Impeller high-shear

High Shear High shear impellers take a variety of proprietary forms and are used primarily for producing emulsions. Their design maximizes the portion of the mixing energy dissipation which is classified as shear. High shear impellers are available for both tank and inline applications. 

Double- and Triple-Shaft Mixers The planetary mixer is an example of a double shaft mixer. However, many different combinations of mixing actions can be achieved with multi-shaft mixers. One variation on planetary motion involves replacing one anchor-style impeller with a high-shear impeller similar to the one shown in Fig. 18-47. The high-shear mixer can be used to incorporate powdered material effectively or create a stable emulsion leading to a final batch of viscous paste or fluid. 

If the application requires high interfacial area (i.e., small drop diameters), a high-shear impeller, such as the Rushton or radial disk turbine (RDT), is a good choice (Fig. 1). Acceptable substitutes include the Scaba and Chemineer s BT-6 and CD-6 impellers, commonly used for gas-liquid dispersion. If moderate, yet gentle shear is required, such as for emulsion polymerization, the retreat-curve impeller is commonly chosen. When larger drops of a narrow size distribution are required, the loop impeller is a reasonable choice. Broad-blade paddles are also used. 

There are numerous options for combining phospholipids with an aqueous phase, but two major methods are used to make liposomal systems for drug delivery. The first is simple hydration (swelling) of the phospholipid. This is followed by high-intensity agitation using sonication or a high-shear impeller. Liposomes are then sized by filtration, extrusion or elutriation. The second method is an emulsion... 

The ratio Rq = Q /Qr will determine how often particles recirculate and pass through the high shear impeller zone. For large values of R the inflow will dominate and relatively few particles will recirculate. If the density difference between the particles and fluid is sufficiently small and the particles themselves are small they will follow the fluid motion. 

There are four types of turbine impellers, which are characterized by the flow patterns and level of shear they create axial flow, radial flow, hydrofoil, and high-shear impellers. They have the widest use in low and medium viscosity liquid applications, solids suspension, liquid-liquid emulsification, and gas dispersion. Turbine impellers can have blades varying from 2 to 12 in number. Two blades are normally unstable mechanically, while it is difficult to install more than six blades on a hub. Axial flow impellers generally have three or four blades, and radial flow impellers are designed with six blades. 

Flow patterns of high-shear impellers, such as the bar turbine, Chemshear, and sawtooth impeller, are similar to those of radial flow impellers. The major difference is in lower pumping at higher shear. Backswept turbine and spring impeller also have similar radial flow patterns. It is important to understand the flow patterns around the impeller blades, where dispersion and attrition processes occur. Changing the blade geometry changes these flow patterns and alters the shear. 

Other high-shear impellers include the tapered blade ChemShear impeller and dispersing disks such as the Cowles impeller. These provide excellent shear, but far less flow than the RDT. They are used primarily in small scale batch applications where dispersion time is not critical. Pitched blade tmbines (PBT) are used when large density differences could lead to a suspension problem. They require higher speed to create the same drop size as the RDT, since they have a lower power number. The flow discharge angle for PBTs varies with Reynolds number and blade angle. 

Figure 21-9 High-shear impeller. (Courtesy of INDCO.)...

Impeller types range from high-flow/low-head impellers to high-shear impellers. Figure 22-7 shows a high-flow, low-head impeller that can be used for a wide variety of applications from mild blending to solid suspensions. This type... 

The impeller in Figure 22-11 is referred to as a high-shear impeller and is used in applications such as paint dispersion where very high shear is required. It has a much lower power number, on the order of 0.45, but it is run at a very high speed to produce the desired dispersion. 

Figure 22-11 High-shear impeller used when a fine dispersion is required Np = 0.45.

The activating solution was prepared immediately prior to use. 5 g of aluminosilicate were added to 100 g of the aetivating solution and stirred in a high shear impeller type mixer, until an increase in viscosity was observed, in this work, samples based on metakaolin are referred to as M series while those based on slag are referred to as S series. In cases where sodium aluminate was added, the sample name bears the suffix + Al . Table 2 presents the theoretical molar ratios for the mixtures used in this study. 

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