Abstract:
A hydrogen generation system is disclosed that includes a fuel reforming reactor generating a hydrogen-rich reformate gas at a temperature greater than 150 C, a pressure swing adsorption (PSA) hydrogen purification unit that separates the reformate gas into a relatively pure hydrogen stream and an off-gas stream, and a catalytic reactor down stream of the PSA unit that converts carbon monoxide (CO) and hydrogen (H 2 ) contained in the relatively pure hydrogen stream into methane (CH 4 ) and water vapor (H 2 O). The method of purification involves generating a hydrogen-rich reformate gas at a temperature greater than 150 C in a fuel reforming reactor, separating the reformate gas into a relatively pure hydrogen stream and an off-gas stream in a pressure swing adsorption (PSA) hydrogen purification unit, and converting carbon monoxide (CO) and hydrogen (H 2 ) contained in the relatively pure hydrogen stream into methane (CH 4 ) and water vapor (H 2 O) in a catalytic reactor down stream of the PSA unit. The hydrogen can be further purified by including a secondary purification stage downstream of the PSA unit and the catalytic reactor wherein the secondary purification stage has a water adsorbent material bed that adsorbs the water vapor H 2 O and a hydrogen absorbent material downstream of the water absorbent material that absorbs hydrogen gas preferentially, thus concentrating the non-hydrogen components, such as CH 4 , into an exhaust stream that exits the bed, wherein the absorbed hydrogen gas is then desorbed to create an exiting very pure hydrogen stream.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of and priority to U.S. Provisional Application Ser. No. 60/781,616, filed Mar. 10, 2006, the contents of which are incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    This disclosure relates to improving the purity of hydrogen gas using a secondary hydrogen purification method downstream of a primary hydrogen pressure swing adsorption unit. 
         [0004]    2. General Background 
         [0005]    A hydrogen generation unit (HGU) is a combination of thermo-chemical processes that convert a fuel-steam mixture into a hydrogen-rich gas mixture typically composed of hydrogen (H 2 ), carbon monoxide (CO), carbon dioxide (CO 2 ), methane (CH 4 ), water vapor (H 2 O) and other gases depending on the composition of the fuel feedstock. Typically this mixture is known as reformate. For many applications this reformate stream is then passed to a hydrogen purification unit in which 60% to 90% of the hydrogen is separated into a relatively pure hydrogen stream (99+% H 2 ) and an off-gas stream consisting of the other species in the reformate mixture. One typical method used to purify the reformate is a pressure swing adsorption (PSA) unit, which consists of a series of bed filled with adsorbent material (typically but not limited to zeolites). As the pressurized reformate flows through the bed gaseous species adsorb on to the active surfaces. Since the H 2  is the least strongly adsorbed species in the reformate stream, a pure H 2  gas exits the bed. After a period of time when the adsorbent sites begin to become saturated, the feed gas is removed and the bed is depressurized forcing the adsorbed species to desorb and exit the bed as the off-gas stream. By cycling several beds through this pressurization and depressurization cycle a continuous H 2  purification process is created. As the capacities of the beds are pushed to their limits with higher flow rates and faster cycle times, non-hydrogen gas species begin to contaminate the relatively pure H 2  gas stream. Typically, the species of concern are the other gases in the reformate stream such as CH 4 , CO, and CO 2 . 
         [0006]    Of the non-hydrogen species typically in the reformate feed to the PSA, H 2 O and CO 2  are strongly adsorbed onto the surfaces of the zeolites and CO and CH 4  are weakly adsorbed. As a result the relatively pure hydrogen stream exiting the PSA typically has CO and CH 4  as the primary contaminates. In fuel cell and hydrogen refueling station applications the most critical of these contaminates is the CO, because is causes performance degradation of the fuel cell or metal hydride hydrogen storage units. CH 4  is relatively non-reactive in the fuel cell and metal hydride materials, and therefore, does not cause performance degradation. It is beneficial to include a reactor between the PSA and the fuel cell that converts the CO back into CH 4 . This reactor allows the capacity of the PSA to be increased substantially without impacting performance of fuel cell units downstream. 
