MX2008001840A

MX2008001840A - Air separation method.

Info

Publication number: MX2008001840A
Application number: MX2008001840A
Authority: MX
Inventors: Neil Mark Prosser; Richard John Jibb
Original assignee: Praxair Technology Inc
Priority date: 2007-03-13
Filing date: 2008-02-07
Publication date: 2009-02-24
Also published as: WO2008112728A2; CN101266095A; WO2008112728A3; EP2122283A2; US20080223077A1

Abstract

An air separation method in which a liquid air stream, produced by vaporizing a pumped liquid oxygen stream, is introduced into a lower pressure column and optionally, a higher pressure column of an air separation unit. The liquid air stream is subcooled by extracting a main air feed to the higher pressure column from a main heat exchanger at a temperature warmer than the liquid air stream to increase argon recovery in an argon column connected to the lower pressure column. 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 0.2K and about 3K.

Description

METHOD OF AIR SEPARATION Field of the Invention A method for separating air in which a pressurized oxygen product is produced by vaporizing a flow of pumped liquid oxygen instead of liquefying a flow of air in a main heat exchanger and an argon product is produced in an area of separation of argon connected to a lower pressure column which is operatively associated in a heat transfer ratio with a higher pressure column. More particularly, the present invention relates to such a method in which a main feed air flow is expelled from the main heat exchanger to a temperature that is hotter than the liquid air flow to subcool the flow of air. liquid air, and in this way increase the recovery of argon. Background of the Invention The separation of air in nitrogen, oxygen fractions and argon have been conducted in air separation units in which the air is compressed, purified and cooled in a main heat exchanger at an appropriate temperature for rectification. The air is introduced into a higher pressure column of a double column arrangement which also has a lower pressure column in a heat transfer ratio with a higher pressure column.

The nitrogen and oxygen products can be extracted from the lower and higher pressure columns. A flow rich in argon can be removed from the lower pressure column and introduced into the argon column to produce the argon-rich column top. The argon-rich column top is condensed, typically with the use of all or a part of crude liquid oxygen flow, produced as lower parts of the column of the higher pressure column, to generate reflux of liquid for the column of argon. A portion of the argon-rich top of the column is taken as an argon product. It is also known that to produce high pressure oxygen product in such an arrangement by pressurizing an oxygen rich flow composed of lower parts of oxygen column produced in the lower pressure column by pumping the flow and vaporizing it in the main heat exchanger in instead of liquefying the air flow that is part of the air that has been compressed at a high pressure. The resulting liquid air flow is expanded and introduced into a lower pressure column or both, the columns of higher and lower pressure. An example of such a plant is shown in U.S. Patent No. 6, 293, 126. In this patent, the main feed air flow is expelled from the main heat exchanger at a temperature warmer than that of the flow. of air that is then compressed and liquefied to produce air flow liquid. In an attempt to simplify the construction of such a plant, the flow of crude liquid oxygen is neither subcooled nor before its use in condensing under argon reflux or in its introduction into the lower pressure column. As a result, there is a higher vapor fraction of the flow of crude liquid oxygen entering the lower pressure column after the expansion that would otherwise have occurred, had the flow of crude liquid oxygen that has been subcooled. In this way, the liquid in vapor ratio at a point in the lower pressure column below the point at which the argon-rich flow is extracted for further refining in the argon column is less than otherwise possible. . In addition, extracting a main air flow at a temperature warmer than the liquid air flow to the extent that it can be close to the temperature of the return flow used to cool the incoming air. As a result, the pressure requirements for the air flow that is then compressed and liquefied are usually greater than the flow and / or pressure that would otherwise have been required and the main feed air flow has not been ejected at a temperature hotter The subsequent subcooling of the liquid air flow tends to compensate for the liquid reduced to the vapor ratio in the lower pressure column. These results in more power have been consumed in one plant without any increase in argon recovery.

