Abstract:
A method and apparatus for operating a pressure swing adsorption process is disclosed that may utilize only a single adsorption stage yet still produce a continuous stream of a concentrated fluid. A portion of the enriched fluid produced during the adsorption cycle in an adsorption chamber is used to purge the adsorption chamber.

Description:
This application is a continuation-in-part application of application Ser. No. 09/240,618, filed on Feb. 1, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a method and apparatus using a single adsorption zone for producing an enriched stream of a first gas from a stream containing the first gas and at least one second gas. In one embodiment, the method and apparatus may be used to obtain a concentrated stream of oxygen from air. 
     BACKGROUND OF THE INVENTION 
     Various different methods have been developed for separating gases and producing a concentrated stream of a selected gas. One particular method which has been used in industry is the pressure swing adsorption process. Generally, these processes use an adsorbent which, under elevated pressure conditions, preferentially adsorbs a targeted gas over other gases present in a gas stream. Accordingly, the adsorbent could be selected to preferentially adsorb an undesirable gas from a gas stream thereby leaving a gas stream having an increased concentration of the gases remaining in the gas stream. An example of such a process would be the use of a pressure swing adsorption process to produce an oxygen enriched air stream. The adsorbent would be selected to preferentially adsorb nitrogen over oxygen. Thus, after the adsorption process is conducted, the pressurized air in contact with the adsorbent contains a higher percentage by volume of oxygen. This oxygen enriched air may then be vented from the adsorption chamber and the adsorbent purged (at reduced pressure conditions) to remove the adsorbed nitrogen. Alternately, such a process may be used to preferentially adsorb a targeted gas (e.g. oxygen) thereby also producing an enriched stream of oxygen. 
     Various different processes have been designed to utilize the selective adsorption ability of zeolite. Examples of these include, Bansal (U.S. Pat. No. 4,973,339), Stanford (U.S. Pat. No. 4,869,733) and Haruna et al (U.S. Pat. No. 4,661,125). 
     The process and apparatus of Bansal, Stanford and Haruna et al each utilize two adsorption chambers. The use of two adsorption chambers is undesirable as it unnecessarily complicates the apparatus since it requires additional valving and control means to cycle each adsorption bed through a pressurization cycle and a purging cycle. Further, this adds to the cost of the apparatus and decreases the reliability of the apparatus. 
     Other disadvantages of existing designs is the requirement to use expensive valve control means. In particular, solenoids are frequently required to switch the adsorption chamber from a pressurization mode to a purging mode. These controls are expensive and also prone to failure after extensive use. 
     Further, existing designs utilize electronics (e.g. micro-processors) to control the cycling of the adsorption chamber. This adds to the cost of the equipment and also requires an electrical power source to operate the process. Further, the electronic components may be damaged in harsh environments and this limits the applications of some existing designs. 
     SUMMARY OF THE INVENTION 
     In accordance with the instant invention, a portion of the enriched fluid produced in an adsorption chamber is used to purge the adsorbed fluid from the adsorption chamber. Thus only a single source of motive force (i.e. the source for pressurizing the adsorption chamber for operating the adsorption cycle) is required. According, the construction of the contactor is simplified and the reliability of the unit may be increased. 
     To this end, in accordance with the instant invention there is provided a fluid concentrator for obtaining an enriched stream of a first fluid from a fluid stream containing the first fluid and at least one second fluid, the concentrator comprising: 
     (a) an adsorption chamber having an inlet for introducing the fluid stream to the adsorption chamber, the adsorption chamber operable to produce the enriched stream during a first cycle and the adsorption chamber having an outlet for venting the enriched stream from the adsorption chamber; 
     (b) a pressurizable storage chamber positioned downstream from the adsorption chamber and in flow communication with the adsorption chamber for receiving at least a portion of the enriched fluid stream; 
     (c) at least one passageway connecting the adsorption chamber and the storage chamber in flow communication through at least one valve to provide a first flow rate of the enriched fluid in the downstream direction and a second flow rate of the enriched fluid upstream to the adsorption chamber; 
     (d) an enriched fluid outlet in flow communication with at least one passageway for delivering a portion of the enriched fluid stream downstream of the concentrator; and, 
     (e) a purge valve in flow communication with the inlet of the adsorption chamber and moveable between a closed position and an open position in which the adsorption chamber is purged during a purging cycle 
     whereby during the first cycle the enriched fluid stream travels in the downstream direction and during the purging cycle a portion of the enriched fluid stream travels in the upstream direction into the adsorption chamber. 
     The first flow rate may be greater than the second flow rate or, alternately, the second flow rate may be greater than the first flow rate. 
     In one embodiment, the fluid stream is at an elevated pressure when introduced to the adsorption chamber and the elevated pressure of the fluid stream provides essentially the only motive force to operate the concentrator. 
     In one embodiment, wherein the concentrator operates on a pressure differential in the adsorption chamber of 5 to 15 psig. 
     In another embodiment, the at least one valve comprises a venting valve moveable automatically from a first position in which the venting valve restricts the venting of the enriched fluid from the adsorption chamber into the passageway to a second position in which the venting valve vents the enriched fluid from the adsorption chamber into the passageway at an increased rate when the pressure in the adsorption chamber reaches a preset level. 
     In another embodiment, the at least one valve seals the adsorption chamber from the at least one passageway when the differential pressure between the adsorption chamber and the passageway is less than a preset level. 
     In another embodiment, the venting valve moves automatically from a first position in which the venting valve restricts the venting of the enriched fluid from the adsorption chamber into the passageway to a second position in which the venting valve vents the enriched fluid from the adsorption chamber into the passageway at an increased rate when the pressure in the adsorption chamber reaches a first preset level. Preferably, the venting valve seals the adsorption chamber from the passageway when the differential pressure between the adsorption chamber and the passageway is less than the first preset level. Further, the venting valve is preferably automatically moveable to a third position in which the venting valve vents enriched fluid from the passageway to the adsorption chamber when the pressure differential between the adsorption chamber and the passageway is less than a second preset level. 
