Patent Publication Number: US-6712087-B2

Title: Rotary valve assembly for pressure swing adsorption system

Description:
This Application is a continuation of Ser. No. 09/925,146 filed Aug. 8, 2001, which is a division of Ser. No. 09/371,464 filed Aug. 10, 1999 U.S. Pat. No. 6,311,719. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to the separation of gases by pressure swing adsorption (PSA), and in particular to a rotary valve assembly for a PSA system. 
     BACKGROUND OF THE INVENTION 
     Cyclic adsorption processes are generally practiced in batteries of adsorption vessels comprised of two or more adsorbent-filled vessels arranged in parallel and operated out of phase such that at least one vessel is in the adsorption mode while at least one other vessel is in the adsorbent regeneration mode. 
     In each cycle of the process a series of sequential steps, including adsorption, equalization and regeneration, are carried out in each vessel. To enable the various streams to flow to and from the vessels, the feed, product, and exhaust lines have been provided with a rotary valve assembly that provides valving action to permit gas flow through these lines at the appropriate time in the adsorption cycle. 
     The rotary-valve assembly also permits communication between the inlet ends of the vessels and the outlet ends of the vessels to permit flow between the vessels during pressure equalization steps. Pressure equalization is the passage of gas from a first vessel that has just completed its adsorption step to a vented or evacuated vessel which has just completed its adsorbent regeneration step. 
     Relevant background art for pressure swing adsorption systems can be found in the following U.S. patents, all of which are hereby incorporated by reference for all they disclose and describe: U.S. Pat. Nos. 5,814,131, 5,814,130, 5,807,423, 5,366,541, 5,268,021, and Re. 35,009. 
     U.S. Pat. Nos. 5,814,130, 5,814,131 and 5,807,423 disclose a rotary valve assembly for use with adsorption vessels that generally includes a valve port disk and rotary valve. The valve port disk and the rotary valve are described as being ground to have highly polished flat finishes to enable the faces of the disks to form a fluid-tight seal with each other. The rotary valve is rotated relative to the stationary valve port disk so that openings on the face of the rotary valve register with holes in the valve port disk, providing valving action to permit appropriate gas flow through the vessels for the adsorption, regeneration and equalization modes. 
     Between the rotary valve and the inside surface of a valve assembly cover are a number of annular channels formed by multiple annular seal rings disposed around the valve. A respective exhaust line, purge fluid supply line and product gas line communicate with these annular channels. The rotary valve includes bores extending from the openings on the flat engagement surface to the periphery of the valve for communicating the openings with the annular channels and fluid lines. 
     There are a number of drawbacks with this proposed design in these patents. First, the multiple annular seal rings are impractical. The seal rings would be expensive to make, difficult to install and service, difficult to make leak-free (even when new), and would be subject to wear and increased leakage over time. The leakage between these various fluid streams could have serious negative effects on the performance of the gas separation device, i.e., the product gas would become contaminated. The seal rings also would greatly increase the torque required to turn the valve and, hence, increase the size of the motor. 
     Second, the rotary valve assembly is not pressure balanced. At the operating pressures needed for the separation cycle, a very heavy pre-load would need to be placed on the valve parts to prevent their separation while operating. 
     Third, the co-location of the purified product gas and feed gas on the faces of the rotary valve and the valve port disk would inevitably lead to leakage of the feed gas into the product gas. Feed gas is at higher pressure than the product gas and, hence, has the multiple driving forces of differential pressure and a large concentration gradient leading to contamination of the high purity product with contaminates from the feed gas. Even though the leak rate can be made very low by producing a valve face interface with sufficient accuracy, i.e., flatness and finish, the leakage can not be eliminated altogether since the valve depends on a thin gas film being established between the flat engagement surfaces of the rotary valve and the valve port disk. In the case of nitrogen separation from air, if the desired product purity is in the range of tenths of percentage points oxygen to PPM (Parts Per Million) levels of oxygen, the rotary valve assembly described in these patents could not be used. 
     SUMMARY OF THE INVENTION 
     The present invention provides a rotary valve assembly for a pressure swing adsorption system having means for inhibiting leakage and contamination between fluid sections of the valve assembly. The rotary valve assembly includes a first valve member and a second valve member relatively rotatable about a common center of rotation to provide valving action for selectively transferring fluids therethrough. The second valve member has a first fluid section with at least one aperture adapted for transferring a first fluid of a first pressure and composition therethrough and a second fluid section with at least one aperture adapted for transferring a second fluid of a second pressure and composition therethrough. The first valve member has a first fluid section with at least one passage for transferring the first fluid in the valve assembly and a second fluid section with at least one passage for transferring the second fluid in the valve assembly. A vent is located between the first fluid sections and the second fluid sections of the valve assembly and is vented to a pressure lower than the pressures of the first and second fluids so as to vent leakage from either of the sections of the valve assembly. The rotary valve assembly further includes means for effecting relative rotation of the first valve member and second valve member. 
