Patent Number: 053125975
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the recovery of hydrogen and the separation of hydrogen isotopes. In particular, the present invention relates to a hydride absorption/desorption apparatus for the recovery of hydrogen and the separation of the isotopes of hydrogen. 2. Discussion of Background Processes for the separation of hydrogen isotopes often rely on hydrogen-absorbing materials (hydrides) for the recovery, storage and supply of the isotopes. Hydrides are capable of absorbing large amounts of hydrogen which can then be desorbed under the appropriate temperature and pressure conditions. They are selective in that they only absorb hydrogen, and also differentially absorb the three isotopes of hydrogen (protium, deuterium, and tritium). When hydrogen contacts a hydride, the temperature of the hydride rises as it absorbs hydrogen in an exothermic reaction. Since the hydrogen equilibrium pressure increases exponentially with increasing temperature, hydrogen absorption decreases with increasing temperature. Absorption ceases when the partial pressure of hydrogen is equal to the equilibrium pressure. Therefore, the hydride must be cooled to maintain the absorption process. To release hydrogen, the reaction is reversed by heating the hydride. The faster the hydride is cooled and heated, the faster the hydrogen is absorbed and released, respectively. Known hydrides include pure metals (Mg, Ti, V, Nb, Pt, Pd, and so forth), alloys (the La-, Ti-, and Co- alloys, rare earth-Ni alloys), and various hydride-containing compositions. The capacity of a particular material to absorb or release hydrogen depends on the temperature, the external hydrogen gas pressure, and the surface area of the material. To maximize surface area and absorption/desorption efficiency, the hydride is often supplied in the form of small-grained particles or pellets. Typical hydrogen separation apparatus includes a column at least partially filled with a hydride. A hydrogen-containing gas mixture is flowed through the column to separate hydrogen from the mixture; the column is heated to recover the hydrogen. A plurality of columns, arranged in "series" or "parallel," may be provided to increase the efficiency of the process. For example, channels might be machined into an aluminum or stainless steel block, filled with a hydride, and covered by a plate welded thereto. Hydride-containing columns may be arranged in parallel within a sealed housing, as in the apparatus described by Konishi, et al. (U.S. Pat. No. 4,859,427). Heat is supplied by applying an electric current to heating coils disposed within the housing. Hydrogen or a hydrogen-containing mixture enters the housing through an inlet and portions thereof are diverted to flow through the individual columns. Known designs of this type generally use straight columns. Since the efficiency of the absorption/desorption process depends in part on how rapidly the column is heated and cooled; the faster the cooling and heating, the higher the efficiency. The cooling and heating rate increases with the column surface area and the heat transfer efficiency between the column surface and the heat transfer medium. The heat transfer efficiency in turn depends on the flow pattern of the heat transfer medium over the external surface of the column. The column surface area is sometimes increased by the use of multiple columns. Designs using multiple columns typically contain a large number of fittings, seams, welds, or couplings. In many cases, it is difficult to examine these to assure the structural strength and integrity of the apparatus. There is a need for an efficient hydrogen isotope separation apparatus having a large column surface area with a minimum of welds or other couplings and a high heat transfer efficiency. SUMMARY OF THE INVENTION According to its major aspects and broadly stated, the present invention is an apparatus for separating a hydrogen isotope from a gaseous mixture or from a mixture of hydrogen isotopes by controlled absorption and desorption using hydride particles. A baffle is disposed within a housing, attached thereto by a bracket. A hollow conduit with an interior, an exterior, one or more inlets, and one or more outlets is coiled about the baffle, in spaced relation to the baffle and the housing so as to confine and accelerate the heat transfer fluid to create turbulent flow over the exterior of the coiled conduit. The coiled conduit is dimensioned for holding a quantity of hydride particles. Depending on the type of hydride particles placed in the conduit, the apparatus may be used to recover hydrogen from a hydrogen-containing gaseous mixture or to separate hydrogen isotopes (protium, deuterium and tritium) from each other. To recover hydrogen from a gaseous mixture, a low pressure hydride such as palladium, uranium or titanium is placed in the conduit. To separate hydrogen isotopes, a hydride with strong isotopic effects, such as palladium, vanadium or a lanthanum-nickel-aluminum alloy, is used. The apparatus is operated in a temperature cycling process, where each cycle consists of a cooling, or separation, phase and a heating, or regeneration, phase. In the cooling phase, the