     The storage chamber may be part of the passage way. Alternately, the storage chamber and the outlet may be separately in flow communication with the venting valve. The storage chamber is preferably drivingly connected to the purge valve (eg. by a mechanical linkage) whereby the storage chamber automatically causes the purge valve to be moved to the open position when the pressure within the storage container reaches a preset pressure. The storage chamber may be expandable due to the pressure of the enriched fluid provided thereto to automatically commence the purge cycle when the storage container reaches a preset pressure. 
     The enriched fluid outlet may have a flow restrictor associated therewith (eg an aperture or it may be a narrower diameter passage) whereby the pressure within the storage chamber is pressurized when the adsorption chamber vents enriched fluid into the passageway. 
     In accordance with the instant invention there is also provided a pressure swing adsorption apparatus for producing an enriched fluid stream of a first fluid from a stream containing the first fluid and at least one second fluid, the apparatus including: 
     (a) reversible adsorption means for reversibly adsorbing the at least one second fluid to produce the enriched fluid stream; 
     (b) pressurizable storage means for receiving and storing a portion of the enriched fluid stream; 
     (c) outlet means in flow communication with the adsorption means for venting a portion of the enriched fluid stream from the apparatus; 
     (d) valve means for alternately providing a first flow rate of the enriched fluid stream downstream from the adsorption means to pressurize the pressurizable storage means, and a second flow rate of the enriched fluid stream upstream into the adsorption means to purge the adsorption means; 
     (e) purging means for removing at least a portion of the at least one second fluid from the reversible adsorption means when the valve means is providing the second flow rate of the enriched fluid stream into the adsorption means. 
     In accordance with the instant invention there is also provided a method for producing an enriched fluid stream having an increased concentration of a first fluid from a fluid stream containing the first fluid and at least one second fluid comprising the steps of: 
     (a) the step of introducing the fluid stream into a vessel containing a member for adsorbing the at least one second fluid; 
     (b) the step of pressurizing the vessel for a time sufficient for the member to adsorb at least a portion of the at least one second fluid to produce the enriched fluid stream; 
     (c) the step of venting enriched fluid at a first flow rate from the vessel; 
     (d) the step of using a portion of the enriched fluid vented from the vessel to pressurize a pressurizable storage container; and, 
     (e) the step of purging the vessel with enriched fluid stored in the pressurizable storage container. 
     In one embodiment, the method further comprises the step of introducing a pressurized fluid stream into the vessel whereby the pressure of the fluid stream is sole motive force for operating the method. 
     In another embodiment, the method further comprises the step of automatically venting enriched fluid from the vessel when the vessel reaches a first preset pressure. 
     In another embodiment, the method further comprises the step of automatically purging the vessel when the pressurizable storage container reaches a second preset pressure. 
     In another embodiment, the method further comprises the step of automatically providing enriched fluid from the pressurizable storage container to the vessel when the pressurizable storage container reaches the second preset pressure. 
     In another embodiment, the method further comprises the step of expanding the storage container as the storage container is pressurized. 
     An advantage of the instant invention is that the expansion of the storage container (i.e. the reservoir for storing enriched gas) may be used to actuate the purging cycle when the storage container expands to a desired level. Accordingly, an electronic controller is not required to time the process. Further, no gas sensors are required to determine when to actuate a particular part of the cycle of the adsorption chamber. 
     A further advantage of the instant invention is that the storage container may be drivingly linked to the purging valve. In this embodiment, simple actuation means may be used to move the purging valve to the open position so as to initiate the purging cycle. Accordingly, solenoids and other complicated switching apparatus are not required. 
     Further, the storage container may be operatively connected to the purge valve by mechanical linkages and, in addition, the purge valve may be a simple mechanical valve (e.g. a seat valve). Accordingly, no electrical power supply is required to initiate the purging cycle. 
     It will be appreciated that, according to the instant invention, a concentrator, and in a preferred embodiment an oxygen concentrator, may be designed wherein a source of pressurized gas (eg. air) which is fed to the adsorption chamber is the driving source of the entire apparatus. Accordingly, the resultant device, which uses only an external motive force, may be manufactured as a lightweight reliable unit. 
     In accordance with the instant invention, the apparatus may be designed to trigger the end of the purging cycle and thereby commence the pressurization (adsorption) cycle as the expandable storage container contracts to a pre-determined position. This position may be pre-determined based upon the volume of the adsorption chamber and the time required to complete the purging cycle as well as the flow rate of enriched gas from the reservoir. 
     Further, the apparatus is energy efficient since the timing of the cycles is based upon the actual completion of a cycle (i.e. the contraction of the expandable reservoir) as opposed to an electronic timing means which would initiate a cycle regardless of the concentration of the enriched gas exiting the apparatus. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other advantages of the instant invention will be more fully and particularly understood in connection with the following description of a preferred embodiment of the invention in which: 
     FIG. 1 is a view illustrating in diagrammatic form an apparatus according to the instant invention wherein the adsorption chamber is supplying enriched gas to the container; 
     FIG. 2 is a further view illustrating in diagrammatic form the apparatus of FIG. 1 wherein the apparatus has commenced the purging cycle; 
     FIG. 3 is a schematic diagram of an alternate purge valve; 
     FIG. 4 is a view illustrating in diagrammatic form an alternate embodiment of the instant invention wherein the apparatus has commenced the adsorption cycle; 
     FIG. 5 is a view illustrating in diagrammatic form the apparatus of FIG. 4 wherein the apparatus has commenced the supplying of enriched gas; 
     FIG. 6 a  is a view illustrating in diagrammatic form the apparatus of FIG. 4 wherein the apparatus has commenced the purge cycle; 
     FIG. 6 b  is a more detailed view illustrating in diagrammatic form the valve between the adsorption chamber and the passageway of FIG. 6 a;    
     FIG. 7 a  is a cross-sectional view of an alternate embodiment of the valve between the adsorption chamber and the passageway of FIG. 4 when the concentrator is in the adsorption cycle; 
     FIG. 7 b  is a cross-sectional view of the valve of FIG. 7 a  when the concentrator is supplying oxygen enriched air; 
     FIG. 7 c  is a cross-sectional view of the valve of FIG. 7 a  when the concentrator is in the purge cycle; 
     FIG. 8 a  is a cross-sectional view of another embodiment of the valve between the adsorption chamber and the passageway of FIG. 4 when the concentrator is in the adsorption cycle and is supplying oxygen enriched gas; and, 
     FIG. 8 b  is a cross-sectional view of the valve of FIG. 8 a  when the concentrator is in the purge cycle. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Concentrator  10  comprises inlet passage  12 , outlet passage  14 , adsorption chamber  16  and container  18 . Passageway  20  extends between adsorption chamber  16  and container  18 . Concentrator  10  is provided with purge valve  22  and valve  24 . 
