Mixing impeller treatment

The above covers most conventional mixers there is another class of mixers, called pump-mix impellers, where the impeller serves not only to mix the fluids, but also to move the fluids through the extraction stages. These are speciahzed designs, often used in the metals extraction industries. For these types of impellers, a knowledge of the power characteristics for pumping is required in addition to that for mixing. For a more detailed treatment of these special cases, the reader is referred to Lo et al. 

A propeller with three blades and a square pitch is installed centrally in a baffled vessel. The vessel is 2.0 m in diameter. The propeller is positioned 60 cm from the bottom of the vessel. The tank is tilled to a depth of 2.0 m and is mixing alum with raw water in a water treatment plant. The water is at a temperature of 25°C and the turbine is running at 3.24 hp at an rpm of 100. What is the diameter of the impeller ... 

In sewage treatment plant processes as well as in chemical and mining processes, gas is sparged in the hquid. The impeller of the mixer is then used to provide dispersion of the gas and circulation of the tank contents. For radial machines, a small radial impeller is installed at the bottom to mix the gas. In the case of hydrofoils, different approaches are used. If the shaft and blades are hollow, gas may be pumped through the shaft and blades. In other apphcations, the impeller is contained within a cylinder that is within the tank. This prevents flooding the impeller with gas. 

Gas-liquid mixing has one key objective the dispersion of gas in liquid with the maximum surface area for mass transfer. As with many multiphase systems, this objective is complicated by the difficulties of multiphase flow. The gas can flood the impeller, dramatically reducing its effectiveness surface properties determine whether the system is coalescing or noncoalescing, and thus whether the surface area created is stable boiling systems require completely different treatment and gas-liquid reactions require consideration of local concentrations of gas. Chapter 11 includes the traditional topics ... 

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