52 ◾  Distillation Control, Optimization, and Tuning
Table 6.2 Number of Theoretical
Stages and Gas Mass Transfer
Units for Rectification
(Absorption) of 1,000 ppm Heavy
Impurity down to 1 ppm
S
NTS
N OG
1.00
999.00
999.00
0.91
47.42
49.72
0.83
28.09
30.73
0.77
20.75
23.59
0.67
14.33
17.43
0.50
8.97
12.43
0.20
4.15
8.36
0.10
2.95
7.56
0.05
2.29
7.22
0.01
1.50
6.97
0.00
0.00
6.91
a comparison of the number of theoretical stages and the
number of gas transfer units required for reducing the heavy
key solute concentration by a factor of 1,000 with a pure
absorption liquid. The number of theoretical stages and transfer units are nearly the same for stripping factor values from
1.0 to 0.5. But the mass transfer becomes severely rate limited
in the gas phase at lower stripping factors for the heavy key in
the rectification section. If the stripping factor is 0.2 or lower
in the rectification section because the relative volatility is
high, then increasing the reflux will not increase the separation power by any significant amount.
When a computer simulation is used for a distillation column, the number of gas transfer units can be calculated for
each theoretical stage in the rectification section with Equation