Highly permeable composite reverse osmosis membrane

Details Highly permeable composite reverse osmosis membrane Highly permeable composite reverse osmosis membrane

1997-01-21

2001-01-09

Fortuna, Ana (Department: 1723)

Liquid purification or separation

Filter

Material

C210S490000, C210S321600, C210S321740, C210S321750

Reexamination Certificate

active

06171497

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a composite reverse osmosis membrane for selectively separating components in a liquid mixture, and more particularly to a composite reverse osmosis membrane having a high salt rejection and a high flux, comprising an active layer or a skin layer referred to as a thin film that mainly contains polyamide. Such a composite reverse osmosis membrane is preferably used for many purposes such as production of ultrapure water and desalination of sea water or brackish water. This membrane can also be used to remove and withdraw contamination source or effective materials from contamination which is the cause of pollution, for example, dyeing baths or electrochemical deposition coating waste. The contaminated water can be cleaned for reuse. Furthermore, it is possible to condense effective components for food and to remove harmful matters for purified water and sewage.
By using such a composite reverse osmosis membrane, operation can be performed at an ultralow pressure which cannot be pratically performed by using a composite reverse osmosis membrane according to the prior art, and the composite reverse osmosis membrane has a high salt rejection performance. Consequently, it is very excellent in economy. For example, the composite reverse osmosis membrane can also be utilized as a domestic water purifier.
BACKGROUND OF THE INVENTION
Conventionally, a composite reverse osmosis membrane, which is obtained by forming an active skin layer substantially having selective separating properties on a microporous support, has been known as a reverse osmosis membrane whose structure is different from that of an assymmetric reverse osmosis membrane.
There have been composite reverse osmosis membranes in which a skin layer comprising polyamide obtained by interfacial polycondensation of polyfunctional aromatic amine and polyfunctional aromatic acid halide is formed on a support (for example, Japanese Laid-Open Patent Publication Nos. 55-147106, 62-121603 and 63-218208, Japanese Patent Publication No. 6-73617, and the like).
The composite reverse osmosis membranes described above have a high desalination performance and a high flux performance. However, it has been desirable that the composite reverse osmosis membranes should further improve the flux performance while keeping a high desalination performance in respect of a reduction in operation and equipment costs, efficiency and the like. For these requirements, various additives have been proposed (for example, Japanese Laid-Open Patent Publication No. 63-12310), which have improved performance but are insufficient. Furthermore, after-treatment of the composite reverse osmosis membrane performed by using chlorine and the like has also been proposed (for example, Japanese Laid-Open Patent Publication Nos. 63-54905 and 1-180208).
However, the reverse osmosis membrane according to the prior art has a salt rejection of 98.9% and a flux of 2.2 m
3
/m
2
·day at an operating pressure 15 kgf/cm
2
as an example of the highest flux. When operation is performed at a pressure of 7.5 kgf/cm
2
, the flux is reduced to about half and the salt rejection is considerably lowered. For this reason, the reverse osmosis membrane could not be practically used at a pressure of 7.5 kgf/cm
2
. Moreover, the flux was rarely obtained at a pressure of about 3 kgf/cm
2
. Therefore, the reverse osmosis membrane could not be used. Accordingly, a composite reverse osmosis membrane having a high desalination performance at a low pressure has been required with respect to economy.
SUMMARY OF THE INVENTION
In order to solve the above-mentioned problems, it is an object of the present invention to provide a composite reverse osmosis membrane having a high salt rejection and a high flux performance. In particular, it is an object of the present invention to provide a composite reverse osmosis membrane which can be practically used at a low pressure of about 3 kgf/cm
2
or less.
In order to achieve the above-mentioned object, a highly permeable composite reverse osmosis membrane of the first aspect of the present invention comprises a thin film, and a microporous support for supporting the thin film, wherein the thin film is a polyamide based skin layer obtained by polycondensation of a compound having two or more reactive amino groups and a polyfunctional acid halide having two or more reactive acid halide groups. The roughness of the polyamide based skin layer on the surface of the composite reverse osmosis membrane is 50 nm or more. The composite reverse osmosis film is treated with an oxidizing agent and provides a salt rejection of 99% or more and a flux of 1.5 m
3
/m
2
·day or more, when evaluated by using a solution having pH 6.5 containing 500 ppm of sodium chloride at an operating pressure of 7.5 kgf/cm
2
and a temperature of 25° C.
Next, a highly permeable composite reverse osmosis membrane of the second aspect of the present invention comprises a high permeable composite reverse osmosis membrane comprising a thin film, and a microporous support for supporting the thin film, wherein the thin film is a polyamide based skin layer obtained by polycondensation of a compound having two or more reactive amino groups and a polyfunctional acid halide having two or more reactive acid halide groups. The specific surface area of the polyamide based skin layer on the surface of the composite reverse osmosis membrane is 3 or more (wherein the specific surface area of the skin layer is represented by the following formula; The specific surface area of the skin layer=(the surface area of the skin layer)/(the surface area of the microporous support)).
The composite reverse osmosis film is treated with an oxidizing agent and provides a salt rejection of 99% or more and a flux of 1.5 m
3
/m
2
·day or more, when evaluated by using a solution having pH 6.5 containing 500 ppm of sodium chloride at an operating pressure of 7.5 kgf/cm
2
and a temperature of 25° C.
It is preferable that the polyamide based skin layer includes as aromatic polyamide.
It is preferable that the polyamide based skin layer is essentially total aromatic polyamide which obtained by polycondensation of a compound having two or more reactive aromatic amino and a polyfunctional aromatic acid halide having two or more reactive acid halide.
It is preferable that the mean roughness of the polyamide based skin layer on the surface of the composite reverse osmosis membrane is 10000 nm or less, and more preferably, 1000 nm or less.
The composite reverse osmosis film should be treated with an oxidizing agent. Also in the case where the treatment is not performed with the oxidizing agent, a composite reverse osmosis membrane which satisfies the mean roughness according to the present invention has a high flux. However, it is necessary to perform the treatment with the oxidizing agent in order to obtain a salt rejection of 99% or more and a flux of 1.5 m
3
/m
2
·day or more when evaluated by using a solution having pH 6.5 containing 500 ppm of sodium chloride at an operating pressure of 7.5 kgh/cm
2
and a temperature of 25° C.
When evaluated by using a solution having pH 6.5 containing 500 ppm of sodium chloride at an operating pressure of 7.5 kgf/cm
2
and a temperature of 25° C., the composite reverse osmosis membrane should have a salt rejection of 99% or more and a flux of 1.5 m
3
/m
2
·day or more.
With the above-mentioned structure, it is preferable that the mean square roughness of the polyamide based skin layer on the surface of the composite reverse osmosis membrane is 60 nm or more.
It is preferable that the mean square roughness of the polyamide based skin layer on the surface of the composite reverse osmosis membrane is 20000 nm or less, and more preferably, 2000 nm or less.
With the above-mentioned structure, it is preferable that the 10-point mean roughness of the polyamide based skin layer on the surface of the composite reverse osmosis membrane is 280 nm or more. It is preferable that the 10-point mean roughness of the polyamide based skin layer on th

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