Patent Abstract:
This invention is directed to a three-stage process for recovering and purifying a gas, and a system for using the three-stage process. The steps comprises a) introducing a gas from a chamber to a particulate removing apparatus to form a particulate-free gas, and recycling a first portion of the particulate-free gas to the chamber; b) passing a second portion of the particulate-free gas to a first compressor prior to passing a selective gas purification membrane to form a purified gas and an exhaust gas, and passing the purified gas to mix with the first portion of particulate-free gas to the chamber; and c) passing a third portion of the particulate-free gas to a liquid separator apparatus and a receiver to form a liquid-free gas, and recycling the liquid-free gas to said chamber.

Full Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to the recovery of gases, and more particularly to the recovery of helium gas in the cold spray forming process.  
         BACKGROUND OF THE INVENTION  
         [0002]    Helium is the gas of choice for cold spray forming (CSF) process. However, the use of helium is economically prohibitive without sufficient helium recovery.  
           [0003]    Generally, high velocities are necessary to accelerate the CSF powder towards the work piece. At 5 mole % nitrogen in helium, the sonic velocity will drop 8%. If the nitrogen concentration increases to 20 mole %, then the sonic velocity will reduce 33%. If a heavier impurity such as carbon dioxide reaches 20 mole % with the balance helium, then the sonic velocity will be reduced 43%. High gas velocities possible with pure helium are a desirable physical property regardless of the specific CSF application.  
                                                                   TABLE 1                           Sonic Velocity as a Function of Gas Composition                        Carbon       Sonic       Helium   Nitrogen   Water   Dioxide       Velocity       Mole %   Mole %   Mole %   Mole %   Temp. (F.)   (Ft/sec)                    100   0   0   0   100   3958       95   5   0   0   100   3467       90   10   0   0   100   3120       85   15   0   0   100   2859       80   20   0   0   100   2653       95   0   5   0   100   3645       95   0   0   5   100   3225       90   0   0   10   100   2788       85   0   0   15   100   2489       80   0   0   20   100   2267                  
 
           [0004]    CSF is a newly developed technology that as of this writing has not been made commercial. CSF can be compared to thermal spraying (TS) with a primary difference being the nozzle gas temperature. TS uses particle velocity combined with thermal heat to form a coating on a work piece. A description of both processes will show a problem associated with TS that is solved with CSF and why helium was not used before and is the gas of choice for CSF.  
           [0005]    [0005]FIG. 1 shows a schematic of the equipment enclosure for CSF and TS. One TS application is plasma spray. Passing gas through an electric arc inside nozzle  14  forms the plasma. Thus, for TS, nozzle  14 , must be water cooled or contain refractory to permit high temperatures. The expected life for nozzle  14  is usually less than 100 hours. Gas and powder pass through nozzle  14  to form spray pattern  16 . Typical nozzle gases could be a mixture of argon and hydrogen. In spray pattern  16  the hydrogen will combust to add additional heat to the powder. The powder will partially or completely melt in spray pattern  16  before hitting work piece  18  and forming a coating on work piece  18 . Care must be taken that work piece  18  does not become too hot or the coating applied too thick. If the coating is too hot or applied too thick, then the coating will crack upon cooling. Care must also be taken in selecting the powder particle size. If the particle size is too small, then losses from vaporization will be economically prohibitive. Spray pattern  16  uses gas velocity and density to accelerate the particle at work piece  18 . High temperatures present in spray pattern  16  decrease gas density which minimizes the impact of gas velocity on particle velocity. TS particle velocities of up to 200 m/s could be expected. Helium can provide higher gas velocities but the density would be substantially lower.  
           [0006]    TS may require that a separate fluid be used to cool work piece  18 . The separate fluid could be liquid carbon dioxide or water. Air is also passed through enclosure  12  through gas inlet  22 . A high volume of air passes over work piece  18  and removes excess powder that did not adhere as the coating. The air and powder exhaust from enclosure  12  through gas discharge port  20 . If helium were used in nozzle  14 , then using air to sweep work piece  18  would make helium recovery and purification difficult and expensive.  
