Short-Term Scheduling of Batch Plants: Reformulation for Handling Material Transfer at the Same Event

This work presents a new variant of a unit-specific event-based model for short-term scheduling of batch plants. The motivation behind the proposed model is to further reduce number of events necessary to efficiently solve a given scheduling model. The main novelty in the proposed formulation is that the production and consumption tasks of each intermediate state are modeled to take place at the same event, in contrast to existing batch scheduling models where the consumption tasks normally start at the next event compared to the production tasks. This is the first time such a feature has been attempted in batch process scheduling, although it has been done in scheduling of continuous plants. Also, this is the first attempt where we are able to model the unit-specific events in their true sense, where the first event in each unit actually takes up the first event. The model is primarily applicable and useful for problems involving no recycle streams, although a hybrid approach has been suggested for problems with recycle streams and different reformulations are presented for both cases. In almost all the cases considered, the proposed model gives a reduction in the number events (on the order of two to three) and fewer constraints and variables compared to recent models in the literature. In 77% of the cases, especially for problems with no recycle streams, the proposed feature also results in better computational performance.