Abstract:
A pressure swing adsorption system for separating components of a gas mixture includes a first adsorbent module, and a second a adsorbent module coupled to the a first adsorbent module. The first adsorbent module includes a first gas inlet for receiving the gas mixture, at least one bed of first adsorbent material in communication with the first gas inlet for adsorbing a gas mixture component from the gas mixture, and a first gas outlet in communication with the first adsorbent beds for receiving a first product gas therefrom. The second adsorbent module includes a second gas inlet coupled to the first gas outlet for receiving the first product gas, at least one second bed of adsorbent material in communication with the second gas inlet for adsorbing a first product gas component from the first product gas, and a second gas outlet in communication with the second adsorbent beds for receiving a second product gas therefrom. The first product gas substantially excludes the adsorbed gas mixture component, and the second product gas substantially excludes the adsorbed first product gas component. Also, the adsorbent modules are configured for transferring the first product gas between the adsorbent modules over a plurality of discrete pressure levels to maintain substantial uniformity of gas flow therebetween.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a method and apparatus for separating gas fractions from a gas mixture having multiple gas fractions. In particular, the present invention relates to a multistage gas separation system having uniform gas flow between each stage.  
         BACKGROUND OF THE INVENTION  
         [0002]    Pressure swing adsorption (PSA) and vacuum pressure swing adsorption (vacuum-PSA) separate gas fractions from a gas mixture by coordinating pressure cycling and flow reversals over an adsorbent bed which preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. The total pressure of the gas mixture in the adsorbent bed is elevated while the gas mixture is flowing through the adsorbent bed from a first end to a second end thereof, and is reduced while the gas mixture is flowing through the adsorbent from the second end back to the first end. As the PSA cycle is repeated, the less readily adsorbed component is concentrated adjacent the second end of the adsorbent bed, while the more readily adsorbed component is concentrated adjacent the first end of the adsorbent bed. As a result, a “light” product (a gas fraction depleted in the more readily adsorbed component and enriched in the less readily adsorbed component) is delivered from the second end of the bed, 20 and a “heavy” product (a gas fraction enriched in the more strongly adsorbed component) is exhausted from the first end of the bed.  
           [0003]    The conventional system for implementing pressure swing adsorption uses two or more stationary adsorbent beds in parallel, with directional valving at each end of each adsorbent bed to connect the beds in alternating sequence to pressure sources and sinks. However, this system is often difficult and expensive to implement due to the complexity of the valving required. Further, it is difficult to obtain a process result (e.g. yield, purity) which is not compromised by the limitations imposed by presently-available adsorbent materials. Furthermore, the conventional PSA system makes inefficient use of applied energy, because feed gas pressurization is provided by a compressor whose delivery pressure is the highest pressure of the cycle. Consequently, energy expended in compressing the feed gas used for pressurization is then dissipated in throttling over valves over the instantaneous pressure difference between the adsorber and the high pressure supply.  
           [0004]    Numerous attempts have been made at overcoming the deficiencies associated with the conventional PSA system. For example, Siggelin (U.S. Pat. No. 3,176,446), Mattia (U.S. Pat. No. 4,452,612), Davidson and Lywood (U.S. Pat. No. 4,758,253), Boudet et al (U.S. Pat. No. 5,133,784) and Petit et al (U.S. Pat. No. 5,441,559) disclose PSA devices using rotary distributor valves whose rotors are fitted with multiple angularly separated adsorbent beds. Ports communicating with the rotor-mounted adsorbent beds sweep past fixed ports for feed admission, product delivery and pressure equalization. However, these prior art rotary devices are impracticable for large PSA units, owing to the weight of the rotating assembly. Furthermore, since the valve faces are remote from the ends of the adsorbent beds, these rotary distributor valves have poor flow distribution, particularly at high cycle frequencies. Also, the gas separation yields and purities are limited by the constraints of the adsorbent material used.  
           [0005]    Hay (U.S. Pat. No. 5,246,676) and Engler (U.S. Pat. No. 5,393,326) provide examples of vacuum pressure swing adsorption systems which reduce throttling losses in an attempt to improve the efficiency of the gas separation process system. The systems taught by Hay and Engler use a plurality of vacuum pumps to pump down the pressure of each adsorbent bed sequentially in turn, with the pumps operating at successively lower pressures, so that each vacuum pump reduces the pressure in each bed a predetermined amount. However, with these systems, the vacuum pumps are subjected to large pressure variations, thereby reducing the efficiency of the gas separation process.  
           [0006]    Accordingly, there remains a need for a PSA system which is suitable for high volume and high frequency production, which reduces the energy losses associated with the prior art devices, and can be more readily configured to obtain the desired process results.  
         SUMMARY OF THE INVENTION  
         [0007]    According to the invention, there is provided a gas separation system and method which addresses deficiencies of the prior art.  
           [0008]    The gas separation system, according to the present invention, includes a first adsorbent module, and a second adsorbent module coupled to the first adsorbent module. The first adsorbent module includes a first gas inlet for receiving a gas mixture, at least one bed of first adsorbent material in communication with the first gas inlet for adsorbing a gas mixture component from the gas mixture, and a first gas outlet in communication with the first adsorbent beds for receiving a first product gas therefrom. The second adsorbent module includes a second gas inlet coupled to the first gas outlet for receiving the first product gas, at least one second bed of adsorbent material in communication with the second gas inlet for adsorbing a first product gas component from the first product gas, and a second gas outlet in communication with the second adsorbent beds for receiving a second product gas therefrom. The first product gas substantially excludes the adsorbed gas mixture component, and the second product gas substantially excludes the adsorbed first product gas component. Also, the adsorbent modules are configured for transferring the first product gas between the adsorbent modules over a plurality of discrete pressure levels to maintain substantial uniformity of gas flow therebetween.  
           [0009]    The gas separation method, according to the present invention, includes the steps of (1) providing a first adsorbent module including at least one bed of a first adsorbent material; (2) providing a second adsorbent module in communication with the first adsorbent module, the second adsorbent module including at least one bed of a second adsorbent material; (3) adsorbing a gas mixture component from the gas mixture with the first adsorbent material; (4) transferring a first product gas from between the first adsorbent module and the second adsorbent module with substantially uniform gas flow, the first product gas substantially excluding the adsorbed gas mixture component; (5) adsorbing a first product gas component from the first product gas with the second adsorbent material, and (6) extracting a second product gas from the second adsorbent module, the second product gas substantially excluding the adsorbed first product gas component.  
