24 at 254nm 151 Figure 5.37 HPLC/PDA chromatograms of DOPSU and DOUP at 254nm 152 Figure 5.38 Comparison of HPLC/PDA chromatograms of DO UP and DO15 at 254nm 153 The HPLC/PDA chromatograms of CF feeds are shown in Figure 5.39; these chromatograms show much better resolutions than those obtained from the decant oil samples. A visual comparison on PAH distribution in CF and DO is given in Figure 5.40. Significant changes in PAH distribution are seen in both the lighter and the heavier ends in the feedstocks, compared to the parent DOs. On the lighter PAH end, phenanthrene and methylphenanthrene show increased peak heights resulting from the shift to less-substituted phenanthrenes distribution in the CFs. Pyrene and chrysene composition also give the same trend as in phenanthrenes. On the heavier end, large increases in the concentration of benzopyrenes and benzoperylenes are seen in these two CF feedstocks. The indistinctive peaks around 25 minutes show similar UV spectra to that of benzoperylene, 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 5.41). 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. It is worth noticing that the reduced phenanthrene peak intensity and the disappearance of chrysene (unsubstituted) in HYD feed. A series of intensified peaks at the longer retention times may be those of partially hydrogenated PAH with higher molecular weights, but this can not be uniquely determined by the PDA detector. HPLC/PDA chromatograms of VTB samples are given in Figure 5.42 and Figure 5.43 respectively. Two clusters of peaks eluted from the HPLC column. The first peak cluster shows a very similar feature to that of their parent decant oils. The second peak 154 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 ( Figure 5.44). 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. Naphthenic compounds (alkanes and cycloalkanes) in needle coke feedstocks can not be analyzed on the PDA detector because of the absence of conjugated π electrons in those molecules. The most important information obtained from HPLC analysis is a clearer picture 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 was found to be the key difference from the decant oil samples. 155 Figure 5.39 HPLC/PDA chromatograms of CF15 and CF24 at 254nm 156 Figure 5.40 Comparison of HPLC/PDA chromatograms of DO15 and CF15 at 254nm 157 Figure 5.41 Comparison of HPLC/PDA chromatograms of HYD15 and DO15 at 254nm 158 Figure 5.42 Comparison of HPLC/PDA chromatograms of VTB15 and DO15 at 254nm 159 Figure 5.43 Comparison of HPLC/PDA chromatograms of VTB15 and VTB245 at 254nm 160 5.3 LC/MS/MS and Laser Desorption Mass Spectrometry Analysis of Needle Coke Feedstocks 5.3.1 LC/MS/MS analysis of decant oil and its derivatives As the HPLC/PDA analysis results showed, there are a series of unresolved peaks at the longer retention time; some of these peaks have shown UV spectra similar to those of alkylated benzo[g,h,i]perylene. To cross check the identities of these heavy PAH compounds, the LC/MS/MS and LD/MS analyses were carried out aiming to seek further information on the heavy PAH composition in decant oil and its derivatives. Figure 5.44 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; there is no chromatographic separation in this experiment. In these spectra, the y-axis represents the normalized counts to the highest peak and the x-axis is the mass-to-charge ratio (m/e). As can be seen in the DO and CF mass spectra, 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). The odd m/e number is due to atmospheric pressure chemical ionization (APCI+) which results in the M+ 1 molecular ion. 161 DO 01-5 4 KV corona APCI+ 03152002_GW_06 10 (0.458) Cm (5:16) Scan AP+ 259 100 2.98e4 295 281 245 214 309 % 231 323 333 335 361 397 409 0 03152002_GW_03 14 (0.632) Cm (11:19) Scan AP+ 231 100 2.44e5 245 % 259 271 281 295 313 335 0 03152002_GW_07 16 (0.718) Cm (14:17) Scan AP+ 285 100 3.72e4 245 250 259 299 % 224 313 325 327 341 367 389 415 437 0 m/z 200 250 300 350 400 450 500 550 600 650 700 750 800 Figure 5.44 LC/MS/MS spectra of DO15 (top), CF15( middle) and HYD15 (bottom) To identify these major (most abundant) peaks in the mass spectra, the selected ions were introduced into the second mass spectrometer on the LC/MS/MS system. The 162 fragmentation patterns, or the daughter ions, of m/e 231, 245, 259, 285 and 295 are shown in Figure 5.45 to Figure 5.47. From the fragmentation pattern, it can be determined that the m/e 231, 245, 259 are C2-, C3-, C4-pyrene and m/e 281, 295, 319 are C2-, C3- and C4-benzopyrene. The highest intensity peak in HYD stream, m/e 285 appears to be the C4-chrysene. However, considering the low intensity of m/e 285 in DO and the easiness of chrysene hydrogenation as shown in HPLC/PDA chromatogram; this high intensity peak in HYD may not really represent the C4-chrysene alone. The daughter ion fragmentation pattern in Figure 5.47 suggests that this ion consists of mostly H4-C2- benzopyrenes. 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 5.45 and Figure 5.46) 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 4- to 6-ring aromatic compounds; the distribution of each compound (i.e., the alkylated aromatics) varies among the decant oil and coker feed samples. 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 from vacuum distillation of decant oil. The partially hydrogenated PAHs can be found from peaks m/e of 211 (H4-C2-PHEN), 225 (H4-C3-PHEN), 237 (H4-C4-PHEN). Peaks with m/e of 251 and 265 (see Figure 5.55) of HYD stream correspond to H8-C2-chrysene and H8-C3-chrysene. 163 One of the assumptions made in the interpretations of mass spectra is that the peak with specific m/e number represents pure PAH. This may be valid for low molecular weight PAHs because in GC/MS, these compounds are very well separated. However, with the increase in fused aromatic rings size (number of benzene rings), the number of structural isomers will be growing rapidly. In other words, the high molecular weight peaks may actually consist of several compounds with the same m/e value. Figure 5.45 Daughter ion spectrum of m/e 231 (LC/MS/MS) 164 Figure 5.46 Daughter ion spectra of m/e 2451 and 259 (LC/MS/MS) 165 Figure 5.47 Daughter ion spectra of m/e 285 and 295(LC/MS/MS) 166 To clarify (or minimize) this ambiguity from the mass spectral analysis, HPLC/MS/MS analyses were performed using the same column as with PDA detector. The HPLC/MS/MS results of decant oils are presented in Figure 5.48 to Figure 5.50. From the HPLC/MS/MS chromatograms, more detailed information can be found on the composition of the heavy PAH in decant oils. Alkylated benzopyrenes and benzo[g,h,i]perylenes comprise the majority of heavy PAHs in decant oil samples. The mass number of these heavy PAH compounds agreed with the major peaks in the whole- range mass spectrum shown in Figure 5.44. Differences in heavy PAHs (four and higher-ring PAH) distribution among decant oil samples can be compared in the retention time window of 12-20 minutes. Both DO93 and DO15, which produced poor optical texture in the resulting semi-cokes, showed lower intensity and featureless chromatogram than the rest of decant oil samples. On the other hand, distinctive peaks with higher intensities were resolved for DO91 and DO24. The difference in HPLC responses may suggest that, in poor needle coke feedstock such as DO93 and DO15, the heavy PAHs are highly alkylated and evenly distributed. In DOUP and DOPSU, heavy PAHs are observed to distribute unevenly: methyl- and dimethylbenzopyrenes constitute the majority of 5-ring PAHs, and the composition of alkylated benzoperylene is simpler than those of other decant oil samples. 167 Figure 5.48 HPLC/MS/MS chromatograms of DO91 and DO93 168 Figure 5.49. HPLC/MS/MS chromatograms of DO15 and DO24 169 Figure 5.50. HPLC/MS/MS chromatograms of DOPSU and DOUP 170 The HPLC/MS/MS chromatograms obtained from CF samples are shown in Figure 5.51. Compared with their parent decant oils, both CF streams show increased intensities of peaks after 13 minutes 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, 16 on Figure 5.51) and six-ring PAH benzoperylene, methylbenzoperylene, dimethylbenzoperylene (peaks 17, 18, 19 on Figure 5.51) show similar patterns in these two CF streams. The most abundant five-ring and six-ring PAHs homologues were found to be the isomers of methylbenzopyrene and dimethylbenzoperylenes. Between CF15 and CF24, the cleaner peaks in this heavy PAH elution time window may suggest the composition of heavy PAHs in CF24 is simpler than that of CF15. LC/MS/MS chromatograms of HYD15 and HYD24 are given in Figure 5.52. The intense peaks (11a, 12, 15 and 15a in HYD24) represent the hydrogenation products of chrysenes and benzopyrenes. Also seen in this figure is the PAH species distribution that follows the same trend as their virgin decant oils (i.e., less alkylated PAHs in HYD24 than in HYD15). The heaviest PAHs found in decant oils are shown in Figure 5.53 and Figure 5.54 in the HPLC/MS/MS chromatograms of VTB samples. The most abundant PAH species in these heavy fractions of decant oils are the alkylated benzo[g,h,i]perylenes (C2- and 171 C3-substituents) and the C1 to C3-dibenzopyrenes. The intense peaks (peaks 14, 16, and peaks 19, 20, 21) reflect the highly alkyl-substituted PAHs dominating the composition of high-boiling-point aromatic compounds in the heavy DO fraction, which is consistent with the results obtained from quantitative GC/MS. As in their parent decant oil composition, the difference in the heavy PAH distribution between two VTB samples is also seen in the simpler heavy PAH composition in VTB24 sample. 172 Figure 5.51. HPLC/MS/MS chromatograms of CF15 and CF24 173 Figure 5.52. HPLC/MS/MS chromatograms of HYD15 and HYD24 174 Figure 5.53 . HPLC/MS/MS chromatograms of VTB15 175 Figure 5.54 HPLC/MS/MS chromatograms of VTB24 176 5.3.2 LD/MS analysis of decant oil and its derivatives HPLC/MS/MS analysis has revealed very important information on the heavy end PAH composition and the semi-quantitative comparisons of the distributions of these major PAHs among decant oil and its derivatives show very close similarities to that of smaller PAHs which can be quantitatively described by GC/MS analysis. However, it may suffer from the same disadvantage of HPLC: only solvent-soluble fractions can be analyzed. In order to overcome this shortcoming, laser desorption mass spectrometry (LD/MS) analysis was used to elucidate the very heavy PAH compounds present in needle coke feedstocks. In this study, decant oil and its derivatives were analyzed without an external matrix, instead, the sample itself was used as its own matrix because of the relatively low molecular weight of the compounds in decant oil sample compared to polymers or biomolecules. Figure 5.55 gives the LD/MS spectra of DO15,CF15 and HYD15. The major peaks are labeled by the mass-to-charge ratios. It can be seen from the LD/MS spectra that the major molecular components in decant oil and its derivatives are in the range of 150-400. Both decant oil and coker feed show a second hump with much lower intensities in the mass range of 400-600; this may be considered as the dimers of the PAHs in the first peak cluster in a broad sense. 177 Figure 5.55. Laser desorption mass spectra of DO15, CF15 and HYD15 178 The most abundant peaks as labeled on the graphs show very similar position to the mass spectra obtained by LC/MS/MS (see Figure 5.44). However, the intensity of four-ring PAHs (such as pyrenes) on LD/MS spectra appears to be underestimated. The possible reason may be that the lower-molecular weight PAH dissolved in THF evaporated before being ionized by the laser beam. Nevertheless, no effort was made to correct this problem, because the focus of LD/MS analysis is to look at the heaviest PAH molecules that were not resolved by GC or HPLC analyses. LD/MS analysis, compared with LC/MS/MS, gives more detailed information on the molecular distribution of the heavy PAHs. Based on the identification of major compounds revealed by LC/MS/MS, most of the major peaks can be identified with confidence and the peak assignments are shown in Figure 5.56 to Figure 5.58. All decant oil samples exhibited almost the same mass spectrum pattern (in terms of major peaks ) but with varying intensities. The results from quantitative GC/MS analysis have shown that the decant oils have almost the same molecular components with varying relative distribution. The same conclusion can be extended to the heavy ends which could not be quantified in such feedstocks. DO93 gave the most complex LD/MS spectrum of all decant oil samples. The primary PAHs in this feedstock contains a wide range of heavy PAHs (from pyrene to benzoperylene). The intensity (or abundance) of these peaks was very close, indicating the heavy PAHs are more evenly distributed. Compared to DO91, there are many less intense peaks between these primary PAH peak groups. This indicated that the severe alkylation (multi-methyl substitution) is prevalent in this feedstock. Four-ring PASH (BNT, peak A, B, and C) were well-resolved in LD/MS in this feedstock; the highest 179 intensity of the peak C (DMBNT) further supports the extensive alkylation of the PAHs in DO93. The heavy PAH components in DO24 (Figure 5.60) were found to be better defined than those in DO91, DO15 and DO93. C2-, C3-benzopyrenes and C2- benzoperylenes constitute the major heavy PAHs. The higher alkylated C3-, C4-and C5- chrysenes (peak 8, 12, and 16) are the lowest among the decant oil samples. DOUP shares a similar LD/MS profile with DOPSU presented to be the least complex in PAH composition. Alkylated pyrene and benzopyrene are the major components in these two decant oils. 180 Figure 5.56. Laser desorption mass spectra of DO91 and DO93 181 Figure 5.57. Laser desorption mass spectra of DO15 and DO24 182 Figure 5.58. Laser desorption mass spectra of DOPSU and DOUP 183 Figure 5.59 and Figure 60 compare the heavy PAH composition in HYD and DO samples. The