9 Synthetic Aromatic Pitch Aromatic Pitch Production Using Steam-Cracker Tar
Downloaded by CORNELL UNIV on August 19, 2016 | http://pubs.acs.org Publication Date: April 14, 1986 | doi: 10.1021/bk-1986-0303.ch009
G. Dickakian Specialties Technology Division, Exxon Chemical Company, Houston, TX 77029
Steam cracker tar (SCT) is a by-product from the steam cracking of naphtha or gas oils to produce ethylene. The characteristics and yield of SCT is dependent on the feed characteristics, the plant design and severity of cracking. SCT, like other heavy aromatic materials, is composed of alkyl substituted low molecular weight polynuclear aromatic oils (Mn = 160) and high molecular fraction (asphaltenes), insoluble in paraffinic solvents (Mn = 700 - 1500). The characteristics of SCT, derived from naphtha, gas o i l and desulfurized gas o i l steam cracking, are given in Table I. SCT can be converted into highly aromatic pitches by physical, thermal and chemical processes such as: vacuum or steam stripping, thermal or catalytic oxidative-polymerization at 229-260°C, or by a thermal process at 370-450°C at atmospheric nitrogen or hydrogen pressure. The physical or chemical characteristics of the pitches produced from SCT depend on the type of process and conditions used. Table II gives the characteristics of SCT pitches produced by distillation, catalytic air-oxidation and thermal process. The most suitable process for transforming SCT into highly aromatic pitch is the thermal process at high temperature (380-430°C) at atmospheric, high or reduced pressure. The main chemical reac tions taking place during the thermal process are dealkylation, aromatization, and the condensation of aromatic rings into high aromaticity pitch. The increase in the aromatic carbon atom (by carbon - NMR) during a typical thermal process of SCT at 380°C at atmospheric pressure is illustrated in Figure 1. When using the thermal process for the production of SCT pitch, the temperature and time are important process parameters. The higher the temperature used, the higher is the aromaticity and condensation of the aromatic rings. The average carbon and proton distributions (determined by Nuclear Magnetic Resonance Spectroscopy) of SCT pitches prepared by thermal process at 390°C and 430°C are presented in Table III. SCT pitches produced by a thermal process at appropriate condi tions have high coking yields, high aromatic carbon, low viscosity, high carbon content and very low content of polar atoms. A compari0097-6156/86/0303-0134$06.00/0 © 1986 American Chemical Society
Bacha et al.; Petroleum-Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
Downloaded by CORNELL UNIV on August 19, 2016 | http://pubs.acs.org Publication Date: April 14, 1986 | doi: 10.1021/bk-1986-0303.ch009
9.
DICKAKIAN
Aromatic Pitch Production Using Steam-Cracker Tar
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son o f t h e p h y s i c a l and c h e m i c a l c h a r a c t e r i s t i c s o f two commercial p e t r o l e u m p i t c h e s , two commercial c o a l t a r p i t c h e s and a SCT p i t c h p r e p a r e d by a t h e r m a l p r o c e s s i s p r e s e n t e d i n T a b l e I V . V i s c o s i t y i s an i m p o r t a n t c h a r a c t e r i s t i c o f p i t c h e s used as b i n d e r s f o r t h e p r o d u c t i o n o f carbon and g r a p h i t e e l e c t r o d e s . We used a Haake b a l a n c e t o measure SCT, p e t r o l e u m and c o a l t a r p i t c h viscosity. SCT p i t c h e s have v i s c o s i t y between 1000-4000 cps a t 160°C. A comparison o f t h e v i s c o s i t y - t e m p e r a t u r e r e l a t i o n s h i p o f two SCT p i t c h e s p r e p a r e d by t h e r m a l and c a t a l y t i c p r o c e s s e s , a commercial p e t r o l e u m and a c o a l t a r p i t c h 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 anodes i s g i v e n i n F i g u r e 2. We used t h e r m a l a