Coke Formation on Metal Surfaces - American Chemical Society

14 Reaction of Steam with Coke on Solid Substrates T. Y. Y A N

and

M. P. ROSYNEK

1

Downloaded by CORNELL UNIV on August 4, 2016 | http://pubs.acs.org Publication Date: October 25, 1983 | doi: 10.1021/bk-1983-0202.ch014

Mobil Research and Development Corporation, Central Research Division, Princeton, NJ 08540

The kinetics of coke gasification are markedly influenced by the nature of the substrate on which the coke i s deposited. Steam gasification of coke deposited on a silica-alumina substrate followed first-order kinetics, with respect to carbon remaining, with an activation energy of 55.5 kcal/mole and a reaction rate constant of 5 x 10 min at 1500°F. Prior impregnation of the substrate with copper, iron, or vanadium oxides had v i r t u a l l y no effect on the rate of subsequent coke gasification. Nickel oxide, on the other hand, caused a three-fold increase in i n i t i a l gasification rate, up to 30% of coke conversion. Coke deposited on an alumina substrate was three to six times more reactive toward steam gasification at 1500°F than that on silica-alumina. Alumina apparently influences both the rate and the structure of coke deposited on it. -3

-1

Reaction of steam w i t h carbon i s one of the b a s i c processes i n v o l v e d i n the g a s i f i c a t i o n of c o a l s or chars to produce clean fuels. The i n d u s t r i a l importance of t h i s r e a c t i o n i s considerable, p a r t i c u l a r l y at t h i s time of s p i r a l i n g energy c o s t s . In a d d i t i o n , the steam-carbon r e a c t i o n f i n d s other important i n d u s t r i a l a p p l i c a t i o n s , such as preventing or minimizing the coking of o l e f i n - p l a n t cracker-tubes. Because of i t s commercial importance, the steam-carbon r e a c t i o n has been studied e x t e n s i v e l y , and e x c e l l e n t reviews are a v a i l a b l e ÇL,2). In a d d i t i o n to r e a c t i o n k i n e t i c s and mechanisms, the e f f e c t s of carbon s t r u c t u r e , c a t a l y s i s by metals, and i m p u r i t i e s (anions) have been i n v e s t i g a t e d (3,.4,5) .

1

Current address: Texas A&M

University, Department of Chemistry, College Station, TX 77843.

0097-6156/82/0202-0283$06.00/0 © 1982 American Chemical Society

Albright and Baker; Coke Formation on Metal Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Except f o r s p e c t r o s c o p i c g r a p h i t e , carbons used in the steam-carbon r e a c t i o n , v i z . , a c t i v e carbon, coke, and char, t y p i c a l l y c o n t a i n v a r y i n g amounts of hydrogen, i . e . , the "carbons" are hydrocarbons in a broader sense. In a d d i t i o n to i m p u r i t i e s , v a r i a t i o n s in the compositions of the carbons (e.g., hydrogen content) complicate the measurements and are a cause of d i s c r e p a n c i e s among previous s t u d i e s . The r e a c t i o n s of steam with c o a l s , cokes and chars have been reported and compared w i t h the corresponding steam-graphite r e a c t i o n s (6-9). References in the open l i t e r a t u r e to the r e a c t i o n of steam w i t h carbon that has been deposited on v a r i o u s i n o r g a n i c s o l i d s u b s t r a t e s , however, are l a c k i n g . In the c a t a l y t i c p r o c e s s i n g of heavy o i l and r e s i d u a over s o l i d c r a c k i n g or hydrocracking c a t a l y s t s , a s i g n i f i c a n t amount of "coke" i s deposited on the c a t a l y s t s , thus lowering the catalytic activities. Such coked c a t a l y s t s are regenerated by burning o f f the coke with a i r . When the l e v e l of coke on the c a t a l y s t i s high, e x c e s s i v e heat can be generated during regeneration, causing temperature run-away, and r e s u l t i n g in p o s s i b l e i r r e v e r s i b l e c a t a l y s t s i n t e r i n g and permanent l o s s of activity. Furthermore, the p o t e n t i a l heating value of the coke i s l o s t , because recovery of waste heat from the f l u e gas i s r e l a t i v e l y expensive. Various schemes have been suggested f o r regenerating s e v e r e l y coked c a t a l y s t s u s i n g steam and oxygen (10,11). We have p r e v i o u s l y reported a process f o r upgrading petroleum r e s i d u a that i n v o l v e s c o n t a c t i n g i t with a s o l i d c a t a l y s t , followed by g a s i f i c a t i o n of the coke-laden s u b s t r a t e with steam and oxygen (12). The present study was undertaken in an e f f o r t to gain a b e t t e r understanding of c e r t a i n aspects of the r e a c t i o n s i n v o l v e d in the g a s i f i c a t i o n of cokes deposited on c a t a l y s t s u b s t r a t e s in these a p p l i c a t i o n s . S o l i d s u b s t r a t e s were coked with a petroleum residuum under simulated c o n d i t i o n s and the substrate-supported coke was then r e a c t e d with steam. Effluent gases were analyzed, and the k i n e t i c s of g a s i f i c a t i o n and the e f f e c t s of s u b s t r a t e s i d e n t i t i e s and metal impregnations were examined. Experimental Methods Materials. P h y s i c a l p r o p e r t i e s of the s o l i d s u b s t r a t e s employed in t h i s study are summarized in Table I. The s i l i c a alumina was an e q u i l i b r a t e d Durabead TCC c a t a l y s t from Mobil O i l Corp., w i t h an a c t i v i t y index of ^50. The b a u x i t e extrudate was of the North American v a r i e t y used in the Claus process f o r s u l f u r recovery, and contained 2.3 and 12.5 wt % of i r o n and S i 0 , respectively. The m o n t m o r i l l o n i t e c a t a l y s t was obtained from Chemetron Corp. (No. K306), and was used in the form of i r r e g u l a r 14/60 mesh granules. I t s wt % composition was S1O2, 71.7; A 1 0 , 12.5; F e 0 , 5.2; CaO, 2.7; MgO, 3.6; i g n i t i o n 2

