Static Mixing Devices

Mixing in extruders occurs not only along the extruder screw but also from the end of the screw to the exit of the die. The flow through the die and possible adapter is a pressure-driven flow where the flow velocities in the center of the channel are high and zero at the wall, see Fig. 7.134. 

This velocity profile results in a non-uniform shear rate profile with high shear rates at the wall and zero shear rate at the center of the channel see Fig. 7.134. For a power law fluid In a circular channel the axial velocity profile can be written as [16] see Fig. 7.110  

Velocity and shear rate profiles in pressure flow through a straight channei 

The shear rate profile is determined by taking the first derivative of the velocity with respect to normal distance r. Thus, the shear rate becomes  

The residence time of a fluid eiement over length L as a function of radial distance is 

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Continuous operation In flow-type reactors, e.g., loop reactors, the space velocity of the reaction is determined through the installed static mixing device that is used to generate the dispersion, together with the velocity of the circiflating medium (catalyst- and substrate/product phase). Knowledge of these parameters allows one to set up a kinetic model for the investigated reaction. 

Eductor An in-line static mixing device that, by flow of water through it creates a vacuum, thereby drawing a solution into the water stream. 

Static mixing devices usually consist of a combination of identical elements, stacked in series with each element turned ninety degrees relative to the next element. The first static mixer, the Multiflux mixer, was described by Sluijters [243]. This mixer developed at AKZO Corporation of the Netherlands splits the flow in rectangular converging and diverging channels see Fig. 7.136. 

There are many more static mixing devices used in the polymer processing industry in fact, the number is so large that it is not possible to list them all. Two characteristics are of critical importance in application of any static mixing device in actual extrusion operations. The first one, obviously, is the mixing capacity. The second one is the resistance that the static mixer offers to flow, i. e., the pressure drop along the static mixer. 

It should be remembered that static mixing devices can also be quite effective in distributive mixing capacity see Section 7.7.2. Thus, if distributive mixing is required, one should consider application of a static mixing device. 

Figure 7-19 Generalized portrayal of simple mixing in a static mixing device (each element dividing the fluid flow into two substreams).

Figure 7-26 Simplified portrayal of dispersive mixing in a static mixing device. A video clip of this process is provided on the Visual Mixing CD affixed to the back cover of the book.

Static mixing device - design, function, and characteristics... 

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