WO2008112728A2

WO2008112728A2 - Air separation method

Info

Publication number: WO2008112728A2
Application number: PCT/US2008/056599
Authority: WO
Inventors: Neil Mark Prosser; Richard John Jibb
Original assignee: Praxair Technology, Inc.
Priority date: 2007-03-13
Filing date: 2008-03-12
Publication date: 2008-09-18
Also published as: WO2008112728A3; EP2122283A2; CN101266095A; MX2008001840A; US20080223077A1

Abstract

An air separation method in which a liquid air stream, produced by vaporizing a pumped (120) liquid oxygen stream, is introduced into a lower pressure column (50) and optionally, a higher pressure column (48) of an air separation unit. The liquid air stream (76) is subcooled (78) by extracting a main air feed to the higher pressure column from a main heat exchanger (40) at a temperature warmer than the liquid air stream to increase argon recovery in an argon column (52) connected to the lower pressure column (48). This temperature is selected such that the liquid air stream approaches an average temperature of the return streams being fed into the main heat exchanger from the higher and lower pressure columns at a range between about.2K and about 3K.

Description

AIR SEPARATION METHOD

Field of the Invention

[0001] A method for separating air in which a pressurized oxygen product is produced by vaporizing a pumped liquid oxygen stream against liquefying an air stream in a main heat exchanger and an argon product is produced in an argon separation zone connected to a lower pressure column that is operatively associated in a heat transfer relationship with a higher pressure column. More particularly, the present invention relates to such a method in which a main feed air stream to the higher pressure column is withdrawn from the main heat exchanger at a temperature that is warmer than the liquid air stream to subcool the liquid air stream, thereby to increase argon recovery.

Background of the Invention

[0002] The separation of air into nitrogen, oxygen and argon fractions have been conducted in air separation units in which air is compressed, purified and cooled in a main heat exchanger to a temperature suitable for its rectification. The air is introduced into a higher pressure column of a double column arrangement also having a lower pressure column in a heat transfer relationship with the higher pressure column. Nitrogen and oxygen products may be extracted from the higher and lower pressure columns. [0003] An argon-rich stream can be removed from the lower pressure column and introduced into an argon column to produce argon-rich column overhead. The argon-rich column overhead is condensed, typically with the use of all or part of a crude liquid oxygen stream, produced as a column bottoms of the higher pressure column, to generate liquid reflux for the argon column. A portion of the argon-rich column overhead is taken as an argon product.

[0004] It is also known to produce a high pressure oxygen product in such an arrangement by pressurizing an oxygen-rich stream composed of a liquid oxygen column bottoms produced in the lower pressure column by pumping the stream and vaporizing it in the main heat exchanger against liquefying an air stream that constitutes part of the air that has been compressed to a high pressure. The resultant liquid air stream is expanded and introduced into the lower pressure column or both the higher and lower pressure columns. [0005] An example of such a plant is disclosed in U.S. Patent No. 6,293,126. In this patent, the main feed air stream is withdrawn from the main heat exchanger at a temperature warmer than that of the air stream that is further compressed and liquefied to produce the liquid air stream. In an attempt to simplify the construction of such a plant, the crude liquid oxygen stream is not subcooled either prior to its use in condensing argon reflux or its introduction into the lower pressure column. As a result, there exists a greater vapor fraction of the crude liquid oxygen stream entering the lower pressure column after expansion than would have otherwise occurred had the crude liquid oxygen stream been subcooled. Thus, the liquid to vapor ratio at a point in the lower pressure column above the point at which the argon-rich stream is extracted for further refinement in the argon column is less than would otherwise have been possible. Moreover, extracting a main air stream at a warmer temperature than the liquid air stream decreases the temperature of the liquid air stream to an extent that it can approach the temperature of the return streams used to cool the incoming air. As a result, the compression requirements for the air stream that is further compressed and liquefied are usually greater than the flow and/or pressure that would otherwise have been required had the main feed air stream not been withdrawn at the warmer temperature. The further subcooling of the liquid air stream tends to compensate for the reduced liquid to vapor ratio in the low pressure column. This results in more power being consumed in such a plant without any increase in argon recovery.

