Scheduling of batch processing plants: constraint model and computer scheduling system

In the process industries batch processing plants are attracting attentions because of their suitability for the manufacturing of small-volume, high-value added products. Pipeless batch plants have also been developed and built to increase plant flexibility to adapt to fast market change. Scheduling batch plants by using computer-aided systems is important for improving the plant productivity since it harmonizes the entire plant operation efficiently to achieve production goals. However, the current scheduling approaches for batch plants are inadequate, particularly for pipeless plants. The main objective of this study is to develop a general-purpose methodology to solve a common class of scheduling problems for batch plants including pipeless plants. This research involves the creation of a general scheduling model based on constraint satisfaction techniques (CST) and the development of a scheduling system that applies the model. The model comprises of three parts, which are concerned with production scheduling, integrating routing and scheduling in pipeless plants, and rescheduling. Production scheduling considers many process constrains on time and resource allocation. One of the novel aspects is to consider finite wait time constraints that limit the unstable intermediate materials to stay in the storage and processing units to a finite time. Another novel aspect is to consider integrating routing and scheduling in pipeless plants. A planning process is applied to generate a possible route between two stations and the feasibility of the route is checked by the scheduling process in which the transfer time and track allocation constraints are imposed. If conflicts take place, the system backtracks to find another route. Recovery-based rescheduling constraints are also proposed in the model to deal with resource breakdown and they can ensure the failed resource cannot be used during its failure period and what has been done before the failure remains unchanged. Plant layout is considered when a pipeless plant is rescheduled. A prototype scheduling system, Batch Processing Scheduler (BPS), was developed in C++ to apply the constraint model based on CST. A number of examples are devised to investigate the performance of the constraint model and BPS, and their feasibility and limitations are demonstrated by the results.