     Adsorption chamber  16  may be of any particular construction which is known in the art for pressure swing adsorption apparatus. In the preferred embodiment, the adsorption chamber  16  comprises a vessel distinct from container  18  that may be subjected to an increased pressure during which a selected fluid is adsorbed into adsorption media provided in adsorption chamber  16  leaving a fluid having an increased concentration of the remaining (unadsorbed) fluids in adsorption chamber  16 . It will be appreciated that adsorption chamber  16  may comprise a bed containing the adsorbent material through which the fluid flows as it passes through adsorption chamber  16 . 
     The fluid may be a liquid or a gas. If the fluid is a liquid, then concentrator  10  may be used, for example, to selectively remove an impurity from a liquid stream (eg. the fluid selectively adsorbed into carbon) such as water or a pesticide. The concentrator may also be used for pressure swing fractional distillation. 
     In a preferred embodiment, the fluid is a gas and, more preferably, the concentrator is an oxygen concentrator. The following description is based upon the use of concentrator  10  as an oxygen concentrator; however, the concentrator may be used for other pressure swing operations of fluids. 
     If concentrator  10  is an oxygen concentrator, then the feed gas which is introduced into adsorption chamber  16  via inlet passage  12  comprises an oxygen containing gas and, more preferably, air. The adsorbent material in adsorption chamber  16  accordingly comprises a material which selectively adsorbs nitrogen (the largest constituent of air) thereby leaving air containing an enriched level of oxygen in adsorption chamber  16 . Such adsorbent material are known in the art. Examples of such material are zeolites and, in a particularly preferred embodiment, the adsorbent is clinoptilolite. 
     It will be appreciated that, in an alternate embodiment, the desired product may be the fluid adsorbed onto the adsorbent media. In such a case, the product exiting purge valve  22  could be fed to a container or other apparatus as may be desired. 
     The remaining part of this description of the preferred embodiment is premised upon concentrator  10  including an adsorbent to remove nitrogen from air thereby producing a stream of oxygen enriched air. It is to be understood that the feed gas stream fed to adsorption stream  16  may comprise at least any two gases and the adsorbent material may be selected to adsorb the one or more of such gases leaving a gas stream having an enhanced concentration of the remainder of such gases. 
     Adsorption chamber  16  operates under pressure. Accordingly, means must be provided to raise adsorption chamber  16  to the desired pressure. In the preferred embodiment, the air fed to inlet passage  12  comprises a stream of pressurized air (eg. at a pressure of 3 to 30 psig, preferably from 5 to 15 psig). It will be appreciated that, in an alternate embodiment, a compressor or other means may be provided as part of apparatus  10  to feed an air stream into adsorption chamber  16  and to pressurize adsorption chamber  16  to the required pressure. The exact pressure which is required for the adsorption media to adsorb the targeted gas, and the length of the adsorption cycle, will depend on the thermodynamics of the adsorption media. 
     Valve  24  is a venting valve which is provided in passageway  20  to alternately (i.e. cyclically), connect adsorption chamber  16  and container  18  in flow communication and to then isolate adsorption chamber  16  from container  18 . Valve  24  may be any valve which is operable to provide a first flow rate of the enriched fluid from adsorption chamber  16  to container  18  and a second flow rate of the enriched fluid from container  18  to adsorption chamber  16 . Preferred examples of valve  24  are shown in FIGS. 4,  7   a  and  8   a.  Further, valve  24  may be positioned at any point between the two vessels. For example, valve  24  could be positioned as part of outlet port  26  of adsorption chamber  16 . Further, if adsorption chamber  16  and container  18  are a single unit separated by a wall (see FIG.  4 ), valve  24  may be positioned in the wall. 
     While adsorption chamber  16  is undergoing the adsorption portion of the cycle, adsorption chamber  16  is sealed sufficiently such that adsorption chamber  16  will be raised to the required pressure. It will be appreciated that valve  24  may allow some gas to exit therethrough so as to provide a more continuous flow of gas through outlet passage  14  (see for example valve  24  shown in FIG. 8 a ). Preferably, no gas flow out of adsorption chamber  16  is permitted during this part of the method (see for example valve  24  in FIG.  4 ). Accordingly purge valve  22  is preferably in the fully closed position shown in FIG.  1  and valve  24  is preferably in the fully closed position shown in FIG.  4 . Adsorption chamber  16  is thus isolated so as to allow pressure to build up therein. Due to the inflow of air through inlet passage  12 , pressure will build up in adsorption chamber  16  and nitrogen will be adsorbed in the adsorbent media. 
     Based upon the volume of adsorption chamber  16 , the adsorption characteristics of the adsorbent in adsorption chamber  16  and the rate of air input into adsorption chamber  16 , the length of time required to achieve the desired concentration of oxygen in the free gas in adsorption chamber  16  may be calculated. Further, a person skilled in the art will be able to determine the pressure at which this desired oxygen concentration will be achieved. 
     Any valve mechanism (either mechanically or electrically operated) may be used for valve  24 . Preferably, valve  24  is a pressure actuated member which will open to bring adsorption chamber  16  into flow communication with container  18  when adsorption chamber  16  reaches the pressure at which the required oxygen concentration will have been achieved (eg. a check valve). One advantage of this approach is that valve  24  may open, and the adsorption cycle therefore terminate, when a desired preset pressure is achieved. Thus no sensors are required to monitor the progress of the adsorption cycle. The adsorption cycle automatically terminates when the requisite pressure is reached. Preferably, valve  24  is a mechanical member which is biased (eg. by a spring  80 ) to the closed position and which will open when the pressure upstream thereof (i.e. in adsorption chamber  16  or in passageway  20 ) reaches a preset pressure (which may be the pressure at which the desired concentration of oxygen is achieved in the free gas in adsorption chamber  16 ). 
