On the reaction of solids with mixed gases
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T H E R E are many systems of practical importance in metals and materials processing operations, where a solid species is made to r e a c t with a gas mixture, one or more components of which participate in the reaction. The reduction of iron oxides with carbon monoxide-hydrogen mixtures in the " d i r e c t reduction processes" and also in the iron blast furnace, when stack gas injection is b e i n g employed may be cited as representative examples. Other systems of considerable practical interest include the reaction of carbon (coal) with oxygen and s t e a m in the L u r g i or the Koppers Totzek coal gasification processes. In s p i t e of the appreciable practical importance of these systems, relatively little fundamental work has been reported on the reaction of solids with gas mixtures. Notable exceptions to this rule are r e c e n t papers by Turkdogan,~'2 the present authors3 and two papers by T s a y , Ray and Szekely,4,5 all of which were conc e r n e d with the effect of gas composition on the rate at which iron oxide pellets r e a c t with a CO + He gas mixture. Turkdogan and Vinters measured the rate at which hematite pellets r e a c t with CO + He mixtures at 900°C in a r e g i m e that was controlled by pore diffusion; they found that the higher was the hydrogen content of the reactant gas mixture, the faster was the overall rate of reaction. In advancing a semiquantitative interpretation these authors suggested that the overall rate of reduction was proportional to a pseudobinary diffusion coefficient, the v a l u e of which increased with an increasing proportion of hydrogen in the gas mixture. This explanation was consistent with the overall t r e n d exhibited by the measurements. In a more recent communication, Szekely and E1Tawil reported findings on the reduction kinetics of hematite pellets with CO + He mixtures which were very s i m i l a r to the data presented e a r l i e r by TurkdoJ. SZEKELY is Professor of Materials Engineeringand M. CHOUDHARY is Graduate Student, Department of Materials Science and Engineering, Massachusetts Instituteof Technology, Cambridge, Mass. 02139. Y. EL-TAWIL, formerly GraduateStudent, Department of Chemical Engineering, State University ofNew York at Buffalo,is presently Assistant Professor, Department of Chemical Engineering, Alexandria University, Alexandria, Egypt. Manuscript submitted February 2, 1977. METALLURGICAL TRANSACTIONS B
gan and Vinters, albeit over a wider temperature range. A quantitative interpretation of these measurements was advanced by T s a y , Ray and Szekely, who used modelling equations b a s e d on the generalization of the three-interface shrinking core model of Spitzer, Philbrook and Mannings in conjunction with rigorously defined pseudobinary diffusion coefficients for the s y s tem. v It has to be stressed, however, that the proper representation of diffusion in multicomponent systems, such as those discussed above, requires a somewhat more sophisticated approach, because the pseudobinary approximation neglects the interaction
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