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Extraction of coal-tar pitch and the effect on carbonization
Carbon, Vol. 32, No. I, pp. G-92, 1994 Copyright 0 1994 Elsevier Science Ltd
Pergamon
Printed in Great Britain. All rights reserved 0008-6223194 $6.00 + .OO
000%6223(93)E0048-P
EXTRACTION
OF COAL-TAR PITCH AND THE EFFECT ON CARBONIZATION and P. J.
J. R. KERSHAW CSIRO
Division
of Materials
(Receiued
Science
10 May
and Technology, Private Victoria, 3169 Australia 1993; accepted
SMART Bag 33, Rosebank
in reuisedform
MDC,
Clayton,
21 July 1993)
Abstract-The extraction of a QI-free coal-tar pitch was investigated using supercritical fluid extraction with aliphatic hydrocarbons at 250-350°C and lo-15 MPa and by liquid extraction at 150-200°C. The softening points, number-average molecular weights, and toluene insoluble (TI) content of the residual pitches increase with the amount of lower molecular weight species extracted. The amount of mesophase formed in a given time and the Alcan coking value of the pitches show good correlations with the molecular weight and TI of the pitches. A process based on heat treatment of pitch under pressure followed by supercritical fluid extraction for preparation of mesophase pitch was investigated. Key Words--Solvent phase.
extraction,
supercritical
fluid extraction,
1. INTRODUCTION
At present, pitch-based carbon fibers account for only about 10% of the worldwide carbon fiber manufacturing capacity. However, pitch has the potential to produce carbon fibers at lower costs than from polyacrylonitrile (PAN), therefore allowing the possibility of considerable growth if suitable processes and products are developed. There are two main types of pitch fibers: (1) high-performance or mesophase fibers, which are produced from mesophase pitch resulting in a high degree of preferred orientation of the graphene layer planes along the fiber axis, and (2) general purpose or pitch fibers, which are produced from isotropic pitch resulting in little, if any, molecular orientation during spinning and no extended graphitic structure in the final fiber[ 11. The low-cost isotropic pitch fibers have much lower tensile strengths and moduli than mesophase fibers, with the tensile moduli of pitch fibers being about 4-20% of those of mesophase fibers, depending on the final carbonisation temperature used. The lower molecular weight species (“lights”) present in pitch are known to inhibit mesophase formation[ l-41. Often sparging with nitrogen to facilitate removal of the volatiles is used during the heat soaking to form mesophase[5,6]. Mesophase formation at 400°C is a slow process utilizing this approach, taking about 24 hours to produce 80 + % mesophase from either coal-tar or petroleum pitch with stirring and nitrogen sparging. However, increasing the temperature to 430-450°C reduces the heat-soaking time considerably. Diefendorf and
coal-tar
pitch,
carbonization,
meso-
Riggs[7] used solvent extraction of petroleum pitch at ambient temperature to remove the lower molecular species and to concentrate the higher molecular species (“heavies”) in the insoluble fraction, resulting in much reduced mesophase formation time. Reduction in the amount of lower molecular weight components is also required to produce suitable precursor pitches for production of isotropic carbon fibers and for binder pitches for carbon-carbon composites. A high carbon yield to minimise the number of densification steps is a requirement for a favourable binder pitch@-101. This property together with a relatively high softening point, to limit stabilization time, are needed in pitches for isotropic fibers[ 11,121. In this paper we report on an investigation of two solvent extraction procedures for removing the “lights” from a QI-free coal-tar pitch and the effect that these extractions have on the carbonization properties. This effect was measured by the percentage of mesophase formed in a given period and the Alcan coking value of the residual (refined) pitches. The extraction procedures were (a) supercritical fluid extraction (SCFE) using aliphatic hydrocarbons at 250-300°C and IO-ISMPa, and (b) liquid extraction at 150-200°C. There is a worldwide trend to supply and transport coal-tar pitch as a hot liquid rather than as a solid due to the potential health risks from pitch dust associated with handling solid pitch. The two extraction techniques are suited to feeding hot liquid pitch to the extraction reactor. Supercritical fluid extraction uses as an extractant a dense gas above its critical temperature and pressure. SCFE appears to be ideally suited to the extraction of the lower molecular weight component from pitch. Although there have been a limited number of reports of SCFE of coal-tar pitch[ 13- 161, the aim of these studies generally has been to extract
86
J. R. KERSHAWand P. J. SMART
the pitch from the undesirable particulate impurities present in pitch, rather than to “strip” off the “lights” from pitch. These pitch extracts, however, could be fractionated to remove the lower molecular weight components. Phase equilibrium data has been reported by Hutchenson et a1.[17,18] for the Ashland A-240 petroleum pitch/toluene system at conditions above the critical temperature and pressure of toluene. Recently Hochgeschurtz et a[.[191 fractionated a heat-soaked Conoco petroleum pitch with supercritical toluene in a region of liquid-liquid equilibrium. The bottom phase, which contains the higher molecular weight components, readily formed mesophase when dried at 357°C for 30 minutes. Subsequent melt-spinning, stabilization, and carbonization gave high-modulus carbon fibers[ 191. It has been recently reported[20,211 that a twostage preparation of mesophase with pressure or reflux treatment as the first stage maintained the volatiles within the reactor. This was followed by sparging or vacuum maturation as the second stage, which results in a mesophase pitch with a lower softening point and improved spinnability compared to a single stage process[20,21]. A variation of this approach based on pressure heat-treatment followed by SCFE is reported in this paper. 2. EXPERIMENTAL 2.1 Materials A coal-tar pitch supplied by Koppers Coal Tar Products, Mayfield, N.S.W., Australia, was filtered through a 1.2-pm glass fiber filter pad at 200°C with Hiflo Super-Cell filter aid, under pressure. Analysis of the filtered pitch is given in Table 1. The petroleum pitches A-240 and Aerocarb 70, 80, and 82 were obtained from Ashland Petroleum. 2.2 Supercritical fluid extraction The extractions were carried out in a 1-L unstirred autoclave. The autoclave was charged with pitch (100 g) and solvent (600 mL) and heated (ca 7°C min-‘). When the extraction temperature was reached, solvent (1 L h-l) was pumped via a dip tube, which acts as a preheater, into the bottom of
Table 1. Analysis of filtered coal-tar pitch Softening point (“C) Quinoline insolubles, QI, (wt%)* Toluene insolubles, TI, (wt%) Alcan Coking Value (wt%) c (wt%) H (wt%) N (wt%) s (wt%) 0 (diff.) (wt%) H/C Aromatic C (%C) Aromatic H (%H) Average MW (VPO) *Filtered through a 0.2 pm filter.
