Distillation - Principles of Chemical Separations with Environmental Applications - page 111

0.3

0.25

Equilibrium line

0.2

y

0.15

0.1

0.05

Operating line

0

0 0.005 0.015 0.02 0.025 0.03 0.035

0.01

x B x F

x

Figure 4.18 Graphical solution for steam injection, Example 4.5.

q -line

y

x B x F x S x D

x

Figure 4.19 McCabe-Thiele analysis for a system with a side stream.

Note 1: If an equilibrium stage does not fall exactly on x S you cannot physically obtain
that side-stream composition. You must adjust the reflux ratio until this is corrected.
Note 2: Yo u can treat multiple feeds in a similar manner.
Withdrawal streams are negative feed streams:

L − L

L − L L L −

q = = = .

F − S S

If S is a saturated liquid, q = 1. If S is a saturated vapor, q = 0.

∴ q / ( q − 1) can be used as the slope of the withdrawal line.