Patent Publication Number: US-2012036892-A1

Title: Air separation method and apparatus

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. patent application Ser. No. 12/855,313, filed on Aug. 12, 2010, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for separating air in which oxygen-rich liquid is pumped to produce a pumped liquid oxygen stream having a supercritical pressure that is in turn warmed to a supercritical temperature through indirect heat exchange with a boosted pressure air stream to produce an oxygen product as a supercritical fluid. More particularly, the present invention relates to such a method and apparatus in which a liquid nitrogen stream is simultaneously vaporized while the pressurized liquid stream is heated, so as to depress the pressure that would otherwise be required of the boosted pressure air stream to heat the pumped liquid oxygen stream alone. 
     BACKGROUND OF THE INVENTION 
     There exists an emerging market for very high pressure supercritical oxygen that is being driven primarily by requirements of gasifiers for such oxygen. Typically, the oxygen is produced from the cryogenic separation of air. Although oxygen produced by such cryogenic rectification can be produced at moderate operational pressures and then compressed, more often, a liquid oxygen stream is pumped to a supercritical pressure within a cryogenic rectification plant and then heated to a supercritical temperature through indirect heat exchange with a boosted pressure air stream to produce an oxygen product as a supercritical fluid. 
     In a cryogenic rectification plant, a feed air stream is compressed and purified of higher boiling contaminants such as moisture, carbon dioxide, carbon monoxide and hydrocarbons to produce a compressed and purified air stream. Part of such air stream can be cooled within a lower pressure heat exchanger of a banked heat exchanger arrangement consisting of at least one higher pressure heat exchanger and at least one lower pressure heat exchanger. In the banked heat exchanger arrangement the higher pressure heat exchanger is provided for heating the pumped liquid oxygen stream to a supercritical temperature through indirect heat transfer with a high pressure boosted air stream. The use of such heat exchanger banking saves fabrication costs in that it is only the higher pressure heat exchanger that must be fabricated to withstand the high oxygen pressure and the even higher pressures of the boosted air stream that are necessary to heat the oxygen. In any case, the cooled air from the lower pressure heat exchanger is then introduced into an air separation unit that has a higher pressure column and a lower pressure column in a heat transfer relationship to rectify the air into nitrogen and oxygen-rich fractions. Such air separation units can also include an argon column connected to the lower pressure column to rectify an argon containing vapor stream into an argon-rich product or an intermediate argon product known in the art as crude argon. 
     The higher and lower pressure columns contain mass transfer contacting elements such as trays or structured packing or a combination of such elements to contact liquid and vapor phases and thereby accomplish a continuous distillation within such columns. The air entering the higher pressure column produces an ascending vapor phase that becomes evermore rich in nitrogen as it ascends the higher pressure column to produce a nitrogen-rich vapor as a column overhead. The nitrogen-rich vapor is then condensed to produce a nitrogen-rich liquid that in part is used to reflux the higher pressure column and initiate a descending liquid phase that contacts the ascending vapor phase within the mass transfer contacting elements and becomes ever more richer in oxygen as such liquid phase descends. As a result, a crude liquid oxygen column bottoms is produced in the higher pressure column that is also known as kettle liquid. Such liquid bottoms is further refined in the lower pressure column and in case an argon column is present, also serves as a heat transfer media to condense the argon-rich vapor in the argon column prior to being introduced into the lower pressure column. The further refinement produces an oxygen-rich liquid column bottoms in the lower pressure column and a tower overhead that is rich in nitrogen. A stream of the oxygen-rich liquid is then removed and pumped to produce the pumped liquid oxygen stream that at least in part is introduced into the higher pressure heat exchanger to form the oxygen product. 
     The heat transfer relationship between the higher and lower pressure column is produced by a condenser reboiler that can be situated in the sump of the lower pressure columns. A stream of the nitrogen-rich vapor column overhead of the higher pressure column is condensed to produce the nitrogen-rich liquid that serves in part as reflux to the higher pressure column. The condensation is through indirect heat exchange with the oxygen-rich liquid column bottoms of the lower pressure column that causes such liquid to boil and produce boilup in the lower pressure column. Part of the nitrogen-rich liquid can be taken as a product and in fact, can be pumped and also introduced into the higher pressure heat exchanger along with the pumped liquid oxygen stream. In U.S. Patent Application Publication No. 2008/0307828 both pumped oxygen and nitrogen streams are passed through a higher pressure heat exchanger of a banked heat exchange arrangement to produce an oxygen product as a supercritical fluid and to vaporize the pumped nitrogen stream and thereby produce a nitrogen vapor product at pressure. 
     In producing oxygen at supercritical pressures, the higher pressure heat exchanger must be built to withstand even higher pressures than the oxygen stream to be heated. For example, if 120 bar absolute oxygen is to be heated to a supercritical temperature, the boosted pressure air stream will optimally have a pressure in the order of 160 bar absolute. The problem with this is that the cost in fabricating such a heat exchanger to withstand the pressure of the boosted air stream can become prohibitively expensive, as well as the cost of the associated pipework and valves which must also be rated to the same very high pressure. In addition, there can be increased energy costs in that an inline barrel compressor might be required, depending on the pressure, that has an efficiency that is less than an integrally geared compressor that could be employed at a lower pressure. Finally, during the startup of such a system the consequences of a failed pressure test at very high pressures can be quite severe. 
     There is therefore, a need to minimize the pressure of the boosted pressure air stream that is required in heating the pumped liquid oxygen to a supercritical temperature. 
