AIR SEPARATION METHOD AND APPARATUS

Abstract

A method and apparatus for separating air in which an oxygen-rich liquid stream is pumped and then heated within a heat exchanger to produce an oxygen product through indirect heat exchange with first and second boosted pressure air streams. The first boosted pressure air stream is cold compressed at an intermediate temperature of the heat exchanger, reintroduced into the heat exchanger at a warmer temperature and then fully cooled and liquefied. The second boosted pressure air stream, after having been partially cooled, is expanded to produce an exhaust stream that is in turn introduced into a lower pressure column producing the oxygen-rich liquid. The second boosted pressure air stream is partially cooled to a temperature no greater than the intermediate temperature at which the cold compression occurs so that both the first and second boosted pressure air streams are able to take part in the heating of the oxygen-rich stream.

Claims

1. (canceled)

2. (canceled)

3. (canceled)

4. (canceled)

5. (canceled)

6. (canceled)

7. (canceled)

8. (canceled)

9. (canceled)

10. (canceled)

11. (canceled)

12. (canceled)

13. (canceled)

14. (canceled)

15. (canceled)

16. (canceled)

17. (canceled)

18. (canceled)

19. (canceled)

20. (canceled)

21. (canceled)

22. (canceled)

23. (canceled)

24. (canceled)

25. (canceled)

26. (canceled)

27. (canceled)

28. (canceled)

29. An apparatus for separating air comprising: one or more main air compressors configured for producing a stream of compressed and purified air, wherein the stream of compressed and purified air is split into a first part of the stream of compressed and purified air, a second part of the stream of compressed and purified air that is further compressed in a first booster compressor to produce a first boosted pressure air stream, and a third part of the stream of compressed and purified air that is further compressed in a second booster compressor to produce a second boosted pressure air stream; a main heat exchange system configured to cool the first part of the stream of compressed and purified air, to partially cool the first boosted pressure air stream, and to partially cool the second boosted pressure air stream; a cold compressor configured to further compress the partially cooled first boosted pressure air stream and form a cold compressed stream, wherein the cold compressed stream is further cooled in the main heat exchange system to form a cold compressed liquid air stream; a turboexpander configured to expand the partially cooled second boosted pressure air stream and form an exhaust stream; a liquid expander disposed between the main heat exchange system of the distillation column system and configured to expand the cold compressed liquid air stream to form a subsidiary air stream; a distillation column system having a higher pressure column and a lower pressure column linked in a heat transfer relationship via a condenser reboiler and configured to produce an oxygen-rich liquid from the lower pressure column through cryogenic rectification of the subsidiary air stream, the exhaust stream, and the first part of the compressed and purified air; and a pump connected to the lower pressure column to pump the oxygen-rich liquid to produce a pumped liquid oxygen stream; wherein the exhaust stream is introduced into the lower pressure column to impart refrigeration and the subsidiary air stream is introduced into at least one of the lower pressure column or higher pressure column to impart additional refrigeration; and wherein at least part of the pumped liquid oxygen stream is warmed in the main heat exchange system to form an oxygen-rich product through indirect heat exchange with the first boosted pressure air stream, the second boosted pressure air stream and the cold compressed stream.

30. The apparatus of claim 29, wherein the main heat exchange system further comprises: a higher pressure heat exchanger configured to warm the pumped liquid oxygen stream through indirect heat exchange with the first boosted pressure air stream and the second boosted pressure air stream; and a lower pressure heat exchanger configured to cool the first part of the compressed and purified air to a temperature suitable for rectification via indirect heat exchange with a stream of waste nitrogen from the distillation column system.

31. The apparatus of claim 30, wherein the higher pressure heat exchanger is further configured to further cool the cold compressed stream.

32. The apparatus of claim 30, wherein the higher pressure heat exchanger is further configured to warm a stream of pumped nitrogen from the higher pressure column to produce a high pressure gaseous nitrogen product stream.

33. The apparatus of claim 30, wherein the distillation column system is configured to produce a stream of gaseous nitrogen and the lower pressure heat exchanger is further configured to warm the stream of gaseous nitrogen to produce a low pressure gaseous nitrogen product stream.

34. The apparatus of claim 33, wherein the stream of gaseous nitrogen is a stream of overhead nitrogen or shelf nitrogen from the lower pressure column.

