Centrally mounted agitators

A stirred tank fermenter consists of a centrally mounted agitation system inside a cylindrical vessel. Typically, the agitation system is composed of either multiple radial flow impellers (see Fig. 4A) or a combination of radial and axial flow impellers as shown in Fig. 4B. A gas sparger is located below the radial gas dispersing impeller. The role of the bottom radial impeller... 

Sand mill n. An apparatus used for preparing pigment dispersions, consisting of a vertical cylinder with a centrally mounted agitator shaft on which are mounted several flat, annular disk impellers. The mill is... 

BLENDING OF MISCIBLE LIQUIDS. Miscible liquids are blended in relatively small process vessels by propellers or turbine impellers, usually centrally mounted, and in large storage and waste-treatment tanks by side-entering propellers or jet mixers. In a process vessel all the liquid is usually well agitated and blending is... 

To prevent gross vortexing behaviour when low-viscosity liquids are agitated in a vertical cylindrical tank with a centrally mounted impeller, baffles are often fitted to the walls of the vessel. Typically four baffles will be used, each having a width equal to about one-tenth of the tank diameter. In some cases the baffles are mounted Hush with the wall, although occasionally a small clearance is left between the baffle and the wall. 

Different types of agitator, and their position in the vessel, can produce substantially different flow patterns. For example, a centrally mounted propeller induces tangential flow and well-established pattern of circulation. A vortex is generally produced at high speeds, but the introduction of wall baffles creates... 

The most common type of agitator is turbine. It consists of several short blades mounted on a central shaft. The diameter of a turbine is normally 35 15% of the tank diameter. There are four to six blades for perfect mixing. Turbines with flat blades give radial flow. This is good for gas dispersion in the media, where the gas is introduced just below the impeller, is drawn up to the blades and broken up into uniform fine bubbles. 

Figure 7.20 shows some of the impellers which are frequently used. Propellers, turbines, paddles, anchors, helical ribbons and screws are usually mounted on a central vertical shaft in a cylindrical tank, and they are selected for a particular duty largely on the basis of liquid viscosity. By and large, it is necessary to move from a propeller to a turbine and then, in order, to a paddle, to an anchor and then to a helical ribbon and finally to a screw as the viscosity of the fluids to be mixed increases. In so doing the speed of agitation or rotation decreases. 

Vertical Mills Vertical mills are, generally speaking, older designs whose chief advantage is that they are inexpensive. They are vertical chambers of various shapes with a central agitator shaft. The media are stirred by discs or pegs mounted on the shaft. Some mills are open at the top, while others are closed at the top. Most mills have a screen at the top to retain media in the mill. 

Anchor-agitated vessels are nearly always operated batchwise in continuous processes, heat transfer to or from viscous liquids is often accomplished in scraped-surface exchangers. Typically these are double-pipe exchangers with a fairly large central tube 100 to 300 mm (4 to 12 in.) in diameter jacketed with steam or cooling liquid. The inside surface of the central tube is wiped by one or more longitudinal blades mounted on a rotating shaft. 

---