           [0010]    In accordance with a preferred embodiment of the present invention, each adsorbent module comprises a rotary pressure swing adsorbent module. Each rotary pressure swing adsorbent module includes a stator and a rotor. The stator includes a first stator valve surface, a second stator valve surface, a plurality of first function compartments opening into the first stator valve surface, and a plurality of second function compartments opening into the second stator valve surface. The rotor is rotatably coupled to the stator and includes a first rotor valve surface in communication with the first stator valve surface, a second rotor valve surface in communication with the second stator valve surface.  
           [0011]    A plurality of flow paths having adsorbent material therein are disposed in the rotors. Each of the flow paths includes a pair of opposite flow path ends. A plurality of apertures are provided in the rotor valve surfaces in communication with the flow path ends and the function compartments for cyclically exposing the flow paths to a plurality of discrete pressure levels to maintain uniformity of gas flow through the function compartments. In this manner, product gas is transferred between the adsorbent modules at the plurality of discrete pressure levels with substantially uniform gas flow, thereby reducing energy losses. Further, the first rotor can be operated at a different speed than the second rotor, and the first and second adsorbent material can be selected independently of each other so as to obtain the desired process results more readily. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The preferred embodiments of the present invention will now be described, by way of example only, with reference to the drawings in which:  
         [0013]    [0013]FIG. 1 is a sectional view of a rotary PSA module according to the present invention, showing the stator, the rotor and the adsorber situated in the rotor;  
         [0014]    [0014]FIG. 2 is a sectional view of the module of FIG. 1, with the stator deleted for clarity;  
         [0015]    [0015]FIG. 3 is a sectional view of the stator shown in FIG. 1, with the adsorbers deleted for clarity;  
         [0016]    [0016]FIG. 4 is an axial section of the module of FIG. 1;  
         [0017]    [0017]FIG. 5 shows a typical PSA cycle attainable with the present invention;  
         [0018]    [0018]FIG. 6 shows one variation of the PSA cycle with heavy reflux, attainable with the present invention;  
         [0019]    [0019]FIG. 7 is a schematic of a vacuum pressure swing adsorption module according to the present invention with a multistage or split stream centrifugal compressor or split stream exhaust pump;  
         [0020]    [0020]FIG. 8 is a schematic of an axial flow rotary PSA module according to the present invention;  
         [0021]    [0021]FIG. 9 shows the first valve face of the axial flow module of FIG. 8;  
         [0022]    [0022]FIG. 10 shows the second valve face of the axial flow module of FIG. 8;  
         [0023]    [0023]FIG. 11 shows an adsorber wheel configuration based on laminated adsorbent sheet adsorbers for the module of FIG. 8;  
         [0024]    [0024]FIG. 12 shows a two stage rotary PSA module according to the present invention having two adsorber wheels in series;  
         [0025]    [0025]FIG. 13 shows a two stage rotary PSA module according to the present invention, showing its adsorber rotors unrolled in a 360° section about its rotary axis, for separating multicomponent mixtures;  
         [0026]    [0026]FIG. 14 shows an alternative two stage rotary PSA module according to the present invention, depicting its adsorber rotor unrolled in a 360° section about its rotary axis, with combined pressure swing and thermal regeneration of the first stage; and  
         [0027]    [0027]FIG. 15 shows a two stage rotary PSA module according to the present invention, showing its adsorber rotor unrolled in a 360° section about its rotary axis, capable of substantially complete separation of a two component mixture. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    [0028]FIGS. 1, 2,  3  and  4   
         [0029]    A rotary adsorbent module  10  according to the present invention is shown in FIGS. 1, 2,  3 ,  4  and  5 . The module includes a rotor  11  revolving about axis  12  in the direction shown by arrow  13  within stator  14 . In general, the apparatus of the invention maybe configured for flow through the adsorber elements in the radial, axial or oblique conical directions relative to the rotor axis. However, for operation at high cycle frequency, radial flow has the advantage that the centripetal acceleration will lie parallel to the flow path for most favorable stabilization of buoyancy-driven free convection, as well as centrifugal clamping of granular adsorbent with uniform flow distribution.  
         [0030]    As shown in FIG. 2, for an example of radial flow, the rotor  11  is of annular section, having concentrically to axis  12  an outer cylindrical wall  20  whose external surface is first valve surface  21 , and an inner cylindrical wall  22  whose internal surface is second valve surface  23 . The rotor has (in the plane of the section defined by arrows  15  and  16  in FIG. 4) a total of “N” radial flow adsorber elements  24 . An adjacent pair of adsorber elements  25  and  26  are separated by partition  27  which is structurally and sealingly joined to outer wall  20  and inner wall  22 . Adjacent adsorber elements  25  and  26  are angularly spaced relative to axis  12  by an angle of [360°/N].  
         [0031]    Adsorber element  24  has a first end  30  defined by support screen  31  and a second end  32  defined by support screen  33 . The adsorber may be provided as granular adsorbent, whose packing voidage defines a flow path contacting the adsorbent between the first and second ends of the adsorber.  
         [0032]    First aperture or orifice  34  provides flow communication from first valve surface  21  through wall  20  to the first end  30  of adsorber  24 . Second aperture or orifice  35  provides flow communication from second valve surface  23  through wall  22  to the second end  31  of adsorber  24 . Support screens  31  and  33  respectively provide flow distribution  32  between first aperture  34  and first end  30 , and between second aperture  35  and second end  32 , of adsorber element  24 .  
         [0033]    Support screen  31  also supports the centrifugal force loading of the adsorbent.  
         [0034]    As shown in FIG. 3, stator  14  is a pressure housing including an outer cylindrical shell or first valve stator  40  outside the annular rotor  11 , and an inner cylindrical shell or second valve stator  41  inside the annular rotor  11 . Outer shell  40  carries axially extending strip seals (e.g.  42  and  43 ) sealingly engaged with first valve surface  21 , while inner shell  41  carries axially extending strip seals (e.g.  44  and  45 ) sealingly engaged with second valve surface  23 . The azimuthal sealing width of the strip seals is greater than the diameters or azimuthal widths of the first and second apertures  34  and  35  opening through the first and second valve surfaces.  
