Abstract:
A pressure swing adsorption (PSA) system using two or more valves for controlling the flow of gases entering or exiting a bed of adsorbents is disclosed, where the two or more valves are opened sequentially (i.e., in at least two actions separated by a delay in time). The sequential opening of the valves may increase the degree to which adsorbed species are purged from the bed, and also facilitates more rapid execution of certain time steps of the PSA cycle, thus increasing adsorbent productivity The sequential opening of the valves may also allow for verification of valve operation by measuring either the absolute value, the slope (derivative) or the rate of change of derivative of the pressure, either in the adsorbent bed, in the downstream manifold, or in a volume of gas held in a buffer vessel.

Description:
FIELD OF THE DISCLOSURE 
       [0001]    Embodiments disclosed herein generally relate to pressure swing adsorption systems. More specifically, embodiments disclosed herein relate to methods and apparatus for controlling the flow of gases entering or exiting a bed of adsorbent. 
       BACKGROUND 
       [0002]    Pressure Swing Adsorption (PSA) is a technique used to fractionate mixtures of gases to provide at least one purified product gas and a raffinate byproduct mixture. PSA has been successfully used to separate hydrogen from other gases, oxygen and nitrogen from air, and helium from natural gas, among others. 
         [0003]    Early PSA systems generally used four adsorbent vessels operated in parallel. An example of this is U.S. Pat. No. 3,430,418 to Wagner. Later improvements to Wagner&#39;s process added an additional pressure equalization step while retaining four adsorbent beds (e.g., U.S. Pat. No. 3,564,816 to Batta) and subsequently added even more pressure equalization steps to seven or more beds in U.S. Pat. No. 3,986,849 to Fuderer et al. These increases in the number of pressure equalizations and the number of adsorbent vessels were implemented to increase the product recovery and the adsorbent productivity. Unfortunately, the increases in performance were accompanied by an increase in the number of valves required from thirty-one for the Wagner process to thirty-three for the Batta process to a minimum of forty-four for the Fuderer et al. process. 
         [0004]    The performance of PSA cycles is commonly measured based upon several criteria. The first is product recovery at a given impurity level, the fraction of the product species in the total feed stream that is delivered as purified product. A second measure is the productivity of the adsorbent, which is related to the proportion of the PSA cycle during which product is delivered compared to the total length of the cycle. In order to maximize one or both of these parameters at fixed feed compositions, a number of approaches have been described in other systems. 
         [0005]    Wagner describes the use of gas stored in the pressurized beds to repressurize one other vessel which had been purged, then to subsequently purge another vessel before the pressure in the first vessel was depleted. Batta subsequently describes that a second pressure equalization could be added to the first, and that this would improve recovery meaningfully. Batta retained the provision of purge gas in his cycle. Fuderer et al. extended this approach to a third pressure equalization, and taught that the purest gas withdrawn from a bed should always be the last gas admitted to any other bed being repressurized. Batta&#39;s four vessel cycle was constituted such that less pure gas was admitted to the vessel being pressurized than was truly desirable. Further, Fuderer et al.&#39;s invention allowed for a higher adsorbent productivity than was achievable with previous cycles, because the fraction of time in the cycle allocated to adsorption was higher due to the details of the valve switching logic. 
         [0006]    Although these methods facilitate excellent product recovery and adsorbent productivity, they do so at the expense of a high degree of complexity. Wagner&#39;s original process employed four vessels and thirty-one valves to facilitate one pressure equalization, and purging of one other vessel. Batta increased this total to thirty-three valves and four vessels for his cycle with two equalizations. Both of these four bed cycles produce gas from a given vessel twenty-five percent of the time. Batta also provided a five vessel system with forty-three valves to re-order the equalizations to provide the desired repressurization with gases increasing continuously in purity. This cycle produced from a given vessel only twenty percent of the time. Fuderer et al.&#39;s most simple cycle providing three equalizations and a purging step required nine vessels and fifty-five valves. This cycle produced thirty-three percent of the time, a significant increase over the cycles of Batta and Wagner. Although these cycles progressed in the critical areas of recovery and productivity, they did so at the expense of much increased mechanical complexity. This increase in complexity is accompanied by increases in system volume, mass, assembly time, and capital cost. Furthermore, the large increase in the number of valves over time significantly reduces the reliability of the PSA system; as such PSA systems are single point of failure systems, which must be shut down even if one valve fails. 
         [0007]    Recent efforts have been made to reduce complexity in order to address its attendant problems. U.S. Pat. No. 4,761,165 to Stocker implemented the process of Wagner using four vessels and eighteen valves, of which four could be proportionally-controlled valves. U.S. Pat. No. 6,146,450 to Duhayer et al. describes a means for reducing complexity by arranging pipe fittings optimally, although this approach does not materially alter the PSA cycle in terms of valve or vessel count. A process including additional mechanical simplification is described in U.S. Pat. No. 6,755,895 to Lomax et al. 
         [0008]    U.S. Pat. No. 6,858,065, also to Lomax et al., discloses a process including a first equalization step having at least two stages where the pressure decreases, and a second equalization step having at least two stages where the pressure increases. 