[0006] As will be discussed, the present invention provides a method for separating air in which argon recovery is increased over that possible in prior art air separation systems, such as discussed above, while minimizing the amount of excess power that is necessarily used in increasing the argon recovery.

Summary of the Invention

[0007] The present invention provides a method of separating air.

[0008] In accordance with the method, a first compressed and purified air stream and a second compressed and purified air stream are produced. The second compressed and purified air stream has a higher pressure than the first compressed and purified air stream. These streams are cooled within a main heat exchanger through indirect heat exchange with return streams that are produced in an air separation unit. The return streams include at least part of a pumped liquid oxygen stream and as a result of the indirect heat exchange, a main feed air stream and a liquid air are produced from the compressed and purified air. [0009] The main feed air stream is introduced into a higher pressure column of the air separation unit and the liquid air stream is expanded and at least part of the liquid air stream is introduced into a lower pressure column of the air separation unit. An argon-rich stream from the lower pressure column is introduced into an argon separation zone formed by at least one column to produce an argon containing column overhead and an argon containing product stream composed of the argon containing column overhead. It is to be noted, that the term "argon separation zone" as used herein and in the claims includes a single argon column, often referred to in the art as a crude argon column, as well as columns in series that provide a sufficient number of separation stages that the argon product has very low levels of oxygen, typically less than about 10 ppm.

[0010] A crude liquid oxygen stream composed of liquid column bottoms of the higher pressure column and a nitrogen-rich liquid stream composed of liquefied nitrogen column overhead of the higher pressure column are subcooled. At least part of the crude liquid oxygen stream and at least part of the nitrogen-rich liquid stream are introduced into the lower pressure column. [0011] The main feed air stream is extracted from the main heat exchanger at a temperature warmer than that of the liquid air stream and introduced into the higher pressure column at least at about such temperature. Preferably, the temperature of the main feed air stream is in a range of between about 6K and about 25K warmer than the liquid air stream and more preferably, in a range of between about 8K and about 15K warmer than the liquid air stream.

[0012] The effect of this is to subcool the liquid air stream, thereby increasing the liquid content thereof after having been expanded to improve the liquid to vapor ratio in the lower pressure column and thereby to increase the argon recovery. It is to be noted that unlike the prior art, there can be no simplification such as by not subcooling the crude liquid oxygen stream. If such stream were not subcooled, argon recovery would suffer in that the liquid to vapor ratio above the point of introduction of the liquid air stream or part thereof would be less due to evolution of vapor during the expansion of the same. Moreover, unlike the prior art, the temperature of the main feed air stream is selected such that the liquid air stream has an approach temperature approaching that of an average temperature of the return streams of no less than a range of between about 0.2K and about 3K, and preferably between 0.4K and 2K. The average temperature is a calculated temperature at which a product of flow and enthalpy of the return streams at a cold end of the main heat exchanger is equal to the product of the flow and the enthalpy of the return streams at their actual temperatures. As will be discussed, it has been found by the inventors herein that if this temperature is made any smaller, given the fact that a main heat exchanger is only of finite size, the compression requirement for the second compressed and purified air stream will increase with no appreciable increase in the argon recovery.

[0013] In order to overcome warm end and heat leakage, as well known in the art, refrigeration must be generated. There are a number of ways to do this that are compatible with the present invention. For example, a third compressed and purified air stream can be produced. The third compressed and purified air stream can be partially cooled within the main heat exchanger and introduced into a turboexpander to produce an exhaust stream for generation of the refrigeration. The exhaust stream can then be introduced into the lower pressure column. A fourth compressed and purified air stream can be produced by extracting the fourth compressed and purified air stream from an intermediate stage of a compressor used in forming the second compressed and purified stream. The fourth compressed and purified stream is expanded within another turboexpander and combined with the first compressed and purified air stream within the main heat exchanger to increase liquid production. [0014] As an alternative method of generating refrigeration, a nitrogen column overhead stream composed of the nitrogen column overhead can be partially warmed within the main heat exchanger and then expanded within a turboexpander to produce an exhaust stream for the generation of the refrigeration. The exhaust stream can then be introduced into the main heat exchanger and then fully warmed therein.

