- Email: [email protected]
Pyrolysis of coal-tar pitches: kinetics of conversion to coke and iodine adsorption by mesophase
Fuel Processing Technology, 20 (1988) 197-205
197
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
PYROLYSIS OF COAL-TAR PITCHES : KINETICS OF CONVERSION TO COKE AND IODINE ADSORPTION BY MESOPHASE R. MENENDEZ, H. MARSH, C. CALVERT and T. TAKEKAWA Northern Carbon Research Laboratories, School of Chemistry, University of Newcastle upon Tyne, Newcastle upon Tyne, NEI 7RU, U.K. SUMMARY Pitch is the parent material for cokes of major commercial importance. These cokes are g r a p h i t i z a b l e as a r e s u l t of formation of l i q u i d crystals (mesophase) during pitch p y r o l y s i s . The concept of molecular cohesion energy to create mesophase is described. Extents of formation of mesophase from blends of d i f f e r e n t pitches do not f o l l o w an a d d i t i v e r ule. Mesophasehas chemical properties s i m i l a r to the parent pitch. Iodine adsorption (up-take) by mesophases from d i f f e r e n t pitches indicates that they s o l i d i f y d i f f e r e n t l y , HTT 500° to 600°C. INTRODUCTION Coke materials derived from petroleum and c o a l - t a r pitches have important i n d u s t r i a l uses.
The delayed coker dominantly converts petroleum pitch of
various q u a l i t i e s to shot-, regular- or sponge-, and needle-cokes(Ref.l}.Although shot coke only has value as a f u e l , regular coke finds major use as a f i l l e r coke in the carbon anodes of the Hall c e l l of aluminium production and needlecoke is the f i l l e r
coke f o r the graphite electrodes of the steel industry.
Coal-
ta r pitch is a suitable binder material fo r the carbon anodes and the graphite electrode with petroleum pitch having a f u r t h e r application as an impregnation pitch to densify the graphite electrode. As discussed in t h i s Conference and elsewhere CRef.2,4), the cokes from petroleum and c o a l - t a r pitch are of g r a p h i t i z a b l e q u a l i t y . The a b i l i t y of cokes to g r a p h i t i z e on heat treatment to >2500°C is dependent upon the existence of a c r y s t a l l i n i t y established within the cokes during pitch pyrolysis. to use pitch as a source of f i l l e r
The a b i l i t y
coke and binder is t o t a l l y dependent upon the
pitch forming a ' c r y s t a l l i n e ' s o l i d coke product.
C r y s t a l l i n i t y is introduced
to the pyrolyzing pitch system as a d i s c o t i c , aromatic, nematic l i q u i d crystal phase created from the molecular constituents of the pitch by association of molecules of appropriate sizes at appropriate temperatures.
The c r y s t a l l i n i t y of
the l i q u i d crystal phase (called mesophase) becomes the c r y s t a l l i n i t y of the so lid green coke by a process of molecular growth (cross-linking=polymerization) associated with time and temperature. The conditions of molecular growth, association to form mesophase and s o l i d i f i c a t i o n to coke are the objectives of this study.
0378-3820/88/$03.50
©
1988 Elsevier Science Publishers B.V.
198
MESOPHASE GENERATION Mesophase is generated only when quite r e s t r i c t i v e chemical and physical conditions are operating within a pyrolyzing pitch: (a) the system must remain f l u i d , with decreasing v i s c o s i t y , at temperatures of about 400°C and r i s i n g (b) the system must be of low v o l a t i l i t y
(c) the molecular constituents must be
planar and r e s i s t a n t to fragmentation (d) molecular growth, by condensation, must occur over a l i m i t e d temperature range, at about 400°C, within a f l u i d phase of low enough v i s c o s i t y to f a c i l i t a t e molecular movement (e) molecular movement and c o l l i s i o n generate molecular associations with s u f f i c i e n t cohesion energy to prevent subsequent thermal dissociation (f) the cohesion energy is responsible fo r the s t a b i l i t y of this newly created nematic l i q u i d crystal (mesophase) system (g) cohesion energy is a function of molecular size, the degree of molecular p l a n a r i t y , the presence of heteroatoms within the molecule so influencing molecular p o l a r i t y and structural defects, i . e . , a t o m vacancies within the molecule. The development of mesophase within a pitch can be described in terms of Figures l a , Ib, which are diagrams i l l u s t r a t i n g how molecular cohesion energy and average molecular weight i n t e r a c t during a pitch pyrolysis.
With isothermal
pyrolysis at e.g.,400°C or non-isothermal treatment (heating rate of 5°C min-1), increasing time and temperature cause an increase in average molecular weight and cohesion energy between molecules, curves AB and CD of Figure la.
At
position P, cohesion energy exceeds k i n e t i c energy, association of two molecules is established and a l i q u i d crystal is generated.
When a v a i l a b l e pitches are
studied, two bands of properties are created, Figure Ib, EF (cohesion energy) i
and GH (a.m.w.) giving an area (or window), P , of conditions of mesophase generation. This window is of l i m i t e d size extending over a few tens of degrees c e n t i grade only. the
Nevertheless, the range is of c r i t i c a l
importance in determining
size, shape and coalescence of the l i q u i d crystals.
