16 Steam Reforming of Natural Gas: Intrinsic Kinetics, Diffusional Influences, and Reactor Design J. C. D E DEKEN, E . F. DEVOS, and G . F. FROMENT Downloaded by UNIV LAVAL on July 11, 2014 | http://pubs.acs.org Publication Date: September 16, 1982 | doi: 10.1021/bk-1982-0196.ch016
Laboratorium voor Petrochemische Techniek, Rijksuniversiteit, Gent, Belgium The intrinsic k i n e t i c s o f the catalytic steam r e f o r ming o f n a t u r a l gas were determined from experiments i n a t u b u l a r r e a c t o r i n the temperature range o f 8 2 3 - 9 5 3 ° K and in the pressure range o f 5-15 b a r . With c a t a l y s t r i n g s o f the s i z e used in i n d u s t r i a l o p e r a t i o n , pronounced c o n c e n t r a t i o n g r a d i e n t s occur i n s i d e the c a t a l y s t . The e f f e c t i v e diffusivity re q u i r e d i n the s i m u l a t i o n o f these g r a d i e n t s was obtained from the molecular and Knudsen diffusivi ties, the i n t e r n a l v o i d f r a c t i o n and the t o r t u o s i t y f a c t o r . The latter was determined by the dynamic gas chromatographic method, u s i n g the Van Deemter equa tion. The t o r t u o s i t y f a c t o r was found t o v a r y between 4.39 and 4.99 and to be independent o f temperature. The reformer tube o p e r a t i o n was simulated on the b a s i s o f a set o f c o n t i n u i t y - , energy- and momentum equations u s i n g one and two dimensional heterogeneous models. I n t r a p a r t i c l e g r a d i e n t s in the r i n g s were accounted f o r by the use o f the g e n e r a l i z e d modulus concept. The steam reforming o f n a t u r a l gas, the main process f o r hydrogen- o r synthesis-gas p r o d u c t i o n i s c a r r i e d out on supported N i c a t a l y s t s i n m u l t i t u b u l a r r e a c t o r s operated a t temperatures v a r y i n g from 500 t o 800°C, pressures ranging from 20 t o 40 bar and molar steam-to-carbon r a t i o s i n the feed between 2.0 and 4.0. Despite the i n d u s t r i a l importance o f the process, t h e design o f the furnace and r e a c t o r tube i s s t i l l c a r r i e d out along very empir i c a l l i n e s . The present work r e p o r t s on the r e s u l t s o f an i n v e s t i g a t i o n o f the k i n e t i c s , i n c l u d i n g the i n f l u e n c e o f i n t r a p a r t i c l e c o n c e n t r a t i o n gradients,and combines t h i s i n f o r m a t i o n with fundamental models f o r the s i m u l a t i o n and design o f reformer tubes i n s e r t e d i n t o g a s - f i r e d furnaces.
0097-6156/82/0196-0181$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.
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I n t r i n s i c k i n e t i c s o f methane steam
reforming
The commercial c a t a l y s t used i n t h i s work c o n t a i n s 12 wt% N i and 83 wt% (1-AI2O3. I t has a BET t o t a l surface area o f 3.4m /g and a unimodal pore s i z e d i s t r i b u t i o n with volume 0.155 cc/g, mean pore r a d i u s 1600 Â and v o i d f r a c t i o n 0.362. I t s a c t i v a t i o n r e q u i r e d a r e d u c t i o n which was c a r r i e d out under atmospheric pressure i n s i t u , f o r 72 hrs a t 850°C by means o f a pure d r i e d hydrogen flow o f 100 Nl/hr. These severe r e d u c t i o n c o n d i t i o n s were r e q u i r e d because 20 wt% of the Ni was present as N1AI2O4-spinel phase, which c o u l d o n l y be reduced above 770°C. I t l e d t o a very a c t i v e c a t a l y s t , with a s p e c i f i c N i - s u r f a c e area o f 0.68 m Ni/g.cat. The k i n e t i c study was conducted i n a bench s c a l e u n i t b u i l t around a t u b u l a r r e a c t o r (HK40;I.D.35mm), operated i n the i n t e g r a l mode i n the ranges 550-675°C, 