35. The apparatus of claim 30, wherein the higher pressure heat exchanger further comprises: a first intermediate outlet positioned to discharge the first boosted pressure air stream at an intermediate temperature about equal to a vaporization or pseudo-vaporization temperature of the oxygen-rich liquid stream; a first intermediate inlet to introduce the cold compressed air stream into the higher pressure heat exchanger at a warmer temperature than the intermediate temperature; and a second intermediate outlet positioned to discharge the second boosted pressure air stream at a temperature no greater than the intermediate temperature so that both the first and second boosted pressure air stream thereby assist in heating the oxygen-rich liquid stream at temperatures within the main heat exchange system above the intermediate temperature.

36. The apparatus of claim 29, wherein the first part of the compressed and purified stream constitutes between about 50 percent to 65 percent of the compressed and purified air stream on a volume basis.

37. The apparatus of claim 36, wherein the second part of the compressed and purified stream constitutes between about 27 percent to 35 percent of the compressed and purified air stream on a volume basis.

38. The apparatus of claim 29, further comprising: a motor coupled to the cold compressor and configured to independently drive the cold compressor; and a variable speed drive connected to the motor to control speed of the motor and therefore, the cold compressor.

39. The apparatus of claim 38 wherein the speed of the cold compressor is reduced during a turndown operation of the apparatus when production of the oxygen-rich product is reduced.

40. A method for separating air comprising the steps of: producing a stream of compressed and purified air in one or more main air compressors; splitting the stream of compressed and purified air into a first part of the stream of compressed and purified air, a second part of the stream of compressed and purified air, and a third part of the stream of compressed and purified air; further compressing the second part of the stream of compressed and purified air in a first booster compressor to produce a first boosted pressure air stream, and further compressing the third part of the stream of compressed and purified air in a second booster compressor to produce a second boosted pressure air stream; cooling the first part of the stream of compressed and purified air and the first boosted pressure air stream in a main heat exchange system and partially cooling cool the second boosted pressure air stream in the main heat exchange system; further compressing the partially cooled first boosted pressure air stream in a cold compressor to form a cold compressed stream; further cooling the cold compressed stream in the main heat exchange system to form a cold compressed liquid air stream; expanding the partially cooled second boosted pressure air stream in a turboexpander to form an exhaust stream; expanding the cold compressed liquid air stream in a liquid expander disposed between the main heat exchange system and the distillation column system to form a subsidiary air stream; rectifying the subsidiary air stream, the exhaust stream, and the first part of the compressed and purified air in a distillation column system comprising a higher pressure column and a lower pressure column linked in a heat transfer relationship via a condenser reboiler to produce an oxygen-rich liquid from the lower pressure column; pumping the oxygen-rich liquid from the lower pressure column to produce a pumped liquid oxygen stream; and warming at least part of the pumped liquid oxygen stream in the main heat exchange system to form an oxygen-rich gaseous product through indirect heat exchange with the first boosted pressure air stream, the second boosted pressure air stream and the cold compressed stream; wherein the exhaust stream is introduced into the lower pressure column to impart refrigeration and the subsidiary air stream is introduced into at least one of the lower pressure column or higher pressure column to impart additional refrigeration.

41. The method of claim 40, wherein the main heat exchange system comprises a higher pressure heat exchanger and a lower pressure heat exchanger and the step of warming at least part of the pumped liquid oxygen stream further comprises warming at least part of the pumped liquid oxygen stream in a higher pressure heat exchanger via indirect heat exchange with the first boosted pressure air stream and the second boosted pressure air stream.

42. The method of claim 40, wherein the main heat exchange system comprises a higher pressure heat exchanger and a lower pressure heat exchanger and the step of cooling the first part of the stream of compressed and purified air, the first boosted pressure air stream, and the second boosted pressure air stream further comprises; cooling the first part of the stream of compressed and purified air in the lower pressure heat exchanger via indirect heat exchange with a stream of waste nitrogen from the distillation column system; and cooling the first boosted pressure air stream and the second boosted pressure air stream in the higher pressure heat exchanger; and further cooling the cold compressed stream in the higher pressure heat exchanger to form the cold compressed liquid air stream;

43. The apparatus of claim 41, wherein the higher pressure heat exchanger is further configured to further cool the cold compressed stream.

44. The method of claim 41, wherein the higher pressure heat exchanger is further configured to warm a stream of pumped nitrogen from the higher pressure column to produce a high pressure gaseous nitrogen product stream.

45. The method of claim 42, wherein the distillation column system is configured to produce a stream of gaseous nitrogen and the lower pressure heat exchanger is further configured to warm the stream of gaseous nitrogen to produce a low pressure gaseous nitrogen product stream.