At constant heating
rates, the mesophase formed at lower temperatures w i l l be of smaller size of optical texture (mosaics)(Ref. 3) with that at l i g h e r temperatures being of larger size, i.e.,domains or n e e d l e - l i k e (Ref. 3). Greinke (Ref.5) Greinke and Singer (Ref.6) using Gel Permeation ChromatoQraphy and high temperature c e n t r i f u g a t i o n have established that the molecules which c o n s t i t u t e mesophase, i n i t i a l l y , are the higher molecular f r a c t i o n of the pyrolyzing pitch, the mesophase growing in extent as larger molecules are generated by condensation reactions. entrapped within the mesophase system.
Smaller molecular weight materials become At 400°C, over 15 hours, the average
molecular weight in mesophase did not change, i l l u s t r a t i n g an essential s t a b i l i t y of the larger sized molecules (Ref.6). However, the mesoDhase as established is
199
Molecular Cohesion Energy
Average Molecular Weight
B!
IIH/ /
//2 !
,
.~//G/"¢'''~ (a)
(b)
Time and Temperature
Figures l a , lb. Diagram of molecular cohesion energy and average molecular weight during mesophase development.
chemically reactive and f u r t h e r polj~nerization/condensation/cross-linkage occurs with increasing time and temperature u n t i l the mesophase ceases to be a f l u i d , becoming p l a s t i c and f i n a l l y becoming a v i s c o - e l a s t i c solid.Mesophasesdiffer in t h e i r s o l i d i f i c a t i o n behaviour as they do in t h e i r generation. Pitch materials as binders, during c a l c i n a t i o n , a l l pass through the stages of a f l u i d i s o t r o p i c pitch, a f l u i d mesophase leading to the s o l i d binder coke bridge.
During c a l c i n a t i o n , the binder pitch and mesophase w i l l flow within the
green anode mix eventually establishing 'anode structure'
Variations of flow
properties of pitches during c a l c i n a t i o n may influence anode and electrode properties.
This paper therefore describes studies of rates of formation of
mesophase from i n d i v i d u a l pitches, blends of pitches as well as f o l l o w i n g the s o l i d i f i c a t i o n process within mesophase. MESOPHASE FROM INDIVIDUAL PITCHES Seven i n d u s t r i a l pitches were used, Nos. I and 2 being petroleum pitches and Nos. 3-7 being c o a l - t a r pitches from several sources. About 0.4 g of each pitch was pyrolyzed in a v e r t i c a l l y held pyrex tube (50 x 5 mm) under nitrogen in an e l e c t r i c a l tube furnace.
The pitches were
heated at a constant rate of 4°C min -I to 450° and 475°C and held at these
200
temperatures f o r up to O, 0.5, I, 2 and 3 hours.
After pyrolysis, samples were
cooled and mounted in a c o l d - s e t t i n g 'Metset' resin. polish ( d i f f i c u l t
An appropriate optical
fo r such soft samples) was generated using alumina powder and
Selvyt cloth laps.
Polished surfaces were examined by optical microscopy using
polarized l i g h t and a half-wave retarder plate to generate appropriate colour contrast (Ref.7)~he extents of mesophase growth were assessed as an area percentage ( i . e . a volume percentage) by an image analysis technique.
Mesophase spheres
<1.5 ~m diameter were excluded because of resolution d i f f i c u l t i e s . Figures 2a, 2b show g r a p h i c a l l y how extents of mesophase development vary with pyrolysis time f o r the seven pitches of the study, at 450° and 475°C. respectively.
Pitch No. I is the most reactive o v e r a l l .
t i v e l y less reactive i n i t i a l l y
Pitch No. 2 is r e l a -
but accelerates at 450°C suggesting that f o r this
pitch r e a c t i v i t y of constituent molecules increases as pyrolysis continues. Pitches Nos. 3-7 are less reactive than the Nos. l and 2 reaching 60-80 vol.% conversion at 475°C a ft e r 2 hours.
The use of molecular separation techniques
and image analysis makes f e a s i b l e k i n e t i c studies of formation of defined molecula r weight f r a c t i o n s in pitch and of mesophase.
A generally a v a i l a b l e value of
the rate constant at 400°C is about 0.7 x 10-5 sec -I with an a c t i v a t i o n energy of about 200 kJ mol -I f o r molecular growth and mesophase development for coaltar pitches and selected petroleum pitch iRef. 5,8).
Mesophase/%
Mesophase/% 100
I00 80
80
60
60
40
40
20
20
HTT=450°C
|
6
120
Residence time/min
Figures 2a, 2b. seven pitches.
180
60
120 Residence time/min
Variation of mesophase development with pyrolysis time f o r
180
201
MESOPHASE FROM PITCH BLENDSCCalvert (Ref. 9)_~ Three i n d u s t r i a l pitches were used, described as binder pitch, i . e . a petroleum pitch No. 8, and two c o a l - t a r pitches,Nos.12andl4 of 6.0 and 8.0 wt.% primary QI.
Also~Ashland A240 pitch was chosen as a pure petroleum pitch of
n e g l i g i b l e QI content.
Blends of these pitches (75:25 wt.% r a t i o s ) were prepared
from 8/12, 12/8, 8/14 and 14/8 from powdered pitch.
Pyrolyses were carried out
as described above to 400 ° and 440°C with pyrolysis times of 0.5, I , 1.5 and 2 hours.
