(137a) Optimal Time-Varying Operation of Nonlinear Process Systems With a Two-Layer Economic Model Predictive Control Scheme

Ellis, M., University of California, Los Angeles
Durand, H., University of California, Los Angeles
Christofides, P. D., University of California, Los Angeles

Time-varying operation is not a new concept within chemical process industry and has been extensively studied within the context of periodically operated chemical reactors [1]-[3]. This research has resulted in numerous demonstrations illustrating that periodic operation of various chemical reactors can result in better economic performance. However, optimal time-varying operation has only been previously studied in the context of determination of the optimal periodic switching pattern [3]. Therefore, it is important to develop systematic methods for determining optimal time-varying operating strategies of these various chemical processes. Economic model predictive control can accomplish this goal by determining the economically optimal time-varying operating strategy on-line and in real-time [4]-[6].

In this work, we propose a two-layer approach to dynamic economic optimization and process control for optimal time-varying operation of nonlinear process systems. The upper layer, utilizing a Lyapunov-based economic model predictive control (LEMPC) system, is used to compute dynamic economic optimization policies for process operation. The lower layer, utilizing a Lyapunov-based MPC (LMPC) system, is used to ensure that the closed-loop system state follows the optimal time-varying trajectories computed by the upper layer over each finite-time operating window. To improve the computational efficiency of the two-layer structure, we allow both the LEMPC and the LMPC to compute control actions for two distinct sets of manipulated inputs thus decreasing the real-time computational demand compared to other one-layer EMPC schemes. Following a rigorous formulation and analysis of the proposed method, we demonstrate boundedness of the closed-loop system state and closed-loop economic performance improvement with the proposed two-layer framework compared to steady-state operation as well as with respect to other existing time-varying operation strategies previously proposed in the literature in the context of a benchmark chemical process application. Lastly, we also perform closed-loop simulations of the benchmark chemical process application with other one-layer EMPC structures [4]-[6] and provide a thorough analysis of each control scheme.


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