aFurther detuning from the Tyreus-Luyben settings.

aFurther detuning from the Tyreus-Luyben settings.

16.5.3 Controller Design

Once the decentralized control structure is in place, dynamic simulations are performed using Aspen Dynamics. A third-order 0.5-min time lag is assumed for temperature measurement13 and a dead time of 5 min is assumed for the composition analyzer. The liquid level is controlled using a proportional-only controller. PI controllers are used for flow, pressure, composition, and temperature controls. Relay-feedback tests are performed on the temperature loops to find the ultimate gains (Ku) and ultimate period (Pu) of each temperature control loop, and initial controller parameters are calculated according to Kc = Ku/3 and tj = 2Pu. In order to obtain an acceptable damping, further detuning from the initial settings are required for some of the loops. Controller settings for all loops in CS1 and CS2 are summarized in Table 16.7.

16.5.4 Performance

The control performance is tested for feedflow and feed composition disturbances (ethanol and acetic acid). Figure 16.42 presents the dynamic responses of CS1 and CS2 for +20% feedflow changes. The results show that CS2 has fast (<5-h settling time) and symmetric responses but CS1 exhibits much slower (^10h) and oscillatory responses. The reason for that is that we have a severe interaction between the two process units (reactive distillation and stripper) for CS1 compared to that of CS2. This can be seen in Figures 16.39 and 16.40 when one compares the reactive distillation temperature sensitivity for the stripper reboiler duty changes or in the RGA values in Table 16.7. It can also be seen that with temperature control (CS1 and CS2) steady-state errors exist for both product compositions (0.05% error for 20% production rate changes). For HAc feed composition disturbances ( + 5 mol%) and EtOH feed composition disturbances (-5 and -10mol%), the CS2 structure

13W. L. Luyben, B. D. Tyreus, and M. L. Luyben, Plantwide Process Control, McGraw-Hill, New York, 1998.

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