Mixer helical blade

With mechanically agitated countercurrent extractors, energy is induced by rotating internals. Agitators are usually mounted on a central column shaft at certain distances from each other (double blade mixer, disc mixer, blade mixer, turbine mixer, helical screw mixers, etc.). Installation of stators (ring discs, punched metal sheets. 

Figure 16-9 Helical blade mixer. A video clip of blending with a helical ribbon is provided on the Visual Mixing CD affixed to the back cover of the book.

Figure 16-39 Combination anchor/helical blade, turbine mixer.

Fig. 23. Some mixers for viscous liquids, (a) Six-blade turbine (b) three inclined-blade paddles (c) helical screw (d) helical ribbon (e) propeller in draught tube (f) anchor. (Reprinted with permission from Chemical Engineering Science, 37, 813, C. J. Hoogendoorn and A. P. Den Hartog, 1967 Pergamon Press pic).

The benefits of an anchor mixer are limited by the fact that the vertical blades provide very little fluid motion between the top and bottom of the tank. Ingredient additions at the surface of the fluid may make many rotations before gradually being spread and circulated to the bottom of the tank. To promote top-to-bottom fluid motion, angled blades on the anchor or helical ribbon blades, described in the next subsection, make better mixers for uniform blending. Significant viscosity differences between fluids may extend mixing times to unacceptable limits with the basic anchor. 

A very useful reactor assembly for pilot-scale polymerization of lactide and glycolide polymers and copolymers is an 8-CV mixer from Design Integrated Technologies (DIT) of Warrington, Virginia. The 8-CV is a dual cone mixer (8 qt. capacity) equipped with double helical mixer blades, a gas inlet line, and a large vacuum port. 

Ribbon Blender nA type of low-intensity mixer comprising helical, ribbon-shaped blades supported by spokes from the horizontal shaft, with the blade edges fitting fairly closely to the lower half of the U-shaped shell of the mixer. The shell is usually jacketed for temperature control and the shaft, spokes and blades may also be cored for circulation of heat-transfer liquid (usually water). They are used in the plastics industry mostly for cooling dry... 

It is important to note that some impellers and mixer configurations do not pump weU. For example, the retreat curve blade impeller in an unbaffled vessel creates solid body rotation and poor pumping. Pumping with close-clearance impellers such as anchors and helical ribbon can be very high or, sometimes, very poor, depending on conditions and the materials being pumped. Turbulent impellers in laminar applications only pump locally. Often, the rest of the tank goes unmixed. 

The power number for helical ribbon mixers depends on Reynolds number blade/wall clearance, e height, h pitch of the helix, p (height of one turn around the helix) blade width, w and number of helical flights, n. The relationship is... 

Related to the helical screw devices are the instrumented batch mixers often called torque rheometers. They consist of a chamber or mixing bowl with two counterrotating blades. There are many different blade configurations. A common one is shown in Figure 5.6.5. These devices are primarily used as simulators of larger batch mixers and extruders and to prepare compounded samples. One of the rotating shafts has a transducer that records torque as polymers melt and mix with other additives. Blyler and Daane (1967) have shown that when the sample has reached steady state, one can obtain a good measure of n (but not m) for power law fluids fi om torque and screw speed. 

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. 

---