Air separation method and apparatus

Abstract

An air separation method and apparatus in which a supercritical oxygen product is produced by heating a pumped liquid oxygen stream having a supercritical pressure, through indirect heat exchange with a boosted pressure air stream. The indirect heat exchange is conducted within a heat exchanger and a liquid nitrogen stream is vaporized in the heat exchanger to depress the pressure that would otherwise be required of the boosted pressure air stream to heat the pumped liquid oxygen stream. The pumped liquid oxygen stream constitutes 90 percent of the oxygen-rich liquid removed from an air separation unit in which the air is rectified, the liquid nitrogen constitutes at least 90 percent of the liquid nitrogen that is not used as reflux and a flow-rate ratio between the liquid nitrogen stream and the oxygen-rich liquid is between about 0.3 and 0.90.

What is claimed is: 
     
         1 . A method of separating air comprising:
 separating the air within a cryogenic rectification process including rectifying compressed, purified and cooled air in an air separation unit having a higher pressure column and a lower pressure column, heating at least part of a pumped liquid oxygen stream and vaporizing a liquid nitrogen stream through indirect heat exchange with at least a boosted pressure air stream within a heat exchanger, pumping at least part of an oxygen-rich stream composed of an oxygen-rich liquid column bottoms produced in the lower pressure column to produce the pumped liquid oxygen stream and producing the liquid nitrogen stream from part of a nitrogen-rich liquid stream formed by condensing a nitrogen-rich vapor column overhead of the higher pressure column against partly vaporizing the oxygen-rich liquid column bottoms that is not used as reflux;   the at least part of the pumped liquid oxygen stream having a supercritical pressure and being heated to a supercritical temperature to produce an oxygen product as a supercritical fluid and constituting at least about 90 percent of the oxygen-rich stream, the at least part of the liquid nitrogen stream having a subcritical pressure and constituting at least about 90 percent of the part of the nitrogen-rich liquid stream; and   the liquid nitrogen stream and the at least part of the pumped liquid oxygen having flow rates in a ratio of between about 0.3 and about 0.90; and   the boosted pressure air stream having a boosted pressure and a flow rate, the boosted pressure lower than that which would otherwise have been required at the flow rate had there been no indirect heat exchange within the heat exchanger with the liquid nitrogen stream.   
     
     
         2 . The method of  claim 1 , wherein the heat exchanger is a higher pressure heat exchanger of a banked heat exchanger arrangement. 
     
     
         3 . The method of  claim 2  wherein:
 an argon containing vapor stream is removed from the lower pressure column and is rectified in an argon column to produce an argon-rich vapor column overhead and an oxygen containing liquid column bottoms; 
 the argon-rich vapor column overhead is condensed to produce an argon reflux stream that is introduced into the argon column; 
 an argon-rich product stream is removed from the argon column; and 
 an oxygen containing liquid stream composed of the oxygen containing liquid column bottoms is introduced into the lower pressure column. 
 
     
     
         4 . The method of  claim 3 , wherein:
 a crude liquid oxygen stream, composed of a crude liquid oxygen column bottoms produced in the higher pressure column is subcooled;   at least part of the crude liquid oxygen stream, after having been subcooled, is valve expanded and introduced into an argon condenser connected to the argon column to condense the argon-rich vapor stream, thereby to partially vaporize the crude liquid oxygen stream and form a vapor phase and a liquid phase;   a vapor phase stream and a liquid phase stream, composed of the vapor phase and the liquid phase, are introduced into the lower pressure column;   a liquid air stream formed from liquefaction of the boosted pressure air stream is expanded and divided into a first subsidiary liquid air stream and a second subsidiary liquid air stream;   the first subsidiary liquid air stream is introduced into the higher pressure column;   the second subsidiary liquid air stream is introduced into the argon condenser and is thereby subcooled; and   the second subsidiary liquid air stream, after having been subcooled, is expanded and introduced into the lower pressure column.   
     
     
         5 . The method of  claim 2  or  claim 4 , wherein:
 the air is compressed and purified by compressing a feed air stream in a main air compressor and purifying the air after the compression thereof in a pre-purification unit to form a compressed and purified air stream; 
 a first part of the compressed and purified air stream is cooled in a lower pressure heat exchanger of the banked heat exchanger arrangement to a temperature suitable for its rectification and introduced into the higher pressure column; and 
 at least a portion of a second part of the compressed and purified air stream is compressed in a booster compressor to form the boosted pressure air stream. 
 
