5 Agitation James K Oldshue 1.0 THEORY AND CONCEPTS Fluid mixing is essential in fermentation processes. Usually the most critical steps in which mixers are used are in the aerobic fermentation process. However, mixers are also used in many auxiliary places in the fermentation process and there are places also for agitation in anaerobic fermentation steps. This chapter will emphasize the aerobic fermentation step, but the principles discussed can be used to apply to other areas of fermentation as well. Table 1 divides the field of agitation into five basic classifications, liquid-solid, liquid-gas, liquid-liquid, miscible liquids and fluid motion. This can be further divided into two parts-on the left are shown those applications which depend upon some type ofuniformity as a criterion, while the processes on the right are typical of those that require some type of mass transfer or chemical reaction as a criterion. On the left-hand side, visual descriptions of flow patterns and other types of descriptions of the flow patterns are helpful and important in establishing the effect of mixing variables on these criteria. In general, they are characterized by a requirement for high pumping capacity rather than fluid shear rate, and studies to optimize the pumping capacity ofthe impellers relative to power consumption are fruitful. 181 182 Fermentation and Biochemical Engineering Handbook Table 1. Classification of Mixing Processes Physical Processing Application Classes Chemical Processing Suspension Liquid-Solid Dissolving Dispersions Liquid-Gas Absorption Emulsions Immiscible Liquids Extraction Blending Miscible Liquids Reactions Pumping Fluid Motion Heat Transfer The other types of processes involve more complicated extensions of fluid shear rates and the determination of which mixing variables are most important. This normally involves experimental measurements to find out exactly the process response to these variables which are not easy to visualize and characterize in terms of fluid mechanics. In order to discuss the various levels of complexity and analysis ofthese mixing systems, some of the fluid mechanics of mixing impellers are examined and then examples of how these are used in actual cases are shown. 2.0 PUMPING CAPACITY AND FLUID SHEAR RATES All the power, P, applied to the systems produces a pumping capacity, Q, and impeller head, H, shown by the equation: PccQH Q has the units of kilograms per second and H has the units of Newton meters per second. Power then would be in watts. The power, P, drawn by mixing impellers in the low and medium viscosity range is proportional to: P cc N3D5 Agitation I83 where D is impeller diameter andN is impeller speed. The pumping capacity of mixing impellers is proportional to ND 3. Q oc ND3 These three equations can be combined to yield the relationship that where This indicates that large impellers running at slow speeds give a high pumping capacity and low shear rates since the impeller head or velocity work term is related to the shear rates around the impeller. High pumping capacity is obtained by using large diameter impellers at slow speeds compared to higher shear rates obtained by using smaller impellers and higher speeds. is the flow to head ratio at constant power. 3.0 MIXERS AND IMPELLERS There is a complete range of flow and fluid shear relationships from any given impeller type. Three types of impellers are commonly used in the low viscosity region, propellers, Fig. 1; turbines, Fig. 2; and axial flow turbines, Fig. 3. Impellers used on small portable mixers shown in Fig. 4, are often inclined at an angle as well as being off-center to give a good top-to-bottom flow pattern in the system, Fig. 5. Large top-entering drives usually use either the axial flow turbine or the radial flow flat blade turbine. For aerobic fermentation, the radial flow disc turbine is most common and is illustrated in Fig. 6. To complete the picture, there are also bottom-entering drives, shown in Fig. 7, which have the advantage of keeping the mixer off the top of all tanks and required superstructure, but have the disadvantage that if the sealing mechanism fails, the mixer is in a vulnerable location for damage and loss of product by leakage. Figure 8 illustrates side-entering mixers which are used for many types of blending and storage applications. 184 Fermentation and Biochemical Engineering Handbook Figure I. Photograph of square-pitch marine type impeller. Agitation 185 Figure 2. Photograph ofradial flow, flat blade, disc turbine. Fermentation and Biochemical Engineering Handbook 186 Figure 3. Photograph of typical 45° axial flow turbine. Agitation 187 Figure 4. Photograph of portable propeller mixer. I88 Fermentation and Biochemical Engineering Handbook I / Agitation 189 Figure 6. Series 800 top-entering mixer. Fermentation and Biochemical Engineering Handbook190 Figure 7. Photograph ofbottom-entering mixer. Figure 8. Photograph ofside-entering propeller mixer.