EP2307835B1 - Nitrogen liquefier retrofit for an air separation plant - Google Patents

Abstract

A method is disclosed for increasing liquid production involving retrofitting an existing air separation plant with a nitrogen liquefier. The nitrogen liquefier liquefies a nitrogen-rich vapor stream withdrawn from the higher pressure column to return a nitrogen-rich liquid stream to the higher pressure column. This increases liquid nitrogen reflux to the higher pressure column to in turn increase the production of liquid oxygen containing column bottoms of the higher pressure column and therefore, the production of oxygen-rich liquid in the lower pressure column. The increased production of the oxygen-rich liquid allows a liquid oxygen product to be taken at an increased rate or for the liquid oxygen product to be taken in the first instance, if the plant is not designed to produce such a product. Also liquid nitrogen and argon products can be produced at an increased rate as a result of the retrofit.

Description

Field of the Invention
• The present invention relates to a method of retrofitting an existing air separation plant with a nitrogen liquefier in which nitrogen-rich vapor produced in a higher pressure column operatively associated with a lower pressure column in a heat transfer relationship is liquefied and reintroduced into the higher pressure column to increase reflux in the higher pressure column and the production of an oxygen-rich liquid column bottoms of the lower pressure column, thereby to allow or to increase liquid production of liquid oxygen products and possibly other liquid products of air separation plants.
Background of the Invention
• Air can be separated into oxygen and nitrogen products within an air separation plant in which the air is cryogenically rectified into oxygen and nitrogen-rich products and also possibly an argon product.
• In such plants, the air is compressed, purified of higher boiling impurities such as carbon dioxide, carbon monoxide and water vapor and then cooled in a main heat exchanger to a temperature suitable for the rectification of air. The air after having been cooled is introduced into a higher pressure column in which an ascending vapor phase is produced that becomes evermore rich in nitrogen. The resulting nitrogen-rich column overhead is condensed to produce a descending liquid phase that becomes evermore rich in oxygen. The liquid and vapor phases are contacted by mass transfer contacting elements that can be trays or structured"packing or possibly, random packing. In any event, the contact produces a crude-rich liquid column bottoms in the higher pressure column that is rich in oxygen.
• A stream of the crude-rich liquid column bottoms is then introduced in the lower pressure to be further refined into an oxygen-rich liquid column bottoms that collects in the lower pressure column and a nitrogen-rich column overhead that is formed in the lower pressure column. The higher pressure column and the lower pressure column are operatively associated in a heat transfer relationship by a condenser reboiler or a main condenser that is typically located within the base of the lower pressure column. The oxygen-rich liquid is in part vaporized with a stream of nitrogen-rich vapor produced from the nitrogen-rich vapor column overhead. The stream of nitrogen-rich vapor is condensed in the condenser against the vaporization of the oxygen-rich liquid to produce a nitrogen-rich liquid stream that is used in refluxing both the higher pressure column and the lower pressure column. Part of such nitrogen-rich liquid stream can be taken as a product. Oxygen and nitrogen products can be removed from the higher and lower pressure columns and pass through the main heat exchanger to help cool the incoming air.
• An argon product can also be produced by extracting an argon-rich stream from the lower pressure column and rectifying such stream in an argon column. The argon-rich product collects as tower overhead and a stream of the same can be extracted. The argon column is refluxed by condensing some of the argon-rich product with the crude liquid oxygen stream extracted from the higher pressure column. Depending upon the number of stages of separation that exist within the argon column or columns, the purity of the argon can be such that a large fraction of the oxygen is separated from the argon. However, argon products can also be produced that are further refined to remove oxygen and residual nitrogen within such an argon product.
• As well known in the art, refrigeration has to be imparted into the cryogenic rectification plant to overcome warm end heat exchanger losses as well as heat leakage through the insulation of a cold box that is used to house the column such as described above. This refrigeration can be imparted by partially cooling part of the air to be rectified within the main heat exchanger and expanding the same in a turboexpander. The work of expansion is extracted from the plant and the resultant cooled air is introduced into the bottom of a higher pressure column. Additionally, refrigeration can be imparted by an expander connected to the lower pressure column. The degree to which refrigeration is imparted to the air separation plant will determine the amount of liquid products that can be produced, typically from the oxygen-rich liquid column bottoms produced in the lower pressure column, but also possibly from the nitrogen-rich liquid stream.