     
     
         6 . The method of  claim 5 , wherein:
 a third part of the compressed and purified air stream is further compressed, partially cooled in the lower pressure heat exchanger and expanded in a turboexpander to produce an exhaust stream; and   the exhaust stream, along with the first part of the compressed and purified air stream, is rectified within the higher pressure column.   
     
     
         7 . The method of  claim 5 , wherein:
 the portion of the second part of the compressed and purified air stream is compressed in the booster compressor in forming the boosted pressure air stream; and   the third part of the compressed and purified air stream is composed of another portion of the second part of the compressed and purified air stream after having been partially compressed in an intermediate stage of the booster compressor and is further compressed in another booster compressor.   
     
     
         8 . The method of  claim 5 , wherein:
 a further part of the nitrogen-rich liquid stream is introduced into the higher pressure column as reflux;   a nitrogen containing reflux stream having a lower nitrogen purity than the nitrogen-rich liquid stream is subcooled, expanded and introduced as reflux to the lower pressure column;   a lower pressure nitrogen vapor stream, composed of column overhead of the lower pressure column subcools the nitrogen containing reflux stream and the crude liquid oxygen stream in a subcooler through indirect heat exchange; and   the lower pressure nitrogen vapor stream is divided into a first and second subsidiary lower pressure nitrogen vapor streams that are introduced, respectively, into the higher pressure heat exchanger and the lower pressure heat exchanger to balance cold end temperatures.   
     
     
         9 . The method of  claim 5 , wherein the liquid nitrogen stream and the nitrogen-rich liquid stream have the same pressure. 
     
     
         10 . An apparatus for separating air comprising:
 a cryogenic air separation plant including an air separation unit having a higher pressure column and a lower pressure column to rectify the air, a heat exchanger configured to indirectly exchange heat from a boosted pressure air stream to at least part of a pumped liquid oxygen stream having a supercritical pressure and a liquid nitrogen stream, thereby to heat the pumped liquid oxygen stream to a supercritical temperature and form an oxygen product as a supercritical fluid and to vaporize the liquid nitrogen stream and form a nitrogen product as a vapor and a pump positioned between the heat exchanger and the lower pressure column such that at least part of an oxygen-rich stream composed of an oxygen-rich liquid column bottoms produced in the lower pressure column is pressurized to the supercritical pressure and the at least part of the pumped liquid oxygen stream constitutes at least about 90 percent of the oxygen-rich stream;   the heat exchanger in flow communication with a condenser reboiler operatively associated with the higher pressure column and the lower pressure column such that the liquid nitrogen stream is composed of at least about 90 percent of a part of a nitrogen-rich liquid stream produced by condensing a nitrogen-rich vapor column overhead of the higher pressure column that is not used as reflux against partly vaporizing the oxygen-rich liquid column bottoms within the condenser reboiler and has a subcritical pressure;   the air separation plant configured such that the liquid nitrogen stream and the at least part of the pumped liquid oxygen stream have flow rates in a ratio of between about 0.3 and about 0.90; and   the boosted pressure air stream produced by a booster compressor configured such that the boosted pressure air stream has a flow rate and a boosted pressure lower than that which would otherwise have been required at the flow rate had there been no indirect heat exchange within the heat exchanger with the liquid nitrogen stream.   
     
     
         11 . The apparatus of  claim 10  wherein the heat exchanger is a higher pressure heat exchanger of a banked heat exchanger arrangement. 
     
     
         12 . The apparatus of  claim 11 , wherein:
 an argon column is connected to the lower pressure column such that an argon containing vapor stream is removed from the lower pressure column and is rectified in the argon column to produce an argon-rich vapor column overhead and an oxygen containing liquid column bottoms and an oxygen containing liquid stream composed of the oxygen containing liquid column bottoms is introduced into the lower pressure column;   an argon condenser connected to the argon column such that the argon-rich vapor column overhead is condensed to produce an argon reflux stream that is introduced into the argon column; and   the argon column having an outlet to discharge an argon-rich product stream from the argon column.   
     