significant difference between DO and HYD is found in the most abundant peaks of 9, 12, 19 and 21. In the HYD mass spectrum, the two-fold higher (relative to tetramethylpyrene, peak 6) peak of m/e 270 (peak 9) is hard to attribute to the response from trimethylchrysenes alone, because chrysenes are more easily to be hydrogenated than pyrenes (Korre, Klein et al., 1995; Korre, Klein et al., 1997). The peak 12 (m/e 284) can not be assigned to C4-chrysene alone either, by the same reasoning. Notice that methylbenzopyrene peak (m/e of 266) was reduced significantly in HYD derivatives, the peaks with m/e of 270 and 284 would probably be the responses mainly from hydrobenzopyrenes. Peak 19 (m/e of 312) is obviously the product of hydrogenation of PAHs in virgin decant oil, and this peak was assigned to tetrahydrotetramethylbenzopyrenes (H4-C4-benzoPY). Similarly, peak 21 (m/e 326) was assigned to H4-C5-benzoPY. CF stream contains relatively high proportions of heavy PAHs than its parent decant oil as Figure 5.61 and Figure 5.62 show. The enrichment of certain heavy PAHs can be attributed to the blending of heavy fraction of HYD; this can be observed by the intense peak 19 and 21 which do not exit in virgin decant oil. In this sense, CF stream differs from the decant oil not only in the concentration and distribution of alkylated PAHs, but also in the hydrocarbon species, i.e., the hydroaromatics. LD/MS spectrum from VTB stream provides more convincing information on the heavy PAH composition in decant oils (see Figure 5.63 and Figure 5.64). The heaviest ends with reasonable abundance in decant oil are the C1-4 alkylated benzoperylene (m/e 184 276) and dibenzopyrenes (m/e 302). Coronene (m/e 300) appears to be in very low concentration in the decant oils. Figure 5.59. Laser desorption mass spectra of DO24 and HYD24 185 Figure 5.60. Laser desorption mass spectra of DO15 and HDY15 186 Figure 5.61. Laser desorption mass spectra of DO24 and CF24 187 Figure 5.62. Laser desorption mass spectra of DO15 and CF15 188 Figure 5.63. Laser desorption mass spectra of DO24 and VTB24 189 Figure 5.64 Laser desorption mass spectra of DO24 and VTB24 (m/e 200~400) 190 5.4 1H and 13C Nuclear Magnetic Resonance Spectroscopy Analysis of Needle Coke Feedstocks Nuclear magnetic resonance spectroscopy (NMR) study of needle coke feedstocks was suggested by Seshadri et al (Seshadri, Albaugh et al., 1982) to reveal the features in feedstock’s chemical composition that could be related to the quality of resultant needle coke. 1H NMR spectroscopy provides quantitative information on the population of aliphatic hydrogen and aromatic hydrogen. Quantitative 13C NMR, facilitated by Fourier transform (FT-NMR) technique, allows the direct measurement of aromatic and non- aromatic carbon atom location and distribution on carbon skeleton of hydrocarbon molecules in petroleum and its fractions (Dickinson, 1980; Rodriguez, Tierney et al., 1994) . The intensities of bands in the NMR spectrum are thought to be directly proportional to the population of contributing hydrogen or carbon atoms. Coupled with other experimental data, such as elemental analysis and molecular weight, the hypothetical average molecular structure for the fractions of petroleum can be established (Knight, 1967; Dickinson, 1980). Three distinctive bands (chemical shift downfield from internal standard, tetramethylsilane, TMS) in the 1H NMR spectrum represent the protons attached to aromatic, aliphatic and olefinic carbons. Each band, depending on the interest in specific functional groups, can be further divided into several sub-regions according to the relative location of proton to aromatic, aliphatic and olefinic carbons. In this study, the classification of protons (Shoorley, 1954; Shoorley and Budde, 1976) in needle coke feedstocks is listed in Table 5.36. 191 Table 5.36 1H NMR downfield shift assignment for functional groups Band, ppm from TMS Assignment 0.5-1.0 CH3 γ or further from aromatic ring 1.0-1.7 CH2 β and CH3 β 1.7-1.9 CH, CH2 β to hydroaromatic rings 1.9-2.1 Hydrogen α to olefinic 2.1-2.4 CH3 α to aromatic rings 2.4-3.5 CH, CH2 α to aromatic rings 3.5- 4.5 CH2 bridge 4.6-6.0 Olefinic proton 6.0-7.0 Single aromatic ring 7.2-8.3 Diaromatic ring 8.3-9.3 Tri-, tetra- and more aromatic rings The 1H NMR spectra of virgin decant oil DO91, DO93 and DOUP are shown in Figure 5.65. Four characteristic regions are shown in the NMR spectra by the bands corresponding to the aromatic hydrogen (6.0-9.0ppm), hydrogen α to aromatics (2.1- 4.0ppm), hydrogen β to aromatic ring (1.0-1.7ppm), and terminal methyl (γ or further) groups and aliphatic hydrogen in alkanes (0.5-1.0ppm). All decant oil samples exhibited similar spectra with regard to peak positions. The only difference among these samples is the variation in peak intensity, which reflects the distribution of hydrocarbon position and population of different compounds. Comparing the peaks of hydrogen in decant oils, it can seen that DO93 contains less of terminal and β –H than the rest of the decant oils. This distribution of hydrogen agrees with the finding from GC/MS, HPLC and LC/MS/MS that DO93 contains more alkylated PAHs with short side chains. 192 Figure 5.65. 1H NMR spectra of DO91, DO93 and DO15 193 The response from decant oil derivatives on 1HNMR is shown in Figure 5.66. HYD shows very similar pattern to its virgin decant oil. Contrary to the decant oil, the spectrum of CF15 gives well-defined peaks in both aromatic and aliphatic proton regions. The intense peaks in 7.2-8.3 ppm indicate the predominant presence of three- and four- ring aromatic compounds with relatively simple compositions. The peaks in 8.3-8.9 ppm also show sharp profile, representing the five and more ring aromatic compounds. Also noticed is the presence of small peaks in the region 8.9-9.3 ppm. This clearly shows the presence of 6-ring aromatic compounds. In the region of hydrogen α to aromatic ring, strong peaks are shown in the downshift range of 2.1-2.4 ppm, which represents the CH3 α to aromatic ring. It suggests the increased amounts of aromatics with methyl substitutes group in the coker feed sample. The most significant feature of the coker feed (CF) is the well-resolved peaks in the band of 1.7-1.9 ppm, indicating methyl-, ethyl- substituted hydroaromatic compounds. The presence of these hydroaromatics in the spectrum of HYD sample confirms the blending of HYD into the coker feed, which can also be found in the LD/MS results. 194 Figure 5.66. 1H NMR spectra of DO15, HYD15 and CF15 195 The heaviest fraction obtained from decant oil (VTB) shows (Figure 5.67) a similar hydrogen distribution to its parent decant oil. The much reduced peak height of terminal and aliphatic hydrogen suggests the side chain length is mostly in the form of methyl and ethyl groups. This can be attributed to the removal of n-alkanes by distillation. Figure 5.67. 1H NMR spectra of DO15 and VTB15 196 The 13C NMR spectra of DO91, DO93, DO15 and decant oil derivatives of DO15 are given in Figure 5.68 and Figure 5.69 respectively. Table 5.37 lists the chemical shift assignment of carbon atoms in hydrocarbons (Shoorley and Budde, 1976). Table 5.37 13C NMR downfield shift assignment for functional groups in petroleum Band, ppm from TMS Assignment 13-17 CH3 γ or further from aromatic ring 17-19 CH3 of ethyl 19-23 CH3 α to aromatic ring 23-30 Naphthenic CH2 30-37 CH2 in alkyl groups 37-60 CH alkyl groups CH naphthenic, Methylene bridge C 108-118 Olefinic 118-129 Protonated aromatic 129-133 Internal aromatic 133-135 Methyl substituted aromatic 135-160 Alkyl (other than methyl) substituted aromatics Decant oil samples show a common feature in the 13C NMR spectra: The signal in the saturated carbon region has 4 bands: 14.2, 19.6 - 20.5, 22, 23, 30 and 32ppm. In addition, a broad, featureless envelope extending from 32 to 45 ppm is also observed. The absorption at 14.2, 22, and 30 ppm is assigned to CH2 of long paraffin chains (Seshadri, Albaugh et al., 1982); the peaks at 19.6 –20.5 and 32 ppm are the isolated methyl group(s) attached in α and β positions to the aromatic rings. The relatively low intensity of peak at 32 ppm compared to that of 19.6-20.5 ppm indicates that the aromatic compounds in decant oil have mainly methyl substitutions and, to a lesser extent, ethyl substitution. 197 The 13C NMR spectra of decant oil derivatives show a very similar profile in saturated carbons. However, the aromatic carbon in CF is well resolved; the strongest peaks at 128-129 ppm indicate the higher content of the unsubstituted polyaromatic ring compounds. The high intensity and good separation of the peaks in this region shows that the constitution of PAHs in CF is much less complex than in decant oil samples. The VTB sample gives lower signal for both types of carbon atoms because of the lower solubility of this heavy decant oil fraction in CS2. The quantitative hydrogen and carbon distribution (based NMR) analyses are listed in Table 5.38 and Table 5.39. The percentages were calculated as integrated band area of interest divided by the total area in the whole range of the spectrum. Table 5.38. Distribution of hydrogen atoms in decant oil and its derivatives Chemical Shift, ppm DO91 DO93 DO15 CF15 VTB15 9-6.0 (Total aromatic H) 30.3% 38.1% 30.5% 39.3% 31.1% 4.0-2 (H α to aromatic) 34.3% 40.3% 32.7% 29.8% 39.8% 2.0-1.0 (H β to aromatic) 23.2% 16.8% 24.2% 21.6% 20.9% 1.0-0.0 (Terminal and aliphatic H) 9.7% 5.6% 10.1% 9.0% 8.8% Table 5.39. Distribution of carbon atoms in decant oil and its derivatives Chemical Shift, ppm DO91 DO93 DO15 CF15 VTB15 148-129 (Aromatic) 16.8% 19.7% 17.1% 18.2% 11.6% 129-118 (Protonated aromatic) 47.1% 49.7% 50.2% 49.7% 51.7% 37-30 (α to ring) 9.3% 7.5% 9.3% 11.8% 10.4% 30-23 (β to ring) 9.2% 6.6% 7.5% 8.6% 10.1% 23-17 (Interior CH) 13.1% 12.3% 12.0% 9.3% 12.9% 17-13 (Terminal CH3) 4.5% 4.1% 3.9% 2.5% 3.3% 198 The hydrogen aromaticity of DO91 and DO15 is very close (~30%) and lower than that of DO93 (38%). About 55% of paraffinic hydrogen in DO91 and DO15 is in the forms of methyl substituents α to aromatic rings. The VTB shows a higher value of α and lower values of β and terminal hydrogen populations than its parent decant oil (DO15), indicating the aromatics in VTB have more methyl substituents. Highest hydrogen aromaticity (38%) is found in DO93, and its paraffinic hydrogens distributed very differently from DO91 and DO15. The high percentage of α H and combined lower percentage of hydrogen β, α further showed the significantly different molecular constitution from DO91 and DO15. The exceptionally lower proportion of 1.0-0.0 ppm signals indicates that the PAHs in this feedstock are highly substituted. CF feed also shows high hydrogen aromaticity, however, the peaks at 2.1-2.4 ppm (Figure 5.64) have significantly high intensity. This means that the methyl group is more abundant in the side chains. The carbon atom distribution is listed in Table 5.39. DO91 and DO15 have similar aromatic carbon distribution (17%) and DO93 has the lowest distribution of carbons in ethyl or longer chain substitutes than both DO91 and DO15. Combined with 1H NMR data, this carbon distribution pattern further supports the conclusion that DO93 has more methyl substituents and fewer long-chain alkanes. The carbon aromaticity in CF stream is slightly higher than its virgin decant oil. However, the relatively small increase in carbon aromaticity in this feedstock is not consistent with the large increase in the hydrogen aromaticity (from 30.5 to 39.3%). This disproportional increase in carbon and hydrogen aromaticity may suggest that the 199 blending stream into the coker charge sample has mainly the two, three and four ring aromatics rather than the larger ring aromatics. The main limitation of these average structures derived from NMR is that the average structures do not necessarily represent the molecules actually present in the hydrocarbon mixtures. However, the parameters derived from averaged molecular structure can be used to emphasize the gross differences in structure between samples with narrower boiling point range or molecular weight range, as in the case of decant oil and its derivatives. 200 Figure 5.68. 13C NMR spectrum of DO91, DO93 and DO15 201 Figure 5.69. 13C NMR spectrum of DO15, HYD15, CF15 and VTB15 202 5.5 Summary The molecular constitution of needle feedstocks consists of three- to six-ring 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% to 40% of total molecular species, which represent the lower to middle boiling range of the molecular components in the feedstocks, was 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 semi-quantitative 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 DOUP, DOPSU, and DO24 decant oil samples, which produced better mesophase development during carbonization than the other decant oil samples. The PAH composition in these better feedstocks 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 within the same decant oil sample set, most probably resulting from the thermodynamic equilibrium composition, which depends on the FCC processing operating conditions. 203 Coker feeds, the actual feedstock to the delayed coker, showed a significantly different molecular composition from 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 methylPAH 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 six-ring aromatics such as benzo[g,h,i]perylene and dibenzopyrene and their methylated homologues constitute the dominating PAH species in this feedstock. 