Gas separation: processes – Solid sorption – Inorganic gas or liquid particle sorbed
Reexamination Certificate
2000-08-28
2002-07-09
Simmons, David A. (Department: 1724)
Gas separation: processes
Solid sorption
Inorganic gas or liquid particle sorbed
C095S126000, C095S143000, C096S132000, C423S239200
Reexamination Certificate
active
06416569
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the purification of gases, and more particularly to the decrease of impurity levels of nitrogen oxides and low molecular weight hydrocarbons in air. Specifically, the invention relates to the simultaneous removal of nitrous oxide and C
2
-C
5
hydrocarbon gases from air by contacting the air with a zeolitic composite which, in its “as crystallized” form, contains both type A crystalline units and type X crystalline units.
BACKGROUND OF THE INVENTION
In cryogenic air separation units (ASUs), atmospheric air is liquefied at cryogenic temperatures and subsequently fractionally distilled into its major components, nitrogen, oxygen and argon. Since water vapor and carbon dioxide freeze at temperatures well above the temperature at which air is liquefied, these compounds must be removed from atmospheric air prior to its introduction to the ASUs to avoid clogging of ASU equipment lines by the accumulation of ice and frozen carbon dioxide in the heat exchange equipment used to chill the air to its liquefaction temperature. ASUs are commonly equipped with air prepurification units (PPUs) to remove water vapor and carbon dioxide from ASU feed air. In modem ASU plants, the PPUs contain one or more layers of adsorbent materials which selectively adsorb water vapor and/or carbon dioxide from air. Such PPUs are generally operated on either pressure swing adsorption (PSA) cycles or temperature swing adsorption (TSA) cycles. Adsorbents suitable for the removal of moisture from air include activated alumina, silica gel and sodium X zeolite, and those typically used for the removal of carbon dioxide from air include type X zeolites.
Atmospheric air also contains trace amounts of nitrogen oxides and low molecular weight hydrocarbons. Since the concentration of these impurities in atmospheric air is much lower than the concentrations of water vapor and carbon dioxide in the air, their presence in air was not considered to be a problem in cryogenic air separation operations. In recent years, however, the concentration of nitrogen oxides and gaseous hydrocarbons in atmospheric air has been steadily growing as the number and size of operating petroleum refineries and chemical process plants in the world increases. Furthermore, the increase in concentration of some of these impurities in air is accelerating because of their extremely long life in the atmosphere. The “lifetime” of nitrous oxide (N
2
O), for example, in the atmosphere is as long as 150 years. Because of the increasing demand for higher purity air separation products, and to avoid the creation of explosion or fire hazards in ASUs, it is now often considered highly desirable or necessary to also remove nitrogen oxide and hydrocarbon impurities from the feed air to ASUs.
Unfortunately, the above-mentioned adsorbents have little or no selectivity for nitrogen oxides and hydrocarbons, particularly in the presence of moisture and carbon dioxide. Consequently, they do not effectively remove these impurities from air. Furthermore, although some adsorbents selectively remove certain low molecular weight hydrocarbons from air, while other adsorbents preferentially adsorb nitrogen oxides and certain other low molecular weight hydrocarbons from air, no single adsorbent material is known to effectively remove both nitrogen oxides and all common low molecular weight hydrocarbons from air. For example, type A zeolites, such as cation-exchanged zeolite A and particularly calcium zeolite A, selectively remove some hydrocarbons from air, but they do not preferentially adsorb nitrogen oxides, while, on the other hand, divalent cation-exchanged type X zeolites, such as calcium X zeolite, readily adsorb nitrogen oxides from air, but do not remove all hydrocarbons from air.
It can be appreciated from the above, that if it is desired to have an air purification system remove substantially all low molecular weight hydrocarbons and nitrogen oxides from atmospheric air using currently practiced adsorption technology, it will be necessary to include multiple adsorbent layers in the purification system. If it is also desired to remove water vapor and carbon dioxide from the air, it may be necessary to additionally include in the system a layer of adsorbent to remove water vapor, and one to remove carbon dioxide.