         [0035]    A set of first function compartments in the outer shell each open in an angular sector to the first valve surface  21 , and each provide fluid communication between its angular sector of the first valve surface  21  and a manifold external to the module. The first function compartments include first feed pressurization compartment  46 , second feed pressurization compartment  50 , first feed compartment  52 , second feed compartment  54 , first countercurrent blowdown compartment  56 , second countercurrent blowdown compartment  58 , and a heavy product compartment  60 . The angular sectors of the compartments are much wider than the angular separation of the adsorber elements. The first function compartments are separated on the first sealing surface by the strip seals (eg.  42 ).  
         [0036]    Proceeding clockwise in FIG. 3, in the direction of rotor rotation, a first feed pressurization compartment  46  communicates by conduit  47  to first feed pressurization manifold  48 , which is maintained at a first intermediate feed pressure. Similarly, a second feed pressurization compartment  50  communicates to second feed pressurization manifold  51 , which is maintained at a second intermediate feed pressure higher than the first intermediate feed pressure.  
         [0037]    For greater generality, module  10  is shown with provision for sequential admission of two feed mixtures, the first feed gas having a lower concentration of the more readily adsorbed component relative to the second feed gas. First feed compartment  52  communicates to first feed manifold  53 , which is maintained at a first feed pressure higher working pressure than that of the second intermediate feed pressure. Likewise, second feed compartment  54  communicates to second feed manifold  55 , which is maintained at a second feed pressure higher than that of the first feed pressure.  
         [0038]    A first countercurrent blowdown compartment  56  communicates to first countercurrent blowdown manifold  57 , which is maintained at a first countercurrent blowdown intermediate pressure. A second countercurrent blowdown compartment  58  communicates to second countercurrent blowdown manifold  59 , which is maintained at a second countercurrent blowdown intermediate pressure above the lower working pressure. A heavy product compartment  60  communicates to heavy product exhaust manifold  61  which is maintained at substantially the lower working pressure. It will be noted that compartment  58  is bounded by strip seals  42  and  43 , and similarly all the compartments are bounded and mutually isolated by strip seals.  
         [0039]    A set of second function compartments in the inner shell each open in an angular sector to the second valve surface  23 , and each provide fluid communication between its angular sector of the second valve surface  23  and a manifold external to the module. The second function compartments are separated on the second sealing surface by the strip seals (e.g.  44 ). The second function compartments include light product compartment  70 , first light reflux exit compartment  72 , first cocurrent blowdown compartment (or third light reflux exit compartment)  76 , third cocurrent blowdown compartment (or fourth light reflux exit compartment)  78 , purge compartment  80 , first light reflux pressurization compartment  82 , second light reflux pressurization compartment  84 , and a third light reflux pressurization compartment  86 .  
         [0040]    Proceeding clockwise in FIG. 3, again in the direction of rotor rotation, light product compartment  70  communicates to light product manifold  71 , and receives light product gas at substantially the higher working pressure, less frictional pressure drops through the adsorbers and the first and second orifices. According to the angular extension of compartment  70  relative to compartments  52  and  54 , the light product may be obtained only from adsorbers simultaneously receiving the first feed gas from compartment  52 , or from adsorbers receiving both the first and second feed gases.  
         [0041]    A first light reflux exit compartment  72  communicates to first light reflux exit manifold  73 , which is maintained at a first light reflux exit pressure, here substantially the higher working pressure less frictional pressure drops. A first cocurrent blowdown compartment  74  (which is actually the second light reflux exit compartment), communicates to second light reflux exit manifold  75 , which is maintained at a first cocurrent blowdown pressure less than the higher working pressure. A second cocurrent blowdown compartment or third light reflux exit compartment  76  communicates to third light reflux exit manifold  77 , which is maintained at a second cocurrent blowdown pressure less than the first cocurrent blowdown pressure. A third cocurrent blowdown compartment or fourth light reflux exit compartment  78  communicates to fourth light reflux exit manifold  79 , which is maintained at a third cocurrent blowdown pressure less than the second cocurrent blowdown pressure.  
         [0042]    A purge compartment  80  communicates to a fourth light reflux return manifold  81 , which supplies the fourth light reflux gas which has been expanded from the third cocurrent blowdown pressure to substantially the lower working pressure with an allowance for frictional pressure drops.  
         [0043]    The ordering of light reflux pressurization steps is inverted from the ordering or light reflux exit or cocurrent blowdown steps, so as to maintain a desirable “last out—first in” stratification of light reflux gas packets. Hence a first light reflux pressurization compartment  82  communicates to a third light reflux return manifold  83 , which supplies the third light reflux gas which has been expanded from the second cocurrent blowdown pressure to a first light reflux pressurization pressure greater than the lower working pressure. A second light reflux pressurization compartment  84  communicates to a second light reflux return manifold  85 , which supplies the second light reflux gas which has been expanded from the first cocurrent blowdown pressure to a second light reflux pressurization pressure greater than the first light reflux pressurization pressure. Finally, a third light reflux pressurization compartment  86  communicates to a first light reflux return manifold  87 , which supplies the first light reflux gas which has been expanded from approximately the higher pressure to a third light reflux pressurization pressure greater than the second light reflux pressurization pressure, and in this example less than the first feed pressurization pressure.  
         [0044]    Each of the first and second function compartments are sequentially exposed to each of the “N” adsorbers  24  as rotor  11  revolves about axis  12 . As a result, substantially uniform gas flow is realized within the first and second function compartments, thereby facilitating use of rotary module  10  in a steady state environment.  
         [0045]    Additional structural details concerning rotary module  10  are shown in FIG. 4. Conduits  88  connect first compartment  60  to manifold  61 , with multiple conduits providing for good axial flow distribution in compartment  60 . Similarly, conduits  89  connect second compartment  80  to manifold  81 . Stator  14  has base  90  with bearings  91  and  92 . The annular rotor  11  is supported on end disc  93 , whose shaft  94  is supported by bearings  91  and  92 . Motor  95  is coupled to shaft  94  to drive rotor  11 . The rotor could alternatively rotate as an annular drum, supported by rollers at several angular positions about its rim and also driven at its rim so that no shaft would be required. A rim drive could be provided by a ring gear attached to the rotor, or by a linear electromagnetic motor whose stator would engage an arc of the rim. Outer circumferential seals  96  seal the ends of outer strip seals  42  and the edges of first valve surface  21 , while inner circumferential seals  97  seal the ends of inner strip seals  44  and the edges of second valve surface  23 . Rotor  11  has access plug  98  between outer wall  20  and inner wall  22 , which provides access for installation and removal of the adsorbent in adsorbers  24 .  
