Short-term scheduling of multistage batch plants with unlimited intermediate storage

This paper presents a new multiple time grid, continuous-time, mixed integer linear program model for the short-term scheduling of multistage, multiproduct batch plants. The formulation is based on the resource-task network process representation and explicitly considers a single intermediate storage unit per stage to keep track of material transfer between processing units belonging to consecutive stages of production. It is compared to other existing unit-specific and sequence-based approaches through the solution of a few example problems taken from the literature. Alternative objective functions such as total cost, makespan, and total earliness minimization, as well as revenue maximization, are considered. The results show that the proposed formulation performs similarly to the one of Castro and Grossmann (2005), which is limited to problems involving a single product batch per stage. In addition, it can be up to 3 or 4 orders of magnitude faster than the one of Shaik and Floudas (2008), due to a lower integrality gap, the use of a smaller number of event points, and fewer constraints.