Abstract:
A gas separation system for separating a feed gas mixture into a first gas component and a second gas component comprises a stator, and a rotor rotatably coupled to the stator. The stator includes a first stator valve face, a second stator valve surface, and a plurality of function compartments opening into the stator valve surfaces. The rotor includes a first rotor valve surface in communication with the first stator valve surface, and a second rotor valve surface in communication with the second stator valve surface. The rotor also includes a plurality of rotor flow paths for receiving gas adsorbent material therein for preferentially adsorbing the first gas component in response to increasing pressure in the rotor flow paths in comparison to the second gas component. Each rotor flow path includes a pair of opposite ends opening into the rotor valve faces for communication with the function compartments. Centrifugal turbomachinery is coupled to a portion of the function compartments, and includes an impeller which has a plurality of impeller flow paths for exposing each rotor flow path to a plurality of discrete pressures as the rotor rotates for separating the first gas component from the second gas component.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to centrifugal turbomachinery for use with gas separation systems. In particular, the present invention relates to split-stream centrifugal compressors, vacuum pumps and expanders in which the flow exiting or entering the centrifugalturbomachinery comprises multiple flows at different total pressures.  
         BACKGROUND OF THE INVENTION  
         [0002]    Gas separation by pressure swing adsorption (PSA) is achieved by coordinated 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 is elevated during intervals of flow in a first direction through the adsorbent bed from a first end to a second end of the bed, and is reduced during intervals of flow in the reverse direction. As the cycle is repeated, the less readily adsorbed component is concentrated in the first direction, while the more readily adsorbed component is concentrated in the reverse direction.  
           [0003]    Many prior art PSA systems have low energy efficiency, because feed gas for adsorber pressurization as well as for the high pressure production step is provided by a compressor whose delivery pressure is the highest pressure of the cycle. Energy expended in compressing the feed gas used for pressurization is then dissipated in throttling across valves over the instantaneous pressure difference between the adsorber and the high pressure supply. Similarly, in vacuum swing adsorption (VSA) where the lower pressure of the PSA 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 occurs in throttling of light reflux gas used for purge, equalization, cocurrent blowdown and product pressurization or backfill steps. The energy dissipation in irreversible throttling becomes more important when such throttling takes place over larger pressure differences between an adsorber and a feed source or an exhaust sink.  
           [0004]    Energy efficiency has been improved in more modern VSA air separation systems, by using feed compressors (or blowers) whose delivery pressure follows the instantaneous pressure of an adsorber being pressurized, and by using vacuum pumps whose suction pressure follows the instantaneous pressure of an adsorber undergoing countercurrent blowdown. In effect, the feed compressor rides each adsorber in turn to pressurize it with reduced throttling losses, and likewise the vacuum pump rides each adsorber in turn to achieve countercurrent blowdown with reduced throttling losses. In such systems, each feed compressor can only supply gas to a single adsorber at any time, and each vacuum pump can only exhaust a single adsorber at a time. The working pressure in each such feed compressor or vacuum pump will undergo large variations, stressing the machinery and causing large fluctuations in overall power demand. Further, compression efficiency is compromised by the unsteady operating conditions.  
           [0005]    Since centrifugal or axial turbomachinery cannot operate under such unsteady conditions, rotary positive displacement machines are typically used in VSA systems. However, such machines have lower efficiency than modern centrifugal turbomachinery working under steady conditions, particularly for larger plant ratings (e.g. 50 tons per day oxygen VSA systems). Further, scale up above single train plant capacities of about 80 tons per day oxygen is inhibited by the maximum capacity ratings of single rotary machines.  
           [0006]    Other modern VSA air separation systems have used multiple individual impellers to increase the enthalpy of the individual streams. However, these latter systems increase system complexity and capital cost. Furthermore, machine efficiency is reduced since the flow rates are smaller for each machine.  
           [0007]    Accordingly, there is a need for centrifugal turbomachinery which can be used in PSA and VSA gas separation processes for maintaining steady conditions of gas flow and pressure, while minimising energy dissipation in irreversible throttling.  
         SUMMARY OF THE INVENTION  
         [0008]    According to the invention, there is provided a gas separation system which addresses the deficiencies of the prior art gas separation systems.  
           [0009]    As herein mentioned, the term “centrifugal turbomachinery” includes centrifugal compressors, vacuum pumps and expanders.  
           [0010]    The gas separation system, according to the invention, separates a feed gas mixture into a first gas component and a second gas component and comprises a stator, and a rotor rotatably coupled to the stator. The stator includes a first stator valve face, a second stator valve surface, and a plurality of function compartments opening into the stator valve surfaces. The rotor includes a first rotor valve surface in communication with the first stator valve surface, and a second rotor valve surface in communication with the second stator valve surface. The rotor also includes a plurality of rotor flow paths for receiving gas adsorbent material therein for preferentially adsorbing the first gas component in response to increasing pressure in the rotor flow paths in comparison to the second gas component.  
           [0011]    Each rotor flow path includes a pair of opposite ends opening into the rotor valve faces for communication with the function compartments. The gas separation system also comprises centrifugal turbomachinery coupled to a portion of the function compartments. The centrifugal turbomachinery includes an impeller which has a plurality of impeller flow paths for exposing each rotor flow path to a plurality of discrete pressures as the rotor rotates for separating the first gas component from the second gas component.  
           [0012]    In a first embodiment of the invention, the centrifugal turbomachinery comprises a split stream centrifugal compressor for delivering the feed gas mixture to the first stator valve surface at a plurality of different feed gas pressure levels. The centrifugal compressor comprises a gas inlet for receiving the feed gas mixture, a plurality of blades extending radially outwards from the axis of rotation of the impeller, and a channel disposed within the impeller in communication with the gas inlet and extending between adjacent pairs of the blades. The blades include a plurality of steps positioned at differing radial distances from the rotational axis and define impeller flow paths for ejecting the feed gas mixture from the channel at a plurality of different angular momentums. The centrifugal compressor also includes a plurality of diffusers in communication with the channel for providing gas flows at a plurality of different pressures. In one variation of the centrifugal compressor, instead of the blades having steps, the blades have respective blade angles which define the impeller flow paths.  
           [0013]    In a second embodiment of the invention, the centrifugal turbomachinery comprises a split stream centrifugal vacuum pump for producing a first product gas from gas flows which are enriched in the first gas component and which are received at a plurality of different sub-atmospheric gas pressure levels from the first stator valve surface. In a third embodiment of the invention, the centrifugal turbo machinery comprises a split stream centrifugal expander for producing a first product gas at atmospheric pressure from gas flows which are enriched in the first gas component and which are received at a plurality of different superatmospheric exhaust gas pressure levels from the first stator valve surface. The centrifugal vacuum pump and the centrifugal expander are structurally similar to the centrifugal compressor except that the direction of gas flow through the impeller flow paths is reversed. In two variations, the centrifugal vacuum pump and the centrifugal expander are coupled to the centrifugal compressor for assisting the centrifugal compressor in delivering the feed gas mixture to the first stator valve surface.  
