Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2002-07-01
2004-04-20
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C095S054000, C096S013000, C096S014000
Reexamination Certificate
active
06723152
ABSTRACT:
FIELD OF INVENTION
The instant invention relates to gas separation membranes and to a process utilizing such membranes. The membranes of the present invention are prepared from blends of perfluorinated polymers. The polymer blend membranes exhibit improved gas separation properties compared to component polymers forming the blend.
BACKGROUND OF THE INVENTION
A number of perfluorinated polymers have been disclosed in the art as membrane materials for gas separation applications. U.S. Pat. Nos. 4,897,457 and 4,910,276, disclose the use of perfluorinated polymers having repeat units of perfluorinated cyclic ethers, and report gas transport properties for a number of such polymers. U.S. Pat. No. 5,051,114 issued to S. M. Nemser and I. C. Roman, discloses a gas separation processes employing 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (BDD) based polymer membranes. European Patent application 1,163,949A2 discloses the preparation of improved composite gas separation membranes from soluble perfluoropolymers, such as BDD and tetrafluoroethylene (TFE) copolymers, and 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxide (TTD) and tetrafluoroethylene (TFE) copolymers. These polymers are commercially available under the tradenames Teflon® AF and Hyflon®.
U.S. Pat. No. 6,406,517 to D. L. Avery and P. V. Shanbhag discloses preparation of permeable membranes from a perfluoropolymer wherein the gas separation selectivity can be increased by blending the perfluoropolymer with a non-polymeric fluorinated adjuvant. Preferred non-polymeric adjuvants have molecular weights below about 10,000 g/mole, and specifically cited adjuvants have molecular weights of 650 and 1200-2400 g/mole.
U.S. Pat. No. 6,361,582, to I. Pinnau et al., discloses the use of perfluorinated polymers with fractional free volume below 0.3 for certain hydrocarbon separation applications.
V. Arcella et al. in an article entitled “Study on a perfluoropolymer purification and its application to membrane formation”, Journal of Membrane Science, Vol. 163, pages 203-209 (1999) reported the use of copolymers of 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxide (TTD) and tetrafluoroethylene (TFE), Hyflon® AD60X and Hyflon® AD80X, as membrane forming materials.
European Patent Applications 969,025, to P. Maccone, et al., and 1,057,521, to V. Arcella et al., disclose the preparation of non-porous and porous membranes prepared from amorphous perfluorinated polymers.
Membrane processes for separation of hydrocarbon vapors from air and other gas mixtures containing hydrocarbon vapor, are known in the art. U.S. Pat. No. 4,553,983 discloses a membrane process for recovering and concentrating organic vapors, including hydrocarbon vapors, from a feed stream of air. The process utilizes a membrane that comprises a microporous support membrane coated with a thin layer of silicone rubber. The organic vapor which has been preferentially concentrated through the membrane is further compressed and condensed to recover the vapor as a liquid.
U.S. Pat. No. 5,089,033 describes a two-step process employing a similar type of membrane for separating hydrocarbons from air, specific mention being made of petroleum product vapors. In both of these processes, the hydrocarbon vapor passes preferentially through the membrane from the high pressure side to the low pressure side, thereby removing the vapor from the air feed stream.
Another approach to the removal of hydrocarbons from air, is to utilize a membrane that preferentially permeates oxygen and nitrogen, while the hydrocarbons remain in the retentate stream. U.S. Pat. Nos. 5,985,002 and 6,293,996 report the application of such a membrane system to the recovery of hydrocarbon fuel vapors. Air containing hydrocarbon vapors is fed from a fuel storage tank to a membrane and the filtered air is withdrawn as a permeate while the hydrocarbon-enriched residue stream is returned to the fuel storage tank. The fluoropolymer membranes of U.S. Pat. No. 5,051,114 cited above can be used to separate hydrocarbons from an air stream, and this membrane system has been incorporated into U.S. Pat. Nos. 5,985,002 and 6,293,996 by reference.
U.S. Pat. No. 6,316,684, to I. Pinnau and Z. He, discloses improved membranes for hydrocarbon vapor separations, including perfluorinated polymers that contain dispersed fine non-porous particles, such as silica or carbon black particles, having an average diameter not greater than about 1,000 Å. It is generally accepted that membrane materials with a high gas selectivity have a relatively low permeation rate or productivity, and vice versa. Thus a trade-off typically exists between the selectivity and the permeability of polymeric materials, and it is an objective of membrane material development to maximize both the separation efficiency or selectivity of a membrane and its productivity.
SUMMARY OF THE INVENTION
The present invention provides for improved gas separation membranes that are fabricated from blends of two or more perfluorinated polymers, such as BDD) based polymers blended with TTD based polymers. The membranes exhibit superior gas separation properties, and are extremely useful in gas separation processes where the feed gas streams contain H
2
, N
2
O
2
, CH
4
, CO, CO
2
, C
3
H
8
or higher molecular weight hydrocarbon vapors, by virtue of the high permeability and selectivity exhibited by membranes made from these blends. The present invention also provides for methods to fabricate such blend membranes.
DETAILED DESCRIPTION OF THE INVENTION
The inventors have found, unexpectedly, that the blends of a first copolymer, preferably 2,2-bis(trifluoromethyl)4,5-difluoro-1,3-dioxole (BDD) and tetrafluoroethylene (TFE), with a second copolymer selected from a number of soluble perfluorinated polymers, such as copolymers of 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole with tetrafluoroethylene, exhibit superior combinations of gas separation and permeation properties. Such blends are extremely useful for gas separation applications, such as the generation of oxygen and nitrogen enriched air, natural gas sweetening, and in particular for separation of volatile organic compounds (VOC) from air and other gases.
Polymer blends can be divided into miscible, homogeneous blends and heterogeneous blends. Miscible homogeneous blends are often referred to as polymer alloys. A typical example of polymer alloy is the blends of poly(phenylene oxide), PPO, and polystyrene, PS. Most polymer blends, however, are heterogeneous with one polymer phase dispersed in another polymer phase.
The gas permeability coefficient, P, when plotted semilogarithmically versus the blend composition in terms of volume fraction, &PHgr;, often shows a linear relationship when the blends are miscible, as discussed by H. B. Hopfenberg, and D. R. Paul, in “Polymer Blends”, Volume 1, D. R. Paul and S. Newman, Eds., Academic Press, New York, 1978, Chapter 10. On the other hand, a number of theoretical models, including the Maxwell model, have been used to predict permeation properties of heterogeneous polymer blends. These models can be found in the following articles: J. A. Barrie and J. B. Ismail, “Gas transport in heterogeneous polymer blends”, Journal of Membrane Science, Vol.13, pages 197-204 (1983); R. J. Li, et al. “Transport of gases in miscible polymer blends above and below the glass transition region”, AIChE Journal, Vol. 39, pages 1509-1518 (1993); A. Senuma, “Generalized equation for the permeability of heterogeneous polymer materials”, Macromolecular Chemistry and Physics, Vol. 202, pages 1737-1742 (2001). Despite some differences, these models predict that the gas transport properties of a blend will fall between the gas transport properties of the component polymers that form the blend.
Surprisingly, the blends of BDD based polymers with other soluble perfluorinated polymers, such as TTD based polymers, exhibited gas transport properties that neither can be predicted by the existing models nor can be anticipated from the existing art. The gas transport properties of membranes formed from such blends,
Bikson Benjamin
Ding Yong
Leroux Johann Katz
Nelson Joyce Katz
Praxair Technology Inc.
Rosenblum David M.
Spitzer Robert H.
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