Abstract:
The present invention is a two stage PSA process for producing high purity oxygen from a feed air stream. Water, carbon dioxide and nitrogen are removed in a first stage. An oxygen selective adsorbent is used to adsorb oxygen in the second stage. High purity oxygen product is recovered during regeneration of the second stage. Importantly, the high purity of the oxygen product is achieved without inclusion of an oxygen rinse step in the process cycle. The high purity oxygen product is obtained by collecting the middle cut of the second stage effluent stream during regeneration.

Description:
This invention was made with United States Government supported under Cooperative Agreement No. 70NANB5H1093, awarded by the Department of Commerce National Institute of Standards and Technology. 
     The United States Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a two stage pressure swing adsorption (PSA) process for producing high purity gas from a mixture of a plurality of gases and more particularly, to a PSA process for producing high purity oxygen from air. 
     BACKGROUND OF THE INVENTION 
     Conventional PSA processes for generating oxygen from an air stream commonly use a fixed bed of adsorbent material adapted to adsorb nitrogen from air, such as zeolite, so that an oxygen-rich product gas exits the bed. The principles of separation involved in such an adsorption system are based upon equilibrium separation, i.e., upon the adsorbent material&#39;s ability to hold nitrogen more strongly than oxygen. Present-day synthetic zeolites used in PSA processes are capable of achieving virtually a complete separation between nitrogen and oxygen. However, the adsorption isotherms of oxygen and argon on these materials are almost identical and a passage of feed air through a zeolite bed has no significant effect on the ratio of oxygen to argon which is typically about 21:1. Thus, the percentage by volume of argon in the oxygen-rich product stream, assuming that all of the nitrogen is adsorbed by the zeolite, is about 5 percent. Therefore, PSA processes which employ nitrogen equilibrium selective materials cannot normally generate a product stream containing an oxygen concentration which is appreciably greater than 95.0 percent. 
     Materials which preferentially adsorb oxygen can also be employed in PSA processes for producing oxygen from an air stream. In such a process, the oxygen-rich product is collected from the adsorbent bed during the regeneration step of each cycle. At the present time the most commonly used oxygen selective adsorbent materials are carbon molecular sieves (CMS). The separation achieved with CMS is a result of the material&#39;s more rapid adsorption of oxygen than of nitrogen—which is known as kinetic selectivity. From the point of view of oxygen/nitrogen separation, the kinetic selectivity of CMS is significantly less efficient than the equilibrium selectivity of zeolite. Further, the oxygen product obtained from an air feed, using CMS as an adsorbent material, contains a considerable portion of unseparated nitrogen. 
     In practice the rates of adsorption of nitrogen and argon on CMS are about the same so that in the case of an air feed, the balance of the oxygen product will contain nitrogen and argon approximately in their atmospheric ratio 78:1. 
     In summary, PSA processes for production of oxygen from air which use nitrogen equilibrium selective adsorbents can give maximum oxygen purity of about 95.0%, with the balance represented virtually entirely by argon. PSA processes for production of oxygen from air, which use CMS as the adsorbent, can give a maximum oxygen purity of about 80%, with the balance represented by nitrogen and argon in their atmospheric ratio, i.e., about 19.75% nitrogen and 0.25% argon. 
     However, oxygen of a purity greater than 95.0% is needed in welding and cutting processes as well as in some medically-related applications. Accordingly, it is desirable to provide a PSA process capable of generating a product stream containing an oxygen concentration which is greater than 95.0 percent from an air feed stream. 
     Several PSA systems are known in the prior art which can produce a product stream containing an oxygen concentration which is greater. than 95.0% from an air feed stream. All such systems utilize a two stage PSA arrangement, i.e., there are two distinct mass transfer zones in the PSA process. 
     One group of two stage PSA processes for production of high purity oxygen from feed air is represented by U.S. Pat. No. 4,190,424 (Armond et al.), U.S. Pat. No. 4,959,083 (Garrett), U.S. Pat. No. 4,973,339 (Bansal) and by publications by Seemann et al. (Chem. Eng. Technol. Vol 11, p 341, 1988) and Hayashi et. al. (Gas Sep. Purif. Vol 10 No. 1, p 19, 1996). The first stage employs one or several beds of a CMS which adsorbs oxygen more rapidly, as compared to nitrogen and argon (i.e., an oxygen kinetically-selective material). A feed stream of air constituents (i.e., oxygen, nitrogen, and argon) is delivered to the first stage where oxygen is adsorbed at a higher rate than nitrogen and argon. The adsorbed oxygen is subsequently desorbed and is fed to a second stage which uses one or several beds of zeolite that adsorbs nitrogen preferentially to oxygen and argon (nitrogen equilibrium-selective material). High purity oxygen is collected at the exit of the zeolite bed. 
     The key to the high purity oxygen product obtained from this PSA process is not just the ability of the first CMS stage to provide an oxygen-enriched feed to the second nitrogen adsorbing zeolite stage. More particularly, it is the ability of the CMS stage to provide a feed which is depleted in argon, the one major constituent of atmospheric air which a zeolite is incapable of separating from oxygen. 
     Another group of two stage PSA processes for production of high purity oxygen from feed air is represented by U.S. Pat. No. 5,395,427 (Kumar et al.), U.S. Pat. No. 5,137,549 (Stanford et al.) and U.S. Pat. No. 4,190,424 (Armond et al.). The first stage comprises two beds of zeolite and separates nitrogen, carbon dioxide and water vapor from atmospheric air and passes oxygen, argon and residual nitrogen to the second stage. The second stage includes a pair of beds with oxygen selective material that adsorb oxygen and pass the argon and the residual nitrogen. The high purity oxygen product is recovered upon depressurization of the second stage. 
     The high purity of the oxygen product is achieved by rinsing the oxygen selective adsorbent with high purity oxygen prior to the depressurization step. 
     Another two stage PSA process for production of high purity oxygen from feed air is disclosed in U.S. Pat. No. 4,959,083 (Garrett). The first stage comprises a bed of CMS which adsorbs oxygen more rapidly than nitrogen. The adsorbed oxygen is desorbed from the first stage and flows to a second stage which comprises another bed of CMS. The adsorbed oxygen in the second stage is subsequently desorbed and collected as high purity oxygen product. 
