Control Structure and Controller Design

For the MeAc system, the heat input is used to control T2 and Tu is maintained by changing the feed ratio (i.e., T2 Qr and Tn-FR), which looks reasonable from a dynamic perspective (note that we are counting the tray numbers from the bottoms up). 

For the type II flowsheets (EtAc and IPAc), a stripper temperature is maintained using the heat input to the stripper and a temperature in the reactive distillation column is controlled using the ratio of fresh feeds into the reactive distUIation column (i.e., 7str,3-Qr,s 7 futc,i5 FR for EtAc, 7str,3—7rdc,i8 FR for IPAc). 

The control stmctures for the type HI flowsheet may look odd from a conventional distillation control perspective, but they have been frequently seen for reactive distillation. Here, the heat input is paired with an upper section tray temperature and a lower section tray temperature is controlled by the feed ratio (e.g., T29-QR and Tg-FR for BuAc, Tss-Qr and Tis-FR for AmAc). The relay-feedback test is used to And the ultimate gain Ku and the ultimate period P followed by the Tyreus-Luyben PI tuning mle. The 

For the MeAc system, two temperature control trays (X2 and Tn) show asymmetric responses and somewhat oscillatory behavior is observed for T (Fig. 13.6a). The product composition, Xo acetate in particular, does not settle down 15 h after the 20% feed flowrate change is introduced. Asymmetrical responses are observed for most of the process variables and steady-state offsets ( 0.002mf Fig. 13.6a). For the type II 

Despite strong nonlinearity (Figs. 13.3 and 13.4), workable temperature control of these reactive distillation systems can be obtained using a systematic design procedure with rather simple control structures. It should be emphasized that the selections of controlled and manipulated variables (control structure design) play a cmcial role for these highly nonlinear processes and that decentralized control provides a better stmcture to cope with steady-state gain variations than model-inversion-based control stmctures. 

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