Abstract:
A process for recovering hydrogen products from a hydrogen/carbon oxide synthesis gas wherein removal of carbon oxides is accomplished in a pressure swing adsorption unit, by operating the pressure swing adsorption unit until carbon oxides break through into the effluent from the pressure swing adsorption unit, followed by passing the effluent from the pressure swing adsorption unit through a methanator to remove breakthrough carbon oxides from the pressure swing adsorption effluent.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention pertains to production of hydrogen from a hydrogen/carbon oxide synthesis gas and/or a hydrogen feedstock. 
     Conventional hydrogen production facilities have a first part or a front end of the process that produces a synthesis gas consisting essentially of hydrogen, carbon monoxide, carbon dioxide and other gases such as nitrogen, argon, methane and water. Steam reformation of hydrocarbons is a well known process for production of the synthesis gas. In the basic process a hydrocarbon or a mixture of hydrocarbons is treated to remove trace contaminants such as sulfur and olefins which would adversely affect the reformer catalyst. Methane is the preferred starting material since it contains a higher proportion of hydrogen than other hydrocarbons. The hydrocarbon, after being treated for removal of adverse components, is combined with steam and the mixture is injected into a reformer furnace. The reformer furnace operates at an elevated temperature and is necessary in order to drive the reaction to completion, the reaction being endothermic. 
     As stated above the effluent from the reformer furnace is principally hydrogen, carbon monoxide and carbon dioxide with minor amounts of methane and other gases. In the conventional process, the effluent from the reformer furnace is introduced into a single or multi-stage shift reactor to form additional hydrogen and carbon dioxide by the conversion of carbon monoxide. In the shift reactor carbon monoxide is converted to carbon dioxide with the liberation of additional hydrogen by reaction in the presence of steam. 
     In one process the effluent from the reformer is subjected, first a high temperature shift reaction which takes place at temperatures of between about 600-850° F. utilizing an iron-chrome catalyst. After the high temperature shift reaction, the effluent is cooled with heat recovery and passed through a low temperature shift reaction where additional amounts of carbon monoxide are converted to carbon dioxide at a temperature of about 385° F. to 500° F. in the presence of a copper-zinc catalyst. The effluent from the low temperature shift reactor is sent to a heat recovery station so the effluent can be subjected to further processing. In the event the desired product is hydrogen, the effluent from the low temperature shift reactor is passed to an acid gas removal unit so that carbon dioxide can be removed by any of the well known carbon dioxide removal techniques such as using methyl ethyl amine (MEA) or methyl diethyl amine (MDEA). The effluent from the CO2 removal unit is subjected to a heat integration step whereby the temperature is adjusted so that the effluent, consisting mainly of hydrogen with minor amounts of carbon monoxide and carbon dioxide, as well as any trace gases present in the original synthesis gas such as argon and nitrogen, is sent to a methanation unit for conversion of the carbon monoxide and carbon dioxide. 
     In an attempt to improve on the process described above it has been proposed to eliminate the CO2 removal and the methanation steps and subject the effluent, after either a high temperature shift followed by a low temperature shift or a high temperature shift and heat recovery, to a pressure swing adsorption (PSA) unit operation for purification. 
     Use of a pressure swing adsorption step in production of hydrogen and carbon dioxide is disclosed in U.S. Pat. Nos. 4,963,339 and 5,000,925 wherein patentees disclose integrating a hydrogen PSA, carbon dioxide PSA and various recycle streams into a reformer process. Patentees discuss and disclose purification and recovery of the hydrogen PSA purge gas using pressure swing adsorption, distillation, compression and recycle. There is no discussion of the optimization of the hydrogen PSA unit operation through manipulation of the product hydrogen stream. 
     J. R. Phillips of the Mississippi Chemical Corporation, Yazoo City, Miss. is noted as the compiler of a section on gas purification in a text book titled Fetitizer, Science and Technology Services, No., 2, (1974) pages 311-319 section II-B entitled Methanation. In the section identified as II-B “methanation” the author discusses the need for methanation of carbon oxide streams in ammonia synthesis. The author explains the need to remove carbon monoxide from a synthesis gas stream to prevent damage to various ammonia and hydrogenation catalysts. 
     In an article titled Methanator Design And Operation which appeared in Volume 69 No. 1, pages 75-79 of Chemical Engineering Progress, the authors provide a general overview of the design and operation of methanation units and the potential hazards of nickel carbonyl. 
     An article entitled Separation Of Hydrogen Mixtures by a Two-Bed Pressure Swing Adsorption Process Using Zeolite 5A, which appeared in Ind. Eng. Chem. Res. Volume 36 No. 7, pages 2789-2798, discusses experimental and theoretical development regarding the bulk separation of hydrogen/carbon monoxide (H 2 /CO) and hydrogen/methane (H 2 /CH 4 ) mixtures using pressure swing adsorption technology. The effects of various process parameters and a mathematical model are disclosed. 
