4.6. Sketch a control concept diagram for the distillation column shown. The objectives of
your control system are:
• Reflux is flow controlled, ratioed to feed rate, and overridden by low reflux drum
level.
Feed
Bottoms
Reflux
Steam
Compressor bypass
Turbine
Compressor
Condenser
D
H.P. steam
Reflux drum
Distillate
FIGURE P4.6
• Steam is flow controlled, with the flow controller setpoint coming from a temperature
controller that controls a tray temperature in the stripping section of the column. Low
base level or high column pressure pinches the steam valve.
• Base level is controlled by bottom product flow rate.
• Reflux drum level is controlled by distillate product flow rate.
• Column pressure is controlled by changing the setpoint of a speed controller on the
compressor turbine. The speed controller output sets a flow controller on the high-
pressure steam to the turbine.
• A minimum flow controller ("antisurge") sets the valve in the compressor bypass line
to prevent the flow rate through the compressor from dropping below some minimum
flow rate.