WO1996015842A1

WO1996015842A1 - Air separation process

Info

Publication number: WO1996015842A1
Application number: PCT/US1995/014758
Authority: WO
Inventors: Orhan Talu; Steven M. Kuznicki
Original assignee: Engelhard Corporation; The Advanced Manufacturing Center At Cleveland State University
Priority date: 1994-11-18
Filing date: 1995-11-14
Publication date: 1996-05-30
Also published as: UY24089A1; TR199501443A2; AU4108896A

Abstract

A process is disclosed for selectively adsorbing nitrogen from a less strongly adsorbed other gas component (preferably oxygen), comprising: contacting the gas mixture in an adsorption zone with an adsorbent comprising a large-pored molecular sieve containing at least one octahedral site comprising titanium and at least silicon as a tetrahedral site. The process is particularly useful in air separation using ETS-10 as the adsorbent.

Description

AIR SEPARATION PROCESS FIELD OF THE INVENTION

The present invention is directed to the separation of nitrogen and/or oxygen from gas streams, such as air. More specifically, the present invention is directed to improved adsorbents for effecting this separation using pressure/vacuum swing adsorption (P/VSA) processes with improved efficiency.

BACKGROUND OF THE PRIOR ART Separations of gas mixtures containing nitrogen and oxygen are important industrial processes. The recovery of oxygen and/or nitrogen from air is practiced on a large scale. In the past, the primary method used for this separation was cryogenic distillation. More recently, pressure/vacuum swing adsorption (P/VSA) processes are being used in applications which have smaller gas requirements. In P/VSA processes, compressed gas is fed through a bed containing an adsorbent material with a preference for one of the components of the gas to produce an exit stream enriched in the other components. A stream enriched in the adsorbed component can be obtained by desorption.

P/VSA processes for selectively adsorbing nitrogen from gas mixtures, such as air, comprise contacting the gas mixture in a zone containing an adsorbent which is selective for the adsorption of nitrogen. Typically, the zone is operated through a series of steps comprising: adsorption, during which the gas mixture contacts the adsorbent, nitrogen is selectively adsorbed and oxygen passes through the zone and can be recovered as product; depressurization, during which the gas mixture contact is discontinued and the zone is reduced in pressure to desorb the nitrogen which can be recovered as product; and repressurization with air or oxygen product to the adsorption pressure.

Although zeolitic molecular sieves have been widely mentioned as adsorbents, other materials have also been mentioned in the patent literature. Thus, U.S. 5,266,102 mentions many zeolites such as zeolites A, X, Y, chabazite and mordenite as well as non-zeolites such as silica-alumina, silica, titanium silicate and phosphates.

The use of zeolitic molecular sieves in PSA processes for air separation is well know. McRobbie in U. S. Pat. No. 3,140,931 claims the use of crystalline zeolitic molecular sieve material having apparent pore sizes of at least 4.6 Angstroms for separating oxygen-nitrogen mixtures at subambient temperatures. Of this group of zeolites, the Na form of X-zeolite (NaX) has often been used to advantage in air separation processes. There have been numerous efforts to develop improved adsorbent materials having high adsorptive capacity for N₂ and high selectivity of N₂ over 0₂. The Ca form of A-zeolite (CaA) , for instance, was the basis of the Batta U. S. Pat. No. 3,636,679 for producing 90 +% 0₂ from air via a PSA process. Later, Sircar and Zondio (U. S. Pat. No. 4,013,429) patented a VSA air separation process using Na-mordenite (NaMOR) . Coe et al in U. S. Pat. Nos. 4,481,081 and 4,544,378 demonstrated the improved performance of faujasite compositions containing divalent cations, such as CaX, provided that they were activated in such a way that a preponderance of the polyvalent cations were in the dehydrated/dehydroxylated state.