           [0014]    In another embodiment of the invention, the centrifugal turbomachinery comprises a double-sided impeller, a plurality of blades extending radially outwards from the impeller, a first gas inlet and a first gas outlet communicating with a first side of the impeller, and a second gas inlet and a second gas outlet communicating with a second side of the impeller. A first channel is disposed within the first side of the impeller for passing gas between the first gas inlet and the first gas outlet, and a second channel is disposed within the second side of the impeller for passing gas between the second gas inlet and the second gas outlet, with the first and second channels each extending between adjacent pairs of the blades. This latter embodiment may be configured as a split stream centrifugal compressor, a split stream centrifugal vacuum pump and a split stream centrifugal expander with the different impeller flow paths being defined either by a stepped impeller or differing blade angles.  
           [0015]    In operation, the feed gas is delivered to the rotor flow paths through the first rotor-stator valve surface pair, and the rotor is rotated at a frequency so as to expose the gas mixture in each rotor flow path to cyclical changes in pressure and direction of flow. These cyclical changes cause the more readily adsorbed component of the feed gas to be exhausted as heavy product gas from the first rotor-stator valve surface pair and the less readily adsorbed component to be delivered as light product gas from the second rotor-stator valve surface pair. To enhance gas separation, light reflux exit gas is withdrawn from the second rotor-stator valve surface pair and is returned after pressure letdown to the second rotor-stator valve surface pair.  
           [0016]    In order for the flowing gas streams entering or exiting the centrifugal turbo machinery at each pressure level to be substantially uniform in pressure and velocity, the feed gas is delivered to the rotor flow paths through a plurality of incremental feed gas pressure levels, and the heavy product gas is exhausted from the rotor flow paths as countercurrent blowdown gas through a plurality of decremental exhaust gas pressure levels. Preferably, the light reflux exit gas is withdrawn from the rotor flow paths through a plurality of decremental light reflux exit pressure levels and returned to the rotor flow paths as light reflux return gas at pressure levels less than the respective light reflux exit pressure level. For thermally boosted energy recovery, heat exchangers may also be provided to reject heat of compression and to heat the countercurrent blowdown and the light reflux gas streams about to be expanded.  
           [0017]    Preferably the rotor also has a large number of adsorbers such that several adsorbers are exposed to each pressure level at any given moment. During pressurization and blowdown steps, the pressures of the adsorbers passing through each of these steps 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 the centrifugal turbomachinery at the nominal pressure level of that step. Hence flow and pressure pulsations seen by the centrifugal turbomachinery at each intermediate pressure level are minimal by averaging from the several adsorbers passing through the step, although each adsorber undergoes large cyclic changes of pressure and flow.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The preferred embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which:  
         [0019]    [0019]FIG. 1 shows a simplified schematic of a gas separation apparatus;  
         [0020]    [0020]FIG. 2 shows a typical gas separation cycle, in the format to which the invention shall be applied;  
         [0021]    [0021]FIG. 3 shows a simplified schematic of a gas separation cycle apparatus using a split stream compressor and a split stream expander;  
         [0022]    [0022]FIG. 4 shows a simplified schematic of a gas separation apparatus for oxygen production, using a split stream air compressor, a split stream vacuum pump, and a split stream light reflux expander powering a product oxygen compressor;  
         [0023]    [0023]FIG. 5 shows a split stream centrifugal compressor according to a first embodiment of the invention, incorporating a cut back impeller, and discrete delivery radii for each stepped section of the impeller;  
         [0024]    [0024]FIGS. 6 and 7 show a variation of the split stream centrifugal compressor shown in FIG. 5, incorporating a compartmentalized impeller with variable impeller blade angle across the blade height, with the impeller sections in each of three compartments having different blade angles;  
         [0025]    [0025]FIG. 8 shows a split stream centrifugal vacuum pump, according to a second embodiment of the invention, incorporating a compartmentalized impeller and with the impeller sections in each of three compartments having different blade angles;  
         [0026]    [0026]FIG. 9 shows a variation of the split stream centrifugal vacuum pump shown in FIG. 8, with a stepped impeller and discrete delivery radii for each stepped section of the impeller;  
         [0027]    [0027]FIG. 10 shows a split stream light reflux expander with a compartmentalized impeller, according to a third embodiment of the invention;  
         [0028]    [0028]FIG. 11 is a meridional view of a split stream or multi-enthalpy machine, according to the invention, incorporating a double sided impeller for different enthalpy level streams;  
         [0029]    [0029]FIG. 12 is a meridional view of a split stream or multi-enthalpy machine, according to the invention, incorporating twin impellers for different enthalpy level streams;  
         [0030]    [0030]FIGS. 13 and 14 show ejectors for combining countercurrent blowdown streams at different streams into inlet nozzles of an expander or inlet nozzles of the split stream vacuum pump; and  
         [0031]    [0031]FIG. 15 shows a diffuser for the split stream compressor, with a slot to divert a lower pressure stream.  
         [0032]    [0032]FIG. 16 shows cross-sectional view of a split-stream compressor, with a tangential discharge including separate diffuses. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    To aid in understanding the invention, the pressure swing adsorption process and associated apparatus will be described first, in association with FIGS. 1 through 4. The invention will then be described, commencing with FIG. 5.  
       FIGS.  1  and  2   
       [0034]    [0034]FIG. 1 shows an elementary PSA apparatus  1  with an adsorber assembly  2  having a plurality of “N” cooperating adsorbers  3  in parallel. Each adsorber  3  has a flow path  4  between first end  5  and second end  6  of the adsorber  3 , with adsorbent material contacting the flow path. Cooperating with the adsorbers are first valve  7  and second valve  8 . Arrow  9  indicates the direction of progression of the adsorbers  3  in being connected to ports of the first and second valves  7 ,  8  as shown in FIG. 1. In the case of a rotary adsorber, as in the preferred embodiments of the invention, adsorber assembly  2  is shown in FIG. 1 unrolled in a  3600  section about its rotary axis so that rotation causes the adsorbers  3  to advance in the direction of arrow  9  to undergo the cycle of FIG. 2.  
         [0035]    [0035]FIG. 2 shows the PSA cycle undergone sequentially by each of the “N” adsorbers  3  over a cycle period “T”. The cycle in consecutive adsorbers is displaced in phase by a time interval of TIN. In FIG. 2 the vertical axis  10  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 here neglected. The higher and lower working pressures of the PSA process are respectively indicated by dotted lines  11  and  12 .  