[0015] In any embodiment of the present invention, the liquid air stream can be introduced into a liquid turbine to expand the liquid air stream to a pressure suitable for its introduction into the intermediate location of the higher pressure column. [0016] The crude liquid oxygen stream and the nitrogen-rich liquid stream can be subcooled through indirect heat exchanger with return streams that are formed from a nitrogen-rich vapor stream composed of column overhead of the lower pressure column and a waste vapor stream enriched in nitrogen to a lesser extent than the nitrogen-rich vapor stream. The nitrogen-rich vapor stream and the waste vapor stream can be introduced into the main heat exchanger after having subcooled the crude liquid oxygen stream and the nitrogen-rich liquid stream.

[0017] A first part of the crude liquid oxygen stream can be expanded and introduced into the lower pressure column and a second part of the crude liquid oxygen stream can indirectly exchange heat with an argon column overhead stream composed of the argon column overhead. As a result, the argon column overhead stream can be condensed and the second part of the crude liquid oxygen stream can be partially vaporized. Liquid and vapor fraction streams resulting from the partial vaporization of the crude liquid oxygen stream can then be introduced into the lower pressure column. Part of the argon column overhead stream after having been condensed can form the argon product stream and a remaining part thereof after condensation can be returned to the argon separation zone as reflux.

Brief Description of the Drawings [0018] While the specification concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings in which :

[0019] Fig. 1 is a schematic process flow diagram of an apparatus for carrying out a method in accordance with the present invention;

[0020] Fig. 2 is a graphical representation of the prior art heating and cooling curves in a main heat exchanger;

[0021] Fig. 3 is a graphical representation of the heating and cooling curves within a main heat exchanger that operates in connection with an air separation method in accordance with the present invention;

[0022] Fig. 4 is a fragmentary, schematic view of an alternative embodiment of Fig. 1 showing an alternative embodiment for a subcooling unit integrated with the main heat exchanger; [0023] Fig. 5 is a fragmentary, schematic view of an alternative embodiment of Fig. 1 employing expansion of a nitrogen-rich stream to generate refrigeration; and [0024] Fig. 6 is a fragmentary, schematic view of an alternative embodiment of Fig. 1 employing further expansion to increase the production of liquid.

[0025] In order to avoid repetition of the explanation of the accompanying figures, the same reference numerals are used and repeated in the figures where the description of particular elements designated by the reference numerals are identical.

Detailed Description

[0026] With reference to Fig. 1, an air separation plant 1 is illustrated that is configured for carrying out a method in accordance with the present invention is illustrated.

[0027] An air stream 10 is compressed by means of a main air compressor 12. The air pressure of the resultant compressed stream is set by the pressure of a higher pressure column 48 to be discussed hereinafter and pressure drop. After cooling in an after cooler 14 to remove the heat of compression, the air stream 10 is purified within a purification unit 16 to remove higher boiling impurities such as carbon dioxide and moisture that could freeze as well as hydrocarbons that could collect to present a safety hazard. Purification unit 16, as well known in the art, can be beds of molecular sieve adsorbent operating out of phase in a known temperature swing adsorption cycle to purify air stream 10. [0028] The compression and purification of the air stream 10 produces compressed and purified air stream 18 that is divided to produce a first compressed and purified air stream 20 that constitutes the largest portion resulting from such division. A part 22 of compressed and purified air stream 18 is further compressed within a booster compressor 24 to produce a second compressed and purified air stream 28. Part 22 of compressed and purified air stream 18 typically has a flow rate in a range of between about 24% and about 40% of compressed and purified air stream 18. The discharge pressure of the booster compressor 24 is set by the pressure of a pumped liquid oxygen stream 122 also to be discussed hereinafter. When the pressure of second compressed and purified air stream 28 is below its critical pressure, the pressure is typically less than about 2.5 times the pressure of pumped liquid oxygen stream 122. The heat of compression of second compressed and purified air stream 28 is preferably removed by after cooler 26.