2

3

2

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l o s s , 4.0. The two γ-alumina s u b s t r a t e s were obtained from C o n t i n e n t a l O i l Co. (Catapal S, in p e l l e t e d form) and from Harshaw Chemical Co. (A1-1706E, 1/8 in. e x t r u d a t e ) . A l l sub s t r a t e s were c a l c i n e d in a i r f o r 16 hrs at 1000°F before use. Table I P h y s i c a l P r o p e r t i e s of S o l i d


Substrate

S.A. Pore V o l . Ave. Pore App. Dens. Real Dens. (m /g) (cc/g) Diam. (Â) (g/cc) (g/cc) 2

Silica-Alumina (Durabead)

150

0.46

92

Silica-Alumina (Montmorillonite)

250

0.54

—

Y-Alumina (Catapal S)

250

0.53

65

γ-Alumina (Harshaw)

218

0.77

—

62

0.44

261

Bauxite

Substrates

1.13

2.35

0.65

0.69

3.32

0.51

1.39

3.55

For experiments i n v o l v i n g metal-containing s u b s t r a t e s , samples of the Durabead s i l i c a - a l u m i n a were impregnated w i t h the n i t r a t e s a l t s of copper, i r o n , or n i c k e l , or w i t h vanadyl s u l f a t e , at a l e v e l of 5 wt % metal, and then c a l c i n e d in a i r f o r 10 hrs at 1000°F. The r e s u l t i n g metal o x i d e - c o n t a i n i n g m a t e r i a l s were then subjected to the coking procedure d e s c r i b e d below without f u r t h e r treatment. Residuum b o i l i n g at >650°F and d e r i v e d from an Agha J a r i crude o i l was used f o r the coking procedure. The residuum had an API g r a v i t y of 17.3°, a pour p o i n t of 75°F, a kinematic v i s c o s i t y of 800 cs. at 100°F, and n i t r o g e n and s u l f u r contents of 2.42 and 0.33 wt %, r e s p e c t i v e l y . Procedures. Coke was deposited on each of the v a r i o u s s o l i d s u b s t r a t e s by i n t i m a t e l y mixing with Agha J a r i residuum at a s o l i d / o i l r a t i o of 0.5 by weight in a r e a c t o r equipped w i t h a s t i r r e r and a r e f l u x condenser> The coking r e a c t i o n was allowed to proceed at 900°F f o r 24 hrs under 1 atm. of n i t r o g e n . The coked s u b s t r a t e was then s t r i p p e d f o r 3 hours w i t h flowing n i t r o g e n in a t u b u l a r r e a c t o r at 950°F, and f i n a l l y cooled under n i t r o g e n to room temperature. The composition of cokes on the s u b s t r a t e s were determined by a combustion method. The composi t i o n of coke on the s i l i c a - a l u m i n a c a t a l y s t was CHQ.25« The r e s u l t i n g l e v e l s of coke deposited on the v a r i o u s s u b s t r a t e s are contained in Table I I .


