New Multiple Time Grid Continuous-Time Formulation for the Cyclic Scheduling of an Industrial Batch Plant

In this paper, a cyclic scheduling problem from a batch pulp plant is used to test a new continuous-time, multiple time grid formulation. It can be employed in multistage multiproduct batch plants with a single unit per stage and complex interactions between immediate orders, which in this case take the form of integrated heating tasks. The results show that the solution of a real life problem can be obtained in under a second, which is some orders of magnitude faster than previous single grid discrete and continuous-time formulations based on the Resource Task Network (RTN). The new formulation is also considerably simpler, allowing to perform capacity expansion studies resulting from the addition of more parallel units. In particular, it is shown that steam availability is a major bottleneck, leading to a much lower productivity increase than the value expected from the increase in capacity.