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Fig. 9.1. Different ways in which agitation can be provided in continuously-mixed, forcefully-aerated bioreactors. (a) Mechanical agitation with an internal agitator, such as a bed with a vertical agitator or a drum with forced aeration and a central stirrer. (b) Agitation action caused by movement of the drum body. (c) Agitation caused by the movement of air

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Fig. 9.1. Different ways in which agitation can be provided in continuously-mixed, forcefully-aerated bioreactors. (a) Mechanical agitation with an internal agitator, such as a bed with a vertical agitator or a drum with forced aeration and a central stirrer. (b) Agitation action caused by movement of the drum body. (c) Agitation caused by the movement of air

The general operating variables that can be manipulated are:

• the temperature, humidity, and flow rate of the inlet air;

• the addition of water and other additives. Such additions create no difficulties, since the bed is already being continuously mixed.

This bioreactor type potentially has a significant advantage over packed beds. The inlet air for packed beds should always remain saturated, in order to minimize evaporation, given that it is not practical to add water to packed beds during the fermentation (Chap. 7). The use of dry air in packed beds would hasten the drying out of the bed to values that restrict growth. In contrast, since it is a relatively simple matter to add water uniformly to the bed in a continuously agitated bioreactor, it is possible to use unsaturated air in order to promote evaporation.

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