     Another group of two stage PSA processes for production of high purity oxygen from feed air is represented by U.S. Pat. No. 5,226,933 (Knaebel et al.) and U.S. Pat. No. 5,470,378 (Kandybin et al.). A first stage utilizes nitrogen equilibrium-selective adsorbent (zeolite) while the second stage utilizes an argon equilibrium selective adsorbent (silver mordenite). The adsorbents can be placed in separate beds or in a single bed. When the feed air is introduced into the system, nitrogen is removed in the first stage, argon is removed in the second stage, and high purity oxygen is collected at the exit of the system as product. 
     There are a number of drawbacks in the prior art PSA processes for producing high purity oxygen from an air feed. 
     1. In the prior art there is an incompatibility between the stage cycle times when one of the stages utilizes an equilibrium selective adsorbent such as zeolite and the other stage utilizes a kinetically selective adsorbent such as CMS. This leads to an asynchronous mode of operation of the stages and complicates the PSA cycle. In addition, a buffer tank must be placed between the stages. 
     2. The mode of operation of a CMS requires relatively high adsorption pressures—typically between 6 atm and 10 atm. For silver mordenite the required adsorption pressures are even higher—between 10 atm and 20 atm. Thus such prior art PSA systems are characterized by high energy consumption. 
     3. The prior art PSA systems which use an oxygen selective adsorbent in the second stage always employ an oxygen rinse prior to the depressurization in order to achieve high purity of the oxygen product. This reduces the productivity of the PSA system because high purity oxygen product is used as the rinse gas. Also, power requirements increase because the high purity oxygen product is obtained at low pressure during depressurization and at least a portion of the high purity oxygen product must be recompressed again to the high adsorption pressure to supply the cocurrent (with respect to the feed) high pressure purging gas. 
     4. The prior art PSA processes which use an oxygen selective adsorbent in the second stage rely on use of oxygen enriched streams from the second stage oxygen selective beds for regeneration of the first stage nitrogen selective beds, resulting in a decrease in the productivity of the second stage beds. 
     Accordingly, it is an object of the invention to provide an improved dual stage PSA process for the production of high purity oxygen, wherein only equilibrium selective adsorbents are employed and the operation of the stages is synchronized. 
     It is another object of the invention to provide an improved dual stage PSA process for the production of high purity oxygen, which employs modest adsorption pressures and thus exhibits reduced power requirements. 
     It is a further object of the invention to provide an improved dual stage PSA process for the production of high purity oxygen, which avoids the need for use of an oxygen rinse step. 
     It is a further object of the invention to provide an improved dual stage PSA process for the production of high purity oxygen, which enables recovery as product, all of the high purity oxygen effluent of the second stage bed, thereby increasing the productivity of the second stage. 
     SUMMARY OF THE INVENTION 
     The present invention is a two stage PSA process for producing high purity oxygen from a feed air stream. Water, carbon dioxide and nitrogen are removed in a first stage. An oxygen selective adsorbent is used to adsorb oxygen in the second stage. High purity oxygen product is recovered during regeneration of the second stage. Importantly, the high purity of the oxygen product is achieved without inclusion of an oxygen rinse step in the process cycle. The high purity oxygen product is obtained by collecting the middle cut of the second stage effluent stream during regeneration. 
     In brief, the method of the invention: 
     i) produces high purity (&gt;95.5% ) oxygen using oxygen equilibrium selective adsorbent; 
     ii) uses no high pressure rinse step (cocurrent displacement step) in the PSA cycle; 
     iii) enables upper and lower stages to be regenerated independently and avoids interaction between the stages during regeneration; and 
     iv) operates the stages in synchronism using the same step times, consequently, avoiding need for buffer tank(s) between the stages. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram illustrating one embodiment of the present invention using serial beds and a single withdrawal conduit. 
     FIG. 2A illustrates the steps during the first half-cycle of the method of the invention. 
     FIG. 2B illustrates the steps during the second half-cycle of the method of the invention. 
     FIG. 3 is a schematic diagram illustrating a second embodiment of the present invention using serial beds and dual withdrawal conduits. 
     FIG. 4 is a schematic diagram illustrating a third embodiment of the present invention using single vessels with multiple bed layers. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The PSA cycle of this invention incorporates an O 2  equilibrium selective adsorbent, which produces an oxygen-enriched product. An adsorbent having an O 2 /N 2  equilibrium selectivity and little O 2 /N 2  rate selectivity is used. A preferred oxygen equilibrium selective adsorbent is designated IC2. The compound designated as IC2, typically abbreviated as Co{3,5-diBu t sal/(EtO) (CO 2 Et)Hmal-DAP}, is the cobalt (II) complex of the dianion of a chelating ligand prepared formally by the 1:1 condensation of ethoxy-methylene diethylmalonate and 3,4-diamino pyridine, followed by schiff base condensation of the 3,5-di-tert-butysalicylaldehyde. Other O 2  equilibrium selective adsorbents may also be used. 
     It is preferred that the nitrogen equilibrium selective adsorbent be a faujasite-type zeolite, at least 80% lithium exchanged with a SiO 2 /Al 2 ) 3  molar ratio, of 2.3. The preferred nitrogen equilibrium selective adsorbent is henceforth referred to as LiX zeolite. 
     The preferred embodiment of the invention will be described in detail with reference to FIGS. 1,  2 A, and  2 B. FIG. 1 is a schematic diagram illustrating the present invention. The system comprises two trains of adsorbers. Each train comprises a first stage adsorber in series with a second stage adsorber. In addition, each train of adsorbers undergoes its respective cycle of steps while collectively operating in parallel with one another. FIG. 2A illustrates the steps during the first half-cycle of the process. FIG. 2B illustrates the steps during the second half-cycle of the process. 
     Table 1 below summarizes the valve sequence for one complete cycle while Table 2 summarizes the time intervals and the step sequence for one complete process cycle. Tables 1 and 2 utilize 80 time units to cover the twelve steps of the cycle so that the relative times for each step can be clearly indicated. 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Valve sequence during the process cycle. 
               
