Nitrogen adsorbent and use thereof

Details Nitrogen adsorbent and use thereof Nitrogen adsorbent and use thereof

2000-06-14

2002-07-23

Spitzer, Robert H. (Department: 1724)

Gas separation: processes

Solid sorption

Inorganic gas or liquid particle sorbed

C095S096000, C095S902000, C502S079000

Reexamination Certificate

active

06423121

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a nitrogen adsorbent which selectively adsorbs nitrogen from a nitrogen-containing gas such as air, and a method of using the same.
BACKGROUND ARTS
A pressure swing adsorption method (PSA method) is one of methods for selective adsorption and separation of nitrogen from a nitrogen-containing gas. This PSA method uses a nitrogen adsorbent such as a zeolite, to obtain oxygen by adsorbing and separating nitrogen from air, for example. The PSA method is conducted by repeating a cyclic pressure change, in other words selectively adsorbing nitrogen by use of a nitrogen adsorbent under a high pressure and then returning to a low pressure to thereby release nitrogen from the nitrogen adsorbent and to regenerate the nitrogen adsorbent.
The zeolite used in the PSA method has cations associated in a crystal thereof and the cations exert an electrostatic attraction on a gas molecule (nitrogen) to be adsorbed. Thus, the zeolite has a property to adsorb more highly polar molecules. By using these properties and changing the cations associated therewith, zeolites with a good adsorption performance have been found. U.S. Pat. No. 3,140,932, for example, discloses an X-zeolite having one cation of Ba, Sr and Ni and exhibiting excellent nitrogen adsorption. U.S. Pat. No. 3,140,933 also discloses a technique concerning nitrogen adsorption which uses a zeolite having an apparent pore size of not less than 4 Å and containing Li
+
as a cation. Further, TOKKOHEI (Japanese published examined patent application) 5-25527 discloses a technique concerning nitrogen adsorption which uses a zeolite having an SiO
2
/Al
2
O
3
ratio of from 2.0 to 2.5 and not less than 88% of Li
+
cations associated. In TOKKOHEI 7-57300, a zeolite is disclosed which has an improved adsorption performance by mixing divalent cations in addition to a Li cation.
However, the aforementioned conventional zeolites require an extremely high association ratio of cations such as Li
+
in order to ensure a high adsorption performance.
Therefore, the zeolites require many cations such as Li
+
for ion-exchange, causing a problem of a high production cost. Further, an X-zeolite has an extremely high affinity with water and once it adsorbs, even a very small amount of adsorbed water remarkably deteriorates its performance, so that activation by dehydration is required. Such activation generally needs a high temperature of not less than 400° C. Thus, zeolites inferior in heat resistance such as an Al-rich zeolite and a zeolite having an Li
+
cation of a high charge density, are difficult to handle in activation, and in the worst case their performances are deteriorated.
In view of the foregoing, it is an object of the present invention to provide a nitrogen adsorbent exhibiting an improved heat-resistance and an improved nitrogen separation factor with a less production cost and a method of using it.
DISCLOSURE OF THE INVENTION
In accordance with an aspect of the present invention in achieving the aforementioned object, provided is a nitrogen adsorbent comprising a crystalline X-zeolite having a faujasite structure with an SiO
2
/Al
2
O
3
ratio of less than 3.0, wherein the crystal contains at least one trivalent element of the group consisting of Fe, B and Ga and has (AlO
4
)
5−
tetrahedral units thereof associated with cations.
Further, in accordance with another aspect of the present invention, provided is a method of using a nitrogen adsorbent wherein the nitrogen adsorbent as described above adsorbs nitrogen after being heated under a vacuum.
The nitrogen-selective adsorption performance of a zeolite has the property of adsorbing more highly polar molecules since the cations associated in a zeolite crystal, as mentioned above, exert an electrostatic attraction on a gas molecule (nitrogen) to be adsorbed. Particularly, the X-zeolite is one kind of zeolite having large pores and the smallest SiO
2
/AlO
2
O
3
ratio. Thus, the X-zeolite has an appropriate space to adsorb gas molecules and a large number of cation sites.
The inventors of the present invention have found that the X-zeolite containing at least one trivalent element selected from the group consisting of Fe, B and Ga, and (AlO
4
)
5−
tetrahedral units associated with cations in the crystal thereof, has a larger number of cation sites contributing to adsorption and a high nitrogen adsorption performance, such as nitrogen-oxygen separation performance. In addition, the zeolite containing at least one trivalent element selected from the group consisting of Fe, B and Ga may have improved heat-resistance. Particularly, the zeolite containing Fe as the trivalent element adsorbs, in terms of adsorption amount of nitrogen and oxygen, much less oxygen than nitrogen. With such a property, during nitrogen-oxygen separation in a PSA method, it is found that an excellent separation performance is exhibited and the oxygen generation ratio becomes extremely high. Further, although some materials have an excellent performance only in nitrogen adsorption, the nitrogen adsorbent of the present invention has an excellent separation factor, and so far there is no other material exhibiting such an excellent performance. Thus, with the zeolite having a high separation factor, the efficiency unit in a PSA method (amount of electricity per one unit of oxygen generation volume) can remarkably be improved and thus oxygen can be generated by less energy in comparison with a conventional one.
In accordance with the present invention, when Li is used as a cation, an excellent nitrogen adsorption performance is exhibited. In other words, among the cations contributing to adsorption, Li
+
has an ion radius of 0.60 Å, which is the smallest among alkali metals. Therefore, Li
+
has the highest charge density among monovalent cations and a strong interaction with a polar substance to thereby attract cations and to create an electrostatic field by its bonding balance. Such an electrostatic field attracts and selectively adsorbs molecules with quadrupole moment such as nitrogen, resulting in the zeolite exhibiting an excellent nitrogen adsorption performance.
Moreover, the inventors have found that when the nitrogen adsorbent of the present invention has not less than 60% to less than 88% of its (AlO
4
)
5−
tetrahedral units associated with Li
+
cations, an excellent adsorption performance is exhibited. Accordingly, the nitrogen adsorbent of the present invention can obtain the excellent adsorption performance even at a relatively lower cation association ratio, thereby decreasing production costs.
Furthermore, the inventors have found that adsorption performance is improved by heating the adsorbent as described above under a vacuum and then adsorbing nitrogen. It has been found that a relatively high temperature, which is not less than 400° C. to not greater than 600° C., is suitable for heating under a vacuum.
BEST MODE FOR CARRYING OUT THE INVENTION
Embodiments of the present invention will next be described in detail.
The nitrogen adsorbent of the present invention selectively adsorbs and thereby separates nitrogen from a nitrogen-containing gas. The nitrogen-containing gas intended is typically air. In this case, nitrogen is adsorbed and separated from air for generating oxygen. The adsorbent also may be used for separating nitrogen from gas mixtures of nitrogen with oxygen, argon, helium, neon, hydrogen and the like as well as from air.
The nitrogen adsorbent of the present invention comprises a crystalline X zeolite. The X zeolite has a faujasite structure which has an SiO
2
/Al
2
O
3
ratio of less than 3.0. The upper limit for the SiO
2
/Al
2
O
3
ratio is preferably not greater than 2.5, more preferably 2.0.
The nitrogen adsorbent has at least one trivalent element of the group consisting of Fe, B and Ga in the crystal thereof. It is considered that, due to the presence of these trivalent elements in the crystal, the adsorbent exhibits a high separation performance in obtainin

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