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MIXING PROCESSES

Mixing of gases

Two or more gases will mix spontaneously and completely by diffusion after a sufficiently long time. The rate of approach to a uniform concentration in such a system can be increased by vigorous mixing of the

Pusher centrifuge

Figure 4.15 Pusher centrifuge: a, slurry feed; b, washing liquid; c, filtrate; d, solids.

components through convection and turbulence because in this way the average size of the domains occupied by the individual components of the mixture is reduced.

Turbulent mixing is effected, for example, with the aid of a free jet of one component which is blown into the other quiescent component (air conditioning). Often the mixing effect of the free jet is part of a complicated mixing process in a mixing device. Examples of such devices are mixing tubes, with or without inserts (bends, shutters etc.), injectors or mixing chambers of various designs.

Mixing of a gas into a liquid (aeration)

Aeration is used in a wide variety of chemical reactions to increase the interfacial area between the gaseous and liquid phases. In the range of lower viscosities stirrers are employed, and kneading machines are used for highly viscous liquids. Two types of stirrer are available: hollow stirrers which suck the gas in as they rotate and distribute it throughout the liquid (Fig. 4.16), and radially transporting fan turbine mixers or disc agitators

Section 4.2 was written with the cooperation of Dr H. Reichert.

Diagram of a hollow stirrer for aerating waste water

Figure 4.16 Diagram of a hollow stirrer for aerating waste water: a, incoming air; b, outgoing air.

into which the gas is introduced separately from below, through, for example, a perforated rim. The simplest and most effective hollow agitators are tubular agitators. Because of their limited sucking capacity hollow agitators are not suitable for very high gas throughputs (Fig. 4.16).

 
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