(58f) Optimal Design of Mixed Refrigerant System in Ethylene Plants for Maximum Energy Savings

Authors: 
Xu, C., Lamar University
Zhang, J., Lamar University
Xu, Q., Lamar University
Refrigeration system is one of the most important and critical operating system in the chemical and petrochemical industries. The tradition cascade refrigeration system (CRS) has been used for decades in ethylene plants. However, the use of the multiple refrigerants in the CRS requires a very number of separate compressors, heat exchangers, pumps and associated pipes for each temperature and pressure level. Mixed refrigerant system (MRS) has been introduced for decades, where the pure working fluids are replaced by mixed working fluids. MRS is more efficient in energy saving and less capital cost than CRS and has been widely applied in liquefaction of natural. However, because of the complexities of ethylene manufacturing process and refrigerant systems aforementioned as well as possible intellectual copyright concerns, the in-depth studies for a complex mixed refrigeration system design and operation are still lack of reporting. In this study, a general methodology has been developed for synthesis of MRS with a tertiary refrigerant, a mixture of methane, ethylene and propylene, to replace the ethylene-propylene cycles of the CRS in an ethylene plant. First, the exergy-temperature chart combined with the exergy analysis is presented to comprehensively analyze the thermodynamic nature of traditional CRS to help to locate and evaluate exergy loss in the system. Opportunities to improve are explored and identified in the exergy-temperature chart. Then, a superstructure is built from the conclusions of thermodynamic analysis to cover all the possible arrangements for the cooling supplies. Moreover, a simulation-assisted mixed-integer linear programming (MILP) model is developed and solved for the optimal process synthesis and heat exchange network (HEN) design. The case study about a mixed refrigerant system used in a typical ethylene plant is employed to demonstrate the efficacy of the developed methodology. The shaft work of compressor has been reduced tremendously. And the capital cost and utilities cost have been saved by 32% and 16%, respectively.