Multiple axial flow impellers

Figure 8.4 Placement of multiple axial flow impellers, (a) good and (b) bad.

A stirred tank fermenter consists of a centrally mounted agitation system inside a cylindrical vessel. Typically, the agitation system is composed of either multiple radial flow impellers (see Fig. 4A) or a combination of radial and axial flow impellers as shown in Fig. 4B. A gas sparger is located below the radial gas dispersing impeller. The role of the bottom radial impeller... 

The choice of impeller for three-phase systems is a compromise between dispersing gas in the liquid and suspending the particles in the liquid. We recall that the axial-flow impellers are usually used for solid-liquid systems, while radial impellers are used for gas-liquid systems. Introducing gas from the bottom of a tank containing a solid-liquid suspension will destroy the flow pattern created by an axial impeller. Therefore, radial impellers are usually more effective in three phase systems even if they require more power for the same level of suspension [65]. Another solution is to apply multiple impellers, one to fulfill the criterion for gas dispersion and another one to fulfill the criterion for solids suspension [87]. The existence of solid particles might also modify the interfacial area between gas and liquid compared to gas-liquid systems. 

This discussion sheds light into the superior performance of a multiple impeller system with a radial-axial flow impeller combination, with a radial flow impeller in the lower position and an axial flow impeller in the upper position. The radial flow impeller is not affected by the sparger type and is able to efficiently disperse small bubbles. The upper impeller is loaded indirectly by the flow field, which it generates, and is able to provide proper mixing conditions. As such, the sparger choice does not affect the performance of the other impellers. If the impellers operate independently, impellers are optimally loaded for gas dispersion and liquid mixing such that progressive reduction in ki a is minimized and the desired process time can be reduced by >30% (Lines, 2000). 

Multiple impellers can be used as shown in Figure 22-12. The configuration shown in this computational fluid dynamics (CFD) simulation uses three high solidity, up-pumping axial flow impellers. The simulation shows the flow pattern by depicting the flow of neutral density particles (Weetman, 1998a). Other impeller types are discussed in Chapter 6. 

The Ekato intermig impeller has reverse pitch on the inner and outer blades and they are almost always used with multiple impellers. They are used at high D/T and promote a more uniform axial flow pattern than other turbine impellers. They are advertised to be very effective for solids suspension, blending, and heat transfer in the medium viscosity range. Lower Nrc limit not given by Ekato (9), perhaps 5. 

Table 4 shows the recommended numbers of impellers used in low viscosity working media. In the case of high viscosity media, the impeller spacing, S, normally needs to be decreased, i.e., S = 0.75-1.0 D. Multiple axial impellers are desirable for a synergistic flow pattern. 

Impellers Radial disk turbine and optional axial flow/ hydrofoil impeller Multiple radial disk turbines and axial flow/hydrofoil impeller for better circulation... 

The Kiihni column employs radial flow impellers located between perforated plates for compartmentalization. The first Scheibel column used wire mesh zones to promote coalescence and limit backmixing between turbine-agitated mixing zones. A later Scheibel column used a shrouded radial impeller and multiple ring baffles to direct most of the rotor s energy towards dispersion and away from axial mixing. 

Curved-blade disc turbines are good choices for gas dispersion (see Figure 9.3(a), second row, right). Note that the open cup advances into the liquid. A second impeller, like an axial-flow hydrofoil, helps improve circulation and gas holdup. Multiple disc turbines should be avoided, since they lead to compartmentalization and poorer overall mixing. 

For dissolving polymer in solvent, the major problem is the small clumps of polymer formed in the viscous fluid. These clumps are difficult to break up. We need sufficient shear combined with axial flow in order to break the polymer quickly and immediately spread the polymer into the liquid for subsequent dissolution. Leave a small gap between the baffle and the tank wall in order to avoid the dead corner of undissolved polymer. If multiple impellers are used, then to save on power consumption, the bottom impeller might supply axial flow plus shear (as an open turbine) with the impeller above supplying axial flow. The Power number for the open turbine might be, for example, 1.2, whereas for axial flow the Power number value might be about 0.3. 

In emulsion polymerization, a high shear rate may cause coagulation. However, a certain amount of turbulence is required for emulsification and to avoid phase segregation. Moreover, high fluid circulation is needed in order to guarantee the macroscopic uniformity and to enhance mass and heat transfer. In this way, mixed-flow turbines with features of both radial and axial flow can be useful. The most common of these impellers is the 45° angled blade turbine. Multiple impellers on the same shaft can also be employed. 

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