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Effect of processing technology on mesophase structure
Effect of processing mesophase structure Zuzana
Weishauptov6
technology
on
and Jifi Medek
Institute of Geotechnics, Czechoslovak Academy Czechoslovakia (Received 18 July 1989; revised 27 April 1990)
of Sciences,
182 09 Prague,
the laboratory scale, mesophase samples were prepared from coal tar pitch by pyrolysis in the bulk phase and from pyrolysis oil by feeding into a reactor. From texture analysis by SEM, it was shown that in the former case the mesophase consists of isolated blocks in which the unidirectionally arranged lamellar formations are contiguous to one another unconformably, thus forming a mosaic structure. In the latter case the mesophase grows layer by layer, the lamellar formations being contiguous to one another conformably to give a mesophase with domain and flow structure. Common systematic features were found in the two types of mesophase: the orientation of lamellae perpendicular to the surface, lamellar packets of 0.1 pm thickness, and the angle of their transverse fracture, 104”. On
(Keywords:
coal structure;
mesophase;
scanning
electron
microscopy)
The decisive phase in the production of synthetic graphite on the basis of coal tar pitch or heavy products from distillation of crude petroleum consists of spontaneous transformation of the original isotropic raw material into anisotropic mesophase within the temperature interval 35G5Oo”C. Polarized light microscopy, electron microscopy, and X-ray diffraction analysis’-4 have shown that the anisotropy of the mesophase is caused by the deposition of planar polyaromatic molecules into parallel stacks whose mutual positioning forms an oriented structure. The primary configurations of the mesophase are individual spherical particles (spherules) with a regular internal structufe’.5*6 which is most often characterized, according to Brooks and Taylor’, by the approximately perpendicular orientation of molecular stacks to the surface of the spherule. The spherules retain their internal structure during both individual growth and mutual coalescence; in the latter process their structures become interconnected and, within the new individual units, either partly or completely reorganized. For large-area formation of mesophase, however, it must be assumed that the structural arrangement is more complex, and that the reorganization of the original structures is not uniform. Moreover, it must be taken into account that the possible action of external factors (magnetic field7-‘l, laminar flow”, percolation of gas bubbles13-16) will exert secondary effects on the structure of the mesophase, mainly on the microscopic scale, which can, under suitable experimental conditions, be utilized for the preparation of mesophase with properties needed in subsequent treatment. Another factor which distinctly influences the nature of the mesophase structure is the technology used in its preparation. In the present study the course of mesophase development under laboratory conditions was examined, using two different preparation methods: (1) in the bulk phase with a single filled reactor; (2) in a thin layer with gradual feeding of the raw material.
0016_236I/91/020235~8 #c! 1991 Butterworth-Heinemann
Ltd.
The formation of mesophase structure was followed by scanning electron microscopy (SEM). EXPERIMENTAL Mesophase was prepared from 50 g of the parent materials whose basic characteristics are given in Table I. For the preparation of mesophase in the bulk phase, industrially produced coal tar pitch was used. The quinoline-insoluble matter (QI) was removed by filtration at 250°C through a combined filter whose inlet section consisted of a porous carbon plate with pores of 60-90 pm diameter prepared by carbonization of filter paper, the outlet section being a sintered glass plate with pores of 160-200 ,um diameter through which the pitch passed in laminar flow’ 7, The filtered pitch was transformed into mesophase by heating to 460°C under nitrogen at normal pressure for 4 h. For the preparation of mesophase in a thin layer by the feed method, pyrolysis oil obtained from distillation of crude petroleum was used. This oil has the advantage of having a low viscosity and negligible QI content. The oil was added under nitrogen atmosphere drop by drop at 20 s intervals into the reactor, whose temperature was
Table 1
Basic characteristics
of raw materials Coal tar pitch ______~
Density (g cmw3) Softening point (KS) (“C) Solidification point (“C) QI (wt%) Conradson carbonization residue Water (wt%) Ash (wt%) C (wt% daf) H (wt% daf) N (WI% daf) S (wt% daf) __ .~__.. _
FUEL,
1.08
1.28 68
(wt%)
._~
1991,
4 _
9.7 52.8 0.3 0.2 93.3 4.1 0.6 0.4 .~__~I-_I
Vol 70,
Pyrolysis oil
25.1 _ 0.005 91.9 7.4 0.1 0.2
February
-
235
Effect
of processing
on mesophase
structure:
Z. Weishauptovi
and J. Medek
RESULTS The mechanism of the transformation of pyrolysis oil into mesophase in a thin layer is illustrated by SEM photographs in Figures l-4, demonstrating the surface structure of a product obtained by the feed method at 460°C. Figure 1 gives a panoramic view of the envelope surface formed after a certain time interval from the moment of feeding of the pyrolysis oil onto the mesophase surface. The voluminous mass of homogeneous mesophase, whose growth has already been interrupted by the
Figure 1 Envelope surface of mesophase (M) covered discontinuous layer of isotropic matrix of pyrolysis oil (IM)
with
a
Figure 3 Various phases of the transformation of isotropic matrix (IM) into mesophase (M) and production of large formations by gradual coalescence
of isotropic matrix (IM) in surface Figure 2 Stepwise transformation layer into mesophase (M). Formation and growth of mesophase islets and their coalescence (denoted by arrow)
kept at 460°C each drop being instantaneously preheated to 420°C. After the last dose the temperature of the reactor was abruptly reduced, interrupting the development of mesophase. The texture analysis was carried out with a microanalyser at an accelerating voltage of 15-25 kV and a current of 10-‘” A. The observations were realized on fractured and enveloping surfaces of lump samples gold-coated in an argon atmosphere. The photomicrographs shown represent the statistically most frequent characteristic structures of the objects analysed.
