Catalyst – solid sorbent – or support therefor: product or process – Zeolite or clay – including gallium analogs – Faujasite type
Reexamination Certificate
2001-06-28
2004-08-24
Dunn, Tom (Department: 1725)
Catalyst, solid sorbent, or support therefor: product or process
Zeolite or clay, including gallium analogs
Faujasite type
C502S064000, C502S075000, C502S060000
Reexamination Certificate
active
06780806
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process and adsorbents for selective adsorption of a gas component, and particularly, selective adsorption of nitrogen.
BACKGROUND OF THE INVENTION
The separation of air for the production of nitrogen and oxygen is a very important operation in the chemical processing industry. Historically, this separation has been done predominately by cryogenic distillation; though, as adsorption systems have become more efficient and new, more effective sorbents have been synthesized, separation by adsorption processes (e.g., pressure swing adsorption (PSA), and vacuum swing adsorption (VSA)) have become increasingly competitive and are already favorable for small-to-medium scale operations. Currently, approximately 20% of air separations are accomplished using adsorption technologies.
Since their introduction in the late 1950's, synthetic zeolites have been used in numerous applications such as catalysis, ion exchange, drying, and separation by selective adsorption. In the separation of air, zeolites of type A and X have typically been used. (See U.S. Pat. No. 5,551,257, Jain). The A and X type zeolites are composed of silica and alumina tetrahedra which are joined together to form the truncated octahedral or sodalite structure. These sodalite units are connected with tertiary units to form the structured zeolite unit cell. While the SiO
2
groups are electroneutral, the A10
2
groups are not, and thus introduce a negative charge to the structure which is offset by the presence of a charge compensating, non-framework cation (e.g., Na
+
, Li
+
, Ca
2+
). Type X zeolites contain between 77 and 96 Al per unit cell. The unit cell, including cation sites, for the X zeolite is shown in FIG.
1
.
The extra-framework cations in the zeolite are largely responsible for the nitrogen selectivity of these materials. These zeolites adsorb nitrogen preferentially to oxygen (usually at a ratio of about 4:1) due primarily a to the interactions between the charge compensating cations of the zeolite and the quadruple moment of the adsorbing gas (N
2
or O
2
). The quadruple moment of N
2
is approximately four times that of 02. Because the extra-framework cations so significantly influence the adsorption properties of the zeolites, numerous attempts have been made to optimize these properties by (1) increasing the number of cation sites (the cation exchange capacity, CEC) by creating zeolites with high aluminum content, and (2) by synthesizing zeolites containing various alkaline, alkaline earth, and combinations of these cations.
Low silica X-type zeolite (LSX) is known. This material is an aluminum saturated X-type zeolite with a silica-to-alumina ratio of 2.0 (or Si/Al=1.0). Commercial X-zeolite, which is typically available as the Na
+
form (known commercially as 13X), is not aluminum saturated and contains 86 aluminum atoms per unit cell, while the low silica X zeolite contains 96 aluminum atoms per unit cell.
Li
+
is among the strongest cations, with respect to its interaction with N
2
, its use was greatly increased with two recent advances. First, it was found that Li
+
ion-exchange in X-type zeolite must exceed an approximate 70% threshold before the Li
+
has any affect on the adsorption properties of the material (U.S. Pat. No. 4,859,217, Chao). Second, a significant increase in the N
2
adsorption capacity was seen in Li
+
ion exchanged low silica X-type zeolites over that of the typical commercial zeolites (Si/Al=1.25). Because of these advances, Li—X (Si/Al=1.0) is now the best sorbent in industrial use for separation of air by adsorption processes (U.S. Pat. No. 5,268,023, Kirner; U.S. Pat. No. 5,554,208, Mullhaupt).
