Abstract:
A rejuvenable ambient temperature purifier is provided. The purifier includes an enclosure with a chamber having an inlet and an outlet. Purifier material comprising a mixture of a transition metal material and a getter material is disposed within the chamber. The transition metal material is in a dispersed form with at least 5% of the transition metal material being in metallic form. The getter material is also in a dispersed form intermixed with the transition metal material. The getter material is selected from the group including Zr, Ti, Nb, Ta, V, and alloys thereof.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to gas purification and more particularly to gas purifiers containing dispersed impurity-sorbing materials. 
     2. Description of the Related Art 
     Ultra-high purity (UHP) gases are used for the manufacture of semiconductor devices, laboratory research, mass spectrometer instruments and other industries and applications. UHP gases are typically defined as at least 99.9999999% pure gas by volume. There are several methods of producing UHP gases. Purifiers are widely used based on the use of solid materials that can bond impurities in the stream of a main gas, by interacting with the impurity molecules according to a variety of mechanisms. 
     An important class of gas purifiers exploits the properties of getter alloys, which include Zr, Ti, Nb, Ta, and V based alloys as active elements. Examples of commonly used alloys are an alloy of weight percent composition Zr 70%-V 24.6%-Fe 5.4%, under the trademark St 707; an alloy of composition Zr 76.5%-Fe 23.5%, under the trademark St 198; an alloy of composition Zr 84%-Al 16%, under the trademark St 101; and certain Ti—Ni alloys, all of which are produced and sold in conjunction with gas purifiers by SAES Pure Gas, Inc. of San Luis Obispo, Calif. 
     The working principle of getter alloys is chemisorption of species such as O 2 , H 2 O, CO, CO 2  and CH 4 , through surface adsorption followed by dissociation and diffusion in the bulk of the getter material of the atoms making up the impurity molecules. Some getter alloys may also fix N 2  according to the same mechanism. The result is the formation of oxides, carbides or nitrides of the metals of the alloy. Because the species formed are very stable, the sorption of the above mentioned gases by getter alloys is essentially irreversible. 
     Because getter alloys do not react with noble or inert gases, they are well suited for purification of these gases. By using these alloys it is possible to remove traces of reactive gases from inert gases. Examples of gases that may be purified by means of getter alloys include noble gases, chloroflourocarbons, which are used in the semiconductor industry, and nitrogen N 2 ). For example, N 2  may be purified by the St 198 alloy, which has a negligible sorption capability for the gas. Examples of purifiers based on the use of getter alloys are disclosed in UK Patents GB 2,177,079 and GB 2,177,080, in European Patent EP 365490, and in U.S. Pat. No. 5,194,233 and 5,294,422. 
     FIG. 1A is a schematic illustration of a getter purifier  10  of the prior art during process gas purification at an elevated temperature. Getter purifier  10  includes a chamber  12 , which is coupled to an inlet  14  and an outlet  16 . Chamber  12  is partially filled with getter material particles  18 . A heater  20  heats getter purifier  10  to at least about 300 degrees Celsius. A process gas with gaseous impurities such as water or carbon oxide is introduced into chamber  12  through inlet  14  where getter material particles  18  absorb the traces of water and carbon oxide. A purified process gas then exits chamber  12  through outlet  16 . 
     While getter materials show essentially irreversible gettering for impurities (e.g. oxygen, water, carbon monoxide, carbon dioxide, methane) normally present in noble or relatively inert gases (such as argon, helium and nitrogen) for semiconductor industry, getter materials behave very differently towards hydrogen. In fact, getter materials show reversible gettering for hydrogen, which undergoes an equilibrium reaction with most getter materials. At about room temperature, the pressure of “free” gas at is very low, but the pressure increases with increasing temperature. 
     FIG. 1B is a schematic illustration of a getter purifier  10  of the prior art during the removal of hydrogen from a process gas. Getter purifier  10  is operational at ambient temperatures (0 to 40 degrees Celsius) to remove traces of hydrogen from process gases. If a process gas with hydrogen is introduced into chamber  12  through inlet  14 , getter material particles  18  will absorb the hydrogen, leaving a purified process gas to exit chamber  12  through outlet  16 . 
