Optimal periodic scheduling of batch plants using RTN-based discrete and continuous-time formulations:: A case study approach

This paper considers the problem of deriving an optimal periodic schedule for an industrial batch plant. Both discrete and continuous-time formulations, based on the general resource task network process (RTN) representation, are employed. For a given cycle time, the proposed discrete-time formulation results in a mixed integer linear programming (MILP) program that can be solved to optimality in a reasonable time, even for a fine discretization of the time grid. The optimal cycle time is then determined by solving a sequence of fixed cycle time problems. On the other hand, the continuous-time formulation results in a mixed integer nonlinear (MINLP) problem that under the assumption of constant throughput becomes a MILP. This can be solved to optimality within reasonable computational effort only for a relatively small number of event points, making it practically impossible to find the global optimum. These results favor the use of the discrete-time formulation over its continuous-time counterpart.