COKE

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FORMATION

Table I I Steam/Coke Reaction Rates At 1500°F

Material

17 59

Durabead Montmorillonite Catapal Pellets Catapal Pellets γ-Alumina Extrudate Bauxite Extrudate

56 58


Basic Substrate

Run

52 60

T o t a l Coke F i r s t Order Rate (wt %) Const, (x 1 0 min" ) 2

4.9 10.0

0.50 0.45

Alumina

7.0

2.17

Alumina

11.4

1.37

Alumina

15.2

1.31

Alumina

8.1

2.83

Silica-Alumina Silica-Alumina

1

Two grams of e x t e r n a l l y - c o k e d s u b s t r a t e were charged to a 1/2" OD χ 12" long Vycor, downflow r e a c t o r that contained a t h i n - w a l l e d l o n g i t u d i n a l thermowell. The r e a c t o r space above and below the s u b s t r a t e bed was packed w i t h 10-20 mesh Vycor chips. Water was fed with a syringe pump at 2 g/hr, and vapor i z e d upon contact w i t h the hot Vycor c h i p s . Unreacted steam was condensed at the r e a c t o r e x i t , and the t o t a l volume of gas flow was measured w i t h a wet t e s t meter. Samples of the e f f l u e n t gas were taken p e r i o d i c a l l y and analyzed mass s p e c t r o m e t r i c a l l y f o r H , CO, C0 , N and 0 . From these analyses, the r a t e s of disappearance of carbon and appearance of carbon monoxide and carbon d i o x i d e were c a l c u l a t e d . The g a s i f i c a t i o n r e a c t i o n s were c a r r i e d out in the temperature range 1400-1600°F. 2

Results

2

and

2

2

Discussion

G a s i f i c a t i o n K i n e t i c s of Coke Deposited on S i l i c a - A l u m i n a . Within the temperature range 1400 to 1600°F and in the presence of excess steam, the g a s i f i c a t i o n r e a c t i o n of coke deposited on the s i l i c a - a l u m i n a c r a c k i n g c a t a l y s t c l o s e l y followed f i r s t order k i n e t i c s with respect to unreacted carbon (Figure 1). F i r s t - o r d e r r a t e constants were c a l c u l a t e d from the slopes of these p l o t s (Table I I I ) , and y i e l d e d an a c t i v a t i o n energy of 55.5 Kcal/mole.



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Figure 1. Kinetics of steam gasification of coke deposited on silica-alumina, for reaction temperatures of 1400°F (O), 1500°F ({J), 1550°F (A), and 1600°F (V).


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Table I I I Rate Constants f o r Carbon + Steam Reaction on S i l i c a - A l u m i n a Substrate

Τ (°F)

F i r s t - O r d e r Reaction Rate Constant (min" ) 1


1400 1500 1550 1600

.00121 .00463 .00906 .01668

3

- 1

The r e a c t i o n r a t e constant at 1500°F of 4.6 χ 10" m i n i s s t r i k i n g l y s i m i l a r to that observed f o r chars from the Hydrane and Synthane processes (^9 χ 1 0 " min" v i a i n t e r p o l a t i o n o f the r e s u l t s o f Fuchs et a l . ) ( 6 ) . The a c t i v a t i o n energy o f 55.5 Kcal/mole i s w i t h i n the range (51.3 to 56.6 Kcal/mole) reported p r e v i o u s l y by T y l e r and Smith f o r g a s i f i c a t i o n o f petroleum coke and e l e c t r o d e m a t e r i a l s (13). In a d d i t i o n , our r e s u l t i s much lower than the 83 ± 5 Kcal/mole found by Long and Sykes (14) f o r g a s i f i c a t i o n o f p u r i f i e d c h a r c o a l , but c l o s e to the 55 ± 7 Kcal/mole that they observed f o r contaminated c h a r c o a l . 3