               
                 (O = open, C = closed) 
               
             
          
           
               
                 Valve 
                 Step Number 
               
             
          
           
               
                 No. 
                 I 
                 II 
                 III 
                 IV 
                 V 
                 VI 
                 VII 
                 VIII 
                 IX 
                 X 
                 XI 
                 XII 
               
               
                   
               
               
                 111 
                 O 
                 O 
                 O 
                 O 
                 O 
                 C 
                 C 
                 C 
                 C 
                 C 
                 C 
                 C 
               
               
                 112 
                 C 
                 C 
                 C 
                 C 
                 C 
                 C 
                 O 
                 O 
                 O 
                 O 
                 O 
                 C 
               
               
                 113 
                 C 
                 C 
                 O 
                 O 
                 O 
                 O 
                 C 
                 C 
                 O 
                 O 
                 O 
                 O 
               
               
                 211 
                 C 
                 C 
                 O 
                 O 
                 O 
                 C 
                 C 
                 C 
                 C 
                 C 
                 C 
                 C 
               
               
                 212 
                 C 
                 C 
                 C 
                 C 
                 C 
                 C 
                 O 
                 O 
                 O 
                 O 
                 O 
                 C 
               
               
                 213 
                 C 
                 C 
                 C 
                 O 
                 O 
                 O 
                 C 
                 C 
                 C 
                 O 
                 O 
                 O 
               
               
                 121 
                 C 
                 C 
                 C 
                 C 
                 C 
                 C 
                 O 
                 O 
                 O 
                 O 
                 O 
                 C 
               
               
                 122 
                 O 
                 O 
                 O 
                 O 
                 O 
                 C 
                 C 
                 C 
                 C 
                 C 
                 C 
                 C 
               