     
         13 . The apparatus of  claim 12 , wherein:
 a subcooling unit is connected to the higher pressure column such that a crude liquid oxygen stream, composed of a crude liquid oxygen column bottoms produced in the higher pressure column is subcooled;   the argon condenser is connected to the subcooling unit and a first expansion valve is positioned between the argon condenser and the subcooling unit such that at least part of the crude liquid oxygen stream, after having been subcooled, is valve expanded in the first expansion valve and introduced into the argon condenser to condense the argon-rich vapor stream and thereby to partially vaporize the at least part of the crude liquid oxygen stream and form a vapor phase and a liquid phase;   the argon condenser connected to the lower pressure column such that a vapor phase stream and a liquid phase stream, composed of the vapor phase and the liquid phase, respectively, are introduced into the lower pressure column;   a liquid expander connected to the higher pressure heat exchanger such that a liquid air stream produced as a result of the liquefaction of the boosted pressure air stream is expanded;   the liquid expander connected to the higher pressure column and the argon condenser such that a first subsidiary liquid air stream composed of part of the liquid air stream is introduced into the higher pressure column and a second subsidiary liquid air stream composed of another part of the liquid air stream is introduced into the argon condenser;   the argon condenser is configured to subcool the second subsidiary liquid air stream and is connected to the lower pressure column such that the second subsidiary liquid air stream, after having been subcooled is introduced into the lower pressure column; and   a second expansion valve positioned between the argon condenser and the lower pressure column to valve expand the second subsidiary liquid air stream.   
     
     
         14 . The apparatus of  claim 13 , wherein:
 a main air compressor compresses a feed air stream and a pre-purification unit is connected to the main air compressor to form a compressed and purified air stream from the feed air stream after having been compressed;   the banked heat exchanger arrangement has a lower pressure heat exchanger positioned between the pre-purification unit and the higher pressure column such that a first part of the compressed and purified air stream is cooled to a temperature suitable for the rectification thereof and is introduced into the higher pressure column; and   a booster compressor is positioned between the pre-purification unit and the higher pressure heat exchanger such that at least a portion of a second part of the compressed and purified air is further compressed in the booster compressor to form the boosted pressure air stream.   
     
     
         15 . The method of  claim 14 , wherein:
 the booster compressor is configured to compress a portion of the second part of the compressed and purified air stream to produce the boosted pressure air stream and to discharge a third part of the compressed and purified air stream, composed of another portion of the second part of the compressed and purified air stream from an intermediate stage of the booster compressor;   another booster compressor is positioned between the intermediate stage and the lower pressure heat exchanger such that the third part of the compressed and purified air stream is further compressed and introduced into the lower pressure heat exchanger;   the lower pressure heat exchanger is configured to partially cool the third part of the compressed and purified air stream;   a turboexpander is connected to the lower pressure heat exchanger to expand the third part of the compressed and purified air stream and thereby produce an exhaust stream;   the turboexpander is in flow communication with the higher pressure column such that the exhaust stream, along with the first part of the compressed and purified air stream, is rectified within the higher pressure column.   
     
     
         16 . The method of  claim 14 , wherein:
 the condenser reboiler is connected to the higher pressure column such that a further part of the nitrogen-rich liquid stream is introduced into the higher pressure column as reflux;   the subcooling unit is connected to the higher pressure column such that a nitrogen containing reflux stream is discharged from the higher pressure column having a lower nitrogen purity than the nitrogen-rich liquid stream and is subcooled in the subcooling unit;   the subcooling unit connected to the lower pressure column such that the nitrogen containing reflux stream is introduced as reflux to the lower pressure column;   a third expansion valve is positioned between the subcooler and the lower pressure column such that the nitrogen containing reflux stream is expanded within the third expansion valve;   the subcooler is also connected to the lower pressure column such that a lower pressure nitrogen vapor stream, composed of column overhead of the lower pressure column, subcools the nitrogen containing reflux stream and the crude liquid oxygen stream through indirect heat exchange; and   the higher pressure heat exchanger and the lower pressure heat exchanger are connected to the subcooler such that first and second subsidiary lower pressure nitrogen vapor streams, composed of the lower pressure nitrogen vapor stream, are introduced, respectively, into the higher pressure heat exchanger and the lower pressure heat exchanger to balance temperatures.   
     
     
         17 . The apparatus of  claim 11  or  claim 16 , wherein the higher pressure heat exchanger is flow communication with the condenser reboiler such that the liquid nitrogen stream and the nitrogen-rich liquid stream have the same pressure.