     U.S. Pat. No. 4,318,711 discloses a process to upgrade low BTU feed gas formed by gasification of carbonaceous materials to a high BTU gas. The process disclosed includes carbon dioxide removal, carbon monoxide removal, hydrogen production via the well known steam-iron process, drying and methanation. There is no disclosure of the utilization of pressure swing adsorption in such a process. Patentees claim it will be unnecessary to purify the hydrogen gas or any of the other gases, except for water removal, before introduction of the gas into the methanation zone to produce the higher BTU product gas. 
     There is no teaching or suggestion in the art that the production of hydrogen from a hydrogen/carbon oxides synthesis gas can be improved by the use of a combination of the well known methanation and pressure swing adsorption processes. 
     BRIEF SUMMARY OF THE INVENTION 
     The main objective of the present invention is to increase hydrogen recovery from synthesis gas using a pressure swing adsorption (PSA) unit. The present invention utilizes a methanator down stream of the PSA unit to treat the entire product hydrogen stream or some portion thereof to remove trace amounts of carbon oxides, e.g. CO and CO2, which are allowed to escape or breakthrough during a normal PSA cycle. According to the present invention the process permits an increase in the cycle time beyond carbon oxide impurity breakthrough by methanating residual carbon oxides to methane. 
     Therefore, in one aspect, the present invention is a method for increasing the recovery of hydrogen from hydrogen/carbon oxide synthesis gas comprising the steps of: (a) passing the synthesis gas stream through one of a high temperature shift reactor, a medium temperature shift reactor, a low temperature shift reactor or a combination thereof to convert carbon monoxide and water vapor in the stream to carbon dioxide and hydrogen; (b) passing the synthesis gas stream exiting the shift reactor(s), after heat recovery, through a pressure swing adsorption unit adapted to remove carbon oxides from the stream; the pressure swing adsorption unit operated for a cycle time beyond which carbon oxide impurities breakthrough the pressure swing adsorption unit and are contained in an effluent from said pressure swing adsorption unit; and (c) passing said effluent from step (b) through a methanator to convert residual carbon oxides in said effluent to methane to produce a purified hydrogen stream with a volumetric increase in hydrogen recovery from the pressure swing adsorption unit. 
     In another aspect, the present invention is a process for recovering hydrogen from a hydrogen/carbon oxide synthesis gas wherein the synthesis gas is subjected to shift reaction to convert carbon monoxide to carbon dioxide followed by treating an effluent from the shift reaction in a pressure swing adsorption unit to yield a purified hydrogen stream containing no carbon oxides, the improvement comprising: operating the pressure swing adsorption unit under a cycle time wherein carbon oxides breakthrough and are present in an effluent from the pressure swing adsorption unit; and passing the effluent from the pressure swing adsorption unit through a methanator to convert residual carbon oxides to methane to produce a purified hydrogen stream with a volumetric increase in hydrogen recovery from the pressure swing adsorption unit. 
     According to yet another aspect, the present invention is a process, wherein the effluent from the pressure swing adsorption step is divided into at least two streams, a first stream being subjected to heat recovery followed by methanation and a second stream split into two sub-streams, a first sub-stream being subjected to heat recovery and methanation and a second sub- stream recovered without further treatment as hydrogen product. 
     In a further aspect, the present invention is a process for recovering hydrogen from a hydrogen/carbon oxide synthesis gas wherein the synthesis gas is subjected to shift reaction to convert carbon monoxide and steam to carbon dioxide and hydrogen followed by treating an effluent from the shift reaction, after heat recovery, in a pressure swing adsorption unit to yield a purified hydrogen stream containing essentially no carbon oxides the improvement comprising operating the pressure swing adsorption unit under a cycle time wherein carbon oxides breakthrough are present in an effluent from the pressure swing adsorption unit; and passing the effluent from the pressure swing adsorption unit through a methanator to convert residual carbon oxides to methane to produce a purified hydrogen stream, whereby there is a volumetric increase in hydrogen recovery from the pressure swing adsorption unit. 
     In a still further aspect, the present invention is a process for recovering hydrogen from a hydrogen/carbon oxide synthesis gas wherein said synthesis gas is subjected to shift reaction to convert carbon monoxide and steam to carbon dioxide and hydrogen followed by treating an effluent from the shift reaction, after heat recovery, in a pressure swing adsorption unit to yield a purified hydrogen stream containing essentially no carbon oxides the improvement comprising operating the pressure swing adsorption unit under a cycle time wherein carbon oxides breakthrough are present in an effluent from the pressure swing adsorption unit; and passing the effluent from the pressure swing adsorption unit through a methanator to convert residual carbon oxides to methane to produce a purified hydrogen stream, whereby there is a volumetric increase in hydrogen recovery from the pressure swing adsorption unit. 
    