Extents of mesophase formation were estimated from polished surfaces by
an image analysis technique. Results are presented in two ways. Fiqures3aand 3b describe development of percentage mesophase in pitch and pitch blends with pyrolysis time at 400°C and 440°C.Figures4a and4b describe development of percentage mesophase in pitch and pitch blends at 400°C with composition of blends of pitches 8/12 and 8/14. Figures 3a, 3b,4a and
4b indicate that extents of mesophase formation at
400° and 440°C from PP No. 8 are greater than from c o a l - t a r pitch Nos.12 and 14. PP No. 8 is therefore more r e a c t i v e ; however, PP A240 is much less reactive than No. 8 and is comparable to the c o a l - t a r pitches.
Extents of mesophase formation
at 400° and 440°C ( i n i t i a l l y ) ( F i g u r e s 3a and 3b)from blends of pitches 8/12 and 8/14 c l o s e l y f o l l o w extents of mesophase formation from the parent pitch No. 8, i.e., No. 8 DOMINATES. Extents of mesophase formation from blends of pitches 14/8 approximate c l o s e l y to extents from CTP No. 14.
For blends of pitches 12/8,
extents of mesophase formation are intermediate between the two parent pitches. For blends of pitches Nos. 8 and 14 at 400°C, Figure 4b, extents of mesophase form development show a broad maximum in extents between 25 and 75 wt.% pitch No. 8. For blends of pitches Nos~and14 at 400°C, Figure 4b, extents of mesophase format i o n are not l i n e a r with respect to blend composition and maximize at 75 wt.% of pitch No. 8. These results indicate that the effects of pyrolysis of pitch blends do not f o l l o w the addition r u l e .
Two possible explanations could be that:
of petroleum and c o a l - t a r pitch i n t e r a c t
(a) molecules
to form a new suite of mesophase
forming molecules (mesogens) which appear at a new position of the cohesion window of Figure Ib, (b) the pitch composed of a mixture of PP and CTP could have d i f f e r e n t s o l u b i l i t y properties compared with the i n d i v i d u a l pitches, and this affects the conditions of appearance of mesophase. These blending effects are complicated and can only be discovered by search-and-find approaches.
Their
relevance to i n d u s t r i a l practice does not appear to be reported upon.
SOLIDIFICATION PROCESSES WITHIN MESOPHASELTakekawa (~ef. I0)_~ In the I n t r o d u c t i o n , the s p e c i f i c and r e s t r i c t i v e properties of pitches leading to mesophase are commented upon.
Deviation from these requirements leads to
the formation of i s o t r o p i c , non-graphitizable carbon.
A v i t a l requirement of
202 mesophase f o r e . g . , needle-coke formation (Ref. I ) , carbon a r t i f a c t s from mesocarbon beads (Ref. I I ) carDon f i b r e s (Ref. 12) is that i t retains f l u i d i t y and p l a s t i c i t y . As such the f i n a l size, shape and structure (optical texture) of a coke or a r t i f a c t can be manipulated by coalescence of growth units of mesophase and manipulation. Mesophase/% 100
Mesophase/% 100
A240 PP 8 .-13- 8114 14/8 14 4 1 - 8/12 12/8 - 0 - 12
80
60
i,,'
PP PP/CTP CTP/PP CTP PP/CTP CTP/PP
60
CTP#
40
20
//'
.~,
(a)
30
80
60
90
< '
120
I
40
20
t
(b)
3O
60
90
I
120
Residence time/min
Residence time/min Figures 3a, 3b.
l
Variation of mesophase development with pyrolysis time f o r pitch and pitch blends at 400°C (a) and 440°C (b). Mesophase/%
Meso )hase/% 25
i
25
20 / 15
-~ ~ -0-
/
90 min soak \ 60 min soak \ 30 min soak ~I
15
10
5 (a) 0 100
25
50
75
75
50 Blend %
25
Figures 4a, 4b.
(b)
100 PP-8
!
0 0 CTP-12 100
25 75
50 50 Blend %
75 25
Variation of mesophase development with pitch blend composition and pyrolysis time at 400°C.
I00 PP-8 0 CTP-14
203
of coalesced mesophase.
These processes of manipulation require a p r a c t ic a l
dimension to the time of s t a b i l i t y of the mesophase. However, to study chemical and physical properties of mesophase at 400°-500°C is not experimentally easy and i t is helpful i f relevant properties can be 'frozen' into the mesophase as i t cools to room temperature to a g l a s s - l i k e m a t e r i a l .
The experimental
approach of sorbing iodine into mesophase at 20°C is to elucidate extents of polymerization w i t h i n a mesophase and to distinguish between mesophases. The concept is a development from a study of iodine sorption by coals (Ref.13) in which iodine sorption responds to cross-linkages in coals, fresh and oxidized, of a rank range. Four pitch materials were used: a Solvent Refined Coal (SRC), a petroleum pitch (PP), Ashland A400 pitch and a c o a l - t a r pitch (CTP).
These materials were
pyrolyzed (carbonized) as 5 g samples to a maximum heat treatment temperature of -I lodine was adsorbed by 0.1 g of carbonization
800°C under nitrogen at 4°C min
product and o r i g i n a l pitch (<250 ~m p a r t i c l e size) from 50 cm3 aqueous potassium iodide solution (I 2 0.055 M; KI 0.15 M) over 20 days at 20°C.
