Scheduling multistage batch plants with parallel units and no interstage storage

Abstract

Scheduling production optimally in multistage multiproduct plants with parallel units is a very difficult but routine problem. While most batch plants avoid interstage storage due to minimize contamination, cleaning, waste, and so forth, the scheduling of plants with no interstage storage and mixed wait policies have received little attention compared to those with unlimited storage/wait. In this paper, we develop and evaluate several different mixed-integer linear programming formulations for scheduling plants with identical and nonidentical parallel processing units and unlimited and zero-wait interstage policies. Because the best approach for handling identical parallel units seems to be sequence-based and that for handling nonidentical units seems to be slot-based, we employ judicious mixes of these approaches to address real plants with mixes of stages with identical and nonidentical units. Our models also allow mixes of unlimited and zero interstage wait policies and scheduling objectives of makespan, tardiness, earliness, and weighted just-in-time. The weighted just-in-time scheduling seems to be more difficult than even the makespan scheduling, and more importantly, a modeling approach that does well for the former does not necessarily suit the latter. While the models presented in this paper do address successfully the scheduling needs of realistic batch plants, considerable future work is warranted for models that can solve large scheduling problems of this type. © 2007 Elsevier Ltd. All rights reserved.