Cyclic Scheduling of Continuous Multiproduct Plants in a Hybrid Flowshop Facility

This paper addresses cyclic scheduling of continuous multiproduct plants operating in a hybrid flowshop, where the operation in the plant envelope is neither strictly sequential nor in a strictly parallel mode. This plant configuration is typically found in many manufacturing industries because of capacity expansion, debottlenecking, and retrofitting, and it poses problems in inventory handling. In addition, resource constraints are considered in the proposed model, where the feed to the hybrid flowshop is a continuous stream with finite intermediate storage (FIS) on the upstream side. It is shown that the resulting scheduling problem is influenced by both the upstream FIS and the downstream demand-related constraints. Rigorous inventory constraints are proposed to handle FIS on the continuous feed and the intermediate inventory. The proposed mixed-integer nonlinear programming model also incorporates a new and practical way of handling product transitions. In the continuous representation of the time domain, an alternate definition of the time slot is proposed to account for the feed loss during transitions. Finally, the proposed model is generalized to account for the case of slots having zero time duration. The novelties of the proposed formulations are demonstrated on a representative scheduling problem of a refinery.