               
                 123 
                 C 
                 C 
                 O 
                 O 
                 O 
                 O 
                 C 
                 C 
                 O 
                 O 
                 O 
                 O 
               
               
                 221 
                 C 
                 C 
                 C 
                 C 
                 C 
                 C 
                 C 
                 C 
                 O 
                 O 
                 O 
                 C 
               
               
                 222 
                 O 
                 O 
                 O 
                 O 
                 O 
                 C 
                 C 
                 C 
                 C 
                 C 
                 C 
                 C 
               
               
                 223 
                 C 
                 C 
                 C 
                 O 
                 O 
                 O 
                 C 
                 C 
                 C 
                 O 
                 O 
                 O 
               
               
                 101 
                 C 
                 C 
                 O 
                 O 
                 O 
                 O 
                 C 
                 C 
                 O 
                 O 
                 O 
                 O 
               
               
                 201 
                 C 
                 C 
                 C 
                 O 
                 O 
                 O 
                 C 
                 C 
                 C 
                 O 
                 O 
                 O 
               
               
                 501 
                 O 
                 C 
                 C 
                 C 
                 O 
                 O 
                 O 
                 C 
                 C 
                 C 
                 O 
                 O 
               
               
                 502 
                 C 
                 O 
                 O 
                 O 
                 C 
                 C 
                 C 
                 O 
                 O 
                 O 
                 C 
                 C 
               
               
                 601 
                 C 
                 C 
                 C 
                 O 
                 O 
                 C 
                 C 
                 C 
                 C 
                 O 
                 O 
                 C 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Time intervals and step sequence of the process cycle. 
               
             
          
           
               
                   
                 Step 
                 Time 
                 Bed Number 
                   
               
             
          
           
               
                   
                 Number 
                 Interval 
                 11 
                 12 
                 21 
                 22 
               
               
                   
                   
               
               
                   
                 I 
                 0-1 
                 RP 
                 EV 
                 ID 
                 EV 
               
               
                   
                 II 
                  1-14 
                 RP 
                 EV 
                 ID 
                 EV 
               
               
                   
                 III 
                 14-16 
                 AD 
                 PG 
                 RP 
                 EV 
               
               
                   
                 IV 
                 16-22 
                 AD 
                 PG 
                 AD 
                 PG 
               
               
                   
                 V 
                 22-28 
                 AD 
                 PG 
                 AD 
                 PG 
               
               
                   
                 VI 
                 28-40 
                 EQ 
                 EQ 
                 EQ 
                 EQ 
               
               
                   
                 VII 
                 40-41 
                 EV 
                 RP 
                 EV 
                 ID 
               
               
                   
                 VIII 
                 41-54 
                 EV 
                 RP 
                 EV 
                 ID 
               
               
                   
                 IX 
                 54-56 
                 PG 
                 AD 
                 EV 
                 RP 
               
               
                   
                 X 
                 56-62 
                 PG 
                 AD 
                 PG 
                 AD 
               
               
                   
                 XI 
                 62-68 
                 PG 
                 AD 
                 PG 
                 AD 
               
               
                   
                 XII 
                 68-80 
                 EQ 
                 EQ 
                 EQ 
                 EQ 
               
               
                   
                   
               
               
                   
                 RP = repressurization  
               
               
                   
                 AD = adsorption  
               
               
                   
                 EQ = pressure equalization  
               
               
                   
                 EV = evacuation  
               
               
                   
                 PG = purge  
               
               
                   
                 ID = idle  
               
             
          
         
       
