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Effects of quinoline-insoluble particles on the elemental processes of mesophase sphere formation
Carbon 42 (2004) 2443–2449 www.elsevier.com/locate/carbon
Effects of quinoline-insoluble particles on the elemental processes of mesophase sphere formation R. Moriyama *, J.-i. Hayashi, T. Chiba Center for Advanced Research of Energy Technology, Hokkaido University, Kita-13, Nishi-8, Kita-ku, Sapporo 060-8628, Japan Received 30 October 2003; accepted 15 April 2004 Available online 26 June 2004
Abstract Quantitative evaluation was made on effects of primary quinoline insolubles (PQI) on the elemental rate processes of mesophase sphere formation from a coal tar pitch. An original coal tar pitch containing 3 wt.% PQI and a PQI free coal tar pitch derived from the original pitch were heated at 703 K for various periods of time. Time dependent changes in number/volume-based concentrations and radius distribution of mesophase spheres were quantified through microscopic observation and an image analysis. The quantified results were further analyzed by a kinetic model considering the three rate processes; generation, growth and coalescence of spheres. The model analysis revealed that PQI increases the total frequency of sphere generation by five times, decreases the rate constant of coalescence by an order of magnitude, and decreases the linear growth rate to less than half. It was also found that PQI delays the commencement of the sphere generation and expands the time period of the generation. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Mesophase, Coal tar pitch; B. Carbonization; C. Optical microscopy, Modeling
1. Introduction Formation of mesophase spheres from coal- and petroleum-derived pitches is recognized to be a process of crystallization from liquid phase, which generally involves three elemental rate processes; generation (nucleation), growth and coalescence of spheres [1–3]. The present authors [4] recently investigated the kinetics of mesophase sphere formation from a coal tar pitch upon heating at 673–723 K, and showed that time dependent changes in the number/volume-based concentration and radius distribution of spheres were quantitatively described by a kinetic model considering the above-described three rate processes. The model analysis can effectively be applied to investigating effects of chemical/physical composition of the initial pitch on the formation of mesophase spheres, since the analysis
* Corresponding author. Present address: The Institute of Applied Energy, Konishi-BLDG, 16-5 Nishishinbashi 1-chome, Minato-ku, Tokyo 105-0003, Japan. Tel.: +81-3-3508-2124; fax: +81-3-3508-2097. E-mail address: [email protected] (R. Moriyama).
0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.04.044
enables to represent such effects as those on the rates of generation, growth in radial direction (linear growth) and coalescence of spheres. In the present paper, a model analysis is applied to evaluating effects of carbonaceous impurity, so called primary quinoline-insoluble fine particles (PQI) on the kinetics of mesophase sphere formation. The effects of PQI on the mesophase formation have been studied since 1960s. Brooks and Taylor [1] found that coal tar pitches containing more PQI tend to yield more spheres with smaller radii. Decrease in the sphere radius with increasing PQI content was also reported by Bhatia et al. [5]. These results, as they concluded, suggest that PQI suppresses the linear growth of spheres, or otherwise, their coalescence. The suppression of coalescence in the presence of PQI may be supported by the results by Romovacek and coworkers [6]. They made in situ observation of mesophase sphere formation from coal tar pitches in the presence and absence of PQI, and found that PQI can surround mesophase spheres and thereby suppress or even prevent coalescence between spheres. Despite of these results, increased population of smaller spheres does not necessarily mean either suppressed growth or coalescence. As far as the net
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CTP-B contained PQI with a content of 3.0 wt.%, while PQI was absent in CTP-A. As shown in Table 1, the chemical composition of CTP-A was nearly identical with that of CTP-B, because the former was prepared by removing PQI from CTP-B by means of filtration at a temperature well above the softening point of CTP-B but sufficiently low as to avoid vaporization and thermochemical reactions. Cross-sectional surfaces of CTP-B were observed by a reflected-light polarized microscopy. The microscopy detected PQI with sizes smaller than 1 lm and no optical anisotropy. PQI in the matrix of CTP-B were homogeneously dispersed. Attempts were made to add PQI that was derived from CTP-B to CTP-A. However, added PQI agglomerated in the matrix of CTP-A, and homogeneous dispersion of PQI as that in CTP-B could not achieved.
formation of mesophase results from the progress of the above-described three elemental processes, the numberbased concentration of spheres is determined by relative extents of generation and coalescence. Moreover, it is impossible to determine the rate of linear growth unless the rate of coalescence is given. Thus, the individual rates of growth and coalescence can be determined only by analyzing time dependent changes in the numberbased concentration and radius distribution of the spheres simultaneously and quantitatively. Tillmanns et al. [7], who estimated the yield of mesophase as that of solvent-insoluble portion of the heat-treated pitch, concluded that PQI accelerates the generation of spheres without any theoretical consideration. On the other hand, Stadelhofer [8] reported no or little accelerating effect of the addition of PQI on the yield of mesophase that was measured by means of a solvent extraction technique. The mass or volume based yield of mesophase spheres, even if given correctly, is not enough to determine the rate of either linear growth or generation of spheres since the rate of increase in the mesophase yield is a function of not only the linear growth rate but also total surface area of the spheres, the latter of which can be determined only from the number-based concentration and radius distribution of spheres. In view of the above, it is difficult to clearly say from the literature how PQI influences the mesophase sphere formation, in other words, how it affects the individual processes of generation, growth and coalescence of spheres. The contribution of each process to the net formation of mesophase spheres has to be evaluated by a kinetic model analysis of time dependent changes in the number-based concentration and radius distribution of the spheres. In the present study, a kinetic model [4] was applied to quantitative description of time dependent changes in the number-based concentration and radius distribution of mesophase spheres during heating of a coal tar pitch before and after PQI removal. The purpose of this study was to evaluate the effects of PQI on the three elemental rate processes quantitatively.
