Abstract: A high pressure nitrogen pipeline, oxygen or plant air is diverted around a pressure letdown station to liquefy the gas or a portion of the gas for storage or air separation assist, with the remaining unused gaseous portion being returned to the pipeline downstream the letdown station. One or more heat exchangers and one or more expanders are used to cool down the gas and liquefy it. A generator or compressor may be coupled to the expanders employing companders for generating power or for further compression of the pipeline gas. In a further embodiment, natural gas is cooled to assist in liquefying the nitrogen by drying the natural gas and forming two streams wherein carbon dioxide is removed from a smaller stream which is applied to cascaded heat exchangers and the larger stream is expanded to further cool it. The two streams are applied to the heat exchangers for cooling and liquefying nitrogen gas or other merchant gas applied to the heat exchangers from a pipeline or other source.

Abstract: A conventional air separator unit separates nitrogen, oxygen or argon merchant gas to be liquefied. Input pressurized natural gas is processed through a pressure letdown liquefaction system including gas separators for removing moisture and for drying the gas. The dried gas is then separated into large and small gas stream portions wherein CO.sub.2 in the smaller portion is removed. The two portions are passed through a first heat exchanger from which the larger portion is expanded which cools it and then passed through two heat exchangers including the first exchanger, used to regenerate the CO.sub.2 separator and drier and then returned to the input stream.