Abstract:
A cryogenic rectification system employing a phase separator, which may include one or more trays, in conjunction with a double column air separation plant, wherein some shelf liquid is subcooled and phase separated to produce crude neon vapor, and the remaining liquid is used to reflux the lower pressure column of the double column plant.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates generally to cryogenic rectification of air and, more particularly, to the cryogenic rectification of air for the production of neon.  
       BACKGROUND ART  
       [0002]     Neon is a valuable inert gas found in low concentrations of about 18 parts per million (ppm) in air. Neon is useful as a filling gas in lamps and luminous sign tubes. In addition, neon is used in airplane beacons because neon light can penetrate fog where other lights cannot. Systems which can improve the recovery of neon would be highly desirable.  
       SUMMARY OF THE INVENTION  
       [0003]     One aspect of the invention is:  
         [0004]     A method for producing crude neon comprising:  
         [0005]     (A) separating feed air by cryogenic rectification in a higher pressure column to produce neon-containing shelf vapor, and condensing at least a portion of the neon-containing shelf vapor to produce neon-containing liquid;  
         [0006]     (B) subcooling the neon-containing liquid, passing the resulting fluid into a separator, and separating the fluid within the separator into neon-containing vapor and remaining liquid; and  
         [0007]     (C) passing remaining liquid from the separator into a lower pressure column, and recovering neon-containing vapor as product crude neon.  
         [0008]     Another aspect of the invention is:  
         [0009]     Apparatus for producing crude neon comprising:  
         [0010]     (A) a higher pressure column, a lower pressure column having a reboiler/condenser, and means for passing feed air into the higher pressure column;  
         [0011]     (B) a subcooler, a separator, means for passing neon-containing fluid from the higher pressure column to the reboiler/condenser, from the reboiler/condenser to the subcooler, and from the subcooler to the separator; and  
         [0012]     (C) means for passing liquid from the separator to the lower pressure column, and means for recovering vapor from the separator as product crude neon.  
         [0013]     As used herein the term “feed air” means a mixture comprising primarily oxygen and nitrogen, and also containing neon, such as ambient air.  
         [0014]     As used herein the term “column” means a distillation or fractionation column or zone, i.e. a contacting column or zone, wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements such as structured or random packing. For a further discussion of distillation columns, see the Chemical Engineer&#39;s Handbook, fifth edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13 , The Continuous Distillation Process.    
         [0015]     Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phases is generally adiabatic and can include integral (stagewise) or differential (continuous) contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out at least in part at temperatures at or below 150 degrees Kelvin (K).  
         [0016]     As used herein the term “indirect heat exchange” means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other.  
         [0017]     As used herein the terms “reboiler” and “reboiler/condenser” mean a heat exchange device that generates column or separator vapor from liquid.  
         [0018]     As used herein the terms “subcooling” and “subcooler” mean respectively method and apparatus for cooling a liquid to be at a temperature lower than the saturation temperature of that liquid for the existing pressure.  
         [0019]     As used herein the terms “upper portion” and “lower portion” mean those sections of a column respectively above and below the mid point of the column.  
         [0020]     As used herein the term “crude neon” means a fluid having a neon concentration within the range of from 400 ppm to 10,000 ppm.  
         [0021]     As used herein the term “tray” means a vapor-liquid contacting stage.  
         [0022]     As used herein the term “phase separator” means a vessel wherein incoming feed is separated into individual vapor and liquid fractions. Typically the vessel has sufficient cross-sectional area so that the vapor and liquid are separated by gravity. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  is a simplified schematic representation of one preferred embodiment of the cryogenic rectification system of this invention wherein the separator includes at least one tray.  
         [0024]      FIG. 2  is a schematic representation of yet another preferred embodiment of the cryogenic rectification system of this invention wherein the separator is a phase separator  
         [0025]      FIG. 3  is a schematic representation of yet another preferred embodiment of the cryogenic rectification system of this invention wherein some of the neon-containing liquid bypasses the subcooler.  
         [0026]      FIG. 4  is a schematic representation of another preferred embodiment of the invention wherein liquid feed air is flashed, the vapor is recovered as part of the crude neon, and the remaining liquid is passed into the lower pressure column. 
