Title of Invention	PRESSURE SWING ADSORPTION PROCESS AND AN APPARATUS
Abstract	A low pressure swing adsorption process and apparatus for the recovery of carbon dioxide from multi-component gas-mixtures, utilizing the simultaneous purge and evacuation of opposite ends of the adsorber(s) to effect controlled depressurization in the absorber bed(s) to maintain the constant purify of a carbon-dioxide enriched product stream recovered from the absorber inlets(s).

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FORM 2 THE PATENTS ACT 1970 [39 OF 1970] COMPLETE SPECIFICATION [See Section 10] "-PRESSURE SWING ADSORPTION PROCESS AND APPARATUS" A PRESSURE SWING ADSORPTION PROCESS AND AN APPARATUS FOR CARRVING OUT THE SAID PROCESS PRAXAIR TECHNOLOGY, INC., Technology Licensors, a corporation organized and existing under the laws of the State of Delaware, United States of America, of 39 Old Ridgebury Road, Danbury, State of Connecticut 06810-5113, United States of America, GRANTED 5-10-2005 The following specification particularly describes the nature of the invention and the manner in which it is to be performed:- 468/MUM/2000 22 MAY 2000 The present invention relates to a pressure swing adsorption process and an apparatus for carrying out the said process. RACKGROUND OF THE INVFNTION Field of the Invention This invention relates to an improved pressure swing adsorption process and apparatus for recovery of the more strongly adsorbed gas component in a multi-component gas mixture. More particularly, it relates to such a process and apparatus for the recovery of carbon dioxide from streams containing more weakly adsorbed components such as nitrogen, oxygen, hydrogen, methane, and carbon monoxide. Description of the Prior Art High-purity liquid carbon dioxide (99.99+%) is commonly produced by direct liquefaction of gas streams containing in excess of 95% C02. These high-concentration sources are available directly as by product streams from chemical processes such as ammonia synthesis. As a result, location of carbon dioxide liquefaction plants is traditionally dictated by. the location and.availability of these sources. Many carbon dioxide customers do not require 99.99 + % purity of liquefied C02. Applications such as pH control and carbonate production can be effectively serviced with gaseous C02 ranging in purity from 8 0% to 90%. Frequently, the sites where lower purity gaseous C02 is. needed have alternative sources of low concentration C02available, i.e., typically containing less than 20% C02. Consequently, there is an opportunity for new technologies that are capable of economical on-site C02 production from low-grade sources such as flue gas from boilers or other combustion sources. Various methods for C02 recovery from low and intermediate concentration sources are known. Chemical D-20657 absorption of C02 from a multi-component gas stream into a liquid absorbent, followed by heating to strip the C02 from solution, is used to recover gaseous C02 at 99+% purity. A variety of liquid amines, or potassium carbonate, can be used as the absorbent medium. The primary disadvantages of these processes are significant energy requirements for thermal regeneration of the absorbent, and reduction in absorbent capacity when modest quantities of oxygen are present in the multi-component gas stream. Membrane separation processes may also be used for C02 recovery, but these processes often require high feed pressures to achieve modest permeability for C02. Expensive multi-staged membrane processes are needed for production of high purity C02 from low concentration sources such as flue gas. Pressure swing adsorption (PSA) separations offer significant potential advantages as compared with other methods for C02 recovery. Thus, PSA offers the potential for lower-cost concentration and delivery of C02 in comparison with the more traditional method of liquefaction and transportation of liquid C02, particularly when transportation costs are high or attractive feed stocks for C02 liquefaction are unavailable. A further and primary advantage of PSA techniques is the flexibility to produce C02 product at variable purity. Adsorption and desorption pressures can be tuned, along with other process parameters, to yield the minimum desired product purity for a particular application. This allows power requirements to be reduced when high-purity product is not needed. A similar reduction in power consumption is difficult to achieve with liquid absorption processes since thermal stripping of the absorbent will always yield product at 99+% purity. PSA does not require a high temperature energy source like steam for regeneration, D-20657 as do absorption processes. As a result, PSA is an attractive alternative for locations where steam is unavailable or expensive. In general, adsorptive separation is a reliable, flexible and potentially lower cost method for recovery of C02, particularly when gas-phase purity in excess of 99% is not required. For C02 production from combustion flue gas, lime kiln off-gas, H2 plant tail gas and other sources, the function of the primary adsorbent(s) is to selectively adsorb C02 while allowing lighter components to pass through. Water, which is typically more strongly adsorbed than C02, may be present but can be effectively removed in a pretreatment layer of adsorbent. Therefore, the production of C02 using PSA requires processes that are effective for heavy component recovery, that is, for recovery of the more strongly adsorbed component in a multi-component mixture. A number of PSA processes for heavy component recovery, including the production of C02 from low concentration sources, have been described in the prior art. See, for example, the following U.S. patents: Werner et al. 4,599,094; Fuderer 4,723,966; Lagree et al. 4,810,265; Hay 4,840,647; Schmidt et al. 4,892,565; Krishnamurthy et al. 4,963,339; Kumar 5,026,406; Knaebel 5,032,150; Kumar 5,248,322 and 5,354,346; LaSala et al. 5,370,728; Leavitt 5,415,683; and Couche 5,669,960. The most common applications for heavy component recovery are N2/02 separation utilizing zeolite adsorbents, and C02/N2, C02/CH4 and C02/H2 separations utilizing zeolites, activated carbons, silica gel or other adsorbents. Typically, prior art processes rely on compression of the feed to an elevated adsorption pressure, evacuation to recover heavy component product and rinsing with heavy D-20657 component. Prior art processes typically use multiple beds to insure continuous utilization of equipment, with surge tanks used to dampen fluctuations in product flow and purity. Prior art processes for heavy component recovery, or combined light and heavy component recovery, can be divided into three general classes: conventional cycles, inverted cycles and reflux cycles. In a conventional cycle, adsorption occurs at higher pressure, with purging and recovery of heavy component product taking place at lower pressure. In an inverted cycle, adsorption occurs at lower pressure, with purging at a higher pressure. Each adsorbent bed in a reflux cycle contains a conventional bed portion and an inverted bed portion, with reflux of light and heavy component between beds. The advantage of the reflux cycle is that both light and heavy component products can be recovered at high purity and high recovery. However, this process is energy intensive and unattractive if recovery of the light component is not desired. The inverted cycle can be used to recover heavy component product at high purity, but requires significant power consumption. A conventional cycle may consume less power, but heavy component product purity varies throughout the cycle. An additional disadvantage of the inverted cycle is that it requires removal of water or other heavy components in a separate vessel before the feed enters the main adsorbent vessels. The individual steps in conventional PSA cycles are well known in the prior art. The first step in the basic cycle is adsorption, in which a multi-component feed gas is passed to the adsorbent bed at an elevated adsorption pressure. During this step the more selectively adsorbed component is retained by the adsorbent while the gas phase is enriched in less selectively adsorbed components. Typically, the adsorption step is D-20657 terminated before the mass transfer front reaches the outlet of the adsorbent bed. Following adsorption, the adsorber vessel is depressurized via countercurrent blowdown and/or evacuation. As the pressure is reduced the gas phase becomes enriched in heavy component. At least a portion of the gas evolved during the depressurization stages is taken as heavy component product. Following