MXPA97008225A - A cryogenic cycle of three columns for the production of impure oxygen and nitrogen p - Google Patents

Abstract

The present invention relates to a method for operating a cryogenic distillation column having a higher pressure stage, a lower pressure stage and a medium pressure stage, to produce at least one nitrogen and an impure oxygen, the method. The method in which the step of condensing the higher stream of higher pressure nitrogen against a liquid of the lower pressure stage includes introducing a higher stream of higher pressure nitrogen to a boiler / intermediate condenser of the stage of Lower pressure for the method is further characterized because it comprises: condensing a second fraction of the first feed air stream in a bottom boiler / condenser of the lower pressure stage to form liquefied feed air, and introducing at least one portion of the liquefied feed air to at least one of the highest pressure stage, the medium pressure stage and the pressure stage plus

Description

A CRYOGENIC CYCLE OF THREE COLUMNS FOR THE PRODUCTION OF IMPURE OXYGEN AND PURE NITROGEN DESCRIPTION OF THE INVENTION The present invention relates to the production of substantially pure nitrogen and impure oxygen in a cryogenic air separation system. The substantially pure nitrogen (mainly, with a nitrogen purity of at least 99.9 mol%) and impure oxygen (mainly purity less than about 98 mol%) are increasingly used in the industry. For example, nitrogen and impure oxygen are used in petrochemical plants, gas turbines for power generation, glass production, and in the pulp and paper industry. Under certain circumstances, only impure oxygen is required as a product of a cryogenic distillation plant and nitrogen is discarded as waste. In other cases, such as with nitrogen generators, impure oxygen constitutes a waste stream and nitrogen is the desired product. Generally, in a cryogenic distillation plant, the production of impure oxygen can be combined with the production of pure nitrogen. Numerous processes are known for the production of impure oxygen and / or nitrogen.

For example, U.S. Patent No. 3,210,951 discloses a dual boiler process wherein a fraction of the air fed is condensed in a boiler / condenser providing reheating to the bottom section of the low pressure column. The upper vapor of the high pressure column is condensed in a second boiler / condenser vaporizing an intermediate liquid stream, which is then supplied to the low pressure column. In comparison with the single-column, double-column, individual kettle cycle, this double-boiler arrangement reduces the irreversible capacity of the distillation process in the low-pressure column and consequently reduces the pressure of the fed air, thus saving energy. U.S. Patent 4,702,757 discloses a dual boiler process wherein a portion of the air fed is only partially condensed, further reducing the pressure of the air fed. U.S. Patent No. 4,453,957 discloses a cryogenic rectification process for the production of nitrogen at a relatively high purity and at a relatively high pressure in a classical double column arrangement with an additional boiler / condenser at the top of the column of low pressure. A waste stream of impure oxygen is vaporized at the top of the kettle / condenser to provide the necessary backflow for the low pressure column. U.S. Patent No. 4,617,036 describes another cryogenic rectification process for recovering nitrogen in large quantities and at a relatively high pressure. In this system, an additional side boiler / condenser is used to condense the high pressure nitrogen gas against the waste oxygen at a reduced pressure. In U.S. Patent No. 5,069,699, a three-column nitrogen generator is disclosed. Specifically, the system includes a classical, two-stage, double-boiler / condenser distillation column, and an additional, discrete third stage having a higher pressure than the high-pressure stage pressure of the two-stage column. In this system, the kettle / bottom condenser in the low pressure stage is used to condense the nitrogen, and it is fed from the unpurified oxygen to the low pressure stage as a liquid. A conventional double-column, double-boiler cycle, which has been used to produce these gases, is shown in Figure 1. The inclusion of a second boiler / condenser in the low-pressure column serves to reduce the specific energy of the cycle of double column. The cycle shown in Figure 1 is considered one of the most efficient cycles for the production of impure oxygen. Nevertheless, the analysis of composition profiles in the low pressure column for this system demonstrates a significant region of procedure irreversibility. This region is graphically represented by the area between the line of operation "0" and the balance line "E" shown in Figure 2. In the strongly competitive market, there is a demand to reduce this irreversibility and the energy required by this cycle , even more . The present invention is directed to a method for operating a cryogenic distillation column having a higher pressure stage, a lower pressure stage, and a medium pressure stage to produce at least one nitrogen and impure oxygen. Preferably, the cycle includes a double-stage column that includes the highest pressure stage and the lowest pressure stage, together with a third discrete column, which is the medium pressure stage having a