MXPA00008381A - Pressure swing adsorption gas separation method, using adsorbents with high intrinsic diffusivity and low pressure ratios - Google Patents

Abstract

A gas separation process incorporating the invention combines use of an adsorbent having high intrinsic diffusivity with a low pressure ratio PSA cycle. Further enhancements to the process are derived from the use of fast cycles, shallow beds and small particles - especially in a radial bed configuration. The combination of low pressure ratio, high rate adsorbents and fast cycles has been found to result in an unexpected simultaneous reduction in bed size factor (BSF) and power consumption. These benefits have been achieved while minimizing a decline in product recovery through use of the high rate adsorbent. The net result is a significant reduction in product cost.

Description

METHOD OF SEPARATION PE PASSES BY ADSORPTION WITH PRESSURE OSCILLATING. WHAT USE ADSORBENTS WITH DIFUSIVITY 1NTRINSECA ELEVATED AND LOW PRESSURE REGIMES FIELD OF THE INVENTION This invention relates to methods of adsorption with oscillating pressure (PSA) and adsorption with vacuum oscillating pressure (VPSA) for gas separation, and more particularly to a method for air separation wherein the cost for the product O2 is reduced by using a process that uses low pressure regimes and uses adsorbents that exhibit high intrinsic diffusivity.

BACKGROUND OF THE INVENTION During the past thirty years, significant developments have taken place in the methods of adsorption by oscillating vacuum (VSA), PSA and VPSA for gas separation, with the main advances that have occurred during the last decade. Such processes have also been named subatmospheric, superatmospheric and transatmospheric, respectively. Unless otherwise specifically noted, PSA will be used later to refer to any or all of these processes. The commercialization of these processes can be attributed to improvements in the adsorbents, process cycles and advances in the design of the adsorber. Highly exchanged lithium molecular weight adsorbents, as illustrated by Chao in U.S. Patent No. 4,859,217, are representative of advanced adsorbents for O 2 production. Such advanced adsorbents are expensive and represent a significant portion of the capital cost of the PSA equipment. A dominant factor in the total energy requirement of the PSA processes is the ratio of adsorption to desorption pressures. Lowering the pressure ratio is a potential method to reduce energy consumption. In addition, a reduction in the PSA cycle time has the potential to reduce the amount of adsorbent required. Unfortunately, the usual consequence of both strategies is a reduced product recovery (v. G., O2). Attempts to operate at lower pressure ratios have been mpanied by substantial decreases in adsorbent productivity, v. g. , Leavitt, E. U., 5,074,892. Smolarek, in the copending U.S. Patent Application Serial No. 08 / 964,293 (Attorney docket D-20335), overcomes some of the low pressure regime compensatory effects through proper selection and operation of machinery. of vacuum and compression, combined with improved flow characteristics of radial flow adsorbers. Ackley, et al., In the co-pending PCT Patent Application Serial No. US99 / 04383 (Attorney docket D-20393), has taught the optimization of product recovery and adsorbent productivity through the use of adsorbents with high intrinsic diffusivity. in fast cycles. Although the invention to be described below is applicable to a wide range of gas separations, air separation processes by PSA with the objective of producing high purity O2 (approximately 88% to 95.7% of O2) are of particular interest. The air separation of the prior art discussed below reflects this range of O2 purity. 5 Advanced adsorbents of the aforementioned types are the result of improvements in equilibrium properties. The working capacity of improved N2 and selectivity of 2 O2 adsorbents have been transformed into large gains in process efficiency - obtaining such benefits at the expense of the higher cost of adsorbent (Smolarek et al., Gas Separation Technology, 1990). The adsorbents of exchanged lithium zeolite (LiX), in particular, have had a major impact on the evolution of air separation processes by PSA. The higher N2 capacity and higher N2 / O2 selectivity resulting from LiX zeolites Highly interchanged SiO2 / AI2O3 ratios of low ratios have been more recently exploited for higher performance in PSA processes for air separation. Other major improvements for these processes have been the introduction of vacuum for desorption, reduction of three-bed processes and four beds to two bed cycles, and the use of modified cycle steps such as pressurization, purging and / or product equalization. These and other advances in the prior art in oxygen production have been summarized by Kumar ("Vacuum Swing Adsorption Process for Oxygen Production - A Historical Perspective", Sep. Sci. Technology, 31: 877-893, 1996).

The improvement of the efficiency of the process and the reduction of the cost of the light component product can be made by decreasing the amount of adsorbent required per unit of product (increasing the productivity of the adsorbent) and increasing the product recovery. The first is generally expressed in terms of bed size factor (BSF) in kg of adsorbent / TPDO (ton per day of O2 content), while the latter is simply the fraction of light component in the feed that is captured as a product . The improvement in adsorbents and the reduction in cycle time are two main methods to reduce the BSF. Considerable attention has been focused on the prior art on process optimization. Reiss (Chem. Ind. XXXV, p689, 1983) emphasizes the importance of the qualities of adsorbents and the high recovery of O2 product on the energy consumption in vacuum processes (VSA) for the enrichment with oxygen from the air. Reiss has shown that there is a minimum in the energy characteristic of the specific vacuum pump as the desorption pressure is increased for a fixed adsorption pressure. More specifically, the energy per unit of O2 produced decreases initially when the pressure regime decreases and then increases such that there is an optimum pressure regime for minimum specific energy consumption. Concurrently with the effect of decreasing the pressure regime on the specific pump energy, there is the uniformly decreasing amount of O2 product or a decrease in the adsorbent productivity.