     As will be discussed, the present invention provides a method and apparatus for separating air involving warming both a pumped liquid oxygen stream at supercritical pressure and a liquid nitrogen stream within a heat exchanger in a manner in which the flow rate of the liquid nitrogen to be vaporized is sufficient to allow for operation at a pressure that is lower than that which would otherwise be required of the boosted pressure air stream. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention provides a method of separating air in which the air is separated in a cryogenic rectification process. In such process, compressed, purified and cooled air is rectified in an air separation unit having a higher pressure column and a lower pressure column and at least part of a pumped liquid oxygen stream is heated and a liquid nitrogen stream is vaporized through indirect heat exchange with a boosted pressure air stream. At least part of an oxygen-rich stream composed of an oxygen-rich liquid column bottoms produced in the lower pressure column is pumped to produce the pumped liquid oxygen stream. The liquid nitrogen stream is produced from part of a nitrogen-rich liquid stream that is formed by condensing a nitrogen-rich vapor column overhead of the higher pressure column against partly vaporizing the oxygen-rich liquid column bottoms and that is not used as reflux. 
     The at least part of the pumped liquid oxygen stream has a supercritical pressure and is heated to a supercritical temperature to produce an oxygen product as a supercritical fluid. The at least part of the pumped liquid oxygen stream constitutes at least about 90 percent of the oxygen-rich stream and the at least part of the liquid nitrogen stream has a subcritical pressure and constitutes at least about 90 percent of the part of the nitrogen-rich liquid stream. The liquid nitrogen stream and the at least part of the pumped liquid oxygen stream have flow rates in a ratio of between 0.3 and 0.90. The boosted pressure air stream has a boosted pressure and a flow rate. The boosted pressure is 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. 
     The inventors herein have found that under certain operation conditions, vaporizing a liquid nitrogen stream together with the heating of the pumped liquid oxygen stream will have a substantial effect on the shape of the composite cooling curve. The upper ratio limit of 0.90 is selected to ensure that there will be sufficient reflux as not to severely effect oxygen recovery and the lower ratio limit of 0.3 represents a limitation where there is not a sufficient depression of the required pressure for the boosted pressure air stream. 
     It is to be noted that the heating and cooling curves represent the aggregate heat transfer from the boosted pressure air stream and any other streams to be cooled to the warming oxygen and warming/vaporizing nitrogen stream and any other streams to be warmed. The composite curves combine multiple cooling streams (“hot” streams) into a single curve and multiple warming streams (“cold” streams) into a single stream. At a given temperature within the heat exchanger, the composite curve is defined such that the sum of the energy change of each hot or cold stream defines the duty for the hot or cold composite curve, respectively. Composite curves are used to simplify and idealize the analysis of heat exchangers with more than two streams transferring heat simultaneously. 
     The effect of simultaneously vaporizing a liquid nitrogen stream in the higher pressure heat exchanger, in addition to heating the pumped liquid oxygen stream is to alter the shape of the composite cooling curve such that it enables the designer to lessen the pressure that would otherwise be required of the boosted pressure air stream to heat an oxygen stream at a supercritical pressure to a supercritical temperature if such oxygen stream were the only stream being heated within the higher pressure heat exchanger. In this regard, in a non-banked heat exchanger arrangement, all of the streams to be warmed and cooled are passed in indirect heat exchange within a single heat exchanger that in most practical applications is a series of heat exchangers run in parallel. Where the designer desires to decrease the pressure of the boosted pressure air stream, a problem that arises is that the flow rate of such stream must be increased. Where the liquid nitrogen stream is vaporized, the change in shape of such curves allows the flow rate of the boosted pressure air stream to be lower than that had such liquid nitrogen stream not been present. The higher flow rate results in more liquid air being produced that will result in a loss of column performance and at an extreme, will not allow column operation. In the banked case in which all of such heat exchange takes place within a higher pressure heat exchanger, the liquid nitrogen vaporization allows such heat exchanger to function at a reasonable approach temperature at the point within the heat exchanger at which the liquid oxygen becomes a supercritical fluid, typically 5 degrees Kelvin or less. If the liquid nitrogen were not present, not only would the heat exchanger not function at the flow rate required with the liquid nitrogen, but at an extreme of operation, the heating and cooling curves would in fact cross preventing any operation of the heat exchanger. 
     As indicated above, the at least part of the pumped liquid oxygen stream can be heated and the liquid nitrogen stream can be vaporized within a higher pressure heat exchanger of a banked heat exchanger arrangement through indirect heat exchange with the boosted pressure air stream. 
     An argon containing vapor stream can be removed from the lower pressure column and 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. In a specific embodiment, 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, respectively, are introduced into the lower pressure column and 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 and the second subsidiary liquid air stream is introduced into the argon condenser and is thereby subcooled. The second subsidiary liquid air stream, after having been subcooled, is expanded and introduced into the lower pressure column. 
     The air can be 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. 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. A third part of the compressed and purified air stream can be further compressed, partially cooled in the lower pressure heat exchanger and expanded in a turboexpander to produce an exhaust stream. The exhaust stream, along with the first part of the compressed and purified air stream, is rectified within the higher pressure column. The portion of the second part of the compressed and purified air stream can be compressed in the booster compressor in forming the boosted pressure air stream. In such case, 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. 
     A further part of the nitrogen-rich liquid stream can be introduced into the higher pressure column as reflux and a nitrogen containing reflux stream having a lower nitrogen purity than the nitrogen-rich liquid stream can be 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, can be used to subcool the nitrogen containing reflux stream and the crude liquid oxygen stream in a subcooler through indirect heat exchange. 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. 