     In a preferred embodiment of the invention, the first valve member is a rotating rotary valve shoe and the second valve member is a stationary valve port plate. The at least one aperture and passage of the first fluid sections are disposed at a first radius and the at least one aperture and passage of the second fluid sections are disposed at a second radius. The vent is comprised of an annular vent groove disposed in an engagement surface of the rotary valve member at a radius between the first radius and the second radius. The annular vent groove is vented to approximately atmospheric pressure. 
     An alternative rotary valve assembly includes a first valve member and a second valve member relatively rotatable about a common center of rotation to provide valving action for selectively transferring fluids therethrough. In this embodiment, a number (N) of concentric fluid sections are adapted to transfer N fluids therethrough. A number of concentric annular grooves equal to N-1 are located respectively between the fluid sections and vented to a pressure lower than the pressures of the fluids in adjacent concentric sections so as to vent any leakage from adjacent sections. The rotary valve assembly further includes means for effecting relative rotation of the first valve member and the second valve member. 
     An alternative rotary valve assembly includes a first valve member and a second valve member relatively rotatable about a common center of rotation to provide valving action for selectively transferring fluids therethrough. The second valve member has a central product fluid aperture at the common center of rotation through which product fluid flows to exit the assembly. A set of equally spaced product fluid apertures are concentrically disposed at a predetermined radius from the common center of rotation and are interconnected to product ends of adsorption vessels. The first valve member includes a cavity, at least one product passage for selectively interconnecting at least two apertures of the set of product apertures with the central product aperture and the cavity, and at least one purge passage interconnected with the cavity for selectively interconnecting the cavity with at least two apertures of the set of product apertures. The rotary valve assembly further includes means for effecting relative rotation of the first valve member and second valve member, whereby registration of the product fluid apertures of the second valve member with the product passage of the first valve member allows product fluid to exit the assembly through the central product aperture and enter the cavity for supplying balancing pressure for the first valve member and second valve member and purge gas via the at least one purge passage for regenerating more than one adsorption vessel. 
     In a preferred embodiment of the invention described immediately above, the rotary valve shoe includes at least one flow control element to control the flow of purge gas from the cavity. 
     An alternative rotary valve assembly for a pressure swing adsorption system having more than one adsorption vessel includes a valve port plate and a rotary valve shoe having respective engaged surfaces generally defining a plane and are relatively rotatable about a common center of rotation to provide valving action for selectively transferring fluids therethrough. The valve port plate has more than one aperture interconnected with the more than one adsorption vessel. The rotary valve shoe has at least one passage adapted to register with two of the apertures for equalizing two of the adsorption vessels and is not coplanar with the engagement surfaces. The rotary valve assembly further includes means for effecting relative rotation of the valve port plate and the rotary valve shoe to enable the valving action. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate both the design and utility of a preferred embodiment of the present invention, wherein: 
     FIG. 1A is a perspective view of a pressure swing adsorption system constructed in accordance with a preferred embodiment of the invention; 
     FIG. 1B is a side-elevational view of the pressure swing adsorption system of FIG. 1A; 
     FIG. 1C is a top plan view of the pressure swing adsorption system of FIG. 1A; 
     FIG. 1D is a cross-sectional view of the pressure swing adsorption system of FIG. 1A taken along line  1 D— 1 D of FIG. 1B; 
     FIG. 1E is a cross-sectional view of the pressure swing adsorption system of FIG. 1A taken along line  1 E— 1 E of FIG. 1C; 
     FIG. 2 is a partial, cross-sectional view of a top portion of the pressure swing adsorption system illustrated in FIGS. 1A-1D; 
     FIG. 3A is a top perspective view of a rotary valve shoe constructed in accordance with a preferred embodiment of the invention; 
     FIG. 3B is a top plan view of the rotary valve shoe illustrated in FIG. 3A; 
     FIG. 3C is a cross-sectional view of the rotary valve shoe taken along line  3 C— 3 C of FIG. 3B; 
     FIG. 3D is a bottom plan view of the rotary valve shoe illustrated in FIG. 3A; 
     FIG. 4A is a top plan view of a valve port plate constructed in accordance with a preferred embodiment of the invention; 
     FIG. 4B is a cross-sectional view taken along lines  4 B— 4 B of FIG. 4A; 
     FIG. 5 is a cross-sectional view of an embodiment of the rotary valve shoe and a first drive shaft; 
     FIG. 6A is a bottom plan view of an alternative embodiment of a rotary valve shoe; 
     FIG. 6B is cross-sectional view of the rotary valve shoe illustrated in FIG. 6A taken along line  6 B— 6 B; 
     FIG. 7A is a top perspective view of a rotary valve shoe constructed in accordance with a further embodiment of the invention; 
     FIG. 7B is a top plan view of the rotary valve shoe illustrated in FIG. 7A; 
     FIG. 7C is a bottom plan view of the rotary valve shoe illustrated in FIG. 7A; 
     FIG. 7D is a side-elevational view of the rotary valve shoe illustrated in FIG. 7A; 
     FIG. 7E is a cross-sectional view of the rotary valve shoe taken along line  7 E— 7 E of FIG. 7A; 
     FIG. 8 is a partial, cross-sectional view of a top portion of a pressure swing adsorption system constructed in accordance with an alternative embodiment of the invention; and 
     FIG. 9 is a perspective view of the valve port plate, top manifold member and bottom manifold member constructed in accordance with an embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIGS. 1A-1E, a pressure swing adsorption (PSA) system  20  including a rotary valve assembly  22 , constructed in accordance with a preferred embodiment of the invention, will now be described. The PSA system  20  is used to fractionate fluids. 