236
FUEL, 1991, Vol 70, February
Figure 4 Advanced phase of transformation of isotropic matrix (IM) into mesophase (M), showing existence of lenses (L) and isolated islets (I) on the mesophase surface
Effect of processing
Figure 5 lamellar
Relief of fracture surface of mesophase structure with respect to the surface
Figure 6 mesophase
Perpendicular arrangement with respect to the surface
and orientation
of lamellar
formations
of
on mesophase
structure:
Z. Weishauptovri
and J. Medek
boundaries between both liquids. During transformation of the pyrolysis oil into mesophase, these holes grow and finally associate to give large-area mesophase formations of various sizes and shapes (Figure 3). At the same time, the layer is gradually exhausted, and its residues form isolated islets on the surface of the mesophase (Figure 4), their relief being mainly determined by surface unevenness of the mesophase. If this residue is sufficiently small, then a ‘lens’ of pyrolysis oil is formed on the mesophase surface by the action of surface tension. Figure 5 represents the relief of a fracture surface at a place where a bulge was formed by gradual growth of mesophase from the surface layers. The photomicrograph shows that lamellar formations of all sizes are orientated perpendicular to the surface, and this orientation remains preserved even when there are alterations of the surface curvature, as illustrated in Figure 6. Figure 7 shows the arrangement of lamellar formations that originated in the surface layer during the transformation of the isotropic matrix into mesophase. The orientation of this formation is perpendicular to the surface of the layer and, at the same time, in accord with the orientation of lamellar formations of the mesophase with which they are interconnected. The unidirectional orientation of the lamellae extends over a depth many times the thickness of one sprayed layer. Figure 8 shows the envelope surface of a macroscopic formation of mesophase formed spontaneously from the coal tar pitch in the bulk phase. The envelope surface is completely compact, and only its unevenness and bulges may suggest the shape of the mesophase periphery. Figure 9 shows the texture of mesophase under this surface, with characteristic individual blocks of different sizes which fill the mesophase volume and have common contact surfaces. Inside these blocks the lamellar formations have a unidirectional arrangement, the orientation, however, being different in the individual blocks, so that at the contact of two blocks an optically
of
cooling, is characterized by rugged surface with numerous bulges of different sizes, showing a smooth face. The mesophase is covered with a discontinuous layer of pyrolysis oil with randomly localized holes filled with newly formed mesophase which protrudes from the layer. Figure 2 shows the same holes of the layer observed from above, perpendicular to the surface. Their shape and size reflect various phases of development of the mesophase in a thin layer and its coalescence with the bulk mesophase. The holes, whose size varies from 0.2 to 140 pm, have approximately circular contours, which indicates the effect of surface tension with formation of
Figure 7 Conformable in the layer of isotropic the mesophase (M)
orientation of lamellar formations matrix (LIM) and the formation
FUEL,
1991,
originating existing in
Vol 70, February
237
Effect of processing
on mesophase
structure:
Z. Weishauptov$
and J. Medek
be identified on fracture surfaces by means of SEM. In contrast to brittle cokes, which fracture predominantly along selected planes with a minimum of cohesive energy, the mesophase with uniform distribution of cohesive energy exhibits random fracture planes, so that the lamellar formations can be detected by SEM only to a limited extent, although typical orientated structures occur quite regularly throughout the whole mesophase volume. The main importance of this orientated structure lies in the fact that it already includes most features of the structure of all products of subsequent thermal
Figure 8 Envelope from coal tar pitch
surface
of macroscopic
formation
of mesophase
Figure 10 Orientation of lamellar formations of mesophase blocks perpendicular to the surface in unconformable planes and their contact in the seam (SM)
Figure 9 individual
Arrangement of bulk mesophase from coal tar pitch blocks (B) separated by seams (SM)
into
distinct boundary is formed as a ‘seam’. Figure 10 illustrates that the lamellar formations in these blocks are also oriented perpendicular to the block surface. Another form of seam is represented in Figure 21, showing the mutual positioning and contact of two blocks whose lamellae are of a disc type. Figure 12 shows their detailed organization in a seam where the lamellae are zipper-like. DISCUSSION The lamellar structure and its orientated arrangement which cause the optical anisotropy of the mesophase can
238
FUEL, 1991, Vol 70, February
Figure 11 Contact of disc-like lamellar formations in seams (SM); (A), perpendicular orientation of the lamellar formations to the surface of mesophase blocks
Effect
of processing
on mesophase
structure:
Z. Weishauptovri
and J. Medek
