US20160025408A1

US20160025408A1 - Air separation method and apparatus

Info

Publication number: US20160025408A1
Application number: US14/444,438
Authority: US
Inventors: Zhengrong Xu; Neil M. Prosser; Yang Luo
Original assignee: Praxair Technology Inc
Current assignee: Praxair Technology Inc
Priority date: 2014-07-28
Filing date: 2014-07-28
Publication date: 2016-01-28
Also published as: WO2016025063A1

Abstract

A method and apparatus for separating air by cryogenic rectification in which cooled, compressed and purified air is separated in a distillation column system having higher and lower pressure columns operatively associated with one another in a heat transfer relationship to produce an oxygen-rich liquid stream from the lower pressure column. The oxygen-rich liquid stream is pumped and heated through indirect heat exchange with a compressed heat exchange stream to form a pressurized oxygen product stream. Part of the air is sequentially and successively compressed in booster compressors driven by turboexpanders to form the compressed heat exchange stream while other parts of the air are expanded in turboexpanders driving the booster compressors to form exhaust streams that are introduced into both the higher and lower pressure columns to generate refrigeration.

Description

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for separating air through a cryogenic rectification process in which a pressurized oxygen product stream is formed by pumping an oxygen-rich liquid stream to produce a pumped liquid oxygen stream and warming the pumped liquid oxygen stream through indirect heat exchange with a compressed heat exchange steam that is composed of part of the air to be separated. More particularly, the present invention relates to such a method and apparatus in which the compressed heat exchange stream is formed by sequentially compressing the air within booster compressors of a first booster loaded expander and a second booster loaded expander that have turboexpanders to expand partially cooled portions of the air to produce exhaust streams that are introduced into thermally linked higher and lower pressure columns to impart refrigeration into the process.

BACKGROUND OF THE INVENTION

Air is separated into its component parts by means of a cryogenic rectification process conducted in distillation columns operated at cryogenic temperatures. The air in such a process is first compressed in a main air compression system that may have a series of compression stages linked to one another by intercoolers to remove the heat of compression between stages. The compressed air is then purified of higher boiling contaminants such as water vapor, carbon dioxide and hydrocarbons within adsorbent beds operated in an out-of-phase cycle where one adsorbent bed is regenerated while another of the beds is adsorbing the impurities. The cycle can be a temperature swing cycle, a pressure swing cycle or a combination of both cycles. After purification the air is cooled to a temperature suitable for its distillation and then separated within distillation columns to produce oxygen-rich and nitrogen-rich streams that are withdrawn from the columns and then used in cooling the incoming air within a main heat exchanger. The warmed streams constitute oxygen and nitrogen-rich products.
The distillation columns can include higher and lower pressure columns. These distillation columns are so designated given that the higher pressure column operates at a higher pressure than the lower pressure column. The incoming air, after having been purified and cooled, is introduced into the higher pressure column and separated to produce a crude liquid oxygen column bottoms, also known as kettle liquid and a nitrogen-rich vapor column overhead. The crude liquid oxygen is further refined in the lower pressure column to produce an oxygen-rich liquid column bottoms and another nitrogen-rich vapor column overhead. The columns are thermally linked by a condenser reboiler in which nitrogen-rich vapor produced as the column overhead of the higher pressure column is condensed through indirect heat exchange with the oxygen-rich liquid column bottoms of the lower pressure column, thereby producing liquid nitrogen reflux for the distillation columns and boilup within the lower pressure column. An argon column can be connected to the lower pressure column to separate and argon from oxygen in a crude argon feed stream fed to the argon column from the lower pressure column. If a pressurized oxygen product stream is desired, a stream of the oxygen-rich liquid column bottoms of the lower pressure column can be pumped to produce a pumped liquid oxygen stream. The pumped liquid oxygen stream can be heated through indirect heat exchange with a compressed heat exchange stream composed of part of the air to be separated. The heat exchange results in liquefaction of the air within the compressed heat exchange stream and resulting liquid air can be introduced as intermediate reflux into both the higher and lower pressure columns.
The compressed heat exchange stream is commonly produced by compressing a portion of the air in a booster compressor after the air has been compressed and purified. Typically, this portion of the air constitutes about 30 percent of the incoming air. The remainder of the air, after having been cooled, is introduced into the higher pressure column. Additionally, the air after having been compressed and purified can be partially cooled and then expanded in a turboexpander to produce an exhaust stream. The exhaust stream is in turn introduced into the higher pressure column to impart refrigeration and thereby balance losses at the warm end of a main heat exchanger used in cooling the air and the export of refrigeration accompanied by the production of liquid products that are discharged from the plant as liquids.
As can be appreciated, the booster compressor, used in compressing the air and thereby forming the compressed heat exchange stream, consumes electrical power and thus, represents part of the ongoing expense in producing a pressurized oxygen product. In order to decrease such expense, it is known in the prior art to compress part or all of the air at a sub-ambient temperature to produce the pressures required in suitably boosting pressure of the compressed air to form the compressed heat exchange stream. In such “cold compression,” all or a portion of the compressed air, after having been partially cooled to a temperature intermediate the warm and cold ends of the main heat exchanger, is compressed at the intermediate temperature and then reintroduced into the main heat exchanger at a temperature level at which the air passes through a phase transition from a liquid to a vapor or a supercritical fluid. Since the air at such point is cold and therefore, has a greater density than the ambient air, less power is expended in compressing the air than had the air been solely compressed at the warm end of the main heat exchanger. For example, U.S. Pat. No. 5,475,980 discloses a cryogenic air separation process to produce a pressurized product stream in which part of the air being cooled is withdrawn from an intermediate location of the main heat exchanger used in cooling the air. The withdrawn air is then compressed by a compressor and reintroduced back into the main heat exchanger at a location thereof at which the oxygen vaporizes. After having been partially cooled, the air that has been reintroduced into the main heat exchanger is then expanded in a turboexpander coupled to the compressor so that no external energy will be required in compressing the air at the intermediate location of the main heat exchanger. The resulting exhaust stream, which is a two phase flow, is then introduced into a phase separator. A liquid phase stream is introduced into the higher pressure column and a vapor phase stream is partially warmed, expanded and then introduced into the lower pressure column to impart additional refrigeration into the process. As can be appreciated, such a process introduces complexity and expense into the main heat exchanger used in carrying out the process because of the intermediate outlets and inlets that are necessarily required to withdraw and reintroduced air back into the heat exchanger.
As will be discussed, the present invention provided a method and apparatus for separating air through cryogenic rectification and producing a pressurized oxygen product that among other advantages is energy efficient and can utilize conventional warm end heat exchange equipment that is less complex and therefore, expensive than prior art equipment discussed above.

