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Pressure effects on the yield and on the microstructure formation in the pyrolysis of coal tar and petroleum pitches
Carbon. 1977, Vol. IS, pp. 69-74.
Pergamon Press.
Printed in Great Britain
PRESSURE EFFECTS ON THE YIELD AND ON THE MICROSTRUCTURE FORMATION IN THE PYROLYSIS OF COAL TAR AND PETROLEUM PITCHES K. J. HOITINGERT and U. ROSENBLATT Schunk & Ebe GmbH, D-63 GieBen, Germany (Received 23 January 1976)
Abstract-The pyrolysis of coal tar and petroleum pitches was studied under nitrogen pressures from 2 to 150bar at temperatures between 300 and 600°C.The resulting residues were investigated by TGA, polarized-light microscopy and X-ray diffraction. It is shown that increasing pressure does not only increase the coke yield but also lowers the temperature at which pyrolysis is completed. This “chemical” effect of pressure is stronger with petroleum pitch. Increasing the pressure also improves preorder and graphitizability of the residues. Simultaneously, the microstructure becomes coarser and more isotropic, i.e. that the enlarged areas of optical anisotropy exhibit no preferred orientation.
1. INTRODUCTION
behaviour of coal tar and petroleum pitch is compared. The comparison was chosen under the aspect that petroleum pitches can generally be distinguished from coal tar pitches by lower coke yield on the one hand and by higher reactivity on the other. Experiments were performed on an extracted coal tar and a petroleum pitch with nearly identical softening points of around 87°C. A conventional coal tar pitch with a softening point of 50.X was also included (see Table I). The pyrolysis behaviour of the extracted coal tar pitch and of the petroleum pitch under normal pressure is well known from an earlier study[7]. Figure I shows in the upper part the change of pyridine and benzene insolubility of the residues with increasing heat-treatment temperature. In the lower part, the hydrogen content of the pyridine soluble and pyridine insoluble fractions as well as of the total residues are given. It is seen that the transformation of the extracted coal tar pitch to mesophase, represented by the increase of pyridine insolubility, occurs in a two phase system of nearly constant composition, if this conclusion is allowed from the hydrogen content of the soluble and insoluble fractions. This property might be attributed to the high aromaticity of the coal tar pitch precursor. The petroleum pitch, less aromatic in nature, forms its mesophase in a dynamic system. This means that the mesophase (represented again by pyridine insoluble fraction) and the “pitch” (represented by the pyridine soluble fraction)
Extensive studies on the influence of very high pressures, approaching 3 kbar, on the carbonization of pure organic
compounds, pitches and coals were performed by Walker et al. [l, 21and by Marsh et al. [3,4]. The structures of the solid products, carbonized whilst sealed in a gold tube and contained in a hydrothermal pressure bomb, were characterized as anisotropic carbonaceous mesophase, whose morphologies change from vesicular to spherical with increasing pressure. Pressure was also observed to hinder or even to prevent coalescence of the mesophase spheres and to enhance graphitizability. The effect of a gas pressure up to 100bar on coke yield from conventional pitches was studied by Fitzer and Terwiesch[5]. It was found that maximum coke yields are achieved at pressures of approx. 100bar maintained up to 550°C. Little information seems to be available in the open literature on the influence of pressures up to some hundred bars on the formation of microstructure in the residues. Some results on studies with conventional coal tar pitches, especially considering the behaviour of the insolubles under pressures up to 200 bar, were given at the London Conference in 1974[6]. It was shown that increasing pressure: (i) does not only increase the coke yield but also seems to lower the temperature at which pyrolysis is completed as measured by weight loss, (ii) changes mesophase microstructure in so far as the orientation of lamellae observable under polarized light is improved, (iii) causes a pronounced segregation of original insolubles of conventional coal tar pitches and of artificial insolubles like carbon black. The separated insolubles accumulate in the upper part of the pyrolysis vessel whereas a highly ordered mesophase without insolubles is found lower down. In this study the effect of pressure on pyrolysis
Table I. Properties of the pitches used
Extacted coal tar pitch (CTP)f Petroleum pitch (PP) Coal tar soft pitch
tNew address: Institut fiir Chemische Technik der Universitit Karlsruhe, Kaiserstr. 12, D-75 Karlsruhe, Germany.
s.p.t (“C)
B.I. (%)
Q.1. (%)
Coke res. (%)
86
23
0.2
40
88
0.2
0.1
30
50.5
-
4.5
37
tKraemer-Sarnow. $Extracted with Quinoline. 69
70
K. J. K~~~NCER and U.
ROSENBLATT
I
I00
so 7 0
;
60-
? 0 2 LO.20 l
01
o-
,
0
4---d
300
1 LOO
500
T,OC
pyrolysis
res.