     Getter based purifiers are highly efficient in removing impurities as shown in FIG. 1A, but they are costly and need to be kept at about 300 to about 450° C. for operation. Therefore, in some circumstances other kinds of purifiers are preferred. An example of lower cost purifiers is the so-called nickel purifiers, which operate at around room temperature. These purifiers include as the active material, metallic nickel, generally supported on a porous substrate such as silica. 
     FIG. 2A is a schematic illustration of a nickel purifier  22  of the prior art during process gas purification. Nickel purifier  22  includes a chamber  24 , which is coupled to an inlet  26  and an outlet  28 . Chamber  24  is partially filled with nickel material particles  30 . Nickel is typically present in metallic form for at least 5% of the overall amount of nickel material particles  30 , with the remainder generally being present as nickel oxide, NiO. Nickel is generally present in a particulate or “dispersed” form, so as to have a high specific area of at least 100 m 2 /g and preferably between about 100 and 200 m 2 /g, but the overall amount of nickel is limited. By “dispersed” it is meant that the material is formed by discrete particles, such as powders, granules, pellets, etc. 
     Nickel purifiers often also contain physical water sorbers, such as molecular sieves, to help remove water vapor and leave nickel material available for removal of oxygen and carbon oxides. As shown, a process gas, water, and trace amounts of oxygen and carbon oxide enter chamber  24  through inlet  26 . During operation of nickel purifier  10 , nickel material particles  18  react with oxygen or water and with CO or CO 2 . The product of the Ni and oxygen or water reaction is NiO. Once the sorbing capacity of nickel material particles  18  has reached its limits, the purifier may be regenerated. 
     FIG. 2B is a schematic illustration of a nickel purifier  22  of the prior art during the process of regeneration. Nickel material particles  30  are regenerated by passing a flow of hydrogen-containing inert gas over the nickel material particles  30  maintained at a temperature of about 200° C. by heater  20 . The inert gas is preferably nitrogen, the amount of hydrogen is preferably below about 20% by volume, and more preferably between about 2 and about 5% by volume of the flowing gas, and the regeneration process is preferably continued for about 14-20 hours. In these conditions NiO and the product of the reaction of Ni and CO/CO 2  are reduced to metallic nickel. Nickel purifiers are disclosed, e.g., in U.S. Pat. No. 4,713,224. 
     Because water and CO are produced during the regeneration step, the operation must be performed with the purifier disconnected from the pure gas line, in order not to pollute the system. A wide range of nickel-based purifiers is sold by Aeronex Inc. of San Diego, Calif. under the name GATEKEEPER®. Further to the application indicated above, another important use of nickel-based purifiers is in gas cabinets, for the purification of gas (generally nitrogen) used to purge gas pipelines during process gas cylinders change out. 
     FIG. 3 illustrates another nickel purifier unit  32  of the prior art. Nickel purifier unit  32  includes a body or enclosure  33  defining a chamber  34 , which is generally made of stainless steel into an essentially cylindrical shape. Chamber  34  is preferably electropolished to at least 10 Ra. At the two opposing bases of nickel purifier unit  32 , a gas inlet  36  and an outlet opening  38  are provided. Gas inlet  36  and outlet opening  38  are typically equipped with suitable fittings  40  for connection to a set of gas lines. Fittings  40  shown are male face seal fittings, but as is well known in the art, compression fittings may also be used. Nickel purifier unit  32  is preferably equipped with particle filters at gas inlet  36  and outlet opening  38 . Particle filters are generally made of sintered stainless steel particles and capable of retaining particles of dimensions of 0.003 μm and larger. 
     The internal volume of nickel purifier  32  is filled with particles of nickel-containing or nickel supporting materials. These materials may be made of formed pieces (spheres or cylinders) of a porous supporting medium, such as silica, over which nickel material is dispersed according to techniques well-known in the field of catalysts production. Nickel may be present in a mixed form, in which part of the metal is present as a compound, generally nickel oxide, NiO, with at least 5% of the metal present in reduced metallic form. 