46. The method of claim 45, wherein the stream of gaseous nitrogen is a stream of overhead nitrogen or shelf nitrogen from the lower pressure column.

47. The method of claim 40, wherein the first part of the compressed and purified stream constitutes between about 50 percent to 65 percent of the compressed and purified air stream on a volume basis wherein the second part of the compressed and purified stream constitutes between about 27 percent to 35 percent of the compressed and purified air stream on a volume basis.

48. The method of claim 40, wherein the cold compressor is coupled to a variable speed motor and a variable speed drive configured to independently drive the cold compressor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] While the specification concludes with claims distinctly pointing out the subject matter that the Applicant regards as his invention, it is believed that the present invention will be better understood when taken in connection with the accompanying drawings in which:

[0031] FIG. 1 is a schematic diagram of an air separation plant designed to carry out a method in accordance with the present invention;

[0032] FIG. 2 is a schematic diagram of an alternative embodiment of an air separation plant designed to carry out a method in accordance with the present invention; and

[0033] FIG. 3 is a schematic, fragmentary diagram of a modification that can be incorporated into embodiment of the air separation plant illustrated in FIG. 2.

DETAILED DESCRIPTION

[0034] With reference to the FIG. 1, an air separation plant 1 is illustrated that is designed to produce an oxygen product stream 98 at pressure. In air separation plant 1, a compressed and purified air stream 10 is divided into a first compressed stream 12, a second compressed stream 14 and a third compressed stream 16. Although not illustrated, compressed and purified air stream 10 can originate from a main air compressor that compresses the air to a pressure of between 4.5 and 7.0 bar(a) and then purified of higher boiling contaminants by means of an adsorption bed system having known adsorbent beds operating in an out-of-phase cycle, typically, a temperature swing cycle. Such higher boiling contaminants are those that would solidify or concentrate at cryogenic temperatures; for instance, carbon dioxide, water vapor and hydrocarbons. The air separation plant 1 could be part of an enclave of such plants or similar plants; and therefore, the compressed and purified air stream 10 could be centrally generated. Alternatively, the air separation plant could be a single plant connected to an adsorption bed system fed by the main air compressor.

[0035] The compressed air is cooled in a main heat exchange system that in the illustrated embodiment is a banked arrangement having a higher pressure heat exchanger 18 and a lower pressure heat exchanger 20. These heat exchangers are so designated in that higher pressure heat exchanger 18 is designed to engage in indirect heat exchange with the use of higher pressure streams than those utilized in the indirect heat exchange conducted in the lower pressure heat exchanger 20. However, the present invention is not limited to such a banked arrangement in that it is also applicable to a series of heat exchangers operating in parallel in which all of the streams to be heated and cooled pass in indirect heat exchange. As would be well known in the art, any such heat exchange system, either a banked or unbanked arrangement, could use heat exchangers of brazed aluminum plate-fin construction. Higher pressure heat exchangers could be spirally wound type heat exchangers.

[0036] After having been cooled, the compressed air is rectified in a distillation column system having higher and low pressure distillation columns 22 and 24 that are thermally linked by a condenser reboiler 78. For illustration purposes only, the higher pressure heat exchanger can be designed to operate at a pressure that typically would be between 38.0 and 120.0 bar(a). The lower pressure heat exchanger 24 can be designed to operate at a pressure of between 4.5 and 7.0 bar(a). The distillation column system is designed to produce an oxygen-rich liquid stream 90 that after having been pumped in a pump 92 to produce a pumped liquid oxygen stream 96 is vaporized in the higher pressure heat exchanger 18. As would occur to those skilled in the art, part of the pumped liquid oxygen stream 96 could be sent to storage at pressure.

[0037] First compressed stream 12 is cooled to a temperature suitable for its rectification within the lower pressure heat exchanger 20 and is introduced into the bottom of the higher pressure column 22. First compressed stream 12 preferably constitutes 50 to 65 percent of the compressed and purified air stream 10 and second compressed stream 14 preferably constitutes 27.0 to 35.0 percent of the compressed and purified air stream 10.

