Liquid purification or separation – Filter – Material
Reexamination Certificate
1998-04-06
2002-07-02
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Filter
Material
C210S490000, C210S500340, C210S652000, C210S653000
Reexamination Certificate
active
06413425
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to improvement in the performance of a reverse osmosis composite membrane such as performance stability and fouling tolerance for selectively separating the components of a liquid mixture. More particularly, the present invention relates to a reverse osmosis composite membrane having a high salt rejection, a high chlorine tolerance, and a high fouling tolerance, which comprises a polymer thin film on the reverse osmosis composite membrane, and to a reverse osmosis treatment method for water using the same.
Such a reverse osmosis composite membrane is suitable for manufacturing ultrapure water, desalinating brackish water, and the like, and it also can contribute to the removal and recovery of the contaminating sources or effective substances from a soil or the like, the cause of pollution in a dyeing waste water system or an electrochemical deposition paint waste water system to implement a waste water recycling system. In particular, it can operate stably for a long period with respect to the quality of water containing various membrane-fouling substances, such as surfactants and transition metal components including iron, which cause a decrease in flux.
BACKGROUND OF THE INVENTION
Examples of the industrially utilized reverse osmosis membranes include Loeb type membranes described in U.S. Pat. Nos. 3,133,132 and 3,133,137 as asymmetric membranes made of cellulose acetate. On the other hand, reverse osmosis composite membranes, in which an active thin film substantially having a selective separation property is formed on a microporous support membrane, are known as reverse osmosis membranes having a different structure from the asymmetric reverse osmosis membranes.
Presently, a number of such reverse osmosis composite membranes, in which a thin film of polyamide obtained by interfacial polymerization of polyfunctional aromatic amine and polyfunctional aromatic acid halide is formed on a support membrane, are known (for example, Publication of Japanese Patent Application (Tokkai Sho) No. 55-147106, Publication of Japanese Patent Application (Tokkai Sho) No. 62-121603, Publication of Japanese Patent Application (Tokkai Sho) No. 63-218208, and Publication of Japanese Patent Application (Tokkai Hei) No. 2-187135). Also, those having a thin film of polyamide obtained by interfacial polymerization of polyfunctional aromatic amine and polyfunctional alicyclic acid halide formed on a support membrane are known (for example, Publication of Japanese Patent Application (Tokkai Sho) No. 61-42308).
In addition, various methods for after treatment of the reverse osmosis membrane are disclosed. For example, methods using various polymers as a protective layer are disclosed (for example, Publication of Japanese Patent Application (Tokkai Sho) No. 51-13388, Publication of Japanese Patent Application (Tokkai Sho) No. 53-16372, Publication of Japanese Patent Application (Tokkai Sho) No. 62-197105, and Publication of Japanese Patent Application (Tokko Hei) No. 7-90152) Recently, it has been expected to apply a reverse osmosis membrane to a treatment for water containing fouling substances such as various surfactants, for example, sewage. In addition to the high performance of the reverse osmosis membrane (a high salt rejection and a high water permeability), a high fouling resistance is required to maintain the desired flux for a long period. The above reverse osmosis membranes and the conventional after treatment methods are not sufficient to satisfy these two requirements. Therefore, a reverse osmosis composite membrane having a higher performance has been sought.
One fouling mechanism includes the charge condition of the membrane. For example, the surface of a cross-linked polyamide reverse osmosis membrane obtained by interfacial polymerization of polyfunctional aromatic amine and polyfunctional alicyclic acid halide has a negative charge due to the residual carboxylic acid. The membrane surface having negative charge adsorbs, for example, cationic fouling substances, decreasing the flux. Therefore, a membrane has been required that is neutral in charge and has a high water permeability and a high salt rejection.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a reverse osmosis composite membrane that has a high salt rejection, a high water permeability, and a high fouling tolerance and permits practical desalination at a low pressure, and a reverse osmosis treatment method for water using the same.
In order to achieve the above object, the present invention provides a reverse osmosis composite membrane comprising a sponge layer, and a separation layer (also referred to as a skin layer) formed on a surface of the sponge layer, wherein at least one substance selected from the group consisting of an electrically neutral organic substance and an electrically neutral polymer is present in the separation layer or a surface of the separation layer is coated with at least one substance selected from the group consisting of an electrically neutral organic substance and an electrically neutral polymer, and wherein a specific surface area of the layer in which the at least one substance is present or the separation layer before the surface coating is in the range of 2 to 1,000. The specific surface area is defined by the following expression: the specific surface area of the separation layer=(the surface area of the separation layer)/(the surface area of the support).
The surface area of the separation layer represents the area of the surface to be in contact with a feed liquid. The surface area of the support indicates the surface area of a support membrane such as polysulfone, with which the sponge layer under the separation layer is in contact. The specific surface area of the separation layer is 2 or more when the surface of the support membrane is relatively flat and the surface of the separation layer is rough and has wrinkles. The surface area and the specific surface area are measured by a general measurement device, for example, a surface area measurement device, a specific surface area measurement device, a scanning electron microscope, a transmission electron microscope, or an atomic force microscope.
In the reverse osmosis composite membrane, it is preferable that the specific surface area of the layer in which the at least one substance is present or the separation layer before the surface coating is in the range of 3 to 500. When the specific surface area is 3 or more, the water permeability is improved. If the specific surface area is more than 500, the strength of the separation layer decreases.
In the reverse osmosis composite membrane, the specific surface area of the separation layer after the surface coating is preferably reduced to 90% or less of the specific surface area of the separation layer before the surface coating, and more preferably, 60% or less. If the specific surface area of the separation layer after the surface coating is more than 90% of that of the separation layer before the surface coating, the surface is not sufficiently covered, preventing a stable fouling tolerance for a long period.
In the reverse osmosis composite membrane, it is preferable that the at least one substance selected from the group consisting of an electrically neutral organic substance and an electrically neutral polymer is an organic substance or a polymer that has a nonionic hydrophilic group. Electrical neutrality is preferred for controlling the electrical adsorption by the membrane of membrane-fouling substances having a charge group present in water. If the organic substance or the polymer has a nonionic hydrophilic group (for example, a —OH group) in addition to the electrical neutrality, adsorption due to a hydrophobic interaction on the membrane is controlled when the membrane-fouling substances have a hydrophobic group.
In the reverse osmosis composite membrane, it is preferable that the organic substance or the polymer that has a nonionic hydrophilic group is polyvinyl alcohol that is water-in
Hachisuka Hisao
Ikeda Kenichi
Fortuna Ana
Nitto Denko Corporation
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