7 Synthetic Aromatic Pitch Aromatic Pitches from the Asphaltene-Free Distillate Fraction of Catalytic Cracker Bottoms
G. Dickakian
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Specialties Technology Division, Exxon Chemical Company, Houston, TX 77029 Catalytic cracking bottoms (CCB) is a widely used aromatic feedstock for carbon production, such as aromatic pitch, carbon black and carbon fibers. We fractionated CCB by a high vacuum d i s t i l l a t i o n into several d i s t i l l a t e fractions and undistillable residue. The distillate fractions were subjected to a two-stage high temperature thermal process to convert them to aromatic pitches. The composition of the pitches produced was determined by a solvent analysis to define their suitability for synthetic carbon production.
T h i s c h a p t e r d e s c r i b e s t h e p r e p a r a t i o n s and c h a r a c t e r i s t i c s o f h i g h l y a r o m a t i c and h i g h l y a n i s o t r o p i c p i t c h e s from t h e d i s t i l l a t e f r a c t i o n of c a t a l y t i c c r a c k e r bottoms ( C C B ) . CCB i s t h e a r o m a t i c r e s i d u e from a c a t a l y t i c cracking process. CCB i s f r a c t i o n a t e d by a h i g h vacuum d i s t i l l a t i o n i n t o a d i s t i l l a b l e f r a c t i o n (around 50% y i e l d ) and a n o n - d i s t i l l a b l e r e s i d u e . These CCB f r a c t i o n s , d i s t i l l a t e and r e s i d u e , v a r y s i g n i f i c a n t l y i n t h e i r physical c h a r a c t e r i s t i c s , chemical s t r u c t u r e , a s p h a l t e n e c o n t e n t , m o l e c u l a r w e i g h t and a r o m a t i c r i n g d i s t r i b u t i o n s . H i g h l y a r o m a t i c and h i g h l y a n i s o t r o p i c p i t c h e s were prepared by a h i g h temperature t w o - s t a g e t h e r m a l t r e a t m e n t of CCB d i s t i l l a t e fraction. A number o f k e y r e a c t i o n parameters e f f e c t i n g p i t c h p r o d u c t i o n , y i e l d , c h a r a c t e r i s t i c s , and t h e f o r m a t i o n o f t h e h i g h l y a n i s o t r o p i c t o l u e n e i n s o l u b l e s were i n v e s t i g a t e d . The t o l u e n e i n s o l u b l e s f r a c t i o n was used f o r t h e p r o d u c t i o n o f c a r b o n f i b e r s . A m i d d l e C C B - d i s t i l l a t e f r a c t i o n (420-520°C/760 mm Hg) was produced by a h i g h vacuum d i s t i l l a t i o n ( 0 . 5 - 1 . 0 mm Hg 1 5 / 5 c o l u m n ) . The CCB feed was s e l e c t e d c a r e f u l l y , o n l y CCB feed w i t h h i g h a r o m a t i c i t y ( a r o m a t i c carbon=65-73 atom % ) , which was produced from a h i g h s e v e r i t y c a t a l y t i c c r a c k i n g o p e r a t i o n , was u s e d . The C C B - d i s t i l l a t e f r a c t i o n c o n t a i n s no a s h o r a s p h a l t e n e s (η-Heptane i n s o l u b l e s a t r e f l u x ) . I t has a l o w and narrow m o l e c u l a r w e i g h t d i s t r i b u t i o n (Mn=290), and c o n t a i n s a v e r y n a r r o w a r o m a t i c r i n g d i s t r i b u t i o n ( 3 , 4, 5 , and 6 r i n g s ) . Table I g i v e s t h e 0097-6156/86/0303-0118$06.00/0 © 1986 American Chemical Society
Bacha et al.; Petroleum-Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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c h a r a c t e r i s t i c s of the CCB m i d d l e f r a c t i o n which was used i n t h i s i n v e s t i g a t i o n f o r our p i t c h p r o d u c t i o n , CCB d i s t i l l a t e was t r a n s f o r m e d i n t o an a r o m a t i c p i t c h by a twostage p r o c e s s . I n i t i a l l y , t h e C C B - d i s t i l l a t e was t h e r m a l l y - t r e a t e d a t h i g h t e m p e r a t u r e s , 400-450°C, under a t m o s p h e r i c ( n i t r o g e n ) p r e s s u r e and f i n a l l y the u n r e a c t e d f r a c t i o n s were vacuum s t r i p p e d a t 0 . 