     As used herein the term “fluids” includes both gases and liquids. Although the present invention will be described in conjunction with the separation of nitrogen from air, it will be readily understood by those skilled in the art that the present invention applies to the fractionation of other fluids such as, but not by way of limitation, the separation of oxygen from air. 
     The adsorption system  20  includes multiple adsorption vessels  24 , each containing a bed of adsorbent material which is selective for a particular molecular species of fluid or contaminant, and the rotary valve assembly  22 . In the preferred embodiment of the system  20 , twelve adsorption vessels  24  are included. However, it will be readily understood by those skilled in the art how other numbers of vessels  24  (two or more) may be used. The adsorption vessels  24  preferably used in the system  20  include straight elongated vessels, as shown. Alternatively, the vessels  24  may have a construction such as, but not by way of limitation, U-shaped or concentric. Each adsorption vessel  24  includes a product end  28  and a feed end  30 . The feed ends  30  communicate with respective feed lines or tubes  32  through passages in a header  34 . Springs  36  are disposed near the bottoms of the adsorption vessels  24  to hold the packed beds of adsorbent material firmly in place. The product ends  28  communicate with passages in a manifold  38  for communication with the rotary valve assembly  22 . 
     With reference additionally to FIGS. 2,  8  and  9 , the manifold  38  is constructed of a top member  201  and a bottom member  202 . The manifold  38  serves to connect the rotary valve assembly  22  with the adsorption vessels  24  at both the feed ends  30  (via the feed tube  32 ) and the product ends  28 . The two-piece construction allows for the creation of passages that connect apertures in either the top member  201  or bottom member  202 . The preferred embodiment of the manifold  38  has provision for flow control elements to be inserted in these internal passages to control the rate of flow of various fluid streams within the system  20 . The preferred embodiment includes both feed orifices  210  and product orifices  211  that control the flow of gas streams into and out of the adsorption vessels  24 . 
     In the case that the density of internal passages is so great that it becomes difficult to route more passages in this manner, one or more external passages may be added. For example, in the preferred embodiment, this is done with a feed U-tube  43  that connects a feed fitting or air feed inlet  41  at a convenient location on the manifold  38  to the appropriate position on the manifold  38  near the valve assembly  22  and a product U-tube  42  that connects a product fitting or product outlet  40  at a convenient location on the manifold  38  to the appropriate position on the manifold  38  near the valve assembly  22 . 
     An annular groove  220  (FIG. 9) in the top member  201  of the manifold  38  is used to introduce feed gas in the preferred embodiment. A single passage communicating to the annular groove  220  is then able to supply a fluid stream to a multiplicity of apertures  112  in a port plate  46  of the rotary valve assembly  22 . 
     In the preferred embodiment, the two members  201 ,  202  making up the manifold  38  are fastened together with a multiplicity of stay bolts to resist the separation force created by the fluid pressure present in the internal passages. Additionally, a sealant is used to seal the various fluid passages so that there is no leakage from one passage to another or from a passage to an external surface. One skilled in the art will recognize that a number of methods exist to provide this sealing function such as using a gasket, O-rings, adhesive material or the like. 
     With reference to FIGS. 1-5,  8  and  9 , to assist the reader in gaining a better understanding of the invention, the PSA system  20  will now be generally described in use. Air flows into the air feed inlet  41  and through the feed U-tube  43 . Air flows through feed inlet aperture  228  and into air feed groove  220  in the manifold  38 . From there, air flows to the rotary valve assembly  22  where it is distributed back through feed apertures  222  in the top member  201  of the manifold  38  to feed orifices  210  in the bottom member  202  via feed channels  232 . The air then flows through multiple feed lines  32  to the header  34 , where it is distributed to the adsorption vessels  24  at the feed ends  30 . 
     The sieve beds (not shown) of the adsorption vessels  24  are a packed particulate adsorbent which preferentially adsorbs oxygen relative to nitrogen in the feed air so that nitrogen is produced as the non-adsorbed product gas. An adsorbent such as a carbon molecular sieve will provide this effect when the adsorption process is carried out on a kinetic basis. 
