In this paper we present an extension of an earlier model for short-term scheduling of continuous/semicontinuous plants based on conditional sequencing, where a consumption task is conditionally aligned to a production task only if it receives material from production task; otherwise, if material is received from storage then sequencing is relaxed. Four-index binary and continuous variables are used to handle conditional sequencing by modifying the material balances and sequencing constraints in the earlier models. Two models with different timing variables are presented, and appropriate lower and upper bounds are postulated for different scenarios involving planned maintenance. The proposed models are demonstrated on different instances of a motivating example and on literature examples related to scheduling of continuous plants under planned maintenance operation, resulting in either improved objective function value and/or the converged solution is found using less number of events.

In this study, we present extensions of earlier models for integrated scheduling of optimal production and planned maintenance for continuous and semicontinuous plants to account for different storage requirements. For handling finite storage, we propose approximate modeling of storage (without loss of generality) based on conditional sequencing without considering storage as a separate task. For dedicated storage tanks, when the "total amount to be stored" at an event is within the storage capacity, then we model an approximation of the amount stored. The conditional alignment of consumption and production tasks is extended for handling different storage policies. These features lead to reduction in the number of events. Two different models, based on task-specific and unit-specific timing variables, are proposed for handling finite intermediate storage (FIS), no intermediate storage (NIS), and zero wait (ZW) policies by modifying different constraints in the earlier models. The proposed models are presented for two cases based on the delivery of products and intermediates (1) as and when required and (2) at the end of the time horizon. The concept of planned maintenance is simultaneously incorporated with inventory management and optimal production. The proposed models are solved on different case studies, resulting in fewer events and better computational performance for small to medium scale problems, when compared with the literature model.