As will be discussed later, the present invention provides a method for separating air in which the argon recovery is augmented as much as possible in the air separation systems of the prior art, as discussed above, while minimizing the amount of excess power that is necessarily used to increase the recovery of argon. Brief Description of the Invention The present invention provides a method of separating air. According to the method, a first flow of compressed and purified air and a second flow of compressed and purified air are produced. The second flow of compressed and purified air has a higher pressure than the first compressed and purified flow. These flows are cooled inside the main heat exchanger through an indirect heat exchange with return flows that are produced in an air separation unit. The return flows include at least part of a liquid oxygen flow pumped and as a result of the indirect heat exchange there is a flow of main supply air and liquid air from the compressed and purified air. The main feed air flow is introduced into a higher pressure column of the air separation unit and the liquid air flow is expanded and at least one part of the liquid air flow is introduced into a lower pressure column of the air separation unit. A flux rich in argon from the lower pressure column is introduced into an argon separation zone formed by at least one column to produce a column top containing argon and a product stream containing argon composed of a top of column that has argon. It should be noted, that the term "argon separation zone" as used herein and in the claims, includes a single argon column, and often referred to in the art as a raw argon column, also as the series columns that provide a sufficient number of separation steps wherein the argon product has very low levels of oxygen, typically less than 10 ppm. A flow of crude liquid oxygen composed of liquid column bottoms of the highest pressure column and a liquid flow rich in nitrogen composed of an upper part of liquid nitrogen column of the higher pressure column are cooled. At least part of the flow of crude liquid oxygen and at least part of the liquid flow rich in nitrogen are introduced into the lower pressure column. The main feed air flow is drawn from the main heat exchanger to a hotter temperature than the liquid air flow and introduced into the higher pressure column at least at this temperature.

Preferably, the temperature of the main feed air flow is in a range of between 6K and approximately 25K hotter than the liquid air flow, and more preferably, in a range of 8K to approximately 15K hotter than the liquid air flow . The effect of this is to subcool the flow of liquid air, and thus increase the liquid content of the liquid after it has been expanded to increase the liquid to vapor ratio in the lower pressure column and thus increase the recovery of argon. It should be noted that like the prior art, there can be no simplification such as to not sub-cool the flow of crude liquid oxygen. If these flows would not be subcooled, the recovery of argon would suffer so that the liquid in vapor ratio below the point of introduction of the liquid air flow or part of it would be smaller due to the evolution of vapor during its expansion. Further, as in the prior art, the temperature of the main feed air flow is selected such that the liquid air flow has the approximate temperature approaching an average temperature of the return flows of not less than a range of between 0.2 K and approximately 3K, and preferably between 0.4K and 2K. The average temperature is a calculated temperature at which the product of flow and enthalpy of the return flows at a cold end of the main heat exchanger is equal to the product of the return flows in its current temperatures. As will be discussed later, it has been found by the inventors here that if the temperature is a little lower, given the fact that a main heat exchanger is only finite in size, the compression requirement for the second flow of compressed and purified air will increase without marked increase in argon recovery. To overcome the hot end and the heat leakage, as is well known in the art, cooling must be generated. There are a number of routes to do this and which are compatible with the present invention. For example, a third flow of compressed and purified air can be produced. The third flow of compressed and purified air can be partially cooled inside the main heat exchanger and introduced into a turbo expander to produce an output flow for the generation of cooling. The outflow can then be introduced into a lower pressure column. A fourth flow of compressed and purified air can be produced by extracting the fourth flow of compressed and purified air from an intermediate stage of a compressor used in forming the second flow of compressed and purified air. The fourth compressed and purified flow is expanded into another turbo expander and combined with the first flow of compressed and purified air into the main heat exchanger to increase the production of liquid.

As an alternative method to generate cooling, a flow from the top of the nitrogen column composed of a nitrogen column top part can be partially heated inside the main heat exchanger and then expanded inside the turbo expander to produce an output flow for the generation of refrigeration. The outflow can then be introduced into the main heat exchanger and then fully heated in the same place. In one embodiment of the present invention, the liquid air flow can be introduced into a liquid turbine to expand the flow of liquid air to an appropriate pressure for its introduction at the intermediate location of the highest pressure column. The flow of crude liquid oxygen and the liquid flow rich in nitrogen can be subcooled through an indirect heat exchanger with return flows that are formed from a flow of nitrogen-rich vapor composed of the top column and column pressure lower and a flow of waste vapor enriched with nitrogen for a smaller measure than the flow of steam rich in nitrogen. The flow of steam rich in nitrogen and the flow of waste vapor can be introduced into the main heat exchanger after the liquid oxygen flow and the liquid flow rich in nitrogen have been subcooled.

