Page 125 - Separation process engineering

appropriate direction. Since the heat of vaporization per mole is usually approximately constant,
condensation of 1 mole of vapor will vaporize approximately 1 mole of liquid. Thus liquid and vapor
flow rates tend to remain constant. Heat exchangers can be used for all other pairs of passing streams: L
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and V , L and V , and L and V .
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Figure 3-5. Countercurrent cascade with intermediate heat exchangers

Note that reflux and boilup are not the same as recycle. Recycle returns a stream to the feed to the
process. Reflux (or boilup) first changes the phase of a stream and then returns the stream to the same
stage the vapor (or liquid) was withdrawn from. This return at the same location helps increase the
concentration of the component that is concentrated at that stage.
The cascade shown in Figure 3-5 can be further simplified by building the entire system in a column

instead of as a series of individual stages. The intermediate heat exchange can be done very efficiently
with the liquid and vapor in direct contact on each stage. The result is a much simpler and cheaper
device. A schematic of such a distillation column is shown in Figure 3-6.
Figure 3-6. Distillation column: (A) schematic of five-stage column (T < T < T < T < T < T ),
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(B) photograph of distillation columns courtesy of APV Equipment, Inc. Tonawanda, NY.