Formed adsorbent particles containing zeolites used for equilibrium air separation also typically contain about 20 wt.% inert inorganic material. The purpose of this material is to bind the zeolite crystallites into agglomerates having high physical strength and attrition resistance in order that the zeolite crystallites can be used in adsorption processing. Those skilled in the art have generally believed that the addition of binder reduces the adsorptive properties of the adsorbent zone. In the past, the trend has been to try to reduce the levels of binder from the typical 20% to as low as possible, often as low as 5%, while at the same time maintaining adequate mechanical properties. For example, Heinze in U. S. Pat. No. 3,356,450 states that it is advantageous to obtain hard formed zeolite particles with the lowest possible binder content to maintain high adsorption capacity. He claims the use of a process which starts with molecular sieve granules bound with silicic acid, which are then treated with aqueous solutions containing alumina and alkali metal hydroxide, whereby the binder is converted to molecular sieve particles. The result is a practically binder-free (and therefore high capacity) shaped material with good abrasion resistance.

At the extreme of this trend toward reduced binder contents is the development of processes for preparing binderless bodies. Flank et al (U. s. Pat. No. 4,818,508) teach the preparation of zeolites, particularly X, Y, and A, in massive bodies from calcined preforms made of controlled- particle-size kaolin-type clay. Kuznicki et al

(U. S. Pat. No. 4,603,040) teach the preparation of low silica X-zeolite (LSX) in the form of essentially binderless aggregates by reaction of calcined kaolin preforms in an aqueous solution of NaOH and KOH. . R. Grace & Co. in GB 1,567,856 teaches a process for converting an extruded mixture of metakaolin and sodium hydroxide to A-zeolite. The advantage stated is that the method does not require the use of a binder such as clay, which usually reduces the activity of the molecular sieve by 15-20%.

Recently, adsorbents produced using these binderless bodies have been stated to have superior adsorptive properties when used for air separation. One such adsorbent is CaLSX, prepared by Coe et al, using the process of Kuznicki et al (Coe et al, "Molecularly Engineered, High-Performance Adsorbent: Self-Bound Low-Silica X Zeolite" in Perspectives in Molecular Sieve Science; Flank, W. H. ; Whyte, Jr., T. E. , Eds.; ACS Symposium Series 368; American

Chemical Society: Washington, D.C., 1988; pp 478- 491) . "The self-bound LSX adsorbents do not have any binder to 'dilute' the active component and lower the gas capacity." In addition, Coe et al in U. S. Pat. No. 4,925,460 prepared chabazite from Y- zeolite extrudate. They state, "This method produces a superior adsorbent, since adsorptive capacity decreases as binder content increases." These materials were converted to the Li form and used for separation of air, among other gas separation processes. Thirdly, Chao in U. S. Pat. No. 4,859,217 claims a process for selectively adsorbing N₂ using X-zeolite having a framework Si/Al molar ratio not greater than 1.5 and having at least 88% of its A102 tetrahedral units associated with Li cations. He converted the bulk of the 20% binder in a zeolite "preform" agglomerate to X-zeolite crystals, obtaining essentially a binderless zeolite prior to ion exchanging into the Li form.

These more recent developments have shown increases in capacity compared to the intrinsic capacity (i.e., capacity of the unbound zeolite) of adsorbents in the prior art. It is noteworthy that even the very high capacity materials described in these more recent developments were prepared in the binderless form. Thus, the prior art teaches that continued increase in capacity is better, there apparently being no upper limit. The desire for higher nitrogen capacity materials is understandable because it lowers the capital investment for the zeolite and adsorbent vessel. Higher nitrogen capacity also decreases the losses of 0₂ in the voids of the bed, which is expected to increase recovery and thereby lower power requirements.