SUMMARY OF THE INVENTION

The present invention provides a method of separating air within a cryogenic rectification process in which the air is separated by cooling the air, after having been compressed and purified and rectifying the air in a distillation column system having a higher pressure column and a lower pressure column operatively associated within one another in a heat transfer relationship. Return streams, enriched in components of the air, are produced that are warmed through indirect heat exchange with the air to help cool the air and to produce product streams. One of the product streams is formed by withdrawing an oxygen-rich liquid stream from a bottom region of the lower pressure column, pumping at least part of the oxygen-rich liquid stream to produce a pumped liquid oxygen stream and heating at least part of the pumped liquid oxygen stream to form a pressurized oxygen product stream. The at least part of the pumped liquid oxygen stream constitutes one of the return streams and the at least part of the pumped liquid oxygen stream is heated through indirect heat exchange with a compressed heat exchange stream composed of part of the air to be cooled and rectified in the distillation column system. The compressed heat exchange stream, a first exhaust stream and a second exhaust stream are formed with the use of a first booster loaded expander and a second booster loaded expander having booster compressors driven by turboexpanders. In this regard, the term, “booster loaded expander” as used herein and in the claims means a turboexpander coupled directly to a booster compressor so that the work of expansion is dissipated in powering the booster compressor. Part of the air is sequentially compressed within the booster compressors of the first booster loaded expander and the second booster loaded expander to form the compressed heat exchange stream. Other parts of the air are partially cooled and then expanded within the turboexpanders to produce a first exhaust stream and a second exhaust stream from expansion of the other parts of the air within in the first booster loaded expander and the second booster loaded expander, respectively. The first exhaust stream is introduced into lower pressure column and the second exhaust stream is introduced into the higher pressure column, thereby to impart refrigeration into the cryogenic rectification process.
As compared with the prior art, since the air need not be extracted and then reintroduced into the main heat exchanger, the design of the main heat exchanger design can be simpler and therefore, less expensive, than prior art heat exchangers where cold compression is utilized. Furthermore, even though cold compression is not used in the present invention, since the energy required to compress the air in forming the compressed heat exchange stream is recovered in turboexpanders coupled to the compressors, the overall energy efficiency of the process is better than or at least equal to that of prior art cold compression techniques to make the present invention attractive from the standpoint of energy consumption.
Preferably, a first compressed air stream, a second compressed air stream and a third compressed air stream can be formed, at least in part, by compressing and purifying the air to produce a compressed and purified air stream and dividing the compressed and purified air stream into the first compressed air stream, the second compressed air stream and the third compressed air stream. The part of the air that is thereby compressed is thus formed from the first compressed air stream and the other parts of the air that are expanded are formed from the second compressed air stream and the third compressed air stream. More specifically, the first compressed air stream is sequentially compressed within a first and second booster compressor of the first booster loaded expander and the second booster loaded expander to form the compressed heat exchange stream. The second compressed air stream is partially cooled and introduced into a first turboexpander of the first booster loaded expander, thereby to produce the first exhaust stream and the third compressed air stream is partially cooled and introduced into a second turboexpander of the second booster loaded expander, thereby to produce the second exhaust stream. The first compressed air stream and the second compressed air stream are partially cooled in a main heat exchanger and the compressed heat exchange stream condensed in the main heat exchanger through indirect heat exchange with the at least part of the pumped liquid oxygen stream to form a liquid air stream. The liquid air stream is divided into first and second subsidiary air streams that are introduced into the higher pressure column and the lower pressure column after having been reduced in pressure compatible with the higher pressure column and the lower pressure column. Preferably, the first compressed stream is further compressed in a third booster compressor located upstream of the first and second booster compressor and the third compressed air stream is further compressed in a forth booster compressor located upstream of the second turboexpander.
The oxygen-rich liquid stream can be divided into a first oxygen-rich liquid subsidiary stream and a second oxygen-rich liquid subsidiary stream. The first oxygen-rich liquid subsidiary stream is pumped by a pump to produce the pumped liquid oxygen stream and the second oxygen-rich liquid subsidiary stream is taken as a liquid product. Additionally, a nitrogen-rich liquid stream can be pumped to produce a pumped liquid nitrogen stream. This stream is also warmed through indirect heat exchange with the compressed heat exchange stream to produce another of the product streams. The pumped liquid oxygen stream can be divided into a first pumped oxygen stream and a second pumped oxygen stream which are warmed through indirect heat exchange with the compressed heat exchange stream. The second pumped oxygen stream can be passed through a valve prior to being warmed so that pressurized oxygen products at two different pressures are produced.
The higher pressure column and the lower pressure column can be thermally linked by a condenser reboiler condensing nitrogen-rich vapor column overhead in the higher pressure column through indirect heat exchange with the oxygen-rich liquid column bottoms of the lower pressure column, thereby producing nitrogen-rich reflux streams introduced, at least in part, into the higher pressure column and the lower pressure column as reflux. The distillation column system can also have an argon column connected to the lower pressure column to separate argon from oxygen containing in a crude argon feed stream withdrawn from the lower pressure column and fed to the argon column for rectification. A kettle liquid stream composed of a crude liquid oxygen column bottoms of the higher pressure column is partially vaporized in an argon condenser connected to the argon column to produce reflux for the argon column and a liquid argon-rich liquid stream. Liquid and vapor phase streams produced as a result of partially vaporizing the kettle liquid stream are introduced into the lower pressure column for further refinement. One of the nitrogen-rich reflux streams and the kettle liquid streams are subcooled in a subcooling heat exchanger and a lower pressure column, nitrogen-rich vapor column overhead stream and a waste nitrogen stream are partially warmed in the subcooling heat exchanger and further warmed within the main heat exchanger to help cool the incoming air.
