Long-term investment and maintenance planning for heat exchanger network retrofit

Abstract Process Optimisation has been the core topic in chemical process industries for decades. However, the performance of the asset strongly influences the production efficiency as well. In the case of Heat Exchanger Network (HEN), the retrofit decisions are often aimed to maximise energy savings and minimise the associated cost. However, the age and performance of heat exchangers have a great influence on the HEN efficiency as well. This study brings innovative retrofit algorithms, integrating exchangers’ lifetime and reliability functions to visualise the benefits of the hybrid process and asset optimisation. The HEN retrofit structures are optimised, considering various heat intensification methods. The model is formulated based on the novel concept of Shifted Retrofit Thermodynamic Grid Diagram (SRTGD). A Mixed Integer Linear Programming (MILP) model is formulated for the investment and maintenance planning. The operating life of the network is discretised into multiple periods, and a decision is made within each period: whether to upgrade the heat exchangers, purchase new heat exchangers – for replacement or adding to the network, maintain the heat exchangers or perform nothing. Two realistic case studies are used to elucidate the application of the method. Under a 20 y planning horizon, the Net Present Value (NPV) obtained are higher than the previous works, i.e. 17% higher for case study 1 with 51% utility savings, and 14% higher for case study 2 with 74% utility savings. The proposed HEN retrofit decisions are proven to be cost-effective. A graphical tool has been developed to track the reliability and maintenance status of the exchangers to provide a significant guide to the economically optimum decisions.

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