         [0036]    The horizontal axis  15  indicates time, with the PSA cycle period defined by the time interval between points  16  and  17 . At times  16  and  17 , the working pressure in adsorber  3  is pressure  18 . Starting from time  16 , the cycle begins as the first end  5  of adsorber  3  is opened by the first valve  7  to first feed mixture supply means  20  at the first intermediate feed pressure  21 .  
         [0037]    The pressure in that adsorber rises from pressure  18  at time  17  to the first intermediate feed pressure  21 . Proceeding ahead, the first end  5  is opened next to second feed supply means  22  at the second intermediate feed pressure  23 . The adsorber pressure rises to the second intermediate feed pressure.  
         [0038]    Then the first end  5  is opened to a third feed supply means  24  at the higher pressure of the PSA process. Once the adsorber pressure has risen to substantially the higher working pressure, its second end  6  is opened by the second valve means to light product delivery conduit  25  to deliver purified light product. While feed gas is still being supplied to the first end of adsorber  3  from the third feed supply means, the second end  6  is next closed to light product delivery conduit  25 , and is opened to deliver “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)  30 . All or some of the feed supply means may be feed compression stages. One of the feed supply means may be an external source, such as the ambient atmosphere for air purification or air separation applications.  
         [0039]    The first end  5  of adsorber  3  is then closed by the first valve  7 , while the second end  6  is opened successively by the second valve  8  to (a) drop the adsorber pressure to the first cocurrent blowdown pressure  32  while delivering light reflux gas to second light reflux pressure letdown means  34 , (b) drop the adsorber pressure to the second cocurrent blowdown pressure  36  while delivering light reflux gas to third light reflux pressure letdown means  38 , and (c) drop the adsorber pressure to the third cocurrent blowdown pressure  40  while delivering light reflux gas to fourth light reflux pressure letdown means  42 . Second end  6  is then closed for an interval, until the light reflux return steps following the countercurrent blowdown steps.  
         [0040]    The light reflux pressure let-down means may be mechanical expansion stages for expansion energy recovery, or may be restrictor orifices or throttle valves for irreversible pressure let-down.  
         [0041]    Either when the second end  6  is closed after the final light reflux exit step (as shown in FIG. 2), or earlier while light reflux exit steps are still underway, first end  5  is opened to first exhaust means  46 , dropping the adsorber pressure to the first countercurrent blowdown intermediate pressure  48  while releasing “heavy” gas (enriched in the more strongly adsorbed component) to the first exhaust means  46 . Next, first end  5  is opened to second exhaust means  50 , dropping the adsorber pressure to the second countercurrent blowdown intermediate pressure  52  while releasing “heavy” gas. Then first end  5  is opened to third exhaust means  54 , dropping the adsorber pressure to the lower pressure  12  of the PSA process while releasing “heavy” gas.  
         [0042]    Once the adsorber pressure has substantially reached the lower pressure while first end  5  is open to the third exhaust means  54 , the second end  6  is opened to receive fourth light reflux gas (as purge gas) from fourth light reflux pressure let-down means  42  in order to displace more heavy gas into the third exhaust means. The heavy gas from the first, second and third exhaust means may be delivered together as the heavy product  56 . All or some of the exhaust means may be mechanical exhauster stages, alternatively either expansion stages if the pressure is to be reduced, or vacuum pumping stages if the pressure is to be increased to ambient pressure, or exhaust compression stages if the exhaust of second product is to be delivered at an elevated pressure. An exhaust means may also be provided by venting to an external sink, e.g. the ambient atmosphere.  
         [0043]    The adsorber is then repressurized by light reflux gas after the first end  5  is closed. In succession, the second end  6  is opened (a) to receive light reflux gas from the third light reflux pressure reduction means  38  to raise the adsorber pressure to the first light reflux pressurization pressure  60 , (b) to receive light reflux gas from the second light reflux pressure reduction means  34  to raise the adsorber pressure to the second light reflux pressurization pressure  62 , and (c) to receive light reflux gas from the first light reflux pressure reduction means  30  to raise the adsorber pressure to the third light reflux pressurization pressure. Unless feed pressurization has already been started while light reflux return for light reflux pressurization is still underway, the process begins feed pressurization for the next cycle after time  17  as soon as the third light reflux pressurization step has been concluded.  
         [0044]    From each feed supply means (e.g.  20 ), the feed flow is delivered by a conduit  70  through an optional surge absorber chamber  71  to a feed compartment  72  opening to a feed port  73  in first valve  7 . Feed compartment  72  may be open to several adsorbers simultaneously, and may have a restricted entrance  74  so as to provide a gradual throttling equalization of pressure as each adsorber is opened to feed compartment  72 .  
         [0045]    To each exhaust means (e.g.  46 ), the exhaust flow is delivered by a conduit  80  through an optional surge absorber chamber  81  from an exhaust compartment  82  opening to an exhaust port  83  in first valve means  7 . Exhaust compartment  82  may be open to several adsorbers simultaneously, and may have a restricted entrance  84  so as to provide a gradual throttling equalization of pressure as each adsorber is opened to exhaust compartment  82 .  
         [0046]    To light product delivery conduit  25 , the light product is delivered through an optional surge absorber chamber  86  from light product exit compartment  87  opening to a light product port  88  in second valve  8 .  
         [0047]    To each light reflux pressure letdown means (e.g.  34 ), the light reflux flow is delivered by a conduit  90  through an optional surge absorber chamber  91  from a light reflux exit compartment  92  opening to a light reflux exit port  93  in second valve  8 . Light reflux exit compartment  92  may be open to several adsorbers simultaneously, and may have a restricted entrance  94  so as to provide a gradual throttling equalization of pressure as each adsorber is opened to light reflux exit compartment  92 .  
         [0048]    From each light reflux pressure letdown means (e.g.  34 ), the light reflux flow is delivered by a conduit  95  through an optional surge absorber chamber  96  to a light reflux entrance compartment  97  opening to a light reflux entrance port  98  in second valve means  8 . Light reflux exit compartment  97  may be open to several adsorbers simultaneously, and may have a restricted entrance  99  so as to provide a gradual throttling equalization of pressure as each adsorber is opened to light reflux entrance compartment  97 .  
         [0049]    The rate of pressure change in each pressurization or blowdown step may thus be restricted by throttling in compartments of the first and second valve means, or by throttling in the ports at first and second ends of the adsorbers, resulting in the typical pressure waveform depicted in FIG. 2. Excessively rapid rates of pressure change would subject the adsorber to mechanical stress, while also causing flow transients which would tend to increase axial dispersion of the concentration wavefront in the adsorber. Pulsations of flow and pressure are minimized by having a plurality of adsorbers simultaneously transiting each step of the cycle, and/or by providing surge absorbers in the conduits connecting to the first and second valve means.  
