Distillation - Principles of Chemical Separations with Environmental Applications - page 106

V , y 1

D , y D

Condenser stage

L, x 0

y D
y 1

y 1

x 0

y 2 x 1

y

x 0

x 1

x

Figure 4.11 Partial condenser.

Partial condensers

A partial condenser, as shown in Figure 4.11, condenses only a portion of the overhead
stream and returns it as reflux, so the distillate product remains vapor. There are only two
things which make the analysis of a column with a partial condenser different from one
with a total condenser. The first is the point where the top operating line intersects the
diagonal on a McCabe-Thiele diagram. The point is still the same, but the notation x D is
no longer useful because the distillate is vapor. Instead, y D is used and the operating line
is plotted as before. The other thing to note is that while stepping off stages, a partial
condenser is counted as one equilibrium stage, while a total condenser is not .Apartial
condenser has both a vapor and liquid exit stream. They are assumed to be in equilibrium,
so the compositions are different from the single vapor stream entering the condenser. A
total condenser, on the other hand, has a vapor at a given composition entering and only
liquid at the same composition exits (i.e., no vapor-liquid equilibrium in the exit stream).

Example 4.4: column with partial condenser (Figure 4.12)

Problem:
A 100 mol hr feed stream containing 30 mol% of a contaminant A is to be distilled

in a column consisting of a reboiler, one plate, and a partial condenser. Each can be
considered as an ideal stage. To concentrate the contaminant prior to further treatment,
the distillate should contain 80 mol% A . The ratio of liquid reflux flowrate to distillate
flowrate is 2.

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