Catalyst – solid sorbent – or support therefor: product or process – Zeolite or clay – including gallium analogs – And additional al or si containing component
Reexamination Certificate
2000-09-28
2001-11-06
Dunn, Tom (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Zeolite or clay, including gallium analogs
And additional al or si containing component
C502S060000, C502S063000, C502S064000, C502S085000
Reexamination Certificate
active
06313059
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to the removal of water from refrigerant mixtures containing difluoromethane (R-32) and relates more particularly to the treatment of such mixtures employed as circulating refrigerant streams of refrigeration systems to sequester water as a means for avoiding freeze-ups and corrosion. The treatment comprises adsorption of these impurities on a zeolitic molecular sieve.
BACKGROUND OF THE INVENTION
In view of the now, well-established relationship between chlorofluorocarbons (CFC's) released into the atmosphere and the depletion of the earth's ozone layer, considerable attention is being directed to finding effective substitutes for these once widely used compounds. It appears that the worst offenders are the fully halogenated CFC's which contain chlorine. These compounds are relatively unreactive with other compounds in the lower atmosphere and thus are able to diffuse into the stratosphere intact and be decomposed by ultraviolet radiation to form inter alia, chlorine-free radicals which readily react with ozone. On the premise that it is the chlorine constituent of the CFC's which ultimately reacts with and destroys the ozone molecules and in the interest of approximating as closely as possible the physical properties of the CFC's already in use, the proposed substitutes in general have been HCFC's containing lesser proportions of chlorine or fluorocarbons containing no chlorine at all. For example, dichlorodifluoromethane, widely used under the trademark Freon 12 as a refrigerant in household refrigerators, in automotive units and in commercial freezers and display cases, has been replaced in many instances by 1,1,1,2-tetrafluoroethane (also known as R-134a) or by chlorodifluoromethane (also known as R-22 or HCFC-22). Because R-134a is not miscible with many commonly used lubricants, mixtures of R-134a and R-22 have been proposed for systems employing lubricants soluble in R-22. See U.S. Pat. No. 5,198,139 (Bierschenk et al.) in this regard. In the recent past, over 90 percent of the chlorodifluoromethane and about a third of the dichlorodifluoromethane manufactured was utilized in air-conditioning and refrigeration.
U.S. Pat. No. 3,536,521 (McKinney et al.) discloses a method of preventing the adsorption of gases other than water, such as fluorinated hydrocarbons, by the coating of Type A zeolite molecular sieves with silicones such as methyl silicone. McKinney et al. further discloses that the fluorinated hydrocarbons used in refrigerant systems react at active sites on the surface of the molecular sieve with subsequent decomposition into halogen acids which react with the basic structure of the molecular sieve.
U.S. Pat. No. 5,514,633 (Noguchi et al.) discloses a method for the production of a desiccant for HFC-32 and blends of HFC-32 which comprises immersing a formed 3A zeolite which has been 20 to 60% ion-exchanged with potassium in an aqueous solution of sodium silicate, potassium silicate, or mixtures thereof to deposit SiO
2
on the formed 3A zeolite. The resulting formed 3A zeolite with the SiO
2
coating is dehydrated and subsequently activated. U.S. Pat. No. 5,514,633 is hereby incorporated by reference.
U.S. Pat. No. 3,625,866 (Conde) discloses a process for preparing composite desiccant materials for applications such as refrigerant drying wherein the pores of the desiccant are less than about 4.9 angstroms in diameter to permit the inclusion of water molecules and to exclude the larger halogenated hydrocarbon molecules. Conde discloses the use of zeolite 3A which he describes as prepared by replacing at least 65 percent of the sodium cations in zeolite A with potassium cations by conventional cation exchange techniques to adsorb molecules having critical diameters up to 3 angstrom units. Conde hardens the zeolite A into an agglomerate optionally with clay, silicates or both and then applies to the surface of the agglomerate a thin coating of diaspore (hydrated alumina) and a clay mineral, wherein the diaspore is the major component. The coated agglomerate is then soaked in an aqueous solution of potassium silicate to impregnate silicate into the agglomerate, dried in air to avoid steaming during calcination, and finally fired to set the binder and activate the molecular sieve. The refrigerant decomposition tests were based on R-22. Conde discloses that clays which may be employed for bonding molecular sieves without substantially altering the adsorptive properties of the molecular sieve are attapulgite, kaolin, sepiolite, palygorskite, kaolinite, plastic ball clays, clays of the attapulgite or kaolin types, bentonite, montmorillonite, illite, chlorite, and bentonite-type clay.
U.S. Pat. No. 5,347,822 (Cannan et al.) discloses the use of a microporous zeolite molecular sieve having the crystal structure of zeolite B and a framework silica-to-alumina molar ratio of at least 2.5 for use in refrigeration systems containing R-32. Cannan et al. discloses that the modified form of zeolite B has pore openings small enough to significantly limit the adsorption of R-32 while retaining a large capacity for water adsorption.
With increasing recognition of the seriousness of atmospheric ozone depletion, stricter limitations on the future use of any chlorine-containing refrigerant continue to be imposed. One of the most suitable replacements for R-22 in stationary refrigeration systems is a non-flammable mixture of the HFC compound difluoromethane, also known as R-32, with other halocarbons or halohydrocarbons such as R-134a and R-125 (C
2
HF
5
). One such mixture known as R-410a has been proposed and consists of 50 percent R-32 and 50 percent R-125. Another proposed mixture (R-407c) consists of 23 percent R-32, 25 percent R-125, and 52 percent R-134a. A significant problem in making this substitution arises from the fact that R-32 is more readily adsorbed than R-22 with zeolite A, commonly employed as an adsorbent-desiccant in the circulating refrigerant stream to protect against freeze-ups and corrosion of the refrigeration unit. Ideally, a purified and dried refrigerant fluid, after having been sealed in a refrigeration unit, would continue to circulate through the compressors, Joule-Thompson nozzles, cooling coils, etc., for the life of the unit without causing any corrosion or freeze-up problems. In practice, however, the system can rarely be so thoroughly sealed or the components so thoroughly dried before sealing to prevent water and other contaminants from entering the sealed system. These contaminating materials must be removed or sequestered to avoid the development of the aforementioned problems. Conventionally, the contaminants are rendered innocuous by adsorption thereof on a suitable adsorbent which is inserted into the sealed system and which is in contact with the circulating refrigerant stream. In the case of halocarbon refrigerants, the contaminants of greatest concern, in addition to water, are attributable to the degradation products of the refrigerant molecules themselves. Halogen acids, notably HCl, can form and cause corrosion. In some instances, the adsorbent composition itself can be a reactant in the chemical reactions which result in the production of corrosive products. Zeolitic molecular sieves generally exhibit this property. Accordingly, in view of the physical and chemical properties of the refrigerant involved, it is necessary to select the particular zeolite adsorbent to minimize harmful reactions. Since essentially all of the active sites of a zeolite are reachable only by molecules which can enter the internal cavities of the crystal structure through its uniform pore system, it is advantageous to employ a zeolite whose pore openings admit water and other small impurity molecules and exclude molecules of the refrigerant. Thus, a commonly used adsorbent for refrigeration systems is a highly exchanged potassium cation form of zeolite A having pore diameters of about 3 angstroms. The effective pore diameters can be further reduced, to a slight degre
Cohen Alan P.
Hurst Jack E.
Lavin MaryEllen
LandOfFree
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