Smolarek et al. (Gas Separation Technology, 1990) achieved the objective of lowering the cost of the unit O2 product by reducing the cost of capital and the consumption of energy. The operating parameters of the process were developed around the advanced characteristics of the adsorbent. The bed size was reduced using shorter cycles, although the optimal cycle time was selected based on the minimum cost. This minimum cost was established as a compromise between decreased bed size and reduced process efficiency as the cycle time was shortened. It was also shown that two beds of adsorbent was optimal. The lower overall energy consumption resulted from the combination of increased O2 product recovery, reduced adsorbent inventory and reduced equipment size. A reduction in the optimal pressure ratio was attributed to the advanced adsorbent. The type of adsorbent (LiX), the BSF (454 kg / TPDO), and the pressure ratio (6: 1), were not originally reported in the publication. In the prior art, process optimization takes advantage of the enhanced properties of the equilibrium adsorbent of the working capacity greater than N2 and the greater selectivity of N2 / O2 to achieve the greater overall product recovery in processes utilizing desorption by empty. The desorption pressure was increased (decreased pressure ratio) to reduce the energy consumption. Ef cycle time was decreased to maintain the bed size and the cost of adsorbent under control. Achieving the minimum pressure ratio was not a primary objective of optimization. In fact, the reduction in pressure ratio was limited by the increase that accompanies it in bed size and the reduction in product recovery. The lowest pressure ratios that correspond to the "optimum performance" achieved in these prior techniques were 5: 1 or greater. Much of the prior art deals with the improvement in the increase of the process efficiency through the modification of the passage of the cycle. A good example of such improvements is given by Baksh and colleagues, (U. U. 5,518,526). The potential benefits of low pressure ratios to achieve the lowest energy consumption have generally been limited, due to the compensating effects of higher BSF and lower product recovery. Although adsorbents with improved equilibrium properties allow process improvement at lower pressure ratios, reductions below a critical or limiting pressure ratio have a more severe impact on processes incorporating advanced, high-cost adsorbents. In other words, the increased inventory of adsorbents that accompanies the lower pressure ratio has a significant impact on the capital investment of the plant. Such relations of critical or limiting pressures were theoretically defined by Kayser and Knaebel (Chem. Eng. Sci. 41, 2931, 1986; Chem. Eng. Sci. 44.1, 1989) for 5A and 13X adsorbents. The recovery characteristics of O2 / pressure ratio are relatively flat at higher pressure ratios (almost constant recovery), but show a sharp decline in recovery below the critical pressure ratio. The critical pressure ratio depends on the type of adsorbent and the operating conditions of the process and these limits have not been well defined in practical applications. However, reduced recovery trends have generally discouraged the operation of O2 process by PSA at pressures below about 4: 1. More recently, Rege and Yang (Ind. Eng. Chem. Res., 36: 5358 -5365, 1997) presented limits for LiX zeolite and revealed a recovery characteristic of O2 / pressure ratio for LiX similar to that previously defined by others for 13X and 5A adsorbents. The theoretical results of Rege and Yang suggest pressure ratios as low as 2: 1 with little punishment in O2 recovery for oscillating vacuum processes. They attribute this behavior to the upper equilibrium properties of the adsorbent and indicate the lowest optimal BSF for its cycle at 18 kg / kg O2 hr (681 kg / TPDO). The pressure drop of the adsorbent bed and the resistance to diffusion of the adsorbent are not taken into account in the theoretical model. The energy consumption was not considered in the analysis. Leavitt (U.S. 5,074,892) proposed cycles of O2 production at a low pressure ratio in the range of 1.4 to 4.0 for adsorbents with advanced equilibrium adsorption properties, v. g., LiX, digested caustic NaX. Leavitt's main motivation was to reduce the overall process costs by reducing energy consumption. Leavitt noted the importance of the high working capacity of N2 and the high selectivity of N2 / 02 of the adsorbent and indicated the need to achieve relatively high product recovery at low pressure ratios in order to limit growth in BSF. Larger amounts of purge were suggested at low pressure ratio to partially compensate for the lower working capacity for N2. Although impressive reductions in energy consumption were indicated, the BSF increased substantially as the pressure ratio decreased. Leavitt did not consider the effect of the intrinsic diffusivity of the adsorbent on the performance of the process. Smolarek (in copending U.S. Patent Application Serial No. 08 / 964,293 (Attorney docket D-20335) has proposed an O2 cycle by two-bed VPSA using a single-stage vacuum device.The adsorption pressure is in The range of 1 .3 to 1 .6 atm, while the desorption pressure level is between 0.4 and 0.55 atm.The preferred pressure ratio is in the range of 2.75 to 3.0.A radial flow adsorber is also used. provide optimum flow distribution and minimum pressure drop.The higher desorption pressure increases the molar production and reduces the pressure differential across the vacuum pump, resulting in the ability to select simplified vacuum equipment (single stage) and The lower pressure ratio results in a reduction in the product recovery which, in turn, requires a greater supply of feed for an equivalent quantity of product, ie compared to a cycle of higher pressure ratio. The cycle time is reduced, but it is limited in order to avoid introducing additional inefficiencies to the process in order to maintain the BSF without significantly increasing it. Smolarek does not claim an increase in the BSF compared to the reference of the higher pressure ratio. The effects of the adsorbent properties, in particular characteristics of high adsorbent ratio, on the performance of the process have not been mentioned in the teachings of Smolarek. The reduction of cycle time is a key strategy to reduce the adsorbent inventory and the adsorbent cost at any pressure ratio. This is even more important for low pressure ratio cycles. Although shorter cycles lead to shorter beds and greater use of adsorbent, product recovery suffers unless the adsorption regime is increased. This phenomenon can be ideally characterized in terms of the size of the mass transfer zone (MTZ), that is, the mass transfer zone becomes a growing fraction of the adsorbent bed as the depth of the bed decreases. Since the use of adsorbent with respect to the heavy gaseous component e-s is much lower in the MTZ than in the equilibrium zone, the working capacity declines as this fraction increases. When resistance to mass transfer is dominated by pore diffusion, a decrease in adsorbent particle size leads to faster adsorption regimes and smaller areas of mass transfer. Unfortunately, the pressure drop across the adsorbent bed increases with decreasing particle size. Armond et al. (Patent Application UK RU 2091 121 A, 1982) demonstrated a short-cycle air separation process (<; 45s) / low pressure ratio (3.0) using a 5A molecular sieve. This cycle was superatmospheric, operating with a desorption pressure close to the environment. Armond apparently achieved a relatively small adsorbent inventory using very small particles (0.5 to 1.2 mm in diameter) to facilitate a short cycle time. However, the pressure drop across the bed (48 kPa / m) was very high as was the energy consumption, 0.7 kWhr / sm3 of O2 (20 kW / TPDO). High energy consumption was presumably the result of low product recovery. Ackley et al., In the co-pending PCT Patent Application Serial No. US99 / 04383 (Attorney docket D-20393), has described improved processes using advanced adsorbents with high intrinsic diffusivities in relation to conventional adsorbents. The increased recovery of O2 product was demonstrated by increasing the adsorption / desorption ratios to create higher N2 mass transfer coefficients at a fixed pressure ratio. This concept was then applied to achieve very short cycles and very low BSF while a only minimal decrease in product recovery is affected. Notaro et al., In the co-pending PCT Patent Application Serial No. US99 / 04388 (Attorney docket D-20270), discloses a process of air separation by PSA, wherein the adsorbent is selected on the basis of relative combinations of intrinsic regime and equilibrium properties.

Accordingly, it is a primary objective of the invention to reduce the product cost, reduce the energy cost and increase the adsorbent productivity of high performance adsorption processes for gas separation. It is a further object of the invention to provide an improved PSA process for air separation.

