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Fig. 11.8. Combined flow-model of a CRDB. Mp is the mass of solids in the plug-flow region while Mm is the mass of solids in the well-mixed region

Fig. 11.8. Combined flow-model of a CRDB. Mp is the mass of solids in the plug-flow region while Mm is the mass of solids in the well-mixed region

Fig. 11.9. Simulation of a Continuous Rotating Drum Bioreactor (CRDB) where a = 0.30 andp= 0.40. Key: (■) y= 0.1; (O) y= 0.3; (•) y= 0.5

Dilution rate (h-1)

Fig. 11.9. Simulation of a Continuous Rotating Drum Bioreactor (CRDB) where a = 0.30 andp= 0.40. Key: (■) y= 0.1; (O) y= 0.3; (•) y= 0.5

• The maximum productivity is not as sensitive to increments in the recycle ratio (x) as it was in the previous case, remaining between 15 and 16 g h-1 kg-dry-matter-1 as the recycle ratio is varied from 0.1 to 0.5.

• In the bioreactor simulated in Fig. 11.9, the maximum of productivity at a value of xof 0.5 is 16% less than that in Fig. 11.7.

11.4.3 Continuous Stirred Tank Bioreactor (CSTB)

A real CSTB has a complex flow-pattern due to the solid nature of the system and the limitations on stirring imposed by the sensitivity of the microorganisms to shear damage. However, given that the flow patterns in such bioreactors have not been studied, a model assuming perfectly mixed flow is used for the simulations. Note that it is assumed that the particles are inoculated as they enter the bioreactor.

In the case in which there is no recycle stream, the dilution rate is the only operating variable. Figure 11.10 shows the results of the simulations for this bioreac-tor as a function of dilution rate, together with simulations of the two former bio-reactors at a recycle ratio of 10%. The behavior of the CSTB is similar to that of the previous bioreactors but some important differences should be noted:

Fig. 11.10. Comparison of the performances of (•) a CSTB, (■) a CTFB (/= 0.1), and (O) a CRDB (y= 0.1)

• Surprisingly, the results of the perfectly-mixed CSTB are better than those the other two bioreactor types. Normally plug-flow bioreactors are better for simple reactions (Fogler 1999) but in the case in which the reaction rate increases with conversion, which could be the case for fermentation processes, which are autocatalytic, the perfectly mixed bioreactor can perform better.

• The CRDB has an intermediate behavior due to the fact that it combines perfectly-mixed flow with plug-flow. The greater the perfectly mixed component is, then the closer the performance of the CRDB will be to that of the CSTB.

• The CTFB tends to behave more and more like a CSTB as prises, which can be seen by comparing Figs. 11.7 and 11.10.

11.4.4 Evaluation of the Various CSSFB Configurations

Figure 11.11 shows the relationship between two important performance criteria, namely the bioreactor productivity and the biomass concentration in the product stream. The relationship is plotted for each of the three configurations of CSSFBs presented in Sects. 11.4.1 to 11.4.3, based on the results of the various simulations performed in those sections. Various points of interest are:

• The CSTB will have maximum productivity when the outlet biomass concentration is a half of Xmax, while the maximum productivity of the CTFB occurs at greater biomass concentrations.

• The maximum productivity of the CSTB with perfectly mixed flow is 30% greater than that of the CTFB. However, the advantage of the CSTB over the CTFB becomes smaller as the recycle ratio of the CTFB is increased.

• For biomass concentrations up to 200 g kg-DM-1, which is very close to the biomass concentration of 220 g kg-DM-1 that gives maximum productivity of the CTFB, the productivity of the CSTB is greater than that of the CTFB.

• The behavior of the CRDB is between these two ideal bioreactors. This is not surprising, because it represents a mixture of the two flow regimes. Indeed, the model of this bioreactor can represent the deviations of flow regimes from the ideal regimes assumed for the CTFB and the CSTB.

In the case of plug-flow through a tubular bioreactor, the reaction rate will be low at the entrance of the bioreactor because of the low concentration of biomass. As the solids flow through the bioreactor, the rate of the reaction will rise to a maximum level at a biomass concentration equal to 0.5Xmax, due to logistic growth kinetics, which cause the growth rate to decelerate as the biomass concentration rises from 0.5Xmax towards Xmax. Therefore at the exit of the plug-flow bioreactor, if the biomass concentration is close to Xmax, the rate of the reaction will tend to be low. This means that the average reaction rate within the plug-flow bioreactor will always be lower than the maximum possible level; hence as a consequence, the overall productivity will never be as high as it would be for a CSTB in which the biomass concentration were maintained at 0.5Xmax.

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