Figure 39. Illustration of the decrease in KLa with increase in apparent viscosity.

One relationship that cannot be changed simply going from a small to large scale is the fact that the Reynolds number normally increases in the large tank over what it was in the small tank. The Reynolds number is typically anywhere from 10 to 50 times higher in the large vessel than in the small. This means that the fluid in the pilot scale will appear much more viscous in terms of flow pattern and many other parameters than it will in the full scale tank. It is usually not practical where conducting a process to change viscosity between pilot plant and full scale, but if one is interested in getting an idea of the flow pattern and some of the macroscale effects, then a synthetic fluid of a lower viscosity than the actual could be substituted in the full scale work to give a better picture of the expected flow pattern.

At this point discussion of the quantitative and qualitative nature of available data is desirable. The user, production, research and engineering, and purchasing department should have discussions with the suppliers and technical personnel to arrive at satisfactory combinations of proposals.

6.3 Data Based on Pilot Plant Work

To keep ratios of impellers, gas bubbles and solid clumps in the fermentation related to full scale, the impeller size and blade width in the small scale must always have a physical dimension two or three times bigger than the particle size of concern.

It is possible to model the fermentation biological process from a fluid mechanics standpoint, even though the impeller is not related properly geometrically to the gas-liquid mass transfer step. Thus, one scale of pilot plant might be usable for one or two of the fermentation mass transfer steps, and/or chemical reaction steps, but might not be suitable for analysis of other mass transfer steps. The decision, then, is based on how suitable existing data are for any steps which are not modeled properly in the pilot plant.

Ideally, data should be taken during the course of the fermentation about gas rate, gas absorption, dissolved oxygen level, dissolved carbon dioxide level, yield of desired product, and other parameters which might influence the decision on the overall process. Figure 41 shows a typical set of data for this situation.

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