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Rheological properties and carbonization of coal-tar pitch
Fuel Vol. 75 No. l, pp. 3-7, 1996 Copyright :~: 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0016-2361/96/$15.00 + 0.00
0016-2361(95)00190-5
ELSEVIER
Rheological properties and carbonization of coal-tar pitch Xuanke Li and Qingtian Li The Department of Chemical Engineering, Wuhan Iron & Steel University, Wuhan, Hubei, 430081 China (Received 5 May 1995) During the carbonization of coal-tar pitch the viscosity of the pitch decreased at 200-360°C, reached a minimum at 360-400°C, and then increased at 400-450°C. The decrease of the viscosityof the pitch after its melting may be attributed to the solubilizationof benzene insolubles (BI) (/3-resin and quinine insolubles) by the solvation of benzene solubles (BS) to BI, in addition to the increase of thermal movementsof pitch with increasing temperature. The viscosity increase above 400°C can be explained by the growth of a high molecular weight mesophase. The addition of extra/3-resin to the pitch resulted in the formation of smaller anisotropic domains, compared with the carbonization of the pitch alone. ¢3-Resinseems to become nuclei for thermal condensation, leading to the rapid solidificationof the mesophase. This hinders the growth of the mesophase to a large anisotropic domain. (Keywords: coal-tar pitch; carbonization; rheology)
Liquid crystal formation in the early stage of carbonization of coal-tar pitch is related to the rheology of the pitch in two aspects. First, the theological properties, viscosity in particular, are a decisive factor for the development of anisotropy during the formation of mesophase. Second, externally applied shearing action affects the growth of mesophase and its flow domain texture during the formation of mesophase. The rheological properties themselves, in turn, depend on the molecular structure, average molecular weight and molecular weight distribution of coal-tar pitch. The study of the mesophase formation in various narrow molecular weight fractions of a quinoline insolubles (QI)-free coal-tar pitch showed that the beginning of mesophase formation below 450°C depends mainly on the average molecular weight I . Isothermal viscosity determination made on mesophase coal-tar pitch shows that, in general, the bulk viscosity depends on the shearing rate, suggesting that the fluid is non-Newtonian. In addition, the amount of polar functional groups and the concentration of free radicals present in the pitch, appear to affect the reaction rate of condensation, which controls the viscosity more decisively than the molecular weight of the pitch during carbonization. It is well known that polar functional groups such as hydroxyl and carboxyl groups induce condensations. Free radicals, which are formed by bond scission above about 350°C, are also a key intermediate for carbonization, since the carbonization process includes addition and decomposition through free radical mechanisms. So, electron spin resonance (e.s.r.) determination of free radical concentration is an effective
way to study the reactivity of mixed aromatic compounds 2-4. Because of the factors considered above and the heterogeneity of the system consisting of the mesophase and the isotropic matrix, and the formation of bubbles during carbonization, the determination of pitch viscosity is very difficult. Especially in the temperature range where condensation and decomposition occur, the accurate and systematic determination of pitch viscosity has rarely been done. In this study the effect of pitch carbonization conditions, such as temperature and time, on pitch viscosity and formation and development of mesophase was investigated. EXPERIMENTAL
Preparation of pitch sample Coal-tar pitch was obtained by moderate temperature coking at Wuhan Iron & Steel Corporation. Analytical data of the pitch are summarized in Table 1. The contents of QI, benzene insolubles-quinoline solubles (BI-QS) (~-resin), and benzene solubles (BS) were determined by solvent extraction with quinoline and benzene, respectively.
