Screw conveyors and belt conveyors, which are examples of continuous tubular flow bioreactors (CTFBs), can move solids with almost zero mixing in the direction of flow (Fig. 11.3). When mixing is desired, which is often the case, static or dynamic mixers can mix the bed in the radial direction and, if desired, also in the axial direction. The current subsection focuses on the situation without axial mixing. Due to lack of back mixing, internal back-inoculation is not possible, however, external inoculation might be done by recycling a part of the fermented product, avoiding the need for a separate process for inoculum production.
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Gibbons et al. (1984, 1986) investigated a continuous screw-type bioreactor for farm-scale fuel ethanol production from various solid substrates such as fodder beets and sweet sorghum. Their system was not aerated and only anaerobic or mi-croaerophilic fermentations could be carried out. Moreover, the void spaces between the solid particles contained significant quantities of liquid, meaning that the process actually represented a borderline case between SSF and a "slurry fermentation". In any case, their bioreactor could be adapted for true SSF processes, although an aeration system would need to be incorporated for the cultivation of aerobic organisms.
Some of the large-scale koji production bioreactors can work in this mode. For example, the rotary disk bioreactor shown in Fig. 10.2 can be operated in a manner in which the rotating disk acts as a circular conveyor belt. As the disks slowly rotate, particles are transferred from the upper disk to the lower disk. The empty space on the upper disk is then filled with freshly inoculated particles. Each particle entering the upper disk spends the same time before being transferred to the lower disk. Each particle entering the lower disk then spends the same time before being harvested. Production rates as high as 4150 kg h-1 have been reported (Yo-kotsuka 1985; Chisti 1999). Tower-type CSSFBs used in certain composting processes operate in a similar manner, with a semi-continuous flow of substrate from one chamber to the next.
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