     A major disadvantage of nickel-based purifiers is that regeneration is not easily accomplished on site, due to the need of keeping for hours the purifier under a hydrogen-containing gas flow that, at the outlet, need be vented outside the system; as a consequence, for the regeneration operation the purifier must generally be returned to the manufacturer. To avoid service interruptions, producers generally offer systems made up of two nickel purifiers in parallel, so that one can operate while the other is regenerated. 
     Also well known are purifiers where both getter and nickel beds are used. These purifiers are disclosed, e.g. in U.S. Pat. Nos. 5,492,682, 5,558,844, 5,556,603 and 5,902,561. These patents show two-stage purifiers, in which the gas first contacts a bed of nickel material kept at room temperature and then a second bed of getter material maintained at a temperature of between about 250 to about 400° C. In these purifiers each bed works according to its normal operation as described before. 
     In view of the foregoing, it is desirable to have a method and apparatus for efficiently and economically rejuvenating a gas purifier, particularly so that it is possible to rejuvenate the purifier on site. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus to purify various gases utilizing a gas purifier capable of operating at room temperature, but such that can easily be rejuvenated at the point of use when saturated by simply isolating it from the gas line it is inserted in and heating the apparatus at a pre-set temperature. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device or a method. Several inventive embodiments of the present invention are described below. 
     In one embodiment of the present invention, a rejuvenable ambient temperature purifier is provided. The purifier includes an enclosure with a chamber having an inlet and an outlet. Purifier material is disposed within the chamber. The transition metal material is preferably in a dispersed form with preferably at least 5% of the transition metal material being in a metallic form. The getter material is also preferably in a dispersed form intermixed with the transition metal material. The getter material is preferably selected from the group including Zr, Ti, Nb, Ta, V, and alloys thereof. 
     In another embodiment of the present invention, a rejuvenable ambient temperature purifier system is provided. The system comprises a purifier including an enclosure with a chamber having an inlet and an outlet. Purifier material comprising a mixture of a transition metal material and a getter material is disposed within the chamber. The transition metal material is preferably in a dispersed form with preferably at least 5% of the transition metal material being in metallic form. The getter material is also preferably in a dispersed form intermixed with the transition metal material. The getter material is selected from the group including Zr, Ti, Nb, Ta, V, and alloys thereof. The purifier system also includes an inlet valve coupled to the inlet and an outlet valve coupled to the outlet. A heater is associated with the purifier for heating the purifier to at least about 200 degrees Celsius. 
     In yet another embodiment of the present invention, a method for rejuvenating an ambient temperature purifier having a mixture of transition metal material and getter material is provided. The method includes sealing a purifier in a working environment. A mixture of a transition metal material and a getter material are disposed within the purifier chamber. The transition metal material is preferably in a dispersed form with preferably at least 5% of the transition metal material being in metallic form. The getter material is also preferably in a dispersed form intermixed with the transition metal material. The getter material is preferably selected from the group including Zr, Ti, Nb, Ta, V, and alloys thereof. The purifier is heated to at least about 200 degrees Celsius, and then cooled so that the purifier can achieve a substantially ambient temperature of its working environment. Finally, the purifier is unsealed, and ready to be used again. 
     In yet another embodiment of the present invention, a method for purifying gases at ambient temperatures is provided. The method includes providing a purifier having a sealable enclosure. The enclosure defines a chamber having an inlet and an outlet. A mixture of a transition metal material and a getter material are disposed within the purifier chamber. The transition metal material is preferably in a dispersed form with preferably at least 5% of the transition metal material being in metallic form. The getter material is also preferably in a dispersed form intermixed with the transition metal material. The getter material is preferably selected from the group including Zr, Ti, Nb, Ta, V, and alloys thereof. Gases flowing into the inlet are purified through the purifier material. Gas then flows out of the outlet at about ambient temperatures, whereby the transition metal material adsorbs water, oxygen and carbon monoxide and the getter material adsorbs hydrogen. The inlet and outlet are then closed to seal the enclosure. 