BRIEF DESCRIPTION OF THE INVENTION A gas separation process incorporating the invention combines the use of an adsorbent having high intrinsic diffusivity with a PSA cycle of low pressure ratio. Additional improvements to the process are derived from the use of fast cycles, shallow beds and small particles - especially in a radial bed configuration. The combination of low pressure ratio, high rate adsorbents and fast cycles has been found to result in an unexpected simultaneous reduction in bed size factor (BSF) and energy consumption. These benefits have been achieved while minimizing a decline in product recovery through the use of high-rate adsorbent. The net result is a significant reduction in product cost. The high adsorption regime partially compensates for the decline in product recovery that accompanies the reduced pressure ratio, thus enabling fast cycle operation in shallow beds which affects an unexpected overall decrease in the BSF. The present invention couples the effects of mass transfer ratios (and associated particle properties), cycle time and bed depth to significantly improve the efficiency of gas separation at low rates of process pressures, i.e. , improvements such as an increase in the productivity of the adsorbent (lower BSF) and a decrease in the energy consumption of the process. Both, reduced cycle time and reduced pressure ratio cause a decrease in product recovery. This occurs in the first due to the increased fraction of the bed dedicated to the mass transfer zone and in the latter due to the decrease in selectivity efficiency or separation of the adsorbent. The reduced separation efficiency is substantial in vacuum desorption cycles using advanced adsorbents such as LiX and where the pressure ratio is commonly reduced by increasing the desorption pressure. The application of high intrinsic diffusivity adsorbents minimizes significantly those undesirable effects during the operation of the process, particularly at low pressure ratios. Although this invention has been demonstrated for the case of air separation, the general methodology is applied to other gas phase separations that: (1) depend on the differences in equilibrium adsorption selectivity; and (2) in which the resistance to mass transfer is dominated by diffusion in the macropores of the adsorbent particles. The methodology is applicable especially to the production of oxygen in PSA processes that incorporate selective adsorbents for N2, v. g. , X-type zeolites or advanced adsorbents such as highly-exchanged Li-type zeolites or other monovalent exchanged cation. The invention is particularly well suited to the use of adsorbents having high capacity and high selectivity (in combination with high intrinsic diffusivity) for the more selectively (weighed) adsorbed component of the gas mixture to be separated. The prior art has focused on the increased recovery of O2 product and has exploited the lower pressure ratios for lower energy consumption only to the extent that it was inherently allowed by the improved equilibrium properties of advanced adsorbents. Thus, with each new improvement in capacity and selectivity of the adsorbent, it was found that a reasonable product recovery could be achieved at a modestly lower pressure ratio. Nevertheless, the lower N2 capacity of work and shorter cycle time -established by lower pressure ratios results in lower adsorbent productivity (higher BSF). Attempts by the prior art to counteract this effect by reducing the cycle time further resulted in a rapid deterioration in product recovery - thereby compensating for the lower energy benefits of the low pressure ratio as well as limiting the gain of potential in adsorbent productivity of the shortened cycle. The use of smaller particles to inhibit the loss of product recovery in faster cycles is limited, because the pressure drop in the adsorbent bed increases with the decrease in particle size, which in turn negatively affects the consumption of the product. Energy. The present invention achieves a higher rate of adsorption through increased intrinsic diffusivity without requiring the use of very small particles (eg, the invention preferably uses particles having an average diameter (dp) >0.8 mm, more preferably = 1 mm). However, the adsorbent particle size suitably selected according to the pore diffusivity can be applied to further increase the benefits of the new invention. The invention also focuses on the decrease of the product cost. This approach does not demand increased product recovery; rather it demands that cycle time, bed depth, pressure ratio, flow rate be selected in such a way to achieve the lowest product cost. It has been found that the potential benefits of the low pressure ratio can be more fully exploited by the use of modified adsorbents to have a high adsorption regime (high intrinsic diffusivity), i.e., in contrast to decreasing particle size. And, it has been found, surprisingly, that the productivity of the adsorbent can be maintained or even increased as the pressure rating decreases.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic of a VPSA system adapted to realize the invention herein.

Figure 2 is a diagram showing a possible set of cycle steps for carrying out the invention. Figure 3 is a cross-sectional view of a radial bed adsorber that is particularly adapted for use with the invention.

DETAILED DESCRIPTION OF THE PREFERRED MODALITY Initially, a description will be given of adsorbents that are preferred for use with a PSA process embodying the invention. Next, a global description of the use of the preferred adsorbents in a PSA process will be considered, followed by specific examples of tests that have been run and a detailed description of a PSA system and the process steps that are performed herein. This invention employs the unexpected results that arise from combining. PSA cycles with low pressure ratios with high intrinsic diffusivity of the adsorbent. The benefits of the invention can be realized in subatmospheric, transatmospheric and superatmospheric pressure ratio cycles and potentially for any bulk gas separation, although the benefits are probably the most attractive for such high performance materials in equilibrium. The benefits of combining low pressure ratio with high intrinsic diffusivity of adsorbent, short cycles and shallow beds in air separation by PSA are as follows: * > Reduced energy consumption * > Increased productivity of adsorbent (reduced BSF). & Reduced container size. & Pressure drop of the equivalent or reduced system. & Reduced cost of product.

The above benefits occur because the integrated effects of the high rate adsorbent used at low pressure ratio with short cycles, shallow beds and simplified compression equipment more than compensate for the lower product recovery that accompanies the lower pressure ratios. In fact, product recovery can be changed by a lower pressure ratio until a minimum product cost is reached. A pore structure of the adsorbent can be manipulated through the modification of the adsorbent processing steps, v. g., zeolite synthesis, ion exchange, calcination, drying, etc. However, the properties of the zeolite are very sensitive to small changes in the manufacturing variables and such changes are frequently damaging the adsorbent. As a result, the manufacture of zeolites is subject to rather strict control of process variables. The commercial zeolite adsorbents shown in Table 1, such as 13XHP, 5AMG, LiX (2.5) and LiX (2.3), available as UOP beds from Des Plaines, III, USA, were all found to have pore diffusivity (Dp). ) of N2 in a rather narrow range (2.6 x 10"6 to 3.2 x 10'6 m2 / s) - as shown in Table 1, where the pore diffusivity of N2 is determined from a decisive experiment conducted with air at 1.5 bar, 300 K. The porosity (ep) of these same materials falls within the range of 0.30 to 0.38 for conventional zeolites Table 1: Comparison of Conventional and High Intrinsic Diffusivity Adsorbents Pore diffusivity = Dp Porosity = ep Digested Caustic = c.d. Diameter of particle = dc Adsorbents with improved intrinsic diffusivity have been produced in accordance with the teachings of Chao in copending application Serial No. PCT US99 / 04219 (Attorney docket D-20658), incorporated herein by reference. Two such samples (Z-1 and Z-2) are shown in Table 1 and represent a minimum improvement of 30% to 70% in pore diffusivity over that of conventional adsorbents. The Z-O sample is an advanced LiX (2.0) adsorbent (as described by Chao in U.S. Patent No. 4,859,217) with conventional pore diffusivity and equilibrium properties and used later in VPSA performance comparisons. Chao in PCT Patent Application Serial No. US99 / 04219 (Attorney docket D-20658), has demonstrated several formulations and methods to produce adsorbents with intrinsic diffusivities greater than those of conventional adsorbents. The pore diffusivities of the adsorbents can be increased by first combining a low amount of binder with zeolite in the bedding step, followed by caustic digestion (ie, "c.d."). The intrinsic regime characteristics of the adsorbent can be further enhanced by the addition of fiber or sub-micron latexes to particles with subsequent burning. Without wishing to be restricted to any method or formulation, the detailed process for producing Z-2 adsorbent of the invention is described herein as an example for making such high-rate adsorbents. The Z-2 method involves four primary steps of: bed formation, caustic digestion, ion exchange and calcination as described below.

Bed formation: 2,640 gm dry weight of zeolite NaKX2.0 (wet weight 4, 190 gm), 360 gm dry weight were heated. of kaolin clay ECCA Tex-61 1 (wet weight 429.1 gm) and 210 gm of corn starch for 15 minutes, while water was pumped at a rate of 10 ml / min. The rate of addition of water was then lowered to 4 ml / min. for 100 min. and the mixture was heated another 35 min. The heated mixture was then transferred to a DBY-10R Nauta Mixer. { supplied by Hosokawa Micron Powder Systems) and mixed for about one hour. The clods were broken to return the mixture to a powder state. Water was then added slowly by means of an atomizer. As the moisture of the mixture increases, the beds begin to form. The bed growth was interrupted by adding dry binder mixture at the point when the highest yield of 8 x 12 beds could be obtained. The beds were air dried overnight and then calcined in a Blue M oven with a dry air purge. The furnace temperature was gradually increased to 600 C in two hours and then maintained at 600 C for two hours during the dry air purge.

