Operating variables are variables that the operator can manipulate in an attempt to control the conditions within the bioreactor. The question of optimum operating strategies for the various bioreactor types is covered in the individual bioreactor chapters (Chaps. 6 to 11) and the modeling case studies (Chaps. 21 to 25). However, it is worthwhile to make some general comments here:
• Regardless of whether the air is blown forcefully through the bed or circulated around the bed, it is possible to control the flow rate, temperature, and humidity of the air supplied at the inlet to the bioreactor or chamber. The costs of supplying air will depend on the volumetric flow rate and the pressure drop in the bed, and the need to heat or refrigerate the air. Pressure drop will be discussed in Chap. 7, which deals with packed-bed bioreactors, since its importance is greatest for this type of bioreactor.
• The conditions in the surroundings of the bioreactor can be controlled. The bio-reactor may be placed in a room or other location where the air temperature, humidity, and circulation are controlled. Alternatively, the bioreactor may be fitted with a water jacket. The flow rate and temperature of the cooling water at the inlet of the jacket can be controlled. Note that if the desired air or water temperatures are different from the temperatures at which they are available, either cooling or heating will be necessary, which entails extra costs.
• Additions can be made to beds that are mixed, even if only intermittently; for example, water can be sprayed onto the bed during mixing.
• In beds that are mixed, it is possible to control the frequency, duration, and intensity (i.e., revolutions per minute of the agitator) of the mixing.
Given the difficulties in controlling the conditions in SSF bioreactors, which were mentioned in Chap. 2 and are discussed in more detail in Chap. 5, it is not a simple matter to maintain the bed conditions at the optimum values for growth and product formation by manipulating these operating variables. The aim therefore is to select combinations of operating conditions that make the best balance in:
• minimizing deviations from the optimum temperature;
• minimizing damage to the organism;
• minimizing deviations of the bed water activity from the optimum value;
• maximizing the supply of O2 to the particle surface.
Chapters 6 to 11 will give some idea of what we already know about how to do this for the various bioreactor types. It must be stressed that, although our knowledge is increasing, it is as yet far from complete.
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