     The purifier is heated to at least 200 degrees Celsius, whereby the getter material releases hydrogen. The hydrogen removes oxygen and carbon from the transition metal material. Excess hydrogen is then adsorbed by the getter material. The purifier is then cooled so that the purifier can return to about ambient temperature of its working environment. Finally, the purifier is unsealed by opening the inlet and outlet, rejuvenated for the purification of gases. 
    
    
     An advantage of the present invention is that it provides a gas purification apparatus and method, that adds the capability of rejuvenation to transition metal material based purifiers. Rejuvenation of the purifier increases efficiency and reduces the number of times a purifier must be returned to the manufacturer for servicing. These and other advantages of the present invention will become apparent to those skilled in the art upon reading the following detailed descriptions and studying the various figures and drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings. To facilitate this description, like reference numerals designate like structural elements. 
     FIG. 1A is a schematic illustration of a getter purifier of the prior art during process gas purification at an elevated temperature. 
     FIG. 1B is a schematic illustration of a getter purifier of the prior art during the removal of hydrogen from a process gas. 
     FIG. 2A is a schematic illustration of a nickel purifier of the prior art during process gas purification. 
     FIG. 2B is a schematic illustration of a nickel purifier of the prior art during the process of regeneration. 
     FIG. 3 illustrates a nickel purifier unit of the prior art. 
     FIG. 4 illustrates rejuvenable purifier during the purification of a process gas in accordance with one embodiment of the present invention. 
     FIG. 5 illustrates rejuvenable purifier during the beginning of the rejuvenation process in accordance with one embodiment of the present invention. 
     FIG. 6 illustrates rejuvenable purifier during the completion of the rejuvenation process in accordance with one embodiment of the present invention. 
     FIG. 7 is a flow chart of a method for purifying gases at ambient temperatures. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A method and apparatus for rejuvenating purifiers is provided. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be understood, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process acts have not been described in detail in order not to unnecessarily obscure the present invention. 
     FIGS. 1-3 were discussed with reference to the prior art. FIG. 4 illustrates rejuvenable purifier unit  42  during the purification of a process gas in accordance with one embodiment of the present invention. Rejuvenable purifier unit  42  includes an enclosure  43  defining a chamber  44 , which is coupled to an inlet  46  and an outlet  48 . It should be noted that enclosure  43  can be of similar configuration and construction to enclosure  33  of prior art nickel purifier unit  32  as illustrated in FIG.  3 . Inlet  46  and outlet  48  are preferably equipped with a particle filter  50  and a male face seal fitting or a compression fitting (see FIG.  3 ). Chamber  44  is partially filled with of getter material particles  54  and a number of transition metal material particles  56 . Examples of transition metal material particles include nickel, iron, manganese, and combinations thereof. Transition metal material particles  56  preferably include nickel as described above with reference to FIG.  2 A. 
     Rejuvenable purifier unit  42  may be operated to remove impurities from a process gas. After opening a valve  45  at inlet  46  and a valve  47  at outlet  48 , the process gas to be purified flows into chamber  44  through inlet  46 . When the process gas contacts the sorbing material bed, transition metal material particles  56  remove impurities from the process gas. A purified process gas then leaves the rejuvenable purifier unit  42  through outlet  48 . 
     The portion of FIG. 4 surrounded by broken line  55  illustrates the reactions of getter material particles  54  and transition metal material particles  56  at ambient temperatures (e.g. about 25° C.) in accordance with one embodiment of the present invention. Transition metal material particles  56  absorbs impurities  58 , leaving a purified process gas  60 . Because rejuvenable purifier unit  42  is kept at between about 0° C. to about 50° C., and preferably between about 10° C. to about 40° C., and most preferably at about 25° C., only transition metal material particles  56  will react in this process by absorbing impurities  58 . Getter material particles  54  will not react and remain inactive. 
     FIG. 5 illustrates rejuvenable purifier unit  42  during the beginning of the rejuvenation process in accordance with one embodiment of the present invention. When rejuvenable purifier unit  42  needs to be regenerated (i.e. when transition metal material particles  56  are saturated), valves  45  and  47  are closed and the purifier is heated to between about 200° C. to about 400° C. by a heater  59  so that getter material particles  54  start to release hydrogen. The regeneration temperature is preferably fixed at about 250° C. to about 350° C., and most preferably fixed to about 300° C. to avoid overpressuring the system in the present embodiment. At higher temperatures, the release of hydrogen has a steep rise, so that the pressure in the purifier chamber can rapidly reach values in the range of tens of bars, which can endanger the mechanical stability of the purifier. 