     When valve  24  is in the open position as represented in FIG. 1, oxygen enriched air will pass from adsorption chamber  16  through passageway  20  and into container  18 . 
     Outlet passage  14  is in flow communication which container  18 . In the preferred embodiment of FIG. 1, when valve  24  is in the open position, outlet passage  14  is also, indirectly, in flow communication with adsorption chamber  16 . When valve  24  opens, container  18  expands so as to receive at least a portion of the oxygen enriched air which exits adsorption chamber  16 . Accordingly, outlet passage  14  provides a flow of oxygen enriched air when valve  24  is open and the oxygen enriched air is passing into container  18 . Further, when the purge cycle commences, valve  24  opens in the other direction (see FIG. 2) such that enriched air may be passed through valve  24  to purge or assist in purging adsorption chamber  16 . Further, container  18  preferably will also have by then stored a sufficient supply of oxygen enriched air so that outlet passage  14  may still provide a flow of oxygen enriched air even while adsorption chamber  16  is undergoing a purge cycle. It will be appreciated that, in an alternate embodiment, chamber  16  and container  18  may be connected by two passageways with a check valve positioned in each passageway (one to permit flow downstream from chamber  16  to container  18  and the other to permit flow upstream from container  18  to chamber  16 ). Thus valve  24  may be replaced by two check valves. 
     As shown in FIG. 1, outlet passage  14  may have an aperture  28  which is open at all times when concentrator  10  is in operation. Aperture  28  is preferable of a pre-set opening size so as to provide a generally continuous flow of oxygen enriched air through outlet passage  14 , at least while container  18  is being charged with enriched air. It will also be appreciated that, if desired, aperture  28  may have a variable opening size so as to vary the flow rate of oxygen enriched air through outlet passage  14 . Further, aperture  28  may be operable so as to seal outlet passage  14  (or alternately a valve to close outlet passage  14  may be provided). This may be desirable if, for example there is back pressure from downstream equipment. 
     Container  18  and aperture  28  may be sized so that outlet  14  provides a continuous flow of oxygen enriched air during both the adsorption cycle (i.e. while adsorption zone is being pressurized and while container  18  is being charged with enriched air) and the purge cycle of adsorption chamber  16 . To this end, outlet passage  14  preferably has a reduced flow rate of gas therethrough than the flow rate of air into adsorption chamber  16  via inlet passage  12 . Preferably, the flow rate of gas through outlet passage  14  is about half that of the flow rate into inlet passage  12 . This flow rate may be achieved by, for example, selecting the cross sectional area of outlet passage  14  or including a flow restriction, such as aperture  28 , in outlet passage  14 , to achieve this result. It will be appreciated that two or more adsorption chambers  16  may be connected to one or more containers  18 . A regulator could also be used to control the output rate from port  14 . 
     Container  18  may be any storage container for storing a portion of the enriched air under pressure. This may be achieved by having a sufficient flow restriction on outlet passage  14  to cause pressure (and therefore gas) to build up in container  18 . Alternately, or in addition, container  18  may have an expandable reservoir  30  for storing at least a portion of the oxygen enriched gas produced in adsorption chamber  16 . In one preferred embodiment, container  18  may be a storage vessel having expandable walls. In this embodiment, container  18  may have at least one wall which will expand when container  18  is subjected to an increased pressure. For example, one or more of the walls of container  18  may be composed of an elastomeric material. Alternately, in another preferred embodiment, container  18  may have a flexible side wall which is movable between a first, compacted portion and a second expanded position when reservoir  30  is filled with oxygen enriched air. For example, container  18  may be in the shape of a bellows. 
     Alternately, or in addition, container  18  may comprise a vessel having a movable member mounted therein and movable between a first position and a second position. The size of reservoir  30  increases as the movable member moves from the first position to the second position. An example of such a construction is shown in FIGS. 1 and 2. In these Figures, the movable member comprises piston  32 . 
     In the embodiment of FIGS. 1 and 2, container  18  is a longitudinally extending member and, is preferably vertically oriented. Piston  32  may be movably mounted in container  18  by any means known in the art. Further, piston  32 , and the means for movably mounting piston  32  in container  18 , preferably isolate reservoir  30  from upper portion  34  of container  18 . In this way, the oxygen enriched gas which enters container  18  will remain in reservoir  30  instead of passing upwardly by piston  32  to upper portion  34 . Piston  32  preferably moves upwardly into upper portion  34  of container  18  due solely to a pressure of the oxygen enriched stream passing through valve  24 . Piston  32  may be so mounted by a plurality of O-rings  36  which are positioned between piston  32  and side wall  38  of container  18 . The O-rings, in conjunction which piston  32  seal reservoir  30  from upper portion  34 . However, it will be appreciated that other means, such as a bellows, bearings or a cam may be used to movably mount piston  32 . 
     When the adsorption cycle of adsorption chamber  16  is completed, valve  24  will preferably open from the fully closed position allowing a stream of oxygen enriched air to pass into container  18  (or, if valve  24  allows some air to pass therethrough during the adsorption cycle, allowing an increased flow rate of oxygen enriched air to pass into container  18 ). At this time, piston  32  may be in approximately the position shown in FIG. 1 (i.e. in a contracted position towards the bottom of container  18 ). Oxygen enriched air will enter container  18  and pass through aperture  28  through outlet passage  14 . However, as the flow of oxygen enriched air through aperture  28  is restricted, pressure will build up in container  18  which will force piston  32  into upper portion  34 . As additional oxygen enriched air enters container  18 , the pressure will be maintained in container  18  and piston  32  will continue to move into upper portion  34 , for example until the position shown in FIG. 2 is reached. 
     As oxygen enriched air exits adsorption chamber  16 , the pressure in adsorption chamber  16  will decrease. For example, the pressure in adsorption chamber  16  may reach 20 to 30 psig at the end of an adsorption cycle. As the pressure is reduced, nitrogen will commence being released by the zeolite thus decreasing the concentration of oxygen in the air exiting adsorption chamber  16 . At this time, it is desirable to purge the zeolite in adsorption chamber  16 . Advantageously, in one embodiment of the instant invention, the purge cycle may be commenced automatically at the end of the charge cycle. 