pressure between the pressures of the highest pressure stage and the lowest pressure stage. The present invention reduces the separation irreversibilities in the lower pressure stage by supplying unpurified oxygen as a vapor to the lower pressure stage. In addition, a portion of the feed air is introduced directly into the medium pressure stage, which results in energy savings as compared to cycles that require a full feed air stream to be pressurized to the highest pressure in the air. Higher pressure stage. According to the present invention, a source of air fed is used to provide, (a) a first stream of air fed, and (b) a second stream of air fed having a pressure less than the pressure of the first stream of air. air fed. The second feed air stream is introduced into the medium pressure stage for rectification in an oxygen enriched liquid, medium pressure and an upper medium pressure nitrogen stream. A first fraction of the first feed air stream is introduced into the higher pressure stage for rectification in an oxygen enriched, higher pressure, and higher pressure nitrogen higher stream. The higher pressure higher nitrogen stream is condensed against a liquid from the lower pressure stage to form a higher pressure nitrogen condensate, a portion of which is returned to the higher pressure stage as reflux. The oxygen-enriched, medium-pressure liquid, and the higher-pressure oxygen enriched liquid (or portions thereof), are reduced in pressure to form a liquid enriched with reduced pressure oxygen.; which is used to condense the upper stream of medium pressure nitrogen, thus forming a steam stream enriched with oxygen and a medium pressure nitrogen condensate. The steam stream enriched with oxygen is introduced to the lower pressure stage as a feed. A portion of the medium pressure nitrogen condensate is returned to the middle pressure stage as reflux. The remaining portions of at least one higher pressure nitrogen condensate and the medium pressure nitrogen condensate are introduced to the lower pressure stage as reflux for the lower pressure stage. Two streams of product are removed: (1) an oxygen enriched product from a position near the bottom of the lowest pressure stage; Y (2) a product enriched with nitrogen from a position closer to the top of the lower pressure stage. It should be understood that both the foregoing general description and the following detailed description are illustrative, but are not restrictive of the invention. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood from the following detailed description when read in conjunction with the accompanying drawings, in which: Figure 1 is a schematic diagram of a conventional dual-column double-boiler cycle. Figure 2 is a McCabe-Thiele diagram showing the equilibrium curve and the operating curve of the system corresponding to Figure 1. Figure 3 is a schematic diagram of a first embodiment of the present invention. Figure 4 is a McCabe-Thiele diagram showing the equilibrium curve and the operating curve of the system corresponding to Figure 3. Figure 5 is a schematic diagram of a second embodiment of the present invention. Figure 6 is a schematic diagram of a third embodiment of the present invention. Figure 7 is a schematic diagram of a fourth embodiment of the present invention. Figure 8 is a schematic diagram of a fifth embodiment of the present invention. Figure 9 is a schematic diagram of a sixth embodiment of the present invention. Figure 10 is a schematic diagram of a seventh embodiment of the present invention. In general, the present invention requests that feed air be introduced to at least one compressor, at least one heat exchanger, and at least one expander to provide, (a) a medium pressure supply air stream, and (b) a higher pressure feed air stream. In the preferred embodiment of the present invention shown in Figure 3, which is an impure, double-boiler, three-column oxygen cycle, a feed air stream in line 10 is compressed in compressor 12, cooled in the heat exchanger 14, cleaned of water and carbon dioxide, preferably in the molecular sieve adsorption unit 16, and divided into two streams: the average pressure fed air stream in line 18 and the current in the line 30. The average pressure fed air stream in line 18 is cooled in a main heat exchanger 20 to a cryogenic temperature and introduced as a feed in line 22 to the middle pressure stage 24. There, the air stream fed of medium pressure (together with another feed discussed below) is rectified in an oxygen-enriched, medium-pressure liquid (withdrawn as a bottom product via line 110) and an upper nitrogen stream of medium pressure (withdrawn as an upper vapor in line 105). The compressed feed air stream in line 30 is further compressed in compressor 32, cooled in heat exchanger 34 against an external cooling fluid, and divided into two: in-line currents 36 and 70. The current in line 36 is cooled in the main heat exchanger 20 near its dew point and divided between two streams: a first fraction of the highest pressure feed air stream in line 38 and a second fraction of the flow of higher pressure feed air in line 40. The first fraction of the highest pressure feed air stream in line 38 is introduced as a feed in the highest pressure stage 60 for rectification (together with another feed discussed below), in a liquid enriched with oxygen, of higher pressure (withdrawn as a bottom product via line 100) and an upper stream of nit Higher pressure structure. The second fraction of the highest pressure feed air stream in line 40 is condensed in the bottom boiler / condenser 42, located at the bottom of the lower pressure stage 62, thereby forming liquefied feed air in line 46 and providing a part of the kettle needed for separation in the lower pressure stage 62. The liquefied feed air in the line 46 can be divided into three streams: a first portion on line 48, a second portion on line 50 and a third portion on line 52, which form liquid air feeds to the highest pressure stage 60, stage of mean pressure 24 and lower pressure stage 62, respectively. Alternatively, the liquefied feed air in line 46 can be directed only to one of the higher pressure stages 60, medium pressure stage 24, or, preferably, lower pressure stage 62, or any combination of any two of this. The operating pressures of the three stages can vary over wide ranges, such as 1.26-12.6 kg / cm2 (18-180 psia) for the lower pressure stage 62, 2.4-17.5 kg / cm2 (35-250 psia) for the middle pressure stage 24, and 3.8-24.6 kg / cm2 (55-350 psia) for the highest pressure stage 60. The portion of the additional compressed feed air stream in line 70 is compressed, then cooled and extended and introduced as a lower pressure supply air stream to the lower pressure stage 62. Specifically, the current in line 70 is compressed in the compressor 72, cooled in the heat exchanger 74 against a fluid in cooling external, cooled in the main heat exchanger 20 and expanded in the turboexpander 76. Then, the current is introduced via the line 78 to the lower pressure stage 62 as a lower pressure feed air stream.

As mentioned above, the first fraction of the highest pressure feed air stream in line 38 and the first portion of liquefied air fed in line 48, are introduced to the highest pressure stage 60, where they are rectified in the oxygen enriched liquid, of higher pressure withdrawn in line 100 and a higher stream of higher pressure nitrogen withdrawn in line 80. The higher pressure nitrogen higher stream in line 80 is condensed against a liquid from the lower pressure stage 62 to form the highest pressure nitrogen condensate in line 84, a portion of which is returned to the highest pressure stage 60 in line 86 as reflux. Specifically, the higher pressure higher nitrogen stream is condensed in an intermediate kettle / condenser 82 located in the lower pressure stage 62 above the bottom condenser / boiler 42. As an alternative to use a boiler / intermediate condenser in the lower pressure stage 62, a separation device, arranged close to, and connected to the lower pressure stage 62 through appropriate steam and liquid lines can be used. The remaining portion of the higher pressure nitrogen condensate is removed via line 88, subcooled in a heat exchanger 90, reduced in pressure through an isenthalpic Joule-Thompson valve 89 and vaporized in a separator 92. The reflux of The resulting low pressure nitrogen is introduced via line 94 near the top of the lower pressure stage 62. As mentioned above, the average pressure supply air stream in line 22 and the second air portion of the liquefied feed in line 50, are introduced to the middle pressure stage 24, so they are rectified in an oxygen enriched, medium pressure liquid, (withdrawn via line 110 as a bottom product) and an upper stream of medium pressure nitrogen, which is condensed in the upper kettle / condenser 106, via line 105. A portion of the medium pressure nitrogen condensate provides reflux for the medium pressure stage 24, and the remaining portion on the line 112 is subcooled in the heat exchanger 90 and reduced in pressure through a Joule-Thompson 91 isentical valve. The stream is then vaporized in the separator 92 for providing an additional reflux to the lower pressure stage 62 via line 94. In all embodiments of the present invention, at least a portion of at least one oxygen enriched liquid, medium pressure and oxygen enriched liquid. of higher pressure is reduced in pressure to form a first liquid enriched with oxygen of reduced pressure, and the first liquid enriched with oxygen, of reduced pressure is used as the cooling medium to condense the upper nitrogen stream of medium pressure in the boiler / upper condenser 106 of the medium pressure stage 24. In the embodiment shown in Figure 3, a liquid is first undercooled. Oxygen-enriched oxygen, of higher pressure in line 100 in heat exchanger 103, is reduced in pressure through a Joule-Thompson 101 isentálpica valve to form a second liquid enriched with reduced pressure oxygen, then combined with the liquid enriched with oxygen of medium pressure from the line 110 coming from the bottom of the middle pressure stage 24 to form a liquid enriched with combined oxygen, and either dividing into two streams in the lines 102 and 104 or directing completely to line 104. The stream in line 104 is reduced in pressure through a Joule-Thompson 107 isentical valve, and then vaporized in an upper kettle / condenser 106, serving as a first enriched, reduced-pressure liquid in the line 104. The cooling provided by the current in line 104 provides the necessary backflow for the middle pressure stage 24. The steam current resulting in line 108 is introduced in the lower pressure stage 62, like a stream of steam