         [0007]    One reaction mechanism that achieves this is known as methanation, which is the reverse of the steam reforming reactions. Typically in methanation reactors, a catalyst is used and ruthenium based catalysts have proven to be very effective, although other catalysts such as nickel, platinum, etc. can be used. For these catalysts to be effective the temperature of the catalyst must be greater than 150 C and preferably greater than 190 C. 
         [0000]      CO+3H 2 →CH4+H 2 O+Heat E-1 Primary 
         [0000]      CO 2 +4H 2 →CH4+2H 2 O+Heat E-2 Secondary 
         [0008]    Methanation reactors have been integrated into hydrogen generation systems in the prior art, but typically they have been used upstream of the PSA unit to minimize the CO concentration entering the PSA unit. Typically, this upstream location is used because the reformate gas is at the appropriate temperature range to maintain the catalyst activity. One issue with this art is the secondary reaction identified above, where CO 2  is also converted into CH 4  since the CO 2  concentration is typically 20% in comparison to CO concentrations in the 2 to 4% range. To manage the methanation reaction a tight temperature range is preferably maintained in the catalytic bed which promotes the reaction of CO but does not promote the reaction of the secondary reaction with CO 2 . Not only is this a source of process inefficiency, but it can also result in thermal run-away in which all the CO 2  is reconverted back into CH 4 . Therefore a system and method are needed which alleviates these inefficiencies and precludes thermal run-away from occurring. 
       SUMMARY OF THE DISCLOSURE 
       [0009]    The present disclosure is directed to a hydrogen generation system and method that fundamentally includes operations of generating a hydrogen-rich reformate gas at a temperature greater than 150 C in a fuel reforming reactor, separating the reformate gas into a relatively pure hydrogen stream and an off-gas stream in a pressure swing adsorption (PSA) hydrogen purification unit, and converting low level concentrations of carbon monoxide (CO) contained in the relatively pure hydrogen stream into methane (CH 4 ) and water vapor (H 2 O) in a catalytic reactor down stream of the PSA unit. Preferably the hydrogen generation method also includes positioning the catalytic reactor such that the temperature of the hydrogen-rich reformate gas maintains the temperature of the catalytic reactor. 
         [0010]    Alternatively, or, in addition, the hydrogen generation system and method includes integrating the fuel reforming reactor with a combustion reactor that uses the off-gas stream to provide thermal energy to the fuel reforming reactor. Here the combustion reactor has a combustion exhaust gas at a temperature greater than 150 C, and includes positioning the catalytic reactor such that the temperature of the exhaust gas maintains the temperature of the catalytic reactor. 
         [0011]    The hydrogen generation method and system may include a secondary purification stage downstream of the PSA unit and the catalytic reactor. The secondary purification stage has a water adsorbent material bed that adsorbs the H 2 O vapor; and concentrates non-hydrogen components into an exhaust stream exiting the bed by providing a hydrogen absorbent (or adsorbent) material downstream of the water absorbent material bed. Finally, the absorbed hydrogen gas is desorbed to generate a very pure hydrogen stream. 
         [0012]    The above-mentioned features and objects of the present disclosure will become more apparent with reference to the following description taken in conjunction with the accompanying drawings. 
     
    
     
       DRAWINGS 
         [0013]      FIG. 1  illustrates one embodiment of a system of the disclosure in which thermal energy of the reformate stream is used to maintain the catalyst at optimum conditions. 
           [0014]      FIG. 2  illustrates a second embodiment of the system in which thermal energy of the combustion exhaust gas is used to maintain the catalyst at optimum conditions. 
           [0015]      FIG. 3  is a graph that illustrates test data collected on a hydrogen generation unit shown in  FIG. 1 . 
