**6. Conclusion**

This chapter discusses the presence of multiple steady states indicating nonlinear reactive distillation process. The presence of multiple steady states urged to fix the operating range for the manipulated variable. For achieving the purity of DPC (y1), more than 99% of the reboiler heat duty of the RD column (u1) must be varied between 886.237 kW and 899.631 kW. Similarly, for purity of MA (y2), more than 99% of the condenser heat duty of the separation column (u2) must be constrained between 293.240 kW and 299.164 kW. This smaller operating range shows that the process is highly sensitive. Furthermore, if we start up the plant avoiding the given operating ranges, we end up at another steady state and hence undesirable product purities. The controller settings, derived from IMC and TL tuning relations, when applied to linear model as well as to non-linear model show proper setpoint tracking and load rejections. From the quantitative performance measures, setpoint tracking, and load rejection tests, the SOPTD–PID controller gives the best performance amongst the FO-PI, FOPTD-PI, FOPTD-PID, and SOPTD-PID controllers. Also, the centralized controller gives better performance, as compared to decentralized controller. Thus, the centralized controller regulates away the interactions more effectively than the decentralized controller. Even for λij > 1, the centralized controller shows better performances. However, the SOPTD model-based controller gives the best performance for any magnitude of setpoint value change and low value of load change. If the load value is high, FO modelbased controller gave the best performance, as indicated by robust stability analysis. In other words, for lower frequency range (10<sup>2</sup> to 1 rad/s), the centralized FO-PI controller gives better robust stability as compared to decentralized FO-PI controller, and SOPTD-PID decentralized controller shows better robustness for higher frequency range (1 rad/s and above). We also conclude that the setpoint changes are tracked effectively for higher order models, whereas the load changes may or may not be regulated by higher order models. Thus, a proper trade-off has to be done between performance and robustness when selecting the control configuration and the model-based controller.
