Abstract:
A method and apparatus for the recovery of crude neon in or as part of a cryogenic air separation system wherein a neon recovery tower recovers crude neon from a nitrogen product stream originating from the top of the high pressure tower, and wherein the cooling for condensing in the neon recovery tower is provided by evaporating the liquefied nitrogen product from the bottom of the neon tower after the nitrogen liquid passes through a pressure reducing valve.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of provisional patent application No. 61/209,011 filed 2009 Mar. 2 by the present inventor and claims the benefit of provisional patent application No. 61/216,879 filed 2009 May 23 by the present inventor. 
     
    
     FEDERALLY SPONSORED RESEARCH 
       [0002]    Not applicable. 
       SEQUENCE LISTING 
       [0003]    Not applicable. 
       BACKGROUND 
       [0004]    1. Field of Invention 
         [0005]    This invention relates to a cryogenic process for recovering crude neon in a double tower cryogenic air separation plant. 
         [0006]    2. Prior Art 
         [0007]    As used herein the term “column” means a distillation or fractionation column, i.e., a contacting column wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column or on packing elements such as structured or random packing. 
         [0008]    As used herein the term “high-pressure tower” means the tower in the cryogenic air separation double tower system which operates at the higher pressure, usually in the range of about 5-6 ATMA. As used herein the term “low pressure tower” means the tower in the cryogenic air separation double tower system which operates at a lower pressure, usually about 1.2-1.6 ATMA. 
         [0009]    As used herein the term “heat exchanger” is a device for effecting indirect heat exchange by bringing two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other. 
         [0010]    As used herein the terms “reboiler” means a heat exchanger device that generates vapor from liquid. 
         [0011]    As used herein the term “condenser” means a heat exchange device that generates column liquid from vapor. 
         [0012]    As used herein the term “reboiler-condenser” means a device that simultaneously performs functions of a reboiler and condenser. 
         [0013]    In the typical cryogenic air separation process described, an expander is a device which produces refrigeration by extracting work from a fluid while lowering the fluid pressure and simultaneously reducing the temperature of the expanded fluid. 
         [0014]    The recovery of crude neon in cryogenic air separation plants is well known and has been widely practiced. The book “Cryogenic Systems” by Randall Barron (published in 1966 by McGraw-Hill) on pages 248-249 and in  FIG. 4-38  describes a conventional “neon-recovery subsystem” where nitrogen-rich gas leaving the condenser at the top of the high pressure tower in a typical double tower air separation system enters a crude neon tower. Condensing refrigeration in the neon tower is provided by liquid nitrogen taken from the top of the high pressure tower, which is reduced in pressure and the evaporated nitrogen vapor joins the nitrogen product leaving that top of the upper (or low pressure) section of the double tower system. A similar description of neon recovery in connection with a typical cryogenic air separation system double tower is given in the book “Separation of Gases” by W H Isalski on pages number 100-101 (published by Clarendon Press-Oxford, 1989). 
         [0015]    This invention relates to the recovery and production of crude neon in connection with a cryogenic air separation plant which produces oxygen and/or nitrogen from ambient air, utilizing a double tower distillation system. The double tower distillation system is the most common system used in the cryogenic separation of air to produce oxygen and/or nitrogen, and is described in the book “Cryogenic Systems” by Randall Barron (published in 1966 by McGraw-Hill) on pages 236-239 and in  FIG. 4-32 . 
         [0016]    Some of the significant features of the prior art relating to Neon recovery cryogenic air separation plants (as, for instance, shown in FIG. 4-38 and the accompanying description in “Cryogenic Systems”, by Randall Barron, published in 1966 by McGraw-Hill), as contrasted to the present invention include: 
         [0000]    1.) Nitrogen vapor from the top of the high-pressure Tower enters the Neon condenser-rectifier, and the condensed nitrogen is returned to the top of the high-pressure Tower.
 
1A.) In this invention, the nitrogen vapor from the top of the high-pressure Tower which enters the crude neon recovery tower is evaporated at a slightly lower pressure, and the nitrogen vapor does not return to the high-pressure Tower, but leaves the ASU system as Nitrogen vapor product.
 