Equilibrium
concentrations of iodine and iodine sorption were determined f o l l o w i n g t i t r a t i o n with 0.I sodium thiosulphate s o l u t i o n .
The v a r i a t i o n of extent of iodine sorp-
t i o n (up-take) from solution with heat treatment temperature of the pitches is -I shown in Figure 5. lodine up-take is expressed in units of mmol g These
Iodine Adsorption/ -I mmol g 6
--0--0-
"v m
i\
-{}---0--
A400
....~ ......~.-.
CTP
•. . - , ~ .
400
Figure 5.
PP
---~-.L~-
"~"~ - - !
SRC
~ , Z - - -m. . . . . . ,
~
.....
!
500 600 Carbonization Temperature/°C
The variation of iodine up-take with heat treatment temperature of several pitches. Equilibration time is 20 days.
204
results indicate (a) extents of iodine up-take by pitches are extensive, equivalent to a surface area of 1,400 m2g-I (~6 mmol g - l )
(b) the iodine is thought to
form charge transfer complexes with constituent pitch molecules (14) (c) extent of iodine up-take by the pitches are in the order S~C > CTP > PP > A400 (d) for mesophase pitch, HTT 450°C, extents of iodine up-take f o l l o w the extents of the parent pitch (e) hence, s i m i l a r molecules are present in the i n i t i a l
mesophase
as in the parent pitch ( f ) extents of iodine up-take do not r e l a t e to opt ic a l texture of mesophase pitch HTT 450°C, (g) fo r mesophase, HTT 500°C, extents of iodine up-take d i f f e r markedly from mesophase, HTT 450°C, increasing in extent with increasing size of optical texture (h) the SRC mesophase cross-links the most r e a d i l y and the CTP the least r e a d i l y , in the HTT range 500° to 600°C ( i ) -I f o r pitch cokes, HTT >600°C, iodine up-take does not exceed 0.7 mmol g CONCLUSIONS This assessment of properties of pyrolyzing pitches indicates how specific pyrolysis chemistry of pitches influences rates of formation of mesophase from i n d i v i d u a l pitches and pitch blends and how this pyrolysis chemistry is carried over into mesophase chemistry. ACKNOWLEDGEMENTS Rosa Menendez is grateful f o r support from the Consejo Superior de ~Investigaciones C i e n t i f i c a s (Spain) to enable her to study in the Northern Carbon Research Laboratories. REFERENCES 1 2 3 4 5 6 7 8
9
R. Debiase, J.D. E l l i o t t and T.E. Hartnett, Delayed-Coking Process Update, "Petroleum-Derived Carbons~IACS Symposium No. 303, Washington D.C., U.S.A., 1986, pp. 155-171. H. Marsh and R. Menendez, Proceedings of ACS Symposium N° 303 (see Ref. l) H. Marsh and C.S. Latham, The Chemistry of Mesophase Formation, Ref. I , pp. 1-28. I. Mochida and Y. Korai, Chemical Characterization and Preparation of the Carbonaceous Mesophase, Ref. I, pp. 29-44. R.A. Greinke, Kinetics of petroleum pitch polymerization by gel permeation chromatography, Carbon 24(6) (1986) pp. 677-686. R.A. Greinke and L.S. Singer, Constitution of c o- ex is t ing phases in mesophase containing pitch during heat treatment, Ext. Abs. of 18th Biennial Conf. on Carbon, American Carbon Society, Worcester, MA, U.S.A. 1987, pp. 179-180. R.A. Forrest, H. Marsh, C. Cornford and B.T. Kelly, Optical properties of anisotropic carbon, Chemistry and Physics of Carbon, Ed. P.A. Thrower, Marcel Dekker, N.Y., 19 (1984) pp. 211-330. H. Marsh and P.L. Walker, J r . , The formation of g r a p h i t i z a b l e carbons via meso phase: chemical and k i n e t i c considerations, Chemistry and Physics of Carbon, Ed. P.L. Walker,Jr. and P.A. Thrower, Marcel Dekker, N.Y., 15 (1979) pp. 229286. C. Calvert, Ph.D. Dissertation "Cokes from Pitches", University of Newcastle upon Tyne, U.K. (1986).
205
I0
T. Takekawa and H. Marsh, Characterization of mesophase by adsorption of iodine, Carbon '86, Extended Abstracts, Deutschen Keramischen Gesellschaft, Augsburg, Baden-Baden (1986), pp. 81-83. 11 M. Inagaki, K. Kuroda, N. Inoue and M. Sakai, Conditions for carbon spherule formation under pressure, Carbon 22(6) (1984) pp. 617-619. 12 D.D. Edie, N.K. Fox and B.C. Barnett, Melt-spun n o n - c i r c u l a r carbon f i b e r s , Carbon 24(4) (1986) pp. 477-482. 13 N. Rodriguez and H. Marsh, Surface chemistry of coals studied by iodine and water adsorption, Accepted f o r p u b l i c a t i o n in FUEL (1987). 14 H. Lopez, T. Yokono, Ko Murakami, Y. Sanada and H. Mars,n, Hydrogea donor a b i l i t y of p i t c h and ESR characterization of iodine complexes, Fuel 66 (1987) pp. 866-867.
