Patent Publication Number: US-2007122329-A1

Title: Purification of raw hydrogen

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit under 35 U.S.C. § 119(e) to provisional application No. 60/741,097, filed Nov. 30, 2005, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND  
      Hydrogen gas can be produced by various processes including the use of steam methane reformers, electrolysis of water, and as a waste gas from chlorine production by electrolysis of an alkali metal chloride solution. The raw hydrogen gas produced by electrolysis of an alkali metal chloride solution often includes impurities such as chlorine (Cl 2 ) and ammonia (NH 3 ). The ammonia impurities are often a result of residue from explosives used in the salt mining process used to obtain the alkali metal chloride. The concentration of chlorine may be as high as tens of parts per million (ppm) while the concentration of ammonia may be as high as hundreds of ppm.  
      Chlorine and ammonia by themselves may cause adverse effects to downstream processes and equipment, such as poisoning of catalysts, destruction of absorbents, and corrosion. When combined, chlorine and ammonia form chloramines, such as monochloramine (NH 2 Cl), dichloramine (NHCl 2 ), and trichloramine (NCl 3 ). The chloramines are highly unstable chemicals which may, as the concentration of the chloramines accumulates and exceeds at least 3%, decompose violently, leading to explosions. Purification of hydrogen gas by using adsorbents, such as activated carbon, promotes the formation and hazardous accumulation of chloramines.  
      Therefore, a need exists for a method and apparatus for removing the chlorine, chloramines, and ammonia from a hydrogen gas stream that is both cost efficient and safe.  
     SUMMARY  
      The embodiments of the present invention generally provide a method for removing chlorine, chloramines and ammonia from a hydrogen gas stream. One embodiment of the invention provides a method for removing impurities from hydrogen gas by passing the hydrogen gas through a first scrubbing unit containing a reducing agent in solution to remove chlorine and chloramines from the hydrogen gas. The first scrubbing unit has a first inlet for receiving the hydrogen gas and a first enclosure in which the hydrogen gas contacts the solution with the reducing agent. The hydrogen gas may then be passed from the first scrubbing unit through a second scrubbing unit containing an acid in solution to remove ammonia from the hydrogen gas. The second scrubbing unit has a second inlet for receiving the hydrogen gas and a second enclosure in which the hydrogen gas contacts the solution with the acid. The hydrogen may further go through an acid removal process by passing the hydrogen gas from the second scrubbing unit through a third scrubbing unit containing water to remove acid traces from the hydrogen gas. The third scrubbing unit has a third inlet for receiving the hydrogen gas and a third enclosure in which the hydrogen gas contacts the water.  
      Another embodiment of the invention provides a method for removing impurities from hydrogen gas by contacting the hydrogen gas with a reducing agent in aqueous solution to remove chlorine and chloramines from the hydrogen gas, wherein the reducing agent is selected from the group consisting of sodium metabisulfite, sodium sulfite, and sodium hyposulfite. After the chlorine and chloramines removal, the hydrogen gas may be contacted with an acid in aqueous solution to remove ammonia from the hydrogen gas. The acid is selected from the group consisting of sulfuric acid and phosphoric acid. The hydrogen gas may, after the ammonia removal, be contacted with demineralized water to remove acid traces from the hydrogen gas.  
      Another embodiment of the invention provides a method for removing chlorine, chloramines, and ammonia impurities from hydrogen gas by contacting the hydrogen gas with an aqueous sodium hyposulfite solution to remove chlorine and chloramines from the hydrogen gas, passing the hydrogen gas after chlorine and chloramines removal through a first demister pad, after passing through the first demister pad, contacting the hydrogen gas with an aqueous sulfuric acid solution to remove ammonia from the hydrogen gas, passing the hydrogen gas after ammonia removal through a second demister pad, after passing through the second demister pad, contacting the hydrogen gas with demineralized water to remove acid from the hydrogen gas, and passing the hydrogen gas after acid removal through a third demister pad. In one embodiment, after passing the third demister pad the hydrogen gas comprises about 0.01 parts per million or less of chlorine, about 0.01 parts per million or less of chloramines, and about 0.1 parts per million or less of ammonia.  
