(337c) Exergy-Analysis Based Refrigeration System Synthesis at Ethylene Plants

A refrigeration system generally works by transferring the heat from low-temperature sources to higher-temperature objects at the expense of electricity or mechanical work, such that the source temperature can be lowered or maintained at a certain value. It is very important for an ethylene plant operation because its performance is closely influence the product quality, energy use efficiency, and the plant profitability. Thus, the system should be studied at the very beginning of the conceptual design stage. However, the design of a chemical plant refrigeration system is a very challenging task. The complexity mainly relies on the following two aspects: i) the refrigeration system of an ethylene plant needs to cover refrigeration requests in a wide temperature range (23 ~ -100.8 Celsius degree). It is thermodynamically impossible for using only one refrigerant as the working media. Thus, multiple refrigerants have to be employed under this situation. ii) The refrigeration system needs to satisfy the cooling demands from many different units. Thus, each refrigerant will have multiple loops and cover different cooling tasks. None of the previous studies have been reported to simultaneously consider the optimal design of a refrigeration system with multiple refrigerants and multiple temperature levels. In this paper, a conceptual design model for ethylene plant refrigeration system synthesis has been developed. The refrigeration system considers multi-refrigerant and cascade multi-loop designs. It also includes the modeling of a multi-stage steam system for accounting the energy consumption to drive the refrigerant compressors. Meanwhile, the concept of exergy is employed to analyze the thermodynamic nature of the refrigeration system and thus to guide its conceptual design. The efficacy of the developed methodology has been demonstrated by designing a cascade refrigeration system for an ethylene plant.