Mixer characteristics

Correlations of nucleation rates with crystallizer variables have been developed for a variety of systems. Although the correlations are empirical, a mechanistic hypothesis regarding nucleation can be helpful in selecting operating variables for inclusion in the model. Two examples are (/) the effect of slurry circulation rate on nucleation has been used to develop a correlation for nucleation rate based on the tip speed of the impeller (16) and (2) the scaleup of nucleation kinetics for sodium chloride crystalliza tion provided an analysis of the role of mixing and mixer characteristics in contact nucleation (17). Pubhshed kinetic correlations have been reviewed through about 1979 (18). In a later section on population balances, simple power-law expressions are used to correlate nucleation rate data and describe the effect of nucleation on crystal size distribution. 

Table 3.1 Summary of mixer characteristics for free-flowing powders (Williams)...

The arrangement of the loop reactor (with perfect mixer characteristics) and the post-reaction coil (approaching plug flow characteristics) will make sure that the maximum conversion of the LAB feedstock is achieved. All equipment is constructed in stainless steel 316L. 

The above covers most conventional mixers there is another class of mixers, called pump-mix impellers, where the impeller serves not only to mix the fluids, but also to move the fluids through the extraction stages. These are speciahzed designs, often used in the metals extraction industries. For these types of impellers, a knowledge of the power characteristics for pumping is required in addition to that for mixing. For a more detailed treatment of these special cases, the reader is referred to Lo et al. 

Packed Tubes Cocurrent flow of immiscible hquids through a packed tube produces a one-stage contact, characteristic of hne mixers. For flow of isobutanol-water through a 0.5-in diameter tube packed with 6 in of 3-mm glass beads, Leacock and Churchill [Am. Jn.st. Chem. Eng. J., 7, 196 (1961)] find... 

Table 19-2 includes impacl velocities for some internal rotating devices in tumblers as well as other mixers. Contamination and wear problems of internal rotating devices are discussed under Performance Characteristics. ... 

In order to be effective in imparting various chosen characteristics, all additives employed in a blend must be homogeneously dispersed throughout the blend. The two most commonly employed pieces of equipment for blending rubber and additives are rubber mills and a special type of mechanical mixer known as the Banbury mixer. A typical rubber mill consists of two rolls which rotate toward each... 

The forces applied by an impeller to the material contained in a vessel produce characteristic flow patterns that depend on the Impeller geometry, properties of the fluid, and the relative sizes and proportions of the tank, baffles and impeller. There are three principal types of flow patterns tangential, radial and axial. Tangential flow is observed when the liquid flows parallel to the path described by the mixer as illustrated in Figure 7. 

A practical method of predicting the molecular behavior within the flow system involves the RTD. A common experiment to test nonuniformities is the stimulus response experiment. A typical stimulus is a step-change in the concentration of some tracer material. The step-response is an instantaneous jump of a concentration to some new value, which is then maintained for an indefinite period. The tracer should be detectable and must not change or decompose as it passes through the mixer. Studies have shown that the flow characteristics of static mixers approach those of an ideal plug flow system. Figures 8-41 and 8-42, respectively, indicate the exit residence time distributions of the Kenics static mixer in comparison with other flow systems. 

The dispersed plug flow model has been successfully applied to describe the flow characteristics in the Kenics mixer. The complex flow behavior in the mixer is characterized by the one-parameter. The Peclet number, Np, is defined by ... 

The Kenics mixer has been shown to provide thorough radial mixing. This results in a reduction in radial gradients in velocity, composition, and temperature. Because the unit possesses nearly plug flow characteristics, both temperature and product quality controls are achieved. 

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