Distillation Control Strategies ◾  43
a condenser with the bottom of the tubes flooded, and the
liquid condensate rate can be manipulated to allow a variable
condenser area to be exposed to the vapors. When the partial
condenser is cooled by an evaporating refrigerant, the liquid
refrigerant level can be manipulated for control of the condenser heat duty.
5.8 Summary
In this chapter, four basic distillation control schemes were
introduced along with a number of variations on each
scheme. The concept was introduced that there can be more
than one way to successfully control a distillation column. In
some schemes, the separation power base is controlled by the
ratio of steam/feed and then the distillate flow rate or reflux
flow rate can be manipulated to control an MRT point in the
distillation column. In other schemes, the separation power
base is controlled by the ratio of reflux/feed, and the steam
to the reboiler or the bottoms flow rate can be manipulated
to control an MRT point in the column. Control of reflux
drum level and column base level was presented as basic to
all control schemes. Control of column pressure was considered to have an overriding effect on the stability of distillation
column control.
Exercises
5.1 What control strategy would be recommended for a
top-fed stripping column?
5.2 What control strategy would be recommended for a
batch rectification column that starts up at total reflux?
5.3 What control strategy would be recommended for a
reflux ratio of 0.1?