         [0009]    U.S. Pat. No. 7,674,319, also to Lomax et al., discloses a PSA system with a control system to monitor the performance and operation of the PSA system, including multiple pressure transducers located at various points in the system. Stocker et al. also disclose use of multiple pressure transducers on the adsorption vessels, feed lines and product lines which are provided in order to progressively control the opening of proportionally-opening valves to prevent adsorbent fluidization. 
         [0010]    U.S. Pat. No. 6,755,895 to Lomax, et al discloses a system of fixed, flow restricting orifices to limit the velocity of gases exiting an adsorbent vessel without using any feedback control or proportional valves. 
         [0011]    It has been found that in the limit of extremely-rapid cyclic operation, that the flowrate achieved through the invention of Lomax &#39;895 may undesirably limit the rapidity with which a pressure equalization step can be executed, thus limiting adsorbent productivity. 
       SUMMARY OF THE DISCLOSURE 
       [0012]    Several of the above-mentioned processes may attain a simplification in the total number of valves used relative to the process of Wagner. Others may provide for multiple measurement devices for monitoring and control of the PSA system to determine valve failure and system performance. 
         [0013]    It has been surprisingly found that simplification of the process and improvements in system performance and monitoring may be attained by using two or more valves for controlling the flow of gases entering or exiting a bed of adsorbents, where the two or more valves are opened sequentially (i.e., in at least two actions separated by a delay in time). The sequential opening of the valves may increase the degree to which adsorbed species are purged from the bed, and also facilitates more rapid execution of certain time steps of the PSA cycle, thus increasing adsorbent productivity The sequential opening of the valves may also allow for verification of valve operation by measuring either the absolute value, the slope (derivative) or the rate of change of derivative of the pressure, either in the adsorbent bed, in the downstream manifold, or in a volume of gas held in a buffer vessel. The resulting system may have the same or reduced piece count (including both valves and sensors) as compared to prior processes, and thus reduced risk of malfunction, while resulting in improvements in both PSA system operation and control. 
         [0014]    In one aspect, embodiments disclosed herein relate to a pressure swing adsorption system, including: a plurality of vessels having one or more layers of adsorbent material therein; a feed gas channel connected to the plurality of vessels; a product recovery channel connected to the plurality of vessels; a purge gas channel connected to the plurality of vessels; a waste gas channel connected to the plurality of vessel; and an equalization channel connected to the plurality of vessels; the product recovery channel being connected to each of the plurality of vessels via a manifold comprising at least two valves in a parallel flow arrangement; and a control system configured to open the at least two valves sequentially. 
         [0015]    In another aspect, embodiments disclosed herein relate to a pressure swing adsorption system, including: a plurality of vessels having one or more layers of adsorbent material therein; a feed gas channel connected to the plurality of vessels; a product recovery channel connected to the plurality of vessels; a purge gas channel connected to the plurality of vessels; a waste gas channel connected to the plurality of vessel; and an equalization channel connected to the plurality of vessels; the purge gas channel being connected to each of the plurality of vessels via a manifold comprising at least two valves in a parallel flow arrangement; and a control system configured to open the at least two valves sequentially. 
         [0016]    In another aspect, embodiments disclosed herein relate to a pressure swing adsorption system, including: a plurality of vessels having one or more layers of adsorbent material therein; a feed gas channel connected to the plurality of vessels; a product recovery channel connected to the plurality of vessels; a purge gas channel connected to the plurality of vessels; a waste gas channel connected to the plurality of vessel; and an equalization channel connected to the plurality of vessels; the waste gas channel being connected to each of the plurality of vessels via a manifold comprising at least two valves in a parallel flow arrangement; and a control system configured to open the at least two valves sequentially. 
         [0017]    In another aspect, embodiments disclosed herein relate to a pressure swing adsorption system, including: a plurality of vessels having one or more layers of adsorbent material therein; a feed gas channel connected to the plurality of vessels; a product recovery channel connected to the plurality of vessels; a purge gas channel connected to the plurality of vessels; a waste gas channel connected to the plurality of vessel; and an equalization channel connected to the plurality of vessels; the equalization channel being connected to each of the plurality of vessels via a manifold comprising at least two valves in a parallel flow arrangement; and a control system configured to open the at least two valves sequentially. 
         [0018]    In another aspect, embodiments disclosed herein relate to a pressure swing adsorption system, including: a plurality of vessels having one or more layers of adsorbent material therein; a feed gas channel connected to the plurality of vessels; a product recovery channel connected to the plurality of vessels; a purge gas channel connected to the plurality of vessels; a waste gas channel connected to the plurality of vessel; and an equalization channel connected to the plurality of vessels; the product recovery channel being connected to each of the plurality of vessels via a manifold comprising at least two valves in a parallel flow arrangement; the purge gas channel being connected to each of the plurality of vessels via a manifold comprising at least two valves in a parallel flow arrangement; the waste gas channel being connected to each of the plurality of vessels via a manifold comprising at least two valves in a parallel flow arrangement; the equalization channel being connected to each of the plurality of vessels via a manifold comprising at least two valves in a parallel flow arrangement; and a control system configured to: open the at least two valves in the manifold connecting the product recovery channel sequentially; open the at least two valves in the manifold connecting the purge gas channel sequentially; open the at least two valves in the manifold connecting the waste gas channel sequentially; and open the at least two valves in the manifold connecting the equalization channel sequentially. 
