Short-Term Scheduling of Batch Processes. A Comparative Study of Different Approaches

The approaches to scheduling problem formulation in chemical engineering can be broadly classified into two categories, namely, the standard recipe approach (SRA) and the overall optimization approach (OOA). In the SRA, first the recipes are standardized either empirically or via single-batch optimization (SBO) and then the production scheduling problem is formulated on the basis of these standardized recipes. However, the standardization of recipes removes degrees of freedom from the system, and because of this, the solutions obtained with this approach can be suboptimal, whereas in the OOA, the process dynamics are directly included in the scheduling problem formulation instead of the standardized recipes. This restores the additional degrees of freedom of the system, and therefore this approach can yield a better solution. However, direct inclusion of the process dynamics in the scheduling problem formulation results in a mixed-integer dynamic optimization (MIDO) problem, the solution of which can be a formidable task. The objective of this paper is to illustrate the advantages and disadvantages of the SRA and OOA for short-term scheduling of batch chemical processes with the help of illustrative examples. It is shown that the results crucially depend on the cost structure of the specific application as well as on the objective function employed.