Air separation method and apparatus

Abstract

A method and apparatus for separating air in which the recovery rate of an oxygen product is increased by increasing the liquid to vapor ratio in a lower pressure column connected to a higher pressure column in a heat transfer relationship. A crude liquid oxygen column bottoms stream produced in the higher pressure column is partially vaporized against condensing argon-enriched vapor of the lower pressure column which is fed back to an argon stripping section of the lower pressure column. A supplemental nitrogen-rich reflux stream and an oxygen-enriched stream having a greater oxygen content than the crude liquid oxygen stream are fed into the lower pressure column. These streams are formed from a two phase stream produced, at least in part, from the crude liquid oxygen stream after the partial vaporization thereof.

1 . A method of separating air to produce an oxygen product, said method comprising:
 separating the air in a cryogenic rectification process having a higher pressure column and a lower pressure column operatively associated with one another in a heat transfer relationship to produce nitrogen-rich reflux for the higher pressure column and the lower pressure column and producing the oxygen product from an oxygen-rich liquid column bottoms of the lower pressure column;   conducting the cryogenic rectification process such that a crude liquid oxygen stream formed of a crude liquid oxygen column bottoms produced in the higher pressure column is partially vaporized against at least partially condensing an argon-enriched vapor produced in the lower pressure column, a supplemental nitrogen-rich reflux stream and an oxygen-enriched stream having a greater oxygen content than the crude liquid oxygen stream are both formed from a two-phase stream produced, at least in part, from the crude liquid oxygen stream after partial vaporization thereof, and the oxygen-enriched stream and at least part of the supplemental nitrogen-rich reflux stream are fed into the lower pressure column;   the argon-enriched vapor at least partially condensed at a location of the lower pressure column where the argon-enriched vapor has a nitrogen concentration of between 0.1 mole percent and 5.0 mole percent and resulting condensate combines with downcoming liquid of the lower pressure column thereby increasing the liquid to vapor ratio in a lowermost section of the lower pressure column; and   the further oxygen-enriched stream and at least a portion of the supplemental nitrogen-rich liquid stream being fed at successively higher locations of the lower pressure column and above the location of the lower pressure column at which the argon-enriched vapor is at least partially condensed.   
     
     
         2 . The method of  claim 1 , wherein the oxygen product is produced from an oxygen-rich liquid column bottoms of the lower pressure column by:
 pumping at least part of an oxygen-rich column bottoms stream withdrawn from the lower pressure column and composed of the oxygen-rich liquid column bottoms to produce a pressurized liquid oxygen stream;   warming at least part of the pressurized liquid oxygen stream through indirect exchange with a high pressure process stream such that at least part of the high pressure process stream is liquefied and the oxygen product is formed from the pressurized liquid oxygen stream after having been warmed; and   introducing the at least part of the high pressure stream after liquefaction into at least one of the higher pressure column and the lower pressure column.   
     
     
         3 . The method of  claim 2 , wherein:
 the crude liquid oxygen stream is valve expanded and partially vaporized and then is passed in indirect heat exchange with the argon-enriched vapor;   the supplemental nitrogen-rich reflux stream and the oxygen-enriched stream are formed by disengaging liquid and vapor phases from the two-phase stream to form a vapor phase stream and a liquid phase stream, valve expanding the liquid phase stream and partially vaporizing the liquid phase stream in indirect heat exchange with the vapor phase stream thereby condensing the vapor phase stream to form the supplemental nitrogen-rich reflux stream and the oxygen-enriched stream from the liquid phase stream after the liquid phase stream has been partially vaporized;   the at least part of the nitrogen-rich reflux stream is valve expanded and at least in part introduced into the lower pressure column; and   the oxygen-enriched stream is introduced into the lower pressure column.   
     
