Abstract:
A system for producing enriched air wherein a multistage compressor is integrated with a cryogenic air separation plant and serves to compress feed air for the plant while also compressing both air and oxygen fluid from the plant to produce the enriched air.

Description:
TECHNICAL FIELD 
     This invention relates generally to cryogenic air separation and, more particularly, to the production of enriched air. 
     BACKGROUND ART 
     Many industrial processes, such as combustion and chemical oxidation, require enriched air as a process input. Often the enriched air is required by the industrial process at a relatively high pressure, typically at a pressure much higher than that at which an air separation plant operates. This creates an inefficiency. 
     Accordingly it is an object of this invention to provide a system for producing enriched air, especially relatively high pressure enriched air, which employs a cryogenic air separation plant and which operates with improved efficiency over conventional systems for providing enriched air. 
     SUMMARY OF THE INVENTION 
     The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is: 
     A method for producing enriched air comprising: 
     (A) passing feed air to a multistage compressor, compressing the feed air in the multistage compressor to produce compressed feed air, and passing a first portion of the compressed feed air into a cryogenic air separation plant; 
     (B) separating compressed feed air in the cryogenic air separation plant by cryogenic rectification to produce oxygen fluid; 
     (C) passing oxygen fluid from the cryogenic air separation plant to the multistage compressor, and mixing oxygen fluid within the multistage compressor with a second portion of the compressed feed air to produce enriched air; and 
     (D) further compressing the enriched air within the multistage compressor and recovering further compressed enriched air from the multistage compressor. 
     Another aspect of the invention is: 
     Apparatus for producing enriched air comprising: 
     (A) a multistage compressor comprising an initial stage and a final stage, and means for passing feed air to the initial stage of the multistage compressor; 
     (B) a cryogenic air separation plant and means for passing feed air from the multistage compressor to the cryogenic air separation plant, said means communicating with the multistage compressor downstream of the initial stage; 
     (C) means for passing oxygen fluid from the cryogenic air separation plant to the multistage compressor at a point upstream of the final stage; and 
     (D) means for recovering enriched air from the final stage of the multistage compressor. 
     As used herein the term “oxygen fluid” means a fluid having an oxygen concentration of at least 40 mole percents preferably at least 80 mole percent, most preferably at least 95 mole percent. 
     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. 
     The term “double column” is used to mean a higher pressure column having its upper portion in heat exchange relation with the lower portion of a lower pressure column. A further discussion of double columns appears in Ruheman “The Separation of Gases”, Oxford University Press, 1949, Chapter VII, Commercial Air Separation. 
     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. Distillation is the separation process whereby heating of a liquid mixture can be used to concentrate the more volatile component(s) in the vapor phase and thereby the less volatile component(s) 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 can be adiabatic or nonadiabatic 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) 
     As used herein the term “enriched air” means a fluid having an oxygen concentration within the range of from 25 to 50 mole percent, with the remainder being primarily nitrogen. 
     As used herein the term “indirect heat exchange” means the bringing of two fluid streams into heat exchange relation without any physical contact or intermixing of the fluids with each other. 
     As used herein the term “feed air” means a mixture comprising primarily oxygen and nitrogen, such as ambient air. 
     As used herein the term “cryogenic air separation plant” means a plant comprising at least one column, which processes feed air and produces oxygen fluid. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified schematic representation of one embodiment of the cryogenic enriched air production system of this invention. 
     FIG. 2 is a representation of one embodiment of a cryogenic air separation plant which may be used in the practice of this invention. 
     FIG. 3 is a representation of another embodiment of the invention wherein the cryogenic air separation plant is integrated with a gas turbine. 
    
