Patent Publication Number: US-10309720-B2

Title: System and method for argon recovery from a feed stream comprising hydrogen, methane, nitrogen and argon

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of and priority to U.S. patent application Ser. No. 62/311,168 filed on Mar. 21, 2016. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a system and method for separating a feed stream comprising hydrogen, nitrogen, methane and argon, and more particularly, a system and method for argon recovery from a feed stream originating from an ammonia production plant via: (i) rectification of the feed stream to produce an argon depleted nitrogen enriched overhead vapor stream, an argon enriched stream, and a methane-rich liquid stream; (ii) separation of argon and nitrogen in an auxiliary rectification column; and (iii) residual argon stripping from the methane rich liquid. 
     BACKGROUND 
     Argon is a highly inert element used in high-temperature industrial processes, such as steel-making. Argon is also used in various types of metal fabrication processes such as arc welding as well as in the electronics industry, for example in silicon crystals production. Still other uses of argon include medical, scientific, preservation and lighting applications. While argon constitutes only a minor portion of ambient air (i.e. 0.93% by volume), it possesses a relatively high value compared to other major atmospheric constituents (oxygen and nitrogen) which may be recovered from air separation plants. Argon is typically recovered in a cryogenic air separation process as a byproduct of high purity oxygen production. In such processes, an argon rich vapor draw from the lower pressure column of a thermally linked dual pressure air distillation system. The stream is directed to an argon rectification column where crude or product grade argon is recovered overhead. 
     The availability of low cost natural gas has led to the restart and construction of numerous ammonia production facilities throughout North America. One of the byproducts of ammonia production plants is a tail gas that may be comprised of methane, nitrogen, argon, and hydrogen. This tail gas is often utilized as fuel to fire various reactors within the ammonia production plant. However, if this argon-containing tail gas can be cost-effectively handled and purified, it could be used as a source of argon production. 
     Ammonia is typically produced through steam methane reforming. In such a process air serves to auto-fire the reaction and to supply nitrogen for the synthesis reaction. In general, the steam methane reforming based process consists of primary steam reforming, secondary ‘auto-thermal’ steam reforming followed by a water-gas shift reaction and carbon dioxide removal process to produce a synthesis gas. The synthesis gas is subsequently methanated and dried to produce a raw nitrogen-hydrogen process gas which is then fed to an ammonia synthesis reaction. In many ammonia production plants, the raw nitrogen-hydrogen process gas is often subjected to a number of purification or additional process steps prior to the ammonia synthesis reaction. In one such purification process, the methane contained in the nitrogen-hydrogen process gas is cryogenically rejected prior to the nitrogen-hydrogen process gas compression. The rejected gas is a tail gas comprising the bulk of the contained methane as well as argon, nitrogen and some hydrogen. This tail gas is often used as a fuel to supply the endothermic heat of reaction to the primary steam reformer. 
     Argon is present in ammonia tail gas generally contains between about 3% to 6% argon. After hydrogen recovery from the tail gas, the relative concentration of argon increases to between about 12% to 20% argon which makes the argon recovery an economically viable process. In an effort to reduce costs and increase process efficiency, the conventional argon recovery processes from ammonia tail gas are typically integrated with the hydrogen recovery process. The conventional argon recovery processes are relatively complex and involves multiple columns, vaporizers, compressors, and heat exchangers, as described for example in W. H Isalski, “ Separation of Gases ” (1989) pages 84-88. Other relatively complex argon recovery systems and process are disclosed in U.S. Pat. Nos. 3,442,613; 5,775,128; 6,620,399; 7,090,816; and 8,307,671. 
     What is needed, however, is a much simpler and cost-effective system and method for the recovery of argon and nitrogen contained within the tail gas of an ammonia production plant as an alternative source of argon production and/or liquid nitrogen production. 
     SUMMARY OF THE INVENTION 
     The present invention may be characterized as a method for separating a feed stream comprised primarily of hydrogen, nitrogen methane and argon comprising the steps of: (a) conditioning the feed stream to a temperature suitable for rectification at pressure less than or equal to about 150 psia; (b) directing the conditioned feed stream to a rectification system which is comprised of at least one rectifying column; (c) separating the conditioned feed stream in the at least one rectification column to produce an argon depleted nitrogen enriched vapor stream, an argon enriched stream, and a methane-rich liquid stream; (d) combining the methane rich liquid stream and a portion of the argon depleted nitrogen enriched stream to form a combined two phase fuel stream; (e) directing the two phase fuel stream to an indirect heat transfer device; and (f) warming the vapor phase and vaporizing the liquid phase of the two phase fuel stream in the indirect heat transfer device to produce a fuel gas stream. 
     In some embodiments, the steps of combining the methane rich liquid stream with the portion of the argon depleted nitrogen enriched stream to form the combined two phase fuel stream and directing the two phase fuel stream to the indirect heat transfer device further comprises: directing the methane rich liquid stream and the portion of the argon depleted nitrogen enriched stream to a phase separator to produce the two phase fuel stream; directing the vapor phase of the two phase fuel stream from the phase separator to a common passage of the indirect heat transfer device; and directing the liquid phase of the two phase fuel stream from the phase separator to the common passage of indirect heat transfer device. 