         [0050]    It will be evident that the cycle shown in FIG. 2 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.  3   
       [0051]    [0051]FIG. 3 shows a simplified schematic of a PSA air separation system  100 , with a split stream centrifugal compressor  101 , and a split stream countercurrent blowdown expander turbine  102 . The PSA system has an inlet filter  103 , and infeed conduit  104  to the suction port  105  of compressor  101 .  
         [0052]    Feed compressor  101  splits its discharge flow to three diffuser discharge ports, the first diffuser discharge port  11  delivering feed air at a first intermediate pressure to feed pressurization conduit  112 , the second diffuser discharge port  113  delivering feed air at a second intermediate pressure to feed pressurization conduit  114 , and the third diffuser discharge port  115  delivering feed air at the higher pressure of the cycle to feed production supply conduit  116 .  
         [0053]    Split stream expander  102  receives the countercurrent blowdown flow in three streams entering three nozzle ports, a first nozzle inlet port  121  receiving exhaust gas at a first countercurrent blowdown intermediate pressure from countercurrent blowdown conduit  122 , a second nozzle inlet port  123  receiving exhaust gas at a second countercurrent blowdown intermediate pressure from countercurrent blowdown conduit  124 , and a third nozzle inlet port  125  receiving exhaust gas at a third countercurrent blowdown intermediate pressure from countercurrent blowdown conduit  126 . The combined exhaust gas is discharged as second product gas from outlet port  127  of expander  102  to conduit  128 , and may be combined with second product gas discharged at the lower pressure of the cycle substantially equal to atmospheric pressure by exhaust conduit  129 .  
         [0054]    Feed compressor  101  is driven by motor  131  through shaft  132 . Expander  102  may be coupled by shaft  133  to assist powering compressor  101 .  
         [0055]    In the option of light reflux pressure let-down without energy recovery, throttle valves  135 ,  136 ,  137  and  138  provide pressure let-down for each of four light reflux stages illustrated. Actuator means  139  is provided to adjust the orifices of the throttle valves.  
       FIG.  4   
       [0056]    [0056]FIG. 4 shows a simplified schematic of a PSA air separation system  150 , with a split stream centrifugal compressor  101  as in FIG. 3, and a split stream countercurrent blowdown vacuum pump  152  powered by motor  131  by shafts  132  and  133  respectively.  
         [0057]    Branching from infeed conduit  104  is first feed pressurization conduit  153 , which defines a first intermediate feed pressurization pressure substantially equal to ambient. Hence the first diffuser discharge port  111  of feed compressor  101  operates at the second intermediate feed pressurization pressure, second diffuser discharge port  113  operates at a third intermediate feed pressurization pressure, and third diffuser discharge port  115  operates at the higher pressure of the cycle as before.  
         [0058]    Split stream vacuum pump  152  receives the countercurrent blowdown and exhaust flow in three streams entering three inlet ports, a first inlet port  161  receiving exhaust gas at a first countercurrent blowdown intermediate pressure from countercurrent blowdown conduit  162 , a second inlet port  163  receiving exhaust gas at a second countercurrent blowdown intermediate pressure from countercurrent blowdown conduit  164 , and a third inlet port  165  receiving exhaust gas at the lower pressure of the cycle from exhaust conduit  166 . The combined exhaust gas is discharged as second product gas from outlet diffuser port  167  of the vacuum pump  152  into discharge conduit  168 .  
         [0059]    A split stream light reflux expander  170  is provided to provide pressure let-down of the illustrated four light reflux stages with energy recovery. As indicated by dashed lines  171 , the stages may 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. However, in most air separation applications, purity of the light product oxygen is not critical, since the presence of argon concentrated with the oxygen will limit oxygen purity to about 95%. Hence, compartmentalization of the split stream light reflux expander is not mandatory for oxygen PSA or VSA systems.  
         [0060]    Unlike the prior examples of the split stream compressor (one feed is split into several compressed streams at different total pressures) or the split stream exhauster (expander or vacuum pump, with three inlet streams at different total pressures combined into a single discharge stream), the split stream light reflux expander has several separate streams entering at different total pressures, with those same streams discharged again at different total pressures after pressure let-down in the expander.  
         [0061]    Light reflux expander  170  is coupled to light product pressure booster compressor  180  by drive shaft  181 . Compressor  180  receives the light product from conduit  24 , and delivers light product (compressed to a delivery pressure above the higher pressure of the PSA cycle) from delivery conduit  182 . Since the light reflux and light product are both enriched oxygen streams of approximately the same purity, expander  170  and light product compressor  180  may be hermetically enclosed in a single housing. This configuration of a “turbocompressor” oxygen booster without a separate drive motor is advantageous, as a useful pressure boost of the product oxygen can be achieved without an external motor and corresponding shaft seals.  
         [0062]    The first embodiment of the invention will now be described with reference to FIG. 5. FIG. 5 is a transverse view of split stream centrifugal compressor  200 . The compressor  200  has a single inlet, but the flow is split to discharge from the single impeller into three separate diffusers and volutes which deliver three compressed gas flows at different total pressures or enthalpies to the PSA apparatus.  
         [0063]    Compressor  200  has a bladed impeller  205  fixed by hub bolt  206  to shaft  207 . The impeller  205  is mounted between casing  208  and housing  209 . Impeller  205  is a semi-open impeller, with a plurality of blades  210  at preferably equal angular spacing and fixed to the hub  211 . The blades  210  extend from impeller eye  212  to impeller tip  213 , and may be radial or inclined rearwards (as shown in FIG. 9). Impeller channels  214  are defined between each adjacent pair of blades  210 , hub  211  and casing  208 . The impeller blades  210  are cut back at steps  215  and  216 , at which the channels  214  are narrowed so as to direct a portion of the flow in each channel  214  out of the impeller  205  at each step  215  and  216 . It will be evident that the energy or enthalpy of the flows leaving the impeller  205  from step  215 , step  216  and tip  213  will be successively increased as increased angular momentum flow is obtained at greater radius within the impeller channels  214 .  
         [0064]    Casing  208  has an inlet flange  230  and inlet  231 . Inlet guide vanes may also be provided. The diffusers and collector scrolls are also formed within casing  208 . Casing  208  and housing  209  are mutually attached by bolts  234 .  
         [0065]    The impeller blades  210  are sealed to minimize leakage on their front edges by narrow clearances or labyrinth seals  235 ,  236  and  237  respectively between the inlet and the first step, between the first and second steps, and between the second step and the tip. A shaft seal  240  is provided between shaft  207  and housing  209 .  