204 Chapter 6 Mesophase Development and the Molecular Composition of Needle Coke Feedstocks 6.1 Vacuum Distillation Product Yields from Decant Oil and Coker Feed In this study, four heavy fractions were obtained from one theoretical plate vacuum distillation of the needle coke feedstocks. The initial boiling points (atmospheric equivalent boiling temperatures, AET) for these residua were 300˚C, 325˚C, 350˚C and 370˚C respectively. Since the heater temperature was controlled under 245˚C (the vapor temperature was less than 180˚C) and distillation time was less than 3 hours, no thermal cracking and condensation reaction was observed on the heavy residua. The residual yields of decant oil and coker feed samples are listed in Table 6.1. Approximately 20% of the compounds in the decant oil samples have the boiling temperature under 350˚C, and approximately 65% of them end up in heaviest fractions. DO24 and DOUP contain more light components (AET boiling-points between 325˚C and 350˚C) than the rest of the DO samples which can also be seen in their LD/MS spectra. Table 6.1 Residua yields of feedstocks under vacuum distillation, wt% DO91 DO93 DO15 DO24 DOPSU DOUP CF15 CF24 300˚C 95.8 96.7 96.1 95.6 90.9 94.9 97.7 97.3 325˚C 89.6 92.8 91.7 89.4 86.7 86.1 89.2 90.0 350˚C 79.0 82.4 81.4 72.7 81.7 74.8 71.9 71.0 370˚C 69.2 65.1 62.5 66.1 69.9 63.4 49.0 51.5 205 Compared with the parent decant oils, the two coker feeds showed a similar boiling point distribution in lighter fractions (<325˚C). However, approximately 50% of CF’s components are found in the heaviest fraction, >370˚C boiling-point. The boiling point distribution in CF feed, in general, agreed with the less-alkylated PAHs composition as revealed by the results from the molecular characterization techniques discussed in the previous sections. 6.2 Optical Texture of Semi-Cokes from Vacuum Residuals of Coker Feedstocks The OTIs of semi-cokes from the residua versus AET are shown in Figure 6.1. As the data show, heavier residues of all decant oils and coker feeds developed progressively better mesophase than lighter residua obtained in vacuum distillation. 100 90 80 whole 3OO+ 325+ 70 OTI 35O+ 37O+ 60 50 40 DO91 DO93 DO_PSU DO_UP DO15 CF15 DO24 CF24 Figure 6.1 Semi-coke OTI from vacuum distillation heavy fractions 206 The OTI increments of the semi-cokes derived from different residua fractions showed a strong dependence upon the starting feedstocks, as shown in Table 6.2. Although the heavier fractions of DO 93 and DO 15 produced enhanced mesophase development over their virgin feedstocks, the overall mesophase development in their heaviest fraction was still inferior to those from better needle coke feedstocks such as DO91 and DO24. On the other hand, compared with the OTI of their decant oils, the mesophase development in DO93 and DO15 was significantly improved. A nearly 30% increase in the OTI was found in the semi-cokes from the heavier residua of the “poor” feeds DO93 and DO15. This observation leads to a closer examination on the difference in molecular composition between the original DOs and their heavier fractions as described in the following sections. Table 6.2 Relative OTI increment in the semi-cokes from vacuum residua DO91 DO93 DOPSU DOUP DO15 CF15 DO24 CF24 300+ 3% 2% 4% 1% -1% 4% -1% 1% 325+ 8% 14% 5% 11% 14% 15% 4% 5% 350+ 17% 32% 8% 11% 26% 18% 11% 9% 370+ 19% 38% 16% 13% 29% 22% 11% 9% 6.3 Changes in Molecular Composition of the Heavier Fractions Obtained by Vacuum Distillation The TICs in Figure 6.2 illustrate the change in PAH composition over various vacuum distillation conditions. A progressive removal of lighter molecular species from the feedstocks can be seen clearly. 207 Figure 6.2 GC/MS TICs of the vacuum residua from DO15 208 Table 6.3 and Table 6.4 list the relative distribution of the major PAH compounds (phenanthrenes and pyrenes) in their residua of DO91and DO93 from GC/MS analyses. There is a slight increase in the proportions of multi-methylated PAH with the increase in the vacuum distillation temperature, which is expected from the fact that multi-alkylation of PAH increases the boiling points of resulting PAH, in general (Morrison and Boyd, 1973). However, the distribution of the isomers of methylated PAHs remained almost the same as in the original decant oil samples (Table 6.5). Table 6.3 Distribution of phenanthrene homologs in vacuum residua, wt% DO91 DO93 Whole 300+ 325+ 350+ 370+ Whole 300+ 325+ 350+ 370+ PHEN 4% 6% 5% 1% 0% 2% 3% 3% 1% 0% MPHEN 21% 23% 24% 15% 8% 19% 23% 23% 18% 7% DMPHEN 37% 35% 39% 42% 41% 42% 41% 41% 42% 43% TMPHEN 33% 32% 29% 36% 45% 34% 31% 31% 38% 49% TEMPHEN 5% 4% 4% 5% 7% 3% 2% 2% 1% 2% Total PHEN 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% Table 6.4 Distribution of pyrene homologs in vacuum residua, wt% DO91 DO93 Whole 300+ 325+ 350+ 370+ Whole 300+ 325+ 350+ 370+ PY 4% 5% 5% 5% 3% 6% 6% 7% 7% 6% MPY 18% 20% 19% 17% 14% 21% 21% 22% 24% 28% DMPY 41% 43% 40% 43% 43% 38% 38% 36% 37% 34% TMPY 38% 32% 36% 35% 40% 35% 35% 35% 33% 33% Total PY 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 209 Table 6.5 Isomer distribution of phenanthrene and pyrene in vac. residua, wt% DO91 DO93 Whole 300+ 325+ 350+ 370+ Whole 300+ 325+ 350+ 370+ 3-MPHEN 28% 31% 30% 30% 29% 25% 27% 26% 26% 26% 2-MPHEN 39% 37% 38% 38% 39% 36% 35% 35% 35% 33% 9-MPHEN 19% 18% 19% 19% 18% 21% 22% 22% 22% 23% 1-MPHNE 14% 14% 14% 14% 14% 17% 16% 17% 17% 18% MPHEN 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 2-MPY 34% 34% 37% 37% 34% 33% 34% 35% 35% 32% 4-MPY 38% 37% 34% 30% 34% 36% 36% 36% 37% 36% 1-MPY 28% 29% 29% 33% 32% 31% 30% 30% 28% 33% MPY 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% The most significant changes in the molecular composition of vacuum residua were found in the total concentration of these major GC-amenable PAHs (phenanthrenes and pyrenes). The concentrations of these PAHs in various vacuum residua of DO91 and DO93 are presented in Figure 6.3 and Figure 6.4. 120000 PHEN 100000 PY 80000 60000 40000 concentration, ppm concentration, 20000 0 DO91 300°C+ 325°C+ 350°C+ 370°C+ Figure 6.3 Total PHEN and PY concentration change in DO91 by vacuum distillation 210 120000 100000 Phen PY 80000 60000 40000 Concentration, ppm 20000 0 DO93 300°C+ 325°C+ 350°C+ 370°C+ Figure 6.4 Total PHEN and PY concentration change in DO93 by vacuum distillation As the figures show, the concentrations of both phenanthrene and pyrene decrease with increasing distillation temperature; however, the degree of the concentration change is significantly different between the major PAHs. At AET 370˚C, phenanthrene in DO91 and DO93 was reduced by 80% while pyrene was reduced only by ~45-50%. A similar trend was also observed in other decant oil samples. The disproportional reduction in the concentrations of different PAHs emphasized that the relative ratio of the major PAHs is the most important compositional change in the vacuum residua that produced enhanced mesophase development. The plot of OTIs of semi-cokes from the original and the residual fractions in DO91 and DO93 versus the total pyrenes to phenanthrenes ratios is presented in Figure 6.5. The plot indicates a good correlation between the mesophase development (or semi- 211 coke texture) and the total pyrenes/phenanthrenes concentration ratios of the residual fractions. 100 90 80 70 60 50 OTI 40 30 20 10 0 0 0.5 1 1.5 2 2.5 PY/PHEN ratio in heavy residua of DO93 and DO91 Figure 6.5 OTI of semi-coke from residua (DO93 and DO91) vs. pyrenes/phenanthrenes concentration ratio This correlation between semi-coke’s OTI with the pyrenes/phenanthrenes ratio was examined over a wide range of decant oil and coker feed samples, Figure 6.6 presents a similar chart that was plotted from a larger set of carbonization experiments. These feedstock samples include the six decant oils and two coker feeds as presented in this thesis, and 32 more decant oil and coker feed samples which were studied in this thesis research but not reported in this thesis. As the figure shows, the OTI or the degree of anisotropy in semi-cokes can be related to the feedstock’s PAH composition. Although the data points are relatively wide scattered, there exists a reasonably good correlation between the OTI value and the pyrene/phenanthrene relative ratios in the coke feedstocks, particularly, for the low pyrenes/phenanthrenes ratios. 212 100 90 80 70 OTI 60 50 40 30 01234567 Pyrene/Phenanthrene (molar ratio) in decant oil and coke feed Figure 6.6 Correlation between the OTI of semi-coke with pyrene to phenanthrene ratio in the feedstock 6.4 Intermediate Product Yields from Lower Temperature Carbonization of Decant Oil and Decant Oil Derivatives Selected decant oils (DO91 and DO93) and a set of decant oil derivatives (DO24, HYD24, CF24 and VTB24) were carbonized at 450˚C from 15 minutes to 4 hours. The yield of intermediate products from this low-temperature experiment was used to establish the initial coking propensity of these feedstocks that produced semi-cokes with varied quality. In this study, semi-coke and asphaltenes were defined as the fraction of dichloromethane-insolubles and the fraction of dichloromethane-soluble and pentane- 213 insolubles in solvent extraction of the carbonization products, respectively. Gases produced during carbonization were not measured. Figure 6.7 shows the asphaltenes and semi-coke yields at various reaction times for decant oils DO91 and DO93. During the reaction, DO93 exhibited a higher rate of semi-coke formation than DO91, indicating a higher reactivity of the PAH compounds found in DO 93. The yield of asphaltenes increased with reaction time for both decant oil samples. However compared with DO91, DO93 gave a higher initial rate of asphaltenes formation in the early stages of carbonization, i.e., within 15 minutes of reaction, DO91 showed a steady increase in the yield of asphaltenes formation over a prolonged reaction period and a lower yield of semi-coke formation. The differences in the relative rates of asphaltenes and semi-coke formation can be related to the differences observed in the mesophase development for DO91 and DO93. The rapid formation of asphaltenes for DO93 in early stages and their fast transformation to semi-coke would hinder the mesophase development. This results from the rapid growth of mesogens in molecular size accompanied by the attendant increase in the viscosity of the carbonizing media in a short time period. Such changes would interfere with the formation of large anisotropic domains and their deformation into needle coke structure by the evolving volatiles. In contrast, the carbonization of DO91 would involve a slow growth of mesogens in a relatively fluid environment to promote the formation of anisotropic flow domains. The difference in the molecular composition of DO91 and DO93 discussed in previous sections can be related to the difference observed in the carbonization reactivity 214 and mesophase development from these feedstocks. This is further discussed in Section 7.4. 40% DO91-semi-coke 35% DO93-semi-coke 30% DO91-Asph DO93-Asph 25% 20% Yield, % 15% 10% 5% 0% 0 50 100 150 200 250 300 Reaction Time, min Figure 6.7 Semi-coke and asphaltene yields during carbonization of DO91 and DO93 at 450˚C The yields of semi-coke and asphaltene from DO24 and its derivatives are shown in Figure 6.8 and Figure 6.9, respectively. 215 80% DO24 70% CF24 HYD24 60% VTB24 50% 40% Coke Yield, % Yield, Coke 30% 20% 10% 0% 0 20 40 60 80 100 120 140 160 180 200 Time, Minutes Figure 6.8 Semi-coke yields from carbonization of DO24 derivatives at 450˚C 40% DO24 35% CF24 30% HYD24 25% VTB24 20% Asph. Yield, % Yield, Asph. 15% 10% 5% 0% 0 20 40 60 80 100 120 140 160 180 200 Time, minute Figure 6.9 Asphaltenes yields from carbonization of DO24 derivatives at 450˚C 216 The DO24 derivatives showed different rates of semi-coke formation under the same experimental conditions: VTB24 fraction, consisting of higher concentrations of heaviest polyaromatic compounds exhibited the highest yield of semi-coke throughout the reaction time periods. On the other hand, the lightest fraction, HYD24, gave the lowest semi-coke yields, as expected from the lower reactivity of the hydrotreated PAH matrix. CF24, which is closer in molecular composition to decant oil, showed a slower and more steady increase in coke formation compared to the decant oil at longer reaction times. All the derivatives except VTB24 showed a similar trend in asphaltenes formation during carbonization. The lowest asphaltenes yield is shown in the carbonization of HYD24, indicating, again, the lowest coking reactivity of this hydrotreated decant oil fraction. CF24 produced much higher content of asphaltenes than its parent decant oil throughout the reaction. It can be suggested that the slower transformation of asphaltenes to semi-coke during carbonization of CF24 explains the better mesophase development during the early stages of carbonization and results in higher degree of orientation or anisotropy in the optical texture of solid coke product. Very high yield of asphaltene from VTB is shown in the early stage of carbonization (within 120 minutes in this case). However, the trend of asphaltene yield is different from that of the other streams. A very rapid initial increase in asphaltene yield is followed by a rapid conversion of asphaltenes to semi-coke, as shown in Figure 6.8 and Figure 6.9. The well-developed mesophase from VTB, despite a fast conversion of asphaltenes into semi-coke, may be attributed to a favorable interaction between the 217 evolution of volatiles and the carbonizing viscous phase (with high asphaltene contents) to form elongated anisotropic domains. 6.5 General Trends in the Molecular Composition Change in the Early Stages of Liquid-Phase Carbonization of Decant Oil and Its Derivatives In parallel to measuring the intermediate bulk products yields (asphaltene and semi-coke) from lower-temperature carbonization, the liquid products (DCM-soluble fraction of the carbonization products) were analyzed by GC/MS. The liquid products were obtained from 15 minutes to 60 minutes of the heat treatment at intervals of 15 minutes. As described in previous section, the semi-coke yields at 60 minute reaction time were less than 5% from all the feedstock samples except VTB stream. This assumes that the reaction is still dominated by the starting material’s molecular composition where the further condensation of mesogenic species is not significant. 