Enhancements to a dual column nitrogen producing cryogenic air separation unit

Abstract

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.

What is claimed is: 
     
         1 . 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 and a lower pressure column linked in a heat transfer relationship via a first condenser-reboiler;   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;   wherein the lower pressure column is configured and produce a lower pressure nitrogen product stream, 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 distillation column system further comprises a second condenser-reboiler operatively associated with the higher pressure column and 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 and a recycle stream from the boil-off of the oxygen-enriched kettle 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 medium/high nitrogen product stream;   wherein the distillation column system further comprises a third condenser-reboiler operatively associated with the lower pressure column and 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; and   wherein the recycle stream is recycled to: (i) the main air compression system and combined with the incoming feed air stream; (ii) a location upstream of the main heat exchange system and combined with the compressed, pre-purified air stream; or (iii) to the main heat exchanger system.   
     
     
         2 . The air separation unit of  claim 1  further comprising a waste expansion turbine configured to expand the waste stream to produce a waste exhaust stream and wherein the waste exhaust stream is directed to the main heat exchange system to provide supplemental refrigeration for the air separation unit. 
     
     
         3 . The air separation unit of  claim 1  wherein the recycle stream is warmed in the main heat exchange system and directed to an inter-stage location of the main air compression system. 
     
     
         4 . The air separation unit of  claim 1  further comprising:
 a recycle compressor configured to compress the recycle stream and direct the compressed recycle stream the location upstream of the main heat exchange system where the compressed recycle stream is combined with the compressed, pre-purified air stream; and 
 the main heat exchange system is configured to cool the compressed, purified air in part via indirect heat exchange with the recycle stream. 
 
     
     
         5 . The air separation unit of  claim 1  further comprising:
 a recycle compressor configured to compress the recycle stream and direct the cold compressed recycle stream to the main heat exchange system where the compressed recycle stream is further cooled; and 
 wherein the further cooled recycle stream is introduced into the higher pressure column of the distillation column system. 
 
     
     
         6 . The air separation unit of  claim 5  wherein the recycle compressor is a cold compressor driven by a booster loaded turbine and wherein the booster loaded turbine is configured to expand a diverted portion of the medium/high nitrogen product stream to produce an exhaust stream from the booster loaded turbine that is combined with the lower pressure nitrogen product stream. 
     
     
         7 . The air separation unit of  claim 1  further comprising a pump configured to pump the reverse reflux stream to the higher pressure column.