Diffusion in mixed-gas reduction
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+ CO2
~MeO+H2
CO + H 2 0 ~-CO2 +H2 It follows that the a p p r o a c h to e q u i l i b r i u m of this r e a c t i o n m u s t m a t c h that of the i n d i v i d u a l o x i d a t i o n - r e duction r e a c t i o n s . The a l t e r n a t i v e is to p o s t u l a t e that specific s u r f a c e a r e a s of the solid a r e e a r m a r k e d to r e a c t only with one r e d u c t a n t . Such i n t e r l i n k e d m e c h a n i s m s p r o v i d e an e x p l a n a t i o n of the r e a c t i o n p r o m o t i n g effect of m e t a L / m e t a l oxide couples a s in, for e x a m ple, the use of a n i c k e l b a s e d c a t a l y s t in the r e f o r m i n g s e c t i o n s of g a s e o u s d i r e c t r e d u c t i o n p r o c e s s e s . In fact for half a c e n t u r y o r so the w a t e r - g a s shift r e a c tion was a m a j o r s o u r c e of h y d r o g e n for the then coal based chemical industry. Thus a m m o n i a - s y n t h e s i s gas (the s t a r t i n g point for n i t r i c acid, f e r t i l i z e r s , e x p l o s i v e s and so forth) was obtained by r e a c t i n g s t e a m with p r o d u c e r - g a s (CO + N2) at the lowest poss i b l e t e m p e r a t u r e in o r d e r to f a v o r the f o r m a t i o n of hydrogen. It was found that a n a c c e p t a b l e c o m b i n a t i o n of yield and r a t e of r e a c t i o n could be a c h i e v e d at ternVICTOR CROFT is Lecturer in Process Engineering, Departmcnt of Metallurgy and Material Science, Imperial College of Science and Technology, University of London, England. Manuscript submitted May 26, 1978. METALLURGICALTRANSACTIONS B
p e r a t u r e s s o m e w h a t below 500~ in the p r e s e n c e of i r o n oxide. T u r k d o g a n and V i n t e r s ~ deduced, f r o m t h e i r r e s u l t s on c h e m i c a l r a t e control, c o n f i r m a t i o n of the g e n e r a l l y held view that the r a t e of a t t a i n m e n t of w a t e r - g a s equil i b r i u m within a bulk gas phase is not s i g n i f i c a n t except at t e m p e r a t u r e s in e x c e s s of 1000~ However the gas c o m p o s i t i o n s at the exit of d i r e c t r e d u c t i o n p r o c e s s e s such a s the M i d r e x c o r r e s p o n d to w a t e r gas e q u i l i b r i u m " f r o z e n i n " at a t e m p e r a t u r e of about 700~ The obvious c o n c l u s i o n to draw is that the n e c e s s a r y w a t e r - g a s r e a c t i o n m u s t be t a k i n g place within the pore s t r u c t u r e of the solid by the i n t e r linked m e c h a n i s m s d e s c r i b e d above. In s p i t e of such e v i d e n c e the effect of c h e m i c a l r e a c t i o n b e t w e e n the diffusing s p e c i e s has b e e n i g n o r e d in the v a r i o u s d i s c u s s i o n s on m i x e d - g a s diffusion (see F i g . 1). Diffusion in m i x e d - g a s r e d u c t i o n is a l s o c o m p l i c a t e d by the fact that when m o r e than two g a s e s a r e p r e s e n t p h y s i c a l i n t e r f e r e n c e b e t w e e n the fluxes r e q u i r e s the use of th
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