enriched with oxygen. The current in line 102 is optional, and for some operating conditions it is not necessary (that is, the flow in line 102 can be zero). When there is a flow in line 102, the current in line 102 is reduced in pressure through a Jentle-Thompson 109 isentical valve and introduced into the lower pressure stage 62. The introduction of the oxygen enriched stream in the line 108 as a vapor, not as a liquid, to the lower pressure stage 62, greatly reduces the irreversibility in the lower pressure stage 62. The corresponding McCabe-Thiele diagram for a system of Figure 3 is shown in Figure 4. When this diagram is compared to Figure 2, it can be seen that the graphical representation of the procedural irreversibilities, particularly the area between the "O" operating line and the equilibrium line "E" is reduced in the Figure 4. In all embodiments of the present invention, two streams are removed: (1) an oxygen enriched product from a position near the bottom of the lowest pressure stage; and a product enriched with nitrogen from a position near the top of the lowest pressure stage. Any product can be withdrawn as a liquid or a gas depending on the particular needs, although the nitrogen is preferably removed as a gas. In the embodiment shown in Figure 3, the gaseous nitrogen product in line 116 is removed from the upper part of the lower pressure portion 62 in line 114, combined with any vaporization gases in separator 92, and heated: (1) in a heat exchanger 90 against the highest pressure nitrogen condensate in line 88 and the medium pressure nitrogen condensate in line 112, (2) a heat exchanger 103 against an oxygen enriched liquid, of highest pressure in line 100, and (3) a main heat exchanger 20 against the medium pressure supply air stream in line 22 and the highest pressure supply air stream in line 36 and the compressor stream 72 and heat exchanger 74. Also, in the embodiment shown in Figure 3, oxygen product 120 is recovered as a bottom vapor from lower pressure stage 62 on line 118, and it is heated in the main heat exchanger 20 against the medium pressure supply air stream in line 22 and the highest pressure supply air stream in line 36, and the compressor 72 current and the heat exchanger 74. Returning to the other embodiments of the present invention shown in Figures 5-10, wherein the like reference numbers refer to the same elements as discussed above in relation to Figure 3, the modalities shown in Figure 5. and in Figure 6 are directed to the use of the middle pressure stage with a nitrogen generator. Such nitrogen plants also produce impure oxygen as a waste. A region of significant irreversibility in the separation section of the lowest pressure stage exists when unpurified oxygen is supplied to the low pressure column as a liquid feed. The irreversibilities are greatly reduced through the introduction of a third column of medium pressure, which allows oxygen to be supplied without purifying the low pressure column in the form of vapor instead of liquid, as discussed above in relation to Figure 3. The mode shown in Figure 5 differs from that of Figure 3 in that there is no intermediate boiler / condenser, but rather there is an upper boiler / condenser 130 of the lower pressure stage 62. Also, in the embodiment shown in Figure 5, all the additional compressed feed air stream in line 36 is directed via line 38 to the highest pressure stage 60. In this embodiment, the condensation stage of the upper stream of higher pressure nitrogen in line 80 against a liquid in the lower pressure stage 62, includes introducing the higher pressure nitrogen stream higher in line 80 to a bottom condenser / kettle 42 of the lower pressure stage 62. In this embodiment, the oxygen enriched stream is withdrawn as a liquid via line 132 from a position close to the bottom of the stage of lower pressure 62 and introduced to the upper boiler / condenser 130 of the lower pressure stage 62 to provide an additional reflow to the lower pressure stage 62 and to vaporize the oxygen enriched stream, which can be classified as a product for some uses, but typically it is a waste stream in this mode. This oxygen enriched stream is heated in the heat exchangers 90 and 103, as well as in the main heat exchanger 20. The mode shown in Figure 6 differs from that in Figure 3 in that there is no intermediate boiler / condenser, but rather there is a boiler / side condenser 134 of the lower pressure stage 62. Also, as in the embodiment shown in Figure 5, all the additional compressed feed air stream in the line 36 is directed via the line 38 to the highest pressure stage 60. In the embodiment shown in Figure 6, the step of condensing the higher pressure nitrogen higher stream includes the steps of introducing a first higher pressure higher pressure nitrogen stream. to the boiler / bottom condenser 42 of the lower pressure stage 62 and to introduce a second portion of the higher pressure nitrogen higher stream to the side boiler / condenser 134 of the lower pressure stage 62. The boiler / side condenser 134 may be contained within the column of the lower pressure stage 62 or located close to it. In addition, the step of removing an oxygen enriched product from a position near the bottom of the lower pressure stage 62 includes first removing an oxygen enriched product as a liquid from a position near the bottom of