Design loading of free flowing and cohesive solids in flighted rotary dryers

Holdup in flighted rotary dryers can be classified according to its loading state as either over, under or at design load. The loading state influences the effectiveness of particle to gas heat and mass transfer as well as the residence time of solids through the dryer. As such, accurate estimation of the design load is critical to the analysis of performance and the optimal design of flighted rotary dryers. In this paper design load experiments carried out in a horizontal, pilot scale flighted rotary dryer at different experimental conditions are described. The design load experiments involved analysis of multiple photographs of the cross sectional area of the solids in the front end of the dryer, at increasing loading conditions. Subsequently, the design load was estimated using conventional criteria based on the saturation of material in the cascading or unloading flights. The study examined both free flowing and cohesive solids with cohesion being controlled through the addition of low volatility fluid to the solids (dynamic angle of repose ranged from 44.7° to 62.3°). The effect of drum rotational speed was also examined (2.5 rpm–4.5 rpm). In order to select an appropriate geometrically derived design load model, comparison with existing design load models from the literature was undertaken. The proportion of airborne to flight borne solids within the drum was characterised through a combination of photographic analysis coupled with Computational Fluid Dynamics (CFD) simulation. In particular, solid volume fractions of the airborne solids with solid flow rate ranging from 0.703 kg/s to 0.134 kg/s were characterised using a CFD technique based on the Eulerian–Eulerian approach. The suitability of using geometric models of flight unloading to predict design loading in flighted rotary dryers is discussed. A modified version of Baker's (1988) design load model is proposed.