Gas separation: apparatus – Apparatus for selective diffusion of gases – Hollow fiber or cylinder
Reexamination Certificate
2000-09-14
2002-05-14
Spitzer, Robert H. (Department: 1724)
Gas separation: apparatus
Apparatus for selective diffusion of gases
Hollow fiber or cylinder
C096S012000
Reexamination Certificate
active
06387163
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to gas separation membranes. In particular, the present invention relates to a surface modified gas separation membrane, wherein the membrane has improved permselective properties to separate a mixture of gases as compared to the unmodified membrane.
BACKGROUND OF THE INVENTION
Fluid permeable membranes fabricated from a wide variety of polymers have been used extensively to separate gases, such as oxygen, nitrogen, carbon dioxide, methane, hydrogen, and other gases from gas mixtures. The efficiency of the fluid separation process is determined by the properties of fluid mixture, the membrane material and its structure. Preferably, the gas separation membrane is highly selective, i.e. the membrane has a high separation factor, high gas permeability, and is resistant to chemicals and temperature variations, and is mechanically strong. However, membranes with high selectivity are generally characterized by low permeability, while membranes with high permeability generally possess unacceptably low separation factors.
A variety of methods for enhancing the selectivity of fluid permeable membranes by modifying the characteristics of the membrane have been described. For example, Kramer et al, U.S. Pat. No. 5,215,554 describe a method for enhancing selectivity by modifying the interstices or recesses of the membrane substantially throughout the thickness of the gas permeable membrane. U.S. Pat. Nos. 4,311,573, 4,589,964 and 4,968,532, disclose graft polymerization of a monomer on to a preformed, saturated polymer substrate which has been subjected to ozonization prior to grafting. See also U.S. Pat. Nos. 4,486,202; 4,575,385; 4,654,055 and 4,728,346. Halogenation treatment techniques have also been used to modify gas separation membranes. See U.S. Pat. Nos. 3,062,905 and 4,828,585.
Although several methods that improve selectivity of specific membranes have been described, including surface modification methods, such methods have several disadvantages—long exposure times, high chemical concentrations and high treatment temperatures, and possibly, degradation of the membrane. Thus there is a need for an improved and cost-effective gas separation membrane which possesses high gas permeability and a high separation factor. The current method provides a cost-effective gas separation membrane with a significant improvement in selectivity with a commercially acceptable loss of permeability.
SUMMARY OF THE INVENTION
The present invention relates to a method for preparing a surface modified gas separation membrane, wherein the membrane has improved permselective properties for separating a mixture of gases as compared to the unmodified membrane. The method comprises coating a surface unmodified gas separation membrane with a solution of a coating material, wherein the coating material is an organic material having at least one site of unsaturation; heating the coated gas separation membrane; and contacting the heated coated gas separation membrane with at least one oxidizing agent for a time effective to surface modify the gas separation membrane to produce the surface modified gas separation membrane having improved permselectivity.
In one aspect, the invention relates to a method for preparing a surface modified gas separation membrane, wherein said membrane has improved permselective properties for separating a mixture of gases as compared to the unmodified membrane, comprising:
(a) providing a surface unmodified gas separation membrane;
(b) providing a solution of a coating material, wherein said coating material is an organic material having at least one site of unsaturation;
(c) contacting the surface unmodified gas separation membrane with the solution of the coating material;
(d) coating said coating material on the surface unmodified gas separation membrane to produce a coated gas separation membrane;
(e) optionally removing substantially all of the residual coating material from the surface of the coated gas separation membrane of step (d);
(f) optionally removing substantially all of the solvent from the surface of the coated gas separation membrane of step (d) or (e) by contacting the surface with at least one anhydrous non-oxidizing gas;
(g) heating the coated gas separation membrane of step (d), (e), or (f) at about 10° C. to about 150° C.;
(h) contacting the heated coated gas separation membrane of step (g) with at least one oxidizing agent for a time effective to surface modify the gas separation membrane between about 5 minutes and 24 hours to produce the surface modified gas separation membrane having improved permselectivity; and
(i) optionally contacting the surface oxidized gas separation membrane of step (h) with a non-oxidizing gas for a time effective to remove substantially all of the oxidizing gas to produce the surface modified gas separation membrane having improved permselectivity.
In an alternative embodiment, the method for preparing a surface modified gas separation membrane further comprises:
(j) contacting the treated gas separation membrane of step (h) or (i) with at least one oxidizing agent for a time effective to surface modify the gas separation membrane between about 5 minutes and 24 hours to produce the surface modified gas separation membrane having improved permselectivity; and
(K) optionally contacting the surface oxidized gas separation membrane of step (j) with a non-oxidizing gas for a time effective to remove substantially all of the oxidizing gas to produce the surface modified gas separation membrane having improved permselectivity.
In a preferred embodiment, the gas separation membrane is a melt-extruded symmetric flat sheet, a solvent cast symmetric flat sheet, an asymmetric flat sheet, or an asymmetric hollow fiber. In a more preferred embodiment, the gas separation membrane is an asymmetric hollow fiber.
In a preferred embodiment, the coating material is an organic material having at least one site of unsaturation. In a preferred embodiment the coating material is a low molecular weight polymeric material or a surfactant. In a more preferred embodiment, the low molecular weight polymeric material has a molecular weight that ranges from about 1000 to about 100,000.
In a preferred embodiment, permselectivity of the surface modified gas separation membrane is increased by about 5% to about 30% as compared to the surface unmodified membrane.
In an alternative embodiment, the invention relates to a method for preparing a surface modified gas separation membrane, wherein said membrane has improved permselective properties for separating a mixture of gases as compared to the unmodified membrane, comprising:
(a) providing a surface unmodified gas separation membrane;
(b) providing a solution of a coating material, wherein said coating material is an organic material having at least one site of unsaturation;
(c) heating the solution of the coating material at about 10°C. to about 150° C.;
(d) optionally heating the surface unmodified gas separation membrane at about 10° C. to about 150° C.;
(e) contacting the surface unmodified gas separation membrane with the heated solution of the coating material;
(f) coating said heated coating material on the surface unmodified gas separation membrane to produce a coated gas separation membrane;
(g) optionally removing substantially all of the residual coating material from the surface of the coated gas separation membrane of step (f);
(h) optionally removing substantially all of the solvent from the surface of the coated gas separation membrane of step (f) or (g) by contacting the surface with at least one anhydrous non-oxidizing gas;
(i) contacting the heated coated gas separation membrane of step (h) with at least one oxidizing agent for a time effective to surface modify the gas separation membrane between about 5 minutes and 24 hours to produce the surface modified gas separation membrane having improved permselectivity; and
(j) optionally contacting the surface oxidized gas separation membrane of step (i) with a non-oxidizing gas for a time effective to
Coan Frederick L.
Jeanes Thomas O.
Jensvold John A.
Leong Pamela J.
MG Generon
Robins & Pasternak LLP
Spitzer Robert H.
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