Cryogenic separation method and apparatus

Abstract

A method and apparatus for separating a mixture, for example air, within a cryogenic rectification plant that utilizes a banked heat exchanger arrangement. In such arrangement, a lower pressure heat exchanger is used to cool part of the mixture and a higher pressure heat exchanger is used to heat one or more pumped liquid streams composed of separated nitrogen-rich and oxygen-rich fractions and thereby produce pressurized product streams. A boosted pressure stream, that can be part of the air, is utilized to supply most of the heat exchange duty in the higher pressure heat exchanger. In addition, a heat exchange stream, that can also be part of the mixture, can be partially cooled in the higher pressure heat exchanger and then further cooled in the lower pressure heat exchanger to decrease the warm end temperature difference of the higher pressure heat exchanger and therefore, the required refrigeration for the plant.

1 . A method of separating a mixture comprising nitrogen and oxygen, said method comprising:
 conducting a cryogenic rectification process comprising: compressing, purifying, cooling and distilling the mixture into oxygen into nitrogen-rich and oxygen-rich fractions, imparting refrigeration into the cryogenic rectification process and producing at least one pressurized product stream by pumping and heating at least part of one of the nitrogen-rich and oxygen-rich fractions in a liquid state;   the cryogenic rectification process being conducted so as to produce a boosted pressure stream and a heat exchange stream and the cryogenic rectification process utilizing a banked heat exchanger arrangement having a lower pressure heat exchanger for the cooling of at least part of the mixture and a higher pressure heat exchanger for the heating the at least part of the one of the nitrogen-rich and oxygen-rich fractions after having been pumped;   introducing the boosted pressure stream and the heat exchange stream into the higher pressure heat exchanger in indirect heat exchange with the at least part of the one of the nitrogen-rich and oxygen-rich fractions;   partially cooling the heat exchange stream in the higher pressure heat exchanger, thereby decreasing a warm end temperature difference within the higher pressure heat exchanger and therefore, the refrigeration required to be imparted to the cryogenic rectification process;   cooling the boosted pressure stream within the higher pressure heat exchanger; and   further cooling the heat exchange stream in the lower pressure heat exchanger after having been partially cooled in the higher pressure heat exchanger.   
     
     
         2 . The method of  claim 1 , wherein the cryogenic rectification process generates a nitrogen-rich vapor stream that is divided into two nitrogen-rich vapor streams that are fully warmed within the higher pressure heat exchanger and the lower pressure heat exchanger so as to balance cold end temperatures of the higher pressure heat exchanger and the lower pressure heat exchanger. 
     
     
         3 . The method of  claim 1  or  claim 2 , wherein:
 the mixture is air; 
 a feed stream composed of the air after having been compressed and purified is divided into a first subsidiary compressed air stream; a second subsidiary compressed air stream and a third subsidiary compressed air stream; 
 the first subsidiary compressed air stream is fully cooled within the lower pressure heat exchanger; 
 at least part of the second subsidiary compressed air stream is further compressed to form the boosted pressure stream and forms a liquid air stream after having been fully cooled within the higher pressure heat exchanger; and 
 the heat exchange stream is the third subsidiary compressed air stream. 
 
     
     
         4 . The method of  claim 3 , wherein:
 a first part of the second subsidiary compressed air stream is further compressed to produce the boosted pressure air stream;   a second part of the third subsidiary compressed air stream is further compressed to a pressure below that of the boosted pressure air stream and is thereafter, yet further compressed, partially cooled within the higher pressure heat exchanger and introduced into a turboexpander to produce an exhaust stream; and   the exhaust stream is introduced into the distillation column to impart at least part of the refrigeration into the cryogenic rectification process.   
     
