abstract
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
1
to 62.3
1
). 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.