[0038] Second compressed stream 14 is compressed in a first booster compressor 26 to produce a first boosted pressure air stream 28. It is understood that first booster compressor 26 is a multi-stage unit having intercoolers to remove the heat of compression between each stage of compression. Preferably, after removal of the heat of compression from the final stage within an after cooler 30, the first boosted pressure air stream 28 is partially cooled in the higher pressure heat exchanger 18 through indirect heat exchange with the pumped oxygen stream 96. The first boosted pressure air stream 28, at such point, is discharged from the higher pressure heat exchanger 18 from a first intermediate outlet 32 situated at an intermediate location of the higher pressure heat exchanger 18 and yet further compressed in a cold compressor 34 to produce a cold compressed air stream 36. The cold compressor 34 is independently driven by an electric motor 35. The cold compressed air stream 36 is reintroduced back into the higher pressure heat exchanger 18 through in inlet 37 and at a warmer temperature thereof than the temperature of the first boosted pressure air stream 28 upon discharge through first intermediate outlet 32 due to the heat of compression. The cold compressed air stream 36 is then further cooled within the higher pressure heat exchanger through indirect heat exchange with the pumped oxygen stream 96 to produce a liquid air stream 38. The resulting liquid air stream 38 is then expanded in an expansion valve 50 and introduced into the higher pressure column 22 as a subsidiary stream 52 and the lower pressure column as a subsidiary stream 54 that is first expanded to the lower column pressure of the lower pressure column 24 by an expansion valve 56. It is understood that depending upon the desired product slate; the liquid air could be introduced solely into the lower pressure column 24 or the higher pressure column 22.

[0039] It is to be noted that the liquid air stream 38 is usually distributed, as illustrated, so that a portion passes to the lower pressure column 24 and the other portion passes to the higher pressure column 22. Such distribution is determined by optimization so that power consumption is minimized. The flow passing to the lower pressure column 24 provides an advantage in that it relieves a compositional pinch that may otherwise occur due to the additional reflux it provides. However, had this flow instead have been passed to the higher pressure column 22, additional nitrogen reflux would have been generated from the higher pressure column, albeit at a lower rate. Hence, an optimal distribution of the liquid air stream 38 between the lower pressure column 24 and the higher pressure column 22 gives the best balance of additional direct liquid air reflux to the lower pressure column 24 and additional nitrogen reflux from the higher pressure column 22. The optimal balance of liquid air can shift to the point where all the liquid air is passed directly to the lower pressure column 24, although this is unusual. This may occur when the product demand is such that there is less nitrogen reflux available to the lower pressure column, or expander flow to the lower pressure column is high. The other extreme, where the entire liquid air stream is passed to the higher pressure column 22 most often will occur when, instead of passing liquid air directly to the lower pressure column 24, a synthetic liquid air stream is withdrawn from the higher pressure column 22 and then passed to the lower pressure column 24. This alternative configuration, known in the art, may be preferred when the liquid air stream otherwise isn't satisfactorily subcooled for direct passage into the lower pressure column 24. In this case, withdrawal of a synthetic liquid air stream of approximately air composition from the higher pressure column 22 reduces the flashoff upon feed to the lower pressure column 24.

[0040] Third compressed stream 16 is used in imparting refrigeration to the air separation plant 1. As known in the art, the addition of refrigeration is necessary to maintain the plant in thermal balance as a result of such factors as heat leakage into the plant through the cold box housing the plant, warm end losses in the heat exchange system and the removal of liquid products. Additionally, refrigeration must also be introduced to compensate for the cold compression of cold compressor 34. For such purposes, the second compressed stream is further compressed in a second booster compressor 40 to produce a second boosted pressure air stream 42. After cooling in an optional after cooler 43, the second boosted pressure air stream 42 is partially cooled in the higher pressure heat exchanger 18, removed from a second outlet 44 thereof, and then introduced into a turboexpander 45. Turboexpander 45 is coupled to the booster compressor 40 by means of a common shaft 46. The advantage of this is that the second compressed air stream 14 is created without the further expenditure of energy. The work of expansion is captured by shaft 46 to drive booster compressor 40. As a result, an exhaust stream 48 is discharged from turboexpander 45 and without further electrical power input into the air separation plant 1. The refrigeration is imparted by introducing the exhaust stream 48 into the lower pressure column 24.

[0041] The use of such a turbine loaded booster arrangement, as described directly above, is advantageous in that is produces a high expansion ratio across the turboexpander 45 without the input of additional electric power. However, there are other possibilities. In this regard, as another means used in forming the first boosted pressure air stream 28 and the second boosted pressure air stream 42, part of the air compressed by the first booster compressor 26 could be taken at an intermediate pressure and then, after aftercooling, could be introduced into the higher pressure heat exchanger 18 for partial cooling. It is also to be noted that the use of electric motor 35 to power cold compressor 34 is preferred in that the compression of the cold dense gas can be accomplished with a very low expenditure of overall energy. While, as in the prior art an expander could be used to power the cold compressor 34, this would not be preferred due to the cost of the additional expander and the more favorable use of the work of expansion to boost the inlet pressure of the expander.