           [0007]    CSF differs from TS in that at ambient temperatures the powder can be accelerated with helium to about 1000 to about 1200 m/s in nozzle  14  to work piece  18 . CSF temperatures in nozzle  14 , typically less than about 400° F., allows the use of particulate less than 20 micron in size and containing volatile alloying elements. The high velocities capable with helium give the particles sufficient energy to fuse into a coating when striking work piece  18 . The resulting coating does not cause work piece  18  substrate to change as could happen if it was exposed to TS temperatures. Helium is also passed through inlet  22  to sweep over work piece  18  and remove excess powder. The helium and powder discharge from enclosure  12  through vent  20  to helium recovery and purification equipment. In CSF, helium ultimately serves two functions. One, it accelerates the coating powder, supplying kinetic energy. Two, it serves as a clean sweeping gas to clean the work piece of extraneous particles.  
           [0008]    No known helium recovery system is believed to exist for CSF. The absence of helium recovery systems for CSF is not surprising because current CSF processes are lab scale and use small quantities of helium. However, other processes that use larger volumes of helium have helium recovery systems.  
           [0009]    U.S. Pat. No. 5,377,491 discloses a coolant gas recovery process for a fiber optic cooling tube that uses a vacuum pump/compressor to remove cooling gas from the cooling tube, remove particulate and contaminants and then return the coolant gas to the fiber optic cooling tube. Purification equipment such as pressure swing adsorption, dryer and membrane are discussed with respect to removing water and oxygen, with the maximum quantity of oxygen in the range of 1 to 50 mole percent, and the cooling tube required to cool gas at 0 to 150 psig.  
           [0010]    U.S. Pat. No. 4,845,334 discloses a plasma furnace gas recovery system where the gas exits the furnace at high temperature (˜700C.) and low pressure (&lt;2 psig). The discharge gas is cooled and then followed by particulate removal equipment. The particulate free gas is then compressed, filtered again and then dried. The dry, compressed helium is then recycled back to the furnace at pressure using gas flows and pressures of 150 SCFM and 100 psig via an oil flooded screw machine.  
           [0011]    U.S. Pat. No. 5,158,625 discloses a process for removing helium from a metal hardening (quenching) chamber, purifying the helium and compressing the helium. The quenching chamber was described as 10 M 3  with helium at 2.5 bar absolute (875 SCF of helium). Helium and impurities may be recovered from the hardening furnace through a vacuum pump. Down stream of the vacuum pump the helium plus impurities would be compressed and stored in one receiver. Once all of the desired helium from the hardening furnace was removed, then helium with impurities was passed through a membrane, dryer, PSA or catalytic oxidation of hydrogen to remove oxygen and water from the process. The purified helium is then compressed again and stored at pressure in another receiver until the next hardening cycle starts. The above process uses higher than atmospheric pressures in the quenching chamber to increase the helium density and thus improve the heat transfer capability.  
           [0012]    The prior art does not teach or suggest the recovery and purification system comprised of three continuous loops involving the strategic placement of the purification equipment. Further, each loop has its own separate function. In addition to purification and recovery, the current invention is capable of pressurizing the helium to achieve the requisite sonic velocity.  
         OBJECTS OF THE INVENTION  
         [0013]    It is therefore an object of the invention to provide a cost effective helium recovery system that will provide acceptable helium purity (&gt;80 mole %), volume and pressure at the CSF nozzle and for the cleansing sweep across the work piece.  
           [0014]    It is another object of this invention to provide for a helium recovery system that will remove contaminants such as oxygen, nitrogen, water, carbon dioxide and particulate from the helium.  
         SUMMARY OF THE INVENTION  
         [0015]    This invention is directed to a three-stage process for recovering and purifying a gas. The steps comprises a) introducing a gas from a chamber to a particulate removing apparatus to form a particulate-free gas, and recycling a first portion of the particulate-free gas to the chamber; b) passing a second portion of the particulate-free gas to a first compressor prior to passing a selective gas purification membrane to form a purified gas and an exhaust gas, and passing the purified gas to mix with the first portion of particulate-free gas to the chamber; and c) passing a third portion of the particulate-free gas to a liquid separator apparatus and a receiver to form a liquid-free gas, and recycling the liquid-free gas to said chamber.  
