System for hydrogen generation
The present invention relates to an improvement in a system for the generation of hydrogen by contacting an aqueous solution of a metal hydride salt with a hydrogen generation catalyst. In particular, the present invention relates to the incorporation within the system of a recycle line of water condensed from the fluid product to the feed line to be contacted with the catalyst. The internal recycle line permits the use of a more concentrated solution of metal hydride as it is diluted by the recycle line prior to contact with the catalyst.
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 INVENTIONHydrogen 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.
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.
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 INVENTIONThere 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 DRAWINGSThe advantages of the present invention will be further understood from the following detailed description when considered with the accompanying drawings wherein:
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.
A conventional system for hydrogen generation from an aqueous metal hydride solution is shown in block diagram in
The metal hydride fuel component of the system illustrated in
BH4−+2H2O=BO2−+4H2
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.
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.
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.
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
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.
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.
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 NaBO2 and NaBH4. 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.
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.
Preferably, the catalyst facilitates both aspects of the reaction of the metal hydride and water, i.e. the availability of a hydrogen site and the ability to assist in the hydrolysis mechanism. Metal hydride solutions are complex systems having multi-step reduction mechanisms. For example, borohydride has four hydrogens and an eight-electron reduction mechanism. Thus, once a single hydrogen atom is removed from a borohydride molecule, the remaining moiety is unstable and will react with water to release the remaining hydrogen atoms. Catalysts that are useful in the system of the invention include, without intended limitation, transition metals, transition metal borides, alloys of these materials and mixtures thereof.
Suitable transition metal catalysts for the generation of hydrogen from a metal hydride solution are known in the art and include metals from Group 1B to Group VIIIB of the Periodic Table, or compounds made from these metals. Representative examples of these metals include, without intended limitation, transition metals represented by the copper group, zinc group, scandium group, titanium group, vanadium group, chromium group, manganese group, iron group, cobalt group and nickel group. These catalyst metals aid in the reaction by adsorbing hydrogen on their surface in the form of the protonic H+. Examples of useful catalyst metals include, without intended limitation, ruthenium, iron, cobalt, nickel, copper, manganese, rhodium, rhenium, platinum, palladium, chromium, silver, osmium, iridium borides thereof, alloys thereof and mixtures thereof. Ruthenium, rhodium and cobalt are preferred.
The catalysts preferably have high surface area, i.e. they have small average particle sizes, for example an average diameter of less than about 100 microns, preferably less than about 50 microns, most preferably less than about 25 microns. The chemical reaction of metal hydrides in water in the presence of the catalyst follows zero order kinetics at all concentrations of metal hydride measured, i.e. the volume of hydrogen gas generated is linear with time. It is, therefore, believed that the reaction rate depends primarily on the surface area of the catalyst. In addition to metal particles having very small average particle size, larger particles, e.g. agglomerates may be utilized provided that they have sufficient porosity to possess the requisite surface area.
In the system improved upon in accordance with the present invention, the generation of hydrogen can be controlled by regulating contact of the solution with the catalyst because little hydrogen will be generated from the stabilized solution in its absence. Control can be effected. for example, by regulating the flow of solution to the catalyst, or by withdrawing the catalyst from the solution to cease production. It has been found that hydrogen generation is increased with increases in temperature and is fairly constant at a given temperature until the metal hydride solution is almost exhausted. It will be appreciated by those of ordinary skill in the art that the desired rate of reaction can be obtained and controlled by factors including regulation of the temperature, the concentration of the alkaline stabilizing agent, the selection of a catalyst, the surface area of the catalyst and the like.
Several methods are available to contact the stabilized metal hydride solution with the catalyst system. When hydrogen is required, the solution can be pumped to a chamber containing the catalyst or the catalyst can be moved into a tank containing the solution. The metal hydride solution can be pumped either in batches or continuously. The instantaneous demand for hydrogen can be met with a small buffer tank, not illustrated, that always contains a supply of available hydrogen gas. The hydrogen gas from this tank can be utilized to meet immediate demand and the resultant pressure drop can trigger the system to produce more hydrogen gas, thereby maintaining a constant supply of hydrogen available to the hydrogen-consuming device.