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating a prior art process for hydrogen purification utilizing methanation. 
     FIG. 2 is a block diagram representing a process for purification of hydrogen using pressure swing adsorption. 
     FIG. 3 is a block diagram of the process according to one aspect of the present invention. 
     FIG. 4 is a block diagram of an embodiment of the present invention wherein multiple hydrogen product streams can be produced using the process of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1, depicts a process scheme  10  which has been used to purify a synthesis gas denoted by arrow  12  which is produced from the front end of a hydrogen production facility or the steam reformation portion of a hydrogen production facility. The synthesis gas  12  consisting mainly of hydrogen and carbon monoxide, and other gases such as carbon dioxide, water vapor, argon and nitrogen, and other trace elements unimportant for the purposes of this invention is sent to a high temperature shift reactor  14  where the synthesis gas is reacted over a temperature range of between about 600° F. and 850° F. and passed over an iron-chrome catalyst so that the major portion or all of the carbon monoxide is converted to carbon dioxide. The effluent from the high temperature shift reactor  14  is subject to heat recovery in a suitable device  16  and passed to a low temperature shift reactor  18  where the effluent is reacted over a temperature range between about 385° F. to 500° F. in the presence of a copper-zinc catalyst to further convert carbon monoxide to carbon dioxide. The effluent from the low temperature reactor  18  is subjected to heat recovery in a suitable heat recovery device  20  and then passed to a carbon dioxide removal unit  22  where the carbon dioxide is removed by such processes as methyl ethyl amine adsorption or methyl diethyl amine adsorption. The hydrogen rich effluent from the carbon dioxide removal step  22  is subjected to a heat integration  24  where the temperature is adjusted so that it can be passed to a methanator or methanation unit  26  for methanation of the remaining carbon oxides to methane and water with trace amounts of nitrogen remaining in the effluent from the methanator  26  which is once again subject to heat recovery  28  and delivered as a product which is shown by arrow  30 . The product  30  is essentially hydrogen with methane, water and nitrogen plus any argon present in the initial mixture, which impurities are acceptable to most users of the hydrogen product stream. 
     Referring to FIG. 2, the synthesis gas  12  is once again subjected to a high temperature shift reaction  14 , heat recovery  16  and optionally a low temperature shift reaction  18  followed by heat recovery  20 . The effluent from the low temperature or the high temperature shift reactor is sent to a pressure swing adsorption unit  32  where the hydrogen is purified by removal of the residual carbon oxides yielding a hydrogen product stream  34  which contains essentially hydrogen with trace amounts of nitrogen, methane and carbon oxides. 
     FIG. 3, shows a process according to the present invention where the synthesis gas  12  is passed through the high temperature shift reactor  14 , heat recovery  16 , optionally a low temperature shift reactor  18 , and heat recovery  20 , a PSA unit  32 , a heat integration unit  36 , a methanator  38 , and a heat recovery unit  40  yielding a hydrogen product shown by arrow  42 . In the process scheme shown in FIG. 3, the PSA unit is followed by a methanation unit so that the PSA unit can be modified to extend the cycle time beyond the time at which carbon oxide impurity breakthrough occurs in the PSA unit. Increasing the cycle time will typically result in increased recovery of hydrogen from the synthesis gas feed to the PSA unit. The methanator is then operated to eliminate breakthrough carbon oxides in the effluent stream from the PSA unit to result in a product hydrogen stream that contains trace quantities of methane and water. The product stream from the methanator may also contain trace quantities of nitrogen, if nitrogen was present in the original feed stream, and is allowed to breakthrough into the PSA hydrogen product stream. 
     Referring to FIG. 4, the process of the present invention can be modified in a number of ways to produce several hydrogen product streams of varying degrees of purity. For example, in one simple modification the effluent from heat recovery operation  40  can be split into two product streams, thus producing hydrogen product streams  44  and  42 . 
     Alternatively, a side stream can be taken from the effluent from the methanation step  38  and split into two streams  46  and  48  with stream  46  exiting the process at the temperature of the methanation step and stream  48  being subject to heat recovery in an additional heat recovery unit  50  to produce a product stream  52 . It is also possible to take a stream  53  after heat addition station  36  and pass stream  53  through a remotely located methanator  54  and heat recovery station  56  to produce a hydrogen product stream  58  at a different location from product stream  42 . 
     A side stream  55  can be removed after the pressure swing adsorption step  32  before heat recovery  36  for transfer to a remote location. In this variation stream  55  can, in one regime be divided into two sub-streams  60 ,  62  one of which ( 60 ) is subject to methanation in a methanation unit  64  followed by heat recovery  66  yielding a hydrogen product stream  68 . Stream  62  can be delivered without methanation as a hydrogen product stream where trace amounts of carbon oxides which are allowed to breakthrough or in the event the PSA unit is operated without carbon oxide breakthrough to deliver a purified hydrogen product stream. In this variation stream  55  can be included to produce either a purified hydrogen product stream ( 68 ) or a stream ( 62 ) with trace amounts of carbon oxides. 
     An additional synthesis gas supply  13  treated to contain substantially hydrogen can be introduced directly into the PSA unit for processing as shown in FIG.  4 . 
     Table 1 sets forth data taken from an existing plant that produces hydrogen utilizing steam reformation. The data presented in Table 1 is for treatment of the synthesis gas subsequent to the steam reformation operation. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 HTS (1) 
                 LTS (2) 
                 CO 2  Removal 
                 Methanation 
                 Hydrogen 
               