presents the texture of a fracture surface of block mesophase formed by lamellar strands of average thickness 0.7 pm, each representing a set of more subtle lamellar formations whose parallelism and compactness are demonstrated on several cleavage faces perpendicular to the fracture surface. According to the proposed classification3, these more subtle formations correspond to the lamellar packets whose arrangement is shown, as a result of their greater ease of cleavage in Figure 15. The average thickness of the packets determined in > 100 instances is 0.10+_0.005 Drn, i.e. with little variance. The
Figure 12 formations
Organization of unidirectionally arranged disc-like lamellar of two blocks (B) in a seam (SM) resembling a zipper
Figure 14 Texture of fracture surface of semicoke from the texture of mesophase in Figure 13
Figure 13 by lamellar substructure
(560°C) resulting
Texture of fracture surface of a block mesophase formed strands of average thickness 0.7 pm. Arrow denotes the of strands from tightly compact plane-parallel lamellae
treatment, as demonstrated by the identical nature of the mesophase (Figure 13) and the semicoke produced at 560°C (Figure 14). The textures determined by SEM give an idea of the different mechanisms of development of mesophase in the bulk phase and in a thin layer. Lamellar
structure
of mesophase
Figures 5,6 and 7 showed that the mesophase is formed of a lamellar system whose individual formations are orientated perpendicular to the surface. Under the magnification used, it was possible to observe a substructure characterized by uniform arrangement and orientation in all the lamellar formations. Figure 13
Figure 15 Orientation of lamellar packets perpendicular to the surface. Packets of 0.1 pm mean thickness (A) are composed of tightly compacted lamellae (E) of 0.01 pm mean thickness. Arrow denotes a loose lamella. (C) denotes places with transverse fractures at an angle of 104’
FUEL,
1991,
Vol 70, February
239
Effect of processing
on mesophase
structure:
2. Weishauptovri
Figure 16 Fanlike arrangement of lamellar packets with transverse fracture (C) at an angle of 104”. Arrow denotes a translucent lamella of ~0.01 q thickness
packets cleaved over their depth are perfectly parallel and orientated perpendicular to the surface of the mesophase block. The packets are composed of closely adjacent lamellae whose thickness can be estimated to be ~0.01 pm. This thinness is also indicated by the translucence exhibited by one loose lamella in the photomicrograph. The uniformity of thickness of all packets indicates that each packet is composed of approximately the same number of thinner lamellae. As the cohesive energy is greater at the basic surfaces of these lamellae than at the cleavage faces of two neighbouring packets, a lamellar packet of 0.1 pm thickness can be considered as a macroscopic texture unit of mesophase. Another arrangement of packets of equal thickness (0.1 pm) is shown in Figure 16, again with a translucent lamella. An interesting phenomenon which is demonstrated in Figures 15 and 16 is the transverse breaking of edges of the packets at practically the same angle. Evaluation of the images of > 150 packets on various fracture surfaces of both samples of mesophase yielded an angle of 104_t3”, the variance being mainly due to planar distortion of the images of lamellae in the microscopic projection. As this value of the angle is repeated systematically and no exception has been found, it can be considered a characteristic textural parameter of the mesophase.
and J. Medek
formation in the form of a mesophase block. It can be presumed that because of the size of the block and owing to cross-linking I*, the structure within the block becomes only slightly mobile or immobile. Therefore the mesophase block increases in size in the same way as the individual spherule, only by means of diffusion of large planar polyaromatic molecules towards its surface, where the action of surface forces results in their orientated sorption”, characterized by conformation of the molecules on the surface with those in the surface, as schematically represented in Figure 17. Their mutual attachment forms a new mesophase layer at the surface of the block, the molecular stacks retaining their orientated position perpendicular to the surface as shown in Figures 10 and IS, and the mesophase block gradually grows. In the final phase of mesophase development the individually growing blocks will approach one another sufficiently closely to begin surface contact. These contacts, however, do not lead to structural interconnections, because the mobility of the structures is minimized by large intermolecular forces; moreover, mutual contact between the block surfaces occurs mainly at unconformable planes, as can be seen in the photomicrographs of fracture surfaces (Figures 11 and 12), where the lamellar structures of the individual blocks are sharply confined by seams at which an instantaneous change of orientation can be observed. This situation is schematically represented in Figure f8. Development of mesophase in a thin layer
The following processes can be considered for the transformation of matrix into mesophase in a layer: (1) the formation of spherules smaller than the actual thickness of the layer, which, after contacting the bulk mesophase, interconnect with it to give new surface formations whose mobile structure is reorganized in accordance with the structure of the mesophase; (2) direct linking of stacks of planar polyaromatic molecules to the structure of bulk mesophase by the same mechanism as in the case of mesophase blocks; (3) combination of (1) and (2). In all cases a local depletion of the matrix occurs, its layer becomes discontinuous, and mesophase islets are formed. This processeontinues until the maximum depletion of the matrix, whose residues form miniature lenses or narrow strands at the surface of the mesophase (Figures 3, 4 and 7). The geometry of these formations isotropic