     The feed stream may be a gas stream, a two phase stream, or a liquid stream. Preferably, the feed stream is a tail gas from an ammonia plant and may generally contain greater than about 50% nitrogen by mole fraction. Conditioning of the feed stream in the refrigeration system may involve cooling the feed stream; warming/vaporizing the feed stream, compressing and/or expanding the feed stream in a plurality of discrete steps. Where the system and method are integrated or coupled to an ammonia plant, recycling of one or more of the streams back to the ammonia plant is contemplated. For example, the argon-depleted, hydrogen-nitrogen gas overhead may be recycled back to the ammonia plant, and preferably recycled back to either a cryogenic purifier in the ammonia plant or other locations within the synthesis gas stream of the ammonia plant. The argon-depleted methane-rich liquid is also preferably recycled back to the ammonia plant, and preferably employed as a high quality fuel gas to be used for example to fire the reformers in the ammonia plant. 
     As indicated above, the rectification column separates the conditioned feed stream to produce an argon depleted nitrogen enriched vapor stream, an argon enriched stream, and a methane-rich liquid stream. The argon-depleted nitrogen enriched rich vapor stream preferably comprises mainly nitrogen and hydrogen vapor. The argon enriched stream, on the other hand may be a liquid stream, a gaseous stream or a two phase stream comprising a fraction of liquid argon and a fraction of gaseous argon. In either state, trace amounts of hydrogen may optionally be removed or rejected from this stream using a hydrogen rejection arrangement such as an evaporator, phase separator, or a hydrogen stripping column. The resulting hydrogen-free, argon enriched stream is then directed to an auxiliary rectification column where it is separated to produce a high purity argon stream and a high purity nitrogen stream. The argon depleted nitrogen enriched vapor is preferably split into two or more portions with a first portion being directed as the nitrogen rich vapor stream used in the argon stripping column. Another portion of the of the argon depleted nitrogen enriched vapor may be combined with the argon depleted methane rich liquid stream. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the specification concludes with claims specifically pointing out the subject matter that Applicant regards as the 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 representation of an ammonia synthesis process used in a typical ammonia plant; 
         FIG. 2  is a schematic representation of the embodiment of a system and method for argon recovery from the tail gas of an ammonia production plant in accordance with the present invention; 
         FIG. 3  is a schematic representation of the refrigeration system suitable for use with the embodiment depicted in  FIG. 2 ; 
         FIG. 4  is a schematic representation of an alternate embodiment of a system and method for argon recovery from the tail gas of an ammonia production plant; 
         FIG. 5  is a schematic representation of another embodiment of a system and method for argon recovery from the tail gas of an ammonia production plant; and 
         FIG. 6  is a schematic representation of yet another alternate embodiment of a system and method for argon recovery from the tail gas of an ammonia production plant. 
     
    
    
     For sake of clarity, many of the reference numerals used in  FIGS. 4-6  are similar in nature such that the same reference numeral in one figure corresponds to the same item, element or stream as in the other figures. 
     DETAILED DESCRIPTION 
     The following detailed description provides one or more illustrative embodiments and associated methods for separating a feed stream comprising hydrogen, nitrogen, methane and argon into its major constituents. The disclosed system and methods are particularly suitable for gas recovery from a tail gas of an ammonia production plant comprising hydrogen, nitrogen, methane and inert gases, such as argon krypton and xenon, and involves four (4) key steps or subsystems, namely: (i) conditioning the feed gas stream in a refrigeration circuit or subsystem; (ii) separating the conditioned feed gas stream in a rectification column to produce a methane-rich liquid column bottoms; an argon-depleted, hydrogen-nitrogen gas overhead; and an argon-rich stream having trace amounts of hydrogen; (iii) stripping the trace amounts of hydrogen from the argon-rich stream to produce an argon depleted stream and a hydrogen-free, nitrogen and argon containing stream; and (iv) separating the argon from the hydrogen-free, nitrogen and argon containing stream in a distillation column system to produce at least an argon product stream and a nitrogen product stream. 
     Turning now to  FIG. 1 , a schematic representation of an ammonia production plant  10  is shown. The production of ammonia from hydrocarbons entails a series of unit operations which include catalytic, heat exchange and separation processes. In general, ammonia synthesis proceeds by steam reforming of a hydrocarbon feed  12  and steam  13  in a primary reformer  14 , typically methane. A secondary reformer  16  is generally employed wherein the synthesis gas mixture  15  is further reformed in the presence of an air feed  17 . The air feed  17  serves to provide a source of oxygen to fire the reforming reaction as well as to supply the necessary nitrogen for subsequent ammonia conversion. After reforming, the synthesis gas  19  is directed to several stages of heat recovery and catalytic water gas shift reaction  22 . The gas  23  is then directed to a carbon dioxide removal process  24  generally known to persons skilled in the art such as MDEA, hot potassium carbonate, etc. to remove carbon dioxide as effluent  21 . The resulting carbon dioxide free gas  25  is then further subjected to methanation  26  to remove residual carbon oxides. A number of further processing arrangements including cryogenic purification  30  and synthesis gas compression  34  are further employed to facilitate final ammonia synthesis  36  which involves a high temperature and high pressure reaction (˜140 bar). Ammonia  40  is then separated or recovered  38  by subsequent cooling and condensation. A recycle stream  39  from the ammonia recovery process is then directed back to the cryogenic purifier  30   
     A common part of the ammonia processing train employs a cryogenic purification process  30  known by those skilled in the art as the “Braun Purifier”. Since the secondary reformer  16  is fed with an air flow that is larger than that required by the stoichiometry of the ammonia synthesis reaction, excess nitrogen and inert gases must be removed or rejected prior to the ammonia synthesis step  36 . In order to reject the excess nitrogen and inerts, a cryogenic purification process  30  is introduced after the methanation  26  reaction. The primary purpose of this cryogenic purification process  30  is to generate an overhead ammonia synthesis gas stream  31  with a stoichiometric ratio of hydrogen to nitrogen (H2:N2) of about 3:1. The cryogenic purification step of the Braun Purifier employs a single stage of refrigerated rectification. The overhead synthesis gas stream from the single stage of refrigerated rectification is free of unconverted methane and a substantial portion of the inerts, such as argon, are rejected into the fuel gas stream-bottoms liquid. In the Braun Purifier process, the feed gas stream  29  is first cooled and dehydrated. The feed gas stream  29  is then partially cooled and expanded to a lower pressure. The feed gas stream  29  may be further cooled to near saturation and then directed to the base of the single stage rectifier. The rectifier overhead is the resulting ammonia synthesis gas  31  that is processed for ammonia synthesis, whereas the rectifier bottoms are partially vaporized by passage through the rectifier condenser and warmed to ambient temperatures. This fuel/waste stream  35  is typically directed back to the reform and serves as fuel. See Bhakta, M., Grotz, B., Gosnell, J., Madhavan, S., “Techniques for Increase Capacity and Efficiency of Ammonia Plants”, Ammonia Technical Manual 1998, which provides additional details of this Braun Purifier process. The waste gas  33  from the Braun Purifier process step is predominantly a mixture of hydrogen (6.3 mole %), nitrogen (76.3 mole %), methane (15.1 mole %) and argon (2.3 mole %) The Braun Purifier waste gas  35  represents a distinct departure from typical ammonia plant tail gas streams and requires new techniques and processes for recovering valuable constituents of the waste gas in a simple, cost effective and efficient manner. 