However, nitrogen capacity is not the only property of the adsorbent that is important for low cost 0₂ production by P/VSA processes. The selective or preferential adsorption of N₂ over 0₂ is also important, because any 0₂ which is coadsorbed on the adsorbent bed with N₂ during the adsorption step is lost during the subsequent desorption step(s) , resulting in lower 0₂ recovery. Selectivity (a) has conventionally been defined at a specific temperature and pressure in the following way:

where ^N _M2^=N2 coadsorbed at N₂ partial pressure in the feed

N₀₂«=O₂ coadsorbed at 0₂ partial pressure in the feed Y_H2 - mole fraction of N₂ in the feed Y_M =« mole fraction of 0₂ in the feed

The very high nitrogen capacities of these recently developed adsorbents have generally been accompanied by higher selectivities. The prior art has recognized the benefits of this higher selectivity. Chao (above) points out the advantages of the high selectivity of the LiX materials and Coe et al (above) point out the advantages of the high selectivity of CaLSX.

Selectivity and recovery impact power costs because they determine the amount of feed gas that must be compressed for the adsorption step per unit of product recovered. The cost of power is as important as the cost of capital in determining commercial viability of a PSA or VSA process. Thus, it is desirable to lower power consumption levels as much as possible.

Thus, despite the previously recited substantial advances in adsorbents for P/VSA air separation of the prior art, there still exists a genuine need for even more efficient air separation processes.

BRIEF SUMMARY OF THE INVENTION The present invention is directed to a process for selectively adsorbing nitrogen from a gas mixture containing nitrogen and at least one less strongly adsorbed other gas component

(preferably oxygen), comprising; contacting the gas mixture in an adsorption zone with an adsorbent comprising a large-pored molecular sieve containing at least one octahedral site comprising titanium and at least silicon as a tetrahedral site. By large- pored is meant a molecular sieve having a pore size of at least about 8 Angstrom units and such materials include ETS-10, disclosed in U. S. Pat. No. 4,853,202 and in U. S. Serial No. 08/248,040 filed May 24, 1994, ETAS-10, disclosed in U. S. Pat. No. 5,244,650, a large-pored molecular sieve containing at least one octahedral site comprising titanium and at least silicon as a tetrahedral site as disclosed and claimed in U. S. Patent No. 5,208,006. Of the above large-pored molecular sieves containing octahedral sites, ETS-10 is the most preferred.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 presents oxygen adsorption isotherms at various pressures for ETS-10 at 23*c, ETS-10 at 45*C, zeolite 5A at 20*C, and zeolite 10X at 0*C, plus it contains an estimation of oxygen adsorption of ETS-10 at O'C. Figure 2 represents the nitrogen adsorption isotherms at various pressures for the same adsorbents as in Figure 1 plus CaLSX at 30'C.

Figure 3 represents the isosteric heat of adsorption of oxygen on ETS-10, zeolite 10X and zeolite 5A.

Figure 4 represents the isosteric heat of adsorption of nitrogen on ETS-10, zeolite 10X and zeolite 5A.

Figure 5 presents oxygen and nitrogen binary adsorption isotherms at various pressures for ETS-10, various forms of 5A and X-type zeolites. The total amount of both oxygen and nitrogen is depicted.

Figure 6 depicts the amount of nitrogen adsorbed from an oxygen/nitrogen binary mixture containing 19.5 vol.% oxygen and 80.5 vol.% nitrogen at 23*C various pressures and isotherms are given for ETS-10, various forms of 5A, calcium X and various zeolites. Figure 7 depicts the oxygen adsorption from an oxygen/nitrogen binary mixture (19.5 vol.% 0₂ at 23*C) and is an isotherm at various pressures with the same zeolites given in Figure 6.

Figure 8 represents the nitrogen selectivity from a oxygen/nitrogen binary mixture (19.5 vol.% 0₂ at 23*C) and various isotherms are given for various zeolites.

In the above Figures and in the specification, the amount of oxygen and nitrogen absorbed was based on the weight of the adsorbent, the weight of the adsorbent being calculated on an anhydrous basis. Since ETS-10 is of higher density than common zeolite adsorbents including those shown in the Figures, compared advantages are even greater when viewed from a volumetric perspective. PREFERRED EMBODIMENTS OF THIS INVENTION

ETS zeolites having a pore size of at least about 8 Angstrom units used in the practice of this invention may be employed in the physical form amenable to the specific process which is being used. This includes fine powders, shaped articles, such as fluidizable microspheres, pellets, honeycombs, wherein composite supported on substrates such as paper.

