Argon and nitrogen coproduction process
The present invention is directed to a process for the production of argon and nitrogen (separately) from air. 
     BACKGROUND OF THE INVENTION 
     Current and projected argon demand currently exceeds the amount of argon available as a product of conventional air separation units (ASUs). Accordingly, Applicants have undertaken a search for new sources of argon and the development of new and economical methods of producing argon. 
     The present invention utilizes pressure swing adsorption at low temperatures to separate argon and nitrogen from air and subsequently utilizes cryogenic distillation to separate argon and nitrogen. This approach avoids the production of oxygen while producing both nitrogen and argon. 
     OBJECTS OF THE INVENTION 
     An object of the present invention is to produce argon and nitrogen from air without simultaneously producing oxygen. 
     A further object of the present invention is the coproduction of argon and nitrogen which is advantageous compared to the production of nitrogen via a nitrogen generator, or to the production of solely argon and nitrogen by an air separation unit (ASU). 
     Yet a further object of the present invention is to coproduce nitrogen and argon utilizing a low temperature PSA separation that provides a high yield of nitrogen and argon. 
     The invention will be more clearly understood by reference to the following description of exemplary embodiments thereof in conjunction with the following drawing in which the sole FIGURE is a diagramatic view of apparatus for performing the process according to the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The Sole FIGURE is a schematic diagram of a process for the coproduction of argon and nitrogen according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the sole FIGURE, a feed air stream 1 is passed to a compressor 2, thereby producing an air stream 3 at an elevated pressure. The air is normally compressed to a pressure of 30-60 psig depending on the pressure at which the nitrogen product in stream 46 is desired. If the product nitrogen is needed at a pressure higher than 50 psig, a nitrogen product compressor (not shown) is optional. Compressed air stream 3 exiting compressor 2 is cooled in an after-cooler 4 to condense some of the contained water. Carbon dioxide and remaining water in the cooled air stream 5 are removed by conventional purification means such as a molecular sieve pre-purification unit 6 containing commercially available 13× molecular sieve. In practice, two adsorbent beds are used to allow continuous operation, wherein the bed not used for purification undergoes thermal regeneration as later described. Having been depleted of carbon dioxide and water, air stream 7 enters a first heat exchanger 14, wherein the air is cooled against returning process streams to be later described. Exiting the heat exchanger 14 at a reduced temperature, the cold air stream 15 is introduced into a pressure swing adsorption unit (PSA unit) 17 containing two or more beds of carbon molecular sieve, as more fully described later. 
     The PSA unit 17 produces a PSA product stream 18 comprising a mixture of nitrogen and argon and a small amount of oxygen, typically 0.05 to 0.1%, and a PSA waste stream 16 enriched in oxygen. 
     The PSA unit 17 comprises a plurality of adsorption beds each of which contains a suitable adsorbent. The adsorbent must have a greater selectivity for both argon and nitrogen than oxygen. Carbon molecular sieve (CMS) is the preferred adsorbent and in particular the CMS commercially available from Kuraray Chemical Company in Japan. Alternatively, carbon molecular sieve manufactured by Bergbau-Forschung in West Germany is suitable. The PSA operates at a temperature depending on the sieve material used, the aim being to maximize the nitrogen and argon yields while keeping oxygen to a very low level in the PSA product. For the PSA embodiment employing Kuraray CMS, an operational temperature range of 60° F. to -40° F. is suitable, with a range of 10° F. to -10° F. being the preferred range for the coproduction process. At a temperature of 0° F., the Kuraray CMS was found to give a nitrogen yield of 55% and an argon yield of 70% in a product containing 0.1% O 2 , as more fully described later. 
     The PSA unit 17 is regenerated periodically in two steps. The pressure of each bed is alternatively reduced to atmospheric pressure and then further reduced to an appropriate vacuum level using a vacuum pump 11. The pressure and vacuum portions of the waste stream 16 are segregated and are appropriately utilized as discussed below. 
     The PSA waste stream 16 is returned to the first heat exchanger 14, where its refrigeration value is recovered. The atmospheric pressure portion of the PSA waste may be warmed in a heater 8 and used to regenerate the bed not undergoing adsorption in the molecular sieve pre-purification unit 6 while stream 10, the vacuum portion of the PSA waste, is drawn through the vacuum pump 11 as discussed above. After being passed through the molecular sieve pre-purification unit 6, the heated waste stream picks up the carbon dioxide and water previously adsorbed and exits the molecular sieve pre-purification unit as stream 12. This waste stream 12 and the waste stream 10 exiting the vacuum pump 11 may be combined and vented directly to atmosphere as stream 13. 