         [0066]    In operation, fluid flow enters the impeller channels  214  through the inlet  231  and is discharged from the impeller  205  via the steps  215 ,  216  and the tip  213 . The flow discharging from the impeller  205  from step  215  enters first diffuser  221  for recovery of pressure head from velocity head, and is then delivered from first collector scroll  222  to first discharge conduit  223  at a first intermediate pressure. The flow discharging from the impeller  205  from step  216  enters second diffuser  224  for recovery of pressure head from velocity head, and is then delivered from second collector scroll  225  to second discharge conduit  226  at a second intermediate pressure. The flow discharging from the impeller from tip  213  enters third diffuser  227  for recovery of pressure head from velocity head, and is then delivered from third collector scroll  228  to third discharge conduit  229  at the final delivery pressure. Diffusers  221 ,  224  and  227  may each be conventional vaned, vaneless or volute diffusers. At each cutback step  215  and  216 , the meridional height of channel  214  is reduced by the width of the step, reflecting the diversion of outward flowing fluid into diffusers  221  and  224 .  
         [0067]    It will be evident that more cutback steps could be provided with more diffusers and volutes to achieve more intermediate pressure flows, or a single cutback step might have been provided with only two diffusers and volute pairs to achieve two delivery flows at the final pressure and one intermediate pressure less than the final pressure.  
       FIGS.  6  and  7   
       [0068]    Whereas a split stream compressor (with a single inlet and three discharge flows at different pressures) was achieved in embodiment  200  by discharging the split flows from the impeller at sequentially increasing radius into separate diffusers, it is also possible to achieve such a compressor by discharging the split flows from the impeller at substantially the same radius but with sequentially steeper blade angles. The impeller is compartmentalized over most of the radial extent of the channels between the impeller blades, so that the flow does not spill between channels having differing blade angle and curvature. However, in one variation, some or even all of the radial extent of the channels is not compartmentalized. In uncompartmentalized sections, preferably the blades are twisted to maintain the axially changing blade angle.  
         [0069]    [0069]FIG. 6 shows the meridional view of split stream compressor embodiment  300 . The gas flow passes through the inlet flange  301 ; inlet  302 ; impeller  303 ; three diffuser channels  304 , 305  and  306 ; three collector scrolls  307 ,  308  and  309 ; and three outlets  310 ,  311  and  312  in outlet flange  313 . Within the impeller  303  there are three compartments  314 , 315  and  316 . Compartments  314  and  315  are separated by partition  318 , while compartments  315  and  316  are separated by partition  317 . Where any portion of partitions  317  and  318  may be deleted, lines  317  and  318  then represent fluid streamlines in the impeller. Partitions  317  and  318  respectively have leading edges  319  and  320  at the inlet and terminate at impeller outer radius  321 . The direction of rotation of the impeller  303  is indicated by arrow  322  in FIG. 7.  
         [0070]    The impeller return path is sealed using labyrinth seals  324 . The impeller is connected to the shaft  325  using a hub bolt connection  326 . The entire volute is connected to the main housing at the housing connection points  327 . Diffuser channels  304  and  305  are separated by diffuser partition  330  which has a narrow sealing clearance to impeller partition  318  at radius  321 , and diffuser channels  305  and  306  are separated by diffuser partition  331  which has a narrow sealing clearance to impeller partition  317 .  
         [0071]    As the flow passes through the impeller  303 , the flow experiences changing blade twist over the blade height, or different blade angle in the separate compartments. In the inlet view of FIG. 7, the blade pressure side  341  and suction side  342  of a single blade are shown for three cut sections of typical blades at the approximate centerline of each compartment. The three blades sectioned are (1) blade  343  in hub side compartment  316 , (2) blade  344  in central compartment  315 , and (3) blade  345  in casing side compartment  314 . The number of blades in each compartment may be similar to conventional centrifugal machines, e.g. about seventeen blades. The blade angle is steeper at the hub, so that a higher total pressure rise is achieved in compartment  316  than on compartment  315 , and in compartment  315  than in compartment  314 . Alternately, the blade angle may be steepest adjacent the casing. In either case, the three exit steams are held separate and exit to differing enthalpy requirements.  
         [0072]    In the split stream compressor  300 , the outlet flow is radially outward. The direction of rotation and of flow may be reversed to operate as an expander, suitable for energy recovery from expansion of countercurrent blowdown gas, with the combined flow exiting at a single total pressure by outlet  303  and outlet flange  302 . It will be evident that the compressor  200  could similarly be operated in reverse as a split stream expander.  
       FIG.  8   
       [0073]    Shown in FIG. 8 is a split stream exhauster or vacuum pump  400  having three compartmentalized inlet eyes. The exhauster meridional view shows the flow entering an inlet volute assembly  401  having a first inlet  402  at the lowest pressure, a second inlet  403  at an intermediate pressure, and a third inlet  404  at a higher pressure still below ambient pressure. Second inlet  403  and third inlet  404  admit their respective streams to volutes  405  and  406 , to convert a portion of inlet static pressure to swirl velocity. Preferably the second inlet  403  and third inlet  404  have guide vanes  407 ,  408  or nozzles, not shown, for assisting in converting a portion of inlet static pressure to swirl velocity. A compartmentalized impeller with variable blade angle similar to impeller  303  of FIGS. 6 and 7 is used in embodiment  400 , and is depicted in FIG. 8 with the same reference numerals. Inlet  402  communicates to compartment  316 , inlet  403  communicates to compartment  315 , and inlet  404  communicates to compartment  314 . The discharge flow from the three compartments exits the impeller at outer radius  321  and is combined in a single volute  410  leading to discharge  412  at substantially atmospheric pressure.  
         [0074]    Impeller  303  is driven by shaft  420  through shaft seal  422 . As the flow passes through the impeller  303 , energy is added to each stream entering the impeller eye. However because the blade angle is steeper along the hub side than the shroud side, greater energy will be transferred from the impeller to the hub side stream, than the shroud side.  