      Another embodiment of the invention provides a system for removing chlorine, chloramines, and ammonia impurities from hydrogen gas. The system has a chlorine and chloramines scrubbing unit, an ammonia scrubbing unit, and an optional acid scrubbing unit. Each scrubbing unit has a packing material, a hydrogen gas inlet tip or diffuser, a liquid distributor directing either a reducing agent in aqueous solution, an acid in aqueous solution, or water onto the packing material. The hydrogen is contacted with the reducing agent, acid, and water to respectively remove chlorine and chloramines, ammonia, and acid from the hydrogen gas. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a further understanding of the nature and objects of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:  
       FIG. 1  illustrates a flow chart of a process for removing chlorine, chloramines, and ammonia from a stream of hydrogen gas, according to one embodiment of the invention;  
       FIG. 2  illustrates the processing units for the removal chlorine, chloramines, and ammonia, according to one embodiment of the invention;  
       FIG. 3  illustrates the processing unit for the removal of chlorine and chloramines, according to one embodiment of the invention;  
       FIG. 4  illustrates the processing unit for the removal of ammonia, according to one embodiment of the invention; and  
       FIG. 5  illustrates the processing unit for the removal of acid, according to one embodiment of the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       FIG. 1  illustrates a flow chart of a process  100  for removing chlorine, chloramines, and ammonia from a stream of hydrogen gas, according to one embodiment of the invention. Process  100  includes a chlorine and chloramines removal process  10 , an ammonia removal process  20 , and an acid removal process  30 .  
       FIG. 2  illustrates a system  200  depicting the processing units in which process  100  takes place, according to one embodiment of the invention. Hydrogen gas is supplied from hydrogen gas source  105  through supply line  106  to scrubbing unit  110  where process  10  takes place.  
      In chlorine and chloramines removal process  10  the hydrogen gas is flowed into contact with an aqueous solution of a reducing agent. The reducing agent may be any compound that will reduce the chlorine gas to chloride ion, and combine with chloramines to form stable, harmless compounds. In one embodiment, the reducing agent is a sulfite, such as sodium metabisulfite (Na 2 S 2 O 5 ), sodium sulfite (Na 2 SO 3 ), and/or sodium hyposulfite (Na 2 S 2 O 3 ). The reducing agent solution may have a concentration of about 20 mole % or less, such as in the range between about 0.01 mole % and about 20 mole %, and more preferably, between about 0.1 mole % and about 10 mole %. In one embodiment the reducing agent concentration is about 0.5 mole %. The optimal concentration may be determined by the maximum (peak) concentration of chlorine and chloramines in the hydrogen feed gas. The reducing agent solution is introduced into scrubbing unit  110  through supply line  113  and liquid distributor  114 . The liquid distributor  114  directs the reducing agent solution onto a packing material  112 . Packing material  112  provides increased surface area for the reaction and increases the rate of diffusion of the chloramines and chlorine in the hydrogen gas into the reducing agent solution. Packing material  112  may be of any shape, and made from metals, metal alloys, or polymers. In one embodiment packing material  112  consists of rings made from polypropylene with diameters in the range between about 1 inch and about 3 inches. In a particular embodiment, the diameter is about 2 inches. The hydrogen gas is introduced into the scrubbing unit  110  through inlet tip  111 . In one embodiment, inlet tip  111  may be in the form of a diffuser. The pressure of the hydrogen gas as it enters scrubbing unit  110  may be between about 1 bar absolute and about 3 bar absolute. In one embodiment the pressure is about 1.7 bars absolute. The flow rate of the hydrogen gas is between about 40 kg/hour and about 1800 kg/hour, preferably, between about 400 kg/hour and about 1200 kg/hour. In one embodiment the flow rate is about 620 kg/hour. The hydrogen gas flows through the packing material  112  and comes into contact with the reducing agent solution so that the chloramines and chlorine gases flowing with the hydrogen gas react with the reducing agent. In a water solution, a particular reaction of sodium hyposulfite and a chloramine, such as monochloramine, is: 
 