     The PSA product stream 18 is passed through the PSA product receiver 19 in order to minimize flowrate fluctuations encountered during bed switch over in the PSA unit 17. Exiting the product receiver 19, the gaseous mixture is subsequently passed to a second heat exchanger 20, where it is cooled close to its dew point against a returning nitrogen product stream 45. The cooled stream 21 is then introduced into distillation column 22 wherein the mixture of nitrogen, argon, and a trace amount of oxygen is separated into pure nitrogen and crude argon products. 
     The refrigeration for the distillation column 22 may be provided by an external heat-pump circuit as shown by dotted lines in the sole FIGURE. A low pressure heat-pump fluid 47 which may be a mixture of argon and nitrogen, is compressed in compressor 48 and exits at an appropriate pressure as more fully described later. Compressed heat-pump fluid stream 49 is then successively cooled in first heat exchanger 14 and second heat exchanger 20 against the cold heat-pump fluid stream 44 returning from the third exchanger 36. The cold high pressure heat-pump fluid stream 43 exiting exchanger 20 is divided into two parts. The first part of this stream, typically 2-10% of the total stream, enters exchanger 36 as stream 42 where it is condensed against the low pressure heat-pump fluid stream 34 and pure nitrogen product stream 29 returning from distillation column 22. The stream 35 leaving exchanger 36 is expanded in a Joule-Thomson (J-T) valve 37 to an appropriate pressure as discussed below. The remaining portion of stream 43 enters a turboexpander 40 as stream 41 where it is expanded to produce a colder stream 39 exiting the turboexpander. The stream 41 is expanded to the same pressure as stream 35 so that the pressures of stream 39 and 38 are the same. The streams 39 and 38 are combined as stream 30 and enter reboiler 23 at the bottom of distillation column 22. 
     In the reboiler, stream 30 is condensed against the reboil vapors leaving the reboiler. The condensed heat-pump fluid stream 31 exiting reboiler 23 is then expanded in a Joule-Thomson valve 32 to yield an even colder low pressure heat-pump fluid stream 33. This latter stream is passed to a condenser 25 where it is used to condense part of the overhead nitrogen product stream 27 leaving the distillation column which divides into reflux stream 28 and nitrogen product stream 29. The reflux stream 28 stream comprising essentially pure nitrogen is condensed in condenser 25 and returned to the distillation column 22 as reflux stream 26. 
     The composition and pressure of stream 30 are chosen in such a way that it condenses against the reboil vapors in reboiler 23. In addition, the stream after being expanded in a J-T valve 32 should condense an appropriate amount of overhead nitrogen product to provide the necessary reflux to the distillation column 22. Also, an overall energy balance across the distillation column, which takes column heat in-leak into account, must be satisfied. For a heat-pump fluid stream consisting of a 50:50 mixture of argon and nitrogen, the pressure of stream 30 may be 78.0 psia for a distillation column operating at a pressure of about 45.0 psia, and the flowrate and temperature of stream 30 may be determined by the amount of material processed in the distillation column and by the overall energy balance across the distillation column, respectively. Once stream 30 is completely characterized, the pressure of the high pressure heat-pump fluid stream 49 may be determined by making an overall plant energy balance. 
     Both the low pressure heat-pump fluid stream 34 leaving the distillation column overhead condenser 25 and the uncondensed portion of nitrogen product stream 29 are warmed in third heat exchanger 36, second heat exchanger 20 and first heat exchanger 14, respectively, in order to recover their refrigeration. The nitrogen product stream is sent via stream 46 to storage, a nitrogen liquefaction plant, or delivered to an industrial user. The low pressure heat-pump fluid stream after being warmed up in exchangers is compressed in compressor 48 and used in the aforementioned heat-pump refrigeration circuit. In addition to the overhead pure nitrogen product stream mentioned before, the distillation column 22 concurrently produces an aforementioned liquid crude argon stream 24 as the bottoms product. This stream may be further purified to produce pure argon in equipment (not shown) such as a conventional DeOxo unit. 
     EXAMPLE 1 
     This example illustrates the low temperature PSA used in Applicant&#39;s argon and nitrogen coproduction process. In contrast with ambient temperature PSA, relatively longer cycle times are needed for the low temperature PSA. Also, since the gases adsorb much more strongly at low temperature, vacuum regeneration is essential for low temperature PSA. For the low temperature PSA discussed in this example, an adsorption pressure of 36.0 psig, a desorption pressure of 100 mbar, and an operating temperature of 0° F. was used. A 3-bed PSA employing a mixture of aforementioned Kuraray CMS, 2 mm+40-60 mesh, was used. 