\ I m.
6
;_+\ 3
PI.
&, 0
, 300
L
“\ LOO
500
\I 600
T.X 3 0
300
LOO
500
600
1.T
Fig. 1. Change of benzene and pyridine solubility (upper part) and of hydrogen content (lower part) of pyrolysis residues at normal pressure from extracted coal tar pitch (left) and from petroleum pitch (right) with increasing pyrolysis temperature.
continuously change in their hydrogen content. However, as already pointed out [7], after hardening of the coalesced mesophase at about SSo”C,the residues of both pitches exhibit the same hydrogen content of approx 3.4 wt%. It has not yet been concluded which of both systems is more suitable for producing a well graphitizing material. Some answer will be given by the following studies under increasing pressures up to 150 bar. 2. EXPERIMENTAL Pressure pyrolysis treatments of the pitches were performed in a laboratory autoclave under constant nitrogen pressures up to 150bar. The samples were placed in a covered porcelain crucible. The temperature was raised at a rate of approx. lO”C/min and the final temperature was maintained for 3 h. The resulting residues were further investigated by thermogravimetric analysis (TGA) at a constant rate of heating, by polarized-light microscopy and by X-ray diffraction (002). X-ray diffraction measurements of the (002)-, (004)- and (006)-reflection were also performed with the residues obtained at 600°C and heat-treated at 2700°Cfor 0.5 hr. From the latter X-ray data, mean defect free distances L in the c-direction, according to Ergun[8], assuming a Gaussian strain distribution, and mean interlayer spacings d2 were calculated.
3. RESULTS The results of the pressure pyrolysis runs are given in Figs. 2 and 3 in terms of weight loss vs heat-treatment temperature. Generally, it can be recognized that pressures up to approx. 100bar have a significant effect on the coke yield whereas pressures higher than 100bar are less effective. Weight loss of the petroleum pitch is significant below 400°C heat-treatment temperature (HTT) even at pressures greater than 100bar. Figures 4 and 5 show weight loss curves during thermogravimetric analysis under flowing nitrogen at normal pressure of the residues obtained from pressure pyrolysis at 400,450 and 500°C H’IT. It can be recognized in Fig. 4(a), for instance, that pressure treatment of the coal tar soft pitch at 400°C only inhibits volatilization, because there is no difference between weight losses of original pitch and pretreated pitch after TGA at 1000°C. This still holds for a pressure pretreatment under 50 bar at 450°C HTT (Fig. 4b), whereas a pretreatment under a pressure of 150bar at 45O’Cmust have caused significant chemical changes in the residue as can be seen from the weight loss during TGA. A similar behaviour is found with petroleum pitch (Fig. 5) with the difference that the chemical changes of petroleum pitch caused by increasing pressure are already observed at lower temperatures. Examples of the effect of pressure on the microstruc-
71
The pyrolysis of coal tar and petroleum pitches
/
'mL7-15O
bar
t
-200
200
600
LOO
LOO
600
T,T
Fig. 2. Weight loss of coal tar pitches after pyrolysis at various nitrogen pressures (residence time 3 hr).
ture of the residues are given in Figs. 6 and 7, showing
200
300
I
I
LOO
500
1
600
T, “C Fig. 3. Weight loss of petroleum pitch after pyrolysis at various nitrogen pressures (residence time 3 hr).
I 60
polarized light micrographs of the coalesced mesophases of extracted coal tar pitch and of petroleum pitch. Corresponding studies of the microstructures of all residues from pressure pyrolysis were performed revealing that increasing pressure improves alignment of lamellae and favours coalescence. Figure 8 finally shows results on a rough estimation of the mean stack heights of the lamellae in the mesophase structures at various temperatures, calculated from the (002)-reflection. Increasing pressure causes an increase of the mean stack height independent of the pitch used. The residues of pressure pyrolysis at 600°C were further heat-treated up to 2700°C in order to study their graphitizability by X-rays. The data are given in Table 2. Pressure pyrolysis slightly influences the mean defectfree distance L of the graphite produced from extracted coal tar pitch.
