External Loop Airlift Reactor

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Loop Reactor

Figure 3 Mechanically driven reactors: (a) stirred reactor, (b) stirred reactor with draught tube, and (c) rotating drum reactor. A, air inlet; G, gas exhaust; M, motor.

External Air Lift Reactor
loop), (c) airlift reactor (external loop), and (d) airlift reactor (draught tube). A, air inlet; G, gas exhaust.

with high turbulence. Three common airlift configurations, the internal loop airlift reactor, the external loop airlift reactor, and the draught tube airlift reactor, are shown in Fig. 4b d. Airlift reactors of types c and d are also available as constructions using suitable foil material (39-41).

E. Packed Bed Reactor

The typical packed bed reactor consists of a vertical column packed with cells and adsorbents such as polymeric beads (Fig. 5a). The nutrient medium can be fed either at the top or bottom of the tube and is circulated through the packed bed. To prevent gas pockets and flow channeling in the packed bed, air is sparged indirectly by aeration of a separately used storage vessel as well as a recycling medium vessel (42) for some applications.

F. Fluidized Bed Reactor

When packed beds are operated in upflow mode, the bed expands at high liquid flow rates and follows the motion of the particles. This is the working principle of the ordinary fluidized bed reactor (Fig. 5d), where the particles are in a constant motion. A special fluidized bed reactor configuration with mixing characteristics similar to those of stirred reactors is depicted schematically in Fig. 5c. Considering the reactor geometry, the fluidized bed reactors with plug flow (Fig. 5d) are taller than fluidized bed reactors with complete backmixing (Fig. 5c).

G. Trickle Bed Reactor

The trickle bed reactor is one variation of the packed bed configuration (Fig. 5b). One or a number of nozzles integrated in the headspace of the column spray the nutrient continuously or periodically onto the top of the packaging. The aeration takes place directly by introducing air at the base. A special version of the trickle bed reactor is the mist reactor. The nutrient medium is introduced as a mist phase produced by an injector or ultrasonic nozzle (43).

H. Membrane Reactor

A few articles describe suitable membrane reactors for plant cell and tissue cultures. They are rarely used in comparison with the other bioreactor systems discussed (22,43). Typical membrane reactor configurations are depicted in Fig. 6. Figure 6a shows a membrane system with external cross-flow. The cells grow in a stirred reactor and will be retained. The membrane filter assembly is responsible for mass exchange, for example, a continuous separation of product in the course of the cultivation process. While the

Fixed Bed Inlet DistributorTrikle Bed ReactorFluidised Reactor Control Loops

Figure 5 Bed reactors: (a) packed bed reactor, (b) trickle bed reactor, (c) fluidized bed reactor (with high energy input), and (d) fluidized bed reactor (with plug flow). A, air inlet; G, gas exhaust; N, nutrient medium.

Basic Diagram Fluidized Bed Reactor

Figure 6 Membrane reactors: (a) membrane reactor with bypass membrane for perfusion mode, (b) typical membrane reactor for immobilized cells (hollow fiber reactor), and (c) membrane reactor with internal dialysis membrane (Bioengineering Ltd., Switzerland). A, air inlet; G, gas exhaust; M, motor; P, permeate.

Figure 6 Membrane reactors: (a) membrane reactor with bypass membrane for perfusion mode, (b) typical membrane reactor for immobilized cells (hollow fiber reactor), and (c) membrane reactor with internal dialysis membrane (Bioengineering Ltd., Switzerland). A, air inlet; G, gas exhaust; M, motor; P, permeate.

product is leaving the reaction system on the permeate side, the culture medium passes the membrane using a bypass on the retentate side. The whole culture medium inside the bioreactor represents permeate. As shown in Fig. 6b, simultaneous nutrient and extracellular product exchange can take place in special membrane modules applied instead of cross-flow assemblies. Here the cells are immobilized on the inside of a hollow fiber module and the mass and gas exchange is realized by hollow fiber membranes. A membrane reactor from the company Bioengineering Ltd. is schematically illustrated in Fig. 6c. The inner and outer chambers are separated by a dialysis membrane. Depending on the molecular weight, medium components or products either diffuse through the membrane or are retained in their respective chamber (44,45).

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