In another aspect, the present invention provides and air separation apparatus that comprises an air separation plant having a main heat exchanger for cooling the air, after having been compressed and purified and a distillation column system connected to the main heat exchanger. The distillation column system has a higher pressure column and a lower pressure column operatively associated within one another in a heat transfer relationship and producing return streams enriched in components of the air that are warmed within the main heat exchanger through indirect heat exchange with the air to help cool the air and to produce product streams. The air separation plant has a pump connected to a bottom region of the lower pressure column to pump at least part of an oxygen-rich liquid stream and thereby to produce a pumped liquid oxygen stream. The pump is also connected to the main heat exchanger so that at least part of the pumped liquid oxygen stream is heated within the main heat exchanger as one of the return streams to form a pressurized oxygen product stream that constitutes one of the product streams. The main heat exchanger is configured so that the at least part of the pumped liquid oxygen stream is heated through indirect heat exchange with a compressed heat exchange stream composed of part of the air to be cooled and rectified in the distillation column system. The air separation plant also has a first booster loaded expander and a second booster loaded expander comprising first and second booster compressors connected to one another and to the main heat exchanger so that part of the air is sequentially compressed within the first and second booster compressors to form the compressed heat exchange stream and first and second turboexpanders that drive the first and second booster compressors, respectively. The first and second turboexpanders are connected to the main heat exchanger so that other parts of the air are expanded after having been partially cooled in the main heat exchanger, thereby producing a first exhaust stream and a second exhaust stream, respectively. The first and second turboexpanders are connected to the distillation column system so that the first exhaust stream is introduced into lower pressure column and the second exhaust stream is introduced into the higher pressure column, thereby to impart refrigeration into the air separation plant.
The air separation plant can have a main air compressor connected to a pre-purification unit to produce a compressed and purified air stream. The first of the booster compressors is in flow communication with the pre-purification unit so that the first compressed air stream is formed from part of the compressed and purified air stream and is sequentially compressed within a first and second booster compressors to form the compressed heat exchange stream. The main heat exchanger is in flow communication with the pre-purification unit so that the second compressed air stream and the third compressed air stream are formed from other parts of the compressed and purified air stream and are partially cooled in the main heat exchanger. The higher pressure column and the lower pressure column are connected to the main heat exchanger so that a liquid air stream, formed from the compressed heat exchange stream indirectly exchanging heat with the at least part of the pumped liquid oxygen stream, divides into first and second subsidiary liquid air streams that are introduced into the higher pressure column and the lower pressure column. Expansion valves are positioned so that the first and second subsidiary liquid air streams are reduced in pressure compatible with that the higher pressure column and the lower pressure column. Further, a third booster compressor can be located between the pre-purification unit and the first of the booster compressors so that the first compressed air stream is further compressed in the third booster compressor. A forth booster compressor can be located between the main heat exchanger and pre-purification unit so that the third compressed air stream is further compressed in the forth booster compressor prior to being partially cooled in the main heat exchanger.
A piping juncture can be located between the pump and the bottom region of the lower pressure column so that the oxygen-rich liquid stream is divided into a first oxygen-rich liquid subsidiary stream and a second oxygen-rich liquid subsidiary stream. The pump is connected to the piping juncture so that first oxygen-rich liquid subsidiary stream is pumped by a pump to produce the pumped liquid oxygen stream and the second oxygen-rich liquid subsidiary stream is able to be taken as a liquid product. The main heat exchanger can also be provided with passages to warm a pumped liquid nitrogen stream and a first pumped oxygen stream and a second pumped oxygen stream through indirect heat exchange with the compressed heat exchange stream to produce other of the product streams and the pump is connected to the passages so that pumped liquid oxygen stream is divided into the first pumped oxygen stream and the second pumped oxygen stream. An expansion valve is located between the pump and one of the passages so that the second pumped oxygen stream is passed through a valve prior to being warmed and pressurized oxygen products at two different pressures are produced. Another pump is located between the higher pressure column and the main heat exchanger to pump a liquid nitrogen stream and thereby form the pumped liquid nitrogen stream.
The higher pressure column and the lower pressure column can be thermally linked by a condenser reboiler condensing nitrogen-rich vapor column overhead in the higher pressure column through indirect heat exchange with the oxygen-rich liquid column bottoms of the lower pressure column, thereby producing nitrogen-rich reflux streams. The higher pressure column and the lower pressure column are connected to the condenser reboiler so that the nitrogen-rich reflux streams are introduced, at least in part, into the higher pressure column and the lower pressure column as reflux. The distillation column system also has an argon column connected to the lower pressure column so that a crude argon feed stream from the lower pressure column is rectified in the argon column to separate argon from oxygen contained in the crude argon feed stream and an argon condenser is connected to the argon column to produce reflux for the argon column and a liquid argon-rich liquid stream. The argon condenser is connected to the higher pressure column so that a kettle liquid stream composed of a crude liquid oxygen column bottoms of the higher pressure column is partially vaporized in the argon condenser. The argon condenser is also connected to the lower pressure column so that liquid and vapor phase streams produced as a result of partially vaporizing the kettle liquid stream are introduced into the lower pressure column for further refinement. A subcooling heat exchanger is in flow communication with the condenser reboiler and the higher pressure column so that one of the nitrogen-rich reflux streams and the kettle liquid streams are subcooled in a subcooling heat exchanger. The subcooling heat exchanger is positioned between the lower pressure column and the main heat exchanger so that a lower pressure column, nitrogen-rich vapor column overhead stream and a waste nitrogen stream are partially warmed in the subcooling heat exchanger and further warmed within the main heat exchanger to help cool the incoming air.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims distinctly pointing out the subject matter that Applicant regards, as his invention, it is believed that the invention will be better understood when takening in connection with the sole figure illustrating a schematic diagraph of an apparatus designed to carry out a method in accordance with the present invention.