6.5.1 Normal alkanes The total concentrations of n-alkanes in the liquid products of short-time carbonization were reduced by 80% and 70% of the original concentrations in DO24 and CF24 respectively within 15 minutes reactions (Figures 6.10). HYD stream appeared to have the same concentration of n-alkanes during the same reaction time. After 45 minutes of reaction time, almost all the n-alkanes disappeared from the liquid products of all samples. 218 25000 20000 DO24 CF24 HYD24 15000 10000 Conc. ppm in Feed Conc. 5000 0 0 min 15min 30 min 45 min 60 min Reaction Time, min Figure 6.10 Alkane concentration in the product from DO24, CF24 and HYD24 within 60 minutes of carbonization at 450˚C The distribution pattern of normal alkanes in liquid products at various reaction times follows a similar trend among decant oil and its derivatives. These normal alkanes are much more reactive than PAH in the feedstock. By 30 minutes of the carbonization reaction, the most abundant normal alkane distribution shifted to lower carbon number range, indicating that the alkanes were cracked extensively into smaller compounds. After 45 minutes, the normal alkanes can barely be detected in the liquid product from the decant oil and its derivatives. The high extent of cracking during carbonization of HYD24 is clearly illustrated in Figure 6.11. 219 3000 2500 0 min 15min 2000 30 min 1500 Conc., ppm Conc., 1000 500 0 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21 C22 C23 C24 C25 C26 C27 C28 C29 C30 C31 C32 500 450 400 0 min 350 15min 30 min 300 250 200 Conc. ppm 150 100 50 0 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21 C22 C23 C24 C25 C26 C27 450 400 0 min 350 15min 300 30 min 250 200 Conc., ppm 150 100 50 0 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21 C22 C23 C24 C25 C26 C27 C28 Figure 6.11 Alkane distribution in the products from DO (top), CF (middle) and HYD (bottom) at 15 and 30 minutes reaction time for carbonization at 450˚C 220 6.5.2 Phenanthrene and pyrene PHEN, PY and their methyl homologues show a steady increase in concentration as the reaction proceeded (within 60 minutes of reaction monitored). In contrast, their dimethyl- and trimethyl- homologues exhibited a continuous decrease in concentration as can be seen from the MICs of PHEN and PY homologues in Figure 6.12. 15min C C 30min C C C C 45min C C C C C C 60min 20.0 30.0 40.0 20.0 30.0 40.0 Retention Time, min Retention Time, min Figure 6.12 MICs of PHEN and PY in the product at various reaction time during carbonization of DO at 450˚C 221 This PAH molecular composition change can be related to the higher thermal stability and lower carbonization reactivity of less-substituted PAHs compared to that of the multi-alkylated homologues. Table 6.6 lists the distribution of phenanthrene and pyrene isomers in the liquid product at different reaction times. With the progress of carbonization, the distribution of isomers of alkyl-PAH’s homologues in decant oil samples shows a similar trend. The distribution of isomers shifted to the compounds dominated by the thermally stable isomers. The isomer composition in CF remained almost unchanged during the short-time carbonization; this observation may indicate that the molecular composition of the PAHs in CF feed is closer to the thermodynamic equilibrium of isomer distribution in these PAH compounds, which may explain the lower carbonization reactivity of this feedstock. 222 Table 6.6 Isomer distribution of major PAHs in the liquid products of short time carbonization (wt,%) DO91 DO93 DO24 Compound 0min 15min 30 min 45 min 60 min 0min 15min 30 min 45 min 60 min 0min 15min 30 min 45 min 60 min 3-MPHEN 28.0% 28.6% 29.1% 36.4% 39.0% 25.0% 24.9% 32.3% 35.7% 40.4% 31.0% 32.0% 33.3% 36.5% 39.0% 2-MPHEN 39.0% 38.1% 39.2% 41.6% 45.6% 36.0% 36.1% 38.8% 43.3% 43.7% 35.0% 35.0% 39.0% 41.3% 45.6% 9-MPHEN 19.0% 19.5% 18.9% 11.8% 7.5% 21.0% 21.1% 16.4% 10.5% 7.4% 19.0% 19.0% 15.7% 11.9% 7.5% 1-MPHNE 14.0% 13.8% 13.2% 10.2% 7.9% 17.0% 17.9% 12.5% 10.4% 8.5% 15.0% 14.0% 12.0% 10.3% 7.9% 2-MPY 33.8% 34.3% 36.0% 52.4% 60.2% 33.0% 34.0% 44.3% 56.5% 66.0% 40.2% 40.4% 42.0% 47.0% 58.4% 4-MPY 38.0% 37.8% 36.9% 28.3% 24.5% 36.0% 36.4% 31.7% 26.5% 21.4% 29.5% 29.8% 28.9% 28.9% 25.3% 1-MPY 28.2% 27.8% 27.1% 19.3% 15.3% 31.0% 29.6% 24.1% 17.0% 12.6% 30.3% 29.7% 29.1% 24.1% 16.3% HYD24 CF24 Compound 0min 15min 30 min 45 min 60 min 0min 15min 30 min 45 min 60 min 3-MPHEN 32.0% 32.1% 33.3% 37.3% 40.8% 39.0% 38.0% 38.0% 38.3% 40.0% 2-MPHEN 38.0% 37.6% 40.2% 43.3% 45.9% 47.0% 47.2% 46.2% 44.9% 45.0% 9-MPHEN 15.0% 14.9% 16.1% 9.5% 6.2% 7.0% 7.4% 7.8% 8.2% 7.9% 1-MPHNE 15.0% 15.4% 10.4% 9.9% 7.1% 7.0% 7.5% 8.0% 7.7% 7.2% 2-MPY 38.0% 40.8% 44.8% 51.7% 54.0% 69.3% 69.0% 68.0% 71.6% 72.7% 4-MPY 30.0% 27.5% 25.7% 23.9% 19.6% 17.6% 18.6% 17.8% 14.1% 11.8% 1-MPY 32.0% 31.6% 29.5% 24.8% 16.9% 13.2% 13.3% 14.2% 13.9% 15.5% 223 It is difficult to quantitatively compare change in the concentration of targeted PAH’s between original feedstock and the products from carbonization. Because, in addition to the actual molecular composition change during the reaction, several other factors also affect the GC measurement of the concentration profiles. These factors include the dilution by the cracking products and the enrichment by removing the even larger PAH species (which are not GC/MS amenable) in the liquid product. Nevertheless, by comparing the relative abundance of different kinds of PAHs in the same liquid product, the overall changes in molecular composition during carbonization can be reasonably monitored. Figure 6.14 shows the relative abundance of unsubstituted phenanthrenes and pyrenes during the carbonization of DO15 and CF15 at 450˚C. The ratio of pyrene to methylpyrene and ratio of phenanthrene to methylphenanthrene both increase over the prolonged carbonization time, indicating the demethylation and/or oligomerization of the more reactive methyl-PAH analogues in the early stages of carbonization. CF and HYD samples show a slower progress in the removal of methylphenanthrenes and methylpyrenes during the carbonization, indicating the overall lower reactivities in the molecular composition in HYD and CF, compared to that of the DO sample. The larger increase in the ratio of PY to MPY (3.5 times higher) than that of Phen to MPhen (1.5 times higher) in DO at 60 minutes reaction shows that larger methylPAHs (i.e., pyrenes) are more reactive than smaller methylPAHs (such as phenanthrenes). 224 1.4 1.2 1 Phen/MPhen_DO 0.8 PY/MPY_DO 0.6 Phen/MPhen_CF Molar ratio PY/MPY CF 0.4 0.2 0 0 10203040506070 Reaction time, minutes Figure 6.13 Ratios of PAHs to their methyl-substituted homologs in the products during carbonization of DO15 and CF15 at 450C. The change of Py/Phen (unsubstituted PAH) ratio in DO and CF is shown in Figure 6.15. 2.5 Py/Phen_DO 2 PY/Phen_CF 1.5 Molar ratio 1 0.5 0 0 15304560 Reaction time, min Figure 6.14 The ratio of unsubstituted pyrene to phenanthrene vs. reaction time during carbonization of DO15 and CF15 at 450C. 225 Both DO and CF show a decreasing ratio of Py/Phen, which can be attributed to the incorporation of the pyrene into asphaltenes or semi-cokes. The rapid decrease in the ratio at the initial reaction stage in DO suggests that more extensive reactions than in CF are taking place during carbonization. 6.6 The Relationship between the Molecular Composition of Feedstocks and Mesophase Development The quality of semi-coke texture from carbonization of feedstock was found to be determined, in essence, by the degree of mesophase development in the narrow temperature range of 450- 500°C during the early stages of reaction (White, 1976; Marsh, 1986; Mochida, Fujimoto et al., 1994; Eser, 1998). The mesophase formation and development from the feedstock (mixture of aromatic and aliphatic hydrocarbons) are the consequences of both chemical reaction, i.e., polymerizing to form mesophase spheres, and physical change, i.e., stacking of the resultant intermediate products by intermolecular forces (e.g., van der Waals forces) or by external forces (e.g., the shearing forces of the eluting gas from the plastic carbonizing matrix) (Brooks and Taylor, 1965; White and Price, 1974; Auguie, Oberlin et al., 1980; Mochida and Korai, 1986). The overall reactivity of the starting compounds in the feedstock, the spatial configuration of molecular intermediates, and the viscosity of the reacting liquid govern the degree of mesophase development, and, thus, the optical texture of the resulting semi-coke. Needle coke feedstocks (DO and its derivatives) have a similar elemental composition (see Table 4.1), consisting of approximately 90% of carbon, 8-10% of hydrogen, and low heteroatom (O, N, S) concentrations. The overall reactivity of carbonization depends on the molecular composition of the feedstock, in particular, on 226 the larger PAH species since these high-molecular-weight aromatic species tend to quickly polymerize into asphaltenes and semi-coke upon heat treatment (see Figure 6.8). In the most general terms, the molecular composition in decant oil derived needle coke feedstocks can be described as the distribution of PAHs and n-alkanes. Normal alkanes, the least thermally-stable species in needle coke feedstocks, can be expected to serve as free radical initiators in the early carbonization reactions. High concentration of alkanes in the feedstock would lead to a rapid reaction rate that could hinder the mesophase development (Filley, 1997). However, as the carbonization experiment results showed (Section 4.5), feedstocks having the highest concentration of normal alkanes such as DOUP and DOPSU produced the highest degrees of mesophase development. This seemingly contrary result may suggest that the role of normal alkanes in the carbonization of feedstock may be complex and depends on the composition of the PAH as well, as far as the formation of aromatic mesogens is concerned. In other words, the interaction of the reactive species (produced by n-alkane cracking) with the constituent PAH is expected to depend on the structure and reactivity of these PAHs. The composition of PAHs (which serve as coke precursor) in the feedstock should play the dominant role in the formation of mesogens and, thereafter, the mesophase development. A sensitive balance between the rate of carbonization, which is controlled by the reactivity of the molecular components, and the viscosity of the carbonizing system, which depends on the molecular growth and alignment processes, is critical for a high degree of mesophase development (Marsh and Walker, 1979; Lewis, 1982). The lower proportion of highly methylated PAHs, or higher proportion of thermally stable unsubstituted PAH (such as pyrene), is shown in this study to promote mesophase 227 development (for example in DOUP, DOPSU). The abundance of thermally stable compounds in the feedstocks leads to an overall low reactivity (for semi-coke formation or carbonization), providing a gradual polymerization of PAH into mesogens in a prolonged fluidity window and result in a high degree of mesophase during carbonization. In contrast, the carbonization of DO93 and DO15, containing high proportions of thermally reactive multi-methylated PAHs, result in an inferior semi-coke indicating a lower degree of mesophase development. One of the molecular characteristics of the coker feeds (CF) that differs from the decant oil is the even higher proportions of unsubstituted PAHs (3 times higher phenanthrene and ~ 3-8 times higher pyrene than in the decant oil) and less multi- methylated PAHs, and, in particular, the dominating presence of thermally stable methylPAH isomers. Coker feeds, therefore, show lower overall rates of semi-coke formation (as shown in Figure 5.8 and Figure 5.9) and result in better mesophase development than DO. Another molecular feature of CF is the presence of hydroaromatic compounds (see Section 5.1 and Figure 5.66). These hydroaromatics have the hydrogen- donor ability. The transferable hydrogens from such hydroaromatic compounds have a significant role in stabilizing the reactive species (free radicals formed either from starting PAH or from intermediate products), therefore, lowering the rate of carbonization, facilitating the formation of polynuclear aromatic molecules leading to better mesophase development (Marsh and Neavel, 1980; Mochida, Shimizu et al., 1988). Hydrotreated decant oil (HYD), the lightest feedstock, is more complex in molecular composition than either decant oil or coker feed. The presence of high concentrations of hydroaromatic compounds from the whole range of PAHs (in decant 228 oil) makes this feedstock the least reactive (because of the higher amount of transferable hydrogens) during carbonization and transforms the PAHs into semi-coke with the highest degree of mesophase development. The high-molecular-weight PAHs enriched feedstock, VTB, was found to produce a better mesophase than DO. A similar result was reported from the carbonization of the heavy fraction of petroleum pitch (Chwastiak and Lewis, 1978). In VTB, the dominant starting molecules are five-ring to six-ring aromatics (benzopyrene, benzo[g,h,i]perylene and dibenzopyrene and their alkylates). It would take a lower degree of thermal polymerization for these large peri-condensed PAHs to form mesogenic molecules (molecular weight ~600-800) than from low-molecular weight PAHs (Greinke, 1994). Therefore, the chance of forming oligomers (among the smaller PAHs and between larger PAH and smaller PAH) that may lead to less planar intermediate PAHs would be greatly reduced during the carbonization of heavy PAHs. This may be the reason why large reactive PAHs enriched feedstock shows a higher anisotropy in the semi-coke product. These results agree with previous reports that both the composition of the starting material and the configuration and reactivity of intermediate products have a significant influence on mesophase development (Marsh, 1986; Mochida, Fujimoto et al., 1994). For all the decant oil and coker feed samples studied in this research, it was found that there exists a good correlation between the optical texture index of semi-cokes and the (total) pyrene to (total) phenanthrene ratios (Figures 6.5 and 6.6). The positive effect of having high concentrations of methylpyrenes on mesophase development was reported (Eser, 1998). Previous studies had shown that phenanthrene, when carbonized, exhibits a 229 longer induction period followed by a rapid rate of carbonization resulting in poor mesophase development (Sasaki, Jenkins et al., 1993a; Sasaki, Jenkins et al., 1993b). Pyrene was found to be stable at relatively high-temperature and it could also serve as a hydrogen shuttler that provides a prolonged fluidity period during the carbonization (Murakami, Okumura et al., 1996; Filley, 1997). The results from this study show that high concentrations of multimethyl substituted phenanthrenes in the feedstocks demotes mesophase development, and lowers the OTI of the resultant semi-coke. Furthermore, the ratio of pyrenes to phenanthrenes can 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. This is demonstrated by an example of free radical addition reaction scheme presented in the next chapter (Figure 7.6) that covers an exploratory quantum chemistry study of interactions between principal PAH found in coker feedstocks. 