(728c) Global Optimization of Batch Direct-Recycle Networks

Direct recycle networks are among the most attractive strategies for promoting sustainable processing. They lead to conserving material resources and mitigating negative environmental impact. While significant progress has been made in developing systematic techniques for the design of direct-recycle network in steady-state systems, much less work has been undertaken for batch systems. The objective of this work is to develop an optimization-based approach to the synthesis and scheduling of direct recycle networks for batch systems. A source-tank-sink representation is developed to embed potential configurations of interest. The representation allows for scheduling within the same cycle or between consecutive cycles. A two-stage approach is adopted: targeting followed by detailed solution. The targeting of fresh material usage and waste discharge is handled through a linear programming formulation. Network synthesis and scheduling involve the development of nonlinear program or a mixed-integer nonlinear program, depending on the choice of cost functions. In either case, the presence of bilinear terms induces non-convexity and necessitates the development of a global optimization approach. A global optimization technique is developed. The approach is based on three concepts: linearization by discretizing nonlinear variables, pre-processing using implicit enumeration of the discretization to form a convex-hull which limits the size of the search space, and application of integer cuts to insure compatibility between the original problem and the discretized formulation. A case study is solved to illustrate the computational aspects of the developed formulation and solution technique.