(70c) Analysis of Infeasiblity in Process Scheduling

Planning and scheduling techniques, which generate feasible, robust, and operationally acceptable schedules, are crucial for a variety of industries. While the planning horizon in most strategic applications spans weeks, we focus on tactical scheduling horizons on the order of hours or days. Imperative to these solutions are requirements that real-time disturbances be accommodated into any proposed schedule. Examples of such disturbances are: equipment failures, resource storage policy change, resource availability etc. In many cases, the optimal schedule obtained a priori is not robust to such changes. On-the-fly rescheduling algorithms try to repair such infeasibilities by employing quick heuristics.

In this work, we present use of infeasibility analysis for a multi-purpose chemical batch process scheduling especially in the context of rescheduling. We isolate multiple irreducible inconsistent sets (IISs) systematically to identify bottle-necks in process scheduling network. An IIS is a subset of an infeasible model, such that the IIS itself is infeasible but any proper subset of the IIS is feasible. Isolating an IIS in an infeasible model provides a set of mutual inconsistencies that must be eliminated to achieve feasibility. The methodology incorporates interactive scheduling interface which provides quick resolution to real-time disturbances. We demonstrate the power of this methodology in successfully analyzing infeasible scheduling models with the well-known Westernberger-Kallrath case study.