A first part of the crude liquid oxygen flow can be expanded and introduced into the lower pressure column and a second part of the raw liquid oxygen flow can indirectly exchange the heat with a flow of the top of the argon column composed of the part top of argon column. As a result, the flow of the upper argon column can be condensed and the second part of the crude liquid oxygen flow can be partially vaporized. The liquid and vapor fraction flows resulting from the partial vaporization of the crude liquid oxygen stream can then be introduced into the lower pressure column. Part of the flow of the upper argon column after it has been condensed can form the flow of argon product and a remaining part of it after condensation can be returned to the argon separation zone as reflux. BRIEF DESCRIPTION OF THE DRAWINGS While the description concludes with the claims that distinctly point to the subject that the applicants consider as their invention, it is believed that the invention will be better understood when taking into account the accompanying drawings in which: Figure 1 is a schematic process flow diagram of an apparatus for carrying out a method according to the present invention; Figure 2 is a graphic representation of the heating and cooling curves of the prior art in a main heat exchanger; Figure 3 is a graphic representation of the heating and cooling curves within the main heat exchanger operating according to an air separation method according to the present invention; Figure 4 is a fragmentary, schematic view of an alternative embodiment of Figure 1 showing an alternative embodiment for a subcooling unit integrated with the main heat exchanger; Figure 5 is a fragmentary, schematic view of an alternative embodiment of Figure 1 employing the expansion of a nitrogen-rich flow to generate cooling; and Figure 6 is a fragmentary, schematic view of an alternative embodiment of Figure 1 employing the subsequent expansion to increase the production of liquid. To avoid repeating the explanation of the accompanying figures, the same references are used and repeated in the figures where the description of particular elements designed by the reference numerals are identical. Detailed Description of the Invention With reference to Figure 1, an air separation plant 1 is illustrated is illustrated so that it is configured to carry out a method according to the present invention. An air flow 10 is compressed by means of a main air compressor 12. The air pressure of the resulting compressed flow is established by the pressure of a higher pressure column 48 to be discussed here and a pressure drop. After cooling in a subsequent cooler 14 to remove the heat of compression, the air flow 10 is purified within a purification unit 16 to remove the high boiling impurities such as carbon dioxide and humectant that could freeze, as well as hydrocarbons that can be collected to present a security risk. The purification unit 16, as is well known in the art, can be molecular sieve adsorbent beds operating out of the phase in a mixed adsorption cycle at known temperature to purify the air flow 10. The understanding and purification of the flow of air 10 produces a flow of compressed and purified air 18 which is divided to produce a first flow of compressed and purified air 20 which constitutes the largest portion resulting from this division. A portion 22 of the compressed and purified air flow 18 is subsequently compressed within a compressor of the impeller 24 to produce a second flow of compressed and purified air 28. The part 22 of the flow of compressed and purified air 18 typically has a flow rate in a range of between 24% and about 40% of the compressed and purified air flow 18. The discharge pressure of the impeller compressor 24 is set by the pressure of the pumped liquid oxygen flow 122 also as discussed above. When the pressure of the flow of compressed and purified air 28 is below the critical pressure, the pressure is typically less than about 2.5 times the pressure of the flow of pumped liquid oxygen 122. The compression heat of the second flow of compressed and purified air 28 it is preferably removed after the cooler 26. As will be discussed later, optionally, another part 30 of the compressed air flow 18 is compressed within the compressor of the impeller 32 to produce a third flow of compressed and purified air 36 for cooling purposes. The flow rate of another part 30 of the flow of compressed and purified air is typically in a range of between 5% and about 20% of the flow of compressed and purified air 18. The compression heat is preferably removed from the third air flow compressed and purified 36 by a subsequent cooler 34. It should be mentioned that the air compressor 12 and the impeller compressor 24 are preferably multi-stage machines with cooling between the stages. The compressor of the impeller 32 is a one-stage machine fed with the turbine 62. The compressors 12 and 24 are usually powered by external sources, usually an electric motor.

