A MIP/RCPSP decomposition approach to short-term planning in chemical batch production with non-identical parallel processing units

We consider the problem of planning and scheduling physical and chemical processes on a multi- product chemical batch production plant. Such a plant consists of several multi-purpose processing units and storage facilities of limited capacity. Given primary requirements for final products, the problem consists in generating an appropriate set of batches for each process and scheduling the processing of those batches on the processing units subject to different types of technological constraints. In the literature the short-term planning problem is generally modeled as a monolithic mixed-integer linear program. Due to the combinatorial nature of the problem, those models generally cannot be used when dealing with problem instances of practical size. In this paper we propose a two-level approach which is based on a decomposition of the problem into a batching and a batch scheduling problem. We formulate the batching problem as a mixed-integer linear program, which allows for considering the execution of processes on alternative processing units with unit-specific lower and upper bounds on the batch sizes. The batch scheduling problem can be solved using a specific method known from the literature. We report on computational results for a sample production process from the chemical engineering literature.