36 ◾  Distillation Control, Optimization, and Tuning
The manipulated distillate scheme is easy to start up on
total reflux simply by closing the distillate flow valve. The
level controller for the reflux drum manipulates the reflux flow
rate to the column. This scheme of controlling temperature
by manipulating the distillate flow rate works well for systems
that run with high reflux ratios, especially when the reflux/
distillate is greater than 5. This is compatible with a general
rule of thumb that the larger stream should be used for level
control and the smaller stream for column temperature control.
This scheme also works well for towers operating at a boilup
rate near to flooding because the vapor traffic can run quite
steady. Environmental disturbances such as day and night
temperature changes, or a rain storm, are shed reasonably well
because the primary effect is on internal reflux in the column.
Another variation on the manipulated distillate scheme is
to add a cascade slave control loop for the distillate flow rate
as shown in Figure 2.1. The temperature control loop then
manipulates the setpoint for the slave distillate flow control
loop. Similarly, a slave flow control loop can be used for the
reflux flow rate and another for the bottoms flow rate. At one
point in time, the use of cascade control loops was called
advanced control; it was compared to SISO (single input single
output) control, because it required the addition of more hardware PID controllers. Modern computer control systems simply
require the addition of software code to the program when
the flow sensors are present.
Another variation on the manipulated distillate scheme is to
use a setpoint for the steam/feed ratio to establish the separation power base. Generally, the feed flow rate signal should
be lagged with an 8 to 20 min capacitance lag (filter), so the
steam flow is proportional to a trailing average of the feed
rate. Sometimes, this falls into the category of model-based
predictive control because the McCabe–Thiele model, or a
computer simulation model shows that the separation power
base can be established by the steam/feed ratio as shown in
Table 3.3.