DETAILED DESCRIPTION

With reference to the drawing, an air separation plant 1 is illustrated that is designed to conduct a cryogenic rectification process in accordance with the present invention. In apparatus 1, a feed air stream 10 is compressed by a main air compressor 12 and then purified in a pre-purification unit 14 (“PP”) to produce a compressed and purified air stream 16. Compressed and purified air stream 16, in a manner that will be discussed in further detail, is in part further compressed and expanded and cooled in a main heat exchanger 17 and then rectified in a distillation column system 18 to produce product streams 125, 127, 145, 141 and 151.
More specifically, main air compressor 12 can be multi-stage, intercooled integral gear compressors with condensate removal between stages. Such a compressor has, in addition to intercoolers, between stages, an after-cooler, not illustrated, for removing the heat of compression. The pre-purification unit 14 is designed to remove higher boiling impurities from the air such as water vapor, carbon dioxide and hydrocarbons. As well known in the art and as discussed above, such purification unit 14 can incorporate adsorbent beds operating in an out of phase cycle that is a temperature swing adsorption cycle or a pressure swing adsorption cycle or combinations thereof.
At a piping junction 19, the compressed and purified air stream 16 is divided into a first compressed air stream 20, a second compressed air stream 22 and a third compressed air stream 24. First compressed air stream 20 is sequentially compressed by compressors in a first booster loaded expander unit 26 and a second booster loaded expander unit 28 to form a compressed heat exchange stream 30 that is condensed through indirect heat exchange with pumped oxygen, specifically first and second pumped oxygen streams 140 and 142 heated to form a high pressure oxygen product stream 141 and a medium pressure oxygen product stream. High pressure only product stream 141 could be a supercritical fluid or a high pressure vapor depending upon the degree to which it was pressurized prior to being heated. The main heat exchanger 17 can be of braised aluminum plate-fin construction. Although on only one main heat exchanger 17 is illustrated, it is understood that the main heat exchanger 17 could be several of such units in parallel. Also, again depending on the pressures of the pressurized oxygen, the main heat exchanger 17 could be divided into two units where one would operate at high pressure and the other at lower pressure in a so called banked arrangement of heat exchangers. The higher pressure heat exchanger in a particularly high pressure application could be a spirally wound unit.
The first booster loaded expander unit 26 has a turboexpander 32 connected to a first booster compressor 34 by means of a shaft 36 and the second booster loaded expander unit 28 has a second turboexpander 38 connected to a second booster compressor 40 by means of a shaft 42. The connecting means, 36 and 42, can also be gear. In the illustrated embodiment, the first compressed air stream 20 is also compressed by a third booster compressor 44. After removal of the heat of compression by an aftercooler 46, the first compressed stream 20 is further compressed by first and second booster compressors 34 and 40 with intermediate removal of the heat of compression by means of an aftercooler 48. The resulting compressed heat exchange stream 30 is also cooled by an aftercooler 50 to remove the heat of compression prior to being introduced into the main heat exchanger 17 for indirect heat exchange with the first pumped oxygen stream 140 and the second pumped oxygen stream 142. The second compressed air stream 22 is partially cooled in the main heat exchanger 17 prior to being expanded in first turboexpander 32. The third compressed air stream 24 can also be compressed in a forth booster compressor 52 and after removal of the heat of compression within an after cooler 54 is partially cooled in the main heat exchanger 17 before being expanded in second turboexpander 38.
The expansion of the second compressed air stream 22 within first turboexpander 32 produces a first exhaust stream 56 and the expansion of the third compressed air stream 24 within second turboexpander 38 produces a second exhaust stream 58. The first exhaust stream 56 is introduced into a lower pressure column 60 of the distillation column system 18 and the second exhaust stream 58 is introduced into a higher pressure column 62 of the distillation column system 18 in order to impart the refrigeration generated by such expansion into the cryogenic rectification process. The compressed heat exchange stream 30 after having been cooled in the main heat exchanger 17 is condensed to form a liquid air stream 64 that is divided into a first subsidiary liquid air stream 66 that is introduced into the higher pressure column 62 and a second subsidiary liquid air stream 68 that is introduced into the lower pressure column 60 after having been expanded in a valve 70 to a pressure compatible with its introduction into the lower pressure column 60. The higher pressure column 62 will operate at a higher pressure than the lower pressure column 60, typically 5.0-6.0 bar(a). The lower pressure column 60 will typically operate at a pressure of 1.1 to 1.5 bar(a).
