Distillation Control Strategies ◾  41
5.6 Side Draw Flow Rate
There are a number of cases where a side draw from a distillation column is used. If a liquid side draw flow rate is a small
stream primarily for purging a concentration bubble of an
impurity, the control scheme may be simply a matter of setting
a small fixed purge rate. However, if a pasteurizing section
is used at the top of a column, the distillate stream may be a
very small lights purge stream with the liquid returning to the
column being close to total reflux. The small distillate purge
can be set at a fixed flow rate. The liquid side draw may be
the main product, so the side draw flow rate may be manipulated by a temperature controller much like the manipulated
distillate scheme described earlier.
When a side draw is used on a distillation column, there is
another degree of freedom introduced into the control scheme.
There are actually two material balance splits to keep in balance. One is the ratio of distillate/side draw, and the other is
the ratio of side draw/bottoms. The separation power base can
be set by the ratio of steam/feed, and then the distillate flow
rate can be manipulated by a temperature controller for the
MRT point above the side draw. The side draw flow rate can
be manipulated by the second temperature controller for the
MRT point below the side draw.
The side draw can be very pure when a DWC (Dividing
Wall Column) is used (Figure 5.1). In that case, there is
another design variable, which is the ratio of liquid to the feed
side of the wall divided by the liquid to the product side of
the wall.
A vapor side draw from a distillation column is not used
very frequently, but, when it is, a large valve in the vapor
side draw line can be used to control the side draw flow
rate. However, the control is very sensitive when such a large
valve is used. An alternative can be to use a fixed large valve
position and a manipulated small valve position in parallel
for control.