Abstract:
A rotary module for implementing a high frequency pressure swing adsorption process comprises a stator and a rotor rotatably coupled to the stator. 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 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, and a plurality of flow paths for receiving adsorbent material therein. Each flow path includes a pair of opposite ends, and a plurality of apertures provided in the rotor valve surfaces and in communication with the flow path ends and the function ports for cyclically exposing each said flow path to a plurality of discrete pressure levels between the upper and lower pressures for maintaining uniform gas flow through the first and second function compartments.

Description:
This application is a continuation of PCT/CA98/01103 filed Dec. 1, 1998 (designating the United States), which claims priority to Ser. No. 60/067,120, filed Dec. 1, 1997. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an apparatus for separating gas fractions from a gas mixture having multiple gas fractions. In particular, the present invention relates to a rotary valve gas separation system having a plurality of rotating adsorbent beds disposed therein for implementing a pressure swing adsorption process for separating out the gas fractions. 
     BACKGROUND OF THE INVENTION 
     Pressure swing adsorption (PSA) and vacuum pressure swing adsorption (VPSA) 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 or VPSA 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, and a “heavy” product (a gas fraction enriched in the more strongly adsorbed component) is exhausted from the first end of the bed. 
     The conventional system for implementing pressure swing adsorption or vacuum 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. 
     Furthermore, the conventional PSA or VPSA 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. In PSA, energy expended in compressing the feed gas used for pressurization is then dissipated in throttling over valves over the instantaneous pressure difference between the absorber and the high pressure supply. Similarly, in VPSA, where the lower pressure of the cycle is established by a vacuum pump exhausting gas at that pressure, energy is dissipated in throttling over valves during countercurrent blowdown of adsorbers whose pressure is being reduced. A further energy dissipation in both systems occurs in throttling of light reflux gas used for purge. equalization. cocurrent blowdown and product pressurization or backfill steps. 
     Numerous attempts have been made at overcoming the deficiencies associated with the conventional PSA or VPSA 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), Petit et al (U.S. Pat. No. 5,441,559) and Schartz (PCT publication WO 94/04249) disclose PSA devices using rotary distributor valves having rotors 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 distributor valves are impracticable for large PSA/VPSA 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 considerable dead volume for flow distribution and collection. As a result, the prior art rotary distributor valves have poor flow distribution, particularly at high cycle frequencies. 
     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, stressing the compression machinery and causing large fluctuations in overall power demand. Because centrifugal or axial compression machinery cannot operate under such unsteady conditions, rotary lobe machines are typically used in such systems. However, such machines have lower efficiency than modern centrifugal compressors/vacuum pumps working under steady conditions. 
     Accordingly, there remains a need for a PSA/VPSA system which is suitable for high volume and high frequency production, while reducing the losses associated with the prior art devices. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a rotary module for implementing a high frequency pressure swing adsorption process with high energy efficiency. 
     The rotary module, in accordance with the invention, comprises a stator and a rotor rotatably coupled to the stator. 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 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, and a plurality of flow paths for receiving adsorbent material therein. Each said flow path includes a pair of opposite ends. and a plurality of apertures provided in the rotor valve surfaces and in communication with the flow path ends and the function ports for cyclically exposing each said flow path to a plurality of discrete pressure levels between the upper and lower pressures for maintaining uniform gas flow through the first and second function compartments. 
     During pressurization and blowdown steps, the several adsorbers passing through the step will converge to the nominal pressure level of each step by a throttling pressure equalization from the pressure level of the previous step experienced by the adsorbers. Flow is provided to the adsorbers in a pressurization step or withdrawn in a blowdown step by compression machinery at the nominal pressure level of that step. Hence flow and pressure pulsations seen by the compression machinery at each intermediate pressure level are minimal by averaging from the several adsorbers passing through the step, although each absorber undergoes large cyclic changes of pressure and flow. 
     During the pressurization steps for each adsorber, either (or both) of the apertures of an adsorber already at a pressure is (are) opened respectively to a first or second pressurization compartment at a stepwise higher pressure. Similarly, during the pressurization steps for each adsorber, either (or both) of the apertures of an adsorber already at a pressure is (are) opened respectively to a first or second pressurization compartment at a stepwise lower pressure. Equalization then takes place by flow through the open aperture(s) from the pressurization/blowdown compartment into the adsorber, which by the end of the pressurization/blowdown step has attained approximately the same pressure as the pressurization/blowdown compartment(s). Each pressurization/blowdown compartment is in communication with typically several adsorbers being pressurized (in differing angular and time phase) at any given time, so the pressure in that compartment and the pressurization flow to that compartment are substantially steady. 
     The flow path through the adsorbers may be radial or axial. If the adsorbers are configured for radial flow, the first valve surface would preferably be radially inward when the less strongly adsorbed gas fraction has much higher density that the more strongly adsorbed fraction, and the first valve surface would preferably be radially outward when the less strongly adsorbed gas fraction has much lower density than the more strongly adsorbed fraction. Hence, for hydrogen purification in a radial flow embodiment, the feed gas would preferably be admitted to (and the higher molecular weight impurity fraction as heavy product is exhausted from) the first valve surface at an outer radius, while the hydrogen as first product gas is delivered from the second valve surface. 
     The present invention also includes the alternatives of (1) layered or laminated thin sheet adsorbers and (2) the centrifugally stabilized fine particle granular adsorbers to enable operation at exceptionally high cycle frequency. PSA cycle frequencies to at least 100 cycles per minute are practicable within the present invention, and will enable process intensification so that high productivity can be realized from compact modules. Cycle frequencies more rapid than about 50 cycles per minute will be achieved preferably with the layered thin sheet adsorbers, with the flow path in flow channels tangential to and between adjacent pairs of adsorbent loaded sheets, to obtain lower frictional pressure drop at high frequency than granular adsorbent. 
     Preferably, the increments between adjacent pressure levels are sized so that the gas flows entering or exiting the module are substantially steady in both flow velocity and pressure. As a result, the module can be operated with centrifugal or axial flow compressors and expanders. for most favourable efficiency and capital cost economies of scale. To reduce throttling losses, it is also preferred that the function compartments are shaped to provide uniform gas flow through the flow paths and/or the valve surfaces include sealing strips having tapered portions for providing uniform gas flow through the flow paths. 
     Since the orifices providing the valving function are immediately adjacent to the ends of the flow paths, the dead volume associated with prior art distribution manifolds is substantially reduced. Also, since the compartments communicating with the first and second valve surfaces are external to the valving function, the compartments do not contribute to dead volume of the adsorbers. As a result, high frequency pressure/vacuum swing adsorption is possible. 
     Also, in contrast to prior art PSA devices whose pressure vessels are subject to pressure cycling and consequent fatigue loading, the pressure vessel of the present invention operates under substantially static stresses, because each of the compartments operates under steady pressure conditions. Mechanical stresses on the rotor and its bearings are relatively small, because only small frictional pressure drops (at most equal to the interval between adjacent intermediate pressures) apply in the flow direction, while transverse pressure gradients between the adsorber elements are also small owing to the large number of elements. These features are important, since pressure vessel fatigue is a major concern and limitation in the design of PSA systems, especially working with corrosive gases or hydrogen at higher pressure or higher cycle frequency. 
     Further, by providing multiple closely spaced intermediate pressure levels, with substantially constant flow and pressure at each level, the present invention facilitates energy efficient application of multistage feed compressors and vacuum pumps (including centrifugal or axial compression machines) for feed compression, heavy product exhaust and heavy reflux compression; as well as multistage expanders (including radial inflow turbines, axial turbines and partial admission impulse turbines). Positive displacement (reciprocating piston, rotary piston, or progressive cavity such as screw or scroll machines) compression and expansion machinery may also be applied within the scope of the invention, particularly when adapted to deliver gas at multiple intermediate delivery pressures and/or to intake gas at multiple intermediate inlet pressures. The invention enables use of single shaft machines to provide all compression and expansion functions for a plurality of modules in parallel, as well as the combined use of motor driven and free rotor machines for more flexible modulezation and splitting of stages. 
