Chemical Reaction EngineeringâBoston - American Chemical Society

38 Coke Deposition on a Commercial Nickel Oxide Catalyst During the Steam Reforming of Methane Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: September 16, 1982 | doi: 10.1021/bk-1982-0196.ch038

STEVE PALOUMBIS

1

and E U G E N E E . PETERSEN

University of California, Department of Chemical Engineering, Berkeley, C A 94720

The steam reforming of methane cycle suffers from the problem of coke deposition on the catalyst bed. The primary objective of this project was to study the stability of a com mercial nickel oxide catalyst for the steam reforming of methane. The theoretical minimum ratios of steam to methane that are required to avoid deposition of coke on the catalyst at various temperatures were calculated, based on equilibrium considerations. Coking experiments were conducted in a tubular reactor at at mospheric pressure in the range of 740-915°C. The quantities of coke deposited on the catalyst were determined by oxidation of coke to CO2, and adsorption on Ascarite. The experimental minimum ratios were obtained graphically from these data. The quantities of coke obtained experimentally were less than the theoretical values, whereas the experimental minimum steam to methane ratios were higher than the theoretical. A simple model of the Voorhies type described the coking data reasonably well. In the course of the coking runs the catalyst did not deactivate to a great extent, the conversion decreasing by not more than 15 percent.

1

Current address: Standard Oil Company of California, Richmond, CA 94802.

0097-6156/82/0196-0489$06.00/0 © 1982 American Chemical Society In Chemical Reaction Engineering—Boston; Wei, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

CHEMICAL REACTION ENGINEERING

Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: September 16, 1982 | doi: 10.1021/bk-1982-0196.ch038

490

Steam reforming of hydrocarbons has become the most widely used process f o r producing hydrogen. One of the c h i e f problems i n the process i s the d e p o s i t i o n of coke on the c a t a l y s t . To c o n t r o l coke d e p o s i t i o n , h i g h steam t o hydrocarbon r a t i o s , n, are used. However, excess steam must be r e c y c l e d and i t i s d e s i r a b l e to minimize the magnitude of the r e c y c l e stream f o r economy. Most of the research on t h i s r e a c t i o n has focused mainly on k i n e t i c and mechanistic c o n s i d e r a t i o n s of the steam-methane r e a c t i o n at high values of η to avoid carbon d e p o s i t i o n 0 ^ 4 ) · Therefore, the primary o b j e c t i v e of t h i s s t u d y i s to determine e x p e r i mentally the minimum value of η f o r the coke-free o p e r a t i o n at v a r i o u s temperatures f o r a commercial c a t a l y s t . Reaction

Equations

The steam-methane system i s known to c o n t a i n CH^, H, CO, C 0 and Η«0 i n the r e a c t i o n mixture, hence, we need two independent chemical r e a c t i o n s to d e s c r i b e the system completely. If i n a d d i t i o n carbon i s deposited, then we need an a d d i t i o n a l r e a c t i o n . Of the many p o s s i b l e r e a c t i o n s i n the steam reforming of methane, a c o n s i d e r a t i o n of the f r e e energies leaves the f o l l o w i n g three r e a c t i o n s at temperatures i n excess of 600°C. 2

2

Kcal I. II.

(Steam Reforming) CH

4

+ H0 2

CO + 3H ,

ΔΗ

2 9 8

-

49.3

C0

+ H,

ΔΗ

2 9 8

=

-10

C + C0 ,

ΔΗ

2 9 8

2

3=U

(Water-Gas S h i f t ) CO + H 0 2

2

2

Kcal I I I . (CO d i s p r o p o r t i o n a t i o n ) 2C0

^Tt

2

- -41.2

g

If we v i s u a l i z e Reaction I t o proceed i n two stages w i t h intermediate formation and removal of carbon as shown below. IV. V.

CH

4

C + H0 2

—

C + 2H ,

ΔΗ

2 9 8

-

=

CO + H ,

ΔΗ

2 9 8

- 31.4

2

2

then i n order that no carbon may mixture i t i s necessary that 2

n

a

a

Kcal ^

appear i n the e q u i l i b r i u m

/

where a^ are the a c t i v i t i e s of the v a r i o u s s p e c i e s and K and Ky are the e q u i l i b r i u m constants f o r Chemical Reactions IV and V r e s p e c t i v e l y . Thus, when the steam/methane r a t i o , n, I V

In Chemical Reaction Engineering—Boston; Wei, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

m

Q

l

38.