Although not illustrated, both the lower pressure column 60 and the higher pressure column 62 contain mass transfer contacting elements in the form of known sieve trays or structured packing or a combination of such types of elements. The mass transfer contacting elements function to bring ascending vapor and descending liquid phases of the air to be distilled in the columns. In case of the higher pressure column 62, the ascending vapor phase is initiated by the introduction of second exhaust stream which becomes successively richer in nitrogen as it ascends. The descending liquid phase becomes ever more rich in oxygen to form a crude liquid oxygen column bottoms 72 also known as kettle liquid. The lower pressure column 60 and the higher pressure column 62 are thermally linked by means of a condenser reboiler 74 that serves to condense a nitrogen-rich vapor stream 76 into a liquid nitrogen stream 78. Nitrogen-rich vapor stream 76 is composed of nitrogen-rich vapor column overhead produced as a result of the distillation occurring within the higher pressure column 62. The liquid nitrogen stream 78 is in turn divided into a first and second subsidiary liquid nitrogen streams 80 and 82. Subsidiary liquid nitrogen stream 80 serves as reflux to the higher pressure column 62 and thus, initiates formation of the descending liquid phase within such column. The lower pressure column 60 serves to further refine the crude liquid oxygen column bottoms 72. For such purposes, a crude liquid oxygen stream 84 after having been subcooled in a subcooling heat exchanger 86 can be partially vaporized in a manner to be discussed and introduced into the lower pressure column 60. This produces an oxygen-rich liquid column bottoms 86 in the lower pressure column 60 and a nitrogen-rich vapor column overhead. The oxygen-rich liquid column bottoms 86 is in turn partially vaporized by condenser reboiler 74 to initiate formation of the ascending vapor phase. The second subsidiary liquid nitrogen stream 82 after having been subcooled in the subcooling heat exchanger is used in initiating formation of the descending liquid phase. As illustrated, part of the second subsidiary liquid nitrogen stream 82 can be reduced in pressure by a valve 88 and taken as a liquid nitrogen product stream 90. Another part of the second subsidiary liquid nitrogen stream 82 can be used in forming the liquid nitrogen reflux stream 92 for the lower pressure column 60. Liquid nitrogen reflux stream 92 is reduced in pressure by means of a valve 94.
Also as illustrated and optionally, the distillation column system 18 can include an argon column 96. An argon and oxygen containing stream 98 is removed from the lower pressure column and then introduced into the argon column 96 for rectification. An oxygen containing column bottoms 100 is produced that is returned to the lower pressure column 60 by means of an oxygen stream 102. Also produced is an argon-rich column overhead that is condensed by removal of an argon-rich vapor stream 104 and condensing the same in an argon condenser 106 having a core 108 surrounded by a shell 110. The argon-rich liquid stream 112 resulting from the condensation of the argon-rich vapor can be divided into a reflux stream 114 and a subsidiary argon-rich liquid stream 116 that can be further processed in a manner known in the art to produce an argon product. For example, such further processing could be conducted in another column to further separate the argon from the oxygen. The condensation of the argon-rich vapor stream 104 is brought about through indirect heat exchange with the crude liquid oxygen stream 84 after having been subcooled. In this regard, the crude liquid oxygen stream 84, after having been expanded by passage through a valve 118, is introduced into the shell 110 to condense the argon-rich vapor. This results in the partial vaporization of the crude liquid oxygen stream 84. Vapor phase and liquid phase streams 120 and 122, respectively, composed of the liquid and vapor phases produced by the partial vaporization of the crude liquid oxygen stream 84, are introduced into the lower pressure column 60 for further refinement of the crude liquid oxygen. It is understood that if argon column 96 were not present, the crude liquid oxygen stream 84 would be directly introduced into the lower pressure column 60. It is to be further pointed out here that subcooling of such a stream is optional.
A nitrogen-rich vapor stream 124, composed of nitrogen-rich vapor column overhead of the lower pressure column 60 and a waste nitrogen stream 126 can be removed from the lower pressure column and then partially warmed in the subcooling heat exchanger 86 and fully warmed in the main heat exchanger 17 to produce a nitrogen product stream 125 and a waste nitrogen product stream 127 which can be used in regenerating adsorbent beds of pre-purification unit 14. Additionally, an oxygen-rich liquid stream 128, composed of residual oxygen-rich liquid 86, can be removed from the lower pressure column 60. By means of a piping juncture 129, a part 130 of such stream can be expanded in a valve 132 and taken as an oxygen-rich liquid product stream. Another part 134 of the oxygen-rich liquid stream 128 can be pressurized by a pump 136 to produce a pumped liquid oxygen stream 138. Pumped liquid oxygen stream 138 can optionally be divided into a first pumped oxygen stream 140 and a second pumped oxygen stream 142. Second pumped oxygen stream 142 can be expanded in a valve 144 to a lower pressure so that when first pumped oxygen stream 140 and second pumped oxygen stream 142 are heated in main heat exchanger 17, high pressure and medium pressure oxygen product streams 141 and 145 are produced. Optionally, a nitrogen-rich liquid stream 146 composed of the liquid nitrogen stream 78 can be removed from the higher pressure column 62 and then pumped by a pump 148 to produce a pumped liquid nitrogen stream 150. Pumped liquid nitrogen stream 150 can be heated to produce a pressurized nitrogen product stream 151.
