Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2000-09-25
2002-11-12
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S103000, C095S130000, C095S902000, C423S710000, C423S718000, C502S079000
Reexamination Certificate
active
06478854
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a low silica X-type zeolite binderless shaped product which is a shaped product with a low SiO
2
/Al
2
O
3
molar ratio and a low X-type zeolite binder content. More particularly, it relates to a high purity, low silica X-type zeolite binderless shaped product which has a high purity and notably high adsorption capacity, which has excellent mechanical strength, and which is suitable for purposes such as separation and concentration of oxygen by adsorption separation from mixed gases, for example, gases composed mainly of nitrogen and oxygen, as well as to a gas separation method employing it. The gas separation method of the invention is, specifically, a gas separation method based on Pressure Swing Adsorption (hereunder abbreviated to PSA), and gases that may be separated and recovered thereby include oxygen gas, nitrogen gas, carbon dioxide gas, hydrogen gas and carbon monoxide gas.
Of these, oxygen gas is one of the particularly important industrial gases, which is widely used especially for ironworks and pulp bleaching. Recently, oxygen enriched combustion is being accomplished in the field of waste combustion and glass melting for the purpose of reducing NO
x
emissions that are unavoidable with combustion in air, and therefore oxygen gas is increasing in importance from the standpoint of environmental problems as well.
Known industrial production processes for oxygen gas include the PSA method, the cryogenic separation method, the membrane separation method, etc., but use of the PSA method is increasing because of its advantages in terms of oxygen gas purity and cost.
Oxygen gas production by the PSA method involves selective adsorption of nitrogen gas in the air onto an adsorbent, extraction of the remaining concentrated oxygen gas and collection thereof as the product. The adsorbent used for this purpose is crystalline zeolite which has a large nitrogen adsorption capacity, and particularly X-type zeolite which has a large porous capacity in the crystals is most widely used as the adsorbent for air separation by the PSA method.
Production of nitrogen gas is also possible by utilizing the selectively adsorbed nitrogen gas.
2. Description of the Related Art
X-type zeolite, like Y-type zeolite, is synthetic zeolite in which the crystalline structure is a faujasite structure; such crystals with a relatively low SiO
2
/Al
2
O
3
molar ratio, i.e. an SiO
2
/Al
2
O
3
molar ratio of 3.0 or lower, are referred to as X-type zeolite. The SiO
2
/Al
2
O
3
molar ratio of synthesized X-type zeolite is generally 2.5, but if NaOH and KOH are added during synthesis it is possible to reduce the SiO
2
/Al
2
O
3
molar ratio to 2.0. Reducing the SiO
2
/Al
2
O
3
molar ratio of zeolite increases the number of aluminum atoms in the crystals, and therefore the number of exchangeable cations increases. Adsorption of molecules of nitrogen and oxygen onto zeolite is generally known as physical adsorption, and a larger number of exchangeable cations offers a greater adsorption capacity.
Hereunder, X-type zeolite with a SiO
2
/Al
2
O
3
molar ratio of lower than 2.5, for example, X-type zeolite with a SiO
2
/Al
2
O
3
molar ratio of from 1.9 to 2.1 inclusive, will be referred to as “low silica X-type zeolite”. Processes for production of low silica X-type zeolite are described in Japanese Unexamined Patent Publications (Kokai) (JP-A-53-8400, JP-A-61-222919, JP-A-01-56112, JP-A-10-310422, JP-A-11-217212, and elsewhere).
For industrial use of X-type zeolite as an adsorbent, clay or the like is usually added as a binder to synthesized X-type zeolite powder, and the mixture shaped into pellets or beads. The amount of clay added is about 20-30 parts, and the adsorption capacity of the shaped zeolite decreases by the amount of binder added with respect to the adsorption capacity of the zeolite powder. In order to overcome this, there have been proposed to date production processes for binderless shaped products, which are shaped with almost no binder. Such low silica X-type zeolite shaped products are described in Japanese Unexamined Patent Publications (JP-A-61-222919, JP-A-5-163015, JP-A-11-076810 and elsewhere).