    
     DETAILED DESCRIPTION 
     The invention will be described in detail with reference to the Drawings. Referring now to FIG. 1, feed air  2  is passed to multistage compressor  102  which comprises an initial stage  60 , a final stage  61  and four intermediate stages designated  62 ,  63 ,  64  and  65 . For the sake of simplicity the intercoolers between the stages are not shown. The feed air is compressed in initial stage  60  and in intermediate stage  62  to produce compressed feed air  66 . A first portion  6  of the compressed feed air is passed to prepurifier  106  wherein it is cleaned of high boiling impurities such as carbon dioxide, water vapor and hydrocarbons. Resulting prepurified feed air  10  is divided into first feed stream  12  which is passed into the cryogenic air separation plant, shown in FIG. 1 in representational form as item  120 , and into second feed stream  14  which is increased in pressure by passage through booster compressor  110  and then passed as stream  16  into cryogenic air separation plant  120 . 
     Within cryogenic air separation plant  120  the feed air is separated by cryogenic rectification to produce oxygen fluid which is withdrawn from the cryogenic air separation plant in stream  26  at a pressure equal to or higher than the pressure of stream  6 . In the embodiment illustrated in FIG. 1 there is also shown the production of nitrogen  24  and argon  22  by the cryogenic air separation plant. Oxygen fluid is passed from cryogenic air separation plant  120  in stream  26  to multistage compressor  102  wherein it mixes with the remaining or second portion  28  of the compressed feed air to form enriched air stream  67 . Oxygen fluid may be withdrawn from the air separation plant as vapor, or it may be withdrawn as liquid, pumped to a higher pressure, vaporized and warmed prior to passage to the multistage compressor. In the embodiment illustrated in FIG. 1, oxygen fluid  26  is shown being passed into multistage compressor  102  at the same stage of compression, i.e. between the same two stages, stages  62  and  63 , from where the feed air  6  was taken for passage into plant  120 . However, this is not necessary and as shown by the dotted lines, stream  26  could pass into multistage compressor  102  at another downstream stage of compression so long as it is upstream of final stage  61 . Enriched air  67  is further compressed by passage through the remaining stages of multistage compressor  102 , which in the embodiment illustrated in FIG. 1 are stages  63 ,  64 ,  65  and  61 , and is recovered from multistage compressor  102  as further compressed enriched air  32 , at a pressure generally within the range of from 150 to 650 pounds per square inch absolute (psia). 
     FIG. 2 illustrates one embodiment of the cryogenic air separation plant which may be used as plant  120  in the practice of this invention Any other suitable cryogenic air separation can also be used as plant  120 . Referring now to FIG. 2, feed air streams  16  and  12  are cooled in heat exchanger  210  by indirect heat exchange with return streams and are withdrawn from heat exchanger  210  as cooled feed air streams  212  and  215 , respectively. A portion  211  of stream  12  is withdrawn from an intermediate point of heat exchanger  210 , expanded by passage through expander  218 , and passed as stream  213  into lower pressure column  224 . Cooled, compressed feed air stream  215  is passed into vaporizer  264  wherein it is liquefied, as will be more fully described below, and from which it emerges as stream  216 . Streams  216  and  212  are passed into higher pressure column  221  of cryogenic air separation plant  120  which also includes lower pressure column  224  and argon sidearm column  232 . Within higher pressure column  221  the feed air is separated by cryogenic rectification into nitrogen-enriched vapor and oxygen-enriched liquid. Nitrogen-enriched vapor is passed in stream  222  into main condenser  223  wherein it is condensed by indirect heat exchange with lower pressure column  224  bottom liquid to form nitrogen-enriched liquid  225 . A portion  226  of nitrogen-enriched liquid  225  is returned to higher pressure column  221  as reflux, and another portion  227  of nitrogen-enriched liquid  225  is subcooled (not shown) and then passed into lower pressure column  224  as reflux. Oxygen-enriched liquid is withdrawn from the lower portion of higher pressure column  221  in stream  228  and a portion  256  is passed into argon column top condenser  229  wherein it is vaporized by indirect heat exchange with argon-richer vapor, and the resulting oxygen-enriched fluid is passed as illustrated by stream  230  from top condenser  229  into lower pressure column  224 . Another portion  257  of the oxygen-enriched liquid is passed directly into lower pressure column  224 . 
     A stream  231  comprising oxygen and argon is passed from lower pressure column  224  into argon column  232  wherein it is separated by cryogenic rectification into argon-richer vapor and oxygen-richer liquid. The oxygen-richer liquid is returned to lower pressure column  224  in stream  233 . The argon-richer vapor is passed in stream  234  into top condenser  229  wherein it condenses by indirect heat exchange with the vaporizing oxygen-enriched liquid as was previously described. Resulting argon-richer liquid is returned in stream  235  to argon column  232  as reflux. Argon-richer fluid, as vapor and/or liquid, is recovered from the upper portion of argon column  232  as product argon in stream  22 . 
     Lower pressure column  224  is operating at a pressure less than that of higher pressure column  221 . Within lower pressure column  224  the various feeds into the column are separated by cryogenic rectification into nitrogen-rich fluid and oxygen-rich fluid. Nitrogen-rich fluid is withdrawn from the upper portion of lower pressure column  224  as vapor stream  240 , warmed by indirect heat exchange with stream  227  (not shown) and by passage through heat exchanger  210  and recovered as product nitrogen in stream  24 . Oxygen-rich fluid is withdrawn from the lower portion of lower pressure column  224  as oxygen fluid stream  258 . Stream  258  is pumped to a higher pressure by passage through pump  262  and resulting pressurized oxygen fluid stream  259  is vaporized in vaporizer  264  by indirect heat exchange with the aforesaid condensing feed air. The resulting vaporized oxygen fluid is withdrawn from vaporizer  264  in stream  260 , warmed by passage through heat exchanger  210  and from there passed as stream  26  into multistage compressor  102 . 
     FIG. 3 illustrates another embodiment of the invention which further includes the integration of a gas turbine. As was the case with FIG. 2, the numerals of FIG. 3 are the same as those of FIGS. 1 for the common elements, and these common elements will not be described again in detail. 
     Referring now to FIG. 3, another feed air stream  40  is compressed in gas turbine compressor  130 . A portion of resulting compressed air  42  is withdrawn via line  44 . Compressed air in stream  44  is cooled first by indirect heat exchange with nitrogen from the cryogenic air separation plant and then by cooling water (not shown). A portion of compressed air  6  is withdrawn at substantially the same pressure as that of cooled air  46  and streams  6  and  46  are combined to produce stream  8  which is then prepurified in prepurifier  106 . Nitrogen streams  24  and  25  (stream  25  is at higher pressure than stream  24 ) are compressed using compressor  122  and then the resulting compressed nitrogen  80  is heated by heat exchange with air in heat exchanger  136 . The compressed and heated nitrogen stream  36  along with the remainder of gas turbine air  48  and fuel  50  are injected into combustor  132  of gas turbine  81 . Fuel is combusted in combustor  132  and hot gas  52  from combustor  132  is expanded in turbine or expander  134 . The turbine exhaust in stream  54  is sent to a heat recovery boiler. 
     Table 1 presents the results obtained in a simulation of the invention in accord with the embodiment illustrated in FIG.  1  and wherein the cryogenic air separation plant produces low purity oxygen. The stream numbers of Table 1 correspond to those of FIG.  1 . The oxygen concentration is presented in volume percent. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Stream 
                 Flow 
                 Temperature 
                 Pressure 
                 O 2  Concen- 
               
