Abstract:
A system and method is presented for reducing a concentration of ambient air used in a feed stream to form an inerting gas in a pressure swing adsorption system. The method includes introducing ambient air into a pressure swing adsorption system to form an inerting gas, introducing the inerting gas to a large volume of atmosphere, thereby inerting at least a portion of the large volume of atmosphere to form an inerted atmosphere, and removing a portion of the inerted atmosphere and introducing the portion of inerted atmosphere to the pressure swing adsorption system to form the inerting gas, thereby reducing an amount of ambient air utilized to form the inerting gas in the pressure swing adsorption system.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. provisional application No. 61/099,974, filed Sep. 25, 2008 which is hereby incorporated herein by reference, in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to a process and apparatus for providing an inert gas to a large volume atmosphere and, more particularly, to a process and apparatus for reducing a concentration of ambient air used in a feed stream of a pressure swing adsorption system that provides the inert gas to the large volume atmosphere. 
       BACKGROUND OF THE INVENTION 
       [0003]    Fires in areas that are partially or totally confined are extremely difficult to extinguish due to a number of factors including, for example, heat buildup, the availability of fuel and the presence of toxic gases, all of which make delivery of fire suppressant material and extinguishing of the fire difficult. Confined areas include, but are not limited to, for example, large furnaces, autoclaves, storage tanks and subsurface structures such as subway and highway tunnels, underground mines and landfills. In coal mine fires, for example, the abundance of fuel in a confined, poorly accessible area practically guarantees that the fire will burn for extremely long periods of time. Historically, many coal mines are abandoned with the onset of a fire because of the great difficulty in extinguishing the fire. One such example is the Jonesville coal mine fire, which started more than 30 years ago and is still burning. 
         [0004]    Efforts have been made to prevent fires within such confined areas by introducing an inert gas to displace oxygen within the atmosphere of the confined area to eliminate, or at least substantially diminish, a required fuel source for the fire. The use of an inert gas to prevent combustion is well known. Similarly, it is well known to use a pressure swing adsorption (PSA) system for providing an enriched gas to inert an atmosphere of interest. 
         [0005]    As is generally known, a gas mixture may be separated using PSA technology by passing the mixture at an elevated pressure through an adsorbent that is selected in accordance with its capacity to adsorb one or more of the components of the mixture. This selectivity is governed by pore size distribution in the adsorbent and the total pore volume. Accordingly, gas molecules with a kinetic diameter less than or equal to the pore size of the adsorbent are retained, or adsorbed, on the adsorbent while gas molecules of larger diameters pass through the adsorbent. The adsorbent, in effect, sieves the gas according to its molecular size. For example, carbon molecular sieves may be used for the production of enriched nitrogen from air as the carbon molecular sieves have a pore structure with a diameter comparable to the kinetic diameter of oxygen. Accordingly, oxygen is adsorbs by the carbon molecular sieve while nitrogen passes through the sieve. 
         [0006]    While PSA systems are known, there are perceived problems with conventional PSA systems and molecular sieve technology such as, for example, low yield of the enriched product gas, requirements for large volumes of sieve materials to provide effective capacity within the absorbent, inefficient regeneration methods for refreshing the absorbent, and the requirement for costly vacuum and air receiver systems to cycle air within the system. Accordingly, the inventors have discovered that use of a PSA system and, in particular, a PSA nitrogen generator for providing an enriched nitrogen gas to inert a large volume atmosphere such as is defined by, for example, a mine, can be optimized to maximize the efficient of the system and achieve the desired effect, while also reducing operating cost. Moreover, the inventors have discovered that an improved PSA nitrogen generator is an effective tool for preventing fires within confined areas such as, for example, mines 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention resides in one aspect in a method for reducing a concentration of ambient air used in a feed stream to form an inerting gas in a pressure swing adsorption system. The method includes introducing ambient air into a pressure swing adsorption system to form an inerting gas, introducing the inerting gas to a large volume of atmosphere, thereby inerting at least a portion of the large volume of atmosphere to form an inerted atmosphere, and removing a portion of the inerted atmosphere and introducing the portion of inerted atmosphere to the pressure swing adsorption system to form the inerting gas, thereby reducing an amount of ambient air utilized to form the inerting gas in the pressure swing adsorption system. The ambient air comprises a readily adsorbable component gas and a less readily adsorbable component gas, and the inerting gas comprises the less readily adsorbable component gas. In one embodiment, the less readily adsorbable component gas comprises nitrogen. In one embodiment, the large volume of atmosphere is defined by an interior of at least one of a mine, a tunnel, a furnace, an autoclave, a storage tank and a subsurface portion of a landfill, and the inerted atmosphere prevents combustion within the interior. 