As illustrated by
There are several advantages realized by the recycle system illustrated in
It will be appreciated that the amount of the metal hydride salt that is not in solution in the fuel supply concentrate is limited by the configuration, of the system, the amount of water that can be added thereto through conduit 29, the time available to affect solubilization thereof and the like. Typically, the fuel supply will contain no more than about 5% of undissolved metal hydride.
A second advantage of the recycle system provided in accordance with the present invention is that the addition of water from the recycle line maintains a dilute fuel feed thereby significantly reducing the possibility of the system becoming clogged as a result of the water being used up to the point where there is insufficient water exiting the catalyst chamber 7 to maintain the product salt, a borate in the case of the fuel being a metal borohydride, in solution. Precipitation of the product salt in the catalyst chamber itself or in any of the associated downstream apparatus of piping will render the system ineffective until disassembled and cleaned. Such a problem can be very significant in terms of the use of such systems as an alternate power source for vehicles.
A further advantage of the system of the invention is that the water exiting the condensate recovery tank 21 is at a significantly lower temperature than in the catalyst chamber 7, hence it functions as an aid in controlling the temperature of the reaction, which is exothermic. This added control of system operating temperature is also significant in the contemplated use of the system to power vehicles. More important, however, is the fact that the regulating capacity of the system assures a substantially constant flow of product hydrogen, a commercially significant advantage. A still further advantage of the system of the invention is the fact that the recycle system is internal of the system. i.e. it can be within the system itself so that there is no need for external apparatus such as tanks and/or conduits to introduce water from an external source.
Another embodiment of the improved system according to the present invention is shown in
In a further embodiment of the present invention shown in
The following example further describes and demonstrates the improved operation of the subject system according to the present invention. The example is given solely for the illustration purposes and is not to be construed as a limitation of the present invention.
EXAMPLE A hydrogen generation test system according to
Claims
1-18. (canceled)
19. A method of generating hydrogen, comprising:
- providing an aqueous composition containing at least one metal hydride;
- contacting the metal hydride with a hydrogen generation catalyst to generate a fluid product stream comprising hydrogen, water and a metal salt; and
- mixing at least part of the water of the fluid product stream with the metal hydride prior to contacting the metal hydride with the catalyst.
20. The method of claim 19, wherein the metal hydride is provided in the form of an aqueous solution.
21. The method of claim 19, further comprising separating the fluid product stream into a gaseous product comprising hydrogen and water vapor, and a liquid product comprising water and the metal salt.
22. The method of claim 21, further comprising condensing water from the gaseous product recovering the condensed water.
23. The method of claim 22, further comprising combining the condensed water with the metal hydride composition prior to contacting the metal hydride with the hydrogen generation catalyst.
24. The method of claim 22, further comprising providing a pump for withdrawing the condensed water from a condensate recovery zone and introducing it into a mixing zone.
25. The method of claim 19, further comprising withdrawing the aqueous composition containing a metal hydride from a fuel supply reservoir before contacting with the catalyst.
26. The method of claim 25, wherein the withdrawing is conducted using a fuel pump.
27. The method of claim 26, wherein the fuel pump is located upstream of a mixing zone for mixing at least part of the water of the fluid product stream with the aqueous composition containing a metal hydride.
28. The method of claim 27, wherein the mixing zone is located in fluid communication with the fuel pump and the hydrogen generation catalyst.
29. The method of claim 28, further comprising providing a valve upstream of the mixing zone and permitting alternative flow of the metal hydride composition and condensed water from a condensate zone into the mixing zone.
30. The method of claim 24, wherein the concentration of metal hydride in the fuel supply reservoir is above the maximum solubility of the hydride and a portion thereof is in suspension.
31. The method of claim 30, wherein sufficient condensed water is added to the mixing zone so that all of the metal hydride is in solution when it contacts the hydrogen generation catalyst.
32. The method of claim 30, further comprising maintaining uniformity of the suspension.
33. The method of claim 19, further comprising providing a sufficient amount of an alkaline stabilizing agent to the aqueous composition containing a metal hydride to provide a pH thereof at about 7.
34. The method of claim 33, wherein the alkaline stabilizing agent is an hydroxide.
35. The method of claim 33, wherein the cation portion of the alkaline stabilizing agent is the same as the cation portion of the metal hydride.
