Continuous Stirred Tank Bioreactors CSTBs for SSF

Continuous Stirred Tank Bioreactors are designed to mix the whole content of the bioreactor thoroughly. In the ideal case, mixing is said to be perfect which means all the properties are identical everywhere inside the vessel at a given time.

In the case of SSF processes, it is impractical to mix perfectly due to two limitations imposed by the solid nature of the system. Firstly, wet solids have limited capacity for flowing and this makes mixing difficult. Secondly, wet solids tend to show a significant degree of flow segregation. The term flow segregation refers to the tendency of particles that have been in the vessel for different periods of time to remain segregated in different groups (Fogler 1999). As a result, any CSTB that is used for a continuous SSF process will behave to some degree as an intermittently mixed bioreactor. Despite these problems, perfect mixing behavior remains as an ideal model that serves as a paradigm for the analysis and design of these systems as we will see later in this chapter.

Note that there is a further limitation on perfect mixing. In perfectly mixed CSTBs for SLF, mixing is perfect even at the molecular level. However, in SSF, the bed of solids cannot be mixed at the molecular level unless the solid substrate particles are completely destroyed. If the solid particles are to remain intact, then perfect mixing can only occur at the "supra-particle" scale, with no mixing at the intra-particle scale. Further, transfer of liquid or biomass between particles will typically be quite limited. As a result, in SSF, even for a perfectly-mixed CSTB, each particle essentially acts as a "batch micro-bioreactor". The main design variables for CSTBs are:

• The geometry of the vessel. Figure 11.1 shows a conical geometry that could favor both the mixing of the solids within the bioreactor and their flow through the bioreactor. The height to diameter ratio of the vessel and the way it is positioned (i.e., vertical, inclined or horizontal) will influence the agitation devices that should be used and also the portions of the flow that will be moved as plug-flow and as perfectly mixed flow.

• The availability of heat transfer devices. Temperature control is easier in this bioreactor because of mixing, so different approaches could be explored, such as the use of water jackets or water-cooled impellers.

• The design of the aeration system. The air can be circulated through the head-space or blown forcefully through the bed. If blown through the bed, the air flow can be in the same direction, in the opposite direction or normal to the solids flow. Of course the aeration system can be designed to allow changes in the direction of air flow during the process.

• The type and number of impellers. The solids mixing efficiency depends strongly upon the type of impeller used (Sastry et al. 1999). A careful study should be conducted to select the appropriate design and positioning.

• The features of the solids addition and removal devices. They may need to be designed to prevent the entry of contaminants into the bioreactor. The solids inlet and outlet should be designed and positioned in order to minimize the possibility of short-circuiting. That is, added solids should be mixed into the bed and should not simply flow directly from the solids inlet to the solids outlet. In the case of external recycling of part of the solids that exit the vessel, the design of the recycling system must prevent contamination and mix the recycled solids well into the fresh solids stream.

• The features of equipment for addition of water and nutrients. A large part of the metabolic heat may be removed via evaporation, in such cases continuous or semi-continuous water replenishment will be required. The equipment for makeup water distribution should be designed to allow an even distribution. Minerals and soluble carbon sources can be added by the same system.

The operating variables include:

• The dilution rate. This is defined as the ratio of the mass flow rate into the bio-reactor to the total mass of solids within the bioreactor. It is a key factor in optimizing the productivity of the process and maximizing the concentration of products. Theoretically washout flow could occur, in a similar manner to that which occurs in continuous CSTB processes in SLF.

• Impeller velocity and frequency of stirring. These factors will influence the quality of mixing and will be very important in determining the distribution of temperatures and concentrations within the vessel.

Flow of solids in

Flow of solids in

Flow of solids out

Fig. 11.1. Schematic representation of a Continuous Stirred Tank Bioreactor (CSTB) that could be used for SSF processes. Note that air could be blown into this bioreactor either at the top or at the bottom. Alternatively, it may even be possible to blow air into a hollow mixing device, with appropriately positioned holes allowing the air to pass into the bed

Flow of solids out

Fig. 11.1. Schematic representation of a Continuous Stirred Tank Bioreactor (CSTB) that could be used for SSF processes. Note that air could be blown into this bioreactor either at the top or at the bottom. Alternatively, it may even be possible to blow air into a hollow mixing device, with appropriately positioned holes allowing the air to pass into the bed

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