Crystallization techniques for making various type X and type A zeolites are described in the patent and technical literature. Typical of such procedures are those described in U.S. Pat. Nos. 2,882,243, 2,882,244, 4,173,622, 4,303,629, 4,443,422, east German Patent 43,221 and British Pat. No. 1,580,928, and in Tatic, M. et al., in “Zeolites: Synthesis, Structure, Technology and Application”,
Studies in Surface Science and Catalysis
, vol. 24, pp. 129-136 (1985).
A procedure for producing alkali or alkaline earth cation-exchanged zeolite A-LSX for use as softeners in detergents is disclosed in U.S. Pat. No. 5,908,823.
Efforts to develop more efficient and less costly methods and equipment for removing all of the above-described impurities from atmospheric air prior to its introduction into an ASU are constantly sought. The present invention provides a method and PPU system which accomplish this goal.
SUMMARY OF THE INVENTION
According to a first broad embodiment, the invention comprises apparatus comprising:
(a) a vessel having a feed air inlet and a purified air outlet;
(b) a water vapor-selective adsorbent positioned within the vessel adjacent the air inlet; and
(c) a composite zeolitic adsorbent selective for at least one nitrogen oxide and at least one low molecular weight hydrocarbon positioned within the vessel between the water vapor-selective adsorbent and the purified air outlet, the composite zeolitic adsorbent, as synthesized, comprising about 5 to about 95% by weight zeolite type A and about 95 to about 5% by weight zeolite type X, and wherein at least part of the exchangeable cations of said zeolite A and at least part of the exchangeable cations of said zeolite X are divalent cations.
In a preferred aspect of the apparatus embodiment of the invention, the composite zeolitic adsorbent is prepared by the process comprising the steps:
(1) forming a uniform aqueous silica- and alumina-containing reaction mixture comprising sodium ions or both sodium and potassium ions, the concentrations of the components in the reaction mixture being such that the SiO
2
/Al
2
O
3
molar ratio is in the range of about 1.3 to about 3.5; the (Na
2
O+K
2
O)/SiO
2
molar ratio is in the range of about 0.25 to about 5.0, the K
2
O/(Na
2
O+K
2
O) molar ratio is in the range of about 0 to about 0.35 and the H
2
O/(Na
2
O+K
2
O) molar ratio is greater than about 10;
(2) subjecting the reaction mixture to a crystallization procedure at least part of which includes maintaining the reaction mixture at a temperature in the range of about 60 to about 100° C., thereby producing a composite zeolitic product; and
(3) at least partially exchanging the composite zeolitic product with divalent cations.
In another preferred aspect of the apparatus embodiment, about 50 to about 100% of the exchangeable cations of the zeolite type A and about 50 to about 100% of the exchangeable cations of the zeolite type X are calcium ions.
In another preferred aspect of the apparatus embodiment, at least 50% of the zeolite type X has a Si/Al atomic ratio in the range of about 0.9 to less than about 1.15. In a more preferred aspect, the zeolite type X has a Si/Al atomic ratio less than about 1.1.
In another preferred aspect of the apparatus embodiment, at least 90% of the exchangeable cations of the zeolite type A and at least 90% of the exchangeable cations of the zeolite type X are calcium ions.
In another preferred aspect of the apparatus embodiment, the water vapor-selective adsorbent comprises activated alumina, silica gel, zeolite sodium X or mixtures thereof.
In another preferred aspect, the apparatus further comprises a carbon dioxide-selective adsorbent positioned within the vessel between the water vapor-selective adsorbent
Brandt Alfons
Bülow Martin
Fitch Frank R.
Ojo Adeola F.
Unger Baldur
Lawrence Frank M.
Neida Philip H. Von
Pace Salvatore P.
Simmons David A.
The BOC Group Inc.
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