40 ◾  Distillation Control, Optimization, and Tuning
predictive control because the McCabe–Thiele model or a
computer simulation model shows that the separation power
base can be established by the reflux/feed ratio; see, for
example, Chapter 3, Table 3.3.
5.5 Temperature Control with
Bottoms Flow Rate
In the bottoms scheme, a column temperature controller
manipulates a control valve in the bottoms line. The reflux
drum level controller manipulates a valve in the distillate line.
The feed and reflux are on flow rate control. The column
base level controller manipulates the valve in the steam line to
the reboiler. With this scheme, the separation power base is
derived from the ratio of reflux/feed. The distillate/feed material balance split is maintained by the MRT point controller.
The manipulated bottoms flow rate scheme is not used
very frequently because of problems with the column base
level control using steam. When a thermosiphon reboiler is
used and the steam flow is increased, there is usually a reversal in the column base level response. The column level first
rises and then falls. The usual case for using this scheme is
when the feed concentration is 95% light key or more. In other
words, most of the feed is distilled overhead from a few percent heavies or tars. The MRT point is usually in the reboiler.
A variation on the scheme is to put the steam on flow rate
control and let the column base level controller manipulate
the feed rate to the column, similar to the way a maple syrup
evaporator is run.
A variation on the bottoms scheme is to add a cascade
slave control loop for the bottoms flow as shown in Figure 2.1.
The temperature control loop then manipulates the setpoint
for the slave bottoms flow control loop. Similarly, a slave
flow control loop can be used for the distillate flow rate and
another for the steam flow rate.