The use of fluidized-bed bioreactors can provide a number of advantages that makes them an interesting alternative for bioprocesses, especially for continuous operation. In comparison with conventional mechanically stirred bio-reactors, fluidized-bed bioreactors provide a much lower attrition of solid particles, and almost any kind of immobilized biocatalyst preparation can be used without physical disruption. Biocatalyst concentration can be significantly higher because of immobilization, and the typical wash-out limitations of continuous bioreactors operating with free cells are overcome because the solid particles are physically retained in the reactor vessel; operation at flow rates higher than the maximum growth rate of the cells can be achieved. As a consequence, the final productivity of the bioreactor can be increased substantially. Compared with packed-bed reactors, fluidized-bed bioreactors can be operated with smaller-size particles and without the drawbacks of clogging, creation of preferential flowpaths, or particle compression due to bed weight. Moreover, the smaller particle size minimizes the internal diffusional resistances, and the higher level of mixing enhances external mass and heat transfer from liquid to solid phase.
The degree of internal mixing in a fluidized bed can vary to a great extent, and it depends on various factors: density and diameter of the solid particles, liquid and gas flow rates at the reactor inlet, endogenous gas production by cell metabolism, and recirculation rate. For some operational conditions, high gas, liquid, or recirculation flow rates, the internal mixing will be very high, and the reactor will approach the behavior of a complete mixed tank. On the other hand, operation with low gas, liquid, or recirculation rates will provide a flow pattern close to plug flow, with some degree of axial mixing. This implies that reaction kinetics is an important factor to consider in the analysis and design of fluidized-bed bioreactors. Configurations favoring liquid mixing will be more appropriate for substrate-inhibited reactions, and configurations approaching plug flow will be indicated for product-inhibited reactions. Another advantage of fluidized-bed bioreactors is the ease of separation of the gas produced in most transformations involving cells (i.e., CO2), or the feed of a gas stream to the reactor, for example, for aeration purposes. Also, fluidized-bed reactors make biocatalyst replacement easy, without disruption of the operation, enabling good control of the overall activity of the reactor. For example, one may replace particles with deactivated enzyme or removing an excess of biomass created by biofilms. On the other hand, solids attrition is higher in fluidized-bed than in packed-bed bioreactors. From the productivity point of view, the advantages of the fluidized bed, especially with respect to mass transfer rates, make it possible to obtain higher levels of overall productivity than in packed-bed reactors, in spite of the fact that the fraction of immobilized biocatalyst particles is lower for a fluidized bed.
Fluidized-bed bioreactors are complex with regard to hydrodynamic aspects, especially taking into account that the properties of the biocatalyst particles may change considerably during the operation time and in the presence of three different phases (solid, liquid, gas) in many cases. In fact, the nature of the particles (for example, their density and size, or their evolution with time, which are especially important with respect to certain kinds of immobilized cells), the liquid and gas flow rates employed, the type of reaction kinetics, and the kinetics of cell growth or enzyme deactivation influence each other and have a direct effect on the reactor design and performance (4).
Using the biocatalyst in immobilized form also contributes to the complexity of a fluidized-bed bioreactor. The behavior of the immobilized biocatalyst, especially when cells are used, can be substantially different than that of free suspensions (5). The behavior must be determined at the kinetic level, the physiological level, and the genetic level, and the biocatalyst's relationship with the diffu-sional restrictions in the particles and the possible direct effects associated with the immobilization itself must be well understood and correctly described in order to build appropriate and reliable models of for reactor design, control, and scaling up.
The potential offluidized-bed bioreactors can be further exploited by considering multistage units and using two solid particles with different properties. Figure 3 gives an example of the concept of multistage operation, in particular, a countercurrent multistage fluidized bed working with immobilized enzymes (6). The main characteristic of this bioreactor is the continuous transport of the solid par ticles of immobilized enzymes from one stage to another in a downward direction. The overall catalytic activity of the reactor remains constant as the exhausted enzyme is removed from the reactor bottom stage while fresh biocata-lyst is added at the top stage. A second advantage of dividing the reactor into compartments is the very low degree of back-mixing of the biocatalyst, and the plug-flow regime attained in the liquid phase. The use of two solid particles with different properties, particularly with different densities, can be used in a fluidized bed to achieve the in situ separation of a product of the reaction; this is a clear advantage in systems with product inhibition or when unfavorable thermodynamic equilibria limit the conversion rate for a reaction. For example, Davison and Scott (7) have proposed a system based in two different types of particles with different densities. As one type of particle, containing the biocatalysts (in this particular example, cells of Lactobacillus delbreuckii), remains fluidized in the bio-reactor, the second type, which is heavier and contains no cells, is introduced from the top of the bioreactor and collected at the bottom. This second type of particle is selected to selectively remove the inhibitory product of the fermentation, for example, lactic acid. Another possibility is to combine both approaches, that is, to design a multistage fluidized bed working with two types of particles to achieve a selective in situ removal of the product. Van der Wielen et al. (8) have used this approach to enhance the enzymatic deacylation of benzylpenicillin, providing the desired product, 6-aminopenicillanic acid, and as a byproduct, phenyl-acetic acid, by means of light particles of immobilized enzymes and heavy particles of an adsorbent of the acidic by-product. Figure 4 presents a scheme of two different possibilities for the design of a two-solid-phase fluidized-bed bioreactor: semicontinous multistage pulsed flow and continuous trickle flow. In the first, the movement of the heavier solid particles downward in the reactor is obtained by periodic pulsations; in the second, the denser particles move continuously from the top to the bottom of the reactor.
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