An optimal control approach for scheduling mixed batch continuous process plants with variable cycle time

Effective scheduling of operations in the process industry has the potential to achieve high economic returns. Process plants containing both batch and continuous units present a difficult scheduling problem. When these processes are modelled with batch cycle times as decision variables, the complexity of the problem is increased significantly. These problems when modelled in the conventional MILP formulation are extremely difficult to solve as they are NP-hard. Many solution methods require unacceptable amounts of time/memory to solve even a simple problem. This paper considers an alternate way to represent the problem in an attempt to improve solution performance. This method is based on optimal control and the hierarchical splitting of the optimisation problem. Results are compared to the traditional MILP formulation. The motivation for this work is an existing scheduling problem in the sugar milling industry. A smaller problem, which has characteristics of the sugar milling problem is considered for performance comparison. A substantial reduction in the complexity required to solve several test-cases of this problem is achieved using the optimal control formulation, while maintaining good quality solutions. (C) 1999 Elsevier Science Ltd. All rights reserved.