Considering risk assessment at the early-stage of Heat Exchanger Network (HEN) synthesis can significantly contribute to obtaining inherently safer design. The development of quantitative safety metrics at the early stage of design are still at the beginning steps of development. The risk is composed of failure frequency and the severity of the consequences. There are numerous methods for determining the severity of the consequences for a certain deviation event and there are mainly index-based methods for determining the severity when inherent safety is analysed. The failure of frequency, however, is usually assumed as a constant during the entire lifetime. Observing Bathtub curve it can be concluded that different type of failures dominates at different periods during a lifetime. Therefore, considering unified failure frequency during the entire lifetime is quite problematic. The aim of this study is to obtain HEN design that exhibits improved safety during the entire lifetime. The previously developed mixed-integer nonlinear programming (MINLP) model for HEN synthesis with embedded risk assessment was upgraded to a model that considers the changing failure frequency during the lifetime. A multi-period MINLP model was developed that accounts for different failure rate within each period of the lifetime. At least three periods should be considered (early stage, random and wear out failures). More reliable results regarding HEN design with enhance safety can be obtained by considering the mentioned different failure rates and different aspects of safety (toxicity, flammability, explosiveness). The designs obtained by the enhanced HEN synthesis method are safer and economically reliable.