Caustic Digestion: 1, 861.8 gm of dry weight of calcined NaKX2.0 beds of size 6 x 16 with 12% binder were used for caustic digestion. To prepare the digestion solution, 360 gm of NaOH (9 moles) and 251.1 gm of KOH (4,475 moles) were dissolved in 7.386 gm of water. To this solution 320 ml of propitiating NaKX2.0 beds were added and stirred at 90 C for 2 hours. The solution was allowed to settle and 6,397.7 gm of supernatant were collected. To this supernatant, 1 477.2 ml of water, 72.0 gm of NaOH and 50.2 gm of KOH were added to prepare the discarded caustic. The resulting solution was used as a digestion solution. The beds were loaded in two stainless steel columns from 7. 62 centimeters in diameter and the solution of a common deposit was recycled through each column, at a flow rate of 30 ml / min and temperature of 88 C for 26 hours. After digestion the beds were washed by pumping 40 liters of NaOH solution (pH = 12, 88 C) through each column. The beds in each column were further washed with 30 liters of NaOH solution (pH = 8.5, 88 C). The product, NaKX2.0CD, was air dried and filtered for fractions of various particle sizes.

Ion exchange: They were charged to a glass column of 7.62 centimeters d.i. 694.5 gm of dry weight of beds of NaKX2.0CD of 8 x 12. A layer of 25.4 centimeters of beds of Pyrex glass of 3 mm was placed in the bottom of the column to serve as a pre-heating zone for the solution. The column was wrapped with a heating tape. The ion exchange solution was first passed through a 15-liter pre-heating flask at 90 ° C to partially remove any dissolved air to avoid the formation of air bubbles that could subsequently be trapped in the column. The hot solution was then pumped to the bottom of the column.

The ion exchange solution was prepared by dissolving 2. 162 gm of LiCI in 80 liters of distilled water (0.64 M) then LiOH solution was added to adjust the pH of the solution to 9. The solution was pumped through the column at a speed of 15 ml / min. up to 10 to 12 times the stoichiometric amount of LiCl, for complete Li exchange of the beds, had been circulated through the column. After the ion exchange was completed, the product was washed with 30 liters of distilled water at 90 C at a flow rate of 60 ml / min. The pH of this water was adjusted to 9 by adding LiOH.

Drying and Calcination: The washed product was first dried with air and then further dried in a low temperature oven with ample air purge for 3 hours to lower the moisture content of the beds to approximately 12-15%. The dry beds they were calcined in a Blue M furnace with ample dry air purge. The oven temperature was gradually raised from (at room temperature to 600 C in two hours and maintained at 600 C for 40 minutes.) The sample was removed from the oven at 450 C and placed in a sealed glass jar for cooling The above procedure was repeated several times in order to produce a sufficient amount of zeolite for pilot plant testing (approximately 1 1 .35 kg) .The different production batches were mixed before being loaded into the plant adsorber vessels. The properties of the adsorbent were determined using the prepared mixture.The effective diffusivity of N2 and O2 were determined at 1.5 bar and 300 K using a combination of decisive experiment and detailed modeling ^ all of which is familiar to an expert in the The effective diffusivity of O2 determined for the adsorbents in Table 1 is about 35% of the effective diffusivity for N2 The PSA improvements in this invention have been correlated with the effective diffusivity of N2. The details of the decisive experiment and empirical method to determine diffusivity are provided by Ackiey et al., in copending patent application Serial No. PCT / US99 / 04383 (Attorney docket D-20393). In addition to the adsorbents described above, one skilled in the art will appreciate that alternative adsorbents with increased pore diffusivity can be applied in a manner similar to that described herein to achieve the corresponding improvements in process performance. The terms pore diffusivity, effective diffusivity and intrinsic diffusivity are used interchangeably in the present. By the term "intrinsic diffusivity" is meant the property of transport that is due to the intrinsic characteristics of the adsorbent particle including, but not limited to, the structure, size, shape and length, etc. , of the macropores. The term "macropores" is intended to include all the intra-particle void volume (that volume that establishes the porosity) that is typically penetrated in a standard Hg porosity test for zeolites. Ideally, an intrinsic or effective diffusivity of material is independent of particle size. CONSIDERATIONS OF THE PSA / ADSORBENT SYSTEM The adsorbents can be deployed by this invention in one or more different adsorption zones, v. g., pre-treatment zones and main adsorbent. In each zone one or more adsorbents may be contained, and the zones do not have to be contained in the same adsorbent vessel. The pre-treatment zone is placed closest to the feed inlet and its purpose is to remove any undesirable contaminants from the feed stream. Typical contaminants in air separation are water and carbon dioxide. Those skilled in the art will appreciate the use of zeolites, activated alumina, silica gel, as well as other suitable adsorbents in the pre-treatment zone. The main adsorbent zone is placed downstream of the pre-treatment zone (relative to the flow through the bed during the adsorption step) and contains selective adsorbent (s) for the component (s) heavy weight (s) in the feed. The pre-treatment zone can be excluded if there are contaminants in the feed stream. The PSA processes described herein are those in which the separation of at least two components of a gas phase mixture is affected by differences in the equilibrium adsorption capacities of the components in the main adsorbent, ie, by At least one component in the mixture is more selectively adsorbed at equilibrium compared to the adsorption of the other components. The invention uses adsorbents having higher intrinsic diffusion rates than conventional adsorbents. In equilibrium separations, a mixture of gases is passed through a bed of adsorbent particles and the gaseous component adsorbed more strongly (heavy) is retained, while the other (light) components arise from the adsorber outlet. At the beginning of the adsorption step, a mass transfer zone is formed and moved through the bed. Almost all adsorption occurs within this zone. The concentration of the gas to be removed decreases from its concentration in the feed mixture to a very low value over the length of this zone. In some separation processes, this zone rapidly reaches a constant length (usually significantly less than the overall depth of the adsorbent bed) and moves through the bed at a constant speed. If light product of relatively high purity is desired, the adsorption step must be stopped (and subsequently followed by a regeneration step) when the front of the area begins just spitting at the exit of the bed. At this time, the bed contains the mass transfer zone near the outlet and the rest of the bed is completely saturated with the component held most strongly in equilibrium with the feed concentration of this component. The part of a bed placed between the entrance of the main adsorption zone and the rear of the mass transfer zone is known as the "equilibrium zone". If the bed becomes shorter than the length of the mass transfer zone, then the component to be removed will burst through the bed immediately at the beginning of the adsorption step. The overall working capacity of the adsorbent for the heavy component is the largest when the size The fractional area of the mass transfer zone remains small relative to the total bed size, that is, most of the bed is saturated (equilibrium zone) at the end of the adsorption step. Faster cycles require shorter beds that result in an increase in the fractional size of the mass transfer zone.

The use of high intrinsic diffusivity adsorbents counteracts this effect and allows the use of fast cycles while avoiding an increase in the fractional size of the mass transfer zone. The size of the mass transfer zone is influenced by the particle size and the gas diffusion rate in the particle.

