A feedback control framework for safe and economically‐optimal operation of nonlinear processes

Abstract

Maintaining safe operation of chemical processes and meeting environmental constraints are issues of paramount importance in the area of process systems and control engineering, and are ideally achieved while maximizing economic profit. It has long been argued that process safety is fundamentally a process control problem, yet few research efforts have been directed toward integrating the rather disparate domains of process safety and process control. Economic model predictive control (EMPC) has attracted significant attention recently due to its ability to optimize process operation accounting directly for process economics considerations. However, there is very limited work on the problem of integrating safety considerations in EMPC to ensure simultaneous safe operation and maximization of process profit. Motivated by the above considerations, this work develops three EMPC schemes that adjust in real‐time the size of the safety sets in which the process state should reside to ensure safe process operation and feedback control of the process state while optimizing economics via time‐varying process operation. Recursive feasibility and closed‐loop stability are established for a sufficiently small EMPC sampling period. The proposed schemes, which effectively integrate feedback control, process economics, and safety considerations, are demonstrated with a chemical process example. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2391–2409, 2016

Process operational safety using model predictive control based on a process Safeness Index

“…Thus, process safety is ensured while the process profit is maximized. Despite the guaranteed closed-loop stability and recursive feasibility properties of this method Albalawi et al (2016), as well as its economic optimization capabilities, it may be unnecessarily restrictive for many processes. For example, regions within which S(x) is below a desired threshold may not be level sets of a Lyapunov function, and trying to find the largest Lyapunov level set within a region where S(x) is less than the threshold may cause the level set to be quite small, which can greatly reduce the economic optimality of process operation within this small region compared to allowing the process to operate within the entire region where S(x) is less than a desired threshold.…”

“…This control design may improve process economic performance and be less conservative than the safety-based control design developed in our prior work Albalawi et al (2016), where safety-based constraints were included within LEMPC that were triggered when a measurement of the closed-loop state was outside a safe Lyapunov level set of operation termed the safety region Ω ρsp ⊂ Ω ρ . The level setbased method of triggering safety-based constraints is conceptually the same as developing a binary Safeness Index function that evaluates to either its value corresponding to safe operation within Ω ρsp (indicating that the process is within a 100% safe operating region and that the safety-based constraints do not need to be activated) or its value corresponding to unsafe operation outside…”

“…Despite many efforts to develop, A c c e p t e d M a n u s c r i p t in the presence of uncertainty Albalawi et al (2016). However, the assumption that safe operating regions are defined by Lyapunov level sets may be restrictive.…”

Responsive Economic Model Predictive Control for Next-Generation Manufacturing

“…In Reference [35], fault-tolerant control for nonlinear switched systems was analyzed in the context of safe parking for model predictive control with a steady-state tracking objective function for actuator faults. For EMPC, Reference [36] handled faults through error-triggered data-driven model updates in the controller, and the uniting of EMPC with driving the state into safety-based regions in state-space (e.g., References [37,38]) also constitutes a form of fault-handling. Despite these advances in handling anomalies and uncertainty, which are critical for addressing moving toward a verification paradigm for EMPC, verifying the controller today would still be expected to be time-consuming; additional work is needed to explore further ways of considering and establishing verification for the control design.…”

On integration of feedback control and safety systems: Analyzing two chemical process applications

“…Several recent works have proposed a systems perspective on process safety (e.g., [3,20,22,36]) which encourages engineers to consider process incidents as events that occur due to a migration of the process state, over time, to conditions at which an accident may occur (this may be applicable, for example, in the case of reactor thermal runaway). Traditional approaches to process safety like process design modifications neglect important aspects impacting process operational safety, such as multivariable interactions of process components and variables, limited control system authority due to limitations on the capacity of control actuators, and the manner in which the safety or relief system response may impact the effectiveness of the process control system [20,40].…”