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Fluidized Bed Reactors

One of the major problems with stirred tank reactors is the attrition of the matrix resulting from the vigorous stirring required for proper suspension of particles, and this becomes more problematic if the particles are heavier, larger and fragile matrices such as gels are used. When high volume fractions of biomass particles are preferred, and this obviously enhances the reactor efficiency, fluidized bed technology offers many possibilities. Such reactors are not very different from bubble columns, except maybe for the higher biomass fraction.

Figure 4. Fluidized bed reactors (a) fluidized bed (b) tapered fluidized bed, (adopted by ref. 5).

In its simplest two-phase operation (Figure 4), a flow of liquid is directed through the particles at velocities above the 'minimum fluidization velocity'. This is the velocity at which the pressure drop over the bed equals the weight of the particles per unit surface and they are lifted off their fixed bed state. At higher velocities, the bed will expand and only at much higher velocities will particles be entrained by the liquid and the fluidized bed organization destroyed. The settling rate drops as the solids fraction in the bed increases, and consequently the minimum fluidization velocity is much lower than the settling velocity of a single particle. Design of such systems in terms of adequate fluid velocities is not very difficult, but in bioreactors of this type the size and density of the aggregates or particles will depend on the growth and hydrodynamic conditions and these are very difficult to predict accurately. The expansion or minimum fluidization velocities are very sensitive to these two parameters. This results in a complex coupled system which is not easily accurately described. If, however, the supporting particles are rather heavy and measures are taken for a stable film thickness, stable operation and easy design will be possible. Excess biomass detached from the particles is entrained by the fluid and can be separated from the effluent.

Since the requirements of fluidization flow rate will seldom match the throughput for complete conversion in continuous systems, recycling is necessary to obtain good fluidization. Using some bed expansion and higher flow rates will give higher mass transfer rates from the liquid to the particles. Clogging and dead zones will also be avoided and attrition may help in controlling the particle film thickness.

Depending on particle size and density, liquid and gas flow rates, the use of recycle and bed geometry, several mixing patterns may be obtained in which the liquid phase and the solid phase are mixed or not. This is important for the micro-organisms as in non-mixed solid systems they will see rather steady conditions, but will rapidly face different conditions of pH, temperature and concentrations of substrate, oxygen and product in the case of mixing. If the liquid is well mixed, the concentrations are equal in all points and if complete conversion is desired, the resulting conversion rates may be low.

With little axial mixing (especially for large height/diameter ratios), a concentration profile may be maintained and high conversion rates in the entrance region may be combined with complete exhaustion of the substrate in the exit part of the reactor.

In some cases a tapered form of the reactor is useful to obtain gradients in the local velocities (Figure 4b). In the narrow bottom zone, a 'spouted bed' operation results and this gives extra mixing, mass transfer and attrition without risk of particle entrainment in the top of the column. As high concentrations in the inlet section may give more gas evolution or require better external mass transfer to the particles, this geometry may be beneficial. Given the complexity of flow and mixing patterns in fluidised bed systems, there is no easy way to predict the performance and at least a few pilot tests on a reasonable scale are needed before designing a full-scale plant. Usually the top of the reactor has to be wider to allow for settling of the particles and keeping the effluent clear of immobilized biomass, but other solutions are possible (screens or settling cones).

In three-phase operation, air is injected into the bed and of course destroys some of the characteristics of two-phase operation, usually resulting in strong backmixing in the system, except for large height to diameter ratios. With high gas rates one reverts to air lift or bubble column operation. In the case of strong gas evolution, also some disruption of the flow patterns can result. When the support particles are lighter than the fluid phase (many polymers), inverted fluid bed operation is necessary and has been proposed as an interesting alternative for some applications.