The first flow of compressed and purified air 20 and the second flow of compressed and purified air 28 are cooled inside the main heat exchanger 40 to produce a flow of main feed air 42 that is just or close to its dew point and a liquid air flow 44. As will be discussed below, the first flow of compressed and purified air 20 and the second flow of compressed and purified air 28 are cooled by the indirect heat exchange with return flows, produced in the air separation unit. 46, which are enriched with oxygen and nitrogen. It should be specified that the present invention contemplates that the second flow of compressed and purified air 28 could be below the critical pressure. In this case, the cooling of such a flow will produce a dense phase vapor in a process called "pseudo liquefaction" since no current liquid phase is produced. Thus, the term "liquefaction" or the term "liquid" when used in connection with the flow of liquid air 44 here and in the claims contemplates both, a pseudo liquefaction that produces a dense phase vapor and a current liquefaction that produces a liquid The main feed air flow 42 is introduced into a lower region of a higher pressure column 48 of the air separation unit 46 which operates at a higher pressure than the lower pressure column 50 of the separation unit. . The air separation unit 46 also it includes an argon column 52 which provides an argon separation zone for argon refining to produce an argon-containing column top from which the argon product is extracted. The argon column 52 in a separate case could be replaced with a series of columns to present a sufficient number of separation steps to substantially separate the oxygen as described above. Although not illustrated, it is understood that the highest pressure column 48, the lowest pressure column 50 and the argon column 52 contain mass transfer elements to contact the liquid and the vapor phases of the mixtures to be separated. inside these columns. These mass transfer elements can be known as structured sieve packages or trays, discharge packaging or combinations thereof. The liquid air flow 44 is introduced into a liquid expansion device 54 and is expanded to an appropriate pressure for its introduction into the intermediate location of the highest pressure column 48 below the main supply air flow 42. The device of liquid expansion 54, as illustrated, is preferably a liquid turbine in which the expansion work can be recovered in an electric generator, used to drive a compressor or dissipate as heat with an oil brake. It is understood that the liquid expansion device 54 could be an expansion valve. After expansion, the liquid air flow 44 is divided into a first subsidiary liquid flow 56 and a second subsidiary liquid flow 58. The second subsidiary liquid flow 58 is introduced into the highest pressure column 48. As such , the discharge pressure of the liquid expansion device 54 is set at a pressure of the highest pressure column 48 plus the pressure drop. The first flow of subsidiary liquid 56 is reduced in pressure by an expansion valve 60 and then introduced into a lower pressure column 50. As would be the case with one skilled in the art, all liquid air flow 44 could be introduced in the lower pressure column 50 and expanded to an appropriate pressure for these purposes. In order to be able to cool the process and thus exceed the hot end losses, the third flow of purified and compressed air 36 after the removal of the compression heat is partially cooled inside the main heat exchanger 40. Being partially cooled, it is intended that the flow is cooled to a temperature that is between hot and cold of the main heat exchanger 40. The third flow of compressed and purified air 36 after it has been partially cooled is then introduced into a turbo expander 62 to produce an outlet flow 64 which is introduced into a lower pressure column. It's obvious from the illustration, that the outlet flow pressure 64 is set at a pressure of the lower pressure column. The air separation within the lower pressure column 48 produces a nitrogen column top that is rich in nitrogen. Additionally, the lower parts of the raw liquid oxygen column is produced within the higher pressure column 48 which is enriched with oxygen. A steam flow enriched with nitrogen 66, composed of a nitrogen-rich column top, is introduced into a condenser reheater 68 which is located within a lower region of the lower pressure column to vaporize the oxygen-rich liquid by collecting the liquid column bottoms within the lower pressure column 50 against condensation of the nitrogen-rich vapor flow 66 to produce liquid flow rich in nitrogen. Part 72 of the nitrogen-rich liquid flow is introduced back into the upper part of the higher pressure column as reflux and part 74 of the nitrogen-rich liquid flow is subcooled along the flow of crude liquid oxygen composed of parts lower of liquid oxygen column of the highest pressure column in a subcooling unit 78. Part 74 of the nitrogen-rich liquid flow 70 is divided into a first and second subsidiary nitrogen flows 80 and 82. The second nitrogen flow subsidiary 82 can be taken as a product The first flow of subsidiary liquid nitrogen 80 is reduced in pressure by an expansion valve 84 and then introduced into the upper part of the lower pressure column 50. As would be understood by those skilled in the art, the entire flow part 74 nitrogen-rich liquid could be introduced into the lower pressure column 50. A flow rich in argon 86 in steam is introduced into the argon column 52. The argon-rich flow 86 will typically contain between 5% and about 20% argon . An argon-rich column top is extracted as a steam flow rich in argon 88 and condensed within a heat exchanger 90 located within a frame 92. The resultant argon-rich liquid flow 94, as a flow 96, is introduced back into the argon column 52 as reflux and the flow of argon product 98 can be extracted as an argon product. The resulting scarce liquid flow of argon 100 is returned to the lowest pressure column 50. Depending on the number of stages of the argon column 52, the argon-rich column top and thus the flow of argon product 98 it can be a crude flow that requires further processing for purification purposes. As is known in the art, such crude flow can also be processed to remove residual oxygen in a deoxo unit and then in a nitrogen column to remove any residual nitrogen.

