Abstract:
A hydrogen purification process is provided. This process includes separating hydrogen from a hydrogen containing stream in at least two sequential palladium membrane purification zones, wherein each purification zone has a permeate side, wherein the permeate side pressure of purification zones are not the same.

Description:
TECHNICAL FIELD 
     The present invention relates to process for producing high pressure hydrogen gas. 
     BACKGROUND 
     Hydrogen is very important feedstock for many chemical and petrochemical processes. It is commonly produced using Steam Methane Reforming, Partial Oxidation, Auto thermal Reforming, and Gasification etc. Carbonaceous feedstock like natural gas, coal, biomass etc. along with oxidizing agent like steam or oxygen undergoes reforming reaction to produce synthesis gas (syngas). Syngas is a mixture of hydrogen, carbon monoxide, carbon dioxide, water and un-reacted methane. Reforming reaction is highly endothermic occurring at very high temperatures of 800-1300 C and high pressures of 20-80 bar. The reforming reaction can be catalytic or non-catalytic process. Excess steam is produced in the process of cooling down syngas and flue gas. The heat required for the highly endothermic reforming reaction is provided by combustion of the carbonaceous feedstock and carbon containing off-gas. The combustion process is associated with generating greenhouse gas, carbon dioxide (CO 2 ) emissions. Syngas from reformer is further sent to water gas shift reactor to produce additional hydrogen from carbon monoxide. Water gas shift reaction produce additional carbon dioxide during the reaction. Syngas rich in hydrogen from water gas shift is further purified in a Pressure Swing Adsorption (PSA) process to produce pure hydrogen and PSA off-gas which is further used as fuel. Hydrogen production is associated with large amounts of carbon dioxide emissions. With current advancement in greenhouse gas regulations, research is underway to capture carbon dioxide from conventional hydrogen plants or reduce emissions from hydrogen plants. 
     Methane can be converted to hydrogen using conventional hydrogen production methods at equilibrium conditions. However, very high temperatures &gt;900° C. and low pressures &lt;30 bar are needed in order to achieve high methane conversions &gt;85%. In order to provide high temperatures required for the reaction the amount of fuel consumed is very high emitting large amounts of carbon dioxide. The use of membrane reactor includes reaction and separation in the same unit allowing methane conversion higher than equilibrium conversion rate at much lower temperatures. 
     Membrane reactors have been used beneficially to produce hydrogen with higher methane conversion at low temperatures and simultaneously produce carbon dioxide rich stream on the retentate side with ease of CO 2  capture. Membranes addition inside the reactor enables the reforming and water gas shift reaction to proceed at rates higher than equilibrium. However, one of the main disadvantages of using membrane reactors is hydrogen production at low pressure &lt;3 bar. The cost associated with compressing hydrogen product is very high and increases the overall cost of membrane reactor. 
     SUMMARY 
     This process includes separating hydrogen from a hydrogen containing stream in at least two sequential palladium membrane purification zones, wherein each purification zone has a permeate side, wherein the permeate side pressure of purification zones are not the same. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic representation of one embodiment of the present invention. 
         FIG. 2  is a schematic representation of one embodiment of the present invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Illustrative embodiments of the invention are described below. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     The present invention deals with using the hydrogen selective membrane in stages on syngas or hydrogen containing gas downstream of a steam methane reformer, pre-reformer, gasifier or any other unit where hydrogen mixture is produced. The flux of hydrogen across the membrane depends on its partial pressure on shell side and permeates side. Hydrogen can permeate through the membrane as long as the partial pressure of hydrogen on shell side is higher than the permeate side. There will be no additional hydrogen permeation with increase in membrane surface area if the partial pressure of hydrogen is same on shell side and permeate side. The concept of staged palladium membrane is to produce hydrogen at high pressure followed by medium and low pressure. This will allow to recover a portion of hydrogen at high pressure as opposed to entire hydrogen stream at low pressure &lt;3 bar. The goal is reduce the compression cost of hydrogen product. The stages of hydrogen selective membrane can be without catalyst in order to avoid methanation reaction or with water gas shift catalyst in order to complete the CO conversion to hydrogen. The last stage with low pressure hydrogen production can be with catalyst in order to complete methane conversion and recover remaining hydrogen. The catalytic reaction stage can be added in between the stages in order to increase the partial pressure of hydrogen in syngas and produce additional hydrogen at high pressure. Sweep gas such as steam can be used in order to increase the pressure of hydrogen from stages. Steam can also be added on the process gas side where water gas shift reaction is desired with WGS catalyst. 
