Patent ID: 7086251

Claim:
A method for liquefying a gas ( 1 ) which comprises cooling a feed gas stream successively through three heat exchange zones ( 310 , 311 , 312 ) in respective first, second, and third temperature ranges to provide a liquefied product ( 13 ), wherein refrigeration for cooling the feed gas stream in the first temperature range is provided by a first vaporizing refrigerant ( 117 ), refrigeration for cooling the stream in the second temperature range is provided by a second vaporizing refrigerant ( 213 ), and refrigeration for cooling the stream in the third temperature range is provided by a third vaporizing refrigerant ( 315 ), the first, second, and third refrigerants being of different compositions from each other, characterized in that additional refrigeration is provided by vaporization ( 357 , 379 ), at temperatures above the lowest temperature in the second heat exchange zone ( 311 ), of an auxiliary refrigerant ( 373 , 377 ) derived from the third vaporizing refrigerant ( 315 ) vaporized in the third (coldest) heat exchange zone ( 312 ).