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High-Surface-Area Active Carbon T. M. O'Grady and A. N. Wennerberg 1
1
2
Amoco Research Center, Amoco Oil Company, Naperville, IL 60566
This paper describes the preparation and properties of a unique active carbon having exceptionally high surface areas, over 2500 m /gm, and extraordinary adsorptive capacities. The carbon is made by a direct chemical activation route in which petroleum coke or other carbonaceous sources are reacted with excess potassium hydroxide at 400° to 500°C to an intermediate product that is subsequently pyrolyzed at 800° to 900°C to active carbon containing potassium salts. These are removed by water washing and the carbon is dried to produce a powdered product. A granular carbon can also be made by further processing the powdered carbon by using specialized granulation techniques. Typical properties of the carbon include Iodine Numbers of 3000 to 3600, methylene blue adsorption of 650 to 750 mg/gm, pore volumes of 2.0 to 2.6 cc/gm and less than 3.0% ash. This carbon's high adsorption capacities make i t uniquely suited for numerous demanding applications in the medical area, purifications, removal of toxic substances, as catalyst carriers, etc. It will be commercially available from Anderson Development Company in mid-1985. 2
A unique a c t i v e carbon h a v i n g v e r y h i g h s u r f a c e a r e a s over 2500 m / gm, and e x t r a o r d i n a r y a d s o r p t i v e c a p a c i t i e s was developed i n o u r laboratories. (1) T h i s paper w i l l d e s c r i b e i t s development, m a n u f a c t u r e , p r o p e r t i e s , and u s e s . U n t i l r e c e n t l y , samples o f t h i s c a r b o n , w h i c h were p r o v i d e d w o r l d w i d e f o r r e s e a r c h and e v a l u a t i o n , were i d e n t i f i e d as Amoco Grades P X - 2 1 , 2 2 , 2 3 , and 24 i n t h e powdered form and Amoco GX-31 and 32 i n g r a n u l a r f o r m . The carbon i s made ( F i g u r e 1) by a d i r e c t c h e m i c a l a c t i v a t i o n r o u t e i n w h i c h p e t r o l e u m coke o r o t h e r carbonaceous sources a r e r e a c t e d w i t h excess p o t a s s i u m h y d r o x i d e , KOH, a t 400° t o 500°C t o an i n t e r m e d i a t e p r o d u c t t h a t i s s u b s e q u e n t l y p y r o l y z e d a t 800°-900°C t o a c t i v e carbon and p o t a s s i u m salts. The s a l t s a r e removed by water w a s h i n g . 2
2
Retired. 0097-6156/86/0303-0302$06.00/0 © 1986 American Chemical Society
In Petroleum-Derived Carbons; Bacha, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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O'GRADY AND WENNERBERG
High-Surface-Area Active Carbon
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Materials A l t h o u g h v a r i o u s carbonaceous sources c a n be u s e d , p e t r o l e u m cokes were p r e f e r r e d because of t h e i r low ash c o n t e n t s and g e n e r a l l y h i g h e r y i e l d s of a c t i v e c a r b o n . The cokes were o b t a i n e d from v a r i o u s sources and w i t h w i d e l y v a r y i n g p r o p e r t i e s . S u l f u r contents ranged from 2.0 to 6.0 w t . % ; m e t a l s , p r i m a r i l y n i c k e l and vanadium, from 500 ppm to 5,000 ppm; v o l a t i l e m a t t e r , from 11% to 20%. The coke q u a l i t y w i t h i n these ranges d i d not appear to a f f e c t a c t i v e carbon p r o p e r t i e s . However, somewhat lower a c t i v e carbon y i e l d s were noted w i t h the h i g h e r v o l a t i l e m a t t e r cokes (58-62 wt% v s . 