           [0016]    In another embodiment, this invention is directed to a three-stage system for recovering and purifying a gas. This system comprises a) a first stage for introducing a gas from a chamber to a particulate removing apparatus to form a particulate-free gas, and recycling a portion of the particulate-free gas to the chamber; b) a second stage for passing a second portion of the particulate-free gas to a first compressor prior to a selective gas purification membrane to form a purified gas and an exhaust gas, and passing the purified gas to mix with the first portion of particulate-free gas to the chamber; and c) a third stage for passing a third portion of the particulate-free gas to a liquid separator apparatus and a receiver to form a liquid-free gas, and recycling the liquid-free gas to the chamber.  
           [0017]    The second stage may comprise adding helium to mix with the second portion of particulate-free gas prior to passing the second portion of particulate-free gas to the first compressor. The first stage may comprise a circulation unit for circulating the flow of gas. The second stage may comprise a gas analyzer to determine the purity of the second portion of particulate-free gas. The selective gas purification membrane may comprise a membrane selected from helium. The third stage may comprise a second compressor, cooler and a liquid separator apparatus. A recovery unit and an adsorption unit may also be added. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0018]    Other objects, features and advantages will occur to those skilled in the art from the following description of preferred embodiments and the accompanying drawings, in which:  
         [0019]    [0019]FIG. 1 is a schematic representation of the CPF process according to this invention; and  
         [0020]    [0020]FIG. 2 is a schematic representation of recovering the gas use in the CPF process according to this invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    There is no disclosure in the prior art disclosing recycling and purifying helium using the volume and pressure requirements for a CSF helium recovery system. The flow rates for the CSF are substantially different for that known in the art. The present invention has three separate loops that operate continuously. Each loop has a different function. First, the invention uses a fan in Loop A to recycle helium from the CSF chamber through particulate removal and back to the CSF chamber to supply the cleansing sweep (FIG. 2). The flow in Loop A must remove particulate from the chamber. The flow rates in Loop A are preferably 1000 SCFM or more. A portion of the gas cycling in Loop A is removed to supply Loop B and Loop C. The invention uses a compressor to remove the gas from Loop A. The amount of gas removed will depend on the number of nozzles and the purity requirements. If the CSF chamber contains one nozzle that requires helium having a purity of at least 90%, preferably at least 95%, then the flows in Loop B and Loop C are approximately 80 SCFM and 125 SCFM respectively. Loop C uses a compressor to increase the pressure and control the flow of gas to the nozzle.  
         [0022]    One could consider providing the nozzle and cleansing sweep flow from a single compressor in Loop A. Having a compressor would eliminate Loop C. However, the low pressure cleansing sweep flow in this example is approximately eight times that of the nozzle flow. The nozzle pressure is at least 20 times or more than the cleansing sweep pressure. Therefore, the capital cost and the operating cost would be several times the respective costs for a fan and compressor combined as described above in Loops A and C.  
         [0023]    The invention must use purification to maintain the desirable properties of helium and remove impurities that would harm the coating or substrate.  
         [0024]    The invention also can use PSA, TSA, membrane, catalytic oxidation and cryogenic separation to remove impurities. However, any impurity in the helium can become a process-limiting agent for CSF. Therefore, depending on the application, the purification system must remove nitrogen, oxygen, water, carbon monoxide, carbon dioxide, hydrogen and possibly light hydrocarbons. The majority of impurities will occur when parts enter and leave the CSF chamber. Some CSF applications will process larger parts that are placed in the CSF chamber one at a time. As the parts are placed in the CSF chamber, helium will escape and air will enter into the chamber. The same will occur when the parts are removed from the CSF chamber. An evacuation of the chamber before and after the parts are placed in the enclosure would minimize the amount of helium lost and the amount of air that enters into the enclosure. Even though an evacuation of the enclosure would improve helium recovery, the time cycle extension and capital required for vacuum capable equipment may not justify the effort.  
         [0025]    Higher molecular weight impurities will significantly lower the sonic velocity of helium. Higher sonic velocities than heavier gases is one of helium&#39;s several unique physical properties that make it the gas of choice for cold spray forming. A typical commercial application is expected to require helium purities of greater than about 85%, preferably greater than about 90% and most preferably greater than about 95%. Helium recoveries of greater than about 90% (based on the flow in Loop C) are expected.  