the pressure stage further low 62 via line 136. This stream is reduced in pressure through a Joule-Thompson 137 isentical valve to form an oxygen-enriched, reduced pressure product, which is supplied to side boiler 134 and used to condense the second portion of the higher current of higher pressure nitrogen. Another embodiment of the present invention is shown in Figure 7. This cycle differs from the cycle presented in Figure 3 in the manner in which the oxygen enriched liquid, of the highest pressure in line 100, is used. Specifically, the higher-pressure oxygen enriched liquid stream in line 100 is reduced in the pressure transverse valve 101 and introduced in the bottom of the medium pressure stage 24 where it is vaporized, thus providing an extra boil for the middle pressure stage 24 and a reflux of additional nitrogen to the lower pressure stage. The oxygen enriched liquid, of medium pressure in the line 110 is cooled in the heat exchanger 103, reduced in pressure in a Joule-Thompson 107 isentical valve in the line 104, then introduced in the upper kettle / condenser 106 of the stage of medium pressure 24. A portion of liquid enriched with oxygen, of medium pressure can be supplied to the lower pressure stage 62 via line 102. The modality shown in Figure 8 differs from the embodiment of Figure 3 in that the Full feed air stream is compressed at a higher pressure to form the highest pressure feed air stream in line 30, then a portion in the higher pressure feed air stream in line 70 is expanded in an expander 76 to form the medium pressure supply air stream in line 22, opposite to that supplied to the lower pressure stage 62. The modality ad shown in Figure 9 differs from the embodiment of Figure 3, in that a small section of stages or packages 150 is added above the upper kettle / condenser 106 of the medium pressure stage 24. With the inclusion of additional stages or 150 packs, the liquid enriched with reduced pressure oxygen is partially separated as it is vaporizing. Specifically, it is separated into two portions: (1) a first portion having a first concentration, which is removed on line 152; and (2) a second portion having a second concentration, less pure in oxygen than the first concentration, which is removed in line 108. The currents in lines 152 and 108 are introduced to the lower pressure stage 62 in different Places. Specifically, the current in line 108 is introduced above the point at which the current in line 152 is introduced to the lower pressure stage 62. This mode also reduces the separation irreversibilities in the lower pressure stage, giving as a result additional energy savings. The embodiment shown in Figure 10 differs from the cycle of Figure 3 in the manner in which the oxygen product is removed. Specifically, the embodiment shown in Figure 10 is desirable if the oxygen product required at a high pressure without the need to include an expensive oxygen compressor in the system. In this embodiment, the oxygen enriched product is withdrawn as a liquid from the bottom of the lower pressure stage 62 via the line 300. This current can be pumped through the pump 310 to the desired higher pressure. Alternatively, pump 310 may not be necessary if a lower oxygen pressure is desired; specifically, several kilograms- (several pounds) of oxygen product pressure can be obtained due to the higher static gain caused by the difference in height between the point at which the liquid oxygen is removed from the lower pressure stage 62 and the where it boils. The product enriched with pressurized oxygen in line 320 is then introduced to a heat exchanger 250, where it is vaporized and heated, leaving as stream in line 330. The current in line 330 is further heated in the heat exchanger 20. The medium directed to the heat exchanger 250, which is used to heat the product enriched with pressurized oxygen from line 320, is a higher pressure feed air stream in line 240. The current in the line 240 is obtained by removing a portion of current in line 70 via line 200, bursting this ratio at a sufficient pressure in auxiliary compressor 210, and cooling the current in a heat exchanger 220 to form the current in line 230, the which is further cooled in the main heat exchanger 20. The current in line 240 is condensed in the heat exchanger 250 to form r liquefied feed air 260, which is coupled with the liquid air stream 48 to form a liquefied air stream 49, which is subsequently supplied in the highest pressure stage 60. Optionally, the liquid air stream 260 it can also be introduced to the currents in lines 46, 50, or 52. Finally, the separate heat exchanger 250 may be unnecessary, since oxygen may boil in the heat exchanger 20 under certain conditions. EXAMPLES In order to demonstrate the efficacy of the present invention, the following example was developed. In Table 1, below, the current parameters for the modality shown in Figure 3 are listed. In Table 2, molar fractions of various streams are provided. The basis of the simulations was to produce a gaseous oxygen at a purity of 95% at an atmospheric pressure of 100 lbmol / h of air at atmospheric conditions. In the simulations, the number of theoretical tests in the highest pressure stage 60 was 25, the number of theoretical tests in the mean pressure stage 24 was 20 and the number of theoretical tests in the lower pressure stage 62 It was 35.

