Introduction

Fluidized-bed bioreactors are directly linked to the use of biocatalysts (cells or enzymes) for transformations in an immobilized form. The solid particles of the immobilized biocatalyst are maintained in fluidization by means of the circulation of a fluid phase (either liquid, gas, or a mixture of both) that compensates their weight. In this way, good liquid mixing and mass transfer between the solid and the liquid phases can be obtained with low attrition. Also, fluidized-bed bioreactors can accommodate a gas phase and can be used to feed solids in suspension. High productivities can be achieved in these systems, but their hy-drodynamic complexity and operational stability have to be well defined for a proper operation.

THE FLUIDIZATION CONCEPT: GENERAL CONSIDERATIONS

The term fluidized-bed is used to define those physical systems composed of a solid phase in the form of individual particles that move within a fluid phase and are not in continuous contact with each other. Fluidization of the solid particles is reached when the flow of fluid through the bed is high enough to compensate their weight. On the other hand, in order to be kept in the fluidized-bed reactor and not be washed out (elutriated), the superficial velocity of the fluid in the bed (that is, the ratio between the flow rate and the bed cross-sectional area) has to be lower than the settling velocity of the particles. These two extreme situations are outlined in Figure 1. When the flow rate of a fluid through a packed bed of solid particles steadily increases, the pressure drop increases proportionally to the flow rate, as long as the bed height remains constant. When the drag force of the fluid equilibrates the weight of

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