Insights the Model Gives into the Operation of Well Mixed Bioreactors

22.3.1 Insights into Operation at Laboratory Scale

The base case simulation is for a small bioreactor of 3.5 L volume, in which the water in the water jacket is maintained constant at the optimum temperature for growth. This bioreactor is of a size comparable to the rocking drum bioreactor of Barstow et al. (1988) and Ryoo et al. (1991). It begins with approximately 800 g of dry substrate. Table 22.3 shows the values used for the key design and operating variables in this and other simulations done in the case study.

Growth is sub-optimal (Fig. 22.4(a)) since, even at this small scale, high solids temperatures will be reached if special efforts are not made to cool the bed (Fig. 22.4(b)). Note that, since the air fed to the bed is almost saturated, the solids do not dry out during the fermentation. The relative importance of temperature and water activity in controlling the growth rate are most easily seem by plotting the fractional specific growth rates, pFT and pWT (Fig. 22.4(c)). At the time of peak heat generation, the value of juFT falls to values around 0.5.

Growth is good for a bioreactor operated under the same conditions but with control of the temperature in the cooling jacket with a value of J of 2 (Fig. 22.4(d)) because the solids temperature is controlled within reasonable limits (Fig. 22.4(e)), that is, the solids temperature is maintained within conditions for which Mft is over 0.9 (Fig. 22.4(f)).

Figure 22.5 shows simulations for a bioreactor of 0.32 m bed height and 0.3 m diameter, which starts with 5 kg of dry solids, the same amount of substrate as Nagel et al. (2001a) used in their bioreactor. Since Nagel et al. (2001a) humidified their inlet air, but did not manage to saturate it, the inlet air water activity is set to 0.9. The simulation is not intended to describe the results of Nagel et al. (2001a) directly, but it is interesting to compare the results.

Growth is poor for a fermentation done without any control of the temperature of the cooling water (Fig. 22.5(a)) because temperatures as high as 47.5°C are reached in the bed (Fig. 22.5(b)). Note that the use of dry air causes the bed to dry out sufficiently to trigger the addition of water, which occurs at 36 h. This alleviates the water-limitation of growth, which becomes quite severe at this time (Fig. 22.5 (c)).

Table 22.3. Design and operating variables changed in the various explorations of performance of a well-mixed forcefully-aerated bioreactora

Figure

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Hb x D

Mo

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