(60a) A Study on Exergy-Based Thermodynamic Analysis and Process Synthesis of Mixed-Refrigerant Systems for Ethylene Plants

Refrigeration system holds an important role in most chemical/petrochemical processes. A refrigerant system transfers heat from low-temperature sources to high-temperature sinks through vapor-compression cycles at the expense of mechanical work, magnetism, laser or other means. In many industrial processes, the refrigeration systems are employed for chilling and freezing the final liquid/solid products or liquefying the gas intermediates/products for the downstream cryogenic separation.  Optimal synthesis of these systems is attractive not only because of their economic benefits but also because they incorporate most of energy transfer operations in widespread use in the chemical industry.  The traditional cascade refrigeration system used in ethylene plants includes multiple refrigerants and multiple temperature and pressure levels. Recently, mixed-refrigerant system (MRS) has shown some advantages comparing to the single-component one. A MRS uses a mixture as the refrigerant instead of several pure components as in conventional cascade refrigeration systems. This system operates within smaller temperature differences at the lower limit; therefore, it leads to a smaller increase in entropy and consequently a smaller loss in energy. Besides, an MRS also requires a simpler unit configuration and fewer maintenance problems.

In this study, MRS used in ethylene plants is studied based on in-depth exergy analysis, a powerful method to explore system energy loss within a large temperature range crossing the ambient temperature due to various compositions of the employed mixed refrigerant.  An exergy-based mixed-integer linear programming (MINLP) model is developed to maximize the energy efficiency of each system, where multiple refrigerant combinations and their recycling loops are simultaneously addressed and all of cooling demands are fulfilled as inputs for a heat exchanger network synthesis. The efficacy of the developed methodology is demonstrated a case study of a mixed-refrigerant system used in a typical ethylene plant.