This invention can be carried out by employing the same type of cyclic systems which are conveniently used in air separation processes. Such processes typically include pressure swing adsorption and pressure vacuum swing adsorption. Processes of this type are disclosed in many U. S. patents such as U. S. Pat. No. 5,266,106, U. S. Pat. No. 4,589,888, U. S. Pat. No. 4,544,378, U. S. Pat. No. 5,266,102, U. S. Pat. No. 4,329,158, U. S. Pat. No. 4,810,265 and U. S. Pat. No. 4,032,150, the entire disclosures of which are herein incorporated by reference.

Vacuum swing adsorption (VSA) is a process generally conducted with large-scale beds (typical bed size 500-2000 lbs. adsorbent/bed) and is generally used to generate nitrogen. In this process, a bed of zeolite which has been evacuated to 50-100 mm total pressure is reloaded to slightly greater than atmospheric pressure with air which has been pre-dried. Air continues to be pumped through the bed at slightly greater than 1 atmosphere until the interstitial gas between the crystals and between the agglomerates containing the crystals has been flushed and is of essentially the same composition as air. During this flushing, an oxygen enriched product is eluted from the bed. The bed contains zeolite in which the nitrogen concentration greatly exceeds that of air and the interstitial gas is air as mentioned. The total number of moles of gas adsorbed is much greater than that in the interstitial space, typically being 10-100 times as much. Once the bed has been flushed with air, it is reflushed with pure nitrogen, generally from a storage tank. This removes the oxygen both from the interstitial air and the small amount adsorbed in the zeolite crystals. The bed contains now essentially pure nitrogen (99-99.9%). The bed is now vacuumed to 50-100 mm total pressure, harvesting the majority of the nitrogen product and returning the bed to a proper state for the next separation cycle.

While VSA has been found to be an economical source of nitrogen generation, it is not generally considered a competitive source for oxygen generation and the oxygen enriched waste stream is generally discarded.

Adsorbents typically employed for VSA generally manifest high nitrogen capacities and selectivities at 1 atmosphere and include zeolites Ca-X, Ca-LSX and ordenite. ETS-10 type adsorbents seem least suited to this type of process at present because the bulk of their nitrogen capacity and selectivity comes at higher pressures. It is, however, possible that modification to increase the interaction energy between the sieve and nitrogen by exchanging highly charged cations such as Li+, Ca++ or Mg++ or transition metal cations such as Zn++ or Cu++ by incorporation of charged sites such as aluminum in ETAS-10 or a combination of these modifications may make them effective VSA agents. Pressure swing adsorption (PSA) is the other major adsorptive air separation process and is generally used to generate oxygen. While PSA generally employs a relatively small adsorbent bed (typically 20-200 lbs. adsorbent/bed) , there are many times as many PSA units as there are VSA units in operation. In a typical PSA cycle, an unpressurized bed of dry zeolite is loaded to approximately 3 atmospheres pressure with air. The air has been pre-dried and most of the adiabatic heat of compression has been removed by an in-line heat exchanger or other such cooling device. Since the adsorbent employed is nitrogen selective, a wave of highly oxygen enriched air is generated in the end of the bed opposite the air inlet. This oxygen enriched product (typically 90-95% 0₂) is bled off at high pressure. Eventually, the gas at this outlet reaches the composition of air. At this point, the bed pressure is dropped to essentially atmospheric and since the preponderance of gas adsorbed on the bed is nitrogen, this depressurization releases a nitrogen enriched waste gas stream. Since the gas remaining in the sieve at one atmosphere is also substantially enriched in nitrogen, a portion of the oxygen enriched product is flowed over the bed at near atmospheric pressure so that the state of the adsorbent is such that it is ready for the next cycle and not contaminated by excess nitrogen. This bed flushing and other problems inherent to current PSA cycles reduce 0₂ recovered from feed air to the range of 55-60%.

