Chemical Reaction EngineeringâBoston - American Chemical Society

3 Direct Reduction of Iron Ore in a Moving-Bed Reactor: Analyzed by Using the Water Gas Shift Reaction R. HUGHES and Ε . Κ. T. K A M Downloaded by YORK UNIV on October 19, 2014 | http://pubs.acs.org Publication Date: September 16, 1982 | doi: 10.1021/bk-1982-0196.ch003

University of Salford, Department of Chemical Engineering, Salford M5 4WT, England A model f o r the d i r e c t r e d u c t i o n of i r o n ore in a moving bed has been developed. The model accounts f o r the water gas shift e q u i l i b r i u m as w e l l as r e d u c t i o n by the species H and CO. I n c l u s i o n o f t h i s e q u i l i b r i u m has been shown t o enhance r e d u c t i o n e s p e c i a l l y a t the h i g h conversions required.Increase of o p e r a t i n g temperature can g i v e decreased conver sions. 2

One of the more important a l t e r n a t i v e s t o the b l a s t furnace f o r the production o f i r o n i s d i r e c t r e d u c t i o n of p e l l e t i s e d o r e i n a s h a f t r e a c t o r . The reducing gas mixture i s u s u a l l y obtained by steam reforming of n a t u r a l gas and flows upward,countercurrent to the downward flow of s o l i d s . Sponge i r o n obtained by d i r e c t r e d u c t i o n may be used d i r e c t l y i n a r c furnaces f o r s t e e l prod uction. Previous s t u d i e s of d i r e c t r e d u c t i o n on i r o n ore p e l l e t s have been reviewed by T h e m e l i s ( l ) , Bogdandy(2) and Huebler(3). Work on r e d u c t i o n by mixtures has been reported by Szekely(4) and Hughes e t a l ( 5 ) . Modelling s t u d i e s on countercurrent moving bed systems have been reported by S p i t z e r ( 6 ) f o r isothermal r e d u c t i o n i n hydrogen, by M i l 1 e r ( 7 ) f o r non-isothermal r e d u c t i o n i n carbon monoxide and more r e c e n t l y by Tsay e t a l ( 8 ) and Kam and Hughes(9) f o r C0/H2 mixtures. However, s i n c e i r o n i s known t o be a c a t a l y s t f o r the water gas s h i f t r e a c t i o n , t h i s r e a c t i o n w i l l i n f l u e n c e the gas composition and t h e r e f o r e the extent of r e d u c t i o n . None of the previous analyses have considered t h i s aspect and the o b j e c t i v e of the present paper i s t o account f o r the o v e r a l l r e d u c t i o n by i n c l u s i o n of t h i s r e a c t i o n . Mathematical

Formulation

The water gas s h i f t r e a c t i o n occurs on or w i t h i n the i r o n oxide p a r t i c l e and t h e r e f o r e a heterogeneous model i s employed u s i n g separate balances f o r the p e l l e t s and the r e a c t o r . 0097-6156/82/0196-0029$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.

30

CHEMICAL REACTION ENGINEERING

S i n g l e p e l l e t r e d u c t i o n . The r e d u c t i o n occurs at high temp eratures and a s h r i n k i n g core model i s appropriate as confirmed experimentally(5). Removal of oxygen occurs a t the advancing i n t e r f a c e while the water gas s h i f t r e a c t i o n occurs i n the outer l a y e r of reduced i r o n . The mechanism of the water gas s h i f t r e a c t i o n i s thought t o be (10,11) between adsorbed oxygen and CO on the a c t i v e surface of the product i r o n , i . e :

K

U

f C0

0(ads)

(1)

'co„

Downloaded by YORK UNIV on October 19, 2014 | http://pubs.acs.org Publication Date: September 16, 1982 | doi: 10.1021/bk-1982-0196.ch003

or i n terms of the e q u i l i b r i u m constant, Κ "CO R

H 0

'CO.

