(738d) Exergy-Based Optimization for Mixed Refrigerant Systems

A refrigeration system generally works by indirectly transferring thermal energy from low-temperature sources to high-temperature sinks at the expense of electricity or mechanical work to lower the source temperature to a certain value.  In nature gas liquefaction process, mixed refrigerant (MR) system is widely used to chill and condense nature gas from gas to liquid phase for easy shipment.  In comparison with pure refrigerant system, an MR system provides refrigerant composite curve that better fits with natural gas cooling curve in the enthalpy-temperature diagram.  Thus, MR system needs less shaft work of compressors.  To quantify the "fitness" of composite curves, the concept of exergy is employed.  Exergy is the maximum useful work possible during a process that brings the system into equilibrium with environment.  The exergy of a stream equals to the enthalpy minus the product of entropy and ambient temperature.

In this work, a novel exergy-based optimization model is introduced.  To disclose the thermodynamic instincts of a refrigeration system, an exergy-temperature diagram (B-T diagram) is employed, where the refrigerant exergy is divided into phase-exergy, pressure-exergy, and temperature-exergy.  Phase-exergy means the exergy change due to solely phase change from liquid to gas.  Pressure-exergy means the exergy change due to solely pressure change from current pressure to the ambient pressure P₀.  Temperature-exergy means the exergy change due to solely temperature change from current temperature to the ambient temperature T₀.  The transition among different types of exergy for a normal refrigeration cycle is perfectly explained.  Based on the understanding, a mixed-integer nonlinear programming (MINLP) model is built.  The model includes all equipment models used in the MR system and exergy related equations.  The objective function is to maximize exergy efficiency, which is the ratio between useful exergy and the total consumed exergy.  An industrial case study of liquefied natural gas (LNG) plant is presented.  Results show that the optimized result could increase the exergy efficiency by about 10%.