Mechanically axial flow impeller

Mechanically stirred hybrid airlift reactors (see Fig. 6) are well suited for use with shear sensitive fermentations that require better oxygen transfer and mixing than is provided by a conventional airlift reactor. Use of a low-power axial flow impeller in the downcomer of an airlift bioreactor can substantially enhance liquid circulation rates, mixing, and gas-liquid mass transfer relative to operation without the agitator. This enhancement increases power consumption disproportionately and also adds other disadvantages of a mechanical agitation system. 

There are four types of turbine impellers, which are characterized by the flow patterns and level of shear they create axial flow, radial flow, hydrofoil, and high-shear impellers. They have the widest use in low and medium viscosity liquid applications, solids suspension, liquid-liquid emulsification, and gas dispersion. Turbine impellers can have blades varying from 2 to 12 in number. Two blades are normally unstable mechanically, while it is difficult to install more than six blades on a hub. Axial flow impellers generally have three or four blades, and radial flow impellers are designed with six blades. 

Although low shear emulsification equipment (mechanical stirrers or impellers) can differ in the type of fluid flow imparted to the mixture (axial-flow propeller or radial-flow turbines), no subclasses have been defined. 

Axial flow devices such as high-efficiency (HE) impellers and pitched blade turbines give better performance than conventional pitched blade turbines. They are best suited to provide the essential flow patterns in a tank that keep the solids suspended. High-efficiency impellers effectively convert mechanical energy to vertical flow... 

The application of draft tubes as related to various mixing operations is showm in Figures 5-231 and 5-24A-5-241. The draft tubes are basically a tube or shell around the shaft of the mixer including the usual axial impeller, which allows a special or top-to-bottom fixed flow pattern to be set up in the fluid system. The size and location of the tube are related to both the mechanical and mixing performance characteristics as well as peculiar problems of the system. Usually they are used to ensure a mixing flow pattern that cannot or w ill not develop in the system. Weber gives the followdng points for draft tubes [23] ... 

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