2.) In the above referenced prior art example, liquid nitrogen evaporated in the crude neon condenser-rectifier is evaporated at low-pressure Tower pressure.
 
2A.) In the present invention, liquid nitrogen evaporated in the crude neon condenser-rectifier is evaporated at an elevated pressure, substantially above the low-pressure Tower pressure and close to the high-pressure Tower pressure.
 
3.) In the above referenced prior art example, nitrogen which is condensed in the neon condenser-rectifier does not provide reboiling at the bottom of the low-pressure Tower, and because of the reduction in low-pressure Tower reboiling, the recovery of Oxygen in the low-pressure Tower is reduced.
 
3A.) In the present invention, the nitrogen vapor which goes to the neon condenser-rectifier is intermediate (high-pressure Tower) pressure nitrogen vapor product, and no additional nitrogen vapor is withdrawn from the high-pressure Tower, so the reboiling and oxygen recovery in the low-pressure Tower is not affected.
 
       ADVANTAGES 
       [0017]    An advantage of this invention is that, in a cryogenic air separation double tower system producing vapor nitrogen product from the top of the high-pressure tower, no liquid nitrogen is required from the double tower system. The use of liquid nitrogen from the double tower system in the neon tower condenser removes liquid nitrogen which would otherwise be used as reflux in the double tower system, and/or reduces the reboiling in the double tower low-pressure tower, thus reducing the separation or recovery of oxygen and/or nitrogen in the cryogenic air separation double tower system. In the improved method and apparatus of this invention, the nitrogen product “self-condenses” in the crude neon tower, without any effect on the flows or recovery in the cryogenic air separation double tower system. An important aspect of the invention is that the only connection between the crude neon tower and the typical cryogenic air separation double tower is the nitrogen product stream leaving the top of the high-pressure tower or leaving the high pressure tower condenser, and there is no other connection between the crude neon tower and the double tower system. The use or addition of the crude neon recovery tower therefore has no effect on the operation of the double tower system. 
         [0018]    Another advantage of this invention is that the product and nitrogen stream originating at the top of the high-pressure tower is only partly reduced in pressure in the crude neon recovery tower, and is available as a nitrogen vapor product at an intermediate pressure, higher than the low pressure, but partly reduced below the high-pressure column pressure. 
         [0019]    In summary, the main elements of this invention which distinguish it from prior art and which are its advantages relative to prior art include:
       A.) The crude neon tower is “self-condensing” and does not require withdrawal of liquid nitrogen from the cryogenic air separation double tower system. Such withdrawal of liquid nitrogen can degrade the separation performance of the double tower distillation system.   B.) The nitrogen product from the top of the high-pressure tower, which is directed to the crude neon recovery distillation tower, is only partly reduced in pressure to provide condensing refrigeration in the crude neon tower. It is not, as in some prior art, reduced to the pressure of the low pressure distillation column, to be delivered as a product at near atmospheric pressure. In this invention, the product nitrogen stream is partly reduced from the pressure of the high-pressure distillation tower by approximately 5-25 PSI, and is available as a nitrogen vapor product at an intermediate pressure, higher than the low pressure column pressure but partly reduced below the high-pressure column pressure.   C.) Other than that nitrogen product stream originating at the top of the high-pressure distillation column, which is the feed stream to the crude neon recovery distillation column, there is no connection of streams to or from the crude neon tower and the double tower distillation system. Because of this lack of connection, the function and operation of the crude neon tower does not have any effect on the normal operation of the double tower distillation system.       
 