     In  FIG. 2 , there is shown an embodiment of the present system and method for argon and nitrogen recovery from a feed stream  35  comprising hydrogen, nitrogen, methane and argon. The stream is typically obtained at low-pressure such as the tail gas of a Braun Purifier based ammonia production plant. The feed stream  35  to the present system and method is preferably a dry, low pressure (e.g., 15 psig to 25 psig) mixture of predominately hydrogen, nitrogen, methane, and argon. The gas is typically derived from a cryogenic purifier positioned just upstream to the synthesis gas compression in an ammonia synthesis or production plant. The low pressure feed gas stream may comprise the waste gas from the Braun purifier, which, as described above constitutes about 6.3% hydrogen, 76.3% nitrogen, 15.1% methane, and 2.3% argon and on a molar basis. Since the feed stream  35  is obtained dry from a previous cryogenic process in the ammonia production plant, pre-purification of the feed gas stream may or may not be required as part of the present argon recovery process and system  50 . 
     The resulting products from the present recovery process and system  50  include: a liquid argon product stream  45 ; and a liquid nitrogen product stream  55 ; a hydrogen-nitrogen product gas stream  65  that may be recycled back to the ammonia plant synthesis section, and more particularly the ammonia synthesis gas stream upstream of the compressor or of the ammonia plant; a high methane content fuel gas  75  that may be recycled back to the ammonia production plant and preferably to the steam reforming section of the ammonia plant, and more specifically to the furnace by which the primary reformer is fired; and a substantially pure nitrogen gaseous overhead stream  85  that is also preferably recycled back to the ammonia plant. 
     Referring again to  FIG. 2 , the basic separation approach entails processing at least a portion of the bottoms/waste from the cryogenic purifier of the ammonia plant as a the feed stream  35 . In order to effectively operate the Braun Purifier, it is often necessary to partially vaporize the bottoms/waste fluid in an overhead condenser to attain an acceptable temperature difference for subsequent heat exchange. After partial vaporization, a substantial portion of the argon or other inerts are contained in the residual/un-vaporized liquid portion of the waste stream. Therefore, an initial step, but not essential step, in the present system and method of argon recovery is to preferably vaporize the residual liquid portion of the feed stream  35  via indirect heat exchange within the refrigeration system  100  to generate a substantially gaseous feed stream  52 . 
     It should be noted that in some instances that residual carbon oxides at levels less than about 10.0 ppm or other unwanted impurities may accompany the feed stream  52  being directed to the auxiliary rectification column  60 . In such circumstances, adsorbents and associated purification systems (not shown) can be employed to further remove such impurities from the feed streams  35 ,  52 . Such purification may be conducted while a portion of the feed stream  35  is in the liquid phase upstream of the vaporization step or when the feed stream  52  in the predominately gas phase downstream of the vaporization step. 
     In a preferred mode of operation, the feed stream  35  exiting the Braun Purifier overhead condenser of the ammonia plant is conditioned in a refrigeration circuit or system  100  by first warming and substantially vaporizing the feed stream  35  and then subsequently cooling the vaporized stream to bring the feed stream to a point near saturation and suitable for entry into the rectification column  60 . Alternatively, the step of conditioning the feed stream may comprise any combination of warming, cooling, compressing or expanding the feed gas stream to a near saturated vapor state at a pressure of less than or equal to about 150 psia and a temperature near saturation. Preferably the pressure is less than or equal to about 50 psia, and more preferably to a range of between about 25 psia and 40 psia. 
     The conditioned and cooled feed gas stream  52  is then directed to an auxiliary rectification column  60  where it is rectified into an argon-depleted, hydrogen-nitrogen gas overhead  62  and a methane-rich liquid column bottoms  64 . The argon-depleted, hydrogen-nitrogen gas overhead  62  contains primarily nitrogen and hydrogen in a molar ratio (N2:H2) of greater than about 3:1 and preferably greater than about 7:1. The exact composition of the argon-depleted, hydrogen-nitrogen gas overhead  62  will depend upon the level of argon recovery desired. In addition, an argon-rich side draw  66  is produced at an intermediate location  67  of the auxiliary rectification column  60 , where it is extracted to form an argon-rich stream  68  having trace amounts of hydrogen. 
