Lot sizing and sequencing optimisation at an animal-feed plant

This paper applies a mixed integer programming model for joint lot sizing and scheduling to a plant for animal feed compounds. A key characteristic of this industry is that certain products can perform a production line cleaning function if a sufficiently large lot is produced between two products that would otherwise require a cleaning setup. Thus the sequence-dependent setup times do not always obey the triangular inequality. Tested on data from the plant, the model takes too long to solve exactly and so several alternative formulations and methods are developed to solve the model more quickly, based on two variants of the Relax and Fix heuristic. The results confirm that the formulations are computationally effective and able to take economic advantage of the intermediate cleaning products. The model schedule substantially improves on that practiced at the feed plant.