a system is set forth to increase the capacity of an lng - based liquefier in a cryogenic air separation unit wherein , in a low production mode , the nitrogen that is fed to the lng - based liquefier consists only of at least a portion of the high pressure nitrogen from the distillation column system while in a high production mode , a supplemental compressor is used to boost the pressure of at least a portion of the low pressure nitrogen from the distillation column system to create additional feed to the lng - based liquefier . a key to the present invention is the supplemental compressor and the associated heat exchange equipment is separate and distinct from the lng - based liquefier . this allows its purchase to be delayed until a capacity increase is actually needed and thus avoid building an oversized liquefier based on a speculative increase in liquid product demand .