         [0046]    It is also possible within the invention to have an integral multiple of “M” groups of “N” adsorbers  24  in a single rotor  11 , so that the angular extent for edge  11   a  to edge  11   b  is 360°. This has the disadvantage of greater complexity of fluid connections to the first and second valve means, but the advantages of slower rotational speed (by a factor of “M” for the same PSA cycle frequency) and a symmetric pressure and stress distribution. With “M”=2, FIG. 5 represents each 360° side of rotor  11 .  
         [0047]    [0047]FIGS. 5 and 6 
         [0048]    [0048]FIG. 5 shows a typical PSA cycle which would be obtained using the gas separation system according to the invention. In particular, it shows a PSA cycle undergone sequentially by each of “N” adsorbers  24  over a cycle period “T”. The cycle in consecutive adsorbers is displaced in phase by a time interval of T/N. In FIGS. 5 and 6, the vertical axis  150  indicates the working pressure in any one of the adsorbers  24  (and the pressure in the first and second function compartments with which the one adsorber  24  is communicating with) at any particular time over the cycle period “T”. Pressure drops due to flow within the adsorber elements are neglected. The higher and lower working pressures are respectively indicated by dotted lines  151  and  152 .  
         [0049]    The horizontal axis  155  of FIGS. 5 and 6 indicates time, with the PSA cycle period defined by the time interval between points  156  and  157 . At times  156  and  157 , the working pressure in a particular adsorber is pressure  158 . Starting from time  156 , the cycle for a particular adsorber  24  begins as the first aperture  34  of that adsorber is opened to the first feed pressurization compartment  46 , which is fed by first feed supply means  160  at the first intermediate feed pressure  161 . The pressure in that adsorber rises from pressure  158  at time  157  to the first intermediate feed pressure  161 . Proceeding ahead, first aperture passes over a seal strip, first closing adsorber  24  to compartment  46  and then opening it to second feed pressurization compartment  50  which is fed by second feed supply means  162  at the second intermediate feed pressure  163 . The adsorber pressure rises to the second intermediate feed pressure.  
         [0050]    First aperture  34  of adsorber  24  is opened next to first feed compartment  52 , which is maintained at substantially the higher pressure  151  by a third feed supply means  165 . Once the adsorber pressure has risen to substantially the higher working pressure  151 , its second aperture  35  (which has been closed to all second compartments since time  156 ) opens to light product compartment  70  and delivers light product  166  which is typically richer in the less readily adsorbed component than that provided by the supply means  160 ,  162 ,  165 .  
         [0051]    In the cycle of FIG. 6, first aperture  34  of adsorber  24  is opened next to second feed compartment  54 , also maintained at substantially the higher pressure  151  by a fourth feed supply means  167 . In general, the fourth feed supply means supplies a second feed gas, relatively richer in the more readily adsorbed component than the first feed gas provided by the first, second and third feed supply means. In the specific cycle illustrated in F  6 , the fourth feed supply means  167  is a “heavy reflux” compressor, recompressing a portion of the heavy product back into the apparatus. In the cycle illustrated in FIG. 5, there is no fourth feed supply means, and compartment  54  could be eliminated or consolidated with compartment  52  extended over a wider angular arc of the stator.  
         [0052]    While feed gas is still being supplied to the first end of adsorber  24  from either compartment  52  or  54 , the second end of adsorber  24  is closed to light product compartment  70  and opens to first light reflux exit compartment  72  while delivering “light reflux” gas (enriched in the less readily adsorbed component, similar to second product gas) to first light reflux pressure let-down means (or expander)  170 . The first aperture  34  of adsorber  24  is then closed to all first function compartments, while the second aperture  35  is opened successively to (a) second light reflux exit compartment  74 , dropping the adsorber pressure to the first cocurrent blowdown pressure  171  while delivering light reflux gas to second light reflux pressure letdown means  172 , (b) third light reflux exit compartment  76 , dropping the adsorber pressure to the second cocurrent blowdown pressure  173  while delivering light reflux gas to third light reflux pressure letdown means  174 , and (c) fourth light reflux exit compartment  78 , dropping the adsorber pressure to the third cocurrent blowdown pressure  175  while delivering light reflux gas to fourth light reflux pressure letdown means  176 . Second aperture  35  is then closed for an interval, until the light reflux return steps following the countercurrent blowdown steps.  
         [0053]    The light reflux pressure let-down means may be mechanical expanders or expansion stages for expansion energy recovery, or may be restrictor orifices or throttle valves for irreversible pressure let-down.  
         [0054]    Either when the second aperture is closed after the final light reflux exit step (as shown in FIGS. 5 and 6), or earlier while light reflux exit steps are still underway, first aperture  34  is opened to first countercurrent blowdown compartment  56 , dropping the adsorber pressure to the first countercurrent blowdown intermediate pressure  180  while releasing “heavy” gas (enriched in the more strongly adsorbed component) to first exhaust means  181 . Then, first aperture  34  is opened to second countercurrent blowdown compartment  58 , dropping the adsorber pressure to the first countercurrent blowdown intermediate pressure  182  while releasing heavy gas to second exhaust means  183 . Finally reaching the lower working pressure, first aperture  34  is opened to heavy product compartment  60 , dropping the adsorber pressure to the lower pressure  152  while releasing heavy gas to third exhaust means  184 . Once the adsorber pressure has substantially reached the lower pressure while first aperture  34  is open to compartment  60 , the second aperture  35  opens to purge compartment  80 , which receives fourth light reflux gas from fourth light reflux pressure let-down means  176  in order to displace more heavy gas into first product compartment  60 .  
         [0055]    In FIG. 5, the heavy gas from the first, second and third exhaust means is delivered as the heavy product  185 . In FIG. 6, this gas is partly released as the heavy product  185 , while the balance is redirected as “heavy reflux”  187  to the heavy reflux compressor as fourth feed supply means  167 . Just as light reflux enables an approach to high purity of the less readily adsorbed (“light”) component in the light product, heavy reflux enables an approach to high purity of the more readily adsorbed (“heavy”) component in the heavy product.  
