Abstract:
A method for the cryogenic separation of air having defined temperatures for condensed feed air passed into a double column system relative to liquid oxygen and preferably to shelf vapor, and wherein kettle liquid is not subcooled from the higher pressure column to the lower pressure column.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates generally to cryogenic air separation and, more particularly, to cryogenic air separation employing a double column and wherein at least some feed air is condensed prior to passage into one or both of the columns.  
       BACKGROUND ART  
       [0002]     Cryogenic air separation is a very energy intensive process because of the need to generate low temperature refrigeration to drive the process. Accordingly, any method which improves the utilization of the available refrigeration in carrying out cryogenic air separation would be very desirable.  
       SUMMARY OF THE INVENTION  
       [0003]     A method for carrying out cryogenic air separation Employing a double column having a higher pressure column and a lower pressure column comprising:  
         [0004]     (A) condensing feed air, passing the condensed feed air into the higher pressure column, and separating feed air within the higher pressure column by cryogenic rectification to produce nitrogen-enriched vapor and oxygen-enriched liquid;  
         [0005]     (B) withdrawing nitrogen-enriched vapor from the higher pressure column, withdrawing oxygen-enriched liquid from the higher pressure column, and passing oxygen-enriched liquid withdrawn from the higher pressure column into the lower pressure column without undergoing subcooling; and  
         [0006]     (C) producing nitrogen-rich vapor and oxygen-rich liquid by cryogenic rectification within the lower pressure column, and withdrawing oxygen-rich liquid from the lower pressure column wherein the temperature of the condensed feed air exceeds the temperature of the oxygen-rich liquid withdrawn from the lower pressure column.  
         [0007]     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 . A double column comprises a higher pressure column having its upper end in heat exchange relation with the lower end of a lower pressure column.  
         [0008]     Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The higher vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the lower 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).  
         [0009]     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.  
         [0010]     As used herein, the term “feed air” means a mixture comprising primarily oxygen, nitrogen and argon, such as ambient air.  
         [0011]     As used herein, the terms “upper portion” and “lower portion” of a column mean those sections of the column respectively above and below the mid point of the column.  
         [0012]     As used herein, the terms “turboexpansion” and “turboexpander” mean respectively method and apparatus for the flow of high pressure fluid through a turbine to reduce the pressure and the temperature of the fluid, thereby generating refrigeration.  
         [0013]     As used herein, the term “cryogenic air separation plant” means the column or columns wherein feed air is separated by cryogenic rectification to produce nitrogen, oxygen and/or argon, as well as interconnecting piping, valves, heat exchangers and the like.  
         [0014]     As used herein, the term “compressor” means a machine that increases the pressure of a gas by the application of work.  
         [0015]     As used herein, the term “subcooling” means cooling a liquid to be at a temperature lower than the saturation temperature of that liquid for the existing pressure. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a schematic representation of one preferred arrangement for the practice of the cryogenic air separation method of this invention.  
         [0017]      FIG. 2  is a schematic representation of another preferred arrangement for the practice of the cryogenic air separation method of this invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]     The invention will be described in greater detail with reference to the Drawings. The cryogenic air separation plant illustrated in the Drawings comprises a double column, having a higher pressure column  260  and a lower pressure column  280 , a low ratio argon column  400 , and a super-staged argon column  410 .  
         [0019]     Referring now to  FIG. 1 , feed air  1  is compressed in compressor  100  and compressed feed air stream  2  is cleaned of high boiling impurities in purifier  110 . Resulting cleaned, compressed feed air  4  is divided into stream  6  and stream  8 . Feed air stream  6  is further compressed in compressor  130  and resulting feed air stream  20  is passed into main heat exchanger  200  wherein it is condensed by indirect heat exchange with return streams such as pumped liquid oxygen, and from which it emerges as condensed feed air stream  22  having a temperature generally within the range of from 92 K to 105 K, preferably within the range of from 93.5 K to 102 K.  
         [0020]     Condensed feed air  22  is divided into a first condensed feed air stream  24 , which is at a temperature essentially the same as that of stream  22  and which is passed through valve  320  and as stream  25  into higher pressure column  260 , and into a second condensed feed air stream  28  which is passed through valve  340  and as stream  30  into lower pressure column  280 . Feed air stream  8  is further compressed by passage through compressor  120  and resulting feed air stream  10  is cooled by indirect heat exchange with return streams in main heat exchanger  200  to form third feed air stream  12 . Third feed air stream  12  is turboexpanded by passage through turboexpander  220  to generate refrigeration bearing third feed air stream  14  having a temperature generally within the range of from 99 K to 117 K. The temperature of condensed feed air stream  24  does not exceed the temperature of turboexpanded third feed air stream  14 . Turboexpanded third feed air stream  14  is passed into the lower portion of higher pressure column  260 .  
