A novel approach to scheduling multipurpose batch plants using unit-slots

Abstract

Several models for scheduling multipurpose batch plants exist in the literature. The models using unit-specific event points have shown better solution efficiency on various literature examples. This article presents a novel approach to scheduling multipurpose batch plants, which uses unit-slots instead of process-slots to manage shared resources such as material storage. We develop two slightly different models that are even more compact and simpler than that of Sundaramoorthy and Karimi, Chem Eng Sci. 2005;60:2679-2702. Although we focus on material as a shared resource, our multigrid approach rationalizes, generalizes, and improves the current multi-grid approaches for scheduling with shared resources. Our models allow nonsimultaneous transfers of materials into and out of a batch. We show by an example that this flexibility can give better schedules than those from existing models in some cases. Furthermore, our approach uses fewer slots (event-points) on some examples than even those required by the most recent unit-specific event-based model. Numerical evaluation using literature examples shows significant gains in solution efficiency from the use of unit-slots except where the number of unit-slots required for the optimal solution equals that of process slots. We also highlight the importance of constraint sequencing in GAMS implementation for evaluating mixed-integer linear programming based scheduling models fairly. © 2009 American Institute of Chemical Engineers.