230 Chapter 7 A Semi-empirical Molecular Orbital Study of Thermal Reactivities of Major PAH in Needle Coke Feedstocks The objective in this computational chemistry study is to investigate the effects of alkylation on the radical formation from PAHs and on the spatial configuration of reactive intermediates. Both factors closely relate to the reaction kinetics in the early stages of carbonization and to the mesophase development. The ease of initiating free radicals from PAH in the feedstocks was quantitatively evaluated in terms of their thermochemical properties. The most probable path of condensation or polymerization reactions is simulated on the intermediate products (such as a dimer of starting PAHs). This would help understand the relationship between molecular composition and carbonization reactivity, and the observed differences in the mesophase development among decant oil and their derivatives. Papers published on the pyrolysis of hydrocarbons established the following major reversible reactions for the initiation of free radicals (Stein, 1981; Poutsma, 1990): Homolysis (breaking a covalent bond to form two radicals) of a hydrocarbon compound is the most common radical formation process. This process depends most strongly on the stability of the free radicals formed, since the reverse reaction (combination of free radicals) is not kinetically activated, i.e., in the absence of major steric effects. Bond breaking takes place at the most susceptible site on a hydrocarbon molecule where the bond (C-H, or C-C) is the weakest. Because C-C bonds tend to be weaker than C-H bonds in most cases, breaking the C-H bond to give a free radical and H radical is seldom 231 competitive with the C-C bond homolysis. One exception is the C-C bond attached to an aromatic ring. For example, Ph-CH3 is much stronger than PhCH2-H in toluene molecule, and the formation of a benzyl free radical is favored over a phenyl radical since the benzyl radical is resonance-stabilized (Stein, 1981). Bimolecular disproportionation produces two radicals in the reaction between a C-H bond in one molecule and a π bond in another. One example is the free radical formation from an anthracene molecule which produces 9,10-dihydroanthracene and 9,10-anthracenyl radical (Stein, 1981; Scaroni, Jenkins et al., 1991). Molecular disproportionation is less competitive to homolysis, and this reaction is only significant under special circumstances where the reactant lacks a weak C-C and the reactant is at a high concentration and relatively low temperatures (McMillan and Golden, 1982; Poutsma, 1990). A third major contributor to the initial activation of hydrocarbon molecules is the cleavage of a C-H bond by hydrogen transfer to an attacking radical (hydrogen abstraction). Parallel to the hydrogen abstraction by a free radical is the β scission where the C-C bond β to the radical center is broken to form an olefin and a smaller radical. The activation energy for the transition state is usually a fraction of the energy needed for breaking the C-H bond and it is the main path of free radical propagation in the cracking of long-chain alkanes. Free radical initiation, chain propagation and termination reactions depend strongly on the temperature, pressure and concentration (phase) of the reactants as they react to the final products. A MOPAC program package, version 6.12 for Windows (from Fujitsu) was used in this study for computing the electronic and thermodynamic properties of the PAHs, the 232 aryl (and aryl-alkyl) radicals and intermediate products. A semi-empirical MNDOd subroutine was chosen for all the quantum chemistry calculations. Two steps are generally involved in a typical computation. Geometry optimization for PAHs and free radicals was found by using the eigenvector following (EF) optimization and thermodynamic properties of the corresponding geometries were calculated by using singlet (for PAHs) and doublet (for free radicals) multiplicity (Stewart, 1989). 7.1 The Overall Reactivity of PAH Molecules As described in Chapter 5, the major PAH components in decant oil consist of alkylated 3-6 ring aromatic compounds. Based on the abundance of these PAH compounds in the feedstocks, five PAH molecules (phenanthrene, pyrene, chrysene, benzo(e)pyrene and benzo[g,h,i]perylene) were selected as input for quantum chemistry calculations. Figure 7.1 shows the structures and numbering conventions of these representative PAH compounds in needle coke feedstocks. 233 10 1 9 10 9 2 8 1 3 8 7 2 7 4 6 5 4 3 6 5 phenanthrene pyrene 2 1 3 12 12 1 11 2 11 4 10 3 10 5 9 4 9 5 8 6 8 7 6 7 chrysene benzo[e]pyrene 10 11 12 9 8 1 2 7 3 6 5 4 benzo[ghi]perylene Figure 7.1 Structures and numbering notations of the predominant PAHs in needle coke feedstocks Chemical reactivity of a molecule consists of two essential aspects, i.e., thermodynamic and kinetic stability (Smith and Savage, 1991; Savage, 2000). Thermodynamic properties are “intrinsic” to molecules themselves and can be 234 determined uniquely by either experimental or computational means. However, kinetic stability is affected by the levels of intermolecular interaction and other factors. Section 7.1.3 provides a preliminary discussion on the kinetics of PAH reactions that would lead to mesophase formation. Hardness and softness as global reactivity indices are well known in chemistry (Pearson and Yang, 1988). They can be derived theoretically from quantum density function theory (TFT). This concept is based on the principle that the frontier molecular orbitals in molecule(s) are most likely to interact between reactants (Fukui, Yonezawa et al., 1952). Frontier orbitals include two consecutive ones: the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). HOMO energy indicates the ease of losing an electron (ionization potential) and LUMO indicates the ease of acquiring an electron (electron affinity). The calculated HOMO and LUMO results for PAHs are listed in Table 7.1. 235 Table 7.1 HOMO and LUMO of PAH and alkylated substitutes using MNDOd level of theory PAH HOMO,eV LUMO,eV LUMO-HOMO,eV Phenanthrene -8.478 -0.481 7.997 Methyl-sub -8.455 ~-8.483 -0.478~-0.528 7.931~8.005 Dimethyl-sub -8.49~ -8.4636 -0.521~ -0.565 7.978~ 7.939 Trimethyl-sub -8.419~ -8.443 -0.559~0.602 7.817~7.884 Tetramethyl-sub -8.404~8.405 -0.632~ -0.639 7.765~7.772 Pyrene -8.038 -0.902 7.138 Methyl-sub -8.019~ -8.035 -0.896 ~ 0.944 7.075~ 7.148 Dimethyl-sub -8.004 ~ 8.005 -0.967~-0.981 7.022 ~7.038 Trimethyl-sub -7.987 ~-8.01 -0.965 ~-1.001 6.986 ~7.044 Chrysene -8.26 -0.719 7.541 Methyl-sub -8.244 ~ 8.263 -0.729 ~-0.762 7.478 ~7.534 Dimethyl-sub --8.218~ -8.24 -0.761~-0.790 7.434~ 7.479 Benzo[e]pyrene -8.108 -0.889 7.219 Methyl-sub -8.039 ~ -8.111 -0.883 ~-0.923 7.116~7.228 Dimethyl-sub -8.076~ -8.083 -0.946 ~-0.954 7.119~7.137 Benz[g,h,i]perylene -7.94 -1.071 6.869 Methyl-sub -7.903 ~-7.940 -1.078 ~-1.106 6.797~6.862 Dimethyl-sub -7.93 ~-7.940 -1.087~ -1.114 6.816~6.858 Trimethyl-sub -7.919 ~-7.936 -1.122~ -1.247 6.672~ 6.814 236 The HOMO energy of PAH molecules increases with increasing number of rings (-8.478 for phenanthrene to -7.94 for benzo[g,h,i]perylene), indicating that the paired electrons, in general, are most likely to unpair to form a radical more readily from large PAH molecules. Also listed in Table 7.1 are the HOMO energies of methylated PAHs. These energy values depend on the specific alkylation position on the PAH ring system, and therefore, the HOMO results are listed as a range of these values to reflect the variation among the different isomers of a given alkyl PAH. In general, the alkylation of a PAH molecule slightly increases the HOMO within a given PAH configuration, suggesting that the multi-alkylation would tend to destabilize the PAH. In Stein and his co-worker’s work (Stein and Brown, 1987), the π –electron properties of very large PAHs (up to 2814 carbon atom) were calculated by Hukel molecular orbital (HMO) theory. In general, the HOMO gives an approximation to the overall reactivity toward free radicals and electrophiles. However, they found that there is only a weak relation between the energy of HOMO and the reactivity of the most reactive position on the PAH. Their results suggest that the thermodynamic stability is strongly influenced by the edge structure (spatial configuration) of the PAH. 7.2 Position of Carbon-centered Free Radicals on PAH Ring Structures As is well-known, the primary reaction pathway of carbonization proceeds via free radical chain reactions (Lewis, 1982; Mochida, Korai et al., 1986). HOMO energy values of a PAH molecule give an approximate indication of the global reactivity. In the condensation reaction of PAH molecules, site selectivity is of greater importance, since the location of the reactive sites on the edge of a PAH molecule can affect the spatial 237 configuration of the intermediate products, and therefore, the subsequent mesophase development. 7.2.1 A thermochemical approach to the reactivity of PAHs One of the descriptors of local reactivity is the ease of forming a free radical from a PAH molecule. Thermodynamically, a stable radical is the easiest to form regardless of the mechanism of radical formation. Therefore, the heat of formation of resultant radical ΔHf (R*) can serve as a local reactivity descriptor (Schaad and Hess, 1972; McMillan and Golden, 1982). When a free radical is formed by thermal homolysis, bond dissociation energy or BDE can serve as a better descriptor. The BDE is defined as follows: BDE (R-H or -CH3)=ΔHf (R*)+ ΔHf (H* or CH3*)- ΔHf (R-H, -CH3) where ΔHf is the heat of formation at standard state. As can be seen from the above definition, BDE normalizes the heat formation of free radical formation with that of the parent molecule, thus, BDE in homolysis reactions provides more useful information on free radical initiation from the PAH molecules (McMillan and Golden, 1982; Poutsma, 2000). Free radical initiation in the homolysis of unsubstituted PAH molecule can be illustrated by following reaction: RHÆ R*+H* (1) When a methyl-PAH is involved in homolysis, either of the following reactions will take place: RCH3ÆR*+CH3* (2) 238 RCH3ÆRCH2*+H* (3) The resulting radicals from methyl-PAH will produce two types of free radicals: the aryl (R*) by breaking the R-CH3 bond in a methylated PAH and aryl-alkyl (RCH2*) radical by cleaving H-C in the methyl group. The heats of formation (expressed at standard state in all the results reported in this study) of forming aryl radicals were also computed using the MNDOd subroutine and are listed in Table 7.2. For the validity of computation methodology in this study, the available experimental data are also listed in this table. The experimental heats of formation are slightly higher than those calculated by the MNDOd method. Several researchers have shown that the computational deviation from experimental data is systematic for a family of structurally related hydrocarbon compounds (Herndon, Paul et al., 1992; Camaioni, Autrey et al., 1996; Ma and Schobert, 2000). Therefore, in this study, no effort was made to pursue the data accuracy since the focus in the thermochemistry analysis here is to compare the differences between the structurally closely related families. In addition, the errors involved in the calculation of BDE will be compensated by the cancellation of the errors in the computed heats of formation of both reactant PAH and the resultant radical. Experimental data for the heat of formation of H radical, 52.10 kcal/mol, and methyl radical, 34.82 kcal/mol (NIST, 2005), were used in BDE calculations. The MNDOd results confirm that ΔHf of PAH molecules increases with the aromatic ring size, as expected. This trend in molecule’s stability agrees with HOMO properties presented in section 7.1. The ΔHf of all possible methylated PAHs listed in Table 7.2 for individual parent compounds show strong position dependence. 