An air separation unit comprising:
- a main air compression system configured for receiving a stream of incoming feed air and producing a compressed air stream;
- an adsorption based pre-purifier unit configured for removing impurities from the compressed air stream and producing a compressed, purified air stream;
- a main heat exchange system configured to cool the compressed and purified air stream to temperatures suitable for fractional distillation;
- a distillation column system comprises a higher pressure column, a lower pressure column linked in a heat transfer relationship via a first condenser-reboiler, a second condenser-reboiler operatively associated with the higher pressure column, and a third condenser-reboiler operatively associated with the lower pressure column;
- wherein the higher pressure column is configured to receive the cooled, compressed, purified air stream and produce a nitrogen enriched overhead and an oxygen-enriched kettle stream and the lower pressure column is configured to produce an overhead stream and an oxygen-enriched bottoms;
- wherein the first condenser-reboiler is configured to condense a first portion of the nitrogen enriched overhead from the higher pressure column against the oxygen-enriched bottoms from the lower pressure column to produce a nitrogen reflux stream for the higher pressure column and an ascending vapor stream in the lower pressure column from the boil-off of the oxygen-enriched bottoms;
- wherein the second condenser-reboiler is configured to condense a second portion of the nitrogen enriched overhead from the higher pressure column against a first split portion of the oxygen-enriched kettle stream from the higher pressure column to produce a liquid nitrogen stream, a recycle stream comprising a first portion of the boil-off of the oxygen-enriched kettle stream, and a diverted stream comprising a second portion of the boil-off of the oxygen-enriched kettle stream;
- wherein the recycle stream is recycled to the main heat exchanger system and the diverted stream is directed via a flow control valve to the lower pressure column and wherein the air separation unit is controlled, in part, by adjusting the relative flows of the recycle stream and the diverted stream;
- wherein a second split portion of the oxygen-enriched kettle stream is introduced into the lower pressure column at an intermediate location;
- wherein a third portion of the nitrogen enriched overhead from the higher pressure column is taken as a high pressure nitrogen product stream;
- and
- wherein the third condenser-reboiler is configured to condense the nitrogen overhead from the lower pressure column against the oxygen bottoms from the lower pressure column to produce a nitrogen reflux stream for the lower pressure column, a reverse reflux stream for the higher pressure column, and a waste stream.