Distillation Control Strategies ◾  39
reflux is on flow rate control. The reflux drum level controller manipulates a valve in the distillate line. The column-base
level controller manipulates a valve in the bottoms line. 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 origins of the boilup scheme may go back to the days
when the feed flow rate and reflux flow rate to a distillation
column were controlled by manual valves and glass rotameters in the control room. A pneumatic temperature controller manipulated an automatic valve in the steam line to the
reboiler. This may be difficult to imagine by those who have
only experienced modern-day equipment.
The manipulated boilup scheme is still used in many cases
and especially for top-fed stripping columns that have no
reflux stream and no rectification section above the feed. A
column temperature controller manipulates the steam flow rate
to the reboiler. The MRT point for a stripper is often the overhead vapor temperature leaving the column. If the overhead
vapor temperature is close to the bottoms temperature, then
an overhead vapor flow rate controller can be used to manipulate the steam flow rate.
The manipulated boilup scheme is quite easy to start up
after some liquid is accumulated in the reflux drum. However,
this scheme does not shed environmental disturbances very
well. When a sudden rain storm hits the distillation tower, the
temperature disturbance is shed by the temperature controller increasing the steam flow rate to the reboiler. This can be
a problem if the distillation tower needs to be running at its
maximum capacity near to flooding.
Another variation on the manipulated boilup scheme is to
use a setpoint for the reflux/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
reflux flow is proportional to a trailing average of the feed
rate. Sometimes, this falls into the category of model-based