108 0.04 21.0 54.5 92.4 4.46 1.47 0.51 1.16 0.58 96 82 429
the autoclave and through the pitch. The pressure was controlled during the extraction by adjusting the throttling valves, and the solvent and extracted pitch were condensed by a water-cooled condenser and collected. After the extraction had been carried out for the desired period, the pump was switched off and the pressure lowered in the autoclave. When the autoclave had cooled, any remaining solvent was removed by blowing compressed air over the residue. The residue was then scaped out of the autoclave, carefully dried in a vacuum oven, and weighed. The solvent was removed from the extraction condensate under reduced pressure using a rotary evaporator. Samples of the extract for analysis were carefully dried in a vacuum oven. 2.3 Liquid solvent extraction of liquid pitch The extractions were carried out in a 450-mL stirred autoclave. The autoclave was charged with pitch and solvent (300 mL) and heated (ca 7°C mini). The stirrer motor was started when the pitch became sufficiently mobile at about 120°C. When the extraction temperature was reached, solvent (1 L h-i) was pumped via a dip tube, which acts as a preheater, into the bottom of the autoclave and through the pitch. The pressure was controlled during the extraction by adjusting the throttling valves, and the solvent and extracted pitch were condensed by a water-cooled condenser and collected. The work-up procedure was similar to that used for SCFE. 2.4 Mesophase formation Samples (3 g) were heated in a glass lined 80-mL stainless steel bomb to 400°C and maintained at that temperature for 6 hours, under a nitrogen atmosphere. 2.5 Analyses Toluene insolubles (TI) were determined by a standard procedure (IS0 6376). Alcan coking values (STPTC PT. 10-79) were determined by Koppers Coal-Tar Products. Softening points were determined using a Mettler instrument. EUMS were carried out on a HP5995A mass spectrometer. The instrument was scanned rapidly and the spectra summed. Average molecular weights were determined by vapour pressure osmometry (VPO) in pyridine at 80°C using a Knauer apparatus[221. Mesophase content of heat-treated pitches was determined by proton magnetic resonance thermal analysis (PMRTA)[23]. 3. RESULTS AND DISCUSSION
3.1 Extraction of QI-free pitch Supercritical fluid extraction (SCFE) of filtered coal-tar pitch was investigated using a number of aliphatic hydrocarbon solvents at temperatures of
87
Extraction of coal-tar pitch and the effect on carbonization Table 2. SCFE conditions, Extraction
conditions
Hexane (T, = 234)” Hexane Hexane Hexane Heptane (T, = 267) Cyclohexane (T, = 280) Petroleum spirit (boiling range 80- 110°C)
Residue
Extract
(MPa)
Time (min)
Yield (wt%)
MW
TI (wt%)
Mesophase (%)$
Alcan Coking Value (wt%)
250 250 300 300 290 300
10 15 1.5 15 15 10
60 60 60 20 60 60
29 32 41 23 46 51
261 NDt ND ND ND 301
31 34 43 26 45 50
36 ND ND ND 54 51
ND ND ND ND ND ND
496 ND ND ND ND 646
171 186 178 147 ND 190
300
15
60
45
ND
46
47
74.8
604
ND
Temp (“Cl
Solvent
yields, and properties of residues and extracts
Pressure
MW
s. Pt. “C
*T, = critical temperature “C. tND = not determined. $Maturation on 3 g, 4OO”C,6 h.
and pressures of lo-15 MPa (see Table 2). The aim was to ascertain conditions for extraction of 30-50% of the pitch. It is apparent from the data in Table 2 that a number of solvents are suitable for extraction of 30-50% of the coal-tar pitch. In a commercial process, an inexpensive mixed hydrocarbon solvent such as a petroleum distillation cut would be preferable to a more expensive single solvent. Extraction with a petroleum spirit shows this approach to be valid. For supercritical fluid extraction, the maximum extraction should occur close to the critical temperature of the solvent where the fluid density is the greatest. However, for hexane (T, = 234°C) extraction, higher yields are obtained at 300 than at 250°C (see Table 2). 250-300°C
The results for the extraction of pitch at 150 or 200°C and 4 MPa using a variety of organic solvents are given in Table 3. Significant extractions could be achieved after 15 minutes with all the solvents investigated. Acetone extractions were carried out at both 150 and 200°C and for various times (see Fig. 1). The results in Fig. 1 indicate that the extraction temperature has only a small effect, while the extraction yield increases with the extraction time. The residual (refined) pitches from both SCFE and liquid extraction had significantly higher softening points than the filtered pitch (see Tables 2 and 3). In some cases foaming, probably caused by residual solvents, prevented the obtaining of reliable softening points. The TI and the average molecular weights of the refined pitches were all considerably
Table 3. Liquid extraction of liquid pitch
Pitch qty (g)
100 50 50 100 50 50 50 50 50 50 50 50 50
Solvent None Acetone Acetone Acetone Acetone Acetone Acetone Acetone Methylene chloride Trichloroethylene Tetrachloroethylene Ethanol Propan-2-01 Cyclohexane
Temperature (“C)
Pressure (MPa)
Time (min)
Extm. (wt%)
TI (wt%)*
200 200 150 150 200 150 200
4 4 4 4 4 4 4
15 15 15 15 30 30 60
0 46 47 49 54 55 57 69
21 36 39 52 54 56 59 73
84 95
150
4
15
75
81
97
200
4
15
200 150 150 150
4 4 4 4
15 15 15 15
>60 63 36 34 42
*Performed on the extraction residue. tMesophase trials performed on extraction residue; 3 g, 400°C. 6 h.
Mesophase (%)+ 12 41
66 31 38 41
100 40
Alcan Coking Value (wt%)*
MW*
s. Pt. (“c)*
429 687 -
108 176 238 -
-
-
-
-
-
-
525 518 -
207 207 -
54.5 78.3 78.4 1 86.2
71.1 -
88
J. R. KERSHAWand P. J. SMART 70
65
0
15
30
45
60
75
Time (mins) Fig.
