High-Intensity Mixers

In the screw conveyor process, solutions of poly(vinyl acetate) and catalyst are mixed in a high intensity mixer and continuously introduced to a screw-type saponification and conveyor system (270). Downstream details are similar to those found in the belt process. 

Banbury Mixer Preeminent in the field of high-intensity mixers, with power input up to 6000 kW/m (30 hp/gal), is the Banbury mixer, made by the Farrel Co. (Fig. 18-43). It is used mainly in the plastics and rubber industries. The top of the charge is confined by an air-operated ram cover mounted so that it can be forced down on the charge. The clearance between the rotors and the walls is extremely small, and it is... 

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. 

FIG. 18-44 High-intensity mixer (a) bottom scraper (h) fluidizing tool (c) horn tool (d) flush-mounted discharge valve. Henschel Mixers America, Inc.)... 

It is usually done by computer controlled electronic weighing scales that supply precise amounts of each ingredient to a high intensity mixer. The still-dry, free-flowing blend is then charged to a feed hopper where it is screw fed into a continuous mixer such as an extruder and/or kneader. Under the action of a mixer s reciprocating screw in the confined volume of the mixer chamber, the blend begins to flux or masticate into the required plastic state. 

Melt-mix Brabender high-intensity flux mixer 63,800 900 4.3 0.1... 

Harding et al. (33) proposed mixing in a Turbula high intensity mixer as a predictive stress test to determine the likelihood of encountering a dissolution problem on scale-up due to lubricant over-mixing. They found a... 

Convection mixers use a different principle for blending. These mixers have an impeller. This class includes ribbon blenders, orbiting screw blenders, vertical and horizontal high-intensity mixers, as well as diffusion blenders with an intensifier bar. Scale-up considerations are similar to those for the tumble blenders. 

High-intensity inline devices are often used to mix fluids in the process industries. Such devices include simple pipes, baffled pipes, tees, motionless mixers, dynamic mixers, centrifugal pumps, ejectors, and rotor/stator mixers. In addition to their traditional application in physical processes such as mixing and dispersion, such devices can provide very effective environments for mass transfer and chemical reaction to take place. Furthermore, combining effective inline mixing with heat transfer is the basis of combined heat exchanger reactors (HEX reactors). 

Gas-liquid reactions form an integral part of the production of many bulk and specialty chemicals, such as the dissolution of gases for oxidations, chlorin-ations, sulfonations, nitrations, and hydrogenations. When the gaseous reactant must be transferred to the liquid phase, mass transfer can become the rate-limiting step. In this case, the use of high-intensity mixers (motionless mixers or ejectors) can increase the reaction rate. Conversely, for slow reactions a coarse dispersion of gas, as produced by a bubble column, will suffice. Because a large variety of equipment is available (bubble columns, sieve trays, stirred tanks, motionless mixers, ejectors, loop reactors, etc.), a criterion for equipment selection can be established and is dictated by the required rate of mass transfer between the phases. 

Motionless mixers and ejectors are useful for applications requiring short residence times (on the order of seconds or less). If long residence times are required, e.g., if the reaction is relatively slow, the use of a motionless mixer alone would lead to a very long mixer, which may not be practical. One way to overcome this problem is to use a loop reactor, which combines a high-intensity mixer, such as a motionless mixer or ejector, with a separation tank. 

The large levels of turbulent energy dissipation produced in high-intensity mixers act to reduce the bubble size, typically from 0.5 to 2.0 mm in high-intensity mixers, compared to 1.0 to 5.0 mm in stirred tanks and bubble columns. In addition, much higher gas-to-liquid ratios can be achieved, and turbulence enhances kL, leading to overall mass transfer coefficients (kLa) 10-100 times greater than for a stirred tank. 

Bourne, J. R., kut, O. M., Lenzner, J., Maire, H., An improved reaction system to investigate micromixing in high-intensity mixers, Ind. Chem. Res. 1992,... 

The continuous mixer (CM) is a counterrotating, nonintermeshing twin-rotor machine. The Farrel Continuous Mixer (FCM) was the first CM developed (1964) by Ahlefeld et al. (8). It has rotor designs along the principles of the Banbury3 high-intensity batch mixer. 

Fig. 10.5 Photograph and cross-sectional schematic representation of a Banbury high-intensity internal batch mixer. The photograph shows the two elements of the drive the electrical motor and the gear reducer. Their large size is due to the very large power requirements of the mixer. [Photograph courtesy of the Farrel Company, Ansonia, CT.]...

Fine coals in wash plant slurries Light hydrocarbon oil for selective flocculation heavy oil for balling. High intensity mixer for flocculate beneficiation, balling on a disc. 

At times, low- or high-intensity blending alone can produce a suitable product for use by the fabricator. An example of this would be a polyvinyl chloride (PVC) blend used for several large-volume extrusion applications. More frequently, however, a compounding process is required to obtain the desired physical property. The five primary compounding processes used in the industry for controlling the above parameters are single-screw extrusion (SSE), twin-screw extrusion (TSE), continuous mixers, batch mixers, and kneaders. Table 18.1 summarizes key aspects of each process. 

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