On the interactions of chemical-process design under uncertainty and maintenance-optimisation

While the impact of uncertainty in process parameters (such as product demands and prices, physical properties, etc.) on the design and operation of a chemical process has been widely recognised and studied over the last fifteen years, relatively little progress has been made in studying the influence of process uncertainty on maintenance scheduling and optimisation. This paper presents a novel optimisation formulation and a decomposition solution strategy for addressing the problem of maintenance scheduling under uncertainty in the context of chemical process design and operation. The proposed formulation features an expected profit objective function which takes into account the transitions between the different states of the chemical system, due to equipment failure; corrective and preventive maintenance policies are explicitly considered. The resulting model corresponds to a large scale mixed-integer nonlinear optimal control problem. By exploiting reliability properties, an effective two-step decomposition solution procedure is then proposed, which as illustrated with a process example problem, depicts an optimal preventive maintenance policy in the presence of process uncertainty: the time instants of required maintenance actions over a time horizon and the optimal sequence of components on which preventive maintenance is to be performed.