2.2. Heat treatment 2.5 g of the pitch sample was loaded into a Pyrexglass-made cylindrical cell with an inner diameter of 15 mm and a depth of 20 mm, which was placed in the middle of a tubular reactor. The sample was then heated in atmospheric N2 flowing at 100 ml-STP/min through the reactor at a heating rate of 10 K min1 from ambient temperature to 703 K. The temperature was maintained for a prescribed period of time, th , within a range from 0 to 90 min. After the temperature holding, the sample was cooled down to ambient temperature at a rate of 70 ± 10 K min1 . The cooling rate was so high as to prevent the mesophase formation during the cooling [3]. Mass fraction of the residual pitch, m, was calculated on a PQI-free basis as m ¼ ðMt M0 mPQI Þ=fM0 ð1 mPQI Þg
ð1Þ
where Mt , M0 and mPQI are mass of the residual pitch, that of initial pitch and mass fraction of PQI, respectively. Observed change in m upon heating was described well by assuming that the ultimate mass fraction of the residual pitch, m1 , was 0.23 for both CTP-A and CTP-B, and that the rate of mass release was first order with respect to (m m1 ). The rate constant was needed for calculating the population and volume densities of spheres on the basis of the initial mass of the pitch [4]. The rate constants for CTP-A and CTP-B were 0.035 and 0.024 min1 , respectively. Thus, PQI seemed to have a role to slow down the mass release while it had no significant effect on m1 .
2. Experimental 2.1. Samples Two different optically-isotropic coal pitch samples, CTP-A and CTP-B, were used for the experiments.
Table 1 Properties of pitch samples Pitch
C (wt.%)
H (wt.%)
O(diff.) (wt.%)
N (wt.%)
S (wt.%)
H/C
SP (K)
QI (wt.%)
CTP-A CTP-B
92.55 92.59
5.11 4.98
0.44 0.58
1.33 1.37
0.57 0.48
0.055 0.054
346 349
– 3.0
R. Moriyama et al. / Carbon 42 (2004) 2443–2449
2.3. Microscopy and image analysis After the heat treatment, the residual pitch in a shape of cylinder was mounted in a polymer resin, and cut equally into two semi-cylinders. The rectangular crosssection of a semi-cylinder (base, 15 mm long; height, 8–12 mm long) was observed by the reflected-light polarized microscopy with a magnification of 400. On the cross-section, 17–28 visual fields, each of which had an area of 45,000 lm2 , were chosen at the same intervals along the symmetry line perpendicular to the base ranging from the base (bottom) to the top, and the images of the fields were taken with a digital camera. In each field, mesophase spheres were detected as optically anisotropic circles on the cross-section with radii of 1 lm and greater. Details of the microscopic observation were reported previously [4]. For the residual pitches from CTP-B at any th examined, there was no clear variation of either the number-based concentration or the radius distribution of anisotropic circles with the position of the visual field on the cross-section, suggesting homogeneous dispersion of mesophase spheres on a scale over the visual fields. On the other hand, for the residual pitches from CTP-A in the case of th P 55 min, the number-based concentration of anisotropic circles was lower in fields nearer to the bottom, where the radius distribution was biased appreciably toward greater radius. This was attributed to segregation of spheres [9,10]. The radii of the detected anisotropic circles distributed in a range from 1 to 64 lm at any th for CTP-B while at th 6 55 min for CTP-A. The radius distribution of the circles was represented in a geometrically discretized form, employing length as the internal coordinate. Circles having radii between ri1 and ri (i ¼ 1; 2; 3; . . . ; 15 and r0 ¼ 1 lm), related as ri =ri1 ¼ 21=3 , were fallen into the ith interval that will be denoted by I-i. The number-based concentration and radius distribution of anisotropic circles were further analyzed assuming random dispersion of mesophase spheres and random cutting of the spheres [4,11]. The analysis gave the number-based concentration and radius distribution of spheres. Details of the analysis were described elsewhere [4]. Hereafter, the number-based concentration of the spheres with radii greater than 2ði1Þ=3 lm but smaller than 2i=3 lm will be denoted by ni on the basis of unit volume of the initial pitch sample. The total numberbased concentration of spheres is then expressed as X NT ¼ ni ð2Þ i
Accordingly the volume-based concentration of the mesophase spheres is given by 4p X 3 VT ¼ ni ðri Þ ð3Þ 3 i
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where ri is the average radius of the spheres in the interval I-i, and defined as ri
ri1 þ ri 2
ð4Þ
2.4. Model analysis Changes in NT , VT and the radius distribution with th , were analyzed by a kinetic model for mesophase formation. Details of the model were given elsewhere [4]. In brief, the model considers three elemental rate processes; generation, growth and coalescence of spheres with some reasonable assumptions. First, spheres are assumed to generate with radii within the first interval, I-1, from their precursor and the generation obeys the following first order kinetics with respect to the precursor concentration. 1 dn1 dCp 4p 3 ð r ¼ Þ 1 3 dt generation dt
ð5Þ
Cp ¼ Cp0 expfkp ðth th;g Þg
ð6Þ
In essence, the initial step of the formation of mesophase spheres is ‘‘nucleation’’ that is believed to be driven by degree of super-saturation of precursor in the isotropic pitch matrix. The radii of any nuclei should be much smaller than 1 lm. It is therefore impossible to detect such nuclei by the present microscopic observation. Thus, the rate of generation defined in the present analysis is recognized as the rate of formation of spheres with radii falling into the first interval I-1. For given rates of growth and coalescence, greater kp means a shorter period of nucleation [4]. In the above equations, Cp0 and Cp are the concentrations of the precursor at the commencement of spheres generation (th ¼ th;g ) and afterward, respectively. The second assumption is that every sphere grows in radial direction at a constant linear growth rate, G, absorbing material from the isotropic pitch matrix. Third, the rate of coalescence is also assumed to be proportional to frequency of collision between two spheres. The coalescence is therefore described as a rate process to be second order with respect to the numberbased concentration of the spheres. The rate constant of coalescence, b, between spheres in the intervals I-i and I-j is generally given by b ¼ b0 ðri þ rj Þ
k
ð7Þ
The exponent k in this equation depends on the mechanism of collision [12,13]. In our previous study on the mesophase formation from a coal tar pitch, the coalescence was described best by employing k ¼ 0. The same value of k was chosen in the present model analysis.
