Liquid purification or separation – Filter – Material
Reexamination Certificate
2000-08-10
2004-03-23
Fortuna, Ana M. (Department: 1723)
Liquid purification or separation
Filter
Material
C210S490000, C210S500370, C427S244000, C427S245000, 62, 62, 62
Reexamination Certificate
active
06709590
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a composite reverse osmosis membrane for selectively separating components in a liquid mixture and a method for producing the same. More specifically, the present invention relates to a composite reverse osmosis membrane having a high rejection property provided by a polyamide-based skin layer (referred to also as an active layer or a thin layer) on a porous support and, and a method for producing such a composite reverse osmosis membrane.
2. Description of the Prior Art
Conventionally, a composite reverse osmosis membrane where a skin layer (active layer or a thin layer) for selective separation is formed on a porous support is known as a reverse osmosis membrane having a structure different than that of an asymmetric reverse osmosis membrane. One such example is a composite reverse osmosis membrane having a polyamide skin layer formed by interfacial polymerization between polyfunctional aromatic amine and polyfunctional aromatic acid halide on a porous support (e.g., JP 55-147106 A, JP 62-121603 A, JP 63-218208 A and JP 2-187135 A).
Some of the conventional composite reverse osmosis membranes have excellent desalination performance, water permeability performance and ionic substance-separation performance. However, the conventional composite reverse osmosis membranes do not have sufficient rejections for nonelectrolyte organic compounds such as isopropyl alcohol (IPA). Furthermore, the conventional composite reverse osmosis membranes do not have sufficient rejections for substances that are not dissociated at the pH range where they are normally used (e.g., boron). IPA is a substance that is widely used in the semiconductor industry and the like, and boron is a problematic substance in the conversion of seawater to freshwater. Therefore, development of a composite reverse osmosis membrane capable of separating these substances at high rejections is desirable.
SUMMARY OF THE INVENTION
Embodiments of the present invention provide a composite reverse osmosis membrane that has excellent desalination performance, water permeability performance and ionic substance-separation performance and is capable of separating nonelectrolyte organic compounds and nondissociative substances at the pH range where these composite reverse osmosis membranes are normally used (hereinafter, referred to also as “general pH range nondissociative substances”) at a high rejection, and a method for producing the membrane.
In one aspect, the present invention relates to a composite reverse osmosis membrane including a porous support and a polyamide skin layer formed on the porous support, wherein the polyamide skin layer is formed by reacting an aromatic compound having at least two reactive amino groups with a polyfunctional acid halide compound having at least two reactive acid halide groups, and the polyamide skin layer contains bromine.
Such a composite reverse osmosis membrane of the present invention has excellent desalination performance, water permeability performance and ionic substance-separation performance and is capable of separating nonelectrolyte organic compounds such as IPA and general pH range nondissociative substances such as boron with high rejections.
In some embodiments of the composite reverse osmosis membrane of the present invention, bromine contained in the polyamide skin layer may be present in the form of bromine atoms or ions. The polyamide skin layer contains bromine, but can also contain other substances. However, when the polyamide skin layer has only chlorine, the rejection of nonelectrolyte organic compounds and general pH range nondissociative substances is not sufficiently improved. In one embodiment of the composite reverse osmosis membrane of the present invention, it is preferable that the aromatic compound portion of the polyamide skin layer has bromine. For example, the bromine atom substitutes for a hydrogen atom so as to be bound to a carbon atom of the aromatic ring of the aromatic compound.
In some embodiments of the composite revere osmosis membrane of the present invention, it is preferable that the ratio (Br/N) of the number of the bromine atoms (Br) to the nitrogen atoms (N) is not less than 0.1 and not more than 1.0. When this ratio is 0.1 or more, the nonelectrolyte organic compounds and the general pH range nondissociative substances can be separated at a higher rejection. When the ratio is 1.0 or less, the nonelectrolyte organic compounds and the general pH range nondissociative substances can be separated at a higher rejection, while the flux is not too low and the operational pressure is not too high, leading to economical advantages. A more preferable range of the ratio is not less than 0.15 and not more than 0.7, and a most preferable range is not less than 0.2 and not more than 0.5. In the specification, “flux” refers to a volume (m
3
) of solution that permeates the membrane a day per m
2
of the area of one surface of the membrane. “Rejection” is expressed by Equation (1) below:
Rejection (%)=(1−(the concentration of the permeate solution/the average concentration of the feed solution))×100 (1)
In some embodiments of the composite reverse osmosis membrane of the present invention, the IPA rejection at a temperature of 25° C., a pH of 6.5, and an operational pressure of 1.5 MPa is at least 98.5%, when a 0.3 wt % IPA aqueous solution is used as the feed solution. When the IPA rejection is at least 98.5%, the IPA concentration in the permeate water can be sufficiently reduced.
In some embodiments of the composite reverse osmosis membrane of the present invention, the salt rejection at a temperature of 25° C., a pH of 6.5 and a flux of 0.6 m
3
/m
2
·day is at least 99%, when a 3.5 wt % salt water containing 5 ppm of boron is used as the feed solution, and the boron rejection is at least 85% under those conditions. Bromine is present at about 4 to 5 mg/l in the seawater. In general, when the conversion from seawater to freshwater is performed by using a reverse osmosis membrane, a recovery ratio (the amount of the permeate water/the amount of the feed water) of 40% is obtained. When the boron rejection is 85% or more in operation at a recovery ratio of 40%, the concentration of boron in the permeate water is 0.7 to 0.9 mg/l when the concentration of boron in the feed seawater is 4 to 5 mg/l. This can be further reduced to a sufficiently low level for drinking water. More preferably, the boron rejection is 88% or more (further more preferably 90% or more). In this case, the concentration of boron in the permeate water is 0.6 to 0.7 mg/l. This embodiment can maintain a sufficiently low concentration of boron for drinking water, even if the performance is degraded by aging of the membrane and so on.
In another aspect, the invention relates to a method for producing a composite reverse osmosis membrane including forming a polyamide skin layer on a porous support by reacting an aromatic compound having at least two reactive amino groups with a polyfunctional acid halide compound having at least two reactive acid halide groups, and treating the polyamide skin layer with a free chlorine aqueous solution containing bromine compound.
In some embodiments, the concentration of free chlorine in the free chlorine aqueous solution is 1 to 100 mg/l. When it is 1 mg/l or more, the rejection of the resultant membrane for the nonelectrolyte organic compounds and the general pH range nondissociative substances is further improved. When it is 100 mg/l or less, the flux of the resultant membrane is acceptable. It is more preferable that the concentration of free chlorine is in the range from 5 to 50 mg/l.
It is preferable that the concentration of bromine in the free chlorine aqueous solution is in the range from 0.5 to 100 mg/l. When it is 0.5 mg/l or more, the rejection is further improved. When it is 100 mg/l or less, the flux is prevented from being reduced. It is more preferable that the concentration of bromine is in the range from 1 to 50 mg/l.
The
Fortuna Ana M.
Nitto Denko Corporation
Rosenthal & Osha L.L.P.
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