197
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
PYROLYSIS OF COAL-TAR PITCHES : KINETICS OF CONVERSION TO COKE AND IODINE ADSORPTION BY MESOPHASE R. MENENDEZ, H. MARSH, C. CALVERT and T. TAKEKAWA Northern Carbon Research Laboratories, School of Chemistry, University of Newcastle upon Tyne, Newcastle upon Tyne, NEI 7RU, U.K. SUMMARY Pitch is the parent material for cokes of major commercial importance. These cokes are g r a p h i t i z a b l e as a r e s u l t of formation of l i q u i d crystals (mesophase) during pitch p y r o l y s i s . The concept of molecular cohesion energy to create mesophase is described. Extents of formation of mesophase from blends of d i f f e r e n t pitches do not f o l l o w an a d d i t i v e r ule. Mesophasehas chemical properties s i m i l a r to the parent pitch. Iodine adsorption (up-take) by mesophases from d i f f e r e n t pitches indicates that they s o l i d i f y d i f f e r e n t l y , HTT 500° to 600°C. INTRODUCTION Coke materials derived from petroleum and c o a l - t a r pitches have important i n d u s t r i a l uses.
The delayed coker dominantly converts petroleum pitch of
various q u a l i t i e s to shot-, regular- or sponge-, and needle-cokes(Ref.l}.Although shot coke only has value as a f u e l , regular coke finds major use as a f i l l e r coke in the carbon anodes of the Hall c e l l of aluminium production and needlecoke is the f i l l e r
coke f o r the graphite electrodes of the steel industry.
Coal-
ta r pitch is a suitable binder material fo r the carbon anodes and the graphite electrode with petroleum pitch having a f u r t h e r application as an impregnation pitch to densify the graphite electrode. As discussed in t h i s Conference and elsewhere CRef.2,4), the cokes from petroleum and c o a l - t a r pitch are of g r a p h i t i z a b l e q u a l i t y . The a b i l i t y of cokes to g r a p h i t i z e on heat treatment to >2500°C is dependent upon the existence of a c r y s t a l l i n i t y established within the cokes during pitch pyrolysis. to use pitch as a source of f i l l e r
The a b i l i t y
coke and binder is t o t a l l y dependent upon the
pitch forming a ' c r y s t a l l i n e ' s o l i d coke product.
C r y s t a l l i n i t y is introduced
to the pyrolyzing pitch system as a d i s c o t i c , aromatic, nematic l i q u i d crystal phase created from the molecular constituents of the pitch by association of molecules of appropriate sizes at appropriate temperatures.
The c r y s t a l l i n i t y of
the l i q u i d crystal phase (called mesophase) becomes the c r y s t a l l i n i t y of the so lid green coke by a process of molecular growth (cross-linking=polymerization) associated with time and temperature. The conditions of molecular growth, association to form mesophase and s o l i d i f i c a t i o n to coke are the objectives of this study.
0378-3820/88/$03.50
©
1988 Elsevier Science Publishers B.V.
198
MESOPHASE GENERATION Mesophase is generated only when quite r e s t r i c t i v e chemical and physical conditions are operating within a pyrolyzing pitch: (a) the system must remain f l u i d , with decreasing v i s c o s i t y , at temperatures of about 400°C and r i s i n g (b) the system must be of low v o l a t i l i t y
(c) the molecular constituents must be
planar and r e s i s t a n t to fragmentation (d) molecular growth, by condensation, must occur over a l i m i t e d temperature range, at about 400°C, within a f l u i d phase of low enough v i s c o s i t y to f a c i l i t a t e molecular movement (e) molecular movement and c o l l i s i o n generate molecular associations with s u f f i c i e n t cohesion energy to prevent subsequent thermal dissociation (f) the cohesion energy is responsible fo r the s t a b i l i t y of this newly created nematic l i q u i d crystal (mesophase) system (g) cohesion energy is a function of molecular size, the degree of molecular p l a n a r i t y , the presence of heteroatoms within the molecule so influencing molecular p o l a r i t y and structural defects, i . e . , a t o m vacancies within the molecule. The development of mesophase within a pitch can be described in terms of Figures l a , Ib, which are diagrams i l l u s t r a t i n g how molecular cohesion energy and average molecular weight i n t e r a c t during a pitch pyrolysis.
With isothermal
pyrolysis at e.g.,400°C or non-isothermal treatment (heating rate of 5°C min-1), increasing time and temperature cause an increase in average molecular weight and cohesion energy between molecules, curves AB and CD of Figure la.
At
position P, cohesion energy exceeds k i n e t i c energy, association of two molecules is established and a l i q u i d crystal is generated.
When a v a i l a b l e pitches are
studied, two bands of properties are created, Figure Ib, EF (cohesion energy) i
and GH (a.m.w.) giving an area (or window), P , of conditions of mesophase generation. This window is of l i m i t e d size extending over a few tens of degrees c e n t i grade only. the
Nevertheless, the range is of c r i t i c a l
importance in determining
size, shape and coalescence of the l i q u i d crystals.
