Distillation Control Strategies ◾  37
A less common variation of the distillate 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 tower. In this case, the bottoms flow rate is
set manually for the controller that manipulates the bottoms
valve. The column-base level controller in the first-column
manipulates the feed rate to the first column.
5.3 Temperature Control with Reflux Flow Rate
In the reflux scheme, a column temperature controller manipulates a control valve in the reflux line. The reflux drum
level controller manipulates a valve in the distillate line. The
column base level controller manipulates a valve in the bottoms line. The feed and reboiler steam are each on flow rate
control. In some cases, there is a controller for the pressure
drop across the trays that manipulates the valve in the reboiler
steam line. However, it is preferred to use a steam flow rate
controller and simply monitor the tower pressure drop. With
this scheme the separation power base is derived from the
ratio of steam/feed. The distillate/feed material balance split is
maintained by the MRT point controller.
The reflux scheme can be difficult to start up because initially there may not be enough light components accumulated
in the reflux drum, and the column temperature may be too
hot; so, the controller may want more reflux flow than is available. This can pump the reflux drum level down until the distillate flow stops and then proceed to pump the reflux drum
empty. However, this situation can be handled with computer
control by using a low-level constraint control that will constrain the reflux flow rate to maintain a low-level constraint
setpoint until the column temperature is low enough, so the
temperature controller calls for less reflux. This reflux scheme
is recommended when the reflux/distillate ratio is less than