I 150 bar __--__--2bar
Ls Ii LO 4 E .F .___.-.--.-
$20
0
al 200
I
I
500
800
200
500
T,
800
zoo
500
50 bar
BOO
“C
Fig. 4. Weight loss curves of pressure pyrolysis residues from coal tar pitch, measured at normal pressure at a heating rate of S”C/min. (a) Pretreatment temperature 400°C; (b) pretreatment temperature 450°C; (c) pretreatment temperature 500°C.
12
K. J. KOTTINGER and LJ. ROSENBLATT
i~~~~
200
500
800
200
500 T. OC
800
200
500
800
Fig. 5. Weight loss curves of pressure pyrolysisresidues from petroleumpitch measuredat normal pressure at a heating rate of S”C/min.(a) Pretreatmenttemperature400°C;(b) pretreatmenttemperature450°C;(c) pretreatment
temperature500°C.
Table 2. Mean defect-freedistance L and interlayer spacingsd2 of the 600°Cpressure pyrolysis residues after graphitizationat 2700°C Pressure during pyrolysis(bar)
Petroleum pitch (PP) d2, nm L, nm Extractedcoal tar pitch (CTP) c/2, nm
i, nm
2
50
150
0.3345 22.2
0.3352 23.1
0.3352 24.5
0.3370 44.7
0.3369 52.8
-
4. DIsCUSION
Gas pressures greater than 100 bar seem to have little influence on coke yield. This limitation of pressure inffuence on coke yield was already found for conventional coal tar pitches by Fitzer and Terwiesch[S] and roughly correlated by these authors with theoretical vapor pressures of pure aromatics, omitting all other parameters. With the exception of benzene, the theoretical vapor pressure of polynuclear aromatics is below 100bar even at 550°c. As seen from this study, the same limitation of pressure influence on coke yield exists for petroleum pitch, although pressure influence on coke yield is less than with coal tar pitches (compare Figs. 2 and 3). Two observations might be mentioned in this connection: (1) the weight loss of petroleum pitch during pressure pyrolysis occurs to a great amount below 400°C (see Fig. 3) and at lower temperatures than with coal tar pitches (see Fig. 2); (2) the softening point of petroleum pitch after pressure pyrolysis at 350°C is lowered by nearly 40°C as compared to the original pitch. This indicates a partial depolymerisation, which may be one factor responsible for the small pressure effect on coke yield. The other factor is seen in the absence of large polynuclear and non-volatile aromatics, typical for coal tar pitches. On the other hand, TGA-runs on petroleum pitch
residues from pressure pyrolysis at 400°C reveal a remarkable decrease in weight loss with increasing pressure applied during pretreatment (see Fig. Sa). This suggests pressure dependent chemical changes have occurred in the residues from pressure pyrolysis. This chemical effect of pressure might be explained as due to increasing pressure enhancing the retention of low molecular weight reactive species which can undergo condensation reactions. This concept is supported by the TGA-runs on coal tar pitch (Fig. 4), if we accept the idea, that these pitches are less reactive due to their higher aromaticity. At 450°C pretreatment, for instance, coal tar pitches require a pressure of 150bar to achieve a chemical effect of pressure whereas for petroleum pitch a pressure of only 50 bar or even less is sufficient. From a technical point of view these results with both pitches can generally be interpreted as meaning that the same coke yield can be achieved by maintaining a lower pressure to higher temperatures or a higher pressure to lower temperatures. As far as petroleum pitch is concerned a pressure of approx. 50 bar up to 450°C would be sufficient for a nearly maximum coke yield. It should be mentioned that these numerical values may depend on heating rate to a certain degree. The pressure effects during pyrolysis on the microstructure formation, as demonstrated in Figs. 6 and 7, are similar for both types of pitches. Areas of optical anisotropy are decisively enlarged by pressure. On the other hand, an analysis of the orientation of individual areas all over the samples shows no preferred orientation, as often found at normal or slightly elevated pressures. This means that the desired needle-like morphologies of the cokes are lost with increasing pressure. This latter phenomenon can be explained by reduced amounts of volatile products, if pyrolysis gas bubble formation and migration is accepted as a predominant factor in generating needle-like morphologies. As far as enlargement of anisotropic regions by pressure is concerned, retardation or even complete hindrance of volatilization of low molecular weight compounds might be an important factor. These compounds form a solvent phase of lower viscosity favouring precipitation, growth, coalescence and especially reorientation of the mesophase spheres after coalescence.