         [0019]    In any of the above embodiments, the system may further include one or more of the following: a pressure sensor for measuring a pressure in each of the plurality of vessels; a pressure sensor for measuring a pressure in the product recovery channel; and a pressure sensor for measuring a pressure in a buffer vessel fluidly connected to the product recovery channel; a pressure sensor for measuring a pressure in the purge gas channel; and a pressure sensor for measuring a pressure in a buffer vessel fluidly connected to the purge gas channel; a pressure sensor for measuring a pressure in the waste gas channel; and a pressure sensor for measuring a pressure in a buffer vessel fluidly connected to the waste gas channel; a pressure sensor for measuring a pressure in the equalization channel; and a pressure sensor for measuring a pressure in a buffer vessel fluidly connected to the equalization channel. 
         [0020]    In any of the above embodiments, the control system may be configured to determine a valve failure based upon at least one of: the pressure measured by at least one of, when present, the pressure sensor in each of the plurality of vessels, the pressure sensor in the product recovery channel, and the pressure sensor in the buffer vessel fluidly connected to the product recovery channel during the sequential opening of the at least two valves in the manifold connecting the product recovery channel; the pressure measured by at least one of, when present, the pressure sensor in each of the plurality of vessels, the pressure sensor in the purge gas channel, and the pressure sensor in the buffer vessel fluidly connected to the purge gas channel during the sequential opening of the at least two valves in the manifold connecting the purge gas channel; the pressure measured by at least one of, when present, the pressure sensor in each of the plurality of vessels, the pressure sensor in the waste gas channel, and the pressure sensor in the buffer vessel fluidly connected to the waste gas channel during the sequential opening of the at least two valves in the manifold connecting the waste gas channel; the pressure measured by at least one of, when present, the pressure sensor in each of the plurality of vessels, the pressure sensor in the equalization channel, and the pressure sensor in the buffer vessel fluidly connected to the equalization channel during the sequential opening of the at least two valves in the manifold connecting the equalization channel. 
         [0021]    In any of the above embodiments, the system may further include the feed gas channel being connected to each of the plurality of vessels via a manifold comprising at least two valves in a parallel flow arrangement; and a control system configured to open the at least two feed valves sequentially. 
         [0022]    In another aspect, embodiments disclosed herein relate to a method of operating a pressure swing adsorption system comprising a plurality of vessels, a feed gas channel connected to the plurality of vessels, a product recovery channel connected to the plurality of vessels, a purge gas channel connected to the plurality of vessels, and a waste gas channel connected to the plurality of vessels, the method comprising at least one of: sequentially opening two or more valves disposed in a parallel flow arrangement and connecting one of the plurality of vessels to the product recovery channel; sequentially opening two or more valves disposed in a parallel flow arrangement and connecting one of the plurality of vessels to the waste gas channel; sequentially opening two or more valves disposed in a parallel flow arrangement and connecting one of the plurality of vessels to the equalization channel; sequentially opening two or more valves disposed in a parallel flow arrangement and connecting one of the plurality of vessels to the feed gas channel; and sequentially opening two or more valves disposed in a parallel flow arrangement and connecting one of the plurality of vessels to the purge gas channel. 
         [0023]    Other aspects and advantages will be apparent from the following description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0024]      FIG. 1  is a simplified flow diagram of a prior art pressure swing adsorption system. 
           [0025]      FIG. 1A  is a simplified diagram of a control scheme used to operate a prior art pressure swing adsorption system. 
           [0026]      FIG. 2  is a simplified flow diagram of a pressure swing adsorption system according to embodiments disclosed herein. 
           [0027]      FIG. 3  is a simplified flow diagram of a pressure swing adsorption system according to embodiments disclosed herein. 
           [0028]      FIG. 4  is a simplified flow diagram of a pressure swing adsorption system according to embodiments disclosed herein. 
           [0029]      FIG. 5  is a simplified flow diagram of a pressure swing adsorption system according to embodiments disclosed herein. 
           [0030]      FIG. 6  is a simplified flow diagram of a pressure swing adsorption system according to embodiments disclosed herein. 
           [0031]      FIG. 7  is a simplified diagram of a control scheme used to operate a pressure swing adsorption system according to embodiment disclosed herein. 
           [0032]      FIG. 8  is a simplified flow diagram of a pressure swing adsorption system according to embodiments disclosed herein. 
           [0033]      FIGS. 9A-9D  are an example valve sequence chart for an 8 vessel pressure swing adsorption system as illustrated in  FIG. 8  using 3 equalization stages. 
           [0034]      FIGS. 10A-10E  are a simplified flow diagram of a pressure swing adsorption system according to embodiments disclosed herein. 
           [0035]      FIG. 11  is an example valve sequence chart for a 9 vessel pressure swing adsorption system as illustrated in  FIGS. 10A-10E  using 4 equalization stages. 
