Process integration and superstructure optimization of Organic Rankine Cycles (ORCs) with heat exchanger network synthesis

Abstract

Low and medium temperature energy utilization is one way to alleviate the energy crisis and environmental pollution problems. In the past decades, Organic Rankine Cycles (ORCs) have become a very promising technology for low and medium temperature energy utilization. When an ORC is used to recover waste heat in chemical plants, heat integration between the ORC and the process streams should be performed to save more utilities and generate more power. This study aims to integrate an ORC into a background process to generate maximum electricity without increasing the hot utility usage. We propose a two-step method to integrate an ORC to a background process, optimally considering the modifications of the ORC to increase the thermal efficiency and heat recovered by the working fluid simultaneously. The first step is to determine the configuration (turbine bleeding, regeneration, superheating) and operating conditions (working fluid flowrate, evaporation and condensation temperatures, turbine bleed ratio, degree of superheat, bleeding pressure). The second step is to synthesize the heat exchanger network by minimizing the number of heat exchangers that keep the hot utility unchanged. A well-studied example from the literature is solved to demonstrate the effectiveness of the proposed model for industrial waste heat recovery. The net power output in this paper is improved by 13% compared with the best known previous literature design for this system. The proposed method is also useful for quickly screening working fluids while considering integration potential. Screening of several working fluids revealed that using R601 (n-pentane) in place of the original working fluid (n-hexane) can increase the power output of the example system by an additional 14%.