Solids Flow Regimes in Rotating Drums

Solids flow in both the radial and axial directions must be considered.

The radial flow regime within the solid bed is important because it affects the heat and mass transfer between the bed and the headspace and the homogeneity within the bed. Transfer of heat, water, and O2 will be most effective when all substrate particles within the bed are regularly brought to the surface. However, this is not necessarily easy to achieve.

In non-SSF applications of unbaffled rotating drums, the various radial solids flow regimes that occur have been characterized (Fig 8.9). The flow regime depends on several factors, including the rotational rate and the percentage filling of the drum. It is convenient to relate the flow regimes to fractions of the critical rotational speed (NC), which is defined as the speed at which the particles are held against the inside of the drum wall by centrifugal action (Ishikawa et al. 1980).

Fig. 8.8. (a) A simple roller bottle system, based on systems used for tissue culture. It is possible to have various layers of roller bars. The system would typically be placed in a temperature-controlled room. For each pair of roller bars that holds several roller bottles, one is a drive bar (solid arrow) and the other a slave bar (dashed arrow). (b) Typically each roller bottle would have a removable lid, with a mesh that allows the exchange of gases

Fig. 8.9. Solids flow patterns within rotating drums without baffles. Relatively slow rotational rates are commonly used in SSF with rotating-drum bioreactors, giving the slumping flow regime. The darker color indicates poorly mixed parts of beds while the lighter color indicates better-mixed areas. Solid arrows indicate movement of the bed or particles within the bed. Adapted from Wightman and Muzzio (1998), with kind permission of Elsevier

Fig. 8.9. Solids flow patterns within rotating drums without baffles. Relatively slow rotational rates are commonly used in SSF with rotating-drum bioreactors, giving the slumping flow regime. The darker color indicates poorly mixed parts of beds while the lighter color indicates better-mixed areas. Solid arrows indicate movement of the bed or particles within the bed. Adapted from Wightman and Muzzio (1998), with kind permission of Elsevier

This is a function of the drum diameter and for a horizontal drum is given by the following equation:

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