       FIG.  9   
       [0075]    Shown in FIG. 9 is the split stream exhauster or vacuum pump  500  having three inlet scrolls at three radii. In the meridional view the flows, at successively higher inlet total pressures below ambient pressure, pass respectively through first inlet  501 , second inlet  502  or third inlet  503 . First inlet  501  leads to the eye inlet  505  of impeller  510 . Second inlet  502  leads by inlet scroll  512  to nozzle  513 . Third inlet  503  leads by inlet scroll  514  to nozzle  515 . Nozzles  513  and  514  are swirl means to impart swirl angular velocity to the flows entering the impeller. Impeller  510  has an eye inlet  520  and an outer radius exit  521 , and is cut out with intermediate steps  523  and  525  at which the depth of impeller channels  530  successively increases with radius to receive additional flow from nozzle  513  at step  523  and from nozzle  515  at step  525 . The combined discharge exits impeller  510  at outer radius  521 , and enters volute diffuser channel  530  in volute  531  leading to discharge flange  532  at substantially ambient pressure. The impeller  501  is connected to the shaft  540  using a hub bolt connection  541 . The entire volute  531  is connected to the main housing  545  at the housing connection points  546 .  
         [0076]    As the flow passes through the impeller  510 , energy is added to each stream entering the impeller  510 . Preferably, nozzles or equivalent nozzle guide vanes are included to accelerate the streams entering the impeller  510  at greater radii. In this manner the fluid entering from outer radius sections will receive less energy transferred, since it has shorter flow path inside the impeller passage than fluid entering at smaller radius portions.  
       FIG.  10   
       [0077]    Shown in FIG. 10 is the split stream expander  170  having four compartmentalized inlet scrolls, a compartmentalized impeller, and compartmentalized outlet diffusers. Four streams of light reflux gas (enriched oxygen in an air separation application) at four stepwise different inlet total pressures are subjected to pressure let-down to four stepwise different exit total pressures. It is desirable (although not mandatory) that distinctness of the four light reflux streams be maintained, since they are ordered as to degree of purity. This stratification of the light reflux is desirable to maintain sharpness of concentration gradients, so that the highest purity of the light product can be achieved with high productivity and yield or fractional recovery.  
         [0078]    The illustrated number off our light reflux streams is arbitrary. With a greater number of light reflux streams drawn over narrower time intervals and from closely spaced pressure intervals, process performance and energy efficiency can theoretically be enhanced. However, a greater number of light reflux streams would make the split stream expander  170  more complex, and with an efficiency loss due to the greater wetted surface area and boundary layer friction of additional compartments. A smaller number of light reflux streams would reduce process performance and efficiency, but would simplify the split stream expander while reducing the wetted surface area of its high velocity flow passages.  
         [0079]    In the split stream expander  170 , the four inlet streams enter four inlet scrolls  601 ,  602 ,  603  and  604  in inlet scroll assembly  605 , each stream having a different enthalpy level or total pressure. The highest pressure light reflux stream, at substantially the higher pressure  11  of the PSA cycle shown in FIG. 2, enters the first inlet scroll  601 . Sequentially lower pressure light reflux streams enter second inlet scroll  602  at substantially pressure  32 , third inlet scroll  603  at substantially pressure  36 , and fourth inlet scroll  604  at substantially pressure  40 .  
         [0080]    From the inlet scrolls the streams flow through four nozzles  611 ,  612 ,  613 , and  614  into four bladed impeller compartments  621 ,  622 ,  623  and  624  of impeller  625 . Impeller  625  is radially stepped at both inlet and exit. As shown in FIG. 10, the compartments are radially staggered, so that compartment  621  passing the stream of highest pressure is positioned radially outward, while compartment  624  passing the stream of lowest pressure is positioned radially inward. Compartment  624  terminates at impeller eye  626 . The depicted radial staggering of the compartments will minimize pressure differences and consequent leakage between compartments at the inner and outer radii of the partitions (e.g.  627 ) between them. Blade angles within the compartments may be identical to simplify structural design and stress analysis of the impeller, or may be different to achieve a further effect in differentiating the enthalpy or total pressure changes between the streams.  
         [0081]    If the blade geometry in each compartment is identical, partitions  627  can be deleted from impeller  625 , so that lines  627  indicate nominal streamline boundaries between the light reflux streams. In this variation, the compartments are no longer separated by physical barriers to mixing, and simply become “zones” for their respective streams. Without partitions between compartments, the impeller is structurally much simpler, and skin friction losses in impeller passages are reduced. However significant mixing between the light reflux streams is expected.  
         [0082]    The four streams flow radially inward though the impeller compartments or zones, losing angular momentum and enthalpy, while exerting torque on the blades with the compartments of impeller  625 . Exiting the impeller compartments at their respective inner radii, three of the streams flow from compartments  621 , 622  and  623  respectively into outlet diffuser scrolls  631 ,  632  and  633 , while the lowest pressure stream flows from compartment  624  past impeller eye  626  into outlet diffuser conduit  634 . The four light reflux streams are discharged from outlets  641 , 642 , 643  and  644  of outlet flange  645  at stepped total pressures  64 ,  62 ,  60  and  12  of the PSA cycle shown in FIG. 2.  
         [0083]    Impeller  625  has shaft  650 , in turn supported on bearing  651  in housing  652 . As the flow passes through the impeller of the split stream expander, energy is extracted from each stream to the shaft. As suggested in PSA process embodiment  150  of FIG. 4, this shaft may be used to drive a light product (e.g. oxygen) compressor to boost the pressure of the light product above the higher pressure of the PSA process. Since the light reflux and light product are almost identical in composition, a common housing may be used to enclose the light reflux expander  170  and the light product compressor  180 , as in typical turbocharger devices.  
       FIG.  11   
       [0084]    [0084]FIG. 11 shows a versatile split stream machine with a double sided impeller. This machine may be used as a split stream compressor, vacuum pump, or expander. The double sided impeller immediately provides for two split streams. It will be evident that either side of the impeller may be stepped to further split impeller streams according to impeller radius transited (as in FIGS. 5 and 9). Alternatively, either side of the impeller may be compartmentalized to further split impeller streams according to blade angle (as in FIGS. 6, 7 and  8 ). Further, one side of the impeller may be stepped, while the other side is compartmentalized.  
         [0085]    Split stream machine  700  has a double sided impeller  701  within casing  702 . Impeller  701  has a first impeller side  705  co-operating with first inlet volute  706  receiving a first gas stream from inlet  707 , and with first discharge diffuser volute  708  delivering the first gas stream to discharge  709 ; and a second impeller side  715  co-operating with second inlet volute  716  receiving a second gas stream from inlet  717 , and with second discharge diffuser volute  718  delivering the second gas stream to discharge  719 . Preferably, the first and second impeller sides  705 ,  715  have different in geometries e.g. by having differing blade angles, to achieve different changes of enthalpy for the two streams.  
         [0086]    The double sided impeller  701  is supported on shaft  730 , here shown with bearing and sealing assemblies  731  and  732  respectively penetrating the first and second inlet volutes  706 ,  716 . The impeller  701  has a disc partition  740  between its first side flow channel  705  and its second side flow channel  715 . The first discharge volute  708  is separated from second discharge volute  718  by a diaphragm partition  742 . A labyrinth or clearance seal  744  is provided between partitions  740  and  742  to minimize leakage between the two streams.  