Japanese Unexamined Patent Publication JP-A-61-222919 describes a process for production of a low silica X-type zeolite shaped product, called a macroscopic monolithic body of self-bonding zeolite, whereby no low silica X-type zeolite powder is used, but rather a shaped product of a kaolin starting material is transformed to metakaolin and then crystallized. According to this process, obtaining low silica X-type zeolite requires adding a large amount of a pore-forming substance (organic) to the shaped kaolin, heating and burning to make a porous metakaolin shaped product, and then crystallizing it.
However, because this process is accompanied by a very large exotherm due to burning of the organic substance, the temperature control is troublesome and it is a very difficult matter to successfully control the pores of the shaped product; moreover, since the pores must be actively formed, this creates the problems of notably impaired crush resistance and attrition resistance of the resulting low silica X-type zeolite shaped product. It is also inadequate in terms of the purity of the low silica X-type zeolite during shaping, and for example, A-type zeolite impurities are sometimes included during shaping, resulting in a low concentration of low silica X-type zeolite.
Conventional low silica X-type zeolite has peak intensities at index 111, 220, 331, 533, 642 and 751+555 in the following order.
Intensity rank
Index
1
111
2
533
3
751 + 555
4
642
5
220
6
331
The macroscopic monolithic body of self-bonding zeolite according to this patent has the same peak intensities in the following order.
Intensity rank
Index
1
111
2
751 + 555
3
642
4
533
5
331
6
220
In Japanese Unexamined Patent Publication JP-A-5-163015 there is described a process for production of a low silica X-type zeolite binderless shaped product 25 wherein a shaped product comprising X-type zeolite powder with an SiO
2
2/Al203 molar ratio smaller than 2.5, kaolin clay transformed to metakaolin, sodium hydroxide and potassium hydroxide, is kept in an aqueous solution of sodium hydroxide and potassium hydroxide at a temperature of 40-100° C. for a few hours to a few days for aging and crystallization.
This process requires admixture of dangerous caustic chemicals during the mixing, kneading and shaping, and workability is poor, while the low silica X-type zeolite binderless shaped product obtained by the process naturally has low strength.
Japanese Unexamined Patent Publication JP-A-11-076810 also describes a low silica X-type zeolite shaped product of which at least 95% has an SiO
2
/Al
2
O
3
molar ratio of 2. The production process is a process in which a mixture obtained by aggregation of low silica X-type zeolite powder with a binder comprising at least 80% clay classified as kaolinite, halloysite, nacrite or dickite which is transformable to zeolite and 15% of montmorillonite as another clay, is shaped and dried and then calcinated at a temperature of 500-700° C., after which the resulting product is contacted for a few hours at 95° C. with at least a 0.5 molar concentration of a caustic solution, which is a solution of sodium hydroxide and potassium hydroxide, wherein the maximum potassium hydroxide content with respect to the total of sodium hydroxide+potassium hydroxide is 30 mole percent, and specifically with the caustic solution at 5.5 moles/liter. The low silica X-type zeolite binderless shaped product obtained by this process has, unsurprisingly, very low crush resistance and attrition resistance and includes A-type zeolite; moreover, since the SiO
2
/Al
2
O
3
molar ratio of the total based on chemical analysis or the SiO
2
/Al
2
O
3
molar ratio of the crystal lattice based on Si-NMR is higher than the theoretically ideal value of 2.0 for low silica X-type zeolite, and particularly the SiO
2
/Al
2
O
3
molar ratio o
Harada Atsushi
Hirano Shigeru
Kotagiri Yasuki
Nakamoto Yoshiyuki
Yoshida Satoshi
Nixon & Vanderhye P.C.
Spitzer Robert H.
Tosoh Corporation
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