[0029] As will be discussed, optionally, a further part 30 of compressed air stream 18 is compressed within a booster compressor 32 to produce a third compressed and purified air stream 36 for refrigeration purposes. The flow rate of the further part 30 of compressed and purified air stream is typically in a range of between about 5% and about 20% of that of compressed and purified air stream 18. The heat of compression is preferably removed from third compressed and purified air stream 36 by an after cooler 34. It is to be pointed out that main air compressor 12 and booster compressor 24 are preferably multi-stage machines with inter-stage cooling. Booster compressor 32 is a single stage machine powered by turbine 62. Compressors 12 and 24 are usually powered by an external source, usually an electric motor.

[0030] First compressed and purified air stream 20 and second compressed and purified air stream 28 are cooled within the main heat exchanger 40 to produce a main feed air stream 42 that is at or near its dew point and a liquid air stream 44. As will be discussed, first compressed and purified air stream 20 and second compressed and purified air stream 28 are cooled by indirect heat exchange with return streams, produced in the air separation unit 46, that are enriched in oxygen and nitrogen. It is to be pointed out that the present invention contemplates that second compressed and purified air stream 28 could be above the critical pressure. In such case, the cooling of such stream would produce a dense phase vapor in a process known as "pseudo liquefaction" in that no actual liquid phase would be produced. Therefore, the term "liquefaction" or the term "liquid" when used in connection with liquid air stream 44 herein and in the claims contemplates both a pseudo liquefaction that produces a dense phase vapor and an actual liquefaction that produces a liquid. [0031] The main feed air stream 42 is introduced into a bottom region of a higher pressure column 48 of air separation unit 46 that operates at a higher pressure than a lower pressure column 50 of air separation unit. Air separation unit 46 also includes an argon column 52 that provides an argon separation zone for refinement of argon to produce an argon containing column overhead from which argon product is extracted. Argon column 52 in a proper case could be replaced with a series of columns to present a sufficient number of stages of separation to substantially separate the oxygen as described above. [0032] Although not illustrated, it is understood that higher pressure column 48, lower pressure column 50 and argon column 52 contain mass transfer elements to contact liquid and vapor phases of the mixtures to be separated within such columns. These mass transfer elements can be known structured packing or sieve trays, dumped packing or combinations thereof. [0033] Liquid air stream 44 is introduced into a liquid expansion device 54 and is expanded to a pressure suitable for its introduction into an intermediate location of higher pressure column 48 above main feed air stream 42. Liquid expansion device 54, as illustrated, is preferably a liquid turbine in which the work of expansion can be recovered in an electric generator, used to drive a compressor or dissipated as heat with an oil brake. It is understood that liquid expansion device 54 could be an expansion valve. After expansion, liquid air stream 44 is divided into first subsidiary liquid stream 56 and a second subsidiary liquid stream 58. The second subsidiary liquid stream 58 is introduced into the higher pressure column 48. As such, the discharge pressure of liquid expansion device 54 is set at a pressure of the higher pressure column 48 plus pressure drop. The first subsidiary liquid stream 56 is reduced in pressure by an expansion valve 60 and then introduced into lower pressure column 50. As would occur to those skilled in the art, all of the liquid air stream 44 could be introduced into the lower pressure column 50 and expanded to a suitable pressure for such purposes.

[0034] In order to refrigerate the process and thus, overcome warm end losses, third compressed and purified air stream 36 after removal of the heat of compression is partially cooled within the main heat exchanger 40. By partially cooled, what is meant is that the stream is cooled to a temperature that is between the warm and cold end temperatures of main heat exchanger 40. The resultant third compressed air stream 36 after having been partially cooled is then introduced into a turboexpander 62 to produce an exhaust stream 64 that is introduced into the lower pressure column 50. As is apparent from the illustration, the pressure of exhaust stream 64 is set at the pressure of the lower pressure column 50. [0035] The separation of the air within the higher pressure column 48 produces a nitrogen column overhead that is rich in nitrogen. Additionally, a crude liquid oxygen column bottoms is produced within higher pressure column 48 that is enriched in oxygen. A nitrogen-rich vapor stream 66, composed of the nitrogen-rich column overhead, is introduced into a condenser reboiler 68 that is located within a bottom region of lower pressure column to vaporize oxygen- rich liquid collecting as liquid column bottoms within lower pressure column 50 against condensing the nitrogen-rich vapor stream 66 to produce the nitrogen- rich liquid stream 70. Part 72 of nitrogen-rich liquid stream 70 is introduced back into the top of higher pressure column 48 as reflux and a part 74 of the nitrogen-rich liquid stream 70 is subcooled along with crude liquid oxygen stream 76 composed of the crude liquid oxygen column bottoms of higher pressure column 48 in a subcooling unit 78.

