Motionless mixer

Solve Equation (8.12) using the methods described in this chapter and ignoring the presence of the mixing elements. 

When an axial position corresponding to four mixing elements is reached, calculate the mixing-cup average composition 

Restart the solution of Equation (8.12) using a uniform concentration profile equal to the mixing-cup average, a(z) = 

Repeat Steps 2 through 4 until the end of the reactor is reached. 

FIGURE 8.6 Coimnerical motionless mixers. (Drawing courtesy of Professor Pavel Ditl, Czech Technical University.) 

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Motionless inline mixers obtain energy for mixing and dispersion from the pressure drops developed as the phases flow at high velocity through an array of baffles or packing in a tube. Performance data on the Kenics (132) and Sul2er (133) types of motionless mixer have been reported. 

Inline motionless mixers derive the fluid motion or energy dissipation needed for mixing from the flowing fluid itself. These mixers iaclude orifice mixing columns, mixing valves, and static mixers. 

Motionless mixers continuously interchange fluid elements between the walls and the center of the conduit, thereby providing enhanced heat transfer and relatively uniform residence times. Distributive mixing is usually excellent however, dispersive mixing may be poor, especially when viscosity ratios are high,... 

The commonly used types of mixing equipment can be placed in the broad categories (1) mechanical agitators, (2) inline motionless mixers, (3) tank jet mixers, and (4) miscellaneous. The nature and type of agitator used depends upon the scale and type of mixing and upon the fluids being mixed. The broad classes of impellers are ... 

Figure 5-45. Lightnin lnliner< > motionless mixer. By permission, Lightnin (formerly Mixing Equipment Co.), a unit of General Signal.

Figure 5-47. Luwa Blendrex motionless mixer. By permission, Luwa Corporation.

For some fluid systems, the motionless mixer may not practically achieve total homogeneity. In some situations of widely diverse fluid densities, the centrifugal motion created may throw some of the fluid to the outside of the flow path when it emerges from the unit. These are concepts to examine with the manufacturer, as only the manufacturer s data can properly predict performance, and the design engineer should not attempt to actually physically design a unit. 

Heywixh). N. I.. Viney, L. J., and Stewart, 1. W. Inst. Chem. Eng. Symposium Series No. 89. Fluid Mixing ]] (1984) 147. Mixing efficiencies and energy requirements of various motionless mixer designs for laminar mixing applications. 

This section considers three special cases. The first is a flat velocity profile that can result from an extreme form of fluid rheology. The second is a linear profile that results from relative motion between adjacent solid surfaces. The third special case is for motionless mixers where the velocity profile is very complex, but its net effects can sometimes be approximated for reaction engineering purposes. 

The models of Chapter 9 contain at least one empirical parameter. This parameter is used to account for complex flow fields that are not deterministic, time-invariant, and calculable. We are specifically concerned with packed-bed reactors, turbulent-flow reactors, and static mixers (also known as motionless mixers). We begin with packed-bed reactors because they are ubiquitous within the petrochemical industry and because their mathematical treatment closely parallels that of the laminar flow reactors in Chapter 8. 

Run an Internet search on static and motionless mixers to learn more about the utility of these devices, but be wary of the hype. 

Web-coating polymerizations (e.g., as used for photographic film and coated abrasives) literally achieve a piston flow reaction environment. Mechanically driven screw devices used as finishing reactors for PET closely approximate piston flow. Motionless mixers can do this as well. However, polymer reactors that closely approximate piston flow are the exception. 

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