matrix
Development of mesophase in the bulk phase
The initial step in the formation of continuous mesophase in the bulk phase is represented by spherules which increase in size by both individual growth and coalescence. As soon as the shear forces acting on the spherule in the gravitational field become equal to the interfacial tension yM,iMat the contact area of mesophase (M) and isotropic matrix (IM), the spherule reaches its limiting size; on further growth, yMI,iM will not be able to maintain the spherical shape of the particle, which is therefore deformed to give a new unsymmetrical
240
FUEL, 1991, Vol 70, February
surface
of flesophase
Figure 17 Scheme of successive orientation of planar polyaromatic molecules near the surface of block mesophase
Effect of processing \
Y
of lamellar
formations
structure:
Z. Weishauptovii
and J. Medek
authors have adopted the simple method of weighing droplets at 420°C to establish the value yIM= 43 mN m- ’ for the pyrolysis oil. This falls well within the range of surface tensions of pitch measured earlier”. The value of interfacial tension YM,,Mat 440°C given by Smith et al 21 varies from 0.003 to 0.16 mN m-‘, depending on the viscosity of the pitch (0.2 to 10 N s mm2). At the viscosity of about 2 N s rnd2 for the pyrolysis oil mesophase system, and a mesophase content of 93 vol%, it can be accepted that yu,+,=0.02 mN m- ‘. Introducing these values mto the simplified relation of Antonow, namely yi” =y,,k2 + y$,,,, leads to yM= 44.9 mN m-‘, which agrees with the relation for spreading, 44.9 > (43 + 0.02). The change of y,Mduring the transformation of matrix into mesophase can be determined from the condition of equilibrium characterizing the stability of the surface formations: y;Mcos 8,, + Y~IM COS eM,IM=Y~ COS eM9 where 8 is the contact angle. Neglecting the term y$,,~ COS &M giVeS y;M = yM COS oM/COS &,. If Y;M > yM,then COS &., > COS f&, and hence From Figures 1-4 it can be seen that this eM
seam
seam
orientation
on mesophase
of blocks
Figure 18 Schematic arrangement of blocks within bulk mesophase. Upper diagram, contact of blocks with differently orientated structures at seams; lower diagram, relative positions of lamellar formations of individual blocks
and of those formed during the transformation itself indicates the existence of a two-component liquid system, and the curvature at the perimeter of contact of the two components provides evidence of the effect of surface tension in the process of their formation. From the surface texture represented in Figures 1-4 it is evident that, immediately after feeding, the isotropic matrix forms a continuous thin layer on the surface of the mesophase whose thickness can be estimated to be 0.3 pm. As the layer appears to be of uniform thickness over the whole surface, it can be presumed that it was formed by the mechanism of spreading, for which the condition is YM> (yIM+ Y~,,~), where JJM,y,M,and yM,,Mare the surface tensions of the mesophase, isotropic matrix and interface, respectively. During the transformation, however, an equilibrium is established between these surface tensions, which must be related to their magnitudes. As the mesophase represents the final product of the transformation, it must be presumed that yM remains constant but that the initial surface tension yIMis changed to yiMso that yM< (y,, + Y;,,~). The experimental data necessary for determination of all the surface tensions of the pyrolysis oil are not available. However, it can be presumed that, as holds for the coal tar pitch, it is true that yhl> yIMat 460°C and that Y~,,~ is also of the same order of magnitude. The
CONCLUSION With the aid of SEM it has been shown that structural differences exist between the mesophase prepared from coal tar pitch by pyrolysis in the bulk phase and that prepared from pyrolysis oil by stepwise feeding. In the former instance, individual mesophase blocks with unidirectionally arranged lamellar formation are produced, the planes of the individual blocks having different orientations, so the mesophase - as a whole - does not possess an unambiguous orientated structure. In the latter instance the mesophase grows layer by layer, and in these layers the uniformly oriented lamellar groupings capable of forming macroscopic units with unidirectionally orientated structure typical of needle coke are gradually linked. Three systematically repeating textural features have been found in the mesophase prepared by both methods: lamellar packets with a mean thickness of 0.1 pm, an angle of transverse fracture of 104, and perpendicular orientation of the packets with respect to the surface. Practically all phases of formation and growth of the mesophase have been observed in the transformation of the isotropic matrix of pyrolysis oil.
ACKNOWLEDGEMENTS The authors are very grateful to Ing. B. Kolman and Mrs Kozumplikova of the Institute of Geology and Geotechnics, Czechoslovak Academy of Sciences, and to Ing. I. Mohyla of the Bateria Slany Research Centre for producing perfect photomicrographs and for valuable collaboration in the selection of characteristic textures.
