Synthesis of Heat Exchangers Network (HEN) and Optimization under Variability and Uncertainty

This paper presents a novel approach to mathematical formulations that takes into consideration the variability of stream properties (such as heat capacity, viscosity and thermal conductivity) as functions of temperature, in order to design and synthesize optimum heat exchanger networks (HEN). The importance of this new method comes from its ability to closely reflect actual performance of a process, where temperature can widely change, and vary stream properties accordingly. This is done in contrast with classical formulations that assume these properties constant. Using the new model, the minimum total annual cost in two case studies has changed in the range of -0.9 % to 7.2 % compared to classical models (where constant stream properties are assumed). It is also observed that investment costs are mostly affected by heat exchanger design variables (heat transfer coefficient, viscosity and thermal conductivity), while utility costs are mostly affected by variable heat capacity. These results have demonstrated that by applying the proposed novel mathematical model and relaxing the assumptions of constant stream properties (unaffected by variable temperature), a considerable effect can be observed on the predictability and accuracy of optimization models.