Part A: Composition control by cascading reflux flow as slave controller (FRC). Part B: Smith predictor with dead time compensation. Part C: Control by triple cascade of ARC to TRC to FRC. Part D: Absorber bottoms composition control (ARC).
Smith Predictor
Controllability of the process is degraded by the dead time between measurement updates. In Part B of Figure 1, the Smith-predictor compensator provides a process model in terms of its time constant and dead time, and predicts what the analyzer measurement should be between analysis updates. The multiplier (AY), first order lag (AY) and dead time (AY) provide the required inputs to the calculation of the predicted analysis. This predicted response is subtracted from the actual measurement (AY+/-) to give a differential of the actual process from its own model. This delta is added to the model (AY∑) without dead time to provide a modified pseudo-measurement to the analyzer controller.
Triple Cascade
The purpose of all cascade systems is to provide slave(s) that will correct for disturbances before they can upset the primary or master controller. Part C in Figure 1 illustrates a triple cascade loop, where a temperature controller is the slave of an analyzer controller, while the reflux flow is cascaded to temperature. Cascade loops will work only if the slave is faster than the master, which adjusts its set point. Therefore, in Part C, the time constants of the flow controller (FRC) must be much smaller than those of the TRC.
Selective Control
Part D illustrates a limit control configuration where the analyzer controller is overruled when the temperature reaches its high limit. The temperature controller is a constraint preventing the temperature from exceeding a limit at the bottom of an absorber stripper. Selective control configurations also require external feedback to protect them from reset windup. Part D shows a combination of selective and cascade systems.The master of the FRC is selected to be either the TRC or the ARC. The external reset (ER) signal is taken from the measurement of the slave controller (FRC).
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