         [0021]     Within higher pressure column  260  the feed air is separated by cryogenic rectification in nitrogen-enriched vapor and oxygen-enriched liquid. Nitrogen-enriched vapor is withdrawn from the upper portion of higher pressure column  260  as stream  50  having a temperature generally within the range of from 94 K to 96 K. Preferably, the temperature of the condensed feed air stream  24  which is ultimately passed into the higher pressure column exceeds the temperature of the nitrogen-enriched vapor in stream  50  withdrawn from the higher pressure column. A portion  54  of stream  50  may be warmed in main heat exchanger  200  and recovered as higher pressure nitrogen product  90 . The remaining portion  52  of the withdrawn nitrogen-enriched vapor is condensed by indirect heat exchange with lower pressure column  280  bottom liquid in main condenser  300 . A portion  58  of the resulting condensed nitrogen-enriched liquid is returned to higher pressure column  260  as reflux. Another portion  60  of the resulting condensed nitrogen-enriched liquid is subcooled in main heat exchanger  200 . Resulting subcooled nitrogen-enriched liquid  62  is passed through valve  360  and as stream  68  into the upper portion of lower pressure column  280 . If desired, a portion  66  of stream  62  may be recovered as liquid nitrogen product.  
         [0022]     Oxygen-enriched liquid is withdrawn from the lower portion of higher pressure column  260  in stream  32 , passed through valve  300  and then passed into lower pressure column  280  without undergoing any subcooling. In the illustrated embodiments the cryogenic air separation plant also includes argon production. In these embodiments the oxygen-enriched liquid  34  from valve  300  is divided into stream  36 , which as previously described is passed without subcooling into lower pressure column  280 , and into stream  38  which is passed into argon column top condenser  430  for processing as will be further described below.  
         [0023]     Within lower pressure column  280  the various feeds are separated by cryogenic rectification into nitrogen-rich vapor and oxygen-enriched liquid. Nitrogen-rich vapor is withdrawn from the upper portion of lower pressure column  280  in stream  70 , warmed by passage through main heat exchanger  200 , and recovered as gaseous nitrogen product in stream  72 . For product purity control purposes waste nitrogen stream  74  is withdrawn from column  280  below the withdrawal level of stream  70 , and after passage through heat exchanger  200  is removed from the process in stream  76 . Oxygen-rich liquid is withdrawn from the lower portion of lower pressure column  280  in stream  78  having a temperature generally within the range of from 93 K to 95 K. The temperature of the condensed feed air stream  24  which is ultimately passed into the higher pressure column exceeds the temperature of the oxygen-rich liquid in stream  78  withdrawn from the lower pressure column. Stream  78  is pumped to a higher pressure by cryogenic liquid pump  240  to form pressurized liquid oxygen stream  80 . If desired, a portion  82  of stream  80  may be recovered as liquid oxygen product. The remaining portion  84  is vaporized by passage through main heat exchanger  200  by indirect heat exchanger with incoming feed air and recovered as gaseous oxygen product in stream  86 .  
         [0024]     A stream comprising primarily oxygen and argon is passed in stream  51  from column  280  into low ratio argon column  400  wherein it is separated into argon-enriched top vapor and oxygen-richer bottom liquid which is returned to column  280  in stream  53 . The argon-enriched top vapor is passed into superstaged argon column  410  in stream  55  wherein it undergoes cryogenic rectification to produce argon top vapor and argon-depleted liquid which is withdrawn from column  410  in stream  57  and pumped by pump  420  into the upper portion of column  400  in stream  59 . Argon top vapor is withdrawn from column  410  in stream  92  and a portion  94  is recovered as product argon. Another portion  96  is condensed in argon top condenser  430  against partially vaporizing oxygen-enriched liquid provided to top condenser  430  in stream  38 . The resulting condensed argon is returned to column  410  in stream  98  as reflux. The resulting oxygen-enriched fluid from top condenser  430  is passed into lower pressure column  280  in vapor stream  40  and liquid stream  42 .  
         [0025]     In the embodiment of the invention illustrated in  FIG. 2 , the numerals are the same as those shown in  FIG. 1  for the common elements, and these common elements will not be described again in detail. Referring now to  FIG. 2 , the second condensed feed air stream  28  undergoes further cooling than does the condensed feed air stream which is passed into the higher pressure column and thus is at a colder temperature than this stream. Moreover, the second condensed feed air stream which is passed into the lower pressure column is at a temperature which does not exceed the temperature of the nitrogen-enriched vapor withdrawn from the higher pressure column.  
         [0026]     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.