       
    
    
     DETAILED DESCRIPTION 
       [0036]    In one aspect, embodiments disclosed herein relate to methods and apparatus for controlling the flow of gases entering or exiting a bed of adsorbent. More specifically, embodiments disclosed herein relate to the use of two or more valves for controlling the flow of gases entering or exiting a bed of adsorbents, where the two valves are opened sequentially (i.e., in at least two actions separated by a delay in time). 
         [0037]    Referring now to  FIG. 1 , a simplified process flow diagram of a prior art PSA system is illustrated. The PSA system  5  includes a first vessel  10 , a second vessel  12 , a third vessel  14 , and a fourth vessel  16 . Each of the vessels  10 ,  12 ,  14 ,  16  typically includes one or more beds of adsorbent material. The vessels  10 ,  12 ,  14 ,  16  are connected in parallel flow relation between a source manifold  18 , which supplies a feed gas mixture, and a product manifold  20 , which provide an outlet for unabsorbed product effluent gas. The vessels  10 ,  12 ,  14 ,  16  are also connected to a waste manifold  22 , which provides an outlet for adsorbed components. Additionally, the vessels  10 ,  12 ,  14 ,  16  are connected to an equalization manifold  24 , providing for equalization of pressure between two or more vessels to conserve pressure energy during operation of the system. These are four manifolds typically discussed in the art, such as in U.S. Pat. No. 6,858,065. U.S. Pat. No. 7,674,319 also discloses connecting vessels  10 ,  12 ,  14 ,  16  to a purge gas manifold  26 . 
         [0038]    Each of the vessels  10 ,  12 ,  14 ,  16  may be connected to the respective manifolds with a valve to control the flow of gas to and from the vessels. In the system of  FIG. 1 , the flow of gases to and from the adsorbent bed may be controlled, for example, using a system as illustrated in  FIG. 1A . Vessel  10 , for example, may be connected to a flow control valve  30 , provided with position control and reporting assemblies (positioners)  32 , used in conjunction with a control system  34  and at least two pressure sensors  36 ,  38  monitoring pressure in vessel  10  to attain targeted difference in pressure and/or rate of pressure decay. The flow rate through flow control valve  30  is continuously varied, and requires a complex control algorithm for tuning of the valve performance. Additionally, there is an inherent lack of reliability in the positioner itself This results in such systems generally being provided with a manual override mode of operation. Further, such a system cannot be operated if either pressure sensor  36 ,  38  fails. This defect necessitates either provision of multiple sensors or of means to isolate the failed adsorbed vessel for repair of the defective components. Mal-operation of the control valve  30  can result in excessively rapid flow out of the vessel  10 , which can result in the aerodynamic fluidization of the individual particles in the one or more adsorbent beds contained in this vessel, which can cause these particles to be carried out of the vessel, to break up due to impact with the vessel or other particles, or to be re-arranged in a non-uniform manner, thus potentially exacerbating the tendency of the particles to subsequently fluidize. If the control valve  30  provides insufficient flow, then the time required to complete the step in the PSA cycle will take longer to complete. If the PSA cycle is being operated at a fixed cycle frequency, this will result in a greater proportion of the desired product gas being disposed of as waste, thus reducing fractional recovery and adsorbent productivity. If the cycle frequency is reduced to compensate for the low flowrate, then the adsorbent productivity is reduced. 
         [0039]    It has been surprisingly found that a PSA system may be improved by using two or more on/off valves in parallel to control the rate at which gas flows during feed, pressure equalization, product pressurization, counter-current blowdown, and/or purge. The use of on/off valves in parallel has also been found to allow a PSA system to operate with a reduction in the reliance on sensors and feedback control loops, both of which are inherently unreliable. 
         [0040]    As an example, increasing the fractional recovery of the desired, less-adsorbing (light) species requires more efficient elimination of the adsorbed species (heavy) from the adsorbent bed at the end of each cycle. This can be achieved by controlling the rate at which pressure changes across the adsorbent bed during the countercurrent blowdown step while not restricting the flowrate from the same vessel to the same waste gas header during the subsequent purge step. 
         [0041]    Referring now to  FIG. 2 , one embodiment of a PSA system according to embodiments disclosed herein is illustrated, using two or more on/off valves for controlling the flow of gases exiting the adsorbent bed to the waste gas channel. PSA system  40  includes a first vessel  42 , a second vessel  44 , a third vessel  46 , and a fourth vessel  48 . Each of the vessels  42 ,  44 ,  46 ,  48  typically includes one or more beds of adsorbent material (not shown). The vessels  42 ,  44 ,  46 ,  48  are connected in parallel flow relation between a feed gas channel  50 , which supplies a feed gas mixture, and a product recovery channel  52 , which provide an outlet for unabsorbed product effluent gas. The vessels  42 ,  44 ,  46 ,  48  are also connected to a waste gas channel  54 , which provides an outlet for adsorbed components. Additionally, the vessels  42 ,  44 ,  46 ,  48  are connected to an equalization channel  56 , providing for equalization of pressure between two or more vessels to conserve pressure energy during operation of the system, and a purge gas channel  57 , supplying a purge gas to the PSA system. 