n a l y s i s t o determine t h e t h e r m o g r a v i m e t r i c a n a l y s i s (TGA) and t h e d i f f e r e n t i a l t h e r m o g r a v i m e t r i c a n a l y s i s (DTG) o f SCT p i t c h e s t o o b t a i n i n f o r m a t i o n on v o l a t i l i t y and coke y i e l d a t v a r i o u s temperatures up t o 1000°C. DTG was found v e r y u s e f u l i n d e f i n i n g p r o c e s s m o d i f i c a t i o n s t o reduce v o l a t i l e s i n t h e p i t c h and i n c r e a s e p i t c h coke y i e l d . F i g u r e 3 g i v e s t h e DTG ( i n n i t r o g e n ) o f s e v e r a l SCT p i t c h e s p r e p a r e d by d i s t i l l a t i o n , t h e r m a l and c a t a l y t i c p r o c e s s , i n comparison w i t h p e t r o l e u m and c o a l t a r p i t c h e s . SCT p i t c h e s l i k e p e t r o l e u m and c o a l t a r p i t c h e s c o n t a i n an a s p h a l t e n e f r e e polycondensed a r o m a t i c o i l w i t h 2-6 a r o m a t i c r i n g s . The a r o m a t i c o i l i n t h e p i t c h c a n be q u a n t i t a t i v e l y d e t e r m i n e d by u s i n g a h i g h vacuum d i s t i l l a t i o n w i t h c o n t i n u o u s a g i t a t i o n t o a v o i d p i t c h c r a c k i n g o r c o k i n g . The o i l i n t h e p i t c h i s i m p o r t a n t as i t e f f e c t s p i t c h v o l a t i l i t y , v i s c o s i t y and t h e development o f a n i s t r o p i c s t r u c t u r e when c o k i n g t h e p i t c h d u r i n g t h e c a r b o n i z a t i o n o f t h e green carbon anodes. F i g u r e 4 g i v e s t h e vacuum d i s t i l l a t i o n c u r v e s o f SCT, p e t r o l e u m and c o a l t a r p i t c h e s . The 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 o f SCT, p e t r o l e u m and c o a l t a r p i t c h e s were determined by G e l P e r m e a t i o n Chromatography a t h i g h temperature u s i n g 1 , 2 , 4 - t r i c h l o r o b e n z e n e as t h e s o l v e n t and a U V s p e c t r o p h o t o m e t e r a t w a v e - l e n g t h 320 mm as t h e d e t e c t o r . A compari son o f t h e 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 c u r v e s o f SCT p i t c h and p e t r o l e u m and c o a l t a r p i t c h e s i s p r e s e n t e d i n F i g u r e 5. I n summary, h i g h s o f t e n i n g p o i n t , h i g h c o k i n g v a l u e and h i g h a r o m a t i c i t y p i t c h e s can be p r e p a r e d from SCT. The p h y s i c a l , t h e r m a l , c h e m i c a l and c o k i n g c h a r a c t e r i s t i c s o f S C T - P i t c h e s i s dependent on the type o f p r o c e s s u s e d , d e s i g n o f p l a n t and t h e p r o c e s s c o n d i t i o n s e s p e c i a l l y , t e m p e r a t u r e , t i m e , p r e s e n c e o f c a t a l y s t o r oxygen and pressure.
Bacha et al.; Petroleum-Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
Bacha et al.; Petroleum-Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
A r o m a t i c Carbon (atom %) A r o m a t i c P r o t o n s (%) B e n z y l i c P r o t o n s (%) P a r a f f i n i e P r o t o n s (%) Carbon/Hydrogen Atomic R a t i o
2. Chemical S t r u c t u r e
V i s c o s i t y e s t a t 210°F C o k i n g V a l u e a t 50°F (%) Toluene I n s o l u b l e s (%) η-Heptane I n s o l u b l e s (%) Pour P o i n t (°C) Ash (%)
65 34 40 25 0. 942
0.003
13.9 12 0.200 3.5
SCT from Naphtha Cracking
72 42 44 14 1.001
19. 16 0 ..200 16 +5 0.003
71 42 46 12 1.079
12.4 24 0.250 20 -6 0.003
SCT from Gas O i l Cracking (2) (1)
74 38 47 15 1.44
25 25 0 .,100 15 +6 0.003
SCT from Desulfurized Gas O i l C r a c k i n g
P h y s i c a l and C h e m i c a l C h a r a c t e r i s t i c s o f Steam C r a c k e r T a r s from Naphtha and Gas O i l C r a c k i n g
1. P h y s i c a l C h a r a c t e r i s t i c s
Table I .
Downloaded by CORNELL UNIV on August 19, 2016 | http://pubs.acs.org Publication Date: April 14, 1986 | doi: 10.1021/bk-1986-0303.ch009
2!
CO Ο
η >
α
m
