Figure 4. Schemes corresponding to two types of operation of a fluidized-bed as countercurrent adsorptive reactors, with two types of solid particles: light/small immobilized biocatalyst (O) and dense/large sorbent (•). Left: one compartment of the semi-continuous, multistage pulsed flow. Right: continuous trickle flow mode. Source: From Ref. 8.

application, the reader is referred to Refs. 6, 9-12 as well as general reviews in Refs. 13 and 14.

The first step in hydrodynamic characterization requires a determination of the fraction of the total volume to be occupied by the different phases (gas, liquid, solid). This is known as phase hold-up and can be determined using different techniques (3,15). Strictly speaking, phase hold-up may not be uniform for the whole reactor, especially with low axial dispersion. However, in systems with appreciable mixing, phase hold-up can often be considered uniform.

With respect to mixing behavior, most attention is generally paid to characterizing the liquid phase because this is, in most cases, the continuous phase in bioreactors, that is, the one in which substrates are fed and products of the reaction accumulate. For a three-phase fluidized-bed bio-reactor with cocurrent up-flow circulation of liquid and gas, the type of flow pattern is very much dictated by the value and the ratio of the liquid and gas superficial velocities in the reactor (3,16). Three main regimes are possible: (1) the dispersed flow regime occurs at high ratios of liquid velocity to the gas velocity and is characterized by the homogeneous dispersion of small gas bubbles in the liquid; (2) the coalesced bubble flow regime occurs at increased gas flow rates and is characterized by the formation of bigger bubbles as a result of the coalescence of smaller ones; the coalesced bubbles have a nonuniform distribution in the liquid; (3) the slug-flow regime is the consequence of a further increase of gas flow rates, occurs at high ratios of gas velocity to liquid velocity, and is characterized by the formation of large gas bubbles that, in small-diameter bio-reactors, tend to completely occupy their cross-sectional area. Slug formation breaks the bed continuity and causes great instability. Figure 5 gives an example of experimental data in a fluidization chart, showing these different regimes. If the gas flow rate is increased dramatically, it will eventually become the continuous phase in the bioreactor. A very detailed study on the characteristics of these dif-

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