     
         5 . The method of  claim 4 , wherein:
 the mixture is distilled in a higher pressure column operatively associated in a heat transfer relationship with a lower pressure column by a condenser-reboiler configured to condense a higher pressure nitrogen-rich column overhead stream, removed from the higher pressure column, by reboiling an oxygen-rich liquid column bottoms of the lower pressure column;   the first subsidiary compressed air stream and the third subsidiary compressed air stream, after having been fully cooled, are introduced into the higher pressure column;   the liquid air stream is expanded and introduced into at least one of the higher pressure column and the lower pressure column;   a crude liquid oxygen stream composed of a liquid column bottoms of the higher pressure column is subcooled, reduced in pressure to that of the lower pressure column and introduced into the lower pressure column for further refinement;   first and second parts of a high pressure nitrogen-rich liquid stream, formed from condensing the higher pressure nitrogen-rich column overhead stream, is used to reflux the higher pressure column and the lower pressure column, respectively;   the second of the parts of the higher pressure nitrogen-rich liquid stream is subcooled, reduced in pressure to that of the lower pressure column prior to being introduced as the reflux into the lower pressure column;   the crude liquid oxygen stream and the second of the parts of the higher pressure nitrogen-rich liquid stream are subcooled through indirect heat exchange with a lower pressure nitrogen-rich column overhead stream withdrawn from the lower pressure column;   the at least one liquid stream is one of an oxygen-enriched stream, composed of the oxygen-rich liquid column bottoms of the lower pressure column and a third part of the high pressure nitrogen-rich liquid stream.   
     
     
         6 . The method of  claim 5 , wherein:
 the lower pressure nitrogen-rich column overhead stream is divided into the two nitrogen-rich vapor streams that are utilized to balance the cold end temperatures of the higher pressure heat exchanger and the lower pressure heat exchanger;   the exhaust stream is introduced into the higher pressure column; and   the liquid air stream is expanded in a liquid expander.   
     
     
         7 . An apparatus for separating a mixture comprising nitrogen and oxygen, said apparatus comprising:
 a cryogenic rectification plant configured to compress, purify, cool and distill the mixture into nitrogen-rich and oxygen-rich fractions;   the cryogenic rectification plant having at least one pump for pumping at least part of a liquid stream composed of one of the nitrogen-rich and oxygen-rich fractions in the liquid state, a banked heat exchanger arrangement having lower pressure heat exchanger configured to cool at least part of the mixture and a higher pressure heat exchanger in flow communication with the at least one pump for heating the at least part of the liquid stream and thereby forming a pressurized product stream, means for producing a boosted pressure stream, means for producing a heat exchange stream, and means for imparting refrigeration into the cryogenic rectification plant;   the higher pressure heat exchanger connected to the boosted pressure stream producing means and the heat exchange stream producing means and configured to partially cool the heat exchange stream by indirectly exchanging heat from the heat exchange stream to the at least part of the liquid stream, thereby decreasing a warm end temperature difference within the higher pressure heat exchanger and therefore, the refrigeration required to be imparted to the cryogenic rectification plant and to cool the boosted pressure stream by indirectly exchanging heat from the boosted pressure stream to the at least part of the liquid stream; and   the lower pressure heat exchanger connected to the higher pressure heat exchanger and configured to further cool the heat exchange stream after having been partially cooled within the higher pressure heat exchanger.   
     
     
         8 . The apparatus of  claim 7 , wherein the cryogenic rectification plant is also configured to generate two nitrogen-rich vapor streams and the higher pressure heat exchanger and the lower pressure heat exchanger are also configured to receive and to fully warm the two nitrogen-rich vapor streams so that cold end temperatures of the higher pressure heat exchanger and the lower pressure heat exchanger are balanced. 
     
     
         9 . The apparatus of  claim 8 , wherein:
 the mixture is air;   the cryogenic rectification plant has a main air compressor and a pre-purification unit connected to the main air compressor to purify the air after having been compressed;   the boosted pressure stream producing means comprises a booster compressor connected to the pre-purification unit;   the lower pressure heat exchanger is also connected to the pre-purification unit so that a feed stream composed of the mixture after having been compressed in the main air compressor and purified in the pre-purification unit is divided into a first subsidiary compressed air stream that is fully cooled in the lower pressure heat exchanger and a second subsidiary compressed air stream that at least in part is further compressed in booster compressor to form the boosted pressure stream and that and forms a liquid air stream after having been fully cooled within the higher pressure heat exchanger; and   the heat exchange stream producing means comprises the higher pressure heat exchanger also connected to the pre-purification unit so that the feed stream after having been compressed and purified is further divided into a third subsidiary compressed air stream that forms the heat exchange stream.   
     