62-65 wt%). I n the p i l o t p l a n t o p e r a t i o n coke was ground to 99% s m a l l e r t h a n 100 mesh. Commercial g r a d e , f i n e c r y s t a l l i n e KOH (90%), was used i n most of the s t u d i e s . P i l o t P l a n t Development and P r o c e s s D e s c r i p t i o n P i l o t p l a n t development was c a r r i e d out to p r o v i d e d a t a f o r p r o c e s s and e n g i n e e r i n g d e s i g n as w e l l as economic e v a l u a t i o n s . The p i l o t p l a n t a l s o s e r v e d to p r o v i d e a c t i v e carbon f o r r e s e a r c h and e v a l u a t i o n s t u d i e s . The major p i e c e s of equipment used to c a r r y out the p i l o t p l a n t process development are d e p i c t e d i n t h e i r f l o w sequence I n F i g u r e 2 . S i n c e i t was d e c i d e d a t the onset to c o n c e n t r a t e on s t u d y i n g each process s t e p r a t h e r t h a n b e i n g d i s t r a c t e d by m a t e r i a l s f l o w p r o b l e m s , t h e r e was no a c t u a l c o n t i n u o u s f l o w o f m a t e r i a l from one p i e c e of equipment to a n o t h e r . Each o p e r a t i o n was c a r r i e d out and s t u d i e d i n d e p e n d e n t l y . Any m a t e r i a l p r o duced was s t o r e d i n s e a l e d b a r r e l s or drums and was moved t o the next o p e r a t i o n when needed. M o r e o v e r , the outputs o f the v a r i o u s p i e c e s of equipment were d i f f e r e n t . I n terms o f pounds of a c t i v e c a r b o n per h o u r , o u t p u t s ranged from 25 to 110. Because of the h y g r o s c o p i c n a t u r e of the KOH and the i n t e r m e d i a t e p r o d u c t s , i t was n e c e s s a r y to p r o v i d e f a c i l i t i e s throughout f o r i n e r t gas ( n i t r o g e n ) b l a n k e t i n g . Water not o n l y a f f e c t e d the h a n d l i n g p r o p e r t i e s o f the v a r i o u s i n t e r m e d i a t e process p r o d u c t s , but a l s o became i n v o l v e d c h e m i c a l l y I n the s h i f t r e a c t i o n w h i c h reduced y i e l d s and d e t r i m e n t a l l y a f f e c t e d a c t i v e carbon p r o p e r t i e s . Because oxygen ( a i r ) had s i m i l a r e f f e c t s and f o r obvious s a f e t y r e a s o n s , oxygen was excluded from h i g h temperature o p e r a t i o n s where r e d u c i n g (H2) atmospheres were n a t u r a l l y produced and m a i n t a i n e d . T y p i c a l p i l o t p l a n t o p e r a t i o n began by p r e p a r i n g an i n t i m a t e m i x t u r e of ground coke and KOH i n a r i b b o n b l e n d e r i n a range of KOH/coke weight r a t i o s o f 2 to 4. The b l e n d was s t o r e d i n N2" b l a n k e t e d and s e a l e d drums. I n the p i l o t p l a n t the coke-KOH m i x t u r e was charged w i t h a screwfeeder to a p r e c a l c i n e r w h i c h was an i n d i r e c t l y - f i r e d r o t a r y kiln. The r e a c t i o n proceeded i n the p r e c a l c i n e r at i n t e r n a l temperatures of 400° to 500°C. The p r o d u c t , c a l l e d p r e c a l c i n a t e , was c o o l e d and t e m p o r a r i l y s t o r e d i n s e a l e d 3 0 - g a l l o n drums. O v e r a l l y i e l d of s o l i d product i n t h i s s t e p was about 95%. The p r e c a l c i n a t i o n r e a c t i o n s produced a gas t h a t c o n t a i n e d m o s t l y hydrogen w i t h s m a l l amounts o f methane and condensables such as w a t e r and t a r s . F o r p r o c e s s and s a f e t y r e a s o n s , a s l i g h t l " - 2 " H2O p r e s s u r e was m a i n t a i n e d i n the k i l n w i t h a c o u n t e r c u r r e n t gas f l o w . The p r e c a l c i n a t e was next p y r o l y z e d or c a l c i n e d i n another
In Petroleum-Derived Carbons; Bacha, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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PETROLEUM-DERIVED CARBONS
F i g u r e 1.
Coke^^OH
Powdered active carbon
Wash/ filtration
Caustic recovery
Granular active carbon
High surface area a c t i v e carbon.
Dry feed / Off gas
Blender _
J
/
\j l o 6 ο 6 ο 6 I | 1
L-To precalciner
Feed preparation
/
y ι ρ ρ ρ ρ ρ ρ ι *=ry
Indirect-fired rotary calciner
Indirect-fired rotary precalciner
To slurry tank
ii
γ φ Water vvdier ,
α
/
Wet filter cake
ID I ^V^* Filtrate
Baghouse
1 Active carbon product
Air heater
\
Air
Slurry tank
Belt filter Figure 2.
j
Drying duct
Wash water
Calcinate
Off gas
Flash dryer
Powdered a c t i v e carbon p i l o t
plant.
