A new model for scheduling of batch process with mixed intermediate storage policies

The problem of short-term production scheduling has been realized to be one of the most important problems in batch process operations. Often different intermediate storage policies may greatly change the scheduling result. Generally, storage policies can be divided into four classes: zero wait (ZW), no intermediate storage (NIS), finite intermediate storage (FIS) and unlimited intermediate storage (UIS). ZW, NIS or UIS are often considered in previous works. However, FIS or the mixed situation of these four storage policies is seldom involved because the state and storage time for storage tank is hard to represent. Though some approaches have been reported in recent literature, they are either limited to multi-product process or too complex to apply in large problems. In this paper, a novel simple mathematical formulation for short-term scheduling of batch process under mixed intermediate storage policies has been presented. It can deal with complex multi-purpose problems, in which processing paths of batches are different and storage policies may be optional so that the utilization of storage tanks is uncertain. The model uses a new kind of time domain representation that dose not rely on the definition of time slot. As a result binary variable and constraint can be effectively reduced. Intermediate storage tank is regarded as a unit with the particularity that material may not pass through it and the time for storage is variable. Consequently different intermediate storage policies can be described by an uniformed format. Finally the formulation results in an MILP problem. The new model has been tested by some case studies to show the good computational efficiency. Optimal result has been got quickly under GAMS/CPLEX.