         [0042]    To achieve improved elimination of adsorbed species from the adsorbent bed during the depressurization step, each vessel  42 ,  44 ,  46 ,  48  may be respectively connected to the waste gas channel via parallel on/off valves  58 ,  60 . While only two valves are shown, three or more valves may also be used. On/off valves  58 ,  60  may include a flow orifice of the same or different effective diameter, where the flow orifice diameters may be selected to tailor the rate at which pressure changes during the depressurization cycle. 
         [0043]    Referring now to  FIGS. 2 and 7 , in operation, the depressurization cycle may be controlled and monitored using a control system  64 , and may begin by opening valve  58 , providing for a first depressurization flow path, followed by the opening of valve  60  after a selected time interval, providing an increase in the size of the flow path during continued depressurization. As one skilled in the art would appreciate, use of n on/off valves, each of different size, may provide for 2′ distinct flow resistances. Selection of the size of the respective flow orifices may be tailored to meet the specific separation process and the desired pressure changes during the depressurization cycle, and may allow for elimination of the adsorbed species to be optimized. In one embodiment of the present invention, the final flow capacity of the combined valves used during the purge step is sufficient that the pressure loss through those valves is less than 2 psi. In another embodiment of the present invention, the valves  58  and  60  are chosen such that by first opening the valve  58  to begin the countercurrent blowdown of the vessel  42  then, after a predetermined time period, which is less than the duration of the countercurrent blowdown step, opening valve  60 , the countercurrent blowdown can be substantially completed before the beginning of the subsequent purge step. In one embodiment of the present invention, substantial completion would be deemed to occur when the pressure within the vessel  42  is less than 5 psi greater than the pressure in waste header  54 . 
         [0044]    It has also been found by the present inventors that on/off valves may improve the reliability of the PSA system as compared to the typical flow control valves, such as illustrated in  FIG. 1A . The PSA system of  FIG. 1  and  FIG. 1A , as mentioned above, requires tuning of the flow valve control and positioning of the valve trim. The control algorithm is complex, may be changed by operators randomly using a digital control system, and requires significant time from startup to achieve the desired control. Further, the variability in valve performance means that the control parameters used for a valve  30  on vessel  10  may not be suitable for a valve  30  on vessel  12 ,  14 , or  16 . This may be due to differences in response times (control lag times, differences in pressure regulator settings and performance, etc.), trim position, and trim size, and other factors. Further, changes in the positioner, the valve itself (i.e. due to erosion, fouling by particles, etc.), or the adsorbent bed (due to settling, plugging, and bulk or local fluidization over time may affect the performance of the valve and the associated feedback control algorithm in controlling flowrate into or out of the vessel 
         [0045]    The on/off valves used in the present invention do not require a control algorithm to provide for varying degrees of flow resistance. Rather, the sequential opening of the two or more on/off valves results in a desired change in the flow resistance. The change in flow resistance is predictable (open or closed) with on/off valves. It has also been found that the length of time to complete a cycle may be decreased with use of two or more on/off valves as compared to a single throttled flow control valve. Further, the on/off valves are significantly more robust than typical flow control valves, are typically less costly to purchase and maintain, and may operate over more cycles before valve failure. 
         [0046]    The sequential opening of the on/off valves, as noted above, may be used to create more than one step change in either pressure itself, rate of pressure change, or the rate of change in the rate of pressure change. This may result in multiple benefits, including one or more of: improved elimination of adsorbed species during depressurization, reduced depressurization cycle time, reduction in the number of pressure sensors required, and less PSA system down time due to increased system reliability using on/off valves. 
         [0047]    While more robust and reliable, as noted above, even on/off valves fail. Still referring to  FIG. 2 , to determine valve failure, a pressure sensor may be located on a vessel  42 ,  44 ,  46 ,  48 , may be located on the waste gas channel, or may be located in a buffer tank connected to the waste gas channel. As illustrated in  FIG. 2 , a pressure sensor  62  is located on the waste gas channel. A single pressure sensor in any of these locations may provide for indirect measurement of valve opening by measuring the step change in pressure, rate of pressure change, or the rate of change in the rate of pressure change. Where the expected change, rate of change, or derivate of change in pressure does not occur, valve failure may be indicated. 
         [0048]    In some embodiments, redundant valves (not shown) may connect a vessel to the waste gas channel. In the event that a valve  58 ,  60  fails, the control system  64  would recognize that the expected the step change in pressure, for example, did not occur, and a complementary algorithm may open one of the redundant valves in order to perform the intended pressure reduction with only a modest delay in action. 
         [0049]    While a majority of the prior art described in the Background above was devoted to reducing the total number of valves, it has been found that although the present invention employs more discrete process valves than the traditional solution of a large single valve (positioned continuously to provide varying degrees of flow resistance), the present PSA systems uses few sensors and little or no feedback action, resulting in an overall more reliable system. Such a system may, in fact, have the same or fewer components due to the reduced need for sensors and actuators to provide for system control. Further, a deviation from intended actuation is easier to rectify automatically than use of a flow control valve, which typically requires manual intervention. 