The flow of crude liquid oxygen 76 after it has been cooled is then divided and the first part 102 of such flow can be expanded within the expansion valve 104 and directly introduced into the lower pressure column 50. A second part 106 can to be expanded within the expansion valve 108 and then introduced into the color exchanger 92 in indirect color exchange with steam flow rich in argon 88 to condense it. The resulting vapor flow 110 can be introduced into the lower pressure column along the liquid flow 112. The flow of crude liquid oxygen 76 and the second part 74 of the liquid flow rich in nitrogen 70 are subcooled within the unit. cooling 78 through the indirect heat exchanger with the flow of the top of nitrogen column 114 and a waste stream 116 having a lower concentration of nitrogen than the flow of the top of nitrogen column 114. At the same time a flow rich in oxygen 118, extracted from the bottom of the lower pressure column 50, can be pumped by a pump 120 to produce a flow of pumped liquid oxygen. The pumped oxygen can also be below its critical pressure and in this way it is a dense or "pseudo liquid" phase. The first part 124 thereof can be introduced into the main heat exchanger 40 for the liquefaction of the second flow of compressed air 28. Also introduced into the exchanger of main heat are other return flows such as the flow of the top of nitrogen column 114 and the waste stream 116. These return flows also serve to cool the incoming compressed and purified first flow 20 to produce the air flow of main feed 42 and partially cool the third flow of compressed air 36. It is worth mentioning that the embodiments of the present invention are possible when the waste stream 116 is not removed. This results in the flow of the top of nitrogen column 114 having a lower concentration of nitrogen and thus forming a waste stream. In the illustrated embodiment, however, the flow of the column top 114, the waste stream 116 and the first portion 124 of the pumped liquid oxygen flow 122 consist of process return flows. The flow of the upper nitrogen column 114 and the first vaporized portion 124 of the pumped liquid oxygen flow form nitrogen and pressurized oxygen products. The second part 126 of the pumped liquid oxygen flow 122 can optionally be taken as a liquid product. As indicated above, the first flow of compressed air 20 is not fully cooled inside the main heat exchanger 40. The idea of producing a main feed air flow 42 having a hotter temperature than the second air flow is discarded. compressed 28 after its liquefaction and discharge as liquid air flow 44 from the main heat exchanger 40. As mentioned above, this causes subcooling of the liquid air flow 44. The temperature of the main feed air flow 42 is preferably in a range between 6K and approximately 25K hotter than liquid air flow 44. A more preferred range is between 8K and approximately 15K. With reference to Figure 2, the temperature profile inside the main heat exchanger 40 is shown in which the first flow of compressed air 20 is completely cooled and thus is expelled after completely crossing the main heat exchanger 40. In this particular operation of the prior art, there is a temperature difference at the cold end of the main heat exchanger of about 6.2K. With reference to Figure 3, the temperature profile within the main heat exchanger 40 is shown according to the present invention. The expulsion of the flow of compressed and purified air 20 and a warmer temperature and in this way, the production of the main feed air flow 42 at a warmer temperature, results in a more pronounced cooling profile because all this remains within of the main heat exchanger 40 to be cooled is the second flow of compressed and purified air which results in the production of liquid air flow 24 at a subcooled temperature. As a result, less vaporization occurs due to the expansion of the liquid air flow 44 within the expander 54 and the first subsidiary liquid flow 56 after the passage through the valve 60 and the second subsidiary liquid flow 58 has a liquid content higher after introduction into the higher pressure column 48. The main feed air flow 42 is hotter entering the high pressure column. This results in a greater liquid vapor traffic and thus in an increase in the production of nitrogen-rich vapor in the upper part of the pressure column 48. The higher liquid content of the first subsidiary air flow 56 produces a liquid increased at vapor ratio below the point of introduction in a lower pressure column 50. Additionally, the higher production of nitrogen-rich vapor in the upper part of the higher pressure column 48 results in more liquid being produced in the column of lower pressure 50 as reflux by virtue of increased production of the second part 74 of the flow rich in liquid nitrogen 70. In the present invention, since the flow of crude liquid oxygen 76 can also be subcooled, a larger liquid fraction of this flow after expansion can also be introduced into the lower pressure column 50. The resulting total liquid higher in vapor