Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2000-12-29
2002-12-31
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S100000, C095S103000, C095S105000, C095S130000, C095S138000, C096S130000, C096S143000
Reexamination Certificate
active
06500235
ABSTRACT:
FIELD OF THE INVENTION
The present invention provides an improved pressure swing adsorption (PSA) process capable of delivering high purity and high recovery of a high purity gas, such as argon, from a feed stream. More specifically, the present invention provides an improved process with high recovery for purification of crude argon available from a cryogenic air separation unit. The high recovery enables the present invention to become a complete process without any additional requirement for purification and recycle back to the cryogenic air separation unit.
BACKGROUND OF THE INVENTION
Currently, oxygen and nitrogen, the two main products of an air separation, can be directly removed from a two-stage cryogenic rectification unit involving a high pressure column and a low pressure column. Argon, which constitutes almost 1% of the feed air, is then enriched in the middle section of the low pressure column. This enriched argon containing about 10 to 12% of argon, 0.1% of nitrogen and the rest of oxygen is fed to the argon low ratio column to produce crude argon containing impurities of about 1 to 5% of oxygen and 1 to 3% of nitrogen. Crude argon is then purified to about 99.999% purity, typically first by catalytic deoxygenation or by a superstaged argon column to remove oxygen, then by rectification in a high ratio column to remove nitrogen.
Catalytic deoxygenation requires the availability of hydrogen, which is not always available and cost effective everywhere in the world. Hydrogen reacts with oxygen to form water, which is then removed from crude argon. Superstaging is another alternative for oxygen removal by adding additional separation stages in the argon column. However, the number of these additional stages could be fairly large, for example, between 115 to about 140, because of the small difference in the relative volatility between oxygen and argon. Furthermore, a high ratio cryogenic column will still be required for additional nitrogen removal if nitrogen is present in the crude argon column.
As compared to the above conventional very elaborate methods of recovering 80 to about 90% argon from air, a PSA process provides a simple and effective alternative for argon purification and recovery. No hydrogen or additional cryogenic stages are required. However, conventional PSA processes suffer from a rather low argon recovery of about 40%. Thus, it is necessary to recycle the PSA waste stream, still containing significant amount of argon, back to the cryogenic air separation unit for additional recovery. Consequently, PSA is much less attractive.
High purity argon is generally produced by purifying crude argon available from an air separation unit. Adsorption is a promising alternative to cryogenic superstaging as disclosed by Bonaquist and Lockett in U.S. Pat. No. 5,440,884 and catalytic deoxygenation as disclosed by Tomita et al. in U.S. Pat. No. 5,783,162.
Jain and Stern in U.S. Pat. No. 5,601,634 and Jain and Andrecovich in AU-A-47537/93 disclose respective cryogenic temperature swing adsorption purification processes. In AU-A-47537/93, the cryogenic TSA is carried out below 150 K in a two layer adsorbent bed. The first layer comprises one or more equilibrium selective adsorbents, such as calcium exchanged type X and A zeolite to preferentially adsorb nitrogen over argon. The second layer comprises one or more rate selective adsorbents, such as CMS and 4A type zeolite, to preferentially adsorb oxygen. Upon completion of adsorption, the bed is regenerated by passing a warm purge gas substantially free of impurities, such as nitrogen and oxygen. This prior art involves low temperature adsorption and argon recycle.
U.S. Pat. No. 5,601,634 discloses a cryogenic TSA process with a liquid-vapor two phase feed. The adsorption bed contains one or more adsorbents selective for nitrogen and/or oxygen at a temperature between the bubble and dew point of the two phase mixture. The advantage of this two phase feed is that any increase in temperature during the adsorption step will evaporate some of the liquid and that the heat of adsorption is offset. This can improve the adsorption capacity. However, because of low operating temperature and the warm purge required, this process is energy relatively intensive.
Nguyen et al. in U.S. Pat. No. 5,730,003, teaches a PSA process for crude argon purification. The process uses oxygen rate or equilibrium selective adsorbent for oxygen removal. In Nguyen et al., the O
2
rate selective adsorbents include CMS, clinoptilolite, type A zeolite, and the O
2
equilibrium selective adsorbents include adsorbents disclosed by Ramprasad et al. in U.S. Pat. No. 5,294,418. A layer of nitrogen equilibrium selective adsorbent such as CaA and type X zeolite is mentioned for nitrogen removal. The process uses the following cycle steps: feed pressurization, adsorption, cocurrent depressurization, countercurrent blowdown, countercurrent purging and product pressurization. This process does not require low temperature as required by a TSA. However, the argon recovery is low (about 40%) and recycling of desorption gas, during bed regeneration, back to the cryogenic air separation plant is necessary to enhanced argon recovery. This prior art uses a Simplex two bed PSA system.
Kumar et al. in U.S. Pat. No. 4,477,265, discloses a two stage PSA process for argon purification. The two layers of adsorbents for oxygen and nitrogen removal are in two separated stages. The two stages are connected in series. This allows the process to be more flexible, for example, it permits possible bed interactions even within a stage and using different number of beds in different stages. In one preferred embodiment, three beds are in fact used in the first stage for nitrogen removal using a nitrogen equilibrium selective adsorbent. Two beds are in the second stage for oxygen removal using an oxygen rate selective adsorbent. The basic cycle steps include adsorption, evacuation and pressurization. Also, argon recovery is low, and recycling the waste stream, still containing considerable amount of argon, back to cryogenic unit is necessary for additional recovery. In addition, recycling of PSA waste stream back to the cryogenic plant makes the air separation unit more complex and a PSA option less attractive.
Also, Kumar et al. in U.S. Pat. No. 5,395,427 discloses a two stage PSA process using oxygen and nitrogen equilibrium selective adsorbents for producing high purity oxygen from air. The oxygen equilibrium selective adsorbent is a cobalt-based material and preferably used in the second stage. Carbon dioxide, water and nitrogen are preferably removed in the first stage filled with one or more adsorbents selective for the impurities. Oxygen is desorbed from the second stage as product, and the effluent is used to regenerate the first stage adsorbent(s).
Wilson, U.S. Pat. No. 4,359,328, discloses an inverted PSA process, in which a strong component is adsorbed at low pressure while a weak component is adsorbed at high pressure. This is just opposite to conventional PSA process and could be used to recover strong component with enhanced purity.
Lee and Paul, U.S. Pat. No. 5,738,709, discloses a nitrogen PSA process with an intermediate pressure transfer. Instead of a conventional end-to-end (bottom, top or both) transfer, a transfer is carried out from an intermediate point of the high pressure bed to a point close to the feed end of the low pressure bed. Such a transfer increases the productivity and recovery of nitrogen.
Leavitt, U.S. Pat. No. 5,085,674, discloses a Duplex PSA process. The setup is similar to a two stage PSA but with two distinguished features: intermediate feed between the two stages rather than at one end (top or bottom) and recycling capability from the low pressure bed to the high pressure bed. Such a process combines both the conventional PSA and the inverted PSA features of U.S. Pat. No. 4,359,328, and could provide high purity and also recovery. However, this process has not been applied to argon purification with removal of both oxygen and nitrogen. In
Baksh Mohamed Safdar Allie
Leavitt Frederick Wells
Notaro Frank
Zhong Guoming
Follett Robert J.
Praxair Technology Inc.
Spitzer Robert H.
LandOfFree
Pressure swing adsorption process for high recovery of high... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Pressure swing adsorption process for high recovery of high..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Pressure swing adsorption process for high recovery of high... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2956290