Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2000-08-28
2002-06-25
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S114000, C095S119000, C095S120000, C095S129000, C095S139000, C095S143000, C095S144000, C095S902000, C096S130000, C096S132000, C096S144000, C096S154000
Reexamination Certificate
active
06409800
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 binderless zeolitic composite containing 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 modern 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 thermal 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).
Procedures for producing alkali or alkaline earth ion-exchanged zeolite A-zeolite X composites are disclosed in U.S. Pat Nos. 4,913,850, 5,075,084 5,908,823 and 5,962,358. U.S. Pat. No. 5,908,823 discloses directly synthesizing a composite zeolitic composition containing both zeolite A and zeolite X , and U.S. Pat. Nos. 4,913,850, 5,075,084 and 5,962,358 disclose agglomerating zeolite X or a mixed zeolite X-zeolite-type A composition with an SiO
2
binder and converting the binder to zeolite type A by contact with sodium aluminate.
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 comprising about 5 to about 95% by weight zeolite A and about 95 to about 5% by weight zeolite X, and wherein at least part of the exchangeable cations of the zeolite A and at least part of the exchangeable cations of the zeolite X are divalent cations.
In a preferred aspect of the apparatus embodiment of the invention, the composite zeolitic adsorbent is made by a process which includes the step of contacting an agglomerate of at least one of zeolite A or zeolite X and an inert binder comprising silica, clay, alumina or combinations thereof with a reagent which effects the conversion of at least part of the inert binder to the other of zeolite A or zeolite X.
In another preferred aspect of the apparatus embodiment, the composite zeolitic adsorbent is made by a process which includes the step of contacting an agglomerate comprising zeolite X and an inert binder comprising silica, clay, alumina or combinations thereof with a reagent which effects the conversion of at least part of the inert binder to zeolite A. In a more preferred aspect, at least 95% of the inert binder is converted to zeolite A.
In another preferred aspect of the apparatus embodiment, the inert binder comprises silica and the reagent comprises sodium aluminate, sodium hydroxide, potassium hydroxide or mixtures thereof. In another preferred aspect, the inert binder is silica-alumina.
In another preferred aspect of the apparatus embodiment, the inert binder comprises clay and the reagent comprises sodium hydroxide, potassium hydroxide or mixtures thereof. In a more preferred aspect, the clay comprises kaolin, metakaolin, kaolinite, nacrite, dickite, halloysite or combinations thereof. Most preferably, the clay comprises kaolin.
In another preferred aspect of the apparatus embodiment, the inert binder comprises alumina and the reagent comprises sodium silicate and sodium hydroxide, potassium hydroxide or mixtures thereof.
In another preferred aspect of the apparatus embodiment, a
Bülow Martin
Fitch Frank R.
Ojo Adeola F.
Neida Philip H. Von
Pace Salvatore P.
Spitzer Robert H.
The BOC Group Inc.
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