     In any embodiment of the present invention, the liquid nitrogen stream and the nitrogen-rich liquid stream may have the same pressure. It is understood, however, that the present invention contemplates that the liquid nitrogen stream may be raised in pressure by liquid head or a pump. 
     In another aspect of the present invention, an apparatus is provided for separating air that comprises a cryogenic air separation plant. Such plant includes an air separation unit having a higher pressure column and a lower pressure column to rectify the air, a heat exchanger in flow communication with the air separation unit and a pump. The heat exchanger is 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. The pump is 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 is 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 against partly vaporizing the oxygen-rich liquid column bottoms within the condenser reboiler that is not used as reflux for the columns. Such liquid nitrogen stream has a subcritical pressure. The air separation plant is configured such that the liquid nitrogen stream and the at least part of the pumped liquid oxygen stream having flow rates in a ratio of between 0.3 and 0.90. The boosted pressure air stream is produced by a booster compressor that is 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. 
     The heat exchanger can be a higher pressure heat exchanger of a banked heat exchanger arrangement also having a lower pressure heat exchanger. 
     An argon column can be 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. An oxygen containing liquid stream composed of the oxygen containing liquid column bottoms is introduced into the lower pressure column. An argon condenser is 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. In a specific embodiment, a subcooling unit can be 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 is also 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 is 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. A second expansion valve positioned between the argon condenser and the lower pressure column to valve expand the second subsidiary liquid air stream. 
     A main air compressor can be provided to compress a feed air stream and a pre-purification unit can be 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. 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. The booster compressor can be 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 and 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. 
     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 is 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. The higher pressure heat exchanger is connected to the low pressure column and the lower pressure heat exchanger is 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. 
     The higher pressure heat exchanger can be in flow communication with the condenser reboiler such that the liquid nitrogen stream and the nitrogen-rich liquid stream have the same pressure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the specification concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed that the present invention will be better understood when taken in connection with the accompanying drawings in which: 
         FIG. 1  is a schematic process flow diagram of an apparatus that is designed to carry out a method in accordance with the present invention; 
         FIG. 2  is a graph illustrating the effect of a ratio of nitrogen to oxygen on the optimal pressure to compress a boosted air stream; 
         FIG. 3  is a graph illustrating the composite heating and cooling curves in a heat exchanger of an air separation plant constructed and operated in accordance with the present invention; 
         FIG. 4  is a graph illustrating the composite heat and cooling curves in a heat exchanger of an air separation plant operated at a nitrogen to oxygen ratio of zero; and 
         FIG. 5  is a graph illustrating the composite heat and cooling curves in a heat exchanger of an air separation plant operated at a nitrogen to oxygen ratio below that specified in the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , a cryogenic rectification plant  1  is illustrated that is designed to separate compressed and purified air and thereby to produce an oxygen product as a supercritical fluid. Cryogenic rectification plant  1  is provided with a banked heat exchanger arrangement  2  and an air separation unit  3 . Air separation unit  3  preferably, for reasons that will be discussed, is provided with an argon column  62  to produce an argon product. The banked heat exchanger arrangement  2  has a lower pressure heat exchanger  22  that operates at a lower average pressure than a higher pressure heat exchanger  28  thereof. The oxygen product is discharged from the higher pressure heat exchanger  28  as an oxygen product stream  132 . Additionally, a nitrogen product stream  134  is also discharged from the higher pressure heat exchanger  28 . It is understood, however, that the present invention has equal application to a cryogenic rectification plant that employs only a non-banked heat exchanger arrangement and one in which an argon column is not used. The banked arrangement is preferred for reasons of lower capital cost as described earlier, but will result in a small energy penalty relative to a fully integrated arrangement which all the streams are in indirect heat exchange relationship because some of the available refrigeration cannot be recovered. In this regard, the present invention in its broader aspects has application to any cryogenic rectification plant utilizing higher and lower pressure columns and that is designed to produce an oxygen product as a supercritical fluid. 
     In cryogenic rectification plant  1 , a feed air stream  10  is compressed in a compressor  12  to produce a compressed air stream  14 . The heat of compression is removed from compressed air stream  14  by an aftercooler  16 . It is understood that compressor  12  may constitute a multi-stage intercooled integral gear compressor with condensate removal and consequently, aftercooler  16  could be part of compressor  12 . In any case aftercooler  16  as well as other aftercoolers mentioned will allow the performance of downstream unit operations such as prepurifiers and heat exchangers to be improved. However, it is possible that an embodiment of the present invention could be constructed without such aftercoolers. 
     The compressed air stream  14  is then introduced into a prepurification unit  18  to remove higher boiling impurities such as water vapor, carbon dioxide and hydrocarbons from the air and thereby produce a compressed and purified air stream  20 . As well known in the art, such unit  18  can incorporate adsorbent beds operating in a cycle that is a combination of temperature and pressure swing adsorption or that is purely a temperature swing adsorption cycle or a pressure swing adsorption cycle. 
