Fig. 11.11. Relation between productivity and biomass concentration in the simulation of a (•) CSTB, (■) a CTFB (y= 0.1), and (O) a CRDB (y= 0.1)
Finally, we should note a difference between the operation of CSTBs in SLF and SSF. In SLF it is practical to have a recycle stream for a CSTB, since it is possible to centrifuge or filter the stream exiting the bioreactor, such that the recycle stream has a higher biomass concentration than the stream exiting the bioreactor while the product stream leaving the process has a lower biomass concentration than the stream exiting the bioreactor. The important point is that in SLF it is possible to separate, at least partially, the biomass and the liquid. In contrast, in SSF the biomass in the stream exiting the bioreactor cannot be separated from the solids. If any recycling is done, then the composition of the recycle stream and the product stream leaving the process will have compositions identical to that of the stream leaving the bioreactor. Therefore, there are no advantages, in terms of productivity, in recycling solids in a CSTB. In fact, solids recycling is only useful for inoculation of the incoming fresh solids, and this will only be effective if there is efficient inter-particle transfer of biomass.
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