depressurization, the adsorber vessel is repressurized to the adsorption pressure and the cycle is repeated. The basic cycle may be modified to include rinsing of the bed with heavy-component product between adsorption and depressurization stages. This displaces a portion of the non-adsorbed gas from the bed and provides increased recovery of the heavy component product. The cycle may also include purging at intermediate or low pressures to further regenerate the adsorbent before the cycle is repeated. Many of the potential on-site applications for gaseous C02 are relatively small, e.g. less than 3 0 tons/day of contained C02 product. On-site C02 plants as small as 1 to 5 tons/day can be envisioned. This small plant size dictates the need for processes that are simple, reliable, and minimize process flow sheet complexity -- and hence minimize capital cost. As plant size decreases, even relatively modest capital expenditures and fixed costs can add significantly to the unit production cost. The capital cost penalty for prior art processes with four or more beds, and associated valves, is significant when plant capacity is very small. The use of expensive surge tanks to dampen fluctuations in product purity adds additional cost to the process. Adsorption at elevated pressure, as in many prior art techniques, requires the compression of large amounts of the light (waste) components; this adds particular energy expense in D-20657 recovering C02 from dilute gas mixtures such as combustion flue gas which may contain as little as 6 to 10% C02, i.e. energy is consumed in compressing 90% or more of the feed gas that is eventually discarded as waste. Typical prior art processes for C02 recovery have relied on adsorbents such as zeolite 13X or BPL activated carbon. For recovery of C02 from low concentration sources such as flue gas, the advantage of using a relatively strong adsorbent such as zeolite 13X is that it retains a significant capacity for C02, even at the low C02 partial pressures present in the feed. The disadvantage is that it requires very low pressure for regeneration. BPL activated carbon is a much weaker adsorbent for C02/ and consequently, does not require such demanding desorption conditions. However, the utility of this adsorbent is diminished for low concentration sources like flue gas because of the weak and nonspecific interaction with C02. At flue gas feed conditions, the equilibrium loadings of N2 and C02 on BPL carbon are nearly identical, resulting in low adsorption selectivity. This low efficiency of separation severely limits the purity and recovery that can be achieved in the process. It is among the objects of the present invention to provide improved PSA processes and apparatus for the recovery of heavy components such as C02 from multi-component gas mixtures at predetermined substantially constant product purity, i.e. a variation of less than plus or minus 10 percent of the desired product purity, utilizing adsorption techniques employing low adsorption pressures and with no bed-to-bed interactions. A further object of the invention is to provide such processes and apparatus, requiring lower capital and operating costs than for prior art techniques, particularly for small scale applications. Yet an additional object of the invention is to provide improved PSA processes for the recovery of C02 from multi-component gas mixtures, by utilizing as adsorbents therein zeolites having particular adiabatic separation factor and dynamic C02 loading characteristics. With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims. SUMMARY OF THE INVENTION According to the present invention there is provided a pressure swing adsorption process for the recovery of at least CO2 from a multi-component gas mixture, which comprises: (1) feeding the multi-component gas mixture into an inlet of and through at least one adsorber at an adsorption temperature of 300° to 400°K and under an adsorption pressure of 90 to 200 kPa, adsorbing CO2 of the gas mixture on an adsorbent within the adsorber and removing an effluent enriched with the light component or components of the mixture from the adsorber through an outlet thereof, at least a portion of the light component-enriched effluent being retained in a pressure zone communicating with the adsorber outlet; (2) blowing down a portion of the light component-enriched effluent cocurrently through the adsorber outlet into a vacuum zone maintained at a pressure less than the adsorption pressure and communicating with the adsorber and, at the same time, removing desorbed gas by countercurrent evacuation from the adsorber through the adsorber inlet, to simultaneously depressurize the adsorber cocurrently from its outlet and countercurrently from its inlet; (3) terminating the flow of the blowdown gas through the adsorber outlet while continuing the countercurrent evacuation of the desorbed gas from the adsorber inlet to further depressurize the adsorber until the pressure in the adsorber is less than that in the vacuum zone; (4) passing the blowdown gas as a purge gas stream from the vacuum zone to the adsorber outlet countercurrently to and through the adsorber to make and recover a product stream from the bed containing the CO2 in at least a predetermined, substantially constant purity, wherein the adsorbent has an adiabatic separation factor DCO2/DN2 in excess of 2.0 and a dynamic CO2 loading in excess of 0.1 mole/kg at said adsorption temperature and pressure, such absorbent performance characteristics determined at the reference conditions of 0.12 CO2 feed mole fraction and a desorption CO2 mole fraction and pressure of 0.8 and 6kPa, respectfully; (5) terminating the flow of the blowdown gas from the vacuum zone and passing light component-enriched effluent from the pressure zone into the outlet of the adsorber to partially repressurize the adsorber; (6) passing an additional portion of the multi-component feed gas mixture into the inlet of the adsorber to complete repressurization of the adsorber; and (7) repeating the foregoing steps to produce the CO2 product stream. Preferably,the PSA process and apparatus of this invention provide an economical system for heavy component recovery such as C02 recovery from gas streams containing up to about 60% C02, e.g., combustion flue gas, lime kiln off-gas or hydrogen plant tail-gas, incorporating more weakly adsorbed, light components such as nitrogen, oxygen, hydrogen, methane and/or carbon monoxide. When so utilized, the process is desirably carried out at about atmospheric pressure, i.e., at less than 2 and, preferably at about 1-1.5 atmospheres. Other applications include nitrogen recovery from air, heavy hydrocarbon recovery from natural gas, and oxygen recovery from air. As indicated above, potential feed sources such as flue gas may contain as little as 6 to 10% C02. Consequently, adsorption at elevated pressure requires the compression of large amounts of light component. If the light component in the feed stream is not a valuable.product, compression should-be minimized-to conserve energy. The process of this invention requires only minimal feed compression to overcome pressure drop in the apparatus. Moreover, as indicated below, the adsorbent utilized is chosen to maximize dynamic capacity and selectivity for the heavy component at the feed concentration and at near-ambient adsorption pressures. Finally, in those cycles described below in D-20657 which partial cocurrent displacement by the heavy-component -enriched product stream is utilized to increase heavy component recovery and provide continuous production, the quantity of displacement gas is minimized, thus further reducing power consumption. Reduced power consumption eliminates the need for the equalization techniques employed in a number of prior art systems to recover compression energy. In the present invention, the further use of simultaneous cocurrent/ countercurrent depressurization of the adsorber by exhausting the light component-enriched effluent as a blowdown gas out of the adsorber and into an external vacuum zone while at the same time removing an evacuation stream from the adsorber inlet (step (2)) rejects void gas from the heavy component product. At the beginning of the depressurization step in a conventional cycle, the concentration of heavy component in the void spaces at the inlet end of the bed is identical to the feed concentration. As the pressure is reduced the heavy-component concentration in the gas phase at the inlet of the bed begins to increase from this initial value. During the initial period of depressurization the exiting gas is enriched in heavy component