Patent ID: 7582223

Claim:
A method of refrigeration, comprising: (a) providing a precooled multistage mixed refrigeration system comprising a compressor, an oil separator for separation of entrained oil from the refrigerant, and one or more refrigerant phase separators for separation of the liquid and vapor phases of the refrigerant; and (b) operating the refrigeration system with a refrigerant composition comprising: more than 36 molar percent but less than 60 molar percent of nitrogen, 15 to 40 molar percent of methane, 5 to 30 molar percent of ethane, ethylene or combinations thereof, up to 20 molar percent of propane, n-butane, iso-butane and combinations thereof, and up to 20 molar percent of neon, helium and combinations thereof; wherein the refrigerant comprises at least one of iso-butane, neon, or helium.