Exergy analysis of multistage cascade low temperature refrigeration systems used in olefin plants

This paper provides an exergy analysis for multistage cascade low temperature refrigeration systems used in olefin plants. The equations of exergy destruction and exergetic efficiency for the main system components such as heat exchangers, compressors and expansion valves are developed. The relations for total exergy destruction in the system and the system overall exergetic efficiency are obtained. Also, an expression for minimum work requirement for the refrigeration systems of olefin plants is developed. It shows that the minimum work depends only on the properties of incoming and outgoing process streams cooled or heated with refrigeration system and the ambient temperature. Using actual work input values, the exergetic efficiency of the low temperature cascade refrigeration system of a typical olefin plant is determined to be 30.88% indicating a great potential for improvements. The novelty of this paper includes the suggestions for increasing efficiencies, along with discussion about the reasons for deviation from reversible processes.