Page 124 - Separation process engineering

The cascade shown in Figure 3-3 has a decreasing vapor flow rate as we go from the feed stage to the top
stage and a decreasing liquid flow rate as we go from the feed stage to the bottom stage. Operation and
design will be easier if part of the top vapor stream V is condensed and returned to stage 1 and if part of
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the bottom liquid stream L is vaporized and returned to stage 5, as illustrated in Figure 3-4. This allows
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us to control the internal liquid and vapor flow rates at any desired level. Stream D is the distillate
product, while B is the bottom product. Stream L is called the reflux while V is the boilup.
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Figure 3-4. Countercurrent cascade of flash chambers with reflux and boilup, p = constant; T < T
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< T < T < T 5
4
3

The use of reflux and boilup allows for a further simplification. We can now apply all of the heat required
for the distillation to the bottom reboiler, and we can do all of the required cooling in the top condenser.
The required partial condensation of intermediate vapor streams and partial vaporization of liquid
streams can be done with the same heat exchangers as shown in Figure 3-5. Here stream V is partially
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condensed by stream L while L is simultaneously partially vaporized. Since L has a higher
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concentration of more volatile component, it will boil at a lower temperature and heat transfer is in the