         [0008]    Another aspect of the invention resides in a system for reducing an amount of ambient air utilized to form an inerting gas in a pressure swing adsorption system. The system includes at least one pressure swing adsorption system adapted to form an inerting gas from ambient air, a large volume of atmosphere in fluid communication with the pressure swing adsorption system, a conduit for introducing the inerting gas to the large volume of atmosphere to form an inerted atmosphere, and a feedback loop for introducing at least a portion of the inerted atmosphere to the pressure swing adsorption system to reduce an amount of ambient air utilized to form the inerting gas in the pressure swing adsorption system. In one embodiment, in the system the ambient air comprises a more readily adsorbable component gas and a less readily adsorbable component gas, and the inerting gas comprises the less readily adsorbable component gas. In one embodiment, the less readily adsorbable component gas comprises nitrogen. In one embodiment, the large volume of atmosphere is defined by an interior of at least one of a mine, a tunnel, a furnace, an autoclave, a storage tank and a subsurface portion of a landfill, and the inerting gas provides means for preventing combustion within the interior. In yet another embodiment, the pressure swing adsorption system includes at least two adsorbent beds. In the two bed embodiment, a second bed is utilized to provide the inerting gas when a first bed reaches it cycle time and is depressurized so as to cleanse an adsorbent material contained in the first bed such that the inerting gas stream is outputted to continually service the large volume atmosphere of interest during cleansing of adsorption system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a simplified block diagram of a pressure swing adsorption system that generates and provides an inert gas to a large volume atmosphere, in accordance with one embodiment of the present invention. 
           [0010]      FIG. 2  illustrates use of nitrogen membrane technology within the inert gas generating system of  FIG. 1 , in accordance with one embodiment of the present invention. 
           [0011]      FIG. 3  illustrates use of two adsorption beds within the inert gas generating system of  FIG. 1 , in accordance with one embodiment of the present invention. 
       
    
    
       [0012]    In these figures like structures are assigned like reference numerals, but may not be referenced in the description of all figures 
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0013]    The present invention provides a novel system and method for providing an enriched gas stream to inert a large volume atmosphere. In accordance with one embodiment of the present invention, a pressure swing adsorption (PSA) nitrogen generating system  100  provides an enriched nitrogen (N 2 ) gas stream  140  to inert an atmosphere of interest  150 . As shown in  FIG. 1 , a feed gas mixture  110  is passed through an air compressor  120  where it is pressurized to a predetermined adsorption pressure (e.g., a pressure in a range from about 120 psig to 140 psig) and passed under pressure (e.g., as pressurized gas  124 ) to an adsorbent bed  130 . The feed gas mixture  110  includes a gas having a more readily adsorbable component and a less readily adsorbable component. The adsorbent bed  130  selectively adsorbs the more readily adsorbable component from the pressurized gas  124 . In one embodiment, the more readily adsorbable component is oxygen, the less readily adsorbable component is nitrogen, and adsorbent bed  130  includes carbon molecular sieves for adsorbing the oxygen molecules and passing the nitrogen molecules therethrough. 
         [0014]    After a predetermined cycle time, the nitrogen gas passed by the sieve material of the adsorbent bed  130  is provided as the enriched nitrogen gas stream  140  to inert the atmosphere of interest  150 . In one embodiment, the cycle time is based upon oxygen saturation of the adsorbent bed  130 . In one embodiment, the atmosphere of interest  150  includes a large volume atmosphere defined by, for example, an interior portion of a mine, a tunnel, a furnace, an autoclave, a storage tank, a subsurface portion of a landfill and the like, and the nitrogen gas stream  140  is provided to inert the large volume atmosphere to, for example, substantially remove oxygen therein to prevent and/or suppress combustion (e.g., a fire) or a subsequent flare up after a fire is thought to have been extinguished. As can be appreciated, the PSA nitrogen generating system  100  is also employed to maintain a relatively dry, oxygen-free atmosphere within the large volume atmosphere to prevent or minimize oxidizing effects as well as to improved cure times for resins and the like. 
