All-At-Once and Step-Wise Detailed Retrofit of Heat Exchanger Networks Using an MILP Model

This paper builds upon the MILP model developed by Barbaro and Bagajewicz (New Rigorous One-Step MILP Formulation for Heat Exchanger Network Synthesis. Comp. Chem. Eng. 2005, 1945-1976), which allows the rigorous one-step grassroots design of heat exchanger networks. For the retrofit, we consider the cases of addition and relocation of heat exchangers allowing control of repiping costs as well as splitting. While previous works considered area reduction in existing exchangers, they very rarely took into account the associated cost, which we now do. We also add all the costs associated with new shells, area addition to existing shells, relocation, and piping changes. We illustrate the power of the formulation with a small example as well as with a crude fractionation unit. Moreover, we discuss step-by-step changes that allow better planning around turnarounds as opposed to an all-at-once solution. Finally, the model also offers a good level of flexibility that opens room for decision making by the users such as allowing/disallowing splitting, considering area and shell additions only (i.e., disallowing relocation), limiting the number of new exchangers and/or the number of relocations, etc. Various design case studies of a same process example are considered to demonstrate the flexibility and versatility of the model.