Secondary Mixing Processes and Devices

Some of the more widely used high-performance screws include the Energy Transfer (ET) screw [19], the Variable Barrier Energy Transfer (VBET) screw [20], the Double Wave screw [21], Stratablend [22] and Stratablend II [23] screws, and the Unimix [24] screw. Other devices can, however, act as a trap for solid materials from the melting section. These devices include Maddock-style mixers [25], spiral dams, and blisters. They will be briefly discussed in this section as many of them have been the subject of many articles on secondary mixing processes. 

Flow ratio control is essential in processes such as fuel-air mixing, blending, and reactor feed systems. In a two-stream process, for example, each stream will have its own controller, but the signal from the primary controller will go to a ratio control device which adjusts the set point of the other controller. Figure 3.17(a) is an example. Construction of the ratioing device may be an adjustable mechanical linkage or may be entirely pneumatic or electronic. In other two-stream operations, the flow rate of the secondary stream may be controlled by some property of the combined stream, temperature in the case of fuel-air systems or composition or some physical property indicative of the proportions of the two streams. 

Kneading Devices These are closely related to the agitated pan but differ as being primarily mixing devices with heat transfer a secondary consideration. Heat transfer is provided by jacketed construction of the main body and is effected by a coolant, hot water, or steam. These devices are applicable for the compounding of divided solids by mechanical rather than chemical action. Apphcation is largely in the pharmaceutical and food-processing industries. For a more complete description, illustrations, performance, and power requirements, refer to Sec. 19. 

---