Extraction
1.
yield vs. time for acetone extraction.
higher than those of the unextracted pitch (see Tables 2 and 3). The TI and number-average molecular weights of the residual pitches correlate well with the weight % of pitch extracted both for liquid extraction at 150-200°C and for supercritical fluid extraction at higher temperatures (see Figs. 2 and 3). Very similar correlations were obtained between the properties of the refined pitches and the weight % extracted
(“lights” removed) by these two extraction techniques. Super~~tical fluid extraction was carried out with nonpolar aliphatic hydrocarbon solvents, while the liquid extraction mainly used polar solvents, but was also carried out with cyclohexane. It appears, therefore, that the properties of the residue, and presumably also of the extract, are not significantly affected by the polarity of the extractant. Coal-tar pitch is a complex mixture of polycyclic
O-
! i
0
SCFE,
0
Liquid extraction.150-200°C,
250-300%.
lo-15MPa
X
Liquid extraction
4MPa
40%
D-
D-
3-
! 3’
20
/
I
40
/
60
Extraction
80
(%)
Fig. 2. Variation of TI with wt% extracted.
Extraction
o/
650
r’
~
600-
s
550
500
-
“7 c
450
20
Liquid
’ 30
3. Variation (an)
15O’C;
4MPa
I
I
40
50
60
Extracted
WT%
Fig.
Extraction;
of number-average molecular with wt% extracted.
89
pitch and the effect on carbonization
of coal-tar
weight
aromatic compounds (PAC). It would be expected that polarity, topology, and possibly intermolecular interactions as well as molecular weight will affect the solubility of pitch components in various solvents. The solubility of isomeric polycyclic aromatic hydrocarbons (PAH) can vary markedly[24]. How-
ever, Fig. 3 indicates that solvent extraction, both supercritical fluid and liquid, extracts predominantly by molecular weight. EUMS of the extracts both from SCFE and from liquid extraction at 150-200°C were similar, and all showed major peaks at 202,228,252, 276, 302, 326, 350,352, and 376. There was a shift of the ion profile to higher molecular weight as the amount extracted increases. Polycyclic aromatic compounds have strong molecular ions in comparison to their fragmentation ions[25]. This allows EUMS to be used to show broad trends in the molecular weight profile of pitch fractions. Variation in molecular ion sensitivities for different PAC and the cumulative effect of fragment ions means that only qualitative profiles of molecular weights can be obtained with EIiMS of these complex mixtures. The carbonization properties of the pitches were studied by monitoring their mesophase formation ability and their Alcan coking values. The latter test measures the coked residue after heating at 550°C for 2.5 hours in a nonoxidizing environment. The pitches were screened for their ability to form mesophase by heating 3-gram samples at 400°C in a small bomb under a nitrogen atmosphere for 6 hours. The results of the mesophase formation studies and the Alcan coking values are given in Tables 2 and 3. In Figs. 4 and 5. the amount of mesophase formed and
1
1
l
Liquid
0
Supercritical
20
Fig. 4. Variation
I
I
40
60
80
Extraction
(wt%)
in mesophase tracted.
formation
J
I
100
20 Extraction
with wt% ex-
Fig.
5. Variation
in Alcan coking tracted.
fluid
extraction
L I
extraction
I
I
40
60
1 I I
(wt%) value
with
wt%
ex-
J. R. KERSHAWand P. J. SMART
90
100
90
80-
SO
P
z
s s
film evaporator. It is noticeable that the petroleum pitches require a higher molecular weight to form the same amount of mesophase as coal-tar pitches. The lower molecular weight species (“lights”) in pitch are known to inhibit mesophase formation[l-41, and therefore an increase in the mesophase formed with an increased in the amount of extracted was expected. Furthermore, “lights” Greinke and Singer’s[26] studies on heat-treated A-240 pitch showed that the probability of a molecule being in the anisotropic phase increases with molecular weight. Nevertheless, the molecular weight profiles of isotropic and anisotropic phases in equilibrium were reported not to vary markedly[26].
-u 70 Q
60
20
I
I
I
40
60
80
50 0
3.2 extraction of beat-treater pitch Except for the low molecular weight components in isotropic pitch, which inhibit mesophase formation, the limited data available indicate that mesophase and isotropic pitch do not markedly differ in their molecular weight profiles[3,4,26]. Nevertheless, the formation of mesophase from refined pitches in which a significant amount of “lights” has been removed is still a relatively slow process, taking about 10 hours to produce 80 + % mesophase compared to 24 hours for filtered pitch under con-
TI (wt%) Fig. 6. Variation in % mesophase formed (r2 = 0.93) and Alcan coking value (r’ = 0.94) with TI. the Alcan coking value are plotted against the weight % extracted for both SCFE and liquid extraction. As the amount of “lights” extracted increases, the amount of mesophase formed and the Alcan coking value increase. The carbonization potential of the pitches as indicated by their mesophase formation ability and Alcan coking value correlate well with the TI and molecular weight of the pitches, as shown in Figs. 6 and 7. There is a strong intercorrelation (rZ = 0.98) between molecular weight and TI for these pitches. The relationships between carbonization properties, TI, and molecular weight may not hold for a wider variety of pitches or indeed for more complex extraction procedures, such as removing the higher as well as the lower molecular weight components, on this pitch. For example, extracting with supercritical toluene (for QI and “heavies” removal) and then extracting the supercritical toluene extract with supercritical hexane gave a pitch with TI = 16% and &&r = 489 from removal of 21% “heavies” and 39% “lights.” (Compare with the filtered pitch, TI = 21% and mn = 429). In Fig. 7, as well as data for solvent-extracted coal-tar pitch, data for commercially available petroleum pitches, Ashland A-240 and Aerocarb, are included. The Aerocarb pitches are obtained from A-240 by stripping the more volatile (lower molecular weight) components from the pitch using a wiped
I 10
I
I
I
I
I
I
I
II
800
600
X&n Fig. 7. Variation in % mesophase formed with numberaverage molecular weight: 0 = coal-tar pitches (r* = 0.86); 0 = petroleum pitches (v’ = 0.99).