     A portion of the argon-depleted, hydrogen-nitrogen gas overhead  62  is preferably directed or recycled back to the ammonia plant as a hydrogen-nitrogen product gas stream  65  while another portion  69  is directed to the refrigeration system  100  where it is condensed and reintroduced as a reflux stream  63  to the auxiliary rectification column  60 . Specifically, the portion of the hydrogen-nitrogen product stream  65  is directed back to the cryogenic purifier (e.g. Braun Purifier) in the ammonia plant or recycled back to the synthesis gas stream in the ammonia plant upstream of the compressor. Similarly, all or a portion of the methane-rich liquid column bottoms  64  is preferably subcooled and directed back or recycled back to fire the reformer as fuel gas stream  75 . 
     A key element of the present recovery process and system  50  is the extraction of an argon rich side draw  66  at a location above the point where methane is present in any appreciable amount, for example a location of the auxiliary rectification column where the methane concentration is less than about 1.0 part per million (ppm) and more preferably less than about 0.1 ppm. The argon-rich liquid stream  68  with trace amounts of hydrogen is extracted from an intermediate location  67  of the auxiliary rectification column  60  and directed to a hydrogen rejection arrangement shown as a hydrogen stripping column  70  which serves to reject trace hydrogen from the descending liquid. The resulting hydrogen free stream  72  exiting the hydrogen rejection arrangement comprises argon and nitrogen containing stream that is free of both methane and hydrogen. 
     An optional feature of the hydrogen rejection arrangement, and more specifically the hydrogen stripping column  70 , is that the resulting overhead vapor  73  or the rejected hydrogen and methane can be returned to the auxiliary rectification column  60 . Alternatively, the rejected hydrogen and methane stream  73  can be vented or combined with virtually any other exiting process stream. 
     The argon-rich liquid stream  72  free of both methane and hydrogen is then directed to a further separation wherein at least an argon stream is generated by way of distillation. Alternatively the argon-rich stream  72  could be taken directly as a merchant product or transported to an offsite refinement process, where it could later be separated into a merchant argon product and optionally nitrogen products. However, in the presently disclosed embodiment shown in  FIG. 2 , the argon rich stream  72  is pressurized via pump  71  and then at least partially vaporized or fully vaporized. The pressurized hydrogen-free, nitrogen and argon containing stream  74 , in a predominately vapor form, is then directed to a thermally linked double column system  80  configured for separating the argon-rich stream  74  and producing a liquid argon product  45  and a pure nitrogen overhead  85 . 
     In the double column distillation system  80 , the hydrogen-free, nitrogen and argon containing stream  74  is first rectified in a higher pressure column  82  to produce a substantially nitrogen rich overhead  81  and an argon enriched bottoms fluid  83 . The nitrogen rich overhead  81  is directed to the condenser reboiler  84  disposed in the lower pressure column  86  where it is condensed to a liquid nitrogen stream  87 . This liquid nitrogen stream  87  from the condenser-reboiler  84  and argon enriched bottoms fluid  83  from the higher pressure column  82  are preferably subcooled in subcooler  91  against a cold stream which could be a low pressure nitrogen rich stream  85  or a separate refrigeration stream. Portions of the liquid nitrogen stream exiting condenser/reboiler  84   88 ,  89  are used as reflux to the lower pressure column  86  and higher pressure column  82  while another portion of the liquid nitrogen stream may be diverted to storage (not shown) as a liquid nitrogen product  55 . A portion of the nitrogen reflux stream  88  and the subcooled argon enriched bottoms fluid  83  are then directed to the lower pressure distillation column  86  where they are distilled into a substantially pure nitrogen overhead gas  85  and an argon rich liquid product  45 . The argon rich liquid product  45  can optionally be further subcooled prior to flashing to storage (not shown). 
     The substantially pure nitrogen overhead  85  may be directed to a warming vent, an expansion circuit, or may be directed as a make-up gas to a refrigeration circuit  100  associated with the present system  50  to produce the refrigeration required for the disclosed process. Alternatively, the substantially pure nitrogen overhead  85  could be directly taken as cold nitrogen gaseous product, liquefied and taken as a cold liquid nitrogen product, or recycled back to the ammonia plant. 
     The resulting substantially pure nitrogen overhead  85  from the lower pressure column  86  can be directed to any number of locations/uses including: (i) to sub-cool the liquid nitrogen reflux streams and/or the argon enriched bottoms fluid; (ii) directly taken as cold nitrogen gaseous product; (iii) to a liquefaction system and taken as a cold liquid nitrogen product; (iii) as a make-up working fluid or component thereof in a refrigeration system; (iv) to the cryogenic purifier (e.g. Braun Purifier) of the ammonia plant. Preferably, the separated nitrogen stream can returned to the point of origin without a substantial portion of the original argon content. In a preferred mode of operation of the present nitrogen-argon separation system  50  depicted in  FIG. 2 , the resulting nitrogen overhead  85  will be of sufficient pressure to be recombined with the methane enriched stream associated with the Braun Purifier. Alternatively, the nitrogen overhead  85  could be recycled or directed back to other locations in the ammonia plant upstream of the cryogenic purifier to be mixed with various feed streams to the ammonia production process or locations downstream of the cryogenic purifier and into the synthesis gas train. 
     Advantageously, the above-described system and method is configured to capture the bulk of the contained argon contained in the feed gas stream and can recover liquid nitrogen or even gaseous nitrogen on an as needed basis. The base level of argon recovery of the presently illustrated and described systems and processes are in the range of about 85% to about 90%. Another advantage of the present system and method is that the initial rejection of methane by way of the auxiliary rectification column and rejection of hydrogen by the hydrogen stripping column is accomplished at or near the feed gas stream pressures (i.e. less than or equal to about 150 psia, and more preferably less than or equal to 50 psia, and still more preferably in the range of about 25 to 40 psia) which promotes the simplicity and cost effectiveness of argon recovery. 
