(657c) Process Design and Control Interactions Under Time-Varying Operating Strategies

Economic model predictive control (EMPC), which incorporates a free-form performance index in its formulation, has been proposed to integrate process control and economic optimization [1, 2, 3]. However, an outcome of this change in the control system is that EMPC may operate a process in a dynamic fashion to optimize its economics (i.e., the plant is not forced to operate at a pre-specified steady-state setpoint). In particular, a concern regarding time-varying operating strategies is how process and equipment designs and instrumentation schemes impact what can be achieved under EMPC. In our recent work [4], we demonstrated that there are considerations beyond process dynamic behavior that should contribute to the design of an EMPC (e.g., an analysis of the temperatures, stresses, or strains in equipment in contact with a process fluid where the process is operated under EMPC). It is necessary to similarly investigate how process design and instrumentation setup should inform EMPC design, and whether the dynamic process behavior under the controller might suggest alternative design and instrumentation plans.

This work performs preliminary analyses that elucidate interactions between process design/instrumentation selection and EMPC design that differ from the considerations which would hold if the process could not be operated in a dynamic fashion by the controller. A simulation study is performed of a reactor for which temperature and exit concentration are controlled by EMPC, and a heat exchanger follows the reactor, with the process fluid from the EMPC on the hot side and a cooling fluid on the cold side. For operating goals on both the hot and cold sides (e.g., maximization of the production rate of the product species in the CSTR and maintaining the temperature at the outlet of the cold side close to a pre-specified value due to subsequent use of that cooling stream for heat integration), we demonstrate that design factors (e.g., the flow rate of cooling fluid on the cold side of the heat exchanger) and instrumentation selection (e.g., whether or not the flow rate of the cooling fluid is adjustable) determine the extent to which the economic objectives of the EMPC can be optimized while meeting the cold side temperature requirement. We elucidate how the process design, control architecture, and EMPC formulation might be considered together at the design stage to develop frameworks that trade-off between computation time, profit, and complexity and suggest principles which underly this combined control/design framework for EMPC for extension to other processes.

References:

[1] M. Ellis, H. Durand, and P. D. Christofides. A tutorial review of economic model predictive control methods. Journal of Process Control, 24:1156–1178, 2014.

[2] J. B. Rawlings, D. Angeli, and C. N. Bates. Fundamentals of economic model predictive control. In Proceedings of the IEEE Conference on Decision and Control, pp. 3851-3861, Maui, Hawaii, 2012.

[3] M. A. Müller, D. Angeli, and F. Allgöwer. Economic model predictive control with self-tuning terminal cost. European Journal of Control, 19:408-416, 2013.

[4] H. Durand. On accounting for equipment-control interactions in economic model predictive control via process state constraints. Chemical Engineering Research and Design, 144:63-78, 2019.