       SUMMARY 
       [0023]    This invention is applicable to the recovery of crude neon in a cryogenic air separation double tower system, where a nitrogen vapor product is produced, or is desired to be produced, originating from the top of the lower or high pressure tower or from the vapor entering or leaving the condenser of the high pressure tower. The nitrogen vapor product from the high pressure tower enters the bottom of the neon recovery tower and is enriched in neon as it flows to the top of the neon tower. The nitrogen condensed in the neon tower condenser flows to the bottom of the neon recovery tower, where it is removed from the neon tower bottom, passes through a pressure reducing valve and enters the cold side of the neon tower condenser, and is evaporated to provide condensing refrigeration in the neon tower. The evaporated nitrogen is then warmed in the air separation plant main heat exchanger or other heat exchanger to become nitrogen vapor product. The crude neon product, comprising a small fraction [approximately 0.033%] of the air entering the double tower system, can be warmed and produced as crude neon product, or can enter a second stage neon purification tower which may produce a higher purity crude neon product. 
     
    
     
       DRAWINGS 
       Figures 
         [0024]      FIG. 1  is the neon recovery process in a typical double tower cryogenic air separation system. 
           [0025]      FIG. 2  is the primary crude neon tower. 
           [0026]      FIG. 3  is the primary and second stage neon towers. 
       
    
    
     REFERENCE NUMERALS 
       [0027]      
         [0000]    
       
         
               
             
               
               
             
           
               
                   
               
               
                 Drawing Reference Numerals 
               
             
          
           
               
                 Part No. 
                 Part Name 
               
               
                   
               
               
                   
                 Stream, N2 Liquid 
               
               
                 101 
                 Valve 
               
               
                 102 
                 Stream, N2 Liquid and Vapor 
               
               
                 103 
                 Stream, Crude Neon Product 
               
               
                 104 
                 Stream, N2, Evaporated 
               
               
                 105 
                 Stream, N2 Enriched 
               
               
                 106 
                 Stream, N2 Enriched Vapor Product 
               
               
                 107 
                 Stream, O2 Enriched 
               
               
                 108 
                 Stream, N2 Enriched 
               
               
                 109 
                 Stream, N2 Reflux 
               
               
                 110 
                 Stream, Air 
               
               
                 111 
                 Stream, O2 Enriched 
               
               
                 112 
                 Stream, N2 Enriched 
               
               
                 113 
                 Stream, Air 
               
               
                 114 
                 Stream, N2 Enriched 
               
               
                 115 
                 Stream, O2 Product 
               
               
                 116 
                 Stream, Air 
               
               
                 117 
                 Stream, O2 Liquid Product 
               
               
                 118 
                 Stream, N2 Vapor Product 
               
               
                 119 
                 Stream, Air 
               
               
                 120 
                 Stream, O2 Vapor Product 
               
               
                 121 
                 Stream, O2 Product 
               
               
                 122 
                 Stream, Air 
               
               
                 123 
                 Stream, N2 Enriched Product 
               
               
                 124 
                 Valve 
               
               
                 125 
                 Stream, O2 Enriched, Pressure-Reduced 
               
               
                 126 
                 Stream, N2 Liquid 
               
               
                 127 
                 Stream, N2 Reflux 
               
               
                 128 
                 Valve 
               
               
                 129 
                 Stream, O2 Enriched, Cooled 
               
               
                 130 
                 Stream, N2 Vapor Product 
               
               
                 131 
                 Stream, N2 
               
               
                 132 
                 Valve 
               
               
                 133 
                 Stream, N2 
               
               
                 134 
                 Stream, Enriched Neon Product 
               
               
                 135 
                 Stream, N2 Vapor 
               
               
                 136 
                 Heat Exchanger 
               
               
                 137 
                 Stream, N2 
               
               
                 138 
                 Vacuum Pump 
               
               
                 139 
                 Stream, Exhaust 
               
               
                 500 
                 Neon Recovery Tower 
               
               
                 501 
                 Stream, N2 Vapor 
               
               
                 502 
                 Heat Exchanger (subcooler) 
               
               
                 503 
                 Second Heat Exchanger (subcooler) 
               
               
                 504 
                 High Pressure Distillation Tower 
               
               
                 505 
                 Reboiler-Condenser 
               
               
                 506 
                 Liquid Oxygen Pump 
               
               
                 507 
                 Air Expander 
               
               
                 508 
                 Main Heat Exchanger 
               
               
                 509 
                 Low Pressure Distillation Tower 
               
               
                 510 
                 Neon Tower (condenser) 
               
               
                 511 
                 Stream, N2 Liquid 
               
               
                 512 
                 Second Stage Recovery Tower 
               
               
                 513 
                 Stream, N2 Vapor 
               
               
                 514 
                 Neon Tower (condenser) 
               
               
                 515 
                 Stream, N2 Liquid 
               
               
                 516 
                 Stream, Supplemental N2 Liquid 
               
               
                 517 
                 Valve 
               
               
                 518 
                 Stream, N2 Liquid 
               
               
                   