Table 1 Table 2 In another example, the selected flow rates and pressures in the three-column double-boiler cycle (shown in Figure 3) and in the conventional double-boiler cycle (as shown in Figure 1) both produced oxygen at the 95%, and they were compared. This comparison is shown in Table 3 below. Using the cycle shown in Figure 3, energy savings were presented. Specifically, since a significant portion of the feed is separated in the medium pressure column in the cycle of Figure 3, a smaller amount of feed needs to be compressed at the high pressure column pressure. In this example, the power of the three-column cycle (from Figure 3) is 4% lower than the energy of the conventional double-boiler cycle (from Figure 1). Table 3 Current or Present Unit Number Cycle Invention Kettle Apparatus Fig. 3 Double Fig. 1 Food 10 mol / s 100 100 Oxygen Product 120 mol / s 21.7 21.7 Nitrogen Product 116 mol / s 78.2 78.2 Compressor Flow 10 mol / s 100 100 Compressor Discharge Pressure 12 KPa 331.3 442.7 Compressor Flow 30 mol / s 70.4 Compressor Discharge Pressure 32 KPa 435.6 Although illustrated and described herein with reference to certain specific embodiments, the present invention, however, is not intended to be limited to the details shown. Rather, various modifications can be made to the details within the scope and ranges of equivalents of the claims and without departing from the spirit of the invention.

Claims (15)

1. A method for operating a cryogenic distillation column having a higher pressure stage, a lower pressure stage and a medium pressure stage, to produce at least one nitrogen and an impure oxygen, the method is characterized in that it comprises the steps of: providing a source of supply air (a) with a first supply air stream having a first pressure, and (b) a second supply air stream having a second pressure lower than the first pressure; introducing the second supply air stream in the medium pressure stage for the rectification in an oxygen enriched medium pressure medium and an upper medium pressure nitrogen stream; introducing a first fraction of the first feed air stream in the higher pressure stage for the rectification in a liquid enriched with higher pressure oxygen, and a higher stream of higher pressure nitrogen; Condense the upper stream of higher pressure nitrogen against a liquid from the lower pressure stage to form a pressure nitrogen condensate plus 2T high and return to a portion of the higher pressure nitrogen condensate to the higher pressure stage as reflux; reducing the pressure of at least a portion of at least one of an oxygen-enriched, medium-pressure liquid and a higher-pressure oxygen-enriched liquid to form a first low-oxygen enriched liquid; condensing the upper pressure medium nitrogen stream against the first low pressure oxygen enriched liquid, resulting in a steam stream enriched with oxygen and a medium pressure nitrogen condensate, and returning a portion of the medium pressure nitrogen condensate to the middle pressure stage as reflux; introducing the remaining portion of at least one of the higher pressure nitrogen condensate and the medium pressure nitrogen condensate to the lower pressure stage as reflux; introduce the steam stream enriched with oxygen to the lower pressure stage as feed; removing an oxygen enriched product from a position near the bottom of the lowest pressure stage; and removing a product enriched with nitrogen from a position near the top of the lowest pressure stage.

2. The method in accordance with the claim 1, wherein the step of condensing the higher pressure nitrogen stream higher against a liquid of the lower pressure stage includes introducing a higher pressure nitrogen higher stream to a boiler / intermediate condenser of the pressure stage. lower pressure, the method is further characterized by comprising: condensing a second fraction of the first feed air stream in a bottom boiler / condenser of the lower pressure stage to form liquefied feed air; and introducing at least a portion of the liquefied feed air to at least one of the higher pressure stage, the medium pressure stage, and the lower pressure stage.