         [0056]    The adsorber is then repressurized by light reflux gas after the first and second apertures close to compartments  60  and  80 . In succession, while the first aperture  34  remains closed at least initially, (a) the second aperture  3   5  is opened to first light reflux pressurization compartment  82  to raise the adsorber pressure to the first light reflux pressurization pressure  190  while receiving third light reflux gas from the third light reflux pressure letdown means  174 , (b) the second aperture  35  is opened to second light reflux pressurization compartment  84  to raise the adsorber pressure to the second light reflux pressurization pressure  191  while receiving second light reflux gas from the second light reflux pressure letdown means  172 , and (c) the second aperture  35  is opened to third light reflux pressurization compartment  86  to raise the adsorber pressure to the third light reflux pressurization pressure  192  while receiving first light reflux gas from the first light reflux pressure letdown means  170 . Unless feed pressurization has already been started while light reflux return for light reflux pressurization is still underway, the process (as based on FIGS. 5 and 6) begins feed pressurization for the next cycle after time  157  as soon as the third light reflux pressurization step has been concluded.  
         [0057]    The pressure variation waveform in each adsorber would be a rectangular staircase if there were no throttling in the first and second valves. In order to provide balanced performance of the adsorbers, preferably all of the apertures are closely identical to each other.  
         [0058]    The rate of pressure change in each pressurization or blowdown step will be restricted by throttling in ports (or in clearance of labyrinth sealing gaps) of the first and second valve means, or by throttling in the apertures at first and second ends of the adsorbers, resulting in the typical pressure waveform depicted in FIGS. 5 and 6. Alternatively, the apertures may be opened slowly by the seal strips, to provide flow restriction throttling between the apertures and the seal strips, which may have a serrated edge (e.g. with notches or tapered slits in the edge of the seal strip) so that the apertures are only opened to full flow gradually. Excessively rapid rates of pressure change would subject the adsorber to mechanical stress, while also causing flow transients which would tend to increase axial dispersion of the concentration wavefront in the adsorber. Pulsations of flow and pressure are minimized by having a plurality of adsorbers simultaneously transiting each step of the cycle, and by providing enough volume in the function compartments and associated manifolds so that they act effectively as surge absorbers between the compression machinery and the first and second valve means.  
         [0059]    It will be evident that the cycle could be generalized by having more or fewer intermediate stages in each major step of feed pressurization, countercurrent blowdown exhaust, or light reflux. Furthermore, in air separation or air purification applications, a stage of feed pressurization (typically the first stage) could be performed by equalization with atmosphere as an intermediate pressure of the cycle. Similarly, a stage of countercurrent blowdown could be performed by equalization with atmosphere as an intermediate pressure of the cycle.  
         [0060]    [0060]FIG. 7 
         [0061]    [0061]FIG. 7 shows a vacuum pressure swing adsorption (VPSA) air separation system  200 , with a multistage or split stream centrifugal compressor  201  and a multistage or split stream exhaust pump  202 . The rotary adsorber module  203  includes rotor  11  and a stator assembly comprising a first valve stator  40  and a second valve stator  41 . Rotor  11  may be configured for radial flow as suggested in FIG. 7, or for axial flow.  
         [0062]    Rotor  11  contains “N” adsorbers  24  with the flow path oriented radially between first end  30  and second end  31  of the adsorbers  24 . The adsorber first ends  30  open by apertures  34  to a sealing face  207  with the first valve stator  40 . Sealing face  207  has ports  209  to define the first valve means  21 . First valve stator  40  has a plurality of functional compartments in fluid communication to sealing face  207  by ports  209 , including a first feed pressurization supply compartment  46 , a second feed pressurization supply compartment  50 , a first countercurrent blowdown exhaust compartment  56 , a second countercurrent blowdown exhaust compartment  58 , and a purge exhaust compartment  60  at substantially the lower pressure.  
         [0063]    The adsorber second ends  31  open by apertures  35  to a sealing face  210  with the second valve stator  41 . Sealing face  210  has ports  212  to define the second valve means  23 . Second valve stator  41  includes, with each compartment in fluid communication to sealing face  210  by ports  212 , a light product delivery compartment  70  at substantially the higher pressure, a first light reflux exit compartment  72  which is, in the embodiment shown, the downstream end of compartment  70 , a second light reflux exit compartment  74 , a third light reflux exit compartment  76 , a fourth light reflux exit compartment  78 , a fourth light reflux return compartment  80  for purge at substantially the lower pressure, a third light reflux return compartment or first light reflux pressurization compartment  86 , a second light reflux return compartment or second light reflux pressurization compartment  84 , and a first light reflux return compartment or third light reflux pressurization compartment  82 . The angular spacing of ports communicating to the compartments in the first and second valve stators  40  and  41  defines the timing of the PSA cycle steps similar to the cycles in FIGS. 5 and 6.  
         [0064]    In this example, sealing faces  207  and  210  are respectively defined by the outer and inner radii of the annular rotor  11 . Fluid sealing between the functional compartments and corresponding sealing faces is achieved by clearance seals. The clearance seals are provided by slippers  220  attached to the first and second valve stators by partitions  27 . Partitions  27  provide static sealing between adjacent compartments. Slippers  220  engage the sealing faces with narrow fluid sealing clearances, which also provide throttling of gas flows between the adsorbers and functional compartments in each pressure-changing step, so that each adsorber may smoothly equalize in pressure to the pressure of the next functional compartment about to be opened to that adsorber. In addition to the functional compartments, static pressure balancing compartments (e.g.  214  and  216 ) are provided behind some clearance seal slippers. The static pressure balancing compartments are disposed in angular sectors of the first and second valve stators not used as functional compartments, in order to establish a controlled pressure distribution behind the clearance slippers so as to maintain their positive sealing engagements without excessive contact pressure and consequent friction.  
         [0065]    Apparatus  200  has a feed air inlet filter  222 , from which feed air is conveyed through optional dehumidifier  224  and conduit  226  to feed compressor inlet  228 . In this example, the first intermediate feed pressurization pressure is selected to be substantially atmospheric pressure, so conduit  226  also communicates to first feed pressurization compartment  46 . The feed compressor  201  has a first discharge port  230  at the second intermediate feed pressurization pressure communicating by conduit  232  and optional dehumidifier  234  to compartment  50  and a second discharge port  236  at substantially the higher pressure of the cycle pressure communicating by conduit  238  and optional dehumidifier  240  to compartment  52 .  