but is not of product quality. To overcome this problem, it has previously been necessary to rinse the adsorbent bed with heavy component product after completion of the adsorption step in order to displace the low quality gas from the bed, and to thereafter recover pressure energy from such operation by using the displaced effluent to repressurize another adsorber. In the present invention the problem is minimized by blowdown protocol which controls pressure and flow in the adsorber to minimize mixing losses or by using partial cocurrent rinsing with heavy component product to D-20657 raise the initial purity before depressurization begins. This latter step allows product to be recovered immediately upon blowdown, eliminating the need for rejecting waste from the inlet end of the bed. Significantly, by thereafter returning the blowdown gas from the vacuum zone countercurrently through the adsorber as a purge stream while continuing to remove the evacuation stream from the adsorber (step (4)), a controlled purge is established in the present invention which not only serves to move the heavy component product toward the adsorber inlet but, moreover, may be utilized to effect controlled reduction in pressure in the adsorber bed during purging to maintain constant purity in the product stream. Fluctuations in heavy component purity that typically occur during depressurization are thereby eliminated, removing the need for a large surge vessel to smooth out such fluctuations. In accordance with a further, preferred form of the invention, at least a portion of the product stream recovered in step (4) above is recycled to the inlet of the adsorber to affect cocurrent displacement of the gas mixture at the inlet and thereby increase the inlet concentration of the heavy component, e.g., C02 to the desired purity of the product stream. The minimum quantity of displacement gas required is dictated by feed and product purity, this minimum quantity of displacement gas is provided to insure compliance with predetermined product purity requirements and also to minimize the consequent power consumption associated therewith. Many prior art processes utilize adsorber bed-to-bed interactions for equalization or purging purposes, which typically require three or more identical adsorbers for efficient cycle operation. As a result, capital costs for multiple adsorbers and associated valving are significant for these processes. The present invention reduces the number of adsorbers and switching valves required by eliminating bed-to-bed interactions. Pressure and vacuum tanks provide repressurization and purge gas, and low pressure adsorption eliminates the necessity for equalization to recover pressure energy. The process of the present invention can thus be implemented with a single adsorber or, preferably, with two independently but sequentially operable adsorbers to permit continuous utilization of vacuum equipment. According to the present invention there is also provided an apparatus for carrying out a pressure swing adsorption process for the recovery of a heavy gas component in a multi-component gas mixture, which comprises: (1) at least one adsorber having an inlet end and an outlet end and incorporating at least one layer of an adsorbent capable of selectively adsorbing the heavy component of the gas mixture relative to the light component or components thereof; (2) a feed blower communicating with the inlet end of the adsorber for feeding the gas mixture into and through the adsorber; (3) a pressure tank communicating with the outlet end of the adsorber for receiving a light component-enriched effluent from the adsorber and for feeding the light component-enriched effluent into the outlet end of the adsorber, (4) a vacuum tank communicating with the outlet end of the adsorber for receiving a further portion of the light component-enriched effluent from the adsorber and for feeding the light component-enriched effluent as a blowdown gas in the adsorber-through its outlet end; (5) outlet valve means for selectively placing the pressure and vacuum tanks in communication with the outlet end of the adsorber for depressurizing and repressurizing the adsorber; (6) a vacuum pump independent of the feed blower and communicating with the inlet end of the adsorber for removing a product stream containing the heavy component of the gas mixture at a predetermined, constant concentration; (7) inlet valve means for selectively placing the feed blower and the vacuum pump in communication with the inlet end of the adsorber to facilitate feed of the gas mixture into and removal of the product stream from the inlet end of the adsorber; and (8) control means for concurrently actuating the outlet valve means and the inlet valve means to facilitate feed of the light component-enriched effluent as a blowdown gas from the vacuum tank into the adsorber and removal of an evacuation stream from the adsorber, to simultaneously depressurize the adsorber cocurrently from its outlet end and countercurrently from its inlet end. As indicated above, the apparatus of the invneiton may utilize a single adsorber with pressure and vacuum tanks communicating with the outlet end thereof to facilitate repressurization and purging of the adsorber, and a desorption effluent wihtdrwal means, e.g. a vacuum pump, communicating with the adsorber inlet for removing the heavy component-enriched avacuation and product streams, the use of such apparatus eliminates the necessity to provide large, expensive surge tanks to dampen fluctuations in product purity, product pressure or product flow rate and thereby reduces capital cost and energy expense in carrying out PSA techniques therewith. While such apparatus of the invention eliminates the need for plural adsorbers and relatively complex switching D-20657 means for effecting bed-to-bed interactions therebetween it is preferred, in accordance with the invention, to employ a pair of independently operable adsorbers which are operated in staged sequence to provide a continuous flow of the product stream from the inlet end of the adsorber. Alternatively but less preferably, portions of the feed stream may be simultaneously fed through the adsorbers and they may operate in parallel. In either event, operation with two adsorbers allows more efficient utilization of any rotating equipment (compressor and vacuum pump) and hence, increased capacity and efficiency. In accordance with a further feature of the present invention, an improved method for selectively recovering C02 from multi-component gas mixtures is provided, comprising adsorbing the C02 at substantially atmospheric pressure in. particular adsorbents. That method comprises adsorbing the gas mixture in an adsorbent at feed conditions of about 250 to 450K, more preferred 300 to 400K, and pressures of about 90 to 200 kPa, depressurizing the adsorbent bed to sub-atmospheric pressure to desorb the C02 from the bed, and recovering a C02-containing product stream from the bed at substantially constant purity, wherein the adsorbent has an adiabatic separation factor ACO2/AN2 in excess of about 1, preferably above 2, and a dynamic DC02 loading of at least 0.1 mol/kg at the process operating conditions. Tailoring of the adsorbent to these process conditions provides additional capital cost reduction through improved efficiency of the adsorbent. Adsorbents that provide high working capacity and selectivity in the preferred low pressure range of operation are critical to reducing the quantity of adsorbent needed for a given production rate. Zeolite D-20657 NaY and zeolite NaX(2.0) are exemplary adsorbents for CO2 that provide reduced bed size factors for production of C02 from combustion flue gas, lime kiln off-gas, hydrogen plant tail gas and other sources containing more weakly adsorbed components such as nitrogen, oxygen, hydrogen, methane, and carbon monoxide. Further reduction in adsorbent inventory is achieved by operating with shallow adsorption beds and short cycle times. This increases productivity of the adsorbent and also works to minimize the size of surge volumes if needed to dampen flow rate variations. BRIEF DESCRIPTION OF THE DRAWINGS The invention is hereinafter described in detail with reference to the accompanying drawings in which: Figure 1 is a flow diagram of a particular embodiment of a single-bed PSA apparatus of the present invention; Figure 2 illustrates the basic cycle sequence of the PSA process of the invention utilizing the single-bed apparatus of Figure 1; Figure 3 is a schematic flow diagram of a further embodiment of a single-bed PSA apparatus of the invention, incorporating a