Optimal design of non-dispersive solvent extraction processes

The purpose of this paper is to carry out the optimal design of a non-dispe rsive solvent extraction (NDSX) process for the removal and recovery of chr omium from waste waters of surface treatment industries. The objective is t o design a process that can separate the waste stream into an environmental ly acceptable stream with a low chromium concentration and a stream in whic h chromium is concentrated for further processing. A superstructure of the problem which is rich enough to account for all potential configurations an d connectivity of the system is proposed. The problem is formulated as a MI NLP optimisation problem to minimise the total cost of the process subject to design specifications and is solved using an Outer Approximation algorit hm. The nonconvexivity of the problem due to bilinear terms in the model eq uations makes necessary the reformulation of the nonconvex equations into l inear inequalities. Three different approaches are tried for this reformula tion. A bound tightening strategy based on the application of branch and bo und methods to the Outer Approximation algorithm is shown to be the most ef fective approach. (C) 2001 Elsevier Science Ltd. All rights reserved.