Carbonization of pitch Pitch (80 g) in a stainless steel tube (diameter 30 mm, height 150 mm) was heated in a vertical electric furnace at a heating rate of 0.5-3°Cmin -1 to a desired temperature under nitrogen atmosphere. Viscosities were determined by a revolving viscometer (NDJ-1) at
Fuel 1996 Volume 75 Number 1
3
Carbonization of coal-tar pitch: Xuanke Li and Oingtian Li Table 1 Characterizationof coal tar pitch
Ultimate analysis (wt%, daf)
Composition (wt%) Ash (wt%)
BI-QS (fl-resin)
QI
H
Atomic ratio H/C
BS
C
Softening point (°c)
93.5
4.4
0.56
0.2
81.8
11.5
6.7
81
each carbonization temperature and at selected temperatures as shown in Table 2. Characterization of carbonized pitch X-Ray diffraction analysis was performed at room temperature using filtered Cu radiation with extra-pure silicon (5%) as reference. Scanning electron microscope (s.e.m.) micrographs were obtained and the diameters of mesophase spheres formed were determined by image analysis. After ultrasonic treatment of the fractionated mesophase spheres suspended in acetone for several minutes, a drop of the suspension was deposited on a s.e.m, sample holder. The number average diameter of mesophase spheres was estimated from the s.e.m, micrographs by point-counting using an image analyser. E.s.r. measurement was carried out at room temperature. Spin concentration was determined using a,adiphenylv3-picrylhydrazyl as a standard. Portions of carbonized pitch were mounted in plastic resin and after conventional polishing, their microstructure was determined by optical microscopy. RESULTS AND DISCUSSION Effect of carbonization temperature and time on pitch viscosity Figure I shows the relationship between pitch viscosity and carbonization temperature in the range of 200450°C. The viscosity of the pitch decreases with increasing temperature, and after reaching its lowest value at 360-400°C, it increases again with an especially rapid increase at 430-450°C. Two reasons are considered for the viscosity decrease of the pitch at 200-300°C. With the increase in temperature, thermal movement of pitch becomes more rapid and more random, and intermolecular interaction forces decrease, leading to the decrease in viscosity. The solubilization of BI (d-resin and QI) by the solvation of BS is considered to be another reason for the decrease in viscosity. Solvation is related to the concept of acid and base properties, such as electrophilic-nucleophilic and electron donor-acceptor properties. Coal-tar pitch has condensed aromatic rings, which can easily become nucleophilic or electrophilic, depending on the substituents (functional groups) on the aromatic ring./3-Resin and QI components which contain functional groups and free radicals, easily associate with BS, which is the major component in the pitch (Table 1). Much evidence for the association between coal-derived components such as coal-tar and coal extract has been reported 5. Shishido et al. 6 reported that the solubility of/3-resin in super critical toluene is enhanced by the solvation (association) of the toluene soluble fraction around/~-resin molecules. Bhata et al. ~ also suggested that TS (toluene solubles) from coal-tar pitch acts as a solvent for TI (toluene
4
Fuel 1996 Volume 75 Number 1
22 -
18
? ~ , 14
"~ 10
\ _\
\.
>
2 200
I
l
250
300
-~'-o-,.o. 350
I 400
450
Temperature (°C) Figure 1 Viscosity of the pitch carbonized at 200-450°C at a heating rate of 3°Cmin -l (closed circle) from room temperature to 400°C and 0.5°Cmin -1 (open circle) from 400°C to 450°C
insolubles) below a critical temperature, which, lying between 380 and 450°C, depends on the TI content in the mixture. Above this temperature the formation of QI increased by condensation. Therefore, the solubilization o f BI (fl-resin and QI) in BS can be considered to be one of reasons for the decrease in viscosity observed at 200360°C. At 360-400°C, the viscosity reaches a minimum value. Above 400°C, thermal condensation occurs preferentially, forming a large amount of mesophase spheres and decreasing BS content, leading to increase the viscosity. Figure 2 shows the change in viscosity with respect to the holding time when the pitch is heated at a heating rate of 2°C min-1 to a desired holding temperature. The slope of the viscosity-time curve depends on the holding temperature and the holding time. The higher the holding temperature and the longer the holding