Distillation Control Strategies ◾  35
the signal to the control valve in the distillate line. The reflux
drum level controller manipulates a valve in the reflux line,
and the column base level controller manipulates a valve in
the bottoms line. The feed rates to the column and reboiler
steam are each on flow rate control. In one variation, there is
a controller for the pressure drop across the trays that manipulates the valve in the reboiler steam line. The trays are used
as the vapor flow rate sensing device. However, it is preferred
to measure the steam flow rate to the reboiler and simply
monitor the tower pressure drop, especially for packed towers. 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 (most responsive temperature) point controller.
In some cases, the temperature controller for this
manipulated distillate scheme has not been tuned well,
so a plant operator changes the controller to manual output. Subsequently, the operators may leave the controller in
manual output and simply adjust the distillate valve position
manually to keep the MRT in a desired range or to keep the
analysis of grab samples of distillate and bottoms in certain
ranges. In other cases, the control system is actually designed
to run with a manual setpoint for a distillate flow rate controller. The plant operators are required to grab samples once or
twice during an 8-hour shift and get them analyzed immediately. There may not be any temperature measurements available in the column for automatic control of the D/F material
balance split. This requires significant operator intervention
to shed disturbances that come at the tower, so one operator can only run a few towers at once. This type of manual
control is sometimes required if the entire column operates
with a low concentration range of impurity. In one case, the
feed contained only 200 ppb impurity that had to be removed
down to 5 ppb at one end of the column and concentrated to
10,000 ppb (10 ppm) concentration at the other end.