     Turning now to  FIG. 3 , an embodiment of the refrigeration circuit or system  100  forming part of the conditioning system is depicted. In order to produce additional refrigeration and to facilitate the above-described separations, an integrated a refrigeration system or liquefaction system can be employed. The preferred conditioning and refrigeration system  100  and process is configured to achieve or produce the following: (1) a low pressure refrigeration stream  102  sufficiently cold to refrigerate the argon-depleted, hydrogen-nitrogen gas overhead  65  of the auxiliary rectification column  60 ; (2) a vaporized refrigerant stream  104 , after having cooled the argon-depleted, hydrogen-nitrogen gas overhead  65 , is then substantially warmed to ambient temperatures in a heat exchanger  106  and the warmed stream  108  is compressed in a single stage or multi-stage compressor  110  to an elevated pressure and cooled in aftercooler  112 ; (3) at least a portion of the elevated pressure refrigerant  118  is expanded in turbo-expander  120  to produce refrigeration; (4) another portion of the elevated pressure refrigerant  116  is cooled to near saturation via indirect heat exchange with at least a portion of the low pressure refrigerant stream in the heat exchanger  106  to produce a cooled, elevated pressure refrigerant stream  122 ; (5) the cooled, elevated pressure refrigerant stream  122  is at least partially condensed against either the incoming feed stream  35  and/or the partially vaporizing hydrogen-free, nitrogen and argon containing stream  72 ; and (6) at least a portion of the partially condensed or fully condensed refrigerant  130  is valve expanded in valve  132  to form the low pressure refrigeration stream  102  used to refrigerate the argon-depleted, hydrogen-nitrogen gas overhead  65  of the auxiliary rectification column  60 . 
     It should also be noted that the above refrigeration circuit or system  100  can also be operated as a liquefaction system. The key difference in the liquefaction system being that a portion of the working fluid may also be delivered as a liquid product  55 . In particular, the use of the substantially pure nitrogen overhead  85  from the lower pressure column  86  of the double column distillation system  80  as a working fluid or make-up gas  152  is ideal. In such liquefaction embodiment, a liquid nitrogen product stream  150  could be extracted from the refrigeration system  100  rather than from the double column distillation system  80  and equivalent volume of make-up refrigerant  152 , such as a portion of the nitrogen overhead  85  from the double column distillation system  80  would be added to the refrigeration system  100 . 
     With respect to the above-described refrigeration system, it is also possible to incorporate multiple stages of compression and/or use multiple compressors arranged in parallel for purposes of accommodating multiple return pressures. In addition, the turbo-expanded refrigerant stream  121  can be configured interior with respect to temperature in the heat exchanger  106  as the turbine discharge or exhaust does not have to be near saturation. The shaft work of expansion can be directed to an additional process stream or may be used to “self-boost” the expansion stream. Alternatively, the shaft work of expansion may also be loaded to a generator or dissipated by a suitable break. 
     As for the composition of the working fluid in the refrigeration circuit or system, a stream of high purity nitrogen is a natural choice. However it may be advantageous to use a combination of nitrogen and argon or even pure argon. It should also be noted that the presence of air compression for secondary reforming in the ammonia plant can be exploited to supply a working fluid for refrigeration, with such working fluid being air or constituents of air. As noted, a liquid product stream can be generated directly from the working fluid of the refrigeration system. Refrigerant makeup for liquid production or turbo-expander leakage may be supplied by the nitrogen-argon separation system or it may be supplied externally from a storage tank or nearby air separation plant. 
     It is also possible to supplement refrigeration generation of the disclosed refrigeration system with the inclusion of a Rankine cycle, vapor compression type refrigeration circuit to provide supplemental warm level refrigeration. Alternatively, a second turbo-expander or warm turbine can be employed which may also use the subject working fluid or a different working fluid, such as carbon dioxide or ammonia to supply yet additional refrigeration (alone and in combination). Such gases can be easily derived from the base ammonia processing sequence in the ammonia plant. 
     With reference again to  FIGS. 2 and 3 , one can appreciate that incorporating or adopting the present nitrogen-argon separation process and system within an ammonia production operation allows the plant operator to also optimize or modify the Braun Purifier operation within the ammonia plant to accommodate the separate nitrogen and methane rich streams from the above-described recovery system as well as any excess nitrogen and argon from the hydrogen free, nitrogen and argon containing stream. For example, when retrofitting an existing Braun purifier based ammonia plant, not all of the feed need be processed for argon recovery and the present system can be sized to recover a desired volume of high purity argon and/or high purity nitrogen. Any nitrogen or argon not recovered as high purity gases or liquids can be directed back to the Braun Purifier for further warming. 
     Alternatively, in a new ammonia production facility, it is possible to design the cryogenic purifier to independently warm the streams returning from the above-described separation process using a customized or specially designed heat exchanger. Furthermore, the ratio of turbo-expansion of the expander used in the Braun Purifier process can be reduced or perhaps even eliminated by way of the refrigeration generated from the present system and method. In essence, the refrigeration systems of the present nitrogen-argon separation process and system may be integrated with the refrigeration system in the Braun Purifier process. 