2

= k

w

(2) C

H

w

with the r a t e constant k given by k=5.6xl0

-15000 -4„ Τ exp

The o v e r a l l r e d u c t i o n scheme can be s i m p l i f i e d to the three reactions:3C0 + F e 0 2

3H

2

CO

Fe 0

+

2

2Fe + 3C0

3

3

At the i n t e r f a c e

H

In the i r o n l a y e r

2

+ H 0

^

2

2

2Fe + 3H 0 0

2

+ C0

' ~

n

2

Since the reducing gas flow i s very high ( t y p i c a l l y 1800 m / tonne of product), i t i s assumed that the bulk of the mass t r a n s f e r r e s i s t a n c e i s w i t h i n the p e l l e t . Under these c o n d i t i o n s , the dimensionless m a t e r i a l balance f o r hydrogen i n the p e l l e t i s 2

v y

=

V

H 2

2

2

The multi-component d i f f u s i v i t i e s i n the gas mixture can be approximated by the m o d i f i e d Stefan-Maxwell equations(8,90 i . e :

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

3.

HUGHES AND K A M

De. i-m

Direct Reduction of Iron Ore

ι—, £ Q _ + τ D.„ £iK

31

(5)

η (y.N.-y.N.)-r y * J J I Η D.. J=l lj J y

1

Λ

At the r e a c t i o n i n t e r f a c e 6* between the ore and i r o n l a y e r s , using the pseudo steady s t a t e assumption the dimensionless m a t e r i a l balances may be represented by y

H 0 ••- w. Η -Η 0

~3δ~


δ=δ*

2

= Da^ δ=δ* 2 \

3δ

2

H

^co Da

= - w.

co-co„

δ=δ*

H 0 2

2

δ=δ*

(6) K e

H

2

yco CO 'CO

2

(7)

K, «C0

The D i r i c h l e t boundary c o n d i t i o n s apply t o eqns (6) and (7) since e x t e r n a l mass t r a n s f e r i s n e g l e c t e d . F i n a l l y , the dimensionl e s s e x p r e s s i o n f o r the r a t e of advance o f the i n t e r f a c e i s : 3y, 36* CO 36 δ = δ * 3τ 36 δ = δ * H -CO 2

yH o

y

2

= Da„

y

-

H

+

D a

W

C 0 H -CO 2

'2-1

r

co

C0

2

(8) C e

C0

Counter-cur rent moving bed I n t h i s r e a c t o r s o l i d s flow i s down ward w i t h the oxide c o n c e n t r a t i o n Cg | a t the top of the reactor. The gaseous s p e c i e s flow upwards w i t h a bottom ( i n l e t ) concent r a t i o n o f Cg Other assumptions a r e : £=0 £=0 1)

Steady s t a t e isothermal o p e r a t i o n ( t h i s may be assumed because of the balance between exothermic CO r e d u c t i o n and endothermic H r e d u c t i o n ) . Plug flow f o r both gas and s o l i d streams. Uniform motion of the s o l i d p e l l e t s w i t h constant voidage. P e l l e t s a r e s p h e r i c a l i n shape and a s h r i n k i n g core,sharp i n t e r f a c e model i s assumed f o r the p e l l e t reduction(8y 9 ) . For the gas s p e c i e s , the dimensionless c o n t i n u i t y eqtns a r e : 2

2) 3) 4)

3y° 9

= σ (δ*)'

yH

2

3ξ

ay,CO 3ξ

(9) δ=δ*

=

σ

_ ,r*>2 H -CO *> W

(6

2

CO 36 δ = δ *


(ΙΟ)

32


and f o r the s o l i d phase

ay,CO

Ή.

Br* = Ω I F

+ w H -CO

do

2

"6=6*

(11) 6=6*

where 3(1-ε') D, eH -m 2

2

(r ) U ο g and


C

D

TO

e

H 2

-m

L

Ω = 2

(Ρ x b ) ( r ) U o o s I t should be noted the σ and Ω a r e n o t constants b u t v a r i a b l e s dependent on D „ 2""^ Method o f s o l u t i o n . A t r i a l and e r r o r method was used t o s o l v e the mass c o n t i n u i t y equations f o r one o f t h e s p e c i e s (e.g. CO) i n the s i n g l e p e l l e t b a l a n c e s . To do t h i s , expressions f o r other s p e c i e s i n terms o f y c o are d e r i v e d through the water gass h i f t r e a c t i o n and the r e a c t i o n s a t t h e i n t e r f a c e , i . e : e

Y

Y

H 0 2

co„

y E

2

( y

H -H 0 2

2

H

ο y

Y 2

)

H 2

w 2

co-co

+ w (y CO-H *C0 X

2

.