     
     The PSA process illustrated in FIGS. 1,  2 A, and  2 B has a first stage comprising two adsorbing beds  11  and  12  each filled with at least two layers of adsorbents. There is at least one layer  11   a ,  12   a  of nitrogen equilibrium selective adsorbent which layer is preceded by at least one layer  11   b ,  12   b  of adsorbent capable of removing carbon dioxide and water from the feed air. 
     A second stage comprises two other adsorbing beds  21  and  22 , each filled with at least one layer of oxygen equilibrium selective adsorbent  21   a ,  22   a , respectively. A feed compressor  31  provides compressed feed air to beds  11  and  12  through valves  111  and  121 , respectively. Beds  11  and  12  are connected to beds  21  and  22 , respectively, through valves  113  and  123  and inlet valves  211  and  221 , respectively. 
     A vacuum pump/compressor  41  serves the purpose of evacuating beds  11  and  12  through valves  112  and  122 , respectively. The effluent of pump  41  is discharged to atmosphere. A vacuum pump/compressor  51  serves the purpose of evacuating beds  21  and  22  through valves  212  and  222 , respectively. The effluent of pump  51  is discharged either to the low purity oxygen line through valve  501  or to high purity oxygen product tank  52  through valve  502 . 
     The upper ends of beds  11  and  12  are connected through a valve  101  and the upper ends of beds  21  and  22  are connected through a valve  201 . The effluent of beds  21  and  22  are discharged through valves  213  and  223 , respectively, and through valve  601  to atmosphere or are collected as argon-enriched product. 
     All of the valves in FIG. 1 are operated automatically via computer system program logic which is not shown. In the description that follows all the valves are assumed closed unless explicitly declared as open. 
     Step I (time units  0 - 1 ): Bed  11  is pressurized with feed air via feed compressor  31  and open valve  111 . Bed  21  is in an “idle” position. Bed  12  is evacuated to atmosphere through open valve  122  and vacuum pump/compressor  41 . Bed  22  is at the beginning of its regeneration sequence and is evacuated through open valve  222  and vacuum pump/compressor  51 . The oxygen purity of the effluent of bed  22  is increasing during Step I, but is less than the minimum purity required for the high purity oxygen product. Consequently, the effluent of bed  22  is discharged to the low purity oxygen line during Step I via open valve  222 , vacuum pump  51  and open valve  501 . Step I is terminated when the effluent of bed  22  reaches the minimum purity required for the higher purity oxygen product. 
     Step II (time units  1 - 14 ): Bed  11  continues to be pressurized with feed air via compressor  31  and open valve  111 . Step II is terminated when bed  11  reaches its adsorption pressure. Bed  21  is still in the “idle” position. Bed  12  continues to be evacuated to atmosphere through open valve  122  and vacuum pump  41 . Bed  22  continues to be evacuated through open valve  222  and vacuum pump  51 . 
     During Step II, the oxygen purity of the effluent of bed  22  is equal or higher than the minimum purity required for the high purity oxygen product. Thus, the effluent of bed  22  during Step II is collected in the product tank  52  via open valve  222 , vacuum pump  51  and open valve  502 . 
     Step III (time units  14 - 16 ): Bed  11  is in its adsorption state. Feed air continues to be fed to bed  11  through feed compressor  31  and open valve  111 . The effluent stream of bed  11  is enriched in oxygen since water, carbon dioxide and nitrogen have been preferentially adsorbed in the bed. The oxygen enriched effluent of bed  11  is introduced into bed  21  through open valves  113  and  211  and is used to pressurize bed  21 . Since the outlet of bed  11  is connected to the inlet of bed  21 , beds  11  and  21  are connected in series. 
     It is important to insure that the mass transfer zone (MTZ) developed in oxygen selective bed  21  has a self-sharpening front. This is achieved by creating a favorable oxygen concentration difference in the oxygen concentration at the outlet of nitrogen selective bed  11  and the oxygen concentration at the inlet of oxygen selective bed  21  at the instant of first communication between the two beds. A favorable oxygen concentration difference for development of a self-sharpening mass transfer zone in oxygen selective bed  21  is created when, at the beginning of pressurization of bed  21 , the oxygen purity of the effluent stream coming from bed  11  and used for pressurization of bed  21 , is higher than the oxygen purity of the gas phase that exists at that moment at the entrance of bed  21 . This favorable oxygen concentration difference creates a self-sharpening MTZ in oxygen selective bed  21  and constitutes an important condition for the optimal operation of the PSA process. If, at the beginning of pressurization of bed  21 , the oxygen concentration difference is unfavorable, that is, the oxygen purity of the effluent stream coming from bed  11  and used for pressurization of bed  21  is less than the oxygen purity of the gas phase that exists at that moment at the entrance of bed  21 , the MTZ formed in the oxygen selective bed is receding, which leads to poor utilization of the oxygen selective adsorbent, and consequently, to poor performance of the PSA process as a whole. 