[0042] Within the higher pressure column 22, an ascending vapor phase becomes ever richer in the more volatile components of the air, mainly nitrogen and a descending liquid phase becomes ever richer in the less volatile components of the air, mainly, oxygen. The ascending and descending vapor and liquid phases are brought into intimate contact with one another through mass transfer contacting elements 58 and 60 that can be structured packing, trays or random packing. This results in a crude liquid oxygen column bottoms, also known as kettle liquid, being created in the bottom of the higher pressure column 22 and a nitrogen-rich vapor column overhead being created in the top of the higher pressure column 22. A crude liquid oxygen stream 62 is then further refined in the lower pressure column by preferably first being subcooled in a subcooling heat exchanger 64 and then expanded to lower pressure column pressure of the lower pressure column 24 by means of an expansion valve 66. Contact between ascending vapor and descending liquid phases is accomplished within the lower pressure column 24 by means of mass transfer contacting elements 68, 70, 72 and 74 to produce an oxygen-rich liquid 76 in the bottom of the lower pressure column 24 and a nitrogen-rich vapor in the top of such column.

[0043] The higher and lower pressure columns 22 and 24 are thermally linked by means of a condenser reboiler 78. A stream 80 of the nitrogen-rich vapor produced in the higher pressure column 22 is condensed in the condenser reboiler 78 to produce a nitrogen-rich liquid stream 82 which is in turn introduced into the higher pressure column 22 as reflux, thereby to initiate formation of the descending liquid phase. A nitrogen reflux stream 84 can be formed from part of the nitrogen-rich liquid stream 82 and introduced into the lower pressure column 24 as reflux thereby to initiate formation of the descending liquid phase within such column. The remaining part of the nitrogen-rich liquid stream 82 is in turn introduced into the higher pressure column 22 as reflux, thereby to initiate formation of the descending liquid phase within such column. Preferably, the nitrogen reflux stream 84 is subcooled in a subcooling heat exchanger 86 and the valve expanded in an expansion valve 88 to a pressure compatible with its introduction in to the lower pressure column 24. The oxygen-rich liquid stream 90 is composed of the oxygen-rich liquid 76 produced in the lower pressure column 24. Such liquid stream is then pumped by pump 92 to produce a pumped liquid oxygen stream 96 which is vaporized in the higher pressure heat exchanger 18 through indirect heat exchange with the first boosted pressure air stream 28 and the cold compressed stream 36 to produce the pressurized oxygen product stream 98 (“GOX”). Part of the oxygen-rich liquid 76 could be taken as a liquid oxygen product stream 100 to a limited extent; and such stream would be valve expanded in a valve 102 prior to storage.

[0044] A nitrogen stream 104 is removed from the top of the lower pressure column and divided into subsidiary nitrogen containing streams 106 and 108. Subsidiary nitrogen containing stream 106 is warmed within the higher pressure heat exchanger 18 to produce a first nitrogen stream 110 (“WN.sub.2”). Subsidiary nitrogen containing stream 108 is successively warmed in subcooling heat exchangers 86 and 84 and then fully warmed within the lower pressure heat exchanger 20 to produce a second waste nitrogen stream 112 (“WN.sub.2”). These waste nitrogen streams serve to balance the cold end temperatures of the higher and lower pressure heat exchangers 18 and 20 so that the effective cooling of the feed streams and warming of the return streams is maximized given the available area of heat exchangers 18 and 20. This is conventional and the control of such balancing takes place by controlling the flow rate of the subsidiary nitrogen containing streams 106 and 108 by such means as appropriate selection of piping and valves.

