The cross-section of a partially-filled cylindrical kiln rotating on its horizontal axis and processing granular solids produces a shear zone (active layer) at the free surface which grows with the kiln's rotational rate. The active layer, although relatively thin, compared with the rest of the bed burden, drives all physical/chemical reactions. This is because of the high rate of surface renewal which, in turn, promotes heat exchange between the exposed surface and the higher temperature feeboard gas. Unlike packed beds, particulate diffusion induced by the flow of granules, adds a significant component to the overall heat transfer in the bed. Problem formulation and modeling of heat conduction using flow fields erived from experiments suggest that at slow kiln speeds the diffusion effect may not be recognized dur to long term duration of particle contacts and hence packed-bed heat conduction models may provide adequate characterization. However, at moderate and high kiln speeds particle collisions are short-termed and kinetic diffusion contributes to the effective thermal conductivity by as much as tenfold thereby resulting in well-mixed conditions and a homogeneous bed temperature. Industrial processing ramifications such as kiln speed control and product quality are discussed hereafter.
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