screw spray nozzle headspace spray nozzle headspace screw auger C bed wire mesh support air box

Fig. 10.3. Intermittently-mixed bioreactor of the type used by Durand and Chereau (1988). There are three motors/agitators mounted across the width of the bioreactor (behind the one shown in the side view given here). The bioreactor dimensions are given in the text. Adapted from Durand and Chereau (1988) with kind permission of John Wiley & Sons.

Durand and Chereau (1988) cultivated Trichoderma, which grows optimally at temperatures around 28°C, on a sugar beet pulp medium for the production of single cell protein. It was possible to maintain the temperature and water content of the bed within acceptable limits by maintaining the relative humidity of the inlet air constant at 90%, and by manipulating the flow rate and temperature of the air supplied to the bioreactor. They describe the operating regime as follows:

• three to four "turnings" (i.e., mixing events) during the 48 h cultivation;

• for the first 10 h, an air flow rate of 750 m3 h-1 (i.e., about 7 vvm) at 29°C;

• as the growth rate accelerates, an increase of the air flow rate to 1000 m3 h-1 (i.e., about 9 vvm) at 26°C.

With this operating regime, during exponential growth the temperatures at 85-cm depth in the bed and at 20 cm depth in the bed ranged from 26.5 to 29.0°C.

Since it was first reported, the use of this bioreactor has been extended successfully to the production of enzymes and biopesticides (Durand 2003).

Xue et al. (1992) adapted the bioreactor of Durand and Chereau (1988) to build a much larger scale process for the production of microbial protein from sugar beet pulp by Aspergillus tamarii. The bioreactor is built from concrete, having a length of 17.6 m, a breadth of 3.6 m, and an overall height of 2.0 m. A perforated stainless steel plate, designed to support the bed, is fixed at a height of 0.6 m. The actual bed height used is 0.7 m, leaving a headspace of 0.7 m, and giving a bed volume of 44 m3. This corresponds to 25 tons of moist substrate, which, given a water content of 80% (wet basis), gives 5 tons of dry substrate. The carriage holding the screw mixers has a linear speed of 30 cm min-1 and rotates the screws at 13.3 rpm. The facility has two such bioreactors. The air system has a maximum capacity of 60,000 m3 h-1. Divided over two bioreactors, this is 11 vvm.

Very little performance data was provided. They used two turning cycles during the first 48 h of the 72 h process. The aeration rate used was 220 m3-wet-air min-1, at a relative humidity of 88% and at a temperature that ranged from 32 to 34°C. However, it was not specified whether this aeration rate was for one or both bioreactors. The outlet air was reported to have "operational parameters" of 100% relative humidity and 33°C, although it is not clear whether this was achieved.

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