[0036] Part 74 of nitrogen-rich liquid stream 70 is divided into first and second subsidiary nitrogen streams 80 and 82. Second subsidiary liquid nitrogen stream 82 can be taken as a product. First subsidiary liquid nitrogen stream 80 is reduced in pressure by an expansion valve 84 and then introduced into the top of lower pressure column 50. As would occur to those skilled in the art, all of part 74 of nitrogen-rich liquid stream 70 could be introduced into lower pressure column 50.

[0037] An argon-rich stream 86 as a vapor is introduced into argon column 52. Argon-rich stream 86 will typically contain between about 5% and about 20% argon. An argon-rich column overhead is extracted as an argon-rich vapor stream 88 and condensed within a heat exchanger 90 located within a shell 92. The resultant argon-rich liquid stream 94, as a stream 96, is introduced back into argon column 52 as reflux and an argon product stream 98 can be extracted as an argon product. The resultant argon lean liquid stream 100 is returned to lower pressure column 50. [0038] Depending upon the number of stages of argon column 52, argon-rich column overhead and therefore the argon product stream 98 can be a crude stream that requires further processing for purification purposes. As known in the art, such a crude stream can be further processed to remove residual oxygen in a de- oxo unit and then in a nitrogen column to remove any residual nitrogen. [0039] Crude liquid oxygen stream 76 after having been subcooled is then divided and a first part 102 of such stream can be expanded within an expansion valve 104 and directly introduced into lower pressure column 50. A second part 106 can be expanded within an expansion valve 108 and then introduced into the heat exchanger 92 in indirect heat exchange with argon-rich vapor stream 88 to condense the same. The resultant vapor stream 110 can be introduced into the lower pressure column 50 along with a liquid stream 112. [0040] Crude liquid oxygen stream 76 and second part 74 of nitrogen-rich liquid stream 70 are subcooled within subcooling unit 78 through indirect heat exchange with nitrogen column overhead stream 114 and a waste stream 116 having a lesser concentration of nitrogen than nitrogen column overhead stream 114. At the same time an oxygen-rich stream 118, extracted from the bottom of the lower pressure column 50, can be pumped by a pump 120 to produce a pumped liquid oxygen stream 122. The pumped oxygen can also be above its critical pressure and therefore is a dense phase or "pseudo liquid." The first part 124 thereof can be introduced into the main heat exchanger 40 for the liquefaction of second compressed air stream 28. Also introduced into main heat exchanger are other return streams such as nitrogen column overhead stream 114 and waste stream 116. These return streams also serve to cool the incoming first compressed and purified air stream 20 to produce the main feed air stream 42 and to partly cool the third compressed air stream 36. It is to be pointed out that embodiments of the present invention are possible in which waste stream 116 is not removed. This results in the nitrogen column overhead stream 114 having a lower concentration of nitrogen and thus forming a waste stream. In the illustrated embodiment, however, column overhead stream 114, waste stream 116 and first part 124 of pumped liquid oxygen stream 122 consist of the return streams of the process.

[0041] Nitrogen column overhead stream 114 and the vaporized first part 124 of the pumped liquid oxygen stream form nitrogen and pressurized oxygen products. The second part 126 of pumped liquid oxygen stream 122 can optionally be taken as a liquid product. [0042] As indicated above, the first compressed air stream 20 is not fully cooled within main heat exchanger 40. Rather, it is withdrawn to produce main feed air stream 42 having a warmer temperature than the second compressed air stream 28 upon its liquefaction and discharge as liquid air stream 44 from main heat exchanger 40. As mentioned above, this causes the subcooling of liquid air stream 44. The temperature of main feed air stream 42 is preferably in a range of between about 6K and about 25K warmer than liquid air stream 44. A more preferred range is between about 8K and about 15K.