Adsorbents typically employed for PSA generally manifest high nitrogen capacities at 3-4 atmospheres and usable selectively (nominally taken to be greater than or equal to 2.0) at that pressure. It is the decline of nitrogen selectivity with pressure which limits the upper pressure regimes of PSA. Ca-A is far and away the most commonly employed zeolite for PSA oxygen generation. As-synthesized ETS-10 seems reasonably well suited for this type of application. This may also be true for potassium versions and perhaps mixed calcium/hydrogen materials where calcium is the dominant species. Other possibilities include large cations such as barium or transition metals such as Zn++ or Cu++ in combination with hydrogen where the metal cation is the preponderant species.

The novel process of this invention can be carried out by (1) comprising air to a pressure greater than about 1 to about 50 atmospheres, preferably 2-20 atmospheres, (2) introducing said compressed air into a bed containing a molecular seive having a pore size of at least about 8 Angstrom units and containing at least one octahedral site comprising titanium and at least silicon as a tetrahedral site so as to preferentially adsorb nitrogen and obtain an oxygen enriched stream and (3) desorbing the nitrogen contained in the bed by reducing the pressure to about 0.05 to about 10 atmospheres, preferably 0.1 to 2 atmospheres and then repeating steps 1-3.

If desired, nitrogen at a pressure of at or slightly lower than the absorbent bed can be used to sweep said absorbent bed before describing the same.

Because of the unique properties of the novel adsorbents of this invention, various modifications can be made to the typical way that pressure swing adsorption processes are carried out. First of all, the pressure swing adsorption can be carried out at even higher pressures that have been typically used in commercial operations. Thus, for example, pressures of about 5-20 atmospheres are particularly advantageous. Secondly, because of the extraordinary selectivity of the molecular sieve adsorbents which are used in the instant process, the use of a purge gas in order to remove the adsorbed nitrogen is not necessary. Quite obviously, this can be used in order to obtain even a more enhanced efficiency.

The above divergences from typical PSA processes can be made because the selectivity of ETS-10 for nitrogen from air does not decline with rising pressure as it does for all other zeolite adsorbents. In fact, the selectivity rises. This indicates that ETS-10 type adsorbents may be better suited for higher pressure cycles than classical zeolites. Such ultra-high pressure (UHPSA) cycles are envisioned as follows: Air is dried and compressed to a pressure in excess of about 3-50 atmospheres. The dry compressed air may then run through a heat exchanger or other cooling device to remove the heat of adiabatic compression, thereby reducing its temperature to about 50' C. or less. This temperature is referred to as near ambient. This stream is then used to pressurize a bed of an ETS-10 type adsorbent to 3-50 atmospheres, preferably, 5-20 atmospheres pressure. An oxygen enriched product is bled off at high pressure from the bed until the oxygen concentration falls to at or below a desired level. The bed pressure is then preferably lowered to approximately 1-2 atmosphere, the eluted gas being a nitrogen enriched waste stream. Because of the declining N₂ selectivity and modest N₂ capacity at 1 atmosphere, the bed may be swept with air (not 0₂) or perhaps not swept at all, and is ready for the next pressurization cycle without wasting product oxygen, perhaps even saving the time of a sweep step. Using this UHPSA type cycle, oxygen recovery can exceed 70% and may even approach or exceed 90% of inlet oxygen.

No classical zeolite adsorbents are appropriate for such UHPSA cycles. ETS-10 type materials which appear appropriate for such cycles include hydrogen exchanged, hydrogen/sodium, hydrogen/potassium and mixtures of hydrogen with multi-valent cations such as calcium, strontium, barium or transition metal cations such as Cu++ and Zn++ where the hydrogen ions predominate.