     The inventive concept of split stream centrifugal machinery is a desirable option for the described PSA process which requires various enthalpies in separate fluid streams at differing total pressures. The split stream machine has multiple inlet flows at multiple enthalpies, and/or multiple exit flows at multiple enthalpies, for a single centrifugal or radial flow impeller. The differing changes in ecthalpy or total pressure are achieved by having a different change in radius, or differing blade angles, for each flow across the impeller. A split stream compressor has one inlet but numerous outlets at different total pressures or enthalpy levels from a single impeller. A split stream exhauster may be a vacuum pump or an expander, and will have multiple inlets and a single outlet at different total pressures or enthalpy levels for a single impeller. Also useful in the present invention is a split stream light reflux expander having a number of inlets and the same number of outlets, at different total pressures or enthalpy levels for a s ingle impeller. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments of the present invention will now be described, by way o f example only, and with reference to the drawings, in which like reference numerals indicate like elements, and in which: 
     FIG. 1 is a sectional view of a rotary PSA module according to the invention; 
     FIG. 2 is the stator of the module of FIG. 1; 
     FIG. 3 is the rotor of the module of FIG. 1; 
     FIG. 4 is an axial section of the module of FIG. 1; 
     FIG. 5 shows an alternative adsorber configuration using layered adsorbent sheets; 
     FIG. 6 shows a typical PSA cycle according to the invention; 
     FIG. 7 shows a PSA cycle with heavy reflux; 
     FIG. 8 shows a PSA apparatus with a single rotary module and energy recovery; 
     FIG. 9 shows a vacuum PSA (VPSA) for oxygen separation from air; 
     FIG. 10 shows a VPSA apparatus without light reflux energy recovery; 
     FIG. 11 shows a PSA apparatus adapted to receive two feed gas mixtures, and with recompression of tall gas; 
     FIG. 12 shows a PSA apparatus with heavy reflux; 
     FIG. 13 shows a PSA apparatus with a free rotor tail gas compressor or vacuum pump, powered by energy recovery; 
     FIG. 14 shows another embodiment of a PSA apparatus with a free rotor compressor; 
     FIG. 15 shows a VPSA apparatus with  4  modules; 
     FIG. 16 shows a PSA apparatus with  5  modules; 
     FIG. 17 shows a simplified schematic of a VPSA cycle for oxygen production, using a split stream air compressor, a split stream vacuum pump as the countercurrent blowdown exhauster, and a split stream light reflux expander powering a product oxygen compressor; 
     FIG. 18 shows a radial flow rotary PSA module; 
     FIG. 19 shows an axial flow rotary PSA module; 
     FIG. 20 shows a double axial flow rotary PSA module; 
     FIG. 21 shows the first valve face of the embodiment of FIG. 19; 
     FIG. 22 shows the second valve face of the embodiment of FIG. 19; 
     FIG. 23 shows an adsorber wheel configurations based on laminated adsorbent sheet adsorbers for the embodiment of FIG. 19; 
     FIG. 24 shows a multistage centrifugal compressor with impulse turbine expanders for the light reflux and countercurrent blowdown; 
     FIG. 25 shows the light reflux impulse turbine runner with four nozzles; 
     FIG. 26 is an unrolled view of the light reflux expander impulse turbine; 
     FIG. 27 is an unrolled view of the countercurrent blowdown expander impulse turbine; 
     FIG. 28 shows a split stream axial compressor with three stages; and 
     FIG. 29 shows a composite pellet with zeolite material coated on a high specific gravity inert core, for centrifugally stabilized granular adsorbers in radial flow embodiments. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1,  2 ,  3  and  4   
     A rotary module  10  according to the invention is shown in FIGS. 1,  2 ,  3  and  4 . The module includes a rotor  11  revolving about axis  12  in the direction shown by arrow  13  within stator  14 . FIG. 4 is an axial section of the module  10 , defined by arrows  15  and  16  in FIG.  1 . FIG. 1 is a cross-section of the module  10 , defined by arrows  17  and  18  in FIG.  4 . FIG. 2 is the sectional view of the rotor  11  repeated from FIG. 1, with the stator deleted for clarity. FIG. 3 is the sectional view of the stator  14  repeated from FIG. 1, with details of the rotor deleted for clarity. 
     In general, the apparatus of the invention may be configured for flow through the adsorber elements in the radial, axial or oblique conical directions relative to the rotor axis. For operation at high cycle frequency, radial flow has the advantage that the centripetal acceleration will lie parallel to the flow path for most favourable stabilization of buoyancy-driven free convection, as well as centrifugal clamping of granular adsorbent with uniform flow distribution. As shown in FIG. 2, 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]. 
     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 float path contracting the adsorbent between the first and second ends of the adsorber. 
     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 between first aperture  34  and first end  30 , and between second aperture  35  and second end  32 , of adsorber element  24 . Support screen  31  also supports the centrifugal force loading of the adsorbent. 
     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. 
     A set of first compartments in the outer shell each open in an angular sector to the first valve surface, and each provide fluid communication between its angular sector of the first valve surface and a manifold external to the module. The angular sectors of the compartments are much wider than the angular separation of the adsorber elements. The first compartments are separated on the first sealing surface by the strip seals (e.g.  42 ). 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 but less than the higher working pressure. 
     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 substantially the higher working pressure. Likewise, second feed compartment  54  communicates to second feed manifold  55 , which is maintained at substantially the higher working pressure. 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. 
     A set of second compartments in the inner shell each open in an angular sector to the second valve surface, and each provide fluid communication between its angular sector of the second valve surface and a manifold external to the module. The second compartments are separated on the second sealing surface by the strip seals (e.g.  44 ). 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. 
     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. 
     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. 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 fight 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. 
     Additional details 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 . 
     A further most important benefit of the invention in radial flow embodiments arises in purification of very low molecular weight gases such as hydrogen and helium to remove higher molecular weight impurities. Here, the light product is separated radially inward, while the heavy impurities are separated radially outward by the centrifugal PSA apparatus of the present invention. In all PSA systems, dispersive effects including axial dispersion. uneven bed packing, thermal gradients and wall flow channeling all tend to spread the concentration gradient in the bed so as to degrade separation performance. But the strong centripetal acceleration field of the present invention will induce a buoyant stratification of the purified light fraction radially inward of the separated heavy fraction, thus opposing dispersive effects and enhancing separation performance. This important desirable effect is present whether granular adsorbent or laminated sheet supported adsorbent is used, as long as the flow direction in the adsorbent bed is radially inward from the first end to the second end of the bed. 
     In air separation with the feed presented to the outer radius of the adsorbers, the buoyancy effect due to the greater molecular weight of oxygen compared to nitrogen would be modestly adverse. The molecular weight difference between hydrogen and its impurities (other than helium) is far greater and in the desired direction. Some process embodiments of the present invention include the feature of heating the oxygen light reflux gas, for the main objects of thermally enhancing expansion energy recovery, improving adsorption/desorption kinetics, and shifting the optimal operating pressure range from vacuum to positive superatmospheric pressure conditions. Heating the light reflux oxygen sufficiently will create a radial thermal gradient, so that the second end of the adsorbers (at an inner radius) will be hotter than the first end of the adsorbers (at an outer radius). In a rapidly rotating rotor of the invention, this thermal gradient will enhance the convective stability of the mass transfer front in the adsorbers, and will tend to compensate the adverse effect of oxygen being more dense than nitrogen at equal temperature. The present invention thus can provide radial stabilization of the mass transfer front by establishing a radial density gradient either of lower molecular weight of the gas contacting the adsorbent radially inward, or by a thermal gradient of higher temperature radially inward. 
     Alternatively, convective stability in air separation applications may be enhanced by operating with the feed applied to an inner radius of radial flow rotating adsorbers, while the oxygen as second product is withdrawn from an outer radius. 
     FIG. 5 
     An attractive alternative to the use of granular adsorbent is obtained by forming the adsorbent material with a suitable reinforcement matrix into thin adsorbent sheets, and layering the adsorbent sheets with spacers to form a layered sheet contactor with flow channels between adjacent pairs of sheets. The adsorber elements may then be installed as angularly spaced rectangular blocks within the rotor and between the first and second valve faces, with the adsorbent sheets as substantially flat sheets extending parallel to the plane defined by the axis of the rotor and a radius from the axis through the rectangular block, and the flat adsorbent sheets being layered with flow channels between them to form the rectangular block. The flow channels also lie in planes parallel to the sheets and to the plane defined by the axis of the rotor and a radius from the axis through the rectangular blocks, and may be configured for either axial flow or radial flow. In the axial flow case, the first and second valve surfaces would be provided as flat discs perpendicular to and concentric with the axis of rotation. In the radial flow case, represented by FIGS. 1-4, the first and second valve surfaces are provided as inner and outer cylindrical surfaces bounding the annular rotor within which the adsorber elements are mounted. 
     A section  110  of rotor  11  has been identified in FIG. 2 between the curved lines with endpoints  111  and  112 , and  113  and  114 . FIG. 5 shows section  110  in detail, with the laminated sheet embodiment of the adsorbers. 