     
         4 . The method of  claim 2 , wherein:
 the crude liquid oxygen stream is valve expanded, is partially vaporized and then is passed in indirect heat exchange with the argon vapor stream;   the high pressure process stream is a high pressure air stream that after liquefaction forms a liquid air stream;   at least part of the liquid air stream is divided into a first subsidiary liquid air stream and a second subsidiary liquid air stream;   the first subsidiary liquid air stream is valve expanded and introduced into the lower pressure column;   the supplemental nitrogen-rich reflux stream and the oxygen-enriched stream are formed by:
 disengaging liquid and vapor phases from the two-phase stream to form a vapor phase stream and a liquid phase stream; 
 introducing the vapor phase stream into a bottom region of an auxiliary column and the second subsidiary liquid air stream into an intermediate location of the auxiliary column to form an oxygen containing bottoms liquid and a supplemental nitrogen-rich vapor column overhead within the auxiliary column; 
 forming a combined oxygen-enriched stream by combining the liquid phase stream with an oxygen containing bottoms liquid stream formed of the oxygen containing bottoms liquid; 
 condensing a supplemental nitrogen-rich vapor column overhead stream formed of the supplemental nitrogen-rich vapor column overhead through indirect heat exchange with the combined liquid stream after the valve expansion thereof, thereby partially vaporizing the combined liquid stream and forming the oxygen-enriched stream and the supplementary nitrogen-rich reflux stream and also an auxiliary column reflux stream from the nitrogen-rich vapor column overhead stream after having been condensed; and 
 refluxing the auxiliary column with the auxiliary column reflux stream; and 
   the at least part of the subsidiary nitrogen-rich reflux stream is valve expanded and introduced into the lower pressure column.   
     
     
         5 . The method of  claim 3 , wherein the high pressure process stream is a high pressure air stream that after liquefaction forms a liquid air stream. 
     
     
         6 . The method of  claim 4  or  claim 5 , wherein the liquid nitrogen reflux to the higher pressure column and the lower pressure column is produced by:
 condensing a higher pressure nitrogen-rich vapor stream composed of higher pressure nitrogen-rich vapor produced in the higher pressure column through indirect heat exchange with the oxygen-rich liquid column bottoms of the lower pressure column, thereby to produce a condensed nitrogen-rich stream; 
 refluxing the higher pressure column with at least part of the condensed nitrogen-rich stream; and 
 removing an impure nitrogen-rich liquid stream from the higher pressure column, subcooling and valve expanding the impure nitrogen-rich liquid stream and refluxing the lower pressure column with the impure nitrogen-rich liquid stream. 
 
     
     
         7 . The method of  claim 6 , wherein:
 a lower pressure nitrogen-rich vapor stream composed of a lower pressure nitrogen-rich vapor produced in the lower pressure column is removed from the lower pressure column;   the lower pressure nitrogen-rich vapor stream passes in indirect heat exchange with the crude liquid oxygen stream and the impure nitrogen-rich liquid stream to subcool the crude liquid oxygen stream and the impure nitrogen-rich liquid stream;   the lower pressure nitrogen-rich vapor stream is passed in indirect heat exchange with a compressed and purified air stream composed of the air to be separated; and   the compressed and purified air stream after having been cooled is introduced into a bottom region of the higher pressure column.   
     
     
         8 . The method of  claim 7 , wherein:
 after the high pressure air stream has been partially cooled, the high pressure air stream is divided into a first subsidiary high pressure air stream and a second subsidiary high pressure air stream;   the first subsidiary high pressure air stream is fully cooled and forms the liquid air stream;   the second subsidiary high pressure air stream after having been partially cooled is expanded in a turboexpander and introduced into the bottom region of the higher pressure column to impart refrigeration into the cryogenic rectification process.   
     
     
         9 . The method of  claim 8 , wherein:
 the higher pressure column is refluxed with a first part of the condensed nitrogen-rich stream;   a second part of the condensed nitrogen-rich stream is passed in indirect heat exchange with the lower pressure nitrogen-rich vapor stream along with the impure nitrogen-rich liquid stream and the crude liquid oxygen steam to form a liquid nitrogen product stream;   a third part of the condensed nitrogen-rich stream is pressurized and warmed through indirect heat exchange with the high pressure air stream along with the oxygen-rich column bottoms stream to form a pressurized nitrogen product stream.   
     
