Mixing impellers Types performance

It is seldom possible, either economically or timewise, to study eveiy potential mixing variable or to compare the performance of many impeller types. In many cases, a process needs a specific fluid regime that is relatively independent of the impeller type used to generate it. Because different impellers may require different geometries to achieve an optimum process combination, a random choice of only one diameter of each of two or more impeller types may not tell what is appropriate for the fluid regime ultimately required. 

This type unit [29] is used for a combination of pumping and mixing purposes. The unit has a closed disk on the top side. The feed flow into the unit comes from directly below the rotating impeller. The performance is dependent on the size of the unit and the physical location with respect to the distance up from the bottom of the vessel. As this clearance increases, the head decreases for constant flow and increases the pow er requirement. 

Ruszkowski, S., A Rational Method for Measuring Blending Performance, and Comparison of Different Impeller Types . Proceedings of the 8th European Conference on Mixing, Cambridge, UK, pp. 283-291 (1994). 

It is seldom possible, either economically or time-wise, to study every potential mixing variable or to compare the performance of many impeller types. 

The oxidation of the PE wax was carried out by babbling air into the polymer melt maintained under stirring at temperatures from 130 C to ITOOC. Due to the low molecular wei t of the PE wax and its low melt viscosity (10-30 cP at 130°C) the oxidation was performed in vessels fitted with a usual impeller-type stirrer. Prom a lab-scale glass vessel of two liters capacity it was scaled up to a stainless-steel reactor of one cubic meter capacity (Figure 1). The main requirement for the oxidation vessel was the need to assure a very intensive mixing of the air bubbles in the wax melt. 

Ruszkowski S. (1994) Rational method for measuring blending performance and comparison of different impeller types. In Proceedings of Eighth European Mixing Conf., IChemE, Rugby, UK. p 283-291. 

Influence of Impeller Type and Bulk Mixing on Xanthan Fermentation Performance... 

Not only is the type of flow related to the impeller Reynolds number, but also such process performance characteristics as mixing time, impeller pumping rate, impeller power consumption, and heat- and mass-transfer coefficients can be correlated with this dimensionless group. 

Figures 5-3 and 5-5 illustrate a few of the types of impellers used for mixing. They may be basically classified as axial, radial and mixed. In general the most generally applicable are the. 3-bladed propeller, the flat-blade turbine, the curved blade turbine, and the paddle. The many other designs are either modifications of these or specially designed for a very special purpose with respect to a fluid system and/or its performance.

One of the complications in stirred tank flows is the presence of macroinstabilities (i.e., low-frequency mean flow variations) that may affect the mixing performance. Various authors have distinguished between various types and investigated their occurrence and their frequencies under varying operating conditions and with several types of vessels and impellers (Yianneskis et al., 1987 Haam et al., 1992 Myers et al., 1997 Hasal et al., 2000 Nikiforaki et al., 2002). 

Another important development which altered our view of crystallization processes was the realization of the importance of secondary nucleation due to contact between crystals and the impeller and vessel. Secondary nucleation of this type has been shown (2-6) to often have a dominant role in determining crystallizer performance. Our understanding of crystal growth, nucleation, fluid mechanics and mixing have all greatly improved. A number of review (2r 101 have appeared in recent years which describe the advances in these and... 

In the research laboratory it is important to recognize the effect of mixing on reaction rate or on other performance criteria. Energy must he supplied to produce fluid motion, thus, to compare mixing with different equipment or with different sizes of the same type impeller, it is essentia] that the comparisons be made on the basis of equal power input. 

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