     The laminate sheets  115  lie in the radial plane and are layered to form the adsorber elements  24  as rectangular blocks. Each sheet  115  comprises reinforcement material, e.g. a glass fiber or metal wire matrix (woven or non-woven) on which the adsorbent material (e.g. zeolite crystallites) is supported by a suitable binder (e.g., clay, silicate or coke binders). Typical thickness of an adsorbent sheet may be about 100 microns. The sheets  115  are installed with spacers on one or both sides to establish flow channels between adjacent pairs of sheets. The flow channels define the flow path approximately in the radial direction between first end  30  and second end  32  of the flow path in each adsorber element. Typical channel height would be about 50% to 100% of the adsorbent sheet thickness. 
     The adsorbent sheets comprise a reinforcement material, preferably glass fibre, but alternatively 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 calcium or lithium cations. The zeolite crystals are bound with silica, clay and other binders within the adsorbent sheet matrix. 
     Satisfactory adsorbent sheets have been made by coating a slurry of zeolite crystals with binder constituents onto the reinforcement material, with successful examples including non-woven 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, fiber glass scrims, and metal foils with etched flow channels in a photolithographic pattern. 
     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 cycle frequencies in the range of 50 to 100 cycles per minute. 
     FIGS. 6 and 7 
     FIG. 6 shows a typical PSA cycle according to the invention, while FIG. 7 shows a similar PSA cycle with heavy reflux recompression of a portion of the first product gas to provide a second feed gas to the process. 
     In FIGS. 6 and 7, the vertical axis  150  indicates the working pressure in the adsorbers and the pressures in the first and second compartments. 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 . 
     The horizontal axis  155  of FIGS. 6 and 7 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 (e.g.  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. 
     First aperture  34  of adsorber  24  is opened next to first feed compartment  52 , which is maintained at substantially the higher pressure by a third feed supply means  165 . Once the adsorber pressure has risen to substantially the higher working pressure, 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 . 
     In the cycle of FIG. 7, first aperture  34  of adsorber  24  is opened next to second feed compartment  54 , also maintained at substantially the higher pressure by a fourth feed supply means  167 . In general, the fourth feed supply means supplies a second feed gas, typically 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 FIG. 7, 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. 6, 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. 
     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 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. 
     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. Either when the second aperture is closed after the final light reflux exit step (as shown in FIGS.  6  and  7 ), 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 . 
     In FIG. 6, the heavy gas from the first, second and third exhaust means is delivered as the heavy product  185 . In FIG. 7, 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. 
     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  35  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. 6 and 7) begins feed pressurization for the next cycle after time  157  as soon as the third light reflux pressurization step has been concluded. 
     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. 
     The rate of pressure change in each pressurization or blowdown step will be restricted by throttling in ports (or in clearance or 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. 6 and 7. 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 transmitting 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. 
     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. 
     FIG. 8 
     FIGS. 8-15 are simplified schematics of PSA systems using the module  10  of FIGS. 1-4 as the basic building block, and showing the connections from the first and second manifolds of the module to machinery for compression and expansion of gases in typical applications. In FIGS. 8-15, the reference numerals of the first and second manifolds are as defined for FIG.  3 . 
     FIG. 8 is a simplified schematic of a PSA system for separating oxygen from air, using nitrogen-selective zeolite adsorbents. The light product is concentrated oxygen, while the heavy product is nitrogen-enriched air usually vented as waste. The cycle lower pressure  152  is nominally atmospheric pressure. Feed air is introduced through filter intake  200  to a feed compressor  201 . The feed compressor includes compressor first stage  202 , intercooler  203 , compressor second stage  204 , second intercooler  205 , compressor third stage  206 , third intercooler  207 , and compressor fourth stage  208 . The feed compressor  201  as described may be a four stage axial compressor or centrifugal compressor with motor  209  as prime mover coupled by shaft  210 , and the intercoolers are optional. With reference to FIG. 6, the feed compressor first and second stages are the first feed supply means  160 , delivering feed gas at the first intermediate feed pressure  161  via conduit  212  and water condensate separator  213  to first feed pressurization manifold  48 . Feed compressor third stage  206  is the second feed supply means  162 , delivering feed gas at the second intermediate feed pressure  163  via conduit  214  and water condensate separator  215  to second feed pressurization manifold  51 . Feed compressor fourth stage  208  is the third feed supply means  165 , delivering feed gas at the higher pressure  151  via conduit  216  and water condensate separator  217  to feed manifold  53 . Light product oxygen flow is delivered from light product manifold  71  by conduit  218 , maintained at substantially the higher pressure less frictional pressure drops. 
     The apparatus of FIG. 8 includes energy recovery expanders, including light reflux expander  220  (here including four stages) and countercurrent blowdown expander  221  (here including two stages), coupled to feed compressor  201  by shaft  222 . The expander stages may be provided for example as radial inflow turbine stages, as full admission axial turbine stages with separate wheels, or as partial admission impulse turbine stages combined in a single wheel as illustrated in FIGS. 17-20 below. 
     Light reflux gas from first light reflux exit manifold  73  flows at the higher pressure via conduit  224  and heater  225  to first light pressure letdown means  170  which here is first light reflux expander stage  226 , and then flows at the third light reflux pressurization pressure  192  by conduit  227  to the first light reflux return manifold  87 . Light reflux gas from second light reflux exit manifold  75  flows at the first cocurrent blowdown pressure  171  via conduit  228  and heater  225  to second light reflux pressure letdown means  172 , here the second expander stage  230 , and then flows at the second light reflux pressurization pressure  191  by conduit  231  to the second light reflux return manifold  85 . Light reflux gas from third light reflux exit manifold  77  flows at the second cocurrent blowdown pressure  173  via conduit  232  and heater  225  to third light reflux pressure letdown means  174 , here the third expander stage  234 . and then flows at the first light reflux pressurization pressure  190  by conduit  235  to the third light reflux return manifold  83 . Finally, light reflux gas from fourth light reflux exit manifold  79  flows at the third cocurrent blowdown pressure  175  via conduit  236  and heater  225  to fourth light reflux pressure letdown means  176 , here the fourth light reflux expander stage  238 , and then flows at substantially the lower pressure  152  by conduit  239  to the fourth light reflux return manifold  81 . 
     Heavy countercurrent blowdown gas from first countercurrent blowdown manifold  57  flows at first countercurrent blowdown intermediate pressure  180  by conduit  240  to heater  241  and thence to first stage  242  of the countercurrent blowdown expander  221  as first exhaust means  181 , and is discharged from the expander to exhaust manifold  243  at substantially the lower pressure  152 . Countercurrent blowdown gas from second countercurrent blowdown manifold  59  flows at second countercurrent blowdown intermediate pressure  182  by conduit  244  to heater  241  and thence to second stage  245  of the countercurrent blowdown expander  221  as second exhaust means  183 , and is discharged from the expander to exhaust manifold  243  at substantially the lower pressure  152 . Finally, heavy gas from heavy product exhaust manifold  61  flows by conduit  246  as third exhaust means  184  to exhaust manifold  243  delivering the heavy product gas  185  to be vented at substantially the lower pressure  152 . 
     Heaters  225  and  241  raise the temperatures of gases entering expanders  220  and  221 , thus augmenting the recovery of expansion energy and increasing the power transmitted by shaft  222  from expanders  220  and  221  to feed compressor  201 , and reducing the power required from prime mover  209  While heaters  225  and  241  are means to provide heat to the expanders, intercoolers  203 ,  205  and  207  are means to remove heat from the feed compressor and serve to reduce the required power of the higher compressor stages. The haters and intercoolers are optional features of the invention. 
     If light reflux heater  249  operates at a sufficiently high temperature so that the exit temperature of the light reflux expansion stages is higher than the temperature at which feed gas is delivered to the feed manifolds by conduits  212 ,  214  and  216 , the temperature of the second ends  35  of the adsorbers  24  may be higher than the temperature of their first ends  34 . Hence, the adsorbers have a thermal gradient along the flow path, with higher temperature at their second end relative to the first end. This is an extension of the principle of “thermally coupled pressure swing adsorption” (TCPSA), introduced by Keefer in U.S. Pat. No. 4,702,903. Adsorber rotor  11  then acts as a thermal rotary regenerator, as in regenerative gas turbine engines having a compressor  201  and an expander  220 . Heat provided to the PSA process by heater  225  assists powering the process according to a regenerative thermodynamic power cycle. similar to advanced regenerative gas turbine engines approximately realizing the Ericsson thermodynamic cycle with intercooling on the compression side and interstage heating on the expansion side. 
