Operating: Rotation rate of the agitator
Fig. 8.2. Design and operating variables that are available with rotating-dram and stirred-drum bioreactors. (a) Design and operating variables that are the same for both types. (b) Design and operating variables that are specific for the bioreactor type
The operating variables that are available include (Fig. 8.2):
• the solids loading used;
• the rotational speed for a rotating-drum bioreactor, the stirring speed for a stirred-drum bioreactor. If rotation or stirring are done intermittently, then the frequency, duration, and speed of rotation or stirring events;
• the flow rate, temperature, and humidity of the air blown into the headspace;
• the timing of water additions;
• the temperature of the cooling water if a jacket is used; if a jacket is not used then whether air is forcefully blown past the drum wall or not.
Most of these operating variables can be changed at will during the fermentation. The solids loading is an operating variable that is fixed at the beginning of each run. Note that it does change during the fermentation as part of the solids is converted into CO2, but once a fermentation has commenced, the solids loading cannot be freely controlled. The solids loading is related to the "fractional filling" of the drum, that is, the fraction of the whole drum volume occupied by the bed, here represented by the symbol ro. Typically, fractional fillings should be kept below 0.4, in order to enable reasonable mixing of the bed.
Since the bed is mixed, water can be replenished by spraying a fine mist onto the bed during mixing. Therefore evaporation can be promoted as part of the cooling strategy, meaning that unsaturated air can be used at the air inlet.
The values chosen for the design and operating variables will be affected by the following considerations:
• the heat production rate in the bed will strongly influence decisions about the loading of the bioreactor, the aeration rate, and the humidity of the inlet air.
• the rotation rate or stirring speed chosen will represent a balance between the promotion of heat and O2 transfer within the bed and between the bed and the headspace and the minimization of shear damage to the microorganism.
• the strength of the particles may affect the maximum diameter and the substrate loading that can be used. Soft particles may be crushed by the weight of a large overlying bed.
This chapter explains what is known, on the basis of experimental studies, about how these design and operating variables influence the performance of ro-tating-drum bioreactors and stirred-drum bioreactors. Chapter 23 shows how mathematical models can be used to explore further the design and operation of rotating-drum and stirred-drum bioreactors.
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