Distillation - Principles of Chemical Separations with Environmental Applications - page 93

V

D, x D

L

n

F, x F y n+1 x n

m

y m+1

x m

L

B, x B

V

Figure 4.3 Distillation column schematic (see Nomenclature section at end of
chapter for variable definition).

a distillation column would operate as a single-stage flash drum with vapor above the feed
location and liquid below.
At the top and the bottom of the column there are two options for condensing the vapor
and vaporizing the liquid, respectively, prior to recycle. A total condenser at the top of
the column simply condenses the entire distillate stream to a liquid. It is not considered to
be an equilibrium stage because although there is a complete phase change the distillate
composition is not changed. A partial condenser, on the other hand, only condenses a
fraction of the distillate such that the compositions of the condensed fraction and the
remaining vapor are altered. It is, therefore, considered to be an equilibrium stage in the
column. The same description applies to the bottoms stream reboiler. A total reboiler is
not an equilibrium stage because the resulting vapor has the same composition as the
liquid bottoms stream. A partial reboiler, however, produces both a liquid fraction and a
vapor fraction which have different compositions than the entering liquid. Thus, it is an
equilibrium stage in the column.
The McCabe-Thiele graphical method uses three important types of lines. First, there
is an equilibrium line which gives the vapor-liquid equilibrium relationship of a binary
system over a range of concentrations. You can obtain information about azeotropes from
this line (where it crosses the diagonal y

x line, vapor and liquid compositions are
equal, as shown in Figure 3.7) and the ease of separation which is demonstrated by the
distance of the equilibrium line from the diagonal. Second, there are the operating lines
which are graphical depictions of the mass balances in each section of the column. There
are different operating lines for the rectifying and stripping sections, respectively. These
lines relate concentrations of liquid and vapor passing streams between stages: L and V in

=