Impeller diameter, influence

It is of interest here to discuss ideas proposed by Rushton and Oldshue (R12), on how to interpret observations of the influence of impeller diameter. Many mixing systems operate in an impeller Reynolds number range such that the power input P is proportional to NBDS ... 

The preceding paragraphs have not considered the possible influences of geometric factors like impeller height, liquid depth, etc. As a starting point it seems reasonable to use the widely adopted square batch, with liquid height equal to vessel diameter, and to set the impeller one impeller-diameter above the vessel bottom. Later, one should check experimentally to see if any of these factors is influencing the process performance. 

Impeller size (DIT ratio) influences whether flow or turbulence governs the process of mixing. Oldshue [5] shows, for equal process results, that impeller diameter affects power and torque characteristics, as shown in Figure 9.5. The values for power and torque are normalized to the values for an impeller with DIT = 0.333. 

Table 62 The influence of the impeller diameter on the temperature distribution in a stirred tank reactor...

With the exception of the density difference, the influence of fluid and particle properties on Njs is not large, as indicated by the small exponents on the kinematic viscosity, v, the particle diameter, dp, and the solid loading parameter, X, in eq. (10-11). The density difference is the property with the largest influence on Njs. Its exponent reflects the effect of the terminal settling velocity of the particles. The exponent on the impeller diameter, D, represents the effect of scale. Note that an exponent of —0.67 on D would imply a scaling rule based on power per volume. 

As can be seen even from the kinetic curves in Fig. 3, the type of impeller has a decisive influence on particle disintegration in stirred tanks. This is particularly clear from a comparison of other impeller systems on the basis of the reference particle diameter dp in Fig. 4. 

Contrary to commonly held opinion (e.g. [62, 63]), this comparison shows that axial-flow impellers such as pitched-blade impellers and propellers lead to particularly high stresses at the same specific impeller power. The impeller geometry, such as the impeller-to-tank diameter ratio d/D and the relative blade height h/d (see Fig. 5), also has a distinct influence. 

Fig. 4. Influence of impeller type on stress Reference floe diameter dpv in dependency on specific impeller power P/V 4 baffles w/D = 0.1 H/D = 1 D = 0.4 m...

A semibatch reaction crystallizer with a profiled bottom and/ or a draft tube was used by Stavek et al. (1988) to study the influence of hydrodynamic conditions on the controlled doublejet precipitation of silver chloride microcrystals. The crystallizer is a cylindrical vessel (14cm in diameter) equipped with four radial baffles and a six-pitched-blade impeller to provide good mixing. Silver nitrate and potassium chloride solutions were added at a constant rate to the crystallizer, which contained aqueous gelatin solution. The temperature and silver ion concentration in the crystallizer were controlled during the run. 

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