The molar flow range of the first, second and third compressed air streams 20, 22 and 24, respectively, as a percentage of all of the incoming air, can be between 25.0 percent to 35.0 percent for the first compressed air stream 20, between 5.0 percent and 8.0 percent for the second compressed air stream and between 60.0 percent and 67.0 percent for the third compressed air stream 24. Thus, most of the air enters the higher pressure column 62 as the second exhaust stream 58 that is produced by expansion of the third compressed air stream 24 after having been expanded. The flow rate of the second compressed air stream 22 and its expansion in the first turboexpander 32 to produce the first exhaust stream 56 that is introduced into the lower pressure column 60 has a far lower flow rate. It is to be noted that the third compressed air stream has to be compressed by the forth booster compressor 54 in order to create a sufficiently large expansion ratio across the second turboexpander 38 that will enable the second exhaust stream to enter the higher pressure column 62 that operates at a higher pressure than the lower pressure column 60. The second compressed stream 22 which is used in forming the first exhaust stream has no compression beyond the pressure imparted by the main air compressor 12 because the resulting exhaust stream 56 enters the lower pressure column 60 which is operated at a lower pressure than the higher pressure column 62. Consequently, the generation of the first exhaust stream 56 is far more efficient than the generation of the second exhaust stream 58 because there is no addition compression required by, for instance, forth booster compressor 52. The reason for the split in compressed air flow rates, set forth above, is that the generation of refrigeration by an expander exhausting into a higher pressure column will have the least effect on the ability of the plant to produce liquids, for example, liquid oxygen stream 130. Specifically, as mentioned above, the distillation column systems 18 functions by separating the nitrogen from the oxygen to produce a crude liquid oxygen column bottoms of the higher pressure column 62 that is further refined in the lower pressure column 60. As more air is diverted directly to the lower pressure column 60, recovery will begin to suffer. Therefore, although there is an energy penalty with the use of forth booster compressor 52, it is a necessary energy penalty if liquid products are to be produced in quantity. This being said, the production of liquid products by the air separation plant 1 is entirely optional. It is to be noted though that the use of the first turboexpander 32 exhausting into the lower pressure column 60 does to a limited extent relieve the degree to which expansion need be generated by the second turboexpander 38 exhausting into the higher pressure column 62; and thus, in this respect an energy efficiency is realized. However, as will be discussed, a process can be conducted in air separation plant 1 in connection with the turboexpander 32 that will realize a more substantial benefit in connection with the reduction of the size of the adsorbent beds used in pre-purification unit 14.
The energy savings of the present invention is brought about by the elimination of a booster compressor that is used in compressing the compressed heat exchange stream 30 to the required pressure from the pressure of the main air compressor 12. The air in any case has to be expanded in turboexpanders to generate refrigeration. The recapture of the work of expansion produced by first and second turboexpanders 32 and 38 in the generation of refrigeration by compressing the first compressed air stream to form the compressed heat exchange stream thus saves energy that would otherwise have been expended in such compression. It is to be noted that the pressure of the pressurized oxygen product stream 141 sets the pressure required of the compressed heat exchange stream 30. In the illustrated embodiment, the production of this pressure requires that some external energy be expended namely in third booster compressor 44. However, this is still less energy that would have otherwise have been required had a separate booster compressor been provided in creating the final pressure. In this regard, depending upon the pressure required of the compressed heat exchange stream, additional compression energy can be added by means of operating main air compressor 12 at a slightly higher pressure than would be normally used. Such higher pressure would allow first booster compressor 44 to extract more energy from first turboexpander 32 and thus create a higher pressure. As mentioned above, this also would have the benefit of allowing for a reduction in the size of the adsorbent beds of the prepurification unit 14.
It is to be noted that embodiments of the present invention can be carried out that are more simple than that of air separation plant 1. For instance, third and forth booster compressors 44 and 52 could be eliminated in such a simplified embodiment. In such an embodiment, the different compressor operating pressure ranges of first and second booster compressors 34 and 44 would be created solely by means of the difference in air flows of the second and third compressed air streams 22 and 24.
While the present invention has been discussed in relation to a preferred embodiment, as would occur to those skilled in the art, numerous additions, omission and changes thereto can be made without departing from the spirit and scope of the invention as set forth in the appended claims.