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3. Results 3.1. Changes in NT , VT and radius distribution of mesophase spheres Plots of Fig. 1(a) and (b) show changes with th of NT and VT for both pitches, respectively. For CTP-A, it is seen that NT decreases monotonically with th at 35–55 min while VT increases. These changes indicate simultaneous occurrence of coalescence and growth of spheres, while there is no convincing evidence of generation. Generation of spheres from CTP-A was first detected around th ¼ 30 min, where the reproducibility of NT , VT as well as the radius distribution was much poorer than that at the other th . The reason for such poor reproducibility will be explained later. It is also noted that NT increases after th ¼ 55 min due to commencement of ‘‘second-stage’’ generation at a significantly high rate. This was a phenomenon peculiar to CTP-A and not observed for CTP-B under the present experimental conditions. The second-stage generation was, however, not analyzed in the present study because of the dispersion of spheres was no more homogeneous at th P 55 min, as stated in Section 2.3. For CTP-B, NT changes with th via a maximum while VT increases monotonically. The increase in NT at th ¼ 40–50 min clearly shows simultaneous occurrence of generation and growth. After th ¼ 50 min, NT decreases due to coalescence. Continuation of the generation in this period cannot be made clear unless the change in NT together with those in VT and the radius distribution are
Fig. 1. Observed and predicted changes in NT (a) and VT (b) with th for CTP-A and CTP-B.
analyzed by the kinetic model. At th between 40 and 55 min, NT for CTP-B is an order of magnitude greater than that for CTP-A. It can be said from this result that PQI played roles in suppression of coalescence and/or enhancement of generation of spheres, which is consistent with the previous reports [1,6]. Figs. 2 and 3 illustrate radius distributions of spheres at different th for CTP-A and CTP-B, respectively. For CTP-B, n1 , n2 and n3 increase with th in a range from 40 to 50 min while n4 to n10 remain negligibly low. The unchanged width of the radius distribution is typical in the absence of coalescence, or otherwise, an extremely slow rate of coalescence compared with that of generation [4]. In our previous study on the mesophase sphere formation from a coal tar pitch containing some PQI [4], the model analysis revealed that the coalescence initiated after termination of the generation. For CTP-A, n1 is negligibly low after th ¼ 40 min. Disappearance of smallest spheres would be explained not only by the progress of coalescence and growth but also termination of generation [4]. 3.2. Results of model analysis Data shown in Figs. 1–3 were analyzed by the model with the aid of the above-mentioned qualitative indication. The mesophase sphere formation from CTP-A was analyzed within a range of th up to 55 min due to inhomogeneous dispersion of spheres afterward. First, the data at th ¼ 35–50 min were analyzed in order to determine G and b temporarily under an assumption of no occurrence of generation, namely, Cp ¼ 0. The result showed that the changes in NT , VT and the radius distribution can be described best by employing G ¼ 0:040 lm min1 for linear growth and b ¼ 600 lm3 min1 for coalescence. Trials were also made assuming occurrence of generation at th ¼ 35–50 min, and then it was found as a result that the model predicts n1 n3 , being much higher than those observed. Thus, sphere generation at th ¼ 35–50 min was unreasonable. Next, characteristics of the generation, which would terminate before th ¼ 35 min, were estimated. Since it was clear from the microscopic observation that the generation began around th ¼ 30 min, th;g was assumed to be in a range from 25 to 35 min. As a result of trial and error, a combination of th;g ¼ 29 min, kp P 5:0 min1 and Cp ¼ 0:011 lm3 /lm3 -initial-CTP-A, together with G ¼ 0:040 lm min1 and b ¼ 600 lm3 min1 , was found to give the best prediction of NT , VT and the radius distribution at th ¼ 35–50 min. The calculated results using these parameters are shown in Figs. 1 and 2. Thus, the model with the optimized kinetic parameters estimates that the generation of spheres from CTP-A began around th ¼ 29 min and then terminated within a minute, while the growth and coalescence began simultaneously with the generation and continued afterward.
R. Moriyama et al. / Carbon 42 (2004) 2443–2449
Fig. 2. Observed and predicted radius distribution of mesophase spheres from CTP-A as a function of th .
Fig. 3. Observed and predicted radius distribution of mesophase spheres from CTP-B as a function of th .
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Such rapid completion of the generation would make it difficult to reproduce NT , VT and the radius distribution around th ¼ 30 min. It is noted in Fig. 2 that the calculated radius distribution at th ¼ 55 min is much different from the observed distribution. As described in Section 2.3, precipitation of coarse spheres occurred at th P 55 min. The segregation of spheres could bring about local concentration of spheres and accelerate their coalescence, thereby causing disappearance of smaller spheres over the intervals from I-1 to I-5. The first analysis of the mesophase formation from CTP-B was carried out in a range of th from 60–90 min, where NT decreased with th monotonically. It was then found that a combination of G ¼ 0:016 lm min1 , b ¼ 55 lm3 min1 and Cp ¼ 0 lm3 /lm3 -initial-CTP-B gives better fit of the predicted NT , VT and the radius distribution to those observed than any other calculations assuming the occurrence of generation. On the other hand, the changes in NT , VT and the radius distribution up to th ¼ 50 min were analyzed by assuming no coalescence of coalescence within that period. This assumption was based on the results of our previous study on the mesophase formation from a coal tar pitch that contained some amount of PQI [4]. During the heat treatment of the pitch at 673–723 K, the coalescence of spheres started after completion of their generation. It was found that spheres were surrounded by PQI particles [4]. In other words, PQI particles were present exclusively at interfaces between the isotropic matrix of the pitch and spheres. The assumption was in fact reasonable for predicting changes in NT , VT and the radius distribution of spheres from CTP-B with th up to 50 min. The model gave the best fit of the prediction to the observation when no coalescence, namely, b ¼ 0, was chosen together with G ¼ 0:016 lm min1 , th;g ¼ 40 min, kp ¼ 0:099 min1 and Cp ¼ 0:057 lm3 /lm3 -initialCTP-B. Finally, the time for the commencement of coalescence (th;c ) was optimized at 55 min. Figs. 1 and 3 show that the model predicts the changes in NT , VT and the radius distribution well over the range of th from 40 to 90 min. The optimized kinetic parameters for CTP-A and CTP-B are summarized in Table 2.