     The banked heat exchanger arrangement  2  has a lower pressure heat exchanger  22  positioned between the pre-purification unit  18  and a higher pressure column  58  of the air separation unit  3  such that a first part  24  of the compressed and purified air stream  20  is cooled to a temperature suitable for the rectification thereof and is introduced into the higher pressure column  58 . A booster compressor  26  is positioned between the pre-purification unit  18  and a higher pressure heat exchanger  28  of the banked heat exchanger arrangement  2  such that a portion of a second part  30  of the compressed and purified air is further compressed in the booster compressor  26  to form a boosted pressure air stream  32 . Booster compressor  26  is a multi-stage integral geared compressor. After removal of the heat of compression by an aftercooler  34 , the boosted pressure air stream  32  is introduced into the higher pressure heat exchanger  28 . The booster compressor  26  is configured to produce the flow rates and pressure of the boosted pressure air stream  32  that are required by the present invention in a manner well known in the art. In this regard, the booster compressor  26  has to be appropriately sized to have the capability of delivering the required pressure and flow and will incorporate suitable controls for controlling its pressure output and flow but such means as inlet guide vanes and downstream controls. 
     It is to be noted that main air compressor  12  and booster compressor  26  are shown as single units. However, as is known in the art, two or more compressors can be installed in parallel to form either the main air compressor  12  or the booster compressor  26 . The two compressors can be of equal size or unequal size. For example, the capacity can be split 70/30 or 60/40 in order to better match customer demand. Typically, the second part  30  of the compressed and purified air stream  20  will have a flow that ranges from between about 25 percent and about 40 percent of the flow of the compressed and purified air stream  20 . 
     Both the higher pressure heat exchanger  28  and the lower pressure heat exchanger  22  are preferably of brazed aluminum construction and consist of layers of parting sheets separated by side bars to produce flow passages for the streams to be heated and cooled. Each of the flow passages are provided with fins as well known in the art to enhance the surface area for heat transfer within said heat exchangers. The higher pressure heat exchanger  28  is so named due to the fact that it has a higher maximum allowable working pressure as compared with lower pressure heat exchanger  22 . The higher pressure heat exchanger  28  is configured to fully cool the boosted pressure air stream  32  to produce a liquid air stream  36  and the lower pressure heat exchanger  22  is configured to fully cool the first part  24  of the compressed and purified air stream  20  to produce a main feed air stream  38 . In this regard, the term “fully cooled” as used herein and in the claims means cooled to a temperature at the cold end of either the lower pressure heat exchanger  22  or the higher pressure heat exchanger  28 . 
     Other types of heat exchangers could be used, for example, higher pressure heat exchanger  28  could be a copper or stainless steel spiral wound, a stainless steel printed circuit or of stainless steel plate-fin construction. Furthermore, as indicated above, the present invention is applicable to a non-banked arrangement in which a heat exchanger or a set of heat exchangers in parallel are each used both in the liquefaction of the boosted pressure air stream  32  and the cooling of the first part  24  of the compressed and purified air stream  20 . Moreover, although each of the higher pressure heat exchanger  28  and the lower pressure heat exchanger  22  are illustrated as single units, in practice, each could consist of several individual heat exchanger blocks or cores linked together in parallel. 
     A third part  40  of the compressed and purified air stream  20 , that constitutes another portion of the second part  30  of the compressed and purified air stream  20 , is partly compressed within booster compressor  26  and then removed from an intermediate stage thereof. The third part  40  of the compressed and purified air stream  20  is then introduced into another booster compressor  42 , cooled within an aftercooler  44  to remove the heat of compression, partially cooled within lower pressure heat exchanger  22  and then introduced into turboexpander  46  to produce an exhaust stream  48 . The term, “partially cooled”, as used herein and in the claims, means cooled to a temperature between the warm and cold end temperatures of the lower pressure heat exchanger  22 . Exhaust stream  48  is introduced into the higher pressure column  58  along with first part  24  of the compressed and purified air stream  20  as a combined stream  50 . Energy is recovered from the turboexpander and applied to the booster compressor  42 . The purpose of this is to generate part of the refrigeration requirements of the cryogenic air separation plant. As known in the art, such refrigeration is imparted due to warm end losses in the lower and higher pressure heat exchangers  22  and  28 , heat in-leak losses, and in order to allow the plant to produce liquids. It is possible to construct an embodiment of the present invention in which the third part  40  of the compressed and purified air stream  20  is partially cooled within the higher pressure heat exchanger  28 . However, this would not be preferable in that more power would have to be supplied to booster compressor  26 . 
     As will be discussed, the liquid air stream  36  produced through liquefaction of the boosted pressure air stream  32  can be introduced into a liquid expander  52  to generate further refrigeration requirements for the plant. Liquid air stream  36 , after the expansion thereof, can be divided into first and second subsidiary liquid air streams  54  and  56 . First subsidiary liquid air stream  54  is introduced into higher pressure column  58  and second subsidiary liquid air stream  56  is introduced into lower pressure column  60  in a manner that will be discussed hereinafter. 
     Air separation unit  3  is provided with a higher pressure column  58 , a lower pressure column  60  and an argon column  62 . All of such columns contain mass transfer contacting elements such as trays or packing, for instance structured packing or a combination of trays and packing. An ascending vapor phase originated from the combined stream  50  that is introduced into the higher pressure column  58  becomes evermore rich in nitrogen to produce a nitrogen-rich vapor column overhead that is withdrawn as a nitrogen-rich vapor stream  64  and condensed within a condenser reboiler  66  located in the sump of the lower pressure column  60  to produce a nitrogen-rich liquid stream  68 . An oxygen-rich liquid column bottoms  70  is in part vaporized in connection with the condensation of the nitrogen-rich vapor stream  64 . A part  72  of the nitrogen-rich liquid stream  68  is returned to the higher pressure distillation column  58  as reflux and thereby establish a descending liquid phase that becomes evermore rich in oxygen through contact with the ascending vapor phase and thereby to produce a crude liquid oxygen column bottoms  74  of the higher pressure column  58 . 
