(596d) Exergy-Loss-Minimization Based Process Synthesis for Cascade Refrigerant System in Ethylene Plants

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. In this study, at first, in-depth 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 and exergy change from one pressure level to another is also demonstrated. Based on this approach, optimizations of single-component cycles are established before they are integrated into the whole CRS model. The achieved methodology is applied in a cascade refrigeration system used in ethylene plants to explore system energy loss below ambient temperature. 

The procedure of process synthesis involves three consecutive steps: Thermodynamic analysis (for both single-loop and cascade refrigeration systems), Synthesis model development and solving, and solution validation and evaluation.  Rigorous steady-state model is firstly constructed and employed to generate Exergy-Temperature (B-T) chart in order to analyze their thermodynamics and energy distributions.  An exergy-based mixed-integer linear programming (MINLP) model is developed to minimize the overall exergy loss of the system, where multiple refrigerants with multiple recycling loops are simultaneously addressed and all of cooling demands are fulfilled. The optimal solution is subsequently examined by conducting rigorous simulations to check their feasibility and consistency. 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.