Power consumption in stirred vessels

From a practical point of view, power consumption is perhaps the most important parameter in the design of stirred vessels. Because of the very different flow patterns and mixing mechanisms involved, it is convenient to consider power consumption in low and high viscosity systems separately. 

In equation (8.4), P is the impeller power, that is, the energy per unit time dissipated wifliin the liquid. Clearly, the electrical power required to drive flie motor will be greater than P on account of transmission losses in the gear box, motor, bearings and so on. 

It is readily acknowledged that the functional relationship in equation (8.4) cannot be estabhshed from first principles. However, by using dimensional analysis, the number of variables can be reduced to give  

In equation (8.6), the Froude munber is generally important only when vortex formation occms, and in single phase mixing can be neglected if the value of the Reynolds number is less than about 300. In view of the detrimental effect of vortex formation on the quality of mixing, tanks are usually fitted 

Power curves for many different impeller geometries, baffle arrangements, and so on are available in the literature [Skelland, 1983 Hamby et al., 1992 Tatterson, 1992, 1994 Ibrahim and Nienow, 1995], but it must always be remembered that though the power cmve approach is applicable to any single phase Newtonian liquid, at any impeller speed, the curve will be valid for only one system geometry. Adequate information on low viscosity systems is now available for the estimation of power requirements for a given duty under most conditions of practical interest. 

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Ascanio, G., Castro, B., and Galindo, E. (2004), Measurement of power consumption in stirred vessels—a review, Chemical Engineering Research and Design Transactions of the Institution of Chemical Engineers Part A, 82(A9) 1282-1290. 

Power consumption for mechanical foam breakdown is smaller in stirred vessels containing antifoams due to decreased hold-up... 

In Figure 11.2 a schematic view of a stirred vessel is given. The vessel is cyhndrical with a height (m) and a diameter T (m). Usually is equal to or greater than 2 T. It is equipped with a stirrer in the lower compartment. TTiis stirrer is mounted near the bottom, usually at a distance equal to the stirrer diameter. At a lower position the stirrer and bottom interact, leading to a decrease in power consumption. At a higher position hquid circulation problems can occur because, at increased gas flow rate in case of aeration, the bubbles will not be recirculated in the lower compartment. Sometimes the upper compartment (s) are also equipped with a stirrer. The vessel is equipped with baffles to prevent rotation of the contents as a whole. For aeration an air sparger is mounted below the stirrer. For mass transfer... 

The power density Py is the characteristic quantity of turbulent flow. It determines the size of the smallest eddies and the intensity of microturbulence. In addition, it is a measure of the shear intensity in laminar flows or the intensity of cavitation in ultrasonic fields (see above). The power input P in the dispersion zone can be derived from the pressure drop (e.g. in pipes and nozzles) or can be measured caloricafly (e.g. for rotor-stator systems and ultrasonication Pohl 2005 Kuntzsch 2004). Additionally, P can be roughly approximated by the electric power consumption of the dispersion machine (e.g. for ultrasonication Mandzy et al. 2005 Sauter et al. 2008), even though the real values may be lower by a factor of 2 to 5. A further source of uncertainty is the volume of the dispersion zone (Vdisp). since the stress intensities are not uniformly distributed in dispersion apparatuses. In particular, this applies to agitated vessels, where the highest dissipation rates are obtained in the vicinity of the stirring instmment (Henzler and Biedermann 1996),... 

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