Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
1999-11-18
2001-10-30
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S106000, C095S120000, C095S126000, C095S139000, C095S902000
Reexamination Certificate
active
06309445
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the removal of carbon dioxide from gas streams, and more particularly to the prepurification of air by the removal of carbon dioxide from air prior to air separation.
BACKGROUND OF THE INVENTION
Gases that occur in nature or which are produced in industrial processes often contain carbon dioxide in small amounts. For example, atmospheric air generally contains about 350 parts per million (ppm) carbon dioxide. Because of certain process constraints or a particular end use that the gas is intended for, it may sometimes be desirable or necessary to remove the carbon dioxide from the gas. For example, air that is separated into various component products by cryogenic separation techniques (cryogenic air separation), such as cryogenic distillation or cryogenic adsorption, must be substantially free of both carbon dioxide and moisture, because these operations are carried out at temperatures below the freezing points of these compounds; consequently, if they are not removed they will freeze in and eventually clog the air separation process equipment.
Small amounts of carbon dioxide and moisture are removed from gas streams by various techniques, such as condensation, reversing heat exchange freezing and adsorption. A particularly preferred method is adsorption using an adsorbent which adsorbs carbon dioxide (and water vapor) more strongly than it adsorbs other components of the gas stream. For example, it is common to remove carbon dioxide from an air stream that is to be cryogenically separated, by passing the gas stream through a bed of zeolite 13X. U.S. Pat. No. 3,885,927, issued to Sherman et al. on May 27, 1975, discloses the use of type X zeolite containing at least 90 equivalent percent barium cations for the removal of carbon dioxide from gas streams containing not more than 1000 ppm carbon dioxide, at temperatures of −40 to 120° F. U.S. Pat. No. 4,775,396, issued to Rastelli et al. on Oct. 4, 1988, discloses the adsorption of carbon dioxide from gas streams by pressure swing adsorption at temperatures of −50 to 100° C., the adsorbent having a SiO
2
/Al
2
O
3
molar ratio of from 2 to 100 and containing at least 20 equivalent percent of one or more cations selected from zinc, rare earth, hydrogen and ammonium cations and not more than 80 equivalent percent of alkali metal or alkaline earth metal cations.
Zeolite 13X efficiently removes small amounts of carbon dioxide (and water vapor) from air streams at low temperatures, i. e., temperatures of about 5° C. or lower, because it more strongly adsorbs these components than it adsorbs nitrogen, oxygen or argon. However, the carbon dioxide adsorption capacity of zeolite 13X diminishes rapidly as the temperature of the gas being separated increases, and the separation process becomes infeasible at temperatures above about 20° C. Since ambient 25 temperatures are often considerably above the preferred 5° C. adsorption temperature, for example ambient temperatures of 40° C. or higher are sometimes encountered, and since, because of the heat of adsorption and the heat of gas compression, there is a tendency for adsorption bed temperatures to increase considerably during the course of an adsorption process, it is usually necessary to cool the air fed to an adsorption-based air prepurification plant by means of external refrigeration to maintain the gas at temperatures below 20° C. This reduces the overall efficiency of the air separation process, since energy must be consumed to provide the necessary refrigeration. U.S. Pat. No. 5,531,808, issued to Ojo et al. discloses the removal of carbon dioxide from gas streams by adsorption using as the adsorbent type X zeolite having as exchangeable cations one or more of various cations, including cations of Group 1A of the periodic table. U.S. Pat. No. 5,300,138, issued to Fischer et al. teaches that the performance of a rotary regeneratable dehumidification wheel is enhanced by using type X molecular sieves in which 20 to 60% of the sodium cations are replaced by potassium. The enhanced performance is attributed to a relatively lower sorption capacity for moisture on the partially potassium exchanged molecular sieve which allows the adsorbent to be regenerated at lower temperatures.
It would be desirable to find improved processes for removing carbon dioxide from gas streams using adsorbents with exceptionally high affinities for carbon dioxide. The present invention provides a carbon dioxide adsorption process which provides such an advantage.
SUMMARY OF THE INVENTION
According to the invention, a gas stream is purified by the removal of carbon dioxide from the gas stream by passing the gas stream through a bed of type X zeolite having a silicon-to-aluminum atomic ratio in the range of about 1.0 to about 1.15 and having as exchangeable cations potassium ions at a temperature in the range of about −50 to about 80° C., wherein at least 75% of the exchangeable cations on the zeolite are potassium ions. The process of the invention can be used to purify any gas that interacts less strongly with the zeolite than does carbon dioxide and which contains carbon dioxide as an impurity. The process is particularly suitable for purifying gases in which the partial pressure of carbon dioxide is in the range of about 3 to about 30 torr. Typical of gases that can be purified by the process of the invention are air, nitrogen, oxygen, argon, hydrogen, helium, neon, xenon, krypton, methane, etc., and mixtures of these.
Preferred adsorbents are X zeolites comprised substantially of potassium ions or potassium and sodium ions. The most preferred adsorbent is potassium X zeolite, i. e., zeolite X having substantially only potassium ions as its exchangeable cation.
In a preferred embodiment of the invention, the type X zeolite has a silicon-to-aluminum atomic ratio of about 1.0 to about 1.1, and in the most preferred embodiment, it has a silicon-to-aluminum atomic ratio of about 1.0.
The adsorption step of the process of the invention is preferably carried out at temperatures in the range of about 0 to about 80° C., and it is most preferably carried out at temperatures in the range of about 20 to about 70° C.
The invention is particularly suitable for removing carbon dioxide from gas streams containing carbon dioxide at partial pressures in the range of about 1 to about 40 torr, and is exceptionally useful for removing carbon dioxide from gas streams containing carbon dioxide at a partial pressure in the range of about 3 to about 30 torr.
The carbon dioxide purification is preferably carried out by a cyclic process, more preferably as pressure swing adsorption (PSA), temperature swing adsorption (TSA), or combinations of these. In the most preferred embodiment, the process is a TSA process.
The process of the invention can comprise the single operation of carbon dioxide adsorption, or it may comprise a combination of separation and purification operations, including carbon dioxide adsorption and one or more of water removal, air separation, hydrogen oxidation, carbon monoxide oxidation, etc. In a preferred procedure carbon dioxide is removed from air by the above-described adsorption method and the purified air is separated by cryogenic distillation into nitrogen, oxygen, argon or mixtures of these.
The carbon dioxide adsorption step with the type X adsorbent can also be used to remove moisture from the gas stream, if present. In a preferred embodiment, moisture is removed prior to carbon dioxide adsorption by passing the gas stream through a desiccant, preferably one of the various types of alumina, silica gel or zeolites, or mixtures of these.
REFERENCES:
patent: 3078639 (1963-02-01), Milton
patent: 3885927 (1975-05-01), Sherman et al.
patent: 3982912 (1976-09-01), Yatsurugi et al.
patent: 4493715 (1985-01-01), Hogan et al.
patent: 4775396 (1988-10-01), Rastelli et al.
patent: 4935580 (1990-06-01), Chao et al.
patent: 4957514 (1990-09-01), Golden et al.
patent: 4964889 (1990-10-01), Chao
patent: 5156657 (1992-10-01), Jain et al.
patent: 5300
Balse Vijay R.
Bulow Martin
Connolly Philip
Fitch Frank R.
Gittleman Craig S.
Neida Philip H. Von
Pace Salvatore P.
Spitzer Robert H.
The BOC Group Inc.
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