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  System for hydrogen generationRelated Patent Categories: Gas: Heating And Illuminating, GeneratorsThe Patent Description & Claims data below is from USPTO Patent Application 20060021279. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] The present invention relates to a system for generating hydrogen gas. In particular, the present invention relates to a hydrogen generation system including a stabilized metal hydride solution and a catalyst system. BACKGROUND OF THE INVENTION [0002] Hydrogen is a "clean fuel" because it can be reacted with oxygen in hydrogen-consuming devices, such as a fuel cell or combustion engine, to produce energy and water. Virtually no other reaction byproducts are produced in the exhaust. As a result, the use of hydrogen as a fuel effectively solves many environmental problems associated with the use of petroleum based fuels. Safe and efficient storage of hydrogen gas is, therefore, essential for many applications that can use hydrogen. In particular, minimizing volume and weight of the hydrogen storage systems are important factors in mobile applications. [0003] Several methods of storing hydrogen currently exist but are either inadequate or impractical for wide-spread consumer applications. For example, hydrogen can be stored in liquid form at very low temperatures. However, liquid hydrogen is neither safe nor practical for most consumer applications. Moreover., the energy consumed in liquefying hydrogen gas is about 60% of the energy available from the resulting hydrogen. [0004] As a result of these and other disadvantages of hydrogen storage and transportation, the art has turned to fuel cells and systems for the generation of hydrogen. Such systems are known, for example Amendola et al, Abstracts ACS National Meeting, August, 1999, pages 864-868, describe such a system that is suitable for use in motor vehicles that is based on the catalyst generation of hydrogen from an aqueous metal hydride solution. In accordance with the present invention, an improvement in the operation of such systems is provided. SUMMARY OF THE INVENTION [0005] There is provided an improvement in a hydrogen generation system including a metal hydride solution and a catalyst that activates the reaction of the metal hydride with water to produce hydrogen gas. The system includes a means for condensing water vapor from the hydrogen product flow. The system is improved in accordance with the present invention by recycling a portion of the condensate water into the feed line to mix with and dilute the metal hydride fuel solution before it is contacted with the catalyst. BRIEF DESCRIPTION OF THE DRAWINGS [0006] The advantages of the present invention will be further understood from the following detailed description when considered with the accompanying drawings wherein: [0007] FIG. 1 is a block diagram of a conventional system for the generation of hydrogen from a metal hydride solution. [0008] FIG. 2 is a block diagram of the improved system of the present invention. [0009] FIG. 3 is a block diagram of an alternative embodiment of the improved system of the present invention. [0010] FIG. 4 is a block diagram of a still further embodiment of the improved system of the present invention. DETAILED DESCRIPTION OF THE INVENTION [0011] The system for the generation of hydrogen in accordance with the present invention is comprised of an aqueous metal hydride solution fuel and a catalyst for promoting the reaction of the metal hydride to produce hydrogen, a byproduct salt of the metal and water in the form of water vapor. This system has been demonstrated to produce hydrogen safely and efficiently for use in a hydrogen fuel cell that possesses many advantages over conventional fuel systems, such as gasoline engines. [0012] A conventional system for hydrogen generation from an aqueous metal hydride solution is shown in block diagram in FIG. 1. Aqueous metal hydride solution is withdrawn from a reservoir 1 through a conduit line 3 by a fuel pump 5 into a catalyst chamber or compartment 7 where it undergoes reaction to form a fluid product stream comprising hydrogen, a salt of the metal and water. The product stream is withdrawn through conduit line 9 into a gas liquid separator 11 where the byproduct salt is withdrawn as a solution through conduit line 13 and the gaseous hydrogen product mixture is withdrawn through conduit line 15. The system is completely inorganic and produces a high quality energy source without polluting emissions. The system is likewise readily controllable since hydrogen is only produced when the solution contacts the catalyst. [0013] The metal hydride fuel component of the system illustrated in FIG. 1 and in the subject improved system is a complex metal hydride having the general formula MBH.sub.4 wherein M is a positive ion selected from those of an alkali metal, such as sodium, potassium or lithium, certain organic groups and ammonium, B is a negative ion of a metal selected from Group 13 (formally Group IIIA) of the Periodic Table, such as boron, aluminum and gallium, and H is hydrogen. Examples of suitable metal hydrides, without intended limitation, include NaBH.sub.4, LiBH.sub.4, NH.sub.4BH.sub.4, LiAlH.sub.4, NaGaH.sub.4 and the like. These metal hydrides may be utilized in mixtures, but are preferably utilized individually. Preferred for such systems in accordance with the present invention are borohydrides, especially sodium borohydride (NaBH.sub.4), lithium borohydride (LiBH.sub.4), potassium borohydride (KBH.sub.4), ammonium borohydride (NH.sub.4BH.sub.4), quaternary ammonium borohydrides and the like, including mixtures thereof A borohydride, such as illustrated above, will react with water to produce hydrogen gas and a borate in accordance with the following chemical reaction: BH.sub.4.sup.-+2H.sub.2O=BO.sub.2.sup.