PALOUMBIS AND PETERSEN

i n the feed i s s u f f i c i e n t l y high so that carbon cannot be present a t e q u i l i b r i u m , the e q u i l i b r i u m composition can be c a l c u l a t e d from c o n s i d e r a t i o n of only Chemical Reactions I and I I . A c c o r d i n g l y , the a c t i v i t i e s of each of the s p e c i e s can be c a l c u l a t e d i f the e q u i l i b r i u m constants, K-j- and K ^ j , and value o f η are known provided the a c t i v i t y r a t i o i n e q u a l i t i e s given i n Equation 1 are met. I f the i n e q u a l i t i e s of Equation 1 are not met, coke d e p o s i t i o n i s p o s s i b l e . Values of the η a t which the e q u a l i t i e s of Equation 1 are met represent the minimum steam/methane r a t i o , îUj^, * carbon d e p o s i t i o n w i l l take p l a c e at e q u i l i b r i u m . Figure 1 d i s p l a y s n ^ versus temperature, T. Extending the a n a l y s i s t o i n c l u d e Chemical Reaction I I I permits the c a l c u l a t i o n of the e q u i l i b r i u m coke laydown as a f u n c t i o n of η and Τ shown i n Figure 2. These curves w i l l be used l a t e r t o compare with the corresponding k i n e t i c curves obtained e x p e r i m e n t a l l y . a


Coke Deposition on NiO Catalyst

t

w

n

c

n

n

o

m

n

Experimental The apparatus c o n s i s t s e s s e n t i a l l y of a steam generator, an Inconel r e a c t o r , a condenser-separator and a chromatograph. D e t a i l s are presented ( 5 ) . A b r i e f d e s c r i p t i o n o f the apparatus f u n c t i o n w i l l be given here. The apparatus was designed t o measure CO, H , and CH^ i n the e f f l u e n t stream. The e f f l u e n t stream was d r i e d , and passed through 1/8 diameter by 6-foot column packed w i t h 80/100 mesh Spherocarb packing. The peaks emerged i n order H , CO, CH^ and C 0 and n i c e l y separated. To determine the amount o f coke deposited on the c a t a l y s t , the flow of methane and steam through the r e a c t o r was i n t e r r u p t e d and the r e a c t o r purged with n i t r o g e n . Then a steam-On mixture o x i d i z e d the coke d e p o s i t s forming CO and C0 . A f t e r d r y i n g the r e a c t o r e f f l u e n t w i t h D r y e r i t e , the dry gaseous mixture passed through a U-tube c o n t a i n i n g H o p c a l i t e which o x i d i z e s carbon monoxide t o carbon d i o x i d e a t ambient temperature. F i n a l l y , the gas stream c o n t a i n i n g only C 0 and H was adsorbed on A s c a r i t e and weighed. In each run we measured f i v e compounds where a minimum of three i s r e q u i r e d . A c c o r d i n g l y , we were able t o a s c e r t a i n accurate m a t e r i a l balances. The c a t a l y s t used was Katalco 23-1 Primary Reforming C a t a l y s t , a commercial n i c k e l reforming c a t a l y s t supported on alumina. I t s chemical composition was r e p o r t e d as 10-14% NiO, 0.2% S i 0 balance Al^O^. I t was s u p p l i e d as hollow c y l i n d e r s of s i z e 5 % - i n O.D., 5/16-in 1.0. and 3/8-in l o n g , and had an apparent bulk d e n s i t y of 66±5 l b / f t . The r i n g s were crushed and s i e v e d t o o b t a i n the 24/32 mesh cut used i n a l l of the experiments. 2

2

2

2

2

2

2


492


Τ

Γ


NO COKING

COKING 0.9

1

Figure 1. Minimum thermodynamic steam-methane ratio vs. temperature.

700 800 TEMP. *C


900

38.

