Shear rate static mixers

Static mixing of immiscible Hquids can provide exceUent enhancement of the interphase area for increasing mass-transfer rate. The drop size distribution is relatively narrow compared to agitated tanks. Three forces are known to influence the formation of drops in a static mixer shear stress, surface tension, and viscous stress in the dispersed phase. Dimensional analysis shows that the drop size of the dispersed phase is controUed by the Weber number. The average drop size, in a Kenics mixer is a function of Weber number We = df /a, and the ratio of dispersed to continuous-phase viscosities (Eig. 32). 

The hydrodynamic enviroiunent in a static mixer is characterized by a more uniform distribution of energy and shear rates than in a conventional agitated tank. Thus, for dispersion processes, a static mixer can produce a narrower drop or bubble size distribution. Also, the narrower residencetime distribution in a static mixer makes it the preferred mixing device for processes involving fast reactions or polymerizations requiring no back-mixing. 

The characteristics of the microstructure formed (such as emulsion droplet size) are dependent on the type of microstructure, type of deformation (shear, extension, or combined), and deformation rate as well as the TSVs. In order to maximize the fluid microstructure/flow field interactions, the flow field must be uniform which requires the generation of the flow field over a small processing volume. There are several types of equipment such as multiple expansion contraction static mixer (MECSM) or its dynamic... 

Flow also induces the formation of micro-emulsions, as it can detach the pinch-offs from the interface. As shown by Kim et al. [76] for a bilayer of reactive PS-end-carboxylic and PMMA-ran-epoxy, micro-emulsions could be observed in a shear fiow under a shear rate of 100 s after less than 50 min, while for static conditions they would form after 15 h [57[. In the case of melt blends prepared in internal mixers, micro-emulsions could be detected after less than 5 min of mixing, thus demonstrating again the importance of fiow [68[. 

Figure 2. Shear stress as a fimction of shear rate at 350 C Coiiq)arison of twin-screw and static mixer mixed PAI-2/LCP blends.

Although density is essentially a static property, absolute (or dynamic) viscosity is a dynamic property and tends to reduce in magnitude as the shear rate in a pipeline increases. Thus, engineers have had to define different forms of viscosity over the years, everything from dynamic viscosity, to kinematic viscosity, to effective pipeline viscosity. The effective pipeline viscosity will be discussed in detail in Chapters 3, 4, and 5. In this chapter, the reader is introduced to basic concepts of the mixture of slurry in a stationary state. This is effectively what the pump, or a mixer, might see at the start-up of a plant. As is often the case, when the driver cannot deliver enough torque to overcome the absolute (or dynamic) viscosity, the operator is forced to dilute the slurry mixture. 

Following the 30-day injectivity test, one polymer product (Polymer Gl) showed superior performance for full-scale use. This product required lower concentrations of polymer and no additional inversion chemicals to meet the design viscosities. However, the solution failed some of the core plugging tests. During the final week of the 30-day test, the failure rate of the solution was improved by applying additional shear to the mixed concentrate stream with an in-line static mixer. With the Incentive that solutions made with Polymer Cl could pass the core plug test by additionally shearing the concentrate. Polymer Cl was selected for full-scale use. 

The flow patterns in static mixers are very complex and only finite element methods can be used to analyze the details of these flows. The shear rates generated in these devices are rather low (on the order of 10 s ), and hence, the pressure drop across the static mixer can be estimated from the expression for flow of a Newtonian fluid through a tube with a correction factor for the given geometry as... 

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