     At the end of the purge cycle, the pressure in adsorption chamber  16  may have been reduced to a pressure from about atmosphere to about 5 psig. Thus concentrator  10  may operate with a pressure swing between the peak pressure of the adsorption cycle and the low pressure of the purge cycle of 5 to 15 psig and, preferably 10 psig. 
     In particular, apparatus  10  may include an actuator which drivingly connects the container (e.g. piston  32 ) to purge valve  22  whereby movement of piston  32  from a first contracted position (as shown in FIG. 1) to a second expanded position (as shown in FIG. 2) actuates the purge valve to move it to the open position. As the flow rate of oxygen enriched air through aperture  28  may be predetermined and as the volume of container  18  is predetermined, a person skilled in the art may determine the distance through which piston  32  will travel as the oxygen enriched air exits adsorption chamber  16 . By designing container  18  so as to permit piston  32  to move this distance, piston  32  may be in the upper position shown in FIG. 2 when the oxygen enriched air has been vented from adsorption chamber  16  and the pressure in adsorption chamber  16  has been reduced to a point wherein it is desirable to purge adsorption chamber  16 . 
     Preferably, piston  32  is drivingly connected to purge valve  22  so as to actuate purge valve  22  when piston  32  is in the upper position shown in FIG.  2 . At that time, purge valve  22  will be in the open position allowing air to exit therethrough (as shown in FIG.  2 ). When purge valve  22  opens, the pressure in passage way  20  will drop to a sufficient degree such that valve  24  (which is preferably pressure operated) will open to permit enriched air in reservoir  30  to flow into adsorption chamber  16  to purge nitrogen from adsorption chamber  16  (see FIG. 6 b ). 
     The driving connection between piston  32  and purge valve  22  may be either mechanical or electrical but is preferably mechanical thus permitting automatic actuation of the purge cycle without the need for any electronic controls. As shown in FIGS. 1 and 2, extension member  40  extends upwardly from upper surface  42  of piston  32 . Movable arm  44  is fixedly mounted to extension member  40 . Accordingly, movable arm  44  moves longitudinal with respect to container  18  as piston  32  moves longitudinally within container  18 . When piston  32  is in the contracted position shown in FIG. 1, moveable arm  44  is positioned adjacent surface  46  of container  18  and, when piston  32  is in the expanded position, as shown FIG. 2, moveable arm  44  is spaced a distance from surface  46  of container  18 . 
     Movable arm  44  may be mechanically linked to purge valve  22  such as by connector member  48 . Connector member  48  comprises a mechanical linkage which extends from movable arm  44  to purge valve  22 . If container  18  extends vertically, then connector member  48  may have a first horizontal portion  50  and a second vertical portion  52  extending downwardly from the end of horizontal portion  50  distal to moveable arm  44 . Connector member  48  has a first end  54  which is operatively connected to purge valve  22  and a second end  56  which is positioned to engage and be actuated by movable arm  44 . 
     If container  18  is vertically disposed, then connector member  48  may be operatively engaged by movable arm  44  so as to move first end  54  upwardly as piston  32  moves upwardly and to move first end  54  downwardly as piston  32  moves downwardly. This may be achieved by having at least a first arm  58  provided on second end  56 . As container  18  expands, piston  32  moves upwardly. At some point, movable arm  44  will engage first arm  58 . Further movement of piston  32  will cause first arm  58  to move upwardly (due to its engagement with movable arm  44 ). As moveable arm  58  moves upwardly, purge valve  22  is moved to the open position. When purge valve  22  has been opened a sufficient amount of time, valve  24  opens in the opposite direction and adsorption chamber  16  will be purged. During this purging cycle, piston  32  will move downwardly into reservoir  30  thereby forcing oxygen enriched air through aperture  28  (if it remains open) and through valve  24  to adsorption chamber  16 . The movement of piston  32  may be due to the pressure of gravity (if container  18  is vertically disposed). In addition, or alternately, a biasing member, such as spring  62  may urge piston  32  downwardly to the contracted position. 
     As air exits reservoir  30 , piston  32  will move downwardly and, accordingly, arm  44  will move downwardly. If purge valve  22  is a vertically operable valve, then purge valve  22  may cause first arm  58  to move downwardly in conjunction with movable arm  44  (such as by the force of gravity and/or a biasing means urging purge valve  22  to the closed position) thus closing purge valve  22 . Alternately, or in addition, second end  56  may have a second arm  60 . In this embodiment, vertical portion  52  is a generally non-compressible member (eg. a rod) and as piston  32  moves downwardly, movable arm  44  will engage second arm  60  thereby driving first end  54  downwardly so as to close purge valve  22 . 
     Preferably, connector member  48  moves essentially only due to movable arm  44  pushing up longitudinally outwardly on first arm  58  and longitudinally inwardly on second arm  60 . Further, arms  58  and  60  are preferably spaced apart. In this way, piston  32  will move upwardly a defined amount before causing purge valve  22  to open thus allowing reservoir  30  to be filled a pre-set amount before the purge cycle commences. Further, piston  32  may move downwardly by a preset amount until it engages second arm  60  thereby closing purge valve  22  and completing the purging cycle. The distance between the arms is preferably sufficient to allow the purging cycle to be conducted while piston  32  is still pumping air from reservoir  30 . In a particularly preferred embodiment, by the time piston  32  is in the contracted position shown in FIG. 1, purge valve  22  has been closed for a sufficient amount of time to allow adsorption chamber  16  to have reached the requisite pressure to have produced an oxygen enriched stream and to cause valve  24  to open in the charging direction (i.e. the direction to introduce enriched air into reservoir  30  from adsorption chamber  16 . 
     In this embodiment, it may be seen that the actuator for purge valve  22  is a mechanical linkage comprising member  40 , movable arm  44  and connector  48 . Purge valve  22  is accordingly actuated by vertical movement of piston  32 . Purge valve  22  is preferably a mechanical valve that is moved to the open position by vertical motion of connector  48 . An example of such a valve is a seat valve which is lifted upwardly by upward motion of first end  54 . However, other valves, such as a gate valve or a ball valve which may be opened to an open position by vertical motion of first end  54  may be utilized. 