• It is also known that refrigeration can be supplied to an air separation plant by nitrogen recycle liquefiers. An example of such a liquefier can be found inU.S. Patent No. 5,231,845. In this patent, a liquefier is illustrated that incorporates dual turbine-booster compressors arranged specifically to provide advantageous machinery design parameter and effective cooling curve characteristics. Medium pressure nitrogen from the higher pressure column, medium pressure nitrogen gas, also from the higher pressure column and after it has been fully warmed within the air separation plant's heat exchange system and low pressure nitrogen product taken from the lower pressure column are all fed to the nitrogen liquefier. The resulting liquid nitrogen can be returned to the top of the higher pressure column to provide refrigeration to produce subcooled liquid products from the air separation plant. InU.S. Patent No. 4,883,518, nitrogen vapor is removed from the higher pressure column and split into two streams, one stream passes through the heat exchangers of a nitrogen liquefier and the other stream passes through the main heat exchangers. The two nitrogen vapor streams are introduced into a recycle compressor and then through dual turbine-booster compressor arrangements to produce a liquid nitrogen stream that is reintroduced into the higher pressure column to produce liquid nitrogen and oxygen products.EP-A-1 873 469, which can be considered as the closest prior art, discloses an air separation unit using a liquefier cooled by LNG to provide refrigeration.
• As is apparent from the above discussion of the two prior art patents, neither is particularly amenable to be used as a retrofit to an existing air separation plant because of the required high degree of integration necessary to employ the liquefiers disclosed in these patents. As will be discussed, the present invention provides a method of retrofitting an existing air separation plant with a nitrogen liquefier that either allows or increases the ability to withdraw a liquid oxygen product and optionally, a liquid nitrogen product and can increase argon production when such a plant is outfitted with an argon column. Moreover, the liquefier is integrated in a manner that does not involve the high degree of integration in the prior art.
Summary of the Invention
• The present invention provides a method of retrofitting an existing air separation plant to produce or to increase production of at least one liquid product according to claim 1.
• In accordance with the method, air is separated within the existing air separation plant. The existing air separation plant has at least higher and lower pressure columns operatively associated with one another in a heat transfer relationship. The existing air separation plant is retrofitted by connecting a nitrogen liquefier to the higher pressure column. The nitrogen liquefier has no components in common with existing components of said existing air separation plant.
• The nitrogen liquefier is connected to the higher pressure column such that the nitrogen liquefier only receives a nitrogen-rich vapor stream from a top portion of the higher pressure column. The nitrogen-rich vapor stream is liquefied in the nitrogen liquefier to produce a nitrogen-rich liquid stream and at least a portion of the nitrogen-rich liquid stream is introduced into the higher pressure column. This increases liquid nitrogen reflux to the higher pressure column, production of a crude liquid oxygen column bottoms formed in the higher pressure column and therefore, an oxygen-rich liquid formed in a bottom region of the lower pressure column.
• The at least one liquid product is withdrawn from the air separation unit and it comprises an oxygen-rich liquid stream composed of the oxygen-rich liquid.
• Preferably, within the nitrogen liquefier, a nitrogen vapor stream, comprising the nitrogen-rich vapor stream is warmed within a heat exchanger, expanded to exhaust stream pressure of a turbine exhaust stream and combined with the turbine exhaust stream to produce a combined stream. The combined stream is compressed in a recycle compressor and after removal of the heat of compression, is divided into a refrigerant fluid stream and a remaining part of the combined stream. The refrigerant fluid stream is compressed in a booster compressor, partially cooled in the heat exchanger and then introduced into the turboexpander to generate the turbine exhaust stream. The turbine exhaust stream is warmed within the heat exchanger and combined with the nitrogen-rich vapor stream. The remaining part of the combined stream is cooled within the heat exchanger and is expanded to higher pressure column pressure. The nitrogen-rich liquid stream is formed from at least part of the combined stream.
• Preferably, the work of expansion of the turboexpander powers the booster compressor. The expansion of the remaining part of the combined stream produces a two-phase stream and the liquid and vapor phases of the two-phase stream are disengaged to form a vapor phase stream and a liquid phase stream. The vapor phase stream is combined with a nitrogen-rich vapor stream to form the nitrogen vapor stream prior to its introduction into the heat exchanger. The liquid nitrogen stream is composed of the liquid phase stream.