     Heater  59  is preferably external to chamber  44  of enclosure  43 , and may be an electro resistive, radiant, or other type of heater in conductive, radiative, or convective communication with enclosure  43 . Heater  59  is preferably provided with an open or closed loop control system to control the temperature of rejuvenable purifier unit  42 . For a closed loop control system, a thermal sensor (such as a thermocouple—not shown) is used to sense the temperature of the purifier, either directly or indirectly, as feedback to the heater  59  control system. 
     Getter material particles  54  preferably have a high equilibrium pressure at 300° C. The high equilibrium pressure is preferably above about 0.1 mbar at about 300° C. An example of such getter material is a zirconium-cobalt intermetallic compound ZrCo, which has a hydrogen equilibrium pressure above 0.25 mbar at 300° C. When the hydrogen contacts transition metal material particles  56  nickel at elevated temperatures, the hydrogen decomposes the species (impurities removed from the process gas) that have been formed on transition metal material particles  56  during the purification of the process gas. Water released in the reaction may be absorbed by hot getter material particles  54 . On the getter surface, water is decomposed into oxygen and hydrogen. The oxygen will be irreversibly fixed as oxide on the getter, while hydrogen can be released again. 
     The portion of FIG. 5 surrounded by broken line  57  illustrates the reactions of getter material particles  54  and transition metal material particles  56  at about 300 degrees Celsius in accordance with one embodiment of the present invention. Therefore in practice, a sort of “hydrogen cycle” is realized, with hydrogen acting as an “oxygen vehicle” from nickel surface to getter bulk. This “hydrogen cycle” is operative only when the temperature is high enough to have hydrogen not sorbed by the getter (and the purifier is isolated from the gas supply line). The main reactions occurring are: 
     
       
         NiO+H 2 →Ni+H 2 O  Equation 1 
       
     
     
       
         H 2 O+getter→getter-O+H 2 (back to NiO reduction reaction)  Equation 2 
       
     
     FIG. 6 illustrates rejuvenable purifier unit  42  during the completion of the rejuvenation process in accordance with one embodiment of the present invention. After regeneration, heater  58  is switched off. As the temperature drops down to ambient, free hydrogen not used during regeneration of transition metal material particles  56  is readsorbed by the getter material particles  54  leaving a negligible pressure of the gas in the system. Alternatively, rejuvenable purifier unit  42  can be actively cooled, with for example, forced air or a water jacket (not shown). Getter material particles  54  preferably have a low hydrogen equilibrium pressure at ambient temperatures. The low hydrogen equilibrium pressure is preferably about 10 −6  mbars at 40° C. An example of such a getter material is zirconium-cobalt intermetallic compound ZrCo, that has hydrogen equilibrium pressure around 1.3×10 −6  mbar at 40° C. 
     The portion of FIG. 6 surrounded by broken line  61  illustrates the reaction of getter material particles  54  during the completion of the rejuvenation process in accordance with one embodiment of the present invention. When the valves are opened for normal purification operations, no hydrogen is introduced in the outlet gas by the purifier. With hydrogen cycling, a build-up of oxygen takes place in getter material particles  54  that eventually “kill” the capability of the getter to absorb and release hydrogen because of the presence of a thick passivating oxide layer on the getter surface. However, the overall result is that the purifier can stand a certain number of regeneration cycles before getter material particles  54  are exhausted. 
     To assure proper rejuvenation of the purifier, the ratio between the transition metal material and getter material within the purifier chamber must be properly designed. If there is too little getter material, then regeneration will not occur because there is not enough hydrogen available. If there is too much getter material, the extra getter material is wasted. Not only does the additional getter material increase costs, but the getter material also takes up space that could be used by the transition metal material, which does all of the purifying. Tests have shown that it is preferable to use a volume of getter material between about 20% and about 50% of the internal volume of the purifier. 
