Gas continuous i

Gas continuous i flow

Coalesced B.F. Slug flow T: Transitional o o

Figure 5. Flow regime diagram for the concurrent gas-liquid-solid fluidized-bed, Ug, gas superficial velocity. U, liquid superficial velocity. Source. From Ref. 3.

ferent regimes and the effect of the density and size of the solid particles on the regime transitions has been conducted by Zhang et al. (17). The liquid flow pattern in the bioreactor is directly influenced by the degree of mixing associated with these different regimes, as well as by other factors such as internal gas generation, bead size distribution, and external liquid recirculation. Basically, the flow will approach plug flow in systems with high velocity in the liquid phase. Quite often, bioreactors experience different degrees of mixing, and complete mixed flow can be observed in fluidized-bed bioreactors.

The influence of the gas phase on the liquid and solid mixing in a fluidized-bed bioreactor has been studied by Gommers et al. (18), considering two extreme situations: a reactor operating without gas and a reactor to which gas was introduced artificially from the bottom. The results showed that the gas phase greatly influences the degree of liquid mixing in the reactor, and that this effect increases sharply with the bed diameter. Another fact to consider is that in systems with gas generation associated with the reaction progress (for example, in most fermentations), the gas flow will change from the bottom to the top of the reactor, proportionally to the substrate consumption, and the axial dispersion will change with reactor height (19). An important aspect to consider in the study of the influence of the gas produced in a fluidized bed on the degree of liquid mixing is that the results will be affected by the experi mental system used for gas injection, as discussed in detail by Buffiere et al. (20). When liquid recirculation is used to promote fluidization because of the slow reaction rate and the long liquid-residence time required, plug flow is usually disrupted, and completely mixed flow is usually achieved.

Other factors influencing the hydrodynamic behavior of fluidized-bed bioreactors are the properties of the solid and liquid phases. In general terms, the weight and size of the solid particles will directly influence the liquid and gas flow rates required for bed fluidization. If the liquid residence time is fixed by criteria of substrate conversion, for example, then the heavier particles will require higher L/D ratios to increase liquid superficial velocity and enhance fluidization, or as an alternative, they will require high recirculation rates. In absence of recirculation, and when particles with a certain distribution in size and weight are used, solid-particle stratification is commonly found. Under stratification conditions, movement of the solid particles in the bed is very limited, and the particles are ordered by decreasing settling-velocities, from the bottom to the top of the bed. One of the consequences of this situation is that, in liquid plug-flow regimes, the particles at different reactor heights will experience different environments. For cells growing as biofilms around a solid particle, it is typical for the particles at the bottom of the bed to provide better conditions for cell growth (for example, substrate availability), and as a consequence, they will decrease their overall density and therefore migrate to the upper part of the reactor. Removal of excess biofilm may require external treatment of the particles. The operation of a stratified fluidized bed is illustrated in Figure 6, where the stratification of a bed with activated carbon as a solid support for cell growth is combined with the capacity to absorb the product of the reaction and, therefore, remove it selectively (21). The crucial effect of particle size and density distribution, as well as gas and liquid superficial velocities, on the phenomena of solid-particle stratification is discussed in detail in various recent studies (22-24). In general, solids mixing has been studied less comprehensively than liquid mixing, and it is less well understood. The possibility of using new experimental techniques (25) has enabled researchers to propose new models for the interpretation of the solids mixing, for example, trajectory length distribution (26,27), in contrast to more classical models that assume that solids mixing can be defined by means of an axial dispersion model (28).

Another important aspect to be considered from the hy-drodynamic point of view is that fluidization properties depend on the difference in densities of the solid and liquid phases in the fluidized bed, and these values change during the operation time. This situation is of particular relevance for those systems where the absolute values for the liquid and solid densities are relatively close, such as for cells immobilized in natural origin polymers (alginate, agarose, etc.) or self-aggregated cells (pellets, flocs, etc.). In this case, relatively low variations in the absolute value of the solid density (usually associated with cell growth or the accumulation of CO2 gas in the particles) and the liquid density (usually associated with substrate consumption) may cause an important percentile variation in the density difference between both phases, producing a relevant impact on the reactor hydrodynamics, as the regime changes from dispersed to coalescing (29), and on reactor stability. This problem can be of particular relevance at the startup of a bioreactor operation, when the density values may change to a greater extent.

A second aspect to be addressed in the characterization of a fluidized-bed reactor is the definition of the flux model. As mentioned previously, most of the attention is focused on the liquid phase, and the flux model is closely connected to the hydrodynamic conditions in the reactor that will generate a given degree of internal mixing, which is somewhere between the two extreme situations of perfectly mixed or plug flow. The determination of the real liquid flux model in the bioreactor is a necessary step for the application of the mass balance equations for the species taking part in the reaction. Stimulus-response techniques are commonly used for such a purpose and are based on the introduction of an inert tracer at the reactor inlet and the analysis ofthe response curve obtained at the outlet, which reflects the type of flux (30). The models that describe the liquid flux in a real reactor (31) can be (1) an axial dispersion model, in which axial dispersion is superimposed on the liquid convective flux; (2) a tank-in-series model, in which the bioreactor is considered as a series of CSTR reactors of the same volume; and (3) a compartmented model, in which the flux model in the bioreactor is described as the combination of different ideal compartments. For fluidized-bed bioreactors, especially when fermentation gas is produced, some interesting contributions have been proposed, such as the consideration of a variable dispersion coefficient, which increases its value in proportion to the fermentation gas generated in the reactor, up to a given point (10).

In addition to hydrodynamic and mixing characteristics, another relevant aspect of a fluidized-bed bioreactor is its biocatalytic activity. Because immobilized biocata-lysts are always used, the definition of the problem must simultaneously include the reaction characteristics (kinetics, stoichiometry, equilibrium) and transport characteristics, including both the transport between the liquid and the solid phase and the transport within the solid phase. When the general situation of a solid biocatalyst particle immersed in a liquid phase, as shown in Figure 7, is studied, different phenomena have to be considered. First, substrate is transported from the liquid phase to the solid ex-

Figure 6. Operation diagram of a fluid-ized-bed bioreactor with simultaneous bioconversion and adsorption/desorption of substrate and product. Source: From Ref. 21.

Figure 6. Operation diagram of a fluid-ized-bed bioreactor with simultaneous bioconversion and adsorption/desorption of substrate and product. Source: From Ref. 21.


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