         [0050]    The use of at least two valves according to embodiments disclosed herein may also provide an advantage relative to the use of a single proportioning flow control valve in that proportioning valves have a reduced rate of opening and closing as compared to on/off valves. This reduced rate causes the possibility of either excessively-extended step time in the PSA cycle to accommodate the valve closing, which is otherwise desirably very rapid, or the provision of an additional actuated valve to provide rapid exhausting. This additional valve adds an additional component which may cause failure. When at least two on/off valves are used in place of a proportioning valve, each of these valves may provide rapid and positive actuation. 
         [0051]    A further advantage of using two or more valves which are opened sequentially in a PSA system is that inevitably, the pressure difference between the vessel and the source or destination of the flowing gas decreases between the first time when the first of the two or more valves open and a second time when further valves are opened. For most types of process valves, the torque and/or force of the actuator is a function of the differential pressure. Thus, though the first valve must be provided with an actuator sufficient to open the valve against the maximum pressure differential, the other valves may be provided with a smaller actuator. This advantageously reduces the size and weight of the actuator, but also reduces the fatigue stress to which the valve is subjected, advantageously reducing the risk of eventual fatigue failure of the valve in service. A related advantage of the provision of a smaller actuator is that inadvertent operation of the valve can be prevented when the differential pressure is undesirably high. In one embodiment of the present invention, the vessels  42 ,  44 ,  46 ,  48  in  FIG. 2  are provided with a valve  58  having a relatively small flow capacity and capable of opening at a first pressure differential. The valve  60  is provided with a second, larger flow capacity, but is provided with a relatively weaker actuator which can only open the valve at a differential pressure appropriate to the intended time of opening. This embodiment prevents unintended opening of the large valve  60  while the vessel  42  is at high pressure, thus preventing an unintended high flowrate to the waste header  54 , which could cause serious operational problems. Further, if the opening pressure differential of the valve  60  is chosen carefully, the opening of this valve would self-compensate for variations in the flowrate through valve  58 , as even though the digital signal to open the valve (often pneumatic) is activated, the valve will not, in fact, open until the desired differential pressure is achieved. 
         [0052]    The improved performance realized with the use of two or more on/off valves connecting the vessels to the waste gas channel may also be realized using two or more on/off valves for performing other steps in the cycle. Accordingly, it may be desired to connect one or more of the feed gas channel, the purge gas channel, the product gas channel, and the equalization channel to the vessels with a valve manifold comprising two or more on/off valves in a parallel flow arrangement. The control system may also be configured to open such valves, when present, sequentially. 
         [0053]    Referring now to  FIG. 3 , where like numerals represent like parts, one embodiment of a PSA system according to embodiments disclosed herein is illustrated, using two or more valves for controlling the flow of gases to or from the adsorbent bed through the purge gas channel. This purge channel may be configured to perform only the purge step, or may be configured to sequentially perform several steps. Likewise, though  FIG. 3  depicts a PSA having four vessels, the present invention can be applied to PSA systems having any number of vessels  2  or greater. In the embodiment of  FIG. 3 , the purge gas channel  57  is connected to each of the vessels  42 ,  44 ,  46 ,  48  via a valve manifold including at least two on/off valves  66 ,  68  in a parallel flow arrangement. Similar to the embodiment of  FIG. 2 , a pressure sensor (not illustrated) may be located on any one of the vessels  42 ,  44 ,  46 ,  48 , the purge gas channel  57 , or on a buffer tank fluidly connected to the purge gas channel. 
         [0054]    Referring now to  FIG. 4 , where like numerals represent like parts, one embodiment of a PSA system according to embodiments disclosed herein is illustrated, using two or more valves for controlling the flow of gases entering or exiting the adsorbent bed to the product recovery channel. In this embodiment, the product recovery channel  52  is connected to each of the vessels  42 ,  44 ,  46 ,  48  via a valve manifold including at least two on/off valves  70 ,  72  in a parallel flow arrangement. Similar to the embodiment of  FIG. 2 , a pressure sensor  74  may be located on any one of the vessels  42 ,  44 ,  46 ,  48 , the product recovery channel  52  (as illustrated), or on a buffer tank (not illustrated) fluidly connected to the product recovery channel  52 . 
         [0055]    The operation of the embodiment of  FIG. 4  differs somewhat from the previous embodiments in that the flow being controlled is into the vessel  42  from the product header  52 . At the beginning of the final pressurization step after the last of the one or more pressure equalization steps, the final pressurization can be achieved by using purified product gas, unpurified feed gas, or both of these. By having a first valve  70  which has a first flow capacity open at the beginning of the final pressurization step, a first flowrate of product gas into vessel  42  is achieved. In one embodiment of the present invention, the flow restriction of valve  70  can be selected so that the flowrate through valve  70  causes a deviation in net product flow out of the PSA  40  which is less than a target value, say 15%. After a first time interval, feed valve  80  could be opened to effect a partial product repressurization. After a second time interval, the valve  72  can be opened. Alternatively, the at least two valves can be provided at the position of the feed valve  80 , instead of the position of the product valves  70  and  72 , or two or more valves can be provided at each of the positions. In an embodiment where two valves are provided at one position and only one valve is provided at the other, five distinct states exist for flow between the headers. No flow from either direction. Flow in only from the feed header  50 , flow in only from the product header  52 , and two combinations of flow resistance across the vessel and through the combination of the open valves. These combinations can permit tailoring the rate of flowrate between the headers to execute the final repressurization without disadvantageous fluidization of the adsorbent bed, and can advantageously subsequently permit production of purified product during what would otherwise nominally constitute the final pressurization step. This advantageously increases the fraction of the PSA cycle during which adsorption separation is occurring, and increases adsorbent productivity. 