               
                 No. 
                 ft 3 /hr 
                 ° F. 
                 psia 
                 tration 
               
               
                   
               
             
             
               
                  2 
                 4689456 
                 70 
                 14.7 
                 20.74 
               
               
                  6 
                 1795303 
                 80 
                 62 
                 20.74 
               
               
                 12 
                 1276138 
                 80 
                 59 
                 20.95 
               
               
                 16 
                  501213 
                 80 
                 164 
                 20.95 
               
               
                 26 
                  386064 
                 75 
                 63 
                 95 
               
               
                 28 
                 2894153 
                 80 
                 62 
                 20.74 
               
               
                 32 
                 3280217 
                 200  
                 650 
                 29.5 
               
               
                   
               
             
          
         
       
     
     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. For example the multistage compressor could have no intermediate stages or any practical number of intermediate stages depending upon the desired recovery pressure of the enriched air. Furthermore a portion of the oxygen-enriched air, either from after or from before the final stage of compression of the multistage compressor, could be prepurified and passed into the cryogenic air separation plant instead of stream  16 . This latter embodiment is particularly useful when oxygen fluid is taken from the cryogenic air separation plant as liquid and the aforesaid enriched air recycle stream is used to vaporize the liquid oxygen fluid. This embodiment will also eliminate the need for booster compressor  110 .