         [0026]    Table 2 shows the different purification technologies to meet different gas specifications. The gas specification is dependent on process conditions and materials used for the coating process. For example, Items #1, 10, and 13 are discussed. Item #1 describes a process where equipment allows very little oxygen to leak into the process but requires gas with high purity. A copper oxide getter will effectively remove oxygen to the low PPMV levels. The process of Item #10 shows a process where two different purification technologies located apart from each other provides the most economical purification strategy. The membrane will remove nitrogen and oxygen in Loop B while the TSA will remove water in Loop D.  
         [0027]    In Item #13, the gas specification is less than 2% air, and 10% air was admitted into Loop A when the parts were placed in to the CSF chamber. The CSF process would start with Loop A and Loop B, while Loop C would not start until the gas specification is reached as measured by oxygen analyzer  19 . If oxygen analyzer  19  signals an acceptable level of oxygen, then Loop C starts up and Loop B would continue to increase the helium purity.  
                                                   TABLE 2                           Purification Vs Impurities in CSF Off-Gas                        Gas       Item   Impurities   Type of   Specifications       #   (FIG. 2, #40)   Purifier/Location   (FIG. 2, #88)                    1   10 PPM to 100   Copper Oxide Getter/   &lt;20 PPM O 2             PPM Oxygen   #68   &lt;20 PPM N 2             Only       &lt;20 PPM H 2 O       2   H 2 O Only   TSA/ #54 or #100   &lt;20 PPM O 2                     &lt;20 PPM N 2                     &lt;20 PPM H 2 O       3   10 PPM H 2 O   Copper Oxide Getter&amp;   &lt;20 PPM O 2             10 PPM O 2     TSA/ #13 or #100   &lt;20 PPM N 2                     &lt;20 PPM H 2 O       4   &lt;2 PPM O 2     Cryogenic Adsorption/   &lt;20 PPM O 2             &lt;2 PPM N 2     #100   &lt;20 PPM N 2             &lt;2 PPM H 2 O       &lt;20 PPM H 2 O       5   &lt;4 PPM O 2     Modified Cryogenic           &lt;4 PPM N 2     Adsorption or PSA/ #100           &lt;4 PPM H 2 O           6   &lt;6 PPM O 2     Modified Cryogenic   &lt;20 PPM O 2             &lt;6 PPM N 2     Adsorption or PSA/ #100   &lt;20 PPM N 2             &lt;6 PPM H 2 O       &lt;20 PPM H 2 O       7   &lt;8 PPM O 2     Modified Cryogenic   &lt;20 PPM O 2             &lt;8 PPM N 2     Adsorption or PSA/ #100   &lt;20 PPM N 2             &lt;8 PPM H 2 O       &lt;20 PPM H 2 O       8   &lt;10 PPM O 2     PSA/ #100   &lt;20 PPM O 2             &lt;10 PPM N 2         &lt;20 PPM N 2             &lt;10 PPM H 2 O       &lt;20 PPM H 2 O       9   &lt;2 PPM O 2  1O% N 2     Membrane/ #54   &lt;20 PPM O 2                     1% N 2         1O   &lt;2 PPM O 2  1O% N 2     Membrane and TSA/ #54   &lt;20 PPM O 2             &lt;2 PPM H 2 O   and #100   1% N 2                     &lt;20 PPM H 2 O       11   &gt;2 PPM O 2  10% N 2     Membrane, TSA and   &lt;20 PPM O 2             &lt;2 PPM H 2 O   Copper Oxide Getter/   1% N 2                 #54 and #100   &lt;20 PPM H 2 O       12   &gt;2 PPM H 2 O   Chiller/ #23   &lt;2000 PPM H 2 O       13   10% air at   membrane   &lt;2% AIR           start of coating               cycle           14   &gt;100 PPMV   TSA or PSA/ #100   &lt;1000 PPMV       15   &gt;2 PPMV   TSA or PSA/ #100   &lt;10 PPMV                  
 
         [0028]    [0028]FIG. 2 provides a schematic of the systems of this invention and the process therefore. The CSF process involves applying a coating to a part inside CSF chamber  30 . The chamber geometry will partly depend on size and geometry. In an embodiment of this invention, the part is loaded into the CSF chamber one at a time and one nozzle is used to coat the part. The opening of the enclosure removes the coated part, and releases about 8 cubic feet (CF) of helium and allows 8 CF of air to enter the enclosure. Furthermore, Loop A contains about 80 CF of gas. At start up, fan  42  will draw on CSF chamber  30  and pull gas through duct  32 , valve  38 , particulate removal  38  and ducts  36  and  40 . Fan  42  will discharge into duct  44  at slightly more than 15 psia. A portion of flow in duct  44  will enter duct  46  while the remaining portion will continue past the entrance of duct  46  to the exhaust of duct  56 . Purified helium from duct  56  will enter duct  44  and continue onto CSF chamber  30 . The helium from Loop A will be used to clean sweep the work piece.  
