The rate of movement of the topochemical interface during gas-solid reactions
- PDF / 751,203 Bytes
- 8 Pages / 583.28 x 777.28 pts Page_size
- 97 Downloads / 148 Views
R (re)
dt
dCSdr r~
where rc is radial position of the interface, R (re) the rate of the chemical reaction at the interface and ( d C , / d r ) l ~ is the gradient of concentration profile of solid reactant in the porous core at the interf~tce. The integration of the above equation can sometimes be achieved analytically but can always be done numerically for even highly complex reaction mechanisms. The equation correctly reduces to previously known solutions when the inner core is impervious to gas diffusion. The utility of the equation is demonstrated by applying it to the industrially important multistage reduction process Mn304 ~ MnO ~ Mn. The predictions from the theoretical model are compared successfully with experimental data for this reaction.
THE topochemical
nature of several important metallurgicaI gas-solid reactions has been demonstrated in many studies. The development of effective mathematical models for the quantitative analysis of such gassolid reactions has received a great deal of attention in the literature and suitable mathematical models have been developed for the description of many practical situations. Much of the theoretical work has been directed towards the problem of developing simplified models under conditions where only one of the various diffusional or chemical kinetic processes control the overall rate of the process. Such simplifications, while often having the advantage of simplicity, are not always appropriate since mixed control and the transition from one control mechanism to another are frequently met. A common feature of all topochemical models is that a definite interface exists which marks the closest distance from the particle surface that a particular solid reactant is found. Such an interface recedes towards the center of the particle as the reaction proceeds finally vanishing at the center at a time which marks the completion of the conversion of the particular solid reactant. Sometimes the interface is very sharp, with the reactant concentration falling very rapidly to zero over a short distance. This occurs when the reaction is localized at or very close to the interface. In other circumstances the reaction zone is diffuse and the concentration of solid reactant falls gradually outwards until it become zero at the topochemical interface. In these diffuse zones the gaseous reactant penetrates into the incompletely reacted solid by diffusion so that chemical reaction and diffusion occur simultaneously. R. P. KING is Professorof ExtractiveMetallurgicalEngineering, Department of Metallurgy,Universityof the Witwatersrand, Johannesburg, South Africa. C. P. BROWN, formerlyStudent in Department of Metallurgy,Universityof Strathclyde,Scotland, is now with Westland Helicopters,Yeovil, England. Manuscript submitted May 3, 1979.
The position of the topochemical interface is a particularly important parameter because the instantaneous rate of conversion in the reacting particle and the total conversion within the particle can be uniquely related to interface position. This
Data Loading...