4. Discussion In this section, the effects of PQI on the elemental rate processes are examined based on the results of the model
analysis. As clearly seen from the results, PQI influenced all of the rate processes of generation, growth and coalescence. First, the generation of spheres was enhanced in the presence of PQI in terms of the total frequency of sphere generation, n T , which can be expressed by 1 4p 3 ðr1 Þ n T ¼ Cp0 ð8Þ 3 Cp0 for CTP-B was larger by more than five times than that for CTP-A. This suggests that some material in the pitch matrix acted as a precursor of the spheres’ nuclei in the presence of PQI, while such material could only participate in the growth of spheres and/or act as solvent in the absence of PQI. The microscopic observation revealed that homogeneously dispersed PQI formed agglomerates in size of 1–10 lm after heat treatment of th ¼ 0 min, and mesophase spheres were first detected among the agglomerated PQI. It was also confirmed that the spheres were surrounded by PQI after they had grown up to sizes of 5 lm. Thus, heterogeneous nucleation at the interface between PQI and the isotropic matrix would be a reasonable explanation of the greater n T in the presence of PQI. On the other hand, the rate constant of sphere generation, kp , seems to be considerably smaller in the presence of PQI. Although it is difficult to quantitatively relate the rate of nucleation with that of generation defined in the present study, it can at least be said that larger kp leads shorter period of time for the nucleation. Thus, PQI would play a role in maintaining super-saturation of the precursor of nuclei at certain levels for longer time. In fact, the generation of spheres from CTP-B continued from th ¼ 40–50 min. In the case of no coalescence, the time period for sphere generation is equal to that of nucleation [4]. In addition, the time period taken for a nuclear to grow in size up to a radius of 1 lm is given as 1=G min assuming that the radius of the nuclear is negligibly small compared with 1 lm [4]. According to the optimized value of G for CTP-B, the time is estimated to be longer than 60 min. This implies that the nucleation started before the temperature of CTP-B reached 703 K. From the comparison of the optimized value of b between CTP-A (600 lm3 min1 ) and CTP-B (55 lm3 min1 ), it is clear that coalescence was largely suppressed by PQI. For CTP-B, it was found from the microscopic observation that every sphere was sur-
Table 2 Optimized kinetic parameters Pitch CTP-A CTP-B
Generation
Growth
Coalescence
th;g (min)
kp (min1 )
Cp0 (lm3 /lm3 -initial-CTP)
G (lm min1 )
th;c (min)
k
b0 (lm3 min1 )
29 40
P5 0.099
0.011 0.057
0.040 0.016
– 50
0 0
600 55
R. Moriyama et al. / Carbon 42 (2004) 2443–2449
rounded by PQI. It is reasonable that such PQI acted as a physical barrier to prevent spheres from coalescing. In our previous study [4], changes in NT , VT and the radius distribution of mesophase spheres were investigated for a coal tar pitch that contained some PQI. It was confirmed that every sphere was surrounded by PQI. It was also reveled from the model analysis that the coalescence was inhibited for a while even after termination of generation. Taken together, it can be concluded that PQI prevented spheres from coalescing for a period of time and afterward suppressed the coalescence to a large extent upon heating of CTP-B. This conclusion is in broad agreement with the results reported by Romovacek et al. [6]. The time for the commencement of coalescence would depend on the concentration of PQI. More PQI would be able to cover larger surface of spheres and therefore prevent spheres from coalescing for longer time. The presence of PQI at sphere–matrix interface would be a more or less resistance for the sphere to incorporate material from the matrix. It is hence reasonable that the linear growth rate G for CTP-B is smaller than that for CTP-A.
5. Conclusions Examination was made on the effects of PQI on the kinetics of mesophase formation from the coal tar pitch upon heating at 703 K. The time dependent changes in NT , VT and the radius distribution of mesophase spheres in the presence and absence of PQI were analyzed by the kinetic model to evaluate the effects of PQI on the elemental rate processes as those on the kinetic parameters. Given the present experimental conditions, the following conclusions were made: 1. PQI increased the total frequency of sphere generation by about five times, decreased the rate constant of coalescence by an order of magnitude, and decreased the rate constant of linear growth by more than half. 2. Coalescence was inhibited until the termination of sphere generation in the presence of PQI while the coalescence began simultaneously with the generation in the absence of PQI.
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3. The time period of sphere generation in the presence of PQI was longer by an order of magnitude than that in the absence of PQI.
Acknowledgements The authors wish to gratefully acknowledge Mr. N. Kashimura and Ms. R. Goda for their assistance in the image analysis.
