80 ◾  Distillation Control, Optimization, and Tuning
proportional gain and insignificant integral (reset) action.
The height between the first peak and valley will be 16%
of the height of the step change in controller setpoint
4. Measure the response time for a self-regulating process
variable to reach the first peak after a step-up change in
setpoint when tuning a self-regulating process response
5. Recognize the response pattern of an integrating process
variable, such as distillation column bottom level or reflux
drum level, when a step change in setpoint is introduced
to the controller with a proportional gain of 1.25 and an
integral time constant of 30 min
6. Measure the response time for a liquid level process variable to reach 95% of the step change in setpoint when
tuning a liquid level control loop for a reflux drum or
distillation column base
The process dynamics of most process variables can be characterized as a self-regulating process response. One example
is the response of a liquid flow rate when a valve position is
opened. The liquid flow rate will increase from the initial flow
rate to a new steady-state flow rate. Another example is the
response of the temperature of a liquid flowing through a heat
exchanger that is heated with steam. When the steam valve
position is increased, the temperature of the liquid outlet will
increase to a new steady-state temperature.
The process dynamics of a few process variables can be
characterized as an integrating process response. One example
is the response of the liquid level in the bottom of a distillation column when liquid is being pumped out with a centrifugal pump and the liquid flow rate is restricted by an automatic
valve after the pump. The column base liquid level may be
steady with an initial automatic valve position, and opening the valve to a new position will increase the liquid flow
rate. The difference between the two flow rates will be integrated over time as seen by the response in liquid level going
down. Eventually, the column base liquid level will go to zero