Page 181 - Separation process engineering

saturated vapor, and the distillate can be very pure but the bottoms will not be very pure. Striping
columns and enriching columns are used when a pure distillate or a pure bottoms, respectively, is not
needed.

Figure 4-24. Stripping and enriching columns; (A) stripping, (B) enriching, (C) McCabe-Thiele
diagram for stripping column

Analysis of stripping and enriching columns is similar. We will analyze the stripping column here and
leave the analysis of the enriching column as a homework assignment (Problem 4.C8). The stripping

column shown in Figure 4-24A can be thought of as a complete distillation column with zero liquid flow
rate in the enriching section. Then the top operating line is y = y . The bottom operating line can be
D
derived as

which is the usual equation for a bottom operating equation with a partial reboiler. Top and bottom
operating lines intersect at the feed line. If the specified variables are F, q, z, p, x , and y , the feed line
B
D
can be plotted and then the bottom operating line can be obtained from its intersection at y = x = x and its
B
intersection with the feed line at y . (Proof is left as Problem 4.C7.) If the boilup rate, /B, is specified,
D
then y will not be specified and can be solved for. The McCabe-Thiele diagram for a stripping column
D
is shown in Figure 4-24C.

4.10 Limiting Operating Conditions

It is always useful to look at limiting conditions. For distillation, two limiting conditions are total reflux