Helical screw mixers

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. 

Entrop (17) reported the process characteristics of the Nauta mixer. The Nauta mixer utilizes the orbiting action of a helical screw, rotating on its own axis, to carry material upward, while revolving about the centerline of the cone-shaped housing near the wall for top-to-bottom circulation, (see the sketch in Fig. 16). Nauta mixers of different sizes are not built geometrically similar to each other, but the diameter of the helical screw and its pitch are kept equal. 

The measurements were executed under the the following conditions mixer volume F= 0.05-10 m diameter of the helical screw [Pg.39]

From the enery point of view, it is therefore advantageous to construct mixers of low heights and to provide them with helical screws of large diameters. 

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).

An in-line measurement is performed in a process line an on-line measurement is performed in a bypass loop from the main process line and the food may be returned to the main process line after measurement is performed. A near-line measurement is performed on a sample taken from a process line which is often discarded after measurement. Because foods are complex materials (e.g., suspensions, emulsions, gels), stmctural changes may take place during sampling (e.g., flow through a valve) for on-line and near-line measurements (Roberts, 2003). Nevertheless, in principle, the previously described capillary flow, and rotational concentric cylinder, plate-cone, and mixer viscometers may be used for in-line, on-line, and near-line measurements. In this respect, Tamura et al. (1989) proposed a helical screw rheometer as an on-line viscometer. The empirical measurement methods described previously are used primarily in near-line measurements. 

Conical mixers are also known as Nauta mixers (Fig. 18-53). Material placed in the conical bin is lifted by the rotation of the helical screw, which in turn is rotated around the wall of the cone. The lifting actions of the screw combined with motion around the cone provide bulk mixing for flowable dry powders, paste materials, and even viscous fluids. The specific energy input is relatively small, and the large volume of the mixers can even provide storage capacity The mixers may have multiple screws, tapered screws, and high-speed dispersers for different applications. At constant speed, both the mixing time and power scale up with the square root of volume. Sizes from 0.1 to 20 m3 (3.3 to 700 ft3) are available. 

Anchors, helical ribbons and screws, are generally used for high viscosity and non-Newtonian hquids. The anchor and ribbon types are arranged with a close clearance at the vessel wall, whereas the helical screw has a smaller diameter and is often used inside a draft tube to promote liquid motion through out the vessel. Helical ribbons or interrupted ribbons are often used in horizontally-moimted cylindrical vessels. A variation of the simple helix mixer is the helicone (Figure 8.27), which has the additional advantage that... 

Another class of devices for solids blending is the stationary shell device, in which the container is stationary and the material displacement is accomplished by single or multiple rotating inner devices. In the ribbon mixer in Fig. 3.4-6b, a shaft with two open helical screws numbers 1 and 2 attached to it rotates. One screw is left-handed and one right-handed. As the shaft rotates, sections of powder move in opposite directions and mixing occurs. Other types of internal rotating devices are available for special situations (PI). Also, in some devices both the shell and the internal device rotate. 

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. 

A vertical helical ribbon blender can be combined with an axial screw of smaller diameter (Fig. 18-25). Such mixers are used in polymerization reactions in which uniform blending is required but in which high-shear dispersion is not a factor. Addition of the inner flight contributes little more turnover in mixing newtonian fluids but significantly shortens the mixing time in nonnewtonian systems and adds negligibly to the impeller power [Coyle etal.. Am. Inst Chem. Eng.J., 15, 903 (1970)]. 

Propeller or helical mixers, including screw conveyors... 

In the Pulsar mixer a helical groove is machined into the root of the screw the helix angle of the groove is greater than the helix angle of the flight. As a result, some... 

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