The pyrolysis of coal tar and petroleum pitches
(b)
1 1
loolrm
I ’
Fig. 6. Comparison between coalesced mesophase microstructures obtained with extracted coal tar pitch at 550°C under nitrogenpressures of 2 (a) and SO bar (b} (reflected polarized light, magni~catjon X 200).
It is suggested that these pressure effects on the microstructure formation are not unlimited. Kinetic studies of weight loss have shown that pressure also causes a chemical effect. which shifts the end of pyrolysis to lower temperatures. This means that low molecular weight species participate in condensation reactions forming larger molecules. Besides this chemical effect which contributes to an increase in viscosity of the melt, the lower temperature in general results in higher viscosity, A certain confirmation of this suggestion is given by results in Fig. 8, which show the limitation of pressure on achieving improved molecular preorder.
Fig. 7. Comparison between coalesced mesophase microstructures obtained with petroleum pitch at 450°C under nitrogen pressures of 2 (a) and 150bar (b) (reflected polarized light, magnificationX2@?).
2
5c
6
p,
‘50
bar
Fig. 8. Mean stack heights of aligned molecules in various pyrolysis residues with increasing pressure.
74
and U. ROS~NBLATT K. J. K~T~INGER
It is further shown by these investigations that the higher preorder as formed under pressure pyrolysis has an important effect on the subsequent degree of graphitization (Table 2). This means that pressure pyrolysis offers a possibility for improving crystalline perfection of synthetic graphites. Another phenomenon is revealed by the crystallite data, namely the remarkable correlation between L and c/2 of petroleum and coal tar pitch graphites. Graphites prepared from petroleum pitch exhibit low C/2-and relatively low L-values, whereas graphites from coal tar pitches combine relatively high F/2-values with high l-values. This relationship is not limited to pressure pyrolysis residues of these precursors [9]. This correlation might be interpreted as meaning smaller but perfect crystallite units typical for petroleum pitch materials and larger but less perfect crystallite units typical especially for extracted coal tar pitch materials. The high distortion of the graphitized pitch coke material can clearly be recognized from the large slope of the plot of half width vs square of the order of the reflection according to Ergun [8]. It was already pointed out in connection with a mesophase study[7] that the layers in chars of coal tar pitches must be curved to a higher degree than they are with chars of petroleum pitches. This curvature prevents the formation of microcracks and gaps especially along the layer stacks. This latter property is well observed with chars from petroleum pitches and it was concluded that these microcracks along the layer planes are responsible
or are a requirement for good graphitizability. It could be
supposed that the dimensions of the gaps and the distances between the gaps observable in polarized light microscopy are one or two orders of magnitude, respectively, larger than those which are responsible for perfect three-dimensional ordering during graphitization treatment. However, there is no reason to assume that similar gaps and layer stacks are also present in the submicroscopic range. In summarizing these considerations, it is concluded that pressure pyrolysis improves the preorder of chars but it does not change the typical structural characteristics of chars prepared from coal tar or petroleum pitches.
REFERENCES
1. S. Hirano, F. Dachille and P. L. Walker, Jr., High Temperatures-High Pressures 5, 207 (1973). 2. P. W. Whang, F. Dachille and P. L. Walker, Jr., High Temperatures-High Pressures 6, 127 (1974). 3. H. Marsh, F. Dachille, J. Melvin and P. L. Walker, Jr., Carbon 9, 159 (1971). 4. H. Marsh, J. M. Foster, G. Hermon and M. Iley, Fuel 52,234, 243, 253 (1973). 5. E. Fitzer and B. Terwiesch, Carbon 11, 570 (1970). 6. K. J. Hiittinger and U. Rosenblatt. Proc. 4th Conference on Industrial Carbon and Graphite, London, 1974. Society of Chem. Ind., London (1976). 7. K. J. Hiittineer. Bitumen. Teere. Asohalte. Peche 24.225 (1973). 8. S. Ergon, P&s. Rev. Bl; 3771(1976);P. A. Thrower and D. d. Nagle, Carbon 11, 663 (1973). 9. M. B. Dowell, Extended Abstracts, 12th Biennal Conf. on Carbon, Pittsburgh, Penn. 1975,p. 31.
