Adsorbents and adsorptive separation process

Details Adsorbents and adsorptive separation process Adsorbents and adsorptive separation process

1999-10-26

2002-02-26

Spitzer, Robert H. (Department: 1724)

Gas separation: processes

Solid sorption

Including reduction of pressure

C095S129000, C095S130000, C095S139000, C095S140000, C095S902000, C096S108000

Reexamination Certificate

active

06350298

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to novel adsorbent compositions and their use, and more particularly, to certain molecular sieve materials such as cation-exchanged zeolites having metal oxides within their pore system. The invention also relates to a method for separating gases such as the separation of nitrogen from air for oxygen production using the novel adsorbent compositions.
BACKGROUND OF THE INVENTION
The separation of nitrogen from other gases, such as oxygen and argon, is of considerable industrial importance. If the separation is conducted on a large scale, fractional distillation is often employed. Distillation is quite expensive, however, because of the large initial capital cost of equipment and the considerable energy requirement involved. In recent times other separation methods have been investigated in efforts to reduce the overall cost of such separations.
An alternative to distillation that has been used to separate nitrogen from other gases is adsorption. For example, sodium X zeolite, described in U.S. Pat. No. 2,882,244 to Milton, has been used with some success for the adsorptive separation of nitrogen from oxygen. A disadvantage of the use of sodium X zeolite for the separation of nitrogen from oxygen is that it has low separation efficiency for nitrogen separation. While this adsorbent strongly adsorbs nitrogen, it also adsorbs substantial amounts of oxygen.
Later research efforts led to the development of adsorbents having considerably improved nitrogen adsorption properties. According to U.S. Pat. No. 3,140,933, type X zeolite in which some of the sodium cations of the zeolite are replaced by lithium cations can be effectively used to separate nitrogen from oxygen at temperatures up to 30° C.
U.S. Pat. No. 4,859,217 discloses that very good adsorptive separation of nitrogen from oxygen can be obtained at temperatures of 15 to 70° C. using a type X zeolite which has more than 88% of its cations present as lithium cations, particularly when a zeolite having an Si/Al atomic ratio of 1 to 1.25 is used.
U.S. Pat. No. 5,179,979 asserts that lithium/alkaline earth metal X zeolites having lithium/alkaline earth metal molar ratios in the range of about 95:5 to 50:50 have thermal stabilities greater than the corresponding pure lithium zeolites and good adsorption capacities and selectivities.
U.S. Pat. No. 5,152,813 discloses the adsorption of nitrogen from gas mixtures using X-zeolites having a zeolitic Si/Al ratio <1.5 and at least binary cation exchange of the exchangeable cation content with between 5 and 95% lithium and between 5 and 95% of a second cation selected from calcium, strontium and mixtures of these, the sum of the lithium cations and second exchangeable cations being at least 60%.
Lithium-exchanged natural mordenite is reported to be a good adsorbent for oxygen pressure swing adsorption (PSA) in papers by H. Minato and M. Watanabe, published in Scientific Paper, University of Tokyo, (1978), 28, 218, and S. Fukuyama and K. Sato in the Journal of Physical Chemistry (1982), 86, 2498-2503.
U.S. Pat. No. 4,925,460 discloses a process for the separation of gas mixtures, where the components differ in heats of adsorption, e.g. nitrogen from air utilizing chabazite (Si/Al ratio 2.1 to 2.8) in which more than 65% of the cations have been exchanged with lithium cations. This patent also discloses a process for the preparation of the novel adsorbent.
U.S. Pat. No. 5,464,467 discloses the preparation of lithium- and trivalent cation-exchanged type X zeolites that are useful for the adsorptive separation of nitrogen from other gases.
Although the above-described zeolites, particularly the lithium-exchanged type X zeolites, have excellent nitrogen adsorption properties, there is still a need in improving separation selectivity by increasing nitrogen sorption and/or decreasing oxygen sorption at given process conditions. Extensive efforts are continuously made to make adsorption-based air separation processes more competitive with cryogenic distillation methods. This invention makes strides in this direction by presenting novel adsorbents which enhance the separation factor of nitrogen-selective adsorbents, with regard to air separation. Further, these novel adsorbent compositions can be used in other gas separation processes.
SUMMARY OF THE INVENTION
According to a first broad embodiment, the invention comprises a composition comprising a molecular sieve material containing more than 50% of the total exchangeable cations being at least one cation selected from the group Li, Ca, Ag, Cu and mixtures thereof and having within its pore system at least one compound which is a metal oxide selected from Li
2
O, Ag
2
O, CuO, Cu
2
O, CaO, MgO, SrO, ZnO, CdO, B
2
O
3
, Al
2
O
3
, Ga
2
O
3
, La
2
O
3
, Ce
2
O
3
, TiO
2
, ZrO
2
, V
2
O
5
, MnO MnO
2
, FeO, Fe
2
O
3
, NiO, a precursor of these metal oxides and mixtures thereof.
In another embodiment of this invention, a method is provided for separating gases by removing a first gas selected from carbon dioxide, carbon monoxide, dinitrogen oxide and nitrogen from a gas stream comprised of one or more of said first gas and other gases with weaker specific interaction with the adsorbent material than said first gas, said method comprising passing said gas stream through at least one adsorption zone containing the adsorbent compositions of this invention, thereby producing an enriched component of one or more of said first gas.
In another embodiment, the invention comprises a method of producing oxygen from an oxygen containing gas mixture comprising passing the gas mixture through at least one adsorption zone containing one of the compositions identified in the first broad embodiment of the invention, thereby yielding an oxygen-depleted adsorbed component and an oxygen-enriched nonadsorbed gas component. In a preferred aspect of this embodiment, the method includes, in addition to the step of passing the gas mixture through the at least one adsorption zone, the step of desorbing the oxygen-depleted adsorbed component from the at least one adsorption zone. Most preferably, the method comprising repeatedly passing the gas mixture through the at least one adsorption zone and desorbing the oxygen-depleted adsorbed component from the at least one adsorption zone as steps of a cyclic adsorption process selected from pressure swing adsorption, temperature swing adsorption or a combination of these.
DETAILED DESCRIPTION OF THE INVENTION
The novel adsorbent compositions of the invention are selected molecular sieve materials such as zeolites or mesopore-structured materials that have metal oxides or metal oxide precursors in their pore systems and having more than 50% of the exchangeable cations being certain cations as described below. As used herein, the term “zeolite” has its ordinary meaning, as defined in W. M. Meier, D. H. Olson, and Ch. Baerlocher, Atlas of Zeolite Structure Types, Elsevier, London, Boston, 1996, but in particular natural or synthetic crystalline aluminosilicates, and the term “mesopore-structured materials” means crystalline or amorphous metal oxides having regularly structured pore systems wherein the average size of the pores is in the range of about 1.5 to about 5 nanometers, for the purposes of this invention. The term “metal oxide precursor” as used herein means a compound which upon activation produces a stable oxide or oxide compound that undergoes no significant further modification upon further treatment in the presence of oxygen, such as activation and calcination.
Although the mechanism involved in the operation of the invention is not fully understood, it appears that the presence of the metal oxide or metal oxide precursor in combination with that of the lattice-charge compensating cations in the adsorbent enhances the ability of the adsorbent towards stronger expressed preferred adsorbability of nitrogen and other gas mixture components that show specific sorption interactions with the adsorbent material. In other words, the metal oxide or precursor appears to enhance the s

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