cocurrent displacement system for feeding a heavy component product to the inlet of the adsorber during a portion of the processing cycle; Figure 4 illustrates the cycle sequence for the single-bed cocurrent displacement system apparatus of Figure 3; Figure 5 is a schematic flow diagram of a further embodiment of a two-bed PSA apparatus of the invention; D-20657 Figure 6 is a schematic flow diagram of a preferred embodiment of a two-bed/cocurrent displacement apparatus of the invention; Figure 7 is a graph of the C02 adsorption isotherms at different pressures for various adsorbents; Figure 8 is a graph of the variation in adiabatic separation factors for various adsorbents at different adsorption temperatures; and Figure 9 is a graph of the variation of dynamic CO2 loading (ACO2) characteristics of various adsorbents at different adsorption temperatures. DETAILED DESCRIPTION OF THE INVENTION As indicated above, the present invention is directed to the recovery of the more strongly adsorbed gas component in a multi-component gas mixture, by separating such component from more weakly adsorbed components admixed therewith. For convenience, the more strongly adsorbed gas component is identified herein as the "heavy" component, and the more weakly adsorbed components are described as the "light" components of the gas mixture. Further, in the following description, principal reference is made to the recovery of C02 as the heavy component of gas mixtures such as combustion flue gas, lime kiln off-gas and/or hydrogen plant tail-gas, containing nitrogen, oxygen, hydrogen, methane, carbon monoxide and/or other light components therein. It will, however, be understood that while the invention is primarily directed to the recovery of C02 from such multi-component gas mixtures, it is not limited to the recovery of C02 or to separations from any particular D-20657 feed-gas source. Thus, it is intended that the processes and apparatus of the invention may be used for any multi-component separation in which the more selectively adsorbed component is a desired product. This includes, but is not limited to, nitrogen recovery from air using nitrogen-selective adsorbents, oxygen recovery from air using oxygen-selective adsorbents, carbon monoxide recovery from syngas using CO-selective adsorbents, and oxygen/argon separation using either oxygen-selective or argon-selective adsorbents. One skilled in the art will appreciate that the process conditions are tailored to the specific separation of interest. D-20657 Single-Bed Systems (Figures 1- 4) The essential elements of the basic apparatus of the invention are illustrated in Figure 1. In the most basic embodiment shown therein, a single adsorber 4 is utilized incorporating a bed or beds of an appropriate adsorbent or adsorbents, and having an inlet end 4A and an outlet end 4B. The inlet end 4A of the adsorber is connected through a two-way valve 3 to a feed blower 2 for supplying a multi-component gas mixture from a feed line 1 to the adsorber, and through a two-way valve 10 to a vacuum pump 11 for evacuating the adsorber to pressures below ambient and for recovering a product stream through line 12. Preferably, feed blower 2 compresses the feed gas only to such an extent as to overcome pressure drop through the system, thereby providing the gaseous feed at a pressure slightly above ambient, atmospheric pressure. The feed blower may be eliminated when the feed gas is supplied at sufficient pressure to overcome system AP. The outlet end 4B of the adsorber is connected through a two-way valve 5 to a pressure tank 6, from which effluent may be removed for recovery as a further product stream, or vented, through line 7, or through a control valve 8 to a vacuum tank 9. The pressure tank 6 is used to store gas at ambient pressure or above; the vacuum tank 9 is used to store gas at pressures below ambient. The pressure tank is preferably a variable volume vessel operating at . approximately ambient pressure. In operation, feed gas is provided to the inlet end 4A of the adsorber 4, light component effluent from the outlet end 4B of the adsorber is collected in D-20657 the pressure tank 6 and subsequently reused as repressurization gas, cocurrent blowdown gas from the outlet end 4B of the adsorber is collected in the vacuum tank 9 and subsequently reused as purge, and heavy component-enriched product is recovered through line 12 from the discharge side of vacuum pump 11. A portion, or all, of the light component effluent is collected in the pressure tank 6. Some of the light component may be exhausted as waste through line 7 without being returned to the system. Two-way switching valves 3, 10 and 5 allow the adsorber 4 to communicate alternately with the blower 2, vacuum pump 11 and pressure tank 6, respectively. A control valve 8 is used to regulate flow between the adsorber and the vacuum tank 9. The single-bed system shown in Figure 1 is suitable for use in the practice of the basic cycle steps shown in Figure 2. During step (1) illustrated in Figure 2, feed gas is passed through line 1 to the inlet side of the feed blower 2. The two-way valve 3 is opened to allow flow from the outlet of blower 2 into the inlet end 4A of the adsorber 4. Effluent from the outlet end 4B of adsorber 4 passes through two-way valve 5 into pressure tank 6. At least a portion of the effluent from the adsorber is stored in pressure tank 6, the fraction of effluent to be removed from the system exiting via line 7. The multi-component feed gas is thus fed at the adsorption pressure to the inlet of the adsorber, the heavy component being selectively retained by the adsorbent and the gas phase being enriched in the light component or components; at least a portion of • D-20657 the light component effluent from the outlet end of the adsorber is collected and retained in the pressure tank during this step. Step (2), simultaneous cocurrent and countercurrent depressurization, is then carried out by closing valve 3 and valve 5, opening control valve 8 to allow flow into vacuum tank 9, and opening the two-way valve 10 to allow flow to vacuum pump 11. During this step, discharge from vacuum pump 11 is recovered in line 12 and may initially be rejected as waste or recycled with the feed. In step (2), the adsorber is thus simultaneously depressurized cocurrently from its outlet end, and countercurrently from its inlet end; the bed pressure provides the driving force for cocurrent depressurization to the vacuum tank, and the bed pressure and vacuum pump provide the driving force for countercurrent depressurization. The cocurrent depressurization gas is collected in the vacuum tank at below ambient pressure for subsequent use as purge gas; the countercurrent depressurization gas in this step consists primarily of void space gas at feed composition from the adsorber inlet and may initially be rejected or recycled with the feed. Enrichment of the heavy component in the countercurrent depressurization gas is minimized in this step by careful balance of the countercurrent and cocurrent blowdown flows. Continued countercurrent evacuation, step (3) , is affected by closing control valve 8 while keeping valve 10 open. The flow is thus terminated from the vacuum tank to the outlet end of the adsorber bed while continuing the countercurrent evacuation with D-20657 the vacuum pump until the purity of the heavy component product meets minimum product purity requirements and the pressure in the adsorber bed is lower than the pressure in the vacuum tank; this countercurrent evacuation gas is enriched in the heavy component and may be rejected as waste or recycled with the feed. At the beginning of the product make-step, step (4), the pressure in adsorber 4 will be lower than in vacuum tank 9. Control valve 8 is opened and used to control the purge stream flowing from vacuum tank 9 into adsorber 4. Valve 10 remains open to allow product to be recovered through vacuum pump 11 into line 12. The heavy component product is thus removed from the inlet end of the adsorber while low pressure purge gas is passed from the vacuum tank to the outlet end of the adsorber. The purge rate is controlled by the control valve so that pressure and product purity remain constant initially. As product purity begins to decrease, the purge rate is decreased as the pressure in the adsorber falls while maintaining constant product purity. Following this constant purity product make-step, the bed is repressurized to the adsorption pressure. If sufficient gas remains in vacuum tank 9, it may be used to partially repressurize the bed in step (5). During this step valve 10 is closed and control valve 