     The resulting product nitrogen gas flows towards the products ends  28  of the adsorption vessels  24 , out product orifices  211  in the bottom member  202 , through product channels  230 , through product apertures  224  in the-upper member  201 , and to the rotary valve assembly  22 , where it is distributed back through the manifold  38  via product aperture  226  of the upper member  201  and product aperture  234  of the lower member  202  to the product U-tube  42 . The product U-tube  42  transfers the nitrogen product gas out to the nitrogen product outlet  40 . As will be described in greater detail below, some of the product gas in the rotary valve assembly  22  may be used to purge or regenerate beds in the adsorption vessels  24 . 
     With reference also to FIGS. 3A-3D and  4 A- 4 B, a preferred embodiment of the rotary valve assembly  22  will now be described in more detail. The rotary valve assembly  22  includes a first valve member such as a rotary valve shoe or disk  44  and a second valve member such as a valve port plate or disk  46 . Both the rotary valve shoe  44  and valve port plate  46  are preferably circular in construction. However, it will be readily understood by those skilled in the art that they may be shaped otherwise, for example, polygonal. The rotary valve shoe  44  and valve port plate  46  are preferably made from a durable material such as ceramic, which can be ground to a highly polished flat finish to enable the faces of the valve shoe  44  and port plate  46  to form a fluid-tight seal when pressed together. 
     With reference specifically to FIGS. 3A-3D, the rotary valve shoe  44  has a flat engagement surface  48  (FIG. 3D) and a cylindrical exterior surface  50 . The valve shoe  44  has several symmetrical arcuate passages or channels cut into the engagement surface  48 , all of which have as its center the geometric center of the circular engagement surface  48 . The passages or channels include opposite feed channels  52 , a first pair of equalization passages  54 , a second pair of equalization passages  56 , and opposite exhaust ports  58  (which open at the side wall  50 ). Although the passages or channels are-generally described below as means for transferring fluid from one part of the engagement surface  48  to another, the passages or channels may also be configured to transfer fluid from the engagement surface  48 , out of the rotary valve shoe  44 . As will be better understood below, the passages or channels related to the feed fluid or the feed end  30  of the adsorption vessels  24  are part of a feed fluid section  59 . 
     Proceeding radially inward from the aforementioned passages or grooves, the engagement surface  48  includes annular vent groove  60  and opposite passages  62  that extend from the vent groove  60  to the side wall  63  of the exhaust ports  58 . Near the center of the rotary valve shoe  44  are arcuate purge channels  64 , purge passages  66  in which flow control elements such as small orifices  68  are inserted, cross-port equalization channels  70 , and product channels  72 A and  72 B which respectively extend radially from a central product passage  74  to symmetrical opposite arcuate product channels or pockets  76 ,  78 . Other flow control elements may be used besides small orifices  68  such as, but not by way of limitation, sintered metal elements or capillary tubes. The above-described passages or channels located at radial positions inside the groove  60  that relate to the product fluid or product ends  28  of the adsorption vessels  24  are part of a product fluid section  79 . 
     Although a pair of purge passages  66  and flow control elements  68  are described, it will be readily understood by those skilled in the art that one or more purge passages  66  and flow control elements  68  may be incorporated into the rotary valve shoe  44 . If more than one flow control element  68  is used, it is desirable to match the flow characteristics of the flow control elements. 
     With reference to FIGS. 6A and 6B, a rotary valve shoe  144  constructed in accordance with an alternative embodiment of the invention, which does not allow for product gas purge flow, is shown. Elements similar to those described above with respect to FIGS. 3A-3D are identified with numbers that include the same last two digits, but with a “1” prefix, i.e., 58 becomes 158, 63 becomes 163, etc. Because the embodiment of the rotary valve shoe  144  does not allow for product gas to be used as purge flow, the following elements described above with respect to FIGS. 3A-3D are not needed: pockets  64 , passages  66 , and flow control elements  68 . This embodiment of the rotary valve shoe  144  is in fact the more common way to operate the nitrogen from air separation cycle when the adsorption vessels  24  are packed with a Carbon Molecular Sieve (CMS). The rotary valve shoe  144  includes opposite arcuate vent grooves  160  and multiple passages  162  that extend from the vent grooves  160  to the side wall  163  of the exhaust ports  158 . 
     With reference back to FIGS. 3A-3E, an upper part of the rotary valve shoe  44  includes an upper annular surface  80  that surrounds a first annular recess  82  and a second surface  84  that surrounds an eccentric recess  86 . An eccentric floor  87  defines a bottom part of the eccentric recess  86 . 
     Equalization routing for the feed ends  30  of the adsorption vessels  24  is done in a plane out of a plane generally defined by the engagement surface  48  of the rotary valve shoe  44  because of the limited amount of room available for this purpose on the engagement surface  48 . A first equalization tube  88  and a second equalization tube  90 , each constructed of formed tubing, are bonded into passages  54  and  56  in the upper surface  80  of the rotary valve shoe  44  for interconnecting the first pair of equalization passages  54  and the second pair of equalization passages  56 , respectively. The first equalization tube  88  and equalization passages  54  form a first passage adapted to communicate with the feed ends  30  of two adsorption vessels  24  for equalization purposes. Likewise, the second equalization tube  90  and equalization passages  56  form a second passage for this purpose. 