               
             
          
         
       
     
       DETAILED DESCRIPTION 
     FIGS.  1  and  2   
     First Embodiment 
       [0028]      FIG. 1  is a simplified schematic representation of one embodiment of the cryogenic rectification system of this invention for recovery of crude neon in a cryogenic air separation double tower system, where a nitrogen vapor product is produced, or is desired to be produced, originating from the top of the lower or high pressure tower or from the vapor entering or leaving the condenser of the high pressure tower. Referring to  FIG. 1 , typical components of a cryogenic air separation system utilizing a double tower are shown, including a main heat exchanger  508  for heat exchange between air feed and products; and further showing a high pressure distillation tower  504 , and low pressure distillation tower  509 , including reboiler-condenser  505 . An air expander  507  to produce low-temperature refrigeration is shown and a liquid oxygen pump  506  is also shown. Heat exchanger  502  and heat exchanger  503  are subcoolers associated with the double tower cryogenic air separation system. It is understood that these components of a “typical cryogenic air separation system” utilizing a double tower distillation system are subject to considerable variation in detail, and may include fewer or more components in the cryogenic air separation system utilizing the double tower distillation process. 
       Operation 
     FIGS.  1  and  2   
     First Embodiment 
       [0029]    In  FIG. 1 , ambient air, compressed to a pressure above the high-pressure tower pressure and after pre-purification to remove condensable components such as carbon dioxide and moisture, enters the feed-product main heat exchanger  508  as stream  122 , and after cooling leaves as stream  113 , and enters the bottom of the high pressure tower  504 . An additional air feed stream  119  may also enter the main heat exchanger  508  and after cooling leave the main heat exchanger  508  as stream  116 , which enters the expander  507  and after reduction in temperature leaves the expander as stream  110 , which enters the low pressure tower  509 . From the top of the high-pressure tower  504 , a nitrogen-rich stream  108  enters the reboiler-condenser  505 , and is condensed to form stream  109  which provides liquid reflux at the top of the high-pressure tower  504 . A nitrogen vapor product stream  130  is withdrawn from the vapor stream  108  leaving the top of the high-pressure tower and entering the reboiler-condenser. Alternately, the nitrogen vapor product stream  130  can be withdrawn as uncondensed vapor at the exit of the reboiler-condenser  505 . In addition to the nitrogen vapor product stream  130 , a liquid nitrogen-rich stream  112  is withdrawn from the high-pressure tower at a point intermediate between the top tray or top of the packing section and the bottom tray or bottom of the packing section in the high-pressure tower. The liquid nitrogen stream  112  is sub-cooled in subcooler heat exchanger  503  to form stream  126 , which is reduced in pressure in valve  128  and enters the top of the low pressure tower  509  as stream  127  to provide reflux liquid at the top of the low pressure tower  509 . A liquid stream, enriched in oxygen relative to the air feed, stream  107  is withdrawn from the bottom of the high-pressure tower  504  and cooled in heat exchanger  502  to become stream  129 , which is reduced in pressure in valve  124  to become stream  125 , which is sent to an intermediate level in the low pressure tower  509 . 
         [0030]    The product streams from the low pressure tower  509  can include an oxygen-rich product, stream  111  originating at the bottom of the low pressure tower and warmed in the feed-product heat exchanger  508  to form an oxygen vapor product stream  120 . Additionally or alternatively, a liquid product stream  117  can be withdrawn from the bottom of the low-pressure tower. In  FIG. 1  an oxygen-rich liquid stream  117  is shown to enter pump  506  which produces a liquid stream elevated in pressure, stream  115 , which can be warmed in the feed-product main heat exchanger  508  to form higher pressure oxygen product stream  121 . Another product from the low pressure tower is a vapor nitrogen-rich product stream  106 , which leaves the top of the low pressure tower and is warmed in the heat exchanger  503  to form stream  105  and further warmed in heat exchanger  502  to form stream  114 , which in turn is warmed in the feed-product main heat exchanger  508  to form a nitrogen-rich product stream  123 . 