         [0066]    Exhaust vacuum pump  202  has a first inlet port  242  at substantially the lower pressure of the cycle in fluid communication with the exhaust compartment  60 , a second inlet port  244  at the second countercurrent blowdown pressure in fluid communication with compartment  56 , and a third inlet port  248  at the first countercurrent blowdown pressure in fluid communication with compartment  56 . Vacuum pump  202  compresses the combined exhaust and countercurrent blowdown gas to form a second product gas enriched in the more readily adsorbed component to substantially atmospheric pressure, and discharges the second product gas from discharge port  248 .  
         [0067]    In the option of light reflux pressure let-down without energy recovery, throttle valves  247  provide pressure let-down for each of four light reflux stages, respectively between light reflux exit and return compartments  72  and  82 ,  74  and  84 ,  76  and  86 , and  78  and  80 . Actuator means  249  is provided to adjust the orifices of the throttle valves.  
         [0068]    [0068]FIGS. 8, 9,  10  and  11   
         [0069]    Referring to FIG. 8, an axial flow rotary PSA module  250  is shown, particularly suitable for smaller scale oxygen generation. The flow path in adsorbers  24  is parallel to axis  251 . The steps of the process and functional compartments are still in the same angular relationship regardless of a radial or axial flow direction in the adsorbers. FIGS. 9, 10, and  11  depict cross sections of module  250  in the planes respectively defined by arrows  252 - 253 ,  254   255 , and  256 - 257  in FIG. 8. FIG. 8 is an axial section of module  250  through compartments  52  and  70  at the higher pressure, and compartments  80  and  117  at the lower pressure. The adsorber rotor  11  contains “N” adsorbers  24  in adsorber wheel  258 , and revolves between the first valve stator  40  and the second valve stator  41 . Compressed feed air is supplied to compartment  52  as indicated by arrow  259 , while nitrogen enriched exhaust gas is exhausted from purge exhaust compartment  60  as indicated by arrow  260 .  
         [0070]    At the ends of rotor  11 , circumferential seals  262  and  264  bound sealing face  207 , and circumferential seals  266  and  268  bound second sealing face  210 . The sealing faces are flat discs. The circumferential seals also define the ends of clearance slippers  220  in the sealing faces between the functional compartments. Rotor . 11  is supported by bearing  270  in housing  272 , which is integrally assembled with the first and second valve stators. Rotor  11  is driven by rim motor  274 , which may have a friction, geared or belt engagement with the outer rim of rotor  11 . By installing rim motor  274  within housing  272 , the module is totally enclosed so as to preclude leakage, either of hazardous process fluids (in this example, enriched oxygen) to the external environment, or of atmospheric contaminants (e.g. humidity which could deactivate the adsorbent) into the apparatus.  
         [0071]    Illustrating the option of light reflux pressure letdown with energy recovery, a split stream light reflux expander  276  is provided to provide pressure let-down of four light reflux stages with energy recovery. The light reflux expander provides pressure let-down for each of four light reflux stages, respectively between light reflux exit and return compartments  72  and  82 ,  74  and  84 ,  76  and  86 , and  78  and  80 .  
         [0072]    Light reflux expander  276  is coupled to a light product pressure booster compressor  278  by drive shaft  280 . Compressor  278  receives the light product from conduit  25 , and delivers light product (compressed to a delivery pressure above the higher pressure of the PSA cycle) to delivery conduit  280 . Since the light reflux and light product are both enriched oxygen streams of approximately the same purity, expander  276  and light product compressor  278  may be hermetically enclosed in a single housing. This configuration of “turbocompressor” oxygen booster without a separate drive motor is advantageous, as a useful pressure boost of the product oxygen can be achieved without an external motor and corresponding shaft seals, and can also be very compact when designed to operate at very high shaft speeds.  
         [0073]    [0073]FIG. 9 shows the first valve face of module  250  of FIG. 8, at section  252 - 253 , with fluid connections to a multistage or split stream feed compressor  201  and to a multistage or split stream countercurrent blowdown expander  280  as in FIG. 8.  
         [0074]    Arrow  281  indicates the direction of rotation by adsorber rotor  11 . The open area of valve face  207  ported to the feed and exhaust compartments is indicated by clear angular segments  46 - 116  corresponding to those functional compartments, between circumferential seals  262  and  264 . The substantially closed area of valve face  207  between functional compartments is indicated by cross-hatched sectors  282  and  283  which are clearance slippers  220 . Typical closed sector  282  provides a transition for an adsorber, between being open to compartment  56  and open to compartment  58 . Gradual opening is provided by a tapering clearance channel between the slipper and the sealing face, so as to achieve gentle pressure equalization of an adsorber being opened to a new compartment. Much wider closed sectors, such as sector  283 , are provided to substantially close flow to or from one end of the adsorbers when pressurization or blowdown is being performed from the other end.  
         [0075]    [0075]FIG. 10 shows the second valve face of module  200  of FIG. 8, at section  254 - 255 , with fluid connections to a split stream light reflux expander  276  and light product booster compressor  278  as in FIG. 5. Fluid sealing principles and alternatives are similar to those of FIG. 9. Similar principles and alternatives apply to radial flow and axial flow geometries, respectively sealing on cylindrical or disc faces.  
         [0076]    [0076]FIG. 11 shows an adsorber wheel configuration for the embodiment of FIG. 8, at section  256 - 257 . The adsorber configuration of FIG. 11 is similar to a radial flow geometry shown in FIG. 1- 4 , and is characterized by seventy-two adsorbers  24  (i.e. N=72). The adsorbers  24  are mounted between outer wall  284  and inner wall  286  of adsorber wheel  258 . Each adsorber comprises a rectangular flat pack of adsorbent sheets  288 , with spacers  290  between the sheets to define flow channels here in the axial direction. Separators  292  are provided between the adsorbers to fill void space and prevent leakage between the adsorbers.  
         [0077]    The adsorbent sheets comprise a reinforcement material, in preferred embodiments glass fibre, metal foil or wire mesh, to which the adsorbent material is attached with a suitable binder. For air separation to produce enriched oxygen, typical adsorbents are X, A or chabazite type zeolites, typically exchanged with lithium, calcium strontium and/or other cations, and with optimized silicon/aluminum ratios as well known in the art. The zeolite crystals are bound with silica, clay and other binders, or self-bound, within the adsorbent sheet matrix.  
