Fig. 9.4. Performance of the 35-L bioreactor of Nagel et al. (2001a), which is of the type shown in Fig. 9.3(a), when Aspergillus oryzae was grown on 8 kg of cooked wheat bran. (a) With wall cooling. Key: (—) bed temperature, (---) wall temperature; (b) with evaporative cooling. As the aeration rate increased the relative humidity of the inlet air decreased since the rate of water vapor addition to the inlet air stream was maintained constant. Key

(—) bed temperature, (---) air flow rate; (c) O2 uptake rate with (—) wall cooling, (---)

evaporative cooling. The sudden decrease in the O2 uptake rate at 50 h with evaporative cooling is due to the drying out of the bed to water activities low enough to restrict growth. IDM = initial dry matter. Adapted from Nagel et al. (2001a), with kind permission from John Wiley & Sons, Inc.

These various bioreactor designs suggest that possibility of a further design, which might be referred to as a "stirred perforated-drum" (Fig. 9.3(d)). This is a stirred drum, similar to that of Nagel et al. (2001a), but rather than blowing air through the ends of the paddles, it can be blown through the base of the drum, in the manner of the Z-blade bioreactor of Ellis et al. (1994). To improve the aeration, air can be introduced across a broad cross-section of the bed. Note that ideally such a bioreactor should be continuously mixed and not intermittently mixed because the different bed heights at different positions mean that during static operation there will be a preferential flow of air through the thinner part of the bed.

The design and the operation of the agitator are crucial for bioreactors with mechanical agitators, since they determine the effectiveness of the mixing. However, it is not a simple matter to establish general principles. Optimal design and operation of agitators will be affected by the properties of the substrate bed, which can vary widely between different substrates. It appears that many agitators have been designed on a best-guess approach, since there are no studies that relate the comparison of various different mixer types in order to select the best design. The work of Schutyser et al. (2003b) shows that the use of discrete-particle models that predict the movement of particles in agitated bioreactors is a powerful tool not only for selecting a particular agitator design amongst the various possibilities but also for optimizing the design and operation of the selected agitator.

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