The kinetics of formation of h 2 o and co 2 during iron oxide reduction
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The k i n e t i c s of the c h e m i c a l r e a c t i o n - c o n t r o l l e d r e d u c t i o n of i r o n oxides by He/H20 and CO/CO2 gas m i x t u r e s a r e d i s c u s s e d . F r o m an a n a l y s i s of the s y s t e m s it is concluded that the d e c o m p o s i t i o n of the oxides takes p l a c e by the two d i m e n s i o n a l n u c l e a t i o n and l a t e r a l growth of oxygen v a c a n c y c l u s t e r s at the g a s / o x i d e i n t e r f a c e . The r a t e s of d e c o m p o s i t i o n of the oxides u n d e r conditions of c h e m i c a l r e a c t i o n c o n t r o l a r e dependent not only on the p a r t i a l p r e s s u r e s of the r e a c t i n g g a s e s at the r e a c t i o n t e m p e r a t u r e but a l s o on the oxygen a c t i v i t y of the p r e v a i l i n g a t m o s p h e r e . A p p l i c a t i o n of this m o d e l to the k i n e t i c data leads to the d e t e r m i n a t i o n of the m a x i m u m c h e m i c a l r e a c t i o n r a t e c o n s t a n t s for the d e c o m p o s i t i o n of the i r o n oxide s u r f a c e s . A s s u m i n g the r e a c t i o n s He(g) + O (ads) - - H20(g) and CO(g) + 0 (ads) - - CO2(g) to be r a t e c o n t r o l l i n g the m a x i m u m c h e m i c a l r e a c t i o n r a t e c o n s t a n t s for the r e d u c t i o n of i r o n oxides a r e given by q% 1-12 =
104.00 exp \ ( - 69,300)R T m o l m -2 s -1 a r m -I
and %CO = 104"4~ exp ( - 103,900RT )
m o l m-2 s -~ arm -1.
The m a x i m u m c h e m i c a l r e a c t i o n r a t e c o n s t a n t s do not n e c e s s a r i l y i n d i c a t e the m a x i m u m r a t e s which can be a c h i e v e d in p r a c t i c e s i n c e t h e s e will depend on the l i m i t a t i o n s i m posed by m a s s t r a n s p o r t in the s y s t e m s . The r a t e c o n s t a n t s a r e i m p o r t a n t however s i n c e they indicate for the f i r s t t i m e the u p p e r l i m i t of any r e d u c t i o n r a t e in these s y s t e m s . The f r a c t i o n s of r e a c t i o n s i t e s which a p p e a r to be a c t i v e on w~istite s u r f a c e s in e q u i l i b r i u m with i r o n a r e c a l c u l a t e d . A d i r e c t r e l a t i o n s h i p b e t w e e n c h e m i c a l r e a c t i o n r a t e s on liquid i r o n s u r f a c e s and r a t e s on a t o m i c a l l y rough i r o n oxide s u r f a c e s is postulated.
THE c h e m i c a l r e a c t i o n s
of CO/CO2 and H2/H20 gas m i x t u r e s with i r o n and i r o n oxide s u r f a c e s have b e e n e x t e n s i v e l y studied in r e c e n t y e a r s . The m e c h a n i s m s of these r e a c t i o n s have been r e v i e w e d by W a g n e r 1 and T u r k d o g a n and V i n t e r s . 2 T h e s e a u t h o r s conclude f r o m isotope exchange and n o n e q u i l i b r i u m e x p e r i m e n t s on i r o n and its oxides that the following r e a c t i o n s a r e r a t e c o n t r o l l i n g in CO/CO2 and H2/H20 s y s t e m s
THE KINETICS OF REDUCTION OF IRON OXIDES
It has b e e n o b s e r v e d 3-7 that the d e
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