     In an alternate embodiment, arms  44 ,  58  and  60  may define electrical connections and contact between arms  44  and  58  may actuate a circuit to open purge valve  22  and connection between arm  44  and arm  60  may consequentially close the circuit to close purge valve  22 . For example, vertical portion  52  may be composed of a bimetal member or a muscle wire which contracts when heated. When movable arm  44  contacts first arm  58 , an electrical connection may be made causing an electrical current to flow through vertical portion  52  thereby heating the vertical portion and causing it to contract. This contraction will cause purge valve  22  to open. When the electrical connection is broken (i.e. arm  44  is no longer in contact with first arm  58  or alternately the circuit is broken when movable arm  44  contacts second arm  60 ) the current flow through vertical portion  52  will be terminated thus allowing vertical portion  52  to cool and expand thereby sealing purge valve  22 . 
     An alternate embodiment of purge valve  22  is shown in FIG.  3 . In this embodiment, the purge valve comprises a bi-metal strip  64  having an outer metal member  66  affixed to an inner metal member  68 . The two metals have different thermal coefficients of expansion. Accordingly, when contact is made between arms  44  and  58 , an electrical connection may be made causing an electric current to pass, eg. via an electrically conductive member  48 , to and through bi-metal strip  64  thereby heating the strip. If the inner metal member  68  has a greater thermal expansion than the outer metal member  66 , the heating of bi-metal strip  64  will cause the bi-metal strip  64  to bend inwardly in the direction of arrow A thereby uncovering opening  70  so that the purge cycle may begin. When movable arm  44  engages arm  60 , or brakes contact with first arm  58 , the circuit may be closed causing the electric heating current to terminate and allowing bi-metal strip  64  to cool. When bi-metal strip  64  cools, inner metal member  68  will contract more than the outer metal member  66  thereby causing the bi-metal strip to curve downwardly and close opening  70 . In an alternate embodiment, it will be appreciated that bi-metal strips  64  may be positioned on the outside passage  12 . 
     In a further alternate embodiment, valve  22  may be actuated by a solenoid. Once again, contact between movable arm  44  and first arm  58  may complete an electric circuit so as to actuate a solenoid to open any desired valve which may function as a purge valve. When movable arm  44  engages arm  60 , or brakes contact with first arm  58 , the circuit may be closed causing the solenoid to move to its starting position thereby closing the purge valve. 
     As previously described, concentrator  10  passes through three distinct cycles, namely an adsorption cycle, a charging cycle, and a purge cycle, which will be more particularly described in relation to FIGS. 4,  5 , and  6 , respectively which show an alternate preferred embodiment. 
     Referring first to FIG. 4, concentrator  10  is shown in its adsorption cycle during which nitrogen from an air feed stream is adsorbed into adsorption media provided in adsorption chamber  16  leaving a fluid having an increased concentration of nitrogen in adsorption chamber  16 . 
     Adsorption chamber  16  communicates with inlet channel  11  through adsorption inlet  71  such that a feed gas stream passing into inlet passage  12  flows through adsorption inlet  71 . Further, inlet channel  11  communicates with purge outlet  13  through purge valve  22 . Accordingly, a feed gas stream passing into inlet passage  12  can also be provided to purge outlet  13  when purge valve  22  is open such that inlet channel  11  is in flow communication with container  18 . 
     Adsorption chamber  16  also includes foam or porous sections  76   a,    76   b  which are positioned within adsorption chamber  16  to retain the adsorbent therein. The feed gas stream which is fed into adsorption chamber  16  through adsorption inlet  71  will pass through foam section  76   a  and that the gas flow out of an adsorption outlet  72  will pass through foam section  76   b  as shown. 
     Valve  24  is a two-way valve which allows a degree of flow communication between adsorption chamber  16  and container  18 , depending upon the pressure differential between the adsorption chamber  16  and the passageway  20 . Valve  24  is associated with adsorption outlet  72  of a common wall  73  between adsorption chamber  16  and passageway  20 . The embodiment of valve  24  shown in FIG. 4 comprises an outer spring  80 , inner spring  82 , plug  84  and annular ring  86 . Outer spring  80  is coupled between the top wall of passageway  20  and the top surface of annular ring  86  such that annular ring  86  is biased against the top surface of common wall  73  of adsorption chamber  16 . Inner spring  82  is coupled between plug  84  and the top surface of annular ring  86  so that plug  84  is biased against the bottom surface of annular ring  86 . 
     Accordingly, valve  24  is moveable between a first position in which adsorption chamber  16  is isolated from container  18 , a second position wherein gas flow passes from adsorption chamber  16  to passageway  20 , and a third position wherein a reverse gas flow passes from passageway  20  to adsorption chamber  16 . Accordingly, valve  24  is a two-way pressure actuated member which will enter into one of the first, second and third positions depending on the differential pressure present between adsorption chamber  16  and passageway  20  in association with a number of preset system parameters (eg. the resistance of outer spring  80  and inner spring  82 ). 
     Purge valve  22  is a one-way valve which is moveable between a first position in which inlet channel  11  is isolated from purge outlet  13  and a second position in which inlet channel  11  is in gas flow communication with purge outlet  13 . The specific mechanics which move purge valve  22  from one position to another may be any of those discussed with respect to FIGS. 1 and 2. 
     In the preferred embodiment of FIG. 4, while adsorption chamber  16  is undergoing the adsorption cycle, valve  24  remains in the first position wherein adsorption chamber  16  is isolated from container  18 . Due to the inflow of air through inlet passage  12 , pressure will build up in adsorption chamber  16  and nitrogen will be adsorbed in the adsorbent media of bed  74 . 
     Further, in the preferred embodiment of FIG. 4, container  18  includes piston  32  and spring  90 . Piston  32  includes a piston head  92 , piston base  94  and piston foot  96 , all of which are rigidly coupled together. Piston  32  may be movably mounted in container  18  by any means known in the art. Piston head  92 , and the means for movably mounting piston  32  in container  18 , serve to isolate reservoir  89  from bottom portion  102  of container  18 . In this way, oxygen enriched gas which enters container  18  will remain in reservoir  89  instead of passing upwardly by piston head  92  to bottom portion  102 . Piston head  92  may be so mounted by one or more O-rings  104  which are positioned between piston head  92  and side wall  106  of container  18  to seal reservoir  89  from bottom portion  102 . 
