Heat exchanger network synthesis with optimal waste heat recovery and multiple hot utilities

Approaches for Heat Exchanger Network (HEN) synthesis have become increasingly significant in recent years because of their potential role in saving energy and tackling climate change. Unlike existing methods that rely on introducing additional stages to accommodate multiple utility options, while often neglecting the potential of waste heat recovery, this work proposes a novel mathematical optimization methodology to tackle the HEN synthesis problem and bring energy benefits of waste heat recovery. The method aims to overcome the drawbacks in existing methods related to low computational efficiency resulting from numerous discrete combinations. An enhanced stage-wise superstructure is presented to automatically optimize selections of stages covering waste heat recovery or heat recovery in hot streams, and multiple hot utilities or heat recovery in cold streams, formulated as a mixed-integer nonlinear programming (MINLP) problem. Grand composite curve (GCC) is adopted to implement preliminary simplifications for the superstructure to solve large-scale HEN problems, by cutting the inappropriate utilities according to the pinch method and minimum temperature driving force to eliminate redundant combinations. The results show that the proposed approach can provide cost-efficient solutions with lower total annual cost (TAC) due to significant reductions in energy cost, compared with previous works. Specifically, the proposed approach achieves TAC savings of 14.5 %, 3.15 %, and 4.5 % for Case 1, Case 2, and Case 3.