Conclusion

This work has formulated a Pinch-Based simultaneous targeting and synthesis procedure for multi-contaminant material recycle-reuse networks, on the example of water networks. It involves the methods to determine the source prioritisation sequence for each sink, classification of sinks to Below/Above Pinch, and determine the limited flowrate of the sources that can be assigned. The allocated sinks and sources are then arranged with ascending order of their concentrations. After the assignment of the sinks and sources to each contaminant cascade, the typical PA can then be performed for each contaminant cascade and for each sink sequentially. If the Pinch Point occurs at the non-limiting contaminant of the sink, a source reduction can be performed to further reduce the freshwater target for that sink. The proposed method is validated with an illustrative example and two real case studies from the paper mill and starch industry. The obtained freshwater targets for both cases studies are 39,836 kt/d and 47.36 kg/h. The results agree well with the solution from the literature and the solutions obtained by solving the superstructure model. The consistent solutions obtained compared with optimisation method and previous works prove the accuracy and legitimacy of the method.
Note that the freshwater requirements determined in this work represent the resource requirement ignoring the interaction between the sinks and sources are. The data used in this work are the maximum flowrates and concentrations of the sources and sinks. Certain sink’s maximum concentration limit might not be reached, and this could affect the concentration of the sources from the same operation of the sink. If fixed contaminant load is assumed for each unit operation, the concentration of the source might be reduced as well, and the overall fresh requirement can be reduced. For fixed load operation, the flowrates of the sink and source can be reduced, as long as the fixed mass load is guaranteed. This could lead to a reduction in a fresh resource. However, future research should incorporate the realistic relationships between the inlet and outlet contaminant concentrations. The interaction between multiple contaminants in either inlet or outlet streams should be taken into account as well. Future research can be focused on a Multi-Contaminant Cascade Table Analysis. It can be developed for automated resource targeting. Water regeneration potential and multiple fresh resources with prioritised objectives should be incorporated into the analysis as well.