           [0016]      FIG. 4  illustrates an exemplary system in which a secondary hydrogen purification process is added downstream of the methanation reactor in either of the systems shown in  FIG. 1  or  2 . 
           [0017]      FIG. 5  is a set of tables showing predicted hydrogen production levels at various stages in the systems shown in  FIGS. 1 ,  2 , and  4 . 
           [0018]      FIG. 6  is a graph of critical CO conversion and hydrogen purity in the purification process shown in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  illustrated one exemplary embodiment of a system in accordance with the disclosure in which a methanation reactor is integrated into a system based on steam reformation and PSA purification. Basically the methanation reactor is integrated into the system such that the reformate gas stream exiting the water gas shift (WGS) reactor at a temperature greater than 150 C is used as the thermal energy source to maintain the methanation reactor at its optimum temperature. Typically the exit temperature of the WGS exit stream is in the range of 150 C to 400 C, and more preferably in the range of 200 C to 300 C. 
         [0020]    The embodiment shown in  FIG. 2  is similar to that of  FIG. 1  except that combustion exhaust gas is used to maintain temperature in the methanation reactor. Both of these embodiments provide the unique advantage of the disclosure that places the methanation reactor downstream of the PSA. Since most if not all the CO 2  is removed from the relatively pure hydrogen product stream from the PSA, there is no need for tight temperature control because the concentration of reactants (CO and CO 2 ) are relatively low (typically below 1000 ppm) and the purification goal is to convert all these reactant species into CH 4 . Therefore, there is no upper limit to the catalytic reactor&#39;s temperature, because the concentration of CO 2  in the relatively pure hydrogen stream is insufficient to allow thermal runway of the methanation reactor. 
         [0021]    Now specific reference is made to the first embodiment of the system shown in  FIG. 1 . This embodiment of the hydrogen generation system  100  consists of a reformer  102 , WGS reactor  103 , condenser  104 , PSA  105 , methanation reactor  106 , and hydrogen gas recuperative heat exchanger  107 . The reformer  102  has an integrated combustor section and steam reforming section (not detailed). Each reactor section or process in the reformer  102  has an optimum temperature range based on the specific catalysts and processes that are occurring. 
         [0022]    The combustion section receiving air  112  and PSA off gas  123  which is typically 40 to 60% hydrogen reacts this fuel air mixture generating heat and operating at a temperature in the 700 to 950 C range. The reformer  102  typically operates in the 650 to 900 C range or at a temperature 50 to 100 C cooler than the combustion section within the reformer  102  to promote the transfer of heat from the combustion section to the reformation section. The WGS typically operates in the 500 to 250 C range depending on the specific catalysts. The PSA&#39;s temperature range is typically in the 40 to 80 C range, and requires no liquid water in the feed. Therefore the condenser  104  is used to cool and dry the reformate stream  136  and can include a reheat function to ensure that the reformate stream  115  entering the PSA  105  has a dew point less than the dry bulb temperature. The ideal methanation reactor temperature is above 190 C. Since the PSA operating temperature is typically lower than the methanation temperature, methanation reactors are typically not utilized downstream of the PSA. 
         [0023]    Fuel  110  and water vapor or steam  111  enters the steam reforming section of the reformer  102  where the steam-fuel mixture is converted into a pre-WGS reformate stream  113  containing H 2 , CO, CO 2 , H 2 O, CH 4 , and other trace species. This stream  113  enters the WGS  103  where additional CO is reacted with H 2 O to form H 2  and CO 2  creating the reformate stream  135  that exits the WGS  103 . In some alternative embodiments the reformer  102  and WGS  103  can be integrated into a single assembly, and in others the WGS can be minimized or even eliminated. The thermal energy contained in the reformate stream  135  is used to heat the methanation reactor  106  by indirect transfer from heat exchanger surface  132  to methanation catalyst  131 . The reformate steam  136  flows to the condenser  104  in which thermal energy is removed to manage the inlet PSA  105  temperature and to condenser water from the reformate mixture. The condensate water  122  is removed. The drier reformate gas  115  is passed to the PSA  105  in which it is separated into a relatively pure hydrogen stream  116  and an off-gas stream  123 . 
