40 Structural Variations as a Tool to Analyze the Mechanism of Noncatalytic Solid-Gas Reactions Chemical Reaction Engineering—Boston Downloaded from pubs.acs.org by YORK UNIV on 12/08/18. For personal use only.
SEVIL U L K U T A N , TIMUR DOGU, and GULSEN DOGU Middle East Technical University, Department of Chemical Engineering, Ankara, Turkey
It is shown that the mechanism of gas-solid noncatalytic reactions can be understood better by following the variations in pore structure of the solid during the reaction. By the investi gation of the pore structures of the limestone particles at different extents of calcination, i t has been shown that the mechanism of this particular system can be successfully repres ented by a two stage zone reaction model below 1000°C. It has also been observed that the mechanism changes from zone reaction to unreacted core model at higher temperatures. A g a s - s o l i d r e a c t i o n u s u a l l y i n v o l v e s heat and mass t r a n s f e r processes and chemical k i n e t i c s . One important f a c t o r which complicates the a n a l y s i s of these processes i s the v a r i a t i o n s i n the pore s t r u c t u r e o f the s o l i d during the r e a c t i o n . Increase or decrease of p o r o s i t y during the r e a c t i o n and v a r i a t i o n s i n pore s i z e s would e f f e c t the d i f f u s i o n r e s i s t a n c e and a l s o change the a c t i v e surface area. These f a c t s i n d i c a t e that the r e a l mechanism of g a s - s o l i d n o n c a t a l y t i c r e a c t i o n s can be understood b e t t e r by f o l l o w i n g the v a r i a t i o n s i n pore s t r u c t u r e during the reaction. Number o f models have been proposed f o r g a s - s o l i d n o n c a t a l y t i c r e a c t i o n s i n the l i t e r a t u r e . Most of the workers have l i m i t e d t h e i r models by n e g l e c t i n g the s t r u c t u r a l changes as the r e a c t i o n proceeds. Microscopic c o n s i d e r a t i o n of pore s i z e change has been considered by Petersen (1), White and Carberry (2), Schechter and G i d l e y ( 3 ) , S z e k e l l y and Evans ( 4 ) , Ramachandran and Smith ( 5 , 6 ) , Dogii ( 7 ) , and Orbey e t a l . (8).
0097-6156/82/0196-0515$06.00/0 © 1982 American Chemical Society
516
CHEMICAL REACTION ENGINEERING
Although number of models proposed i n recent years consider the s t r u c t u r a l v a r i a t i o n s , there are v e r y few works t r y i n g to p r e d i c t the a c t u a l mechanism of such r e a c t i o n s from experimental pore s t r u c t u r e data. The major aim of t h i s work i s the under standing of the mechanism of g a s - s o l i d n o n c a t a l y t i c r e a c t i o n s and p r e d i c t i o n of the best model, using the experimental pore s i z e d i s t r i b u t i o n data and v a r i a t i o n of pore s t r u c t u r e during the reaction. C a l c i n a t i o n of limestone has been chosen as a model r e a c t i o n and pore s i z e d i s t r i b u t i o n s of the limestone p a r t i c l e s are d e t ermined at d i f f e r e n t extents of c a l c i n a t i o n at d i f f e r e n t temperatures. Although the c a l c i n a t i o n r e a c t i o n s have been i n v e s t i g a t e d f o r ages there a r e s t i l l questions about the a c t u a l mechanism of such r e a c t i o n s . The l i t e r a t u r e does not i n v o l v e the structural variations. The mechanism of many of the n o n c a t a l y t i c f l u i d - s o l i d r e a c t i o n s can be described by a model i n between unreacted core and homogeneous r e a c t i o n s models. I s h i d a and Wen (9) formulated such a model using the zone r e a c t i o n concept of Ausman and Watson (10). In t h i s model the r e a c t i o n i s not r e s t r i c t e d to the s u r f a c e of the core as i n the unreacted core model but occurs homogeneously w i t h i n a r e t r e a t i n g core of r e a c t a n t . Wen and I s h i d a (11) combined the g r a i n concept w i t h the zone r e a c t i o n model and analyzed the r e a c t i o n of SO2 w i t h CaO p a r t i c l e s . In the study conducted by M a n t r i , Gokarn and Doraiswamy (12) the concept of f i n i t e r e a c t i o n zone model was f u r t h e r developed. In t h i s work i t i s shown from the v a r i a t i o n s of pore s t r u c ture that a zone r e a c t i o n model s i m i l a r to the one suggested by Ishida and Wen (9) can e x p l a i n the mechanism of c a l c i n a t i o n reaction studied. Experimental The limestone p a r t i c l e s of about one cm equivalent diameter are c a l c i n e d i n a tubular furnace. These p a r t i c l e s c o n t a i n 99.5% CaC03 and have an i n i t i a l p o r o s i t