time, the greater the increase in viscosity. This suggests that thermal condensation, which increases with temperature and time, increases the formation of mesophase and the viscosity of the pitch. Figure 3 shows the viscosity and BI content of the pitch carbonized at 400-450°C. BI content of the pitch increases with increasing temperature. This indicates that the rate for thermal condensation of the pitch increases with temperature. Kremer 7 reported that with increasing heating temperature, heavy components in pitch, i.e. BI and QI portions increase due to thermal condensation, and the pitch becomes more and more pseudoplastic. A similar relationship between BI and viscosity was observed when holding time was varied at 405°C, as shown in Figure 4. Table 2 shows that the
Carbonization of coal-tar pitch. Xuanke Li and Qingtian Li Table 2 Viscosity and QI of the pitch carbonized at various temperaturesand times
26 --
22 --
•
14--
b
°>
•
•
.~
a
•
Time (h)
Viscosity/temperature~ (Pa s) (°C)
QI (wt%)
420 450 390, 4206 390 430
5 0.5 3, 1b 6 2
64.0/370 14.5/350 25.8/350 10.7/350 38.0/350
74.8 48.0 54.3 43.8 65.3
"The temperature at which viscosity was measured b Carbonization at 390°C for 3 h, and then at 420°C for 1 h
t° K 6
Table 3 The diameter of the mesophase formed by the carbonization of the pitch at various temperaturesa and times
2
I 0
1
2
3
4
5
Mesophase diameter (#m)
6
T i m e (h)
Figure 2 Relationship between viscosity of the pitch carbonized and carbonization time: (a) 390°C; (b) 450°C
Temperature (°C) Time(h)
Minimum Maximum Average
405 405 405 390, 420 a 390, 420 a
5 6 6 7 7
4 5 6 3, 1b 3, 2 b
/
18
, ~ 14 --
•m
60
•
•~ tO
L
Table 4 X-Ray diffraction parameters of the pitch carbonized at various temperaturea and time
oo
Temperature (°C)
Time (h)
d002 (,&.)
Lc (002) (A)
405 405 405 405 405 390,420 b 390, 420 b 390, 420 b
2 3 4 5 6 3, 1a 3, 2 a 3, 3"
3.50 3.50 3.46 3.46 3.46 3.46 3.45 3.45
17 19 25 26 28 29 30 33
40
> 6
2 400
410
420
430
440
30 450
Temperature ( ° C ) Figure 3 Viscosity (a) and BI (fl-resin and QI) content (b t of the pitch carbonized at 400-450°C at a heating rate of 0.5°C min-" from room temperature to 400°C and then at 2°Cmin -l from 400°C to 450°C
/
22 --
18 -14 --
/
10 --
./ ./
o i/~* ° i 30
40
50
and 3 h,
Effect of carbonization temperature and time on the growth of mesophase Table 3 shows the diameter o f the mesophase formed
6 -2
a Heating rate of 2°C m i n - t b C a r b o n i z a t i o n at 390°C for 3 h, and then at 420°C for 1, 2, respectively
viscosity also relates with QI wt%, when the pitch was heated at a heating rate o f 2 o C m i n - 1 , and after holding at a desired temperature and time, it was cooled at a rate of 8°Cmin -l to the temperature o f viscosity measurement. Since, QI and BI contents are k n o w n to be related with the mesophase content, it may be reasonable to consider that the formation o f the mesophase increases the viscosity.
26 --
8
11 14 22 22 24
a Heating rate of 2°C min l b Carbonization of 390°C for 3 h, and then at 420°C for 1 and 2 h, respectively
50
-?
43 35 125 73 84
- 70
22 -
-?
Temperature (°C)
I
I
60
70
BI c o n t e n t ( w t % )
Figure 4 Relationship between viscosity and BI (fl-resin and QI) content o f the pitch carbonized for various holding time at 405°C at a heating rate o f 2°C min -~ from r o o m temperature to 405°C
by the carbonization o f the pitch at a heating rate o f 2°Cmin -1. Table 3 shows that with increasing holding time, the diameter o f mesophase spheres increases. This indicates that at a given temperature, the increase of holding time favours the growth o f optically anisotropic structure, i.e. the growth o f mesophase. Table 4 shows d002 and L c parameters determined at r o o m temperature by X-ray diffraction for the carbonized pitch at different temperatures and times. It
Fuel 1996 Volume 75 Number 1
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Carbonization of coal-tar pitch: Xuanke Li and Qingtian Li shows that the interlamellar distance, d002 decreased slightly with holding time. d002 for the pitch carbonized at 390°C and then at 420°C is similar to that at 405°C for the same holding time. With increasing holding time, Lc increases, i.e. mesophase spheres become larger. The higher the temperature at which mesophase is formed, the larger the size of the mesophase spheres. At high temperatures, large spheres can merge with each other.
