In-line dynamic mixers

Only a selection of commercially available mixing equipment has been described here. Indeed, the devices described all exist in a variety of configurations. In a ition, there are many items of equipment based on altogether different principles typical examples include jet mixers, in-line dynamic mixers, mills, valve homogenisers, ultrasonic homogenisers, etc. These, as well as many other types, have been discussed by Harnby et alp-K Oldshue and Nagata ... 

Impellers 120 Jet mixers 125 In-line static mixers 126 In-line dynamic mixers 127 Mills 128... 

For mixing operations which require the continuous production of finely dispersed solids, emulsions, stable foams, etc., the in-line dynamic mixer in one of its several forms could be used. These usually consist of a rotor which spins at high speed inside a casing and the feed materials are pumped continuously to the unit. Inside the casing the fluid is subjected to extremely high shear forces which are required for the dispersing operation, see Figure 7.10. 

Often the dispersing unit itself produces little or no pressure differential and the unit has to be fed with its input materials by separate. pumping units. However, some in-line dynamic mixers do incorporate a pumping action and in such cases additional pumping may not be necessary. 

Figure 7.10 In-line dynamic mixer (Courtesy ofE. T. Oakes, Macclesfield, UK)...

This type of equipment is similar to the in-line dynamic mixer but in this case it is used in a vessel. The mixing head consists of a high-speed rotor inside a... 

When the flow is laminar, either single or multiphase, there is only one design class option static or motionless mixers. Other pipeline mixing devices described for turbulent flow are not usable for even the simplest mixing applications in the laminar regime. All rely on turbulence and cannot function at low Reynolds numbers. The only alternative technology is in-line dynamic mixers, which include extruders, rotor-stator mixers, and a variety of rotating screw devices. None of these has the benefits of simplicity and the little or no maintenance characteristic of static mixers. In-line mechanical mixers are discussed briefly later in the chapter. 

Sfatic mixers offer certain advantages over dynamic in-line mixers and continuous stirred tanks. Table 9.18 is a summary of the characteristics of a static mixer compared with a conventional mechanically stirred vessel. 

Dynamic in-line mixers. These are capable of generating high shear rates suitable for producing a high gas content ... 

Preparation of a concentrated emulsion. This stage consisted in mixing the heavy crude oil and a fresh-water surfactant solution, first in static mixers, then in a dynamic in-line turbine-type blender. The oil/water proportion was quite high (about 85% of heavy erode oil). Consequently, mixing was carried out very near the inversion A /B" branch. Inversion to W/O was delayed by the use of a relatively high surfactant concentration (the new one from the surfactant solution plus the remaining one from the primary emulsion). 

In plastics extrusion they have advantages of ease of installation with no requirement to be attached to, or incorporated with, the extruder screw. As they need to be positioned between the extruder output flange and the die, the requirement for extra space may place them with the same retrofit disadvantages as add-on dynamic mixers. However, with no screw connection required they can be installed within any available space between extruder outlet and die inlet, even at right angles to the axial line of the extruder barrel, e.g., vertically for a blown film extrusion line. 

The newest in this series of add-on mixing torpedoes is the Dynamic Melt Mixer, DMX. The device consists of a series of mixing modules inside a tubular housing. Two types of modules are available. Each module is made of two components a stationary ring fixed to the sleeve, and veined rotor keyed to an axial shaft either attached to the SSE screw or independently powered. Part of the rotor that does not enter the sleeve, provides the cutting action across the flow lines that induces the distributive action. The primarily distributive one consist of a stator and a rotor... 

Proof of the success and reliability of dynamic on-line mixers is shown by their extensive use. Each mixer offers its own advantages and disadvantages (see Chapter 2) with different machines and materials. Unfortunately there is no one system that solves all melting problems. The data available from the different equipment suppliers can be used in comparative studies. 

Problems due to piston turnaround can often be relieved by proper output hose selection. Hose diameters should be selected so that the dynamic component pressures at the mixer will be equal. Hose line pressure drop is affected by viscosity, flow rate, and hose diameter. The hose diameter usually is the only one of these factors that the designer can change and in order to compensate for differences in flow rate or viscosity between components, hoses should be selected that will yield the desired balanced pressures. The formula for pressure drop in hoses is given by Eq. (1). 

The ramp should not start until the reagent pipe and injection lines are assured to be completely full of reagent. The ramp should go beyond the pH set point so the curvature is identified for overshoot. After the ramp is completed, the reagent flow should be initialized to the value identified to correspond with the desired pH set point to minimize the upset. If the ramp is fast compared to the size of the volume, the disturbance to the final pH should be negligible. The ramp can be periodically scheduled or initiated as a phase in a start-up or batch sequence. The accuracy of the curve is greatly improved by the use of Coriolis mass flowmeters and linear control valves. Figure 8-5b shows the use of a static mixer and the dynamic compensation of the ratio to identify a titration curve online. 

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