     Hydrogen selective membrane reactors have been used to undergo reforming or water gas shift reaction along with simultaneous hydrogen separation. Palladium or its alloys are permeable to H2 but not to other gases. When H 2  contacts the Pd membrane, the hydrogen molecule is adsorbed on the surface where it dissociates and hydrogen atoms diffuse into the membrane Thus H 2  can selectively pass from one surrounding atmosphere through the palladium membrane. The selectively separated hydrogen atoms then re-combine into H 2  gas on the opposite side of the membrane. The difference in hydrogen partial pressure drives the permeation of hydrogen through the membrane. At a specific point on the membrane, the hydrogen flow rate is equal to the permeance multiplied by the membrane surface area multiplied by the difference of the square root of the H 2  feed partial pressure and the square root of the H2 permeate partial pressure, as shown by the following formula:
 
 Q=P·S· ( P   H2,feed   1/2   −P   H2,permeate   1/2 )
 
     Where Q is the hydrogen molar flow rate, P, the permeance, S the membrane surface area, P H2,feed  the hydrogen partial pressure in the feed, and P H2,permeate  the hydrogen partial pressure in the permeate. Permeance increases with increasing operating temperature. 
     Palladium membrane can be manufactured using several different techniques. US 20100132546 and US2009277331A describe the composite gas separation module including the porous metal substrate, intermediate layer and a dense gas selective metallic membrane. The intermediate layer is used to prevent porous metal support diffusion into the dense metallic membranes at operating conditions. U.S. Pat. No. 5,366,712A describes the use of porous ceramic material support for hydrogen selective membrane. 
     Hydrogen selective membrane can be manufactured from various different alloys including but not limited to palladium, silver, gold, platinum, yttrium, ruthenium, copper etc. The thickness of hydrogen selective dense metal layer can vary from 3 microns to 20 microns preferably from 5 micron to 10 micron. The operating temperature of hydrogen selective membrane can vary from 300° C. to 700° C. preferably from 450° C. to 550° C. The operating pressure of hydrogen selective membrane can vary from 10 bar to 90 bar. 
     Membrane reactors have the advantage of driving the reaction rates beyond chemical equilibrium at lower temperatures and enable gas separation in the same unit. In conventional SMR, the reaction takes place in Steam Methane Reformer (SMR) followed by Water Gas Shift (WGS) reactor and the hydrogen purification occurs in Pressure Swing Adsorption (PSA) unit. Membrane reformers can replace the SMR, WGS and PSA in a single unit. However, the hydrogen purity from membrane reformers is still questionable and a small Thermal Swing Adsorption (TSA) unit or a small PSA unit may still be needed to achieve very high hydrogen purity required by customers. 
