16 ◾  Distillation Control, Optimization, and Tuning
Table 3.2 Mole Percent Cyclohexane in Liquid
and Vapor at Total Reflux
Stage No.
Mol% C6
in Liquid
Mol% C6
in Vapor
Total Separation
Power
4
36.12
49.30
8.75
3
24.74
36.12
5.09
2
16.04
24.74
2.96
1
10.00
16.04
1.72
of pure component vapor pressures would be about (1.036) 17.7
= 1.87.
3.2 Separation Power with Multiple Stages
When a distillation column is run at total reflux for a long
enough period of time to reach steady state, all of the vapor
leaving a stage and going up the column will be coming back
down the column as liquid from the stage above at the same
concentration. For example, when the relative volatility is constant at 1.72, the separation power of four theoretical stages at
total reflux is (1.72) 4 = 8.75 (Table 3.2).
The separation power in a distillation column can be
defined by the ratio of light key to heavy key component in
the distillate divided by the ratio of these components in the
bottoms (Equation 3.2):
Separation power = (x 1 /x 2 ) D /(x 1 /x 2 ) B
(3.2)
where:
subscript D = distillate
subscript B = bottoms
For example, a distillation column running at total reflux
that achieves the equivalent of four theoretical stages of