y of 0.09. Hot N gas flows over the p a r t i c l e s during the c a l c i n a t i o n and the gas flow r a t e i s adjusted such that the f i l m mass t r a n s f e r l i m i t a t i o n s are n e g l i g i b l e (13). Conversion-time data i s determined g r a v i m e t r i c a l l y . The pore s i z e d i s t r i b u t i o n s of samples at d i f f e r e n t conversions are determined by a mercury i n t r u s i o n porsimeter and a l s o i n v e s t i g a t e d with an e l e c t r o n microscope. In order to ob t a i n samples with c e r t a i n degree of c o n v e r s i o n , c a l c i n e s are suddenly cooled and the r e a c t i o n i s stopped i n a system through which c o l d Ν 2 gas i s flowing through. C a l c i n a t i o n r e a c t i o n s a r e repeated a t 8 d i f f e r e n t temperatures i n the range 700 °C-1040 °C. The v a r i a t i o n s i n the pore s t r u c t u r e during the c a l c i n a t i o n are examined and used to analyze the r e a c t i o n mechanism. 2
V a r i a t i o n s i n Pore S t r u c t u r e During C a l c i n a t i o n . Typical cumulative pore volume d i s t r i b u t i o n data obtained f o r d i f f e r e n t
40.
ULKUTAN ET A L .
517
Noncatalytic Solid-Gas Reactions
degrees of conversion o f CaCÛ3 t o CaO a t 860 °C a r e shown i n F i g u r e 1. During the f i r s t stages o f the r e a c t i o n , o n l y t h e macropore volume i s found to i n c r e a s e and the pore s i z e d i s t r i butions o f samples a t small conversion have a monodisperse c h a r a c t e r . A f t e r a c e r t a i n degree o f conversion (which i s found to be around 0.15 i n t h i s system) micropores begin t o form, and the b i d i s p e r s e pore s i z e d i s t r i b u t i o n o f the samples i s developed. T h i s b i d i s p e r s e c h a r a c t e r o f the samples can be seen i n F i g u r e 2. I n v e s t i g a t i o n o f the c a l c i n e s w i t h a Scanning E l e c t r o n Microscope a l s o showed t h i s b i d i s p e r s e pore s t r u c t u r e (13). As can be seen from F i g u r e 2 the pores with r a d i i g r e a t e r than 0.5 microns can be considered as macropores. The o r i g i n a l limestone used has a beige c o l o r . I f the c r o s s s e c t i o n o f a sample with small conversions i s examined the c o l o r seen throughout the stone i s gray. The pore s i z e d i s t r i b u t i o n s of these samples g i v e monodispersed curves. A t higher conv e r s i o n s the c r o s s s e c t i o n s o f the samples have two l a y e r s . The outer l a y e r i s white w h i l e the i n n e r core i s gray. The pore s i z e d i s t r i b u t i o n s of these samples show b i d i s p e r s e c h a r a c t e r . Separate i n v e s t i g a t i o n s o f pore s t r u c t u r e s o f gray (core) and white ( s h e l l ) s e c t i o n s have shown that the s h e l l c o n t a i n s both macro and micropores w h i l e there a r e only macropores i n the i n n e r core. These o b s e r v a t i o n s suggest that the mechanism o f the c a l c i n a t i o n o f t h i s p a r t i c u l a r limestone can be examined i n two stages. In the f i r s t stage, r e a c t i o n s t a r t s a t every i n t e r i o r p o i n t o f the stone. The time a t which the formation o f micropores s t a r t s i s considered as the end o f the f i r s t stage. With the assumption t h a t s h e l l s e c t i o n i s completely c a l c i n e d (the j u s t i f i c a t i o n o f t h i s assumption i s g i v e n l a t e r ) and the pore s i z e d i s t r i b u t i o n obtained a t complete conversion i s the charact e r i s t i c d i s t r i b u t i o n o f the ash l a y e r the f o l l o w i n g r e l a t i o n can be w r i t t e n to p r e d i c t the dimensionless r a d i u s (ζ ) o f the inner core from the pore s i z e d i s t r i b u t i o n curves: ιη
1/3
X > ε + ξ - { 1 'm "
V
V
, 2- 1
2 x=l -Ε ε "f
}
(1)
Some of the v a l u e s of t o t a l p o r o s i t y (ε), cumulative pore volumes o f macropores ( V ^ and micropores (V*2) a t d i f f e r e n t degrees of c a l c i n a t i o n a t 860 °C a r e given i n Table IA. C a l c u l ated v a l u e s o f ζ u s i n g Equation 1 a r e r e p o r t e d i n Table IB. As can be seen from Table IB average r a d i u s of micropores i s e s s e n t i a l l y the same a t d i f f e r e n t v a l u e s o f f r a c t i o n a l conversion of CaC03. The s u r f a c e area o f micropores ( p r e d i c t e d from the pore s i z e d i s t r i b u t i o n curves) i n c r e a s e with degree o f c a l c i n a t i o n as expected. The s u r f a c e area of micropores d i v i d e d by the v o l ιη
Figure 1. Cumulative pore size distributions of Goynuk limestone at different conversions at 860°C.