5 t o 15 bar t o t a l pressure and molar steam-to-methane r a t i o s o f 3 t o 5. Methane, water and hydrogen were preheated and mixed p r i o r to e n t e r i n g the r e a c t o r , c o n s i s t i n g o f preheat-, r e a c t i o n - and a f t e r - z o n e s and e l e c t r i c a l l y heated by 5 independently c o n t r o l l e d s e c t i o n s . The r e a c t i o n s e c t i o n contained 6 grams o f c a t a l y s t , crushed t o 350 Mm t o e l i m i n a t e i n t e r n a l mass t r a n s f e r l i m i t a t i o n s and d i l u t e d with i n e r t r e f r a c t o r y m a t e r i a l s to ensure i s o t h e r m i c i t y . A f t e r condensation o f the steam, the dry e x i t gas was analyzed by two gas chromatographs c o n t a i n i n g Porapack Q and Ν columns and connected with a PDP-8A process computer. Heated l i n e s a l s o permit bypassing the r e a c t o r t o prevent a l t e r i n g the c a t a l y s t d u r i n g s t a r t - u p and shut-down o p e r a t i o n s . Molar H^/CH^ feed r a t i o s between 1.0 and 3.25 were maintained d u r i n g the expe r i m e n t a t i o n (1_) , t o prevent any carbon b u i l d up and r e o x i d a t i o n o f the c a t a l y s t and t h e r e f o r e d e a c t i v a t i o n . By way of example a small p o r t i o n of the experimental r e s u l t s i s shown i n F i g u r e 1. These r e s u l t s l e d t o the f o l l o w i n g r e a c t i o n mechanism : CH +S ^z±S-C+2H (1) H 0+S* ^ ± S - 0 + H (2) S-C+S-0 ^ ± S - C 0 + S : r.d.s. (3) S-CO ^±C0+S (4) S-C+2S-0^z±S-C0 +2S* : r . d . s . (5) s-co ^±co +s* (6) Since the gas phase c o n t a i n s f i v e components which should s a t i s f y three elementary mass balances, two a r b i t r a r i l y chosen, but inde pendent, conversions are r e q u i r e d to d e f i n e i t s composition, e.g. the t o t a l methane conversion, X^H-r and the conversion o f methane i n t o C0 , Xco p r e d i c t i o n o f these conversions i n any p o i n t o f the r e a c t o r t h e r e f o r e n e c e s s i t a t e s two r a t e equations, each d e r i v e d under the assumption o f a t l e a s t one r a t e determining step ( r . d . s . ) . A number o f authors have used one r a t e equation only, thereby assuming the watergas s h i f t r e a c t i o n ( C 0 + H 0 ^ ± C 0 2 + H ) t o be a t e q u i l i b r i u m a t any p o i n t i n the r e a c t o r (2/3,4) , but others have c o n t r a d i c t e d t h i s assumption (5/6) . From t h i s mechanism and a f t e r d i s c r i m i n a t i o n between more than 150 r i v a l models (1_) , the 2
Downloaded by UNIV LAVAL on July 11, 2014 | http://pubs.acs.org Publication Date: September 16, 1982 | doi: 10.1021/bk-1982-0196.ch016
2
X
4
2
2
2
X
X
2
2
2
T
2
h
e
2
2
In Chemical Reaction Engineering—Boston; Wei, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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Steam Reforming of Natural Gas
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DE DEKEN ET AL.
Figure 1. Total methane conversion versus W/F° H at 5 BAR, steam-to-carbon ratio of 5.0, and different temperatures. Key: · , experimental; and , model prediction. C
k
In Chemical Reaction Engineering—Boston; Wei, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
184
CHEMICAL REACTION ENGINEERING
following
Langmuir-Hinshelwood type r a t e equations were obtained : P
P
W CH H,O r
=
CO
/P
H i
= — ί
W
( p
cH
p H
n
/ p
2
H
;*CO'\)
2
R
(7)
—
2
2
(8)
3
( t
Vco'
Downloaded by UNIV LAVAL on July 11, 2014 | http://pubs.acs.org Publication Date: September 16, 1982 | doi: 10.1021/bk-1982-0196.ch016
with the p a r t i a l p r e s s u r e s g i v e n by t h e r e l a t i o n s : P
p
P
CH,
= (i-'W/N
co
= < CH- CO,
C0
X
X
X
2
» < C0 ^
P P
) / N
) / S
N
2
H
H
=