In Petroleum-Derived Carbons; Bacha, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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Ο'GRADY AND WENNERBERG
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i n d i r e c t l y - f i r e d k i l n a t about 850°C. Somewhat l e s s gas was produced a l s o c o n t a i n i n g m o s t l y hydrogen. The a c t i v e carbon p r o d u c t c o n t a i n e d o c c l u d e d p o t a s s i u m s a l t s , p r i m a r i l y c a r b o n a t e w i t h some s u l f i d e s and s u l f a t e and u n c o n v e r t e d KOH. The c a l c i n a t e was quenched i n water to produce a pumpable slurry. Carbon washing was c a r r i e d out on a h o r i z o n t a l b e l t vacuum f i l t e r w h i c h reduced the s a l t c o n t e n t to 4 to 5%. To f u r t h e r reduce the s a l t c o n t e n t , i f needed, a d d i t i o n a l s l u r r y i n g and washing on the b e l t f i l t e r was r e q u i r e d . I n commercial o p e r a t i o n the f i l t r a t e c o n t a i n i n g p o t a s s i u m s a l t s c o u l d be p r o c e s s e d to r e c o n s t i t u t e the KOH f o r r e c y c l e . I n the l a s t p r o c e s s s t e p , the wet f i l t e r cake c o n t a i n i n g about 70% water was f l a s h d r i e d to a f i n a l w a t e r c o n t e n t of 2-5%. The powdered a c t i v e carbon was c o l l e c t e d i n p o l y e t h y l e n e bags w h i c h were s e a l e d and s t o r e d i n f i b r e drums. By u s i n g s u i t a b l e b i n d i n g agents and t e c h n i q u e s , the powdered carbon c o u l d be produced i n g r a n u l a r f o r m . From p r o c e s s s t u d i e s c a r r i e d out i n t h i s p i l o t p l a n t over s e v e r a l y e a r s , parameters were d e f i n e d to p e r m i t p r o c e s s and e n g i n e e r i n g d e s i g n s of f u l l commercial o p e r a t i o n s . Process eco nomics i n d i c a t e d the c o s t of t h i s a c t i v e carbon would be g r e a t e r than c u r r e n t l y a v a i l a b l e commercial c a r b o n s . However, hundreds of e v a l u a t i o n s i n a f u l l spectrum o f a p p l i c a t i o n s showed t h a t t h i s a c t i v e carbon would be c o m p e t i t i v e i n many of those a p p l i c a t i o n s on a cost-performance b a s i s . I t w i l l be c o m m e r c i a l l y a v a i l a b l e m i d - 8 5 i n v a r i o u s f o r m s , trademarked as SUPER-Α, from Anderson Development Company i n A d r i a n , M i c h i g a n . Properties T a b l e s I and I I l i s t major t y p i c a l p h y s i c a l and a d s o r p t i v e p r o p e r t i e s of the powdered a c t i v e c a r b o n . E f f e c t i v e s u r f a c e a r e a , measured by the BET method u s i n g a D i g i s o r b 2500, i s c o n s i s t e n t l y i n the range of 3000 to 3400 m^/gm. T h i s exceeds the t h e o r e t i c a l a r e a of about 2600 m^/gm as c a l c u l a t e d by the a r e a of one gram of a g r a p h i t i c p l a n e because o f m u l t i l a y e r a d s o r p t i o n and pore f i l l i n g i n a h i g h l y microporous s t r u c t u r e . As shown by H . M a r s h , et a l ( 2 ) . u s i n g phase c o n t r a s t , h i g h r e s o l u t i o n e l e c t r o n m i c r o s c o p y (JEOL 1 0 0 C ) , t h i s carbon has a c a g e l i k e s t r u c t u r e i n w h i c h the i n d i v i d u a l cages a r e so s i z e d t h a t they e x h i b i t p r o p e r t i e s o f super m i c r o p o r o s i t y , i . e . , e s s e n t i a l l y complete f i l l i n g o f the i n d i v i d u a l cages by the a d s o r b a t e a t low c o n c e n t r a t i o n to g i v e h i g h e f f e c t i v e s u r f a c e a r e a s and l a r g e m i c r o pore volumes. The cages a r e a l s o s u b s t a n t i a l l y homogeneous i n s i z e as can be seen by low m a g n i f i c a t i o n image photomicrographs ( x l 4 2 , 0 0 0 ) i n F i g u r e 3. A t h i g h e r m a g n i f i c a t i o n ( x 3 , 1 1 6 , 0 0 0 , F i g u r e 4) the i n d i v i d u a l cages a r e c l e a r l y e v i d e n t and appear to be formed by w a l l s o f f o l d e d carbonaceous g r a p h i t i c - t y p e l a m e l l a e 1-3 carbonaceous s h e e t s i n t h i c k n e s s . U s i n g X - r a y d i f f r a c t i o n t e c h n i ques, K o n n e r t e t a l . (3) s i m i l a r l y d e s c r i b e s the s t r u c t u r e as b e i n g composed of d i s t o r t e d r i b b o n s of one or v e r y few g r a p h i t e - l i k e l a y e r s . More d e t a i l e d d i s c u s s i o n s on t h i s s u b j e c t can be found i n these papers by Marsh and K o n n e r t . N o t w i t h s t a n d i n g the h i g h m i c r o p o r o s i t y of t h i s c a r b o n , i t
In Petroleum-Derived Carbons; Bacha, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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PETROLEUM-DERIVED CARBONS
Table I .
Typical Physical Properties Powdered A c t i v e Carbon
S u r f a c e A r e a , BET m /gm Average P o r e D i a m e t e r , S P o r e Volume, >2θ8 D i a m . , cc/gm < 2 0 & D i a m . , cc/gm Total
3000-3400 23-25 0.7-0.9 1.3-1.7 2.0-2.6
B u l k D e n s i t y , gm/cc P a r t i c l e Size vl)