         [0015]    Moreover, while described above as a PSA system, it should be appreciated that in one embodiment nitrogen membrane technology may be employed such that the adsorbent bed  130  is replaced by a container  230  ( FIG. 2 ) having an inlet  232  and an outlet  234 , and including permeable membrane  240  disposed therein. In one embodiment, the permeable membrane  240  is comprised of a plurality of hollow fibers  242 . The pressurized gas  124  (e.g., compressed air) enters the container  230  at the inlet  232  and is passed to a first end  244  of the permeable membrane  240  (e.g., each of the plurality of hollow fibers  242 ). As shown in a detailed portion of  FIG. 2  labeled Detail  2 A, which is an enlarged partial cross sectional view of a portion of a hollow fiber, oxygen (O 2 ) and other components (e.g. carbon monoxide (CO), carbon dioxide (CO 2 ), and the like) pass through holes  246  in a wall of the fibers  242  and exit the pressurized gas  124  stream at, for example, a secondary outlet  236 . The nitrogen N 2  in the pressurized gas  124  travels a length of the hollow fibers, and exits at a second end  248  of the membrane, opposite from the first end  244 . The second end  248  of the membrane is disposed in proximity to the outlet  234  so that the nitrogen N 2  is passed as the enriched gas stream  140 . As shown in  FIG. 2 , the oxygen O 2  in the pressurized gas  124  passes through sidewalls of the hollow fibers and exits through sides of the membrane apart from the enriched gas stream  140 . In one embodiment, a pressurized gas  124  is heated in the permeable membrane  240  to excite the molecules and increase, for example, a rate in which the oxygen and other components permeate out through the holes  246 . In one embodiment, hollow fibers may be purchased from UBE America Inc., Air Products and Chemicals, Inc., or GENERON® brand membranes sold by Generon IGS, Inc. (GENERON is a registered trademark of Generon IGS, Inc.). 
         [0016]    Referring again to  FIG. 1 , once the enriched gas stream  140  is drawn off, the adsorbent bed  130  is depressurized to a predetermined desorption pressure (e.g., typically ambient pressure) such that the more readily adsorbable component may be removed from the adsorbent bed  130  to cleanse the adsorbent bed  130  for a next cycle. In one embodiment, illustrated in  FIG. 1 , the enriched nitrogen gas stream  140  drawn off from the adsorbent bed  130  is passed to a surge or storage tank  160  prior to use in the desired application, such as is described above to inert the large volume atmosphere  150 . 
         [0017]    In accordance with one aspect of the present invention, a feedback loop  170  is employed to draw a portion  152  of atmosphere within the atmosphere of interest  150  back into the feed stream  110  of the PSA system  100 . As shown in  FIG. 1 , the portion of atmosphere  152  passed through the feedback loop  170  is provided at the intake of the air compressor  120  to form or mix with the feed gas mixture  110 . It should be appreciated that by using at least a portion of the atmosphere of interest  150  in the feed stream  110  of the PSA system  100 , oxygen saturation of the adsorbent bed  130  is reduced proportionally to the reduced oxygen content of the atmosphere of interest  150 . As such, the cycle time of the PSA system  100  is increased as oxygen saturation time of the adsorbent bed  130  is extended. By extending the saturation time, and ultimately the process cycle time, work required to refresh the adsorbent bed  130  between cycles is reduced providing a cost savings for inerting an atmosphere of interest and/or maintaining an atmosphere at a target purity. It should also be appreciated that when a combination (e.g., mix) of ambient air and the portion  152  of the atmosphere of interest  150  are provided at the intake of the compressor  120 , it is desirable to minimize a concentration of the ambient air to realize more of the aforementioned extending effects of a feed stream having reduced oxygen content. In one embodiment, illustrated in  FIG. 1 , a buffer  180  (e.g., a storage tank) is disposed in the feedback loop  170 . The buffer  180  is fed by the large volume atmosphere  150 . Accordingly, the feed gas  110  is drawn from the buffer  180  and not directly from the large volume atmosphere  150 . In this way, the inerting process may begin with a higher purity feed gas. 
         [0018]    In accordance with one embodiment of the present invention illustrated in  FIG. 3 , two adsorbent beds  330  and  340  (substantially similar to adsorbent bed  130 ) are utilized. In this two adsorbent bed embodiment, a second bed (e.g., bed  340 ) is utilized to provide the enriched gas stream  140  when a first bed (e.g., bed  330 ) reaches it cycle time and is depressurized so as to cleanse the adsorbent material contained in the first bed. In this two bed embodiment, the enriched nitrogen gas stream  140  outputted by the PSA system  100  continually services the large volume atmosphere of interest  150  even during the cleansing process. 
         [0019]    It should be understood that the present invention is not limited with regard to the number of adsorbent beds employed within the PSA system, nor the size of the large volume atmosphere receiving the enriched gas stream outputted by the novel PSA configuration. Accordingly, although the invention has been described with reference to particular embodiments thereof, it will be understood by one of ordinary skill in the art, upon a reading and understanding of the foregoing disclosure, that numerous variations and alterations to the disclosed embodiments will fall within the spirit and scope of this invention and of the appended claims.