17 Transient Kinetics of the Fischer-Tropsch Synthesis Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 3, 2016 | http://pubs.acs.org Publication Date: September 16, 1982 | doi: 10.1021/bk-1982-0196.ch017
E . PH. K I E F F E R
1
and H . S. V A N D E R B A A N
Eindhoven University of Technology, Laboratory of Chemical Technology, Eindhoven, The Netherlands
A comparison o f the r e s u l t s of a t h e o r e t i c a l t r e a t ment o f the t r a n s i e n t behaviour o f i r o n c a t a l y s t s w i t h experimental data shows t h a t the low turnover frequencies found f o r those c a t a l y s t s cannot be the r e s u l t o f a low r a t e constant f o r the propagation r e a c t i o n . To o b t a i n accurate data f o r the t r a n s i e n t p e r i o d , which l a s t e d l e s s than 20 s, a r e a c t i o n system w i t h very little a x i a l d i s p e r s i o n was built.
According to the I n t e r n a t i o n a l Union o f Pure and A p p l i e d Chemistry (IUPAC ( O ) the turnover frequency o f a c a t a l y t i c r e a c t i o n i s d e f i n e d as the number o f molecules r e a c t i n g p e r a c t i v e s i t e i n u n i t time. The term a c t i v e s i t e s i s a p p l i e d t o those s i t e s f o r adsorption which are e f f e c t i v e s i t e s f o r a p a r t i c u l a r heterogeneous c a t a l y t i c r e a c t i o n . Because i t i s o f t e n impossible to measure the amount o f a c t i v e s i t e s , some i n d i r e c t method i s needed t o express the r a t e data i n terms o f turnover f r e q u e n c i e s . In some cases a r e a l i s t i c measure o f the number o f a c t i v e s i t e s may be the number o f molecules o f some compound that can be adsorbed on the c a t a l y s t . T h i s measure i s f r e q u e n t l y used i n the l i t e r a t u r e of the F i s c h e r - T r o p s c h s y n t h e s i s , where the amount o f a d s o r p t i o n s i t e s i s determined by carbon monoxide a d s o r p t i o n on the reduced c a t a l y s t . However, i t i s questionable whether the number o f adsorption s i t e s on the reduced c a t a l y s t i s r e a l l y an i n d i c a t i o n of the number o f s i t e s on the c a t a l y s t a c t i v e during the s y n t h e s i s , because the m e t a l l i c phase o f the F i s c h e r - T r o p s c h c a t a l y s t s i s o f t e n carbided o r o x i d i z e d during the process. The turnover frequencies reported f o r the F i s c h e r - T r o p s c h s y n t h e s i s a r e s m a l l . In the p u b l i c a t i o n o f Vannice (2) the t u r n over frequencies f o r CO-conversion to hydrocarbons range from
1
Current address: Royal Dutch Shell Laboratory (KSLA), Amsterdam, The Netherlands.
0097-6156/82/0196-0199$06.00/0 © 1982 American Chemical Society Wei and Georgakis; Chemical Reaction Engineering—Boston ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
200
CHEMICAL REACTION ENGINEERING
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1
1
0.325 s " f o r ruthenium to 0.002 s " f o r i r i d i u m a t 550 Κ and 0.1 MPa. At the same process c o n d i t i o n s f o r i r o n a value of 0.16 s""* i s given. Dautzenberg e t a l . (3) have determined the k i n e t i c s o f the Fischer-Tropsch synthesis with ruthenium c a t a l y s t s . The authors showed, that because the synthesis can be described by a consecu t i v e mechanism, the non steady s t a t e behaviour o f the c a t a l y s t can give information about the k i n e t i c s of the process. On ruthenium they found that not only the o v e r a l l r a t e of hydrocarbon production per a c t i v e s i t e i s s m a l l , but a l s o that the r a t e con stant of propagation i s low. Hence, Dautzenberg e t a l . f i n d that the low a c t i v i t y of Fischer-Tropsch c a t a l y s t s i s due to the low i n t r i n s i c a c t i v i t y o f