         [0031]    The nitrogen vapor product stream  130 , from the top of the high-pressure distillation tower  504 , enters the bottom of the neon recovery tower  500  and is enriched in neon as it flows to the top of the neon tower. The nitrogen vapor stream  501 , leaving the top of the distillation tray or packing section of the neon recovery tower  500 , is condensed in the neon tower condenser  510  to form the liquid stream  511 , which returns to the top of the distillation tray or packing section and flows to the bottom of the neon recovery tower  500 , where it is removed from the neon tower bottom as stream  100 , passes through a pressure reducing valve  101  and, as stream  102 , enters the cold side of the neon tower condenser  510 , and is evaporated to provide condensing refrigeration in the neon tower. The evaporated nitrogen, stream  104 , is then warmed in the air separation plant main heat exchanger  508  or other heat exchanger to become nitrogen vapor product, stream  118 . The crude neon product is stream  103 , the un-condensed vapor exiting the condenser  510 . The crude neon product, stream  130  is a small fraction [approximately 0.033%] of the air entering the double tower system, and can be warmed and produced as crude neon product, or can first enter a second stage neon purification tower (as shown in  FIG. 3 ) which may produce a higher purity crude neon product. 
         [0032]    An important advantage of the invention is that the nitrogen evaporated in the neon tower condenser  510  is only partially reduced in pressure and is not reduced to the pressure level of the upper or low-pressure tower. Depending on the nitrogen product requirements, the pressure reduction of the nitrogen product in valve  101  may be in the range of 5-25 PSIA. 
         [0033]      FIG. 2  shows the crude neon tower portion only. In the embodiment of this invention, referring to  FIG. 2 , the neon tower feed stream  130  is a nitrogen product stream originating from the top of the high-pressure tower in a cryogenic air separation double tower system, either as a vapor from the topmost stage or as a portion of the vapor entering or leaving the high-pressure tower condenser. In the embodiment of this invention, the product streams from the neon tower, nitrogen vapor product stream  104  and crude neon product stream  103 , have no connection to the double tower system and are warmed in appropriate heat exchangers to be delivered as products, except that the crude neon product may optionally be directed to a second stage neon enrichment tower, as shown in  FIG. 3 . 
         [0034]    In  FIG. 2 , the nitrogen vapor product, stream  130 , from the top of the high-pressure distillation tower enters the bottom of the neon recovery tower  500  and is enriched in neon as it flows to the top of the neon tower. The nitrogen vapor stream  501 , leaving the top of the distillation tray or packing section of the neon recovery tower  500 , is condensed in the neon tower condenser  510  to form the liquid stream  511 , which returns to the top of the distillation tray or packing section and flows to the bottom of the neon recovery tower  500 , where it is removed from the neon tower bottom as stream  100 , passes through a pressure reducing valve  101 , and enters the cold side of the neon tower condenser  510  as stream  102 , then is evaporated to provide condensing refrigeration in the neon tower condenser  510 . The evaporated nitrogen, stream  104 , is then warmed in the air separation plant main heat exchanger or other heat exchanger to become nitrogen vapor product. The crude neon product is stream  103 , the un-condensed vapor exiting the condenser  510 . The crude neon product, stream  130 , is a small fraction [approximately 0.033%] of the air entering the double tower system, and can be warmed and produced as crude neon product, or can first enter a second stage neon purification tower (as shown in  FIG. 3 ) which may produce a higher purity crude neon product. 
         [0035]    A computer simulation of the embodiment of the invention illustrated in  FIG. 2  was carried out and the results are presented in Table 1, below. These results are presented for illustrative purposes and are not intended to be limiting. The stream numbers correspond to those of  FIG. 1  and  FIG. 2 . 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 N2 Vapor 
                   
