Conclusions on Group IVb Bioreactors

Intermittently-mixed, forcefully-aerated bioreactors appear to have some potential, judging by the fact that several processes involving bioreactors that operate in this mode have been demonstrated at a reasonably large scale. They appear to offer some benefits in control of the conditions within the bed, while minimizing the deleterious effects that continuous mixing can have, at least for fungal processes.

Based on what is known to date, it would seem that the best strategy is not to try to use mixing of the bed directly as a temperature control strategy. For fungal fermentations such a strategy would lead to intolerably frequent mixing events. Rather, the mixing events should be used to:

• prevent undue aggregation of substrate particles, unduly high pressure drops, and the appearance of cracks and channels in the bed;

• replenish water in the bed in order to prevent low water activities in the bed from being one of the factors that limit growth.

Attempts to control the temperature in such bioreactors therefore should be focused on manipulation of the temperature, humidity, and flow rate of the inlet air. These have not been explored to any great extent, but Chap. 25 will present a mathematical model of an intermittently-mixed, forcefully-aerated bioreactor that can be used to explore the question of how best to operate such bioreactors in order to control the temperature.

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