At constant heating
rates, the mesophase formed at lower temperatures w i l l be of smaller size of optical texture (mosaics)(Ref. 3) with that at l i g h e r temperatures being of larger size, i.e.,domains or n e e d l e - l i k e (Ref. 3). Greinke (Ref.5) Greinke and Singer (Ref.6) using Gel Permeation ChromatoQraphy and high temperature c e n t r i f u g a t i o n have established that the molecules which c o n s t i t u t e mesophase, i n i t i a l l y , are the higher molecular f r a c t i o n of the pyrolyzing pitch, the mesophase growing in extent as larger molecules are generated by condensation reactions. entrapped within the mesophase system.
Smaller molecular weight materials become At 400°C, over 15 hours, the average
molecular weight in mesophase did not change, i l l u s t r a t i n g an essential s t a b i l i t y of the larger sized molecules (Ref.6). However, the mesoDhase as established is
199
Molecular Cohesion Energy
Average Molecular Weight
B!
IIH/ /
//2 !
,
.~//G/"¢'''~ (a)
(b)
Time and Temperature
Figures l a , lb. Diagram of molecular cohesion energy and average molecular weight during mesophase development.
chemically reactive and f u r t h e r polj~nerization/condensation/cross-linkage occurs with increasing time and temperature u n t i l the mesophase ceases to be a f l u i d , becoming p l a s t i c and f i n a l l y becoming a v i s c o - e l a s t i c solid.Mesophasesdiffer in t h e i r s o l i d i f i c a t i o n behaviour as they do in t h e i r generation. Pitch materials as binders, during c a l c i n a t i o n , a l l pass through the stages of a f l u i d i s o t r o p i c pitch, a f l u i d mesophase leading to the s o l i d binder coke bridge.
During c a l c i n a t i o n , the binder pitch and mesophase w i l l flow within the
green anode mix eventually establishing 'anode structure'
Variations of flow
properties of pitches during c a l c i n a t i o n may influence anode and electrode properties.
This paper therefore describes studies of rates of formation of
mesophase from i n d i v i d u a l pitches, blends of pitches as well as f o l l o w i n g the s o l i d i f i c a t i o n process within mesophase. MESOPHASE FROM INDIVIDUAL PITCHES Seven i n d u s t r i a l pitches were used, Nos. I and 2 being petroleum pitches and Nos. 3-7 being c o a l - t a r pitches from several sources. About 0.4 g of each pitch was pyrolyzed in a v e r t i c a l l y held pyrex tube (50 x 5 mm) under nitrogen in an e l e c t r i c a l tube furnace.
The pitches were
heated at a constant rate of 4°C min -I to 450° and 475°C and held at these
200
temperatures f o r up to O, 0.5, I, 2 and 3 hours.
After pyrolysis, samples were
cooled and mounted in a c o l d - s e t t i n g 'Metset' resin. polish ( d i f f i c u l t
An appropriate optical
fo r such soft samples) was generated using alumina powder and
Selvyt cloth laps.
Polished surfaces were examined by optical microscopy using
polarized l i g h t and a half-wave retarder plate to generate appropriate colour contrast (Ref.7)~he extents of mesophase growth were assessed as an area percentage ( i . e . a volume percentage) by an image analysis technique.
Mesophase spheres
<1.5 ~m diameter were excluded because of resolution d i f f i c u l t i e s . Figures 2a, 2b show g r a p h i c a l l y how extents of mesophase development vary with pyrolysis time f o r the seven pitches of the study, at 450° and 475°C. respectively.
Pitch No. I is the most reactive o v e r a l l .
t i v e l y less reactive i n i t i a l l y
Pitch No. 2 is r e l a -
but accelerates at 450°C suggesting that f o r this
pitch r e a c t i v i t y of constituent molecules increases as pyrolysis continues. Pitches Nos. 3-7 are less reactive than the Nos. l and 2 reaching 60-80 vol.% conversion at 475°C a ft e r 2 hours.
The use of molecular separation techniques
and image analysis makes f e a s i b l e k i n e t i c studies of formation of defined molecula r weight f r a c t i o n s in pitch and of mesophase.
A generally a v a i l a b l e value of
the rate constant at 400°C is about 0.7 x 10-5 sec -I with an a c t i v a t i o n energy of about 200 kJ mol -I f o r molecular growth and mesophase development for coaltar pitches and selected petroleum pitch iRef. 5,8).
Mesophase/%
Mesophase/% 100
I00 80
80
60
60
40
40
20
20
HTT=450°C
|
6
120
Residence time/min
Figures 2a, 2b. seven pitches.
180
60
120 Residence time/min
Variation of mesophase development with pyrolysis time f o r
180
201
MESOPHASE FROM PITCH BLENDSCCalvert (Ref. 9)_~ Three i n d u s t r i a l pitches were used, described as binder pitch, i . e . a petroleum pitch No. 8, and two c o a l - t a r pitches,Nos.12andl4 of 6.0 and 8.0 wt.% primary QI.
Also~Ashland A240 pitch was chosen as a pure petroleum pitch of
n e g l i g i b l e QI content.
Blends of these pitches (75:25 wt.% r a t i o s ) were prepared
from 8/12, 12/8, 8/14 and 14/8 from powdered pitch.
Pyrolyses were carried out
as described above to 400 ° and 440°C with pyrolysis times of 0.5, I , 1.5 and 2 hours.
