Optimal scheduling of a multiproduct batch chemical plant with preemptive changeover tasks

Abstract

Most short-term scheduling formulations in Process Systems Engineering assume a time horizon without interruptions. However, local working patterns may prevent execution of activities at nights or during weekends. One such problem is reported in Egli and Rippin (1986) and deals with a multiproduct batch chemical plant featuring recipes with stable and unstable intermediates, shared resources with limited availability of electricity and steam, non-instantaneous transfer times, and the possibility of interrupting changeover tasks when encountering a pre-defined break period. The objective function balances sequence-dependent changeovers with product holding costs. In this paper, we model these features with a Resource-Task Network (RTN) formulation based on a discrete-time representation. After solving the original problem to optimality for savings of 5%, we evaluate the alternative of forcing complete execution of changeover tasks before or after a break, for a couple more scenarios. The results show that preemption is always beneficial, with improvements ranging from 0.57% to 36.4%.