FUEL, 1991, Vol 70, February
241
Effect of processing on mesophase structure: Z. Weishauptob REFERENCES 1
2 3 4 5 6 7 8 9 10
242
Brooks, J. D. and Taylor, G. H. In ‘Chemistry and Physics of Carbon’ (Ed. P. L. Walker, Jr), Marcel Dekker, New York, 1968, Vol. 4, p. 243 Oberlin, A. Carbon 1985, 23, 522 Weishauptova, Z. and Medek, J. Fuel 1985,64, 999 Bourrat, X., Oberlin, A. and Escalier, J. C. Fuel 1986,65,1490 Imamura, T. and Nakamizo, M. Carbon 1979, 17, 507 Marsh, H., de Lopez, H. and Qian, Z. Fuel 1984, 63, 1594 Kovac, C. A. and Lewis, I. C. Carbon 1978, 16,433 Imamura, T., Yamada, Y., Shoichi, 0. and Honda, H. Carbon 1978, 16,481 Delhaes, P., Rouillon. J. C., Fug, G. and Singer, L. S. Carbon 1979, 17,435 Yamada, Y., Shiraishi, M. and Yamakawa, T. In ‘Carbon ‘86’,
FUEL, 1991, Vol 70, February
11 12 13 14 15 16 17 18
19 20
21
and J. Medek Baden-Baden 1986, Extended Abstracts, p. 82 Zimmer, J. E. and Weitz, R. L. Cm-bon 1988,26, 579 Jenkins. J. C. and Jenkins. G. M. Carbon 1983. 21. 473 White, J. L. and Price, R.‘J. Carbon 1974, 12, 321 Singer, L. S. Fuel 1981, 60, 839 Jenkins, J. C. and Jenkins, G. M. Fuel 1981, 60, 883 Mochida, I., Oyama, T. and Korai, Y. Carbon 1988,26,49 Czechoslovak Patent No. 247846/1986 Marsh, H. and Smith, J. In ‘Analytical Methods for Coal and Coal Products’ (Ed. C. Karr, Jr), Academic Press, New York, 1978, Vol. II, p. 371 Dell, M. B. Carbon 1986,24,437 Greenalgh, E. and Moyse, M. In ‘3rd Conference on Industrial Carbon and Graphite’, Society of Chemical Industry, London, 1971, p. 539 Smith, W. G., White, J. L. and Buechler, M. Carbon 1985,23,117
Weishauptov6
technology
on
and Jifi Medek
Institute of Geotechnics, Czechoslovak Academy Czechoslovakia (Received 18 July 1989; revised 27 April 1990)
of Sciences,
182 09 Prague,
the laboratory scale, mesophase samples were prepared from coal tar pitch by pyrolysis in the bulk phase and from pyrolysis oil by feeding into a reactor. From texture analysis by SEM, it was shown that in the former case the mesophase consists of isolated blocks in which the unidirectionally arranged lamellar formations are contiguous to one another unconformably, thus forming a mosaic structure. In the latter case the mesophase grows layer by layer, the lamellar formations being contiguous to one another conformably to give a mesophase with domain and flow structure. Common systematic features were found in the two types of mesophase: the orientation of lamellae perpendicular to the surface, lamellar packets of 0.1 pm thickness, and the angle of their transverse fracture, 104”. On
(Keywords:
coal structure;
mesophase;
scanning
electron
microscopy)
The decisive phase in the production of synthetic graphite on the basis of coal tar pitch or heavy products from distillation of crude petroleum consists of spontaneous transformation of the original isotropic raw material into anisotropic mesophase within the temperature interval 35G5Oo”C. Polarized light microscopy, electron microscopy, and X-ray diffraction analysis’-4 have shown that the anisotropy of the mesophase is caused by the deposition of planar polyaromatic molecules into parallel stacks whose mutual positioning forms an oriented structure. The primary configurations of the mesophase are individual spherical particles (spherules) with a regular internal structufe’.5*6 which is most often characterized, according to Brooks and Taylor’, by the approximately perpendicular orientation of molecular stacks to the surface of the spherule. The spherules retain their internal structure during both individual growth and mutual coalescence; in the latter process their structures become interconnected and, within the new individual units, either partly or completely reorganized. For large-area formation of mesophase, however, it must be assumed that the structural arrangement is more complex, and that the reorganization of the original structures is not uniform. Moreover, it must be taken into account that the possible action of external factors (magnetic field7-‘l, laminar flow”, percolation of gas bubbles13-16) will exert secondary effects on the structure of the mesophase, mainly on the microscopic scale, which can, under suitable experimental conditions, be utilized for the preparation of mesophase with properties needed in subsequent treatment. Another factor which distinctly influences the nature of the mesophase structure is the technology used in its preparation. In the present study the course of mesophase development under laboratory conditions was examined, using two different preparation methods: (1) in the bulk phase with a single filled reactor; (2) in a thin layer with gradual feeding of the raw material.
0016_236I/91/020235~8 #c! 1991 Butterworth-Heinemann
Ltd.
