Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
1998-10-22
2002-11-05
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S101000, C095S102000, C095S105000, C095S118000, C095S130000, C095S138000
Reexamination Certificate
active
06475265
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a two stage pressure swing adsorption (PSA) process for producing high purity gas from a mixture of a plurality of gases and more particularly, to a PSA process for producing high purity oxygen from air.
BACKGROUND OF THE INVENTION
Conventional PSA processes for generating oxygen from an air stream commonly use a fixed bed of adsorbent material adapted to adsorb nitrogen from air, such as zeolite, so that an oxygen-rich product gas exits the bed. The principles of separation involved in such an adsorption system are based upon equilibrium separation, i.e., upon the adsorbent material's ability to hold nitrogen more strongly than oxygen. Present-day synthetic zeolites used in PSA processes are capable of achieving virtually a complete separation between nitrogen and oxygen. However, the adsorption isotherms of oxygen and argon on these materials are almost identical and a passage of feed air through a zeolite bed has no significant effect on the ratio of oxygen to argon which is typically about 21:1. Thus, the percentage by volume of argon in the oxygen-rich product stream, assuming that all of the nitrogen is adsorbed by the zeolite, is about 5 percent. Therefore, PSA processes which employ nitrogen equilibrium selective materials cannot normally generate a product stream containing an oxygen concentration which is appreciably greater than 95.0 percent.
Materials which preferentially adsorb oxygen can also be employed in PSA processes for producing oxygen from an air stream. In such a process, the oxygen-rich product is collected from the adsorbent bed during the regeneration step of each cycle. At the present time the most commonly used oxygen selective adsorbent materials are carbon molecular sieves (CMS). The separation achieved with CMS is a result of the material's more rapid adsorption of oxygen than of nitrogen—which is known as kinetic selectivity. From the point of view of oxygen
itrogen separation, the kinetic selectivity of CMS is significantly less efficient than the equilibrium selectivity of zeolite. Further, the oxygen product obtained from an air feed, using CMS as an adsorbent material, contains a considerable portion of unseparated nitrogen.
In practice the rates of adsorption of nitrogen and argon on CMS are about the same so that in the case of an air feed, the balance of the oxygen product will contain nitrogen and argon approximately in their atmospheric ratio 78:1.
In summary, PSA processes for production of oxygen from air which use nitrogen equilibrium selective adsorbents can give maximum oxygen purity of about 95.0%, with the balance represented virtually entirely by argon. PSA processes for production of oxygen from air, which use CMS as the adsorbent, can give a maximum oxygen purity of about 80%, with the balance represented by nitrogen and argon in their atmospheric ratio, i.e., about 19.75% nitrogen and 0.25% argon.
However, oxygen of a purity greater than 95.0% is needed in welding and cutting processes as well as in some medically-related applications. Accordingly, it is desirable to provide a PSA process capable of generating a product stream containing an oxygen concentration which is greater than 95.0 percent from an air feed stream.
Several PSA systems are known in the prior art which can produce a product stream containing an oxygen concentration which is greater. than 95.0% from an air feed stream. All such systems utilize a two stage PSA arrangement, i.e., there are two distinct mass transfer zones in the PSA process.
One group of two stage PSA processes for production of high purity oxygen from feed air is represented by U.S. Pat. No. 4,190,424 (Armond et al.), U.S. Pat. No. 4,959,083 (Garrett), U.S. Pat. No. 4,973,339 (Bansal) and by publications by Seemann et al. (Chem. Eng. Technol. Vol 11, p 341, 1988) and Hayashi et. al. (Gas Sep. Purif. Vol 10 No. 1, p 19, 1996). The first stage employs one or several beds of a CMS which adsorbs oxygen more rapidly, as compared to nitrogen and argon (i.e., an oxygen kinetically-selective material). A feed stream of air constituents (i.e., oxygen, nitrogen, and argon) is delivered to the first stage where oxygen is adsorbed at a higher rate than nitrogen and argon. The adsorbed oxygen is subsequently desorbed and is fed to a second stage which uses one or several beds of zeolite that adsorbs nitrogen preferentially to oxygen and argon (nitrogen equilibrium-selective material). High purity oxygen is collected at the exit of the zeolite bed.
The key to the high purity oxygen product obtained from this PSA process is not just the ability of the first CMS stage to provide an oxygen-enriched feed to the second nitrogen adsorbing zeolite stage. More particularly, it is the ability of the CMS stage to provide a feed which is depleted in argon, the one major constituent of atmospheric air which a zeolite is incapable of separating from oxygen.
Another group of two stage PSA processes for production of high purity oxygen from feed air is represented by U.S. Pat. No. 5,395,427 (Kumar et al.), U.S. Pat. No. 5,137,549 (Stanford et al.) and U.S. Pat. No. 4,190,424 (Armond et al.). The first stage comprises two beds of zeolite and separates nitrogen, carbon dioxide and water vapor from atmospheric air and passes oxygen, argon and residual nitrogen to the second stage. The second stage includes a pair of beds with oxygen selective material that adsorb oxygen and pass the argon and the residual nitrogen. The high purity oxygen product is recovered upon depressurization of the second stage.
The high purity of the oxygen product is achieved by rinsing the oxygen selective adsorbent with high purity oxygen prior to the depressurization step.
Another two stage PSA process for production of high purity oxygen from feed air is disclosed in U.S. Pat. No. 4,959,083 (Garrett). The first stage comprises a bed of CMS which adsorbs oxygen more rapidly than nitrogen. The adsorbed oxygen is desorbed from the first stage and flows to a second stage which comprises another bed of CMS. The adsorbed oxygen in the second stage is subsequently desorbed and collected as high purity oxygen product.
Another group of two stage PSA processes for production of high purity oxygen from feed air is represented by U.S. Pat. No. 5,226,933 (Knaebel et al.) and U.S. Pat. No. 5,470,378 (Kandybin et al.). A first stage utilizes nitrogen equilibrium-selective adsorbent (zeolite) while the second stage utilizes an argon equilibrium selective adsorbent (silver mordenite). The adsorbents can be placed in separate beds or in a single bed. When the feed air is introduced into the system, nitrogen is removed in the first stage, argon is removed in the second stage, and high purity oxygen is collected at the exit of the system as product.
There are a number of drawbacks in the prior art PSA processes for producing high purity oxygen from an air feed.
1. In the prior art there is an incompatibility between the stage cycle times when one of the stages utilizes an equilibrium selective adsorbent such as zeolite and the other stage utilizes a kinetically selective adsorbent such as CMS. This leads to an asynchronous mode of operation of the stages and complicates the PSA cycle. In addition, a buffer tank must be placed between the stages.
2. The mode of operation of a CMS requires relatively high adsorption pressures—typically between 6 atm and 10 atm. For silver mordenite the required adsorption pressures are even higher—between 10 atm and 20 atm. Thus such prior art PSA systems are characterized by high energy consumption.
3. The prior art PSA systems which use an oxygen selective adsorbent in the second stage always employ an oxygen rinse prior to the depressurization in order to achieve high purity of the oxygen product. This reduces the productivity of the PSA system because high purity oxygen product is used as the rinse gas. Also, power requirements increase because the high purity oxygen product is obtained at low pressure during depressurization and at least a portion of the high purity oxy
Baksh Mohamed Safdar Allie
Leavitt Frederick Wells
Notaro Frank
Serbezov Atanas
Follett Robert J.
Praxair Technology Inc.
Spitzer Robert H.
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