         [0087]    As discussed above, the split stream machine  700  may be used as a compressor, a vacuum pump or an expander. When used as a split stream compressor or vacuum pump, the direction of flow is as indicated by arrow  750 . A split stream compressor based on split stream machine  700  connects inlets  707  and  717  in parallel to receive feed gas at the same inlet pressure, while delivering the discharge streams at different total pressures from discharges  709  and  719 . A split stream vacuum pump or exhauster based on split stream machine  700  maintains separate inlets  707  and  717  to receive inlet exhaust gas at differing total pressures, while delivering the discharge streams at the same total pressures from discharges  709  and  719  now connected in parallel.  
         [0088]    When used as a split stream expander, the direction of flow is reversed from that indicated by arrow  750 . Terminology is also changed, so that impeller first side  705  receives flow from inlet  709  into nozzle volute  708 , and discharges flow to diffuser  706  and discharge  707 ; while impeller second side  715  receives flow from inlet  719  into nozzle volute  718 , and discharges flow to diffuser  716  and discharge  717 . For a two stream light reflux expander, inlets  709  and  719  remain distinct, and likewise discharges  707  and  717  remain distinct. For a countercurrent blowdown expander (e.g.  102  of FIG. 3), inlets  709  and  719  remain distinct to receive streams of differing total pressure or enthalpy, while discharges  707  and  717  are connected in parallel to discharge a combined flow at typically ambient pressure.  
         [0089]    It will be evident that either or both sides of the double sided split stream machine  700  may be further compartmentalized to provide additional split streams, according to any of the above embodiments illustrated in FIGS.  5 - 10 .  
       FIG.  12   
       [0090]    [0090]FIG. 12 shows a split stream machine  770  similar to split stream machine  700 . Split stream machine  770  includes a twin impeller having a first impeller  771  supporting the first impeller side  705 , and a second impeller  772  supporting the second impeller side  715 . First and second impellers  771  and  772  are mounted on a common shaft  773 , with bearing and fluid seal assemblies  774  and  775  for each impeller. Between the bearing and fluid seal assemblies  774 ,  775 , shaft  773  has a drive pinion  776  engaging bull gear  777  on shaft  778 , in turn driven by motor  779 .  
         [0091]    The split stream machine  770  may be applied to the PSA system embodiment  100  of FIG. 3. Thus, the first impeller  771  (with cooperating inlet and diffuser discharge components) may be replaced by a split stream feed compressor (e.g. FIGS.  5  or  6 ) equivalent to compressor  101 , while second impeller  772  (with cooperating inlet and diffuser discharge components) may be replaced by a split stream countercurrent blowdown expander (e.g. FIGS.  5  or  6 ) equivalent to expander  102 .  
         [0092]    Likewise the split stream machine  770  may be applied to the PSA system embodiment  150  of FIG. 4. Again, the first impeller  771  (with cooperating inlet and diffuser discharge components) would be replaced by a split stream feed compressor (e.g. FIGS.  5  or  6 ) equivalent to compressor  101 , while second impeller  772  (with cooperating inlet and diffuser discharge components) may be replaced by a split stream vacuum pump (e.g. FIGS.  8  or  9 ) equivalent to exhauster  152 .  
       FIG.  13   
       [0093]    [0093]FIG. 13 shows an ejector  800  for combining a pair of countercurrent blowdown streams into a single stream prior to entering an inlet nozzle of an expander or an inlet nozzle of a split stream vacuum pump. This embodiment allows up to twice the number of countercurrent streams to exit the first valve means, relative to the number of such streams entering inlet nozzles of the expander or vacuum pump stages. It is desirable to provide the total countercurrent flow at the first valve means as a relatively large number of separate streams at more closely spaced intermediate countercurrent blowdown pressures, so as to reduce the pressure intervals over which irreversible expansion takes place in the first valve means and associated restrictor orifices. Any stream entering the expander or vacuum pump may thus be a combination of two countercurrent blowdown streams leaving the first valve means. For example, the apparatus of FIG. 13 could be used in the apparatus shown in FIG. 3 to combine streams through conduits  122  and  124  prior to entry into expander  102 .  
         [0094]    Ejector  800  has a first inlet  801  to which a first stream “A” of countercurrent blowdown gas is admitted from the second valve means at a first total pressure, and a second inlet  802  to which a second stream “B” of countercurrent blowdown gas is admitted from the second valve means at a second total pressure lower than the first pressure. The first flow is conveyed from inlet  801  to first nozzle  803 , while the second stream is conveyed from the second inlet to a second nozzle  804 , here depicted as an annulus around first nozzle  801 . The area of nozzle  801  is relatively small, so as to achieve a high velocity of the first stream from nozzle  803 . The area of nozzle  804  is relatively large, so that the velocity of the second stream from nozzle  804  is relatively low.  
         [0095]    Nozzles  803  and  804  deliver the first and second streams into a mixing section  85 , in which the streams mix into a combined stream “A+B” at a third total pressure intermediate between the first and second pressures. If the mass flows of the first and second streams are similar, the third total pressure may be approximately equal to the first total pressure plus ¼ the difference between the first and second total pressures.  
         [0096]    In a conventional ejector, the combined stream would normally enter a diffuser after mixing section  85 . Here, the combined stream will be kept at the relatively high velocity after mixing, for admission to an expansion turbine or an intermediate inlet of a split stream vacuum pump. This ejector is accordingly a mixing nozzle.  
       FIG.  14   
       [0097]    [0097]FIG. 14 shows a nozzle assembly  850  with multiple ejectors for combining countercurrent blowdown streams at different streams into inlet nozzles of an expander or inlet nozzles of a split stream vacuum pump.  
         [0098]    Nozzle assembly  850  has an impeller  865  which may comprise (but is not limited to) the impeller  205  shown in FIG. 5, the impeller  303  shown in FIG. 6, the impeller  510  shown in FIG. 9 and the impeller  625  shown in FIG. 10. The nozzle assembly has a number “N” (here N=2) first nozzles  851  with inlets  852  to which first stream “A” is admitted in parallel, and also an equal number of “N” second nozzles  853  with inlets  854  to which second stream “B” is admitted in parallel. Preferably, the first and second nozzles  851 ,  853  are equally spaced about nozzle ring  860 . The first and second nozzles  851 ,  853  deliver their streams tangentially into annular nozzle chamber  862  defined between nozzle ring  860  and impeller inlet radius  864  at which the combined stream “A+B” will enter impeller  865 . The impeller  865  rotates about axis  867  as indicated by arrow  868 . As in ejector  800 , nozzle areas are selected so that the first stream enters the nozzle chamber  862  at a significantly higher velocity than the second stream. Nozzle chamber  862  will serve as a mixing chamber, in which the combined stream is mixed to approach a combined velocity and a third intermediate total pressure, as in ejector  800 . Further mixing may take place within the impeller itself.  