     A crude liquid oxygen stream  75 , composed of the crude liquid oxygen column bottoms  74  is further refined in the lower pressure column  60 . For such purposes, the crude liquid oxygen stream  75  is subcooled within a subcooling unit  76  and then divided. A part  78  of the crude liquid oxygen stream  75 , after expansion within expansion valve  80 , is introduced into lower pressure column  60 . A part  82  of the crude liquid oxygen stream  75  is then valve expanded in expansion valve  84  and introduced into argon condenser  102  where it partially vaporizes into vapor and liquid phases. Liquid and vapor phase streams  86  and  88 , composed of the liquid and vapor phases, respectively, are introduced into the lower pressure column  60 . Lower pressure column  60  is refluxed with an nitrogen containing reflux stream  90  removed from the higher pressure column  58  at a level at which such stream has a lower nitrogen content than the part  72  of the nitrogen-rich liquid stream  68 . Nitrogen containing liquid stream  90  is subcooled in the subcooling unit  76 , expanded in an expansion valve  92  and then introduced into lower pressure column  60 . 
     The advantage of argon column  62  is that oxygen recovery will be improved because argon is being separated from the downcoming liquid phase. An argon containing vapor stream  94  is removed from the lower pressure column  60  and introduced into the argon column  62  and rectified to produce an argon-rich column overhead and an oxygen containing liquid column bottoms  96 . An oxygen containing liquid stream  98  composed of the oxygen containing liquid column bottoms  96  is returned to the lower pressure column  60 . The argon-rich vapor column overhead is removed as an argon-rich vapor stream  100  and condensed in an argon condenser  102  to produce reflux for the argon column  62 . The argon condenser illustrated herein has a shell  104  and a heat exchanger  106  within the shell  104 . The part  82  of the crude liquid oxygen stream  75 , after having been subcooled and expanded is introduced into the shell  104  where it is partially vaporized into liquid and vapor phases against condensing the argon-rich vapor containing in the argon-rich vapor stream  100  that is passed through the heat exchanger  106 . Liquid phase and vapor phase streams  86  and  88  composed of such liquid and vapor phases, are reintroduced into the lower pressure column  60 . The resulting argon-rich liquid stream  108  is passed into a phase separator  110  to produce a vapor phase that is discharged as a vapor phase stream  112  and a liquid phase that is discharged from the phase separator  110  as a liquid reflux stream  114  that is reintroduced into the argon column  62 . The purpose of this is to prevent the build-up of nitrogen in stream  100  in case of an operational excursion. Too much nitrogen could result in ceasing operation of condenser  106  due to a reduction of the temperature difference, as is known in the art. An argon product stream  116  can be removed from the argon column as a liquid or a vapor. 
     In the illustrated embodiment, the heat exchanger  106  is provided with a set of passages to subcool second subsidiary liquid air stream  56 . The resulting subcooled second subsidiary liquid air stream  118  is valve expanded to the pressure of lower pressure column  60  in an expansion valve  120  and introduced into the lower pressure column  60 . The advantage here is that oxygen recovery will be improved along with argon recovery. 
     An oxygen-rich liquid stream  122 , composed of the oxygen-rich liquid column bottoms  70 , can be divided into first and second oxygen-rich liquid streams  124  and  126 . First oxygen-rich liquid stream  124  is pumped in a pump  128  to a supercritical pressure to produce a pumped liquid oxygen stream  130 . Second oxygen-rich liquid stream  126  is optional and can be taken as a product. Alternatively and/or in addition, part of the pumped liquid oxygen could be taken as a product. Pumped liquid oxygen stream  130  is thereafter heated within the higher pressure heat exchanger  28  to a supercritical temperature so that an oxygen product stream  132  is discharged as a supercritical fluid. 
     As mentioned above, in order to reduce the pressure required for the boosted pressure of air stream, a liquid nitrogen stream  133  formed of part of the nitrogen-rich liquid stream  68  is vaporized within the higher pressure heat exchanger  28  to produce a nitrogen product stream  134  at pressure. Although such stream is illustrated as having the same pressure as nitrogen-rich liquid stream  68 , the liquid nitrogen stream  133  could be pumped to a pressure below the supercritical pressure of the nitrogen contained in such stream if higher pressure nitrogen were required. However, it is important that the nitrogen be at a pressure so that it in fact vaporizes during heating. It is also to be noted that another liquid nitrogen stream  136  composed of another part of the nitrogen-rich liquid stream  68  can be subcooled within subcooling unit  76  and taken as an optional liquid nitrogen product stream  138 . However, liquid nitrogen stream  133  should constitute at least 90 percent of the part of the nitrogen-rich liquid stream  68  that is not returned to the higher pressure column  58  as the reflux stream  72 . Moreover, flow rates of the liquid nitrogen stream  133  and the pumped liquid oxygen stream  130  should be in a ratio of between about 0.3 and about 0.90. In this regard, pumped liquid oxygen stream or the part thereof that is heated within the higher pressure heat exchanger  28  should constitute at least 90 percent of the flow rate of the oxygen-rich liquid stream  122 . Above a ratio of 0.90, oxygen production will fall to about 94 percent of that which would otherwise be produced without the production of vaporized nitrogen. As will be discussed, the lower limit is a limit where there will not be a meaningful effect of being able to reduce the boosted pressure of the boosted pressure air stream  32 . It is to be pointed out that such nitrogen flow rates can be controlled by means of a valve, with appropriately sized equipment for the flow path, such as piping and heat exchangers. When the nitrogen is pumped, the pump flow and head characteristics control the nitrogen flow rates. Further, the liquid nitrogen stream  133  could be pumped so long it was not pumped beyond a supercritical pressure which is about 34 bar absolute. 