     Piston base  94  and piston foot  96  are also adapted to function as part of purge valve  22 . Purge valve  22  includes spring  98  which is coupled between the top surface of platform  100  (which itself is fixed in position in container  18 ) and the top surface of piston foot  96 . Accordingly, piston foot  96  is spring biased to the first (e.g. closed) position shown in FIG.  4  and will open when the pressure in reservoir  89  exceeds the force exerted by springs  98  on piston foot  96 . 
     Since outlet passage  14  is open at all times when concentrator  10  is in operation, the dimensions of flow restrictors  15   a,    15   b  and the volume of container  18  may be chosen so that the concentrator provides a generally continuous flow of oxygen enriched air through outlet passage  14  during the charging cycles and, alternately, during all three cycles. This necessitates that outlet passage  14  have a reduced flow rate of gas therethrough than the flow rate of air into adsorption chamber  16  via inlet passage  12 . 
     When pressure within adsorption chamber  16  reaches the pressure at which the desired oxygen concentration will have been achieved, the pressure at adsorption outlet  72  on the bottom surfaces of annular ring  86  and plug  84  will be enough to overcome the combined resistance of outer and inner springs  80  and  82 . Accordingly, valve  24  will move from the adsorption position (FIG. 4) to the charging position (FIG.  5 ). 
     Passageway  20  also includes volume chamber  88  and is in open flow communication with both container  18  and outlet passage  14  through flow restrictors  15   a  and  15   b.  The specific dimensions of flow restrictors  15   a  and  15   b  may be chosen so that optimal gas flows are provided into container  18  and through outlet passage  14 . 
     When valve  24  moves to its second (charging) position, valve  24  provides flow communication between adsorption chamber  16  and passageway  20  such that oxygen enriched air is provided along passageway  20  through volume chamber  88  and through flow restrictor  15   b  to outlet passage  14  and through flow restrictor  15   a  to container  18 . At this time, piston  32  may be in approximately the position shown in FIG. 5 (i.e. in a contracted position towards the top of container  18 ). Oxygen enriched air will enter container  18  through flow restrictor  15   a  and flow out of outlet passage  14  through flow restrictor  15   b.  However, as the flow of oxygen enriched air through flow restrictor  15   b  is restricted, pressure will build up in container  18  which will force piston  32  towards bottom portion  102 . As additional oxygen enriched air enters expandable reservoir  89  of container  18 , the pressure will be maintained in container  18  and piston  32  will continue to move into bottom portion  102 , for example until the position shown in FIG. 6 a  is reached (i.e. the pressure on the top surface of piston head  92  is sufficient to overcome the resistance of springs  90  and  98  and the pressure from inlet channel  11  against the bottom surface of piston foot  96 ). Due to the flow restrictors, volume chamber  88  (if provided) will also pressurize to provide an additional reservoir of enriched air for the purge cycle. 
     FIG. 6 a  shows concentrator  10  in its purge cycle during which the nitrogen rich air is purged from adsorption chamber  16  through adsorption inlet  71  into inlet channel  11  and through purge valve  22  to purge outlet  13 . 
     When purge valve  22  is at least partially open, air in inlet channel  11  is allowed to exit therethrough (as shown in FIG. 6 a ) thus bypassing adsorption chamber  16 . When purge valve  22  is open, the pressure in inlet channel  11  and within adsorption chamber  16  will drop to a sufficient degree such that the differential pressure between adsorption chamber  16  and passageway  20  will cause valve  24  to move from its second position where gas flow passes from adsorption chamber  16  to passageway  20 , into its third (purge) position where enriched air passes from passageway  20  to adsorption chamber  16 . 
     Specifically, when the pressure in adsorption chamber  16  reaches a low enough level as compared to the pressure within passageway  20 , outer spring  80  will bias annular ring  86  against the floor of valve  24  over adsorption outlet  72  as shown. Further, at this point the force exerted on the top surface of plug  84  will be enough to overcome the resistance of inner spring  82  such that plug  84  is biased away from the floor of valve  24  towards foam  76   b  so that a passageway is provided for gas flowing from passageway  20  into adsorption chamber  16  (as shown in more detail in FIG. 6 b ), through adsorption inlet  71 , out to purge outlet  13 , through inlet channel  11  and purge valve  22  thereby removing the nitrogen which was releasably adsorbed by the adsorption media of bed  74  from concentrator  10 . This embodiment provides for a reverse gas flow from, eg., volume chamber  88  and reservoir  89  into adsorption chamber  16  to assist with the purging cycle. 
     When purge valve  22  has been opened a sufficient amount of time for the purge cycle to be completed, the pressures within adsorption chamber  16  and passageway  20  will start to equalize such that valve  24  moves from its third position (FIG. 6 b ) to its first position (FIG. 4) whereby the adsorption cycle recommences. 
     It should be noted that as the relative strength of outer spring  80  combined with inner spring  82  will always be greater than the strength of inner spring  82  alone, it will always take a greater pressure differential to move valve  24  from its first position to its second position then to move valve  24  from its second position to its third position. The relative ratio between these two pressure differentials can be adjusted by suitably choosing the resistance values of outer spring  80  and inner spring  82 . 
     Further, since outlet passage  14  is open at all times, it may be desirable to design flow restrictors  15   a,    15   b,  the volume of reservoir  89 , and other system dimensions and spring strengths so as to provide a generally continuous flow of oxygen enriched air through outlet passage  14 . 
     Accordingly, it will be appreciated that container  18  is pressurizable (eg. by having an outlet producing a back pressure sufficient to allow pressure to build up in a container  18  having fixed walls or by having at least a portion of one wall of container  18  moveable in response to the pressure build up in container  18 ). The enriched air in container  18  provides a source of air to purge, or assist in purging, adsorption chamber  16 . Further, and more preferably, the pressure increase in container  18  is used to trigger the start of the purge cycle at the end of the charge cycle without any electronic monitoring or controls. 
     FIGS. 7 a,    7   b  and  7   c  show an alternate embodiment of valve  24  as a two-way valve illustrated in cross section, which includes a first plug  120 , a second plug  122 , a first spring  124  and a second spring  126 . Common wall  73  between adsorption chamber  16  and passageway  20  around adsorption outlet  72  has a “step” configuration as shown, and includes first recessed annular surface  125   a,  first cylindrical surface  125   b,  second cylindrical surface  125   c,  and second annular surface  125   d.  First cylindrical surface  125   b  defines a first port  127   a  and second cylindrical surface  125   c  defines a second port  127   b.    