     
    
     DETAILED DESCRIPTION  
       [0027]     The invention will be described in detail with reference to the Drawings. Referring now to  FIG. 1 , feed air  1  is passed into higher pressure column  20  which is operating at a pressure generally within the range of from 60 to 220 pounds per square inch absolute (psia). In the embodiment of the invention illustrated in  FIG. 1 , feed air  1  is a gaseous stream, and liquid feed air in stream  11  is also provided into the system. Stream  11  is divided into stream  12  which is passed through valve  13  and into higher pressure column  20  as stream  14 , and into stream  6  which is passed through valve  15  and into lower pressure column  21  as stream  16 .  
         [0028]     Within higher pressure column  20  the feed air is separated by cryogenic rectification into oxygen-enriched liquid and nitrogen-enriched vapor. Oxygen-enriched liquid is withdrawn from the lower portion of column  20  in stream  3 , cooled by passage through heat exchanger  17  to form stream  18 , and passed through valve  19  and into lower pressure column  21  as stream  22 . Nitrogen-enriched vapor or shelf vapor, containing from 30 to 70 ppm neon, is withdrawn from the upper portion of higher pressure column  20  in stream  23  and passed into reboiler/condenser  24  wherein it is condensed by indirect heat exchange with lower pressure column bottom liquid. This neon-containing liquid is withdrawn from reboiler/condenser  24  in stream  25 . A portion  26  of stream  25  is passed back into the upper portion of higher pressure column  20  as reflux. Another portion of the neon-containing fluid from reboiler/condenser  24  is passed in stream  27  to subcooler  28 .  
         [0029]     Within subcooler  28  the neon-containing liquid is subcooled by indirect heat exchange with nitrogen streams from the lower pressure column, and the resulting fluid is withdrawn from subcooler  28  as subcooled neon-containing liquid in stream  2 . Stream  2  is passed through valve  29  and then into separator  30  in stream  31 .  
         [0030]     In the embodiment of the invention illustrated in  FIG. 1 , separator  30  contains at least one tray  32 . That is, in the embodiment of the invention illustrated in  FIG. 1  separator  30  is a small rectification column. Separator  30  also contains reboiler  33  which is driven by a portion of the shelf vapor passed to reboiler  33  in stream  4 . Within separator  30  the neon-containing liquid is separated into neon-containing vapor and remaining liquid. The remaining liquid is passed from separator  30  in stream  34  into the upper portion of lower pressure column  21 . This liquid yields high purity nitrogen product containing very low concentrations of light components owing to the removal of much of the light components with the crude neon. The neon-containing vapor is recovered from separator  30  in stream  5  as product crude neon. Typically the crude neon is provided to a neon refinery for the production of high purity or refined neon. The neon-containing shelf vapor in stream  4 , which is condensed in reboiler  33 , is passed out of reboiler  33  in stream  35 . Preferably, as illustrated in  FIG. 1 , stream  35  is passed into stream  27  and then passed to subcooler  28  and ultimately into separator  30  for subsequent recovery of the neon in this fluid as part of the crude neon in stream  5 .  
         [0031]     Lower pressure column  21  is operating at a pressure less than that of higher pressure column  20  and generally within the range of from 16 to 75 psia. Within lower pressure column  21  the various fluids passed into that column are separated by cryogenic rectification into oxygen-rich liquid and nitrogen-rich vapor. Oxygen-rich liquid is withdrawn from the lower portion of column  21  in stream  36  for recovery as product oxygen having an oxygen concentration of at least 90 mole percent. If desired, as shown in  FIG. 1 , the oxygen-rich liquid may be increased in pressure by pump  37  prior to recovery as high pressure liquid and/or gaseous oxygen. Nitrogen-rich vapor is withdrawn from the upper portion of column  21  in stream  9 , warmed by passage through subcooler  28  and heat exchanger  17 , and recovered as product nitrogen  38  having a nitrogen concentration of at least 99.9 mole percent. For product purity control purposes a nitrogen-containing waste stream  39  is withdrawn from column  21  below the withdrawal level of stream  9 , warmed by passage through subcooler  28  and heat exchanger  17 , and removed from the system in stream  40 .  