Y 2

(12)

H

.ο

co

= y *E

J

W

2

_ r

0

° H 0

y

co ~ co y

(13)

) + δ*(δ*-ΐ)

2

86

6=6* CO-H„ +w

CO 36 6=6*

(14)

In order t o s i m p l i f y the procedures f o r s o l v i n g the water gass h i f t r e a c t i o n i n the s i n g l e p e l l e t , an average value o f the con c e n t r a t i o n f o r each o f t h e reducing gases i s employed, i . e : Y

H 2

=

H

*bo"

°-

5

(

Y

+

H 2

Y

H

0 . 5 2

(15)

H

0

+y

c o

)

(16)

Further s i m p l i f i c a t i o n can be achieved by l i n e a r i s i n g the water g a s - s h i f t r e a c t i o n r a t e , and u s i n g T a y l o r ' s s e r i e s expansion the flowing expression f o r the s h i f t r e a c t i o n can be obtained \

y

= *1 *C0 - *2 H

+ 2

*3


(17)

3.

33


HUGHES AND K A M

where t h e l i n e a r i s a t i o n constants a r e

y ο

»coco

H

(18)

2 r

CO

(19) y~ 2 H

y

1


W

co (20)

+

y

* i co

Hence, the l i n e a r i s e d form o f eqtn.(4)

V

y

C0

=

ν

Η ^0

*lV*2 H y

Φ

2

+ ψ 2

i n terms o f CO becomes (21)

3

An a n a l y t i c a l s o l u t i o n o f t h e above equation can be obtained as

y

C0

a ^ i n h i i ^ 6*)+a cosh(^a^ δ*) 2

δ*

Ύο-Ι —

Y, ±

Ύ

(22)

1

where *1

CO-Η,

+

W

H -

C 0

*

2

2

y

γ

CO-Η,

2

φ

{ψ (δ*)(δ*-1)

H2

9

δ=δ*

+ wH„-C0

*y,CO 36

δ=δ*

and W

CC-H

and 0 ^ and a

2

2

*

W Î ,

+

W

H 2

C

0

^

0

)

are i n t e g r a t i o n constants which can be d e r i v e d from

the boundary c o n d i t i o n s a t the i n t e r f a c e . The procedure f o r the s o l u t i o n o f the above s e t o f equations i s as f o l l o w s : (1) (2)

v a l u e s o f δ* a r e s e l e c t e d a value o f y a t δ* i s assumed

(3)

y„ » y . and y a r e c a l c u l a t e d from eqtns.(12-14) 2 2° °2 the multi-component d i f f u s i v i t i e s i n the bulk, a t the i n t e r f a c e and the mean v a l u e s are c a l c u l a t e d y i s c a l c u l a t e d from eqtn.(22) and compared w i t h the assumed value o f y i n step (1). Steps (2) t o (5) a r e repeated u n t i l agreement i s a t t a i n e d the time r e q u i r e d f o r the i n t e r f a c e advancement v i a eqtn. (8) i s obtained, and steps (1-6) a r e repeated u n t i l t h e process i s completed.

C

n

H

(4) (5)

(6) (7)

Q

H

c

C

o


34


The s o l u t i o n procedure f o r the moving bed has been d e s c r i b e d i n d e t a i l elsewhere(9). The two p o i n t boundary v a l u e problem i s s o l v e d by a p r e d i c t o r - c o r r e c t o r procedure on the m i s s i n g boundary a t the top o f the r e a c t o r u n t i l agreement with the i n l e t gas composition a t the base o f t h e r e a c t o r i s achieved. R e s u l t s and D i s c u s s i o n . Some experimental r e s u l t s on H2/CO mixtures with no added CO2 o r H2O, were a v a i l a b l e from p r e v i o u s work (12) u s i n g a h i g h p u r i t y p e l l e t i s e d ore (Carol Lake). A comparison o f t h e experimental and p r e d i c t e d r e s u l t s u s i n g t h e water gas s h i f t r e a c t i o n a t a s o l i d conversion o f 50% i s g i v e n i n Table I below. Downloaded by YORK UNIV on October 19, 2014 | http://pubs.acs.org Publication Date: September 16, 1982 | doi: 10.1021/bk-1982-0196.ch003