36. The method of claim 35, wherein the cation is a sodium ion.
37. The method of claim 33, wherein the alkaline stabilizing agent is sodium hydroxide and the metal hydride is sodium borohydride.
38. The method of claim 19, further comprising providing a containment system for the catalyst.
39. The method of claim 38, wherein the containment system comprises a cylinder having the catalyst therein.
40. The method of claim 19, wherein the hydrogen generation catalyst comprises a transition metal selected from the group consisting of ruthenium, iron, cobalt, nickel, copper, manganese, rhodium, rhenium, platinum, palladium, chromium, silver, osmium, iridium, borides thereof, alloys thereof, and mixtures thereof.
41. The method of claim 19, wherein the metal hydride is selected from the group consisting of sodium borohydride, lithium borohydride, potassium borohydride, ammonium borohydride, and mixtures thereof.
42. The method of claim 19, wherein the catalyst is a supported catalyst.
43. A method of generating hydrogen, comprising:
- providing a fuel supply reservoir containing an aqueous solution of at least one metal hydride;
- withdrawing the aqueous solution from the reservoir and contacting the aqueous solution with a hydrogen generation catalyst to generate a fluid product stream comprising hydrogen, water and a salt of the metal;
- separating the fluid stream into a gaseous product comprising hydrogen and water vapor, and a liquid product comprising water and the metal salt;
- recovering at least part of the water vapor to obtain recovered water; and
- mixing the recovered water with the aqueous solution of metal hydride prior to contacting the catalyst.
44. The method of claim 43, wherein mixing is conducted in a separate mixing zone prior to contacting the metal hydride with the hydrogen generation catalyst.
45. The method of claim 43, wherein withdrawing the solution from said fuel supply reservoir is conducted with a fuel pump.
46. The method of claim 45, wherein the fuel pump is located upstream of the mixing zone.
47. The method of claim 43, wherein the mixing zone is located in fluid communication with the fuel pump and the hydrogen generation catalyst.
48. The method of claim 43, further comprising providing a pump for withdrawing the recovered water from a recovery zone and introducing it into the mixing zone.
49. The method of claim 43, further comprising providing a valve upstream of the mixing zone for permitting alternative flow of the metal hydride solution and the recovered water into the mixing zone.
50. The method of claim 43, wherein the concentration of the metal hydride in the fuel supply reservoir is above the maximum solubility of the hydride and a portion thereof is in suspension.
51. The method of claim 50, further comprising adding sufficient recovered water to the mixing zone so that all of the metal hydride is in solution when it contacts the hydrogen generation catalyst.
52. The method of claim 50, further comprising agitating the fuel supply to maintain uniformity of the suspension.
53. The method of claim 50, further comprising providing a sufficient amount of an alkaline stabilizing agent to the aqueous solution of metal hydride to provide a pH thereof at about 7.
54. The method of claim 53, wherein the alkaline stabilizing agent is an hydroxide.
55. The method of claim 53, wherein the cation portion of the alkaline stabilizing agent is the same as the cation portion of the metal hydride.
56. The method of claim 55, wherein the cation is a sodium ion.
57. The method of claim 53, wherein the alkaline stabilizing agent is sodium hydroxide and the metal hydride is sodium borohydride.
58. The method of claim 43, further comprising providing a containment system for the catalyst.
59. The method of claim 58, wherein the containment system comprises a cylinder having the catalyst therein.
60. The method of claim 43, wherein the hydrogen generation catalyst comprises a transition metal selected from the group consisting of ruthenium, iron, cobalt, nickel, copper, manganese, rhodium, rhenium, platinum, palladium, chromium, silver, osmium, iridium, borides thereof, alloys thereof, and mixtures thereof.
61. The method of claim 43, wherein the metal hydride is selected from the group consisting of sodium borohydride, lithium borohydride, potassium borohydride, ammonium borohydride, and mixtures thereof.
62. The method of claim 43, wherein the catalyst is a supported catalyst.
Type: Application
Filed: Sep 22, 2005
Publication Date: Feb 2, 2006
Inventors: Richard Mohring (East Brunswick, NJ), Michael Strizki (Hopewell, NJ)
Application Number: 11/232,008
International Classification: B01J 7/00 (20060101);