Basic Features of Group IVb Bioreactors

The basic design features of intermittently mixed bioreactors are similar to those of the various continuously mixed designs (Chap. 9), the difference being in the mode of operation. Since the mixing is only intermittent and the bioreactor spends periods in the static mode of operation, designs should be preferred that give a uniform aeration of the bed when it is static. Forced aeration may or may not be applied during the mixing period, depending on the design. Figure 10.1 shows possible basic designs for intermittently mixed bioreactors.

Intermittently mixed bioreactors have the same design and operating variables as packed-bed bioreactors (Sect. 7.2), which affect the performance during the periods of static operation. In addition to this, the type of agitation is an extra design variable for intermittently mixed bioreactors. The bed may be mixed by a mechanical stirrer, by rotation of the whole bioreactor or by the air flow.

In addition to having the operating variables for packed-bed bioreactors, intermittently mixed bioreactors have several extra operating variables available.

Basic Bioreactor Design

Fig. 10.1. Two basic options for mixing of intermittently mixed bioreactors. (a) The bed may be agitated by a mechanical agitator inserted into the substrate bed. In this case forced aeration can be applied during the mixing period. The agitator may simply rotate around its axis, in which case it will need to be almost as wide as the bioreactor, it may rotate with a planetary action (as shown) or it may travel from side to side across the bioreactor. (b) The bed may be agitated through rotation of the bioreactor around its central axis (Toyama 1976). There is no mechanical structure within the bed itself. In this case it is not practical to aerate the bed forcefully during the mixing period

Fig. 10.1. Two basic options for mixing of intermittently mixed bioreactors. (a) The bed may be agitated by a mechanical agitator inserted into the substrate bed. In this case forced aeration can be applied during the mixing period. The agitator may simply rotate around its axis, in which case it will need to be almost as wide as the bioreactor, it may rotate with a planetary action (as shown) or it may travel from side to side across the bioreactor. (b) The bed may be agitated through rotation of the bioreactor around its central axis (Toyama 1976). There is no mechanical structure within the bed itself. In this case it is not practical to aerate the bed forcefully during the mixing period

Firstly, there is the strategy for initiating mixing events, which will determine the frequency of the mixing events. Secondly, the duration and intensity of mixing events can be varied. Thirdly, unlike packed beds, the relative humidity of the inlet air is potentially available as an operating variable. Since water can be added to the bed in a reasonably uniform manner during the mixing events, unsaturated air can be used to aerate the bed in order to promote evaporative cooling.

The values selected for these extra design and operating variables will be most affected by:

• the temperatures reached in the bed during static operation (e.g., mixing could be triggered by high temperatures at the outlet end of the bed);

• the water activities reached in the bed during static operation (e.g., mixing could be triggered when the outlet-air relative humidity falls below a set point);

• the pressure drop through the bed (e.g., mixing could be triggered when the pressure drop reaches unacceptably high values);

• the sensitivity of the organism to damage during mixing events, which will affect the frequency, intensity, and duration of mixing events.

Considerations affecting the selection of appropriate values of other operating variables, such as the air flow rate and temperature, will be similar to those for packed-beds (Sect. 7.2).

Channeling should be less of a problem for intermittently mixed beds than for static packed beds. The mixing events will tend to break up the bed so that the particles remain separate and these will tend to settle as the bed shrinks, rather than being knitted together and pulled away from the wall as happens with packed beds. However, channeling might be caused by an imperfect bed structure at the end of the mixing event. For example, in the case that the agitator stays in the bed during the static periods, it may leave a hole behind or around it as it comes to a stop. Alternatively, in the case that it is withdrawn from the bed, it may leave a hole as it leaves. In either case, if nothing is done to close the hole, the air will flow preferentially through it during the period of packed-bed operation.

This chapter explains what is known, on the basis of experimental studies, about how these design and operating variables influence bioreactor operation. Later, Chap. 25 will show how mathematical models can be used to explore further the design and operation of intermittently mixed bioreactors.

Was this article helpful?

0 0

Post a comment