3. The method of compliance with the claim 2, further characterized in that it comprises: cooling and expanding a third fraction of the first supply air stream to form a third supply air stream having a third pressure lower than the second pressure; and introduce the third feed air stream to the lowest pressure stage. The method according to claim 1, further characterized in that it comprises: heating the oxygen enriched product against the first supply air stream and the second supply air stream in a first heat exchanger; heating the product enriched with nitrogen against: (a) the first feed air stream and the second feed air stream in the first heat exchanger; (b) the highest pressure nitrogen condensate and the medium pressure nitrogen condensate in a second heat exchanger; and (c) a liquid enriched with oxygen of higher pressure in a third heat exchanger. 5. The method of compliance with the claim 1, characterized in that: the step of reducing the pressure of at least a portion of at least one of a liquid enriched with oxygen of medium pressure, and the liquid enriched with oxygen of higher pressure, comprises: (a) reducing first the pressure of the liquid enriched with oxygen of higher pressure to form a second liquid enriched with oxygen of reduced pressure; (b) combining the second low pressure oxygen enriched liquid with the medium pressure oxygen enriched liquid to form a combined oxygen enriched liquid; and (c) reducing the pressure of a first portion of the liquid enriched with combined oxygen to form the first liquid enriched with oxygen of reduced pressure; the step of condensing the medium pressure higher nitrogen stream includes introducing the first reduced pressure oxygen enriched liquid to an upper boiler / condenser of the medium pressure stage to form a steam stream enriched with oxygen and to condense the upper stream of medium pressure nitrogen; the method further comprises: reducing the pressure of a second portion of the liquid enriched with combined oxygen to form a fourth liquid enriched with oxygen of reduced pressure; and introduce the fourth liquid enriched with oxygen of reduced pressure to the lower pressure stage. The method according to claim 1, characterized in that: the step of reducing the pressure of at least a portion of at least one of a liquid enriched with oxygen of medium pressure, and liquid enriched with oxygen of higher pressure , comprising: (a) first reducing the pressure of the liquid enriched with oxygen of higher pressure to form a second liquid enriched with oxygen of reduced pressure; (b) combining the second low pressure oxygen enriched liquid with the medium pressure oxygen enriched liquid to form a liquid enriched with combined oxygen; and (c) reducing the pressure of all the liquid enriched with combined oxygen to form the first liquid enriched with oxygen of reduced pressure; and the step of condensing the medium pressure nitrogen overhead stream includes introducing the first reduced pressure oxygen enriched liquid to an upper boiler / condenser of the medium pressure stage to form a vapor stream enriched with oxygen and to condense the current upper of nitrogen of medium pressure. The method according to claim 1, characterized in that: the step of condensing the higher current of higher nitrogen against a liquid of the lower pressure stage includes introducing the higher current of higher nitrogen to a lower boiler / condenser of the lowest pressure stage; and the step of removing an oxygen enriched product from a position near the bottom of the lower pressure stage, comprises removing the oxygen enriched product as a liquid and introducing the oxygen enriched product to an upper stage condenser / condenser. of lower pressure to provide an additional reflux to the lower pressure stage and vaporize the product enriched with oxygen. The method according to claim 1, characterized in that: the step of condensing the upper stream of the higher pressure nitrogen against a liquid of the lower pressure stage includes the steps of: (a) introducing a first portion of the higher pressure nitrogen higher stream to a bottom boiler / condenser of the lower pressure stage; and (b) introducing a second portion of the higher pressure nitrogen higher stream to a side boiler / condenser of the lower pressure stage; and the step of removing an oxygen enriched product from a position near the bottom of the lower pressure stage comprises the steps of: (a) removing the oxygen enriched product as a liquid; (b) reducing the pressure of the product enriched with oxygen to form an oxygen enriched, reduced pressure product; and (c) introducing the reduced pressure oxygen enriched product to the boiler / side condenser to vaporize the product enriched with reduced pressure oxygen. 9. The method of compliance with the claim 1, characterized in that: the step of reducing the pressure of at least a portion of at least one of a liquid enriched with oxygen of medium pressure and the liquid enriched with oxygen of higher pressure comprises first reducing the pressure of the liquid enriched with higher pressure oxygen to form a second liquid enriched with reduced pressure oxygen; the method further comprises introducing the second liquid enriched with reduced pressure oxygen to the medium pressure stage; the step of reducing the pressure of at least a portion of at least one of a medium pressure oxygen enriched liquid and the higher pressure oxygen enriched liquid, further comprising reducing the pressure of the medium pressure oxygen enriched liquid to form the first liquid enriched with oxygen of reduced pressure; and the step of condensing the medium pressure nitrogen top stream includes introducing at least a portion of the first reduced pressure oxygen enriched liquid to an upper boiler / condenser of the medium pressure stage to form the steam stream enriched with oxygen and to condense the upper nitrogen stream of medium pressure. The method according to claim 1, characterized in that it comprises the step of compressing and cooling of feeding comprising: first compressing the feed air at the first pressure to form the first supply air stream; and expanding a portion of the first feed air stream to form the second feed air stream. 