Refrigeration system synthesis by continuous temperature level optimization considering the sub-cooler configuration

Abstract

Synthesizing a multi-level refrigeration system by considering the optimization of pressure levels results in a complex Mixed Integer Nonlinear Programming (MINLP) problem. Existing studies mostly adopted fixed pressure levels or optimized them in series of fixed alternative levels. In this study, a more comprehensive methodology was developed for the simultaneous synthesis of a refrigeration system and its heat exchanger network, wherein the pressure/temperature levels were optimized as continuous variables. A series configuration of connected sub-coolers and expansion valves was used in our superstructure to reduce the inlet temperatures of expansion valves, thus reduce the energy consumption. A Generalized Disjunctive Programming model was used and reformulated as a MINLP problem. Our methodology was validated by three case studies of an ethylene refrigeration system, with total shaft work saving of compressors reached 7.17%. Besides, refrigerant from refrigeration system, together with cold process streams, was verified to suitable cooling capacity for sub-coolers.