239 Table 7.2 Heat of formation of PAH and their alkylated analogues ΔHf, kcal/mol Parent PAH MNDOd Exptl *. phenanthrene 54.9 49.7 1-methylphenanthrene 52.4 N/A 2-methylphenanthrene 48.0 N/A 3-methylphenanthrene 48.2 N/A 4-methylphenanthrene 57.0 N/A 9-methylphenanthrene 51.3 N/A pyrene 60.7 54.0 1-methylpyrene 56.7 N/A 2-methylpyrene 53.2 N/A 4-methylpyrene 55.9 N/A chrysene 74.9 64.5 1-methylchrysene 71.6 N/A 2-methylchrysene 67.1 N/A 3-methylchrysene 67.4 N/A 4-methylchrysene 84.2 N/A 5-methylchrysene 80.2 N/A 6-methylchrysene 70.6 N/A benz[e]pyrene 80.2 N/A 1-methylenz[e]pyrene 90.6 N/A 2-methylenz[e]pyrene 72.8 N/A 3-methylenz[e]pyrene 76.6 N/A 4-methylenz[e]pyrene 77.1 N/A 9-methylenz[e]pyrene 90.9 N/A 10-methylenz[e]pyrene 72.6 N/A benz[g,h,i]perylene 82.5 N/A 1-methylbenz[g,h,i]perylene 79.1 N/A 3-methylbenz[g,h,i]perylene 77.9 N/A 4-methylbenz[g,h,i]perylene 79.3 N/A 5-methylbenz[g,h,i]perylene 77.7 N/A 6-methylbenz[g,h,i]perylene 75.1 N/A 7-methylbenz[g,h,i]perylene 87.2 N/A * experimental data were obtained from NIST Standard Reference Database 69. The data can be accessed online at (http://webbook.nist.gov/chemistry/) For most methyl-PAH isomers (e.g., 2-, 3-, 9-, 1-methylphenanthrene, 1-, 2-, 3-, 6-methylchrysene), the ΔHf. is lower than their parent PAH. This result seems to be contrary to the general reactivity trend predicted by the global molecule orbital 240 properties. The most probable reason for this discrepancy is the structural change between the unsubstituted and alkylated PAH compound: As a first approximation, forming sigma bonds (in the case of alkylation) will need less energy than forming C-C double bonds in the aromatic structure. Some methyl-substituted PAH isomers show even higher heats of formation than their parent PAHs. These isomers have the alkyl substitution take place on the more sterically hindered site as in the 4-methylphenanthrene, 4- and 5-methylchrysene, and 7- benzperylene. Therefore, it can be intuitively inferred that the reactivity of such sterically hindered compounds should be the highest. However, the presence of these energetic isomers is expected to be in lower concentration, if at all present in decant oil and coker feed samples. This is true at least for the distribution of 4-MPhen (trace amount) and 4-, 5-MChry (<5%). The computed BDE forming an aryl radical from PAHs is shown in Table 7.3. BDE of unsubstituted PAH shows a similar range (104 to 109 kcal/mol) regardless of the starting molecule. While forming exactly the same free radicals from methylated PAHs, the energy needed is much lower (80-90 kcal/mol) than that required from unsubstituted PAHs. The difference in BDE values suggests that in a mixed system (both unsubstituted and substituted PAHs), the cleavage of methyl group from methyl-PAH is the predominant reaction pathway to form aryl free radicals. 241 Table 7.3 BDE of PAHs to form aryl radical BDE, kcal/mol Aryl Radical From PAH From M-PAH 1-phenanthrenyl 108.2 93.4 2-phenanthrenyl 109.8 99.5 3-phenanthrenyl 109.3 98.7 4-phenanthrenyl 105.1 86.5 9-phenanthrenyl 107.8 94.1 1-pyrenyl 107.8 94.5 2-pyrenyl 108.4 98.6 4-pyrenyl 107.0 94.5 1-chrysenyl 107.5 93.6 2-chrysenyl 109.1 99.6 3-chrysenyl 108.4 98.7 4-chrysenyl 104.6 78.1 5-chrysenyl 104.1 81.4 6-chrysenyl 107.0 94.0 1-benz[e]pyrenyl 104.9 76.7 2-benz[e]pyrenyl 108.5 98.6 3-benz[e]pyrenyl 108.0 94.2 4-benz[e]pyrenyl 107.1 92.9 9-benz[e]pyrenyl 105.1 76.1 10-benz[e]pyrenyl 109.0 99.3 1-benz[g,h,i]perylene 107.5 93.6 3-benz[g,h,i]perylene 107.4 94.8 4-benz[g,h,i]perylene 107.1 93.1 5-benz[g,h,i]perylene 108.0 95.5 6-benz[g,h,i]perylene 108.4 98.6 7-benz[g,h,i]perylene 104.9 82.9 Within the methylPAH homologues, some free radicals are more thermodynamically favorable than others. Analyzing the BDE data of radicals and the position on the PAH ring leads to the following observation. In an unsubstituted PAH, stronger C-H bonds are adjacent to two secondary benzene-like carbon atoms, examples are the 2-,3-methylphenanthrene, 2-methylpyrene, 2-, 3-methylchrysene, and 6- 242 methylbenz[g,h,i]perylene. Weaker bonds are adjacent to one secondary carbon atom and one tertiary carbon atom, and examples are 1-,9-methylphenanthrene, 1-,4- methylpryene,1-,6-methylchrysene, 3-,4-methylbenz[e]pyrene, and 4-,5- methylbenz[g,h,i]perylene. The weakest bonds are located at arm-chair carbon atom, and the steric effect promotes the cleavage of the H from the substrate. The order in the ease of free radical initiation on methylated PAH also follows the aforementioned observation, for example, 1-, 9-phenanthryl’s BDE is less than that of the 2- or 3-phenanthryls. Multi-methylation slightly lowers the BDE of aryl-C bonds, but the position of the most likely aryl radical remains the same on the unsubstituted PAHs as the data show in Table 7.4. Table 7.4 BDE of forming aryl radicals at various sites on multi-alkylated PAHs Radical position -H from PAH -CH3 from MPAH -CH3 from DMPAH -CH3 from TMPAH 1-aryl of Phen 108.2 93.4 93.4 93.3 2-aryl of Phen 109.8 99.5 99.5 98.8 3-aryl of Phen 109.3 98.7 98.7 98.8 9-aryl of Phen 106.8 94.1 94.1 93.4 1-aryl of Py 107.8 94.5 94.5 2-aryl of Py 108.4 98.6 98.5 4-aryl of Py 107.0 94.5 93.1 Note: BDE in kcal/mol; MPAH, DMPAH and TMPAH refer to mono, di and tri- methyl substitutes; Phen=phenanthrene, Py=pyrene. A parallel reaction to forming aryl radical from methylPAHs is the formation of aryl-alkyl radical by cleaving the H from methyl group. The BDEs (in Table 7.5) associated with forming aryl-alkyl radical clearly show that the formation of aryl-alkyl radical is more favorable than forming aryl radicals. The reason for the preferred 243 formation of aryl-alkyl radicals is the dominant conjugation effect of the unpaired electrons on the methyl group with π-electrons over the large aromatic substrate. The ease of aryl-alkyl radical initiation with respect to the position of the edge carbon atoms on the aromatic ring is found to follow the same rule as in forming the aryl free radical. Table 7.5 BDE of forming aryl-alkyl radical from methyl-PAHs BDE, kcal/mol Parent PAH Aryl-alkyl radical Aryl radical 1-methylphenanthrene 87.8 93.4 2-methylphenanthrene 89.6 99.5 3-methylphenanthrene 89.6 98.7 4-methylphenanthrene 81.0 86.5 9-methylphenanthrene 88.1 94.1 1-methylpyrene 87.5 94.5 2-methylpyrene 90.3 98.6 4-methylpyrene 87.6 94.5 1-methylchrysene 87.6 93.6 2-methylchrysene 89.7 99.6 3-methylchrysene 89.6 98.7 4-methylchrysene 78.6 78.1 5-methylchrysene 82.8 81.4 6-methylchrysene 88.9 94.0 1-methylbenz[e]pyrene 76.8 76.7 2-methylbenz[e]pyrene 90.4 98.6 3-methylbenz[e]pyrene 88.4 94.2 4-methylbenz[e]pyrene 86.8 92.9 9-methylbenz[e]pyrene 77.4 76.1 10-methylbenz[e]pyrene 90.0 99.3 1-methylbenz[g,h,i]perylene 89.7 93.6 3-methylbenz[g,h,i]perylene 88.8 94.8 4-methylbenz[g,h,i]perylene 88.6 93.1 5-methylbenz[g,h,i]perylene 89.5 95.5 6-methylbenz[g,h,i]perylene 90.4 98.6 7-methylbenz[g,h,i]perylene 83.1 82.9 244 7.2.2 Molecular orbital calculation approach to the study of reactivity of PAHs Chemical reactions of large molecules, such as condensed aromatic ring system, involve not only the higher molecular orbitals (i.e. LUMO and HOMO), but also a set of active orbitals near the HOMO and LUMO. Fukui (Fukui, Yonezawa et al., 1952) introduced a quantity called the frontier electron density that is the weighted sum of the squares of the molecular coefficients. N N 2 −λ (εHOMO −ε j ) 2 −λ (εLUMOo −ε j ) ∑v jφ j (x) e ∑(2 − v j )φ j (x) e 2 − v j=1 v j=1 f (x) = N + N 2 2 −λ (εHOMO −ε j ) 2 −λ (εLUMO −ε j ) ∑v j (x) e ∑(2 − v j )e j=1 j=1 where • λ is a scale factor that is usually set to 3.0, • ν is a number indicating the type of reaction: 0 for an electrophilic reaction, 1 for a radical reaction, and 2 for a nucleophilic reaction; • N is the total number of orbitals; • νj is the number of electrons in orbital j; • φj(x) is the value of the orbital j at point x. • εj is the energy of orbital j. According to the frontier molecular orbital theory, the most reactive position (the carbon on which a free radical attack occurs most likely) has the highest frontier electron density. The radical attack frontier electron densities of perimeter carbon atoms in the five primary PAHs are listed in Table 7.6. The numbering of those carbon atoms is consistent with the conventional numbering system (Figure 7.1). Within each unsubstituted PAH, 245 the allocation of closely related molecular orbitals shows a large difference. The 9- and 10- carbon atoms on phenanthrene, 1-, 3-, 6-, 8- carbon atoms on pyrene, 1-, 6-, 7-, 12- carbon atoms on chrysene, 3-, 4-, 5-, 6- carbon atoms on benzo[e]pyrene and 4-, 5-, 10-, 11- carbon atoms on benz[g,h,i]perylene show the highest free radical susceptibility. The common feature of these reactive carbon atoms is that they are all adjacent to one secondary carbon atom and one tertiary carbon atom on the aromatic ring, similar to the observation from the BDE analyses. Methylation of PAH slightly altered the frontier density of the center carbon and its adjacent carbons as shown in Table 7.7, but the order of the radical frontier density remains the same as in unsubstituted PAH. Figure 7.2 gives a graphic representation of frontier electron density for a free radical attack reaction of the five unsubstituted PAHs. The contour is colored with the relative positive value of frontier orbitals density in the decreasing order of light blue> blue. 246 Table 7.6 Radical frontier density of edge carbon atoms on unsubstituted PAH Radical Frontier Density Atom No. Phen. Py. Chry. BeP BPery 1 0.144 0.209 0.108 0.167 0.078 2 0.113 0.044 0.082 0.053 0.078 3 0.142 0.209 0.086 0.17 0.072 4 0.132 0.143 0.107 0.119 0.122 5 0.132 0.143 0.105 0.119 0.145 6 0.142 0.209 0.177 0.17 0.045 7 0.113 0.044 0.108 0.053 0.133 8 0.144 0.209 0.082 0.167 0.133 9 0.193 0.143 0.086 0.074 0.045 10 0.193 0.143 0.107 0.048 0.145 11 0.105 0.048 0.122 12 0.177 0.074 0.072 Table 7.7 Effect of methylation on the radical frontier density of edge carbon in PAH Radical Frontier Density Atom No. Phen. 9M-Phen. Py. 1MPy. 1 0.144 0.141 0.209 0.234 2 0.113 0.108 0.044 0.042 3 0.142 0.133 0.209 0.196 4 0.132 0.129 0.143 0.135 5 0.132 0.124 0.143 0.144 6 0.142 0.148 0.209 0.206 7 0.113 0.106 0.044 0.04 8 0.144 0.141 0.209 0.203 9 0.193 0.203 0.143 0.142 10 0.193 0.215 0.143 0.142 247 Figure 7.2 Frontier electron density map for radical attack (molecular structures refer to Figure 7.1) 248 7.3 Oligomerization of PAH into Mesogens Section 7.2 discussed the relative tendencies of free radical initiation from both unsubstituted and methylated PAH molecules. The most probable free radical attack position can be predicted by the local frontier electron density descriptor. Mesophase formation follows the free radical reactions that lead to growth in the molecule size. The heterogeneity of the PAH molecular constitution in the feedstocks leads to complex reaction pathways. However, the reactivity indices and site-selectivity descriptors can be used to assess the important reaction routes and eliminate others. In this study, only the oligomerization of PAH molecules with same ring structure were considered. This argument is also supported by the softness matching criteria which states that the attack of free radical will mostly take place at the molecules with the closest softness to the radical (Pal and Chandrakumar, 2000; Chattaraj, 2001). 7.3.1 Aryl radical addition to PAH and aryl-radical coupling One of the possible routes in thermal polymerization of reactant PAH will be the free radical addition to PAH or the coupling of two radicals. Based on the free radical initiation and the susceptibility of free radical attack, following reaction pathways can be proposed (Figure 7.3 and Figure 7.4): 249 C A + D B AB + C D Figure 7.3 Proposed oligomerization and condensation pathways for phenanthrene and pyrene 250 A C + D B E + A B + A B Figure 7.4 Proposed oligomerization and condensation pathways for chrysene, benzopyrene and benz[g,h,i]pyrene The main postulate for the proposed reaction pathways is that the predominant free radical (most easily formed one) attacks the most vulnerable carbon site on the edge of the aromatic ring structure. When an aryl radical, for example, 9-phenanthrenyl, attacks another PAH molecule, the most likely position will be on the carbon 9 of the 251 attacked phenanthrene molecule, since carbon at 9 position has the highest radical frontier electron density of all other edge carbons. The intermediate product radical, which can be taken as π-cyclohexandienyl-like radical (Stein, 1981; Poutsma, 1990), delocalizes over the attacked phenanthrene substrate. This radical can take two forms (A, B) that lead to final condensation products. Depending on the position of intra-molecular addition and dehydrogenation, the products end up with dimers with different configurations, such as six-member ring oligomer (D) or five-member ring oligomer (C). The formation of oligomers that contain five-member ring can be related to the conformation of the starting PAH: cata-condensed starting PAHs (such as phenanthrene and chrysene) show higher probability of producing a five-member-ring-containing oligomers than peri-condensed PAHs (such pyrene and benz[g,h,i]perylene). Due to the strain and the non-planar configuration, these five-member-ring bridged PAHs intermediates are less preferred than the planar six-member ring bridged PAHs in terms of carbonaceous mesophase development. A fundamental prerequisite for the reactions proposed above to proceed is the presence of aryl-PAH radical. This is true when starting material consists of unsubstituted PAHs, so that aryl radical would be initiated by homolysis or bimolecular disproportiontation. However, when the alkylated PAH is also present with the unsubstituted PAH in the reactants such as needle coke feedstock, forming aryl-methyl radical from the alkylated PAH is thermodynamically more competitive than forming aryl radical. Although some experimental data had been reported to show the significance of side-chain cleavage from large PAH molecules (alkylpyrene and alkylperylene) at higher temperature (425-500°C) cracking reaction (Freund, Matturro et al., 1991; Smith 252 and Savage, 1991), when the free radical reactions are considered in the alkylated PAH dominant system, the reactions via aryl-alkyl radical should provide the primary pathways that lead to the mesophase formation. 