8 is fully opened to equalize the pressure in vacuum tank 9 and adsorber vessel 4. Ideally, the exact quantity of gas stored in step (2) is used as purge in step (4) and this, equalization step (5) may be eliminated. If necessary, step (5) would thus partially repressurize the adsorber countercurrently D-20657 from its outlet end, using the remaining cocurrent depressurization gas from the vacuum tank. Note that the end of step 4 and step 5, if required, serve to restore the vacuum tank to the low pressure required in subsequent steps. In step (6), partial repressurization with light component effluent is accomplished by opening valve 5 while control valve 8 and valve 10 are closed. Light component effluent that was stored in pressure tank 6 during step (1) repressurizes adsorber 4. Partial countercurrent repressurization is thus effected at the outlet end 4B of the adsorber with light component from the pressure tank. The final step in the cycle, step (7), is repressurization to the adsorption pressure. Valve 5 is closed and valve 3 is opened to allow feed gas to enter adsorber 4. The adsorber is thus repressurized to the adsorption pressure in the cocurrent direction using feed gas passed to the adsorber inlet 4A. At the end of repressurization the cycle sequence is complete, and a subsequent cycle may then begin with step (1) . An alternative embodiment of the invention employs cocurrent displacement with heavy component product during a portion of the process cycle. This requires an additional tank for collecting a portion of the heavy component product for subsequent feed to the adsorber. The essential elements of the apparatus for cocurrent displacement are illustrated in Figure 3. Elements 21-32 of the VPSA system shown therein correspond to the like elements 1-12 of the embodiment of the invention illustrated in Figure 1. In addition, in order to provide partial cocurrent rinsing with the D-20657 product stream recovered from adsorber 24, a heavy component storage tank 33 is provided which is connected to the discharge side of vacuum pump 31, and is provided with a discharge line 34 and a recycle line 35 connected to the inlet side of blower 22. Valve 36 is a two-way valve that is opened during cocurrent displacement to allow displacement gas to be passed to the inlet side of blower 22. Valve 37 is a two-way valve used to stop flow of feed gas during the cocurrent displacement step. The cocurrent displacement gas may be passed directly to the inlet of adsorber 24 without passing through blower 22 if the discharge pressure of the vacuum pump is high enough to overcome the pressure drop in the system. A portion of the heavy component product recovered from the discharge side of the vacuum pump may thus be temporarily stored in the heavy, component storage tank, and thereafter fed to the suction side of the feed blower during a later step in the cycle. This heavy component storage tank operates at approximately ambient pressure and may be a variable volume vessel. Only a portion of the heavy component product is recycled back to the adsorber; the remaining fraction is removed from the system as product. The single-bed system shown in Figure 3 is suitable for use in the cocurrent displacement cycle shown in Figure 4. During the first step, step (1), feed gas 21 is passed through valve 37 to the inlet side of feed blower 22. The two-way valve 23 is opened to allow flow from the outlet of blower 22 into the inlet end 24A of adsorber 24. Effluent from the outlet end 24B of adsorber 24 passes through two-way valve 25 D-20657 into pressure tank 26. At least a portion of the effluent from the adsorber is stored in pressure tank 26, the fraction of effluent to be removed from the system exiting via line 27. The multi-component feed gas is thus passed at the adsorption pressure to the adsorber inlet, during which the heavy component is selectively adsorbed by the adsorbent material and the gas phase is enriched in the lighter component or components; at least a portion of the light component effluent from the outlet end of the adsorber is collected and retained in the pressure tank during this step. Partial cocurrent rinsing with heavy component, step (2), is accomplished by passing product from the heavy component storage'tank 33 through line 35 and valve 36 to the inlet side of blower 22. Valves 23 and 25 remain open. Valve 37 is closed. A portion or all of the effluent from adsorber 24 is stored in pressure tank 26, the fraction of effluent to be removed from the system exiting via line 27. The heavy component product is thus passed from the heavy component storage tank to the inlet of the adsorber; this displaces a portion of the nonadsorbed gas containing a large fraction of light component from the inlet end of the adsorber, increasing the gas phase concentration of heavy component at the inlet end, to the minimum desired product purity. The amount of displacement gas is chosen so that the concentration of heavy component recovered from the inlet end of the bed during subsequent depressurization does not drop below the minimum product purity requirements. A portion or all of the effluent from the outlet end of the adsorber may be stored in the pressure tank during D-20657 this step. Simultaneous cocurrent and countercurrent depressurizations, step (3), are then conducted by closing valves 23 and 25, while opening control valve 28 to allow flow into vacuum tank 29, and opening two-way valve 30 to allow flow to vacuum pump 31. During this step, discharge from vacuum pump 31 is passed through line 32 into the heavy component storage tank 33. The portion of product not retained for subsequent cocurrent displacement is removed from the system through line 34. In this step the adsorber is simultaneously depressurized cocurrently from its outlet end and countercurrently from its inlet end, the bed pressure providing the driving force for cocurrent depressurization to the vacuum tank, and the bed pressure and vacuum pump providing the driving force for countercurrent depressurization. The cocurrent depressurization gas is collected in the vacuum tank at sub-ambient pressure for subsequent use as purge gas; the countercurrent depressurization gas is recovered as heavy component product. Continued countercurrent evacuation, step (4), is accomplished by closing control valve 28 while keeping valve 30 open. Control valve 28 remains closed until the pressure in the adsorber bed reaches a predetermined level that is lower than the pressure in the vacuum tank; this countercurrent evacuation gas is recovered as heavy component product. At the beginning of step (5), the pressure in adsorber 24 will be lower than in vacuum tank 29. Control valve 28 is opened and used to control flow from vacuum tank 29 into the adsorber. Valve 30 remains open to allow product to be recovered through D-20657 vacuum pump 31 into line 32. In this step the heavy component product from the inlet end of the adsorber is recovered while passing low pressure purge gas from the vacuum tank to the outlet end of the adsorber 24. The purge rate is controlled by the control valve so that both pressure and product purity remain constant initially; as product purity begins to decrease, the purge rate is decreased to allow pressure in the adsorber to fall in order to restore and maintain constant product purity. Following the constant-purity product make-step (5), the bed may be partially repressurized in step (6). If all of the cocurrent depressurization gas that was stored in vacuum tank 29 during step (3). is not used as purge gas during step (5), it may be used to partially repressurize the bed in step (6). During step (6), valve 30 is closed and control valve 28 is fully opened to equalize the pressure in vacuum tank 29 and adsorber 24. Ideally, however, the exact quantity of gas stored in step (3) is used as purge in step (5) and the equalization step (6) may be eliminated. In step (7), partial repressurization with light component effluent is accomplished by opening valve 25 while control valve 28 and valve 30 are closed. Light component effluent that was stored in pressure tank 26 during step (1) repressurizes adsorber 24. The final step in the cycle, step (8), is repressurization to the adsorption pressure. Valve 25 is closed and valve 23 is opened to allow feed gas to enter the adsorber 24. The adsorber is thus repressurized to its operating pressure in a cocurrent direction using feed gas passed from the feed ■ D-20657 compressor to the adsorber inlet. At the end of repressurization the cycle sequence is complete, and a subsequent cycle may then begin with step (1). Two-Bed Systems (Figures 5 and 6) Preferred embodiments of the PSA systems of the invention utilize two identical, independently operable adsorbers through which the multi-component gas mixture is sequentially fed and in which the adsorption/desorption operations are carried out in staged sequence such that the product streams recovered from the individual adsorbers are combined to provide a continuous flow of the product stream containing the heavy gas component at the desired purity. Each adsorber interacts alternately with the pressure tank, vacuum tank, feed compressor and vacuum pump. There are no direct adsorber-to-adsorber interactions during the cycle. The essential elements of the basic two-bed apparatus are shown in Figure 5. Elements 41-48 and 50-53 are the elements of a first adsorber train A, corresponding to elements 1-12 of the single-bed apparatus of Figure 1, and elements 54-58 represent a second adsorber train B cycled with adsorber train A (thus elements 54-58 correspond to elements 43-45, 48 and 51, respectively). Control valve 49 is incorporated in the system to control blowdown and purge flows between vacuum tank 50 and the respective adsorber trains A and B. The process cycle steps for each bed in the basic two-bed apparatus of Figure 5 are identical to those for the single adsorber in Figures 1 and 2. The cycle D-20657 for the first adsorber is operated out of phase with respect to the second adsorber as indicated in Table I below: Time Table I: Two-Bed Basic Cycle Bed A Bed B 5) V Repress. 2) Blowdown 6) P Repress. 3) Evacuation 7) Feed Repress. 4) Product/Purge 1) Adsorption t 1/2 cycle 2) Blowdown 3) Evacuation 5) V Repress. 6) P Repress. 7) Feed Repress. 4) Product/Purge 1) Adsorption tcycle Preferably, the cycle steps are balanced such that adsorber train A goes through steps(5), (6), (7) and (1) while adsorber train B is cycled through steps (2), (3) and (4). This provides balanced cycle operation which eliminates idle time for the adsorbers, allows more efficient utilization of the rotating equipment, and provides increased capacity. If the optional equalization step (5) is unnecessary the cycle can be balanced so that the vacuum pump is in continuous operation. Otherwise, an idle period equal to the time required for step (5) is necessary to prevent simultaneous interaction between both adsorbers and the vacuum surge tank. D-20657 The most preferred embodiment of the invention, illustrated in Figure 6, employs two adsorbers and cocurrent rinsing with the heavy component product. Each adsorber undergoes the same cyclic operation as for the single adsorber in Figures 3 and 4. Each adsorber interacts alternately with the pressure tank, vacuum tank, feed compressor and vacuum pump. Again, there are.no direct adsorber-to-adsorber interactions during the cycle. Two-adsorber operation with cocurrent rinsing allows continuous production of heavy component product, as well as more efficient utilization of the rotating equipment, resulting in the best process efficiency and highest production capacity. The essential elements of the cocurrent displacement, two-adsorber bed apparatus are shown in Figure 6. The elements 61-78 correspond to elements 41-58 of the two-bed basic apparatus illustrated in Figure 5. An additional heavy component storage tank 79 receiving discharge from vacuum pump 72 through line 73 and provided with removal line 80 and recycle line 81 communicating with the inlet side of blower 62 is additionally provided for cocurrent displacement (rinsing). Elements 72, 73 and 79-83 correspond, respectively, to elements 31-37 of the single-bed cocurrent displacement apparatus shown in Figure 3, and operate in the same manner as described in connection therewith. The process cycle steps for each bed in the cocurrent displacement, two-bed apparatus of Figure 6 are identical to those for the single adsorber in Figures 3 and 4. The cycle for one vessel is operated out of phase with respect to the other vessel as D-20657 indicated in Table II below. Table II: Two-Bed Cocurrent Displacement Cycle Time Bed A Bed B 6) V Repress. 3) Blowdown 7) P Repress. 4) Evacuation 8) Feed Repress. 5) Product/Purge 1) Adsorption 2) Cocurrent Disp. t 1/2cycle 3) Blowdown 6) V Repress. 4) Evacuation 7) P Repress. 8) Feed Repress. 5) Product/Purge 1) Adsorption 2) Cocurrent Disp. lcycle Preferably, the cycle steps are balanced so that adsorber train A goes through steps (6), (7) and (8), (1) and (2) while adsorber train B executes steps (3), (4) and (5). This provides balanced cycle operation which eliminates idle time for the adsorbers, allows more efficient utilization of the rotating equipment, provides increased capacity, and provides continuous production of heavy component product. If optional step (6) is eliminated the cycle can be balanced so that the vacuum pump is in continuous operation. Otherwise, an idle period equal to the time required for step (6) is necessary to D-20657 prevent simultaneous interaction between both adsorbers and the vacuum surge tank. Compared to the basic cycle, the cocurrent displacement cycle can provide higher throughput and higher purity product — a surprising result since throughput and product purity are usually inversely related. Two-adsorber operation is preferred, as it provides continuous utilization of vacuum equipment. Two-adsorber operation with partial cocurrent displacement is particularly preferred, as it provides continuous utilization of vacuum equipment and continuous production of heavy component product, in addition to higher throughput. Adsorbents Utilized (Figures 7-9) Adsorbents useful in the practice of the present invention include the known types of molecular sieve adsorbents, such as those of the zeolite A, X and Y types disclosed, for example, in Milton U.S. Patent Nos. 2,882,243 and 2,882,244. The adsorbent(s) utilized are chosen to maximize both the degree of dynamic loading of the heavy component on the adsorbent under the adsorption pressure, temperature and composition, and the degree of separation at the conclusion of the adsorption/ desorption cycle between the heavy component to be recovered from the multi-component gas mixture and the lighter component or components thereof. The adsorbent may be provided in a single bed, or in multi-layer adsorbent beds such as described in copending application Serial No. 08/837,411, filed April 17, 1997(Case D-20347). The most preferred adsorber configuration D-20657 comprises a pretreatment layer plus a main adsorbent layer in the adsorber or adsorbers utilized. One or more adsorbents may be contained in each layer. The pretreatment layer is located nearest the feed inlet and its purpose is to remove any undesirable contaminants from the feed stream. Typical contaminants include water, S0X, N0X, and other strongly adsorbed species. Those skilled in the art will appreciate the use of zeolites, activated alumina, silica gel as well as other appropriate adsorbents in the pretreatment zone. The pretreatment zone may be eliminated if there are no contaminants in the feed stream. The most preferred adsorption pressure is near ambient or slightly above to provide reduced power consumption, particularly for low concentrations of the heavy component in the feed. The preferred desorption pressure is sub-atmospheric pressure. The most preferred desorption pressure is below the partial pressure of heavy component in the feed. For C02 production from combustion flue gas, lime¬kiln gas, H2 plant tail gas and other sources, the duty of the primary adsorbent(s) is to selectively adsorb CO2 while allowing lighter components to pass through. Although C02 adsorbs strongly on most porous adsorbents, particularly in comparison to many other permanent gases, the strength of adsorption is quite variable. Figure 7 presents pure component isotherms at approximately 27°C (300°K) for C02 adsorption on several adsorbents. These isotherms demonstrate the varying degree of interaction of C02 with solid adsorbents, ranging from weak (e.g., on alumina) to D-20657 very strong (e.g., on zeolite 5A). The strength of adsorption is primarily indicated by the slope of the isotherms at low pressure. For zeolite 5A, the equilibrium loading at 20 kPa is roughly 90 percent of the equilibrium loading at 200 kPa. Steep isotherms such as this require very low desorption pressures in order to adequately desorb CO2 and to achieve good C02 working capacity or dynamic loading between the adsorption and desorption steps. The minimal change in loading at higher pressures discourages feed compression. Isotherms with less significant slopes at lower pressure may make compression more