     With reference to FIGS. 7A-7E a rotary valve assembly  244  constructed in accordance with an alternative embodiment of the invention, which interconnects each set of equalization passages  54 ,  56  of the rotary valve shoe  44  in a different way, will now be described. As shown in FIGS. 7C and 7E, the equalization passages  54 ,  56  extend from the engagement surface  48  to the sidewall  50  of the rotary valve shoe  44 . Each equalization passage  54 ,  56  is routed around the outer circumference or sidewall  50  of the rotary valve shoe  44 , underneath a ring  91  that is shrunk or bonded onto the sidewall  50  of the rotary valve shoe  44 , to interconnect each respective set of equalization passages  54 ,  56 . It will be readily apparent to those skilled in the art that other ways exist to interconnect each set of equalization passages  54 ,  56  of the rotary valve shoe  44 . 
     The purge passages  66  extend from the second surface  84  to the arcuate passages  64  of the rotary valve shoe  44 . The central product passage  74  extends from the floor  87  of the eccentric recess  86  to the product channels  72 A,  72 B. 
     The first and second annular recesses  82 ,  86  are configured to receive a first drive shaft  92  as shown in FIGS. 1,  2  and  5 . A drive motor  94  has a second drive shaft  95  which extends through a top wall of a valve assembly cover  97  and extends into a recess  99  in a top part of the first drive shaft  92 . The motor  94  is connected to a source of electric power and its shaft  95  drives the shaft  92 . 
     As the motor shaft  95  rotates, it causes the rotary valve shoe  44  to rotate, to cycle the adsorption vessels  24  through the various steps of the adsorption process. The motor  94  can impart continuous or stepwise rotation to the rotary valve shoe  44  around its center of rotation. Although the rotation of the valve  22  preferably includes the first valve member  44  rotating and the second valve member  46  remaining stationary, in an alternative embodiment of the invention, the opposite may be true or both valve members  44 ,  46  may rotate in the same or opposite directions. The valve members  44 ,  46  rotate at different speeds in the event they rotate in the same direction. 
     A cavity  96  in the first drive shaft  92  receives a spring  98  for applying a preload force against the rotary valve shoe  44 . The spring  98  ensures that the rotary valve shoe  44  is in contact with the valve port plate  46  even at start-up, before a balance pressure (discussed below) is established. An O-ring  100  is located in a groove  102  of the drive shaft  92 . 
     With reference to FIGS. 4A and 4B, the valve port plate  46  will now be described in greater detail. The valve port plate  46  has a flat engagement surface  104  and a smooth cylindrical side wall  106  with opposite notches  108  therein. 
     The valve port plate  46  also includes multiple sets of symmetric concentrically disposed ports or openings. The openings preferably extend completely through the valve port plate  46  in a direction generally perpendicular to the engagement surface  104 . Although some of the openings are described as having different configurations, it will be readily apparent to those skilled in the art that the openings may have alternative configurations such as, but not by way of limitation, round-shaped, square-shaped, sector-shaped and elongated holes. The openings may also extend through the port plate  46  at a variety of angles. Preferably, all of the openings of each set have the same configuration. 
     Each set of openings will now be described. A first set of twelve obround openings  110  are concentrically disposed at a first radius from the geometric center of the valve port plate  46  and interconnected with the feed ends  30  of the twelve adsorption vessels  24 . Although sets of twelve openings are described herein, it will be readily apparent to those skilled in the art that other numbers of openings may be used. Further, the number of openings in each set need not match the number of absorption vessels  24 , the number could be more or less. 
     A second set of twelve round feed openings  112  concentrically disposed at a second radius from the geometric center of the valve port plate  46  are interconnected with the feed air inlet for delivering feed fluid to the valve assembly  22 . The feed openings  112  have a first bore  118  and a smaller diameter second bore  120 . 
     A third set of twelve round openings  114  concentrically disposed at a third radius from the geometric center of the valve port plate  46  are interconnected with the product ends  28  of the twelve adsorption vessels  24 . 
     A round central product opening  116  disposed at the geometric center of the valve port plate  46  and the center of rotation of the valve assembly  22  is interconnected with the product U-tube  42  and outlet  40  for withdrawing product fluid. 
     The openings  110 ,  112  are located in a feed fluid section  115  of the valve port plate  46 . The feed fluid section  115  is the radial region of the valve port plate  46  outside of the vent groove  62  when the rotary valve shoe  44  is engaged in position with the valve port plate  46 . 
     The openings  114  and central product opening  116  are located in a product fluid section  117  of the valve port plate  46 . The product fluid section  117  is the radial region of the valve port plate  46  inside of the vent groove  60  when the rotary valve shoe  44  is engaged in position with the valve port plate  46 . 