1

E f f e c t o f Impregnated Metals on Coke G a s i f i c a t i o n Rate. The steam-carbon r e a c t i o n i s known to be c a t a l y z e d by metals, p a r t i c u l a r l y t r a n s i t i o n metals (_3,4· ). In an e f f o r t to improve the r a t e o f g a s i f i c a t i o n , separate samples o f the s i l i c a alumina (Durabead) c a t a l y s t were impregnated with one of v a r i o u s metals p r i o r to coke d e p o s i t i o n , and the r e s u l t s f o r the subse quent steam-carbon r e a c t i o n at 1500°F over these m a t e r i a l s a r e shown in F i g u r e 2 and Table IV. The e f f e c t s o f the deposited metal oxides can be summarized as f o l l o w s : Cu, V, Fe: Ni:

No e f f e c t on the g a s i f i c a t i o n r e a c t i o n r a t e . A c c e l e r a t i o n o f the i n i t i a l g a s i f i c a t i o n r a t e ( f o r coke conversions up to ^30%).



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Figure 2. Kinetics of steam gasification at 1500°F of coke deposited on silicaalumina impregnated with oxides of copper (A), vanadium (Q), iron (V), and nickel (O, 3, Q, # [various preparations]).


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Table IV Coke/Steam Reaction Rates on Metal-Impregnated Durabead ( S i l i c a - A l u m i n a ) TCC C a t a l y s t s at 1500°F 1


Run 17 39 40 41 38 42 43

Metal

Wt %

_

_

Fe Cu V Ni Ni Ni

5 5 5 5 5 5

Rate Constant (min" ) 0-30% Conv. >30% Conv. .0046 .0058 .0049 • 0046 .0139 .0137 .0140

__

— — — .0059 .0063 .0067

R e l a t i v e Rate Constant 0-30% Conv. >30% Conv. 1.00 1.25 1.05 1.00 3.00 2.96 3.02

— — — 1.28 1.37 1.46

Impregnated Cu, V, and Fe oxides had no e f f e c t on the r a t e of subsequent coke g a s i f i c a t i o n . H a l l and Rase (15) found that f o r a s i l i c a - a l u m i n a c r a c k i n g c a t a l y s t contaminated w i t h metals up to a l e v e l of MD.2 wt %, the r a t e of o x i d a t i o n o f deposited coke d i d not i n c r e a s e in comparison w i t h that over the uncontaminated c a t a l y s t . These metals are known to a c c e l e r a t e the r e a c t i o n , however, when they are admixed i n t i m a t e l y w i t h the coke. I t i s apparent in t h i s case that contact between the impregnated metals and the coke was not s u f f i c i e n t l y c l o s e to be catalytically significant. Haldeman and Botty (16) found that on a s i l i c a - a l u m i n a c r a c k i n g c a t a l y s t c o n t a i n i n g 6% coke by weight, the dimensions of the average coke p s e u d o c r y s t a l l i t e were 17 and 10-12 Â f o r Le (normal to l a y e r s ) and La ( i n p l a c e of l a y e r s ) , r e s p e c t i v e l y . For an i n t e r l a y e r s p a c i n g of 3.47 Â, t h i s represents about 5 p a r a l l e l l a y e r groups. Coke d e p o s i t s were a l s o found to be l o c a t e d p r e f e r e n t i a l l y on the metal s i t e s . At h i g h e r coke l e v e l s , the p s e u d o c r y s t a l l i t e s should be l a r g e r and c o n t a i n more l a y e r s . Thus, at the coking l e v e l o f ^10% used in t h i s study, the metals could be completely covered w i t h coke, be impermeable to steam, and, hence, i n e f f e c t i v e c a t a l y t i c a l l y . Since the g a s i f i c a t i o n r e a c t i o n followed f i r s t - o r d e r k i n e t i c s up to coke conversion l e v e l s of 50%, i t i s apparent t h a t , assuming uniform g a s i f i c a t i o n , even p s e u d o c r y s t a l l i t e s one-half the o r i g i n a l s i z e are s t i l l too l a r g e f o r the metals to be e f f e c t i v e . An a l t e r n a t i v e , but l e s s l i k e l y , e x p l a n a t i o n f o r the c a t a l y t i c i n e f f e c t i v e n e s s of deposited Fe, Cu, and V i s that these metals r e a c t w i t h s i l i c a - a l u m i n a at the h i g h g a s i f i c a t i o n temperatures to form c a t a l y t i c a l l y i n a c t i v e s i l i c a t e s , aluminates, or s o l i d mixtures. There i s another p o s s i b l e e x p l a n a t i o n f o r the apparent l a c k of e f f e c t of metals on the steam g a s i f i c a t i o n r e a c t i o n r a t e . The metals deposited on s u b s t r a t e s could c a t a l y z e the dehydrogenation r e a c t i o n during the coking step l e a d i n g to "cokes" of lower hydrogen content. Such low-hydrogen "coke" i s known to react w i t h steam more slowly. Thus, the metals could a f f e c t the