Extents of mesophase formation were estimated from polished surfaces by
an image analysis technique. Results are presented in two ways. Fiqures3aand 3b describe development of percentage mesophase in pitch and pitch blends with pyrolysis time at 400°C and 440°C.Figures4a and4b describe development of percentage mesophase in pitch and pitch blends at 400°C with composition of blends of pitches 8/12 and 8/14. Figures 3a, 3b,4a and
4b indicate that extents of mesophase formation at
400° and 440°C from PP No. 8 are greater than from c o a l - t a r pitch Nos.12 and 14. PP No. 8 is therefore more r e a c t i v e ; however, PP A240 is much less reactive than No. 8 and is comparable to the c o a l - t a r pitches.
Extents of mesophase formation
at 400° and 440°C ( i n i t i a l l y ) ( F i g u r e s 3a and 3b)from blends of pitches 8/12 and 8/14 c l o s e l y f o l l o w extents of mesophase formation from the parent pitch No. 8, i.e., No. 8 DOMINATES. Extents of mesophase formation from blends of pitches 14/8 approximate c l o s e l y to extents from CTP No. 14.
For blends of pitches 12/8,
extents of mesophase formation are intermediate between the two parent pitches. For blends of pitches Nos. 8 and 14 at 400°C, Figure 4b, extents of mesophase form development show a broad maximum in extents between 25 and 75 wt.% pitch No. 8. For blends of pitches Nos~and14 at 400°C, Figure 4b, extents of mesophase format i o n are not l i n e a r with respect to blend composition and maximize at 75 wt.% of pitch No. 8. These results indicate that the effects of pyrolysis of pitch blends do not f o l l o w the addition r u l e .
Two possible explanations could be that:
of petroleum and c o a l - t a r pitch i n t e r a c t
(a) molecules
to form a new suite of mesophase
forming molecules (mesogens) which appear at a new position of the cohesion window of Figure Ib, (b) the pitch composed of a mixture of PP and CTP could have d i f f e r e n t s o l u b i l i t y properties compared with the i n d i v i d u a l pitches, and this affects the conditions of appearance of mesophase. These blending effects are complicated and can only be discovered by search-and-find approaches.
Their
relevance to i n d u s t r i a l practice does not appear to be reported upon.
SOLIDIFICATION PROCESSES WITHIN MESOPHASELTakekawa (~ef. I0)_~ In the I n t r o d u c t i o n , the s p e c i f i c and r e s t r i c t i v e properties of pitches leading to mesophase are commented upon.
Deviation from these requirements leads to
the formation of i s o t r o p i c , non-graphitizable carbon.
A v i t a l requirement of
202 mesophase f o r e . g . , needle-coke formation (Ref. I ) , carbon a r t i f a c t s from mesocarbon beads (Ref. I I ) carDon f i b r e s (Ref. 12) is that i t retains f l u i d i t y and p l a s t i c i t y . As such the f i n a l size, shape and structure (optical texture) of a coke or a r t i f a c t can be manipulated by coalescence of growth units of mesophase and manipulation. Mesophase/% 100
Mesophase/% 100
A240 PP 8 .-13- 8114 14/8 14 4 1 - 8/12 12/8 - 0 - 12
80
60
i,,'
PP PP/CTP CTP/PP CTP PP/CTP CTP/PP
60
CTP#
40
20
//'
.~,
(a)
30
80
60
90
< '
120
I
40
20
t
(b)
3O
60
90
I
120
Residence time/min
Residence time/min Figures 3a, 3b.
l
Variation of mesophase development with pyrolysis time f o r pitch and pitch blends at 400°C (a) and 440°C (b). Mesophase/%
Meso )hase/% 25
i
25
20 / 15
-~ ~ -0-
/
90 min soak \ 60 min soak \ 30 min soak ~I
15
10
5 (a) 0 100
25
50
75
75
50 Blend %
25
Figures 4a, 4b.
(b)
100 PP-8
!
0 0 CTP-12 100
25 75
50 50 Blend %
75 25
Variation of mesophase development with pitch blend composition and pyrolysis time at 400°C.
I00 PP-8 0 CTP-14
203
of coalesced mesophase.
These processes of manipulation require a p r a c t ic a l
dimension to the time of s t a b i l i t y of the mesophase. However, to study chemical and physical properties of mesophase at 400°-500°C is not experimentally easy and i t is helpful i f relevant properties can be 'frozen' into the mesophase as i t cools to room temperature to a g l a s s - l i k e m a t e r i a l .
The experimental
approach of sorbing iodine into mesophase at 20°C is to elucidate extents of polymerization w i t h i n a mesophase and to distinguish between mesophases. The concept is a development from a study of iodine sorption by coals (Ref.13) in which iodine sorption responds to cross-linkages in coals, fresh and oxidized, of a rank range. Four pitch materials were used: a Solvent Refined Coal (SRC), a petroleum pitch (PP), Ashland A400 pitch and a c o a l - t a r pitch (CTP).
These materials were
pyrolyzed (carbonized) as 5 g samples to a maximum heat treatment temperature of -I lodine was adsorbed by 0.1 g of carbonization
800°C under nitrogen at 4°C min
product and o r i g i n a l pitch (<250 ~m p a r t i c l e size) from 50 cm3 aqueous potassium iodide solution (I 2 0.055 M; KI 0.15 M) over 20 days at 20°C.