     In use, the flat engagement surface  48  of the rotary valve shoe  44  engages the flat engagement surface  104  of the valve port plate  46  so that the surfaces  48 ,  104  have the same geometric center. This center serves as the center of rotation of the rotary valve shoe  44 . The notches  08  in the port plate  46  receive stop members  208  (FIG. 9) of the manifold  38  to prevent the port plate  46  from rotating or moving during rotation of the valve shoe  44 . 
     Although not shown, conventional equipment may be used to supply feed fluid, monitor and automatically regulate the flow of product fluid from the system so that it can be fully automated to run continuously in an efficient manner. 
     With reference generally to FIGS. 1-4, the pressure swing adsorption system and particularly the rotary valve assembly  22  will now be described in use as it applies to the separation of nitrogen from air with the adsorption vessels  24  being packed with a particulate adsorbent, e.g., Carbon Molecular Sieve (CMS), which, based on a kinetic effect, preferentially adsorbs oxygen relative to nitrogen so that nitrogen is produced as the nonadsorbed product gas. During use of the pressure swing adsorption system  20 , the rotary valve shoe  44  rotates in the valve assembly  22 . Although the rotary valve shoe  44  preferably rotates with respect to the valve port plate  46  during use so that each cycle described below is sequentially and continuously established for each vessel  24 , to help the reader gain a better understanding of the invention the following description describes the relationship between what occurs in the valve assembly  22  and the adsorption vessels  24  while the rotary valve shoe  44  is in a single position because at any given position all of the adsorption vessels are at some point in the PSA cycle. 
     It should be noted, with each revolution of the rotary valve shoe  44 , the adsorption vessels  24  undergo two complete PSA cycles. For each cycle, the steps include: 1) adsorption, 2) equalization down, 3) regeneration, and 4) equalization up. As the rotary valve shoe  44  rotates over the valve port plate  46 , each step described below is sequentially and continuously established for each vessel  24 . 
     It will be readily apparent to those skilled in that art that the rotary valve assembly may be designed so that different number of cycles may be completed with each revolution of the rotary valve shoe  44 . Further, although in the rotary valve assembly  22  illustrated the duration of the adsorption stages and purge stages is constructed to be the same, it will be readily apparent to those skilled in the art that the timing of these stages (as well as the equalization stage) may be varied by changing the configuration and/or location of the openings and/or channels in the rotary valve assembly  22 . 
     Compressed air is supplied to the system  20  at the air feed inlet  41 . The air may be pre-treated to remove particulates and liquid water. Feed air flows through the feed air U-tube  43  and into the manifold  38 , where it is distributed via the feed air groove  220  to the valve assembly  22 . The valve assembly  22  distributes the feed air to the multiple adsorption vessels  24 . 
     Feed air enters the valve assembly  22  through the openings  112  (FIG. 4A) of the valve port plate  46 . Air flows through the inner feed openings  112  and is blocked by the engagement surface  48  of the rotary valve shoe  44  except at opposite feed channels or pockets  52 . Air flows into the feed channels  52 , which communicate at least two of the openings  112  with at least two of the openings  110 , and through the corresponding openings  110  of the valve port plate  46 . The feed air then flows through feed apertures  222 , feed orifices  210  and feed channels  232  in the manifold  38  and through the feed lines  32 , into passages of the header  34 . The air then flows from the feed ends  30  of the respective adsorption vessels  24  towards the product ends  28 , and adsorption takes place in the adsorbent beds. 
     The nonadsorbed product gas flows out of the product ends  28  of the respective vessels  24 , through product orifices  211 , product channels  230  and product apertures  224  in the manifold  38  and through openings  114  in the valve port plate  46 . It should be noted, the number of openings  114  through which product gas flows in the valve port plate  46  typically corresponds with the number of aforementioned openings  110  through which the initial air flowed through the valve port plate  46 . The product gas flows into the arcuate channels  76 ,  78  of the rotary valve shoe  44  and is channeled towards the center of the rotary valve shoe  44  through the product channels  72 A,  72 B. 
     From the center of the rotary valve shoe  44 , some of the product gas flows through central product opening  116 , manifold  38 , and out product U-tube  42  to the product outlet  40 . The product gas not withdrawn flows through the passage  74  of the rotary valve shoe  44  and into a space  96  between the shoe  44  and shaft  92 . 
     The pressure of the product gas in the space  96  produces a pressure-balancing effect on the rotary valve shoe  44 . During use of the PSA system  20 , various pressure forces in the system e.g., compressed feed air pressure, nitrogen product gas pressure, act to separate the shoe  44  from the port plate  46  at the engagement surface  48 . The pressure of the product gas in the space  96  imparts a force on the rotary valve shoe  44  equal to or slightly greater than the pressure forces acting on the engagement surface  48  of the rotary valve shoe  44 , causing the engagement surface  48  of the rotary valve shoe  44  to be pressed firmly against the engagement surface  104  of the valve port plate  46  so as to inhibit leakage at this interface. Balance is maintained over a broad range of operating pressures because pressure forces are all related and proportional to the inlet pressure. The O-ring  100  is pressure actuated for ensuring a good seal between the rotary valve shoe  44  and the first shaft  92 . 