         [0024]    The relatively pure hydrogen stream  116  is raised in temperature by the recuperative heat exchanger  107 . The temperature of the relatively pure hydrogen stream  116  is dependant on the sizing and configuration of the recuperative heat exchanger  107 , but typically temperatures in the 100 C to 150 C are achievable depending on the temperature of the methanation reactor. The hotter relatively pure hydrogen stream  117  then enters the methanation reactor  106  in which any CO and CO 2  contaminates are reacted with H 2  to form CH 4  and H 2 O. The temperature of the methanation reactor  106  is managed by the indirect heat transfer with the reformate stream  135  and the heat exchange surface  132 . Although the methanation reactions (E-1 and E-2) are exothermic, the heat generated is not sufficient to control the temperature of the methanation reactor  106  because the concentrations of CO and CO 2  in the feed stream  117  are relatively low, typically less than 1000 ppm or 0.1%. The low CO concentration, relatively pure hydrogen stream  118  exits the methanation reactor  106  and provides thermal energy to the recuperative heat exchanger  107 . Typically the temperature of the exit hydrogen stream  119  is in the range of 100 to 160 C depending on the surface area and design of heat exchanger  107 . 
         [0025]    The advantage of this system  100  and the method of its operation is to produce a relatively pure hydrogen exit stream  119  in which the only impurities are CH 4 , H 2 O and other non-reactive species, such as nitrogen, argon, etc. The reactive species CO and CO 2  have been converted back into CH 4 , which is non-reactive at the operating temperatures of a proton exchange membrane (PEM) fuel cell. This relatively pure hydrogen exit stream  119 , for example, can be sent directly to a PEM fuel cell for power generation. The PEM fuel cell consumes the hydrogen to generate power and the CH 4  and other non-reactive species will build up concentration in the dead headed flow path. Periodically a valve will open forcing the collected CH 4  and other non-reactive species out of the fuel cell to maintain performance while achieve 99% hydrogen utilization in the fuel cell. 
         [0026]      FIG. 2  illustrates a second embodiment  200  of the system in accordance with the present disclosure. In this embodiment  200  like numbers are used as in  FIG. 1  for the same components. Embodiment  200  differs specifically from system  100  in that exhaust gas  120  from the reformer  102  feeds directly into and through the heat exchanger surface  132  rather than the discharge of the WGS  103  feeding the heat exchanger  132 . Thus the thermal energy used to maintain the temperature of the methanation reactor  106  is energy from the exhaust gas  120  from the combustion section of the reformer  102 . Since this temperature source is used, the reformate stream  214  from the WGS  103  flows directly into the condenser  104 . The combustion exhaust  120  from the reformer  102  is typically used for feed air  112  preheating and exits the reformer  102  in the temperature range of 150 to 300 C depending on the preheating characteristics. 
         [0027]      FIG. 3  illustrated actual test data graph  300  collected from a HGU with a methanation reactor in accordance with the present disclosure. The HGU was an integrated steam reformer and water gas shift reactor system  100  as illustrated in  FIG. 1 . The reformate gas  135  was used to provide the thermal energy for the methanation reactor  106  prior to flowing to the condenser  104  and the PSA unit  105 . The CO and CH 4  concentration after the PSA unit  105  and after the methanation reactor  106  are plotted. The relatively pure hydrogen stream  119  exiting the test hardware had a CO concentration of less than 0.5 ppm as illustrated by the open square data set  341  and a CH 4  concentration as illustrated by the downwardly pointed open triangle data set  344 . Both data sets are plotted with hydrogen purity as the x-axis. The open circle data set  342  illustrates the CO concentration in stream  116  exiting the PSA  105  and the upwardly pointed open triangle data set  343  illustrates the CH 4  concentration in stream  116 . This data set indicates the increase in CH 4  concentration after the methanation reactor  106  which corresponds to the decrease in CO concentration. 