Table 5 g-value and spin concentration of BS, ~3-resin and QI of the pitch
g-value spin/g
BS
/3-Resin
QI
2.0032 4.5 × 1018
2.0031 6.8 x 1019
2.0029 4.5 x 1019
Characterization and structure of fl-resin Figure 5a-e illustrates the s.e.m, micrographs of raw fl-resin (Figure 5a, b) which show it is composed of spherical particles, and the fl-resin (Figure 5c) carbonized
Figure 6 Optical micrographs of the pitch carbonized at 450°C for 1.5h with a heating rate of 3°Cmin -I, with (a) and without (b) the addition of 50 wt% of fl-resin
at 410°C for 1.5h, shows the merging of spherical particles during carbonization. X-Ray analysis shows d002 and L c of raw 13-resin are 3.566 and 14.6 A, respectively indicating that the average number of crystalline layer stacks is about three to four for fl-resin. It shows that polynuclear aromatic rings of /3-resin have a certain extent of crystallite growth, which is a precursor for mesophase formation.
Effect of the addition of l3-resin on the growth of mesophase
Figure 5 S.e.m. photographs of raw (a, b) and carbonized (c)/3-resin. Carbonization was carried out at 410°C for 1.5h. (b) is part magnification of (a)
6
Fuel 1996 Volume 75 Number 1
G-values and spin concentrations of the BS, 13-resin and QI portions of the pitch are shown in Table 5. Table 5 shows that g-values are in the range 2.00292.0032 with the fl-resin having the highest free radical concentration. The pitches were heated to 450°C at a heating rate of
Carbonization of coal-tar pitch: Xuanke Li and Qingtian Li 3°Cmin -l and held at the temperature for 1.5 h with or without the addition of 50 wt% of/3-resin. Figure 6a, b show the micrographs of their carbonization products. The carbonized pitch without fl-resin gives larger anisotropic domains than that with fl-resin. The latter (Figure 6a) contains mainly mesophase spheres. Since the free radical concentration in the/3-resin is one order of magnitude higher than that in the BS, high 13-resin concentration certainly accelerates thermal condensation and its mesophase solidifies too rapidly to merge together, resulting in a smaller anisotropic domains./3Resin dissolved in BS is considered to become the nuclei for thermal condensation.
alone, fl-Resin seems to become the nuclei for thermal condensation, leading to the rapid solidification of the mesophase which hinders the growth of mesophase to form large anisotropic domains.
ACKNOWLEDGMENT The authors thank Dr Masashi Iino of Tohoku University, Japan for his helpful discussion and suggestion for this work.
REFERENCES CONCLUSION During the carbonization of a coal-tar pitch the viscosity of the pitch decreased at 200-360°C, reached a minimum at 360-400°C, and then increased at 400-450°C. The decrease of the viscosity of the pitch after its melting may be attributed to the solvent action of BS toward BI (/3resin and QI), in addition to the increase of thermal movements of the pitch. The viscosity increase above 400°C can be explained by the growth of mesophase of high molecular weight. The addition of extra/3-resin to the pitch resulted in the formation of smaller anisotropic domains, compared with the carbonization of the pitch
1 2 3 4 5
6 7
Bhatia,G., Fitzer, E. and Kompalik, D. Carbon 1986, 24, 489 Lewis,I. C. and Singer, L. S. in 'Chemistryand Physicsof Carbon' (Eds P. L. Walker,Jr. and P. A. Thrower),Vol. 17, MarcelDekker, New York, 1981,pp. 1 and 73 Lewis,I. C. and Singer, L. C. Carbon 1967,5, 373 Fitzer,E., Mueller,K. and Schaefer,W. in 'Chemistryand Physics of Carbon' (Ed. P. L. Walker, Jr.), Vol. 7, Marcel Dekker, New York, 1987,p. 237 Stenberg,V. I., Baltisberger, R. I., Patal, K. M., Raman, K. and Woolsey, N. F. In 'Coal Science' (Eds M. L. Gorbaty, J. W. Larsen and I. Wender), Vol. 2, Academic Press, New York, 1983, p. 125 Shishido,M., Yamada, S., Arai, K. and Saito, S. Fuel 1990, 69, 1490 Kremer,H. A. Chem. Ind. 1982,702
