Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
1999-03-02
2001-01-09
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S103000, C095S130000, C095S902000, C096S130000, C502S079000
Reexamination Certificate
active
06171370
ABSTRACT:
The present invention relates to an adsorbent for separating gases to separate a gas mixture having a component which is easily adsorbed (adsorbable component) and a component which is hardly adsorbed (adsorption resistant component), and a process for producing it. Particularly, the adsorbent for separating gases of the present invention relates to an adsorbent for separating gases to separate a gas mixture by a pressure swing adsorption method (hereinafter referred to as PSA method for short). The gas to be separated and recovered by PSA method by using the adsorbent for separating gases of the present invention may, for example, be oxygen gas, nitrogen gas, carbon dioxide, hydrogen or carbon monoxide.
Among these, oxygen gas is one of the particularly important industrial gases, and widely used for steel making, bleaching of pulp or the like. Particularly, in recent years, in order to reduce generation of NOx which is inevitable by combustion in air, oxygen-enriched combustion is used practically in the field of refuse burning, glass melting or the like, and oxygen gas is becoming important in view of environmental problems.
As an industrial method for producing oxygen gas, PSA method, a cryogenic distillation processing or a membrane separation method has been known. Among these, PSA method which is advantageous in view of the purity of oxygen gas and cost, is increasingly used. The method for producing oxygen gas by PSA is to adsorb nitrogen gas in air on an adsorbent, and obtain the remaining concentrated oxygen gas as a product. As the adsorbent to be used, an adsorbent capable of selectively adsorbing nitrogen gas is employed.
In the case of separating a gas mixture by using a crystalline zeolite, the adsorbable component is selectively adsorbed on the crystalline zeolite. For example, in the case of producing oxygen gas from air by using PSA method, nitrogen in the air is selectively adsorbed on the crystalline zeolite to carry out separation of the air. Selective adsorption of nitrogen on the crystalline zeolite is due to the strong interaction between quadruple moment of nitrogen and electrostatic force of attraction of cations in the zeolite. Therefore, for PSA method, a crystalline zeolite is used wherein electrostatic force of attraction of cations is high and the amount of nitrogen adsorbed is large, and an adsorbent having A-type or X-type zeolite ion-exchanged with e.g. lithium cations, calcium cations, strontium cations or barium cations is used. Particularly, lithium-exchanged crystalline zeolite X which is ion-exchanged with lithium cations, is excellent in selective adsorption of nitrogen, and used as a crystalline zeolite to obtain concentrated oxygen by PSA method.
For example, U.S. Pat. No. 3,140,933 proposes a lithium-exchanged crystalline zeolite X which is excellent in the equilibrium amount of nitrogen adsorbed and the separation factor calculated from the adsorption isotherm of nitrogen and oxygen, and JP-B-5-25527 reconfirms its performance.
In general, separation of a gas mixture is conducted in a packed bed, and it is preferred that the pressure drop in the packed bed is small. For example, in the case of separating gases by PSA method, in order to decrease the pressure drop in the packed bed and reduce the load to the vacuum pump or the blower constituting the PSA apparatus, the crystalline zeolite is formed into beads or pellets by using a binder, e.g. an inorganic binder such as silica sol or alumina sol. An organic additive may be used depending upon the purpose. In the agglomerate, a network of macropores is formed by the crystalline zeolite and the binder. When the adsorbable component is adsorbed on the adsorption site of the crystalline zeolite existing at the center of the agglomerate, the adsorbable component diffuses in the macropores and reaches to the adsorption site at the center of the agglomerate. And the adsorbable component desorbed from the adsorption site diffuses in the macropores and is evacuated to the exterior of the agglomerate. In order to obtain the expected performance of an adsorbent, some adsorbents have been proposed wherein even the adsorption site existing at the center of the agglomerate can be effectively used, so that the efficiency for adsorption of the adsorbable component can be made high.
For example, a zeolite agglomerate for separating gases wherein A-type zeolite exchanged with calcium cations is used, and the macropore volume is at least 0.3 ml/g (JP-A-58-124539), or a zeolite agglomerate wherein the macropore volume/micropore volume ratio is from 1 to 4.5, and the macropore volume is from 0.3 to 0.7 ml/g (JP-A-62-283812) has been proposed. Further, a process for separating air by using an adsorbent wherein the porosity of the adsorbent and the average diameter of macropores are controlled (JP-A-9-308810) has been proposed.
Such an adsorbent is to improve the adsorption rate when the adsorbable component such as nitrogen in the case of separating air, is adsorbed on the crystalline zeolite, and to reduce the time of contacting the gas mixture with the adsorbent, and no consideration is given to the phenomena when the adsorbed adsorbable component is desorbed under reduced pressure. As adsorption and desorption are repeated in PSA method, in order to obtain the performance of the adsorbent, it is necessary to improve not only the diffusion rate of the adsorbable component when adsorbed, but also the diffusion rate of the adsorbable component when desorbed. Accordingly, none of the conventional adsorbents can be regarded as an adsorbent for separating gases, of which the utilization rate has been sufficiently improved.
The shape of the adsorbent for separating gases is usually cylindrical type pellet or in the form of beads. The cylindrical type pellet and the beads are prepared usually by extrusion and tumbling granulation utilizing centrifugal force, respectively. Macropores in the inside of the agglomerate are usually collapsed, and such a problem in forming the agglomerate is more serious when using, as a binder, a clay having a plate structure such as kaolin clay or bentonite clay. Such an adsorbent has a higher resistance against gas diffusion in the inside of the agglomerate, and it is not possible to effectively use the center portion of the adsorbent.
Adsorption of the adsorbable component on the crystalline zeolite is an exothermic process. However, desorption of the adsorbable component from the crystalline zeolite is an endothermic process, and a higher energy is required for desorption of the adsorbable component than adsorption of the adsorbable component. Accordingly, to obtain the desired gas separation performance, it is required to quickly evacuate the adsorbable component desorbed from the crystalline zeolite to the exterior of the adsorbent. Particularly, in the case where the amount of the adsorbable component adsorbed on the crystalline zeolite is large, in order to evacuate a larger quantity of the adsorbable component to the exterior of the adsorbent during desorption, a higher desorption rate is required. A high adsorption rate when adsorbing the adsorbable component is also an important factor. Further, although the adsorbent for separating gases is used in a state where water is removed (activated state), the crystalline zeolite has a strong affinity with water, and there is a fear that water in the atmosphere may be re-adsorbed. If water remains in the adsorbent due to water adsorption, the adsorption site for gas is occupied with water, thereby gas separation deteriorates, and the desired performance is less likely to be obtained.
For example, in the case of using lithium-exchanged faujasite type zeolite as a crystalline zeolite for separating air by PSA method, since the amount of nitrogen adsorbed is large, it is necessary to adsorb and desorb a larger amount of nitrogen as compared with the case of using a crystalline zeolite exchanged with e.g. calcium, and unless the diffusion rate of nitrogen during adsorption and desorption is adequately improved, the adequate perform
Hirano Shigeru
Kawamoto Taizo
Nishimura Toru
Yoshimura Keiji
Nixon & Vanderhye
Spitzer Robert H.
Tosoh Corporation
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