The performance of rotating-drum and stirred-drum bioreactors will depend strongly on the effectiveness of the exchange of water and energy between the bed and the headspace gases. The effectiveness of this exchange will be affected by the flow patterns within the bed and headspace. It is unlikely that rotating- or stirred-drum bioreactors will be well mixed, unless specific attention is paid at the design stage to the promotion of mixing. Rather, air flow patterns and solids flow
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 patterns are likely to be complex. The flow patterns within the bed and the head-space of Group III bioreactors have only recently started to be explored. To date, the attention has been largely focused on rotating drum bioreactors. Note that quantitative approaches for determining bed-to-headspace transfer coefficients will be discussed in Sect. 20.5.
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