Gregory T. Benz - Agitator Design for Gas-Liquid Fermenters and Bioreactors

Figure 4.4 Available motor power curve using VFD.

Impellers operating in turbulent flow have variable loads created on each blade, and a variable net side load, which is transmitted to the shaft and creates a bending moment. This was described in Chapter 3, under the heading of Dimensionless Force. In general, fermenters have very high agitation intensity, raising the value of the Dimensionless Force, even when ungassed.

When gas is introduced, the forces for radial impellers typically double. For axial impellers in up‐pumping mode, the forces can more than double. In down‐pumping mode, or when liquid level passes through an impeller, the loads can be triple; sometimes even more. We will discuss these issues in more detail in Chapters 5and 17.

This chapter has presented the basics of power draw, flooding, and mechanical effects exhibited by impellers under gassed conditions. More specific information will be presented in Chapter 5, which is about impellers used in gas dispersion.

aInterfacial area/volumeklLiquid film coefficientklaOverall mass transfer coefficient (usually treated as one variable)DImpeller diameterHFractional gas holdupNShaft speedNaAeration NumberNCDShaft speed for complete dispersionNFlShaft speed for flooding emergenceNFrFroude numberNReReynolds numberOUROxygen uptake rate.PPower draw, generalPgGassed power drawPuUngassed power drawTTank diameterUsSuperficial gas velocityVgGas volumeVlLiquid VolumeZLiquid heightρLiquid density

1 1 Private communication.

2 2 Sensel, M.E., Myers, K.J., and Fasano, J.B. (1992). “Gas dispersion at high aeration rates in low to moderately viscous Newtonian liquids.” Paper 1071, AIChE 1992 Annual Meeting.

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