Patent Publication Number: US-8978413-B2

Title: Rare gases recovery process for triple column oxygen plant

Description:
BACKGROUND 
     In recent years the demand for rare gases, in particular Krypton and Xenon, is becoming very important. New applications and advances in electronics, medical, glass insulation etc. are greatly contributing to this high demand. 
     Krypton and Xenon are produced as the by-products of a cryogenic air separation plant. The basic recovery scheme is well known in the art. Since Kr and Xe are heavier than oxygen and will accumulate in liquid oxygen, the recovery technique usually calls for the refining of a liquid oxygen purge stream of the low pressure column of a double column cycle. The rare gases contained in the purge stream are further concentrated in a first concentrating column along with other heavy components in liquid oxygen such as hydrocarbons, CO2, nitrogen oxide etc. For safety considerations, the limit of this first concentrating operation corresponds to about 10% of the limit of flammability of hydrocarbons in oxygen. The first concentrated stream is then either treated in an on-site purification plant or transported to a central purification center where it is vaporized, heated and treated in a catalytic reactor at high temperature of about 500° C. to remove the hydrocarbons. This oxidation reaction forms CO2 and moisture. The mixture is then dried, its CO2 content is removed in an adsorber. The dried and CO2-free mixture is then cooled and distilled to yield the product which is usually a mixture of Kr and Xe. The product is then further refined to remove oxygen, argon and some other impurities such as CFC compounds, green house gases, remaining traces of hydrocarbons etc. and to yield pure Krypton and pure Xenon as final products. 
     Kr and Xe are present in very small concentration in atmospheric air (1.14 ppm Kr and 0.086 ppm Xe by volume). Therefore it is currently only economically viable to produce Kr—Xe in large oxygen plants, preferably above 1000 T/D and even larger. 
     If the purification portion of the process can be a standardized process to refine different types of first concentrated streams, either from an oxygen plant, nitrogen plant, low purity or high purity oxygen plant etc. then the same remark cannot be applied for the process involved to extract a stream containing Krypton and Xenon from the air separation columns. Indeed, because of the above-mentioned variety of air separation plants/processes, it is not possible to have one type of extraction process applicable for all types of air separation plants. For example, a plant producing gaseous oxygen product from the low pressure column would require a different type of rare gases extraction from a plant producing liquid oxygen product for pumping from the low pressure column. 
     Heavy industrial demand for oxygen for gasification, IGCC, GTL, oxyfuels have increased significantly the size of trains of oxygen plants. Because of the limitation of the size of distillation columns by transport regulations the technological trend in cryogenic process is shifting toward elevated air pressure plants wherein the feed air and the columns&#39; pressure are at higher pressure than traditional oxygen plants. The triple column process is designed to address this type of application and there is a need to provide a technique for extracting rare gases from this type of process. 
     This triple column process is described in details in several patent such as U.S. Pat. No. 5,231,837, and U.S. Pat. No. 5,341,646. 
     The technique of recovering Krypton and Xenon from an oxygen plant have been covered extensively in several patents: 
     U.S. Pat. No. 6,776,004: this prior art taught the technique of recovering rare gases of a mixing column plant for oxygen production. The liquid purge of the low pressure column is treated in an enrichment column reboiled by the top gas of the mixing column to recover the rare gases. 
     PCT WO 2004/023054: air feeds to the high pressure column is separated into a nitrogen rich stream and 2 oxygen rich liquid streams: rare gases rich liquid and rare gases lean liquid. The rare gases-rich stream is treated in a column located above the crude argon column to yield a Krypton Xenon concentrate at the bottom. 
     U.S. Pat. No. 6,662,593: the rare gases in the feed air are confined in a rare gases rich liquid stream of the high pressure column and then its oxygen content is stripped in a side column to yield the rare gases concentrate stream. By extracting the rare gases prior to the final distillation in the low pressure column the oxygen product can be quite lean in rare gases and can then be pumped and vaporized to high pressure as final product without incurring losses of rare gases. 
     U.S. Pat. No. 6,612,129: Krypton and Xenon containing liquid from the high pressure column is partially evaporated in the top condenser of the side-arm argon column of the double column plant. The liquid purge and the vaporized streams of the condenser are then treated in an enrichment column to yield the Krypton Xenon concentrate at the bottom. 
     U.S. Pat. No. 6,220,054: a column is used to treat the bottom liquid of the crude argon column to yield final oxygen product which is depleted of Krypton and Xenon since the feed to the crude argon column is also depleted in Krypton and Xenon. A stream concentrated in Krypton and Xenon is extracted at the bottom of the low pressure column. 