The formation of mesophase structure was followed by scanning electron microscopy (SEM). EXPERIMENTAL Mesophase was prepared from 50 g of the parent materials whose basic characteristics are given in Table I. For the preparation of mesophase in the bulk phase, industrially produced coal tar pitch was used. The quinoline-insoluble matter (QI) was removed by filtration at 250°C through a combined filter whose inlet section consisted of a porous carbon plate with pores of 60-90 pm diameter prepared by carbonization of filter paper, the outlet section being a sintered glass plate with pores of 160-200 ,um diameter through which the pitch passed in laminar flow’ 7, The filtered pitch was transformed into mesophase by heating to 460°C under nitrogen at normal pressure for 4 h. For the preparation of mesophase in a thin layer by the feed method, pyrolysis oil obtained from distillation of crude petroleum was used. This oil has the advantage of having a low viscosity and negligible QI content. The oil was added under nitrogen atmosphere drop by drop at 20 s intervals into the reactor, whose temperature was
Table 1
Basic characteristics
of raw materials Coal tar pitch ______~
Density (g cmw3) Softening point (KS) (“C) Solidification point (“C) QI (wt%) Conradson carbonization residue Water (wt%) Ash (wt%) C (wt% daf) H (wt% daf) N (WI% daf) S (wt% daf) __ .~__.. _
FUEL,
1.08
1.28 68
(wt%)
._~
1991,
4 _
9.7 52.8 0.3 0.2 93.3 4.1 0.6 0.4 .~__~I-_I
Vol 70,
Pyrolysis oil
25.1 _ 0.005 91.9 7.4 0.1 0.2
February
-
235
Effect
of processing
on mesophase
structure:
Z. Weishauptovi
and J. Medek
RESULTS The mechanism of the transformation of pyrolysis oil into mesophase in a thin layer is illustrated by SEM photographs in Figures l-4, demonstrating the surface structure of a product obtained by the feed method at 460°C. Figure 1 gives a panoramic view of the envelope surface formed after a certain time interval from the moment of feeding of the pyrolysis oil onto the mesophase surface. The voluminous mass of homogeneous mesophase, whose growth has already been interrupted by the
Figure 1 Envelope surface of mesophase (M) covered discontinuous layer of isotropic matrix of pyrolysis oil (IM)
with
a
Figure 3 Various phases of the transformation of isotropic matrix (IM) into mesophase (M) and production of large formations by gradual coalescence
of isotropic matrix (IM) in surface Figure 2 Stepwise transformation layer into mesophase (M). Formation and growth of mesophase islets and their coalescence (denoted by arrow)
kept at 460°C each drop being instantaneously preheated to 420°C. After the last dose the temperature of the reactor was abruptly reduced, interrupting the development of mesophase. The texture analysis was carried out with a microanalyser at an accelerating voltage of 15-25 kV and a current of 10-‘” A. The observations were realized on fractured and enveloping surfaces of lump samples gold-coated in an argon atmosphere. The photomicrographs shown represent the statistically most frequent characteristic structures of the objects analysed.
236
FUEL, 1991, Vol 70, February
Figure 4 Advanced phase of transformation of isotropic matrix (IM) into mesophase (M), showing existence of lenses (L) and isolated islets (I) on the mesophase surface
Effect of processing
Figure 5 lamellar
Relief of fracture surface of mesophase structure with respect to the surface
Figure 6 mesophase
Perpendicular arrangement with respect to the surface
and orientation
of lamellar
formations
of
on mesophase
structure:
Z. Weishauptovri
and J. Medek
boundaries between both liquids. During transformation of the pyrolysis oil into mesophase, these holes grow and finally associate to give large-area mesophase formations of various sizes and shapes (Figure 3). At the same time, the layer is gradually exhausted, and its residues form isolated islets on the surface of the mesophase (Figure 4), their relief being mainly determined by surface unevenness of the mesophase. If this residue is sufficiently small, then a ‘lens’ of pyrolysis oil is formed on the mesophase surface by the action of surface tension. Figure 5 represents the relief of a fracture surface at a place where a bulge was formed by gradual growth of mesophase from the surface layers. The photomicrograph shows that lamellar formations of all sizes are orientated perpendicular to the surface, and this orientation remains preserved even when there are alterations of the surface curvature, as illustrated in Figure 6. Figure 7 shows the arrangement of lamellar formations that originated in the surface layer during the transformation of the isotropic matrix into mesophase. The orientation of this formation is perpendicular to the surface of the layer and, at the same time, in accord with the orientation of lamellar formations of the mesophase with which they are interconnected. The unidirectional orientation of the lamellae extends over a depth many times the thickness of one sprayed layer. Figure 8 shows the envelope surface of a macroscopic formation of mesophase formed spontaneously from the coal tar pitch in the bulk phase. The envelope surface is completely compact, and only its unevenness and bulges may suggest the shape of the mesophase periphery. Figure 9 shows the texture of mesophase under this surface, with characteristic individual blocks of different sizes which fill the mesophase volume and have common contact surfaces. Inside these blocks the lamellar formations have a unidirectional arrangement, the orientation, however, being different in the individual blocks, so that at the contact of two blocks an optically
of