• A liquid nitrogen product stream can be withdrawn that is made up of a further part of the nitrogen-rich liquid stream. The air separation unit can also be provided with an argon column connected to the lower pressure column to purify an argon-rich stream and thereby to produce an argon product stream. The further part of the nitrogen-rich stream is withdrawn at a rate that does not increase oxygen concentration within the argon-rich stream. Where, the further part of the nitrogen-rich stream is not produced, argon recovery can be increased by increased production of the oxygen-rich liquid and removal of the oxygen-rich liquid stream.
• The nitrogen liquefier can be operated intermittently so that the at least one liquid product stream is able to be stored for future utilization.
• Furthermore, the existing air separation plant can be configured such that attachment points exist within the higher pressure column of the existing air separation plant for connection to the nitrogen liquefier if the same is to be retrofitted.
Brief Description of the Drawings
• 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:
• Fig. 1 is a schematic process flow diagram of an existing air separation plant that is used in carrying out a method in accordance with the present invention; and
• Fig. 2 is a schematic process flow diagram of a nitrogen-rich liquefier that is to be retrofitted and connected to the higher pressure column of the air separation plant illustrated inFig. 1.
Detailed Description
• With reference toFig. 1 an existing air separation plant 1 is illustrated for exemplary purposes. As will be discussed, it includes a higher and lower pressure column, an ultra high purity oxygen column and argon columns to produce liquid argon as a product. However, this is for exemplary purposes only and the present invention has applicability to an air separation plant that has solely a higher pressure column and a lower pressure column or one that also includes an argon column.
• Anair stream 10 after filtration in afiltration unit 12 is compressed in amain air compressor 14. After the heat of compression is removed in an after cooler 16, theair stream 10 is purified within aprepurification unit 18.Prepurification unit 18 typically contains beds of adsorbents that are operated in an out of phase cycle to purify the air stream of higher boiling contaminants such as carbon monoxide, carbon dioxide and water vapor. Typically, the cycle can be pressure swing adsorption cycle or a temperature swing adsorption cycle.
• The resulting compressed andpurified air stream 20 is then divided into afirst portion 22 and asecond portion 24.First portion 22 is utilized in generating refrigeration for the plant. Anexhaust stream 26 is combined withfirst portion 22 and introduced into arecycle compressor 28. After the removal of the heat of compression in an after cooler 30, the resultant compressed stream is divided into afirst subsidiary stream 32 and asecond subsidiary stream 34.First subsidiary stream 32 is fully cooled within themain heat exchanger 36 andsecond subsidiary stream 34 is introduced into aturbine booster compressor 38. After removal of the heat of compression within an after cooler 40, the resulting compressed stream is cooled withinmain heat exchanger 36 and introduced into aturbine 42 in which the work of expansion can be utilized to drive theturbine booster compressor 38. The expansion occurring withinturbine 42 produces the cooledexhaust stream 26 which is warmed withinmain heat exchanger 36 to impart the refrigeration into the air separation plant 1.
• Air separation plant 1 is provided with a higher pressure column 44 that is operatively associated with alower pressure column 46 in a heat transfer relationship by means of acondenser reboiler 48. In addition, air separation plant 1 is also provided with alow ratio column 50 associated with asuperstage column 52 to separate argon in a manner that will be discussed. Additionally, an ultra highpurity oxygen column 54 is provided to produce an ultra high purity oxygen product that will also be discussed. Each of the higher pressure column 44, thelower pressure column 46, thelow ratio column 50, thesuperstage column 52 and the ultra high purityliquid oxygen column 54 contain mass transfer elements such as structured packing or trays to bring liquid and vapor phases of the mixtures that are introduced therein to be separated into intimate contact and thereby to rectify such mixtures.
• Second portion 24 of the compressed air stream is fully cooled withinmain heat exchanger 36 and divided into afirst subsidiary stream 60 that is introduced directly into the higher pressure column 44 and asecond subsidiary stream 62 that is introduced into areboiler 64 placed in the bottom of ultra highpurity oxygen column 54 to produce aliquid stream 66.
• First subsidiary stream 32 is fully cooled within themain heat exchanger 36 and is divided into first andsecond portions 68 and 70.First portion 68 is introduced directly into thelower pressure column 46 andsecond portion 70 is combined with theliquid stream 66 to form a combinedstream 72 that is introduced into the higher pressure column 44. The introduction of combinedstream 72 along withfirst portion 60 initiate the formation of an ascending vapor phase within higher pressure column 44 that becomes evermore lean in nitrogen to produce a nitrogen-rich vapor column overhead.