NH 2 Cl+2Na 2 S 2 O 3 +H 2 O=NaCl+Na 2 S 4 O 6 +NaOH+NH 3   Equation 1. 
 
      Chlorine is reduced to chloride by the sodium hyposulfite as in Equation 2: 
 
Cl 2 +2Na 2 S 2 O 3 =2NaCl+Na 2 S 4 O 6   Equation 2. 
 
      As seen in Equations 1 and 2, the gaseous chloramines and chlorine react with the sodium hyposulfite resulting in sodium chloride (NaCl), sodium tetrathionate (Na 2 S 4 O 6 ), and sodium hydroxide (NaOH, Equation 1). These resulting compounds stay in solution with the water and exit scrubbing unit  110  through outlet  116 , and are thus separated from the hydrogen gas which exit scrubbing unit  110  through outlet supply line  115 . Upon exiting scrubbing unit  110 , chlorine and chloramines are at least partially removed, and the hydrogen gas may have less than about 0.01 ppm chlorine and less than about 0.01 ppm chloramines contained within the hydrogen gas. To avoid any water droplets containing the reducing agent or any reaction products from flowing with the hydrogen gas and into the ammonia removal process  20 , the hydrogen gas is flowed through demister pads  117 . Illustratively, the demister pads  117  may be fabricated from knitted wire of stainless steel, polypropylene, high density polyethylene or nylon.  
       FIG. 3  depicts more details of process  10 , according to one embodiment of the invention. Water is supplied from water source  140 , and is pumped by water pump  142  through water supply line  143  into scrubbing unit  110 . In one embodiment the water is demineralized. Pump  142  is controlled to keep a constant fluid level  120  in the bottom of scrubbing unit  110 . Outlet line  116  removes the fluid from the bottom of scrubbing unit  110 , and pump  124  pumps the fluid to supply line  113  and through liquid distributor  114 . The reducing agent is stored in storage tank  130 . Concentrated reducing agent is stored under an inert gas to prevent any potential oxidation of the reducing agent in air. In one embodiment the inert gas is nitrogen. In one embodiment the reducing agent solution is also stabilized by sodium hydroxide, sodium sulfite, or sodium carbonate to keep the solution neutral or slightly basic and to prevent sodium hyposulfite from decomposing into sulfite, sulfur, and sulfur dioxide. Dosage pump  132  pumps the reducing agent from storage tank  130  and dilutes it into the fluid of supply line  113 . After the reducing agent solution has been used in the chlorine and chloramines removal process  10 , the solution is recycled through outlet line  116 . Part of the solution is removed from the solution cycle in waste line  126  in order to keep the concentration of reaction products sodium chloride, sodium hydroxide, and sodium tetrathionate low enough to avoid precipitation and flaking causing fouling and plugging. The amount of solution removed through waste line  126  is controlled by valve  128 . The solution is monitored by measuring the pH values and oxidation reduction potentials of the solution at various locations of the fluid cycle. The measurements are used to determine whether valve  128  needs to be opened to remove any reaction products such as sodium chloride, sodium hydroxide, and sodium tetrathionate.  
      As seen in Equation 1, the reaction of sodium hyposulfite with the chloramines produces ammonia. The ammonia formed, or a fraction of the ammonia formed, escapes the solution and flows with the hydrogen gas. It is therefore beneficial that, in at least one embodiment, the chlorine and chloramines removal process  10  is performed before the ammonia removal process  20 . However, in one embodiment the hydrogen gas does not pass through ammonia removal process  20  after coming out of the chlorine and chloramines removal process  10 . In this embodiment, after coming out of the chlorine and chloramines removal process  10 , the hydrogen gas may be used in downstream processes where the presence of ammonia in the hydrogen gas may be acceptable, or where there are alternative methods for removing the ammonia in the downstream process.  
      In ammonia removal process  20  the hydrogen gas is flowed into contact with an aqueous solution of an acid. The acid may be any compound that will combine with ammonia to form stable, harmless compounds. In an embodiment the acid is phosphoric acid (H 3 PO 4 ). In another embodiment the acid is sulfuric acid (H 2 SO 4 ). In one embodiment, the acid solution has a concentration range between about 1 mole % and about 30 mole %, preferably, between about 5 mole % and about 20 mole %. In a particular embodiment the acid concentration is about 10 mole %. The optimal acid concentration is determined by the maximum concentration of ammonia in the hydrogen feed gas. The acid solution is introduced into scrubbing unit  210  through supply line  213  and liquid distributor  214 . The liquid distributor  214  directs the acid solution onto a packing material  212 . The hydrogen gas is introduced via outlet supply line  115 , coming out of scrubbing unit  110 , and through inlet tip  211  into the scrubbing unit  210 . In one embodiment, inlet tip  211  may be in the form of a diffuser. The hydrogen gas flows through the packing material  212  and comes into contact with the acid solution so that the ammonia gas flowing with the hydrogen gas reacts with the acid. The primary reaction of ammonia with the sulfuric acid forms ammonium sulfate: 
 
2NH 3 +H 2 SO 4 =(NH 4 ) 2 SO 4   Equation 3. 
 
