A process for cryogenic air separation and liquefaction, including purifying and compressing an inlet air stream, thereby producing a compressed inlet air stream, dividing the compressed inlet air stream into an ASU portion and a liquefaction portion, introducing the ASU portion into an air separation unit, thereby producing a gaseous oxygen steam and a gaseous nitrogen stream, and introducing the liquefaction portion, the gaseous oxygen steam and the gaseous nitrogen stream into a liquefaction unit, thereby producing a liquid nitrogen stream and a liquid oxygen stream. Wherein, the air separation unit is located more than 200 meters from the liquefaction unit, and there is no compression driven by external energy within 200 meters of the liquefaction unit.

A nitrogen generating device comprises: a main heat exchanger; a nitrogen distillation column; at least one nitrogen condenser; a compressor; an expansion turbine; a rotation control unit for controlling rotation with respect to a rotating shaft connecting the compressor and the expansion turbine; a pressure measuring unit for measuring a pressure value of product nitrogen gas; and an optimum rotational speed calculation command unit which inputs the pressure value measured by the pressure measuring unit into a pre-installed rotational speed calculation function to calculate the rotational speed of the rotating shaft, and issues a command to the rotation control unit.