s
The overall mass targets identified using the techniques of Chapter 3, Benchmarking Process Performance Through Overall Mass Targeting, can be attained when there are no technical or financial limitations on the solutions. The implementations involve several strategies such as rerouting of streams, manipulation of design and operating variables, addition of new process units, substitution of solvents and chemicals, and alteration of technologies. These strategies provide different levels of cost and benefit. In this chapter, focus is given to direct-recycle networks. Recycle refers to the utilization of a process stream (e.g., a waste or a low-value stream) in a process unit (a sink). While reuse is distinguished from recycle by emphasis that reuse corresponds to the reapplication of the stream for the original intent, we will use the term recycle in a general sense that includes reuse. Specifically, this chapter will focus on direct recycle where the streams are rerouted without the addition of new pieces of equipment. As such, direct recycle is a no/low-cost strategy since it essentially involves pumping and piping and in some cases may even be achieved without the need for additional pumping or piping. In this chapter, a holistic approach is presented for the synthesis of direct-recycle networks with the objective of determining rigorous targets for minimum usage of fresh materials, maximum recycle of process streams, and minimum discharge of waste materials. First, the design problem is stated. Then, a graphical approach (the material-recovery pinch diagram) is described to identify the targets. Next, an algebraic technique is presented to determine the same targets obtained from the graphical approach. Finally, the source-sink mapping diagram is used to identify the detailed rerouting implementations that achieve the targets.
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