     Step III is terminated when bed  21  reaches its adsorption pressure. Part of the oxygen rich effluent that comes out of bed  11  is introduced via open valves  101  and  123  to bed  12  and is used for low pressure countercurrent purge of bed  12 . The effluent of bed  12  is discharged to atmosphere through open valve  122  and vacuum pump/compressor  41 . Bed  22  continues to be evacuated through open valve  222  and vacuum pump  51 . 
     During Step III the oxygen purity of the effluent of bed  22  is equal or higher than the minimum purity required for the high purity oxygen product. Thus, the effluent of bed  22  during Step III continues to be collected in the product tank  52  via open valve  222 , vacuum pump  51  and open valve  502 . 
     It is to be noted that the oxygen enriched stream necessary for the regeneration of the first stage bed (bed  12 ) is provided by the other first stage bed (bed  11 ) which is in its adsorption state. Thus, the entire oxygen enriched effluent that comes out of second stage bed  22  can be collected as product, increasing the productivity of the process. 
     Step IV (time units  16 - 22 ): Beds  11  and  21  are both in their adsorption state. Feed air is introduced to bed  11  through compressor  31  and open valve  111 . The oxygen enriched effluent of bed  11  is introduced into bed  21  through open valves  113  and  211 . oxygen is preferentially adsorbed in bed  21  and an oxygen depleted effluent is discharged as waste or collected as argon enriched product from bed  21 , via open valves  213  and  601 . 
     Part of the oxygen rich effluent that comes out of bed  11  is introduced via open valves  101  and  123  to bed  12  and is used for low pressure countercurrent purge of bed  12 . The effluent of bed  12  is discharged to atmosphere through open valve  122  and vacuum pump/compressor  41 . Part of the oxygen depleted effluent that comes out of bed  21  is introduced via open valves  201  and  223  to bed  22  and is used for low pressure countercurrent purge of bed  22 . 
     The oxygen purity of the effluent of bed  22  is decreasing during Step IV but is higher than the minimum purity required for the high purity oxygen product. Consequently, the effluent of bed  22  continues to be collected in the product tank  52  via open valve  222 , vacuum pump  51  and open valve  502 . Step IV is terminated when the effluent of bed  22  reaches the minimum purity required for the high purity oxygen product. 
     Step V (time units  22 - 28 ): Beds  11  and  21  continue to be in adsorption phases. Feed air continues to be fed to bed  11  through feed compressor  31  and open valve  111 . The oxygen enriched effluent of bed  11  is introduced into bed  21  through open valves  113  and  211 . Oxygen is preferentially adsorbed in bed  21  and an oxygen depleted effluent is discharged from bed  21  via open valves  213  and  601 . 
     Part of the oxygen rich effluent that comes out of bed  11  is introduced via open valves  101  and  123  to bed  12  and is used for low pressure countercurrent purge of bed  12 . The effluent of bed  12  is discharged to atmosphere through open valve  122  and vacuum pump/compressor  41 . 
     Part of the oxygen depleted effluent that comes out of bed  21  is introduced via open valves  201  and  223  to bed  22  and is used for low pressure countercurrent purge of bed  22 . The oxygen purity of the effluent of bed  22  is decreasing further during Step V and is now below the minimum purity required for high purity oxygen product. Consequently, the effluent of bed  22  is discharged to the low purity oxygen line during Step V, via open valve  222 , vacuum pump  51  and open valve  501 . Step V is terminated when the mass transfer zones in beds  11  and  21  reach the effluent ends of the beds and are about to break through. 
     An important feature of the present invention is the synchronized operation of the beds so that the mass transfer zones reach the ends of the beds at the same time. This synchronization leads to a better utilization of the adsorbent material in the beds and eliminates the necessity of a buffer tank between the stages. 
     Step VI (time units  28 - 40 ): First stage bed  11  which is at high adsorption pressure and first stage bed  12  which is at low regeneration pressure are connected through open valves  113 ,  101  and  123  to equalize their pressures. At the same time the second stage bed  21  which is at high adsorption pressure and the second stage bed  22  which is at low regeneration pressure equalize their pressures through open valves  213 ,  201  and  223 . 