Fuel 1996 Volume 75 Number 1
7
0016-2361(95)00190-5
ELSEVIER
Rheological properties and carbonization of coal-tar pitch Xuanke Li and Qingtian Li The Department of Chemical Engineering, Wuhan Iron & Steel University, Wuhan, Hubei, 430081 China (Received 5 May 1995) During the carbonization of coal-tar pitch the viscosity of the pitch decreased at 200-360°C, reached a minimum at 360-400°C, and then increased at 400-450°C. The decrease of the viscosityof the pitch after its melting may be attributed to the solubilizationof benzene insolubles (BI) (/3-resin and quinine insolubles) by the solvation of benzene solubles (BS) to BI, in addition to the increase of thermal movementsof pitch with increasing temperature. The viscosity increase above 400°C can be explained by the growth of a high molecular weight mesophase. The addition of extra/3-resin to the pitch resulted in the formation of smaller anisotropic domains, compared with the carbonization of the pitch alone. ¢3-Resinseems to become nuclei for thermal condensation, leading to the rapid solidificationof the mesophase. This hinders the growth of the mesophase to a large anisotropic domain. (Keywords: coal-tar pitch; carbonization; rheology)
Liquid crystal formation in the early stage of carbonization of coal-tar pitch is related to the rheology of the pitch in two aspects. First, the theological properties, viscosity in particular, are a decisive factor for the development of anisotropy during the formation of mesophase. Second, externally applied shearing action affects the growth of mesophase and its flow domain texture during the formation of mesophase. The rheological properties themselves, in turn, depend on the molecular structure, average molecular weight and molecular weight distribution of coal-tar pitch. The study of the mesophase formation in various narrow molecular weight fractions of a quinoline insolubles (QI)-free coal-tar pitch showed that the beginning of mesophase formation below 450°C depends mainly on the average molecular weight I . Isothermal viscosity determination made on mesophase coal-tar pitch shows that, in general, the bulk viscosity depends on the shearing rate, suggesting that the fluid is non-Newtonian. In addition, the amount of polar functional groups and the concentration of free radicals present in the pitch, appear to affect the reaction rate of condensation, which controls the viscosity more decisively than the molecular weight of the pitch during carbonization. It is well known that polar functional groups such as hydroxyl and carboxyl groups induce condensations. Free radicals, which are formed by bond scission above about 350°C, are also a key intermediate for carbonization, since the carbonization process includes addition and decomposition through free radical mechanisms. So, electron spin resonance (e.s.r.) determination of free radical concentration is an effective
way to study the reactivity of mixed aromatic compounds 2-4. Because of the factors considered above and the heterogeneity of the system consisting of the mesophase and the isotropic matrix, and the formation of bubbles during carbonization, the determination of pitch viscosity is very difficult. Especially in the temperature range where condensation and decomposition occur, the accurate and systematic determination of pitch viscosity has rarely been done. In this study the effect of pitch carbonization conditions, such as temperature and time, on pitch viscosity and formation and development of mesophase was investigated. EXPERIMENTAL
Preparation of pitch sample Coal-tar pitch was obtained by moderate temperature coking at Wuhan Iron & Steel Corporation. Analytical data of the pitch are summarized in Table 1. The contents of QI, benzene insolubles-quinoline solubles (BI-QS) (~-resin), and benzene solubles (BS) were determined by solvent extraction with quinoline and benzene, respectively.
