(88e) Simultaneous Targeting and Scheduling for Water Minimisation in Batch Plants: A Polyvinyl Chloride (PVC) Manufacturing Case Study

Batch processing has received considerable attention from both industry and academia for its flexibility and suitability for the production of low volume, high value-added products, also as a result of the recent trend towards product-centred manufacturing globally. With inherent complexity, the design and synthesis of batch processes necessitates concurrent development of effective scheduling techniques. Common objectives include profit maximisation and cost minimisation in the time horizon of interest. In addition to process scheduling, process integration may be considered to improve energy utilisation and/or reduce waste production. A particular case commonly found in the process industries is water integration for minimum fresh water consumption and wastewater generation. Various process integration techniques and tools have been developed for resource conservation in batch plants. Most early developed techniques assume the schedule to be fixed, while many of recent ones are combined with a scheduling model to exploit the flexibility of batch processes. The automated targeting model – ATM – is one of the established tools for batch water network synthesis. Its formulation is based on pinch analysis and thus enables flow targets to be identified prior to detailed network design. A great advantage of the ATM is its linearity, for which global optimality is guaranteed. Furthermore, it is more flexible than conventional pinch methods being able to incorporate cost considerations. However, the ATM was only developed for cases with fixed schedules. This drawback will be overcome in the present work by embedding the ATM in a scheduling formulation to allow flow/cost targeting and process scheduling to be performed simultaneously, and the true minimum flow/cost targets to be located. A case study on polyvinyl chloride (PVC) manufacturing is presented to demonstrate the application of the proposed simultaneous approach. In the batch-operated PVC plant, the upstream reactor is scheduled to match the downstream separator, while water minimisation through direct reuse/recycling is considered. The trade-off between fresh water (operating) and storage (capital) costs will also be analysed.