t h e i r s i t e s . On the other hand, Rautavuoma (4) s t a t e s that the low a c t i v i t y of c o b a l t c a t a l y s t s i s due to a small amount of a c t i v e s i t e s , the amount being much smaller than the number of adsorption s i t e s measured. In an attempt to c o n t r i b u t e to the d i s c u s s i o n we have a p p l i e d the t r a n s i e n t response method (5) to i r o n c a t a l y s t s i n order to get f i r s t hand information about the r a t e of some r e a c t i o n steps i n the s y n t h e s i s . K i n e t i c model f o r pulse s i m u l a t i o n The t r a n s i e n t response o f the c a t a l y s t to a step f u n c t i o n i n the c o n c e n t r a t i o n of reactant gases i s simulated from the k i n e t i c s of the F i s c h e r - T r o p s c h s y n t h e s i s . The f o l l o w i n g features a r e g e n e r a l l y accepted f o r the mecha nism of the Fischer-Tropsch s y n t h e s i s : i. the r e a c t i o n i s i n i t i a t e d by a mono carbon s p e c i e s : ii. the chain growth takes p l a c e by a stepwise a d d i t i o n of mono carbon u n i t s to the growing chain; i n . the r a t e of termination and the r a t e of propagation a r e i n dependent of the chain length. According to these assumptions the r e a c t i o n can be represented by thé f o l l o w i n g simple scheme:
Because i n some cases the production of methane does not obey the S c h u l z - F l o r y d i s t r i b u t i o n , k j i s allowed to d i f f e r from I C 3 . The S c h u l z - F l o r y constant i s d e f i n e d as α = r / r - j , f o r η > 3. A number of mechanisms a r e claimed to d e s c r i b e the propaga t i o n step of the F i s c h e r - T r o p s c h s y n t h e s i s . These mechanisms are: n
n
Wei and Georgakis; Chemical Reaction Engineering—Boston ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
17.
KIEFFER AND VAN DER BAAN
201
Fischer-Tropsch Synthesis
i.
c h a i n growth by carbon monoxide i n s e r t i o n i n t o the metalhydrocarbon bond; ii. c h a i n growth by condensation of oxygen c o n t a i n i n g s p e c i e s ; i i i . c h a i n growth by a d d i t i o n of oxygen f r e e C H - s p e c i e s . For a l l these mechanisms the propagation r e a c t i o n can be expressed as: x
**
*
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C
*
. + C. — • C n-1 1 η
** where Cj i s the one carbon atom c o n t a i n i n g species that i s i n serted as the b u i l d i n g b l o c k f o r the growing hydrocarbon c h a i n . With respect to the propagation mechanisms mentioned, a number of p o s s i b i l i t i e s a r i s e f o r Cj . - For C O - i n s e r t i o n , C j * (=C0 ) w i l l e i t h e r be the p r e c u r s o r A of the i n i t i a t i n g s p e c i e s , o r i t may be a p r e c u r s o r o f A . In the f i r s t case Ci = A , while i n the l a t t e r case i t i s reasonable to assume that Cj i s i n e q u i l i b r i u m w i t h A , hence 6 ** = Κθ^*. - For condensation, C j * i s i d e n t i c a l to C|. - For C H - a d d i t i o n , C j * need not be i d e n t i c a l to C*. However, i n case that Cj and Cj are not the same s p e c i e s , the b u i l d i n g b l o c k i s e i t h e r a precursor f o r C j , o r a hydrogenated form o f C j . In the f i r s t case Cj = A*. F o r the second case i t i s a s sumed here that C**and Cj are i n e q u i l i b r i u m , hence 6 ** = K6 *. 1 With these d e f i n i t i o n s , the r a t e equations that have t o be solved become : - For the C O - i n s e r t i o n model, and the C l ^ - a d d i t i o n model, f o r the cases where Cj = A o r QQ** • Α* C
x
C
C
Κ Θ
Γ
1
-
k
l *C*>
d
e C
- Κ
/ d t
;
V
-
( k
+
2 V
k
+
k
l
*
9
-> C
for η > 1 r
k
d
n - 3 V ; η
where k
2
V
/
d
t
=
k
η
6
2 C*
V n-1
( k
+
2 V
k
θ
3> 0
i n c l u d e s the e q u i l i b r i u m constant Κ when
η QQ**
= Κθ^*.