• A stream of the nitrogen-rich column overhead as a stream 74 is condensed within acondenser reboiler 48. A first portion 76 is returned as a reflux stream to higher pressure column 44 and asecond portion 78 is subcooled withinmain heat exchanger 36 and used to reflux thelower pressure column 46. Aportion 80 can be optionally taken as a liquid nitrogen product and the remainingportion 82 can then be introduced as a reflux stream into thelower pressure column 46.
• Within the higher pressure column 44, as the liquid phase descends it becomes evermore rich in oxygen to produce a crude liquid oxygen column bottoms. A crudeliquid oxygen stream 84 composed of the crude liquid oxygen column bottoms can be introduced into aheat exchanger 86 that is used in generating reflux for the superstageargon separation column 52. This partially vaporizes crudeliquid oxygen stream 84 to produce aliquid phase stream 88 and avapor phase stream 89 that is introduced into thelower pressure column 46 for further refinement. Additionally, another crudeliquid oxygen stream 87 can be introduced into thelower pressure column 46. Although not illustrated, but as known in the art, both crude liquid oxygen streams 84 and 87 would be valve expanded prior to their introduction into the lower pressure column so that the streams are at a pressure suitable for introduction into such column.
• The descending liquid phase withinlower pressure column 46 produces an oxygen-rich liquid that is vaporized bycondenser reboiler 48. Residual liquid can be taken as a liquidoxygen product stream 90. The resulting nitrogen-rich vapor can be taken as a nitrogenvapor product stream 92. Nitrogenvapor product stream 92 can have a concentration of less than about 2 ppm. Additionally, awaste nitrogen stream 94 can also be removed.Waste nitrogen stream 94 can be used in regenerating adsorbents withinprepurification unit 18. Both nitrogenvapor product stream 92 and waste nitrogen stream are first warmed in a superheater and then in themain heat exchanger 36 to near ambient temperatures. Additionally, a gaseousoxygen product stream 96 can also be removed fromlower pressure column 46 that consists of vaporized oxygen-rich liquid that is produced by the vaporization of the liquid phase at the bottom oflower pressure column 46 bycondenser reboiler 48. Both gaseousoxygen product stream 96 and liquidoxygen product stream 90 can have a purity of about 99.5 percent by volume.
• An argon containingvapor stream 98 that can contain greater than about 10 percent by volume argon and less than 1 ppm nitrogen can be removed from thelower pressure column 46 and introduced into thelow ratio column 50. This creates an argon-rich column overhead and an oxygen-rich column bottoms withinlow ratio column 50. The oxygen-rich column bottoms can be returned as an oxygen-richliquid stream 100 back to thelower pressure column 46. The argon-rich column overhead can be taken as an argon-rich stream 102 and introduced intosuperstage column 52 for separation of oxygen to very low levels to thereby produce an oxygen-rich column bottoms that can be removed as an oxygen-rich stream 106 that is pumped by apump 108 back to thelow ratio column 50 as a pumped stream 110. The removal of the oxygen produces an argon-rich column overhead. An argon-rich stream 112 can be introduced intoheat exchanger 86 to produce anargon reflux stream 114, anargon vent stream 116 is taken to prevent build up of non-condensable nitrogen and a liquidargon product stream 120 can be removed from thesuperstage argon column 52 as a liquid argon product stream that can contain less than about 1 ppm nitrogen and about 1 ppm oxygen.
• Anoxygen liquid stream 122 that is essentially hydrocarbon and nitrogen free can be removed from the lowratio argon column 50 and introduced into the ultra highpurity oxygen column 54 as feed to produce an ultra high purity liquidoxygen product stream 124 from residual liquid that is not reboiled byreboiler 64 that has a purity of about 99.99999 percent oxygen. The vapor overhead within ultra highpurity oxygen column 54 can be removed as avapor stream 126 that is reintroduced into thelow ratio column 50.
• The air separation plant 1 produces an ultra high purityliquid oxygen product 124, aliquid oxygen product 90 and potentially a liquidnitrogen product stream 80. The degree to which liquids are produced is dependent upon the total refrigeration that is imparted into air separation plant 1. During turn down conditions of air separation plant 1 the aforesaid liquid products are produced at a lower rate. In order to increase the production of the liquid products, both during turn down conditions and during normal operation of air separation plant 1, aliquefier 2 can be retrofitted into air separation plant 1.Liquefier 2 is illustrated inFig. 2. A nitrogen-rich vapor stream 130 is introduced intoliquefier 2 that liquefies the nitrogen-rich vapor stream 130 and returns the resulting nitrogen-richliquid stream 132 back to the higher pressure column 44. It is to be noted that it is solely that nitrogen-rich vapor stream 130 that is removed and the nitrogen-richliquid stream 132 that is reintroduced into the higher pressure column 44.