[0045] As mentioned above, the pumped liquid oxygen stream 96 is heated through indirect heat exchange with both the first boosted pressure air stream 28 and the second boosted pressure air stream 42. In this regard, the first outlet 32 from which the first boosted pressure air stream 28 is removed from the higher pressure heat exchanger 18 is situated at an intermediate temperature that will be about equal to the temperature at which the pumped liquid oxygen stream 96 either vaporizes or pseudo-vaporizes in cases where the pumped liquid oxygen stream 96 has been pressurized to a supercritical pressure. In this context, the term “about” means between 5.0_K below to about 15.0_K above and preferably, in a range of between 3.0 K below and 10.0 K above the vaporization or pseudo-vaporization temperature. The resulting cold compressed stream 36 is introduced into an inlet 37 that is situated at a temperature that is consistent with the increase in temperature due to the heat of compression which would be warmer than the intermediate temperature of first outlet 32. The second outlet 44 from which the second boosted pressure air stream 42 is removed from the higher pressure heat exchanger 18 is situated so that the second boosted pressure air stream 42 has been cooled to a temperature no greater than the intermediate temperature achieved at the first outlet 32. Preferably the temperature at the second outlet 44 is at or no greater than 30.0 K below the intermediate temperature at first outlet 32. What this allows is for both first and second boosted pressure air streams 28 and 42 to both heat the pumped liquid oxygen stream 98. When this is coupled with the cold compression, the pressure and flow of the first booster compressor 26 will be able to be decreased to in turn decrease the overall power consumed by the air separation plant. As can be appreciated, the greatest benefit of the present invention over the prior art will be obtained where the oxygen product is a vapor rather than a supercritical fluid. In such case, over certain pressure ranges of the oxygen, it is possible to compress the air in the first booster compressor 26 to a subcritical pressure and the cold compressor 35 to a supercritical pressure. Even where the first booster compressor 26 is required to compress the air to a supercritical pressure, such pressure will be less than that otherwise required had the present invention not have been practiced.

[0046] With reference to FIG. 2, an air separation plant 2 is illustrated having many of the same components as the air separation plant 1 shown in FIG. 1. In order to avoid needless repetition, components of air separation plant 2 having the same description as those in FIG. 1 will have the same reference numbers and will not be further described below.

[0047] In FIG. 2, additional refrigeration can be imparted with the use of a liquid expander 120 that is connected to an electrical generator or oil-brake system, generally shown by reference number 122 to dissipate the work of expansion. This added refrigeration will compensate for the work of compression added by the cold compressor 34 and thereby allow even less air flow to the turboexpander 45 and therefore the low pressure column 24. As a result, oxygen recovery will increase. It will usually be most efficient to reduce the pressure of stream 28 without changing the pressure of stream 36 so cold compressor 34 is generating a higher pressure ratio. Such an increase in efficiency will be at the acquisition cost of the liquid expander 120. A yet further increase in oxygen production can be obtained by an argon rejection column 124. An argon and oxygen containing stream 126 is withdrawn from the low pressure column and introduced into the argon rejection column 124 for rectification. The rectification produces an oxygen containing column bottoms that is returned as an oxygen containing stream 128, lean in argon, to the low pressure column 24 to increase oxygen recovery. It is understood, therefore, that argon rejection column 124 is simply designed to separate the argon from the oxygen and not to produce an argon product. However, if an argon product were desired, either liquid or vapor, an appropriately staged argon column could be used for such purpose. Unlike air separation plant 1, the crude liquid oxygen stream, after having been subcooled in subcooling heat exchanger 64 and valve expanded in valve 66, is partially vaporized in argon condenser 130 to generate reflux for the argon rejecting column 124. The partial vaporization produces vapor and liquid phases of the crude liquid oxygen that are introduced into the lower pressure column 24 by way of vapor and liquid phase streams 132 and 134, respectively. Preferably, an argon-rich vapor stream 136, composed of the argon-rich vapor column overhead produced in the argon column 124, is condensed in the argon condenser 130 to produce a liquid argon stream 138 that is passed into a separation vessel 140 to produce an argon reflux stream 142 and a vapor stream 144, rich in argon, that is introduced into the waste nitrogen stream to produce a waste nitrogen stream 104′ that has a slightly lower nitrogen purity than nitrogen stream 104 shown in FIG. 1. Waste nitrogen stream 104′ is similar divided into subsidiary waste nitrogen containing streams 106′ and 108′ for balancing cold end temperatures of the higher and lower pressure heat exchangers 20 and 18. The subsidiary waste nitrogen containing streams 106′ and 108′ are withdrawn from the high and low pressure heat exchangers 18 and 20 as waste nitrogen streams 110′ and 112′ (“WN.sub.2”).