                 N2 Vapor 
                   
               
               
                   
                 Product 
                 N2 Liquid 
                 Product from 
               
               
                   
                 Feed to 
                 Leaving Neon 
                 Neon Tower 
                 Crude Neon 
               
               
                   
                 Neon Tower 
                 Tower Bottom 
                 Condenser 
                 Product 
               
               
                   
                 (Stream 130) 
                 (Stream 100) 
                 (Stream 104) 
                 (Stream 103) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Flow, MSCFH 
                 225.016 
                 224.817 
                 224.817 
                 2.957 
               
               
                 Pressure, PSIA 
                 79.196 
                 79.196 
                 69.196 
                 78.70 
               
               
                 Temperature, ° F. 
                 −288.36 
                 −288.36 
                 −291.44 
                 −290.49 
               
               
                 O2, Mole Fraction, % 
                 0.0012238 
                 0.0012249 
                 0.0012249 
                 0.0000194 
               
               
                 N2, Mole Fraction, % 
                 99.9152 
                 99.9969 
                 99.9969 
                 92.7967 
               
               
                 Ar, Mole Fraction, % 
                 0.0004915 
                 0.0004919 
                 0.0004919 
                 0.0000173 
               
               
                 Ne, Mole Fraction, % 
                 0.06272 
                 0.00128 
                 0.0012782 
                 5.6281129 
               
               
                 H2, Mole Fraction, % 
                 1.72987E−03 
                 3.35827E−05 
                 3.35827E−05 
                 0.1477826 
               
               
                 Helium, Mole Fraction, % 
                 1.85895E−02 
                 6.01340E−05 
                 6.01340E−05 
                 1.4273668 
               
               
                   
               
             
          
         
       
     
       Detailed Description 
     FIG.  3   
     Second Embodiment 
       [0036]      FIG. 3  shows an optional embodiment of the invention in which the crude neon product from neon recovery tower  500 , stream  103 , is directed to a second stage neon enrichment tower in which the condenser is cooled by liquid nitrogen evaporating at a pressure below atmospheric pressure. The nitrogen vapor stream  513 , leaving the top of the distillation tray or packing section of the second neon recovery tower  512 , is condensed in the neon tower condenser  514  to form the liquid stream  515 , which returns to the top of the distillation tray or packing section and flows to the bottom of the neon recovery tower  512 , where it is removed from the neon tower bottom as stream  131 . An uncondensed portion of the stream leaving condenser  514  is the enriched product neon stream  134 . 
       Operation 
     FIG.  3   
     Second Embodiment 
       [0037]    In the second stage neon enrichment tower  512  the liquid nitrogen at the bottom of neon enrichment tower  512 , stream  131 , is reduced in pressure in valve  132  so that the resulting stream  133  is at a pressure below atmospheric pressure and the resulting vapor stream  135 , resulting from the evaporation of the liquid nitrogen in the cold side of neon tower condenser  514 , is directed to a vacuum pump  138  or other means of maintaining stream  135  at a pressure level below atmospheric pressure. In  FIG. 3 , the vapor nitrogen stream  135 , evaporated from the second stage neon tower condenser  514 , is warmed in heat exchanger  136 , and the resulting stream  137  is maintained at a suitable pressure below atmospheric pressure by vacuum pump  138  and the resulting stream  139  is exhausted to the atmosphere. In the operation of the second stage neon enrichment tower  512 , a supplemental liquid nitrogen stream  516  may be supplied through the valve  517  to provide an additional liquid nitrogen stream  518  to the cold side of the second stage neon tower condenser. Again, none of these streams resulting from the first and second stage neon enrichment towers in  FIG. 3  has any further connection to the double tower system, except for the connection through the nitrogen product feed stream  130  entering the first stage neon recovery tower  500 .