Batch intensive mixers

In this section we discuss the following counterrotating, twin rotor-based equipment the fully intermeshing TSE, the tangential TSE, the multilobal compounders, the CM FCMs, and the Banbury-type batch intensive mixer. We will comment on all the elementary steps taking place in them, and do so quantitatively, when possible. 

Figure 5.3 Overview of morphology development for a PA/EP-MA blend system with 20wt.% EP-MA in a batch intensive mixer [5]...

Figure 5.9 Number-average diameter of polypropylene particles in a polystyrene matrix as a function of weight percent polypropylene. Results fiom twin-screw extmders compare well to the batch intensive mixer, The bars show one standard deviation. The Taylor limit is also indicated [33]...

Our knowledge of rheological changes during reactive blending is based primarily on experiments conducted in batch intensive mixers with transducers capable of measuring... 

Figure 5.17 [5] compares the torque as a function of time in a batch intensive mixer for non-reactive and reactive blends of polystyrene/ethylene-propylene rubber. These results are typical for a relatively slow interfacial reaction. As the room temperature pellets of the blend are added to the hot mixer, the mixing torque rises rapidly in the melting regime. The torque for both blends then begins to fall as the temperature increases and the polymers soften. In the case of the non-reactive blend, the torque continues to fall and levels out to a reasonably constant value. However, in the case of the reactive blend there is a second peak in the torque due to the chemical reaction. The interfacial chemical reaction builds molecular weight, and in some cases may result in local crosslinking. This increases the viscosity of the blend relative to a non-reactive blend. 

High-Intensity Mixer. Mixers such as that shown in Fig. 18-44 combine a high shear zone with a fluidized vortex mixing action. Blades at the bottom of the vessel scoop the batch upward at peripheral speeds of about 40 m/s (130 ft/s). The high shear stress (to 20,000 s" ) and blade impact easily reduce agglomerates and aid intimate dispersion. Since the energy input is high [200 kW/m (about 8 hp/fE)h even powdery material is heated rapidly. 

Banbury Mixers The dominant high-intensity mixer, with power input up to 6000 kW/m (30 hp/gal), is the Banbury mixer made by Farrel Co. (Fig. 18-51). It is used primarily in the plastics and rubber industries. The batch charge of material is forced into the mixing chamber by an air-operated ram at the top of the mixer. The clearance between the rotors and the walls is extremely small. The mixing action takes place in that small gap. The rotors of the Banbury mixer operate at different speeds, so one rotor can drag material against the rear of the other and thus clean ingredients from behind and Between the rotors. 

BMC is different again in that bulky high-strength compounds are batch mixed in low-intensity mixers as a first step in the process. The mixing procedure is carefully monitored to achieve the highest possible mechanical properties with the least amount of fiber degradation. These compounds are also available in a rope form in any length or diameter. 

Tables 15-2 to 15-5 show different parameters of significance for agitator mixers. Table 15-2 shows different classes in batch mixers followed by the mode of operation and Froude number and respective capacity ranges and their power requirements. Note that only the high intensity mixers have Fr 1, and centrifugal types with Fr > 1 otherwise, most of the mixers operate with Fr < 1. Sections to follow will treat each of the classes of mixers above.

Even if the mixer is mounted to an adequate support, if that support is independent of the tank, relative motion between the tank and mixer can cause safety problems. Large, high intensity mixers are often provided with a change can arrangement that allows the use of different tanks for batch processing. These tanks should be secured by a means that does not allow the tank to move relative to Ihe mixer when in use. 

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