Gas separation: apparatus – Apparatus for selective diffusion of gases
Reexamination Certificate
2002-08-26
2004-03-23
Spitzer, Robert H. (Department: 1724)
Gas separation: apparatus
Apparatus for selective diffusion of gases
C096S013000, C096S014000, C055S524000, C055SDIG005
Reexamination Certificate
active
06709491
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a gas separation membrane which is usable for separating each gas component from a gas mixture containing two or more kinds of gas, and a method of producing the same.
DESCRIPTION OF THE RELATED ART
Gas separation membrane technologies are used in a wide range of fields such as separation of nitrogen and oxygen from air, separation of carbon dioxide and methane from natural gas, as well as dehumidifying treatment of gas for semiconductors. Further, studies have recently been made on other applications such as separation of carbon dioxide from combustion gas discharged from a power plant etc. Such gas separation membranes are produced by various methods.
As a gas separation membrane which is excellent in properties such as heat resistance, solubility and membrane fabricability, there has conventionally been known a cellulose acetate membrane, but the cellulose acetate membrane is not practically satisfactory since it has poor chemical resistance and poor heat resistance. In addition, as a separation membrane having an improved heat resistance, a polysulfone semipermeation membrane is commercially produced, though it is not satisfactory due to its insufficient permeability. Further, a silicone membrane is known as an oxygen selective permeation membrane, but silicone is not industrially satisfactory since a sufficient mechanical strength cannot be obtained. Further recently, study has been started on an aromatic polyimide resin which has high elastic modulus, high strength, low expansibility and excellent heat resistance. However, from a practical standpoint, importance is placed on selective separation ability and heat resistance, and thus an aromatic polyimide resin which satisfies solubility to various solvents and workability has not been obtained. Therefore, a practical gas separation membrane has not yet been obtained because of difficulties in effecting a defectless thin membrane.
As practical properties of a gas separation membrane, high permeability rate and high selective separation ability are particularly required, but at present there has been no report on a gas separation membrane which simultaneously satisfies these two conditions.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a novel gas separation membrane having high gas permeability and high gas selectivity, and a method of producing the same.
The inventors of the present invention have made intensive studies to achieve the above object. As a result, they have found that both high gas permeability and high gas selectivity can be simultaneously accomplished by using a membrane which is formed from polymeric material soluble in an organic solvent and has a surface modified by ion bombardment, for separating a gas mixture. Thus, they have completed the present invention.
According to the present invention, there is provided a gas separation membrane comprising a polymeric material soluble in an organic solvent, wherein at least a part of the surface is modified by ion bombardment.
Preferably, the polymeric material is polyimides, polyethers, polyesters, or polyamides. Particularly preferably, the polymeric material is fluorine-containing polyimides.
Preferably, the gas separation membrane of the present invention has an asymmetric structure composed of a non-defective surface skin layer and a porous layer which supports the surface skin layer, and at least a part of the surface of the surface skin layer is modified by ion bombardment.
The asymmetric structure composed of a non-defective surface skin layer and a porous layer which supports the surface skin layer in the gas separation membrane of the present invention is preferably formed by casting on a support a solution obtained by dissolving the polymeric material in a solvent mixture composed of a good solvent with a low boiling point, a good solvent with a high boiling point and a poor solvent, evaporating the solvent from a surface thereof, and conducting immersion into a coagulation bath to induce phase separation.
Preferably in the present invention, the modification by ion bombardment is conducted by ion implantation in a dose &phgr; range of 1×10
10
ions/cm
2
≦&phgr;≦1×10
16
ions/cm
2
.
According to another aspect of the present invention, there is provided a method of producing a gas separation membrane which comprises conducting ion implantation in a dose &phgr; range of 1×10
10
ions/cm
2
≦&phgr;≦1×10
16
ions/cm
2
on at least a part of a surface of a polymeric material soluble in an organic solvent.
Preferably, the method of producing a gas separation membrane according to the present invention comprises the steps of:
casting on a support a solution obtained by dissolving the polymeric material in a solvent mixture composed of a good solvent with a low boiling point, a good solvent with a high boiling point and a poor solvent;
evaporating the solvents from a surface thereof;
conducting immersion into a coagulation bath to induce phase separation, thereby forming a membrane having an asymmetric structure composed of a non-defective surface skin layer and a porous layer which supports the surface skin layer; and
conducting ion implantation in a dose &phgr; range of 1×10
10
ions/cm
2
≦&phgr;≦1×10
16
ions/cm
2
on at least a part of a surface of the membrane.
DETAILED DESCRIPTION OF INVENTION
Hereinafter, embodiments of the present invention will be explained in detail.
A gas separation membrane of the present invention is characterized in that it comprises a polymeric material soluble in an organic solvent and at least a part of the surface thereof is modified by ion bombardment.
The gas separation membrane of the present invention can be used for separating and/or concentrating a specific component such as hydrogen gas, carbon dioxide gas, oxygen gas, nitrogen gas, water vapor, organic matter gas, and organic matter vapor, from a gas mixture.
The polymeric material to be used in the present invention is not particularly limited and any material may be employed, as long as the material is soluble in an organic solvent and is fabricable to form a membrane. Preferred examples of the polymeric material in the present invention include polyimides (including fluorine-containing polyimides), polyethers (polyether, polyether sulfone, polyether ketone and the like), polyesters, and polyamides. Fluorine-containing polyimides are particularly preferred.
Polyimides are obtained generally by polycondensation of tetracarboxylic acid dianhydride and diamine in equimolar amount, and are polymers having a repeating unit of a structure formed by imide-binding of a quadrivalent structural portion derived from tetracarboxylic acid dianhydride and a divalent structural portion derived from diamine component. In the present invention, it is particularly preferable to use a fluorine-containing polyimide containing at least one —CF
3
group in the repeating molecular structure unit. Since aromatic polyimides are generally insoluble and infusible, the formation of a membrane of aromatic polyimides is difficult. The fluorine-containing polyimide exhibits excellent solubility in an organic solvent, and therefore it has an advantage to readily form a microthin membrane.
In the present invention, the fluorine content of the fluorine-containing polyimide resin is not particularly limited. The number of fluorine atoms in the repeating molecular structural unit is preferably 6 to 12 in order to obtain a gas separation membrane having substantially stable high quality.
Examples of polyimides (including fluorine-containing polyimides) which can be used in the present invention include polymers having repeating units represented by the following structures, wherein the subscript n represents the number of repeating units.
The fluorine-containing polyimides used in the present invention can be prepared using tetracarboxylic acid dianhydride and a diamine compound by a known polymerization method. For example, almost equimolar amounts of tetraca
Iwaki Masaya
Kawakami Hiroyoshi
Nagaoka Shoji
Suzuki Yoshiaki
Greenblum & Bernstein P.L.C.
Riken
Spitzer Robert H.
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