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23.2.3 Scale-up of Well-Mixed Rotating-Drum Bioreactors

Explorations with the model give insights into how to operate well-mixed rotat-ing-drum bioreactors, both at laboratory scale and at large scale.

The model can be used to show that the air flow rates of around 0.01 vvm that were used by Stuart et al. (1999) and that were used in the base case simulation in the previous section, were simply insufficient. For good performance, they should have used flow rates of around 1 vvm. Figure 23.6(a)) shows a simulation done using the base case values (with n = 10) but with an aeration rate of 1 vvm. Note that at this higher flow rate, the substrate dries out during the fermentation. Therefore the fermentation is started with a high inlet air water activity of 0.99, and the inlet air water activity is only decreased to 0.15 when the bed temperature exceeds 38°C. Note also that the addition of water is triggered due to the drying out of the substrate bed, with water being added at 29 h and at 35 h.

For this simulation, the predicted peak bed temperature is 42.4°C at 27 h (Fig. 23.6(a)). At this time the overall metabolic heat generation rate is 62.8 W. The heat removal rates are 4.5 W by convection to the headspace, 27.0 W by conduction to the bioreactor wall and 31.1 W by evaporation (Fig. 23.7(a)). Therefore the increase in the air flow rate to 1 vvm allows evaporation to contribute around 50% of the heat removal, whereas at 0.01 vvm it was contributing only 2%.

In order for the bed temperature not to exceed 40°C, aeration rates higher than 1 vvm are needed. Figures 23.6(b) and 23.7(b) show the predicted results for a fermentation undertaken under the same conditions but with an aeration rate of 10 vvm. Due to the effectiveness of the cooling, the inlet air temperature is set at 35°C. In this case the predicted maximum temperature is 39.0°C at 29 h.

The model can also be used to explore scale-up of rotating-drum bioreactors. The simulations are done with n = 10 in order to simulate mixed operation. The inlet air temperature is maintained at 30°C. In the simulations described here, the aim is to prevent the bed temperature from exceeding 40°C. Readers can use the accompanying program to explore the predictions in greater depth.

Several bioreactors of ever-increasing size are compared. For each increase in scale the diameter and length are both doubled. The volumes and initial bed weights of these bioreactors are shown in Table 23.4. Note that the "large-scale" rotating drum has a bed capacity equal to the large-scale rotating drum used in koji production that was mentioned in Chap. 8, which has a capacity for 1500 kg of cooked substrate (Sato and Sudo 1999).

Table 23.4. Comparison of drum sizes and initial bed weights in the simulations done to investigate the effect of increase in scale on the performance of a rotating-drum bioreactor

Scale of bioreactor

Diameter

Length

Volume

Initial wet bed

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