favorable. The performance of adsorbents in a cyclic PSA process is strongly related to the ratio of the dynamic loadings of the strongly held and weakly held components. This ratio, computed as an adiabatic separation factor, is most accurately calculated from process simulation results by integration of the light and heavy component loadings on the adsorbent bed at the end of the adsorption and desorption stages. A reasonable equilibrium approximation of this separation factor can be made directly from isotherms. Initially, the temperature of the feed, gas mixture, and the pressure and gas phase compositions at the end of adsorption and the end of desorption are chosen. Once these conditions have been specified, the corresponding adiabatic temperature rise for each adsorbent is then determined by experiment or by iterative solution of a sOimplified energy balance. Application of the adiabatic separation factor analysis is described in the above-referenced copending application Serial No. 08/837,411 (D-20347), which is incorporated herein by reference. D-20657 The most preferred adsorption pressures utilized in the practice of this invention for CO2 recovery from flue gas are near ambient pressure so as to minimize unnecessary compression of waste gas. The adsorbers can be economically evacuated as low as 6 kPa with standard vacuum equipment, so that value is chosen as the minimum desorption pressure. A feed composition of 12% CO2 and 88% N2 is selected as representative of a typical flue gas stream. A desired product purity of 80% CO2 is chosen for this analysis. The results of the adiabatic separation factor analysis based on the foregoing factors are presented in Figure 8. These results show the approximate variation in equilibrium adiabatic separation factor as a function of temperature in the bed at the end of the adsorption step, for various adsorbents of potential interest. Each adsorbent is characterized by a different thermal swing (AT) that occurs between the adsorption and desorption steps. Zeolite NaY adsorbent maintains moderate capacity and selectivity for CO2 yet can be regenerated at modest conditions, and provides a significant improvement over prior art adsorbents. At a feed temperature of 330°K, the separation factor for zeolite NaY is approximately 4 times greater than that of BPL activated carbon. For moderate feed temperatures (up to 350°K) , zeolite NaY offers an advantage in C02 selectivity over the other prior art adsorbents noted. Depending on the source and proximity of the C02 PSA plant to the flue gas or other multi-component feed gas mixture, the flue gas may be available at higher temperatures. As adsorption temperatures increase, the stronger adsorbents for C02 are preferred D-20657 as their isotherms become less steep and more linear. At higher temperatures (greater than 370°K) , the performance for zeolite 13X can exceed that of zeolite NaY. However, zeolite NaX(2.0), or NaX zeolite with a Si02/Al203 ratio equal to 2.0, demonstrates substantially improved performance as compared with zeolite 13X or NaX(2.5). The additional cation sites that are provided by the lower Si/Al ratio provide NaX(2.0) with significantly higher capacity for C02 and increased separation factors. At temperatures above 350°K, the results shown in Figure 8 demonstrate that NaX(2.0) is a superior adsorbent in comparison to the other noted prior art adsorbents. Adiabatic separation factors characterize the selectivity of the adsorbent.s and give a relative measure of the recovery and purity that can be achieved with the process cycle conditions. The change in C02 loading between the adsorption and desorption stages provides a relative measure of the productivity of the adsorbent. in other words, it determines how much adsorbent win be required for a given C02 production rate (referred to as bed size factor). Variations in these dynamic loadings with different adsorbents are presented in Figure 9. At lower temperatures, zeolite NaY has the highest dynamic loading and hence the smallest bed size factor. At higher temperatures, zeolite NaX(2.0) offers the smallest bed size factor. Smaller bed size factors allow the quantity of adsorbent and the adsorber size to be reduced, providing cost reduction in comparison to the prior art. As indicated above, the characteristics of those adsorbents useful in the practice of the present D-20657 invention will depend upon the pressure, temperature and compositions of the heavy and light gas components in the adsorption and desorption steps of the process. In the recovery of CO2 it is preferred to utilize zeolite adsorbents having an adiabatic separation factor, ACO2/AN2, in excess of about 2 and a dynamic C02 loading in excess of 0.1 mol/kg, at adsorption temperatures of about 300 to 400_K and under adsorption pressures of about 90 to 200 kPa. The two most preferred adsorbents for C02 recovery, zeolite NaY and zeolite NaX(2.0) offer significant potential improvement over adsorbents traditionally employed in the prior art. For a typical flue gas feed, zeolite NaY has the highest separation factor and highest CO2 working capacity at moderate adsorption temperatures (less than 340°K). At elevated temperatures NaX(2.0) offers higher separation factors while maintaining relatively high C02 working capacity. These adsorbents may be employed as the main adsorbent layer in the PSA process of the present invention or other pressure swing, vacuum swing or vacuum/pressure swing processes for C02 recovery from mixtures with a variety of more weakly adsorbed components such as nitrogen, oxygen, hydrogen, methane, or carbon monoxide. Examples Simulation and pilot plant experiments have been used to verify performance of the process and adsorbents of this invention for recovery of C02. D-20657 Examples 1 and 2 Representative examples involve the use of zeolite NaY to recover approximately 80% CO2 product from flue gas containing 12% CO2 and 88% N2. Simulation results for the two-bed basic cycle and the two-bed cocurrent displacement cycle are presented in Table III below. The basic cycle produces 80% product purity at a recovery of 66%. Recovery is defined herein as the fraction of CO2 in the feed that is recovered as product. The bed size factor (BSF) of 341 pounds of zeolite NaY per metric ton per day of contained C02 product (lb/mtpd) was achieved for this example. Higher recovery and productivity are obtained by utilizing the two-bed cocurrent displacement cycle of the present invention. With cocurrent displacement, product recovery increases to 75 % while the BSF decreases to 319 pounds of zeolite NaY per metric ton per day of product. TABLE III Feed: 12% C02, 88% Basic Cycle Cocurrent N2 Adsorbent: NaY (Example 1) Displacement zeolite Cycle (Example 2) Product Purity 80 80 (%C02) Recovery (%) 66 75 Bed Size Factor 341 • 319 (lb/mtpd) Cycle Time (sec) 92 98 D-20657 Examples 3 and 4 Additional representative examples involve the use of zeolite NaY in a process to recover approximately 92% C02 product from a hydrogen plant tail gas stream containing 54% C02, 16% CH4 and 30% H2. Simulation results for the two-bed basic cycle and two-bed cocurrent displacement cycle are presented in Table IV. The basic cycle produces 92%C02 product at a recovery of 87%. The bed size factor is 228 lb/mtpd. Higher recovery and -slightly higher productivity are obtained by utilizing the two-bed cocurrent displacement cycle. With this cycle, product recovery increases to 92% while the bed size factor is essentially unchanged at 227 lb/mtpd. TABLE IV Feed: 54% C02, 30% Basic Cycle Cocurrent H2 16% CH4 (Example 3) Displacement Adsorbent: NaY Cycle (Example 4) zeolite Product Purity 92 92 (%C02) Recovery (%) 87 92 Bed Size Factor 228 227 (lb/mtpd) Cycle Time (sec) 194 196 It will be understood that various changes may be made in the process and apparatus described above or illustrated in the accompanying drawings without departing from the scope of the present invention. Thus, the process, although preferably operated with two adsorbers and one or more storage tanks of any desired configuration, i.e., either constant volume or D-20657 constant pressure tanks, may utilize more than two adsorbers and multiple storage tanks. Moreover, the invention may be employed with axial flow, radial flow, lateral flow or other flow patterns through the adsorbers. With respect to the individual adsorbers, each may comprise multiple main adsorbent layers, either without any or with one or more pretreatment layers for the adsorption of other components, e.g., water vapor. Also, each adsorbent layer may contain a single adsorbent or a mixture of two or more adsorbents. In