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steam carbon r e a c t i o n in two opposite ways; v i z . , to i n c r e a s e the r a t e by c a t a l y z i n g the r e a c t i o n on one hand and to decrease the r e a c t i o n r a t e by forming slow r e a c t i n g cokes on the other hand. Except f o r n i c k e l , these two e f f e c t s counteract each other so that the deposited metal shows no o v e r a l l e f f e c t on the r a t e of steam coke r e a c t i o n . In marked c o n t r a s t to the e f f e c t s observed w i t h i r o n , vanadium and copper oxides, p r i o r impregnation of s i l i c a - a l u m i n a w i t h n i c k e l oxide l e d to a c o n s i d e r a b l e i n c r e a s e in the i n i t i a l r a t e of subsequent coke g a s i f i c a t i o n , up to a coke conversion l e v e l of V30% (Figure 2). The k i n e t i c s of carbon d e p o s i t i o n on n i c k e l have been s t u d i e d in d e t a i l p r e v i o u s l y (17,18,19), and a mechanism which e x p l a i n s most observations has been advanced. Coke p r e c u r s o r s proceed through a s e r i e s of dehydrogenation steps on the n i c k e l s u r f a c e , r e s u l t i n g e v e n t u a l l y in carbonaceous s p e c i e s . These s p e c i e s d i s s o l v e in, and p r e c i p i t a t e from, the metal phase detaching n i c k e l c r y s t a l l i t e s from the s u r f a c e of the bulk n i c k e l or the s i l i c a - a l u m i n a surface. Further coke d e p o s i t i o n i n c o r p o r a t e s a d d i t i o n a l n i c k e l i n t o the growing carbon l a y e r , as shown by e l e c t r o n m i c r o s c o p i c examination of the d e p o s i t s (18). In the experiments reported here, some n i c k e l may, theref o r e , be t r a n s p o r t e d by the growing carbon c r y s t a l l i t e s to the s o l i d s u r f a c e d u r i n g coke d e p o s i t i o n . When the carbon-steam r e a c t i o n i s subsequently begun, t h i s n i c k e l i s p a r t i c u l a r l y a c c e s s i b l e and r e a c t i v e , and r e s u l t s in an i n c r e a s e d i n i t i a l r e a c t i o n r a t e . In a d d i t i o n , however, there i s apparently a second f r a c t i o n of coke, deposited on the s i l i c a - a l u m i n a subs t r a t e , which i s not a s s o c i a t e d with the n i c k e l . T h i s coke i s not a f f e c t e d by impregnated n i c k e l and r e a c t s more slowly, p a r a l l e l i n g the behavior of coke on metal-free s i l i c a - a l u m i n a . The presence of these two types of coke r e s u l t s in the observed k i n e t i c behavior. Our data i n d i c a t e that about 30% of the coke r e a c t s at a r a t e approximately 3 times greater than that deposited in the absence of n i c k e l , w h i l e the remaining 70% of the coke r e a c t s at the same r a t e as that without n i c k e l (Table III). This r e s u l t suggests that the f r a c t i o n of coke a s s o c i a t e d with the n i c k e l i s about 30%. E f f e c t of Substrate on Coke G a s i f i c a t i o n Rate. The e f f e c t of s u b s t r a t e v a r i a t i o n s on the steam-carbon r e a c t i o n r a t e at 1500°F i s shown in Table I I and in F i g u r e 3. The s u b s t r a t e s can be c l a s s i f i e d i n t o two c a t e g o r i e s according to t h e i r e f f e c t on coke g a s i f i c a t i o n r e a c t i o n r a t e s : Low

activity:

S i l i c a - a l u m i n a ( c r a c k i n g c a t a l y s t and Montmorillonite) High a c t i v i t y : Alumina, ( c a l c i n e d Catapal, alumina and b a u x i t e ) .


