38 ◾  Distillation Control, Optimization, and Tuning
0.8. The distillation column can be run very close to flooding because the boilup and vapor traffic can be run steady
at maximum rate. This manipulated reflux scheme can also
handle environmental disturbances well such as day and night
temperature variations and rain storms.
Another variation on the manipulated reflux scheme is
to add a cascade slave control loop for the reflux flow rate
as shown in Figure 2.1. The temperature control loop then
manipulates the setpoint for the slave reflux flow control loop.
Similarly, a slave flow control loop can be used for the distillate flow rate and another for the bottoms flow rate.
Another variation on the manipulated reflux 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.
A less common variation on this control scheme is used on
the occasion where the first column in a distillation train is for
removing light components, and the bottoms flow rate needs
to be very steady as the flow rate to a large diameter fractionator as the second column. In this case the bottoms flow rate
from the first column is the manually adjusted setpoint for the
controller that manipulates the bottoms valve. The columnbase level controller for the first column manipulates the feed
rate to the first column.
5.4 Temperature Control with
Boilup (Steam Flow Rate)
In the boilup scheme, a column temperature controller manipulates a control valve in the steam line to the reboiler. The