     The lower limit of 0.3 can be best explained with reference to  FIG. 2 . As shown in  FIG. 2 , an 80 bar absolute pumped liquid oxygen stream is heated to a supercritical temperature with the vaporization of a liquid nitrogen stream at varying ratios. In all cases, low liquid product rates were assumed for purposes of calculation. As is evident, below 0.30, a point of inflection exists in which the curve is no longer linear. As such, at nitrogen to oxygen ratios of below about 0.30, there is progressively less effect of vaporizing liquid nitrogen. 
       FIG. 3  illustrates the composite heating and cooling curves within a heat exchanger, such as the higher pressure heat exchanger  28  at a ratio of about 0.85 and pumped oxygen being heated to a supercritical temperature at a pressure of 80 bar absolute. The optimal pressure of the boosted air stream is 68 bar absolute. In  FIG. 4 , the ratio has been reduced to 0.0. As is evident, the optimal air pressure is 110 bar absolute. As shown in  FIG. 5 , a nitrogen to oxygen ratio of 0.2 was used. The optimal air pressure is 92 bar absolute. It is to be pointed out that in  FIGS. 4 and 5 , although the pressure is higher than in  FIG. 3 , the energy required to compress the air and form the boosted air stream is less than that of  FIG. 3  because there is a penalty associated with vaporizing the liquid nitrogen and more boosted air flow is required. However, to deliver the same rate of pressurized nitrogen when the nitrogen is withdrawn as a vapor from the high pressure or low pressure column and is not pumped and vaporized within heat exchanger  28 , the nitrogen must instead be compressed externally. When this is the case, much of the energy penalty associated with the higher boosted air flow is eliminated. The energy penalty must be balanced against the costs and availability of heat exchangers that would otherwise be required to withstand the high pressure of the boosted air stream such as boosted pressure air stream  32 , as well as the cost for an additional nitrogen compressor, if necessary. Thus, all else being equal, the use of the liquid nitrogen will allow for a lower pressure of the boosted pressure air stream  32 . 
     Referring to  FIG. 3 , for example, the liquid nitrogen stream vaporizes at a constant temperature causing the flat section of the cold composite curve near the cold end. The pressure of the pumped liquid oxygen equals or exceeds the critical pressure of oxygen (about 51 bar absolute). Hence, unlike the liquid nitrogen stream, the pumped oxygen stream does not boil as it is warmed. However, there is a zone where the supercritical oxygen “pseudo-boils”, where its heat capacity is markedly higher. This can be observed for the cold composite curve of  FIG. 3  in the flatter zone that begins near the critical temperature of oxygen (155 K). From  FIGS. 3 ,  4 , and  5 , the flow of the liquid nitrogen stream has a large effect on the cold composite temperature profile. For example, in  FIG. 3  the pumped oxygen stream begins “pseudo-boiling” at a duty of about 0.45. In  FIG. 4 , where the flow of the liquid nitrogen stream is zero, the pumped oxygen stream begins “pseudo-boiling” at a duty of about 0.30. At the optimal pressure of the boosted pressure air stream (represented by the hot composite curve in  FIGS. 3-5 ), there is a minimum temperature difference approach near the critical temperature of oxygen. Preferably, this approach temperature difference is about 1.0 K, but may be as low at 0.5 K or as high as 5.0 K. Depending on the oxygen pressure, this minimum temperature difference occurs in a range between 150 K and 180 K. Other minimum temperature differences may also occur simultaneously within the heat exchanger, for example, near the warm end. This optimal design condition minimizes total power consumption for a given total heat exchanger surface area. Also, the area between the hot composite curve and cold composite curve is approximately minimized. Power consumption is minimized under this condition because the heat transfer between the hot composite and cold composite streams is most thermodynamically reversible. To achieve this condition, the pressure of the boosted air stream is optimized. Hence, the optimal pressure of the boosted air stream is lower for higher rates of the liquid nitrogen stream, affecting the shape of the hot composite curve. For  FIG. 3 , the optimal pressure of the boosted air stream is 68 bar absolute, for  FIG. 4  it is 110 bar absolute, for  FIG. 5  it is 92 bar absolute. Lower pressures of the boosted air stream cause more inflection in the hot composite curve, related to the “pseudo-condensing” of the supercritical boosted pressure air stream. This allows the approach temperature difference at 150 K to 180 K to be reduced to an optimal value at higher rates of the liquid nitrogen stream. Upon further reduction of the pressure of the boosted air stream below the optimum, it becomes impossible to maintain a positive temperature difference at 150 K to 180 K because there is too much inflection in the hot composite curve for banked heat exchangers. Hence, the required heat transfer becomes infeasible for banked heat exchangers when the pressure of the boosted air stream is reduced appreciably below its optimal value. For non-banked heat exchangers, it is possible to further reduce the pressure of boosted pressure air stream, but its requisite flow then increases such that power consumption increases. 
     A nitrogen vapor stream  140  composed of the nitrogen-rich vapor column overhead of the lower pressure column  60  is preferably divided into first and second nitrogen vapor streams  142  and  144 . First nitrogen vapor stream  142  is introduced into the subcooling unit  76  for the subcooling duty required by such unit and then is fully warmed within the lower pressure heat exchanger  22 . Second nitrogen vapor stream  144  is introduced into the higher pressure heat exchanger  28 , fully warmed and discharged as a waste nitrogen stream  146 . The flow rate of the first and second nitrogen streams  142  and  144  should be selected in a known manner to optimally balance the temperature profiles of the higher and lower pressure heat exchangers  28  and  22 . This is done with the goal of avoiding tight temperature approaches in heat exchangers  28  and  22 , and to minimize the heat transfer surface required. The first nitrogen stream  142  after having been fully warmed can be divided into a regeneration stream  148  that is used to regenerate adsorbents within pre-purification unit  18  with the remainder discharged as a waste nitrogen stream  150 . It is to be noted that the second nitrogen stream  144  within the higher pressure heat exchanger will have no effect on the nitrogen to oxygen ratios set forth above. 
     As illustrated, the resulting nitrogen product stream  132 , if desired at higher pressure can be in part compressed by a compressor  152  to produce a high pressure nitrogen stream  154 . The remaining part of the nitrogen product stream  132  can therefore be taken as a lower pressure nitrogen stream  156 . 
     The following table is a simulated example of the operation of air separation plant  1  in accordance with the present invention. 
     Stream Summary 
       