     In the instance of PSA applied to oxygen separation from air, the total light reflux flow is much less than the feed flow because of the strong bulk adsorption of nitrogen. Accordingly the power recoverable from the expanders is much less than the power required by the compressor, but will still contribute significantly to enhanced efficiency of oxygen production. By operating the adsorbers at moderately elevated temperature and using strongly nitrogen-selective adsorbents such as Ca—X, Li—X or calcium chabazite zeolites, a PSA oxygen generation system can operate with favourable performance and exceptional efficiency. While higher temperature of the adsorbent will reduce nitrogen uptake and selectivity, the isotherms will be more linear. Effective working capacity in superatmnospheric pressure PSA cycles may be enhanced by operation in TCPSA mode with an elevated temperature gradient in the adsorbers. Working with adsorbents such as Ca—X and Li—X, recent conventional practice has been to operate ambient temperature PSA at subatmospheric lower pressures in so-called “vacuum swing adsorption” (VSA), so that the highly selective adsorbents operate well below saturation in nitrogen uptake, and have a large working capacity in a relatively linear isotherm range. At higher temperatures, saturation in nitrogen uptake is shifted to more elevated pressures, so the optimum PSA cycle higher and lower pressures are also shifted upward. For satisfactory operation of the apparatus of FIG. 8, the typical operating temperature of the second ends of the adsorbers may be approximately 50° C. for Ca—X or Li—X, and 100° to 150° C. for calcium chabazite. 
     If high energy efficiency were not of highest importance, the light reflux expander stages and the countercurrent blowdown expander stages could be replaced by restrictor orifices or throttle valves for pressure letdown, as illustrated in FIG.  10 . The schematic of FIG. 8 shows a single shaft supporting the compressor stages, the countercurrent blowdown or exhaust expander stages, and the light reflux stages, as well as coupling the compressor to the prime mover. However, it should be understood that separate shafts and even separate prime movers may be used for the distinct compression and expansion stages within the scope of the present invention. 
     It should also be understood that the number of compression stages and the number of expansion stages (as well as the number of vacuum pump stages in the embodiment of FIG. 9 below) may be varied within the scope of the invention. Generally and for equal stage efficiency of the compressor or expander type chosen, a larger number of stages will improve the PSA process efficiency, since the irreversible equalization expansions over the first and second orifices will be performed over narrower pressure intervals. However, the improvement in efficiency for each additional stage will diminish as the number of stages is greater. 
     FIG. 9 
     FIG. 9 shows a vacuum PSA (VPSA) system for oxygen separation from air. Intermediate pressure  158  of FIG. 6 is now nominally atmospheric pressure. Lower pressure  152  and higher pressure  151  may respectively be approximately 0.5 and 1.5 times atmospheric pressure. Feed compressor first stage  202  becomes directly the first feed means feeding manifold  48 . Likewise, compressor second stage  204  and third stage  206  operate as the second feed means  162  and third feed means  165  respectively. The condensate separators are omitted for simplicity. 
     A multistage vacuum pump  260  is driven by shaft  222 , and assisted by light reflux expander  220 . The vacuum pump may for example be a multistage centrifugal or axial compression machine, or it may be provided by rotary positive displacement machinery adapted to accept inlet gas at multiple suction pressures. First stage vacuum pump  261  (acting as third exhaust means  184 ) draws nitrogen-enriched air from the heavy product exhaust manifold  61  at substantially the lower pressure, and delivers this gas through intercooler  262  at the second countercurrent blowdown pressure  182  to second stage vacuum pump  263  (acting as second exhaust means  182 ) which also draws heavy gas from the second countercurrent blowdown manifold  59  at the same pressure. The combined heavy gas discharged from vacuum pump  260  is combined with heavy gas discharged by conduit  240  (acting as first exhaust means  181 ) to form the heavy product  185  delivered to atmosphere (equal to the first countercurrent blowdown pressure) by conduit  243 . 
     FIG. 10 
     FIG. 10 shows a VPSA apparatus similar to that of FIG. 9, but with the light reflux pressure letdown means  170 ,  172 ,  174  and  176  provided respectively by throttle orifices  270 ,  272 ,  274 , aid  276 . The throttle orifices may be fixed orifices, or may be throttle valves with a control actuator  277  for coordinated adjustment of their orifice aperture. Control actuator  277  provides means to adjust the rate of pressure letdown for each light reflux step, so that the process may be adjusted for operation at a different cycle frequency or a different ratio of the higher and lower working pressures. It should be noted that adjustable nozzles (similar to the above adjustable throttles with controller  277 ) may be alternatively used in conjunction with expansion turbines used for each of the light reflux (or countercurrent blowdown expander stages. 
     FIG. 11 
     FIG. 11 shows a PSA apparatus adapted to receive two feed gas mixtures, and with recompression of the heavy product gas. A suitable application would be hydrogen recovery from petroleum refinery offgases, e.g. hydrotreater purge gases typically containing light hydrocarbon gases with 30% to 70% hydrogen. Frequently, several offgases of differing hydrogen concentration are available at elevated feed pressures in the range of 10 to 20 atmospheres. Using typical adsorbents, e.g. activated carbon or zeolites, the hydrocarbon impurities will be much more readily adsorbed than hydrogen, so the purified hydrogen will be the light product delivered at the higher working pressure which may be only slightly less than the feed supply pressure, while the impurities will be concentrated as the heavy product and will be exhausted from the PSA process as “PSA tail gas” at the lower working pressure. The tail gas is often either flared or used as fuel gas. 
     For hydrogen duty, positive displacement expansion and compression machinery (e.g. twin screw machines) may be preferred because of the low molecular weight of the gas. Such machines may be adapted in accordance with the invention with extra inlet and/or discharge ports to accept and deliver gas at multiple intermediate pressures. 
     Performance and productivity of PSA hydrogen recovery from refinery offgases (with the adsorbers working at near ambient temperature) will be greatly enhanced by operating with the lower working pressure as low as possible and preferably near atmospheric pressure. However, the tail gas is usually delivered at a pressure of at least 5 or 6 atmospheres, for disposal to the refinery fuel gas header. Compression costs, particularly for combustible gases under refinery safety constraints, may be prohibitively high. 
     The apparatus of FIG. 11 is configured to accept first and second feed gas mixtures, the first having a higher concentration of the less readily adsorbed component (e.g. hydrogen) while the second is more concentrated than the first feed gas mixture in the more readily adsorbed fraction. The first feed gas is supplied at substantially the higher working pressure by first infeed conduit  280  to first feed manifold  53 , while the second feed gas is supplied at substantially the higher working pressure by second infeed conduit  281  to first feed manifold  35 . Each adsorber receives the second feed gas after receiving the first feed gas. so that the concentration profile in the adsorber is monotonically declining in concentration of the more readily adsorbed component along its flow path from first end  34  to second end  35  of the flow path in adsorber element  24 . All but the final pressurization steps are here achieved with light reflux gas. The final feed pressurzation (from the third light reflux pressurization pressure  192  directly to the higher pressure  151  ) is achieved as the first end of each adsorber is opened to compartment  52  communicating to manifold  53 . Additional feed pressurization steps could readily be incorporated as in the embodiment of FIG.  8 . 
     In this embodiment, the tail gas (heavy product) is discharged from second product delivery conduit at a higher pressure than the lower working pressure, in this example being approximately the first countercurrent blowdown pressure  180  of FIG. 6 with conduit  240  being first exhaust means  181 . Tail gas is recompressed by tail gas compressor  290 , with compressor first stage  291  being the third exhaust means  184  compressing the first product gas from exhaust manifold  61  via conduit  246 , and delivering the first product gas after first stage compression through intercooler  292  to compressor second stage  293  which itself is the second exhaust means compressing second countercurrent blowdown gas from manifold  59  via conduit  244 . 
     FIG. 12 
     FIG. 12 shows a PSA apparatus with heavy reflux to obtain either higher enrichment and purity of the more readily adsorbed component into the heavy product, or higher yield (recovery) of the less readily adsorbed component into the light product. This apparatus may also be configured to deliver the heavy product at elevated pressure, here approximately the higher working pressure so that both product gases are delivered at about the higher pressure. 
     The apparatus of FIG. 12 has infeed conduit  300  to introduce the feed gas at substantially the higher pressure to first feed manifold  53 . As in the example of FIG. 11, adsorber pressurization is achieved mainly by light reflux, with a final feed pressurization step through manifold  53 . 
     A multistage heavy reflux compressor  301  has a first stage  302  as third exhaust means  184  of FIG. 7, drawing heavy gas by conduit  246  from first product exhaust manifold  61 . and compressing this gas through intercooler  303  to second stage  304 . Heavy reflux compressor second stage  304  as second exhaust means  183  also draws heavy gas from second countercurrent blowdown manifold  59  through conduit  244 , and delivers this gas by intercooler  305  to third stage  306  which as first exhaust means  181  also draws heavy gas from first countercurrent blowdown manifold  57  through conduit  240 , and delivers this gas by intercooler  307  to fourth stage  308  which attains substantially the higher working pressure of the PSA cycle. The heavy reflux compressor is driven by prime mover  209  through shaft  210 , and by light reflux expander  220  through shaft  309 . 
