114 ◾  Distillation Control, Optimization, and Tuning
Skogestad 4 reported a modification of the IMC rules to
SIMC rules for improved disturbance rejection. The recommendation for PI control is to adjust the integral time constant to
the smaller value of either T i = TFO or T i = 8 ADT. The TFO of
20 min is smaller than the calculated T i = (8)(3.6) = 28.8 min
in this case.
10.6 Integrating Process Response
The process flow diagram in Figure 10.3 shows the concept of
an open-loop process response for an integrator. The level in
the tank can be envisioned as the integrator or accumulator
of the difference in flow rate between the process fluid flow
rate into the tank and the flow rate out of the tank. There may
be little dead time, if any, between the time the valve position
changes and the level change starts to integrate the difference
in flow rates.
Another example of an integrating process response is the
storage of an inert gas in a pressure vessel. The inert gas can
accumulate or can be depleted from a storage tank as indicated by the gas pressure in the vessel.
An integrating process response, for example, the level in a
reflux drum or distillation column base, will eventually overflow or run empty in an open-loop step test without further
manual intervention. One example of an open-loop step test
Vent
Process
Fluid In
Process
LT = Level Transmitter
LT
Fluid Out
Figure 10.3 Process concept for modeling an integrator.