Airlift Reactorselection And Design Scaleup of Airlift Bioreactors

The problems encountered in the scale-up of bioreactors can be concentrated into two groups. The first includes the cases in which a high power input per unit volume is used on a laboratory scale, but cannot be maintained on an industrial scale due to economic or mechanical limitations. This is not the case for plant or animal cultures, for which a very high specific power input cannot be used because of cell fragility. The second group of problems can be generalized as lack of knowledge about the hydrodynamics of large-volume vessels.

The methods available for scale-up of bioreactors have been reviewed by Oosterhuis (177), Kossen and Oosterhuis (178), Sweere et al. (179), and Sola and Godia (180), among others. Because, in general, design from first principles cannot be undertaken because of the lack of basic knowledge about the hydrodynamics of the bioreactors, one possible solution is the semifundamental method, which comprises using approximate simple models for fluid dynamics and integrating them with basic known kinetics and heat and mass transfer rates.

It does not happen very often that all this information is available with a degree of certainty that allows safe design of a large system. Thus, the designer must usually resort to dimensional analysis. This method requires a knowledge of all the variables affecting the process, which can be obtained from a qualitative, but realistic, model (181).

A simplified version of this method is to limit the number of variables to one or two and to follow rules of thumb (182), which, depending on the specific case, may be constant P/V, constant kLa, etc. The literature shows, however, many cases of inconsistency for this method. For example, design of a scaled-up bioreactor in which the oxygen transfer rate is kept constant can lead to a better performance than expected, as in the example reported by Tag-uchi et al. (183) for glucoamylase production by Endomyces sp., or worse than expected, as in the case of protease production by Streptomyces sp. (184). These two examples are shown in Figure 34.

The method of regime analysis and scale down, proposed by Kossen and Oosterhuis (178), combines two tools to overcome the problem posed by the complexity of biochemical reactors. Their method is based on considering the regime of the full-scale process as the objective and planning the strategy of process development from this point on. This method is therefore applicable only to conventional and well-studied bioreactors that are to be used in a new bioprocess.

Regime analysis is based on the consideration that, generally, biochemical processes involve a series of steps, some being mass or heat transfer by convection, some being diffusive mechanisms (activated or not), and others being chemical reaction steps. In the latter case, a mass-transfer mechanism is superimposed, since molecules must encounter one another in order to react, and usually a heat effect will accompany the reaction. Depending on whether these steps take place in parallel or in series and on the relaxation time for each step (185), the rate of the total process is often given as the rate of one single step. But the equilibrium between all the individual rates can be (and usually is) upset by a change in scale. This is to be expected, because a change in scale will not bring a change in the physicochemical or kinetic parameters (scale-insensitive variables), but will affect the overall convective mass and

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