     The compressed heavy gas is conveyed from compressor fourth stage  308  by conduit  310  to condensate separator  311 , from which the heavy product is delivered by conduit  312  which is externally maintained at substantially the higher pressure less frictional pressure drops. Condensed vapours (such as water or liquid hydrocarbons) are removed through conduit  313  at substantially the same pressure as the heavy product in conduit  312 . The remaining heavy gas flow, after removal of the first product gas, flows by conduit  314  to the second feed manifold  55  as heavy reflux to the adsorbers following the feed step for each adsorber. The heavy reflux gas is a second feed gas, of higher concentration in the more readily adsorbed component or fraction than the first feed gas. 
     FIG. 13 
     FIG. 13 shows a PSA apparatus with a free rotor tail gas compressor or vacuum pump, powered by energy recovery expanders analogous to a multistage turbocharger. Free rotor compressor  320  includes, on shaft  321 , tail gas compressor  322  (or vacuum pump  322 , if the lower pressure is subatmospheric) which is the third exhaust means  184  drawing heavy product gas or tail gas from exhaust manifold  61 . In this example, the heavy product gas is discharged from conduit  243  at the second countercurrent blowdown pressure  182 , which is above the lower pressure. Pressure  182  may here be atmospheric pressure, in which case the third exhaust means is a vacuum pump. Conduit  244  is the second exhaust means  183 . The first exhaust means  181  is expander  323  coupled to shaft  321  of free rotor compressor  320 . Expander  323  expands heavy gas flowing from manifold  57  through conduit  240  and optional heater  241 , and releases that gas to exhaust conduit  243 . 
     The light reflux expander  220  and the countercurrent blowdown expander  323  are both coupled to drive the tail gas compressor  322  by shaft  321 , with no other source of mechanical power required. The application of energy recovery (from light reflux and countercurrent blowdown) provides the alternative benefits of reducing the lower pressure so as to improve PSA (or VPSA) cycle performance, or raising the first product delivery pressure as may be required e.g. for tail gas disposal, without the requirement for an electric motor driven compressor. This feature would be particularly useful for hydrogen separation, where reducing the lower pressure greatly improves performance, while elevated tail gas pressures may be desired. Alternatively, a hydrogen PSA system could be operated with a subatmospheric lower pressure, while the tail gas is discharged at sufficiently above atmospheric pressure for combustion in a flare or furnace. 
     FIG. 14 
     FIG. 14 shows another embodiment using a free rotor compressor or turbocharger. In this embodiment, applied to oxygen separation from air, a motor driven first feed compressor  330  is driven by prime mover  209  through shaft  210 . Using the same nomenclature and reference numerals of feed compression stages as FIG. 8, feed compressor  330  includes feed compression first stage  202  and third stage  206  on shaft  210  driven by motor  209 . Free rotor second compressor  340  includes feed compression second stage  04  and fourth stage  208  on shaft  222 , driven by countercurrent blowdown expander  221  and light reflux expander  220  through shaft  222 . This configuration enables operation of a motor driven feed compressor with a limited number of stages (here 2 stages) to operate a PSA cycle with a larger number of feed supply pressures (here the three pressures  161 ,  163  and  151  of FIG.  6 ), since the free rotor compressor has dual functions as means to boost feed pressure by application of thermally boosted expansion energy recovery, and means to split the stage intermediate pressures for supply to the PSA module. 
     FIG. 15 
     FIG. 15 shows a VPSA oxygen generation plant with  4  modules in parallel, each having a free rotor booster compressor powered by energy recovery expanders, and the entire apparatus having a single prime mover  350  which may for example be an electric motor or a gas turbine. Prime mover  350  drives first feed compressor  351  on shaft  352 . Feed compressor  351  has a first stage  353  drawing feed gas from infeed conduit  200 , and a third stage  354 . The second stage of feed compression is provided by the free rotor compressors of each module. The first feed compressor  351  in this embodiment also includes an exhaust vacuum pump  355  likewise coupled to shaft  352 . 
     The plant includes four identical modules  10 A,  10 B,  10 C and  10 D. In FIGS. 15 and 16, component nomenclature and reference numerals follow that established for FIGS. 1-14, with a suffix A to D appended to the reference numerals for module components, and each component so identified with reference to any one module will be identically found in each of the other modules. The first manifolds are identified with reference to module  10 D as  48 D and  51 D for feed pressurization,  53 D for feed supply at the higher pressure,  57 D and  59 D for countercurrent blowdown, and  61 D for exhaust at the lower pressure. The second manifolds are identified with reference to module  10 C as  71 C communicating to light product delivery manifold  360  and delivery conduit  218 , light reflux exit manifolds  73 C,  75 C,  77 C and  79 C, and light reflux return manifolds  81 C,  83 C,  85 C and  87 C. 
     The identical free rotor compressor for each module will be described with reference to module  10 B. Free rotor compressor assembly  370 B includes feed compression second stage  371 B and vacuum pump  372 B, both coupled by shaft  373 B to light reflux expander  220 B. Feed gas compressed by feed compressor first stage  353  is conveyed by feed manifold  376  in parallel to the first feed pressurization manifold (e.g.  48 D) of each module, and to the inlet of feed compression second stage (e.g.  371 B) of the free rotor compressor assembly (e.g.  370 B) of each module which delivers further compressed feed pressurization gas to the second feed pressurization manifold (e.g.  51 D) of each module. Feed gas compressed to the higher pressure by third feed compressor stage  354  is conveyed by feed manifold  377  in parallel to the first feed supply manifold (e.g.  53 D) of each of the modules. Heavy gas at the lower pressure is drawn from the heavy compartment (e.g.  61 D) of each module through vacuum exhaust manifold  378  to exhaust vacuum pump  355  as the third exhaust means. Countercurrent blowdown gas from the first countercurrent blowdown manifold (e.g.  57 D) of each module is discharged by e.g. conduit  240 B as first exhaust means, while countercurrent blowdown gas from the second countercurrent blowdown manifold (e.g.  59 D) of each module is exhausted by vacuum pump (e.g.  372 B) of the free rotor compressor assembly as second exhaust means, delivering the heavy tail gas to the module heavy product or waste gas exhaust, e.g.  243 B. 
     FIG. 16 
     FIG. 16 shows a PSA apparatus with  5  modules  10 A- 10 E. In this embodiment, the prime mover, all compressor stages and all expander stages are directly mechanically coupled (e.g. on a single shaft) following the embodiment and component descriptions of FIG. 8, with the only difference being the connection in parallel of multiple modules. 
     FIG. 17 
     In this example, sealing faces  21  and  23  are respectively provided as hard-faced ported surfaces on the first and second valve stators  40  and  41 . Sliding seals  380  are provided on rotor  115  between each adsorber  24  and its neighbours, to engage both sealing faces  21  and  23  in fluid sealing contact. Seals  380  may have a wear surface of a suitable composite material based on PTFE or carbon, and should be compliantly mounted on rotor  11  so as to compensate for wear, deflections and misalignment. Ports  381  may be sized, particularly at the leading edge of each compartment, to provide controlled throttling for smooth pressure equalization between adsorbers and that compartment, as each adsorber in turn is opened to that compartment. 
     Split stream vacuum pump  260  receives the countercurrent blowdown and exhaust flow in three streams receiving exhaust gas at incrementally reduced pressures from countercurrent blowdown compartment  56 , compartment  58  and compartment  60 . The combined exhaust gas is discharged as heavy product gas. In this example, initial feed pressurization is performed from atmosphere, so a first feed pressurization conduit  382  admits feed air directly from inlet filter  200  to first feed pressurization compartment  46  at substantially atmospheric pressure. The first discharge port of feed compressor  201  now communicates to second feed pressurization compartment  50 . The compressor is shown as a split stage machine with inlet  391 , and three discharges  392 ,  393  and  394  at incrementally higher pressures. 
     In the option of light reflux pressure letdown with energy recovery, a split stream light reflux expander  220  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 f our light reflux stages as illustrated. As indicated by dashed lines  395 , the stages may optionally be compartmentalized within the light reflux expander to minimize mixing of gas concentration between the stages. The light product purity will tend to decline from the light reflux stages of higher pressure to those of lower pressure, so that a desirable stratification of the light reflux can be maintained if mixing is avoided. 