[0043] With reference to Fig. 2, the temperature profile within the main heat exchanger 40 is shown in which the first compressed air stream 20 is fully cooled and is thus withdrawn after having fully traversed the main heat exchanger 40. In this particular prior art operation, there exists a temperature difference in the cold end of main heat exchanger of about 6.2K. [0044] With reference to Fig. 3, the temperature profile within main heat exchanger 40 is shown in accordance with the present invention. Withdrawal of compressed and purified air stream 20 at the warmer temperature and therefore, production of main feed air stream 42 at the warmer temperature results in a steeper cooling profile because all that remains within the main heat exchanger 40 to be cooled is second compressed and purified air stream 28 which results in the production of liquid air stream 24 at a subcooled temperature. As a result, less vaporization occurs due to the expansion of liquid air stream 44 within expander 54 and the first subsidiary liquid stream 56 after passage through valve 60 and second subsidiary liquid stream 58 has a greater liquid content upon its introduction into higher pressure column 48. Main feed air stream 42 is warmer entering the high pressure column. This results in greater liquid-vapor traffic and therefore an increase in the production of nitrogen-rich vapor in the top of higher pressure column 48. The greater liquid content of first subsidiary air stream 56 produces an increased liquid to vapor ratio below the point of introduction into the lower pressure column 50. Additionally, the greater production of nitrogen-rich vapor at the top of higher pressure column 48 results in more liquid being produced into lower pressure column 50 as reflux by virtue of increased production of second part 74 of liquid nitrogen-rich stream 70. In the present invention, since the crude liquid oxygen stream 76 is also subcooled, a greater liquid fraction of this stream after expansion is also able to be introduced into lower pressure column 50. The resultant overall greater liquid to vapor ratio within lower pressure column 50 results in more argon being present within argon-rich stream 86 and therefore, a greater rate argon recovery. It is to be noted, that the same also will increase the oxygen recovery, albeit to a lesser extent. However, since, typically, the oxygen is being supplied to customers under supply contracts, the plant can be operated to meet commercial needs by decreasing the degree of main air compression to also lower the overall power requirements of a method conducted in accordance with the present invention while still taking advantage of the increased argon recovery possible in the inventive method disclosed herein .

[0045] However, as main feed air stream 20 becomes progressively warmer, the temperature of the liquid air stream 42 becomes progressively lower. In order to prevent the heating and cooling curves within the main heat exchanger 40 from crossing, more air will have to be compressed within booster compressor 24 thereby increasing the power requirements of the plant. Increasing flow of 30 is another way of compensating for the smaller temperature difference at the cold end of heat exchanger 40. This tends to increase total power and decrease argon recovery. It has been found by the inventors herein that the withdrawal of the main feed air stream 20 from the main heat exchanger 40 at a specific, predefined temperature, allow the temperature of liquid air stream to be controlled so as to approach the temperatures of the return streams, namely, nitrogen column overhead stream 114, waste stream 116 and pumped liquid oxygen stream 124. Such control thereby allows for an increase in argon recovery without unnecessary increases in the power requirements for the compression of the air. In a typical plate-fin heat exchanger, the main feed air stream 42 should be withdrawn from the main heat exchanger 40 at a temperature such that liquid air stream 44 has a temperature that approaches that of the average temperature of the return streams by no less than a range of between about 0.2K and 3K, and preferably between 0.4K and 2K. Below this range in temperature, power requirements rapidly increase without any appreciable increase in argon recovery. As mentioned above, this "average temperature" is calculated to be a temperature at which the flow times the enthalpy is equal to the flow times the enthalpy of such return streams at their actual temperature at the cold end of the main heat exchanger 40. In the illustrated embodiment, the return streams at the cold end of main heat exchanger 40 are first part 124 of pumped liquid oxygen stream 122, and nitrogen column overhead stream 114 and waste stream 116 at the warm end of subcooling unit 78. It is to be noted that if any additional streams are withdrawn from the column system and then fed to main heat exchanger 40, then such streams would be counted in such calculation of the average temperature. As would be known, the control of such temperature of main feed air stream 44 is effectuated by design of the main heat exchanger 40 and more specifically, the location of an outlet thereof to discharge main feed air stream 42 therefrom. [0046] With reference to Fig. 4, in an alternative embodiment of the air separation plant shown in Fig. 2, main heat exchanger 40 and subcooling unit 28 can be combined into a single unit 40' . The air separation plant illustrated in Fig. 4 otherwise functions in a manner set forth for the apparatus of Fig. 1.