PALOUMBis A N D P E T E R S E N



Results The experimental r e s u l t s are presented i n Figure 3· In t h i s f i g u r e the amount of coke deposited on the c a t a l y s t has been p l o t t e d versus the volume of methane (at STP) f e d i n t o the r e a c t o r . Both absolute amounts o f coke on a 5 g charge of c a t a l y s t , and percent coke by weight are reported. Since the feed flow r a t e o f methane was maintained constant a t 0.31 l i t e r s (STP)/min, the a b s c i s s a a l s o represents time. Each p o i n t on Figure 3 represents an experimental run of approximately 12-15 hours d u r a t i o n i n c l u d i n g the r e d u c t i o n and subsequent o x i d a t i o n o f the c a t a l y s t . F i g u r e 3 supports s e v e r a l q u a l i t a t i v e o b s e r v a t i o n s . For a p a r t i c u l a r temperature and steam r a t i o , the amount of coke deposited on the c a t a l y s t i n c r e a s e s w i t h process time and the r a t e of d e p o s i t i o n decreases monotonically w i t h process time. The coke d e p o s i t i o n curve appears t o reach a "plateau" or that the coke d e p o s i t i o n r a t e i s extremely slow a t long process times. Decreasing η a t any temperature l e v e l leads to higher l e v e l s of coke on the c a t a l y s t . These observations permit one t o determine a " k i n e t i c minimum" value of η by p l o t t i n g the " p l a t e a u " value of the deposited coke versus η at constant temperature as shown on Figure 4. The p o i n t s are w e l l - r e p r e s e n t e d by l i n e a r curves analogous t o those shown on Figure 2. The l i n e a r e x t r a p o l a t i o n t o zero coke production g i v e s the k i n e t i c minimum n, n ^ , p l o t t e d on Figure 5. F i g u r e 5 then i s the d e s i r e d p l o t that gives the k i n e t i c boundary o f the cokef r e e r e g i o n at v a r i o u s o p e r a t i n g temperatures f o r the steam reforming r e a c t i o n of methane corresponding t o the s i m i l a r thermodynamic boundary shown i n Figure 1. m

n

Discussion The thermodynamic a n a l y s i s p r e d i c t e d that the minimum steam r a t i o should decrease w i t h i n c r e a s i n g temperature above 650°C and g r a d u a l l y l e v e l o f f at round 900°C. S i m i l a r l y , the experimentally e s t a b l i s h e d coke boundary curve e x h i b i t s very much the same behavior, however, the experimental values o f the minimum r a t i o are c o n s i d e r a b l y h i g h e r than the t h e o r e t i c a l ones a t a l l temperatures. The reason why the minimum steam r a t i o goes down w i t h temperature i s not known with c e r t a i n t y . One p o s s i b i l i t y i s that the competing r e a c t i o n s o f carbon p r o d u c t i o n and consumption have such k i n e t i c s that the r a t e of coke consumption i n c r e a s e s f a s t e r w i t h temperature than the r a t e of coke generation, which suggests that the carbon-steam r e a c t i o n has a higher a c t i v a t i o n energy than the methane c r a c k i n g and carbon monoxide d i s p r o p o r t i o n a t l o n r e a c t i o n .


494



>·

! III

8

0

10 20 30 40 50 60 70 LITERS OF CH FED (STP) 4

Figure 3.

Experimental coke on catalyst vs. methane fed. Parameter is steammethane ratio.




PALOUMBIS AND PETERSEN

0.8

1.0

1.4

1.2 4

2 Figure 4.

Experimental coke deposition vs. steam-methane ratio. Key: Q V , 780°C; 0,815°C; Δ, 885°C;and O, 9 i 5 ° C .

1

γ

"

495

\

1

740°C;

1

NO COKING

"

COKING « 700

ι

ι

800 TEMP. C

ι 900

e

Figure 5.

Experimental minimum steam methane ratio vs. temperature.



