34 ◾  Distillation Control, Optimization, and Tuning
Table 5 .1 Four Basic Control Strategies
Ratio That Establishes
Separation Power Base
Manipulated Variable for
D/F (Temperature) Control
Steam/feed
Distillate
Steam/feed
Reflux
Reflux/feed
Steam
Reflux/feed
Bottoms
Tolliver and McCune 1 described four alternative material balance
control schemes for distillation columns. Each scheme manipulates a different variable to control a temperature point in the
column, that is, the D/F ratio, and this provides a structure for
categorizing distillation control strategies (Table 5.1). There can
be a number of variations of the four basic control schemes.
5.1 Column Pressure Control
One common strategy for controlling column pressure is to
manipulate a valve in a vent line from the reflux drum. If
necessary, another line and automatic valve can be added to
inject inerts, for example, nitrogen, before the vent valve. If
the distillate is taken as a vapor stream, then the condenser
may need to be run as a partial condenser with temperaturecontrolled cooling liquid on the condenser.
5.2 Temperature Control with
Distillate Flow Rate
In the manipulated distillate scheme, a column temperature
controller manipulates a control valve in the distillate line. In
other words, the column temperature is the process variable
that is controlled to a setpoint, and the controller output is