Finally, the novel air separation process of this invention can be operated with the same conditions which are typically used in commercial pressure or vacuum swing operations. It is to be understood that although this invention has been described in connection with its preferred embodiment, i.e., air separation, it is applicable to gaseous mixtures other than nitrogen and oxygen. The use of the instant adsorbents would allow separation of other gaseous components. This may impact many commercial separation processes including the purification of natural gas or hydrogen streams.

_________ In all of the examples which follow, a sample of as-synthesized ETS-10 powder is used. The

ETS-10 typically has the properties set forth below:

Physical Properties of ETS-10

Elemental Analysis - Wt.% of Oxides (Volatile Free Basis.

Si0₂ 61.1

Ti0₂ 17.4

Na0₂ 10.0

K₂0 4.8 A1₂0_ 0.29

Si/Ti 5.0

(Na+K)/Ti 1.9

Crystal Properties Framework Density - about 2.5 g/cc. Effective Pore Size - -8 Angstrom units.

EXAMPLE 1 ETS-10 is activated under vacuum at 325*C on a Cahn-1000 microbalance system. After cooling, isotherms were collected for oxygen at two different temperatures, namely 23'C and 45*C. The results are shown in Fig. 1 along with isotherms for zeolites 5A and 10X taken from the literature.

As can be seen, ETS-10 adsorbs about 75% more oxygen than 5A around room temperature throughout the investigative pressure range.

Extrapolation of the ETS-10 data indicates that oxygen adsorption on ETS-10 is almost 100% more than 10X at O'C.

EXAMPLE 2

The procedure of Example l was repeated with the exception that the adsorption of nitrogen was determined and the results are shown in Fig. 2.

ETS-10 nitrogen adsorption is about 35% higher than zeolite 5A around room temperature.

Again by extrapolation, ETS-10 absorbs 83% more nitrogen than 10X at O'C.

Fig. 3 also includes literature data for CaLSX in addition to 5A and 10X.

The nitrogen isotherm on CaLSX rises very sharply, its adsorption at 30*C is about 54% higher than ETS-10 at 23*C around atmospheric pressure. Given the shape of the isotherms, ETS-10 nitrogen adsorption is expected to catch up with CaLSX at about 3 atmospheres of pressure.

In attempting to evaluate the isotherms of Figs. 1 and 2, two additional points must be kept in mind.

1. ETS-10 has a higher crystal density than most conventional zeolites and

2. Sometimes, zeolites adsorption capacity can differ from that reported in the open literature.

With the above precautions, it can be stated that:

1. Oxygen and nitrogen adsorption on ETS- 10 is higher than A zeolites by about 67% and 35%. 2. Oxygen and nitrogen capacity of ETS-10 is at least comparable to X zeolites at higher pressures and X zeolites adsorb more than ETS-10 at atmospheric pressure, due to the shape of the isotherms.

Isosteric Heat of Adsorption The heat of adsorption has obvious importance in any separation process. Heat released during adsorption lowers the isotherms resulting in lower loading than isothermal operation at the same pressure. In addition, the variation of isosteric heat of adsorption with surface coverage carries important information about the primary molecular interactions in the surface phase (O. Talu et al, AIChE J., V. 33, p. 510 (1987)).

The isosteric heat of oxygen calculated from the isotherms is shown in Fig. 3. The limiting heat of adsorption at 0 loading indicative of a solids affinity for guest molecules is slightly higher for ETS-10 than for 5A and 10X. The difference is considered to be insignificant given the uncertainties involved in calculating the heat of adsorption from isotherm data.

More importantly, the isosteric heat does not vary appreciably during the data range indicating a homogeneous or near-homogeneous surface with respect to oxygen. The isosteric heat of nitrogen is shown in Fig. 4. The limiting heat of adsorption on 5A is about 15% higher than ETS-10. It is considerably higher on 10X by about 44%. Combined with a higher nitrogen loading on 10X, the temperature increase with 10X would be much higher than with ETS-10 during adsorption.