496


T h i s proposed explanation i s , of course, c o n s i s t e n t with the o b s e r v a t i o n that the amount of coke laydown measured experimentally decreases with i n c r e a s i n g temperature. However, Figure 4 a l s o shows that f o r a given n, the amount of deposited coke goes through a minimum with i n c r e a s i n g tempeature. The shape of the carbon d e p o s i t i o n curves of Figure 3 shows that the r a t e of coking f a l l s o f f r a p i d l y with time, a frequent o b s e r v a t i o n w i t h coking behavior. However, i t was a l s o observed that the c a t a l y s t does not d e a c t i v a t e to a great extent even at the h i g h e s t coke l e v e l s . These observations suggest that most of the coke deposits on the support s i t e s and not on the a c t i v e metal. As the amount of deposited carbon on the c a t a l y s t i n c r e a s e s , the support s i t e s a v a i l a b l e f o r more d e p o s i t i o n w i l l decrease. Thus, at r e l a t i v e l y long process times, some s o r t of support s a t u r a t i o n would be expected. It i s a l s o of i n t e r e s t to observe that the coke laydown observed experimentally i s more than an order of magnitude, l e s s than would be p r e d i c t e d from e q u i l i b r i u m c a l c u l a t i o n s . That i s , the amount of coke on the c a t a l y s t per l i t e r of methane on Figure 4 a t a given temperature and steam methane r a t i o i s about 5% of that shown on Figure 2 formed under e q u i l i b r i u m c o n d i t i o n s suggesting that coke formation i s r a t e l i m i t e d . The carbon d e p o s i t i o n curves are w e l l - r e p r e s e n t e d by Voorhies (6) type coking curves, i . e . , curves given by the equation C

« AF

c

n

(2)

where F « l i t e r s of methane passed through r e a c t o r (STP) and where η and In A represent the slope and i n t e r c e p t , r e s p e c t i v e l y , of a p l o t of l o g C versus In F. The data of Figure 3 give good Voorhies p l o t s where η i s approximately 1/3 and suggests a carbon formation equation of the form c

dC dF

3 A 2 A

c S Q r

(3)

The constant A depends upon η and T. F i n a l l y , i t should be mentioned that some thermal c r a c k i n g on the ceramic packing m a t e r i a l was observed a t temperatures above 850°C. Whenever such an e f f e c t was detected, an attempt was made to c o r r e c t f o r i t i n the coking data. A t y p i c a l value f o r an experimental run at 810°C f o r 160 minutes was 35 mg coke on 5 gram of reforming c a t a l y s t , or about 0.7 percent by weight. The o b s e r v a t i o n that no massive coking occurred when only alumina was charged i n t o the r e a c t o r even at low steam


38.

PALOUMBis A N D

PETERSEN


r a t i o s seems t o suggest that massive carbon formation r e q u i r e s (under normal reforming c o n d i t i o n s and i n the absence of an a c i d c a t a l y s t ) the presence of both the high s u r f a c e area support and the n i c k e l c a t a l y s t . However, the f a c t that there was some coking on the alumina bed leads us to conclude that coking may take p l a c e by r e a c t i o n on the n i c k e l s u r f a c e , by c r a c k i n g on the support m a t e r i a l , and homogeneously i n the gas phase (thermal p y r o l y s i s ) , the c r a c k i n g being more pronounced a t e l e v a t e d temperatures.


Acknowledgments

T h i s work was supported i n p a r t funds from the N a t i o n a l Science Foundation by Grant ENG-75-06559. The authors a p p r e c i a t e the work o f Ms. Yang i n checking the theromochemical c a l c u l a t i o n s .

Literature Cited 1. 2. 3. 4. 5. 6.

Akers, W. W., Camp, D. P., AIChE J. 1955, 1, No. 4, 471 Bodrov, I. M., Appel'baum, A. O., Temkin, M. I . , Kinetics and Catalysis, 1967, 8, No. 4, 821. Lavrov, Ν. V., Petrenko, I. G., Dokl. AN SSSR, 1964, 15B, No. 3, 645. Allen; D. W., Gerhard, E. R. and Likins, M. R., I&EC Process Des Dev., 1975, 14, No. 3. Paloumbis, S. S., M. S. Thesis, University of California, Berkeley, CA, 1978. Voorhies, Α., I&EC, 1945, 37, 319.

R E C E I V E D April 27,

1982.


497

Chemical Reaction EngineeringâBoston - American Chemical Society

Recommend Documents

Chemical Reaction EngineeringâBoston - American Chemical Society