Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Utility Patent
1998-12-09
2001-01-02
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C095S051000, C095S054000, C096S009000
Utility Patent
active
06168649
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the use of semi-permeable membranes derived from polycarbonates and polyestercarbonates to separate xenon and other noble gases from diluent gases such as oxygen and nitrogen. This invention also includes a method used to separate these gases.
The use of membranes to separate gases is well known in the art. Membranes have been used to recover or isolate a variety of gases, including hydrogen, helium, oxygen, nitrogen, carbon dioxide, methane and light hydrocarbons. Applications of particular interest include the separation of air to provide an enriched nitrogen stream for use as an inert atmosphere over flammable fluids or for food storage. Conversely, the separation of air can provide an enriched oxygen stream for use in enhanced combustion processes. Other embodiments include the separation of carbon dioxide from methane and other light hydrocarbons such as ethane and propane, as part of the tertiary oil recovery process. Still other embodiments include the recovery of nitrogen or helium or hydrogen from natural gas streams. Still further embodiments include the recovery of rare or expensive gases from less expensive diluent gases.
The membrane separation processes described above are based on the relative permeability of two or more gaseous components through the membrane. To separate a gas mixture into two streams, one richer in at least one component and one leaner in at least one component, it is necessary to bring the gas mixture into contact with one side of a semi-permeable membrane through which at least one of the gaseous components selectively permeates. A gaseous component, which selectively permeates through the membrane, passes through the membrane more rapidly than the other component(s) of the mixture. The gas mixture is thereby separated into a stream which is enriched in the selectively permeating component(s) (the permeate) and a stream which is depleted of the selectively permeating component (the retentate). The non-permeating component(s) is concentrated in the retentate stream. A membrane material is chosen for a mixture so that an appropriate degree of separation of the gas mixture can be achieved.
Membranes for gas separation have been fabricated from a wide variety of polymeric materials including cellulose triacetate (CTA); polyolefines such as polyethylene, polypropylene and poly-4-methyl-pentene-1; polycarbonates; polyesters; polysulfones; and polyimides. An ideal gas-separation membrane is characterized by the ability to operate under high temperature and/or pressure while possessing a high separation factor (permselectivity) and a high gas permeability or flux. The problem is finding membrane materials which possess all the desired characteristics. Typically, membrane materials that possess high permselectivity have low gas permeabilities; conversely, those membrane materials which possess high gas permeability or flux, have low permselectivity. Materials that cannot offer a good balance of properties, i.e. permselectivity and permeability, are unlikely to provide sufficient economic justification for commercial use.
Xenon is a very rare and expensive noble gas. It has been successfully used as a relaxant and anesthetic for surgery. The value of xenon is a strong driving force for devising a method for recovering xenon gas in high purity. The current method of recovery involves a cryogenic separation of xenon, oxygen and nitrogen, but this process incurs significant losses of xenon due to the close proximity of the boiling points of the gases. Membranes offer a practical alternative to the cryogenic process and can provide a high purity stream with a lower loss of xenon to the waste stream.
The present invention provides a membrane material which possesses high permselectivity with respect to xenon, and other rare and expensive gases, and a convenient method of using such a membrane to recover xenon or other rare and expensive gases.
SUMMARY OF THE INVENTION
The present invention comprises a gas separation membrane which includes a thin discriminating layer selected from the group consisting of polycarbonate, polyester, and polyestercarbonate. In the preferred embodiment, the thin discriminating layer includes material selected from the bisphenolic group consisting of 2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane, 2,2-bis(3,5-dichloro-4-hydroxyphenyl)propane, and 9,9-bis(3,5-dibromo-4-hydroxyphenyl)fluorene.
The gas separation membrane of the present invention functions as a semi-permeable membrane capable of separating a noble gas from oxygen, nitrogen, carbon dioxide or a mixture thereof. The invention is especially useful in the separation of xenon. The semi-permeable membrane of the present invention exhibits an oxygen
oble gas permselectivity of at least 21.
The devices made with the membranes described above may include a single unit, to produce a product gas, or several units manifolded together so as to operate in a parallel fashion, to provide an increased volume of product gas at the same desired purity. Alternatively, two or more membrane devices may be connected in a series or staged configuration to provide a smaller volume of highly purified product gas. The invention also includes the method of using the membrane of the present invention, either as a single-stage process or as a multiple-stage process, to provide the desired product gas.
The present invention therefore has the primary object of providing a semi-permeable membrane which is capable of economically separating a noble gas from oxygen, nitrogen, carbon dioxide, or a mixture thereof.
The present invention has the further object of providing a semi-permeable membrane which effectively and economically separates xenon from oxygen, nitrogen, carbon dioxide, or a mixture thereof.
The invention has the further object of providing a semi-permeable membrane which separates xenon from other gaseous components, with a permselectivity of at least 21.
The invention has the further object of providing a method of separating a noble gas from oxygen, nitrogen, carbon dioxide, or a mixture thereof.
The invention has the further object of providing various multi-stage processes for separating a noble gas from oxygen, nitrogen, carbon dioxide, or a mixture thereof.
The invention has the further object of making it more economical to separate noble gases, especially xenon, from other gaseous components, by non-cryogenic means.
The reader skilled in the art will recognize other objects and advantages of the present invention, from a reading of the following brief description of the drawings, the detailed description of the invention, and the appended claims.
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Jeanes Thomas O.
Jensvold John A.
Eilberg William H.
MG Generon, Inc.
Spitzer Robert H.
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