1
w w
«
ζ
w ο
S
H
w > ο
2
w
a
00
H-*
Figure 2. Differential pore size distributions of Goynuk limestone at different conversions at 860°C.
4*
VO
Ut
s
S'
!
««·«·
?
Ο*
1
ι
> ρ
W H
25
>
r
Cl
Ο
520
CHEMICAL REACTION ENGINEERING
ume of the s h e l l s e c t i o n , ( l a s t column i n Table I B ) , i s a l s o independent of degree of c a l c i n a t i o n and e s s e n t i a l l y same as the v a l u e obtained a t complete c o n v e r s i o n . These observations together w i t h the experimental f i n d i n g showing that micropores are present only i n the s h e l l s e c t i o n i n d i c a t e that the complete c o n v e r s i o n assumption f o r the s h e l l s e c t i o n i s j u s t i f i a b l e . These i n f o r m a t i o n from the pore s t r u c t u r e data show that a simple two-stage zone r e a c t i o n model can be s u c c e s s f u l l y used to des c r i b e the mechanism o f the c a l c i n a t i o n of t h i s p a r t i c u l a r limestone and i t i s not necessary to c o n s i d e r much more complex models such as three-zone and p a r t i c l e - p e l l e t models. Table IA S t r u c t u r a l V a r i a t i o n s During C a l c i n a t i o n of Limestone a t 860 °C Fractional Conversion of CaC0 3
Cumulative Macropore Volume V ml/g r
0.25 0.59 0.82 1.00
0.0800 0.0919 0.0959 0.1000
Cumulative Micropore Volume V , ml/g
Porosity ε
2
0.0407 0.1339 0.1866 0.2825
0.235 0.377 0.419 0.520
Table IB S t r u c t u r a l V a r i a t i o n s During C a l c i n a t i o n of Limestone a t 860 °C Fractional Conversion of CaC0
£
0.25 0.59 0.82 1.00
0.926 0.748 0.653 0
m
3
Average Micropore Radius, um 0.0258 0.0266 0.0272 0.0267
Micropore Surface Area Sg, cm /g 2
4
3.2 χ Ι Ο 10.1 χ Ι Ο 13.7 χ Ι Ο 21.4 χ Ι Ο
4
4
4
S ν (1 - £ ) p^ τη 3.01 2.89 2.83 2.91
χ χ χ χ
5
ΙΟ ΙΟ 1θ5 ΙΟ 5
5
Zone Reaction Model Experimental f i n d i n g s show that c a l c i n a t i o n r e a c t i o n s t a r t s at every i n t e r i o r p o i n t o f the r e a c t a n t and a f t e r a c e r t a i n time the r e a c t a n t s o l i d near the e x t e r n a l s u r f a c e i s completely ex hausted forming a b i d i s p e r s e i n e r t product l a y e r . The p e r i o d of r e a c t i o n p r i o r to the formation of the ash l a y e r i s designated as the f i r s t stage and the p e r i o d f o l l o w i n g the formation o f the ash l a y e r as the second stage. During the c a l c i n a t i o n , the gaseous product CO2 d i f f u s e s out through the pores. The r a t e o f CO2 e v o l u t i o n depends upon whether d i f f u s i o n or s u r f a c e r e a c t i o n i s the c o n t r o l l i n g mechanism. Since c a l c i n a t i o n r e a c t i o n i s r e v e r s i b l e c o n c e n t r a t i o n p r o f i l e o f CO2 w i t h i n the pores would s t r o n g l y e f f e c t the apparent r a t e of
40.
ULKUTAN ET A L .