Carbonization of pitch Pitch (80 g) in a stainless steel tube (diameter 30 mm, height 150 mm) was heated in a vertical electric furnace at a heating rate of 0.5-3°Cmin -1 to a desired temperature under nitrogen atmosphere. Viscosities were determined by a revolving viscometer (NDJ-1) at
Fuel 1996 Volume 75 Number 1
3
Carbonization of coal-tar pitch: Xuanke Li and Oingtian Li Table 1 Characterizationof coal tar pitch
Ultimate analysis (wt%, daf)
Composition (wt%) Ash (wt%)
BI-QS (fl-resin)
QI
H
Atomic ratio H/C
BS
C
Softening point (°c)
93.5
4.4
0.56
0.2
81.8
11.5
6.7
81
each carbonization temperature and at selected temperatures as shown in Table 2. Characterization of carbonized pitch X-Ray diffraction analysis was performed at room temperature using filtered Cu radiation with extra-pure silicon (5%) as reference. Scanning electron microscope (s.e.m.) micrographs were obtained and the diameters of mesophase spheres formed were determined by image analysis. After ultrasonic treatment of the fractionated mesophase spheres suspended in acetone for several minutes, a drop of the suspension was deposited on a s.e.m, sample holder. The number average diameter of mesophase spheres was estimated from the s.e.m, micrographs by point-counting using an image analyser. E.s.r. measurement was carried out at room temperature. Spin concentration was determined using a,adiphenylv3-picrylhydrazyl as a standard. Portions of carbonized pitch were mounted in plastic resin and after conventional polishing, their microstructure was determined by optical microscopy. RESULTS AND DISCUSSION Effect of carbonization temperature and time on pitch viscosity Figure I shows the relationship between pitch viscosity and carbonization temperature in the range of 200450°C. The viscosity of the pitch decreases with increasing temperature, and after reaching its lowest value at 360-400°C, it increases again with an especially rapid increase at 430-450°C. Two reasons are considered for the viscosity decrease of the pitch at 200-300°C. With the increase in temperature, thermal movement of pitch becomes more rapid and more random, and intermolecular interaction forces decrease, leading to the decrease in viscosity. The solubilization of BI (d-resin and QI) by the solvation of BS is considered to be another reason for the decrease in viscosity. Solvation is related to the concept of acid and base properties, such as electrophilic-nucleophilic and electron donor-acceptor properties. Coal-tar pitch has condensed aromatic rings, which can easily become nucleophilic or electrophilic, depending on the substituents (functional groups) on the aromatic ring./3-Resin and QI components which contain functional groups and free radicals, easily associate with BS, which is the major component in the pitch (Table 1). Much evidence for the association between coal-derived components such as coal-tar and coal extract has been reported 5. Shishido et al. 6 reported that the solubility of/3-resin in super critical toluene is enhanced by the solvation (association) of the toluene soluble fraction around/~-resin molecules. Bhata et al. ~ also suggested that TS (toluene solubles) from coal-tar pitch acts as a solvent for TI (toluene
4
Fuel 1996 Volume 75 Number 1
22 -
18
? ~ , 14
"~ 10
\ _\
\.
>
2 200
I
l
250
300
-~'-o-,.o. 350
I 400
450
Temperature (°C) Figure 1 Viscosity of the pitch carbonized at 200-450°C at a heating rate of 3°Cmin -l (closed circle) from room temperature to 400°C and 0.5°Cmin -1 (open circle) from 400°C to 450°C
insolubles) below a critical temperature, which, lying between 380 and 450°C, depends on the TI content in the mixture. Above this temperature the formation of QI increased by condensation. Therefore, the solubilization o f BI (fl-resin and QI) in BS can be considered to be one of reasons for the decrease in viscosity observed at 200360°C. At 360-400°C, the viscosity reaches a minimum value. Above 400°C, thermal condensation occurs preferentially, forming a large amount of mesophase spheres and decreasing BS content, leading to increase the viscosity. Figure 2 shows the change in viscosity with respect to the holding time when the pitch is heated at a heating rate of 2°C min-1 to a desired holding temperature. The slope of the viscosity-time curve depends on the holding temperature and the holding time. The higher the holding temperature and