         [0029]    Gas entering duct  46  will feed the suction of compressor  48 . Compressor  48  will discharge into duct  50  at approximately 180 psig. A portion of gas in duct  50  will enter duct  62  and pass through regulator  64  to oxygen analyzer  68 . Oxygen analyzer  68  will sound an alarm if the oxygen content of the gas is above specification. If the gas is above specification, then the operator or software will decide if the coating process should start or continue. If the coating process is not started then the most economical operation of the equipment is to not start Loop C until gas is within specification. However, for alternate equipment configurations where purification occurs in Loop C or Loop D then operation of the compressor in Loop C is needed.  
         [0030]    The remaining portion in duct  50  after passing duct  62  will continue to duct  52  and duct  68 . Duct  52  is the inlet to Loop B. Gas passes through duct  52  to membrane  54 . Retentate leaves the membrane through back pressure regulator  58  to vent  60 . The purified helium leaves the membrane as permeate (low pressure side) through duct  56  and enters duct  44  as described above. Table 3 shows the results when 10% dry air in helium is the feed to a membrane. The permeate stream that will enter duct  56  will have 97.5% pure helium. As the gas from duct  56  mixes with the gas in duct  44 , the impurity concentration will drop.  
                                                                 TABLE 3                       Membrane Simulation 1, 10% Dry Air in Feed                                MOD.   FIBER OD   FIBER ID   ACTIVE   POTTED   AREA       NO.   mils   mils   LENGTH ft   LENGTH ft   ft 2                 1   14.00   7.50   5.667   0.267   5005.1                    CALCULATED PROCESS PARAMETERS                FEED   RAFF   PERM       Stream #   #1   #2   #3               F, MMSCFD (60F)   1   0.08531   0.9147       PRESS, psia   195.00   195.00   16.00       TEMP, F.   108.00   108.00   108.00       Molec. Weight   6.49   26.09   4.66       Viscos, cp   0.0212   0.0190   0.0208       CONCENTRATIONS, mol %       HELIUM   90.0000   10.0000   97.4611       NITROGEN   7.9000   77.9749   1.3646       OXYGEN   2.1000   12.0251   1.1744                                          
 
         [0031]    Table 4 shows that as the feed to the membrane increases in purity, then the gas entering duct  56  will also increase in purity. In addition, as the feed helium purity increases, then the retentate flow rate decreases. Decreasing the retentate flow rate improves helium recovery.  
