Page 199 - Fluid mechanics, heat transfer, and mass transfer

MIXING EQUIPMENT 177

TABLE 6.7 Advantages and Disadvantages of Selected Liquid Mixers
Type of Mixer Advantages Disadvantages
Paddle mixer Good radial and rotational flow; cheap Poor perpendicular flow; high vortex formation risk at higher speeds
Multiple paddle mixer Good flow in all three directions More expensive; higher energy requirements
Propeller mixer Good flow in all three directions More expensive than the paddle mixer
Turbine mixer Very good mixing Expensive; blockage risks

& The upflow in the annulus around the tube has
6.2.5 Draft Tubes
sufficient velocity to keep particles in suspension.
. What is a draft tube? What is its purpose in an agitator? & Figure 6.9 illustrates a draft tube agitator.
& The draft tube directs the flow to the regions of the . What are the advantages of a draft tube?
vessel that otherwise would not be agitated by the & The advantages of a draft tube are as follows:
liquid stream.
➢ Increases mixing efficiency by preventing short-
& It is a tube or shell around the shaft of a mixer,
circuiting of the fluid.
enclosing the usual axial flow impeller, which allows
➢ Minimizes areas of inadequate turbulence.
a special or top-to-bottom fixed flow pattern to be set
➢ Amplifies mixing action by effectively increasing
up in the fluid system.
ratio of mixer to container diameter.
& Draft tubes are employed to improve the mixing of
large quantities of liquids by directing the motion of
the liquid. 6.2.6 Other Types of Mixers
& Size and location of the tube are related to both the
. What is a vortex mixer? What are its applications?
mechanical and mixing performance characteristics.
& A vortex mixer is used for turbulent processes.
& Usually these tubes are used to ensure a mixing flow
& These mixers consist of a series of baffles or tabs
pattern that cannot or will not develop in the system
otherwise. These are favorable for large ratios of inclined relative to one another and at an angle
liquid depth to mixer diameter. relative to the pipe axis. The mixer elements are
rotated by 90 and arranged successively in the pipe.

& With a draft tube inserted in the tank, no sidewall
& Mixing is achieved by controlled vortex structures
baffles would be required.
generated by the baffle geometry that requires a
& Flow into the axial impeller mounted inside the tube
mixer length less than two pipe diameters.
is flooded to give a uniform and high flow pattern into
& This design can be used in all turbulent flow mixing
the inlet to the impeller.
applications regardless of line shape or size and has
pressure losses 75% less than conventional static
mixers.
& The vortex created by the impeller travels to the
bottom of the tank and into the nooks and corners
of the container. Such an impeller and flow field are
excellent for solids suspension.
& Water-borne paints and many other solids are diffi-
cult to suspend in solution and keep from stratifying.
The V-impeller and its associative flow field are
excellent for solids suspension.
& Typical applications include low-viscosity liquid–li-
quid blending processes, as well as gas–gas mixing.
. What is a rotor–stator mixer?
& A rotor–stator mixer is similar in concept to an
electric motor and consists of a high-speed rotor
surrounded in close proximity by a stator.
& Rotor is a rapidly spinning device nested inside a
stationary stator. Typical rotor tip speeds range from
Draft tube agitator. 10 to 50 m/s.
FIGURE 6.9