[0048] As illustrated, a liquid nitrogen stream 148 can be removed from the liquid nitrogen produced by condenser reboiler 78 and divided into first and second subsidiary liquid nitrogen stream 150 and 152. First subsidiary liquid nitrogen stream 150 can be pressurized by a pump 154 to produce a pumped liquid nitrogen stream 156. Pumped liquid nitrogen stream 156 can in turn be subdivided into a first part 158 and a second part 158. The first part 158 can be fully warmed in the higher pressure heat exchanger 18 to produce a high pressure gaseous nitrogen product 162 (“HPGN.sub.2”). Second part 160 can be expanded in a valve 164 and then fully warmed in the higher pressure heat exchanger 18 to produce a low pressure gaseous nitrogen product stream 166 (“LPGN.sub.2”). The second subsidiary liquid nitrogen stream 152 can be subcooled in subcooling heat exchanger 86′ that differs from subcooling heat exchanger 86 by provision of heat exchange passages for such purpose. The resulting subcooled stream can be expanded in an expansion valve 168 and then taken as a liquid nitrogen product 170 (“LN.sub.2”).

[0049] It is appropriate to point out here that an impure nitrogen reflux stream 84′ is withdrawn from the higher pressure column 22, subcooled in subcooling heat exchanger 86′ and then introduced as reflux into the lower pressure column 24. The use of the impure nitrogen reflux stream 84′ is particularly preferred because it also increases recovery of oxygen production. The flow rate of the impure nitrogen reflux stream 84′ from the higher pressure column 22 is greater than the flow rate would be had that stream been formed from part of the nitrogen-rich liquid stream 82. The lower draw point of stream 84 enables a greater withdrawal rate from the higher pressure column 22 without compromising the nitrogen purity attained in the liquid nitrogen product 170. The larger flow rate of the reflux stream improves the separation from low pressure column 24 and hence, the oxygen recovery. The draw point of impure nitrogen reflux stream 84′ is selected such that its composition does not appreciably degrade the composition of waste nitrogen vapor stream 104′ that is withdrawn from low pressure column 24, yet its flow is maximized within that limitation. As can be appreciated, impure nitrogen reflux stream 84′ could be used in connection with the air separation plant 1 shown in FIG. 1 with the taking of a liquid nitrogen product in a similar manner as has been described with respect to air separation plant 2.

[0050] It is understood, that both air separation plant 1 and air separation plant 2 are designed to principally supply gas. Therefore, the amount of liquid that could be removed from such a plant would be limited. For instance, the rates at which such liquid products would be removed would be about five percent of the removal of the gaseous oxygen product stream 98. It also is to be noted, that the pumping of the nitrogen product has been found to be usually less efficient than withdrawing nitrogen as a vapor and compressing it after warming in the main heat exchange system or in case of the illustrated embodiments, high pressure heat exchanger 18. However, it may be desirable to eliminate a nitrogen compressor by pumping the nitrogen to its required delivery pressure. In such case the high pressure air provides energy for heating both the oxygen and the nitrogen. The benefit of application of the present invention to a system that uses liquid pumping of nitrogen in addition to oxygen is relatively unaffected. This benefit occurs primarily due to the improved temperature profile in the heat exchange system used in heating the nitrogen at temperatures at and above where the oxygen is boiling or pseudo-boiling. Nitrogen, when it is pumped, is usually no more in flow than fifty percent of that of the oxygen. When the pumped nitrogen is of relatively low pressure, the flat temperature profile where it boils usually produces a pinch point near the cold end of the heat exchanger used for such boiling. However, since this occurs at a temperature below where oxygen boils, its effect on the efficiency of the air separation unit is similar for both the present invention and a prior art design. When the pumped nitrogen pressure is high, its effect on the composite cooling curve of the present invention as compared to a prior art is also very small. That is because nitrogen becomes supercritical at about 490 psia (34 bars) and 126 K. Above this it no longer produces a flat section in the temperature profile and its existence becomes virtually indiscernible in the heat exchanger temperature profile. Consequently, the pumping of nitrogen will have virtually no effect on the present invention and the present invention as set forth in the pending claims is not meant to exclude such an option.

