Packed-bed bioreactors are the natural choice when the microorganism does not tolerate mixing well. The major challenge in developing large-scale packed-bed bioreactors for new applications will be to minimize the axial temperature gradients. There are two main strategies by which this can be done:
• to use traditional packed-beds but use a low bed height
• to use a Zymotis-type bioreactor, with internal heat transfer plates.
If the organism can tolerate infrequent mixing events, of the order of once every few hours, or even as infrequent as once per day, then the traditional design should be chosen. These mixing events allow the pressure drop to be decreased, and also the addition of water to replenish the water lost in evaporation. This mode of operation is discussed in more depth in Chap. 10. Agitation is not a feasible option in the Zymotis packed-bed due to the presence of the heat transfer plates.
If a traditional packed-bed is chosen, then the bed height will need to be no more than say 20 cm to 1 m, in order to prevent high temperatures at the outlet end of the bed. The other possibility, of using tall water-jacketed columns of 15 cm or less in diameter, is unrealistic, since, to hold large amounts of substrate, the biore-actors will either need to be very tall or a large number of bioreactors will be needed.
If it is not desired to mix the bed at all, due to the sensitivity of either the organism or the substrate to damage by mixing, then the Zymotis design should be strongly considered, on the basis of considerations of the water balance. The contribution of conduction to heat removal will decrease the axial temperature gradient, and this will decrease the evaporation rate, as long as saturated air is used at the air inlet. Further, the greater the sensitivity of the process to high temperatures, the more the Zymotis bioreactor is indicated. For the same bioreactor height, the maximum temperature reached in a Zymotis bioreactor is lower than for the traditional bioreactor. This will be explored in the modeling case study in Chap. 24.
However, the Zymotis bioreactor does have some disadvantages in its operabil-ity compared to the traditional bioreactor. Both bioreactors have a potential problem with water condensing from the saturated outlet air onto the exposed bioreac-tor surfaces above the substrate bed, and this condensate can flood the top of the bed, causing O2 limitations in this region. This problem will be greater with the Zymotis bioreactor than for traditional packed-beds if the cooling plates extend above the top of the bed.
Additionally, the traditional packed-bed will be easier to load and unload than the Zymotis packed-bed. For example, for the traditional packed-bed it will probably be possible to (1) have a hinged base plate, in which the substrate can be dropped into a screw conveyor or (2) open a side and use a backhoe or (3) insert a pneumatic conveying tube to suck the substrate out. These operations will not be so easy in the Zymotis packed-bed due to the presence of the heat transfer plates.
A more detailed comparison of these two designs will require more work than is currently in the literature. The Zymotis design has not received much experimental attention since the early 1990s. However, as Chap. 24 shows, mathematical models can be used in a preliminary evaluation.
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