     The aforementioned compression spring  98  also biases the rotary valve shoe  44  against the valve port plate  46 . The spring  98  is the only balancing force provided upon initial start-up of the PSA system, i.e., there is no balancing pressure in the space  96 . 
     The balancing medium, i.e., product gas, in the space  96  is also a convenient, controllable source of purge gas for the separation cycle. Product gas in the space  96  flows out of the space  96  through the purge passages  66 . The flow out of the space  96 , i.e., the purge flow, is controlled by the small orifices  68  in the purge passages  66 . Product gas flows through the purge passages  66  and into the opposite purge channels  64 . In turn, product gas flows from the purge channels  64  through respective openings  114  in the valve port plate  46 . Product gas then flows through appropriate through product apertures  224 , product channels  230  and product orifices  211  in the manifold  38  and into the product ends  28  of the respective adsorption vessels  24 . Product gas flows through the adsorbent material in the vessels  24 , regenerating the adsorbent beds of the vessels  24  and sweeping out oxygen adsorbed therein. 
     As indicated above, in an alternative embodiment of the invention, as shown in FIGS. 6A and 6B, purging may not take place. Consequently, the purge channels  64  and purge passages  66 ,  68  may not exist. 
     Resulting exhaust gas flows out of the feed ends  30  of the vessels  24  and into lines  32 . The exhaust gas flows out of the lines  32 , through feed orifices  210 , feed channels  232  and feed apertures  222  of the manifold  38 , through outer feed and exhaust openings  110  of the valve port plate  46 , and out of opposite exhaust ports  58 . The exhaust gas exiting the rotary valve shoe  44  enters a chamber  118  between the rotary valve shoe  44  and the valve assembly cover  97 . The exhaust gas exits the system  20  through an exhaust outlet and an optional silencer (not shown) that is in communication with the chamber  118 . The exhaust gas may be vented to the atmosphere or withdrawn for further use. Also, a vacuum may be interconnected to the exhaust fitting to improve the withdrawal of the exhaust gas and assist in regeneration. Vacuum desorption may also be used if there is no purge option. The cover  97  may be sealed at its interface with the manifold  38  and the shaft  92  of the rotary valve assembly  22  may be sealed at its penetration through the cover  97  with an O-ring or similar device to facilitate applying a vacuum or collecting waste gas for further use. Separation of nitrogen from air may be accomplished without a purge or regeneration stage. However, purging or regenerating the adsorbent beds is done to improve the purity of the product gas where a high purity product level is important. 
     Equalization of the adsorption vessels  24  will now be discussed. It is well known to equalize the pressure between adsorption vessels transitioning between the adsorbing and desorbing cycles to enhance product concentration and high product flow rates. This is done by equalizing the pressure between adsorption vessels that have just completed the adsorption step and adsorption vessels that have just completed the regeneration step. 
     Cross-port equalization channels  70  are used to equalize the pressure between the product ends of two adsorption vessels  24  where adsorption has just occurred with the product ends of two respective adsorption vessels  24  where regeneration has just occurred. This is accomplished upon rotation of the rotary valve shoe  44  where one end of a cross-port equalization channel  70  communicates with an opening  114  corresponding to a vessel  24  that just completed the adsorption phase and the other end of the same cross-port equalization channel  70  communicates with a corresponding opening  114  corresponding to a vessel  24  that just completed the regeneration phase. Each cross-port equalization channel  70  serves as a bridge to communicate and, hence, equalize the pressures in the product ends  28  of the vessels  24 . 
     In a similar fashion, the first equalization passages  54  and equalization tubes  90  and second equalization passages  56  and equalization tubes  88  serve to communicate and equalize the feed ends  30  of respective adsorption vessels  24  that have just completed the adsorption and regeneration steps. The first equalization passages  54  and second equalization passages  56  communicate with the feed ends  30  of the vessels  24  through the openings  110  of the valve port plate  46 . 
     It should be noted, most of the communication that exists between the vessels  24  during the adsorption, regeneration and equalization steps results from the ports, passages and channels being generally parallel with the engagement surface  48  of the rotary valve shoe  44 . An exception is the communication that exists between feed ends  30  of the adsorption vessels  24  for equalization purposes. This communication results from the first equalization passages  54  and tubes  90  and the second equalization passages  56  and tubes  88  which extend vertically through the rotary valve shoe  44 , out of the upper annular surface  80  of the rotary valve shoe  44 , and around a majority of the periphery of the upper annular surface  80 . The equalization tubes  88 ,  90  are generally coplanar with each other and parallel, but not coplanar with the plane of the engagement surface  48  of the rotary valve shoe  44 . 