     It is important to note that at the end of the equalization step, the oxygen purity of the gas phase that exists (at that moment) at the bottom end of bed  22  is lower than the oxygen purity of the oxygen enriched stream that will be supplied to bed  22  later in the cycle (Step IX) from first stage bed  12 . As pointed out above, this creates a favorable difference in oxygen concentration at the outlet of nitrogen selective bed  12  and the oxygen concentration at the inlet of oxygen selective bed  22 , at instant of first communication between the two beds. This action leads to a sharp mass transfer zone in bed  22 . Also, the effluent gas coming out of bed  21  that is used to partially repressurize bed  22  is depleted in oxygen, which also leads to a sharpening of the mass transfer zone in the second stage during subsequent adsorption steps. 
     Steps VII-XII (time units  40 - 80 ): Steps VII-XII constitute the second half-cycle of the process. In the second half-cycle beds  11  and  21  repeat the steps of beds  12  and  22  in the first half-cycle, respectively, and vice versa. The steps of the second half-cycle are shown in FIG.  2 B. 
     Tables 3 and 4 below give examples of the operating conditions and PSA process performance, using nitrogen and oxygen equilibrium selective adsorbents in the lower and upper beds, respectively. In the tables, the symbols have the following meaning: TPD=ton (2000 lb) per day of oxygen, kPa=1000 Pa=S.I. unit for pressure (1.0 atm.=101.323 kPa, s=time unit in seconds, kW=kilowatt). Also, in the tables, the nitrogen equilibrium selective adsorbent is a faujasite-type zeolite, at least 80% lithium exchanged, and the oxygen equilibrium selective adsorbent is IC2, as described above. 
     Table 3: Gives an example using two trains of two beds in series for production of high purity (&gt;95%) oxygen; wherein, the lower bed of each train contains a faujasite-type zeolite, at least 80% lithium exchanged, and the upper bed of each train contains IC2. The results shown below were obtained from PSA simulation results for the case where all of the oxygen is recovered from the upper bed during the regeneration step(s) of the PSA process, and feed (air) enters the lower bed. In this case, the desorption pressure is high enough to facilitate the use of a single stage machine for the evacuation step(s) of the PSA process. 
     Table 3: An example using the PSA process of the invention. 
     Adsorbent in Lower Bed: LiX zeolite 
     Adsorbent in Upper Bed: IC2 
     Feed Composition: 79% N 2 , 21% O 2    
     High Pressure: 160 kPa 
     Low Pressure: 45 kPa 
     Feed Rate: 2.15×10 5  NCFH 
     Amount of O 2  Produced: 15.37 TPD 
     Oxygen Purity: 98.10% 
     Overall oxygen Recovery: 45.7% 
     Bed Size Factor: 286.5 lb/TPD O 2    
     Power: 6.35 kW/TPD 
     Temperature: 300 K 
     Table 4: An example using two trains of two beds in series for production of high purity (&gt;95%) oxygen; wherein, the lower bed of each train contains a faujasite-type zeolite, at least 80% lithium exchanged, and the upper bed of each train contains IC2. The results shown below were obtained from PSA simulation results for the case where all of the oxygen is recovered from the upper bed during the regeneration step(s) of the PSA process, and feed (air) enters the lower bed. In this case, the lower desorption pressure requires use of a two stage machine for the evacuation step(s) of the PSA process. Table 4: A further example using the PSA process of the invention: 
     Adsorbent in Lower Bed: LiX zeolite 
     Adsorbent in Upper Bed: IC2 
     Feed Composition: 79% N 2 , 21% O 2    
     High Pressure: 150 kPa 
     Low Pressure: 30 kPa 
     Feed Rate: 2.15×10 5  NCFH 
     Amount of O 2  Produced: 17.05 TPD 
     Oxygen Purity: 98.24% 
     Overall oxygen Recovery: 54.3% 
     Bed Size Factor: 287.3 lb/TPD O 2    
     Power: 6.96 kW/TPD 
     Temperature: 300 K 
     Table 5: An example using two trains of two beds in series for production of medium purity (&lt;95%) oxygen; wherein, the lower bed of each train contains a faujasite-type zeolite, at least 80% lithium exchanged, and the upper bed of each train contains IC2. The results shown below were obtained from PSA simulation results for the case where all of the oxygen is recovered from the upper bed during the regeneration step(s) of the PSA process, and feed (air) enters the lower bed. In this case, the lower desorption pressure requires use of a two stage machine for the evacuation step(s) of the PSA process. 
     Table 5: An example for producing medium purity oxygen. 
     Adsorbent in Lower Bed: LiX zeolite 
     Adsorbent in Upper Bed: IC2 
     Feed Composition: 79% N 2 , 21% O 2    
     High Pressure: 150 kPa 
     Low Pressure: 30 kPa 
     Feed Rate: 2.15×10 5  NCFH 
     Amount of O 2  Produced: 23.80 TPD 
     Oxygen Purity: 93.65% 
     Overall oxygen Recovery: 75.9% 
     Bed Size Factor: 205.6 lb/TPD O 2    
     Power: 4.98 kW/TPD 
     Temperature: 300 K 
     Table 6: An example using two trains of two beds in series for production of medium purity (&lt;95%) oxygen; wherein, the lower bed of each train contains a faujasite-type zeolite, at least 80% lithium exchanged, and the upper bed of each train contains IC2. The results shown below were obtained from PSA simulation results for the case where a portion of the oxygen is recovered from the lower bed during the adsorption step, additional oxygen is recovered from the upper bed during the regeneration step(s) of the PSA process, and feed (air) enters the lower bed. This example used serial beds with a dual withdrawal of product. 
