Optimal Integration of Organic Rankine Cycles into Process Heat Exchanger Networks

Abstract

This paper presents a mathematical programming model for the synthesis of an optimal heat exchanger network (HEN) in a process incorporating an organic Rankine cycle (ORC), which is used for improved heat recovery. The model is based on a modified stage-wise superstructure, where the process and ORC streams can exchange heat in all the stages. In addition, the evaporation and condensation temperatures of the ORC working fluid are treated as variables, and its thermodynamic properties (e.g. enthalpies and temperatures) are correlated as functions of evaporation and condensation temperatures. This allows the ORC operating conditions and the HEN to be optimised simultaneously, with the objective of maximising net power output or minimising overall energy cost. A literature example is presented to demonstrate the application of the proposed model. Results show that, besides producing shaft work, the ORC can further reduce the cold utility requirement. Furthermore, the overall energy cost can be minimised by increasing ORC power production, at the expense of increased hot and cold utility requirements. The proposed approach is thus useful in assessing the benefits from optimal ORC integration in the background process.