In many cases, the greatest resistance for this diffusion is in the macrosporos of the particles, that is, the gas molecules must travel through narrow and sinuous empty passages within the particle to reach the active surfaces of the adsorbent. If the particle size is reduced, this transfer occurs much more quickly (since the length of the path is shortened) - which results in a shorter mass transfer zone. There are both limitations and disadvantages to this approach as small particles lead to increased pressure drop per unit bed length, difficulty in bed retention, and increased tendency to fluidize. This approach is further limited because it ignores the possibility of achieving process performance improvements by increasing the intra-particle diffusivity directly without any reduction in particle size. The pressure drop across the adsorbent mass in a fixed bed adsorber is dependent on the gas velocity across the bed, the size of the particles in the bed, the packing density of the particles and the depth of the bed. The relationship between these variables is established by the well-known Ergun equation (Chem. Engng, Progress, 1952), which is widely used to determine the pressure drop across a bed of fixed adsorbent. Simply, the pressure drop increases for small particles, deeper beds, higher gas flows and denser packing. While the particular adsorber depends on the characteristics of the separation to be made, the pressure drop of the adsorbent bed less than or equal to 56 mbar / m and bed depths of 1.2 m to 1.8 m have been very common in the production of O2, using selective adsorbents for N2, as well as in other conventional PSA processes. To compensate for the higher pressure drops that result from reduced particle size, and to minimize the increase in energy and fluidization tendency, it is necessary to decrease the depth of the bed and / or the flow velocity through the bed. These changes lead to reduced recovery and compensation in the use of the bed for a fixed particle size, ie shorter beds need faster cycles leading to reduced recovery and possibly some improvement in bed utilization, although the although the reduced speed counteracts this increase in bed utilization (for a fixed input area) due to the resulting lower feed efficiency. Although this latter problem can be counteracted by increasing the flow area, there are practical limits to the size of the adsorber vessels - particularly in the case of conventional, cylindrical adsorbers with axial flow. Ackley et al., (Co-pending Application Serial No. PCT / US99 / 04383 (Attorney docket D-20393) is incorporated herein by reference in its entirety, and teaches a method for improving efficiency (optimizing product recovery) and to reduce the product cost of the VPSA process at a moderate pressure ratio (5: 1), using fast cycles and shallow beds.The method of Ackley et al., is practiced using high mass transfer rates achieved with modified adsorbents with high intrinsic pore diffusivity. Examples of such adsorbents are defined by Chao as described in copending application Serial No. PCT / US99 / 04219 (D-20658), the content of which is incorporated herein by reference. The particle size can also be selected in combination with high intrinsic diffusivity to tailor mass transfer regimes for high product recovery. Although this guide applies to any relationship of process pressures, Ackley and colleagues do not teach the integration of these concepts with a low pressure ratio. In particular, there is no indication that a decreasing pressure ratio could be achieved in conjunction with these teachings without suffering the usual expected loss in adsorbent productivity. Smolarek (D-20335, co-pending Application Serial No. 08/964293, the contents of which are incorporated herein by reference), teaches the importance of desorption pressure level, bed flow and pressure drop, and the use of equipment of simplified vacuum to minimize the fall in product recovery that accompanies the low ratio of process pressures. This simplified equipment integration and low pressure ratio leads to a reduced product cost. Smolarek also notes the limitations of reducing cycle time without "engendering additional inefficiencies ..." in the process. Smolarek does not teach particular adsorbents or high-rate adsorbents. EXAMPLES: VPSA Performance Comparisons - Production of O? A series of pilot plant tests were performed with Z-0 and Z-2 adsorbents (described in Table 1 above) to demonstrate the benefits of the invention for air separation by VPSA. The adsorption pressure was maintained at approximately 1.5 bar in all tests, while the desorption pressure was varied to reach pressure ratios of approximately 5.1, 3.3 and 2.6. Note that the term "pressure ratio" defines the relationship of adsorption to desorption process pressures, when measured at the end of adsorption and desorption, respectively. The pilot plant consisted of two beds operating out of phase and with a cycle and steps similar to those described by Baksh et al., In U.S. Patent 5,518,526, which will be described in detail below. The yield results for 90% purity O2 production are summarized in Table 2 below.

Table 2 The improvement in process performance achieved by replacing a high-regime Z-2 adsorbent with a Z-O adsorbent can be evaluated from the results of Tests 1 and 3 for 1.0 m deep beds. Product recovery increased from 71.5% to 74.5%, the specific energy (total process energy / unit of product O2) decreased by only about 1% and there was a large decrease (22%) in BSF for the process using adsorbent Z -2. The bed size factor (BSF) and the total process energy have been normalized for the values obtained in Test 1.