         [0028]    In some refueling station and industrial gas applications a pure hydrogen product of greater than 99.9% is required. To achieve this level of purity, typically the PSA unit is increased in size and the hydrogen recover in the PSA unit is decreased until there are very few non-hydrogen species in the PSA product gas  116 . A hydrogen concentration of 99.99+% is equivalent to less than 100 ppm non-hydrogen concentration and a 99.999+% is equivalent to less than 10 ppm non-hydrogen concentration. This requirement can force the PSA unit  105  to be large and costly. 
         [0029]    The systems  100  and  200  illustrated in  FIG. 1  and  FIG. 2  can also be used in these applications with an additional secondary purification system  400  positioned downstream of the relatively pure hydrogen exit stream  119 . The secondary purification system  400  allows the primary PSA unit  105  to be decreased in size and cost and operated at a faster cycle time, which also helps decrease pressure pulsations in both the off-gas and product gas. The secondary purification subsystem  400  is illustrated in  FIG. 4 . 
         [0030]    The objective of subsystem  400  is to purify the reformate stream with a PSA unit  105  allowing CH 4 , CO, and CO 2  as contaminates at concentrations less the 0.1% or 1000 ppm. This allows the PSA unit  105  to be considerably downsized and a very rapid cycle can be used to maximize the capacity of the PSA unit  105 . Downstream of the PSA unit  105  a methanation reactor  106  as indicated in  FIG. 1  or  2  is used to convert the CO and CO 2  into additional CH 4  and H 2 O. Downstream of the methanation reactor  106  this hydrogen stream is cooled and passed through a desiccant material to remove the water vapor produced in the methanation reactor  106 . This dry stream is then passed into a H 2  absorbent bed such as a metal hydride in which the H 2  is absorbed into the material and stored for later release. 
         [0031]    Removal of the water vapor and CO is important in this secondary purification subsystem  400 , because these species can damage the performance and endurance of the metal hydride bed. The absorbent bed is designed to have a high length over width ratio such that as the hydrogen gas is adsorbed the CH 4  and other non-reactive, non-condensable species are concentrated as they move down the bed. Finally, the gases exit the bed as a hydrogen-rich mixture with higher concentrations of CH 4  and other non-reactive species. These gases are vented from the secondary purification system  400  and may be returned to the combustion section of the reformer  102 . Valves in the system  400  are used to isolate the beds and to open the H 2  absorbent to the product H 2  supply line. The absorbed H 2  is desorbed resulting in a very pure H 2  stream, typically in the 99.995+% range because the only non-hydrogen is the contaminate CH 4  and other non-reactive, non-condensable species in the gas volume around the absorbent material. 
         [0032]    In reference to  FIG. 4  one embodiment of the secondary hydrogen purification system  400  consists of four beds, first desiccant bed  401 , first H 2  absorbent bed  402 , second desiccant bed  403  and second H 2  absorbent bed  404 . The system  400  also has a series of valves that manage the flow of relatively pure hydrogen stream from the methanation reactor  106  and feed line  440  to the product hydrogen supply line  442  and the vent line  441 . In this embodiment first and second parallel paths  408  and  409  are illustrated which cycle on and off in sequence to create a continuous process. Other embodiments may include three or more parallel paths to optimize purity and process integration. The open valves of the first parallel path  408  are shown as white or open symbols, while the closed valves of the second parallel path  409  are shown as dark or solid symbols. The valves are shown as on-off solenoid valves, but could also be rotary valve assemblies similar to the valves used in a rapid cycle PSA unit. Rotary valves assemblies provide advantages of reliability, lower cost, and simple process control. 
