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Figure 34. Results for a scaled-up bioreactor with a constant oxygen transfer rate (183,184). OD 660 is the optical density measured at wavelength of 660 nm.

Figure 34. Results for a scaled-up bioreactor with a constant oxygen transfer rate (183,184). OD 660 is the optical density measured at wavelength of 660 nm.

heat rates (scale-sensitive variables). A new equilibrium will be established, and the interplay of all the parameters of the system may lead to a regime in which a different step becomes the step-controlling the process rate.

The method of Kossen and Oosterhuis (178) starts with an analysis of the operation of the large-scale system. Once the regime is clarified, a small-scale system is designed in such a way that it simulates the operation regime of the larger one. Optimization studies can be done on the smaller model, and conclusions will then be extrapolated to the full-scale process. This concept is depicted in Figure 35. An example of this method is given by Oosterhuis (177), in which a large-scale stirred tank reactor is simulated on a laboratory scale by two interconnected vessels, one with a small liquid volume, high agitation rate, and high rate of oxygen supply (representing the zone near the impeller-sparger in the reactor), and a second vessel, with a much larger liquid volume, oxygen consumption (simulated by nitrogen sparging), and gentle agitation. The success of this model suggests that large vessels must be carefully analyzed and their internal structure studied for proper modeling and design to be performed. The analysis starts with the definition of the characteristic time constants of the system.

In the case of low gas recirculation, the relationship between the total circulation time and the residence time in

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