oa South African Journal of Chemical Engineering - Diffusion effects in rotating rotary kilns



A novel approach to the modeling of mass transfer in rotary kilns has been descriptionbed (Heydenrych <i>et al</i>, 2001). It considers the mass transfer to occur by the inclusion of gas in the interparticle voids in between the particles that move concentrically with the kiln. By doing so, the rate of mass transfer was found to be dependent on bed fill and the ratio of reaction rate constant to angular velocity (k/ ). The model was found to be valid at slow to medium fast reactions. For fast reactions it under-predicted mass transfer. Therefore in this paper, the model will be extended to include diffusion effects. An additional dimensionless number is necessary then to descriptionbe the system. This can either be a Peclet number (R<sup>2</sup>/D<sub>e</sub>) or a Thiele modulus (kR<sup>2</sup>/D<sub>e</sub>)<sup>1/2</sup>. The solution of the 2-dimensional partial differential equations that descriptionbe the extended model gives a handle on the effect of scale-up in rotary kilns. For industrial-scale kilns, the Peclet number is large, which means that diffusion within the lower (passive) layer of the bed is unimportant for slower rates. With high reaction rates, iso-concentration lines are closely stacked near the surface of the bed, implying that it is important to model the active layer rather than the bed as a whole in these circumstances. However, the stiff differential equations are not easily solved then, and other methods of solution are advisable.


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