         [0033]    Process gases from the methanation reactor  106  flow into the system  400  through supply line  440 . In  FIG. 4  the first parallel path  408  supply valve  431  is open allowing the hot gas from the methanation reactor  106  to enter the first heat exchanger coil  410  of first desiccant bed  401 , while second parallel path supply valve  451  is closed. The hot gas from the methanation reactor  106  transfers heat to the desiccant material  412  of desiccant bed  401 . The gas flows from coil  410  to second heat transfer coil  411  embedded in first H 2  absorbent bed  402  again transferring heat to the first H 2  absorbent material  413 . Absorbed H 2  from the prior cycle in the first material  413  is desorbed and flows through open first exit valve  436  and out of the system through product line  442 . 
         [0034]    H 2  desorption is endothermic which decreases the temperature of the first absorbent material  413  and helps transfer heat from second coil  411  cooling the gas temperature in coil  411 . The gas flows from the second coil  411  through open valve  132  and into second desiccant bed  403  and past the second desiccant material  462  which adsorbs any water vapor in the gas stream generated by the methanation reactions in bed  106 . The dry hydrogen rich gas flows through open valve  433  and into second H 2  absorbent bed  404  and H 2  absorbent material  463 . The hydrogen in the stream is absorbed as the gas flow through second absorbent bed  463  thereby increasing the concentration of CH 4  and other non-reactive species in the process flow. This gas then exits the second H 2  absorbent bed  404  through open valve  434  and into first desiccant bed  401  in contact with first desiccant material  412 . The heat transferred from first coil  410  causes the adsorbed water vapor in first desiccant material  412  to evolve and flow out of the system through open valve  435  to system vent  441 . 
         [0035]    Once the second desiccant material  462  or the second absorbent material  463  nears saturation the process cycle is reversed and flow is diverted through the second parallel path  409  by closing the open valves  431 ,  432 ,  433 ,  434 ,  435 , and  436  and opening the closed valves  451 ,  452 ,  453 ,  454 ,  455 , and  456 . When the secondary parallel path  409  is active, heat transferred in third coil  457  and fourth coil  458 , while water vapor is adsorbed in first desiccant material  412  and hydrogen is absorbed in first absorbent material  413 . Similarly, product hydrogen is desorbed from second absorbent material  463  flowing through valve  456  toward product line  442 . 
         [0036]    In one embodiment of the secondary purification system  400  the hydrogen absorbent material  413  and  463  is a metal hydride type material. The material absorbs H 2  molecules into the metal lattices and has the capacity to absorb approximately 150 to 300 times the volume of hydrogen at standard conditions in comparison to the volume of metal. Any absorbent or adsorbent materials can be used which preferentially remove H 2  gas from a mixture. 
         [0037]    The method described is very simple in that all the open valves are closed and all the closed valves are open uniformly. More complex and potentially more effective cycles are embodied. For example the purge valve  434  positioned between the second H 2  absorbent material  463  and the first desiccant material  412  can be opened after the starting the process parallel path  408 , to maintain the pressure of the process gases in absorbent material  463  and thereby, enhance the percentage of the process hydrogen gas in the flow stream that is absorbed in the material  463 . 
         [0038]    Similarly, the open valves of parallel path  408  can be closed except for purge valve  434  and exhaust valve  435  which are left open to allow a brief period during which process gases in the gas volumes of absorbent material  463  are purged due to depressurization and desorption of pure hydrogen. The pure hydrogen pushes any process gas with contaminates out through valve  434 . After the brief period parallel path  463  valves are closed and process flow is initiated through parallel path  409 . As a result the hydrogen gas allowed to flow through product valves  436  and  456  is extremely pure achieving purities one magnitude greater than without this delay. 