7.3.2 Kinetics of aryl-alkyl radical addition to PAH to form mesogenic precursor Computational modeling has offered a valuable contribution to the in-depth understanding the molecular growth process during carbonization. Kinetic parameters, such as pre-exponential factor and activation energy of addition and cyclization of small hydrocarbons, can be obtained accurately via transition state theory by high-level ab initio calculations (Heuts, Gilbert et al., 1996; Richter, Mazyar et al., 2001; Speybroeck, Martele et al., 2001). However, when the same procedure is applied to large aromatic ring systems such as the large aromatic systems in carbonization of the decant oil, the ab initio computation will be prohibitively expensive to carry out. Therefore, a semi-empirical computation was used in this study to investigate the kinetic characteristics during the carbonization of the major PAH compounds. Figure 7.5 shows the conformations of the aryl-alkyl free radicals of primary PAHs found in decant oil and coker feed. A similar conformational geometry (the minimum-energy geometry located by MNDOd method) was found among these methyl- PAH radicals. The ending radical (methyl radical) attached to the ring structure has two hydrogen atoms that lie in the same plane of the PAH substrate, and the orbital of the unpaired electron is perpendicular to the substrate plane. Since this benzylic radical is stabilized due to its resonance with the π-electrons over the aryl ring, this conformation of methyl-PAH free radical prevents the free rotation of the methyl group. 253 Figure 7.5 The conformation of aryl-alkyl radicals of PAHs (From left to right, top to bottom: 9-methylphenanthrenyl, 4-methylpyrenyl, 6-methylchrysenyl, 3-methylbenz[e]pyrenyl, and 5-methylbenz[g,h,i]perylenyl) The reaction kinetic parameters can be calculated from the properties of transition state (Willems and Froment, 1988; Barrow, 1996). The activation energy, Ea, and Arrhenius factor, A, are related to the enthalpy and entropy change between the activated complex (transition state) and the reactant. For PAH free radical addition reaction AR*+RÆ AR-R, ° ° * Ea = ΔH f (TS) − ∑ΔH f ,reac tan t + (1−υ )RT kT ≠ A = e(ΔS / R) h 254 where ν* is the change in number of moles from reactants to TS state; k is Boltzman constant, 1.3807x10-23 J/K and h is Plank constant, 6.62x10-34 J/K ΔS≠ is the entropy change from reactant to TS state. In the proposed methylPAH radical addition reaction scheme (in Figure 7.6), the attacking free radical approaches to the PAH parallel to the substrate plane but with certain displacement. Since the radical is actually a non-rotating radical, the parallel approximation will ensure the maximum overlap between the orbital of this radical and the π electrons on the edge carbon of the attacked PAH substrate. + TS of inter-molecular addition Six-member ring product TS of intra-molecular addition Five-member ring product Figure 7.6 Proposed PAH free radical addition scheme leading to carbonaceous mesophase 255 The transition state (TS) was first obtained by gradually decreasing the length of the forming C-C bond until the energy reached a maximum. The final TS was refined for a full geometry optimization and verified by the “one imaginary (negative wavenumber) transmittance on infrared transition spectrum” criterion. This imaginary absorption band is associated with the translational motion along the reaction coordinate. Figure 7.7 gives an example of such an IR spectrum of a TS from 9-methylphenyl-phenanthrene radical addition reaction. Figure 7.7 IR spectrum and motion vector of 9MPhen+Phen TS 256 The geometric characteristics of TS (inter-molecular addition) of the primary radical-PAH addition reactions are listed in Table 7.8. The definition of parameters is shown in Figure 7.8. Table 7.8 The geometric parameters of inter-molecular addition of PAH Reactants TS Forming bond length:2.105Å Bond angle (at attacking C*): C1-C*-C: 105.35°; H1-C*-C1-C2: 25.87° 9MPhen+Phen (9) Bond angle (at attacked C): C3-C-C*: 101.14° H1-C-C3-C4: 17.13° Forming bond length:2.110Å Bond angle (at attacking C*): C1-C*-C: 110.44°; H1-C*-C1-C2: 11.13° 4MPy+Py (1) Bond angle (at attacked C): C3-C-C*: 102.97° H1-C-C3-C4: 10.44° Forming bond length:2.171Å Bond angle (at attacking C*): C1-C*-C: 110.82°; H1-C*-C1-C2: 10.99° 6MChry+Chry (6) Bond angle (at attacked C): C3-C-C*: 102.86° H1-C-C3-C4: 13.47° Forming bond length:2.142Å Bond angle (at attacking C*): C1-C*-C: 119.28°; H1-C*-C1-C2: 14.96° 3MBeP+BeP (3) Bond angle (at attacked C): C3-C-C*: 103.16° H1-C-C3-C4: 12.75° Forming bond length:2.019Å Bond angle (at attacking C*): C1-C*-C: 105.20°; H1-C*-C1-C2: 24.16° 4MBenzPery+BenzPery (4) Bond angle (at attacked C): C3-C-C*: 111.14° H1-C-C3-C4: 17.37° 257 C2 H1 C1 C* H1' H2 C C3 C4 Figure 7.8 Definition of the geometric parameters of the TS As can be seen from the data in Table 7.8, the length of newly formed bonds between attacking radical and the attacked carbon is about 2.15 Å, still longer than that of the aromatic C-C bond (1.4 Å). The electron orbitals on both attacking and attacked carbons have to transforming from trigonal (sp2) to tetrahedral (sp3) hybridization in order to form C-C linkage between the two PAHs, which can be seen from the changes in the angles of the existing bonds of the approaching carbon atoms. For instance, the dihedral angle (H1-C*-C1-C2) increases from 0° (in reactant) to 25° in the TS of 9- methylphenanthryl-phenanthrene radical as shown in Figure 7.8. 258 Figure 7.9 Conformations of TS of 4MPY-1PY, 6MCHRY-6CHRY, 3MBeP-3BeP and 4MBenzPery-4BenzPery (from left to right, top to bottom) 259 A graphic representation of the conformation of transition states of the most reactive methylPAH radical addition reactions is shown in Figure 7.9 and the related thermodynamic and kinetic properties obtained from MOPAC calculation are listed in Table 7.9. Table 7.9 Transition state properties and the kinetics parameters of PAH radical addition reaction ΔHf, of TS ΔH , of radical, ∆S≠, E , PAH radical addition f ΔH , of PAH Arrehnuis a kcal/mol f cal/mol kcal/m kcal/mol ol 9MPhen*+Phen_9 174.3 88.1 54.9 -36.2 1.1E+09 31.1 4MPy+Py_1 184.6 91.4 60.7 -40.2 4.1E+08 32.7 6MChry*+Chry_6 213.8 107.4 74.9 -34.9 1.5E+09 31.5 3MBeP*+BeP_3 223.6 112.9 80.2 -35.4 1.3E+09 30.5 4MBenzPery*+BenzPery_4 211.7 108.2 82.5 -33.5 2.1E+09 20.9 The calculated activation energies for the PAH radical addition reactions showed a clear trend with the reacting substrates. Heavy PAH molecule (such as six-ring benzoperylene) promotes the addition reaction rates. This result can be expected due to the increased resonance stability of the intermediate product (di-aryl PAH). The calculated pre-exponential factors A were found to be similar in all the reactions except in pyrene radical addition, which is almost one order of magnitude lower than the other A value. Comparing the kinetics parameters of pyrene with its structural isomer, peri-condensed chrysene, the calculated results suggest that pyrene is more stable 260 than chrysene, which is in agreement with the GC/MS analysis of the short-time carbonization experiments. Once the intermediate product forms (actually the di-aryl PAH free radical), the most possible route to forming condensed PAH would be the intra-molecular addition and aromatization (see Figure 7.6). The kinetics results for the intra –molecular addition of methyl-di-aryl radicals (9MPhen_Phen(9) and 4MPY_PY(1)) are listed in Table 7.9. Table 7.10 Kinetics data for intra-molecular addition of PAH compounds ΔH , of TS ΔH , of radical, E , f f ≠ a PAH radical addition ∆S , cal/mol Arrehnuis kcal/mol kcal/mol kcal/mol 9MPhen*+Phen_9 To 6 Ring product 171.2 137.0 -11.0 4.5E+11 34.197 To 5 Ring product 168.6 137.0 -7.8 9.7E+11 31.532 4MPY*+PY_1 To 6 Ring product 188.4 145.0 -6.6 1.3E+12 43.345 To 5 Ring product 185.2 145.0 -6.3 1.4E+12 40.144 Compared with the inter-molecular addition reaction, the intra-molecular additions for both phenanthrene and pyrene showed an increase in both pre-exponential factor and activation energy. The relative ratio of reaction rate constant (at 298K) between inter- and intra-molecular addition reaction for phenanthrene and pyrene is calculated to be 1:30 and 1: 0.7 respectively. This result suggests that during addition 261 reaction to form condensed PAH product, the ring-closing reaction is more rapid than that of pyrenes. The five-member PAH intermediate product seems to be kinetically favored, because of the partial release in the steric hindrance between hydrogen atoms on the condensed ring products. The energy diagrams for some PAH radical addition reactions are presented in Figure 7.10. As can be seen, the six-member ring product is less energetic and more thermodynamically favored over the five-member ring PAHs. Although the reaction of di-aryl radicals to form five-member ring PAH is much faster, the reverse reaction of the five-member ring PAH product is more significant than that of six-member ring PAH products. In addition to the energy difference, the spatial configuration of the products from phenanthrene and pyrene addition reactions is also quite different in terms of planarity. As can be seen on the 3-D images in Figure 7.11, both six-member ring and five-member ring PAH product from methylphenanthrene addition reaction show a significant deviation from the planarity. While in the case of pyrene addition reaction, almost planar configuration, the products from methylpyrene addition take an almost planar configuration. 262 60 TS(6R) of PAH intra-PAH addition TS of PAH addition 50 TS(5R) of PAH intra-PAH addition 40 30 20 Relative Energy to Product 6R, kcal/mol 6R, to Product Energy Relative 10 Product 5R Product 6R 0 02468101214 TS(6R) of PAH intra-PAH addition 60 TS of PAH addition TS(5R) of PAH intra-PAH addition 50 40 30 Relative Energy, kcal/mol 20 10 Product 5R Product 6R 0 024681012 Figure 7.10 Energy diagram of reactant, TS and product of 9-MPhen and 4-MPy alkyl- radical addition and condensation reaction. 263 Figure 7.11 Spatial configurations of the products from intra-molecular addition reactions of phenanthrene (four at top) and pyrene (four at the bottom) 264 7.4 Summary The reactivities of the major PAH compounds present in needle coke feedstock were investigated by using a semi-empirical molecular modeling software. The ease of free radical initiation (therefore the thermal stability) of a PAH compound can be predicted by the bond dissociation energy and/or the frontier electron density. The local environment of the peripheral carbon atom in a PAH compound determines its reactivity toward a free radical reaction. A reactive carbon site was found to be an atom which is adjacent to a secondary carbon and a tertiary carbon on the aromatic ring. The oligomerization and condensation of PAHs in needle coke feedstocks may proceed mostly through aryl-alkyl PAH radical addition to the alkylPAH compounds because of the relative ease of forming a benzylic radical than forming an aryl radical. The kinetic data for proposed PAH radical addition reaction showed significantly different behaviors for cata-condensed methylphenanthrenes and peri- condensed methylpyrenes. An initial slower reaction in forming a di-aryl radical and a faster reaction toward a condensed product was found in phenanthrene addition reaction from the transition state properties. The product (oligomer) of methylphenanthrene addition reaction was found to deviate greatly from a planar configuration, which is of critical importance for mesophase development. In contrast to the behavior of methylphenanthrenes, methylpyrenes addition reaction proceeds gradually toward increasing the molecular weight of the intermediate products which have planar configurations. 265 Chapter 8 Summary, Conclusions and Suggestions for Future Work The two principal objectives of this thesis research were to analyze the molecular composition of the needle coke feedstocks and to investigate the relationship between the molecular composition of the complex feedstocks and mesophase development during carbonization. A wide range of commercial needle coke feedstocks, six decant oil samples (DO91, DO93, DO15, DO24, DOPSU and DOUP), and six samples of decant oil derivatives, including the actual feeds to the coke drum (CF15 and CF24), hydrotreated fractions (HYD15 and HYD24) and vacuum bottoms (VTB15 and VTB24), were obtained from different commercial delayed coking operations in the United States. All the samples were carbonized at 500°C for 3 hours (with the exception of 5 hours for HYD samples) in a tubing bomb reactor to obtain a semi-coke product for optical texture characterization to measure the degree of mesophase development. The characterization of the semi-cokes by polarized-light microscopy shows that there is a wide range of variation in the optical texture, as represented by an optical texture index (OTI). This observation reflects the different degrees of mesophase development from these feedstocks during carbonization under the same conditions. The highest OTI values, which correspond to the highest degree of mesophase development, are found in the semi-cokes produced from DOUP, DOPSU and DO24, followed by DO91, DO15 and DO93. The coker feed (CF) gives a much better mesophase development than the parent decant oil sample, although the mesophase development in 266 semi-coke from CF 15 is still inferior to that obtained from CF24. The heaviest feedstock, the vacuum tower bottom (VTB) samples, gave even better mesophase development than the CF samples. Hydrotreated decant oils (HYD) gave the highest degree of mesophase development among all the feedstocks. The optical texture of semi-cokes from HYD samples appeared to consist exclusively of large domain and flow domain units, including the largest domain sizes observed in this study. In an attempt to relate the estimated OTI values of the semi-cokes to the graphite crystallite structures, selected semi-coke samples were calcined and heat treated to initial graphitization temperature (2270°C). The resultant coke/carbon samples were analyzed by x-ray diffraction. All the semi-cokes, when calcined at 1000°C, show a rudimentary characteristic of pre-graphitic structure: decreasing the d002 spacing and increasing