The decrease in BSF occurs as a result of the combined effects of increased product recovery, decreased time. The decrease in BSF occurs as a result of the combined effects of increased product recovery, decreased cycle time, lower Z-2 density (approximately 8). lower%) compared to ZO and the higher production of feed gas that is possible with a high-regime adsorbent. The significant reduction in recovery of O2 product (71.5% to 60.5%) that accompanies a reduction in pressure ratio of 5.3 to 3.4 is shown by the results of Tests 1 and 2 for an advanced adsorbent with diffusivity (Z-0). ) conventional. These tests were conducted with an adsorbent bed depth of approximately 1.0 m. The lower working capacity of N2 resulting from the lower pressure ratio required the shortest cycle time. The BSF decreased slightly in contrast to the significant increases in BSF reported by Leavitt (E. U. 5,074,892). A modest decrease is also made, but this is limited due to the great reduction in product recovery. The performance of Exhibit 2 is representative of processes taught by Smolarek et al. (Copending Application Serial No. 08/964293). The same reduction in pressure ratio was imposed for the high-regime Z-2 adsorbent between Tests 3 and 4. Here again, the reduction in product recovery O2 is substantial (74.5% to 58.5%), although in this case the BSF remains unchanged due to the beneficial effects of the high-rate adsorbent. The ability to maintain the BSF as it decreases the pressure ratio is the result of the much shorter cycle time that can be achieved with the adsorbent of higher intrinsic diffusivity. The recovery punishment of O2 product as it decreases the cycle time is minimized by replacing the high regime adsorbent as is evident by comparing the results of Test 2 with those of Test 4 in Table 2. The high intrinsic diffusivity of the Z- adsorbent 2 also allows the benefits of the lower pressure ratio to be more fully exploited by reducing the energy consumption of the process. Comparing results from Test 1 and Test 4, a reduction of 9% in specific energy was achieved from the combination of low pressure ratio and high diffusivity of N2. The 22% reduction in BSF for Test 4 (compared to Test 1) results from the combination of factors cited above in addition to the fact that the Z-2 adsorbent has approximately a N2 12% higher working capacity compared to the 2-0 adsorbent at a pressure ratio of 3.1. This difference in working capacity results from the conversion of binder to zeolite in the process step of caustic digestion. This greater working capacity of N2 will not necessarily occur in other treatment strategies used to achieve high intrinsic regimen. In fact, although advantageous, such greater work capacity is not essential for the practice of the basic invention. The pressure ratio was further reduced from 3.1 to 2.6 for the Z-2 adsorbent. The results of Test 5 show that the recovery of product O2 decreased to 51.0% (from 58.5% at a pressure ratio of 3.1 in Test 4), while the BSF increased substantially and the energy continued to decrease compared to the results of Test 4. Here the potential benefits of the lower pressure ratio are diminished due to the overwhelming reduction in product recovery, that is, the ratio of Pressures have been reduced too much for this bed depth and adsorbent diffusivity. The use of an adsorbent with intrinsic diffusivity greater than that of the Z-2 adsorbent, v. g., the adsorbent Z-1, could have preserved the desired simultaneous reduction of the BSF and energy at pressure ratios less than 3.1. However, the performance in Test 5 at a pressure ratio of 2.6 represents a substantial benefit in both energy and BSF compared to performance at a pressure ratio of 5.3 in Test 1. The above results demonstrate the advantages of combining low pressure ratio with high intrinsic diffusivity of adsorbent, while maintaining a constant bed depth. The cycle time was only decreased as much as necessary to compensate for the reduced N2 working capacity of the adsorbent due to the lower pressure ratio. The low pressure ratio can be further exploited to gain additional performance advantage by employing faster cycles in shorter beds, that is, as long as the intrinsic diffusivity of the adsorbent is sufficiently high. Test 6 was carried out with a bed depth of 0.9 m at the pressure ratio of 3.3. Surprisingly, the product recovery increased from 58.5% to 60.0% while the BSF and energy continued to decrease, that is, compared to the results for Test 4 at approximately the same pressure ratio. These improvements derive from the shorter cycle / shallow bed and are partially due to a lower bed pressure drop and improved flow characteristics - the combination of which is enabled by the high regime characteristics of the adsorbent. The results in Table 2 are not intended to represent a complete definition of desirable conditions in terms of cycle time, bed depth and pressure ratio for the Z-2 adsorbent. Shorter beds and faster cycles can still produce additional advantages for this adsorbent at a pressure ratio of 3.3. In addition, lower pressure ratios (<; 3: 1) can be combined with even greater intrinsic diffusor adsorbents (such as Z-1 in Table 1) to achieve additional yield improvements over those obtained with the Z-2 adsorbent. Thus, the higher intrinsic diffusivity allows both improved performance at a fixed pressure ratio and / or extension of lower pressure ratios to achieve even lower product cost. The substantial reductions in BSF and specific energy consumption represented by the results of Tests 4, 5 and 6, using adsorbent Z-2, provide a significant reduction in the cost of O2 product - particularly when coupled with simplifications of equipment of vacuum (eg, single-stage vacuum pump) possible at lower pressure ratios. The higher possible cost of the adsorbent due to specialized processing to achieve higher intrinsic diffusivity is expected to be more than compensated by the lower BSF. The lower product recovery experienced at lower pressure ratios results in higher air compression costs for a given plant capacity. These additional costs are more than offset by savings in the cost of energy and evacuation equipment at these lower pressures due to the higher suction pressure in the process. This invention represents a significant advance in air separation PSA performance because the processes represented by Tests 4 and 6 simultaneously reach the BSF < 227 kg / TPDO and an energy consumption < 7.5 kW / TPDO. It is evident from the above examples of air separation that a minimum product cost can be achieved by tailoring the pressure ratio, cycle time and bed depth for a specific intrinsic diffusivity. The best combination of conditions will vary for adsorbents with different intrinsic diffusivities. In addition, this invention can be applied to other gas separations to affect lower product costs through lower energy consumption by impregnating lower pressure ratios. Thus, the exact combination of process conditions for the minimum cost depends on the properties of the adsorbent and the separation of interest. For air separation by PSA, the preferred range of pressure ratios is from about 2.0 to 5.0 for adsorbents with an intrinsic diffusivity of N2 equal to or greater than 3.5 x 10"6 m2 / s.A more preferred range of pressure ratios is from 1.5 to 3.5 for adsorbents with a diffusivity equal to or greater than 4.5 x 10"6 m2 / s. VPSA PROCESS / SYSTEM DESCRIPTION Referring now to Figures 1 and 2, the operation of a VPSA system according to the invention will be described. Referring to Figure 1, a VPSA system 10 includes a feed air inlet 12 that allows the feed air to enter through the inlet filter 14 and the inlet squelch 16. The supply air fan 18 compresses the air for delivery to the system via the discharge silencer 20 and the conduit 22. During periods of discharge, the supply fan is vented via the valve 24. The air enters the adsorbers 26 and 28 via conduit 30 and fas open valves 32 and 34. The N2 and waste contaminants are removed from the adsorbers via the open valves 38 and 40, ßl conduit -42 and the vacuum pump 44. The waste gas is silenced before vent it by eff silencer 46. During periods of discharge, the vacuum pump 44 is recirculated through the open valve 48. The reflow steps of the product and product make are carried out via conduit 50 and valves 52 and 54. Product make is conducted through conduit 56 and valve 58. The storage of final product is contained within equalizing tank 60 and delivered to point of use 62 via valve 64 and conduit 66. Each of the adsorber beds 26 and 28 includes a high rate adsorbent which is selective for N2, assuming that the VPSA system is going to be used for O2 production. The process steps performed by the VPSA System 10 during the implementation of the invention will be described in conjunction with the process step diagram shown in Figure 2. Table 3 indicates the elapsed cycle time, start pressure and final pressure for each step of the representative cycle. One skilled in the art will recognize that the essential elements of the invention can be practiced using other cycle configurations. For the purpose of the cycle description below, the "bottom" of the container means the feed inlet while the "top" of the container is the product extraction point. Note that while the adsorber bed 26 undergoes steps 1-6, the adsorber bed 28 undergoes steps 7-12.

TABLE 3. SINGLE STAGE CYCLE Description of step time pressure step pressure initial seconds final (kg / cm2 abs) * Step # 1 2.0 8.0 14.5 Feeding with increasing pressure with equalization in overlap Step # 2 2.0 14.5 19.0 Feeding with increasing pressure with pressurization of prod. in overlap Step # 3 2.0 19.0 22.0 Feeding with increasing pressure Step # 4 2.0 22.0 23.0 Constant feed pressure with product making Step # 5 4.0 23.0 23.0 Feeding at constant pressure with prod. and purge Step # 6 2.0 23.0 19.0 Equalization at decreasing pressure half cycle - Step # 7 2.0 19.0 13.0 Declining pressure evacuation with overlap matching Steps # 8 & # 9 & # 10 6.0 13.0 7.0 Declining pressure evacuation Step # 1 1 4.0 7.0 7.0 Constant pressure evacuation with oxygen purge with oxygen purge Step # 12 2.0 7.0 8.0 Increased pressure evacuation with overlap equalization Subsequently, additional details are given for each of the steps listed in Table 3 and illustrated in Figure 3.

Step # 1 Increasing pressure feed with overlap equalization: This step initiates the period of feed air pressurization. The air is fed at the bottom of the adsorber bed 26 (for example) of the compressor 18 and via the conduit 22. The pressure rises rapidly within the adsorber bed 26 from about 0.563 kg / cm2 abs to about 1.02 kg / cm2 abs. The step lasts two seconds. The oxygen-rich equalization gas is introduced simultaneously to the upper part of the adsorber bed 26 from the adsorber bed 28 during this step.

Step # 2 Feeding at increasing pressure with pressurization of product with overlap: This step continues the period of pressurization of the supply air. The air is fed to the bottom of the adsorber bed 26 from the compressor 18. The pressure continues to rise during this step from 1.02 kg / cm2 abs to approximately 1.337 kg / cm2 abs, step 2 lasts two seconds. Oxygen repressurization gas taken from the product matching tank 60 is simultaneously introduced to the upper part of the adsorber bed 26 during this step.

Step # 3 Feeding at increasing pressure: Feeding air is introduced only to the adsorber bed 26 and the upper part of the container is closed. The pressure increases from 1,337 to approximately 1,548 kg / cm2 abs during this two-second step. Feed air is supplied by the compressor 18 during this step.