Pergamon Press.
Printed in Great Britain
PRESSURE EFFECTS ON THE YIELD AND ON THE MICROSTRUCTURE FORMATION IN THE PYROLYSIS OF COAL TAR AND PETROLEUM PITCHES K. J. HOITINGERT and U. ROSENBLATT Schunk & Ebe GmbH, D-63 GieBen, Germany (Received 23 January 1976)
Abstract-The pyrolysis of coal tar and petroleum pitches was studied under nitrogen pressures from 2 to 150bar at temperatures between 300 and 600°C.The resulting residues were investigated by TGA, polarized-light microscopy and X-ray diffraction. It is shown that increasing pressure does not only increase the coke yield but also lowers the temperature at which pyrolysis is completed. This “chemical” effect of pressure is stronger with petroleum pitch. Increasing the pressure also improves preorder and graphitizability of the residues. Simultaneously, the microstructure becomes coarser and more isotropic, i.e. that the enlarged areas of optical anisotropy exhibit no preferred orientation.
1. INTRODUCTION
behaviour of coal tar and petroleum pitch is compared. The comparison was chosen under the aspect that petroleum pitches can generally be distinguished from coal tar pitches by lower coke yield on the one hand and by higher reactivity on the other. Experiments were performed on an extracted coal tar and a petroleum pitch with nearly identical softening points of around 87°C. A conventional coal tar pitch with a softening point of 50.X was also included (see Table I). The pyrolysis behaviour of the extracted coal tar pitch and of the petroleum pitch under normal pressure is well known from an earlier study[7]. Figure I shows in the upper part the change of pyridine and benzene insolubility of the residues with increasing heat-treatment temperature. In the lower part, the hydrogen content of the pyridine soluble and pyridine insoluble fractions as well as of the total residues are given. It is seen that the transformation of the extracted coal tar pitch to mesophase, represented by the increase of pyridine insolubility, occurs in a two phase system of nearly constant composition, if this conclusion is allowed from the hydrogen content of the soluble and insoluble fractions. This property might be attributed to the high aromaticity of the coal tar pitch precursor. The petroleum pitch, less aromatic in nature, forms its mesophase in a dynamic system. This means that the mesophase (represented again by pyridine insoluble fraction) and the “pitch” (represented by the pyridine soluble fraction)
Extensive studies on the influence of very high pressures, approaching 3 kbar, on the carbonization of pure organic
compounds, pitches and coals were performed by Walker et al. [l, 21and by Marsh et al. [3,4]. The structures of the solid products, carbonized whilst sealed in a gold tube and contained in a hydrothermal pressure bomb, were characterized as anisotropic carbonaceous mesophase, whose morphologies change from vesicular to spherical with increasing pressure. Pressure was also observed to hinder or even to prevent coalescence of the mesophase spheres and to enhance graphitizability. The effect of a gas pressure up to 100bar on coke yield from conventional pitches was studied by Fitzer and Terwiesch[5]. It was found that maximum coke yields are achieved at pressures of approx. 100bar maintained up to 550°C. Little information seems to be available in the open literature on the influence of pressures up to some hundred bars on the formation of microstructure in the residues. Some results on studies with conventional coal tar pitches, especially considering the behaviour of the insolubles under pressures up to 200 bar, were given at the London Conference in 1974[6]. It was shown that increasing pressure: (i) does not only increase the coke yield but also seems to lower the temperature at which pyrolysis is completed as measured by weight loss, (ii) changes mesophase microstructure in so far as the orientation of lamellae observable under polarized light is improved, (iii) causes a pronounced segregation of original insolubles of conventional coal tar pitches and of artificial insolubles like carbon black. The separated insolubles accumulate in the upper part of the pyrolysis vessel whereas a highly ordered mesophase without insolubles is found lower down. In this study the effect of pressure on pyrolysis
Table I. Properties of the pitches used
Extacted coal tar pitch (CTP)f Petroleum pitch (PP) Coal tar soft pitch
tNew address: Institut fiir Chemische Technik der Universitit Karlsruhe, Kaiserstr. 12, D-75 Karlsruhe, Germany.
s.p.t (“C)
B.I. (%)
Q.1. (%)
Coke res. (%)
86
23
0.2
40
88
0.2
0.1
30
50.5
-
4.5
37
tKraemer-Sarnow. $Extracted with Quinoline. 69
70
K. J. K~~~NCER and U.
ROSENBLATT
I
I00
so 7 0
;
60-
? 0 2 LO.20 l
01
o-
,
0
4---d
300
1 LOO
500
T,OC
pyrolysis
res.
