Insights into Mixing and Transport Phenomena in Group IVb Bioreactors

Intermittently-mixed bioreactors are typically static for most of the fermentation and therefore the principles of heat and mass transfer in them have many similarities to those of packed-bed bioreactors, or namely, axial and possibly radial temperature gradients will be established, the magnitude of which will depend on the combination of bed height, superficial air velocity and microbial growth rate (see Sect. 7.3). As pointed out in Sect. 10.2, the operating variables that intermittently mixed bioreactors have in addition to those of packed-bed bioreactors include the humidity of the inlet air, the strategy for initiating mixing events (which affects their frequency), and the duration and intensity of mixing events.

Little work has been done to characterize quantitatively the damage that intermittent mixing causes to the microorganism and the speed of recuperation, or not, of the microorganism after mixing. Schutyser et al. (2003a) reported a decrease of about 10% in the O2 consumption rate immediately after mixing events in their intermittently agitated bioreactor, although they did not actually show the results.

Schutyser et al. (2003a) also investigated the timing of the first agitation event, concluding, at least in the case for fungi that produce significant amounts of aerial hyphae, that an early mixing event should be scheduled to prevent the formation of bound aggregates of substrate particles. If such aggregates are allowed to form, then they will be difficult to break apart in subsequent mixing events and O2 supply to the particle surfaces within the aggregates will be greatly restricted. They showed that for Aspergillus oryzae growing on wheat, this "hyphae-disrupting" mixing event will be needed before it is necessary to make the first water addition, even if evaporation is the sole cooling mechanism.

There has been little effort to characterize experimentally the heat and mass transfer phenomena associated with the intermittent mixing mode of operation, although the modeling study of Ashley et al. (1999) suggests that this mode of operation can potentially lead to temperatures being reached that are higher than those that would be obtained in completely static (i.e., packed bed) operation (Fig. 10.7(a)). Immediately before a mixing event, the temperature profile in the biore-actor is identical to that which would be expected for packed-bed operation. In this situation the rate of heat removal is uniform at the different heights within the bed.

Immediately after a mixing event, due to the absence of an axial temperature gradient, the cooling effect is concentrated at the bottom of the bioreactor. As a result there is significant heat transfer to the air, warming it up to such a degree that it is ineffective in cooling the top of the bed. This allows the top of the bioreactor to heat up since in this region the metabolic heat is not being removed as fast as it is produced. The cooling effect travels up the bioreactor like a "wave-front" (indicated by the region within the dotted ellipse in Fig. 10.7(b)). Under the conditions simulated, it takes around 20 min for the cooling effect to reach the top of the bioreactor, during which time the temperature has risen to a value over 2 °C higher than the value for packed-bed operation. Once this cooling "wave-front" arrives, the temperature returns to the value for packed-bed operation.

Pressure drops will typically not be a crucial problem in intermittently-mixed, forcefully-aerated bioreactors, since the intermittent mixing will tend to disrupt the inter-particle hyphae that develop during static periods and squash aerial hy-phae onto the surface of the substrate particles. After a mixing event the pressure drop through the bed will typically be significantly smaller than the pressure drop before the mixing event. In some cases the mixing event has been triggered exactly for this reason, that is, to reduce the magnitude of the pressure drop across the bed.

Fig. 10.7. Predictions of the modeling study of Ashley et al. (1999) about the temperatures reached in the intermittent-mixing mode of operation in a bioreactor or 34.5 cm height with a superficial air velocity of 0.0236 m s-1. (a) Temperature profile predicted for a bioreactor mixed approximately every hour. At each mixing time the sensible energy in the bed is distributed evenly amongst the bed contents. The hollow symbols (o) represent the temperature profile expected for the absence of mixing events, that is, for simple packed-bed operation. (b) More detail of the temperature profiles at different heights in the bed, showing why the maximum bed temperature exceeds the value expected for packed-bed operation, which in this case is the value of 40.7 °C at the top of the bed immediately before the mixing event. The arrows mark the timings of the mixing events. The dashed oval shows how the "cooling wave-front" moves up the bed after a mixing event. Adapted from Ashley et al. (1999), with kind permission of Elsevier

Fig. 10.7. Predictions of the modeling study of Ashley et al. (1999) about the temperatures reached in the intermittent-mixing mode of operation in a bioreactor or 34.5 cm height with a superficial air velocity of 0.0236 m s-1. (a) Temperature profile predicted for a bioreactor mixed approximately every hour. At each mixing time the sensible energy in the bed is distributed evenly amongst the bed contents. The hollow symbols (o) represent the temperature profile expected for the absence of mixing events, that is, for simple packed-bed operation. (b) More detail of the temperature profiles at different heights in the bed, showing why the maximum bed temperature exceeds the value expected for packed-bed operation, which in this case is the value of 40.7 °C at the top of the bed immediately before the mixing event. The arrows mark the timings of the mixing events. The dashed oval shows how the "cooling wave-front" moves up the bed after a mixing event. Adapted from Ashley et al. (1999), with kind permission of Elsevier

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