             
          
           
               
                   
                 Inlet 
                 Outlet 
                 Inlet 
                 Outlet 
                 Inlet 
                 Outlet 
                 Inlet 
                 Outlet 
                 Product 
               
               
                   
                   
               
             
          
           
               
                 Pressure 
                 psia 
                 310 
                 307 
                 307 
                 305 
                 305 
                 303 
                 303 
                 300 
                 296 
               
               
                 Temperature 
                 F. 
                 722 
                 772 
                 431 
                 442 
                 144 
                 108 
                 570 
                 590 
                 90 
               
               
                 Hydrogen 
                 mol % 
                 37.790 
                 40.370 
                 39.240 
                 39.800 
                 77.010 
                 91.610 
                 91.610 
                 91.460 
                 92.590 
               
               
                 Carbon Monoxide 
                 mol % 
                 3.212 
                 0.630 
                 0.612 
                 0.055 
                 0.107 
                 0.127 
                 0.127 
                 0.000 
                 0.000 
               
               
                 Carbon Dioxide 
                 mol % 
                 5.350 
                 7.932 
                 7.712 
                 8.269 
                 15.930 
                 50 ppm 
                 50 ppm 
                 0.000 
                 0.000 
               
               
                 Nitrogen 
                 mol % 
                 0.056 
                 0.056 
                 0.054 
                 0.054 
                 0.104 
                 0.124 
                 0.124 
                 0.125 
                 0.126 
               
               
                 Methane 
                 mol % 
                 3.041 
                 3.041 
                 2.955 
                 2.955 
                 5.719 
                 6.803 
                 6.803 
                 6.947 
                 7.033 
               
               
                 Water 
                 mol % 
                 50.540 
                 47.960 
                 49.420 
                 48.860 
                 1.122 
                 1.335 
                 1.335 
                 1.465 
                 0.248 
               
               
                 Total 
                 mol % 
                 99.988 
                 99.988 
                 99.993 
                 99.993 
                 99.992 
                 99.998 
                 99.998 
                 99.996 
                 99.997 
               
               
                   
               
               
                 Note:  
               
               
                 Values do not total to exactly 100% due to lack of significant figures in the hydrogen and water mole fractions.  
               
