Proceedings of the 1 st Annual Gas Processing Symposium H. Alfadala, G.V. Rex Reklaitis and M.M. El-Halwagi (Editors) © 2009 Elsevier B.V. All rights reserved. 1 Development in Mixed Refrigerant Cycles Used in Olefin Plants M. Mafi, M. Amidpour, and S.M. Mousavi Naeynian Department of Mechanical Engineering, K.N.Toosi University of Technology, No. 15, Pardis Street, Mollasadra Avenue, Vanak Suare, P.O. Box: 19395-1999, Tehran, Iran Abstract Numerous mixed refrigerant cycles for separation systems of petrochemical industries were developed in the past several decades. In this paper, two sets of low temperature mixed refrigerant cycles were developed for a typical olefin plant utilizing a mixture of methane, ethane, propane and nitrogen as cycle working fluid to replace the pure ethylene refrigeration cycle which is used in conjunction with propylene refrigeration in conventional plants. The key parameters of the two cycles were compared, and the matching of the heating and cooling curves in heat exchangers was also analyzed. The obtained results reveal that mixed refrigerant cycles can improve thermodynamic performance of refrigeration systems in the case of using optimal working fluid mixture composition, optimal high and low operating pressures and optimal arrangement of the cycle components. Keywords: Mixed refrigerant; Cascade refrigeration system; Olefin plant 1. Introduction The recovery of ethylene from crude light hydrocarbon gas mixture is an economically important but highly energy intensive process. Cryogenic separation methods are commonly used which require large amounts of refrigeration at low temperatures and the development of the methods to reduce net power to provide this refrigeration is important in the petrochemical industry. Essentially all olefin plants use an ethylene-propylene cascade refrigeration system to provide the major portion of refrigeration required in the olefin plant. Most of the propylene (high level) refrigeration is utilized at several pressure/temperature levels in the initial feed precooling and fractionation sections of the plant to cool the feed from ambient temperature to about -35°C and to condense the ethylene refrigerant at about -30°C. Similarly, the ethylene (low level) refrigeration is utilized at several pressure/temperature levels in the cryogenic section of the plant to cool the feed from -35°C to about -100°C in order to condense the bulk of the ethylene in the form of liquid feeds to a demethanizer column, and in the demethanizer column overhead condenser at about -101°C to provide reflux to that column. Refrigeration below -101°C, to condense the remaining ethylene from the feed, is provided primarily by work of expansion or Joule-Thomson expansion of rejected light gases, H 2 and methane, and/or by vaporization of methane refrigerant which has been condensed by the