References [1] Brooks JD, Taylor GH. In: Walker Jr PL, editor. Chemistry and physics of carbon, vol. 4. New York: Marcel Dekker; 1968. p. 243–86. [2] Taylor GH, Pennock GM, Fitz Gerald JD, Brunckhorst LF. Influence of QI on mesophase structure. Carbon 1993;31(2):341– 54. [3] Moriyama R, Kumagai H, Hayashi J-I, Yamaguchi C, Mondori J, Matsui H, et al. Formation of mesophase spheres from a coal tar pitch upon heating and subsequent cooling observed by an in situ 1 H-NMR. Carbon 2000;38(5):749–58. [4] Moriyama R, Hayashi J-I, Suzuki K, Hiroshima T, Chiba T. Analysis and modeling of mesophase sphere generation, growth and coalescence upon heating of a coal tar pitch. Carbon 2002;40(l):53–64. [5] Bhatia G, Aggarwal RK, Punjabi N, Bahl OP. Formation of mesophase spherules in low-QI coal tar pitches and development of monolithic carbons therefrom. J Mater Sci 1994;29:4757–63. [6] George R, Romovacek GR, McCullough JP, Perrotta AJ. Formation of mesophase in coal tar pitches. Influence of pyrolytic carbon particles. Fuel 1983;62:1236–9. [7] Tillmanns H, Pietzka G, Pauls H. Influence of the quinolineinsoluble matter in pitch on carbonization behavior and structure of pitch coke. Fuel 1978;57:171–3. [8] Stadelhofer JW. Examination of the influence of natural quinoline-insoluble material on the kinetics of mesophase formation. Fuel 1980;59:360–1. [9] Yamada Y, Sakawaki K, Ida S. Accumulation of mesophase spheres by sedimentation. Carbon 1985;23(5):597–600. [10] H€ uttinger KJ, Wang JP. Kinetics of mesophase formation in a stirred tank reactor and properties of the products. Carbon 1991;29(3):439–48. [11] Underwood EE. Quantitative stereology, particle and grain-size distributions. Addison-Wesley; 1970. p. 109–47. [12] Hounslow MJ. A discretized population balance for continuous systems at steady state. AIChE J 1990;36(1):106–16. [13] Hartel RW, Randolph AD. Mechanisms and kinetic modeling of calcium oxalate crystal aggregation in a urinelike liquor. Part I: Mechanisms. AIChE J 1986;32(7):1186–95.
Effects of quinoline-insoluble particles on the elemental processes of mesophase sphere formation R. Moriyama *, J.-i. Hayashi, T. Chiba Center for Advanced Research of Energy Technology, Hokkaido University, Kita-13, Nishi-8, Kita-ku, Sapporo 060-8628, Japan Received 30 October 2003; accepted 15 April 2004 Available online 26 June 2004
Abstract Quantitative evaluation was made on effects of primary quinoline insolubles (PQI) on the elemental rate processes of mesophase sphere formation from a coal tar pitch. An original coal tar pitch containing 3 wt.% PQI and a PQI free coal tar pitch derived from the original pitch were heated at 703 K for various periods of time. Time dependent changes in number/volume-based concentrations and radius distribution of mesophase spheres were quantified through microscopic observation and an image analysis. The quantified results were further analyzed by a kinetic model considering the three rate processes; generation, growth and coalescence of spheres. The model analysis revealed that PQI increases the total frequency of sphere generation by five times, decreases the rate constant of coalescence by an order of magnitude, and decreases the linear growth rate to less than half. It was also found that PQI delays the commencement of the sphere generation and expands the time period of the generation. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Mesophase, Coal tar pitch; B. Carbonization; C. Optical microscopy, Modeling
1. Introduction Formation of mesophase spheres from coal- and petroleum-derived pitches is recognized to be a process of crystallization from liquid phase, which generally involves three elemental rate processes; generation (nucleation), growth and coalescence of spheres [1–3]. The present authors [4] recently investigated the kinetics of mesophase sphere formation from a coal tar pitch upon heating at 673–723 K, and showed that time dependent changes in the number/volume-based concentration and radius distribution of spheres were quantitatively described by a kinetic model considering the above-described three rate processes. The model analysis can effectively be applied to investigating effects of chemical/physical composition of the initial pitch on the formation of mesophase spheres, since the analysis
* Corresponding author. Present address: The Institute of Applied Energy, Konishi-BLDG, 16-5 Nishishinbashi 1-chome, Minato-ku, Tokyo 105-0003, Japan. Tel.: +81-3-3508-2124; fax: +81-3-3508-2097. E-mail address: [email protected] (R. Moriyama).
0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.04.044
enables to represent such effects as those on the rates of generation, growth in radial direction (linear growth) and coalescence of spheres. In the present paper, a model analysis is applied to evaluating effects of carbonaceous impurity, so called primary quinoline-insoluble fine particles (PQI) on the kinetics of mesophase sphere formation. The effects of PQI on the mesophase formation have been studied since 1960s. Brooks and Taylor [1] found that coal tar pitches containing more PQI tend to yield more spheres with smaller radii. Decrease in the sphere radius with increasing PQI content was also reported by Bhatia et al. [5]. These results, as they concluded, suggest that PQI suppresses the linear growth of spheres, or otherwise, their coalescence. The suppression of coalescence in the presence of PQI may be supported by the results by Romovacek and coworkers [6]. They made in situ observation of mesophase sphere formation from coal tar pitches in the presence and absence of PQI, and found that PQI can surround mesophase spheres and thereby suppress or even prevent coalescence between spheres. Despite of these results, increased population of smaller spheres does not necessarily mean either suppressed growth or coalescence. As far as the net
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R. Moriyama et al. / Carbon 42 (2004) 2443–2449
CTP-B contained PQI with a content of 3.0 wt.%, while PQI was absent in CTP-A. As shown in Table 1, the chemical composition of CTP-A was nearly identical with that of CTP-B, because the former was prepared by removing PQI from CTP-B by means of filtration at a temperature well above the softening point of CTP-B but sufficiently low as to avoid vaporization and thermochemical reactions. Cross-sectional surfaces of CTP-B were observed by a reflected-light polarized microscopy. The microscopy detected PQI with sizes smaller than 1 lm and no optical anisotropy. PQI in the matrix of CTP-B were homogeneously dispersed. Attempts were made to add PQI that was derived from CTP-B to CTP-A. However, added PQI agglomerated in the matrix of CTP-A, and homogeneous dispersion of PQI as that in CTP-B could not achieved.