5 - 1 - 0 mm/760 mm H g , l e a v i n g the a r o m a t i c p i t c h i n 30-40% y i e l d i n w h i c h t h e r e i s up to 74% of a h i g h l y a n i s o t r o p i c t o l u e n e i n s o l u b l e s fraction. A number of i m p o r t a n t p r o c e s s parameters were i n v e s t i g a t e d to f i n d out t h e i r e f f e c t on p i t c h c h a r a c t e r i s t i c s and y i e l d of the t o l u ene and q u i n o l i n e i n s o l u b l e s . The p i t c h e s produced were c h a r a c t e r i z ed by s o l v e n t a n a l y s i s , NMR, t h e r m a l , and e l e m e n t a l a n a l y s i s . In s o l u b l e s i n t o l u e n e , p y r i d i n e and q u i n o l i n e were used because these f r a c t i o n s r e p r e s e n t the f u s a b l e and i n f u s a b l e a n i s o t r o p i c l i q u i d c r y s t a l f r a c t i o n formed i n the p i t c h . A r o m a t i c p i t c h e s were produced by t r e a t i n g C C B - d i s t i l l a t e a t 400°C, 410°C, 420°C, 430°C, and 440°C. We found t h a t p r o c e s s tem p e r a t u r e i s a v e r y Important parameter i n d e t e r m i n i n g the r a t e of t o l u e n e and p y r i d i n e i n s o l u b l e f o r m a t i o n . I t was a l s o found t h a t a r e l a t i v e l y h i g h t e m p e r a t u r e (around 430°C) i s r e q u i r e d to produce a p i t c h w i t h a h i g h l i q u i d c r y s t a l c o n t e n t . T a b l e I I g i v e s the c o m p o s i t i o n of p i t c h e s produced a t 400-440°C. F i g u r e 1 I l l u s t r a t e s g r a p h i c a l l y , t h e e f f e c t of p r o c e s s temperature on the r a t e of t o l u e n e , p y r i d i n e , and q u i n o l i n e i n s o l u b l e s f o r m a t i o n . The e f f e c t of r e a c t i o n t i m e was i n v e s t i g a t e d by t h e r m a l l y t r e a t i n g CCB d i s t i l l a t e a t 420°C and 430°C f o r 1, 2, 3, and 4 h o u r s . We found t h a t a t b o t h temperatures i n v e s t i g a t e d i n c r e a s i n g r e a c t i o n time r e s u l t e d i n i n c r e a s i n g t h e r a t e of t o l u e n e , p y r i d i n e , and q u i n o l i n e i n s o l u b l e s . T a b l e I I I g i v e s the c o m p o s i t i o n of p i t c h e s produced a t 420°C and 430°C f o r v a r y i n g time (1 t o 4 h o u r s ) , and F i g u r e 2 i l l u s t r a t e s , g r a p h i c a l l y , t h e e f f e c t of r e a c t i o n time on the f o r m a t i o n of the t o l u e n e , p y r i d i n e and q u i n o l i n e i n s o l u b l e s . The c h e m i c a l s t r u c t u r e of C C B - d i s t i l l a t e p i t c h e s produced a t v a r y i n g t e m p e r a t u r e s (410-430°C) were determined by s o l i d - s t a t e NMR, p r o t o n NMR, and c a r b o n / h y d r o g e n atomic r a t i o . We found t h a t i n c r e a s i n g the t h e r m a l t r e a t m e n t temperature r e s u l t e d I n i n c r e a s i n g the a r o m a t i c c a r b o n c o n t e n t i n the p i t c h , i n c r e a s i n g t h e a r o m a t i c p r o t o n s , d e c r e a s i n g the b e n z y l i c and t o t a l a l i p h a t i c p r o t o n s , and i n c r e a s i n g the c a r b o n / h y d r o g e n atomic r a t i o . These changes i n the c h e m i c a l s t r u c t u r e of the p i t c h e s a r e produced as a r e s u l t of the v a r y i n g degree of d e a l k y l a t i o n of the a l k y l a l i p h a t i c s i d e c h a i n s , p o l y m e r i z a t i o n , and c o n d e n s a t i o n of the a r o m a t i c r i n g s . T a b l e IV g i v e s the NMR d a t a and c a r b o n / h y d r o g e n atomic r a t i o of C C B - d i s t i l l a t e p i t c h e s produced a t 410°C-430°C. The t h e r m a l c h a r a c t e r i s t i c s of the a r o m a t i c p i t c h i s one of the key c h a r a c t e r i s t i c s of a p i t c h which i s used f o r h i g h temperature a p p l i c a t i o n s such as c a r b o n o r g r a p h i t e anode o r c a r b o n f i b e r p r o d u c tion. We determined the TGA thermograms i n n i t r o g e n ( 1 0 ° C / m i n ) . TGA d a t a i n d i c a t e s t h a t as t h e t h e r m a l t r e a t m e n t t e m p e r a t u r e used f o r p r o d u c i n g the p i t c h i s i n c r e a s e d , t h e p i t c h becomes more t h e r m a l l y s t a b l e as i n d i c a t e d by the decreased v o l a t i l e c o n t e n t and i n c r e a s i n g coke y i e l d . T a b l e V g i v e s TGA d a t a and coke y i e l d a t 550°C f o r C C B d i s t i l l a t e p i t c h e s produced a t 400°C-440°C. F i g u r e 3 g i v e s a t y p i c a l DTG thermogram ( n i t r o g e n , 10°C/min) of a C C B - d i s t i l l a t e p i t c h prepared a t 430°C.