Claims

I claim:

1. A method of separating air within a cryogenic rectification process, said method comprising:

separating the air in the cryogenic rectification process by cooling the air, after having been compressed and purified and rectifying the air in a distillation column system having a higher pressure column and a lower pressure column operatively associated within one another in a heat transfer relationship to produce return streams enriched in components of the air that are warmed through indirect heat exchange with the air to help cool the air and to produce product streams;

one of the product streams formed by withdrawing an oxygen-rich liquid stream from a bottom region of the lower pressure column, pumping at least part of the oxygen-rich liquid stream to produce a pumped liquid oxygen stream and heating at least part of the pumped liquid oxygen stream to form a pressurized oxygen product stream, the at least part of the pumped liquid oxygen stream constituting one of the return streams and the at least part of the pumped liquid oxygen stream heated through indirect heat exchange with a compressed heat exchange stream composed of part of the air to be cooled and rectified in the distillation column system;

forming the compressed heat exchange stream, a first exhaust stream and a second exhaust stream with the use of a first booster loaded expander and a second booster loaded expander having booster compressors driven by turboexpanders by sequentially compressing the part of the air within the booster compressors of the first booster loaded expander and the second booster loaded expander to form the compressed heat exchange stream and partially cooling and then expanding other parts of the air within the turboexpanders to produce a first exhaust stream and a second exhaust stream from expansion of the other parts of the air within in the first booster loaded expander and the second booster loaded expander, respectively; and

introducing the first exhaust stream into lower pressure column and the second exhaust stream into the higher pressure column, thereby to impart refrigeration into the cryogenic rectification process.

2. The method of claim 1, wherein:

a first compressed air stream, a second compressed air stream and a third compressed air stream are formed, at least in part, by compressing and purifying the air to produce a compressed and purified air stream and dividing the compressed and purified air stream into the first compressed air stream, the second compressed air stream and the third compressed air stream, thereby to form the part of the air from the first compressed air stream and the other parts of the air from the second compressed air stream and the third compressed air stream;

the first compressed air stream is sequentially compressed within a first and second booster compressor of the first booster loaded expander and the second booster loaded expander to form the compressed heat exchange stream;

the second compressed air stream is partially cooled and introduced into a first turboexpander of the first booster loaded expander, thereby to produce the first exhaust stream;

the third compressed air stream is partially cooled and introduced into a second turboexpander of the second booster loaded expander, thereby to produce the second exhaust stream; and

the first compressed air stream and the second compressed air stream is partially cooled in a main heat exchanger and the compressed heat exchange stream condensed in the main heat exchanger through indirect heat exchange with the at least part of the pumped liquid oxygen stream to form a liquid air stream;

the liquid air stream is divided into first and second subsidiary liquid air streams that are introduced into the higher pressure column and the lower pressure column after having been reduced in pressure compatible with the higher pressure column and the lower pressure column.

3. The method of claim 2, wherein:

the first compressed stream is further compressed in a third booster compressor located upstream of the first and second booster compressor; and

the third compressed air stream is further compressed in a forth booster compressor located upstream of the second turboexpander.

4. The method of claim 1 or claim 2, wherein:

the oxygen-rich liquid stream is divided into a first oxygen-rich liquid subsidiary stream and a second oxygen-rich liquid subsidiary stream;

the first oxygen-rich liquid subsidiary stream is pumped by a pump to produce the pumped liquid oxygen stream; and

the second oxygen-rich liquid subsidiary stream is taken as a liquid product.

5. The method of claim 4, wherein:

a nitrogen-rich liquid stream is pumped to produce a pumped liquid nitrogen stream and is warmed through indirect heat exchange with the compressed heat exchange stream to produce another of the product streams; and

the pumped liquid oxygen stream is divided into a first pumped oxygen stream and a second pumped oxygen stream which are warmed through indirect heat exchange with the compressed heat exchange stream and the second pumped oxygen stream is passed through a valve prior to being warmed so that pressurized oxygen products at two different pressures are produced.

6. The method of claim 5, wherein:

the higher pressure column and the lower pressure column are thermally linked by a condenser reboiler condensing nitrogen-rich vapor column overhead in the higher pressure column through indirect heat exchange with the oxygen-rich liquid column bottoms of the lower pressure column, thereby producing nitrogen-rich reflux streams introduced, at least in part, into the higher pressure column and the lower pressure column as reflux;

the distillation column system also has an argon column connected to the lower pressure column to separate argon from oxygen containing in a crude argon feed stream withdrawn from the lower pressure column and fed to the argon column for rectification;

a kettle liquid stream composed of a crude liquid oxygen column bottoms of the higher pressure column is partially vaporized in an argon condenser connected to the argon column to produce reflux for the argon column and a liquid argon-rich liquid stream;

liquid and vapor phase streams produced as a result of partially vaporizing the kettle liquid stream are introduced into the lower pressure column for further refinement;

one of the nitrogen-rich reflux streams and the kettle liquid streams are subcooled in a subcooling heat exchanger; and

a lower pressure column, nitrogen-rich vapor column overhead stream and a waste nitrogen stream are partially warmed in the subcooling heat exchanger and further warmed within the main heat exchanger to help cool the incoming air.

