Axial-flow impellers, viscosity

High-Viscosity Systems A axial-flow impellers become radial flow as Reynolds numbers approach the viscous region. Blending in... 

FIGURE 1 Three typical impellers for low and medium viscosity (a) Axial flow, 45° blade (A200), (b) radial flow, disc turbine (R100), and (c) fluidfoil axial flow impeller (A300). 

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. 

High-Viscosity Systems All axial-flow impellers become radial flow as Reynolds numbers approach the viscous region. Blending in the transition and low-viscosity system is largely a measure of fluid motion throughout the tank. For close-clearance impellers, the anchor and helical impellers provide blending by having an effective action at the tank wall, which is particularly suitable for pseudoplastic fluids. 

The motion of the vessel contents is influenced by the agitator and any baffles. The agitator, or impeller, provides the power to move the vessel contents. There are two types of impellers radial-flow impellers, which move the liquids along the radius of the impeller, and axial-flow impellers, which move the liquids parallel to the axis of the agitator shaft. Without baffles, low-viscosity fluids tend to create... 

Viscosity, hence Reynolds number, affects the performance of axial-flow impellers. The discharge angle (measured from the horizontal) decreases with increasing viscosity, causing flow patterns to change. Propellers discharge flow in a similar pattern to the 45° PBT as shown in Figure... 

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. 

Axial flow impellers used in low bottom clearance tanks can also create radial flow if the direction of the flow is downward. In this case, the flow can leave the impeller zone only by flowing in the radial direction (Tatterson, 1991). This phenomenon is usually not observed since the standard bottom clearance for low viscosity impellers in gas-liquid dispersions is between one impeller diameter and one half the tank diameter (Ulbrecht and Patterson, 1985). The hydrofoil impellea-... 

Draft Tubes. A draft tube is a tube installed centrally within the vessel. Axial flow impellers located inside a draft tube are used to provide an efficient top-to-bottom circulation pattern, which is important for flow-controlled processes. Draft tubes reduce the standard deviations in process variables such as concentration, density, and viscosity. They are also useful in tanks with a high ratio of height to diameter. 

There are four types of turbine impellers, which are characterized by the flow patterns and level of shear they create axial flow, radial flow, hydrofoil, and high-shear impellers. They have the widest use in low and medium viscosity liquid applications, solids suspension, liquid-liquid emulsification, and gas dispersion. Turbine impellers can have blades varying from 2 to 12 in number. Two blades are normally unstable mechanically, while it is difficult to install more than six blades on a hub. Axial flow impellers generally have three or four blades, and radial flow impellers are designed with six blades. 

Slow speed close-clearance impellers are used when mixing high viscosity materials. Helical or anchor type close-clearance impellers are used in this application at speeds from 5 to 20 rpm. Table 1 compares the pow er required and cost for conventional axial flow turbines and the helical type. 

Figure 5-52. A-410 Composite Impeller. Strong axial flow at very high flow efficiency. Operates through a wide range of viscosities. By permission, Lightnin, (Formerly Mixing Equipment Co.) a unit of General Signal.

The three basic types of impeller which are used at high Reynolds numbers (low viscosity) are shown in Figures 10.55a, b, c. They can be classified according to the predominant direction of flow leaving the impeller. The flat-bladed (Rushton) turbines are essentially radial-flow devices, suitable for processes controlled by turbulent mixing (shear controlled processes). The propeller and pitched-bladed turbines are essentially axial-flow devices, suitable for bulk fluid mixing. 

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