"Table I "

V

100

80

50

C0% 0 20 50 E x p t l (min) 12 21 31 P r e d i c t e d (min) 14 19 25 Experimental r e s u l t s were not a v a i l a b l e f o r CO r i c h mixtures, but the agreement i s seen t o be adequate. B e t t e r agreement might have been obtained i f t h e experimental gas mixture had contained both CO2 and H2O, i n s t e a d o f j u s t CO and H 2 . Because i n s i n g l e p e l l e t s experiments, there i s l i t t l e o p p o r t u n i t y f o r an e q u i l i b r i u m i n the gas mixture t o be a t t a i n e d , s i n g l e p e l l e t r e s u l t s are not g e n e r a l l y i n d i c a t i v e o f o v e r a l l r e a c t o r behaviour. A parametric study o f moving bed behaviour has been undertaken. The s o l i d p e l l e t s a r e assumed t o be preheated t o the appr o p r i a t e r e d u c t i o n temperatures b e f o r e e n t e r i n g t h e r e a c t i o n zone of the r e a c t o r . Although t h i s n e g l e c t s the s o l i d s preheat zone, t h i s can e a s i l y be i n c l u d e d i n the model i f r e q u i r e d . The present study t h e r e f o r e i s focussed on the r e a c t i o n zone i t s e l f where the important parameters o f gas and s o l i d flow r a t e s , gas i n l e t tempe r a t u r e and gas mixture composition a r e c o n s i d e r e d . Reactor l e n g t h i s a l s o o f major importance but i n the present paper t h i s has been f i x e d a t lm i n order t o o b t a i n comparative d a t a . Modelling s t u d i e s f o r the moving bed were made a t two gas compositions, a hydrogen r i c h composition c o n t a i n i n g 50% H2 and 20% CO with 10% H2O and 5% C02# and a CO r i c h gas mixture cont a i n i n g 50% CO and 20% H2 w i t h 5% H2O and 20% C02. Most r e s u l t s were obtained w i t h the l a t t e r mixture, which i s r e p r e s e n t a t i v e o f gas produced from c o a l g a s i f i c a t i o n , which i s l i k e l y t o have a major a p p l i c a t i o n f o r r e d u c t i o n processes i n the f u t u r e . P e l l e t s of 8mm diameter were modelled u n l e s s otherwise indicated.Temperatures were v a r i e d from 873 t o 1273K while gas flows and s o l i d flow rates are t y p i c a l o f those used commercially. F i g u r e 1 shows the e f f e c t o f gas flow r a t e p r e d i c t e d by the model on the s o l i d c o n v e r s i o n f o r a CO r i c h gas mixture. Three gas flow r a t e s o f 9,7 and 5 m/s a r e shown. A l s o i l l u s t r a t e d i s the p r e d i c t e d conversion f o r the model which does n o t i n c l u d e the



3.

