Agitators high viscosity liquids

The various methods of agitation to produce emulsions have been described recently (18). In addition, the emulsions of smaller droplets can be produced by applying more intense agitation to disrupt the larger droplets. Therefore, the liquid motion during the process of emulsification is generally turbulent (9) except for high viscosity liquids. 

It should be emphasized that stirred gas/liquid contactors are not practical for high viscosity liquids. There is the risk that bubbles are dispersed to a degree where they rise with a very low velocity, resulting in the formation of viscous foams, and eventual flooding. For the intermediate viscosity range multi-impeller agitators appear feasible (Linden et al., 1991). 

Typical equipment for low viscosity liquids consists of a vertical cylindrical tank, with a height to diameter ratio of l.5 to 2, fitted with an agitator. For low viscosity liquids, high-speed propellers of diameter about one-third that of the vessel are suitable, running at 10-25 Hz. Although work on single-phase mixing of low viscosity liquids is of limited... 

Small blade high speed agitators are used to mix low to medium viscosity liquids. Two of the most common types are the six-blade flat blade turbine and the marine type propeller shown in Figures 5.1 and 5.2 respectively. Flat blade turbines used to mix liquids in baffled tanks produce radial flow patterns primarily perpendicular to the vessel wall as shown in Figure 5.3. In contrast marine type propellers used to mix liquids in baffled tanks produce axial flow patterns primarily parallel to the vessel wall as shown in Figure 5.4. Marine type propellers and flat blade turbines are suitable to mix liquids with dynamic viscosities up to 10 and 50 Pa s, respectively. 

Propeller A propeller is a three- or four-bladed flow impeller, having helically shaped blades. The flow is primarily axial (discharge flow parallel to the agitator shaft) and is most effective in low-viscosity fluids. The marine-type propeller (Figure 3.15) is characteristically operated at relatively high speed, particularly in low-viscosity liquids (Treybal, 1980). 

Liquid ADMET monomers often become so viscous that magnetic stirring is impossible. There are several solutions to this. One is to attach a strong horseshoe or other magnet to a mechanical stirrer and rotate it very slowly under the vessel and temperature bath to slowly agitate the polymer melt. Another solution is to use a high viscosity mechanical stirrer if the polymer is a viscous melt Vacuum adapters for the stirring shaft are available, so that the vacuum method may be used with mechanical stirrers. 

This chapter deals with the agitation of liquids of low to moderate viscosity and the mixing of liquids, liquid-gas dispersions, and liquid-solid suspensions. Agitation and mixing of highly viscous liquids, pastes, and dry solid powders are discussed in Chap. 28. 

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