46 ◾  Distillation Control, Optimization, and Tuning
traffic in the column. This is driven by the boilup rate from the
reboiler and creates a pressure drop across the trays or packing
in the column. Above a certain pressure drop, liquid is held
up in the packing or in the downcomers or trays. This condition is also associated with a maximum liquid flow that can be
allowed for a column. Also, above certain velocities, the vapor
can entrain liquid droplets and reduce tray efficiency or packing efficiency because of backmixing. This can limit or constrain the maximum separation power that is achievable.
All of the process flow rate variables are constrained
because of the trim size in the control valves and may also
be constrained because of the range of the flow rate sensors.
Also, the liquid level in the reflux drum and the column base
must be constrained to run between 0% and 100% of the
level signals.
Another unique constraint can occur when the system to be
separated has a high relative volatility, for example, 10 to 50,
and the column is designed with, say, four times the minimum
number of theoretical stages required at total reflux. The distillation column may be designed this way because one of the
components is highly toxic and the project design team wants
plenty of safety margin for removing the toxic component. Or,
the condenser may need to run extremely cold or with vapor
compression, so the project team wants to be certain that an
absolute minimum of condenser duty is used. In these cases,
the column may be able to run in the range of 1.01 times the
minimum reflux ratio. However, the problem is that many
of the extra stages in the column operate at a pinch condition around the feed tray. In other words, there is a series of
trays that run at the same concentration and temperature. The
inventory of material on these trays in the column can isolate
one end of the column from the other end for a long period
of time regarding process control of the D/F material balance
split. In this rare case, the solution could be to install and run
two temperature control loops. The boilup is manipulated by a