A New General Continuous-Time State Task Network Formulation for Short-Term Scheduling of Multipurpose Batch Plants

A new continuous time MILP model for the short-term scheduling of multipurpose batch plants is presented. The proposed model relies on the State Task Network (STN) and addresses the general problem of batch scheduling, accounting for resource (utility) constraints, variable batch sizes and processing times, various storage policies (UIS/FIS/NIS/ZW), batch mixing/splitting, and sequencedependent changeover times. The key features of the proposed model are the following: (a) a continuous time representation is used, common for all units, (b) assignment constraints are expressed using binary variables that are defined only for tasks, not for units, (c) start times of tasks are eliminated; thus, time matching constraints are used only for the finish times of tasks, and (d) a new class of valid inequalities that improves the LP relaxation is added to the MILP formulation. Compared to other general continuous STN formulations, the proposed model is significantly faster. Compared to event-driven formulations, it is more general as it accounts for resources other than equipment and often gives better solutions being equally fast. The application of the model is illustrated through four example problems.