ratio within the column of lower pressure 50 results in more argon being present within the argon-rich flow 86 and in this way, a greater argon recovery in relation. It should be noted that it will increase oxygen recovery, albeit to a lesser extent. "However, since, typically, oxygen is being supplied to customers under supply contracts, the plant can be operated to meet the needs commercial applications by decreasing the degree of primary air compression to also lower the total power requirements of a method conducted in accordance with the present invention while taking advantage of the possible increased argon recovery in the inventive method demonstrated in this invention. However, as the main feed air flow 20 becomes hotter, the temperature of the liquid air flow 42 becomes progressively lower.To prevent the heating and cooling curves within the main heat exchanger 40 of the junction, more air would have to be compressed inside the impeller compressor 24 and in this way increase the power requirements of the plant. Increasing the flow of the 30 is another way to compensate for the lower temperature difference at the cold end of the heat exchanger 40. That tends to increase the total feed and decrease the recovery of argon. The inventors here found that the expulsion of air flow from main feed 40 at a specified, predefined temperature, allows the temperature of the liquid air flow that is controlled to reach the temperatures of the return flows, specifically, the flow of the top of nitrogen column 114, the waste stream 116 and the flow of pumped liquid oxygen 124. This control allows an increase in argon recovery without unnecessary increases in power requirements for air compression. In a fin heat exchanger, the main feed air flow 42 must be expelled from the main heat exchanger 40 at a temperature such that the flow of liquid air 44 has a temperature that reaches an average temperature of the return flows not less than a range between 0.2K and approximately 3K, and preferably between 0.4K and 2K. Under this temperature range, the feed 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 these return flows at their current temperature at the cold end of the main heat exchanger 40. In the illustrated embodiment, the return flows at the cold end of the main heat exchanger 40 are the first part 124 of the pumped liquid oxygen stream 122, and the flow of an upper part of the nitrogen column. 114 and the flow of waste 116 at the hot end of the subcooling unit 78. It should be noted that if any additional flow is expelled from the column system and then fed to the main heat exchanger 40, then these flows will be counted in this average temperature calculation . As will be known, the control of this temperature of the main feed air flow 44 is effected by design of the main heat exchanger 40 and more specifically, the location of an outlet thereof for discharging the main feed air flow 42. With Referring to Figure 4, in an alternative embodiment of the air separation plant shown in Figure 2, the main heat exchanger 40 and the subcooling unit 28 can be combined into a single unit 40 '. The air separation plant illustrated in Figure 4 functions in the manner established for the apparatus of Figure 1. With reference to Figure 5, an alternative embodiment of the air separation plant shown in Figure 1 is illustrated. A steam stream enriched with nitrogen 130 can be extracted from a nitrogen-rich steam stream 66 and a remaining portion 67 of a nitrogen-rich steam stream 66 can be introduced into a condenser reheater 68. The steam flow enriched with nitrogen 130 is introduced into the main heat exchanger 40"in which it is partially heated and then introduced into a turbo expander 132 coupled to a generator 134. The resultant cooled exit flow 136 is introduced into the main heat exchanger 40"which is supplied by a passage to fully heat this flow and thereby cool the process.Another method than the alternative generation method for cooling, the plant illustrated in Figure 5 is otherwise identified to that shown in Figure 1. With reference to Figure 6, still another alternative embodiment of the air separation plant is illustrated in Figure 1 shown. In this embodiment, a fourth flow of compressed air 150 is taken from an intermediate stage of the driving compressor 24, preferably the first or second stage thereof, the resulting fourth compressed air flow 150 is then compressed within a compressor 152 to produce compressed air flow 154 which, after removal of heat of compression within a subsequent cooler 156, is introduced into a turbine 158 to produce an output flow 160 that is combined with the first flow of compressed air 20 at an intermediate location and the temperature level of the main heat exchanger 40"has an input provided for this purpose. This results in an ability to produce more liquid than the plant shown in Figure 1. Another modification other than that described in this paragraph, the remainder of the plant would be otherwise identical to the air separation plant shown in Figure 1 .

