The reduction of hematite pellets with carbon monoxide-hydrogen mixtures
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Fig. 1 - - C h e m i c a l c o n v e r s i o n for uniform diameter pellets r o a s t e d in e q u a l - s i z e d fluidized b e d s (ideal b a c k m i x r e a c t o r s ) in s e r i e s with p e l l e t shell-diffusion rate controlling.
iI i
1
99
95
90
F(t,Rp)
85
PERCENT
80
75
Pellet diana, in.
3/16 in.
1/8 in.
average
First stage exit Conversion
91.3 Pct
95.9 Pct
93.6 Pct
99.1
99.7
99.4
(t = 25 min)
T h e s e c u r v e s a r e not l i m i t e d to the l i m e - c o n c e n t r a t e p e l l e t r o a s t p r o b l e m but have g e n e r a l u t i l i t y for fluidized beds or o t h e r b a c k m i x r e a c t o r s in s e r i e s with p a r t i c l e s of c o n s t a n t s i z e . The r o a s t e r d e s i g n c u r v e s of Fig. 1 w e r e used to compute c o n v e r s i o n s for s e l e c t e d a v e r a g e r e s i d e n c e t i m e s to o b t a i n the d e s i r e d a v e r a g e r e s i d e n c e t i m e to gain n e a r l y c o m p l e t e r o a s t i n g ( c o n v e r s i o n ) of the l i m e - c o n c e n t r a t e p e l l e t s . F o r the + 0.32 - 0.47 cm p e l l e t s a 25 m i n a v e r a g e r e s i d e n c e t i m e in each of two e q u a l - s i z e d fluidized beds i n s e r i e s will give m o r e than 99 pct a v e r a g e c o n v e r s i o n . T h e r e s u l t s a r e s u m m a r i z e d in T a b l e I. It should be noted that o v e r 90 pct of the r o a s t i n g c h e m i c a l r e a c t i o n and heat r e l e a s e o c c u r s in the f i r s t s t a g e while h e a t i n g a m b i e n t a i r to about 600~ The s e c o n d stage would r e s u l t in a f u r t h e r t e m p e r a t u r e i n c r e a s e of only about 30~ Hence, to p r e v e n t pellet o v e r c o o l i n g , exit gas f r o m the f i r s t s t a g e m u s t be u s e d f o r r o a s t i n g i n the s e c o n d s t a g e . Heat t r a n s f e r c o r r e l a t i o n s s for fluidized beds u n d e r the conditions for l i m e - c o n c e n t r a t e p e l l e t r o a s t i n g have b e e n used to c a l c u l a t e heat t r a n s f e r r a t e s to the gas in the f i r s t s t a g e and the r e s u l t s show that the i n 490-VOLUME 7B, SEPTEMBER 1976
65
60
CONVERSION
Table I. Conditions for Two Stage Fluidized Bed Pellet Roasting with 99 Pct Conversion
Second stage exit Conversion (t~ = 50 mill)
70
c o m i n g a i r is v e r y quickly heated to n e a r the pellet t e m p e r a t u r e . The gas t e m p e r a t u r e is r a i s e d to within 25~ of the a v e r a g e p e l l e t t e m p e r a t u r e a f t e r t r a v e l i n g only 5 cm into the bed of the f i r s t s t a g e . P e l l e t t e m p e r a t u r e s tend to be v e r y close to the a v e r a g e pellet t e m p e r a t u r e b e c a u s e of the i d e a l s o l i d s m i x i n g within the bed. 1. R. W. Bartlett and H. H. Haung: Z Metals, 1973, vol. 25, no. 12, pp. 28-34. 2. H. H. Haung and R. W. Bartlett: Met. Trans. B, 1976, vol. 7B, p. 369. 3. D. Kunii and O. Levenspiel: Fluidization Engineering, p. 328, J. Wiley and Sons, New Yor
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