7. An air separation apparatus comprising:

an air separation plant having a main heat exchanger for cooling the air, after having been compressed and purified and a distillation column system connected to the main heat exchanger and having a higher pressure column and a lower pressure column operatively associated within one another in a heat transfer relationship and producing return streams enriched in components of the air that are warmed within the main heat exchanger through indirect heat exchange with the air to help cool the air and to produce product streams;

the air separation plant having a pump connected to a bottom region of the lower pressure column to pump at least part of an oxygen-rich liquid stream to produce a pumped liquid oxygen stream and the pump also connected to the main heat exchanger so that at least part of the pumped liquid oxygen stream is heated within the main heat exchanger as one of the return streams to form a pressurized oxygen product stream constituting one of the product streams;

the main heat exchanger configured so that the at least part of the pumped liquid oxygen stream is heated through indirect heat exchange with a compressed heat exchange stream composed of part of the air to be cooled and rectified in the distillation column system; and

the air separation plant also having a first booster loaded expander and a second booster loaded expander comprising first and second booster compressors connected to one another and to the main heat exchanger so that part of the air is sequentially compressed within the first and second booster compressors to form the compressed heat exchange stream and first and second turboexpanders drive the first and second booster compressors, respectively;

the first and second turboexpanders connected to the main heat exchanger so that other parts of the air are expanded after having been partially cooled in the main heat exchanger, thereby producing a first exhaust stream and a second exhaust stream, respectively; and

the first and second turboexpanders connected to the distillation column system so that the first exhaust stream is introduced into lower pressure column and the second exhaust stream is introduced into the higher pressure column, thereby to impart refrigeration into the air separation plant.

8. The apparatus of claim 7, wherein:

the air separation plant has a main air compressor connected to a pre-purification unit to produce a compressed and purified air stream;

the first of the booster compressors in flow communication with the pre-purification unit so that the first compressed air stream is formed from part of the compressed and purified air stream and is sequentially compressed within a first and second booster compressors to form the compressed heat exchange stream;

the main heat exchanger is in flow communication with the pre-purification unit so that the second compressed air stream and the third compressed air stream are formed from other parts of the compressed and purified air stream and are partially cooled in the main heat exchanger;

the higher pressure column and the lower pressure column connected to the main heat exchanger so that a liquid air stream, formed from the compressed heat exchange stream indirectly exchanging heat with the at least part of the pumped liquid oxygen stream, divides into first and second subsidiary liquid air streams that are introduced into the higher pressure column and the lower pressure column; and

expansion valves are positioned so that the first and second subsidiary liquid air streams are reduced in pressure compatible with that the higher pressure column and the lower pressure column.

9. The apparatus of claim 7, wherein:

a third booster compressor is located between the pre-purification unit and the first of the booster compressors so that the first compressed air stream is further compressed in the third booster compressor; and

a forth booster compressor is located between the main heat exchanger and pre-purification unit so that the third compressed air stream is further compressed in the forth booster compressor prior to being partially cooled in the main heat exchanger.

10. The apparatus of claim 7 or claim 8, wherein:

a piping juncture is located between the pump and the bottom region of the lower pressure column so that the oxygen-rich liquid stream is divided into a first oxygen-rich liquid subsidiary stream and a second oxygen-rich liquid subsidiary stream; and

the pump connected to the piping juncture so that first oxygen-rich liquid subsidiary stream is pumped by a pump to produce the pumped liquid oxygen stream and the second oxygen-rich liquid subsidiary stream is able to be taken as a liquid product.

11. The apparatus of claim 10, wherein:

the main heat exchanger also has passages to warm a pumped liquid nitrogen stream and a first pumped oxygen stream and a second pumped oxygen stream through indirect heat exchange with the compressed heat exchange stream to produce other of the product streams and

the pump is connected to the passages so that pumped liquid oxygen stream is divided into the first pumped oxygen stream and the second pumped oxygen stream;

an expansion valve is located between the pump and one of the passages so that the second pumped oxygen stream is passed through a valve prior to being warmed and pressurized oxygen products at two different pressures are produced; and

another pump is located between the higher pressure column and the main heat exchanger to pump a liquid nitrogen stream and thereby form the pumped liquid nitrogen stream.

12. The apparatus of claim 11, wherein:

the higher pressure column and the lower pressure column are thermally linked by a condenser reboiler condensing nitrogen-rich vapor column overhead in the higher pressure column through indirect heat exchange with the oxygen-rich liquid column bottoms of the lower pressure column, thereby producing nitrogen-rich reflux streams;

the higher pressure column and the lower pressure column are connected to the condenser reboiler so that the nitrogen-rich reflux streams are introduced, at least in part, into the higher pressure column and the lower pressure column as reflux;

the distillation column system also has an argon column connected to the lower pressure column so that a crude argon feed stream from the lower pressure column is rectified in the argon column to separate argon from oxygen contained in the crude argon feed stream;

an argon condenser is connected to the argon column to produce reflux for the argon column and a liquid argon-rich liquid stream;

the argon condenser is connected to the higher pressure column so that a kettle liquid stream composed of a crude liquid oxygen column bottoms of the higher pressure column is partially vaporized in the argon condenser;

the argon condenser connected to the lower pressure column so that liquid and vapor phase streams produced as a result of partially vaporizing the kettle liquid stream are introduced into the lower pressure column for further refinement;

a subcooling heat exchanger in flow communication with the condenser reboiler and the higher pressure column so that one of the nitrogen-rich reflux streams and the kettle liquid streams are subcooled in a subcooling heat exchanger; and

the subcooling heat exchanger positioned between the lower pressure column and the main heat exchanger so that a lower pressure column, nitrogen-rich vapor column overhead stream and a waste nitrogen stream are partially warmed in the subcooling heat exchanger and further warmed within the main heat exchanger to help cool the incoming air.