Abstract In the first part of this paper, we present the results of a survey of production scheduling problems occurring in a broad range of industries, including the manufacture of polymers and aromas, and the processing of minerals. The surprising result of interviews with plant managers and production schedulers is the remarkable similarity in the process structures, product sequencing constraints, and scheduling objectives important to these diverse industries. Scheduling to reduce tardiness of customer orders was found to be the most important objective. A key requirement was the ability to handle realistic constraints on product sequencing and restrictions on the number of units which can make a given product. Thus, a class of industrial scheduling problems important to a broad range of process industries is defined. This is significant, since a great deal of research effort has, in the past, been devoted to problems with other scheduling objectives, without sequencing constraints and with identical units. In the second part, a solution method is presented which is shown to obtain optimal or near-optimal solutions for processes involvin stage of nonidentical parallel units. Sequence-dependent cleanouts, unit- and product-dependent processing times and product-to-unit assignment constraints are modeled. Test problems with up to 12 process units and up to 100 customer orders were solved, which is the typical size of industrial-scale problems. An evolutionary strategy was developed which generates an initial solution using heuristics, then systematically improves the solution until certain necessary conditions for optimality are satisfied. The method found optimal or near-optimal schedules for highly-constrained and difficult test problems with modest computational requirements. These results are significant since an effective solution method for single-stage industrial problems of this complexity has not heretofore been demonstrated.
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