The temperature profiles plotted in Fig. 23.6 show that as scale increases at a constant aeration rate of 1 vvm, the peak temperature reached in the bed increases, peaking near 50°C in the largest bioreactor. On the other hand, as scale increases with a constant aeration rate of 10 vvm, the maximum bed temperature stays below 40°C. It is clear that high aeration rates, of the order of magnitude of 10 vvm, will be required for adequate temperature control in large-scale rotating-drum bio-reactors. An analysis of the contributions to cooling (Fig. 23.7) shows that heat removal to the drum wall removes a significant proportion of the metabolic heat in the 25 L bioreactor but becomes less and less significant as scale increases. At large scale the majority of the metabolic heat is removed by evaporation.
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