A GENERAL ALGORITHM FOR SHORT-TERM SCHEDULING OF BATCH-OPERATIONS .2. COMPUTATIONAL ISSUES

The first part of this paper (p. 211) presented a general mathematical framework for describing a wide variety of scheduling problems arising in multiproduct/multipurpose batch chemical plants. The problem is formulated as a large mixed integer linear programming model (MILP). We describe a variety of techniques that exploit the characteristics of the problem in order to reduce the amount of computation required. These include reformulation of some of the constraints, derivation of an alternative and much more compact linear programming relaxation of the MILP, and reduction of the non-integrality of the solutions of relaxed LPs through their a posteriori analysis. The combination of the three measures results in a significant improvement in computational performance without compromising the optimality of the solution obtained. A case study is presented to illustrate the applicability of the method to the scheduling of multipurpose plants under a variety of operational constraints.