Gas-solid reaction-rate enhancement by pressure cycling
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I.
INTRODUCTION
G A S - s o l i d reactions are of considerable industrial importance and readily found in chemical and metallurgical industries. Examples are the reduction of metal oxides, the roasting of ores, and the combustion of solid fuels, to mention but a few. Although most gas-solid reactions involve a rather complex set of steps and may require individual treatments, there are certain aspects of the overall reaction that are common to a wide range of reactions. The overall reaction process may involve the following individual steps: (1) diffusion of the fluid reactants across the fluid film surrounding the solid, (2) diffusion of fluid reactants through porous solid, (3) chemical reaction on the solid surface, and (4) diffusion of the fluid products away from the reaction surface through the porous solid and through the fluid film surrounding the solid. Gas-solid reactions are, in general, carried out under the conditions in which the pore diffusion of gaseous reactants strongly affects or entirely controls the overall rate. Pore diffusion i s a slow process, especially compared with bulk flow. Pore diffusion occurs due to the molecular movement of matter under a concentration gradient. The pumping action created by the pressure cycling increases the transport rate of the reducing gas into and the product gas out of the solid pellet. The effect is to charge the solid with fresh reactant as pressure rises and to purge the product gas as the pressure drops. These bulk flows result in enhanced intraparticle transport and thereby increase the overall rate. The utility of periodic external pressures as a means to increase reaction rates was studied by Hamer and Cormack tl] for catalytic reactions in porous catalyst pellets. Sohn and Chaubal t23 theoretically investigated the effect of operating gas-solid reaction systems under a cy-
H.Y. SOHN, Professor, is with the Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112-1183. M.B. ABOUKHESHEM, formerly Graduate Student, Department of Metallurgical Engineering, University of Utah, is an Engineer with the Iron and Steel Complex, Tripoli, Libya. Manuscript submitted Auguest 13, 1991. METALLURGICAL TRANSACTIONS B
cling external pressure. No previous experimental verification of the effect of pressure cycling on the rate of a gas-solid reaction has been reported. It was the objective of this research to study this effect using nickel oxide reduction with hydrogen, by carrying out laboratoryscale experiments under a cycling pressure, and to develop a mathematical model from the first principles of reaction kinetics and mass transfer through a porous solid. II.
MATHEMATICAL MODELING
Let us consider a porous pellet of nickel oxide reacting with hydrogen according to H2 (g) + NiO (s) = H20 (g) + Ni (s)
[1]
which can be represented in a general form as A (g) + bB (s) = cC (g) + dD (s)
[2]
In order to describe this system, it is assumed that the macroscopic structure of the pellet is unchanged by reaction. Szekely et a/. [3] have