Extraction of coal-tar pitch and the effect on carbonization Table 4. Data for pressure maturation followed by supercritical fluid extraction Maturationt Run
Pitch*
Tetralin (g)
1
A-240 A-240
None None
450 410
None
450
None 30 30 30 100 100 100
450 450 430 420 420 430 400
2 3
4 5 6 7 8 9 10
:; CT CT CT CT CT CT
Temp. (“C)
Extraction yield (wt%)$
Mesophase content in residue (%)
1 1 I
24 57
78 95
36
60
0.75 3 2
44 41 61 67 75 79 75
88 79 84 89 83 91 71
Time (hi
1 3 1.5 2
*lo0 g; A-240 = A-240 petroleum pitch; CT = filtered coal-tar pitch. tin an enclosed autoclave under Nz, cold pressure 3 MPa. *Extraction with cyclohexane, 35O”C, 10 MPa, 1 hour.
ventional maturation (stirring at 400°C with nitrogen sparging) conditions[27]. Chemical reactions, such as polymerisation, rearrangement, and cyclisation occur during mesophase formation and probably give rise to PACs similar to those removed during extraction. Therefore, it appears potentially advantageous to have the required chemical reactions take place prior to extraction. The procedure used was to heat the pitch at 400-450°C for 0.75-3 hours in an enclosed autoclave under nitrogen (cold pressure 3 MPa) to carry out the chemical reactions necessary for mesophase formation. The autoclave is then allowed to cool to 35O”C, and the pitch is extracted with a supercritical fluid to remove low-molecularweight isotropic pitch, which inhibits mesophase formation. The study was carried out with both petroleum and filtered coal-tar pitch. For the latter pitch, the heat treatment step was also carried out in the presence of the hydrogen donor solvent, tetrahydronaphthalene (tetralin). Mild hydrogenation of coal-tar pitch enables high mesophase content to be achieved at relatively low viscosity, facilitating the spinning process[28,29]. The potential advantages of this approach are as follows: 1. Single “pot” process for refining/mesophase formation. 2. Can be combined with hydrogenation by addition of a hydrogen donor solvent such as tetralin . 3. Low molecular weight components are present during the heat treatment stage and may be incorporated into the mesophase, possibly improving the properties of the resultant mesophase pitch. 4. Short time to form mesophase. 5. Process is simple and novel. It has been recently reported that two-stage maturation with pressure or reflux treatment at the first
stage, thus maintaining the volatiles within the reactor, followed by sparging or vacuum maturation as the second stage, results in a mesophase pitch with a lower softening point and improved spinn~~biijty compared to single stage maturation[20,2i]. High mesophase content pitches have been obtained under these conditions (see Table 4), and these results indicate the potential of this approach. EUMS of the extract from run 5 showed naphthalene but no tetralin. indicating that pitch hydrogenation had occurred. However, unfortunately the lack of stirring in the autoclave and the slow heating and especially cooling cycles limited the flexibility of this study. 4. CONCLUSIONS
Extraction of QI-free coal-tar pitch with supercritical fluids at 250-300°C or with liquid solvents at 150-200°C gave refined (residual) pitches with (a) significantly higher softening points, (b) high carbon yields, and (c) formation of mesophase much more rapidly than the QI-free pitch. These properties of the refined pitches should make them valuable as precursors for pitch and mesophase carbon fibers and as impregnating pitches for carbon-carbon composites. High mesophase content pitches can be obtained in a relatively short time by first heat treating the pitch and then super-critically extracting the heat-treated pitch. Acknowledgements-We are grateful to Krista Black for TI and molecular weight determinations and to Tim Parks, CSIRO Division of Coal and Energy Technology, for mesophase determinations by PMRTA.
REFERENCES 1. D. D. Edie. In Curhon Fibers, Filuments and Composites (Edited by J. L. Figueiredo, C. A. Bernardo, R. T. K. Baker, and K. J. Huttinger), pp. 43-72. Kluwer Academic Publishers, Dordrecht (1990).
92
J. R. KERSHAWand P. J. SMART
2. W. C. Stevens and R. J. Diefendorf, Carbon ‘86, Proc. 4rh In?. Carbon Conf p. 58. Baden-Baden (1986). 3. I. C. Lewis, Fuel 66, 1527 (1987). 4. L. S. Singer, Faraday Discuss. Chem. Sot., 79, 265 (1985). 5. I. C. Lewis, E. R. McHemy, and L. S. Singer, U.S. Patent 4, 017, 327 (1977). 6. R. T. Lewis and I. C. Lewis. U.S. Patent 4.303.631 (1981). 7. R. J. Diefendorf and D. N. Riggs, U.S. Patent 4,208,267 (1980). 8. E. Fitzer, Carbon 25, 163 (1987). 9. J. L. White and P. M. Sheaffer, Carbon, 27, 697 (1989). 10. G. Rellick, Carbon, 28, 589 (1990). 11. W. R. Sawran, F. H. Turrill, J. W. Newman, and N. W. Hall, U.S. Patent 4,497,789 (1985). 12. I. Mochida, Y. Korai, M. Nakamura, S. M. Zeng, and M. Kameyana, Carbon 27,498 (1989). 13. Z. Lisicki, W. Majewski, J. Kwiatkowski, A. Dunalewicz, and J. Polaczek, Fuel Processing Technol. 20, 103 (1988). 14. H. Beneke and S. Peter, Chem. Zng. Tech. 60, 700 (1988); U.S. Patent 4,806,228 (1989). 15. M. Shishido, S. Yamada, K. Arai, and S. Saito, Fuel 69, 1490 (1990). 16. J. R. Kershaw and P. J. Smart, J. Supercrit Fluids, 6, 155 (1993).
17. K. W. Hutchenson, J. R. Roebers, and M. C. Thies, Carbon 29, 215 (1991). 18. K. W. Hutchenson, J. R. Roebers, and M. C. Thies, J. Supercrit. Fluids 4, 7 (1991). 19. T. Hochgeschurtz, K. W. Hutchenson, J. R. Roebers, G.-Z. Liu, J. C. Mullins, and M. C. Thies, In Supercritical Fluid Engineering Science (Edited by E. Kiran and J. F. Brennecke). ACS Svmuosium Series 514. American Chemical Society, Washington, pp 347-362 (1993). 20. Y. D. Park and I. Mochida, Carbon 27, 925 (1989). 21. B. Rhee, D. H. Chung, S. J. In, and D. D. Edie, Carbon 29, 343 (1991). 22. J. R. Kershaw and K. J. T. Black, Energy Fuels 7, 420 (1993). 23. T. J. Parks, L. F. Cross, and L. J. Lynch, Carbon 29, 291 (1991). M. Zander, Fuel 66, 1459 (1987). M. L. Lee. M. V. Novotnv.< and K. D. Bartle. Analvti_ cal Chemistry of Polycyclic Aromatic Compounds, Academic Press, New York (1981). 26. R. A. Greinke and L. S. Singer, Carbon 26,665 (1988). 27. J. R. Kershaw and P. J. Smart, Fuel Processing Technol., in press. 28. H.-G. Franck and J. W. Stadelhofer, Industrial Aromatic Chemistry, Springer-Verlag, Berlin (1988). 29. K. Yudate, Y. Ohsugi, and M. Kamisita, Ext. Abstr. Znt. Symp. on Carbon (Tsukuba), p. 38 (1990).