Additionally, the nitrogen isosteric heat on ETS-10 stays almost constant with increasing loading indicating a "near-homogeneous" system. The heat of adsorption on other zeolites, especially on X zeolites drops sharply with coverage indicating a highly heterogeneous system. Analysis of the isosteric heat of adsorption leads to the following: 1. The interaction of the oxygen molecule with the ETS-10 surface is similar to other zeolites typical of a non-polar molecule.

2. The interaction of nitrogen molecules with ETS-10 surface is very different than other zeolites with lower limiting heat at 0 loading and almost no variation with increasing coverage.

EXAMPLE 3

This example uses a binary mixture containing about 20% oxygen and 80% nitrogen by volume. This example was carried out by a cyclic saturation volumetric system wherein samples are saturated with a predetermined mixture of oxygen and nitrogen at a predetermined pressure and analyzed for adsorbed gas content following this equilibration. Isotherms were collected at 23*C and the isotherms are presented as Fig. 5.

As can be seen from Fig. 5, adsorption on ETS-10 and on 5A zeolites is similar in character with the ETS-10 being about 38% higher than 5A. Limited data at atmospheric pressure for X zeolites are also shown in this figure. Adsorption on ETS-10 is lower than X zeolites, especially lower than CaLSX by about 43% at atmospheric pressure. On the other hand, ETS-10 adsorption capacity is expected to reach and exceed that of X zeolites at higher pressures as can be deduced from the sharp bend in the nitrogen isotherm of CaLSX in Fig. 2. Furthermore, the higher density of ETS-10 must be considered while interpreting Fig. 5 which shows the capacities on a per weight basis.

The larger adsorption capacity at low pressures by X zeolites compared to ETS-10 is primarily due to the adsorption of nitrogen from the gas mixture. The trends in Fig. 6 for the partial nitrogen amount adsorbed are very similar to the total amount shown in Fig. 5. The unique feature of air separation on ETS-10 is clearly displayed in Fig. 7 for the amount of oxygen adsorbed when in competition with nitrogen. Oxygen adsorption from the binary mixture is higher on ETS-10 at low pressure and it levels off sharply at about 2 atmospheres reaching a plateau. The oxygen adsorption on 5A and X zeolites steadily increases with pressure crossing the ETS curve at about 2.2 or 1.6 atmospheres total pressure respectively.

Nitrogen Selectivity of ETS-10 Nitrogen selectivity is observed for all zeolites which are capable of adsorbing air. This fact forms the basis for air separation processes. The value of selectivity and its variation with other properties is one of the major differences which defines the utililty of various types of zeolite adsorbents.

The nitrogen selectivity decreases with increasing surface coverage (or pressure) for all aluminosilicate zeolites as displayed for X- and A- zeolites in Fig. 8. Although there is no rigorous thermodynamic explanation of this commonly observed behavior, it is mechanistically explained in reference to surface-molecule interactions. The high-energy/high-selectivity sites in heterogeneous porous structures are preferentially occupied at low loading. As the pressure (and loading) increases, more of the low-energy/low-selectivity locations are filled, resulting in a decrease in the overall selectivity of the system. The nitrogen selectivity of ETS-10 increases with increasing pressure which is extremely unusual and fundamentally different from the common behavior of all other zeolites. ETS-10 data shown in Fig. 8 is the first observation of an increasing selectivity with pressure reported on any adsorbent. One mechanistic explanation is based on the "near-homogeneous" nature of the ETS-10 surface although it is microporous with comparable porosity to aluminosilicates. Regardless of the mechanisms responsible for this unusual behavior, this discovery is very significant for air separation processes. Advantages of ETS-10 in Air Separation Processes Almost all adsorption processes, including air separation, are cyclic in nature. That is, the adsorbent is regenerated for reuse within a cycle. Often the regeneration step dictates the feasibility; thus the economics of a process. Most air separation units utilize a "pressure swing adsorption" (PSA) cycle. By some counts, there are more than 400,000 PSA air separation units worldwide with widely ranging production capacities. There are many commercial PSA cycles operating under different conditions. All PSA cycles include two main steps to complete the cycle, adsorption and desorption. Typically, PSA cycles include:

1. Feed/adsorption step where high pressure air is passed through the column. Oxygen is produced since nitrogen is preferentially adsorbed in the solid and retained in the column. 2. Blowdown/purge step at a lower pressure where most of the nitrogen is removed and the solid is regenerated. Part of adsorbed nitrogen is released during blowdown due to the decrease in pressure. The nitrogen can be recovered as a secondary product. Much of the remainder of nitrogen is desorbed during purge by passing part of the oxygen product which results in a waste stream containing both nitrogen and oxygen. In commercial processes, numerous other steps are involved to tailor the process for a specific product, or for a specific solid, or to increase energy efficiency, etc. Regardless of these intermediate steps, the impact of ETS-10 in air separation can be readily deduced from its equilibrium characteristics. The oxygen/nitrogen equilibrium on ETS-10 is fundamentally different than on other zeolites during the adsorption and desorption steps which form the heart of a PSA cycle. The key feature of ETS-10 is the variation in selectivity with pressure.

1. At the high pressures of the adsorption step where nitrogen is retained in the column, ETS-10 shows its highest nitrogen selectivity while all other zeolites show their lowest. Therefore, columns packed with ETS-10 can produce either a higher purity oxygen, or a larger amount of same purity during the feed step.

2. At the low pressure desorption step where the nitrogen must be removed to regenerate the solid, ETS-10 nitrogen selectivity is lowest while it is highest with other zeolites. It is easier to regenerate the ETS-10 packed columns. This results in a higher purity secondary product during blowdown as well.

3. Because of the unusual nitrogen selectivity for ETS-10, a purge gas is not necessary and, if used, a smaller proportion of the oxygen product is wasted in the purge step effluent.

Claims

WHAT IS CLAIMED IS :

1. A cyclic process of separating nitrogen and oxygen from air which comprises the following steps: (1) compressing air to a pressure of greater then about 1 to about 50 atmospheres;

(2) introducing said compressed air into a bed containing a large-pored molecular sieve having a pore size of at least about

8 Angstrom units and containing at least one octahedral site comprising titanium and at least silicon as a tetrahedral site so as to preferentially adsorb nitrogen and obtain an oxygen-enriched stream;

(3) desorbing the nitrogen contained in the bed by reducing the pressure to about 0.05 to about 10 atmospheres; and (4) repeating steps 1-3.

2. The process of claim 1 wherein said molecular sieve is ETS-10.

3. The process of claim 1 wherein the pressure employed in step 1 ranges from about 2 to 20 atmospheres and the desorption pressure in step 3 ranges from 0.1 to 2.0 atmospheres.

4. The process of claim 3 wherein desorption is carried out at essentially atmospheric pressure.

5. The process of claim 2 wherein a purge gas is used to remove the nitrogen adsorbed on the zeolite ETS-10.

6. The process of claim 2 wherein nitrogen desorption is accomplished simply by the reduction of pressure and no purge gas is used.

7. The process of claim 5 wherein nitrogen at a pressure of at least the pressure of the absorbent bed is used to sweep the said absorbent bed before desorbing the same.

8. The process of claim 7 wherein desorbed nitrogen is recovered.

9. A cyclic process of separating nitrogen and oxygen from air which comprises the following steps

(1) compressing air to a pressure in excess of 3 to about 50 atmospheres; (2) cooling said compressed air to near ambient temperature;

(3) introducing said cooled compressed air into a bed containing ETS-10 so as to preferentially adsorb nitrogen and obtain an oxygen-enriched stream by purging said bed with air of a pressure slightly greater that the pressure of said bed;

(4) desorbing the nitrogen containing in the bed by reducing the pressure to about 1-2 atmospheres; and (5) repeating steps

1-4.

10. The process of claim 9 wherein the purging of said bed in step 3 is continued until the oxygen concentration falls at or below a desired level.