20 ◾  Distillation Control, Optimization, and Tuning
Table 3.3 Energy Consumption to Double the Key
Impurity Separation Power for C 6 /C 7 Distillation with 50
Theoretical Stages
Minimum
Separation
Power
Design
Separation
Power
X D =
0.9990
0.9993
Weight fraction C6 in distillate
X B =
0.0010
0.0007
Weight fraction C6 in bottoms
R/D =
3.22
3.41
kg reflux/kg distillate
R/F =
1.61
1.71
kg reflux/kg feed
Q r /F = 400
415
btu reboiler duty/kg feed
doubles the key impurity separation power, that is, from 1 million to 2 million in this case. This required the reboiler duty to
be increased 4% and the reflux flow rate to be increased 6%
per pound of feed (Table 3.3).
The separation power base for distillation control can
be set either by the ratio of reflux/feed or by the ratio of
boilup/feed. If the separation power base is set by the ratio
of boilup/feed, then either the reflux or the distillate flow
rate can be manipulated to control the distillate/feed material balance split. If the separation power base is set by the
ratio of reflux/feed, then the steam, that is, boilup, flow rate
can be manipulated to control the distillate/feed material
balance split.
3.4 Summary
This chapter has derived the separation power for a distillation
column from its origin in vapor–liquid equilibrium to the key
impurity separation power for a distillation column with a stripping and a rectification section. The stripping section primarily
removes light key impurity from the bottoms liquid stream, and
the rectification section primarily removes heavy key impurity