Mixing axial-flow impellers

Axial flow impeller (propeller) or mixed flow type (horizontal or vertical)... 

J. Y. Oldshiie, T. A. Post, R. J. Weetman, Comparison of Mass Transfer Characteristics of Radial and Axial Flow Impellers, BHRA Proc. 6th European Conf. on Mixing, 5/88. 

Figure 5-3 highlights the most commonly used radial and axial flow impeller styles for process applications. Other styles/designs are used for special specific applications to accomplish the mixing objectives (Figures 5-4 and... 

Centrifugal pumps can also be classified by the design of the impeller. Centrifugal pumps may have radial-flow impellers, axial-flow impellers, and mixed-flow impellers (both radial-flow and axial-flow). 

Axial flow impellers Impellers that pump the fluid primarily in an axial direction when installed in a baffled mixing tank. 

Axial-flow impellers may also be mounted near the bottom of the cylindrical wall of a vessel as shown in Fig. 18-10. Such side-entering agitators are used to blend low-viscosity fluids [<0.1 Pa s (100 cP)] or to keep slowly settling sediment suspended in tanks as large as some 4000 m (10 gal). Mixing of paper pulp is often carried out by sideentering propellers. 

Mechanically stirred hybrid airlift reactors (see Fig. 6) are well suited for use with shear sensitive fermentations that require better oxygen transfer and mixing than is provided by a conventional airlift reactor. Use of a low-power axial flow impeller in the downcomer of an airlift bioreactor can substantially enhance liquid circulation rates, mixing, and gas-liquid mass transfer relative to operation without the agitator. This enhancement increases power consumption disproportionately and also adds other disadvantages of a mechanical agitation system. 

Examples of axial impellers are marine propeller and pitched blade turbine, shown in Fig 7.2. A three bladed marine t3rpe propeller is similar to the propeller blade used in driving boats. The propeller can be a side entering type in a tank or be clamped on the side of an open vessel in an off-center position. Axial flow impellers are used in blending and mixing of miscible liquids. 

In a tank with radial impellers, suitable baffles will produce strong top-to-bottom currents from the radial discharge. The installation of baffles generally increase the power consumption [65]. For axial flow impellers, the need for baffling is not as great as for radial flow impellers, thus axial flow impellers also consume less power than radial impellers. Baffles are normally used in turbulent mixing only. 

Naturally, the patterns of liquid movements will vary with the type of impeller used. When marine propeller-type impellers (which often have two or three blades see Figure 7.7c) are used, the liquid in the central part moves upwards along the tank axis and then downwards along the tank wall. Hence, this type of impeller is categorized as an axial flow impeller. This type of stirrer is suitable for suspending particles in a liquid, or for mixing highly viscous liquids. 

Figure 8.2 Particle concentration as a function of height in a tank mixed with an axial flow impeller according to Shamlou and Koutsakos (1989).

Axial flow impellers produce less radial flow, and much more axial flow for similar power input (as the name suggests). They therefore produce much more vertical mixing when so oriented. They also do a much better job mixing between the region above and below the impeller. 

Since radial flow impellers generally produce a higher turbulence zone and axial flow impellers produce more vertical vessel flow, a good combination in applications where two or more impellers are used is to use a radial flow impeller near the bottom where the feed is introduced, and one or more axial flow impellers on the same shaft above it to provide vertical mixing and induce flow through the otherwise partially segregated region under the radial flow impeller. 

An aerobic fermentation is to be carried out in a 200-m reactor (4-m dia. X 16 m) with a normal liquid depth of 12 m and atmospheric pressure at the top. A fiat-blade turbine will be used to disperse the air, and two axial-flow impellers will be installed on the same shaft to promote end-to-end mixing. Air will be supplied below the turbine at a superficial velocity of 3 cm/sec (based on 30°C and 1 atm). The Ffenry s law constant for oxygen is 5.2 x 10" atm/m.f. (10% greater than for pure water), and the peak oxygen demand is estimated to be 45 mmol/L-hr. Tests in a small unit show that kj a for this solution is 70% of the value for oxygen absorption in sodium sulfite solution. The solution viscosity is about 1.5 cp. 

Pumping the liquid volume pumped by the impeller = kp N where kp is a constant dependent upon the impeller Dj is the diameter of the impeller and N = rpm. For a three-bladed marine propeller whose pitch = Dj, kp = 0.5 many other types of impellers have smaller values, and therefore the marine propeller is one of the good axial flow impellers. In general, impellers giving axial flow are used in about 70 % of the mixing applications. 

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