Simulation and optimization of refrigeration cycle in NGL recovery plants with exergy-pinch analysis

In this study, a combination between pinch and exergy analysis was applied for refrigeration cycle in NGL recovery plant. Pinch analysis is an established systematic tool for the optimal design and retrofit of energy recovery systems. Unfortunately, one of the main limitations of pinch analysis technique in optimizing energy recovery systems is that it can only deal with heat transfer processes, not processes involving power utilization. Hence, for optimization of shaft work, an exergy-pinch method should be used. A commercial simulator was used to obtain thermodynamic properties of the process streams and to perform mass and energy balances. The equations of exergy destruction and exergetic efficiency for the main system components such as heat exchangers, compressors and throttle valves were developed. First, the refrigeration cycle of the NGL plant was analyzed and optimized by combined exergy-pinch analysis. In doing so, the work of the compressor was reduced to 170 kw. In the last part, the refrigerant was replaced with R-600a which resulted in the reduction of work of compressor and refrigerant mass flow rate by about 570 kw and 11.5%, respectively.