Pergamon
Printed in Great Britain. All rights reserved 0008-6223194 $6.00 + .OO
000%6223(93)E0048-P
EXTRACTION
OF COAL-TAR PITCH AND THE EFFECT ON CARBONIZATION and P. J.
J. R. KERSHAW CSIRO
Division
of Materials
(Receiued
Science
10 May
and Technology, Private Victoria, 3169 Australia 1993; accepted
SMART Bag 33, Rosebank
in reuisedform
MDC,
Clayton,
21 July 1993)
Abstract-The extraction of a QI-free coal-tar pitch was investigated using supercritical fluid extraction with aliphatic hydrocarbons at 250-350°C and lo-15 MPa and by liquid extraction at 150-200°C. The softening points, number-average molecular weights, and toluene insoluble (TI) content of the residual pitches increase with the amount of lower molecular weight species extracted. The amount of mesophase formed in a given time and the Alcan coking value of the pitches show good correlations with the molecular weight and TI of the pitches. A process based on heat treatment of pitch under pressure followed by supercritical fluid extraction for preparation of mesophase pitch was investigated. Key Words--Solvent phase.
extraction,
supercritical
fluid extraction,
1. INTRODUCTION
At present, pitch-based carbon fibers account for only about 10% of the worldwide carbon fiber manufacturing capacity. However, pitch has the potential to produce carbon fibers at lower costs than from polyacrylonitrile (PAN), therefore allowing the possibility of considerable growth if suitable processes and products are developed. There are two main types of pitch fibers: (1) high-performance or mesophase fibers, which are produced from mesophase pitch resulting in a high degree of preferred orientation of the graphene layer planes along the fiber axis, and (2) general purpose or pitch fibers, which are produced from isotropic pitch resulting in little, if any, molecular orientation during spinning and no extended graphitic structure in the final fiber[ 11. The low-cost isotropic pitch fibers have much lower tensile strengths and moduli than mesophase fibers, with the tensile moduli of pitch fibers being about 4-20% of those of mesophase fibers, depending on the final carbonisation temperature used. The lower molecular weight species (“lights”) present in pitch are known to inhibit mesophase formation[ l-41. Often sparging with nitrogen to facilitate removal of the volatiles is used during the heat soaking to form mesophase[5,6]. Mesophase formation at 400°C is a slow process utilizing this approach, taking about 24 hours to produce 80 + % mesophase from either coal-tar or petroleum pitch with stirring and nitrogen sparging. However, increasing the temperature to 430-450°C reduces the heat-soaking time considerably. Diefendorf and
coal-tar
pitch,
carbonization,
meso-
Riggs[7] used solvent extraction of petroleum pitch at ambient temperature to remove the lower molecular species and to concentrate the higher molecular species (“heavies”) in the insoluble fraction, resulting in much reduced mesophase formation time. Reduction in the amount of lower molecular weight components is also required to produce suitable precursor pitches for production of isotropic carbon fibers and for binder pitches for carbon-carbon composites. A high carbon yield to minimise the number of densification steps is a requirement for a favourable binder pitch@-101. This property together with a relatively high softening point, to limit stabilization time, are needed in pitches for isotropic fibers[ 11,121. In this paper we report on an investigation of two solvent extraction procedures for removing the “lights” from a QI-free coal-tar pitch and the effect that these extractions have on the carbonization properties. This effect was measured by the percentage of mesophase formed in a given period and the Alcan coking value of the residual (refined) pitches. The extraction procedures were (a) supercritical fluid extraction (SCFE) using aliphatic hydrocarbons at 250-300°C and IO-ISMPa, and (b) liquid extraction at 150-200°C. There is a worldwide trend to supply and transport coal-tar pitch as a hot liquid rather than as a solid due to the potential health risks from pitch dust associated with handling solid pitch. The two extraction techniques are suited to feeding hot liquid pitch to the extraction reactor. Supercritical fluid extraction uses as an extractant a dense gas above its critical temperature and pressure. SCFE appears to be ideally suited to the extraction of the lower molecular weight component from pitch. Although there have been a limited number of reports of SCFE of coal-tar pitch[ 13- 161, the aim of these studies generally has been to extract
86
J. R. KERSHAWand P. J. SMART
the pitch from the undesirable particulate impurities present in pitch, rather than to “strip” off the “lights” from pitch. These pitch extracts, however, could be fractionated to remove the lower molecular weight components. Phase equilibrium data has been reported by Hutchenson et a1.[17,18] for the Ashland A-240 petroleum pitch/toluene system at conditions above the critical temperature and pressure of toluene. Recently Hochgeschurtz et a[.[191 fractionated a heat-soaked Conoco petroleum pitch with supercritical toluene in a region of liquid-liquid equilibrium. The bottom phase, which contains the higher molecular weight components, readily formed mesophase when dried at 357°C for 30 minutes. Subsequent melt-spinning, stabilization, and carbonization gave high-modulus carbon fibers[ 191. It has been recently reported[20,211 that a twostage preparation of mesophase with pressure or reflux treatment as the first stage maintained the volatiles within the reactor. This was followed by sparging or vacuum maturation as the second stage, which results in a mesophase pitch with a lower softening point and improved spinnability compared to a single stage process[20,21]. A variation of this approach based on pressure heat-treatment followed by SCFE is reported in this paper. 2. EXPERIMENTAL 2.1 Materials A coal-tar pitch supplied by Koppers Coal Tar Products, Mayfield, N.S.W., Australia, was filtered through a 1.2-pm glass fiber filter pad at 200°C with Hiflo Super-Cell filter aid, under pressure. Analysis of the filtered pitch is given in Table 1. The petroleum pitches A-240 and Aerocarb 70, 80, and 82 were obtained from Ashland Petroleum. 2.2 Supercritical fluid extraction The extractions were carried out in a 1-L unstirred autoclave. The autoclave was charged with pitch (100 g) and solvent (600 mL) and heated (ca 7°C min-‘). When the extraction temperature was reached, solvent (1 L h-l) was pumped via a dip tube, which acts as a preheater, into the bottom of
Table 1. Analysis of filtered coal-tar pitch Softening point (“C) Quinoline insolubles, QI, (wt%)* Toluene insolubles, TI, (wt%) Alcan Coking Value (wt%) c (wt%) H (wt%) N (wt%) s (wt%) 0 (diff.) (wt%) H/C Aromatic C (%C) Aromatic H (%H) Average MW (VPO) *Filtered through a 0.2 pm filter.