     Table 6: Another example for medium purity oxygen 
     Adsorbent in Lower Bed: LiX zeolite 
     Adsorbent in Upper Bed: IC2 
     Feed Composition: 79% N 2 , 21% O 2    
     High Pressure: 150 kPa 
     Low Pressure: 30 kPa 
     Feed Rate: 2.15×10 5  NCFH 
     Amount of O 2  Produced: 21.20 TPD 
     Oxygen Purity (LiX Bed): 89.81% 
     Oxygen Purity (IC2 Bed): 90.7% 
     Oxygen Recovery (LiX Bed): 56.05% 
     Oxygen Recovery (IC2 Bed): 86.2% 
     Overall Oxygen Recovery 54.39% 
     Bed Size Factor (LiX Bed): 251.7 lb/TPD O 2    
     Bed Size Factor (IC2 Bed): 25.20 lb/TPD O 2    
     Power: 4.59 kW/TPD 
     Temperature: 300 K 
     In an alternative mode of operation illustrated in FIG. 3, oxygen product of modest purity (˜90%) is collected in the beginning of adsorption from the effluent stream of the nitrogen selective bed  11  or  12  via open valves  113  and  701 , or  123  and  701 , respectively. As adsorption in the nitrogen selective bed continues with decreasing oxygen purity, the lower purity effluent of the bed(s) is passed to the oxygen selective bed(s) wherein oxygen is recovered upon regeneration via vacuum pump  51 . This mode is referred to as a serial beds dual withdrawal (SBDW) mode, and the PSA simulation results for this case are shown in Table 6. Note, that results shown in Table 6 were obtained using a well defined cycle. However, it should be noted that other PSA cycles could be used without deviating from the scope of this alternative mode of operation, i.e., serial beds, dual withdrawal. 
     In another mode of operation as illustrated in FIG. 4, the N 2  and O 2  equilibrium selective adsorbents are placed in the same bed. In this arrangement, the nitrogen adsorbent layer is placed near the feed end, and the O 2  selective adsorbent layer is placed above it in the same vessel. In this mode of operation, feed air enters the bed, passes through the nitrogen selective layer, then through the oxygen selective adsorbent layer to produce Ar rich effluent during the high pressure adsorption step. After a predetermined time, the adsorption step is terminated and the bed is regenerated. 
     During the regeneration step(s), the adsorbed oxygen in the oxygen selective adsorbent bed is recovered at one end of the bed (not the feed end), and the desorbed gas at the other end (the feed end) of the bed can be discarded as waste. Also, if desired, an additional oxygen-enriched stream may be obtained by evacuating the vessels through a side port at the oxygen selective section of the vessels. In this mode of operation different PSA cycles can be used without deviating from the key features of this invention. 
     In an alternative mode of operation, Step I may be modified so that the effluent of bed  22  is used to repressurize bed  21 . In the same mode of operation the effluent of bed  21  in Step VII is used to pressurize bed  22 . 
     In a further alternative mode of operation, Step I may be modified so that the effluent of bed  22  is recycled to bed  11 . The effluent of bed  22  can be recycled either to the feed of bed  11  or it can be introduced at an intermediate point of bed  11  since the effluent of bed  22  in Step I is free of water and carbon dioxide and is partially enriched in oxygen. In the same mode of operation, the effluent of bed  21  in Step VII is recycled to bed  12 . 
     In still another alternative mode of operation Step V may be modified so that the effluent of bed  22  is used to purge bed  12 . In the same mode of operation the effluent of bed  21  in step XI is used to purge bed  11 . 
     In further alternative modes of operation (i) Steps VI and XII are modified so that the equalization of the second stage oxygen selective beds is carried out not only by connecting their top ends but by simultaneously connecting their bottom ends as well; and (ii) carbon molecular sieve may be used as an oxygen selective adsorbent in the second stage. 
     Preferably, the highest adsorption pressure in the two stages is in the range of 1 atm to 4 atm. 
     Preferably, the lowest desorption pressure in the two stages is in the range of 0.02 atm to 0.75 atm. 
     Preferably, the average purity of the oxygen enriched stream in the first stage is in the range of 35 percent oxygen to 85 percent oxygen. 
     In all of the aforementioned PSA processes of this invention, a prepurifier section e.g., a layer of alumina, is placed at the upstream end of the zeolite bed to remove water and carbon dioxide from the feed air. 
     In other modes of operation, other adsorbents can be used with this invention. For example, 5A, 13X, and mixed cations zeolites can be used as the N 2  selective adsorbent in the lower bed, and carbon molecular sieve, clinoptilolite, and mordenite can be used as the O 2  selective adsorbent in the upper bed of the two stage PSA process. 
     Other oxygen equilibrium selective adsorbents can be used instead of IC2. Examples of such oxygen equilibrium selective adsorbents are disclosed in U.S. Pat. No. 5,735,938 and the references therein. Oxygen rate selective adsorbents, such as carbon molecular sieves or zeolites (e.g., 4A, clinoptilolite, mordenite, etc.) can be employed as well). 
     It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.