Feedback loops state tables

Relay-feedback tests are run individually on both temperature loops with the other loop on manual. Three 1-min temperature measurement lags are included. Temperature transmitter spans are 50 K. Valves are half open at steady state. Controller tuning constants are given in Table 11.1. Tyreus-Luyben tuning constants are used with some reductions in controller gains to give less oscillatory behavior. 

To eliminate steady-state offsets in product compositions, dual composition control is implemented. Figure 12.56 shows that the distillate composition is controlled by changing the reflux ratio, and the bottoms composition is maintained by adjusting vapor boilup. Because the composition analyzer has a slower response, a measurement dead time of 4 min is assumed in the simulation. Relay-feedback tests are performed to find settings for the PI controllers. Table 12.5 shows that because of the dead time in the composition measurement, the reset times for the top and bottoms loops now become 120 and 102 min, compared to 4.3 and 2.9 min in the case of temperature control. 

To eliminate the steady-state offset in acetate purity, we also consider composition control. A parallel cascade control structure is used in which the setpoint of the stripper temperature controller is reset by the output of the composition controller. Thus, we have one composition control loop xb iac-TItr and two ternperamre control loops 2r,str-Tstr and 2/j,rd-Trd, with constant R/F on (CS3, as shown in Fig. 16.38). Relay-feedback tests are performed to find the ultimate gains and ultimate period of each control loop and initial controller parameters are calculated according to = Ku/2> and T/= 2F . Controller settings for aU loops are summarized in Table 16.7. The control performance is tested for feedflow disturbances. Figure 16.44 shows the dynamic responses of CS3 for +20% feedflow changes. The control performance of CS3 is quite similar to that of the temperature control (CS2), except that steady-state error in product purity is eliminated. 

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