-+4H.sub.2 This reaction takes place very slowly in the absence of a catalyst. It has further been found that the solution of metal hydride salt is stable without appreciable generation of hydrogen at alkaline pH. The salt formed in the reaction, borate in the instance of a metal borohydride, is non-toxic and environmentally safe. In addition, borate can be regenerated into borohydride for future use. It is important to note that all of the hydrogen atoms present in borohydride and water are converted to hydrogen gas, and that half of the hydrogen atoms in the hydrogen gas produced by the reaction given above actually come form the water. [0014] In general, the various borohydride salts are soluble in water up to about 35%, lithium borohydride has only about 7% solubility, potassium borohydride about 19% and sodium borohydride about 35%. It will be appreciated that sodium borohydride is preferred for the practice of the present invention due to its comparatively high solubility. Where the concentration of the metal hydride in the fuel system exceeds the maximum solubility of the particular salt utilized, it will be in the form of a slurry or suspension. This is acceptable provided that only a minor portion of the metal hydride is not in solution and the fuel system includes a means of maintaining the uniformity of the slurry or suspension withdrawn to be exposed to the catalyst. As will be detailed below, the present invention is advantageous in that a slurry of the fuel borohydride may be utilized for greater economy of operation. [0015] Since two molecules of water are consumed for each borohydride molecule during the reaction illustrated above, the product stream containing the borate salt is more concentrated than the borohydride fuel mixture. Stoichiometrically, twice as many water molecules as borohydride molecules are required to sustain a constant rate of reaction. In practice, water in excess of even that requirement is necessary for the efficient conversion of the sodium borohydride to hydrogen. [0016] This excess water has heretofore been provided in two ways: charging the initial metal hydride solution with excess water, i.e. starting with a dilute solution, or adding more water from a separate source during or after the reaction. The second alternative is clearly preferable for reasons of economy since utilizing a dilute fuel solution would substantially increase the size of the fuel tank 3 in FIG. 1. It has been proposed in co-pending application Ser. No. 09/479,362 to utilize a separate source of water from the hydrogen-consuming device, e.g. a fuel cell, combustion engine or a gas turbine. Since these devices consume hydrogen, a main by-product is water and it is proposed to utilize some of this water to maintain a constant rate of reaction in the subject hydrogen generators. However, such use still represents a source of water external to the system. It is often the case that such water is utilized in a humidification loop to maintain the membrane in a proton exchange membrane (PEM) fuel cell and is not available for recycle to other parts of the system. [0017] The concept of recycling water from the device, e.g. a fuel cell, has been proposed as well in U.S. Patent Application Publication No. US 2002/0025462, published Feb. 28, 2002. The disclosed system includes a condenser to remove water from the hydrogen gas stream by radiative cooling as well. Further, MacCarley, Symposium on Alternative Fuel Resources, Santa Monica, Calif., March, 1976, pages 315-320, in discussing hydrogen systems for automotive application describes condenser loops for the removal of water from the generated hydrogen gas stream. However, the paper does not specifically mention the recycling of water and gives no detail as to how or where the recycling would be carried out. As will be shown below, the present invention improves on this concept by providing a recycle of water within the hydrogen generator itself thereby significantly enhancing the economies of its operation. [0018] The metal hydride solution utilized as the fuel for the system is stabilized against decomposition by being at an alkaline pH, i.e. a pH of at least above pH 7. This is carried out by the addition of a suitable alkaline stabilizing agent, preferably a hydroxide, most preferably an alkali metal hydroxide. It is particularly preferred that the cation portion of the alkaline stabilizing agent be the same as the cation of the metal hydride salt. For example, if the metal borohydride is sodium borohydride, the alkaline stabilizing agent would be sodium hydroxide, both of which are preferred in the practice of the present invention. The concentration of the alkaline stabilizing agent is typically greater than about 0.1 molar, preferably greater than 1.0 molar or about 4% by weight. The alkaline stabilizing agent is typically added to the water prior to the addition of the borohydride thereto. Sodium hydroxide is a particularly preferred stabilizing agent due to its high solubility in water (about 44%) which allows stability of the solution without adversely affecting the solubility of the metal borohydride. The presence of the alkaline stabilizing agent prevents premature reaction and degradation of the metal hydride salt before it contacts the catalyst. [0019] The catalyst in the subject system is present in a containment means so that it can be separated from the reacted metal hydride solution which, in the instance of a sodium borohydride fuel mixture, would contain a mixture of NaBO.sub.2 and NaBH.sub.4. Containment may be any physical, chemical, electrical and/or magnetic means of securing the catalyst. Containment systems are preferably a tube or cylinder retaining the catalyst between mesh or porous ends such that the solution can flow through during the reaction and the product liquid/gas mixture is withdrawn from the downstream end. Other similar means will be readily apparent to those of ordinary skill in the art. [0020] The catalyst can also be attached or bound to a suitable substrate, i.e. a supported catalyst, and thereby be contained in that the substrate is held in place while the solution of metal hydride passes over it. Thus, hydrogen production can be controlled by either contacting or separating the bound catalyst from the metal hydride solution. An example of such a catalyst is one entrapped by physical or chemical means onto and/or within a porous or nonporous substrate. Nonlimiting examples of porous substrates include ceramics and ion exchange resins. Nonlimiting examples of nonporous substrates include metallic meshes, fibers and fibrous materials. The preparation of such supported catalysts is taught, for example in copending application Ser. No. 09/999,226, the disclosure of which is incorporated herein by reference. Continue reading... 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