A synthesis plant (309A) includes a compression device (360A) that increases a pressure differential between the bottom product pressure of a cryogenic separator (340A) and the delivery pressure of the bottom product to a downstream plant component. Such increased pressure differential is employed to increase cooling in the separator (340A) to thereby significantly reduce the volume of excess air. In most preferred aspects, at least part of the energy required for the compression device is provided by expansion (320A) of the separator feed.