Rotary kiln transport processes

Slow rotation of a cylindrical kiln, partially filled with powder or granular solid, produces a planar interface between the bulk of the solid and the gas phase, tilted at the angle of repose. Particles cascade downward on this plane, moving relatively freely and randomly, providing the principal mechanism for particulate diffusion and mixing in the kiln. The plane is also the most favorable location for chemical reactions that require efficient gas–solid heat and mass transfer. Conservation equations are constructed for planar and bulk regions, coupled by the particulate exchange produced by rotation. Sample problems of mixing and heat exchange illustrate problem formulation and useful simplifications. A penetration model predicts wall-to-bed heat transfer coefficients, leading to better agreement with experiment than is obtained when planar processes are omitted.