the longer the holding time, the greater the increase in viscosity. This suggests that thermal condensation, which increases with temperature and time, increases the formation of mesophase and the viscosity of the pitch. Figure 3 shows the viscosity and BI content of the pitch carbonized at 400-450°C. BI content of the pitch increases with increasing temperature. This indicates that the rate for thermal condensation of the pitch increases with temperature. Kremer 7 reported that with increasing heating temperature, heavy components in pitch, i.e. BI and QI portions increase due to thermal condensation, and the pitch becomes more and more pseudoplastic. A similar relationship between BI and viscosity was observed when holding time was varied at 405°C, as shown in Figure 4. Table 2 shows that the
Carbonization of coal-tar pitch. Xuanke Li and Qingtian Li Table 2 Viscosity and QI of the pitch carbonized at various temperaturesand times
26 --
22 --
•
14--
b
°>
•
•
.~
a
•
Time (h)
Viscosity/temperature~ (Pa s) (°C)
QI (wt%)
420 450 390, 4206 390 430
5 0.5 3, 1b 6 2
64.0/370 14.5/350 25.8/350 10.7/350 38.0/350
74.8 48.0 54.3 43.8 65.3
"The temperature at which viscosity was measured b Carbonization at 390°C for 3 h, and then at 420°C for 1 h
t° K 6
Table 3 The diameter of the mesophase formed by the carbonization of the pitch at various temperaturesa and times
2
I 0
1
2
3
4
5
Mesophase diameter (#m)
6
T i m e (h)
Figure 2 Relationship between viscosity of the pitch carbonized and carbonization time: (a) 390°C; (b) 450°C
Temperature (°C) Time(h)
Minimum Maximum Average
405 405 405 390, 420 a 390, 420 a
5 6 6 7 7
4 5 6 3, 1b 3, 2 b
/
18
, ~ 14 --
•m
60
•
•~ tO
L
Table 4 X-Ray diffraction parameters of the pitch carbonized at various temperaturea and time
oo
Temperature (°C)
Time (h)
d002 (,&.)
Lc (002) (A)
405 405 405 405 405 390,420 b 390, 420 b 390, 420 b
2 3 4 5 6 3, 1a 3, 2 a 3, 3"
3.50 3.50 3.46 3.46 3.46 3.46 3.45 3.45
17 19 25 26 28 29 30 33
40
> 6
2 400
410
420
430
440
30 450
Temperature ( ° C ) Figure 3 Viscosity (a) and BI (fl-resin and QI) content (b t of the pitch carbonized at 400-450°C at a heating rate of 0.5°C min-" from room temperature to 400°C and then at 2°Cmin -l from 400°C to 450°C
/
22 --
18 -14 --
/
10 --
./ ./
o i/~* ° i 30
40
50
and 3 h,
Effect of carbonization temperature and time on the growth of mesophase Table 3 shows the diameter o f the mesophase formed
6 -2
a Heating rate of 2°C m i n - t b C a r b o n i z a t i o n at 390°C for 3 h, and then at 420°C for 1, 2, respectively
viscosity also relates with QI wt%, when the pitch was heated at a heating rate o f 2 o C m i n - 1 , and after holding at a desired temperature and time, it was cooled at a rate of 8°Cmin -l to the temperature o f viscosity measurement. Since, QI and BI contents are k n o w n to be related with the mesophase content, it may be reasonable to consider that the formation o f the mesophase increases the viscosity.
26 --
8
11 14 22 22 24
a Heating rate of 2°C min l b Carbonization of 390°C for 3 h, and then at 420°C for 1 and 2 h, respectively
50
-?
43 35 125 73 84
- 70
22 -
-?
Temperature (°C)
I
I
60
70
BI c o n t e n t ( w t % )
Figure 4 Relationship between viscosity and BI (fl-resin and QI) content o f the pitch carbonized for various holding time at 405°C at a heating rate o f 2°C min -~ from r o o m temperature to 405°C
by the carbonization o f the pitch at a heating rate o f 2°Cmin -1. Table 3 shows that with increasing holding time, the diameter o f mesophase spheres increases. This indicates that at a given temperature, the increase of holding time favours the growth o f optically anisotropic structure, i.e. the growth o f mesophase. Table 4 shows d002 and L c parameters determined at r o o m temperature by X-ray diffraction for the carbonized pitch at different temperatures and times. It
Fuel 1996 Volume 75 Number 1
5
Carbonization of coal-tar pitch: Xuanke Li and Qingtian Li shows that the interlamellar distance, d002 decreased slightly with holding time. d002 for the pitch carbonized at 390°C and then at 420°C is similar to that at 405°C for the same holding time. With increasing holding time, Lc increases, i.e. mesophase spheres become larger. The higher the temperature at which mesophase is formed, the larger the size of the mesophase spheres. At high temperatures, large spheres can merge with each other.