hydride is cooled to a low temperature by circulating a low temperature fluid over the exterior of the coil. In the heating phase, the hydride is heated to a high temperature by circulating a high temperature fluid over the coil. The faster the temperature is cycled, the better the separation efficiency of the apparatus. To separate hydrogen from a gaseous mixture, the mixture is pumped through the conduit in the cooling phase of the separation cycle. At least a portion, and preferably most of the hydrogen in the mixture is absorbed by the hydride, and largely hydrogen-free gas is withdrawn from the coiled conduit at the outlet. In the heating phase of the cycle, the hydrogen is desorbed from the hydride and collected at the outlet. To separate hydrogen isotopes from each other, one of two types of hydride is used. The first type, such as palladium, absorbs the lighter hydrogen isotopes better than the heavier isotopes; that is, it absorbs protium better than deuterium and tritium, and deuterium better than tritium. The second type of hydride, such as vanadium, absorbs the heavier isotopes better than the lighter isotopes, that is, it absorbs tritium better than deuterium and protium, and deuterium better than protium. An important feature of the present invention is the hollow coiled conduit, disposed about the cylindrical baffle and at least partially filled with a hydride. The coiled conduit can contain no seams or welds along its length, as opposed to a series of parallel conduits, but has the same efficient use of space that parallel conduits have compared to a single long conduit. The conduit is dimensioned for holding a quantity of hydride particles, but the particular dimensions and configuration of the coiled conduit depend on the dimensions of the cylindrical baffle and the housing. The inside diameter and length of the coiled conduit also affect the gas flow characteristics. The optimum dimensions of the coiled conduit are best determined by a modest degree of computation and experiment for each particular apparatus. The heat energy exchange between fluid and hydride is enhanced by the combination of the cylindrical baffle and the coiled conduit. These features confine or restrict the fluid and accelerate its velocity, causing turbulent flow for better heat transfer. Another important feature of the present invention is the combination of the housing, the cylindrical baffle and the exterior of the coiled conduit. During absorption, the hydride temperature is cooled by flowing a fluid at low temperature over the coiled conduit. When the fluid is pumped into the housing inlet, it is diverted by the cylindrical baffle to flow over the coiled conduit to the housing outlet. Fluid flow is largely confined to the annular region between the cylindrical baffle and the housing, where turbulent flow facilitates efficient heat transfer from the hydride and the coiled conduit to the fluid. The spacing between the coiled conduit and the cylindrical baffle, and the spacing between the coiled conduit and the housing, are preferably selected to induce turbulent flow for efficient heat transfer, but not to be too restrictive and cause too much pressure drop and loss of flow. The optimum spacings depend on the particular dimensions of the components of the apparatus. A further feature of the present invention is the fluid. The fluid is preferably nitrogen, but may be any convenient stable gas (including the inert gases) or liquid, which does not react with the process gas or the apparatus materials, and can efficiently transfer heat in the desired operating temperature range. The fluid temperature can be regulated so as to be a higher or lower temperature than the hydride temperature. The larger the temperature difference between the fluid temperature and the hydride temperature, the better heat transfer and the more effective the absorption or desorption process. Still another feature of the present invention is the hydride. Depending on the choice of hydride, the apparatus can be used to separate hydrogen--or a particular isotope of hydrogen--from a gaseous mixture containing hydrogen, or to separate an isotope of hydrogen from a mixture of hydrogen isotopes. To maximize the efficiency of hydrogen absorption/desorption, the surface area of the hydride is maximized by supplying the hydride in the form of small, porous particles. Preferably, the hydride is a granular, dimensionally-stable metal hydride or hydride composition such as the stable hydrogen-absorbing composition described in commonly assigned and recently issued U.S. Pat. No. 5,248,649 titled Palladium/Kieselguhr Composition and Method, described below in the Detailed Description of a Preferred Embodiment. With this composition, the apparatus effectively separates &gt;99.9 vol. % deuterium (D) from a process gas containing 50 vol. % each protium (H) and deuterium, and &gt;99.9 vol. % H.sub.2 from a mixture containing 20 vol. % H.sub.2 and 80 vol. % other gases such as N.sub.2. Other features and advantages of the present invention will be apparent to those skilled in the art from a careful reading of the Detailed Description of a Preferred Embodiment presented below and accompanied by the drawings.