Impeller Speed and Power

The impeller speed recommended will in general be higher than Njs, the speed required for the just suspended state estimated by the Zwietering correlation. The speed required should be based on experimental data. For quick estimates of the speed and power requirements for complete uniformity, the ratios in Table 10-2 may be applied to the estimated value of Njs. 

Design Tip. For multiprocess batch reactors, mixers equipped with variable speed drives permit the mixer to be operated at different impeller speeds to accommodate the different mixing needs of the various steps in the process. 

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

Impeller- fluid-solid interactions Impeller speed and power input solid tree or fixed. If free, are the particles suspended and if so, what are their spatial and velocity distributions What is the liquid flow pattern and the distribution of turbulence intensity throughout the vessel Does surface aeration occur What is the effect of solid inventory ... 

Westerterp et al, 1963), there is a minimum impeller speed for efficient operation above this, kLaL can be expressed as the sum of two terms, one related to impeller speed and diameter and vessel dimensions, and the other depending on the power per unit volume and the gas rate. The correlations for kLaL presented in this study are outlined in Table XXI. 

For equal tip speed and impellers with geometric similarity, the rotational speed remains the same in spite of the added liquid level. Thus, the power and torque remain the same for the same impellers, speed, and fluid properties. However, the volume change affects power per volume 1000(2.66 hp)/7000gal = 0.38 hp/1000 gal (75.0 W/m3). Similarly, the torque per volume is reduced to 1000(3193 in-lb)/7000 gal = 456 in-lb/1000 gal (13.6 N-m/m3). Both changes represent a substantial reduction in agitation intensity from the pilot-scale results. 

These ideas of impeller flow, head and power input as related to operating variables have some merit for a qualitative description of the effects of the operating variables on the process. However, it requires extensive experience, and usually actual experiments, to decide whether a system performance is favored by a particular combination of flow and head. (Rushton and Oldshue (R12) note that high values of Q/3Care preferred for blending and solid suspension, low ratios for liquid-liquid and gas-liquid operations.) This approach still requires the systematic study of impeller speed and diameter as process variables. 

Impeller power consumption can be calculated as a product of the direct torque, rotational impeller speed, and a coefficient (usually equal to 2n times a unit conversion factor, if required). 

In 2001, Holm, Schaefer, and Larsen have applied the Leuenberger method to study various processing factors and their effect on the correlation between power consumption and granule growth. They have found that such a correlation did indeed exist but was dependent, as expected, on the impeller design, the impeller speed, and the type of binder. The conclusion was that it was possible to control the liquid addition by the level detection method whereby the liquid addition is stopped at a predetermined level of power consumption. An alternative approach involves an inflection point (peak of the signal derivative with respect to time). 

A fully turbulent, baffled vessel is to be scaled up by a factor of 512 in volume while maintaining constant power per unit volume. Determine the effects of the scaleup on the impeller speed and mixing time. 

Parameters such as impeller speed and shaft power (in a stirred bioreactor) and fluid velocity are indicators of the degree of mixing and thus play an important role in the control of mass transfer. Impeller speed is easily monitored with a tachometer (electronic or mechanical) [39], but the measurement of shaft power input is not as straightforward. The most common method utilizes a torsion dynamometer attached to the impeller drive however, this technique includes losses due to friction in the drive shaft. Better data can be obtained from balanced strain gauges mounted on the impeller [37]. On-line measurement of the liquid velocity in a flowing or stirred system can be obtained by a heat-pulse method in which a resistance thermometer is used to measure a brief temperature increase caused by an upstream pair of electrodes [43]. Use of this sensor system has been limited to laboratory applications. 

It is worth noting that the calculation of the power requirement demands a knowledge of the impeller speed and it is this value which is difllcult to determine, e.g. ... 

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