                                                                 TABLE 4                       Membrane Simulation 2, 10% Air in Feed                                MOD.   FIBER OD   FIBER ID   ACTIVE   POTTED   AREA       NO.   mils   mils   LENGTH ft   LENGTH ft   ft 2                 1   14.00   7.50   5.667   0.267   3752.4                    CALCULATED PROCESS PARAMETERS                FEED   RAFF   PERM       Stream #   #1   #2   #3               F, MMSCFD (60F)   1   0.01178   0.9882       PRESS, psia   195.00   195.00   16.00       TEMP, F.   108.00   108.00   108.00       Molec. weight   4.66   27.63   4.39       Viscos, cp   0.0208   0.0191   0.0207       CONCENTRATIONS, mol %       HELIUM   97.4600   5.0000   98.5619       NITROGEN   1.3600   74.4417   0.4890       OXYGEN   1.1800   20.5585   0.9490                                          
 
         [0032]    Duct  68  starts Loop C. Gas enters Loop C at from about 100 psig to about 270 psig, preferably about 155 psig to about 195, and most preferably about 175 psig and is further compressed by compressor  70  to from about 270 psig to about 1130 psig, preferably from about 300 psig to about 1100 psig depending on the application. Higher pressures will allow for higher velocities at the nozzle in CSF chamber  30 . High pressure gas passes through cooler  72  and water separator  74 , to remove any condensed water. The water separator is placed after the compressor since water will condense first at higher pressures keeping gas temperature the same. For additional water removal the water separator can be augmented with a chiller to lower the gas temperature. Duct  76  delivers gas to receiver  78 , which is sufficiently large to dampen any pulsation in the gas flow that could come from a diaphragm or reciprocating compressor. Gas flows from receiver  78  into duct  80  and through valve  82 , which opens when the operator is ready to start coating the part in CSF chamber  30 . Gas flows into duct  84  through regulator  86 . Regulator  86  ensures that the pressure entering duct  88  is the desired nozzle pressure. The gas from here enters the CSF chamber in order to assist in coating the work piece. The helium gas picks up the powder and speeds it up to supply the kinetic energy required for coating. As discussed in Table 2 the gas specification will depend on the application.  
         [0033]    Helium make-up comes from helium storage  90  through duct  92  and valve  96  into duct  46 . Valve  96  opens when pressure as measured at pressure indicator  94  falls below set point.  
         [0034]    This invention also contemplates placing the membrane in duct  68  and feeding the permeate to the suction of compressor  20 . Putting the membrane in duct  68  is desirable if impurities continuously entered duct  32  from CSF chamber  30 . A continuous supply of parts into CSF chamber  30  by a conveyor belt or some other mechanism would be an example of when impurities would continuously enter duct  32 .  
         [0035]    As the gas specification becomes significantly less than 2% impurities in helium, a pressure swing adsorption or cryogenic adsorption unit may optionally be needed. Duct  98  would take a portion of gas from duct  68  pass it through the pressure swing adsorption unit  100 . The pure helium would then enter duct  102  and mix with impure gas in duct  68 . The mixture of gases from duct  68  and duct  102  would meet the gas specification. Regulator  104  would create the needed pressure drop in duct  68  to force gas through pressure swing adsorption unit  100 . As the level of gas purity specification increases, the percentage of gas that passes through Loop D also increases. For specifications of less than 2 ppm, then the pressure swing adsorption unit or cryogenic adsorption unit would be placed in duct  68 .  
         [0036]    If CSF chamber  30  uses evacuation to recover helium and remove air impurities before a part is ready for the coating process Loop B could be removed and Loop D remain for high purity helium applications. If CSF chamber  30  is evacuated and the gas specification is maintainable by a membrane then Loop B would remain.  
         [0037]    If CSF chamber  30  uses evacuation and oxygen must be kept to low levels, then a copper oxide getter could control the oxygen. A membrane would then maintain other impurities to acceptable levels. Similarly, if oxygen or other chemical content must be controlled to a certain level, such as to control a reaction with the coating, then a getter or catalytic oxidation could be used. An example would be a slight oxide layer on an aluminum coating to passivate the metal. Such a system may use hydrogen and a catalyst to react some part or all of the oxygen. The hydrogen would be maintained in the system to a certain level or hydrogen would be introduced into the system at particular level of oxygen. The water from the hydrogen, oxygen reaction would then be removed in separator  74 .  
         [0038]    Flow rate in recovery systems described above assumed one nozzle in the CSF chamber. However, a particular application may have several nozzles in one chamber with each requiring more than about 100 SCFM of helium. For multi-nozzle applications the flows will significantly increase over what was described above.  
         [0039]    Specific features of the invention are shown in one or more of the drawings for convenience only, as each feature may be combined with other features in accordance with the invention. Alternative embodiments will be recognized by those skilled in the art and are intended to be included within the scope of the claims.

Technology Classification (CPC): 2