cooling, is characterized by rugged surface with numerous bulges of different sizes, showing a smooth face. The mesophase is covered with a discontinuous layer of pyrolysis oil with randomly localized holes filled with newly formed mesophase which protrudes from the layer. Figure 2 shows the same holes of the layer observed from above, perpendicular to the surface. Their shape and size reflect various phases of development of the mesophase in a thin layer and its coalescence with the bulk mesophase. The holes, whose size varies from 0.2 to 140 pm, have approximately circular contours, which indicates the effect of surface tension with formation of
Figure 7 Conformable in the layer of isotropic the mesophase (M)
orientation of lamellar formations matrix (LIM) and the formation
FUEL,
1991,
originating existing in
Vol 70, February
237
Effect of processing
on mesophase
structure:
Z. Weishauptov$
and J. Medek
be identified on fracture surfaces by means of SEM. In contrast to brittle cokes, which fracture predominantly along selected planes with a minimum of cohesive energy, the mesophase with uniform distribution of cohesive energy exhibits random fracture planes, so that the lamellar formations can be detected by SEM only to a limited extent, although typical orientated structures occur quite regularly throughout the whole mesophase volume. The main importance of this orientated structure lies in the fact that it already includes most features of the structure of all products of subsequent thermal
Figure 8 Envelope from coal tar pitch
surface
of macroscopic
formation
of mesophase
Figure 10 Orientation of lamellar formations of mesophase blocks perpendicular to the surface in unconformable planes and their contact in the seam (SM)
Figure 9 individual
Arrangement of bulk mesophase from coal tar pitch blocks (B) separated by seams (SM)
into
distinct boundary is formed as a ‘seam’. Figure 10 illustrates that the lamellar formations in these blocks are also oriented perpendicular to the block surface. Another form of seam is represented in Figure 21, showing the mutual positioning and contact of two blocks whose lamellae are of a disc type. Figure 12 shows their detailed organization in a seam where the lamellae are zipper-like. DISCUSSION The lamellar structure and its orientated arrangement which cause the optical anisotropy of the mesophase can
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Figure 11 Contact of disc-like lamellar formations in seams (SM); (A), perpendicular orientation of the lamellar formations to the surface of mesophase blocks
Effect
of processing
on mesophase
structure:
Z. Weishauptovri
and J. Medek
presents the texture of a fracture surface of block mesophase formed by lamellar strands of average thickness 0.7 pm, each representing a set of more subtle lamellar formations whose parallelism and compactness are demonstrated on several cleavage faces perpendicular to the fracture surface. According to the proposed classification3, these more subtle formations correspond to the lamellar packets whose arrangement is shown, as a result of their greater ease of cleavage in Figure 15. The average thickness of the packets determined in > 100 instances is 0.10+_0.005 Drn, i.e. with little variance. The
Figure 12 formations
Organization of unidirectionally arranged disc-like lamellar of two blocks (B) in a seam (SM) resembling a zipper
Figure 14 Texture of fracture surface of semicoke from the texture of mesophase in Figure 13
Figure 13 by lamellar substructure
(560°C) resulting
Texture of fracture surface of a block mesophase formed strands of average thickness 0.7 pm. Arrow denotes the of strands from tightly compact plane-parallel lamellae
treatment, as demonstrated by the identical nature of the mesophase (Figure 13) and the semicoke produced at 560°C (Figure 14). The textures determined by SEM give an idea of the different mechanisms of development of mesophase in the bulk phase and in a thin layer. Lamellar
structure
of mesophase
Figures 5,6 and 7 showed that the mesophase is formed of a lamellar system whose individual formations are orientated perpendicular to the surface. Under the magnification used, it was possible to observe a substructure characterized by uniform arrangement and orientation in all the lamellar formations. Figure 13
Figure 15 Orientation of lamellar packets perpendicular to the surface. Packets of 0.1 pm mean thickness (A) are composed of tightly compacted lamellae (E) of 0.01 pm mean thickness. Arrow denotes a loose lamella. (C) denotes places with transverse fractures at an angle of 104’
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Effect of processing
on mesophase
structure:
2. Weishauptovri
Figure 16 Fanlike arrangement of lamellar packets with transverse fracture (C) at an angle of 104”. Arrow denotes a translucent lamella of ~0.01 q thickness
packets cleaved over their depth are perfectly parallel and orientated perpendicular to the surface of the mesophase block. The packets are composed of closely adjacent lamellae whose thickness can be estimated to be ~0.01 pm. This thinness is also indicated by the translucence exhibited by one loose lamella in the photomicrograph. The uniformity of thickness of all packets indicates that each packet is composed of approximately the same number of thinner lamellae. As the cohesive energy is greater at the basic surfaces of these lamellae than at the cleavage faces of two neighbouring packets, a lamellar packet of 0.1 pm thickness can be considered as a macroscopic texture unit of mesophase. Another arrangement of packets of equal thickness (0.1 pm) is shown in Figure 16, again with a translucent lamella. An interesting phenomenon which is demonstrated in Figures 15 and 16 is the transverse breaking of edges of the packets at practically the same angle. Evaluation of the images of > 150 packets on various fracture surfaces of both samples of mesophase yielded an angle of 104_t3”, the variance being mainly due to planar distortion of the images of lamellae in the microscopic projection. As this value of the angle is repeated systematically and no exception has been found, it can be considered a characteristic textural parameter of the mesophase.