     It will be readily apparent to one skilled in the art that the following exemplary equalization combinations are possible, depending on the application: 
     1) No equalization (either end); 
     2) Equalization (feed end only); 
     3) Equalization (product end only); 
     4) Equalization (both ends, i.e., feed—feed, product—product); 
     5) Equalization (feed end to product end); 
     6) Equalization (product end to feed end); and 
     7) Equalization (product end to both ends of regenerated bed). 
     The purpose of the annular vent groove  60  will now be described. As discussed above, feed gas and product gas flow through the feed fluid sections and product fluid sections of the valve port plate  46  and the rotary valve shoe  44 . The vent groove  60  prevents leakage from one section from reaching the other. Of main concern is leakage from the feed fluid section into the product fluid section, i.e. leakage from the outer annular section into the inner annular section. Leakage from the feed section into the product section occurs because feed gas, i.e., air, is at higher pressure than the product gas, i.e., nitrogen, and, hence, has a driving force of differential pressure in combination with the driving force caused by a large concentration gradient. These driving forces may lead to contamination of the high purity nitrogen product gas with oxygen from the feed gas. 
     Even though the leak rate can be made very low by producing a valve face interface, i.e., the engagement surfaces  48 ,  104  of the rotary valve shoe  44  and valve port plate  46 , with sufficient accuracy (flatness and finish) and applied contact force (through spring  98  and balance pressure in the space  96 ), the leakage cannot be eliminated altogether since the valve assembly  22  depends on a thin fluid film being established between engagement surfaces  48 ,  104  of the rotary valve shoe  44  and valve port plate  46 . If the desired product purity is in the range of tenths of percentage points oxygen to PPM (parts per million) levels of oxygen, it is not practical to use many of the rotary valve constructions proposed in the prior art. 
     The annular vent groove  60  is ported to the relatively low pressure of the valve assembly chamber  118 , where the exhaust gas normally flows, via the opposite passages  62 . Because the valve assembly chamber is in communication with the atmosphere via the exhaust outlet, the pressure within the chamber  118  is at approximately atmospheric pressure. Leaking gases that would normally flow from the feed section to the product section at the valve interface stop at the annular vent groove  60  and are withdrawn through the passages  62  to the chamber  118 , where they are then expelled to the atmosphere. Thus, leakage of feed gas into product gas is prevented. 
     In an alternative embodiment, the vent groove  60  may communicate with a vacuum, e.g., via chamber  118 , to further reduce the pressure in the groove  60  and improve the ability of the groove  60  to prevent contamination. With reference to FIG. 8, in a further embodiment, the groove  60  may be vented by passages  250 ,  252 ,  254  extending through the port plate  46 , upper manifold member  201  and lower manifold member  202 , respectively, to the external atmosphere (or a vacuum connection). 
     Although two passages  62  are shown, it will be readily apparent to those skilled in the art that other numbers of passages, e.g.,  1 ,  3 ,  4 , etc., may be used. For example, with reference to FIGS. 6A and 6B, multiple passages  162  may be used to increase the flow area for leaking fluid to limit pressure in the groove  160  to near atmospheric pressure (or pressure maintained in chamber  118 , e.g., with a vacuum). This limits the pressure drop through the passages and keeps the pressure in the vent groove  160  as low as possible. 
     Although the annular vent groove  60  has been described as being located on the engagement surface  48  of the rotary valve shoe  44 , it may also be located on the engagement surface  104  of the valve port plate  46 . In this instance, the opposite passages  62 , i.e., vent ports, would preferably be made to pass through the port plate  46 . Alternatively, both the rotary valve shoe  44  and valve port plate  46  may include opposing annular vent grooves on their respective engagement surfaces and venting could be done through either or both the rotary valve shoe  44  or the valve port plate  46 . 
     In an alternative embodiment of the invention, where the number of fluid sections in the rotary valve is N and is greater than two, N- 1  annular vent grooves may exist in the rotary valve assembly, one between each pair of fluid sections where leakage between fluid sections is a concern. 
     In a further embodiment of the invention, the vent groove  60  may comprise one or more vent grooves, not necessarily annular, located between different fluid sections and vented to a lower pressure than the pressure of the fluids in the adjacent sections. For example, with reference to FIGS. 6A and 6B, instead of an annular vent groove  60 , two arcuate vent grooves  160  may be located between the product fluid section  179  and the feed fluid section  159 . 
     Although some of the elements described above and below are referred to by order, i.e., “first,” “second,” etc., it should be noted that this is done to facilitate the reader&#39;s understanding of the invention and is not intended to limit the invention. Further, the foregoing description and drawings were given for illustrative purposes only, it being understood that the invention is not limited to the embodiments disclosed, but is intended to embrace any and all alternatives, equivalents, modifications and rearrangements of elements or steps falling within the scope of the invention as defined by the following claims.