Noncatalytic Solid-Gas Reactions
521
decomposition. During the f i r s t stage o f the r e a c t i o n and i n the r e a c t i o n zone during the second stage the pore d i f f u s i o n of C0 i s c o n t r o l l e d by Equation 2. I n w r i t i n g t h i s equation the r e a c t i o n i s considered t o be zeroth order i n t h e forward d i r e c t i o n and f i r s t order with r e s p e c t to 0 0 c o n c e n t r a t i o n i n the r e v e r s e direction. 2
2
0 = -±r _ L 2 r
d_ άξ
u
2 d± άζ>
+ 1
(2)
where k
1/2 (3) eA
On the other hand, m a t e r i a l balance f o r the c o n c e n t r a t i o n o f the s o l i d r e a c t a n t ( C ) can be w r i t t e n a s , s
dc
s
(4) e Simultaneous s o l u t i o n o f these equations w i t h the assumption o f n e g l i g i b l e f i l m mass t r a n s f e r r e s i s t a n c e y i e l d the f o l l o w i n g r e l a t i o n s f o r f r a c t i o n a l conversion a t the end o f f i r s t stage (Xj) and the d u r a t i o n o f f i r s t stage ( T J ) :
ν
11
φ
coth
(φ )
(5)
C„ (6) T l
=
k
C
-l A
For the second stage, Equation 2 (which holds f o r 0 < ξ < ξ ) and the d i f f u s i o n equation f o r the outer l a y e r m
0 =\
J-
2
(ξ d £
) f
r
1 -
CaO
(ID
CaCO,
In c o n c l u s i o n , i t i s shown that the mechanism of a nonc a t a l y t i c g a s - s o l i d r e a c t i o n can be analyzed i n d e t a i l by observing the v a r i a t i o n s i n pore s t r u c t u r e during the r e a c t i o n .
Legend o f Symbols C
Ae
e q u i l i b r i u m c o n c e n t r a t i o n o f the product gas
D
eA
e f f e c t i v e d i f f u s i v i t y i n the core s e c t i o n
D*
e f f e c t i v e d i f f u s i v i t y i n the s h e l l s e c t i o n
A
f i r s t order r e v e r s e r a t e constant cumulative macropore volume, ml/g V2
cumulative micropore volume, ml/g
Vp
s p e c i f i c volume o f samples, ml/g
V
Ca0'
molar volumes o f CaO and CaC03
V
CaC0 ε ε^
ψ ω ξ
3
p o r o s i t y a t a c e r t a i n degree o f conversion p o r o s i t y a t complete conversion dimensionless c o n c e n t r a t i o n o f C O 2 a t t h e f i r s t stage and i n the core s e c t i o n a t the second stage, O^/^e dimensionless c o n c e n t r a t i o n of C 0 a t the second stage, i n the s h e l l s e c t i o n dimensionless r a d i a l c o o r d i n a t e dimensionless r a d i u s o f the core 2
duration of f i r s t
stage
time r e q u i r e d f o r complete c o n v e r s i o n
524
CHEMICAL REACTION ENGINEERING
1.0
TEMPERATURE
• • • Δ
Ο
8I0°C 860 °C 905 °C 950°C I000°C 1040°C
ém I—
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
CONVERSION Figure 3.
Variation of total porosity with fractional conversion of CaCO to CaO at different calcination temperatures. s
40. ULKUTAN ET A L .
Noncatalytic Solid-Gas Reactions
525
Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
Petersen, E . E . AIChE J. 1957, 3, 443. White, Α . ; Carberry, J.J. Can. J. Chem. Eng. 1965, 43, 334. Schechter, R . S . ; Gidley, J.L. AIChE J. 1969, 15, 339. Szekelly, J.; Evans, J.W. Chem. Eng. S c i . 1970, 25, 1091. Ramachandran, P . Α . ; Smith, J . M . AIChE J. 1977, 23, 353. Ramachandran, P . Α . ; Smith, J . M . Chem. Eng. J. 1977, 14, 137. Dogu, T. Chem. Eng. J. 1981, 21, 213. Orbey, N . ; Dogu, G . ; Dogu, T. Can. J. Chem. Eng., i n press. Ishida, M . ; Wen, C.Y. AIChE J. 1968, 14, 311. Ausman, J . M . ; Watson, C.C. Chem. Eng. S c i . 1962, 17, 323. Wen, C . Y . ; Ishida, M. Env. S c i . Technol. 1973, 7, 703. Mantri, V . B . ; Gokarn, A . N . ; Doraiswamy, L . K . Chem. Eng. S c i . 1976, 31, 779. Ulkutan, S. M.S. Thesis, Middle East Technical University, Ankara, Turkey, 1979. H i l l , K.J.; Winter, E.R.S. J. Phys. Chem. 1956, 60, 1361. Ingraham, T . R . ; Marier, P. Can. J. Chem. Eng. 1963, 41, 170. Freeman, E . S . ; Carrol B. J. Phys. Chem. 1958, 62, 394.
Received April 27, 1982.