     
         10 . An apparatus for separating air to produce an oxygen product, said apparatus comprising:
 an air separation plant configured to separate the air through cryogenic rectification and having a higher pressure column and a lower pressure column operatively associated with one another in a heat transfer relationship to produce nitrogen-rich reflux for the higher pressure column and the lower pressure column and means for producing the oxygen product from an oxygen-rich liquid column bottoms of the lower pressure column;   the operative association of the lower pressure column and the higher pressure column including: means for partially vaporizing a crude liquid oxygen stream composed of a crude liquid oxygen column bottoms of the higher pressure column against at least partially condensing an argon-enriched vapor produced in the lower pressure column, means for producing a supplemental nitrogen-rich reflux stream and an oxygen-enriched stream having a greater oxygen content than the crude liquid oxygen column bottoms from a two-phase stream produced, at least in part, from the crude liquid oxygen stream after the partial vaporization thereof;   the argon-enriched vapor at least partially condensed at a location of the lower pressure column where the argon-enriched vapor has a nitrogen concentration between 0.1 and 5.0 mole percent and the resulting condensate will combine with downcoming liquid of the lower pressure column and increase a liquid to vapor ratio in a lowermost section of the lower pressure column; and   the supplemental nitrogen-rich reflux stream and the oxygen-enriched stream producing means connected to the lower pressure column such that the oxygen-enriched stream and at least part of the supplemental nitrogen-rich liquid stream are fed at successively higher locations of the lower pressure column and above the location of the lower pressure column at which the argon-enriched vapor is at least partially condensed.   
     
     
         11 . The apparatus of  claim 10 , wherein the oxygen product producing means comprises:
 a pump connected to the lower pressure column such that at least part of an oxygen-rich column bottoms stream withdrawn from the lower pressure column and composed of the oxygen-rich liquid column bottoms is pressurized by the pump to produce a pressurized liquid oxygen stream;   a main heat exchanger in flow communication with the pump and configured to warm at least part of the pressurized liquid oxygen stream through indirect exchange with a high pressure process stream such that at least part of the high pressure process stream is liquefied and the oxygen product is formed from the pressurized liquid oxygen stream after having been warmed; and   at least the lower pressure column is in flow communication with the main heat exchanger such that at least part of the high pressure process stream after having been liquefied is introduced into at least one of the higher or lower pressure column.   
     
     
         12 . The apparatus of  claim 11 , wherein:
 the means for partially vaporizing the crude liquid oxygen stream comprises an intermediate heat exchanger in flow communication with the lower pressure column and configured to pass the crude liquid oxygen stream in indirect heat exchange with the argon enriched vapor;   
       the supplemental nitrogen-rich reflux stream and the oxygen-enriched stream producing means comprises:
 a phase separator connected to the intermediate heat exchanger to disengage liquid and vapor phases from the two-phase stream and thereby to form a vapor phase stream and a liquid phase stream; and 
 an auxiliary heat exchanger connected to the phase separator and configured to partially vaporize the liquid phase stream in indirect heat exchange with the vapor phase stream thereby condensing the vapor phase stream and forming the supplemental nitrogen-rich reflux stream from the vapor phase stream after condensation thereof and the oxygen-enriched stream from the liquid phase stream after partial vaporization of the liquid phase stream; 
 
       the auxiliary heat exchanger is connected to the lower pressure column such that the oxygen-enriched stream and the at least part of the supplemental nitrogen-rich reflux stream are introduced into the lower pressure column; and 
       an arrangement of expansion valves are positioned for expanding: the crude liquid oxygen stream before being partially vaporized; the liquid phase stream prior to the indirect heat exchange with the vapor phase stream; and that at least part of the nitrogen-rich reflux stream before being introduced into the lower pressure column. 
     
     
         13 . The apparatus of  claim 11 , wherein:
 the means for partially vaporizing the crude liquid oxygen stream comprises an intermediate heat exchanger in flow communication with the lower pressure column and configured to pass the crude liquid oxygen stream in indirect heat exchange with the argon enriched vapor;   the high pressure process stream is a high pressure air stream that after liquefaction forms a liquid air stream;   the lower pressure column is in flow communication with the main heat exchanger such that the at least part of the liquid air stream is divided into a first subsidiary liquid air stream that is introduced into the lower pressure column and a second subsidiary liquid air stream;   the supplemental nitrogen-rich reflux stream and the oxygen-enriched stream producing means comprises:
 a phase separator connected to the intermediate heat exchanger such that liquid and vapor phases from the two-phase stream are disengaged to form a vapor phase stream and a liquid phase stream; 
 an auxiliary column connected to the phase separator and the main heat exchanger such that the vapor phase stream is introduced into a bottom region of the auxiliary column and the second subsidiary liquid air stream is introduced into an intermediate location of the auxiliary column to form an oxygen containing bottoms liquid and a supplemental nitrogen-rich vapor column overhead within the auxiliary column; and 
 an auxiliary column heat exchanger connected to the auxiliary column and the phase separator such that: the liquid phase stream is combined with an oxygen containing bottoms liquid stream formed of the oxygen containing bottoms liquid; the combined liquid stream passes in indirect heat exchange with a supplemental nitrogen-rich vapor column overhead stream formed of the supplemental nitrogen-rich vapor column overhead, thereby condensing the supplemental nitrogen-rich vapor stream and partially vaporizing the combined liquid stream to form the oxygen-rich reflux stream and the supplemental nitrogen-rich reflux stream and also, an auxiliary column reflux stream; and the auxiliary column reflux stream refluxes the auxiliary column; and 
   an arrangement of expansion valves is positioned for expanding: the crude liquid oxygen stream before being partially vaporized; the first subsidiary liquid air stream prior to being introduced into the lower pressure column; the combined liquid stream prior to passing into the auxiliary column heat exchanger; and the at least part of the subsidiary nitrogen-rich liquid stream prior to introduction into the lower pressure column.   
     
