Stirring stirrer power, determination

Fluid flow in a stirred tank with anchor stirrer can be characterized as flow past a horizontal plane [310] induced by the vertical arms of the close-clearance stirrer. A numerical algorithm of the two-dimensional Newtonian flow past a horizontal plane was established using an iterative method for the determination of the boundary values of the stream function. The flow profile was determined with measurements and the stirrer power derived therefrom. The correspondence between the measurements and calculations was excellent. 

In experiments [141, 566] in three baffled stirred tanks with turbine stirrers (V = 1 20 100 1), the position of the feed point for the add and the stirrer power were varied. The micro-mixing was determined according to the ViUermaux s method. The results in Fig. 1.26 show a fairly weak dependence of the degree of micromixing a upon thepower per unit mass e ... 

The uneasiness involved in basing the design of very large stirred tanks for homogenization on the expression n3 = const, had already been mentioned years ago by Kipke. In his paper [277] over outstanding problems in stirring, he pointed out, that the macro-scale of turbulence depended upon the size of the stirrer, whereas micro-scale of turbulence was merely a function of the dissipated stirrer power per unit volume and hence independent of scale (see Section 1.4.2). In scale-up of a stirred tank to industrial-scale (e.g. 12 0 x 26 m V = 3000 m ), if P/V = const the micro-scale of turbulence does not change (micro-eddies with dimensions A % 0.2 mm), whereas its macro-scale, which was characterized by the size of the primary eddy A, which is A w 0.08 D here, determines the primary eddy of ca. 0.7 m diameter, whose size would therefore far exceed the size of the experimental tank. 

In order to design and dimension stirrers for the homogenization of liquid mixtures - and this is by far the most common task when it comes to stirring - it is vital to know the power characteristic and the mixing time characteristic of the type of stirrer in question. If this information is available for various types of stirrers, it is possible to determine both the best type of stirrer for the given mixing task and the optimum operating conditions for this particular type. 

The rotational speeds of the six-bladed stirrers that were used corresponded to power inputs of 0.02 and 0.18 watt/liter at speeds of 125 and 250 rpm, respectively. Concentrations of oxalic acid during precipitation and crystal growth in the stirred tank reactors were varied by changing the flow ratio of oxalic acid-to-waste solution while maintaining the nitric acid concentration constant at 0.9 fl. Permissible nitric acid concentrations for the 0PIX process (4, 6, 7) are 0.5 to about 1.0 M HNO3. Yields of precipitate were determined on the basis of praseodymium recovery. Tracer 142pr (half-life = 19.2 day, 1.6-MeV y-ray) was used to measure yields. 

The emulsifying capacity is represented by the volume of oil (cm3) that is emulsified in a model system by 1 g of protein when oil is added continuously to a stirred aliquot of solution or dispersion of the tested protein. It is determined by measuring the quantity of oil at the point of phase inversion. The latter can be detected by a change in color, viscosity, or electrical resistance of the emulsion, or the power taken by the stirrer engine. The emulsifying capacity decreases with an increasing concentration of protein in the aqueous volume. It is affected by the parameters of emulsification, depending on the equipment, as well as by the properties of the oil. 

Ne, the so-called Newton number, is tabulated in the standard literature for numerous types of agitators [58,59]. The revolution rate of the stirrer determines to the first power the conductive heat removal rate and to the third power the dissipated stirring power, respectively. 

If the correlations are well established to determine the agitation power in absence of a gas phase, Pq> for different stirrers (71), the situation is more complex for the case of aerated tanks. Most authors propose to represent the decrease of the agitation power in presence of a gas phase at the same stirring velocity, by ccjrrelations between Pg/P aerated number... 

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