Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2001-04-04
2003-03-25
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S114000, C095S139000, C095S902000
Reexamination Certificate
active
06537348
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a method of removing carbon dioxide by adsorptive separation from a gaseous mixture containing carbon dioxide and gases less polar than carbon dioxide.
(2) Description of the Related Art
Carbon dioxide is contained in natural gas, exhaust gas from combustion, as well as atmospheric air although in a minor amount. Further, carbon dioxide is produced as by-product in industrial processes, for example, at a step of steam-reforming natural gas, naphtha, coke or methanol to produce hydrogen. In recent years carbon dioxide industrially produced as by-product including that contained in combustion gas has been attracting widespread attention because it causes global warming. Further, cryogenic separation of air has a problem such that a trace amount, i.e., about 300 to 400 ppm, of carbon dioxide in air is solidified upon cooling, leading to clogging an equipment such as a heat exchanger.
As the method of separating and removing carbon dioxide contained in a gas, there can be mentioned a method of chemically absorbing carbon dioxide in a solution of an alkali or amine, and a method of physically adsorbing carbon dioxide by an adsorbent such as active carbon or a zeolite. The methods of physical adsorption of carbon dioxide using a zeolite adsorbent include a temperature/pressure swing adsorption (PTSA) method and a pressure swing adsorption (PSA) method. In these methods, adsorption of carbon dioxide by a zeolite adsorbent is effected at a low temperature and a high pressure and desorption thereof from the zeolite adsorbent for the reproduction of the zeolite adsorbent is effected at a temperature higher and a pressure lower than those for adsorption. Upon desorption, the zeolite adsorbent may be purged with a gas which contains no carbon dioxide and is less adsorbed than carbon dioxide.
It is known that a zeolite adsorbent adsorbs a molecule by the interaction between the cation present in the zeolite adsorbent and the molecule. The interaction is enhanced and the amount of the molecule adsorbed increases with an increase of polarity of the molecule. For example, the descending order of interaction between the zeolite and ingredients in air is water, carbon dioxide, nitrogen, oxygen and then argon. The descending order of the amount of adsorption is also the same.
Assuming that carbon dioxide in atmospheric air is removed by adsorption, the content of carbon dioxide in air is about 300 ppm and the content of nitrogen in air is about 78%, and hence, when air is brought into contact with an adsorbent at a pressure of 5 to 10 atm., the partial pressure of carbon dioxide is about 2 mmHg and the partial pressure of nitrogen is about 4 to 8 atm. Carbon dioxide has a large polarity and, even when the partial pressure thereof is low, it should be adsorbed in a large amount. But, the amount of carbon dioxide adsorbed is small because its adsorption is hindered by the presence of a large amount of nitrogen. Note, water in air can be substantially completely removed by previously treating with an adsorbent such as alumina.
The zeolite adsorbent hitherto used for removing carbon dioxide from a gaseous mixture such as air includes a type A zeolite and a type X zeolite having an SiO
2
/Al
2
O
3
molar ratio of at least 2.5. When air is subjected to cryogenic separation by using these zeolite adsorbents, a large amount of the zeolite adsorbents must be used because the amount of air treated for cryogenic separation is very large. Therefore, to reduce the size of equipment or reduce the energy consumption, an adsorbent exhibiting a high adsorption for carbon dioxide even in the co-presence of a large amount of nitrogen is eagerly desired.
A method of removing carbon dioxide from a gas stream by using a zeolite adsorbent has been proposed in Japanese Unexamined Patent Publication (abbreviated to “JP-A”) No. H8-252419 (corresponding to U.S. Pat. No. 5,531,808) wherein the gas stream is contacted with a type X zeolite having a silicon/aluminum atomic-ratio of about 1.0 to about 1.15 and having been ion-exchanged with a cation selected from the ions of group 1A, group 2A, group 3A, group 3B, the lanthanide series and mixtures thereof at a temperature of about −50° C. to about +80° C. It is noted that the change of the uptake of carbon dioxide depending upon the pressure of carbon dioxide is examined (see table on page 6 of JP-A '419 [in col. 6 of U.S. '808]), but, the selective adsorption of carbon dioxide in a gaseous mixture containing carbon dioxide and nitrogen is not examined therein.
Further, as preferable exchangeable cations, sodium and lithium falling in the ions of group 1A and calcium falling in the ions of group 2A are mentioned in JP-A '419 (U.S. '808) More specifically a sodium-exchanged type X zeolite (NaLSX) and a lithium- and calcium-exchanged type X zeolite (Li,CaLSX) (the amounts of lithium and calcium are 95 equiv. % and 5 equiv. %, respectively) are examined, and it is shown that Li,CaLSX is superior to NaLSX in the uptake of carbon dioxide, but, the adsorption selectivity between carbon dioxide and nitrogen is not examined.
A method of removing water vapor and carbon dioxide from a gas wherein water vapor is first removed and then carbon dioxide is removed by using sodium LSX zeolite as adsorbent has been proposed in JP-A H8-179,137 (corresponding to U.S. Pat. No. 5,914,455). However, this patent is silent on the removal of carbon dioxide from a gaseous mixture containing carbon dioxide and nitrogen.
A method of removing carbon dioxide from a gas such as atmospheric air wherein carbon dioxide is adsorbed by type X zeolite having an Si/Al atomic ratio of 1 to 1.5 and having been ion-exchanged with calcium, sodium and potassium has been proposed in JP-A H11-253,736.
A zeolite adsorbent for gas purification comprising a sodium-type low-silica faujasite having an SiO
2
/Al
2
O
3
of about 1.8 to 2.2 with a residual content of potassium ions less than about 8.0 equiv. % and a binder has been proposed in WO 00/01478. The zeolite adsorbent used therein has a low zeolite crystal purity and contains a binder, and hence, the adsorption performance inherently possessed by zeolite crystal is not sufficiently manifested.
In general zeolite adsorbents are used in the form of a shaped product prepared by incorporating a binder in zeolite and shaping the mixture of zeolite and the binder into a desired shape. However, a binder has no adsorption performance, and hence, the zeolite adsorbent containing a binder has a relatively poor adsorption performance. Therefore, proposals of using a binder capable of being converted to zeolite have been made. For example, a method of producing a low-silica type X zeolite binderless shaped product has been proposed in JP-A H5-163,015 wherein a shaped product comprised of a type X zeolite powder having an SiO
2
/Al
2
O
3
molar ratio of smaller than 2.5, metakaolin converted from kaolin clay, sodium hydroxide and potassium hydroxide is maintained in an aqueous solution containing sodium hydroxide and potassium hydroxide at a temperature of 40 to 100° C. for several hours to several days whereby metakaolin is converted to zeolite to give a low-silica type X zeolite binderless shaped product.
A shaped product comprised of at least 95% of a low-silica type X zeolite having an SiO
2
/Al
2
O
3
molar ratio of at least 2, which is prepared by using a binder capable of being converted to a zeolite, is described in JP-A H11-76810. This shaped product is prepared by a process wherein a low-silica type X zeolite is agglomerated by using a binder containing at least 80% of a clay capable of being converted to a zeolite; the thus-obtained agglomerate is shaped; the shaped product is dried and then calcined at a temperature of 500 to 700° C.; and the thus-obtained solid product is placed in contact with an aqueous alkali solution containing an alkali at least 0.5 molar concentration comprising sodium hydroxide and potassium hydroxide, wherein the proporti
Harada Atsushi
Hirano Shigeru
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Spitzer Robert H.
Tosoh Corporation
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