     
       
         
           
               
               
               
            
               
                   
                   
               
               
                   
                 Molar 
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Flow 
                 Pressure 
                 Temperature 
                 Vapor 
                 Mole Fraction 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Name 
                 [mol/hr] 
                 [psia] 
                 [K] 
                 Fraction 
                 Nitrogen 
                 Argon 
                 Oxygen 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                  20 
                 1000.0 
                 87.6 
                 285.9 
                 1.0 
                 0.7811 
                 0.0093 
                 0.2096 
               
               
                  24 
                 353.1 
                 87.6 
                 285.9 
                 1.0 
                 0.7811 
                 0.0093 
                 0.2096 
               
               
                  40 
                 185.3 
                 320.0 
                 308.2 
                 1.0 
                 0.7811 
                 0.0093 
                 0.2096 
               
               
                  30 
                 538.3 
                 87.6 
                 285.9 
                 1.0 
                 0.7811 
                 0.0093 
                 0.2096 
               
               
                 122 
                 197.4 
                 20.8 
                 93.6 
                 0.0 
                 0.0000 
                 0.0040 
                 0.9960 
               
               
                     142 1   
                 469.0 
                 16.5 
                 294.0 
                 1.0 
                 0.9757 
                 0.0036 
                 0.0207 
               
               
                 144 
                 159.5 
                 18.8 
                 80.1 
                 1.0 
                 0.9758 
                 0.0036 
                 0.0207 
               
