Knowledge of solids transportation within rotary dryers is required for dryer modelling and design. In this paper, the mechanism of solids flow in rotary dryers is first examined. Based on the underlying principles of the flow of solids through rotary dryers, solids transportation models were classified into seven models: empirical model, solid bed motion model, rheological model, cascading solid model, two-stream model, tanks-in-series model, and dynamic model. Each category of models is briefly presented and discussed.

Some experimental results obtained within pilot-scale and industrial rotary dryers are then presented. These results clearly show that the values of the solids mean residence time, predicted using most literature correlations, are much smaller than those obtained experimentally. The empirical correlations largely under-predict the mean residence time mainly because they fail to consider the varying characteristics of wet materials along the dryer. Experimental results clearly indicate that the solids residence time distribution (RTD) is strongly influenced by solids moisture content. For some solids, the presence of two distinct humps in the RTD curve is observed. The second hump becomes predominant for solids with higher moisture contents. Further research is needed to develop a new model in order to incorporate those varying characteristics.