Distillation - Principles of Chemical Separations with Environmental Applications - page 115

Stepping off stages
Figure 4.23 reveals that there are four-and-a-half steps on the graph, so five equilibrium
stages will be used. With an efficiency of 0.7, seven equilibrium stages will be required.
Note that since x D is so close to the point where the y

x and equilibrium lines meet,
the accuracy in the graphing technique here is low, and anywhere from five to eight
steps could be considered a correct answer.
The 12 M HNO 3 stream which will be re-used for plutonium stabilization will reduce
the cost of the process; currently the nitrates are discarded as waste. The savings in
waste storage and disposal costs is estimated to be US $100,000 annually.

=

V-1

B2

F-1

B1

Preheater
(V-1 = 0)

D-1

F-2

B-1

Figure 4.22 Flowsheet schematic, Example 4.6.

4.5 Remember

Distillation uses an energy-separating agent: heat. It is an equilibrium-limited separation.
Vapor-liquid equilibrium (VLE) is the type of equilibrium by which distillation columns
separate, and vapor pressure is the primary property difference that forms the basis for
separation.
Distillation is a UNIT OPERATION. Regardless of what chemicals are being separated,
the basic design principles for distillation are always similar.
The assumption that stages in a distillation column are in equilibrium allows calculations
of concentrations and temperatures without detailed knowledge of flow patterns and heat
and mass transfer rates. This assumption is a major simplification.