11. The method according to the claim 1, further characterized in that it comprises partially separating the enriched liquid with reduced pressure oxygen as the liquid enriched with reduced pressure oxygen vaporizes to form a first portion of the steam stream enriched with oxygen having a first concentration and a second concentration. portion of the vapor stream enriched with oxygen, having a second concentration, and wherein the step of introducing the vapor stream enriched with oxygen to the lower pressure stage as feed comprises: introducing the first portion of the vapor stream enriched with oxygen to a first location of the lowest pressure stage; and introducing the second portion of the vapor stream enriched with oxygen to a second location of the lower pressure stage. The method according to claim 1, characterized in that: the step of removing the oxygen enriched product from a position near the bottom of the lower pressure stage comprises removing the product enriched with oxygen as a liquid; the method further comprises pressurizing the product enriched with oxygen to form a product enriched with pressurized oxygen; the step of compressing and cooling the supply air includes further compressing a second fraction of the first supply air stream to form a fourth supply air stream having a fourth pressure higher than the first pressure; and vaporizing and heating the product enriched with pressurized oxygen against the fourth supply air stream. The method according to claim 1, characterized in that the step of compressing and cooling the supply air comprises: compressing a first portion of the supply air at the first pressure to form the first feed air stream and compressing a second portion of the feed air at a second pressure to form the second supply air stream; and cooling the first supply air stream and the second supply air stream in a first heat exchanger. 1

4. The method according to the claim 1, characterized in that the step of condensing the upper stream of higher nitrogen against a liquid of the lower pressure stage, includes introducing the higher stream of higher pressure nitrogen to a lower boiler / condenser of the lower pressure stage , the method further comprises: condensing a second fraction of the first feed air stream in a bottom boiler / condenser of the lower pressure stage to form liquefied feed air; introducing a first portion of the liquefied feed air to the higher pressure stage; introducing a second portion of the liquefied feed air to the medium pressure stage; and introducing a third portion of liquefied feed air to a lower pressure stage. 1

5. A method for operating a cryogenic distillation column having a higher pressure stage, a lower pressure stage and a medium pressure stage, to produce at least one nitrogen and an impure oxygen, the method comprises the steps of: (a) compressing and cooling the supply air to provide (i) a first supply air stream having a first pressure, and (ii) a second supply air stream having a second pressure lower than the first Pressure; (b) introducing the second supply air stream to the medium pressure stage for rectification in a liquid enriched with medium pressure oxygen and a higher medium pressure nitrogen stream; (c) introducing a first fraction of the first feed air stream in the higher pressure stage for rectification in a liquid enriched with higher pressure oxygen and a higher stream of higher pressure nitrogen; (d) condensing the higher pressure nitrogen higher stream against a liquid from the lower pressure stage to form a higher pressure nitrogen condensate and returning a first portion of the higher pressure nitrogen condensate to the pressure stage higher as reflux, and introducing a second portion of the higher pressure nitrogen condensate to the lower pressure stage as reflux; (e) removing the oxygen enriched product from a position near the bottom of the lowest pressure stage; and (f) removing a nitrogen enriched product from a position near the top of the lowest pressure stage, characterized in that the method comprises: (g) reducing the pressure of at least a portion of at least one liquid enriched with oxygen, of medium pressure and a liquid enriched with oxygen of higher pressure, to form a first liquid enriched with oxygen, of reduced pressure; (h) condensing the upper medium pressure nitrogen stream against the first low pressure oxygen enriched liquid, resulting in a steam stream enriched with oxygen and a medium pressure nitrogen condensate, and returning a portion of the nitrogen condensate from medium pressure to the middle pressure stage as reflux and introducing a second portion of the medium pressure nitrogen condensate to the lower pressure stage as reflux; e (i) introducing the vapor stream enriched with oxygen to the lower pressure stage as feed.