\ I m.
6
;_+\ 3
PI.
&, 0
, 300
L
“\ LOO
500
\I 600
T.X 3 0
300
LOO
500
600
1.T
Fig. 1. Change of benzene and pyridine solubility (upper part) and of hydrogen content (lower part) of pyrolysis residues at normal pressure from extracted coal tar pitch (left) and from petroleum pitch (right) with increasing pyrolysis temperature.
continuously change in their hydrogen content. However, as already pointed out [7], after hardening of the coalesced mesophase at about SSo”C,the residues of both pitches exhibit the same hydrogen content of approx 3.4 wt%. It has not yet been concluded which of both systems is more suitable for producing a well graphitizing material. Some answer will be given by the following studies under increasing pressures up to 150 bar. 2. EXPERIMENTAL Pressure pyrolysis treatments of the pitches were performed in a laboratory autoclave under constant nitrogen pressures up to 150bar. The samples were placed in a covered porcelain crucible. The temperature was raised at a rate of approx. lO”C/min and the final temperature was maintained for 3 h. The resulting residues were further investigated by thermogravimetric analysis (TGA) at a constant rate of heating, by polarized-light microscopy and by X-ray diffraction (002). X-ray diffraction measurements of the (002)-, (004)- and (006)-reflection were also performed with the residues obtained at 600°C and heat-treated at 2700°Cfor 0.5 hr. From the latter X-ray data, mean defect free distances L in the c-direction, according to Ergun[8], assuming a Gaussian strain distribution, and mean interlayer spacings d2 were calculated.
3. RESULTS The results of the pressure pyrolysis runs are given in Figs. 2 and 3 in terms of weight loss vs heat-treatment temperature. Generally, it can be recognized that pressures up to approx. 100bar have a significant effect on the coke yield whereas pressures higher than 100bar are less effective. Weight loss of the petroleum pitch is significant below 400°C heat-treatment temperature (HTT) even at pressures greater than 100bar. Figures 4 and 5 show weight loss curves during thermogravimetric analysis under flowing nitrogen at normal pressure of the residues obtained from pressure pyrolysis at 400,450 and 500°C H’IT. It can be recognized in Fig. 4(a), for instance, that pressure treatment of the coal tar soft pitch at 400°C only inhibits volatilization, because there is no difference between weight losses of original pitch and pretreated pitch after TGA at 1000°C. This still holds for a pressure pretreatment under 50 bar at 450°C HTT (Fig. 4b), whereas a pretreatment under a pressure of 150bar at 45O’Cmust have caused significant chemical changes in the residue as can be seen from the weight loss during TGA. A similar behaviour is found with petroleum pitch (Fig. 5) with the difference that the chemical changes of petroleum pitch caused by increasing pressure are already observed at lower temperatures. Examples of the effect of pressure on the microstruc-
71
The pyrolysis of coal tar and petroleum pitches
/
'mL7-15O
bar
t
-200
200
600
LOO
LOO
600
T,T
Fig. 2. Weight loss of coal tar pitches after pyrolysis at various nitrogen pressures (residence time 3 hr).
ture of the residues are given in Figs. 6 and 7, showing
200
300
I
I
LOO
500
1
600
T, “C Fig. 3. Weight loss of petroleum pitch after pyrolysis at various nitrogen pressures (residence time 3 hr).
I 60
polarized light micrographs of the coalesced mesophases of extracted coal tar pitch and of petroleum pitch. Corresponding studies of the microstructures of all residues from pressure pyrolysis were performed revealing that increasing pressure improves alignment of lamellae and favours coalescence. Figure 8 finally shows results on a rough estimation of the mean stack heights of the lamellae in the mesophase structures at various temperatures, calculated from the (002)-reflection. Increasing pressure causes an increase of the mean stack height independent of the pitch used. The residues of pressure pyrolysis at 600°C were further heat-treated up to 2700°C in order to study their graphitizability by X-rays. The data are given in Table 2. Pressure pyrolysis slightly influences the mean defectfree distance L of the graphite produced from extracted coal tar pitch.
