• Temperature limitations will more difficult to prevent, since infrequent mixing events do little to control the temperature in the bed, with the temperature gradient that was present before the mixing event being re-established relatively quickly after the mixing event (the speed at which it is re-established depending on the bed height and the flux of air). To minimize temperature limitations, it will be necessary to optimize the combination of air flux, bed height, and inlet air temperature. Of course, the optimal combination will also depend on the specific growth rate of the organism. There appears to be relatively little advantage in blowing unsaturated air into the bioreactor, as this will cause large moisture gradients within the bed during the periods of static operation.
The operation of intermittently-mixed forcefully-aerated bioreactors certainly deserves more attention, as they represent a middle ground between the advantages and disadvantages of continuous mixing. Attention needs to be given to the following issues:
• determination of heat and mass transfer coefficients between the solids and gas phases. For example, in the case study shown in this chapter, the coefficients were borrowed from a study of the drying of corn;
• determination of the deleterious effects of mixing on the organism, and how it recovers after a mixing event;
• optimization of the agitation regime (frequency, duration, and intensity).
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