Bacha et al.; Petroleum-Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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C C B - d i s t i l l a t e p i t c h e s produced u s i n g C C B - d i s t i l l a t e and our h i g h temperature two-stage process are h i g h l y a n i s o t r o p i c i n s t r u c t u r e as determined by p o l a r i z e d l i g h t m i c r o s c o p y . The a n i s o t r o p i c c o n t e n t of t h e p i t c h i s v e r y much dependent on the t h e r m a l t r e a t m e n t t e m p e r a t u r e . We found t h a t a n i s o t r o p i c s t r u c t u r e b e g i n s d e v e l o p i n g a t a low r a t e when t h e r m a l l y t r e a t i n g the d i s t i l l a t e a t 400° - 410°C. We a l s o found t h a t a temperature between 430° - 440°C i s r e q u i r e d to produce a d i s t i l l a t e p i t c h w i t h 90-100% of o p t i c a l a n i s o t r o p y . Fig u r e s 4, 5, and 6 p r e s e n t the m i c r o g r a p h s of C C B - d i s t i l l a t e p i t c h e s produced a t 410°C, 420°C and 430°C, r e s p e c t i v e l y . R h e o l o g i c a l p r o p e r t i e s of p i t c h e s a r e key i n d e f i n i n g t h e i r u s e f u l n e s s f o r h i g h temperature a p p l i c a t i o n s o r f o r s p i n n i n g i n t o c a r b o n f i b e r s . We determined the g l a s s t r a n s i t i o n temperature (Tg) of C C B - d i s t i l l a t e p i t c h e s p r e p a r e d a t 400°C, 410°C, 420°C, 430°C, and 440°C u s i n g a d i f f e r e n t i a l s c a n n i n g c a l o r i m e t e r ( D S C ) . We found t h a t a l l the p i t c h e s p r e p a r e d have a low Tg (217 to 2 2 9 ° C ) . I t was a l s o found t h a t i n c r e a s i n g the p r o c e s s temperature l e d to i n c r e a s i n g the Tg of the p i t c h . DSC d a t a a r e p r e s e n t e d i n T a b l e V I . The a r o m a t i c p i t c h e s produced from C C B - d i s t i l l a t e a r e b e i n g developed f o r p i t c h c a r b o n f i b e r p r o d u c t i o n .
Table I .