     The getter material is also preferably pre-charged with hydrogen in production, otherwise, one will rely on the hydrogen intake by the getter during the first purification run. However, such intake (if hydrogen content in the gas under purification is low) might not be enough for assuring good regeneration performance. On the other hand, the pre-charging level mustn&#39;t be too high, or the pressure increase in the purifier during regeneration may be excessive, with the risk of mechanical failure of the enclosure. Preferably, the amount of hydrogen absorbed in the getter bulk should span from about 10 −3  to about 5×10 −3  moles of hydrogen per gram of alloy. 
     As is the case for conventional nickel purifiers, the purifiers of the present invention may contain water sorbing materials. The water sorbing materials may be physical sorbers such as molecular sieves, alumina, silica, etc. or chemical sorbers, and are preferably molecular sieves 13×. It is preferred that the water sorbing material be able to release water at regeneration conditions to ensure that water released by NiO reduction will reach the getter materials giving rise to free hydrogen. This increases the amount of hydrogen taking part in the “hydrogen cycle”, thereby increasing the probability of having effective regeneration operations in the future. If a water sorber is added to the purifier, then the volume of getter material is preferably between about 15% and about 40% of the internal volume of the purifier chamber. 
     As a practical example, a purifier of internal volume 200 cc may include a three-materials mixture comprising particulate ZrCo, nickel supported on porous silica, and molecular sieves 13×(not shown). The sorbing capacity of absorbing beds in this field of art is often measured in liters/liters (l/l), and indicates the liters of gas, as measured at standard temperature and pressure, that can be removed by one litre of the absorbing bed, measured as actual volume occupied by particles of that material. The nickel material bed may have a capacity of 20 l/l for oxygen and a capacity of 15 l/l for water. The molecular sieves 13× have 25 l/l of capacity for water. To assure that the purifier has the same removing capability for oxygen and water during purification, it will preferably contain nickel material and molecular sieves in a volume ratio between of about 5:1, while the volume of ZrCo is preferably between about 35 and 75 cc. 
     FIG. 7 is a flow chart of a method  62  for purifying gases at ambient temperatures. Method  62  begins with an act  64  where a purifier is provided. The purifier is used in an act  66  to purify gases at an ambient temperature by introducing a process gas into the purifier chamber. Impurities in the process gas are removed by transition metal material particles inside the purifier chamber so that a purified process gas may exit the purifier chamber. An act  68  then determines whether there is a need to rejuvenate the purifier. If not, the purification continues with act  66 . If the transition metal material particles have been saturated (“exhausted”), then method  62  proceeds to an act  70 , which determines whether rejuvenation is possible. If rejuvenation is not possible, then a new purifier should be provided in act  64 . 
     It is possible to determine whether rejuvenation is possible in act  68  by a number of techniques. For one, the purifier may be rejuvenated at set time intervals, after a certain amount of gas has been purified, etc. Alternatively, detection equipment at the outlet of the purifier can be used to detect when the trace gases are not being removed by the purifier, indicating that it is time to regenerate or replace the purifier. 
     If rejuvenation is possible, then a method  72  for rejuvenating the purifier begins by sealing the purifier, typically by closing the inlet and outlet valves in an act  74 . The purifier is then heated to about 300 degrees Celsius in an act  76 . At the higher temperature, getter material particles inside the purifier chamber begin releasing hydrogen gas. The hydrogen then decomposes the impurities that formed on the transition metal material particles during purification of the process gas in act  66 . The purifier is then cooled in an act  78 . As the purifier cools back to an ambient temperature, hydrogen that was not used during regeneration is absorbed by the getter material particles. The purifier is then unsealed in an act  80 , and method  62  is ready to return to act  66  to begin the purification of process gases. 
     It will therefore be appreciated that the present invention provides a method and apparatus of purifying gases, and then rejuvenating the gas purifier by utilizing a transition metal material and a getter material in the purifier chamber. The invention has been described herein in terms of several preferred embodiments. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention. Furthermore, certain terminology has been used for the purposes of descriptive clarity, and not to limit the present invention. The embodiments and preferred features described above should be considered exemplary, with the invention being defined by the appended claims.