         [0032]      FIG. 2  illustrates another embodiment of the invention wherein the separator is a phase separator. The numerals in  FIG. 2  are the same as the numerals in  FIG. 1  for the common elements and these common elements will not be described again in detail. In the embodiment of the invention illustrated in  FIG. 2 , the phase separator  50  does not contain a reboiler so that the phase separation is essentially totally as a result of flashing through valve  29  and gravitational separation within the phase separator. However, phase separator  50  could contain a reboiler in which case the fluid flow employing streams  4  and  35  illustrated in  FIG. 1  would also be employed with the embodiment of the invention illustrated in  FIG. 2 .  
         [0033]     The numerals in the embodiment of the invention illustrated in  FIG. 3  are the same as those of  FIG. 2  for the common elements, and these common elements will not be described again in detail. Referring now to  FIG. 3 , a portion  60  of stream  27  is not subcooled but rather is passed through valve  61  and as stream  62  is combined with flashed stream  31 . This increases the amount of vapor produced in phase separator  50  thus increasing the recovery of the more volatile neon which preferentially concentrates in the vapor rather than in the remaining liquid which is passed from the separator into the lower pressure column.  
         [0034]     The numerals in the embodiment of the invention illustrated in  FIG. 4  are the same as those of  FIG. 2  for the common elements, and these common elements will not be described again in detail. Referring now to  FIG. 4 , liquid air stream  6  which is flashed through valve  15  is passed in stream  16  to feed air phase separator  45 . Vapor from feed air phase separator  45  is passed in stream  46  to crude neon stream  5  to form part of the crude neon product. Liquid from feed air phase separator  45  is passed in stream  47  into lower pressure volume  21 . This embodiment of the invention serves not only to increase the recovery of neon but also enhances the purity of the nitrogen product because light impurities, which would otherwise be in the nitrogen product, are removed from the system in stream  46 .  
         [0035]     A computer simulation of the embodiment of the invention illustrated in  FIG. 1  was carried out and the results are presented in Table 1. These results are presented for illustrative purposes and are not intended to be limiting. The stream numbers correspond to those of  FIG. 1 .  
                                                                                                             TABLE 1                                               Liquid Air to   Oxygen-               Main Air   Total Liq.   Lower Pressure   Enriched   Subcooled           Stream   Air Stream   Column   Liquid   Liquid           (1)   (11)   (6)   (3)   (2)                        F, MCFH   715   305   183   530.6   306.4       P, psia   87.45   754.6   754.6   87.45   84.0       T, K   100.7   94.27   94.28   100.0   82.14       XN2   0.78110   0.78110   0.78110   0.65479   0.99980       XAr   0.009343   0.009343   0.009343   0.014666   0.000125       XO2   0.209530   0.209530   0.209530   0.330544   1.5 ppm       XH2, ppm   3.0   3.0   3.0   0.0415   8.12       XNe. ppm   18.2   18.2   18.2   0.298   49.19       XHe ppm   5.2   5.2   5.2   0.022   14.16       XCO, ppm   1.0   1.0   1.0   1.13   0.778                                Nitrogen   Reboiler           Neon Crude   Liquid   Product   Flow           (5)   (34)   (9)   (4)                        F, MCFH   11.01   295.4   587.4   5.0       P, psia   20.22   20.22   20.02   84.0       T, K   80.23   80.23   80.16   95.96       XN2   0.997997   0.99987   0.999822   0.99980       XAr   5.247E−05   1.282E−04   1.629E−04   0.000125       XO2, ppm   0.44   1.539   1.0   1.5       XH2, ppm   221.3   0.177   0.813   8.12       XNe. ppm   1333.5   1.33   5.09   49.19       XHe ppm   394.2   0.003   1.22   14.16       XCO ppm   0.533   0.788   0.986   0.778                  
 
         [0036]     Although the invention has been described in detail with reference to certain preferred embodiments, those skilled in the art will recognize that there are other embodiments of the invention within the spirit and the scope of the claims.