         [0039]    An analytical model was developed to assess the performance characteristics and sensitivities of the process to the various parameters. One set of results from this model is shown in  FIG. 5 . This  FIG. 5  is divided into six blocks of data. The first block (Table 5.A) shows a typical reformate composition entering the PSA primary hydrogen purification unit. The analysis was conducted for one multiple of 100 moles at this point, and the concentrations of gas species are shown in ppm and percent in the third and fourth column of each block of data, respectively. 
         [0040]    The second block (Table 5.B) illustrates an assumed relatively pure hydrogen stream  116  leaving the primary purification unit  105  as in  FIG. 1 . Assuming that 70% of the hydrogen was recovered, the CO concentration was set at 200 ppm and the CH 4  concentration was set at 800 ppm, with 10 ppm of CO 2  and 3 ppm or other non-reactive species such as argon or nitrogen. This composition is representative of the relatively pure hydrogen stream  116 . Assuming the initial 100 moles of gas and 74 moles of H 2  the relatively pure hydrogen stream has 51.8 moles of H 2  at the 70% recovery rate. The hydrogen purity of this stream is only 99.899%, which is acceptable for a fuel cell from a bulk characteristic except for the 200 ppm of CO which would degrade PEM fuel cell performance very rapidly. 
         [0041]    To address this issue a methanation reactor  106  is integrated into the system ( FIG. 1  or  2 ). The methanation reaction converts 99.975% of the CO and CO 2  into CH 4  and H 2 O and the outlet conditions are shown in the third block of data (Table 5.C). The H 2  concentration has decreased slightly because of the H 2  losses, but the output of the methanation reactor  106  is acceptable for direct fuel cell consumption because the CO concentration is 0.05 ppm well below the required 1 ppm level. Actually if only 99.5% of the CO is converted in the methanation reaction the CO concentration would be 1 ppm in the outlet. 
         [0042]    The water content after the methanation reaction is 220 ppm, and because the assumed H 2  absorbent is a metal hydride, this concentration of water would damage the hydride. The desiccant material removes this water in a process assumed to be 99.99% effective as shown (Table 5.D). As the gas stream flow through the hydride, H 2  is absorbed. If 95% of the hydrogen is absorbed, the outlet concentration of CH 4  and CO has increased as shown (Table 5.E). The CO level is still approximately 1 ppm and the CH 4  concentration has increased from 1010 ppm to 19830 ppm or just under 2%. Since only 95% of the hydrogen is absorbed, the hydrogen concentration is still 98% and 2.59 moles of H 2  leave the secondary system with the CH 4  and CO. The 49.18 moles of H 2  absorbed results in the net H 2  recovery being 66.5% (94.9% of the initial 70% value). Assuming that the metal hydride had only a 150 times H 2  gas volume to vessel volume, when the desorbed H2 during the next parallel path cycle this volume of H 2  is mixed with one volume of absorbent bed inlet gas (with 1010 ppm CH 4  and H 2  concentration of 99.9%). The result is an H 2  purity of 99.9993% after the secondary purification process as shown (Table 5.F). 
         [0043]    One aspect of the durability of the secondary purification process  400  is the concentration of CO at the end of the H 2  absorbent beds  413  or  463  as the H 2  is absorbed. An example of this analysis is presented in  FIG. 6  illustrating the CO concentration at the end of the absorption bed as a function of the percent of H 2  recovered in the secondary bed (x-axis) and the level of CO conversion in the methanation reactor  106 . With decreased activity in the methanation reactor ( 106 ) the H 2  recovery in the secondary purification process  100  must be decreased to maintain CO concentrations at an acceptable level. If H 2  absorbent materials  413  and  463  with higher tolerance to CO are used optimization of these parameters can be adjusted. Higher activity of the methanation reactor  106  is achieved by increasing the temperature of the catalyst. The innovations defined in  FIGS. 1 and 2  are needed to passively achieve reactor  106  temperatures without too much system complexity or process inefficiencies. 
         [0044]    While the apparatus and method have been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the disclosure need not be limited to the disclosed embodiments. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structures. The present disclosure includes any and all embodiments of the following claims.