     First plug  120  comprises a first plug stem  128  and a first plug plate  130  and has a longitudinal bore  132  which extends through the body of plug  120 . In its first position (FIG. 7 a ), first plug stem  128  extends along first and second cylindrical surfaces  125   b  and  125   c  and is sized such that sufficient room is left between the outside surface of first plug stem  128  and the first and second cylindrical surfaces  125   b  and  125   c  to accommodate first spring  124 . First plug plate  130  has a larger diameter than that of second port  127   b  such that first plug plate  130  extends over second annular surface  125   d  to seal adsorption chamber  16  from passageway  20 . First spring  124  is coupled between first annular surface  125   a  and the top surface of first plug plate  130  and urges first plug plate  130  into sealing arrangement with second annular surface  125   d  thus preventing fluid flow through adsorption outlet  72 . 
     Second plug  122  comprises a second plug stem  134  and a second plug plate  136 . Second plug stem  134  is adapted to fit within bore  132  of first plug  120 . Second plug plate  136  has a smaller diameter than that of first port  127   a  such that sufficient clearance is available between the outer surface of second plug plate  136  and the first cylindrical surface  125   b  for movement of second plug plate  136  within adsorption outlet  127   a.  Second plug plate  136  is also sized to extend over the top surface of second valve stem  134  such that second spring  126  can be coupled to the bottom surface of second plug plate  136  and to the top surface of first plug plate  130 . Accordingly, second spring  126  urges second plug plate  136  into sealing arrangement with the top surface of second valve stem  134  so as to prevent fluid flow through bore  132 . 
     Referring specifically to FIG. 7 a,  while concentrator  10  is in its adsorption cycle, valve  24  will be in its first position. That is, the pressure differential between adsorption chamber  16  and passageway  20  will be such that first spring  124  prevents air from flowing from passage  20  into adsorption chamber  16  (plug  122  is fully engaged with first plug  120  to block bore  132  to prevent gas flow therethrough) and second spring  126  prevents air from flowing from adsorption chamber  16  to passageway  20  (first plug plate  130  will be sealably engaged over second port  127  to prevent gas flow through adsorption outlet  72 ). 
     Referring now to FIG. 7 b,  when a sufficiently high pressure forms within adsorption chamber  16  (e.g. 1 psig), valve  24  will be caused to move from its first position (FIG. 7 a ) to its second position (FIG. 7 b ). Specifically, once the pressure exerted on the bottom surface of second plug stem  128  of second plug  122  through bore  132  is larger than the resistance of second spring  126  and the pressure on the top surface of second plug plate  136 , second spring  126  will be extended and second plug  122  will be lifted out of its engagement with first plug  120  such that second plug stem  128  will be removed from bore  132 . Accordingly, gas will flow from adsorption chamber  16  into passageway  20  through bore  132  and around second plug  122 . 
     Referring now to FIG. 7 c,  when the pressure within passageway  20  reaches a high enough value (e.g. 4-6 psig), valve  24  will be caused to move from its second position (FIG. 7 b ) to its third position (FIG. 7 c ). Specifically, once the pressure exerted on the top surface of second plug plate  136  of second plug  122  and the top surface of the first plug plate  130  is larger than the resistance of first spring  124  and the pressure on the bottom surface of first plug plate  130 , second spring  126  will no longer be extended and first spring  124  will be extended such that first plug  120  will be forced away from adsorption outlet  72 . Accordingly, enriched gas will flow from passageway  20  into adsorption chamber  16  through the space around engaged first and second plugs  120  and  122 . 
     FIGS. 8 a  and  8   b  show an alternate embodiment of valve  24  as a two-way valve with a single spring configuration. In this embodiment, valve  24  comprises only first plug  120 . While concentrator  10  is in its adsorption and charging cycles, valve  24  will be in a first position (FIG. 8 a ). During the adsorption cycle, as pressure builds up within adsorption chamber  16 , plug plate  146  is further urged against annular surface  125   d.  It should be noted that in this position a first gas flow passes through bore  148  into passageway  20 . During the charging cycle, valve  24  will remain in the first position as no amount of pressure in adsorption chamber  16  will cause an increased gas flow through valve  24  due to the fact that plug plate  146  extends over second port  127   b.    
     Referring specifically to FIG. 8 b,  while concentrator  10  is in its purge cycle, valve  24  will move to the second position wherein gas will flow into adsorption chamber  16  from passageway  20  to effect a quick and efficient purge cycle. Once the purge cycle has been completed, the pressure of passageway  20  and adsorption chamber  16  will start to equalize until the pressure differential therein is such that spring  124  will no longer be caused to extend into adsorption chamber  16  causing plug to reassume its position within adsorption outlet  72  as shown in FIG. 8 a.    
     It should be noted that the gas flow rate from adsorption chamber  16  into passageway  20  may be greater than, the same as or less than the gas flow rate in the reverse direction. The gas flow rates may be altered by varying the ratio of the surface area of the opening of valve  24  during the charging cycle compared to the surface area of the opening of valve  24  during the purge cycle (i.e. during the reverse flow). 
     By constructing a concentrator according to the instant invention, a concentrator may be constructed whereby the pressurized air fed to adsorption chamber  16  may be the only motive force to open purge valve  22  and valve  24 . Further, it provides the requisite motive force to cause container  18  to expand. Thus, by using simple mechanical linkages and movable or expandable elements, a gas concentrator having a simple, rugged construction may be developed. 
     In addition, aperture  28  may be in an open position at all times so as to provide a continual supply of enriched gas to outlet  14  even when adsorption chamber  16  is being purged. This is due to reservoir  30  contracting during the purge cycle thereby driving the enriched stored gas from reservoir  30  to aperture  28 . 
     Another advantage of the instant invention is that the expansion of container  18  may be used to time the purging cycle of adsorption chamber  16 . Accordingly, electronic timers or concentration sensors are not required to provide input to a controller to determine when the purge cycle should be commenced or terminated.