and J. Medek
formation in the form of a mesophase block. It can be presumed that because of the size of the block and owing to cross-linking I*, the structure within the block becomes only slightly mobile or immobile. Therefore the mesophase block increases in size in the same way as the individual spherule, only by means of diffusion of large planar polyaromatic molecules towards its surface, where the action of surface forces results in their orientated sorption”, characterized by conformation of the molecules on the surface with those in the surface, as schematically represented in Figure 17. Their mutual attachment forms a new mesophase layer at the surface of the block, the molecular stacks retaining their orientated position perpendicular to the surface as shown in Figures 10 and IS, and the mesophase block gradually grows. In the final phase of mesophase development the individually growing blocks will approach one another sufficiently closely to begin surface contact. These contacts, however, do not lead to structural interconnections, because the mobility of the structures is minimized by large intermolecular forces; moreover, mutual contact between the block surfaces occurs mainly at unconformable planes, as can be seen in the photomicrographs of fracture surfaces (Figures 11 and 12), where the lamellar structures of the individual blocks are sharply confined by seams at which an instantaneous change of orientation can be observed. This situation is schematically represented in Figure f8. Development of mesophase in a thin layer
The following processes can be considered for the transformation of matrix into mesophase in a layer: (1) the formation of spherules smaller than the actual thickness of the layer, which, after contacting the bulk mesophase, interconnect with it to give new surface formations whose mobile structure is reorganized in accordance with the structure of the mesophase; (2) direct linking of stacks of planar polyaromatic molecules to the structure of bulk mesophase by the same mechanism as in the case of mesophase blocks; (3) combination of (1) and (2). In all cases a local depletion of the matrix occurs, its layer becomes discontinuous, and mesophase islets are formed. This processeontinues until the maximum depletion of the matrix, whose residues form miniature lenses or narrow strands at the surface of the mesophase (Figures 3, 4 and 7). The geometry of these formations isotropic
matrix
Development of mesophase in the bulk phase
The initial step in the formation of continuous mesophase in the bulk phase is represented by spherules which increase in size by both individual growth and coalescence. As soon as the shear forces acting on the spherule in the gravitational field become equal to the interfacial tension yM,iMat the contact area of mesophase (M) and isotropic matrix (IM), the spherule reaches its limiting size; on further growth, yMI,iM will not be able to maintain the spherical shape of the particle, which is therefore deformed to give a new unsymmetrical
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FUEL, 1991, Vol 70, February
surface
of flesophase
Figure 17 Scheme of successive orientation of planar polyaromatic molecules near the surface of block mesophase
Effect of processing \
Y
of lamellar
formations
structure:
Z. Weishauptovii
and J. Medek
authors have adopted the simple method of weighing droplets at 420°C to establish the value yIM= 43 mN m- ’ for the pyrolysis oil. This falls well within the range of surface tensions of pitch measured earlier”. The value of interfacial tension YM,,Mat 440°C given by Smith et al 21 varies from 0.003 to 0.16 mN m-‘, depending on the viscosity of the pitch (0.2 to 10 N s mm2). At the viscosity of about 2 N s rnd2 for the pyrolysis oil mesophase system, and a mesophase content of 93 vol%, it can be accepted that yu,+,=0.02 mN m- ‘. Introducing these values mto the simplified relation of Antonow, namely yi” =y,,k2 + y$,,,, leads to yM= 44.9 mN m-‘, which agrees with the relation for spreading, 44.9 > (43 + 0.02). The change of y,Mduring the transformation of matrix into mesophase can be determined from the condition of equilibrium characterizing the stability of the surface formations: y;Mcos 8,, + Y~IM COS eM,IM=Y~ COS eM9 where 8 is the contact angle. Neglecting the term y$,,~ COS &M giVeS y;M = yM COS oM/COS &,. If Y;M > yM,then COS &., > COS f&, and hence From Figures 1-4 it can be seen that this eM
seam
seam
orientation
on mesophase
of blocks
Figure 18 Schematic arrangement of blocks within bulk mesophase. Upper diagram, contact of blocks with differently orientated structures at seams; lower diagram, relative positions of lamellar formations of individual blocks
and of those formed during the transformation itself indicates the existence of a two-component liquid system, and the curvature at the perimeter of contact of the two components provides evidence of the effect of surface tension in the process of their formation. From the surface texture represented in Figures 1-4 it is evident that, immediately after feeding, the isotropic matrix forms a continuous thin layer on the surface of the mesophase whose thickness can be estimated to be 0.3 pm. As the layer appears to be of uniform thickness over the whole surface, it can be presumed that it was formed by the mechanism of spreading, for which the condition is YM> (yIM+ Y~,,~), where JJM,y,M,and yM,,Mare the surface tensions of the mesophase, isotropic matrix and interface, respectively. During the transformation, however, an equilibrium is established between these surface tensions, which must be related to their magnitudes. As the mesophase represents the final product of the transformation, it must be presumed that yM remains constant but that the initial surface tension yIMis changed to yiMso that yM< (y,, + Y;,,~). The experimental data necessary for determination of all the surface tensions of the pyrolysis oil are not available. However, it can be presumed that, as holds for the coal tar pitch, it is true that yhl> yIMat 460°C and that Y~,,~ is also of the same order of magnitude. The
CONCLUSION With the aid of SEM it has been shown that structural differences exist between the mesophase prepared from coal tar pitch by pyrolysis in the bulk phase and that prepared from pyrolysis oil by stepwise feeding. In the former instance, individual mesophase blocks with unidirectionally arranged lamellar formation are produced, the planes of the individual blocks having different orientations, so the mesophase - as a whole - does not possess an unambiguous orientated structure. In the latter instance the mesophase grows layer by layer, and in these layers the uniformly oriented lamellar groupings capable of forming macroscopic units with unidirectionally orientated structure typical of needle coke are gradually linked. Three systematically repeating textural features have been found in the mesophase prepared by both methods: lamellar packets with a mean thickness of 0.1 pm, an angle of transverse fracture of 104, and perpendicular orientation of the packets with respect to the surface. Practically all phases of formation and growth of the mesophase have been observed in the transformation of the isotropic matrix of pyrolysis oil.
ACKNOWLEDGEMENTS The authors are very grateful to Ing. B. Kolman and Mrs Kozumplikova of the Institute of Geology and Geotechnics, Czechoslovak Academy of Sciences, and to Ing. I. Mohyla of the Bateria Slany Research Centre for producing perfect photomicrographs and for valuable collaboration in the selection of characteristic textures.
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