     Turning now to  FIG. 4 , there is shown an alternate embodiment of the present system  200  and method for argon and nitrogen recovery from a low-pressure tail gas of an ammonia production plant. In a broad sense, this alternate embodiment also includes the basic steps of: (i) conditioning the feed gas stream in a refrigeration circuit or subsystem; (ii) separating the conditioned feed gas stream in a rectification column to produce a methane-rich liquid column bottoms; an argon-depleted, hydrogen-nitrogen gas overhead; and an argon-rich stream containing nitrogen and argon with trace amounts of hydrogen; (iii) stripping the trace amounts of hydrogen from the argon-rich stream to produce an argon depleted stream and a hydrogen-free, nitrogen and argon containing stream; and (iv) separating the argon from the hydrogen-free, nitrogen and argon containing stream in a distillation column system, with liquefaction to produce liquid products, namely liquid argon and liquid nitrogen. 
     The refrigeration circuit or system of the embodiment of  FIG. 4  comprises a heat exchanger  210  that cools the feed stream  235  via indirect heat exchange with a low pressure nitrogen waste stream  285 , the hydrogen-nitrogen product stream  265 , and the high methane content fuel gas  275 . The feed stream  235  is preferably cooled in the heat exchanger  210  to near saturation and then directed to a low pressure auxiliary rectification column  260  where the feed stream  235  is subjected to a rectification process. Within the refrigeration circuit or system, an integrated nitrogen based heat pump or recycle and compression circuit may also be provided to supply the necessary refrigeration to produce the liquid products, namely a liquid argon product stream  245  and a liquid nitrogen product stream  255 . Specifically, the recycle compression circuit  250  compresses a portion of the waste nitrogen stream  285  from a pressure of about 24 psia to a pressure of about 650 psia. A partially compressed side nitrogen draw  222 A may be extracted at a pressure of about 78 psia from an intermediate location of the recycle compressor train  250  or alternatively from the discharge of the turbine  220 . The side nitrogen draw  222  is subsequently cooled in heat exchanger  210 . In the illustrated embodiment, the subject pressure and temperature of the side nitrogen draw  222 A must be is sufficient to reboil the liquids at the bottom of distillation column  280 . Also, in order to attain high liquefaction efficiency, supplemental refrigeration is further provided via the use of a cryogenic nitrogen turbine configured to operate between the recycle discharge and the moderate pressure required of the reboiler  284 . 
     In the embodiment of  FIG. 4 , the configuration of the turbine outlet temperature is ideally above the cold end temperatures of the heat exchanger  210 . The vaporization of the auxiliary rectification column bottoms allows a substantial warming of the turbine  220  and an increase in overall liquefaction efficiency. It should be noted, however, that the turbine  220  need not be directly coupled to a recycle booster compressor  215  as illustrated, but rather, the turbine shaft work may be directed to a generator or other process compression. The turbine pressure levels may also be configured across lower pressure recycle compression stages; however this would increase the size of heat exchanger  210  and increase the associated power consumption. 
     A stream of liquid nitrogen  224  is generated from the heat exchanger  210  by cooling and condensing a fraction of the higher pressure nitrogen recycle stream. The liquid nitrogen stream is extracted from the cold end of the heat exchanger  210  and, as described in more detail below, serves to refrigerate condenser  225  associated with rectification column  260 . Alternatively, a portion of the condensed liquid nitrogen stream from the heat exchanger  210  may be directed to storage or used as reflux  289  in the distillation column  280 . 
     In some applications of the present system and methods, where liquid nitrogen production exceeds the local demand, the excess liquid nitrogen can be directed to condenser  225  (shown as the dotted line) and vaporized in condenser  225  with a resulting decrease in overall power consumption. Conversely, depending upon local gaseous nitrogen product demands, it is possible to configure the recycle compression circuit  250  to provide gaseous nitrogen product at a range of pressures. 
     Within the methane removal subsystem, methane is removed from the ascending vapor within auxiliary rectification column  260  and extracted as a bottoms liquid  264 . The extracted methane-rich bottoms liquid  264  comprising about 84% methane is preferably subcooled and the subcooled methane-rich liquid stream  275  directed back to the heat exchanger  210  where it is vaporized. Cold end refrigeration is thus effectively generated by way of the vaporization of the methane-rich (e.g., ˜84% methane) bottoms liquid of auxiliary rectification column  260 . The vaporized methane-rich stream  275  is then preferably recycled as a fuel gas back to the steam reforming section of the ammonia product plant (not shown). 
     The auxiliary rectification column  260  is further staged to remove most all of the argon from the feed gas stream leaving a nitrogen-rich overhead gas  262 . A portion of the nitrogen-rich overhead gas  269 , which contains roughly 90% nitrogen, is directed to a condenser-reboiler  215  where it is condensed against a liquid nitrogen stream to produce a nitrogen rich reflux  263  that is re-introduced to rectification column  260 . Another portion of the nitrogen-rich overhead gas from column  260  is diverted as the hydrogen-nitrogen product gas  265  that warmed in the heat exchanger  210  and then may be recycled back to the ammonia synthesis section of the ammonia product plant. The vaporized portion of the nitrogen stream  233  from the condenser-reboiler  215  is combined with the waste nitrogen gas  285  and directed to the heat exchanger  210  where it is warmed to about ambient temperature. 
     Given sufficient staging in the rectification column  260 , argon accumulates above the methane removal section, which are generally the bottommost 15 to 20 stages in column  260 . A side liquid argon draw  266  is extracted from a point above the methane removal section approximately midway up the rectification column  260  to form an argon-rich stream  267 . The argon-rich stream  267  is preferably in liquid form and will typically contain trace amounts of hydrogen. The argon recovery can be enhanced even further by way of reboiling within rectification column, albeit at the expense of additional operating costs associated with the additional compression power required. 