     As can be seen, most of the prior art addressed the rare gases recovery for oxygen plant equipped with argon production for high purity oxygen and in some cases, mixing column. Those processes operate at relatively low pressure at about 1.5 to 2 bar in the low pressure column which would yield an air pressure of about 6 to 7.5 bar. Higher pressure than these values would deteriorate the distillation performance especially for the argon recovery. Elevated pressure plant in contrary produces low purity oxygen and operates at about 10 to 16 bar air pressure with the low pressure column operates at about 4 to 6 bar. In order to maintain a good oxygen recovery rate an intermediate column is used to generate more liquid nitrogen reflux from the top of the intermediate column. 
     SUMMARY 
     The present invention is a process for recovering rare gases from a multiple column oxygen plant, wherein the multiple column oxygen plant comprises a higher pressure column, a lower pressure column, a middle pressure intermediate column, and a low pressure intermediate column, said middle pressure intermediate column comprising a first bottom reboiler and said low pressure intermediate column comprising a second bottom reboiler. The process includes providing a first oxygen rich liquid stream containing rare gases from the higher pressure column, wherein said first oxygen rich liquid stream is introduced to the first bottom reboiler. The process also includes removing a second oxygen rich liquid stream rich in rare gases from the bottom of the middle pressure intermediate column, wherein said second oxygen rich liquid stream is introduced to the low pressure intermediate column. The process also includes removing a first liquid purge stream concentrated in rare gases is removed from the low pressure intermediate column, wherein said first liquid purge stream is further concentrated downstream. And the process includes removing a third oxygen rich liquid stream lean in rare gases at a location that is at least one tray above the first bottom reboiler, wherein said third oxygen rich liquid stream is introduced to the lower pressure column. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein: 
         FIG. 1  is a schematic representation of one embodiment of the present invention; 
         FIG. 2  is a schematic representation of another embodiment of the present invention; 
         FIG. 3  is a schematic representation of another embodiment of the present invention 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Illustrative embodiments of the invention are described below. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     As illustrated in  FIG. 1 , elevated pressure air  7  at about 10 to 16 bar is fed to a high pressure column  100  to form a nitrogen rich gas at the top and an oxygen rich liquid  10  at the bottom. A liquid air stream  8  is fed to an intermediate tray location of column  100 . A liquid stream  20  with a composition close to liquid air is extracted from the liquid of the tray above the feed tray of liquid air stream  8 . Nitrogen rich gas is condensed to yield a first reflux  40  to the low pressure column  200 . The oxygen rich liquid  10  is then fed to the bottom reboiler of an intermediate column  300  wherein it is distilled to form a second nitrogen rich gas at the top and a second oxygen rich liquid  31  at the bottom. The second nitrogen rich gas is condensed to yield a second reflux  44  to the low pressure column  200 . Stream  20  is fed to column  200  or to both columns  200  and  300 . It can be seen that most of the Kr and Xe contained in the air feeds  7  and  8  of the high pressure column is collected in stream  10 . Column  300  operates at a pressure lower than column  100 &#39;s pressure but higher than column  200 &#39;s pressure. In order to balance out the system a third oxygen rich liquid  32  is extracted at a tray location above the bottom reboiler of column  300 . By adopting an adequate tray location and flow of stream  32 , it is possible to yield a stream  32  very lean in Kr and Xe and therefore almost all Kr and Xe of the feed stream  10  can be captured in stream  31 . Stream  31  is then fed to the bottom reboiler  72  of a column  400 , which is reboiled by condensing nitrogen from the top of the intermediate column. This column  400  contains about 5 to 15 theoretical trays and operates at about the same pressure as column  200 . A portion  33  of stream  32  is used as reflux for column  400 . A liquid purge  50  rich in Kr and Xe is then extracted at the bottom of column  400  for further concentrating operation. 
     In some plants low pressure air expander  12  expanding air feed into the low pressure column  200  is used. This expanded stream  15  also contained rare gases which would be lost if sent to the low pressure column  200 . In this case it is possible to send the expanded air  15  to the bottom of column  400  in order to wash out the contained rare gases and maintaining high recovery of Kr and Xe. 
     In the process without rare gases production, the bottom stream of the intermediate column  300  is normally divided into 2 portions: the first one is vaporized in the overhead condenser of the intermediate column, the second one is fed as liquid feed to the low pressure column  200 . If the same process is applied for rare gases production, the Kr—Xe contained in the second portion of bottom liquid feeding the low pressure column  200  would have been lost in the liquid oxygen product  30 . In order to remedy this situation a liquid stream  32  free of Kr—Xe is extracted at a tray located above the bottom reboiler to substitute this second portion of bottom liquid. By doing so the process efficiency is essentially unchanged, and the bottom stream  31  containing the rare gases can be isolated and treated, either in a column or a vaporizer, to recover the rare gases prior to sending it to the low pressure column  200  to produce oxygen. A fraction  33  of stream  32  is used to reflux the KrXe column  400  to further improve the recovery of rare gases. 
     In reference to the process described in  FIG. 1 , for a total feed air of 1000 containing 1.14 ppm Kr and 0.086 ppm Xe: 
     