     Light reflux expander  220  is coupled to drive light product pressure booster compressor  396  by shaft  397 . Compressor  396  receives the light product from compartment  70 , and delivers light product (compressed to a delivery pressure above the higher pressure of the PSA cycle) from delivery conduit  218 . Since the light reflux and light product are both enriched oxygen streams of approximately the same purity, expander  220  and light product compressor  396  may be hermetically enclosed in a single housing similar to a turbocharger. 
     FIG. 18 
     FIG. 18 shows a radial flow rotary PSA module  500 , contemplated for tonnage oxygen generation. With reference to FIG. 17, this view may be interpreted as an axial section through compartments  54  and  70  at the higher pressure, and compartments  80  and  60  at the lower pressure. Arrows  501  and  502  respectively indicate the feed and exhaust flows. Rotor  11  has a first end plate  510  with stub shaft  511  supported by bearing  512  in first bearing housing  513 , which is integral with first valve stator  40 . Rotor  11  is attached at assembly joint  514  to a second end plate  515  with stub shaft  516  supported by bearing  517  in second bearing housing  518 , which is attached at assembly joint  519  to first valve stator  40 . 
     Rotor  11  is driven by motor  95  connected to stub shaft  511  by shaft  94  penetrating housing  513  through shaft seal  522 . First end plate  510  has no perforations that might compromise purity of the light product gas by leakage from the first valve surface to the second valve surface. Second end plate  515  is penetrated at bushing  530  by the second valve stator. Second valve stator  41  is a stationary pintle within rotor  11 , with guide bushings  530  and  532 , and is attached to the second bearing housing  518  at assembly face  534 . Bearings  512  and  517  may be much smaller in diameter than the outer diameter of rotor  11  at sealing face  21 . A shaft seal  535  is provided between shaft  516  and bearing  517 , to prevent contamination of the light product gas by leakage from chamber  536  adjacent the first valve sealing face  21  to chamber  537  adjacent the second valve sealing face  23 . 
     Preferably, seal  535  is tight against leakage so that product purity is not compromised. By configuring this seal at smaller diameter than the valve sealing faces, frictional torque from shaft seal  535  is greatly reduced than if this seal were at the full rotor diameter. Leakage across seals in the first valve face is much less important, because moderate leakage across those seals simply reduced the volumetric efficiency of the process. Similarly, moderate leakage across the seals in the second valve face may be tolerated, as the concentration of light reflux gases and the light product gas that may leak across those seals is almost identical. Because moderate leakage across seals in the first valve surface (including circumferential seals  96 ), and across seals in the second valve surface (including circumferential seals  97 ), can be accepted, all of those seals may be designed for relatively light mechanical engagement to minimize frictional torque. In fact, use of narrow gap clearance seals or labyrinth seals with zero mechanical rubbing friction is an attractive option especially for larger capacity modules operating at high cycle frequency (e.g. 50 or 100 cycles per minute) where seal leakage flows would have a minimal effect on overall efficiency. Preferably, the seals in the first and second valve faces have consistent performance and leakage, so that all “N” adsorbers experience the same PSA cycle flow and pressure regime as closely as possible, without being upset by variations in leakage between the adsorbers. 
     Hence an important benefit of the present invention is that close tolerance sealing is only required on one dynamic rotary seal, shaft seal  535 , whose diameter has been made much smaller than the rotor diameter to reduce the sealing perimeter as well as mechanical friction power loss. For a given rotary seal section and loading, rubbing friction power loss at given RPM is proportional to the square of the sealing face diameter. 
     Because of the compactness (similar to an automotive turbocharger) of a “turbocompressor” oxygen booster as described for FIG. 17 above, it is possible to install a split stream light reflux expander  220  with close-coupled light product compressor  396  inside the light valve stator. Compressed oxygen product is delivered by conduit  218 . 
     FIG. 19 
     FIG. 19 shows an axial flow rotary PSA module  600  for smaller scale oxygen generation. The flow path in adsorbers  24  is now parallel to axis  601 . A better understanding will be obtained from FIGS. 20,  21  and  22 , which are cross sections of module  600  in the planes respectively defined by arrows  602 - 603 ,  604 - 605 , and  606 - 607 . FIG. 19 is an axial section of module  600  through compartments  54  and  70  at the higher pressure, and compartments  60  and  80  at the lower pressure. The adsorber rotor  11  contains the “N” adsorbers  24  in adsorber wheel  608 , and revolves between the first valve stator  40  and the second valve stator  41 . Compressed feed air is supplied to compartment  54  as indicated by arrow  501 , while nitrogen enriched exhaust gas is exhausted from compartment  60  as indicated by arrow  502 . 
     At the ends of rotor  11 , circumferential seals  6081  and  609  bound first sealing face  21 , and circumferential seals  610  and  611  bound second sealing face  23 . The sealing faces are flat discs. The circumferential seals also define the ends of seals between the adsorbers, or alternatively of dynamic seals in the sealing faces between the stator compartments. Rotor  11  has a stub shaft  511  supported by bearing  512  in first bearing housing  513 , which is integral with first valve stator  40 . Second valve stator  41  has a stub shaft engaging the rotor  11  with guide bushing  612 . 
     A flanged cover plate  615  is provided for structural connection and fluid sealing enclosure between the first valve stator  40  and the second valve stator  41 . Rotor  11  includes seal carrier  618  attached at joint  619  to adsorber wheel  608 , and extending between the back of second valve stator  41  and cover plate  615  to sealing face  621  which is contacted by dynamic seal  625 . Seal  625  prevents contamination of the light product gas by leakage from chamber  626  adjacent the first valve sealing face  21  to chamber  627  adjacent the second valve sealing face  23 . 
     Seal  625  needs to be tight against leakage that could compromise product purity. By this seal to a smaller diameter than the valve faces outer diameter, frictional torque from this seal is greatly reduced than if this seal were at the full rotor diameter. The circumferential perimeter exposed to leakage is also reduced. As in FIG. 18, the light reflux pressure letdown means, illustrated as a split stream light reflux expander  220  with close-coupled light product compressor  396 , may be installed inside the light valve stator. 
     FIG. 20 
     FIG. 20 shows an axial flow rotary PSA module  650  with twin adsorber wheels. The same reference numerals are used as in FIG. 19 for the first adsorber wheel  608 , and primed reference numerals are used for the second adsorber wheel  608 ′, which are assembled together as rotor  11 . Module  650  has two first valve faces  21  and  21 ′, each with a full set of feed supply and second product exhaust compartments. External manifolds will be provided to supply feed fluid to each pair of feed compartments and to withdraw exhaust fluid from each pair of exhaust compartments. Module  650  has two second valve faces  23  and  219 ′, which share a common set of compartments for light product delivery, light reflux exit and return, and purge. Arrows  651  indicate the flow direction in compartment  221 , and arrows  652  indicate the flow direction in compartment  70 . 
     Rotor  11  is driven by shaft  94  coupled to the first adsorber wheel  608 . The adsorber wheels  408  and  608 ′ are attached at joint  655 . Flanged cover plate  615  of FIG. 19 is here replaced by the first valve stator  40 ′ for the second adsorber wheel  608 ′, in the role of connecting the first valve stator  40  and second valve stator  41  to form an enclosed housing for the module. A small diameter dynamic seal  660  is mounted adjacent bushing  612 ′, to prevent leakage between the first and second valve faces. 
     Embodiment  650  enables a doubled capacity rating for the twin axial wheel configuration compared to the single wheel embodiment  600 . 
     FIG. 21 
     FIG. 21 shows the first valve face  21  of embodiment  600  of FIG. 19, at section  602 - 603 , with fluid connections to a split stream feed compressor  201  and a split stream countercurrent blowdown expander  221 . Arrow  670  indicates the direction of rotation by adsorber rotor  11 . The open area of valve face  21  ported to the feed and exhaust compartments is indicated by clear angular segments  46 ,  50 ,  52 ,  56 ,  58 ,  60  corresponding to those compartments, between circumferential seals  608  and  609 . The closed area of valve face  21  between compartments is indicated by cross-hatched sectors  675  and  676 . Typical closed sector  675  provides a transition for an adsorber, between being open to compartment  56  and open to compartment  58 . Gradual opening is provided at the leading edges  677  and  678  of compartments, so as to achieve gentle pressure equalization of an adsorber being opened to a new compartment. Much wider closed sectors (e.g.  676 ) 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. 
     Sealing between compartments at typical closed sectors (e.g.  675 ) may be provided by rubbing seals on either stator or rotor against a ported hard-faced sealing counter face on the opposing rotor or stator, or by narrow gap clearance seals on the stator with the area of the narrow sealing gap defined by the cross hatched area of the nominally closed surface. Rubbing seals may be provided as radial strip seals, with a self-lubricating solid material such as suitable PTFE compounds or graphite, or as brush seals in which a tightly packed brush of compliant fibers rubs against the counter face. 
