Studies in gas-solid reactions: Part I. A structural model for the reaction of porous oxides with a reducing gas
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A structural model is presented for describing the reaction of a porous metal oxide pellet with a reducing gas. It is suggested that the pellet is made up of a large number of grains and the overall rate of reaction is computed by summing the contributions of all these individual grains. The model thus incorporates structural parameters, such as grain size, porosity (pore size distribution) and allows a quantitative assessment of the role played by these quantities in determining the rate of progress of the reaction. r e a c t i o nH of porous m e t aE l oxide a g g r e g a t e s with a T r e d u c i n g gas is of c o n s i d e r a b l e t e c h n o l o g i c a l i m p o r taslce arid has c o n s e q u e n t l y r e c e i v e d a g r e a t deal of attention. A l a r g e p r o p o r t i o n of t h e s e studies was c o n c e r n e d with iron oxide r e d u c t i o n but many o t h e r s y s t e m s have a l s o been e x t e n s i v e l y studied. Most of the i n v e s t i g a t o r s to data have i n t e r p r e t e d t h e i r r e s u l t s by using a s u i t a b l e v a r i a n t of the " s h r i n k ing c o r e m o d e l " . 1"8 T h i s m o d e l , sketched s c h e m a t i c a l l y in Fig. 1 is based on the a s s u m p t i o n that a f t e r s o m e r e a c t i o n had o c c u r r e d the solid p h a s e c o n s i s t s of an u n r e a c t e d c o r e , s u r r o u n d e d by a r e a c t e d s h e l l . T h e s e two z o n e s a r e s e p a r a t e d by a s h a r p phase bounda r y w h e r e the c h e m i c a l r e a c t i o n takes p l a c e . As the r e a c t i o n p r o c e e d s , the r e a c t e d s h e l l expands and diffusion of the r e a c t a n t s and p r o d u c t s through this r e g i o n may b e c o m e one of the r a t e l i m i t i n g f a c t o r s . The shrinking c o r e m o d e l {model based on " t o p o c h e m i c a l r e a c t i o n s " ) has been r e m a r k a b l y s u c c e s s f u l for the i n t e r p r e t a t i o n of e x p e r i m e n t a l r e s u l t s ; in g e n e r a l , the equations b a s e d on this r e p r e s e n t a t i o n d e s c r i b e d the o v e r a l l r a t e in t e r m s of a " ' d r i v i n g f o r c e " and t h r e e s e t s of r e s i s t a n c e s , n a m e l y : i) Gas phase m a s s t r a n s f e r . if) Diffusion of r e a c t a n t s and p r o d u c t s through the reacted shell. iii) C h e m i c a l r e a c t i o n o c c u r r i n g at the i n t e r f a c e s e p a r a t i n g the r e a c t e d and the u n r e a c t e d r e g i o n s . Of t h e s e q u a n t i t i e s , the gas phase m a s s t r a n s f e r c o e f f i c i e n t and the r a t e of p o r e diffusion may be p r e d i c t e d , o r at least e s t i m a t e d . The p a r a m e t e r s c h a r a c t e r i z i n g the c h e m i c a l k i n e t i c s , h o w e v e r , have to be m e a s u r e d e x p e r i m e n t a l l y . Indeed, the f o r m of the r a t e equation and the n u m e r i c a l v a l u e s of the r a t e constant s e r v e d as the p r i n c i p a l a d j u s t a b l e p a r a m e
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