On the Minimum Number of Units in Heat Exchanger Networks

Abstract

The minimum number of units in a network has been proposed to be obtained using a very simple formula using graph theory (Hohmann, E. C. Ph.D. Thesis, University of Southern California, Los Angeles, CA, 1971.; Linnhoff, B. et al. Comput. Chem. Eng. 1979, 3, 295). This is done, usually, assuming that thermodynamic feasibility holds, especially in Pinch technology, where it is applied above and below the pinch, but also for cases where the pinch is ignored. While the failure of this formula is informally known in the community, to our knowledge no realistic counterexample was presented before. We provide such an industrial counterexample. In addition, we propose to use a recently developed MILP model (Barbaro, A., and Bagajewicz, M. Comput. Chem. Eng. 2005, 29, 1945) that guarantees finding the global minimum number of exchangers. Finally, we point out the large number of alternative solutions that real industrial problems may exhibit.