Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2001-01-19
2002-10-15
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C096S008000, C096S010000, C096S014000
Reexamination Certificate
active
06464755
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gas separation membrane with an asymmetric structure composed of a skin layer and a porous layer, the gas separation membrane being characterized in that the membrane permeation rate of the permeate gas (membrane permeating component) is increased by lowering the resistance of the permeate gas as it permeates the porous layer, and in that it has at least a practical level of mechanical strength as a hollow fiber gas separation membrane. The invention further relates to a gas separation membrane with excellent water resistance and hot water resistance. The invention still further relates to a dehumidification method and a humidification method characterized by employing the aforementioned gas separation membrane.
2. Description of the Related Art
Gas separation membranes are used in a variety of gas separation methods. Most of them are formed of glassy polymers that have high selectivity. Generally speaking, glassy polymers have high selectivity (degree of separation), but have the drawback of a low gas permeation coefficient. Most gas separation membranes formed of glassy polymers are therefore used with an asymmetric structure composed of a porous layer (support layer) and a thin skin layer (selective layer), i.e., the selective layer that produces permeation resistance against the gas is reduced in thickness so that the gas permeation rate is not too low.
Gas separation membranes are generally used as hollow fiber gas separation membrane modules constructed by bundling a large number of hollow fiber membranes (for example, from a hundred to a few hundred thousand) into a hollow fiber bundle, and anchoring at least one end of the hollow fiber bundle with a thermosetting resin such as an epoxy resin or with a thermoplastic resin, in such a manner that the hollow fiber membrane is open at that end, to construct a hollow fiber separation membrane element, and then inserting one or a plurality of these hollow fiber separation membrane elements into a container with at least a mixed gas inlet, a permeating gas outlet and a non-permeating gas outlet in a manner which partitions the space passing inside the hollow fiber membranes from the space passing outside the hollow fiber membranes. In hollow fiber gas separation membrane modules, the mixed-gas is supplied to the space contacting the inside or outside of the hollow fiber membranes, and specific components (permeate gases) in the mixed gas selectively permeate the membrane while it flows in contact with the hollow fiber membranes and are recovered through the permeating gas outlet, while the gas depleted of those specific components (permeate gases) is recovered through the non-permeating gas outlet, thus accomplishing gas separation.
As concerns gas separation membranes made of polymer blends, U.S. Pat. No. 5,055,116 discloses a gas separation membrane comprising a blend of two different polyimides with specific molecular structures, and it shows that the permeation rate for oxygen and nitrogen can be linearly controlled by the polyimide blend ratio. Also, U.S. Pat. No. 5,248,319 discloses a gas separation membrane comprising a blend of a polyimide with a phenylindane residue and a specific polyimide, polyamide or polyamideimide. U.S. Pat. No. 5,608,014 discloses a gas separation membrane comprising a blend of a specific polyethersulfone, a specific aromatic polyimide and a specific aromatic polyimide or polyamide or polyamideimide, and U.S. Pat. No. 5,917,137 discloses a gas separation membrane comprising a blend of a specific polyethersulfone and a specific aromatic polyimide. These publications, however, do not mention the water vapor permeation rate or the gas permeation resistance of the porous layer. They also contain no disclosure or suggestion regarding a gas separation membrane with a very high water vapor permeation rate while also having mechanical strength suitable for practical use as an asymmetric hollow fiber membrane in an industrial module.
Water resistance and hot water resistance are important properties for dehumidifying membranes and humidifying membranes. However, membranes with improved permeation rates for gases containing water vapor have often exhibited inferior water resistance and hot water resistance. Japanese Unexamined Patent Publication No. 2-222717 discloses a polyimide separation membrane with excellent water resistance and hot water resistance, but it is used for the dewatering of organic vapor and has a low water vapor permeation rate.
For membranes with asymmetric structures, the rate-determining parameter of the permeation rate at which the permeate gas passes through the membrane is the process by which the permeate gas passes through the skin layer of the membrane. In the process in which the permeate gas passes through the porous layer of the membrane, there is a relatively low permeation resistance. In most cases, therefore, it is possible to substantially ignore the effect, on the process, that the permeate gas passing through the porous layer of the membrane has on the permeation rate.
However, in cases where the skin layer is exceedingly thin so that the permeation rate at which the permeate gas passes through the membrane is very high, or in cases where the membrane permeating component is a gas that permeates very easily through the membrane, the permeation rate at which the permeate gas passes through the membrane is sometimes notably affected by the rate of the permeate gas passing through the porous layer. In such cases, a membrane with an asymmetric-structure can still be improved in the permeation rate at which the permeate gas passes through the membrane, and efforts have been made to develop a more compact, high performance gas separation membrane with higher efficiency through such an improvement. When the component passing through the membrane is water vapor, since water vapor has a much higher permeation rate through membranes than other inorganic gases (from a few hundred times to a few thousand times greater), the permeation rate of water vapor through the membrane is particularly affected by the permeation resistance of the porous layer. Consequently, it has been considered that reducing the permeation resistance for passage of water vapor through the porous layer could increase the permeation rate of water vapor passing through the membrane, and efforts have been made to carry out this improvement to develop compact, high performance dehumidifying membranes and/or humidifying membranes with high efficiency, due to an increased permeation rate of water vapor through the membranes.
For membranes with an asymmetric structure, however, when it is attempted to further reduce the permeation resistance for membrane permeating components passing through the membrane, by simply reducing the porous layer thickness or increasing the. porosity of the porous layer, in order to increase the permeation rate of the permeate gases passing through the membrane, the permeation rate is successfully increased but at the expense of the membrane support function performed by the porous layer, i.e., the mechanical strength. For this reason, it has been difficult to obtain practical, high performance gas separation membranes having both an improved permeation rate for permeation of permeate gases through the membrane, and mechanical strength at a level suitable for actual use as an asymmetric hollow fiber membrane for an industrial module, i.e., a practical level of mechanical strength.
In addition, when gas separation membranes have been used for dehumidifying or humidifying, membranes with inferior water resistance and hot water resistance are problematic as they cannot be used stably overlong periods and their uses are limited. Efforts have therefore also been directed toward developing dehumidifying membranes and/or humidifying membranes with excellent water resistance and hot water resistance.
BRIEF SUMMARY OF THE INVENTION
The present invention has been achieved in light of the circumstances descri
Ito Kenji
Kusuki Yoshihiro
Nakanishi Shunsuke
Yoshinaga Toshimune
Morgan & Lewis & Bockius, LLP
Spitzer Robert H.
Ube Industries Ltd.
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