Control loop, dynamic elements properties

Fortunately, a process need not be very complicated before the properties of the typical feedback loop make their appearance. A rapid introduction to loop behavior may be presented using the simplest dynamic element found in the process-dead time. This chapter is devoted exclusively to discussion of the control of simple dynamic ele-... 

The dynamic properties of the rest of the loop set a lower limit on P. The natural period of the temperature-control loop is found by equating the sum of the phase lags of all dynamic elements to 180 . Since the phase of a negative lag is between -90 and —180°, there is little room for other elements. This clearly rules out integral control. 

Process design modifications usually have a bigger impact on operability (dynamic resilience). Dynamic resilience depends on controller structure, choice of measurements, and manipulated variables. Multivariable frequency-response techniques have been used to determine resilience properties. A primary result is that closed-loop control quality is limited by system invertability (nonmin-imum phase elements). Additionally, it has been shown that steady-state optimal designs are not necessarily optimal in dynamic operation. 

In this exercise, we will evaluate the controllability of a CSTR with heating jacket (Fig. 12.10). The reaction y4 —> 5 is first-order, irreversible and moderate exothermic. Since the heat of reaction is not enough to achieve a temperature that gives high conversion, heat is provided by pressurised hot water (inlet temperature 383 K). Temperature measurements follow a first order dynamics with a time constant of 60 s. Valve dynamics is represented by first order elements with a time constant of 30 s. Study the controllability property of the SISO loop keeping the reactor temperature at set-point by manipulating the hot-water flow rate. Disturbances in reactor inlet temperature and reactor inlet concentration are expected. 

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