For the condensation model and the CH - a d d i t i o n model, f o r the ** * ** * cases where Cj = Cj o r QQ* Ί l x
β
κ
θ
C
r
-
k
e
d
e
l G/ 1 for η > 1
r
l
k
n " 3 V ; η
d
c: 1
6
/ d t
C*
/ d t
n
=
k
+ V
k
- 2 V
"
(
Σ
k
, 2 V n=l
+
k
, c ! - 2 c* n-1 1 e
( k
e
+
l
+
k
e
-> c ! η
k
1
6
3> C * 1
η
Wei and Georgakis; Chemical Reaction Engineering—Boston ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
202
CHEMICAL REACTION
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where k
ENGINEERING
i n c l u d e s the e q u i l i b r i u m constant Κ when θ^** = Κθ^*.
2
To be able to c a l c u l a t e the p r o d u c t i o n r a t e of hydrocarbons as a f u n c t i o n of time, an assumption has to be made w i t h r e s p e c t to the s u r f a c e coverage of carbon c o n t a i n i n g s p e c i e s . We have as sumed that during the pulse the t o t a l s u r f a c e coverage of carbon c o n t a i n i n g intermediates, i n c l u d i n g A , i s constant i n time and that these carbon species cover almost the e n t i r e a c t i v e s u r f a c e . Both the propagation and the t e r m i n a t i o n r e a c t i o n w i l l r e q u i r e a c e r t a i n amount of hydrogen. In p r i n c i p l e the coverage of hydrogen w i l l be a f u n c t i o n of the s u r f a c e coverage of carbon c o n t a i n i n g species and the pressure of hydrogen, hence CO
ν
-
F
(
\
+
ν
n=l
ν
+
e
c «
p 1
η
H
> 2
Because i t i s assumed that the s u r f a c e coverage of carbon con t a i n i n g species i s independent of the time, the same a p p l i e s to the coverage of hydrogen at a constant hydrogen p a r t i a l pressure. Since the hydrogen pressure does not change d u r i n g the p u l s e , i t s coverage i s not a v a r i a b l e i n the s i m u l a t i o n of the p r o d u c t i o n r a t e of hydrocarbons as a f u n c t i o n of time. In the case of a steady s t a t e p r o d u c t i o n , the coverages of hydrocarbon intermediates with η > 1 are r e l a t e d to the s u r f a c e coverage of C| by: 11
θ * = ο " n(stst)
1
θ * l(stst)
The t o t a l s u r f a c e coverage of hydrocarbon i n t e r m e d i a t e s , e x c l u s i v e of A , can be expressed as: CO
Σ η=1
θ * = (1-α)" n(stst)
1
θ * l(stst)
We now d e f i n e k£ as the r a t e constant of propagation excluding^the e q u i l i b r i u m constant K. Hence k§ = ko when C. = A or C. • C.; and k§ = k /K when 9 ** - Κ Θ * or 9 **= K6 *. 2
C
Α
C
C
By u s i n g the steady s t a t e k i n e t i c equations, i t i s then p o s s i b l e to express k and k3 as a f u n c t i o n of the o v e r a l l turnover f r e quency f o r CO-conversion to hydrocarbons ( N Q Q ) , the o v e r a l l t u r n over frequency f o r methane formation ( N ), the p r o b a b i l i t y f o r chain growth ( a ) , the steady s t a t e coverage of the p r e c u r s o r A and the value of the e q u i l i b r i u m constant K. In t a b l e I the ex p r e s s i o n s f o r the k and kg are given. Dautzenberg et a l . c l a i m low r a t e constants f o r the propa g a t i o n r e a c t i o n . With the r a t e constant k^ shown i n t a b l e I i t i s p o s s i b l e to c a l c u l a t e the lowest v a l u e of t h i s constant at a g i v e n value f o r the turnover f r e q u e n c i e s ( N and N ) and f o r the 2
C H
2
m
C H /
Wei and Georgakis; Chemical Reaction Engineering—Boston ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
Wei and Georgakis; Chemical Reaction Engineering—Boston ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
C
0
Α
** Λ * 1 "
a
- C.
6
1
C?* "
K 6
1
C?
CH a d d i t i o n χ
C.
Condensation nu ΑΛ'*or CH a d d i t i o n
CO i n s e r t i o n A ** = * C * A
r
S""2^ · or CH a d d i t i o n
Mechanism
A
V (stst)
(
N
A
4
N
2
>ί -°>
W
-Ν CO CH, 4
A
N
A
CO- CH ° 4 2
+ K ( ,
"
a ) ) 2
Κ(1-θ* )