[0047] With reference to Fig. 5, an alternative embodiment of the air separation plant shown in Fig. 1 is illustrated. A nitrogen enriched vapor stream 130 can be extracted from nitrogen-rich vapor stream 66 and a remaining portion 67 of nitrogen-rich vapor stream 66 can be introduced into condenser reboiler 68. Nitrogen enriched vapor stream 130 is introduced into main heat exchanger 40'' in which it is partially warmed and then introduced into a turboexpander 132 coupled to a generator 134. The resultant cooled exhaust stream 136 is introduced into the main heat exchanger 40'' that is provided with a passage to fully warm such stream and thereby refrigerate the process. Other than the alternative method of generating refrigeration, the plant illustrated in Fig. 5 is otherwise identical to that shown in Fig. 1. [0048] With reference to Fig. 6, a yet further alternative embodiment of the air separation plant illustrated in Fig. 1 is shown. In such embodiment, a fourth compressed air stream 150 is taken from an intermediate stage of the booster compressor 24, preferably, the first or second stage thereof. The resulting fourth compressed air stream 150 is then compressed within a compressor 152 to produce compressed air stream 154 that, after removal of heat of compression within an after cooler 156, is introduced into a turbine 158 to produce an exhaust stream 160 that is combined with first compressed air stream 20 at an intermediate location and temperature level of a main heat exchanger 40' ' ' having an inlet provided for such purpose. This results in a capability to produce more liquid than the plant shown in Fig. 1. Other than the modification outlined in this paragraph, the remainder of the plant would otherwise be identical to the air separation plant 1 shown in Fig . 1.

[0049] The following are calculated examples of the operation of air separation plant 1, as illustrated in Fig. 1, that is conducted in accordance with a method of the present invention (Table 1) and a prior art method in which the main feed air stream 42 is withdrawn from the main heat exchanger 40 at the cold end temperature of the main heat exchanger 40 (Table 2) . In both examples, the plants are designed to produce a unitized gaseous oxygen flow of 1000 (first part 124 of pumped liquid oxygen stream 122 after vaporization in main heat exchanger 40) and a unitized liquid oxygen flow of 34 (second part 126 of pumped liquid oxygen stream 122) .

TABLE 1

[0050] By way of comparison, the argon recovery of the present invention, as shown in Table 1, is 78.1%. The argon recovery for a prior art method, represented in Table 2, is 74.1%. Likewise, the oxygen recovery from Table 1 is 99.3%, the oxygen recovery in Table 2 is 98.9%. The lower degree of flash off of streams 56 and 58 as they enter the higher and lower pressure distillation columns 48 and 60, for the present invention as shown in Table 1 (percent vapor) , and the warmer temperature of main feed air stream 42, lead to the improved product stream recoveries . The reduced flash off is a result of the lower temperature of liquid air stream 44 in the present invention. In Table 1, the flow of second compressed and purified air stream 28 is required to be 1.9% higher than in Table 3. As a result, the power consumption for the present invention is slightly higher than in the prior art.

[0051] While the invention has been described with reference to a preferred embodiment, as will occur to those skilled in the art, numerous changes, additions and omissions can be made without departing from the spirit and the scope of the present invention as recited in the appended claims.

Claims

We Claim :

