Impeller Intermig

These types of agitator are used in low-viscosity systems (ji < 50 kg m 1 s-1) with high rotational speed. The typical tip speed velocity for turbine and intermig is in the region of 3 m s 1 a propeller rotates faster. These impellers are classified as remote clearance type, having diameters in the range 25-67% of the tank diameter. 

Twelve common impeller types are illustrated in Figure 10.2. Impellers (a) through (i) and (k) in Figure 10.2 are available worldwide. Impellers (j) (the Intermig) and (1) (the Coaxial [Paravisc Outside and Viscoprop inside]) are available only from Ekato. Key factors to aid in selection of the best impeller to enhance desired process result(s) are as follows ... 

The Ekato intermig impeller has reverse pitch on the inner and outer blades and they are almost always used with multiple impellers. They are used at high D/T and promote a more uniform axial flow pattern than other turbine impellers. They are advertised to be very effective for solids suspension, blending, and heat transfer in the medium viscosity range. Lower Nrc limit not given by Ekato (9), perhaps 5. 

Ekato MIG Ekato INTERMIG Fig. 2 Axial flow impellers. (From Ref... 

The workhorse impeller is the pitched-blade mrbine because of its ability to create good circulation at relatively low shear. These attributes help reduce secondary nucleation and crystal breakage while achieving good suspension and circulation. The flat-blade turbine is less versatile because of high shear and less overall circulation. The Ekato Intermig has proven to have superior performance in some crystaUization operations because of its combination of excellent circulation and low shear. 

The final process utilizes simultaneous addition of A and B over a 5 hour period to a seed bed from the previous batch ( -—15%). The impeller selected was the Ekato Intermig for the same reasons as in Example 10-1. 

Kipke [276] investigated the effect of different stirrer types and their d/D ratios on the droplet size and its distribution. This study utilized the already mentioned coalescence-prone camauba wax/water system (type 2442 p = 825 kg/m, pj = 2 mPa s at 95°C). The wax/water dispersion could be frozen in with ice water and the droplet size distribution determined by sieving. The stirrer types investigated were 2-stage Intermig d/D — 0.5 0.6 0.7) propeller stirrer (d/D = 0.31 0.37) pitched-blade turbine d/D = 0.31) 6-blade turbine stirrer, Pfaudler impeller stirrer (d/D = 0.575). The experimental data are presented in Fig. 6.7. It is evident that in this material system the droplet size distribution extended to dp/d32 = 0.4-1.5. 

Figure 8.20 Composite impellers used for gas-liquid dispersions (Left) The Intermig, (right) The Scaba SRGT impeller...

This scaling, however, introduces a factor of (D/T). This may work well where the bulk characteristics of the flow dominate, but it is not an accurate measure of turbulence if local characteristics are needed. For the same power input per unit tank volume, or holding eq. (2-18) constant with variations in impeller type, diameter, and off-bottom clearance, Zhou and Kresta (1996a) provided an extensive set of data and showed that the local dissipation can vary by up to a factor of 100. This is illustrated for the Intermig on the Visual Mixing CD affixed to the back cover of the book. The best order-of-magnitude estimate of the maximum dissipation uses the swept volume of the impeller instead of the total tank volume ... 

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