A State Task Network model for scheduling operations in cascaded continuous processing units

The paper proposes a mathematical model based on State Task Network representation for generating optimal schedule for a sequence of n continuous processing units responsible for processing m products. Each product line has fixed capacity storage tanks before and after every unit. Each unit can process only one product line at any point of time. However, the inputs for all the product lines arrive simultaneously at the input side of the first unit, thus giving rise to the possibility of spillage. There can be multiple intermediate upliftments of the finished products. An optimal schedule of the units attempts to balance among spillage, changeover and upliftment failures. The paper develops an MINLP model for the problem. The model has been linearized using standard linearization techniques and the resulting MILP model can output, in reasonable time, optimal schedule for 2 weeks for a 3-unit 3-product scenario encountered by the authors.