A general continuous state task network formulation for short term scheduling of multipurpose batch plants with due dates

A new continuous-time MILP model for the short term scheduling of multipurpose batch plants with due dates is presented. The proposed model is a general State Task Network (STN) formulation that accounts for variable batch sizes and processing times, various storage policies (UIS/FIS/NIS/ZW), utility constraints (other than units), and allows for batch mixing and splitting. Its key features are: (a) a continuous, common for all units time partitioning, (b) assignment constraints are expressed using only the binary variables defined for tasks, (c) start times of tasks are eliminated, (d) a new class of tightening valid inequalities is added to the MILP formulation, and (e) a new disjunctive programming formulation is used for the matching of due dates with time points. The proposed model is more general than the previously reported models and is computationally efficient.