Medium-pressure nitrogen production with high oxygen recovery

Abstract

A cryogenic air separation process for the production of oxygen and nitrogen products uses a distillation system having at least a higher pressure column and a lower pressure column. A nitrogen-enriched liquid stream is recovered from the higher pressure column and is eventually at least partially vaporized by indirect heat exchange at a pressure intermediate that of the higher pressure column and the lower pressure column. A vapor stream is withdrawn from an intermediate location of the stripping section of the lower pressure column and is at least partially condensed by indirect heat exchange with the nitrogen-enriched liquid stream. At least some of the nitrogen product is recovered from the vapor that results from the at least partial vaporization of the nitrogen-enriched liquid. The process is appropriate for the production of nitrogen in quantities up to 40 mole % of the incoming air flow.

1 . A process for separating air to produce an oxygen product and a nitrogen product, said process using a distillation column system having at least two distillation columns, including a higher pressure column at a first pressure and a lower pressure column at a second pressure lower than the first pressure, the lower pressure column having a stripping section and being in thermal communication with the higher pressure column, wherein each distillation column and the stripping section have a top, a bottom, and at least one intermediate location between the top and the bottom, comprising the steps of: 
 providing a stream of compressed air;    feeding at least a portion of the stream of compressed air to the distillation column system;    withdrawing a nitrogen-enriched liquid stream from the higher pressure column;    withdrawing a vapor stream from the lower pressure column at an intermediate location of the stripping section of the lower pressure column; and    heat exchanging at least part of the vapor stream with at least part of the nitrogen-enriched liquid stream, thereby at least partially condensing at least part of the vapor stream to form a condensate and at least partially vaporizing at least part of the nitrogen-enriched liquid stream to form a nitrogen-enriched vapor, at an intermediate pressure between the first pressure and the second pressure.    
     
     
         2 . A process as in  claim 1 , comprising the further step of feeding at least a portion of the condensate to the lower pressure column at or above the intermediate location.  
     
     
         3 . A process as in  claim 1 , comprising the further step of feeding at least a portion of the condensate to an argon distillation column at or above the bottom of the argon distillation column.  
     
     
         4 . A process as in  claim 1 , wherein the nitrogen-enriched liquid stream is withdrawn from the top of the higher pressure column.  
     
     
         5 . A process as in  claim 1 , wherein the nitrogen-enriched liquid is withdrawn from an intermediate location of the higher pressure column.  
     
     
         6 . A process as in  claim 1 , wherein at least a portion of the nitrogen-enriched vapor is withdrawn from the distillation column system as at least a portion of the nitrogen product.  
     
     
         7 . A process as in  claim 6 , comprising the further step of compressing said nitrogen-enriched vapor at a temperature near an ambient temperature.  
     
     
         8 . A process as in  claim 6 , wherein the portion of the nitrogen product that is the nitrogen-enriched vapor is less than or equal to about 40 mole % of the stream of compressed air.  
     
     
         9 . A process as in  claim 6 , wherein the nitrogen product has a third pressure between about 1.5 times the second pressure and about 0.9 times the first pressure.  
     
     
         10 . A cryogenic air separation unit using a process as in  claim 1 .  
     
     
         11 . A process for separating air to produce an oxygen product and a nitrogen product, said process using a distillation column system having at least three distillation columns, including a higher pressure column at a first pressure, a lower pressure column at a second pressure lower than the first pressure, and an additional column, the lower pressure column having a stripping section and being in thermal communication with the higher pressure column, wherein each distillation column and the stripping section have a top, a bottom, and at least one intermediate location between the top and the bottom, comprising the steps of: 
 providing a stream of compressed air;    feeding at least a portion of the stream of compressed air to the distillation column system;    withdrawing a nitrogen-enriched liquid stream from the higher pressure column;    withdrawing a vapor stream from the lower pressure column at an intermediate location of the stripping section of the lower pressure column;    feeding a first part of the vapor stream to a feed location at or adjacent the bottom of the additional column; and    heat exchanging a second part of the vapor stream with at least part of the nitrogen-enriched liquid stream, thereby at least partially condensing at least part of the second part of the vapor stream to form a condensate and at least partially vaporizing at least part of the nitrogen-enriched liquid stream to form a nitrogen-enriched vapor, at an intermediate pressure between the first pressure and the second pressure.    
     
     
         12 . A process as in  claim 11 , comprising the further step of feeding at least a portion of the condensate to the additional column.  
     
     
         13 . A process as in  claim 11 , comprising the further step of feeding at least a portion of the condensate to the lower pressure column at or above the intermediate location.  
     
     
         14 . A cryogenic air separation unit using a process as in claim  11 .