     The complete cycle for this low temperature PSA is shown in Table I below. In each half of this cycle (separated by the dotted line in the Table), one of the primary beds (bed A or B) is first pressurized (step a or h) and then set to produce nitrogen product. During this time, the other primary bed undergoes regeneration by either atmospheric or vacuum venting (steps b and c or i and j). Subsequently, while the first primary bed continues to produce, the secondary bed (bed C) is regenerated through the primary bed undergoing regeneration (step d or k). For part of each half cycle, the product from the first primary bed is passed through the secondary bed (bed c) for further purification, while the other primary bed continues to be regenerated by vacuum venting (step e or l). After the end of production in each half cycle, pressure equalization between the secondary bed and the regenerated primary bed is effected and then the pressure equalization between the primary beds is effected. 
     In comparison with ambient temperature PSA, where a total cycle time of 4 to 8 min. will normally be used, the cycle time for this low temperature PSA is 2 to 4 times longer. For the conditions discussed in this example, a nitrogen yield of 55% and an argon yield of 70% for a product containing 0.1% oxygen was obtained. 
     
                       TABLE I                                                     
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FULL CYCLE SEQUENCE FOR 3-BED LOW                                         
TEMPERATURE CARBON MOLECULAR SIEVE PSA                                    
                                   Typical                                
Step Bed A          Bed B          Time (sec)                             
______________________________________                                    
a    Pressurize     Vent to atm.   14                                     
b    Produce nitrogen                                                     
                    Vent to atm.   100                                    
     product                                                              
c    Produce nitrogen                                                     
                    Vent through the                                      
                                   40                                     
     product        vacuum pump                                           
d    Produce nitrogen                                                     
                    Vent Bed C through                                    
                                   80                                     
     product        Bed B under vacuum                                    
e    Purify nitrogen                                                      
                    Vent through the                                      
                                   240                                    
     product through                                                      
                    vacuum pump                                           
     Bed C                                                                
f                   Pressure equalize                                     
                                   2                                      
                    with Bed C                                            
g    Pressure equalize                                                    
                    Pressure equalize                                     
                                   4                                      
     with Bed B     with Bed A                                            
h    Vent to atm.   Pressurize     14                                     
i    Vent to atm.   Produce nitrogen                                      
                                   100                                    
                    product                                               
j    Vent through the                                                     
                    Produce nitrogen                                      
                                   40                                     
     vacuum pump    product                                               
k    Vent Bed C through                                                   
                    Produce nitrogen                                      
                                   80                                     
     Bed A under vacuum                                                   
                    product                                               
l    Vent through the                                                     
                    Purify nitrogen                                       
                                   240                                    
     vacuum pump    product through                                       
                    Bed C                                                 
m    Pressure equalize             2                                      
     with Bed C                                                           
n    Pressure equalize                                                    
                    Pressure equalize                                     
                                   4                                      
     with Bed B     with Bed A                                            
                                   16.0                                   
                                   min/cycle                              
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     EXAMPLE 2 
     Typical pressure, temperature and composition of various process streams for Applicant&#39;s argon and nitrogen coproduction process are illustrated in this example and are listed in the accompanying Table II. The stream numbers correspond to the sole FIGURE. Only compositions of nitrogen, oxygen and argon in various streams are shown. Other components such as carbon dioxide and water may be present in small amounts in some of the streams. This example is for the case in which the feed air is compressed to a pressure of 49.0 psia and the low temperature PSA is operated at a temperature of 0° F. The nitrogen product leaving the distillation column is at a pressure of 44.0 psia while the crude argon product leaving the distillation column is a saturated liquid. A total of 40 theoretical distillation column trays produce nitrogen and argon products whose compositions are listed in the accompanying Table II. 
     
                                           TABLE II                                
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TYPICAL PROCESS STREAM CONDITIONS FOR THE ARGON AND NITROGEN              
COPRODUCTION PROCESS                                                      
(N2 = NITROGEN, O2 = OXYGEN and Ar = ARGON)                               
                                STREAM    STREAM                          
STREAM                                                                    
      STREAM    STREAM STREAM   TEMPERATURE                               
                                          PRESSURE                        
NUMBER                                                                    
      NAME      STATE  COMPOSITION                                        
                                (Deg. F)  (PSIA)                          
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 1    Feed air  Superheated                                               
                       0.781 N2 80.0      14.7                            
                vapor  0.209 O2                                           
                       0.010 Ar                                           
 5    Compressed feed                                                     
                Superheated                                               
                       0.781 N2 80.0      49.0                            
      leaving after-                                                      
                vapor  0.209 O2                                           
      cooler 4         0.010 Ar                                           
13    PSA waste Superheated                                               
                       0.648 N2 77.0      15.0                            
                vapor  0.348 O2                                           
                       0.004 Ar                                           
15    Cold feed to PSA                                                    
                Superheated                                               
                       0.781 N2 0.0       47.0                            
      unit 17   vapor  0.209 O2                                           
                       0.010 Ar                                           
16    Cold nitrogen PSA                                                   
                Superheated                                               
                       0.648 N2 -5.0      5.0                             
      waste     vapor  0.348 O2                                           
                       0.004 Ar                                           
18    Nitrogen product                                                    
                Superheated                                               
                       0.981 N2 5.0       46.0                            
      from low  vapor  0.018 Ar                                           
      temperature PSA  0.001 O2                                           
21    Nitrogen PSA                                                        
                Superheated                                               