HUGHES AND K A M


35

water gas s h i f t r e a c t i o n ( f o r a gas flow r a t e o f 7 m/s) , The f i n a l conversion i s seen t o be 58.5% when the water gas s h i f t r e a c t i o n i s n e g l e c t e d but 70% when t h i s i s included.Furthermore, the shape o f the curve i s d i f f e r e n t ; the curve i n which the water gas s h i f t r e a c t i o n i s n e g l e c t e d being convex towards the conversion a x i s , whereas when the water gas s h i f t r e a c t i o n i s i n c l u d e d t h i s does not happen and indeed a t higher flow r a t e s the curve becomes con cave t o the conversion a x i s . T h i s i s e s p e c i a l l y pronounced f o r the 5 m/s flow r a t e and demonstrates the e f f i c i e n c y o f the water gas s h i f t r e a c t i o n i n promoting c o n v e r s i o n . An i n c r e a s e i n gas flow r a t e g i v e s a g r e a t e r f r a c t i o n a l con v e r s i o n o f the i r o n o r e . T h i s e f f e c t i s not due t o i n c r e a s e d mass t r a n s p o r t with i n c r e a s i n g flow as the c a l c u l a t e d Sherwood number i s 500, j u s t i f y i n g n e g l e c t o f t h i s i n the model. The most probable reason f o r i n c r e a s e d conversion with i n c r e a s e d flow r a t e i s t h a t as the gas flow i n c r e a s e s , the amount o f r e a c t a n t gases a t a higher r e l a t i v e c o n c e n t r a t i o n c o n t a c t i n g the ore i s i n c r e a s e d . Hence, a f a s t e r r a t e o f r e d u c t i o n ensues. The e f f e c t o f s o l i d flow r a t e i s i l l u s t r a t e d i n Figure 2 f o r 3 s o l i d flow r a t e s o f 1.5, 2.0 and 2.5xlO~ m/s r e s p e c t i v e l y . A l s o shown by the broken curves are r e s u l t s when the water gas s h i f t r e a c t i o n i s not i n c l u d e d . I t can be seen t h a t when the s o l i d con v e r s i o n i s l a r g e ( s o l i d s flow 1.5xlO~ m/s) the enhancement o f conversion by the water gas s h i f t r e a c t i o n i s c o n s i d e r a b l e g i v i n g 99% conversion o f s o l i d under these c o n d i t i o n s , compared t o o n l y 75% i f the water gas s h i f t process i s n e g l e c t e d . A t l a r g e r flow r a t e s , where the conversion i s l e s s , the e f f e c t o f the water gas s h i f t r e a c t i o n becomes l e s s important. Again, i t can be noted t h a t f o r the water gas s h i f t , the X vs ξ curves, a f t e r , an i n i t i a l convex behaviour (up t o ξ= .1) become concave t o the X a x i s whereas when t h i s r e a c t i o n i s not i n c l u d e d the X vs ξ curves a r e convex t o the X a x i s throughout. For both models, i n c r e a s e i n s o l i d s flow r a t e r e s u l t s i n a decreased s o l i d s conversion as would be expected. The i n f l u e n c e o f gas i n l e t temperature on the r e d u c t i o n was a l s o s t u d i e d . I n s t u d i e s o f s i n g l e p e l l e t r e d u c t i o n by e i t h e r pure gases o f gas mixtures an i n c r e a s e i n r e a c t i o n temperature w i l l normally r e s u l t i n an i n c r e a s e d oxide conversion.However, i n the present study, i n a moving bed with e i t h e r a H2 r i c h o r CO r i c h r e a c t i o n mixture the r e v e r s e e f f e c t was observed. That f o r a CO r i c h mixture i s shown i n F i g u r e 3 where the broken curves a l s o show the p r e d i c t e d curves when the water gas s h i f t r e a c t i o n i s ignored. The l a t t e r r e s u l t s confirm the c o n c l u s i o n s a l r e a d y made,that when conversions are h i g h the water gas s h i f t r e a c t i o n enhances the reduction.However,for both cases, a more g e n e r a l c o n c l u s i o n i s a l s o obtained i . e . , the conversion decreases with i n c r e a s i n g op e r a t i n g temperature. The extent o f the decrease i n conversion with temperature was found t o be l e s s f o r the H2 r i c h mixture, as shown i n Fig.4 where a comparison i s made with the CO-rich mixture f o r 10mm diameter p e l l e t s . I f a H2 r i c h mixture had no CO2 p r e s e n t and 4

4


36



Figure 2. Effect of solids velocity, U , on conversion in α CO rich mixture. Num bers on curves are solid velocities (ΧίΟ' m/s). Key: , water gas shift reaction included; and , water gas shift reaction excluded. a

4



HUGHES AND KAM


Figure 3. Effect of T on conversion in a CO rich mixture. Key: , water gas shift reaction included; and , water gas shift reaction excluded. 0