crystallite stacking height and diameter. Upon increasing the temperature to 2270°C (for 1 hour), the d002 spacing of all the carbon products approached 3.370 Å, along with the stacking height and unit diameter of over 250 Å, and 900 Å, respectively, depending on the starting semi-coke samples. Heat-treated carbon product from the highest OTI semi- coke (DOUP) showed the lowest d002 spacing (3.370 Å) and those of lower OTI semi- coke (DO93, DO15) exhibited the highest d002 spacing of 3.380 Å. Furthermore, the growth of graphite crystallites from the semi-cokes tested in this study exhibited different aspect ratios. It was found that higher OTI semi-cokes show a preferred growth along the stacking layers (a-direction) over that in the perpendicular direction (c-direction). There appears to be a good correlation between the La (diameter of graphite crystallite) and OTI value (mesophase development) of the semi-cokes. Although the d002 spacing is just one factor that affects the properties of solid carbons, particularly the coefficient of thermal 267 expansion (CTE), this study did show that semi-cokes with higher OTI tend to lead to crystallites with larger diameter than those obtained from the lower OTI semi-cokes associated with a lower degree of anisotropy. Extensive effort in this study was focused on analyzing the molecular composition of the needle coke feedstocks. A sequence of chromatographic separation methodologies and spectrometric techniques was applied to the samples of decant oils and their derivatives. Quantitative GC/MS was first used to analyze the GC-amenable fraction of the feedstocks, followed by the HPLC/PDA and LC/MS/MS, LD/MS and NMR, to highlight the higher boiling molecular species present in the feedstocks that do not, or poorly, resolve on GC/MS. The GC/MS analysis results reveal the GC/MS-amenable fraction of the decant oil samples consists of 2- to 4-ring condensed aromatics as the major components, and alkanes (C12-C30) as the minor components. The PAH compounds comprise naphthalene (two-ring), phenanthrene (three-ring), pyrene and chrysene (four-ring) as dominating compounds. These PAHs are generally found to have a high degree of alkyl substitution. The overall composition of the PAH in the decant oil can be divided into two broad categories, the cata-condensed PAH and the peri-condensed PAH. The linear PAH compounds with lower thermal stability such as anthracene were not observed in any significant concentrations. The quantitative GC/MS provides a fine analysis of the molecular constitution of the volatile fraction in the decant oil samples. It was found in this study that each decant oil sample contains almost the same molecular components, but the relative distribution 268 of alkylPAHs and the isomeric distribution of the alkylated PAHs varied substantially among the decant oil samples. Depending on the crude oil source and FCC operating conditions, the GC- amenable PAH and alkanes (that are positively identified) account for from 16% to 40% of the starting decant oil samples. In a broad comparison of molecular composition, the decant oil samples that gave the highest degree of mesophase development during carbonization (DOUP, DOPSU and DO24) contain relatively high concentration of normal alkanes, and low concentrations of sulfur-containing polyaromatics. In view of a more detailed PAH composition, better needle coke feedstocks were found to have lower proportions of multi-methylated PAHs (particularly, the alkylphenanthrenes) and higher proportions of heavier PAHs (particularly, the alkylpyrenes). In contrast to the large variations in the distribution of PAH ring systems, the isomeric distribution of methylPAH was found to be constant within the same decant oil set (i.e., DO91, DO93, DO15 and DO24 in one set, DOPSU and DOUP in another set). Without exception, the most abundant methylPAH isomers are those with the least steric hindrance and the highest thermal stability. Coker feeds, (heavy fraction of decant oil + recycle stream + heavy fraction of hydrotreated decant oil), were found to have a different molecular composition than that of their parent decant oil. The coker feeds have very low sulfur content (<2 wt%) and extremely low normal alkane (1%) concentrations. In addition, CF contains higher proportions of unsubstituted and less substituted PAHs. Furthermore, there is a shift of the composition of alkylated PAH isomers to the dominating presence of thermally stable 269 isomers. This shift is readily observed from the methylPAH isomer distribution profile, (34% of 9-, 1-MPhen in DO15 and 12% of 9-, 1-MPhen in CF15). HYD (hydrotreated decant oil sample) shows a more complex GC/MS chromatogram than those for both decant oil and coker feed samples, because of the new molecular species (the hydroaromatics) introduced during the hydrotreating process. The most important molecular compositional feature of the HYD feeds is the large decrease of angular PAHs concentration (phenanthrene and chrysene) in favor of their corresponding hydroaromatics. This hydrogen-rich feedstock produced semi-cokes with the highest degrees of mesophase development, as can be expected from its very low propensity to form coke. It should be noted that carbonization experiments in this study were carried out in sealed reactors. Therefore, hydroaromatic compounds (the hydrogen donors) have the opportunity for staying in the reacting medium for the duration of the experiments. The high degree of mesophase development from HYD may not be duplicated in the commercial delayed coking process because of the shorter residence time of volatiles in the reaction zone. The VTB samples contain almost the same isomeric distribution of the major alkyl PAH isomers found in corresponding decant oil samples, but the concentrations of lower boiling point PAH compounds in VTB are much lower than in DO sample, as expected. The high-boiling PAHs that poorly resolved or did not resolve in GC/MS show a better resolution on HPLC/PDA, LC/MS/MS and LD/MS spectrometry analyses. The major high-boiling PAHs were found to be benzopyrene (five-ring), perylene (five-ring), benzoperylene (six-ring) and their alkyl-substituted analogs. The higher alkylated pyrenes 270 and alkylated chrysenes that could not be resolved on GC were also shown in these chromatograms. A quantitative analysis of these heavy PAH could not be achieved. However, a qualitative and semi-quantitative comparison suggests that the heavy PAH compounds share a similar distribution pattern with their lower molecular weight counterparts. For example, the peri-condensed pyrene (four-ring condensed) and benzopyrene (five-ring condensed) show a closer distribution pattern in their multi- methyl homologues, and the same is true for the cata-condensed and structurally related phenanthrene and chrysene homologues. This observation indicates a continuity in the molecular composition of the decant oil over the whole molecular weight, or volatility, range. Therefore, the results on the homologous PAH distribution obtained from GC/MS can be extended to these heavy PAHs with confidence. Some characteristics of the molecular composition of the coker feedstocks were found to correlate with the degree of mesophase development during carbonization of the feedstock. From the view point of reactivity, the feedstocks having relatively low reactivity would lead to a relatively low rate of molecular growth and extend the time period for maintaining the fluidity of the reacting matrix. These factors would favor a high degree of mesophase development. The better needle coke feedstocks, such as DOUP, DOPSU and DO24, have a lower degree of alkylation of PAH in common. In contrast to the reactive feedstocks such DO93, which contains high concentrations of highly alkylated PAHs, DOPSU and DOUP exhibit a low carbonization reactivity, leading to a higher degree of mesophase development toward a highly anisotropic coke texture. 271 In order to study the carbonization reactivity of the major molecular species in the feedstocks during carbonization, decant oil samples which produced different degrees of mesophase development, along with their derivatives, were carbonized at a lower temperature (450°C) for 15 to 180 minutes. These experiments were carried out to monitor the coke yield and asphaltenes yield from different feedstocks as measures of carbonization reactivity. The low-temperature carbonization has clearly showed the differences in reactivity between the feedstocks that lead to a good mesophase development (e.g., DO91) and those that gave a low degree of mesophase development (e.g., DO93). During the carbonization, DO93 produced higher yields of asphaltenes and semi-coke than DO91. In addition, DO93 also gave a higher initial rate of asphaltenes formation in the early stage of carbonization (within 60 minutes), whereas DO91 showed a steady rate of asphaltenes formation over a long reaction period. The higher initial rates of asphaltenes formation and their fast transformation to semi-coke (for DO93), hinders the mesophase development. Except for VTB, an inverse relationship was noted between the carbonization reactivity and mesophase development. Carbonization of decant oils and their heavy residua obtained by vacuum distillation has definitively shown the effect of molecular composition change on the mesophase development during carbonization. By progressively removing the low boiling point PAHs (phenanthrenes in this case), the mesophase development is enhanced. A strong correlation was observed between the OTI of the semi-cokes and the pyrenes to phenanthrenes weight ratio in the starting feeds. When the correlation was extended to the decant oil and coker feed samples, a similar relationship was observed, particularly in the range of relatively low degrees of mesophase development (low OTI). 272 In order to elucidate the roles of pyrenes and phenanthrenes during carbonization, the molecular composition of the product from short time (15 to 60 min) and lower temperature (450°C) carbonization of DO and CF was monitored. The constant increase in the ratio of unsubstituted PAH to methylPAH over the reaction time period clearly shows that the methyl PAH are removed by incorporation into higher molecular weight species and /or by undergoing demethylation reactions in the early stages of carbonization. It appears that methylphenanthrenes disappear faster than methylpyrenes. Using a semi-empirical MOPAC approach to study the PAH molecules in the needle coke feedstocks, the most reactive carbon site on a molecule can be predicted by the frontier electron density. The most reactive sites involve the carbon atoms that are adjacent to secondary and tertiary carbons on the aromatic ring system. A possible reaction pathway was simulated via the methylPAH free radical addition to PAH, followed by intra-molecular addition and dehydrogenation, leading to a condensed PAH oligomer. Phenanthrene free radical addition reaction was found to be faster than pyrene free radical addition. The spatial configuration of the intermediate products and the condensed products from phenanthrene radical addition reaction was also found to take a non-planar configuration. The proposed reaction scheme and the calculated thermodynamic and kinetic parameters agree with the experimental observations. These results suggest that the effects of pyrenes to phenanthrenes ratio in needle coke feedstocks on the OTI of the resultant semi-coke 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. 273 This work confirms that the molecular composition of needle coke feedstock has a strong effect on mesophase development, and hence, the structure and properties of the resulting cokes and carbons, including the degree of graphitizability and coefficient of thermal expansion (CTE). In particular, the pyrenes to phenanthrenes ratio in the feedstock can be used as a predictor for the resultant coke’s texture quality. Qualitative analysis of the higher-boiling fractions of the feedstocks by HPLC and LDMS suggest that the pyrenes/phenanthrenes ratio can be directly related to the overall ratio of peri-/cata- condensed compounds in the heavier fractions. An exploratory quantum chemistry modeling (MNDO level of theory) was conducted to examine reaction pathways of alkyl PAH oligomerization and condensation, using phenanthrenes and pyrenes as models. The kinetic data for proposed PAH radical addition reaction showed significantly different behaviors for cata-condensed methylphenanthrenes and peri-condensed methylpyrenes. An initially slower reaction in forming a di-aryl radical and a faster reaction toward a condensed product was found in phenanthrene addition reaction from the transition state properties. The product (oligomer) of methylphenanthrene addition reaction was found to deviate greatly from a planar configuration, which is of critical importance for mesophase development. 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Yokono, T., Miyazawa, K., Sanada, Y. and Marsh, H. (1979). "Relations between free valence and thermal reactivity of polynuclear aromatic hydrocarbons." Fuel 58: 692-694. Vita Guohua Wang was born on April 1 1965 in a port city in Liaoning province, China. He attended Dalian Institute of Technology and received a Bachelor of Science degree in Chemical engineering in 1986, a Master of Science in Chemical Engineering in 1989 from Dalian University of Technology in China. He worked at Baoshan (Shanghai, China) Steel Complex as an engineer in 1990-1991. He was employed by Anshan Institute of Iron and Steel Technology from 1991 to 1995 to conduct researches on carbonization of coking coal for metallurgical coke operation. He entered the Material Science and Engineering department in Pennsylvania State University in 1997. He is a member of American Chemical Society and a member of Chinese Metallurgical Society. He married Weidong Gu of Anshan, Liaoning and has a daughter Amy who was born in 1999.