     With current regulations on carbon dioxide emissions it is important to capture CO 2  from the hydrogen plants. In a conventional SMR unit carbon dioxide can be captured from syngas or PSA off-gas. The CO 2  content in PSA off-gas is less than 50% and is available at low pressure (&lt;2 bar). The CO 2  content in syngas is less than 25% and is available at syngas pressure 20-30 bar. In the case of a membrane reformer the shell-side gas is rich in CO 2  with more than 80% concentration on dry basis and is available at high pressure (feed pressure) 30-40 bar. It is advantageous to capture CO 2  from membrane reformer as it is available at higher content and high pressure making the capture process much simpler than the capture process from conventional SMR. However, the hydrogen product on the permeate side is at low pressure &lt;10 bar mostly &lt;3 bar. Hydrogen product cannot be produced at high pressure in the catalytic membrane reactor because the hydrogen partial pressure always remain low on the shell side and it permeates on the permeate side as soon as it is formed. Hydrogen product is mostly needed at high pressure &gt;20-40 bar for most of the chemical processes. The cost of hydrogen product compression from 1-10 bar to 20-40 bar requires multiple compression stages and increases the overall cost of hydrogen production making the use of membrane reformers uneconomical. Another disadvantage of membrane reformers with catalyst and membrane together is that the reaction kinetics and hydrogen permeability rate is very crucial. If the reaction kinetics is too fast catalyst will act intermittently with surges waiting for the hydrogen to permeate through the membrane underutilizing the catalyst area. If the hydrogen permeation is too fast the flux of hydrogen will be low as the hydrogen partial pressure on the shell side will be low underutilizing the membrane area. The material compatibility of catalyst with membrane can also be an issue. In few cases catalyst are known to scratch the membrane surface causing defects on the membrane. In order to better utilize the catalyst or membrane area and avoid compatibility issues staged process has been proposed in the past. The concept of staged membrane process has been described in patent application US2008/0311013A1 where the catalytic reactor is separated from membranes in order to better utilize the catalyst and membrane area. However the proposed solutions produce hydrogen at low pressure on the permeate side. All the permeate streams are mixed from the stages to deliver hydrogen at low pressure. 
     The current invention deals with the use of staged membrane process with hydrogen product withdrawal at different pressures in order to reduce cost of hydrogen product compression. The staged membrane process can be used on a mixture of gases with hydrogen from refinery or any other chemical processes. The proposed solution can be used on syngas obtained from Steam Methane Reformer. The SMR can operate at 650-950° C. and 20-45 bar pressure in order to produce hydrogen at partial pressure from 7 bar to 22 bar. The proposed solution can be used on syngas produced from pre-reformer at 500-700 C at 30-45 bar in order to produce hydrogen at 3 to 13 bar partial pressure. 
     Gasifiers using coal, biomass or other carbon containing feed stock can also be used at 900-1400 C and 20-80 bar in order to produce hydrogen at 8 to 30 bar partial pressure. The high partial pressure of hydrogen is favorable to produce hydrogen at high pressure. Syngas with high hydrogen partial pressure can also be produced using high pressure partial oxidation or auto thermal reforming. The staged membrane process can be used to produce hydrogen at different pressures in order to reduce the hydrogen product compression cost. The number of stages can vary from 2 to 6. Each staged membrane process unit can be with or without catalyst. Water gas shift catalyst may be used in order to complete the CO conversion to hydrogen and increase the partial pressure of hydrogen in syngas. Reforming or pre-reforming catalyst may not be used in the first stage because of potential reverse methanation reaction. Reforming or pre-reforming catalyst can be added in a stage where hydrogen partial pressure is low and methanation reaction is not favorable in order to complete the methane conversion and produce additional hydrogen. Hydrogen product from different stages with same pressure can be mixed and directed to a compression stage. 
     For example, the staged membrane process can be used as shown in  FIG. 1  in order to produce hydrogen at high pressure followed by medium and low pressure. This allows reducing the compression energy of hydrogen product and overall compression cost of hydrogen. The hydrogen permeate from first stage can be produced at 5 bar to 30 bar. The hydrogen permeate from second stage can be produced at 3 bar to 20 bar. The hydrogen permeate from third stage can be produced from 1 bar to 10 bar. Multi-stage reforming, pre-reforming or gasification can also be used by recycling the hydrogen depleted syngas to the second stage of reforming to increase partial pressure of hydrogen. 