     
         14 . The apparatus of  claim 12  wherein the high pressure process stream is a high pressure air stream that after liquefaction forms a liquid air stream. 
     
     
         15 . The apparatus of  claim 13  or  claim 14 , wherein:
 a condenser reboiler is connected to the higher pressure column and the lower pressure column such that a higher pressure nitrogen-rich vapor stream composed of high pressure nitrogen-rich vapor produced in the higher pressure column is condensed through indirect heat exchange with the oxygen-rich liquid column bottoms of the lower pressure column, thereby to produce a condensed nitrogen-rich stream; 
 the higher pressure column is connected to the condenser reboiler such that the higher pressure column is refluxed with at least part of the condensed nitrogen-rich stream; and 
 the lower pressure column is connected to the higher pressure column such that an impure nitrogen-rich liquid stream is removed from the higher pressure column and introduced into the lower pressure column; 
 a subcooling unit is positioned between the higher pressure column and the lower pressure column such that the impure nitrogen-rich liquid stream is subcooled prior to being introduced into the lower pressure column; and 
 the arrangement of expansion valves also includes an expansion valve positioned between the subcooling unit and the lower pressure column such that the impure nitrogen-rich liquid stream is subcooled prior to being introduced into the lower pressure column. 
 
     
     
         16 . The apparatus of  claim 15 , wherein:
 the subcooling unit is connected to the lower pressure column and the higher pressure column and is configured such that a lower pressure nitrogen-rich vapor stream composed of a lower pressure nitrogen-rich vapor produced in the lower pressure column passes in indirect heat exchange with the crude liquid oxygen stream and the impure nitrogen-rich liquid stream to subcool the crude liquid oxygen stream and the impure nitrogen-rich liquid stream;   the main heat exchanger connected to the subcooling unit such that the lower pressure nitrogen-rich vapor stream, after having passed through the subcooling unit, is passed in indirect heat exchange with a compressed and purified air stream composed of the air to be separated; and   the main heat exchanger is connected to the higher pressure column such that a compressed and purified air stream after having been cooled in the main heat exchanger is introduced into a bottom region of the higher pressure column.   
     
     
         17 . The apparatus of  claim 15 , wherein:
 the main heat exchanger is configured such that after the high pressure air stream has been partially cooled, the high pressure air stream is divided into a first subsidiary high pressure air stream that is fully cooled and forms the liquid air stream and a second subsidiary high pressure air stream that is partially cooled prior to discharge from the main heat exchanger; and   a turboexpander is positioned between the main heat exchanger and the higher pressure column such that the second subsidiary high pressure air stream is expanded in a turboexpander and introduced into the bottom region of the higher pressure column to impart refrigeration into the air separation plant.   
     
     
         18 . The apparatus of  claim 17 , wherein:
 the higher pressure column is connected to the condenser reboiler such that the higher pressure column is refluxed with a first part of the condensed nitrogen-rich stream;   the subcooling unit is connected to the condenser reboiler such that a second part of the condensed nitrogen-rich stream is passed in indirect heat exchange with the lower pressure nitrogen-rich vapor stream along with the impure nitrogen-rich liquid stream and the crude liquid oxygen steam to form a liquid nitrogen product stream;   another pump is positioned between the main heat exchanger and the condenser reboiler such that a third part of the condensed nitrogen-rich stream is pumped and then warmed in the main heat exchanger through indirect heat exchange with the high pressure air stream along with the oxygen-rich column bottoms stream to form a pressurized nitrogen product stream.