       
         
           
               
               
            
               
                   
                   
               
               
                   
                 Stream 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 10 
                 32 
                 31 
                 36 
                 33 
                 KrXe 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Flow 
                 454 
                 150 
                 225 
                 254 
                 34 
                 5 
               
               
                 ppm Kr 
                 2.49 
                 0.26 
                 4.9 
                 0.04 
                 0.26 
                 219 
               
               
                 ppm Xe 
                 0.19 
                 0.00012 
                 0.38 
                 0 
                 0.00012 
                 17.2 
               
               
                   
               
               
                 ppm: parts per million by volume 
               
            
           
         
       
     
     In this process simulation, stream  32  is extracted at 2 trays above the bottom reboiler. In another embodiment, stream  32  may be extracted at least one tray above the bottom reboiler. Range of composition of stream  31 :
         about 5.5 ppm to 3 ppm Kr   about 0.5 ppm to 0.3 ppm Xe       

     Stream  32  has very low content of Kr and Xe, preferably a maximum at about 1.5 ppm of Kr and 0.01 ppm Xe. The rich liquid  10  is fed to the bottom of the intermediate column. 
     In another embodiment described in  FIG. 2 , the vaporized stream  36  from condenser  72  is treated in a short column  401  to recover the Kr and Xe carried over in stream  36 . Column  401  operates at about the same pressure as the low pressure column  200 . Column  401  is refluxed by a portion  33  of stream  32 . The reboil of column  401  can be supplied by heating the bottom reboiler  75  with any suitable stream such as air, nitrogen, oxygen rich liquid, liquid air etc. The liquid purge stream  50  of the top condenser can be optionally sent to the bottom of column  401  and the combined collected Kr and Xe is recovered is bottom stream  53 . Again, the expanded air stream (not shown), if existed, can be fed to the bottom of column  401  to recover its rare gases content. 
     It is also possible to just vaporize the bottom liquid  31  in the condenser  72  without the use of the column  400  or  401  as illustrated in  FIG. 3 . The Krypton recovery will be reduced significantly because of the carry-over of Kr and, at a lesser proportion, of Xenon in the vaporized stream  36 . This process is slightly simpler and can be used in cases when Krypton recovery does not need to be very high. 
     A Krypton recovery higher than 96% and a Xenon recovery higher than 99% in the liquid purge bottom are expected for this type of process as illustrated in  FIGS. 1 and 2 .