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 unit arrangement 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; (iii) a waste expansion cycle, and (iv) a recycle stream of a portion of the boil off vapor from one or more of the condenser-reboilers. The improved air separation cycle minimizes the backpressure of the lower pressure column and yields improvements in the nitrogen recovery as well as reductions in unit 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, 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.   
     
     
         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 pump configured to pump the reverse reflux stream to the higher pressure column. 
     
     
         5 . 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, a first waste stream taken from an intermediate location of the lower pressure column, 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 all or a portion of the boil-off of the oxygen-enriched kettle stream that is recycled to the main heat exchanger system;   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 second waste stream.   
     
     
         6 . The air separation unit of  claim 5  further comprising a waste expansion turbine configured to expand the first 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 and the second waste stream is combined with the waste exhaust stream. 
     
     
         7 . The air separation unit of  claim 5  further comprising:
 a first waste expansion turbine configured to expand the first waste stream to produce a first waste exhaust stream; 
 a second waste expansion turbine configured to expand the second waste stream to produce a second waste exhaust stream; 
 and wherein the first waste exhaust stream and second waste exhaust stream are directed to the main heat exchange system to provide supplemental refrigeration for the air separation unit. 
 
     
     
         8 . The air separation unit of  claim 5  further comprising:
 a waste expansion turbine configured to expand the first waste stream or a mixture of the first waste stream and the second waste stream to produce a waste exhaust stream; and 
 a plurality of flow control valves operatively configured to direct the second waste stream to mix with the waste exhaust stream downstream of the waste expansion turbine or to mix with the first waste stream upstream of the waste expansion turbine; 
 wherein the waste exhaust stream is directed to the main heat exchange system to provide supplemental refrigeration for the air separation unit. 
 
     
     
         9 . The air separation unit of  claim 5  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. 
     
     
         10 . The air separation unit of  claim 5  further comprising a pump configured to pump the reverse reflux stream to the higher pressure column.