               
                 (1) High Temperature Shift  
               
               
                 (2) Low Temperature Shift  
               
             
          
         
       
     
     From the foregoing data it is apparent that the production of hydrogen utilizing the prior art technique results in a hydrogen purity of about 93 mole percent. 
     Table 2 sets forth data for carbon oxide removal and methanation of a synthesis gas used in the production of ammonia (NH 3 ). 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 HTS (1) 
                 LTS (2) 
                 CO 2  Removal 
                 Methanation 
                 Hydrogen 
               
             
          
           
               
                   
                 Inlet 
                 Outlet 
                 Inlet 
                 Outlet 
                 Inlet 
                 Outlet 
                 Inlet 
                 Outlet 
                 Product 
               
               
                   
                   
               
             
          
           
               
                 Pressure 
                 psia 
                 408 
                 399 
                 388 
                 374 
                 366 
                 365 
                 361 
                 359 
                 356 
               
               
                 Temperature 
                 F. 
                 650 
                 798 
                 400 
                 408 
                 190 
                 110 
                 525 
                 574 
                 100 
               
               
                 Hydrogen 
                 mol % 
                 45.185 
                 52.905 
                 51.920 
                 53.502 
                 74.807 
                 98.364 
                 98.364 
                 97.971 
                 97.971 
               
               
                 Carbon Monoxide 
                 mol % 
                 10.640 
                 2.920 
                 2.866 
                 0.200 
                 0.279 
                 0.367 
                 0.367 
                 0.000 
                 0.000 
               
               
                 Carbon Dioxide 
                 mol % 
                 5.582 
                 13.302 
                 13.055 
                 15.406 
                 21.486 
                 0.005 
                 0.005 
                 0.000 
                 0.000 
               
               
                 Nitrogen 
                 mol % 
                 0.033 
                 0.033 
                 0.032 
                 0.032 
                 0.044 
                 0.058 
                 0.058 
                 0.058 
                 0.058 
               
               
                 Methane 
                 mol % 
                 0.461 
                 0.461 
                 0.452 
                 0.444 
                 0.620 
                 0.815 
                 0.815 
                 1.197 
                 1.197 
               
               
                 Water 
                 mol % 
                 38.085 
                 30.365 
                 31.662 
                 30.405 
                 2.746 
                 0.367 
                 0.367 
                 0.750 
                 0.750 
               
               
                 Argon 
                 mol % 
                 0.013 
                 0.013 
                 0.013 
                 0.013 
                 0.018 
                 0.023 
                 0.023 
                 0.023 
                 0.023 
               
               
                 Total 
                 mol % 
                 100.000 
                 100.000 
                 100.000 
                 100.000 
                 100.000 
                 100.000 
                 100.000 
                 100.000 
                 100.000 
               
               
                   
               
               
                 (1) High Temperature Shift  
               
               
                 (2) Low Temperature Shift  
               
             
          
         
       
     
     From the data presented in Table 2, it is apparent that when the content of the synthesis gas is higher in hydrogen the hydrogen product will be of improved purity. 
     Table 3 sets forth a simulation of a pressure swing adsorption unit integrated into an existing hydrogen production facility. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 HTS (1) 
                 LTS (2) 
                 PSA 
                 Hydrogen 
               
             
          
           
               
                   
                 Inlet 
                 Outlet 
                 Inlet 
                 Outlet 
                 Inlet 
                 Outlet 
                 Product 
               
               
                   
                   
               
             
          
           
               
                 Pressure 
                 psia 
                 524 
                 520 
                 515 
                 510 
                 492 
                 488 
                 488 
               
               
                 Temperature 
                 F. 
                 610 
                 728 
                 413 
                 442 
                 72 
                 73 
                 73 
               
               
                 Hydrogen 
                 mol % 
                 45.432 
                 51.697 
                 51.697 
                 53.191 
                 74.085 
                 99.995 
                 99.995 
               
               
                 Carbon Monoxide 
                 mol % 
                 7.957 
                 1.692 
                 1.692 
                 0.198 
                 0.276 
                  1 ppm 
                 1 ppm 
               
               
                 Carbon Dioxide 
                 mol % 
                 5.433 
                 11.698 
                 11.698 
                 13.192 
                 18.314 
                 0.000 
                 0.000 
               
               
                 Nitrogen 
                 mol % 
                 0.263 
                 0.263 
                 0.263 
                 0.263 
                 0.367 
                 50 ppm 
                 50 ppm  
               
               
                 Methane 
                 mol % 
                 4.933 
                 4.933 
                 4.933 
                 4.933 
                 6.870 
                  2 ppm 
                 2 ppm 
               
               
                 Water 
                 mol % 
                 35.982 
                 29.717 
                 29.717 
                 28.223 
                 0.088 
                 0.000 
                 0.000 
               
               
                 Total 
                 mol % 
                 100.000 
                 100.000 
                 100.000 
                 100.000 
                 100.000 
                 100.000 
                 100.000 
               