Sicar et al., U.S. Pat. No. 4,557,736 and Coe et al., U.S. Pat. No. 4,481,018 reported the use of a binary exchanged X-zeolite having lithium and calcium and/or lithium and strontium ions in a ratio of 5% to 50% calcium and/or strontium and 50% to 95% lithium. This zeolite provided for enhanced nitrogen adsorption over those of the Na—X, Li—X and Ca—X zeolites. They also reported the use of mixed ion-exchanged A and X zeolites with lithium and an alkaline earth metal (e.g., Ca
2+
, Sr
2+
). In this case the zeolite contained lithium and the alkaline earth cations in a mixture of 10% to 70% alkaline earth and 30% to 90% lithium. These mixed cation zeolites provide good adsorption capacity and good thermal stability. However, the cost of separation still remains high. Therefore, there remains the need for improved methods and adsorbents to effectively and economically separate nitrogen from a gaseous mixture.
SUMMARY OF THE INVENTION
The invention provides new methods for separating nitrogen from a mixture. The invention provides adsorbents specifically for accomplishing. nitrogen separation. The adsorbents and separation methods are particularly useful for the selective adsorption of nitrogen from air. In one aspect, the adsorbent comprises an ion exchange zeolite X and preferably zeolite LSX (low silica zeolite X). The zeolite is most preferably a lithium-based zeolite. Further, the zeolite has exchangeable cationic sites, with silver cation or copper cation occupying at least some of the exchangeable cationic sites. The presence of the silver cation or copper cation at any of the sites will provide an improvement over the non-exchanged zeolite. Therefore, the minimum amount of silver cation or copper cation is greater than zero. The inclusion of silver cation and/or copper cation at the exchangeable cationic sites provides such an improvement in strength of adsorption of nitrogen, that any amount is helpful. However, consideration is given to the strength of such adsorbent capacity when optimizing the amount, in view of subsequent desorption. Since Ag
+
and Cu
+
strongly hold nitrogen, it is desirable that the amount of such cation be up to about 20% of the exchangeable cationic sites. It is preferred that the silver or copper cation occupy about 10% of the exchangeable cationic sites. Such optimization leads to a good balance between strength of adsorption and facilitating subsequent desorption. Therefore, it is evident that not all of the ion exchangeable cationic sites of the zeolite will contain copper or silver and preferably less than half of such sites will be so exchanged.
Zeolites are known and have been used as adsorbents due to their selectivity. Crystalline zeolite Y, zeolite A, and zeolite X are examples and are described in U.S. Pat. Nos. 3,130,007; 2,882,243; 3,992,471; and 2,882,244; each of which is incorporated by reference in its entirety. Type 5A zeolite, and type 13X zeolite are described for nitrogen adsorption in U.S. Pat. No. 5,551,257, also incorporated herein by reference in its entirety. Low silica X zeolite (LSX) having Si/Al ratio less than or equal to 1.25, desirably less than or equal to 1.2, and preferably about 1, is described in U.S. Pat. No. 5,268,023. Each of the aforementioned patents is incorporated herein by reference in its entirety. Consistent with the features described in these patents, zeolite characteristics are also described in the reference book entitled “Gas Separation by Adsorption Processes” by R. T. Yang (1987 Butterworth Publishers). To the extent that zeolite characteristics are pertinent to the present invention, they will be described further hereinbelow.
In the practice of the invention, the important characteristic desired is imparted by the presence of silver and/or copper cation in a zeolite which has been previously exchanged to provide a lithium X zeolite or a lithium LSX zeolite. The desirable X zeolite has a silicon to aluminum ratio (Si/Al) of about 1 to about 1.3. The more desirable lithium LSX has the preferred silicon to aluminum ratio of 1.0. Therefore, the adsorbents of the invention are essentially silver or copper ion exchanged Li
+
zeolites. The presence of the silver cation or the presence of the copper cation in combination with the lithium cation provides t
Hutson Nick D.
Yang Ralph T.
Dierker & Associates P.C.
Dunn Tom
Ildebrando Christina
The Regents of the University of Michigan
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