               
                     40 2   
                 185.3 
                 508.8 
                 308.2 
                 1.0 
                 0.7811 
                 0.0093 
                 0.2096 
               
               
                     52 3   
                 185.3 
                 506.8 
                 179.4 
                 1.0 
                 0.7811 
                 0.0093 
                 0.2096 
               
               
                  48 
                 185.3 
                 84.9 
                 112.6 
                 1.0 
                 0.7811 
                 0.0093 
                 0.2096 
               
               
                  38 
                 353.1 
                 84.4 
                 107.8 
                 1.0 
                 0.7811 
                 0.0093 
                 0.2096 
               
               
                     32 4   
                 461.7 
                 1160.3 
                 308.2 
                 1.0 
                 0.7811 
                 0.0093 
                 0.2096 
               
               
                  36 
                 461.7 
                 1157.1 
                 100.5 
                 0.0 
                 0.7811 
                 0.0093 
                 0.2096 
               
               
                 130 
                 194.3 
                 1750.3 
                 98.8 
                 0.0 
                 0.0000 
                 0.0040 
                 0.9960 
               
               
                 126 
                 3.0 
                 15.5 
                 90.7 
                 0.0 
                 0.0000 
                 0.0039 
                 0.9961 
               
               
                 132 
                 194.3 
                 1740.5 
                 292.6 
                 1.0 
                 0.0000 
                 0.0040 
                 0.9960 
               
               
                 146 
                 159.5 
                 16.7 
                 292.6 
                 1.0 
                 0.9758 
                 0.0036 
                 0.0207 
               
               
                  75 
                 435.6 
                 84.2 
                 100.0 
                 0.0 
                 0.6223 
                 0.0144 
                 0.3632 
               
               
                  78 
                 163.1 
                 84.1 
                 94.2 
                 0.0 
                 0.6223 
                 0.0144 
                 0.3632 
               
               
                  82 
                 272.4 
                 84.1 
                 94.2 
                 0.0 
                 0.6223 
                 0.0144 
                 0.3632 
               
               
                  88 
                 258.8 
                 20.0 
                 86.6 
                 1.0 
                 0.6381 
                 0.0143 
                 0.3476 
               
               
                  86 
                 13.6 
                 20.0 
                 86.6 
                 0.0 
                 0.3233 
                 0.0177 
                 0.6591 
               
               
                  54 
                 221.6 
                 87.0 
                 98.6 
                 0.0 
                 0.7811 
                 0.0093 
                 0.2096 
               
               
                  56 
                 240.1 
                 87.0 
                 98.6 
                 0.0 
                 0.7811 
                 0.0093 
                 0.2096 
               
               
                 118 
                 240.1 
                 87.0 
                 88.0 
                 0.0 
                 0.7811 
                 0.0093 
                 0.2096 
               
               
                  50 
                 538.3 
                 84.2 
                 109.4 
                 1.0 
                 0.7811 
                 0.0093 
                 0.2096 
               
               
                 138 
                 0.0 
                 — 
                 — 
                 0.0 
                 — 
                 — 
                 — 
               
               
                  90 
                 156.6 
                 81.3 
                 95.5 
                 0.0 
                 0.9882 
                 0.0049 
                 0.0068 
               
               
                     90 5   
                 156.6 
                 81.3 
                 81.1 
                 0.0 
                 0.9882 
                 0.0049 
                 0.0068 
               
               
                 133 
                 167.8 
                 80.5 
                 95.3 
                 0 
                 1.0000 
                 34 ppm 
                 5 ppm 
               
               
                 134 
                 167.8 
                 75.6 
                 292.6 
                 1 
                 1.0000 
                 34 ppm 
                 5 ppm 
               
               
                  94 
                 185.2 
                 19.9 
                 92.8 
                 1.0 
                 1 ppm 
                 0.1040 
                 0.8960 
               
               
                  98 
                 178.9 
                 19.6 
                 92.6 
                 0.0 
                 0.0000 
                 0.0726 
                 0.9274 
               
               
                 116 
                 6.2 
                 16.7 
                 88.5 
                 0.0 
                 2 ppm 
                 1.0000 
                 1 ppm 
               
               
                 112 
                 0.1 
                 16.7 
                 88.5 
                 1.0 
                 0.0006 
                 0.9994 
                 1 ppm 
               
               
                 140 
                 628.6 
                 18.8 
                 80.1 
                 1.0 
                 0.9758 
                 0.0036 
                 0.0207 
               
               
                   
               
               
                 Notes: 
               
               
                   1 Stream 142 after having been fully heated within lower pressure heat exchanger 22 
               
               
                   2 Stream 40 after booster compressor 42 and before entering lower pressure heat exchanger 22 
               
               
                   3 Stream 40 after having been partially cooled within lower pressure heat exchanger 22 
               
               
                   4 Stream 32 after aftercooler 34 and before entering higher pressure heat exchanger 28 
               
               
                   5 Stream 90 after having been subcooled within subcooling unit 76 
               
            
           
         
       
     
     While the present invention has been described with reference to a preferred embodiment, as will occur to those skilled in the art, numerous changes, additions and omissions can be made without departing from the spirit and scope of the invention as set forth in the appended claims.