A A A

stationary sprayer/mixer A A A A

solids carried in this direction by the conveyer belt

Fig. 11.5. Mixing schemes in intermittently-mixed, forcefully-aerated bioreactors operated in (a) batch mode, in which the mixer moves back and forth along the whole length of the bioreactor; (b) plug flow continuous mode, in which the mixer stays in place and the bed moves past it

Fig. 11.5. Mixing schemes in intermittently-mixed, forcefully-aerated bioreactors operated in (a) batch mode, in which the mixer moves back and forth along the whole length of the bioreactor; (b) plug flow continuous mode, in which the mixer stays in place and the bed moves past it

In batch bioreactors of this type, mixing is often performed by a moving mixer, although in some cylindrical koji bioreactors the mixing system is stationary and the bed is moved past it via rotation of the base plate. In both cases, simultaneous mixing of the whole bed is difficult to achieve. In a system such as that shown in Fig. 11.5(a), parts of the bed located to the left of the mixer are mixed soon after the start of mixing while parts located to the right of the mixer are mixed only after a lag time that depends on the bed length and the speed with which the mixer travels back and forth along the bed. For large-scale bioreactors the lag time may become considerable, especially in cases in which the mixer travels slowly in order to enable homogeneous distribution of added water. Such lag times could have adverse effects on the product uniformity. Use of several mixers could reduce time lags in the batch mode but will imply a more expensive mixing system. On the other hand, in the continuous system shown in Fig. 11.5(b), the mixer stays in place as the substrate is moved past it and all of the fermenting solids are mixed or wetted at the same time interval after their entrance into the bioreactor, leading to more uniform product.

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