formation of mesophase results from the progress of the above-described three elemental processes, the numberbased concentration of spheres is determined by relative extents of generation and coalescence. Moreover, it is impossible to determine the rate of linear growth unless the rate of coalescence is given. Thus, the individual rates of growth and coalescence can be determined only by analyzing time dependent changes in the numberbased concentration and radius distribution of the spheres simultaneously and quantitatively. Tillmanns et al. [7], who estimated the yield of mesophase as that of solvent-insoluble portion of the heat-treated pitch, concluded that PQI accelerates the generation of spheres without any theoretical consideration. On the other hand, Stadelhofer [8] reported no or little accelerating effect of the addition of PQI on the yield of mesophase that was measured by means of a solvent extraction technique. The mass or volume based yield of mesophase spheres, even if given correctly, is not enough to determine the rate of either linear growth or generation of spheres since the rate of increase in the mesophase yield is a function of not only the linear growth rate but also total surface area of the spheres, the latter of which can be determined only from the number-based concentration and radius distribution of spheres. In view of the above, it is difficult to clearly say from the literature how PQI influences the mesophase sphere formation, in other words, how it affects the individual processes of generation, growth and coalescence of spheres. The contribution of each process to the net formation of mesophase spheres has to be evaluated by a kinetic model analysis of time dependent changes in the number-based concentration and radius distribution of the spheres. In the present study, a kinetic model [4] was applied to quantitative description of time dependent changes in the number-based concentration and radius distribution of mesophase spheres during heating of a coal tar pitch before and after PQI removal. The purpose of this study was to evaluate the effects of PQI on the three elemental rate processes quantitatively.
2.2. Heat treatment 2.5 g of the pitch sample was loaded into a Pyrexglass-made cylindrical cell with an inner diameter of 15 mm and a depth of 20 mm, which was placed in the middle of a tubular reactor. The sample was then heated in atmospheric N2 flowing at 100 ml-STP/min through the reactor at a heating rate of 10 K min1 from ambient temperature to 703 K. The temperature was maintained for a prescribed period of time, th , within a range from 0 to 90 min. After the temperature holding, the sample was cooled down to ambient temperature at a rate of 70 ± 10 K min1 . The cooling rate was so high as to prevent the mesophase formation during the cooling [3]. Mass fraction of the residual pitch, m, was calculated on a PQI-free basis as m ¼ ðMt M0 mPQI Þ=fM0 ð1 mPQI Þg
ð1Þ
where Mt , M0 and mPQI are mass of the residual pitch, that of initial pitch and mass fraction of PQI, respectively. Observed change in m upon heating was described well by assuming that the ultimate mass fraction of the residual pitch, m1 , was 0.23 for both CTP-A and CTP-B, and that the rate of mass release was first order with respect to (m m1 ). The rate constant was needed for calculating the population and volume densities of spheres on the basis of the initial mass of the pitch [4]. The rate constants for CTP-A and CTP-B were 0.035 and 0.024 min1 , respectively. Thus, PQI seemed to have a role to slow down the mass release while it had no significant effect on m1 .
2. Experimental 2.1. Samples Two different optically-isotropic coal pitch samples, CTP-A and CTP-B, were used for the experiments.
Table 1 Properties of pitch samples Pitch
C (wt.%)
H (wt.%)
O(diff.) (wt.%)
N (wt.%)
S (wt.%)
H/C
SP (K)
QI (wt.%)
CTP-A CTP-B
92.55 92.59
5.11 4.98
0.44 0.58
1.33 1.37
0.57 0.48
0.055 0.054
346 349
– 3.0
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2.3. Microscopy and image analysis After the heat treatment, the residual pitch in a shape of cylinder was mounted in a polymer resin, and cut equally into two semi-cylinders. The rectangular crosssection of a semi-cylinder (base, 15 mm long; height, 8–12 mm long) was observed by the reflected-light polarized microscopy with a magnification of 400. On the cross-section, 17–28 visual fields, each of which had an area of 45,000 lm2 , were chosen at the same intervals along the symmetry line perpendicular to the base ranging from the base (bottom) to the top, and the images of the fields were taken with a digital camera. In each field, mesophase spheres were detected as optically anisotropic circles on the cross-section with radii of 1 lm and greater. Details of the microscopic observation were reported previously [4]. For the residual pitches from CTP-B at any th examined, there was no clear variation of either the number-based concentration or the radius distribution of anisotropic circles with the position of the visual field on the cross-section, suggesting homogeneous dispersion of mesophase spheres on a scale over the visual fields. On the other hand, for the residual pitches from CTP-A in the case of th P 55 min, the number-based concentration of anisotropic circles was lower in fields nearer to the bottom, where the radius distribution was biased appreciably toward greater radius. This was attributed to segregation of spheres [9,10]. The radii of the detected anisotropic circles distributed in a range from 1 to 64 lm at any th for CTP-B while at th 6 55 min for CTP-A. The radius distribution of the circles was represented in a geometrically discretized form, employing length as the internal coordinate. Circles having radii between ri1 and ri (i ¼ 1; 2; 3; . . . ; 15 and r0 ¼ 1 lm), related as ri =ri1 ¼ 21=3 , were fallen into the ith interval that will be denoted by I-i. The number-based concentration and radius distribution of anisotropic circles were further analyzed assuming random dispersion of mesophase spheres and random cutting of the spheres [4,11]. The analysis gave the number-based concentration and radius distribution of spheres. Details of the analysis were described elsewhere [4]. Hereafter, the number-based concentration of the spheres with radii greater than 2ði1Þ=3 lm but smaller than 2i=3 lm will be denoted by ni on the basis of unit volume of the initial pitch sample. The total numberbased concentration of spheres is then expressed as X NT ¼ ni ð2Þ i
Accordingly the volume-based concentration of the mesophase spheres is given by 4p X 3 VT ¼ ni ðri Þ ð3Þ 3 i
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where ri is the average radius of the spheres in the interval I-i, and defined as ri
ri1 þ ri 2
ð4Þ
2.4. Model analysis Changes in NT , VT and the radius distribution with th , were analyzed by a kinetic model for mesophase formation. Details of the model were given elsewhere [4]. In brief, the model considers three elemental rate processes; generation, growth and coalescence of spheres with some reasonable assumptions. First, spheres are assumed to generate with radii within the first interval, I-1, from their precursor and the generation obeys the following first order kinetics with respect to the precursor concentration. 1 dn1 dCp 4p 3 ð r ¼ Þ 1 3 dt generation dt
ð5Þ
Cp ¼ Cp0 expfkp ðth th;g Þg
ð6Þ
In essence, the initial step of the formation of mesophase spheres is ‘‘nucleation’’ that is believed to be driven by degree of super-saturation of precursor in the isotropic pitch matrix. The radii of any nuclei should be much smaller than 1 lm. It is therefore impossible to detect such nuclei by the present microscopic observation. Thus, the rate of generation defined in the present analysis is recognized as the rate of formation of spheres with radii falling into the first interval I-1. For given rates of growth and coalescence, greater kp means a shorter period of nucleation [4]. In the above equations, Cp0 and Cp are the concentrations of the precursor at the commencement of spheres generation (th ¼ th;g ) and afterward, respectively. The second assumption is that every sphere grows in radial direction at a constant linear growth rate, G, absorbing material from the isotropic pitch matrix. Third, the rate of coalescence is also assumed to be proportional to frequency of collision between two spheres. The coalescence is therefore described as a rate process to be second order with respect to the numberbased concentration of the spheres. The rate constant of coalescence, b, between spheres in the intervals I-i and I-j is generally given by b ¼ b0 ðri þ rj Þ
k
ð7Þ
The exponent k in this equation depends on the mechanism of collision [12,13]. In our previous study on the mesophase formation from a coal tar pitch, the coalescence was described best by employing k ¼ 0. The same value of k was chosen in the present model analysis.