     If the rubbing seals are on the rotor (between adjacent adsorbers), cross-hatched sectors  675  and  676  would be non-ported portions of the hard-faced sealing counter face on the stator. If the rubbing seals are on the stator, the ported hard-faced counter face is on the rotor valve face. Those rubbing seals could be provided as full sector strips for narrow closed sectors (e.g.  675 ). For the wider closed sectors (e.g.  676 ), narrow radial rubbing seals may be used as the edges  678  and  679 , and at intervals between those edges, to reduce friction in comparison with rubbing engagement across the full area of such wide sectors. 
     Clearance seals are attractive, especially for larger scale modules with a very large number “N” of adsorbers in parallel. The leakage discharge coefficient to or from the clearance gap varies according to the angular position of the adsorber, thus providing gentle pressure equalization as desired. The clearance gap geometry is optimized in typical nominally closed sectors (e.g.  675 ) so that the leakage in the clearance gap is mostly used for adsorber pressure equalization, thus minimizing through leakage between compartments. The clearance gap may be tapered in such sectors  675  to widen the gap toward compartments being opened, so that the rate of pressure change in pressure equalization is close to linear. For wide closed sectors (e.g.  676 ) the clearance gap would be relatively narrow as desired to minimize flows at that end of adsorbers passing through those sectors. 
     For all types of valve face seals described above, it is preferable that consistent performance be achieved over time, and that all “N” adsorbers experience the same flow pattern after all perturbations from seal imperfections. This consideration favours placing rubbing seals on the stator so that any imperfections are experienced similarly by all adsorbers. If the seals are mounted on the rotor between adsorbers, it is preferable that they are closely identical and highly reliable to avoid upsetting leakages between adjacent adsorbers. 
     To compensate for misalignment, thermal distortion, structural deflections and wear of seals and bearings, the sealing system should have a suitable self-aligning suspension. Thus, rubbing seal or clearance seal elements may be supported on elastomeric supports, bellows or diaphragms to provide the self-aligning suspension with static sealing behind the dynamic seal elements. Rubbing seals may be energized into sealing contact by a combination of elastic preload and gas pressure loading. 
     Clearance seals require extremely accurate gap control, which may be established by rubbing guides. Clearance seal gap control may also be achieved by a passive suspension in which the correct gap is maintained by a balance between gas pressure in the gap of a clearance seal segment, and the pressures of adjacent compartments loading the suspension behind that segment. For seal elements between blowdown compartments, a simple passive self-adjusting suspension should be stable. Active control elements could also be used to adjust the clearance seal gap, with feedback from direct gap height measurement or from the pressure gradient in the gap. Active control may be considered for seal elements between pressurization compartments, for which the simple passive control may be unstable. 
     FIG. 22 
     FIG. 22 shows the second valve face  23  of embodiment  600  of FIG. 19, at section  604 - 605 , with fluid connections to a split stream light reflux expander  220  and light product booster compressor  396 . Fluid sealing principles and alternatives are similar to those of FIG.  21 . Similar principles and alternatives apply to radial flow and axial flow geometries, respectively sealing on cylindrical or disc faces. 
     FIG. 23 
     Adsorber wheel  608  may use radially aligned rectangular flat packs of adsorbent laminate, as shown in FIG. 5 for radial flow. FIG. 23 shows an alternative adsorber wheel configuration for the embodiment of FIG. 19, at section  606 - 607 . As in FIG. 5, the adsorbers  24  are again formed of a pack of rectangular adsorbent sheets with spacers, but with the sheets here curved arcs rather than flat. With this configuration, the ports and seals in valve faces  21  and  23  would desirably be configured as corresponding curved arcs. Voids between the circularly curved adsorber packs are filled by separators  684 . Such circularly curved adsorber packs may be made by forming the adsorbent sheets with spacers in a spiral roll on a circular cylindrical mandrel, and then cutting the spiral roll longitudinally to obtain the desired packs. Packing density may be further improved by forming the adsorber sheets to a spiral rather than circular curve. 
     FIGS. 24-27 
     FIG. 24 shows a multistage centrifugal compressor  400  with impulse turbine expanders for the light reflux and countercurrent blowdown, configured to provide the feed compressor stages  202 ,  204 ,  206  and  208 , the countercurrent blowdown expander stages  242  and  245 , and the light reflux expander stages  226 ,  230 ,  234 , and  238  of FIG.  8 . Prime mover  209  drives shaft  402 , supported in compressor casing  403  by bearings  404  and  405  on axis  406 . Shaft  402  carries compressor first stage impeller  411 , second stage impeller  412 , third stage impeller  413  and fourth stage impeller  414 , exhaust impulse turbine runner  415  and light reflux impulse turbine runner  416 . 
     Feed air from PSA plant inlet  200  enters suction port  420  to suction scroll  421  to the inlet  422  of impeller  411 . Impeller  411  discharges the air to first stage diffuser  425  and first stage collector scroll  426 , which directs the first stage compressed air to the inlet of the second stage impeller  412 . Impeller  412  discharges the air to second stage diffuser  430  and second stage collector scroll  431 , from which second stage delivery port  432  discharges a portion of the feed air as pressurization gas at the second stage pressure to conduit  212 . Similarly, the feed air is compressed by the third and fourth stage impellers  413  and  414 , discharging air at the third stage pressure from third stage delivery port  436  communicating to conduit  214 , and at the fourth stage pressure from fourth stage delivery port  440 . 
     The multistage centrifugal compressor  400  provides the stages of feed compressor  201  in FIG.  8 . Multistage vacuum pumps, as required in the embodiment of FIG. 9, may similarly be provided as conventional centrifugal stages. For a large multiple module plant, for example as described in FIG. 16, the exhaust and light reflux expander turbines may be provided as multistage radial inflow turbines whose stages would be mechanically similar to the centrifugal stages of FIG.  24 . In larger plants, expander stages may also be provided full admission axial turbine stages, similar to gas turbine stages. 
     For particular advantage in smaller plant capacities, considerable simplification is obtained in the embodiment of FIGS. 24-27 by using partial admission impulse turbines for countercurrent blowdown and light reflux expansion, with each expander stage occupying a sectorial arc of the corresponding turbine on a single runner wheel. This approach is practicable because the stages for each turbine expand gases of approximately similar composition across adjacent pressure intervals. 
     FIG. 25 is a section of FIG. 24, defined by arrows  451  and  452 , across the plane of light reflux impulse turbine runner  416 . FIG. 24 is a section of FIG. 25, in the plane indicated by arrows  453  and  454 . Runner  416  rotates about axis  406  in the direction indicated by arrow  455 . Runner  416  has blades  456  mounted on its rim. FIG. 26 is a projected view of the light reflux expander impulse turbine, unrolled around 360° of the perimeter of the impulse turbine as indicated by the broken circle  458  with ends  459  and  460  in FIG.  25 . 
     The light reflux turbine has four nozzles serving the four  900  quadrants of the runner to provide the four expansion stages, including first nozzle  461  receiving flow from port  462  communicating to conduit  224 , second nozzle  463  receiving flow from port  464  communicating to conduit  228 , third nozzle  465  receiving flow from port  466  communicating to conduit  232 , and fourth nozzle  467  receiving flow from port  468  communicating to conduit  236 . 
     The first stage light reflux flow from nozzle  461  impinges blades  456 , and is collected in diffuser  471  and discharged at the reduced pressure by port  472  communicating to conduit  227 . Similarly the light reflux flow from nozzle  463  is collected in diffuser  473  and flows by port  474  to conduit  231 , the light reflux flow from nozzle  465  is collected in diffuser  475  and flows by port  476  to conduit  235 , and the light reflux flow from nozzle  467  is collected in diffuser  477  and flows by port  478  to conduit  239 . To minimize interstage leakage losses, the channel gap  479  between the casing  403  and blades  456  of runner  416  is appropriately narrow between quadrants. 
     The exhaust expander turbine, or countercurrent blowdown expander turbine, has two stages. Its sectional arrangement is similar to that depicted in FIG. 25, except that two rather than four nozzles and diffusers are required for the two exhaust stages. FIG. 27 is an unrolled projection around exhaust turbine runner  415  as indicated by broken circle  458  for the light reflux turbine. The exhaust turbine has impulse blades  480  on runner  415 . Nozzle  481  receives the first countercurrent blowdown stream by port  482  communicating to conduit  240 , while nozzle  483  receives the second countercurrent blowdown stream by port  484  communicating to conduit  244 . Nozzles  481  and  483  have guide vanes  485  and  486 , and direct the countercurrent blowdown flows to impinge on blades  480  in opposite half sectors of the turbine  415 . After deflection by blades  480 , the expanded flow from nozzle  481  is collected in diffuser  491 , and is passed to collector ring manifold  492 . The flow from nozzle  483  likewise passes the blades  480  and is collected in diffuser  493  joining manifold  492  to deliver the combined low pressure exhaust flow by exhaust port  494  which is connected to the discharge  243 . 