Figure 4. Effect of gas composition and T on conversion. Key: mixtures; and , H rich mixtures. 0

x


, CO rich

38



the water gas s h i f t r e a c t i o n was neglected,then an i n c r e a s e d c o n v e r s i o n w i t h i n c r e a s i n g temperature was predicted.The deccreased c o n v e r s i o n a t h i g h e r temperatures observed i n F i g s . 3 and 4 i s due t o the i n f l u e n c e of temperature on the e q u i l i b r i u m con s t a n t f o r the CO r e d u c t i o n . T h i s decreases w i t h temperature (CO r e d u c t i o n i s exothermic) while the H2 r e d u c t i o n e q u i l i b r i u m constant i n c r e a s e s w i t h temperature (reduction i s endothermic). Thus, a t h i g h e r temperatures, the r e a c t i o n o f any C02 p r e s e n t with the reduced i r o n t o produce oxide i s favoured and t h i s r e s t r i c t s the o v e r a l l r e d u c t i o n by both H2 and CO i n the mixture and hence the f r a c t i o n a l conversion i s reduced. Legend of Symbols b Ci

stoichiometric coefficient concentration of species i r

De

o

r K

r

De

DaH2/Daco Damkohler number,defined as kH2 o/ H2-m CO o/ CO-m De^ e f f e c t i v e d i f f u s i v i t y of species i iK Knudsen d i f f u s i v i t y o f s p e c i e s i i - m molecular d i f f u s i v i t y , s p e c i e s i i n mixture m k r a t e constant K water gas s h i f t e q u i l i b r i u m constant KecO'^Ho e q u i l i b r i u m constant f o r CO or H2 r e d u c t i o n r , r * p e l l e t radius, radius of r e a c t i o n i n t e r f a c e i n p e l l e t W' H2' C0 © of water gas s h i f t r e a c t i o n , H 2 reduction,CO reduction,respectively 5 surface a r e a o f p e l l e t g^ s 9 solids velocity, respectively y dimensionless c o n c e n t r a t i o n X s o l i d s conversion i n bed ξ dimensionless bed length δ,δ* dimensionless p e l l e t r a d i u s , r e a c t i o n r a d i u s r e s p e c t i v e l y ρ p e l l e t density ε' bed voidage Literature Cited 1. Themelis,N.J. and Gauvin,W.H.,AIChE Jl.8, 437 (1962). 2. Von Bogdandy,L. and Engell,H.J.'The Reduction o f I r o n Ores' Springer V e r l a g , B e r l i n , 1 9 7 1 . 3. Huebler,J.,Iron Ore Reduction Proc.Symp.Chicago,Pergamon Press,Oxford (1962). 4. S z e k e l y , J . and E l - T a w i l , Y . Met.Trans.7B, 490 (1976). 5. Hughes,R., Mogadamzadeh,H. and Kam,Ε.Κ.Τ., 2nd European Symposium on Thermal A n a l y s i s , Aberdeen (1981)-(in p r e s s ) . 6. S p i t z e r , R.H.,Manning,F.S. and Philbrook,W.O., TMS-AIME, 236, 726 (1966). 7. M i l l e r , R . L . , Ph.D.Thesis,Mellon U n i v e r s i t y , P i t t s b u r g h (1972). 8. Tsay,Q.T.,Ray,W.H. and Szekelv,J.,AIChE Jl,22,1072 (1976). 9. Kam,Ε.Κ.Τ. and Hughes,R.,Trans.Inst.Chem.Engrs.59,196 (1981). 10. Kaneko,Y. and Oki,S.,J.Res.Inst.Catalysis,Hokkaido Univ., 13, N o . l , 55 (1965). 11. Meschter,P.J. and Grabke,H.J.,Met.Trans.,10B, 323 (1979). D

D

w

0

R

R

u

12.

u

R

r a t

a s

Mogadamzadeh,Η.,

RECEIVED May

11,

Ph.D.Thesis,Salford U n i v e r s i t y

(1977).

1982.


Chemical Reaction EngineeringâBoston - American Chemical Society

Recommend Documents

Chemical Reaction EngineeringâBoston - American Chemical Society