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3. Results 3.1. Changes in NT , VT and radius distribution of mesophase spheres Plots of Fig. 1(a) and (b) show changes with th of NT and VT for both pitches, respectively. For CTP-A, it is seen that NT decreases monotonically with th at 35–55 min while VT increases. These changes indicate simultaneous occurrence of coalescence and growth of spheres, while there is no convincing evidence of generation. Generation of spheres from CTP-A was first detected around th ¼ 30 min, where the reproducibility of NT , VT as well as the radius distribution was much poorer than that at the other th . The reason for such poor reproducibility will be explained later. It is also noted that NT increases after th ¼ 55 min due to commencement of ‘‘second-stage’’ generation at a significantly high rate. This was a phenomenon peculiar to CTP-A and not observed for CTP-B under the present experimental conditions. The second-stage generation was, however, not analyzed in the present study because of the dispersion of spheres was no more homogeneous at th P 55 min, as stated in Section 2.3. For CTP-B, NT changes with th via a maximum while VT increases monotonically. The increase in NT at th ¼ 40–50 min clearly shows simultaneous occurrence of generation and growth. After th ¼ 50 min, NT decreases due to coalescence. Continuation of the generation in this period cannot be made clear unless the change in NT together with those in VT and the radius distribution are
Fig. 1. Observed and predicted changes in NT (a) and VT (b) with th for CTP-A and CTP-B.
analyzed by the kinetic model. At th between 40 and 55 min, NT for CTP-B is an order of magnitude greater than that for CTP-A. It can be said from this result that PQI played roles in suppression of coalescence and/or enhancement of generation of spheres, which is consistent with the previous reports [1,6]. Figs. 2 and 3 illustrate radius distributions of spheres at different th for CTP-A and CTP-B, respectively. For CTP-B, n1 , n2 and n3 increase with th in a range from 40 to 50 min while n4 to n10 remain negligibly low. The unchanged width of the radius distribution is typical in the absence of coalescence, or otherwise, an extremely slow rate of coalescence compared with that of generation [4]. In our previous study on the mesophase sphere formation from a coal tar pitch containing some PQI [4], the model analysis revealed that the coalescence initiated after termination of the generation. For CTP-A, n1 is negligibly low after th ¼ 40 min. Disappearance of smallest spheres would be explained not only by the progress of coalescence and growth but also termination of generation [4]. 3.2. Results of model analysis Data shown in Figs. 1–3 were analyzed by the model with the aid of the above-mentioned qualitative indication. The mesophase sphere formation from CTP-A was analyzed within a range of th up to 55 min due to inhomogeneous dispersion of spheres afterward. First, the data at th ¼ 35–50 min were analyzed in order to determine G and b temporarily under an assumption of no occurrence of generation, namely, Cp ¼ 0. The result showed that the changes in NT , VT and the radius distribution can be described best by employing G ¼ 0:040 lm min1 for linear growth and b ¼ 600 lm3 min1 for coalescence. Trials were also made assuming occurrence of generation at th ¼ 35–50 min, and then it was found as a result that the model predicts n1 n3 , being much higher than those observed. Thus, sphere generation at th ¼ 35–50 min was unreasonable. Next, characteristics of the generation, which would terminate before th ¼ 35 min, were estimated. Since it was clear from the microscopic observation that the generation began around th ¼ 30 min, th;g was assumed to be in a range from 25 to 35 min. As a result of trial and error, a combination of th;g ¼ 29 min, kp P 5:0 min1 and Cp ¼ 0:011 lm3 /lm3 -initial-CTP-A, together with G ¼ 0:040 lm min1 and b ¼ 600 lm3 min1 , was found to give the best prediction of NT , VT and the radius distribution at th ¼ 35–50 min. The calculated results using these parameters are shown in Figs. 1 and 2. Thus, the model with the optimized kinetic parameters estimates that the generation of spheres from CTP-A began around th ¼ 29 min and then terminated within a minute, while the growth and coalescence began simultaneously with the generation and continued afterward.
R. Moriyama et al. / Carbon 42 (2004) 2443–2449
Fig. 2. Observed and predicted radius distribution of mesophase spheres from CTP-A as a function of th .
Fig. 3. Observed and predicted radius distribution of mesophase spheres from CTP-B as a function of th .
