Solids suspension impeller diameter

The larger the ratio of impeller diameter to tank diameter, the less mixer power required. Large, slow speed impellers require a low er horsepow er for a given pumping capacity, and solid suspension is governed by the circulation rate in the tank. 

Expert opinions differ somewhat on this factor. As a first approximation, the impeller can be placed at 1/6 the liquid level off the bottom. In some cases there is provision for changing the position of the impeller on the shaft. For off-bottom suspension of solids, an impeller location of 1/3 the impeller diameter off the bottom may be satisfactory. Criteria developed by Dickey (1984) are based on the viscosity of the liquid and the ratio of the liquid depth to the tank diameter, h/D,. Whether one or two impellers are needed and their distances above the bottom of the tank are identified in this table ... 

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. 

Mass-transfer characteristics on large-scale equipment seem to be quite similar, but the fluidfoil impellers tend to release a larger-diameter bubble than is common with the radial-flow turbines. The blend time is one-half or one-third as long, and solid-suspension characteristics are better so that there have been notable improved process results with these impellers. This is particularly true if the process requires better blending and there is solid suspension. If this is not the case, the results from these impellers can be negative compared to radial-flow turbines. 

There are analogies between Vjs, the minimum agitation intensity to suspend solids, and the corresponding variable, Vmm, for drop suspension. Both depend on density difference, continuous phase viscosity, and impeller diameter. However, Vjs depends directly on particle size, while Amin depends instead on interfacial tension and the other physical properties that determine drop size. 

In mixing process design for solid suspension, normally, once the proper type of impeller is selected, the major job is to tradeoff the impeller diameter with rpm to get desire vertical flow velocity. 

Geometry For a stirred tank, the geometry is cylindrical, with a small aspect ratio (the height of fluid in the tank [H], is one to three times the tank diameter [T]). Although many industrial vessels have a dished bottom (especially if solids suspension is involved), simulations to date have used the simpler flat bottom geometry. The impeller, of diameter D = T/4 to T/2, is placed at the desired off-bottom clearance (C). Around the tank walls, two to four rectangular baffles are evenly... 

Solid Suspension. Characteristic dissolution times increase by an order of magnitude if suspension does not occur. There exists a critical agitation speed for suspension, Nm- This critical suspension speed depends on the densities of the solid and liquid. A well-known correlation for suspension appears in equation (16), where is the solution viscosity, ps is the density of solid, pi, is the density of liquid, g is gravity, dp is the particle diameter, w is the weight percent solids and di is the impeller diameter (67). The critical agitation speed for high density... 

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