I 150 bar __--__--2bar
Ls Ii LO 4 E .F .___.-.--.-
$20
0
al 200
I
I
500
800
200
500
T,
800
zoo
500
50 bar
BOO
“C
Fig. 4. Weight loss curves of pressure pyrolysis residues from coal tar pitch, measured at normal pressure at a heating rate of S”C/min. (a) Pretreatment temperature 400°C; (b) pretreatment temperature 450°C; (c) pretreatment temperature 500°C.
12
K. J. KOTTINGER and LJ. ROSENBLATT
i~~~~
200
500
800
200
500 T. OC
800
200
500
800
Fig. 5. Weight loss curves of pressure pyrolysisresidues from petroleumpitch measuredat normal pressure at a heating rate of S”C/min.(a) Pretreatmenttemperature400°C;(b) pretreatmenttemperature450°C;(c) pretreatment
temperature500°C.
Table 2. Mean defect-freedistance L and interlayer spacingsd2 of the 600°Cpressure pyrolysis residues after graphitizationat 2700°C Pressure during pyrolysis(bar)
Petroleum pitch (PP) d2, nm L, nm Extractedcoal tar pitch (CTP) c/2, nm
i, nm
2
50
150
0.3345 22.2
0.3352 23.1
0.3352 24.5
0.3370 44.7
0.3369 52.8
-
4. DIsCUSION
Gas pressures greater than 100 bar seem to have little influence on coke yield. This limitation of pressure inffuence on coke yield was already found for conventional coal tar pitches by Fitzer and Terwiesch[S] and roughly correlated by these authors with theoretical vapor pressures of pure aromatics, omitting all other parameters. With the exception of benzene, the theoretical vapor pressure of polynuclear aromatics is below 100bar even at 550°c. As seen from this study, the same limitation of pressure influence on coke yield exists for petroleum pitch, although pressure influence on coke yield is less than with coal tar pitches (compare Figs. 2 and 3). Two observations might be mentioned in this connection: (1) the weight loss of petroleum pitch during pressure pyrolysis occurs to a great amount below 400°C (see Fig. 3) and at lower temperatures than with coal tar pitches (see Fig. 2); (2) the softening point of petroleum pitch after pressure pyrolysis at 350°C is lowered by nearly 40°C as compared to the original pitch. This indicates a partial depolymerisation, which may be one factor responsible for the small pressure effect on coke yield. The other factor is seen in the absence of large polynuclear and non-volatile aromatics, typical for coal tar pitches. On the other hand, TGA-runs on petroleum pitch
residues from pressure pyrolysis at 400°C reveal a remarkable decrease in weight loss with increasing pressure applied during pretreatment (see Fig. Sa). This suggests pressure dependent chemical changes have occurred in the residues from pressure pyrolysis. This chemical effect of pressure might be explained as due to increasing pressure enhancing the retention of low molecular weight reactive species which can undergo condensation reactions. This concept is supported by the TGA-runs on coal tar pitch (Fig. 4), if we accept the idea, that these pitches are less reactive due to their higher aromaticity. At 450°C pretreatment, for instance, coal tar pitches require a pressure of 150bar to achieve a chemical effect of pressure whereas for petroleum pitch a pressure of only 50 bar or even less is sufficient. From a technical point of view these results with both pitches can generally be interpreted as meaning that the same coke yield can be achieved by maintaining a lower pressure to higher temperatures or a higher pressure to lower temperatures. As far as petroleum pitch is concerned a pressure of approx. 50 bar up to 450°C would be sufficient for a nearly maximum coke yield. It should be mentioned that these numerical values may depend on heating rate to a certain degree. The pressure effects during pyrolysis on the microstructure formation, as demonstrated in Figs. 6 and 7, are similar for both types of pitches. Areas of optical anisotropy are decisively enlarged by pressure. On the other hand, an analysis of the orientation of individual areas all over the samples shows no preferred orientation, as often found at normal or slightly elevated pressures. This means that the desired needle-like morphologies of the cokes are lost with increasing pressure. This latter phenomenon can be explained by reduced amounts of volatile products, if pyrolysis gas bubble formation and migration is accepted as a predominant factor in generating needle-like morphologies. As far as enlargement of anisotropic regions by pressure is concerned, retardation or even complete hindrance of volatilization of low molecular weight compounds might be an important factor. These compounds form a solvent phase of lower viscosity favouring precipitation, growth, coalescence and especially reorientation of the mesophase spheres after coalescence.