108 0.04 21.0 54.5 92.4 4.46 1.47 0.51 1.16 0.58 96 82 429
the autoclave and through the pitch. The pressure was controlled during the extraction by adjusting the throttling valves, and the solvent and extracted pitch were condensed by a water-cooled condenser and collected. After the extraction had been carried out for the desired period, the pump was switched off and the pressure lowered in the autoclave. When the autoclave had cooled, any remaining solvent was removed by blowing compressed air over the residue. The residue was then scaped out of the autoclave, carefully dried in a vacuum oven, and weighed. The solvent was removed from the extraction condensate under reduced pressure using a rotary evaporator. Samples of the extract for analysis were carefully dried in a vacuum oven. 2.3 Liquid solvent extraction of liquid pitch The extractions were carried out in a 450-mL stirred autoclave. The autoclave was charged with pitch and solvent (300 mL) and heated (ca 7°C mini). The stirrer motor was started when the pitch became sufficiently mobile at about 120°C. When the extraction temperature was reached, solvent (1 L h-i) was pumped via a dip tube, which acts as a preheater, into the bottom of the autoclave and through the pitch. The pressure was controlled during the extraction by adjusting the throttling valves, and the solvent and extracted pitch were condensed by a water-cooled condenser and collected. The work-up procedure was similar to that used for SCFE. 2.4 Mesophase formation Samples (3 g) were heated in a glass lined 80-mL stainless steel bomb to 400°C and maintained at that temperature for 6 hours, under a nitrogen atmosphere. 2.5 Analyses Toluene insolubles (TI) were determined by a standard procedure (IS0 6376). Alcan coking values (STPTC PT. 10-79) were determined by Koppers Coal-Tar Products. Softening points were determined using a Mettler instrument. EUMS were carried out on a HP5995A mass spectrometer. The instrument was scanned rapidly and the spectra summed. Average molecular weights were determined by vapour pressure osmometry (VPO) in pyridine at 80°C using a Knauer apparatus[221. Mesophase content of heat-treated pitches was determined by proton magnetic resonance thermal analysis (PMRTA)[23]. 3. RESULTS AND DISCUSSION
3.1 Extraction of QI-free pitch Supercritical fluid extraction (SCFE) of filtered coal-tar pitch was investigated using a number of aliphatic hydrocarbon solvents at temperatures of
87
Extraction of coal-tar pitch and the effect on carbonization Table 2. SCFE conditions, Extraction
conditions
Hexane (T, = 234)” Hexane Hexane Hexane Heptane (T, = 267) Cyclohexane (T, = 280) Petroleum spirit (boiling range 80- 110°C)
Residue
Extract
(MPa)
Time (min)
Yield (wt%)
MW
TI (wt%)
Mesophase (%)$
Alcan Coking Value (wt%)
250 250 300 300 290 300
10 15 1.5 15 15 10
60 60 60 20 60 60
29 32 41 23 46 51
261 NDt ND ND ND 301
31 34 43 26 45 50
36 ND ND ND 54 51
ND ND ND ND ND ND
496 ND ND ND ND 646
171 186 178 147 ND 190
300
15
60
45
ND
46
47
74.8
604
ND
Temp (“Cl
Solvent
yields, and properties of residues and extracts
Pressure
MW
s. Pt. “C
*T, = critical temperature “C. tND = not determined. $Maturation on 3 g, 4OO”C,6 h.
and pressures of lo-15 MPa (see Table 2). The aim was to ascertain conditions for extraction of 30-50% of the pitch. It is apparent from the data in Table 2 that a number of solvents are suitable for extraction of 30-50% of the coal-tar pitch. In a commercial process, an inexpensive mixed hydrocarbon solvent such as a petroleum distillation cut would be preferable to a more expensive single solvent. Extraction with a petroleum spirit shows this approach to be valid. For supercritical fluid extraction, the maximum extraction should occur close to the critical temperature of the solvent where the fluid density is the greatest. However, for hexane (T, = 234°C) extraction, higher yields are obtained at 300 than at 250°C (see Table 2). 250-300°C
The results for the extraction of pitch at 150 or 200°C and 4 MPa using a variety of organic solvents are given in Table 3. Significant extractions could be achieved after 15 minutes with all the solvents investigated. Acetone extractions were carried out at both 150 and 200°C and for various times (see Fig. 1). The results in Fig. 1 indicate that the extraction temperature has only a small effect, while the extraction yield increases with the extraction time. The residual (refined) pitches from both SCFE and liquid extraction had significantly higher softening points than the filtered pitch (see Tables 2 and 3). In some cases foaming, probably caused by residual solvents, prevented the obtaining of reliable softening points. The TI and the average molecular weights of the refined pitches were all considerably
Table 3. Liquid extraction of liquid pitch
Pitch qty (g)
100 50 50 100 50 50 50 50 50 50 50 50 50
Solvent None Acetone Acetone Acetone Acetone Acetone Acetone Acetone Methylene chloride Trichloroethylene Tetrachloroethylene Ethanol Propan-2-01 Cyclohexane
Temperature (“C)
Pressure (MPa)
Time (min)
Extm. (wt%)
TI (wt%)*
200 200 150 150 200 150 200
4 4 4 4 4 4 4
15 15 15 15 30 30 60
0 46 47 49 54 55 57 69
21 36 39 52 54 56 59 73
84 95
150
4
15
75
81
97
200
4
15
200 150 150 150
4 4 4 4
15 15 15 15
>60 63 36 34 42
*Performed on the extraction residue. tMesophase trials performed on extraction residue; 3 g, 400°C. 6 h.
Mesophase (%)+ 12 41
66 31 38 41
100 40
Alcan Coking Value (wt%)*
MW*
s. Pt. (“c)*
429 687 -
108 176 238 -
-
-
-
-
-
-
525 518 -
207 207 -
54.5 78.3 78.4 1 86.2
71.1 -
88
J. R. KERSHAWand P. J. SMART 70
65
0
15
30
45
60
75
Time (mins) Fig.
