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.
In this paper, an integrated multilevel, control-theoretic framework has been proposed for effectively handling integration of planning, scheduling, and rescheduling. A general resourceconstrained, multistage, multiproduct plant operating as a hybrid flowshop facility has been considered. The proposed approach is based on the inherent hierarchical decomposition of the overall decision-making process that is a typical characteristic of large enterprises. The overall problem is segregated into three levels with different horizons, wherein planning over multiperiods is at the top level followed by scheduling for a single period at the middle level and a detailed inventory management schedule for the operator at the lower level. In the hierarchical decomposition, the upper levels are equipped with abstractions of the lower levels and proactiveness for reactive scheduling. The integration of reactive scheduling is motivated by some of the process control principles like cascade control and the concepts of receding horizon. Using the philosophy of decentralized decision-making, it is demonstrated that the lower levels with accurate models have the flexibility and amenability for rescheduling without upsetting the global performance. As an illustrative case study, cyclic scheduling of a simple refinery flow sheet involving continuous lube production in a resource constrained hybrid flowshop is presented to demonstrate the proposed methodology.
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