The pyrolysis of coal tar and petroleum pitches
(b)
1 1
loolrm
I ’
Fig. 6. Comparison between coalesced mesophase microstructures obtained with extracted coal tar pitch at 550°C under nitrogenpressures of 2 (a) and SO bar (b} (reflected polarized light, magni~catjon X 200).
It is suggested that these pressure effects on the microstructure formation are not unlimited. Kinetic studies of weight loss have shown that pressure also causes a chemical effect. which shifts the end of pyrolysis to lower temperatures. This means that low molecular weight species participate in condensation reactions forming larger molecules. Besides this chemical effect which contributes to an increase in viscosity of the melt, the lower temperature in general results in higher viscosity, A certain confirmation of this suggestion is given by results in Fig. 8, which show the limitation of pressure on achieving improved molecular preorder.
Fig. 7. Comparison between coalesced mesophase microstructures obtained with petroleum pitch at 450°C under nitrogen pressures of 2 (a) and 150bar (b) (reflected polarized light, magnificationX2@?).
2
5c
6
p,
‘50
bar
Fig. 8. Mean stack heights of aligned molecules in various pyrolysis residues with increasing pressure.
74
and U. ROS~NBLATT K. J. K~T~INGER
It is further shown by these investigations that the higher preorder as formed under pressure pyrolysis has an important effect on the subsequent degree of graphitization (Table 2). This means that pressure pyrolysis offers a possibility for improving crystalline perfection of synthetic graphites. Another phenomenon is revealed by the crystallite data, namely the remarkable correlation between L and c/2 of petroleum and coal tar pitch graphites. Graphites prepared from petroleum pitch exhibit low C/2-and relatively low L-values, whereas graphites from coal tar pitches combine relatively high F/2-values with high l-values. This relationship is not limited to pressure pyrolysis residues of these precursors [9]. This correlation might be interpreted as meaning smaller but perfect crystallite units typical for petroleum pitch materials and larger but less perfect crystallite units typical especially for extracted coal tar pitch materials. The high distortion of the graphitized pitch coke material can clearly be recognized from the large slope of the plot of half width vs square of the order of the reflection according to Ergun [8]. It was already pointed out in connection with a mesophase study[7] that the layers in chars of coal tar pitches must be curved to a higher degree than they are with chars of petroleum pitches. This curvature prevents the formation of microcracks and gaps especially along the layer stacks. This latter property is well observed with chars from petroleum pitches and it was concluded that these microcracks along the layer planes are responsible
or are a requirement for good graphitizability. It could be
supposed that the dimensions of the gaps and the distances between the gaps observable in polarized light microscopy are one or two orders of magnitude, respectively, larger than those which are responsible for perfect three-dimensional ordering during graphitization treatment. However, there is no reason to assume that similar gaps and layer stacks are also present in the submicroscopic range. In summarizing these considerations, it is concluded that pressure pyrolysis improves the preorder of chars but it does not change the typical structural characteristics of chars prepared from coal tar or petroleum pitches.
REFERENCES
1. S. Hirano, F. Dachille and P. L. Walker, Jr., High Temperatures-High Pressures 5, 207 (1973). 2. P. W. Whang, F. Dachille and P. L. Walker, Jr., High Temperatures-High Pressures 6, 127 (1974). 3. H. Marsh, F. Dachille, J. Melvin and P. L. Walker, Jr., Carbon 9, 159 (1971). 4. H. Marsh, J. M. Foster, G. Hermon and M. Iley, Fuel 52,234, 243, 253 (1973). 5. E. Fitzer and B. Terwiesch, Carbon 11, 570 (1970). 6. K. J. Hiittinger and U. Rosenblatt. Proc. 4th Conference on Industrial Carbon and Graphite, London, 1974. Society of Chem. Ind., London (1976). 7. K. J. Hiittineer. Bitumen. Teere. Asohalte. Peche 24.225 (1973). 8. S. Ergon, P&s. Rev. Bl; 3771(1976);P. A. Thrower and D. d. Nagle, Carbon 11, 663 (1973). 9. M. B. Dowell, Extended Abstracts, 12th Biennal Conf. on Carbon, Pittsburgh, Penn. 1975,p. 31.