addition, although preferably operated at adsorption pressures near ambient pressure, the adsorption pressure may be at or above atmospheric pressure. Similarly, the minimum desorption pressure may be above, at or below atmospheric pressure. Nor are the processes disclosed in this invention limited to the use of zeolite NaY, zeolite NaX(2.0) or any other specific adsorbent as the primary adsorbent for CO2 recovery. These processes could be used for CO2 recovery with other adsorbents deployed in one or more main adsorbent layers. The adiabatic separation factor method used to evaluate and select adsorbents, for CO2 recovery is general and may be applied equally well to other multi-component separations. Although this invention is primarily addressed to CO2 recovery from multi-component feed streams, the concepts disclosed herein can be applied to many other separations. Thus, the processes disclosed may be used for other multi-component separations, with any combination of appropriate adsorbents in which the more selectively adsorbed component is a desired product. This includes, but is not limited to, D-20657 nitrogen recovery from air using nitrogen-selective adsorbents, oxygen recovery from air using oxygen-selective adsorbents, carbon monoxide recovery from syngas using CO-selective adsorbents, and oxygen/argon separation using either oxygen-selective or argon-selective adsorbents. Additionally, this invention may be used for co-production of both light and heavy products, for example, the production of enriched N2 and enriched CO2 from flue gas, or enriched N2 and O2 from air. Accordingly, specific features of the present invention are shown in one or more of the drawings or disclosed as illustrative above for convenience only, as such features may be combined with other features in accordance with the invention. Those skilled in the art will recognize other embodiments which may be utilized in the practice of the invention and which are intended to be included within the scope of the claims appended hereto. WE CLAIM: 1. A pressure swing adsorption process for the recovery of at least CO2 from a multi-component gas mixture, which comprises: (1) feeding the multi-component gas mixture into an inlet of and through at least one adsorber at an adsorption temperature of 300° to 400°K and under an adsorption pressure of 90 to 200 kPa, adsorbing CO2 of the gas mixture on an adsorbent within the adsorber and removing an effluent enriched with the light component or components of the mixture from the adsorber through an outlet thereof, at least a portion of the light component-enriched effluent being retained in a pressure zone communicating with the adsorber outlet; (2) blowing down a portion of the light component-enriched effluent cocurrently through the adsorber outlet into a vacuum zone maintained at a pressure less than the adsorption pressure and communicating with the adsorber and, at the same time, removing desorbed gas by countercurrent evacuation from the adsorber through the adsorber inlet, to simultaneously depressurize the adsorber cocurrently from its outlet and countercurrently from its inlet; ... (3) terminating the flow of the blowdown gas through the adsorber outlet while continuing the countercurrent evacuation of the desorbed gas from the adsorber inlet to further depressurize the adsorber until the pressure in the adsorber is less than that in the vacuum zone; (4) passing the blowdown gas as a purge gas stream from the vacuum zone to the adsorber outlet countercurrently to and through the adsorber to make and recover a product stream from the bed containing the CO2 in at least a predetermined, substantially constant purity, wherein the adsorbent has an adiabatic separation factor DCO2/DN2 in excess of 2.0 and a dynamic CO2 loading in excess of 0.1 mole/kg at said adsorption temperature and pressure, such absorbent performance characteristics determined at the reference conditions of 0.12 CO2 feed mole fraction and a desorption CO2 mole fraction and pressure of 0.8 and 6kPa, respectfully; (5) terminating the flow of the blowdown gas from the vacuum zone and passing light component-enriched effluent from the pressure zone into the outlet of the adsorber to partially repressurize the adsorber; (6) passing an additional portion of the multi-component feed gas mixture into the inlet of the adsorber to complete repressurization of the adsorber; and (7) repeating the foregoing steps to produce the CO2 product stream. 2. The pressure swing adsorption process as claimed in claim 1, wherein the adsorption in step (1) is carried out at about atmospheric pressure. 3. The pressure swing adsorption process as claimed in claim 1, wherein nitrogen, oxygen, hydrogen, methane, carbon monoxide or other light gases are the light gas component(s) contained in the mixture from which the CO2 is separated. 4. The pressure swing adsorption process as claimed in claim 3, wherein the adsorbent is zeolite NaY or NaX (2.0). 5. The pressure swing adsorption process as claimed in claim 1, wherein portions of the multi-component gas mixture are fed in step (1) through two or more independently operable adsorbers, steps (2) -(7) are thereafter separately carried out in the respective adsorbers, and the product streams recovered in step (4) from each adsorber are combined to provide a continuous flow of product containing the CO2. 6. The pressure swing adsorption process as claimed in claim 1 wherein at least a portion of the product stream recovered in step (4) is thereafter recycled to the inlet of the adsorber to effect cocurrent displacement of the gas mixture at the inlet and thereby increase the concentration of the CO2 at the inlet after step (1) of the process. 7. The pressure swing adsorption process as claimed in claim 1 wherein, after terminating the flow of blowdown gas to make the product stream in step (4), additional blowdown gas from the vacuum zone is passed into the adeorber outlet to augment countercurrent repressurization of the adsorber. 8. An apparatus for carrying out a pressure swing adsorption process for the recovery of a heavy- ga$ component in a multi-component gas mixture, which comprises: (1) at least one adsorber having an inlet end and an outlet end and incorporating at least one layer of an adsorbent capable of selectively adsorbing the heavy component of the gas mixture relative to the light component or components thereof; (2) a feed blower Communicating with the inlet end of the adsorber for feeding the gas mixture into and through the adsorber; (3) a pressure tank communicating with the outlet end of the adsorber for receiving a light component-enriched effluent from the adsorber and for feeding the light component-enriched effluent into the outlet end of the adsorber, (4) a vacuum tank communicating with the outlet end of the adsorber for receiving a further portion of the light component-enriched effluent from the adsorber and for feeding the light component-enriched effluent as a blowdown gas in the adsorber through its outlet end; (5) outlet valve means for selectively placing the pressure and vacuum tanks in communication with the outlet end of the adsorber for depressurizing and repressurizing the adsorber; (6) a vacuum pump independent of the feed blower and communicating with the inlet end of the adsorber for removing a product stream containing the heavy component of the gas mixture at a predetermined, constant concentration; (7) inlet valve means for selectively placing the feed blower and the vacuum pump in communication with the inlet end of the adsorber to facilitate feed of the gas mixture into and removal of the product stream from the inlet end of the adsorber; and (8) control means for concurrently actuating the outlet valve means and the inlet valve means to facilitate feed of the light component-enriched effluent as a blowdown gas from the vacuum tank into the adsorber and removal of an evacuation stream from the adsorber, to simultaneously depressurize the adsorber cocurrently from its outlet end and countercurrently from its inlet end. 9. A pressure swing adsorption process for the recovery of at least CO2 from a multi-component gas mixture substantially as hereinbefore described with reference to the accompanying drawings. 10. An apparatus for carrying out a pressure swing adsorption process for the recovery of a heavy gas component in a multi-component gas mixture substantially as hereinbefore described with reference to the accompanying drawings. Dated this 22nd day of May, 2000 RITUSHKA NEGI OF REMFRY & SAGAR ATTORNEY FOR THE APPLICANTS

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