(410b) Exergetic Analysis in Performance Evaluation and Process Synthesis of Refrigeration System in an Ethylene Plant

Exergy is defined as the maximum amount of useful energy that a process needs in order to bring the system to equilibrium with its surrounding environment. Exergetic analysis has been employed in several studies to address the quality instead of the quantity of energy usage in chemical processes. Refrigeration system holds an important role in most chemical/petrochemical processes. The traditional cascade refrigeration system (CRS) used in ethylene plants includes multiple refrigerants, each of which operates at multiple temperature and pressure levels. Each component subsystem includes a compression section, a condensing section and an evaporation section containing sub-coolers, expansion valves, evaporators and flash drums.  Therefore, exergetic analysis is a suitable tool to evaluate such complicated and energy-intensive process.

In this study, at first, in-depth exergy-based thermodynamic analysis for a single-loop refrigeration system is carried out to identify exergy loss in the heat exchanger network with non-isothermal phase changes, especially at sub-coolers and evaporators in the refrigerant system. The relationship between compressor work consumption, exergetic coefficient of performance (COP), and the amount of exergy change from one pressure level to another is also taken into consideration. Based on this approach, optimization of an integrated CRS containing multiple cycles is established. The procedure of process synthesis involves three consecutive steps: Exergetic analysis (for both single-loop and cascade refrigeration systems), Synthesis model development and solving, and Exergy-based solution validation and evaluation.  Rigorous steady-state model is firstly constructed and employed to analyze their thermodynamics and energy distributions. In a mathematical programming problem, the overall exergy loss of the system is minimized while the exergetic COPs are increased, where multiple refrigerants with multiple recycling loops are simultaneously addressed and all of cooling demands of the whole process are fulfilled. The efficacy of the developed methodology is demonstrated by a case study of a cascade refrigeration system used in an ethylene plant that employs two refrigerants, ethylene and propylene.