Rigorous graphical targeting for resource conservation via material recycle/reuse networks

Material recycle/reuse is one of the key strategies in reducing the consumption of fresh resources in the process industries. Over the past decade, several techniques have been developed to reduce the consumption of specific material utilities such as water and hydrogen. To date, none of the published techniques provides a noniterative, systematic, and graphical technique for identifying a target for minimum usage of the fresh resources ahead of detailed design of the recycle/reuse network. In this paper, we present a rigorous graphical targeting approach to minimize the use of fresh resources by using segregation, mixing, and direct recycle/reuse strategies. First, the problem is formulated mathematically to provide a systematic basis for its solution. Then, dynamic programming techniques are employed to derive the mathematical conditions and characteristics of an optimal solution strategy. These conditions and characteristics are transformed into a graphical form that can be readily used to identify rigorous targets for minimum usage of fresh resources. The graphical technique is also useful in locating a material recycle/reuse pinch point, which provides insightful information on the use of fresh resources, the discharge of unused materials, and the relationships between process streams (sources) and units (sinks). Several test problems are solved to illustrate the ease, rigor, and applicability of the developed targeting technique.