Impeller speed

Some studies (6) have been carried out to measure distribution of soHds in mixing tanks. Local soHds concentrations at various heights are measured at different impeller speeds. Typical data (Fig. 16) demonstrate that very high mixer speeds are needed to raise the soHds to high levels. At low levels, soHds concentration can exceed the average concentration at low mixer speeds. These soHds distributions depend on the impeller diameter, particle size, and physical properties. 

Fig. 16. SoHds concentration vs impeller speed at different heights.

N = impeller speed Breakage occurs when N > Nc,-Maximum enhancement before breakage was factor of 2.0. 

Constant impeller diameter Constant impeller speed... 

Where Q = volumetric flow of gas, ftVs N = impeller speed, rev/s D = impeller diameter, ft and T = tank diameter, ft. 

When macro-scale variables are involved, every geometric design variable can affect the role of shear stresses. They can include such items as power, impeller speed, impeller diameter, impeller blade shape, impeller blade width or height, thickness of the material used to make the impeller, number of blades, impeller location, baffle location, and number of impellers. 

It turns out that in low-viscosity blending the acdual result does depend upon the measuring technique used to measure blend time. Two common techniques, wliich do not exhaust the possibilities in reported studies, are to use an acid-base indicator and inject an acid or base into the system that will result in a color change. One can also put a dye into the tank and measure the time for color to arrive at uniformity. Another system is to put in a conductivity probe and injecl a salt or other electrolyte into the system. With any given impeller type at constant power, the circulation time will increase with the D/T ratio of the impeller. Figure 18-18 shows that both circulation time and blend time decrease as D/T increases. The same is true for impeller speed. As impeller speed is increased with any impeller, blend time and circulation time are decreased (Fig. 18-19). 

FIG. 18-19 Effect of impeller speed and power for the same diameter on circulation time and blend time for a particular impeller. 

FIG. 18-21 Effect of impeller speed on circulation time for a helical impeller in the Reynolds niimher arranged less than 10. 

Most paste and high-viscosity mixtures are nonnewtonian, with viscosity dropping with shear rate. Consequently, increasing impeller speed may be counterproductive, as the transmitted shear drops rapidly and an isolated hole may be created in the central mass without circulation of the bulk material in the remainder of the vessel. 

Increase solids mixing. Improve powder flowahihty of feed. Increase agitation intensity (e.g., impeller speed, fluidization gas velocity, or rotation speed). 

Decrease load to reduce wear Lower-formulation density. Decrease hed-agitation and compaction forces (e.g., mixer impeller speed, fliiid-hed height, bed weight, fluid-hed excess gas velocity, drum rotation speed). 

The common indices of the physical environment are temperature, pressure, shaft power input, impeller speed, foam level, gas flow rate, liquid feed rates, broth viscosity, turbidity, pH, oxidation-reduction potential, dissolved oxygen, and exit gas concentrations. A wide variety of chemical assays can be performed product concentration, nutrient concentration, and product precursor concentration are important. Indices of respiration were mentioned with regard to oxygen transfer and are particularly useful in tracking fermentation behavior. Computer control schemes for fermentation can focus on high productiv-... 

Fluid viscosity—For Newtonian fluids (a constant viscosity at all impeller speeds) approximate viscosities up to 5,000 centipoises are satisfactory. Above 5,000 centipoises. estimating errors of 20 to 50% can mean undersizing or oversizing the agitator. 

For the set vessel geometry and approximate impeller diameter, impeller speed is then calculated to satisfy process requirements. For process requirements as stated as a tip speed, impeller speed is given by the following relationship ... 

For a process requirement stated as a power unit per unit volume, impeller speed can be computed from ... 

Seleet impeller diameter for the larger system to aeeom-modate the system being mixed. This leaves the impeller speed N as the only independent variable. 

The models presented correctly predict blend time and reaction product distribution. The reaction model correctly predicts the effects of scale, impeller speed, and feed location. This shows that such models can provide valuable tools for designing chemical reactors. Process problems may be avoided by using CFM early in the design stage. When designing an industrial chemical reactor it is recommended that the values of the model constants are determined on a laboratory scale. The reaction model constants can then be used to optimize the product conversion on the production scale varying agitator speed and feed position. 

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