     
         10 . The apparatus of  claim 9 , wherein:
 the booster compressor is a multi-stage machine;   a first part of the second subsidiary compressed air stream is discharged from a final stage of the booster compressor and forms the boosted pressure air stream; and   the refrigeration imparting means, at least in part, comprises a further booster compressor connected to an intermediate stage of the booster compressor to further compress a second part of the second subsidiary compressed air stream, the higher pressure heat exchanger connected to the further booster compressor so that the second part of the third subsidiary compressed air stream, after having been further compressed, is partially cooled within the higher pressure heat exchanger, a turboexpander connected to the higher pressure heat exchanger to expand the first part of the second subsidiary compressed air stream and thereby to produce an exhaust stream and the turboexpander connected to the distillation column so that the exhaust stream is introduced into the distillation column.   
     
     
         11 . The apparatus of  claim 10 , wherein:
 the cryogenic rectification plant has a higher pressure column and a lower pressure column to distill the mixture, the higher pressure column operatively associated with the lower pressure column in a heat transfer relationship by a condenser-reboiler configured to condense at least part of a higher-pressure nitrogen-rich column overhead stream, discharged from the higher pressure column, by reboiling an oxygen-rich liquid column bottoms of the lower pressure column;   the lower pressure heat exchanger is connected to the higher pressure column so that the first subsidiary compressed air stream and the second subsidiary compressed air stream are introduced into the higher pressure column;   the higher pressure heat exchanger is in flow communication with at least one of the higher pressure column and the lower pressure column so that the liquid air stream is introduced into at least one of the higher pressure column and the lower pressure column;   an expansion device positioned between the higher pressure heat exchanger and the at least one of the higher pressure column and the lower pressure column to expand the liquid air stream;   the higher pressure column connected to the lower pressure column so that a crude liquid oxygen stream composed of liquid column bottoms of the higher pressure column is introduced into the lower pressure column so as to be further refined and first and second parts of a high pressure nitrogen-rich liquid stream, formed from condensing the higher pressure nitrogen-rich overhead stream are introduced into the higher pressure column and the lower pressure column, respectively, as reflux;   a subcooler, positioned between the lower pressure column and the lower pressure heat exchanger or incorporated into the lower pressure heat exchanger, configured to subcool the crude liquid oxygen stream and the second of the parts of the higher pressure nitrogen-rich liquid stream;   expansion valves located between the subcooler and the lower pressure column to expand the crude liquid oxygen stream and the second of the parts of the higher pressure nitrogen-rich liquid stream prior to their introduction into the lower pressure column;   the subcooler connected to the lower pressure column so that a lower pressure nitrogen-rich column overhead stream discharged from the lower pressure column passes in indirect heat exchange with the crude liquid oxygen stream and the second of the parts of the higher pressure nitrogen-rich liquid stream; and   the at least one liquid stream is one of an oxygen-enriched stream, composed of the oxygen-rich liquid column bottoms of the lower pressure column and a third part of the higher pressure nitrogen-rich liquid stream.   
     
     
         12 . The apparatus of  claim 11 , wherein:
 the higher pressure heat exchanger and the lower pressure heat exchanger are connected to the subcooler so that the lower pressure nitrogen-rich column overhead stream divides into the two nitrogen-rich vapor streams that are utilized to balance the cold end temperatures of the higher pressure heat exchanger and the lower pressure heat exchanger;   the turboexpander is connected to the higher pressure column so that the exhaust stream is introduced into the higher pressure column; and   the expansion device is a liquid expander.