COKE


292

FORMATION

At 1500°F, the steam-coke r e a c t i o n on alumina occurs approximately three to s i x times f a s t e r than on s i l i c a - a l u m i n a type s u b s t r a t e s . Comparison of r e s u l t s from Runs 56 and 58 i n d i c a t e s the extent of r e p r o d u c i b i l i t y of the experiments. In comparison with t h i s r e p r o d u c i b i l i t y and in c o n s i d e r a t i o n of the observed consistency, the increased g a s i f i c a t i o n r a t e s on the alumina substrates are s i g n i f i c a n t . A l l alumina-based m a t e r i a l s t e s t e d , v i z . , γ-alumina, c a l c i n e d Catapal, and bauxite, showed e q u a l l y high a c t i v i t i e s . The a c t i v a t i o n energies f o r coke g a s i f i c a t i o n on the three substrates shown in Figure 4 were ^33 Kcal/mole f o r the three alumina-based m a t e r i a l s and 54 Kcal/mole, f o r the s i l i c a - a l u m i n a catalysts. The increased a c t i v i t y and lower a c t i v a t i o n energy f o r the coke deposited on the aluminas (compared to that on the s i l i c a - a l u m i n a s ) cannot be due to a d i r e c t c a t a l y t i c e f f e c t of alumina on the g a s i f i c a t i o n r e a c t i o n , but r a t h e r to an i n d i r e c t e f f e c t o f the alumina that c o n t r o l s the nature and s t r u c t u r e (surface area and s t r u c t u r a l d i s o r d e r ) of the coke during i t s deposition. The coke formed on alumina appears to be more s t r u c t u r a l l y disordered. I t i s l e s s g r a p h i t i c , more porous, higher in surface area, and lower in d e n s i t y than that deposited on the silica-alumina catalyst. This type of coke w i l l be more r e a c t i v e in the g a s i f i c a t i o n r e a c t i o n (6). Q u a l i t a t i v e l y , i t was a l s o observed that the v a r i o u s aluminas coked more q u i c k l y and at lower temperatures during the coking procedure than d i d the silica-aluminas. This i s in agreement with the r e s u l t s of Tantarov et a l . , who found that during c a t a l y t i c c r a c k i n g o f α-methyl styrene at 450°C, the i n i t i a l coking r a t e on alumina was twice that on s i l i c a - a l u m i n a (20). The alumina c a t a l y z e s the coke d e p o s i t i o n on the s u b s t r a t e and, perhaps, promotes formation of more r e a c t i v e coke. The enhanced r e a c t i v i t y o f coke on alumina could be the r e s u l t of increased s u r f a c e area (higher p o r o s i t y ) , but the nature o f the coke could be a l t e r e d as w e l l , as suggested by the lower a c t i v a t i o n energy (54 v s . 33 Kcal/mole) of g a s i f i c a t i o n . This c o n c l u s i o n i s c o n s i s t e n t with the observation that coke which i s deposited q u i c k l y i s g e n e r a l l y l e s s dense and more r e a c t i v e to g a s i f i c a t i o n . An i n v e s t i g a t i o n of the nature and p h y s i c a l p r o p e r t i e s o f cokes deposited on v a r i o u s substrates may be h e l p f u l in e l u c i d a t i n g the mechanism. The high carbon r e a c t i o n r a t e on alumina makes t h i s m a t e r i a l the s u b s t r a t e o f choice f o r the proposed petroleum upgrading scheme (12). For example, i f a r e a c t i o n r a t e constant of 0.011 m i n i s r e q u i r e d (corresponding to about 210 min residence time f o r 90% coke removal), a temperature advantage of 65°F (1495 v s . 1560°F) would e x i s t f o r alumina over s i l i c a alumina. - 1



14.

YAN AND ROSYNEK

0.21

0

Reaction

. 20

of Steam with

.

40

. 60

Coke

. 80

. 100

293

. 120

Reaction Time, min.

Figure 3. Kinetics of steam gasification at 1500°F of coke deposited on montmorillonite (A), silica-alumina ( Α λ American Cyanamid alumina (\Zi)> Catapal alumina, Run 58 (Φ), Catapal alumina, Run 56 (O), and fresh bauxite

/// k

1000/T, K

-

1

Figure 4. Arrhenius plots for steam gasification of coke deposited on American Cyanamid alumina (A), fresh bauxite ([J), and silica-alumina (O). Corresponding activation energies are 32.6, 33.8, and 53.1 kcal/mol, respectively.


294

COKE

FORMATION

Acknowledgment The authors wish to thank Dr. W. K. B e l l f o r v a l u a b l e discussions.

Literature Cited 1.


2.

3.

4. 5.

6.

7.

8. 9.

10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

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RECEIVED

June 28, 1982.


Coke Formation on Metal Surfaces - American Chemical Society

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