[0051] With reference to FIG. 3, a modification of the air separation plant 2 is shown in which the cold compressor 34 is operated at variable speed to enhance the ability of the air separation plant 2 to be turned down. As will become apparent, the same modification could be incorporated into air separation plant 1. Turndown of an air separation plant is desirable during times in which electrical power is at a peak demand and is therefore, the most expensive. During a turndown operation of an air separation plant, less product will be produced and lower electrical power consumption in the compression of air will be realized. However, turndown of centrifugal compressors in general is very ineffective, below about 75% capacity, where the compressor power consumption is practically fixed. This 75% level corresponds to the point where the surge control line is reached and the compressor must then operate in recirculation to avoid surge. In such a mode of operation, part of air after having been compressed is recirculated back to the inlet of the compressor. For a cold compressor, the extra energy in such an operation is rejected in the main heat exchanger, as opposed to cooling water for a warm compressor. Therefore, more turbine refrigeration is required to balance it. This means that, in addition to the cold compressor recirculation flow, the turbine flow must be maintained at the same level when the plant is below 75% capacity. In reality, recirculation is not practical for a cold compressor and operation will be maintained at 75% capacity even though it is not necessary. The power penalty incurred in the operation of the main air compressors and possibly booster compressors, depending on plant design, can make such a turndown operation with cold compression not very attractive, if not impractical. It is to be mentioned that the turbine will usually have a greater flow range and ability to operate in a turndown mode than the cold compressor. However, the full range of operation cannot be utilized due to limitations on compressor turndown capability.

[0052] As illustrated, two feed air streams 180 and 182 are compressed by two main air compressors 184 and 186, respectively. Each of the main air compressors 184 and 186 can be multistage installations with interstage cooling between stages that feed a common after cooler 188 to remove the heat of compression. The resulting compressed air is feed to a prepurification unit 188 (“PPU”) that incorporates beds of adsorbent to remove higher boiling contaminants such as carbon dioxide and water vapor. The beds of adsorbent are operated in an out of phase cycle, commonly a temperature swing adsorption cycle or a pressure swing adsorption cycle or a combination of the two cycles. The result is compressed and purified air stream 10. The compressors 184 and 186 are preferably provided with inlet guide vanes 192 and 194 to allow the flow to be independently reduced to each of the compressors. Additionally, the booster air compressor 26 can also be provided with guide vanes 196. The use of two compressors 184 and 186 allow for a turndown operation of less than 50 percent. If less of a turndown operation is required one of such compressors could be used and in any case, turndown could be accomplished by the use of inlet guide vanes 192, 194 and 196 alone. A variable speed motor 35′ is used to drive the cold compressor 34 and the speed of the variable speed direct drive motor 35′ is controlled by a variable frequency drive 198 (“VFD”). Motor 35′ can be a permanent magnet motor or a high speed induction motor. The variable speed drive 198 allows the speed of the motor 35′ to be controlled and therefore, the speed of the compressor 34. The enablement of a wide speed range for the cold compressor 34 will in turn allow for a wide turndown range.

[0053] When the air separation plant 2, incorporating the features illustrated in FIG. 3, is to be turned down, the guide vanes 192, 194 and 196 of the main air compressors 184 and 186 and the booster air compressor 26 are gradually closed. This reduces the flow from the air compressors. Although not illustrated, cold compressor 34 could similarly incorporate such guide vanes. However, in the illustrated embodiment the speed of the electric motor 35′ is solely adjusted to decrease the speed of cold compressor 34. At the same time, the gaseous oxygen product flows of oxygen-enriched liquid streams 90 and 100 shown in FIG. 2 are decreased by means of, for example control valves not illustrated. Eventually, as the compressors 184, 186 and 26 are turned down, they will reach the low end of their flow capabilities. This is set by the surge limit. Below this level, surging operation will occur. To avoid this, booster air compressor 26 will have a recirculation line 200 and a control valve 202 that can be opened to return the cooled discharge flow to the feed end of the booster air compressor 26. This prevents surging operation so that the machine is not damaged, but operation at this capacity and lower will no longer reduce power. For main air compressors 184 and 186, vents 204 and 206 to atmosphere are used by opening control valves 208 and 210 to protect the compressors from surging operation, again without reducing power as the capacity. As indicated above, where turndown is to be effectuated below 75% of capacity or possibly, at or below 50% of capacity, one of the two compressors 184 and 186 can be shut down. Since, however, cold compressor 34 is turned down by means of a speed reduction, there is no extra energy that must be rejected in the main heat exchanger 18 and therefore, a resulting refrigeration requirement that must be met for the turndown of cold compressor 34. As such, the reduction of flow of the compressed and purified air stream 10 will reduce the flow and the refrigeration imparted by the turboexpander 45 in line with the reduction of refrigeration requirements due to the turndown operation of air separation plant 2.

[0054] Although the present invention has been described with reference to preferred embodiments, as will occur to those skilled in the art, numerous changes, additions and omission can be made without departing from the sprit and scope of the present invention as set forth in the appended claims.