1. A method of separating air comprising: producing a first compressed and purified air stream and a second compressed and purified air stream having a higher pressure than the first compressed and purified air stream; cooling the first compressed and purified air stream and the second compressed and purified air stream in a main heat exchanger, through indirect heat exchange with return streams produced in an air separation unit that include at least part of a pumped liquid oxygen stream, thereby to produce a main feed air stream and a liquid air stream; introducing the main feed air stream into a higher pressure column of the air separation unit, expanding the liquid air stream and introducing at least part of the liquid air stream into a lower pressure column of the air separation unit; introducing an argon-rich stream from the lower pressure column into an argon separation zone formed by at least one column to produce an argon containing column overhead and an argon containing product stream composed of the argon containing column overhead; subcooling a crude liquid oxygen stream composed of liquid column bottoms of the higher pressure column and a nitrogen-rich liquid stream composed of liquefied nitrogen column overhead of the higher pressure column and introducing at least part of the crude liquid oxygen stream and at least part of the nitrogen-rich liquid stream into the lower pressure column; and the main feed air stream being extracted from the main heat exchanger at a temperature warmer than the liquid air stream and introduced into the higher pressure column at least at about said temperature, thereby subcooling the liquid air stream and increasing the liquid content thereof after having been expanded to improve the liquid to vapor ratio in the lower pressure column and thereby to increase the argon recovery, the temperature being selected such that the liquid air stream has an approach temperature approaching that of an average temperature of the return streams of no less than a range of between about 0.2K and about 3K, the average temperature being a calculated temperature at which a product of flow and enthalpy of the return streams at a cold end of the main heat exchanger is equal to the product of the flow and the enthalpy of the return streams at their actual temperatures .

2. The method of claim 1, wherein the range is between about 0.4K and about 2K.

3. The method of claim 1, wherein the temperature of the main feed air stream is in a range of between about 6K and about 25K warmer than the liquid air stream.

4. The method of claim 1, wherein the temperature of the main feed air stream is in a range of between about 8K and about 15K warmer than the liquid air stream.

5. The method of claim 4, wherein the range is between about 0.4K and about 2K.

6. The method of claim 1, wherein: the liquid air stream is expanded to a pressure suitable for its introduction into an intermediate location of the higher pressure column; the liquid air stream is divided into a first subsidiary liquid stream and a second subsidiary liquid stream; the first subsidiary liquid stream is introduced into the higher pressure column; and the second subsidiary liquid stream is expanded and introduced into the lower pressure column above a point of discharge of the argon-rich stream to the argon column.

7. The method of claim 1, wherein: a third compressed and purified air stream is produced; the third compressed and purified air stream is partially cooled within the main heat exchanger and introduced into a turboexpander to produce an exhaust stream for generation of refrigeration; and the exhaust stream is introduced into the lower pressure column.

8. The method of claim 5, wherein: a fourth compressed and purified air stream is produced by extracting the fourth compressed and purified air stream from an intermediate stage of a compressor used in forming the second compressed and purified stream; and the fourth compressed and purified stream is expanded within another turboexpander and combined with the first compressed and purified air stream within the main heat exchanger to increase liquid production .

9. The method of claim 1, wherein a nitrogen column overhead stream composed of the nitrogen column overhead is partially warmed within the main heat exchanger, expanded within a turboexpander to produce an exhaust stream for generation of refrigeration and the exhaust stream is introduced into the main heat exchanger and fully warmed therein.

10. The method of claim 1 or claim 5 or claim 6 or claim 7 or claim 8 or claim 9, wherein the liquid air stream is introduced into a liquid turbine to expand said liquid air stream to the pressure suitable for its introduction into an intermediate location of the higher pressure column.

11. The method of claim 1, wherein the crude liquid oxygen stream and the nitrogen-rich liquid stream are subcooled through indirect heat exchange with the return streams that are formed from a nitrogen-rich vapor stream composed of column overhead of the lower pressure column and a waste vapor stream enriched in nitrogen to a lesser extent than the nitrogen-rich vapor stream, the nitrogen-rich vapor stream and the waste vapor stream being introduced into the main heat exchanger after having subcooled the crude liquid oxygen stream and the nitrogen-rich liquid stream.

12. The method of claim 1, wherein: a first part of the crude liquid oxygen stream is expanded and introduced into the lower pressure column and a second part of the crude liquid oxygen stream indirectly exchanges heat with an argon column overhead stream composed of the argon column overhead, thereby condensing the argon column overhead stream and partially vaporizing the second part of the crude liquid oxygen stream; liquid and vapor fraction streams resulting from partial vaporization of the crude liquid oxygen stream are introduced into the lower pressure column; and part of the argon column overhead stream after having been condensed forms the argon product stream and a remaining part thereof after condensation is returned to the argon separation zone as reflux.