A method and device to produce oxygen by the low-temperature separation of air at variable energy consumption. A distillation column system comprises a high-pressure column, a low-pressure column and a main condenser, a secondary condenser and a supplementary condenser. Gaseous nitrogen from the high-pressure column is liquefied in the main condenser in indirect heat exchange with an intermediate liquid from the low-pressure column. A first liquid oxygen stream from the bottom of the low-pressure column is evaporated in the secondary condenser in indirect heat exchange with feed air to obtain a gaseous oxygen product. The supplementary condenser serves as a bottom heating device for the low-pressure column and is heated by means of a first nitrogen stream from the distillation column system, which nitrogen stream was compressed previously in a cold compressor.

Enhancements to a dual column, nitrogen producing cryogenic air separation unit with waste expansion are provided. Such enhancements include an improved air separation cycle that uses: (i) three condenser-reboilers; (ii) a reverse reflux stream from the condenser-reboiler associated with the lower pressure column to the higher pressure column; and (iii) a recycle stream of a portion of the vapor from one or more of the condenser-reboilers that is recycled back to the incoming feed stream and or the compressed purified air streams to yield improvements in the performance of such dual column, nitrogen producing cryogenic air separation units in terms of overall nitrogen recovery as well as power consumption compared to conventional dual column, nitrogen producing cryogenic air separation units employing waste expansion.

Feed air is compressed to a first pressure in a main air compressor. A first sub-stream of the air compressed to the first pressure is cooled and fed at least in part to the distillation column system. A second sub-stream of the air compressed to the first pressure is cooled and at least partially liquefied in a low-pressure column bottom evaporator. The at least partially liquefied second sub-stream is introduced at least in part into the distillation column system. A liquid oxygen-enriched fraction is introduced into the evaporation chamber of a high-pressure top condenser. An argon-containing oxygen stream from an intermediate point in the low-pressure column is introduced into an argon column. The second sub-stream is introduced at least in part into an argon top condenser and partially evaporated therein. The second sub-stream is then introduced at least in part into the high-pressure column and/or into the low-pressure column.

This invention is about an air separation apparatus to produce oxygen and nitrogen through isobaric separation, which is based on the Rankine cycle system of similar thermal energy power circulation apparatus at cryogenic side, a liquid pump is used to input work and the cold is made up to the air separation apparatus with refrigerating media, so as to realize the isobaric separation of air to produce nitrogen and oxygen. The air separation apparatus of this invention can save energy by over 30% as compared with the traditional advanced apparatus with the identical refrigerating capacity, and it can also realize centralize gas supply via the air separation apparatus, therefore it constitutes a breakthrough to the traditional air separation technology and refrigeration theory, with substantial economic, social and environmental protection benefits.

A method for separating air is provided, in which a flow of oxygen-rich liquid is sent to a top of a pure oxygen column, having a pure oxygen reboiler, in which said flow is purified in order to form a vessel liquid containing at least 98 mol % of oxygen and the vessel liquid is drawn off as a product. A supercharged airflow at a second pressure is sent to the pure oxygen reboiler and to a liquid oxygen vaporizer; a nitrogen-rich gas is drawn from the top of the medium-pressure column and sent to an intermediate reboiler of the low-pressure column and the condensed gas is sent to the top of the medium-pressure column; and a nitrogen-rich gas or air is sent to a vessel reboiler of the low-pressure column and the liquid that condenses therein is sent to the medium-pressure column.

An air separation plant is provided that includes a plurality of air compressors, a plurality of air purification units, and one or more cold boxes. In a first mode of operation, a first air compressor is configured to compress air to a higher pressure than a second air compressor. In a second mode of operation, the second air compressor is configured to compress air to a higher pressure than during the first mode of operation.