C h a r a c t e r i s t i c s of M i d d l e Cut C C B - D i s t i l l a t e F r a c t i o n
B o i l i n g Range (°C/760mm) A s h (Wt. %) A s p h a l t e n e s (η-Heptane I n s o l u b l e s ) C o k i n g V a l u e (Wt. % a t 550°C) Number Average M o l . Weight Carbon/Hydrogen Atomic R a t i o
(Wt.%)
420-520 0.0005 nil 1.0 290 0.90
NMR-Data A r o m a t i c c a r b o n (%) B e n z y l i c Protons (%) T o t a l A l i p h a t i c P r o t o n s (%) Aromatic Ring 1 2 3 4 5 6+ 3+4 4+5 3+4+5
Ring Rings Rings Rings Rings Rings Rings Rings Rings
72.0 34.3 37.2
Distillation 1.0 15.0 30.0 45.0 7.0 1.0 75.0 52.0 82.0
Bacha et al.; Petroleum-Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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T a b l e I I . E f f e c t of P r o c e s s Temperature on C C B - D i s t i l l a t e P i t c h C o m p o s i t i o n P i t c h Process Conditions Temperature ( ° C ) Time (Hours)
Feed
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η-Heptane I n s o l u b l e s (%) Toluene I n s o l u b l e s (%) P y r i d i n e I n s o l u b l e s (%) Q u i n o l i n e I n s o l u b l e s (%)
0 0 0 0
400 1.0
410 1.0
420 1.0
430 1.0
440 1.0
78.0 0.1 0 0
95.3 1.9 0 0
75.9 4.9 0.4 0.1
97.2 32.2 18.7 3.3
99.0 60.0 38.1 13.4
T a b l e I I I . E f f e c t of R e a c t i o n Time on C C B D i s t i l l a t e P i t c h Composition P i t c h Process Conditions Temperature ( ° C )
3
< 4
98.1 38.0 17.3 2.7
430 1
>
2
97.2 99.3 42.2 53.5 18.7 2 5 . 0 3.3 6.0
3
4
98.4 62.4 35.9 11.2
98.8 73.5 54.0 22.2
E f f e c t of P r o c e s s Temperature on C C B - D i s t i l l a t e P i t c h Chemical S t r u c t u r e
Temperature Time (Hours)
Feed
410 1.0
430 1.0
72.0
83.0
90.7
28.0
16.2
9.3
34.3 37.2
46.5 39.0
58.7 34.1
28.5
14.5
7.2
Carbon D i s t r i b u t i o n A r o m a t i c Carbon (%) Total Aliphatic Carbon (%) Proton D i s t r i b u t i o n Aromatic Protons B e n z y l i c Protons Total Aliphatic P r o t o n s (%)
(%) (%)
Carbon/Hydrogen Atomic Ratio
1.03
1.23
1.62
Bacha et al.; Petroleum-Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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Table V .
Thermogravimetric A n a l y s i s of C C B - D i s t l l l a t e
Pitches
P i t c h Process Conditions Temperature ( ° C ) Time (Hours)
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Temperature
400 1.0
Feed
(°C)
C u m u l a t i v e L o s s , Wt.%
200 300 400 500 600
Coke Y i e l d (Wt. @ 500°C
440 1.0
430 1.0
420 1.0
410 1.0
1.2 40.5 95.6 96.4 97.0
0.4 2.1 17.2 56.4 66.9
0.3 1.3 20.8 50.0 60.0
0 1.8 10.0 43.3 56.4
0 0.5 3.7 18.8 37.9
0 0.3 2.7 16.9 30.0
5.6
43.6
50.0
56.7
81.2
83.1
%)
Table V I .
DSC D a t a of
CCB-Distillate
Process Conditions Temperature ( ° C ) Time (Hours)
400 1.0
410 1.0
420 1.0
430 1.0
440 1.0
I n i t i a t i o n Temperature ( ° C ) G l a s s T r a n s i t i o n Temperature (Tg) (°C) T e r m i n a t i o n Temperature ( ° C )
187 217
189 218
191 219
194 223
198 229
242
247
251
254
258
Bacha et al.; Petroleum-Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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+ 60
+ 30
20 +
10 +
400
410 420 430 TEMPERATURE IN DEGREES CENTIGRADE
F i g u r e 1. E f f e c t o f p r o c e s s temperature on the r a t e o f p y r i d i n e , and q u i n o l i n e i n s o l u b l e s f o r m a t i o n .
F i g u r e 2 . E f f e c t o f r e a c t i o n time on t o l u e n e , quinoline insolubles formation.
440
toluene,
p y r i d i n e , and
Bacha et al.; Petroleum-Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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F i g u r e 4.
Micrograph
of
C C B - d i s t i l l a t e p i t c h p r e p a r e d a t 410
Bacha et al.; Petroleum-Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
C.
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DICKAKIAN
Figure 5.
Figure 6.
Aromatic Pitches from the Asphaltene-Free Distillate Fraction
Micrograph of C C B - d i s t i l l a t e pitch prepared at 420 C.
Micrograph of C C B - d i s t i l l a t e pitch prepared at 430 C
RECEIVED September 10, 1985
Bacha et al.; Petroleum-Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