     As seen in  FIG. 4 , the argon-rich stream  267  is then directed to the hydrogen removal arrangement which is shown as a small side stripper column  270  where the trace amounts of hydrogen in the argon-rich stream  267  are removed. The small side stripper column  270  preferably includes between about 4 and 7 stages of separation, with the stripped hydrogen being returned to the auxiliary rectification column  260  via stream  273 , discharged to vent or sent to a fuel header while the nitrogen and argon containing stream  272 , substantially free of hydrogen, is removed from small side stripper column  270 , valve expanded in valve  271  and then introduced as stream  274  to the argon and nitrogen distillation column  280 . The staging of the side stripper column  270  may vary depending upon the specification of product nitrogen. In some applications, the hydrogen separation may even be performed using any available hydrogen removal technologies including, for example, a falling film type evaporator or even a combination of the hydrogen stripping column and an evaporator. 
     The hydrogen-free, argon and nitrogen containing liquid is then directed to a distillation column  280  which serves to separate the nitrogen and argon. This distillation column  280  is preferably comprised of both a stripping section and a rectification section. The distillation column  280  produces a pure nitrogen overhead stream  285  a portion of which is preferably recycled to the heat exchanger  210  and then returned to the ammonia production plant. Distillation column  280  also includes a reboiler  284  configured to reboil the argon with a moderate pressure nitrogen gas stream to produce an ascending argon vapor and a liquefied nitrogen stream  287 . A first portion of the liquefied nitrogen stream may be depressurized via valve  292  and then directed to combined phase separator-subcooler vessel  294  or outside use. A second portion of the liquefied nitrogen  289  is employed as reflux to distillation column  280 . An additional fraction of the liquid nitrogen may be used supplement the refrigeration for the condenser  225 . A liquid argon product stream  245  is extracted from a location near the bottom of distillation column  280 . The liquid argon  245  may be further subcooled prior to being directed to suitable storage means or outside use. Also, while distillation column  280  typically operates at low pressure of between about 25 psia to about 30 psia, it is possible to operate distillation column  280  at an even lower pressure with an increase in the complexity and size of the recycle compression circuit. 
     In some embodiments, the methane, nitrogen, hydrogen and argon containing feed stream  235  may be pre-purified and/or compressed prior to entry to the heat exchanger. Similarly, the methane-rich bottoms liquid  264  may be adjusted in pressure prior to vaporization in the heat exchanger, by way of a pump, valve or static head. Also, depending upon the reforming train in the ammonia production plant, the hydrogen-nitrogen overhead from rectification column could be recombined with the methane-rich bottoms liquid and recycled back to the ammonia production plant as a fuel gas to fire the primary steam reformer. This mixing of the hydrogen-nitrogen overhead stream with the methane-rich stream can be done prior to or after warming in the primary or main heat exchanger. Alternatively, the hydrogen-nitrogen overhead stream may be compressed and reintroduced into synthesis gas train. 
     Another alternative embodiment of the present system and method of argon recovery from the tail gas of an ammonia production plant is contemplated wherein the hydrogen stripping or rejection column  270  may be simplified or even replaced with a phase separator or phase separation supplemented with a small amount of heat. It is also conceivable that the refrigeration circuit composition can be made to be independent from the distillation column  280  overhead composition. However, this will require an additional condenser associated with distillation column  280  as well as a reconfiguration of the liquid nitrogen process draw. Although not preferred, the operating pressure of distillation column  280  can be higher than the operating pressure of rectification column  260  if a liquid pump is used to direct the hydrogen free, argon and nitrogen containing liquid stream from side stripping column  270  to distillation column  280 . 
     Turning now to  FIG. 5 , there is shown another embodiment of the present system and method for argon and nitrogen recovery from a feed stream comprising primarily hydrogen, methane, nitrogen and argon, such as a moderate-pressure tail gas of an ammonia production plant of between roughly 50 psia and 500 psia. Hydrogen rejection, argon recovery and nitrogen recovery in the embodiment of  FIG. 5  is in many ways the same as or similar to the hydrogen rejection, basic argon recovery and nitrogen recovery systems and processes disclosed above with reference to  FIG. 4  to produce a pumped (via pump  330 ) and subcooled, high purity liquid argon stream  245  and high purity liquid nitrogen product stream  255 . The main differences between the embodiment of  FIG. 5  and the previously described embodiment of  FIG. 4  relates to the power benefits associated with the embodiment of  FIG. 5  and the production of a high quality fuel gas stream at sufficient pressure for return to the ammonia synthesis process, preferably as reformer  14  fuel or feed. In addition, while the above described embodiments of  FIGS. 1-4  prefer the feed stream to be a tail gas from an ammonia plant having a Braun Purifier, such is not the case with the embodiments of  FIGS. 5 and 6 . Rather, the system and method for argon recovery from a feed stream comprising hydrogen, methane, nitrogen and argon is suitable for use with classic ammonia synthesis tail gas processes in addition to Braun Purifier based ammonia tail gases. 
     As seen in  FIG. 5 , a pressurized feed stream  235 , preferably a feed gas stream between 50 psia and 400 psia, is directed to a turbine driven booster compression stage. The compressed feed gas stream  235  is compressed in compressor  230 , partially cooled in heat exchanger  210  and expanded, generally in the range of about 40 psia to about 80 psia in turboexpander  238 . The expanded feed gas stream  237  is then directed to the rectification column  260  where it is rectified to produce an argon depleted nitrogen-rich overhead gas  262 , a methane rich column bottoms  264  and an argon/nitrogen rich side draw  267  from an intermediate location  266  of the rectification column  260 . The side draw  267  is preferably in liquid form but alternatively may be in gaseous form or a two-phase stream. 