Equilibrium
concentrations of iodine and iodine sorption were determined f o l l o w i n g t i t r a t i o n with 0.I sodium thiosulphate s o l u t i o n .
The v a r i a t i o n of extent of iodine sorp-
t i o n (up-take) from solution with heat treatment temperature of the pitches is -I shown in Figure 5. lodine up-take is expressed in units of mmol g These
Iodine Adsorption/ -I mmol g 6
--0--0-
"v m
i\
-{}---0--
A400
....~ ......~.-.
CTP
•. . - , ~ .
400
Figure 5.
PP
---~-.L~-
"~"~ - - !
SRC
~ , Z - - -m. . . . . . ,
~
.....
!
500 600 Carbonization Temperature/°C
The variation of iodine up-take with heat treatment temperature of several pitches. Equilibration time is 20 days.
204
results indicate (a) extents of iodine up-take by pitches are extensive, equivalent to a surface area of 1,400 m2g-I (~6 mmol g - l )
(b) the iodine is thought to
form charge transfer complexes with constituent pitch molecules (14) (c) extent of iodine up-take by the pitches are in the order S~C > CTP > PP > A400 (d) for mesophase pitch, HTT 450°C, extents of iodine up-take f o l l o w the extents of the parent pitch (e) hence, s i m i l a r molecules are present in the i n i t i a l
mesophase
as in the parent pitch ( f ) extents of iodine up-take do not r e l a t e to opt ic a l texture of mesophase pitch HTT 450°C, (g) fo r mesophase, HTT 500°C, extents of iodine up-take d i f f e r markedly from mesophase, HTT 450°C, increasing in extent with increasing size of optical texture (h) the SRC mesophase cross-links the most r e a d i l y and the CTP the least r e a d i l y , in the HTT range 500° to 600°C ( i ) -I f o r pitch cokes, HTT >600°C, iodine up-take does not exceed 0.7 mmol g CONCLUSIONS This assessment of properties of pyrolyzing pitches indicates how specific pyrolysis chemistry of pitches influences rates of formation of mesophase from i n d i v i d u a l pitches and pitch blends and how this pyrolysis chemistry is carried over into mesophase chemistry. ACKNOWLEDGEMENTS Rosa Menendez is grateful f o r support from the Consejo Superior de ~Investigaciones C i e n t i f i c a s (Spain) to enable her to study in the Northern Carbon Research Laboratories. REFERENCES 1 2 3 4 5 6 7 8
9
R. Debiase, J.D. E l l i o t t and T.E. Hartnett, Delayed-Coking Process Update, "Petroleum-Derived Carbons~IACS Symposium No. 303, Washington D.C., U.S.A., 1986, pp. 155-171. H. Marsh and R. Menendez, Proceedings of ACS Symposium N° 303 (see Ref. l) H. Marsh and C.S. Latham, The Chemistry of Mesophase Formation, Ref. I , pp. 1-28. I. Mochida and Y. Korai, Chemical Characterization and Preparation of the Carbonaceous Mesophase, Ref. I, pp. 29-44. R.A. Greinke, Kinetics of petroleum pitch polymerization by gel permeation chromatography, Carbon 24(6) (1986) pp. 677-686. R.A. Greinke and L.S. Singer, Constitution of c o- ex is t ing phases in mesophase containing pitch during heat treatment, Ext. Abs. of 18th Biennial Conf. on Carbon, American Carbon Society, Worcester, MA, U.S.A. 1987, pp. 179-180. R.A. Forrest, H. Marsh, C. Cornford and B.T. Kelly, Optical properties of anisotropic carbon, Chemistry and Physics of Carbon, Ed. P.A. Thrower, Marcel Dekker, N.Y., 19 (1984) pp. 211-330. H. Marsh and P.L. Walker, J r . , The formation of g r a p h i t i z a b l e carbons via meso phase: chemical and k i n e t i c considerations, Chemistry and Physics of Carbon, Ed. P.L. Walker,Jr. and P.A. Thrower, Marcel Dekker, N.Y., 15 (1979) pp. 229286. C. Calvert, Ph.D. Dissertation "Cokes from Pitches", University of Newcastle upon Tyne, U.K. (1986).
205
I0
T. Takekawa and H. Marsh, Characterization of mesophase by adsorption of iodine, Carbon '86, Extended Abstracts, Deutschen Keramischen Gesellschaft, Augsburg, Baden-Baden (1986), pp. 81-83. 11 M. Inagaki, K. Kuroda, N. Inoue and M. Sakai, Conditions for carbon spherule formation under pressure, Carbon 22(6) (1984) pp. 617-619. 12 D.D. Edie, N.K. Fox and B.C. Barnett, Melt-spun n o n - c i r c u l a r carbon f i b e r s , Carbon 24(4) (1986) pp. 477-482. 13 N. Rodriguez and H. Marsh, Surface chemistry of coals studied by iodine and water adsorption, Accepted f o r p u b l i c a t i o n in FUEL (1987). 14 H. Lopez, T. Yokono, Ko Murakami, Y. Sanada and H. Mars,n, Hydrogea donor a b i l i t y of p i t c h and ESR characterization of iodine complexes, Fuel 66 (1987) pp. 866-867.