     FIG. 28 
     FIG. 28 shows a three stage axial flow split stream compressor  700 . While it is known in the prior art to divert minor bleed flows between stages of multistage axial flow compressors or expanders, compressor  700  has nested annular diffusers for splitting fractionally large intermediate flows from the main flow between stages. 
     Compressor  700  may represent split stream compressor  201  of FIG. 4, and has a scroll housing  701  with feed inlet  391 , first discharge port  392 , second discharge port  393  and third discharge port  394 . Rotor  702  is supported by bearings  703  and  704  with shaft seals  705  and  706  within housing  701 , and is driven by motor  209  through shaft  210 . The rotor supports first stage rotor blades  711 , second stage rotor blades  712 , and third stage rotor blades  713 . 
     Housing  701  includes an inlet scroll  721  distributing feed gas from inlet  391  to annular feed plenum  722 , with the flow direction indicated by arrow  723 . The feed flow enthalpy is increased by first stage blades  711 , with static pressure recovery by first stage stator blades  724  mounted in first stage stator ring  725 . The feed flow enthalpy is further increased by second stage blades  712 , with static pressure recovery by second stage stator blades  726  mounted in second stage stator ring  727 ; and finally by third stage blades  713 , with static pressure recovery by third stage stator blades  728  mounted in third stage stator ring  729 . 
     Second stage stator ring  727  has a smaller diameter than first stage stator ring  725 , defining an annular area of annular first stage diffuser  731  which delivers the first intermediate feed pressurization flow to collector scroll  732  and thence to first discharge port  392  as indicated by arrow  733 . Similarly, third stage stator ring  729  has a smaller diameter than second stage stator ring  727 , defining an annular area of annular second stage diffuser  734  which delivers the first intermediate feed pressurization flow to collector scroll  735  and thence to second discharge port  393  as indicated by arrow  736 . The fraction of flow entering the first and second stage annular diffusers is substantially equal to the ratio of the annular area of those diffuser inlets to the annular flow area of that stage between its stator ring and the rotor  702 . 
     The flow delivered by the third stage passes stator blades  728  into third stage diffuser  737 , and in collector scroll  738  into discharge port  394  as indicated by arrow  739 . Stator rings  725 ,  727  and  729  are respectively supported by partitions  741 ,  742  and  743  separating the inlet and discharge scrolls. 
     It will be evident that additional stages could be added with more paired sets of rotor blades and stator blades, with the option of including or not including an annular diffuser for diverting an intermediate flow stream between any adjacent pair of stages. It will also be evident that the structure of compressor  700  could be applied to a split stream axial flow vacuum exhauster or expander, by reversing the flow directions indicated by arrows  723 ,  733 ,  736 , and  739 , so that port  394  would be a first inlet, port  393  a second inlet, and port  392  a third inlet for each of three inlet streams at incremental total pressures, and with port  391  the discharge port for the combined total flow. 
     FIG. 29 
     FIG. 29 shows a composite adsorbent pellet  800 , useful in the practice of the present invention with the radial flow configuration of FIGS. 4,  5 ,  6  and  18 , in the alternative of using granular packed bed adsorbers  24 . 
     Granular adsorbent beds cannot be operated in prior art PSA devices at very high cycle frequency without excessive pressure drops leading to incipient fluidization and resulting attrition. The present apparatus in the radial flow configuration provides a centripetal acceleration field which may be greater than the ordinary gravitational field. This provides a desirable “centrifugal clamping” effect to stabilize the adsorbent bed, and thus facilitate safe operation at higher cycle frequency. However, the specific gravity of conventional macroporous zeolite adsorbent pellets is only about 0.75, thus limiting the effect of centrifugal clamping. While the use of rotating granular adsorbent beds in radial flow configurations is well known in the above cited prior art, operating conditions that would provide useful centrifugal clamping have not been disclosed. Thus, Boudet et al in U.S. Pat. No. 5,133,784 contemplate a maximum cycle frequency and rotor speed of 20 RPM, which with their mentioned rotor outer radius of 1 meter would provide a maximum centripetal acceleration of less than half the acceleration of gravity at the outer radius. The adsorbent beds, within the rotor and closer to the axis, are subject to a much smaller centripetal acceleration. 
     Ballasted composite pellet  800  has an inert core  801  of a dense material, surrounded by a coating  802  of macroporous zeolite material similar to the material of conventional adsorbent pellets. The core material may be selected for high density, high heat capacity, high thermal conductivity and compatibility for adhesion to zeolite binders as well as for thermal expansion. Suitable core materials include transition metal oxides, most simply iron oxide, as well as solid iron or nickel-iron alloys. 
     If the diameter of core  801  is e.g. 790 microns, and the radial thickness of coating  802  is e.g. 105 microns so that the overall diameter of a spherical pellet  800  is 1 mm, the volume of the pellet is then 50% inert and 50% active macroporous adsorbent. In a packed bed using such composite pellets, the active volume of adsorbent has been reduced by 50%, while the fractional bed voidage of the active material has been increased from the typical 35% of spherical granular media to approximately 50%. This might seem to be an inferior packed bed, with half as much useful material and reduced effective selectivity performance because of the high effective void fraction. Unexpectedly, this can be a superior packed bed, because pressure drop and mass transfer resistance are both reduced. so that the PSA cycle can be operates at higher cycle frequency without excessive pressure drop and without risk of fluidization. At the same cycle frequency, pressure drops are reduced by the smaller flows in proportion to the smaller active adsorbent inventory for the same voidage channels, while mass transfer through the macropores only has to take place through a relatively thin shell. The inert material also acts as thermal ballast to isothernalize the adsorber against thermal swings due to heat of adsorption. 
     While the higher void fraction will reduce product yield at specified purity in the uneconomic regime of very low cycle frequency, product yield and productivity are actually enhanced in the economic regime of higher cycle frequency. Degradations of product yield and process energy efficiency (at specified product purity) will result from mass transfer resistance and pressure drop, and those degradations are more severe for the conventional bed than for the present inventive granular adsorber of composite pellets. 
     Such composite pellets are very useful in the radial flow embodiment of the rotary adsorber module, since the heavy composite pellets are centrifugally stabilized very positively, even as mass transfer resistance and pressure drop are reduced. Such composite pellets will also be very useful in axial flow embodiments, as well as non-rotary adsorbers, with vertically oriented flow path. Again, cycle frequency can be increased, while performance can be enhanced in terms of productivity, yield and efficiency at the most economic operating point. Consider FIGS. 4 and 18 to be vertical views of radial flow embodiments. The vertical axis embodiment of FIG. 4 will benefit from centrifugal stabilization if its rotor radius and cycle frequency are high enough. The horizontal axis embodiment of FIG. 18 will have centripetal acceleration assisting the gravitational field to suppress fluidization in the feed production step with upward flow from compartment  54  to compartment  70  at higher pressure, while the centripetal acceleration will assist pressure drop in the purge step with upward flow from compartment  80  to compartment  60  at lower pressure to prevent downward collapse of the adsorbers at the top of their rotational orbit. The adsorbent beds are supported at their first end (radially outside) by a first set of screens, and retained against collapsing when the rotor is stopped by a second set of screens at their second end (radially inside). Hence, the adsorbent beds are centrifugally clamped on the first screens by centripetal acceleration with the rotor acting as a centrifuge. 
     While composite pellets  800  are shown in FIG. 29 as spherical, other geometries are also attractive. For example, cylindrical composite pellets might be made by dip-coating the zeolite and binder slurry onto steel rods, which are then cut into short lengths. 
     The centrifugal clamping aspect of the present invention allows operation of granular adsorbent beds with much higher than conventional flow friction pressure gradients while still positively preventing any particle movement and attrition. In turn, this allows use of smaller adsorbent gram sizes, also enabling a very shallow radial bed depth which reduces total pressure drop. With the small adsorbent granule size reducing the mass transfer diffusional resistance, high PSA cycle frequencies become practicable. Closing the logical argument, high cycle frequencies correspond to the high rotational speed needed for centrifugal clamping. 
     The foregoing description of the preferred embodiments of the invention is intended to be illustrative of the present invention. Those of ordinary skill will be able to make certain additions, deletions or modifications to the described embodiments which although not explicitly diclosed herein, do no depart from the spirit or scope of the invention as defined by the appended claims.