• The introduction of the liquid nitrogen as nitrogen-richliquid stream 132 increases the amount of liquid oxygen that collects within crude liquid oxygen column bottoms and therefore the amount of oxygen that is being introduced into thelower pressure column 46. This has the effect of allowing liquidoxygen product stream 90 to be withdrawn at a greater rate in that more oxygen-rich liquid is produced. Additionally, liquidnitrogen product stream 80 and ultra high purityliquid oxygen stream 124 can also be withdrawn at a greater rate.
• As can be appreciated by those skilled in the art, the liquidnitrogen product stream 80 should not be withdrawn in an excessive rate that would affect the purity of argon-rich stream 98. However, the increased amount of liquid nitrogen that is being introduced into thelower pressure column 46 without any production of liquidnitrogen product stream 80 will increase the argon concentration within argon-rich stream 98 and thereby increase the recovery and rate at which liquidargon product stream 120 can be removed fromsuperstage column 52. Consequently, in one mode of operation,liquid nitrogen stream 80 is removed at a rate that will not effect the argon concentration of argon-rich stream 98 or alternatively can be removed at a lesser rate or not removed at all to increase the argon concentration of argon-rich stream 98. It is to be noted that it is also possible to take a liquid nitrogen product stream from part of the nitrogen-richliquid stream 132 and the same discussion as above would apply to removal of such a stream as a product.
• Preferably, air separation plant 1, as well as any other air separation plant that is retrofitted withnitrogen liquefier 2 in accordance with the present invention, can be constructed with standard attachment points 128 and 129 that allow for the simple connection of thenitrogen liquefier 2 to such a plant. For example, attachment points 128 and 129 could be capped pipes or closed and capped valves that would be built into a standard plant design. A product line of plants could thereby be designed with such attachment points 128 and 129. This would allow the retrofit ofnitrogen liquefier 2 to be carried out in an expeditious and cost effective manner if the same were desired on any such plant within the product line. Once employed, the air separation plant 1 could be utilized to operate in a mode in which liquid production of products were increased to meet an increased demand. Alternatively, thenitrogen liquefier 2 could be employed to increase oxygen production during periods in which electrical power could be obtained at lower cost to allow the liquid products produced at the enhanced rate to be stored for later use during periods of higher cost electrical power. Additionally, the nitrogen liquefier could also be employed during turn-down conditions of the plant to produce liquid products at such time. A yet further mode of operation is to employnitrogen liquefier 2 in connection with a plant not designed to produce liquid products and retrofit such plant to produce liquid products.
• With reference toFig. 2, thenitrogen liquefier 2 to be retrofitted to air separation plant 1 is illustrated. Innitrogen liquefier 2, nitrogen-rich vapor stream 130 combined with avapor phase stream 134 that is expanded in anexpansion valve 136 to the pressure of nitrogen-rich vapor stream 130 and combined therewith to form anitrogen stream 138.Nitrogen stream 138 is warmed within aheat exchanger 140 and is then reduced in pressure within anexpansion valve 142. After the reduction in pressure, thenitrogen stream 138 is combined with anexhaust stream 144 which imparts refrigeration to the liquefier by being fully warmed within theheat exchanger 140. This produces a combinedstream 146 that is compressed within arecycle compressor 148. After removal of the heat of compression with an after cooler 150 afirst portion 152 is introduced into abooster compressor 154 to produce acompressed stream 156. After removal of the heat of compression within an after cooler 157, compressedstream 156 is partially cooled withinheat exchanger 140, reduced in pressure by means of anexpansion valve 158 and is then introduced intoturbine 160 to produceexhaust stream 144. The work of expansion provided byturbine 160 can be applied to the compression withinbooster compressor 154. Theother portion 162 of combinedstream 146 after having been compressed incompressor 148 and cooled within after cooler 150, is fully cooled inheat exchanger 140 and then expanded by means of anexpansion valve 164 into a two-phase fluid that is introduced into aphase separator 166. The vapor phase is disengaged from the liquid withinphase separator 166 to produce nitrogenvapor phase stream 134. Aliquid phase stream 168 is reduced in pressure by means of anexpansion valve 170 to a pressure of the higher pressure column 44 and introduced as the nitrogen-richliquid stream 132 back intolower pressure column 46.
• As can be appreciated, other designs fornitrogen liquefier 2 could be utilized in accordance with the present invention. For example a very simple nitrogen liquefier could be used in which the hitrogen-rich vapor stream 130 alone was compressed, expanded and liquefied at of course a much higher energy cost. However, it is believed thatnitrogen liquefier 2 is a particularly advantageous design in its simplicity and strikes a balance between simplicity and efficiency for retrofit applications.