      In addition to ammonium sulfate, some ammonium bisulfate (NH 4 HSO 4 ) may also be formed by the reaction. Both the ammonium sulfate and ammonium bisulfate stay in solution with the water and exit scrubbing unit  210  through outlet  216 , and are thus separated from the hydrogen gas which exit scrubbing unit  210  through a second set of demister pads  217  and outlet supply line  215 . Upon exiting scrubbing unit  210 , ammonia is at least partially removed, and the hydrogen gas may have less than about 0.1 ppm ammonia contained within the hydrogen gas.  
       FIG. 4  depicts more details of process  20 , according to one embodiment of the invention. Water is supplied from water source  240 , and is pumped by water pump  242  through water supply line  243  into scrubbing unit  210 . In one embodiment the water is demineralized. Pump  242  is controlled to keep a constant fluid level  220  in the bottom of scrubbing unit  210 . Outlet line  216  removes the fluid from the bottom of scrubbing unit  210 , and pump  224  pumps the fluid to supply line  213  and through liquid distributor  214 . Concentrated acid is stored in storage tank  230 . Dosage pump  232  pumps the concentrated acid from storage tank  230  and dilutes it into the fluid of supply line  213 . After the acid solution has been used in the ammonia removal process  20  the solution is recycled through outlet line  216 . Parts of the solution are removed from the solution cycle in waste line  226  in order to keep the concentration of reaction products such as ammonium sulfate and ammonium bisulfate low enough to avoid precipitation and flaking. The amount of solution removed through waste line  226  is controlled by valve  228 . The solution is monitored by measuring the pH values of the solution at various locations of the fluid cycle. The measurements are used to determine whether valve  228  needs to be opened to remove ammonium sulfate and ammonium bisulfate.  
      For some down stream processes it may be important to ensure complete removal of any traces of acid from the hydrogen stream leaving the ammonia removal process  20 . Accordingly, acid removal process  30  is an optional process for removing any potential acid not trapped by demister  217  of process  20 .  FIG. 5  depicts acid removal process  30 , according to an embodiment of the invention. Hydrogen gas, having exited ammonia removal process  20  through outlet supply line  215 , is introduced to scrubbing unit  310  through inlet tip  311 . Demineralized water is supplied from water source  330  and pumped by pump  332  through supply line  313  to liquid distributor  314  within scrubbing unit  310 . The liquid distributor  314  directs the water onto a packing material  312 . The hydrogen gas flows through the packing material  312  and comes into contact with the water so that the acid flowing with the hydrogen gas is absorbed by the water. The hydrogen gas then passes through demister  317  to remove any droplets of water before the hydrogen gas is passed through outlet supply line  315  for further down stream processing. The water, having passed the packing material  312  accumulates in the bottom of scrubbing unit  310  and is kept at a constant level  320 . Excess water is recycled through outlet line  316  at a rate controlled by pump  324  to supply line  313 . The water is monitored by measuring the pH values of the water at various locations of the water cycle. The water is removed from the cycle through waste line  326 . The amount of water removed through waste line  326  is controlled by valve  328  to ensure no acidity in the water cycle. In an embodiment of the invention, the acidic waste water of process  30  is used as the water source  240  for ammonia removal process  20 .  
      In another embodiment of the invention, acid removal may be obtained by using a high efficiency demister  217  in addition to a liquid distributor  214  having a design so as not to create excessive mist. In one embodiment, liquid distributor  214  is a trough type distributor.  
      Another embodiment of the invention includes an optional process which is used to monitor the efficiency of the purification process  100 . In this embodiment, the hydrogen gas, having been exposed to process  100 , is fed by outlet supply line  315  to a vessel packed with activated carbon. Activated carbon adsorbs any potential residual chlorine and chloramines not removed by process  100 . In a preferred embodiment, the activated carbon is impregnated with potassium hydroxide (KOH) to adsorb traces of acid. The level of any potential chlorine and chloramines is continuously monitored so as to avoid accumulation of chloramines to a concentration above about 3% in order to avoid violent reactions.  
      In another embodiment of the invention, scrubbing units  110 ,  210 , and  310  may be combined into a single unit containing the individual scrubbing units for removing chorine and chloramines, ammonia, and acid as described above. In another embodiment of the invention, scrubbing unit  110  may be kept separate and scrubbing units  210  and  310  may be combined into a single unit containing the individual scrubbing units for ammonia and acid as described above.  
     EXAMPLES  
      In an embodiment of the invention, water saturated hydrogen gas with 19.9 ppm ammonia, 1.5 ppm chlorine, and 1.5 ppm monochloramine is flowed through inlet tip  111  into scrubbing unit  110  at a rate of 623 kg/hour, a pressure 1.5 bars absolute, and at 32° C. Scrubbing unit  110  has a 3.0 feet diameter and a height of 30 feet. A 0.5 mole % solution of sodium hyposulfite is flowed at a rate of 19000 kg/hour out of liquid distributor  114 . Scrubbing unit  210  has a 2.5 feet diameter and a height of 30 feet. A 10 mole % solution of sulfuric acid is flowed at a rate of 23636 kg/hour out of liquid distributor  214 . Scrubbing unit  310  has a 3.0 feet diameter and a height of 10 feet. Demineralized water is flowed at a rate of 8075 kg/hour out of liquid distributor  314 . Upon exiting scrubbing unit  310 , the hydrogen gas has less than about 0.01 ppm chlorine, less than about 0.01 ppm chloramines, and less than about 0.1 ppm ammonia contained within the hydrogen gas.  
      It will be understood that many additional changes in the details, materials, steps, and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above and/or the attached drawings.