Mixing close-clearance impellers

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. 

Close-Clearance Impellers There are two close-clearance impellers. They are the anchor impeller (Fig. 18-6) and the helical impeller (Fig. 18-7), which operate near the tank wall and are particularly effective in pseudoplastic fluids in which it is desirable to have the mixing energy concentrated out near the tank wall where the flow pattern is more effective than with the open impellers that were covered earlier. 

Cabaret et al. (2008) and Gagnon et al. (1998) concluded that better mixing and higher product conversion can be achieved if a close clearance impeller, such as the helical ribbon, is used in conjunction with a radial flow impeller such as the RT in a highly viscous system. The Rushton-type turbine provides proper gas dispersion, while the close clearance impeller attempts to contact most of the reactor volume and provides proper bulk mixing, shear distribution, lower apparent viscosity, and minimal stagnant zones (Tecante and Choplin, 1993). These effects also lead to higher reactor utilization and can decrease power requirements. 

Low viscosity mixing applications can usually be handled efficiently with impeller systems consisting of one or more turbines. To obtain adequate mixing under the laminar flow conditions encountered in high viscosity applications, on the other hand, close-clearance impellers such as anchors and helical ribbons are required. These impellers sweep the whole wall surface of the vessel and agitate most of the fluid batch through physical contact. Helical ribbon impellers are typically used for industrial applications where the viscosity is in the range 20 000 to 25 000 Pa s. Wall scrapers can be mounted on the impeller blades to improve heat transfer. 

A number of mixing system designs are used for mixing of viscous fluids in laminar regime. They include (1) close-clearance impellers, (2) planetary impellers that move throughout the tank, and (3) fixed impellers in tanks that move to expose the material to the impeller. All of these systems generate the necessary three dimensional flows required for mixing. 

For laminar mixing other impellers are used, some laminar impellers are sketched in Fig. 7.4. To bring the fluid in the entire tank in motion, the diameter of these impellers usually approach the tank diameter since the laminar bulk flow is otherwise relatively low. Laminar impellers with diameter approaching the tank diameter are also called close-clearance impellers. Laminar mixers often have complex geometries, characterized by geometrical variables as the impeller diameter D, the blade width W, the pitch p, the impeller wall clearance C, and the off-bottom clearance Cb- In most applications, baffles are not needed and can in fact cause poor mixing behavior [87], Examples of laminar impellers are helical ribbons, screws, helical ribbon screws and anchor impellers. 

Figure 5-39. Close-clearance anchor and helical impellers. By permission, Oldshue, J. Y., Fluid Mixing Technology, 1983, Chemical Engineering McGraw-Hill Publications Co., Inc. [29].

For turbine agitators, impeller to tank diameter ratios of up to about 0.6 are used, with the depth of liquid equal to the tank diameter. Baffles are normally used, to improve the mixing and reduce problems from vortex formation. Anchor agitators are used with close clearance between the blades and vessel wall, anchor to tank diameter ratios of... 

This type unit [29] is used for a combination of pumping and mixing purposes. The unit has a closed disk on the top side. The feed flow into the unit comes from directly below the rotating impeller. The performance is dependent on the size of the unit and the physical location with respect to the distance up from the bottom of the vessel. As this clearance increases, the head decreases for constant flow and increases the pow er requirement. 

The clearance, expressed as C or as a ratio C/T or C/D, is the distance from the bottom edge of an impeller to the bottom of the vessel. C/D for an impeller in the middle is 0.5. General-purpose vessels used for multiple operations use an impeller placed very close to the bottom of the tank. This enables solids to be handled more efficiently as well as loading and unloading operations. When the impeller is some distance from the bottom, a small impeller tickler can help solve mixing problems with settling solids. 

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