         [0056]    Referring now to  FIG. 5 , where like numerals represent like parts, one embodiment of a PSA system according to embodiments disclosed herein is illustrated, using two or more valves for controlling the flow of gases exiting the adsorbent bed to the equalization channel. In this embodiment, the equalization channel  56  is connected to each of the vessels  42 ,  44 ,  46 ,  48  via a valve manifold including at least two on/off valves  76 ,  78  in a parallel flow arrangement. Similar to the embodiment of  FIG. 2 , a pressure sensor  80  may be located on any one of the vessels  42 ,  44 ,  46 ,  48 , the equalization channel  56  (as illustrated), or on a buffer tank (not illustrated) fluidly connected to the equalization channel  56 . As before, the equalization channel need not be used only for equalization, and can be of the type known in the art, where the same channel is used to execute at least two sequential steps, such as a first and a second equalization step. 
         [0057]    Referring now to  FIG. 6 , where like numerals represent like parts, one embodiment of a PSA system according to embodiments disclosed herein is illustrated, using two or more valves for controlling the flow of gases flowing to or from the adsorbent bed to each of the product recovery channel, the purge channel, the waste gas channel, and the equalization channel. 
         [0058]    Cyclic adsorption processes, as mentioned above, use several steps to achieve the desired separation, including adsorption, equalization, and depressurization, among others. These steps may be performed upflow or downflow, depending upon the vessel and piping configuration used. For example, as illustrated in  FIG. 5 , the PSA system  40  includes an equalization channel  56  disposed above the vessels  42 ,  44 ,  46 ,  48 . The cyclic adsorption process may recover pressure energy by equalizing the pressure between one vessel at a higher initial pressure with another vessel at a lower initial pressure to achieve a final, intermediate pressure. A vessel that performs the pressure equalization in an upflow direction has the potential to fluidize and dust downstream valves and piping if the velocity is not carefully controlled. Thus, care must be used when selecting the amplitude of the flow restriction for the two or more valves  76 ,  78 . To limit the potential for fluidization when pressure equalization begins, where the pressure differential and hence velocities are greatest, a first valve  76  with a limited flow area may open first, limiting the maximum velocity achievable to a fraction of the velocity required to fluidize the upflowing vessel. At a later point in the pressure equalization step, a second valve  78  is opened while the first valve  76  remains open, to increase the total area for flow and to ensure the intermediate pressure is achieved in the desired period of time. The interval at which the second valve  78  opens may be programmed, for example, to occur at a point where the pressure differential between the vessels is smaller and the resulting velocity in the upflowing vessel will not fluidize the adsorbent. For greater velocity control, this concept can be extended to a plurality (three or more) of valves that are programmed to open at different intervals during pressure equalization to achieve the desired velocity profile in the upflowing vessel. 
         [0059]    While only four vessels are shown in  FIGS. 2-6 , PSA systems according to embodiments herein may include any number of vessels, such as 2, 3, 4, 5, 6, 7, 8, 9, or more vessels. Single vessel PSA may also benefit from embodiments herein, although a pressure equalization channel may not be necessary. 
         [0060]    Referring now to FIGS.  8  and  9 A- 9 D, where like numerals represent like parts, a valve sequence for a PSA system according to embodiments disclosed herein is illustrated. The PSA system of FIGS.  8  and  9 A- 9 D includes 8 vessels with the valve configuration shown in  FIG. 8 , including two parallel on/off valves connecting the vessel to each of the feed gas channel  50 , the product recovery channel  52 , the waste gas channel  54 , the equalization channel  56 , and the purge gas channel  57 . Although only one vessel is illustrated, it is appreciated that the 8 vessels of the system are connected similar to the embodiments illustrated in  FIGS. 2-6 . 
         [0061]    The valve sequence illustrated in  FIGS. 9A-9D  use 3 equalization steps in the PSA cycle, which includes: adsorption (“Adsorb”), where the vessel is at high pressure and preferably making enriched product gas; “Pry EQ 1 ,” “Pry EQ 2 ,” and Pry EQ 3 ,” where the vessel provides gas for a pressure equalization stage; blowdown (“blowdown”), denoting depressurization to the lowest pressure of the system; “Rcv EQ 1 ,” “Rcv EQ 2 ,” and “Rcv EQ 3 ,” where the vessel receives gas during a pressure equalization stage; product repressurization (“Prod Repress”); and “Pry Purge” and “Rcv Purge” where purge gas is fed from or to the vessel, respectively. 
