Key Words

Bioreactors Fluidized-bed Hydrodynamics Immobilized biocatalysts

Superficial velocity

Figure 1. Illustration of the basic concept of fluidization. Variation of the pressure drop (DP) and the height of the solid particles in a bed (L), with increasing fluid superficial velocity. vOM, minimum fluidization velocity.

Superficial velocity

Figure 1. Illustration of the basic concept of fluidization. Variation of the pressure drop (DP) and the height of the solid particles in a bed (L), with increasing fluid superficial velocity. vOM, minimum fluidization velocity.

the particles, the bed starts to expand and, after a transition period, reaches fully developed fluidization. At this point, further increments in the flow rate do not produce an increase in pressure drop, but instead lead to an increase of the height occupied by the solid particles in the reactor. If the flow rate is increased significantly, the elu-triation of the solid particles occurs when the fluid's superficial velocity is higher than the solid's settling velocity. The fundamentals of the fluidization phenomena are discussed comprehensively in the chemical engineering literature (1,2).

Figure 2 represents the basic scheme of a fluidized-bed bioreactor. Although various configurations are possible (3), the most extensively used is the gas-liquid cocurrent up-flow reactor. In it, liquid usually comprises the continuous phase and is fed from the reactor bottom. Its flow upward in the reactor promotes fluidization of the solid particles. Usually, the reactor will have two or three phases. In addition to the liquid and solid phases, the occurrence of a gas phase is quite common in those systems using cells as biocatalysts, either for aeration requirements (in which case, an air or oxygen stream is fed to the reactor, as shown in Fig. 2) or because cell metabolism produces a gas product (for example, CO2, CH4). In systems using enzymes as biocatalysts, the most common situation is two-phase fluidization, without any gas phase. Very often, due to the low reaction rates of most biological transformations, long liquid residence times are needed for the completion of the reaction, and therefore the drag force created by the low liquid flow rate in a single pass reactor is not enough to promote fluidization of the solid particles. Fluidization is obtained either by external liquid recirculation or by the gas loaded to the reactor, as depicted in Figure 2. In systems where a gas is produced by cell metabolism, the gas can also be an additional factor contrib uting to solid particle fluidization, although other effects are also observed in this case, such as internal liquid recirculation patterns. Fluidization at relatively low liquid flow rates is also favored in tapered fluidized-bed configurations; the liquid superficial velocity at the bottom of the reactor is higher due to the reduced cross-sectional area. In general, one can distinguish three main sections in fluidized-bed bioreactors: (1) the bottom section, where feed (liquid, gas, or both) and recirculation are provided; (2) the central main section, where most of the reaction takes place; (3) and the top section, with a wider diameter that serves to decelerate the movement of the particles by decreasing the superficial velocity of the liquid, thus enhancing the retention of the solid phase and at the same time allowing gas disengagement from the liquid phase. It is a common trend for fluidized-bed bioreactors to use bio-catalysts, either cells or enzymes, in the form of immobilized preparations. In general, the particles can be ofthree different types: (1) inert cores on which a biofilm is created by cell attachment, or in the case of enzymes, by adsorption or covalent binding immobilization; (2) porous particles in which the biocatalysts are entrapped; (3) cell aggregates obtained by self-immobilization caused by the ability of some cell strains to form flocs, pellets, or aggregates. Fluidized-bed bioreactors are usually differentiated from air-lift bioreactors by the fact that the latter do not specifically require the use of immobilized biocatalysts. Indeed, they were developed for free cell suspensions. In addition, air-lift bioreactors have different compartments, created by physical internal divisions, with different degrees of aeration.

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