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

TABLE 1 No. Ref. Flow Flow Temperature Pressure, Composition Percentage, K psia steam 18 4948 282.0 88.0 air 100 20 2815 282.0 88.0 air 100 28 (after 1453 305.4 1100 air 100 cooling in rear cooler 26) 42 2815 108.9 84.0 air 100 44 1453 97.9 1099 air 0 58 436 96.2 83.7 air 0 56 (after valve 1017 82.0 20.1 air 14.8 60) 36 (after discharge of heat exchanger 679 144.9 136.8 air 100 main 40) 64 679 89.2 20.2 air 100 82 34.0 81.9 17.8 99.9998% 0 N2 + Ar 98 36.1 89.1 450 99.9998 Ar 0 126 34.0 96.3 450 99.6% 02 0 124 (after 1000 291.0 446 99.6% 02 100 vaporization within the main heat exchanger 40) 116 (after having 815 291.0 17.2 98.6N2 100 been fully heated in the main heat exchanger 40). 114 (after having 3029 291.0 16.9 99.9999% 100 been fully N2 + Ar heated inside the main heat exchanger 40) TABLE 2 By way of comparison, the argon recovery of the present invention, as shown in Table 1, is 78.1%. The recovery of argon for a prior art method, shown in Table 2, is 74.1%. Likewise, the Oxygen recovery from Table 1 is 99.3%, the oxygen recovery of Table 2 is 98.9%. The lower degree of drying of flows 56 and 58 as they enter the lower and higher low pressure distillation columns 48 and 60, for the present invention as shown in Table 1 (vapor in percent) and temperature Hotter of the main feed air flow 42, leads to improved product flow recoveries. The reduced drying is a result of the lower temperature of the liquid air flow 44 in the present invention. In Table 1, the flow of the second flow of compressed and purified air 28 is required to be 1.9% higher than in the prior art. As a result, the power consumption for the present invention is a little higher than in the prior art. While the invention has been described with reference to the preferred embodiment, as would occur to those skilled in the art, various changes, additions and omissions can be made without departing from the spirit and scope of the present invention as mentioned in the appended claims.