Claims (8)

1. A method of retrofitting an existing air separation plant (1) to produce or to increase production of at least one liquid product, comprising:

   separating air within an existing air separation plant having at least higher and lower pressure columns (44, 46) operatively associated with one another in a heat transfer relationship;

   retrofitting the existing air separation plant (1) by connecting a nitrogen liquefier (2) to the higher pressure column (44), the nitrogen liquefier having no components in common with existing components of said existing air separation plant;

   the nitrogen liquefier (2) being connected to the higher pressure columns (44) such that the nitrogen liquefier only receives a nitrogen-rich vapor stream (130) from a top portion of the higher pressure column, the nitrogen-rich vapor stream is liquefied in the nitrogen liquefier to produce a nitrogen-rich liquid stream (132) and at least a portion of the nitrogen-rich liquid stream is introduced into the higher pressure column, thereby to increase liquid nitrogen reflux to the higher pressure column, production of a crude liquid oxygen column bottoms formed in the higher pressure column (44) and therefore, an oxygen-rich liquid formed in a bottom region of the lower pressure column (46); and

   withdrawing the at least one liquid product (90, 124) from the air separation plant (1), the at least one liquid product comprising an oxygen-rich liquid stream (90) composed of the oxygen-rich liquid.
2. The method of claim 1, wherein, within the nitrogen liquefier (2) :

   a nitrogen vapor stream (138) comprising the nitrogen-rich vapor stream (130) is warmed within a heat exchanger (140), expanded to exhaust stream pressure of a turbine exhaust stream (144) and combined with the turbine exhaust stream to produce a combined stream (146);

   the combined stream (146) is compressed in a recycle compressor (148) and after removal of the heat of compression is divided into a refrigerant fluid stream (152) and a retaining part (162) of the combined stream_;

   the refrigerant fluid stream (152) is compressed in a booster compressor (154), partially cooled in the heat exchanger (140) and then introduced into the turboexpander (160) to generate the turbine exhaust stream (144);

   the turbine exhaust stream (144) is warmed within the heat exchanger (140) and combined with the nitrogen-rich vapor stream (130) ;

   the remaining part (162) of the combined stream (146) is cooled within the heat exchanger (140) and expanded to higher pressure column pressure; and

   the nitrogen-rich liquid stream (132) is formed at least in part from the combined stream (146).
3. The method of claim 2, wherein:

   the work of expansion of the turboexpander (160) powers the booster compressor (154);

   the expansion of the remaining part (162) of the combined stream (146) produces a two-phase stream;

   liquid and vapor phases of the two-phase stream are disengaged to form a vapor phase stream (134) and a liquid phase stream (168);

   the vapor phase stream (134) is combined with the nitrogen-rich vapor stream (130) to form the nitrogen vapor stream (138) prior to its introduction into the heat exchange (140); and

   the liquid nitrogen stream (132) is composed of the liquid phase stream (168).
4. The method of claim 1, wherein a liquid nitrogen product stream (80) is withdrawn that is made up of a further part of the nitrogen-rich liquid stream (132).
5. The method of claim 4, wherein:

   the air separation plant (1) also has an argon column (50) connected to the lower pressure column (46) to purify an argon-rich stream (98) and thereby to produce an argon product stream (102); and

   the further part of the nitrogen-rich stream (132) is withdrawn at a rate that does not increase oxygen concentration within the argon-rich stream (98).
6. The method of claim 1, wherein:

   the air separation plant (1) also has an argon column (50) connected to the lower pressure column (46) to purify an argon-rich stream (98) and thereby to produce an argon product stream (102);

   the argon recovery is increased by increased production of the oxygen-rich liquid and removal of the oxygen-rich liquid stream (90).
7. The method of claim 1, wherein the liquefier (2) is operated intermittently so that the at least one liquid product stream (80, 90, 124) is able to be stored for future utilization.
8. The method of claim 1, wherein the existing air separation plant (1) is configured such that attachment points (128, 129) exist within the higher pressure column (44) of the existing air separation plant for connection of the nitrogen liquefier (2).

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