Production scheduling of flexible continuous make-and-pack processes with byproducts recycling

This work considers the scheduling problem of continuous make-and-pack industries, including flexible intermediate storage vessels, aiming to provide better synchronisation of the production stages. A novel continuous-time, precedence-based, MILP model is developed for the problem under consideration. Mass balance constraints are cleverly satisfied using a continuous-time representation. Extending previously proposed precedence-based frameworks, flexible vessels are used for storing multiple intermediates of the same recipe. Furthermore, new efficient resource-constraints, related to generation and recycling of byproduct waste are introduced to consider additional benefits by their utilisation in the plant. A two-stage MILP-based solution strategy is proposed for the solution of real-life, large-scale industrial problem instances. Several case studies, inspired by consumer goods industries, are used to illustrate the applicability of the proposed framework. Results illustrate that the utilisation of intermediate buffers leads to a better synchronisation of the production stages and increased productivity, as unnecessary idle times, total waste and total plant costs are reduced.