Laminar Blending

To achieve a defined blending result, it is a prerequisite that the entire volume of the vessel is involved in the laminar flow. The Reynolds number calculated with the impeller diameter represents only an inadequate description of the flow conditions in the entire vessel, particularly in the case of impellers with small zones of agitation. The dimensionless blend time in nO for these impellers therefore starts to increase at a point where the reducing Reynolds number theoretically still indicates turbulent flow (see Figpre 13.6, propeller D/T = 0.3). 

Multiple impeller systems with a large diameter ratio, such as Viscoprop impellers arranged one above another, are suitable for Re 50. 

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Fig. 30. Mechanism for laminar blending in Kenics static mixer (a) after one element, (b) two elements, (c) three elements, (d) four elements, and (e) five...

Figure 4. Hydrocarbon permeability in containers from polyethylene, polyethylene—nylon homogeneous blend and polyethylene—nylon laminar blend. (Reprinted with permission from Ref. 1. Copyright 1984 Technical Association of Pulp and Paper Industry.)...

Long-Term Permeability in Polyethylene-Nylon Laminar Walled Containers. Polyolefin containers often are subject to loss of properties with aging. These result from plasticization, crystallization, and stress cracking. Figure 8 shows the permeability properties of 66/6 copolyamide based "laminar" blends. The permeability performance was found to be unchanged during over 5 years of storage in the two separate samples shown here. 

In the case of liquid/liquid and gas/liquid dispersion, the above mixing mechanism does not hold and the operation is usually carried out in the turbulent region. The break-up of gas bubbles or liquid droplets to give a system of high interfacial area for mass transfer is brought about by the shear stresses in the system. These stresses are related to the pressure drop and hence flow rate through the mixer. Thus to get a smaller droplet size the fluid flow rate must be increased. It will not be effective to merely increase the number of elements (as was the case in laminar blending). 

Figure 7.9 Laminar blending in a Chemineer-Kenics in-line static mixer simplified mixing mechanism...

All the above analyses have considered the laminar blending of components of identical viscosities. This restriction was removed by Mohr et al who postulated that continuity of stress must be maintained across the interface between components and thus derived a modified version of equation (11.9), i.e. 

Whilst in-line static mixers act on the distributive mixing principle for their effectiveness in laminar blending, it has been mentioned earlier in this chapter that laminar shear mixing can be enhanced by redistribution, see Figure 11,7. Thus redistribution of the two components to be mixed between periods of laminar shear or elongation is very beneficial. 

So far the analyses in this chapter have centred upon laminar blending operations. In the following section the stresses generated in laminar flows will be considered and related to the dispersive mixing in solid/liquid and liquid/ liquid systems. 

In-line static mixing units operate by a redistributive mixing action when used for laminar blending operations. As discussed in section 11.4 this mechanism is essentially independent of flow rate and rheology. However, for dispersion duties it is the stress field which is important and here both flow rate and viscous properties will be critical. Further consideration of these mixing devices is given in Chapter 12. 

In order to assess the efficiency of mixing equipment which is used for laminar blending and dispersion duties it is necessary to obtain a measure of the... 

Although the concept of the static mixer was developed with laminar blending in mind there are probably now more applications in turbulent flow. It could be argued that an empty tube could provide a very suitable environment for the turbulent mixing process but a variety of applications have been found for the static mixer. It is often claimed that the static mixer offers a better solution than the empty pipe but the relationship between the two alternatives has not been much explored. For the present It would seem sensible to regard the static mixer as a device for raising the level of turbulence without changing pipe diameter or flow rate. 

Fig. 19. Permeability of toluene in nylon-polyethylene barriers. The data ( ) for a laminar blend is compared to theoretical permeabilities in a true laminate (upper curve) and in a homogeneous blend (lower curve). The permeability factor is the permeability in pure polyethylene divided by the permeability in the mixture.

Two methods have been used to identify the boundary between transitional and laminar blending, and they give similar results for the value of Reynolds number at the boundary. Wichterle and Wein (1981) made visualization studies of the flow in vessels agitated by Rushton and pitched blade turbines. They deflned two Reynolds numbers the value when motion first appears and the value when all stagnant zones disappear. The second definition is used here ... 

The most commonly used impeller for laminar blending applications is the helical ribbon. Other impeller types have been studied, including anchors and helical screws, but the helical ribbon is most effective. A helical ribbon impeller will have a large diameter, typically 90 to 95% of the vessel diameter. This ensures that the fluid is positively displaced by the ribbons. This is important because there is no mixing due to entrainment by eddies in the laminar regime. 

PE bottles exhibit a very poor gasoline permeation resistance, where about 25% of the filled gasoline penetrates out of a PE bottle in 14 days at 40 °C. Xylene, white spirit, cleaning naphtha, and many other pure and/or mixed hydrocarbon solvents easily penetrate PE containers such permeation results in pollution, safety, and health problems. The laminar blends of PE and PA, polyvinyl alcohol, and/or EVOH (with 32% ethylene), in the presence of modified PA or zinc-neutralised ethylene-acrylic acid copolymer as compatibilisers, exhibit significantly higher barrier properties as compared with PE or the conventional homogeneous hlends associated with uniform dispersed PA within a PE matrix [11]. 

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