     Turning to  FIG. 1 , syngas  101  generated from a pre-reformer, reformer or gasifier (not shown) enters first stage  102  Pd membrane. This generates high pressure hydrogen permeate stream  103 , and retentate stream  104 . Retentate stream  104  then enters second stage  105  Pd membrane. This generates medium pressure hydrogen permeate stream  106 , and retentate stream  107 . Retentate stream  107  then enters third stage  108  Pd membrane. This generates low pressure hydrogen permeate stream  109 , and carbon dioxide rich retentate stream  110 . 
     Low pressure hydrogen permeate stream  109  may be sent to first compressor  111 . 
     This produces a second medium pressure hydrogen stream  112 , the pressure of which is approximately the same as that of medium pressure hydrogen permeate stream  106 . Medium pressure hydrogen permeate stream  106  and medium pressure hydrogen stream  112  may be combined and then may be sent to second compressor  113 . This produces a second high pressure hydrogen stream  114 , the pressure of which is approximately the same as that of high pressure permeate stream  103 . High pressure hydrogen permeate stream  103  and high pressure hydrogen stream  114  may be combined and then may be sent to booster  115  to further increase the pressure of the stream. Boosted hydrogen stream  116  may then be used in subsequent processes (not shown). 
     This hydrogen rich syngas can be sent to staged membrane process in order to recover additional hydrogen at high pressure as shown in  FIG. 2 . Sweep gas  123  such as steam can be used in the stages in order to increase the permeate pressure of the hydrogen. The permeate gas which is a mixture of hydrogen and steam can be further cooled down and condensed in order to separate hydrogen from steam. 
     In order to minimize ambiguity, the numbering convention is maintained for all figures. Turning to  FIG. 2 , syngas  101  generated from a pre-reformer, reformer or gasifier  117  enters first stage  102  Pd membrane. This generates high pressure hydrogen permeate stream  103 , and retentate stream  104 . Retentate stream  104  then enters second stage  105  Pd membrane. This generates medium pressure hydrogen permeate stream  106 , and retentate stream  107 . Retentate stream  107  is returned to a second stage  119  of pre-reformer, reformer or gasifier  117 . Second syngas stream  120  then enters third stage  108  Pd membrane. This generates second high pressure hydrogen permeate stream  122 , and retentate stream  121 . Retentate stream  121  then enters fourth stage  110  Pd membrane. This generates low pressure hydrogen permeate stream  109 , and carbon dioxide rich retentate stream  118 . 
     Low pressure hydrogen permeate stream  109  may be sent to first compressor  111 . This produces a second medium pressure hydrogen stream  112 , the pressure of which is approximately the same as that of medium pressure hydrogen permeate stream  106 . Medium pressure hydrogen permeate stream  106  and medium pressure hydrogen stream  112  may be combined and then may be sent to second compressor  113 . This produces a third high pressure hydrogen stream  114 , the pressure of which is approximately the same as that of high pressure permeate stream  103  and second high pressure hydrogen permeate stream  122 . High pressure hydrogen permeate stream  103 , second high pressure permeate steam  122 , and high pressure hydrogen stream  114  may be combined and then may be sent to booster  115  to further increase the pressure of the stream. Boosted hydrogen stream  116  may then be used in subsequent processes (not shown). The palladium membrane may also be catalytic. Typically, the last stage is catalytic in order to increase the methane conversion ( 108  in  FIGS. 1 and 110  in  FIG. 2 ). 
     The TSA system can be used to completely dry hydrogen gas. TSA or PSA unit can be used on hydrogen product gas in order to remove any trace impurities like carbon monoxide, carbon dioxide, methane or water and produce high purity hydrogen. The carbon dioxide rich stream from the shell side of membrane can be further purified using cryogenic process or any other purification process. The pure carbon dioxide can be captured and used for enhanced oil recovery or sent for geological sequestration, deep saline aquifer etc.