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Such rapid completion of the generation would make it difficult to reproduce NT , VT and the radius distribution around th ¼ 30 min. It is noted in Fig. 2 that the calculated radius distribution at th ¼ 55 min is much different from the observed distribution. As described in Section 2.3, precipitation of coarse spheres occurred at th P 55 min. The segregation of spheres could bring about local concentration of spheres and accelerate their coalescence, thereby causing disappearance of smaller spheres over the intervals from I-1 to I-5. The first analysis of the mesophase formation from CTP-B was carried out in a range of th from 60–90 min, where NT decreased with th monotonically. It was then found that a combination of G ¼ 0:016 lm min1 , b ¼ 55 lm3 min1 and Cp ¼ 0 lm3 /lm3 -initial-CTP-B gives better fit of the predicted NT , VT and the radius distribution to those observed than any other calculations assuming the occurrence of generation. On the other hand, the changes in NT , VT and the radius distribution up to th ¼ 50 min were analyzed by assuming no coalescence of coalescence within that period. This assumption was based on the results of our previous study on the mesophase formation from a coal tar pitch that contained some amount of PQI [4]. During the heat treatment of the pitch at 673–723 K, the coalescence of spheres started after completion of their generation. It was found that spheres were surrounded by PQI particles [4]. In other words, PQI particles were present exclusively at interfaces between the isotropic matrix of the pitch and spheres. The assumption was in fact reasonable for predicting changes in NT , VT and the radius distribution of spheres from CTP-B with th up to 50 min. The model gave the best fit of the prediction to the observation when no coalescence, namely, b ¼ 0, was chosen together with G ¼ 0:016 lm min1 , th;g ¼ 40 min, kp ¼ 0:099 min1 and Cp ¼ 0:057 lm3 /lm3 -initialCTP-B. Finally, the time for the commencement of coalescence (th;c ) was optimized at 55 min. Figs. 1 and 3 show that the model predicts the changes in NT , VT and the radius distribution well over the range of th from 40 to 90 min. The optimized kinetic parameters for CTP-A and CTP-B are summarized in Table 2.
4. Discussion In this section, the effects of PQI on the elemental rate processes are examined based on the results of the model
analysis. As clearly seen from the results, PQI influenced all of the rate processes of generation, growth and coalescence. First, the generation of spheres was enhanced in the presence of PQI in terms of the total frequency of sphere generation, n T , which can be expressed by 1 4p 3 ðr1 Þ n T ¼ Cp0 ð8Þ 3 Cp0 for CTP-B was larger by more than five times than that for CTP-A. This suggests that some material in the pitch matrix acted as a precursor of the spheres’ nuclei in the presence of PQI, while such material could only participate in the growth of spheres and/or act as solvent in the absence of PQI. The microscopic observation revealed that homogeneously dispersed PQI formed agglomerates in size of 1–10 lm after heat treatment of th ¼ 0 min, and mesophase spheres were first detected among the agglomerated PQI. It was also confirmed that the spheres were surrounded by PQI after they had grown up to sizes of 5 lm. Thus, heterogeneous nucleation at the interface between PQI and the isotropic matrix would be a reasonable explanation of the greater n T in the presence of PQI. On the other hand, the rate constant of sphere generation, kp , seems to be considerably smaller in the presence of PQI. Although it is difficult to quantitatively relate the rate of nucleation with that of generation defined in the present study, it can at least be said that larger kp leads shorter period of time for the nucleation. Thus, PQI would play a role in maintaining super-saturation of the precursor of nuclei at certain levels for longer time. In fact, the generation of spheres from CTP-B continued from th ¼ 40–50 min. In the case of no coalescence, the time period for sphere generation is equal to that of nucleation [4]. In addition, the time period taken for a nuclear to grow in size up to a radius of 1 lm is given as 1=G min assuming that the radius of the nuclear is negligibly small compared with 1 lm [4]. According to the optimized value of G for CTP-B, the time is estimated to be longer than 60 min. This implies that the nucleation started before the temperature of CTP-B reached 703 K. From the comparison of the optimized value of b between CTP-A (600 lm3 min1 ) and CTP-B (55 lm3 min1 ), it is clear that coalescence was largely suppressed by PQI. For CTP-B, it was found from the microscopic observation that every sphere was sur-
Table 2 Optimized kinetic parameters Pitch CTP-A CTP-B
Generation
Growth
Coalescence
th;g (min)
kp (min1 )
Cp0 (lm3 /lm3 -initial-CTP)
G (lm min1 )
th;c (min)
k
b0 (lm3 min1 )
29 40
P5 0.099
0.011 0.057
0.040 0.016
– 50
0 0
600 55
R. Moriyama et al. / Carbon 42 (2004) 2443–2449
rounded by PQI. It is reasonable that such PQI acted as a physical barrier to prevent spheres from coalescing. In our previous study [4], changes in NT , VT and the radius distribution of mesophase spheres were investigated for a coal tar pitch that contained some PQI. It was confirmed that every sphere was surrounded by PQI. It was also reveled from the model analysis that the coalescence was inhibited for a while even after termination of generation. Taken together, it can be concluded that PQI prevented spheres from coalescing for a period of time and afterward suppressed the coalescence to a large extent upon heating of CTP-B. This conclusion is in broad agreement with the results reported by Romovacek et al. [6]. The time for the commencement of coalescence would depend on the concentration of PQI. More PQI would be able to cover larger surface of spheres and therefore prevent spheres from coalescing for longer time. The presence of PQI at sphere–matrix interface would be a more or less resistance for the sphere to incorporate material from the matrix. It is hence reasonable that the linear growth rate G for CTP-B is smaller than that for CTP-A.
5. Conclusions Examination was made on the effects of PQI on the kinetics of mesophase formation from the coal tar pitch upon heating at 703 K. The time dependent changes in NT , VT and the radius distribution of mesophase spheres in the presence and absence of PQI were analyzed by the kinetic model to evaluate the effects of PQI on the elemental rate processes as those on the kinetic parameters. Given the present experimental conditions, the following conclusions were made: 1. PQI increased the total frequency of sphere generation by about five times, decreased the rate constant of coalescence by an order of magnitude, and decreased the rate constant of linear growth by more than half. 2. Coalescence was inhibited until the termination of sphere generation in the presence of PQI while the coalescence began simultaneously with the generation in the absence of PQI.
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3. The time period of sphere generation in the presence of PQI was longer by an order of magnitude than that in the absence of PQI.
Acknowledgements The authors wish to gratefully acknowledge Mr. N. Kashimura and Ms. R. Goda for their assistance in the image analysis.
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