Fig. 11.6. Flow-model for screw conveyor and belt conveyor bioreactors with recycling, these being examples of Continuous Tubular Flow Bioreactors (CTFBs). M represents the mass of solids in the bioreactor, F represents the inlet flow rate, fR the recycle flow rate, and X represents the biomass concentration

Fig. 11.6. Flow-model for screw conveyor and belt conveyor bioreactors with recycling, these being examples of Continuous Tubular Flow Bioreactors (CTFBs). M represents the mass of solids in the bioreactor, F represents the inlet flow rate, fR the recycle flow rate, and X represents the biomass concentration

Fig. 11.7. Simulation of a Continuous Tubular Flow Bioreactor (CTFB) with recycling at different dilution rates and recycling ratios. Key: (■) y= 0.1; (O) y= 0.3; (•) y= 0.5

Dilution rate (h-1)

Fig. 11.7. Simulation of a Continuous Tubular Flow Bioreactor (CTFB) with recycling at different dilution rates and recycling ratios. Key: (■) y= 0.1; (O) y= 0.3; (•) y= 0.5

• For dilution rates less than the optimal one, the fraction of mass-flow recycled back to the entrance has no influence on the productivity of the bioreactor.

• If high levels of the product are the main objective, the system can operate at a low dilution rate and with a low recycle ratio. In these cases the task is to find the optimal dilution rate, this being constrained by the minimum acceptable product concentration. The recycle ratio will not be of great importance.

• When high productivities are necessary at high dilution rates, the system will demand greater recycling ratios. The design problem in this case is more complicated and would include finding a combined optimum for both variables, namely the dilution rate and recycle ratio.

11.4.2 Continuous Rotating Drum Bioreactor (CRDB)

One of the many possible flow models for describing the micro-mixing inside a CRDB has been presented by Ramos-Sánchez et al. (2003). The pattern that describes the micro-mixing and, consequently, the behavior of this bioreactor, is a combination of a plug-flow reactor, a perfectly mixed reactor, and a recycle stream (Fig. 11.8). There are three main operating variables for such a system: The fraction of the flow that passes through the plug-flow bioreactor (a = f/fm), the fraction of the "in-bioreactor" mass that is contained by the plug-flow bioreactor (P = M/Mm), and the fraction of the flow that it is recycled back to the entrance of the CRDB (j=fR/F).

Figure 11.9 shows the simulations for a given set of a and p at different values of the dilution rate and recycled fraction y. The behavior is similar to that shown in Fig. 11.7, but some important differences should be pointed out:

• Above the productivity maximum, the decrease in productivity with dilution rate is less pronounced than it was in the case of the former bioreactor (compare the profiles in Figs. 11.7 and 11.9). This means that the operation in this region is more stable, which is more desirable for practical purposes. In fact, for high dilution rates, for example, greater than 0.15 h-1, the CRDB will have higher productivities than the CTFB.

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