     Specifically, the nitrogen-rich overhead gas  262 ,  269 , which preferably contains roughly 90% nitrogen, is directed to a condenser-reboiler  215  where it is partially condensed against a liquid nitrogen stream. The partially condensed nitrogen-rich stream  263  that is directed to a phase separator  340  which separates the stream  263  into a liquid fraction  344  which is re-introduced to rectification column  260  as reflux and a vapor fraction  342  which is directed to another phase separator  350  where it is mixed with the methane rich liquid stream  275  to form the two phase fuel stream. The vapor portion of the two-phase fuel stream  354  is directed to a common passage in the multi-passage heat exchanger  210 . The liquid portion of the two phase fuel stream  352  exiting the phase separator  350  is also directed to the same common passage in the heat exchanger. If necessary, the liquid can be combined with the vapor stream after partial warming. The vaporized portion of the nitrogen stream  233  from the condenser-reboiler  215  is combined with the waste nitrogen gas  285  and directed to the heat exchanger  210  where it is warmed to about ambient temperature. In lieu of the phase separator  350 , it is contemplated that a static in-line mixer could be used to mix the vapor fraction  342  of the partially condensed nitrogen-rich stream  342  and the methane rich liquid stream  275  to form the two phase fuel stream. Alternatively, stream  342  and stream  275  may undergo partial warming in separate passages of primary heat exchanger  210 , at a temperature lower than the bubble point temperature of stream  275  after which the two streams may then be mixed. 
     The effect of this fluid mixing in either the phase separator  350  or a static in-line mixer is the drastic lowering of the dewpoint of the two phase fuel stream mixture and thus the vaporization region of the mixture within the primary heat exchanger  210 . This, in turn, allows the combined fuel gas stream  265  exiting the primary heat exchanger  210  to be at a higher pressure compared to separate warming of the streams in the primary heat exchanger  210  (as shown in  FIG. 4 ). The resulting mixed fuel gas stream may be further compressed as necessary. The higher pressure fuel gas stream  265  exiting the primary heat exchanger  210  is preferably delivered to or directed to the reforming process of the ammonia plant for purposes of firing the reformer. In some instances it may be used as regeneration gas for prepurification units (not shown). 
     In applications where higher recovery of argon is needed or desired, one can boost overall argon recovery by supplemental argon recycle from the methane rich fuel gas stream as shown generally in  FIG. 6 . The embodiment shown in  FIG. 6  is similar in many regards to the embodiment of  FIG. 5 . Since the methane rich liquid bottoms  264  taken from the rectification column  260  is comprised primarily of methane with some argon impurities of perhaps between 5% and 25% of the argon contained in the incoming feed stream  235 . The preferred approach is to ‘strip’ the argon impurities from the methane rich liquid bottoms  264  in a stripping column arrangement  300 . Gas stripping is a term used to describe the countercurrent contacting of vapor and liquid streams wherein a component of the descending liquid is ‘stripped’ from the descending liquid and carried with the ascending vapor flow to the column overhead. 
     As shown in  FIG. 6 , argon is stripped form the descending liquid methane rich liquid bottoms  264  in the stripping column  310  with the ascending vapor being an argon-depleted vapor stream  315  such as a portion of the argon depleted nitrogen-rich overhead gas  262  from the rectification column  262 . Alternatively, the argon-depleted vapor stream  315  may originate as a stream from nitrogen refrigeration circuit, or the argon column rectification overhead. Whatever the source, the argon-depleted vapor stream  315  may be conditioned by means of any combination of warming, cooling, prepurification, compressing or expanding the argon-depleted vapor stream. 
     The argon-depleted vapor stream  315  has an argon concentration less than the argon concentration of the methane-rich liquid stream and a dewpoint lower than the methane rich liquid bubble point. The resulting liquid bottoms in the stripping column  310  consists of liquid methane substantially free of argon. A stream  325  of the liquid bottoms is directed to the phase separator  350  where it is mixed with the vapor portion  342  of the partially condensed nitrogen rich stream  263  to form the two phase fuel stream. The vapor fraction  354  and liquid fraction  352  of the two phase fuel stream are directed to the common passage in a multi-passage heat exchanger  210  where it cools the incoming compressed feed gas stream  236  and forms the methane rich fuel gas stream  265 . Alternatively, stream  325  and vapor portion  342  of the partially condensed nitrogen rich stream  263  may be warmed separately (and perhaps at different pressures prior to directing the streams to the common passage in a multi-passage heat exchanger  210 . The overhead gas  326  from the stripping column  310  contains the stripped argon together with the bulk of the ascending argon-depleted vapor and is recycled back and mixed with the incoming feed stream  235  via the primary heat exchanger  210  to increase the overall argon recovery of the system  200 . 
     In the illustrated embodiment of  FIG. 6 , the incoming feed stream  235  is optionally compressed in one or more compressors  205 ,  230 , preferably turbine driven compressors, and cooled with intercoolers/aftercoolers  231  to form a high pressure feed stream  236  that is partially cooled in the primary heat exchanger  210 . After partial cooling in the primary heat exchanger  210 , the feed stream  236  shown in  FIG. 6  is then expanded in turbo-expander  238  to generate refrigeration with the resulting lower pressure feed stream  237  being directed to the base of the rectification column  260 . Alternatively, the incoming feed stream  235  may be cooled directly and refrigeration is generated solely from the refrigeration circuit. 
     While the present invention has been described with reference to one or more preferred embodiments and operating methods associated therewith, it should be understood that numerous additions, changes and omissions to the disclosed system and method can be made without departing from the spirit and scope of the present invention as set forth in the appended claims.