Optimal retrofit of heat exchanger networks: A stepwise approach

- A new approach for the retrofit of heat exchanger networks is presented.
- End-user intervention is possible at each step of the procedure.
- MINLP models support polynomial and piecewise linear enthalpy functions.
- Graph theoretical algorithms remove superfluous options in superstructures.
- Calculation time and result improvements observed for two literature case studies.

This paper introduces a new stepwise approach for the optimal retrofit of heat exchanger networks. At each iteration, the proposed method involves superstructures which embed retrofit alternatives as well as numerical optimization to minimize the project's total annualized cost. In order to reduce calculation times, new algorithms are proposed to thin out superstructures. These are based on thermodynamic criteria, namely on the concept of heat path, and on new graph theoretical results. To carry out the numerical optimization tasks, novel mixed integer nonlinear models are developed. These models support any combination of constant, polynomial and (continuous) piecewise linear enthalpy functions of temperature. Numerical examples are presented for the retrofit of the preheat train of an atmospheric crude unit and for the retrofit of a fluid catalytic cracking plant. The new method is shown to reduce calculation times and total annualized costs reported in the literature.