Pérez-Correa and Agosin (1999) built a bioreactor with a capacity for a bed of 200 kg. The bioreactor has three sections (Fig. 10.4). The bottom section, which remains stationary, is simply the air box. The 150 cm diameter bed is held by the second section, which is rotated in its entirety by a motor. The top of the bioreac-tor represents a third section, which is stationary, and on which the agitators are mounted. The thermocouples can be withdrawn from the bed into the headspace during the mixing event. The bioreactor was designed to enable a bed height of 80 cm, although in practice the bed height was kept at 60 cm or below.
mechanical seal • (sealing ability not crucial)
motor for turning the agitators
►ir helical mixing blades toothed skirt attached to
middle section m water seal "' (sealing ability crucial)
the middle section that holds the bed rotates during a mixing event air out thermocouples perforated base plate thermocouples perforated base plate bottom "air box"
thermocouples are raised out of the bed during mixing periods and lowered into it during static operation bottom "air box"
Fig. 10.4. The 200-kg capacity intermittently-mixed bioreactor used by Pérez-Correa and Agosin (1999). The upper cover and air box are maintained stationary by an outer frame while the middle section is rotated by the motor
The advantage of holding the agitators stationary and rotating the bed is that this simplifies the design of the agitator device. However, it also brings a disadvantage: The seal between the bottom and middle sections of the bioreactor, which move relative to one another, must not allow air to escape, otherwise air will leave the bioreactor without passing through the bed. The bioreactor shown in Fig. 10.4 has a water seal. However, since the depth of water in the seal is only 10 cm, this means that the pressure drop across the bed cannot be more than the equivalent of 10 cm of water; otherwise the air will simply bubble through the water in the seal and leave the bioreactor without passing through the bed. This limits the height of the bed that can be used and means that often mixing events are necessary simply to prevent the pressure drop from becoming too high, rather than being prompted by a need for water replenishment or temperature control.
Figure 10.5 shows typical data obtained from this bioreactor, for the growth of Gibberella fujikuroi on extruded wheat bran granules for the production of gibber-ellic acid. A bed height of 40 cm was used. It was possible to control most of the bed within the range of 25 to 30°C most of the time. However, there were hotspots formed, in which the bed temperature exceeded the temperature of the outlet air (compare Fig. 10.5(b) with Fig. 10.5(c)); these hotspots most likely represent regions that were receiving poor aeration due to channeling. The CO2 production rate peaked at around 40 h (Fig. 10.5(c)). In an attempt to control the temperature in the bed at 28°C, the temperature (Fig. 10.5(c)), humidity (Fig. 10.5(e)), and flow rate (Fig. 10.5(f)) of the inlet air were manipulated. The pressure drop was kept well below 10 cm of water by the mixing events (Fig. 10.5(g)). These 30-min-long mixing events occurred, on average during a fermentation, once every 6 to 10 h, although during periods of high heat production they were as frequent as once every 4 h. Water needed to be replenished to replace evaporated water (Fig. 10.5(h)). This was done over the 30-min period of the mixing event, with the amount of water necessary being calculated from a set of mass balance equations.
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