Claims

1. An air separation method comprises: producing a first flow of compressed and purified air and a second flow of compressed and purified air having a higher pressure than the first flow of compressed and purified air; cooling the first flow of compressed and purified air and the second flow of compressed and purified air in a main heat exchanger, through an indirect heat exchange with return flows produced in an air separation unit including at least one part of a flow of liquid oxygen pumped, and in this way produce a flow of main feed air and a flow of liquid air; introducing the main feed air flow into a higher pressure column of an air separation unit, expanding the liquid air flow and introducing at least a part of the liquid air flow into a lower pressure column of the unit of air separation; introduce an argon-rich flow from the lowest pressure column in an argon separation zone in at least one column to produce an upper section of argon-containing column and a product flow containing argon composed of a column top containing argon; Sub-cool a flow of compound liquid liquid oxygen of the lower parts of the liquid column of the highest pressure column and a liquid flow rich in nitrogen composed of a top part of liquid nitrogen column of the highest pressure column and introducing at least a part of the flow of liquid oxygen crude and at least a part of the liquid flow rich in nitrogen in the lower pressure column; and the main feed air flow being extracted from the heat exchanger at a temperature warmer than the liquid air flow and introduced into the higher pressure column at least at this temperature, and thereby sub-cool the liquid air flow and increasing the liquid content thereof after having expanded to improve the liquid at vapor ratio in the lower pressure column and thus increase the argon recovery, the temperature being selected so that the liquid air flow has a temperature that reaches the average temperature of the return flows of not less than a range between 0.2 K and about 3K, the average temperature being a calculated temperature e which a product of flow and enthalpy of the return flows at a cold end of the exchanger The main heat is equal to the flow product and the enthalpy of the return flows at their current temperatures.

2. The method according to claim 1, wherein the range is between 0.4K and approximately 2K.

3. The method according to claim 1, wherein the temperature of the main feed air flow is in a range of between 6K and up to about 25K hotter than the liquid air flow. The method according to claim 1, wherein the temperature of the main feed air flow is in a range of between 8K and approximately 15K hotter than the liquid air flow. 5. The method according to claim 4, wherein the range is between 0.4K and approximately 2K. The method according to claim 1, wherein: the liquid air flow is expanded to an appropriate pressure for its introduction at an intermediate location of the highest pressure column; the liquid air flow is divided into a first subsidiary liquid flow and a second subsidiary liquid flow; the first subsidiary liquid flow is introduced into the highest pressure column; and the second subsidiary liquid flow is expanded and introduced into the lower pressure column below the discharge point of the argon-rich flow to the argon column. The method according to claim 1, wherein: a third flow of compressed and purified air is produced; the third flow of compressed and purified air is partially cooled inside the main heat exchanger and introduced in a turbo expander to produce an output flow for the generation of cooling; and the outflow is introduced into the lower pressure column. The method according to claim 5, wherein: a fourth flow of compressed and purified air is produced by extracting the fourth flow of compressed and purified air from an intermediate stage of a compressor used to form the second compressed and purified flow; and the fourth flow of compressed and purified air is expanded into another turbo expander and combined with the first flow of compressed and purified air into the main heat exchanger to increase the production of liquid. The method according to claim 1, wherein the top of the nitrogen column composed of the top of the nitrogen column is partially heated inside the main heat exchanger, expanded within the turbo expander to produce an output flow for the generation of cooling and the output flow is introduced into the main heat exchanger and fully heated inside it. 10. The method according to claim 1, or 5, or 6, or 7, or 8, or 9, wherein the liquid air flow is introduced into a liquid turbine to expand the flow of liquid air to an appropriate pressure for its introduction in an intermediate location of the highest pressure column. The method according to claim 1, wherein the crude liquid oxygen and the liquid flow rich in nitrogen are subcooled through the indirect heat exchanger with the return flows that are formed from a steam flow rich in compound nitrogen of a lower pressure column and a waste vapor stream enriched in nitrogen for a smaller size than the nitrogen rich vapor flow, the nitrogen rich vapor flow and the waste vapor stream being introduced into the exchanger of Main heat after having been sub-cooled the flow of crude liquid oxygen and the liquid flow rich in nitrogen. The method according to claim 1, wherein: a first part of the crude liquid oxygen stream is expanded and introduced into a lower pressure column and a second part of the raw liquid oxygen stream indirectly exchanges heat with a part upper of argon column, and in this way condensing the upper part of argon column and partially vaporizing the second part of the flow of crude liquid oxygen; the liquid and vapor fraction flows result from the partial vaporization of the flow of crude liquid oxygen introduced in the lowest pressure column; and part of the flow of the upper part of the argon column after it has been condensed as an argon product flow and the remaining part after the condensation is returned to the separation zone as reflux.