Extraction
1.
yield vs. time for acetone extraction.
higher than those of the unextracted pitch (see Tables 2 and 3). The TI and number-average molecular weights of the residual pitches correlate well with the weight % of pitch extracted both for liquid extraction at 150-200°C and for supercritical fluid extraction at higher temperatures (see Figs. 2 and 3). Very similar correlations were obtained between the properties of the refined pitches and the weight % extracted
(“lights” removed) by these two extraction techniques. Super~~tical fluid extraction was carried out with nonpolar aliphatic hydrocarbon solvents, while the liquid extraction mainly used polar solvents, but was also carried out with cyclohexane. It appears, therefore, that the properties of the residue, and presumably also of the extract, are not significantly affected by the polarity of the extractant. Coal-tar pitch is a complex mixture of polycyclic
O-
! i
0
SCFE,
0
Liquid extraction.150-200°C,
250-300%.
lo-15MPa
X
Liquid extraction
4MPa
40%
D-
D-
3-
! 3’
20
/
I
40
/
60
Extraction
80
(%)
Fig. 2. Variation of TI with wt% extracted.
Extraction
o/
650
r’
~
600-
s
550
500
-
“7 c
450
20
Liquid
’ 30
3. Variation (an)
15O’C;
4MPa
I
I
40
50
60
Extracted
WT%
Fig.
Extraction;
of number-average molecular with wt% extracted.
89
pitch and the effect on carbonization
of coal-tar
weight
aromatic compounds (PAC). It would be expected that polarity, topology, and possibly intermolecular interactions as well as molecular weight will affect the solubility of pitch components in various solvents. The solubility of isomeric polycyclic aromatic hydrocarbons (PAH) can vary markedly[24]. How-
ever, Fig. 3 indicates that solvent extraction, both supercritical fluid and liquid, extracts predominantly by molecular weight. EUMS of the extracts both from SCFE and from liquid extraction at 150-200°C were similar, and all showed major peaks at 202,228,252, 276, 302, 326, 350,352, and 376. There was a shift of the ion profile to higher molecular weight as the amount extracted increases. Polycyclic aromatic compounds have strong molecular ions in comparison to their fragmentation ions[25]. This allows EUMS to be used to show broad trends in the molecular weight profile of pitch fractions. Variation in molecular ion sensitivities for different PAC and the cumulative effect of fragment ions means that only qualitative profiles of molecular weights can be obtained with EIiMS of these complex mixtures. The carbonization properties of the pitches were studied by monitoring their mesophase formation ability and their Alcan coking values. The latter test measures the coked residue after heating at 550°C for 2.5 hours in a nonoxidizing environment. The pitches were screened for their ability to form mesophase by heating 3-gram samples at 400°C in a small bomb under a nitrogen atmosphere for 6 hours. The results of the mesophase formation studies and the Alcan coking values are given in Tables 2 and 3. In Figs. 4 and 5. the amount of mesophase formed and
1
1
l
Liquid
0
Supercritical
20
Fig. 4. Variation
I
I
40
60
80
Extraction
(wt%)
in mesophase tracted.
formation
J
I
100
20 Extraction
with wt% ex-
Fig.
5. Variation
in Alcan coking tracted.
fluid
extraction
L I
extraction
I
I
40
60
1 I I
(wt%) value
with
wt%
ex-
J. R. KERSHAWand P. J. SMART
90
100
90
80-
SO
P
z
s s
film evaporator. It is noticeable that the petroleum pitches require a higher molecular weight to form the same amount of mesophase as coal-tar pitches. The lower molecular weight species (“lights”) in pitch are known to inhibit mesophase formation[l-41, and therefore an increase in the mesophase formed with an increased in the amount of extracted was expected. Furthermore, “lights” Greinke and Singer’s[26] studies on heat-treated A-240 pitch showed that the probability of a molecule being in the anisotropic phase increases with molecular weight. Nevertheless, the molecular weight profiles of isotropic and anisotropic phases in equilibrium were reported not to vary markedly[26].
-u 70 Q
60
20
I
I
I
40
60
80
50 0
3.2 extraction of beat-treater pitch Except for the low molecular weight components in isotropic pitch, which inhibit mesophase formation, the limited data available indicate that mesophase and isotropic pitch do not markedly differ in their molecular weight profiles[3,4,26]. Nevertheless, the formation of mesophase from refined pitches in which a significant amount of “lights” has been removed is still a relatively slow process, taking about 10 hours to produce 80 + % mesophase compared to 24 hours for filtered pitch under con-
TI (wt%) Fig. 6. Variation in % mesophase formed (r2 = 0.93) and Alcan coking value (r’ = 0.94) with TI. the Alcan coking value are plotted against the weight % extracted for both SCFE and liquid extraction. As the amount of “lights” extracted increases, the amount of mesophase formed and the Alcan coking value increase. The carbonization potential of the pitches as indicated by their mesophase formation ability and Alcan coking value correlate well with the TI and molecular weight of the pitches, as shown in Figs. 6 and 7. There is a strong intercorrelation (rZ = 0.98) between molecular weight and TI for these pitches. The relationships between carbonization properties, TI, and molecular weight may not hold for a wider variety of pitches or indeed for more complex extraction procedures, such as removing the higher as well as the lower molecular weight components, on this pitch. For example, extracting with supercritical toluene (for QI and “heavies” removal) and then extracting the supercritical toluene extract with supercritical hexane gave a pitch with TI = 16% and &&r = 489 from removal of 21% “heavies” and 39% “lights.” (Compare with the filtered pitch, TI = 21% and mn = 429). In Fig. 7, as well as data for solvent-extracted coal-tar pitch, data for commercially available petroleum pitches, Ashland A-240 and Aerocarb, are included. The Aerocarb pitches are obtained from A-240 by stripping the more volatile (lower molecular weight) components from the pitch using a wiped
I 10
I
I
I
I
I
I
I
II
800
600
X&n Fig. 7. Variation in % mesophase formed with numberaverage molecular weight: 0 = coal-tar pitches (r* = 0.86); 0 = petroleum pitches (v’ = 0.99).