Steps # 4 - # 5 Constant pressure feed with product make-up: Feeding air is introduced to the bottom of the adsorber bed 26 while removing oxygen product from the top. The pressure remains relatively constant during this period of six seconds at 1,548 - 1,618 kg / cm2 abs. The feed air is supplied by the compressor 18. The oxygen product is supplied to the equalizing oxygen tank 60 as well as to the adsorber bed 28 as an oxygen purge during Step 5. The purity of the oxygen product remains relatively constant (90). % &during the product making steps.

Equalization at decreasing pressure: The residual oxygen product in the upper part of the adsorber bed 26 is extracted during this step from the upper part of the container. There is no flow from the bottom of the adsorber bed 26. The pressure of the container drops from 1,618 to approximately 1,337 kg / cm 2 abs during this two-second step. The compressor 18 is vented during this step.

Step # 7 Declining pressure evacuation with overlap equalization: Valve 38 is opened and waste nitrogen is removed from the bottom of adsorber bed 26 through vacuum pump 44. The pressure drops from 1.337 to about 0.915 kg / cm2 abs during this two second step. The oxygen concentration begins at about the purity of air and falls rapidly. The equalization drop step continues as the oxygen rich gas is removed from the top of the adsorber bed 26.

Steps # 8-10 Declining pressure evacuation: Waste nitrogen is removed from the bottom of the adsorber bed 26 through the vacuum pump 44. The pressure drops from 0.915 to approximately 0.492 kg / cm2 abs during this period of 6 seconds. The end of the upper part of the absorber bed 26 is closed during this step. The concentration of oxygen in the waste gas reaches its minimum at the end of step 10.

Step # 1 1 Evacuation at constant pressure with oxygen purge: The minimum evacuation pressure is reached and oxygen purge is introduced to the top of the adsorber 26. The pressure remains constant during this step from 4 seconds to 0.492 kg / cm2 abs due to the equalization of the purge flow with the evacuation flow.

Step # 12 Increasing Pressure Evacuation with Overlap Equalization: Vacuum pump 44 continues to remove waste gas from the bottom of adsorber bed 26 while oxygen equalizing gas is added to the top of it. The pressure rises during this step because the equalization oxygen flow is larger than the evacuation flow. The pressure rises from 0.492 to approximately 0.563 kg / cm2 abs during the two second step. The cycle described above is illustrative only, and the essential aspects of the invention can be practiced using other adsorption cycles.

Radial Bed Configuration The VPSA method of the invention is particularly suitable for use with a radial bed structure due to the low ratios for use with a radial bed structure due to the low pressure ratios that are employed. The radial bed configuration is shown in Figure 3. Feeding and waste gas is supplied through a conduit 80 to a radial flow distribution assembly 82 where the inlet gas flows to the external walls of the container 84. The gas , which is now evenly distributed in the lower head 86, is supplied to the adsorption bed 88 via external vertical flow paths 90, which flow upward through straight or tapered flow passages. The gas then flows through the adsorber bed 88 in a radially inward manner. The gas leaving the product end of the adsorber bed 88 is collected within the vertical flow paths 92 and flows downward. The product gas is collected in a conical collection assembly 94 at the bottom of the container 84. The product gas collected leaves the container through conduit 96, contained within the supply conduit 80. Alternatively, the conduit 96 may be oriented such that the product gas is withdrawn at the top of the container in Figure 3. The flow distribution of the container is critical to the successful operation of a VPSA process and a major contributor to the distribution Flow is the channel pressure differential between the feed ends and the adsorber product. This pressure differential is a combination of frictional pressure losses and velocity changes in the head in the flowing gases. These two effects tend to cancel out when flow is entering a channel, and they are cumulative when a channel is flowing out. The degree of cancellation and accumulation is affected by the internal geometry of the camera, that is, through the design of tapered channels. All VPSA processes by nature reverse the gas flow direction periodically to achieve the subsequent adsorption and desorption process steps.

SUMMARY The main aspects of the invention are as follows: 1) low pressure rebound combined with a high pore diffusivity adsorbent; 2) low pressure ratio combined with a high pore diffusivity adsorbent, such that BSF < 227 kg TPDO and total energy < 7.5 kW / TPDO, more preferably such that BSF < 136 kg / TPDO and total energy <7.0 kW / TPDO; 3) shallow beds combined with 1) above, followed by shallow beds and shorter cycles combined with 1) above, and with increasing preferred combinations of bed depth / cycle time < 1.2 m / < 40s, < 0.9 m / < 30s and < 0.6 m / < 20s 4) single stage vacuum device combined with 1) above; 5) radial flow adsorber vessel combined with 1) or with 4) above; 6) a pressure drop across the adsorbent bed is preferred that does not exceed 0.105 kg / cm2 in desorption and adsorption, and a pressure drop through the adsorbent bed that does not exceed 0.070 kg / cm2 in desorption and adsorption , is most preferred in combination with 1) above; 7) Custom particle size for intrinsic diffusivity to maximize the mass transfer rate without undergoing significant increase in pressure drop combined with 1) above, the most preferred range of particle size is for an average particle diameter from about 0.8 mm to about 1.6 mm; 8) adsorbents with increased capacity and / or selectivity combined with 1) above. The fundamental objective of the invention is to reduce the product cost by combining adsorbents with high intrinsic diffusivity with low pressure ratios in PSA processes. The invention is directed to processes of adsorption separation based on equilibrium with mass transport dominated by pore intra-particle diffusion. Although the examples have been directed to the separation of air using a single main adsorbent, the invention is not limited to binary mixtures, neither to air as a feed nor to a single main adsorbent. When more than a simple separation is to be achieved, it is feasible to include one or more adsorbents as main adsorbents - each adsorbent responsible for a different separation or a different level of the same separation. Thus, the properties (particularly those related to the adsorption regime) of the different adsorbent materials in the main adsorbent zone are selected to maximize all the required separations of the process. Examples of such processes include recovery of H2 from H2 / CO / CO2 / CH4 mixtures, removal of H2O and CO2 from air, separation of Ar from air or N2 or O2, drying of process streams and recovery of CO of the flue gases or H2 of the tail gas of the PSA. The invention can be applied to adsorption processes intended to recover either the light or heavy product or both. In all cases, the pore diffusivities of the key adsorbates are to be minimized. The benefits of the low pressure regime are likely to be the largest in bulk separations in which the heavy component is also the main component of the feed gas, particularly in processes incorporating vacuum desorption. Proper matching of the desorption pressures with suction pressures from the existing vacuum equipment provides a means to simplify and reduce the cost of the equipment, v. g. , reducing the number of compression stages, eliminating post-cooling, etc. Air separation using selective adsorbents for N2 is a primary example where the low pressure ratio represents a substantial potential reduction in energy consumption. Adsorbents of type X zeolite are suggested for air separation, most preferably highly exchanged LiX as described by Chao (cited above). Zeolites type X, type A and occurring in nature containing monovalent, multivalent or mixed cations are also applicable to the present invention when properly produced to achieve high intrinsic pore diffusivity. It should also be clear that the present invention can be practiced with several deployments of adsorbents in the main adsorbent zone, v. g. , layers and mixtures of adsorbents of various types or of the same type but with varying adsorption and / or physical characteristics. For example, the pressure / high diffusivity regime concepts of this invention can be applied to the layer beds suggested by Ackley in co-pending U.S. Patent Application Serial No. 837,411 (Attorney docket D-20347). Although the invention has been described for adsorbent zones consisting of a fixed bed of adsorbent particles, the invention can be practiced using alternative adsorbent deployment configurations, v. g. , monoliths, agglomerates of adsorbent dispersed in a fibrous substrate, etc. Additional improvements can be achieved by the appropriate selection of adsorbent particle size in conjunction with the intrinsic diffusivity of the adsorbent and by combining them in a low pressure regime cycle. For air separation and for the ranges of preferred pressure regime and intrinsic diffusivities of N2, the preferred average particle size (diameter) is between 0.8 mm and 1.6 mm. The latter issue can be better referenced using radial flow, where the flexibility to select flow area, bed depth and flow distribution is the greatest with respect to controlling the pressure drop to acceptable levels. Radial flow adsorbers provide an inherent restriction of the adsorbent such that fluidization can be prevented when using small particles. This invention is generally applied to a full range of cycle steps and process conditions, v. g., temperatures, pressures, feed speed, etc. Similarly, their concepts can be applied to single-bed processes as well as multiple-bed processes that operate with sub-atmospheric (VSA), transatmospheric cycles. { VPSA) or superatmospheric (PSA). The concepts described herein are not limited to any particular adsorber configuration and can be effectively applied to adsorbers of axial tether, radial flow, lateral flow, etc. The adsorbent may be restricted or unrestricted within the adsorber vessel. It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications may be conceived by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to encompass all such alternatives, modifications and variations that fall within the scope of the appended claims.

