Impellers systems

Critical speed the mixer shaft speed which matches the first lateral natural frequency of the shaft and impeller system. Excessive vibrations and shaft deflections are present at this speed. 

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. 

Figure 11 shows the reference floe diameter for viscometers as a function of shear stress and also the comparison with the results for stirred tanks. The stress was determined in the case of viscosimeters from Eq. (13) and impeller systems from Eqs. (2) and (4) using the maximum energy density according to Eq. (20). For r > 1 N/m (Ta > 2000), the disintegration performance produced by the flow in the viscosimeter with laminar flow of Taylor eddies is less than that in the turbulent flow of stirred tanks. Whereas in the stirred tank according to Eq. (4) and (16b) the particle diameter is inversely affected by the turbulent stress dp l/T, in viscosimeters it was found for r > 1.5 N/m, independently of the type (Searle or Couette), the dependency dp l/ pi (see Fig. 11). 

Fig. 22. Dependency of average particle diameter dp on maximiun energy dissipation of impeller systems with baffles by stirring of biological and model particle systems explanations see Tables 3 and 4...

Gogate, P.R., Beenackers, A.A.C.M. and Prandit, A.B. (2000) Multiple impeller systems with a special emphasis on bioreactors a critical review. Biochemical Engineering Journal, 6, 109-144. 

For single-impeller systems, the correlation of Calderbank can be used (Bouaifi et al., 2001 Laakkonen et al., 2005) ... 

The ratio Pq/Pq for flat-blade turbine impeller systems can also be estimated by Equation 7.35 [8]. 

In multiple impeller systems, it appears that conditions in the region around the impeller nearest to the point of gas injection have the greatest influence on system performance. In view of the work of Tereshkevitz, which showed a large influence of bubble size (as compared with liquid shear) on mass transfer, a tentative conclusion is that the average bubble size is determined mostly by the conditions near the gassed impeller. It follows that primary emphasis in process development and scale-up should be placed upon selection of the impeller and the gas-inlet conditions variations in the over-all system geometry, with the possible exception of liquid depth, may be of secondary importance. 

Fig. 8 k- Factor of multiple impeller system (Rushton -I- PBT) as a function of aeration number. (View this art in color at h>h>h>. dekker.com.)... 

Typical values for the impeller clearance off the vessel bottom are 7/4 for hydrofoil impellers and 7/3 for pitched-blade turbines. Hydrofoil impellers may be a poor choice when solid suspension is accompanied by other mixing duties, such as liquid-liquid dispersion or gas dispersion. For such cases, a multiple impeller system consisting of a high-efficiency impeller in combination with a 45"" pitched-blade impeller should be evaluated. Small pitched blade impellers with a... 

This has led to the common practice today of using the distribution of power in a three-impeller system, for example, 40% to the lower impeller and 30% to each of the two upper impellers. Fig. 35. 

For typical reactive mixing systems in a stirred tank, a dual impeller system, Figure 9.23, with a radial-flow impeller at the bottom and an axial flow on the top is recommended [31-33], We recommend the following ratios as initial starting points for a typical batch with liquid height equal to tank diameter ... 

Figure 9.24 shows that for a typical 1-hp/lOOO-gal (0.2 kW/m ) power input, the blend time would go up from about 0.1 min in a 1-gal reactor to 0.8 min in a 10,000-gal reactor. Would this type of increase have any detrimental effect on the reaction system Of course the answer depends on the specific chemistry. Comparative mixing times are given for different impellers. These are shown in the following equation for the correction factor, CF. If the mix time is known for a given impeller system, multiplying this time by the CF gives the mix time for a different system ... 

Vortex and large cavities on upper impellers (multiple impeller systems) develop when FlgFr X 0.2 (Figure 9.30(c))... 

The flow in an anchor agitated vessel has been studied in detail by Peters and Smith . The impeller promotes fluid motion close to the vessel wall but the region near the shaft may be relatively stagnant. In addition, there is little top-to-bottom turnover. In order to promote a top-to-bottom motion a helical ribbon may be used and often a screw is added to the shaft to obtain motion in the central regions of the vessel. This combined impeller system would have a ribbon pumping upward near the wall of the vesel with the screw, twisted in the opposite sense, pumping downwards near the shaft. 

Thus if this model is correct equal values of Pe give equal concentration profi les. Magelli et al. have extended this theory to multiple impeller systems in vessels of aspect ratio greater than one. 

Multiple impeller systems are able to distribute energy throughout the reactor more efficiently, which leads to a more homogeneous shear rate distribution. Liquid circulation and gas dispersion are also improved, leading to longer gas-phase residence times. These factors lead to better gas utilization, higher gas-liquid mass transfer coefficients (Bouaifi and Roustan, 2001 Cabaret et al., 2008 Fujasova... 

The impeller is the key component for proper STR operation, especially for multiple impeller systems. A proper selection procedure has to consider numerous options and their applicability to the particular process of interest. A mixed... 

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