Extraction of coal-tar pitch and the effect on carbonization Table 4. Data for pressure maturation followed by supercritical fluid extraction Maturationt Run
Pitch*
Tetralin (g)
1
A-240 A-240
None None
450 410
None
450
None 30 30 30 100 100 100
450 450 430 420 420 430 400
2 3
4 5 6 7 8 9 10
:; CT CT CT CT CT CT
Temp. (“C)
Extraction yield (wt%)$
Mesophase content in residue (%)
1 1 I
24 57
78 95
36
60
0.75 3 2
44 41 61 67 75 79 75
88 79 84 89 83 91 71
Time (hi
1 3 1.5 2
*lo0 g; A-240 = A-240 petroleum pitch; CT = filtered coal-tar pitch. tin an enclosed autoclave under Nz, cold pressure 3 MPa. *Extraction with cyclohexane, 35O”C, 10 MPa, 1 hour.
ventional maturation (stirring at 400°C with nitrogen sparging) conditions[27]. Chemical reactions, such as polymerisation, rearrangement, and cyclisation occur during mesophase formation and probably give rise to PACs similar to those removed during extraction. Therefore, it appears potentially advantageous to have the required chemical reactions take place prior to extraction. The procedure used was to heat the pitch at 400-450°C for 0.75-3 hours in an enclosed autoclave under nitrogen (cold pressure 3 MPa) to carry out the chemical reactions necessary for mesophase formation. The autoclave is then allowed to cool to 35O”C, and the pitch is extracted with a supercritical fluid to remove low-molecularweight isotropic pitch, which inhibits mesophase formation. The study was carried out with both petroleum and filtered coal-tar pitch. For the latter pitch, the heat treatment step was also carried out in the presence of the hydrogen donor solvent, tetrahydronaphthalene (tetralin). Mild hydrogenation of coal-tar pitch enables high mesophase content to be achieved at relatively low viscosity, facilitating the spinning process[28,29]. The potential advantages of this approach are as follows: 1. Single “pot” process for refining/mesophase formation. 2. Can be combined with hydrogenation by addition of a hydrogen donor solvent such as tetralin . 3. Low molecular weight components are present during the heat treatment stage and may be incorporated into the mesophase, possibly improving the properties of the resultant mesophase pitch. 4. Short time to form mesophase. 5. Process is simple and novel. It has been recently reported that two-stage maturation with pressure or reflux treatment at the first
stage, thus maintaining the volatiles within the reactor, followed by sparging or vacuum maturation as the second stage, results in a mesophase pitch with a lower softening point and improved spinn~~biijty compared to single stage maturation[20,2i]. High mesophase content pitches have been obtained under these conditions (see Table 4), and these results indicate the potential of this approach. EUMS of the extract from run 5 showed naphthalene but no tetralin. indicating that pitch hydrogenation had occurred. However, unfortunately the lack of stirring in the autoclave and the slow heating and especially cooling cycles limited the flexibility of this study. 4. CONCLUSIONS
Extraction of QI-free coal-tar pitch with supercritical fluids at 250-300°C or with liquid solvents at 150-200°C gave refined (residual) pitches with (a) significantly higher softening points, (b) high carbon yields, and (c) formation of mesophase much more rapidly than the QI-free pitch. These properties of the refined pitches should make them valuable as precursors for pitch and mesophase carbon fibers and as impregnating pitches for carbon-carbon composites. High mesophase content pitches can be obtained in a relatively short time by first heat treating the pitch and then super-critically extracting the heat-treated pitch. Acknowledgements-We are grateful to Krista Black for TI and molecular weight determinations and to Tim Parks, CSIRO Division of Coal and Energy Technology, for mesophase determinations by PMRTA.
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J. R. KERSHAWand P. J. SMART
2. W. C. Stevens and R. J. Diefendorf, Carbon ‘86, Proc. 4rh In?. Carbon Conf p. 58. Baden-Baden (1986). 3. I. C. Lewis, Fuel 66, 1527 (1987). 4. L. S. Singer, Faraday Discuss. Chem. Sot., 79, 265 (1985). 5. I. C. Lewis, E. R. McHemy, and L. S. Singer, U.S. Patent 4, 017, 327 (1977). 6. R. T. Lewis and I. C. Lewis. U.S. Patent 4.303.631 (1981). 7. R. J. Diefendorf and D. N. Riggs, U.S. Patent 4,208,267 (1980). 8. E. Fitzer, Carbon 25, 163 (1987). 9. J. L. White and P. M. Sheaffer, Carbon, 27, 697 (1989). 10. G. Rellick, Carbon, 28, 589 (1990). 11. W. R. Sawran, F. H. Turrill, J. W. Newman, and N. W. Hall, U.S. Patent 4,497,789 (1985). 12. I. Mochida, Y. Korai, M. Nakamura, S. M. Zeng, and M. Kameyana, Carbon 27,498 (1989). 13. Z. Lisicki, W. Majewski, J. Kwiatkowski, A. Dunalewicz, and J. Polaczek, Fuel Processing Technol. 20, 103 (1988). 14. H. Beneke and S. Peter, Chem. Zng. Tech. 60, 700 (1988); U.S. Patent 4,806,228 (1989). 15. M. Shishido, S. Yamada, K. Arai, and S. Saito, Fuel 69, 1490 (1990). 16. J. R. Kershaw and P. J. Smart, J. Supercrit Fluids, 6, 155 (1993).
17. K. W. Hutchenson, J. R. Roebers, and M. C. Thies, Carbon 29, 215 (1991). 18. K. W. Hutchenson, J. R. Roebers, and M. C. Thies, J. Supercrit. Fluids 4, 7 (1991). 19. T. Hochgeschurtz, K. W. Hutchenson, J. R. Roebers, G.-Z. Liu, J. C. Mullins, and M. C. Thies, In Supercritical Fluid Engineering Science (Edited by E. Kiran and J. F. Brennecke). ACS Svmuosium Series 514. American Chemical Society, Washington, pp 347-362 (1993). 20. Y. D. Park and I. Mochida, Carbon 27, 925 (1989). 21. B. Rhee, D. H. Chung, S. J. In, and D. D. Edie, Carbon 29, 343 (1991). 22. J. R. Kershaw and K. J. T. Black, Energy Fuels 7, 420 (1993). 23. T. J. Parks, L. F. Cross, and L. J. Lynch, Carbon 29, 291 (1991). M. Zander, Fuel 66, 1459 (1987). M. L. Lee. M. V. Novotnv.< and K. D. Bartle. Analvti_ cal Chemistry of Polycyclic Aromatic Compounds, Academic Press, New York (1981). 26. R. A. Greinke and L. S. Singer, Carbon 26,665 (1988). 27. J. R. Kershaw and P. J. Smart, Fuel Processing Technol., in press. 28. H.-G. Franck and J. W. Stadelhofer, Industrial Aromatic Chemistry, Springer-Verlag, Berlin (1988). 29. K. Yudate, Y. Ohsugi, and M. Kamisita, Ext. Abstr. Znt. Symp. on Carbon (Tsukuba), p. 38 (1990).