Patent Application
20070122329
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 (Cl2) and ammonia (NH3). 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 (NH2Cl), dichloramine (NHCl2), and trichloramine (NCl3). 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.
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.
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:
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 (Na2S2O5), sodium sulfite (Na2SO3), and/or sodium hyposulfite (Na2S2O3). 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:
NH2Cl+2Na2S2O3+H2O=NaCl+Na2S4O6+NaOH+NH3 Equation 1.
Chlorine is reduced to chloride by the sodium hyposulfite as in Equation 2:
Cl2+2Na2S2O3=2NaCl+Na2S4O6 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 (Na2S4O6), 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.
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 (H3PO4). In another embodiment the acid is sulfuric acid (H2SO4). 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:
2NH3+H2SO4=(NH4)2SO4 Equation 3.
In addition to ammonium sulfate, some ammonium bisulfate (NH4HSO4) 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.
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.
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.
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.
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.
| Number | Date | Country | |
|---|---|---|---|
| 60741097 | Nov 2005 | US |
