Power solids suspension

The overall superficial fluid velocity, mentioned earlier, should be proportional to the settling velocity o the sohds if that were the main mechanism for solid suspension. If this were the case, the requirement for power if the setthng velocity were doubled should be eight times. Experimentally, it is found that the increase in power is more nearly four times, so that some effect of the shear rate in macro-scale turbulence is effec tive in providing uphft and motion in the system. 

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. 

A. For blending and solid suspension, use Z/T for minimum power at about 0.6 to 0.7, but recognize that this may not be the most economical. 

It is well known that the critical impeller speed for solid suspensions is higher in the presence of a gas, depending mainly on the superficial gas velocity (Rewatkar et al., 1991). This is because of a decrease in the impeller power draw due to the formation of ventilated cavities behind the impeller blades on gassing. For example, for Rushton turbines, /)T//)a - 2-3.3 ... 

Suspension of solids is maintained by upward movement of the liquid. In principle, use of a draft tube and an axial flow impeller will accomplish this flow pattern most readily. It turns out, however, that such arrangements are suitable only for low solids contents and moderate power levels. In order to be effective, the cross section of the draft tube must be appreciably smaller than that of the vessel, so that the solids concentration in the draft tube may become unpractically high. The usually practical arrangement for solids suspension employs a pitched blade turbine which gives both axial and radial flow. 

For transport of a gas-solid suspension, it is important to be able to evaluate the pressure drop in order to estimate the power consumption. In a pneumatic conveying system, particles are usually fed into the carrying gas and accelerated by the gas flow. The power consumption (or pressure drop per unit pipe length) for the acceleration of particles can be significant compared with that for a fully developed flow condition. Moreover, the pressure... 

Much of the literature correlations for solids suspension are based on the so-called critical impeller speed. Attempts to duplicate experiments between various investigators often yield deviations of 30-50% from the critical speed shown by other investigators. Because power is proportional to speed cubed, power varies on the order of 2 to 3 times, which is not sufficiently accurate for industrial full-scale design. Therefore, many approximate, conservative estimates have been made in the literature as general guidelines for choosing mixers for solids suspension. Table IV is one such guideline for solid particles of a closely sized nature. 

The use of the new type of fluidfoil impeller has reduced the power required for solids suspension to about one-half to two-thirds of the values formerly used with 45° pitch blade turbines. 

FIGURE 18 Typical comparison of power required for axial flow impeller compared to radial flow impellers in solids suspension. 

As mentioned previously, axial flow impellers are typically used for solids suspension. It is also typical to use radial flow impellers for gas-liquid mass transfer. In combination gas-liquid-solid systems, it is more common to use radial flow impellers because the desired power level for mass transfer normally accomplishes solids suspension as well. The less effective flow pattern of the axial flow impeller is not often used in high-uptake-rate systems for industrial mass transfer problems. There is one exception, and that is in the aeration of waste. The uptake rate in biological oxidation systems is on the order of 30 ppm/hr, which is about to the rate that may be required in industrial processes. In waste treatment, surface aerators typically use axial flow impellers, and there are many types of draft tube aerators that use axial flow impellers in a draft tube. The gas rates are such that the axial flow characteristic of the impeller can drive the gas to whatever depth is required and provide a very effective type of mass transfer unit. 

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. 

P Power input to impeller Pc Critical power input for solids suspension... 

Dilatant Fluids. Dilatant fluids or shear-thickening fluids are less commonly encountered than pseudoplastic (shear-thinning) fluids. Rheological dilatancy refers to an increase in the apparent viscosity with increasing shear rate (3). In many cases, viscometric data for a shear-thickening fluid can be fit by using the power law model with n > 1. Examples of fluids that are shear-thickening are concentrated solids suspensions. 

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