               
                   
               
               
                 (1) High Temperature Shift  
               
               
                 (2) Low Temperature Shift  
               
             
          
         
       
     
     From the data set forth in Table 3, simulation points to the fact that utilization of a pressure swing adsorption process can further enhance purity of hydrogen in the final product. 
     Table 4 sets forth a simulation of the process of the present invention integrated into the same process used for the simulation in FIG.  3 . 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
             
             
               
                   
                   
               
               
                   
                 HTS (1) 
                 LTS (2) 
                 CO 2  Removal 
                 Methanation 
                 Hydrogen 
               
             
          
           
               
                   
                 Inlet 
                 Outlet 
                 Inlet 
                 Outlet 
                 Inlet 
                 Outlet 
                 Inlet 
                 Outlet 
                 Product 
               
               
                   
                   
               
             
          
           
               
                 Pressure 
                 psia 
                 524 
                 520 
                 515 
                 510 
                 492 
                 488 
                 488 
                 488 
                 488 
               
               
                 Temperature 
                 F. 
                 610 
                 729 
                 413 
                 442 
                 72 
                 73 
                 525 
                 525 
                 100 
               
               
                 Hydrogen 
                 mol % 
                 45.524 
                 51.817 
                 51.817 
                 53.326 
                 74.151 
                 99.935 
                 99.935 
                 99.930 
                 99.930 
               
               
                 Carbon Monoxide 
                 mol % 
                 8.002 
                 1.709 
                 1.709 
                 0.200 
                 0.278 
                  50 ppm 
                  50 ppm 
                 0.000 
                 0.000 
               
               
                 Carbon Dioxide 
                 mol % 
                 5.422 
                 11.715 
                 11.715 
                 13.225 
                 18.330 
                 0.000 
                 0.000 
                 0.000 
                 0.000 
               
               
                 Nitrogen 
                 mol % 
                 0.263 
                 0.263 
                 0.263 
                 0.263 
                 0.366 
                 500 ppm 
                 500 ppm 
                 500 ppm 
                 500 ppm 
               
               
                 Methane 
                 mol % 
                 4.881 
                 4.881 
                 4.881 
                 4.881 
                 6.788 
                 100 ppm 
                 100 ppm 
                 150 ppm 
                 150 ppm 
               
               
                 Water 
                 mol % 
                 35.907 
                 29.614 
                 29.614 
                 28.105 
                 0.088 
                 0.000 
                 0.000 
                  50 ppm 
                  50 ppm 
               
               
                 Total 
                 mol % 
                 100.000 
                 100.000 
                 100.000 
                 100.000 
                 100.000 
                 100.000 
                 100.000 
                 100.000 
                 100.000 
               
               
                   
               
               
                 (1) High Temperature Shift  
               
               
                 (2) Low Temperature Shift  
               
             
          
         
       
     
     From Table 4 it can be seen that with the process according to the present invention carbon oxides can be entirely eliminated from the process stream. However, there is an increase in nitrogen, methane and water content using the process of the present invention. These impurities can usually be tolerated in most processes that would utilize the hydrogen product stream to which the invention would be applied. 
     Thus it is apparent that the present invention, combining pressure swing adsorption followed by methanation in series, is of great value to the production of hydrogen where low levels of carbon oxide impurities are particularly detrimental but low levels of methane and water may be accepted. Refinery hydrogen supply to an isomerization unit is one example of such an application. 
     According to the present invention the synthesis gas consisting essentially of hydrogen and carbon oxides and other gases such as nitrogen, argon, methane and water can be reacted over temperature of between about 600° F. and 850° F. over an iron-chrome catalyst (high temperature shift), reacted over a temperature of between about 400° F. to 650° F. over a copper-zinc catalyst (medium temperature shift), reacted over a temperature of between about 385° F. to 500° F. over a copper-zinc catalyst (low-temperature shift), or a combination of such reactors in series followed by a heat recovery after each shift reaction. Thereafter the effluent from the shift reactor or the last of the series of shift reactors can be sent to the pressure swing adsorption unit, followed by heat addition and methanation. 
     The process of the present invention will typically benefit existing hydrogen production facilities by improving hydrogen production efficiency and increasing the maximum hydrogen production capacity of the facility. The process of the present invention can be installed in new facilities to improve hydrogen production efficiency and reduce overall hydrogen plant capital cost. 
     Having thus described our invention what is desired to be secured by Letters Patent of the United States is set forth in the appended claims which should be read without limitation.