         [0078]    Satisfactory adsorbent sheets have been made by coating a slurry of zeolite crystals with binder constituents onto the reinforcement material, with successful examples including nonwoven fiber glass scrims, woven metal fabrics, and expanded aluminum foils. Spacers are provided by printing or embossing the adsorbent sheet with a raised pattern, or by placing a fabricated spacer between adjacent pairs of adsorbent sheets. Alternative satisfactory spacers have been provided as woven metal screens, non-woven fiber glass scrims, and metal foils with etched flow channels in a photolithographic pattern.  
         [0079]    Typical experimental sheet thicknesses have been 150 microns, with spacer heights in the range of 100 to 150 microns, and adsorber flow channel length approximately 20 cm. Using X type zeolites, excellent performance has been achieved in oxygen separation from air at PSA  
         [0080]    [0080]FIG. 12 
         [0081]    Referring to FIG. 12, a longitudinal cross-sectional view of a two-stage gas separation module  300  is shown having a first stage module  301 , and a second stage module  302  both configured for axial gas flow, with the first module having a first adsorber wheel and the second module having a second adsorber wheel, and the two modules being integrated with both wheels in a single housing  272 . However, it should be understood that the invention is not limited to axial flow configurations. Accordingly, in one variation (not shown), the modules  301 ,  302  are configured for radial flow with one of the modules  301 ,  302  being disposed within the inner radius of the other of the modules  301 ,  302 .  
         [0082]    The first stage  301  is a chemical desiccant dryer having alumina gel as an adsorbent material, and includes a plurality of first feed gas function compartments corresponding to pressurization compartments  46 ,  50 ,  52  of the rotary module  10 , a plurality of first product function compartments corresponding to light reflux exit compartments  72 ,  74 ,  76 ,  78 , a plurality of second feed gas function compartments corresponding to light reflux return compartments  80 ,  82 ,  84 ,  86 , and a plurality of second product function compartments, which correspond respectively to blowdown compartments  56 ,  58 ,  60 .  
         [0083]    The second stage  302  is an axial flow oxygen-PSA concentrator, similar to the axial flow rotary PSA module  250  shown in FIG. 8, including lithium and/or calcium exchanged low silica faujasite adsorbents. As in FIG. 8, the oxygen-PSA concentrator includes a plurality of first feed gas function compartments, a plurality of light reflux exit function compartments, a light product compartment  70 , a plurality of light reflux return function compartments (such as light reflux return compartment  80 ), and a plurality of countercurrent blowdown compartments. The first product function compartments of the first stage  301  communicate with the first feed gas function compartments of the second stage  302  through respective connecting compartments, such as compartment  304 . Similarly, the countercurrent blowdown function compartments of the second stage  302  communicate with the second feed gas function compartments of the first stage  301  through respective connecting compartments, such as compartment  305 . In addition, a split stream light reflux expander  276  is provided to provide pressure let-down for the light reflux stages of the second stage module  302  with energy recovery.  
         [0084]    In operation, compressed humid air is introduced into the first module  301  in the sector open to compartment  52 . A product gas comprising dehydrated compressed air exits module  301  and flows through connecting compartment  304  into the second module  302 . Gas entering the second module  302  is further purified to produce a relatively pure oxygen stream flowing out of module  302  and into compartment  70 . Simultaneously, the exhaust step at the lower pressure is conducted with purge oxygen entering the second adsorber wheel of module  302  in the sector open to compartment  80 , via the light reflux expander  276 . Enriched nitrogen is exhausted from the second adsorber wheel to the first adsorber wheel through connecting compartment  305 , and humid nitrogen enriched air is exhausted from the first adsorber wheel to compartment  60 .  
         [0085]    Preferably, the rotational frequencies, angular interval for each step, and other characteristics of each module  301 ,  302  are tailored to suit the contemplated gaseous separation. Accordingly, for effective removal of water from the feed air received by the first module  301 , and for effective separation of oxygen gas from the dry air received by the second module  301  from the first module  301 , preferably the rotor in the first module  301  is rotated at a speed of approximately 10 to 20 RPM, and the rotor in the second module  302  is rotated at a speed of approximately 50 to 100 RPM.  
         [0086]    It will be appreciated that by operating the first module  301  and the second module  302  with different rotational frequency and angular intervals, both of the modules  301 ,  302  will be exposed to pressure variations which can stress the associated compression machinery and reduce the overall efficiency of the chemical separation occurring in each module  301 ,  302 . Accordingly, preferably the first module  301  and the second module  302  each comprises a rotary module  10  so that the first product function compartments and the second feed gas function compartments are maintained at substantially constant pressure levels and, therefore, the rate of gas flow between the first stage module  301  and the second stage module  302  is substantially constant. However, other gas separation modules, besides the rotary module  10 , may be used for maintaining constant pressure levels across the connecting compartments  304 ,  305 .  
         [0087]    It will also be appreciated that by employing different adsorbers in the first and second module  301 ,  302 , the apparatus  300  can be configured to obtain results previously not possible with only a single adsorbent. For instance, nitrogen selective lithium zeolites are a preferred adsorbent for separating oxygen gas from air. However, it is known that such adsorbent material are prone to deactivation when exposed to humid air. Accordingly, by employing a chemical desiccant dryer as the first stage  301 , the apparatus  300  is able to achieve favorable separation without deactivation of the expensive lithium zeolites.  
         [0088]    However, it should be understood that the invention is not limited to a first stage comprising a desiccant dryer. Rather, other adsorbent materials maybe used in the first stage  301  without departing from the scope of the invention. Further, the first stage  301 , and the second stage  302  may employ similar adsorbent materials for improved concentration of product gases. In addition, the invention may employ more than two stages, with each stage delivering a different product gas or with each stage delivering the same product gas but with different levels of purity. Alternately, any of the stages may deliver a product gas to another stage for further processing.  
         [0089]    [0089]FIG. 13 
         [0090]    [0090]FIG. 13 shows a two stage apparatus  300 ′ according to the invention, comprising two  30  rotary PSA modules  301 ′ and  302 ′, for separating multicomponent mixtures. In the embodiment shown, each of the rotary PSA modules  301 ′ and  302 ′ comprise the radial flow rotary PSA module illustrated in FIGS. 1 through 4 and having its rotor unrolled in a 360° section about its rotary axis. Alternatively, modules  301 ′ and  302 ′ can each be an axial flow rotary PSA module illustrated in FIGS. 8 through 11. The modules  301 ′ and  302 ′ are connected via connecting compartments  304 ′ and  305 ′ such that compartment  304 ′ feeds product gas from modules  301 ′ and  302 ′ and compartment  305 ′ feeds product gas from module  302 ′ to  301 ′.  