                       0.981 N2 -296.8    45.0                            
      product leaving                                                     
                vapor  0.018 Ar                                           
      exchanger 20     0.001 O2                                           
24    Crude argon                                                         
                Saturated                                                 
                       0.00132 N2                                         
                                -281.0    44.0                            
      product leaving                                                     
                liquid 0.05513 O2                                         
      distillation     0.94355 Ar                                         
      column 22                                                           
26    Pure nitrogen                                                       
                Saturated                                                 
                       0.99908 N2                                         
                                -301.2    44.0                            
      product leaving                                                     
                liquid 0.000001 O2                                        
      reflux condenser 0.000919 Ar                                        
      25                                                                  
27    Overhead vapors                                                     
                Saturated                                                 
                       0.99908 N2                                         
                                -301.2    44.0                            
      leaving   vapor  0.000001 O2                                        
      distillation     0.000919 Ar                                        
      Column 22                                                           
28    Pure nitrogen                                                       
                Saturated                                                 
                       0.99908 N2                                         
                                -301.2    44.0                            
      product entering                                                    
                vapor  0.000001 O2                                        
      reflux condenser 0.000919 Ar                                        
      25                                                                  
29    Pure nitrogen                                                       
                Saturated                                                 
                       0.99908 N2                                         
                                -301.2    44.0                            
      product entering                                                    
                vapor  0.000001 O2                                        
      exchanger 36     0.000919 Ar                                        
30    High pressure                                                       
                Vapor-liquid                                              
                       0.50 N2  -276.8    78.0                            
      heat-pump fluid                                                     
                mixture                                                   
                       0.50 Ar                                            
      entering                                                            
      condenser 23                                                        
31    High pressure                                                       
                Vapor-liquid                                              
                       0.50 N2  -280.6    78.0                            
      heat-pump fluid                                                     
                mixture                                                   
                       0.50 Ar                                            
      entering J-T                                                        
      valve 32                                                            
33    Low pressure                                                        
                Vapor-liquid                                              
                       0.50 N2  -310.4    18.0                            
      heat-pump fluid                                                     
                mixture                                                   
                       0.50 Ar                                            
      entering                                                            
      condenser 25                                                        
34    Cold low  Saturated                                                 
                       0.50 N2  -306.4    18.0                            
      pressure heat-                                                      
                vapor  0.50 Ar                                            
      pump fluid                                                          
      entering ex-                                                        
      changer 36                                                          
35    Heat-pump fluid                                                     
                Subcooled                                                 
                       0.50 N2  -280.7    100.0                           
      stream leaving                                                      
                liquid 0.50 Ar                                            
      exchanger 36                                                        
38    Heat-pump fluid                                                     
                Saturated                                                 
                       0.50 N2  -280.7    78.0                            
      stream leaving                                                      
                liquid 0.50 Ar                                            
      J-T valve 37                                                        
39    Heat-pump fluid                                                     
                Vapor-liquid                                              
                       0.50 N2  -276.7    78.0                            
      stream leaving                                                      
                mixture                                                   
                       0.50 Ar                                            
      turboexpander 40                                                    
43    High pressure                                                       
                Superheated                                               
                       0.50 N2  -269.7    101.0                           
      heat-pump fluid                                                     
                vapor  0.50 Ar                                            
      stream leaving                                                      
      exchanger 20                                                        
44    Low pressure                                                        
                Superheated                                               
                       0.50 N2  -299.1    17.0                            
      heat-pump fluid                                                     
                vapor  0.50 Ar                                            
      stream leaving                                                      
      exchanger 36                                                        
45    Pure nitrogen                                                       
                Superheated                                               
                       0.99908 N2                                         
                                -299.1    43.0                            
      product leaving                                                     
                vapor  0.000001 O2                                        
      exchanger 36     0.000919 Ar                                        
46    Warm pure Superheated                                               
                       0.99908 N2                                         
                                77.0      41.0                            
      nitrogen product                                                    
                vapor  0.000001 O2                                        
                       0.000919 Ar                                        
47    Warm low  Superheated                                               
                       0.50 N2  77.0      15.0                            
      pressure heat-                                                      
                vapor  0.50 Ar                                            
      pump fluid                                                          
49    Warm high Superheated                                               
                       0.50 N2  80.0      103.0                           
      pressure heat-                                                      
                vapor  0.50 Ar                                            
      pump fluid                                                          
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     The foregoing and other various changes in form and details may be made without departing from the spirit and scope of the present invention. Consequently, it is intended that the appended claims be interpreted as including all such changes and modifications.