Table 5 g-value and spin concentration of BS, ~3-resin and QI of the pitch
g-value spin/g
BS
/3-Resin
QI
2.0032 4.5 × 1018
2.0031 6.8 x 1019
2.0029 4.5 x 1019
Characterization and structure of fl-resin Figure 5a-e illustrates the s.e.m, micrographs of raw fl-resin (Figure 5a, b) which show it is composed of spherical particles, and the fl-resin (Figure 5c) carbonized
Figure 6 Optical micrographs of the pitch carbonized at 450°C for 1.5h with a heating rate of 3°Cmin -I, with (a) and without (b) the addition of 50 wt% of fl-resin
at 410°C for 1.5h, shows the merging of spherical particles during carbonization. X-Ray analysis shows d002 and L c of raw 13-resin are 3.566 and 14.6 A, respectively indicating that the average number of crystalline layer stacks is about three to four for fl-resin. It shows that polynuclear aromatic rings of /3-resin have a certain extent of crystallite growth, which is a precursor for mesophase formation.
Effect of the addition of l3-resin on the growth of mesophase
Figure 5 S.e.m. photographs of raw (a, b) and carbonized (c)/3-resin. Carbonization was carried out at 410°C for 1.5h. (b) is part magnification of (a)
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G-values and spin concentrations of the BS, 13-resin and QI portions of the pitch are shown in Table 5. Table 5 shows that g-values are in the range 2.00292.0032 with the fl-resin having the highest free radical concentration. The pitches were heated to 450°C at a heating rate of
Carbonization of coal-tar pitch: Xuanke Li and Qingtian Li 3°Cmin -l and held at the temperature for 1.5 h with or without the addition of 50 wt% of/3-resin. Figure 6a, b show the micrographs of their carbonization products. The carbonized pitch without fl-resin gives larger anisotropic domains than that with fl-resin. The latter (Figure 6a) contains mainly mesophase spheres. Since the free radical concentration in the/3-resin is one order of magnitude higher than that in the BS, high 13-resin concentration certainly accelerates thermal condensation and its mesophase solidifies too rapidly to merge together, resulting in a smaller anisotropic domains./3Resin dissolved in BS is considered to become the nuclei for thermal condensation.
alone, fl-Resin seems to become the nuclei for thermal condensation, leading to the rapid solidification of the mesophase which hinders the growth of mesophase to form large anisotropic domains.
ACKNOWLEDGMENT The authors thank Dr Masashi Iino of Tohoku University, Japan for his helpful discussion and suggestion for this work.
REFERENCES CONCLUSION During the carbonization of a coal-tar pitch the viscosity of the pitch decreased at 200-360°C, reached a minimum at 360-400°C, and then increased at 400-450°C. The decrease of the viscosity of the pitch after its melting may be attributed to the solvent action of BS toward BI (/3resin and QI), in addition to the increase of thermal movements of the pitch. The viscosity increase above 400°C can be explained by the growth of mesophase of high molecular weight. The addition of extra/3-resin to the pitch resulted in the formation of smaller anisotropic domains, compared with the carbonization of the pitch
1 2 3 4 5
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Bhatia,G., Fitzer, E. and Kompalik, D. Carbon 1986, 24, 489 Lewis,I. C. and Singer, L. S. in 'Chemistryand Physicsof Carbon' (Eds P. L. Walker,Jr. and P. A. Thrower),Vol. 17, MarcelDekker, New York, 1981,pp. 1 and 73 Lewis,I. C. and Singer, L. C. Carbon 1967,5, 373 Fitzer,E., Mueller,K. and Schaefer,W. in 'Chemistryand Physics of Carbon' (Ed. P. L. Walker, Jr.), Vol. 7, Marcel Dekker, New York, 1987,p. 237 Stenberg,V. I., Baltisberger, R. I., Patal, K. M., Raman, K. and Woolsey, N. F. In 'Coal Science' (Eds M. L. Gorbaty, J. W. Larsen and I. Wender), Vol. 2, Academic Press, New York, 1983, p. 125 Shishido,M., Yamada, S., Arai, K. and Saito, S. Fuel 1990, 69, 1490 Kremer,H. A. Chem. Ind. 1982,702
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