         [0091]    The embodiment shown in FIG. 13 illustrates that the cycles for the first and second stages  301 ′ and  302 ′ need not be identical as to basic flow pattern. In this embodiment, the first stage  301 ′ achieves initial pressurization by a feed pressurization step via throttle orifice  350 ′ and compartment  351 ′, whereas the second stage  302 ′ achieves initial pressurization by light reflux from expander  276 . As a further example of how flow patterns can be tailored for each module, the first stage  301 ′ of this embodiment achieves initial blowdown via throttle orifice  360 ′ and compartment  361 ′, whereas the second stage  302 ′ achieves initial blowdown cocurrently by light reflux into expander  276 .  
         [0092]    [0092]FIG. 14 
         [0093]    [0093]FIG. 14 shows a two stage rotary PSA apparatus  400  with combined pressure swings and thermal regeneration of the first stage  401 . In the embodiment shown, modules  401  and  402  each comprise the radial flow rotary PSA module illustrated in FIGS. 1 through 4 and having its rotor unrolled in a 360° section about its rotary axis. Alternatively, modules  401  and  402  can each be an axial flow rotary PSA module illustrated in FIGS. 8 through 11. First stage  401  achieves initial pressurization by a feed pressurization step via throttle orifice  350  and compartment  351 , and achieves initial blowdown via throttle orifice  360  and compartment  361 .  
         [0094]    Vacuum pump  202  is provided to pull a vacuum for desorbing adsorbent  24  in module  402 , thereby effecting vacuum regeneration. The exhaust of the vacuum pump  202 , already heated by compression, is further heated in heat exchanger  410 , and then used to purge the first module  301  at substantially atmospheric pressure. While vacuum regeneration is operative with respect to the second module  402 , the first module  401  of this embodiment does not operate under vacuum and hence operates with a lower overall upper to lower pressure ratio.  
         [0095]    Regeneration in the first module  401  is achieved in part by heating gas used to purge first module  401  with heat exchanger  410 . Since the thermal swing operation requires heat exchange with the adsorbent in module  401 , the rotor in module  401  operates at a lower rotational speed, of about 0.5 to 3 RPM, relative to the rotor of module  402 .  
         [0096]    [0096]FIG. 15 
         [0097]    [0097]FIG. 15 shows a two stage apparatus  500 , comprising two rotary PSA modules  501  and  502 , capable of substantially complete separation of a two component mixture. In the embodiment shown, end of modules  501  and  502  comprise the rotary PSA module illustrated in FIGS. 1 through 4 and having its rotor unrolled in a  360  section about its rotary axis. Alternatively, modules  501  and  502  can each be an axial flow rotary PSA module illustrated in FIGS. 8 through 11.  
         [0098]    Light reflux is used in the second stage module  502  to provide a high purity light product. A heavy reflux compressor  511  is used in the first stage module  501  to provide a high purity heavy product, or equivalently to achieve very high recovery of the light product. The heavy product is delivered from conduit  510 , which may be connected to the inlet or any delivery port of the heavy reflux compressor  511  according to the desired delivery pressure of the heavy product.  
         [0099]    The feed is introduced to connecting manifolds  521 ,  522  and  523  communicating between compartments of the first and second stage modules  501  and  502 . A purge is also released from conduit  550  communicating to a connecting compartment between the first and second stages modules  501  and  502 . This purge allows higher parities to be achieved when it is desired to purify both light and heavy products simultaneously.  
         [0100]    It will be appreciated that any of the two-stage systems illustrated in FIGS. 12, 13, or  14  can be used as air separators to produce oxygen from humid or contaminated air. In such cases, the adsorbent  24  of the first stage rotor is a desiccant for removing water, carbon dioxide, and any vapor contaminants from the feed air. The second stage rotor removes nitrogen for air separation. The first stage preferably operates at a lower frequency, particularly if thermal swing regeneration is used as in the case of the embodiment shown in FIG. 14. During shut-down, isolation valves in each of the conduits interconnecting the first and second stage rotors can be closed, in order to prevent diffusive migration of water vapor out of the desiccant and into the air separation zeolite adsorbent which could thereby be deactivated.  
         [0101]    However, as discussed above, the invention has applications not limited to oxygen separation. For instance, in one variation, the embodiment shown in FIG. 13 is applied to hydrogen separation from syngas, syngas being those gaseous products produced from natural gas by steam methane reforming. The first stage rotor removes water and carbon dioxide. The second stage rotor removes carbon monoxide, methane and nitrogen impurities from the hydrogen.  
         [0102]    In another variation, the apparatus of FIG. 13 is used to separate hydrogen from refinery offgases, such as hydrotreater purge gas or catcracker gas. The first stage rotor removes heavier hydrocarbon vapors and hydrogen sulfide. The second stage rotor removes light hydrocarbon impurities from the hydrogen. In either of these embodiments, the adsorbent used in the rotor for each stage is different.  
         [0103]    In another variation, the apparatus shown in FIG. 14 is used for the enrichment of methane from landfill gas, with the first stage removing water vapor and contaminant vapors, and the second stage removing carbon dioxide.  
         [0104]    In yet another variation, the apparatus illustrated in FIG. 15 is used as an air separator to produce nitrogen, or to produce oxygen and nitrogen simultaneously. The air feed is introduced to the first end of the second stage rotor, which has light reflux to produce purified oxygen. The first stage rotor has heavy reflux to produce purified nitrogen at its first end.  
         [0105]    In still another variation, the apparatus depicted in FIG. 15 is used to separate hydrogen from steam reformate syngas, to produce purified hydrogen and carbon dioxide simultaneously. The syngas feed is introduced to the first end of the second stage rotor, which has light reflux to produce purified hydrogen. The first stage rotor has heavy reflux to produce purified carbon dioxide at its first end.  
         [0106]    The present invention is defined by the claims appended hereto, with the foregoing description being illustrative of the preferred embodiments of the present invention. Those of ordinary skill may envisage certain additions, deletions or modifications to the described embodiments which, although not explicitly disclosed herein, do not depart from the spirit or scope of the invention as defined by the appended claims.