Claims (24)

1. REVIVAL NAMES 1. An oscillating pressure adsorption method comprising repeats of a N-step cycle, said method adapted to separate components of a gas mixture in at least a first component and a second component by selective adsorption of said first component in a bed of adsorbent, said method comprising the steps of: a) during the adsorption steps of a cycle, raising a pressure of a feed of said gas to said bed at an adsorption pressure to allow adsorption of said first component by said adsorbent, said adsorbent exhibiting an intrinsic diffusivity for said first component that is equal to or greater than 3.5 x 10"6 m2 / sec; b) during the desorption steps of said cycle, depressurize said bed at a desorption pressure to desorb said first component of said said adsorbent, a pressure ratio of said adsorption pressure to said desorption pressure that falls within a range of less than about 5.0.
2. 2. The oscillating pressure adsorption method as recited in claim 1, wherein said pressure ratio falls within a range of about 3.5 to 5.
3. 3. The oscillating pressure adsorption method as recited in claim 1 , wherein said intrinsic diffusivity of said adsorbent for said first component is equal to or greater than 4.0 x 10 ~ 6 m2 / sec and said pressure ratio falls in a range of about 1.5 to 3.5.
4. 4. The oscillating pressure adsorption method as recited in claim 3, wherein a lower pressure during a cycle is within 0.422 to 0.633 kg / cm2 abs.
5. 5. The oscillating pressure adsorption method as recited in claim 3, wherein said gas is air, said first component is nitrogen and said second component is oxygen.
6. 6. The oscillating pressure adsorption method as recited in claim 1, wherein said adsorbent is particulate and has an average particle diameter between 0.8 mm and about 1.6 mm.
7. 7. The oscillating pressure adsorption method as recited in claim 1, wherein said adsorbent is a type X zeolite with a ratio of SIO2 / AI2O3 less than or equal to 2.5 and exchanged with Li (> 70%).
8. 8. The oscillating pressure adsorption method as recited in claim 1, wherein said adsorbent bed has a dimension, in a flow direction of said gas through said adsorbent bed, of less than about 1. 2 m and said method performs steps a) and b) in a time of less than about 40 seconds.
9. 9. The oscillating pressure adsorption method as recited in claim 1, wherein said adsorbent bed has a dimension, in a flow direction of said gas through said adsorbent bed, of less than about 0.9 m and method performs steps a) and b) in a time of less than about 30 seconds.
10. The oscillating pressure adsorption method as recited in claim 1, wherein said adsorbent bed has a dimension, in a flow direction of said gas through said adsorbent bed, of less than about 0.6 m and method performs steps a) and b) in a time of less than about 20 seconds.
11. 1 1. An oscillating pressure adsorption system for performing a gas separation method comprising repetitions of a N-step cycle, said separation method for separating components of a gas mixture in at least a first component and a second component by selective adsorption of said first component in a bed of adsorbent particles, said system comprising: an -absorbent that is selective for said first component, said adsorbent exhibiting an intrinsic diffusivity for said first component that is equal to or greater than 3.5 x 10"6 m2 / sec, said system that exhibits both: a reduction in the bed size factor (BSF) and specific energy consumption in relation to an oscillating pressure adsorption system that incorporates lower intrinsic diffusivity adsorbents; control to control an adsorption pressure ratio of desorption pressure through said bed within a range of less than about 5.0.
12. 12. The system as mentioned in claim 1, wherein said control means controls said ratio to fall within a range of more than about 3.5 to 5.
13. The system as mentioned in claim 1, wherein said means The control system controls a pressure drop through said bed during said method so as not to exceed approximately 0.07 kg / cm2 during desorption and during adsorption.
14. The system as recited in claim 1, wherein said control means controls a pressure drop across said bed during said method so as not to exceed approximately 0.105 kg / cm2 during desorption and during adsorption.
15. The system as recited in claim 1, wherein said intrinsic diffusivity of said adsorbent particles for said first component is equal to or greater than 4.5 x 10 * e m2 / sec and said control means controls said relationship to fall within from a preferred range of approximately 1.5 to 3.5.
16. The system as recited in claim 1, wherein said bed is arranged in a radial annular configuration, and said gas mixture exhibits a flow pattern that is transverse to said annular configuration.
17. The system as recited in claim 16, further comprising: a single pass vacuum pump coupled to said bed to extract an adsorbed component from said bed during a desorption phase of said gas separation method.
18. 18. The system as recited in claim 1, wherein said gas is air, said first component is nitrogen and said second component is oxygen.
19. 19. The system as recited in claim 18, wherein the system exhibits a bed size factor < 227 kg / TPDO and a specific energy consumption < 7.5 kW / TPDO.
20. The system as recited in claim 1, wherein said adsorbent is particulate and has an average particle diameter between 0.8 mm and about 1.6 mm. twenty-one .
21. The oscillating pressure adsorption method as recited in claim 1, wherein said adsorbent is a type X zeolite with an SiO2 / AI2? 3 ratio less than or equal to 2.5 and exchanged with Li (> 70%) .
22. 22. The system as recited in claim 1, wherein said adsorbent bed has a dimension, in a flow direction of said gas through said adsorbent bed, of less than about 1.2 m and said medium The control system controls said system to carry out the steps of adsorption and desorption in a time of less than about 40 seconds.
23. 23. The system as recited in claim 1, wherein said adsorbent bed has a dimension, in a flow direction of said gas through said adsorbent bed, of less than about 0.9 m and said control means controls said system for performing the adsorption and desorption steps in a time of less than about 30 seconds.
24. 24. The system as recited in claim 1, wherein said adsorbent bed has a dimension, in a flow direction of said gas through said adsorbent bed, of less than about 0.6 m and said control means controls said system. to perform the adsorption and desorption steps in a time of less than about 20 seconds.