         [0099]    It is seen that ejector  850  is really a mixing nozzle, so that two streams at different in let total pressures may be combined into a single stream in the turbomachinery. This is a relatively inexpensive way to increase the number of split streams in the PSA process, while having a smaller number of streams in the turbomachinery.  
       FIGS.  15  and  16   
       [0100]    [0100]FIG. 15 shows a diffuser  900  for a compressor, with a slot to divert a lower pressure stream. Diffuser  900  has an inlet  901 , and a main channel  902  with a first wall  903 , a second wall  904 , and an exit  905 . Walls  903  and  904  are divergent from inlet  901  to exit  905 , so as to achieve recovery of static pressure from the high velocity inlet stream entering inlet  901  from a compressor impeller.  
         [0101]    A primary stream “A” is delivered from exit  905 . A slot  906  is provided in the first wall  903  to divert boundary layer fluid from the main channel  902  into a side channel  907 . Slot  906  may equivalently be provided as a plurality of small apertures or pores in wall  903 . The flow in channel  907  is a secondary stream “B”, which is delivered from exit  908 . Hence, the flow from the compressor impeller into inlet  901  is a combined flow “A+B”.  
         [0102]    Diffusers are typically an inefficient component of centrifugal compressors, owing to the unfavourable expansion which tends to cause stall as boundary layers thicken and begin to recirculate. This is particularly true if the diffuser wall is convex toward the flow, as here illustrated for first wall  903 . It is well known that suction from the wall of diffusers can improve efficiency, by removing the “tired” boundary layer. However, suction requires a sink to which the secondary flow “B” can be delivered at a lower total pressure than the final delivered total pressure of the primary flow “A”.  
         [0103]    Because the diffuser  900  delivers multiple split streams from the feed compressor to the first valve means, primary stream “A” will be a feed stream admitted to the first valve means at a particular feed pressure, and secondary stream “B” will be a feed stream admitted to the first valve means at an intermediate pressure lower than the said feed pressure. Alternatively, stream “B” could be injected into a second diffuser of a split stream compressor, with the second diffuser working at an inlet enthalpy or total pressure less than the enthalpy or total pressure of stream “A” in diffuser  950 .  
         [0104]    [0104]FIG. 16 illustrates diffuser  900 , when integrated into a split-stream compressor  920 . Split stream compressor  920  includes a casing  922 , with an interior curvilinear wall  924  and an impeller  926  with an impeller eye  927 . Interior curvilinear wall  924  includes an inner curvilinear wall section  9241  and an outer curvilinear wall section  9242 . Outer wall section  9242  is disposed radially outwards from inner wall section  9241 , relative to impeller eye  927 . Wall section  9241  includes first and second ends  9241   a,    9241   b.  Wall section  9242  includes first and second ends  9242   a,    9242   b.  Wall section  9241  second end  9241   b  merges with wall section  9242  first end  9242   a.  Wall section  9241  first end  9241   a  is joined to wall section  9242  second end  9242   b  by diffuser  900 . Impeller  926  is disposed within casing  922  and is radially spaced inwardly from wall  924 , thereby defining volute  928 .  
         [0105]    Compressor further includes an inlet  930  and a tangential discharge or diffuser  900 , extending tangentially from interior wall  924 . Tangential discharge  900  comprises a first diffuser  934  and a second diffuser  936 . First diffuser  934  is configured to discharge a gas flowing immediately adjacent wall  924  (hereinafter referred to as “the boundary layer flow”) as a boundary layer bleed  940 . Second diffuser  936  is configured to discharge gas flowing in flowpaths which are disposed further radially inwards from interior wall  924 , relative to the boundary layer flow, as main discharge  942 .  
         [0106]    First diffuser  934  is disposed radially outwards from the second diffuser  936 , relative to the impeller eye  927 . In one embodiment, first diffuser  934  merges with outer curvilinear section  9242  at second end  9242   b  and second diffuser  936  merges with inner curvilinear wall section  9241  at first end  9241   a.  In another embodiment, first diffuser  934  includes an outer wall  934   a  which is tangential to wall section  9242  at second end  9242   b.    
         [0107]    First diffuser  934  includes an aperture or slot  944 . Likewise, second diffuser  936  includes an aperture  946 . Aperture  944  is disposed earlier in the flowpath  948  of gas flowing within volute  928  than is aperture  946 , to improve the likelihood that boundary layer flow is discharged through first diffuser  936 .  
         [0108]    As impeller  926  rotates in the direction of the arrow  938 , velocity is imported to gas introduced through inlet  930 , thereby forcing such gas to discharge through diffusers  934 ,  936 . As velocity head. Gas flowing immediately adjacent to interior wall  924  includes relatively less energy than gas whose flowpaths are further radially spaced inwardly from the interior wall  924 . This relatively low energy gas flow enters first diffuser  934 . As such, gas flowing through first diffuser  934  includes lower energy relative to gas flowing through second diffuser  936 . In other words, a boundary layer bleed flows through first diffuser  934 .  
         [0109]    In one embodiment, gas flowing from each of diffusers  934  and  936  is coupled to a PSA apparatus, such as PSA apparatus  1  illustrated in FIG. 1, to sequentially deliver separate feeds to an adsorber  3  at progressively increasing pressure.  
         [0110]    In one embodiment, gas flowing from diffuser  934  is coupled to first feed mixture supply means  20  of PSA apparatus  1 , thereby delivering a feed mixture at the first intermediate pressure  21 . Similarly, gas flowing from diffuser  936  is coupling to second feed supply means  22  or third feed supply means  24 , thereby delivering a feed mixture to the adsorber  3  at a higher pressure then the first intermediate pressure  21  of the feed mixture being delivered through first feed mixture supply means  20 .  
         [0111]    Likewise, in another embodiment, gas flowing from diffuser  934  is coupled to second feed supply means  22 , thereby delivering a feed mixture at the second intermediate feed pressure  23 . Gas flowing from diffuser  936  is then coupled to third feed supply means  24 , thereby delivering a feed mixture to adsorber  3  at a higher pressure then the second intermediate pressure  23  of the feed mixture being delivered through the second feed mixture supply means  22 .  
         [0112]    The foregoing description is intended to be illustrative of the present invention. Those of ordinary skill will be able to envisage certain additions, deletions or modifications to the described embodiments which do not depart from the spirit or scope of the invention as defined by the appended claims.