         [0062]    The two valves connecting the vessel to each channel are differentiated in  FIGS. 9A-9D  by indicating one as the “pilot,” which is typically the first valve to open during initiation of a stage, the “pilot” valve having a smaller flow orifice to control initial flow from the vessels as described above. For example, valve  58  in  FIG. 8  may be the pilot valve connecting the vessel to the waste gas channel  54 , and valve  60  may be the second valve connecting the vessel to the waste gas channel  54 . The sequential opening of the second valve is indicated in  FIGS. 9A-9D  by “delay,” where the time delay between opening of the pilot valve and the second valve is appropriate for the stage of the PSA cycle. The valve sequence illustrated in  FIGS. 9A-9D  uses  3  equalization steps in the PSA cycle, which includes: Adsorption (“adsorb”), where the vessel is at high pressure and preferably making enriched product gas; “Pry EQ 1 ,” “Pry EQ 2 ,” and “Pry EQ 3 ,” where the vessel provides gas for a pressure equalization stage; blowdown (“blowdown”), denoting depressurization to the lowest pressure of the system; “Rcv EQ 1 ,” “Rcv EQ 2 ,” and “Rcv EQ 3 ,” where the vessel receives gas during a pressure equalization stage; product repressurization (“Prod Repress”); and “Pry Purge” and “Rcv Purge” where purge gas is fed from or to the vessel, respectively. 
         [0063]    Referring now to  FIGS. 10A-10E  and  11 , where like numerals represent like parts, a valve sequence for a PSA system according to embodiments disclosed herein is illustrated. The PSA system of  FIGS. 10A-10E  and  11  includes  9  vessels with the valve configuration shown in  FIGS. 10A-10E , including two parallel on/off valves connecting the vessel to each of the feed gas channel  50 , the product recovery channel  52 , the waste gas channel  54 , a first equalization channel  56 ( 1 ), a second equalization channel  56 ( 2 ), and the purge gas channel  57 . Although only one vessel is illustrated, it is appreciated that the  9  vessels of the system are connected similar to the embodiments illustrated in  FIGS. 2-6 . 
         [0064]    The valve sequence illustrated in  FIG. 11  uses  4  equalization steps in the PSA cycle, which includes: adsorption (“Adsorb”), where the vessel is at high pressure and preferably making enriched product gas; “Pry EQ 1 ,” “Pry EQ 2 ,” “Pry EQ 3 ,” and “Pry EQ 4 ,” where the vessel provides gas for a pressure equalization stage; blowdown (“blowdown”), denoting depressurization to the lowest pressure of the system; “Rcv EQ 1 ,” “Rcv EQ 2 ,” “Rcv EQ 3 ,”and “Rcv EQ 4 ,” where the vessel receives gas during a pressure equalization stage; product repressurization (“Prod Repress”); and “Pry Purge” and “Rcv Purge” where purge gas is fed from or to the vessel, respectively. 
         [0065]    The two valves connecting the vessel to each channel are differentiated in  FIG. 11  by indicating one as the “pilot,” which is typically the first valve to open during initiation of a stage, the “pilot” valve having a smaller flow orifice to control initial flow from the vessels as described above. For example, valve  58  in  FIGS. 10A-10E  may be the pilot valve connecting the vessel to the waste gas channel  54 , and valve  60  may be the second valve connecting the vessel to the waste gas channel  54 . The sequential opening of the second valve is indicated in  FIGS. 9A-9D  by “delay,” where the time delay between opening of the pilot valve and the second valve is appropriate for the stage of the PSA cycle. The valve sequence illustrated in  FIGS. 9A-9D  uses 3 equalization steps in the PSA cycle, which includes: Adsorption (“adsorb”), where the vessel is at high pressure and preferably making enriched product gas; “Pry EQ 1 ,” “Pry EQ 2 ,” and Pry EQ 3 ,” where the vessel provides gas for a pressure equalization stage; blowdown (“blowdown”), denoting depressurization to the lowest pressure of the system; “Rcv EQ 1 ,” “Rcv EQ 2 ,” and “Rcv EQ 3 ,” where the vessel receives gas during a pressure equalization stage; product repressurization (“Prod Repress”); and “Pry Purge” and “Rcv Purge” where purge gas is fed from or to the vessel, respectively.  FIG. 11  shows another example of the application of some aspects of the present invention, and differs from the example illustrated in  FIGS. 9A-9D  by the addition of a 4 th  pressure equalization, where an additional two time steps are included in the cycle in order to execute “Pry EQ 4 ” and “Rcv EQ 4 ,” respectively. 
         [0066]    As described above, PSA systems according to embodiments include two or more on/off valves in a parallel flow arrangement to provide for enhanced control of flow to or from adsorbent beds during the respective cycles. While using multiple valves for each step, total piece count for the PSA systems herein may be decreased or minimized by properly locating pressure sensors. 
         [0067]    Advantageously, embodiments disclosed herein may provide for one or more of: enhanced process performance (higher fractional recovery of product gas); a reduction in the reliance on sensors and feedback control loops; equivalent or better technical performance with fewer parts, greater simplicity, as well as the potential for permitting a “self-healing” mode where the failure of a single valve or sensor does not interrupt the function of the PSA system. 
         [0068]    While the disclosure includes a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments may be devised which do not depart from the scope of the present disclosure. Accordingly, the scope should be limited only by the attached claims. 
         [0069]    All documents cited herein, including testing procedures, are herein fully incorporated by reference, for all jurisdictions in which such incorporation is permitted, to the extent such disclosure is consistent with the description of the present invention.