JP3210733B2 - Multistage reverse osmosis system and method for producing ultrapure water using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for membrane-treating a liquid using a plurality of membrane modules connected in series.
More particularly, the present invention relates to a multistage reverse osmosis (hereinafter, sometimes referred to as RO) system capable of stably obtaining ultrapure water essential for semiconductor production for a long period of time.

[0002]

2. Description of the Related Art Ultrapure water used for cleaning LSIs in the manufacture of semiconductors has been required to have a higher level of water quality as the integration density has further increased in recent years.
For this reason, when the water quality level of ultrapure water rises, elution of organic compounds from ion exchange resins, which has conventionally been responsible for removing impurities by adsorption, has become a problem.

As a means for solving this problem, a two-stage RO system using a reverse osmosis (RO) membrane module instead of the regenerative ion exchange resin used in the preceding stage of the ultrapure water production line has recently been developed. Is coming. This system changes the charge of the first and second stage membranes,
A method for efficiently blocking ions is known (for example, JP-A-61-287492).

[0004]

In general, as a reverse osmosis membrane, various composite semipermeable membranes in which an active layer and a supporting layer for supporting the active layer are manufactured from different materials have been proposed. What consists of a crosslinked polyamide has become mainstream because of its ease of use. These have negative fixed charge groups, and these are often used in the first stage of a two-stage RO system, and the second stage requires a membrane having a positive charge. For this reason, there has been proposed a composite semipermeable membrane in which the surface of a conventional reverse osmosis membrane having negative chargeability is coated with an organic polymer having a positive fixed charge group (for example, Japanese Patent Application Laid-Open No. Sho 62-62). 266103).

However, since such a composite semipermeable membrane mainly has an adsorption mechanism in view of its production method, the desired performance may be reduced due to a certain amount of removal of the charged group upon repeated use. Point was insufficient. Therefore, even if a two-stage RO system is manufactured using such a membrane module, the quality of the permeated water deteriorates in a short period of time, and when this system is used in an ultrapure water production line, The subsequent non-regeneration type ion exchange resin has a disadvantage that the load on the ion exchange resin is increased and the resin needs to be frequently replaced.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in order to solve the problems of the prior art, and has a multi-stage RO system capable of maintaining stable performance for a long period of time and an ultrapure water system using the same. And a method for producing the same.

That is, according to the present invention, there is provided a method of treating a liquid with a plurality of membrane modules connected in series, wherein at least one membrane module is provided with a negatively chargeable composite semipermeable membrane.
Is a crosslinked polymer layer formed by crosslinking a polymer having a positive charge
The present invention relates to a multi-stage reverse osmosis system characterized by using a composite semipermeable membrane coated with a polymer.

Hereinafter, the present invention will be described by taking a two-stage RO system as an example. However, the present invention is not limited to a two-stage type RO system, and can be applied to a two-stage type or more multi-stage type.

In the present invention, a specific composite semipermeable membrane having positive charge may be used for at least one of a plurality of membrane modules connected in series. For example, a two-stage RO system In the case of, either composite positive or negatively chargeable semipermeable membrane may be used in the first stage, but the composite semipermeable membrane in the second stage has the opposite chargeability to the first stage. And Normally, it is preferable to use a negatively chargeable composite semipermeable membrane in the first stage, since NaOH is added to raw water to convert carbon dioxide into HCO ₃ ^- or CO ₃ ^2- to remove carbon dioxide.

The negatively chargeable composite semipermeable membrane used herein is:
For example, an active layer having negative charge is formed on a microporous support layer made of, for example, polysulfone. Such an active layer is composed of a crosslinked organic polymer having groups such as carboxylic acid, sulfonic acid, phosphoric acid, and sulfuric acid as negatively fixed charged groups, and is synthesized by interfacial polycondensation, particularly from the viewpoint of desalination performance and water permeability. Crosslinked polyamide-based polymers are preferred.
Specifically, a composite semipermeable membrane having piperazine polyamide, aromatic polyamide, or the like as an active layer can be given.

The composite semipermeable membrane coated with a crosslinked polymer layer obtained by crosslinking a polymer having a positive charge used in the present invention means that the surface of the above-mentioned various negatively fixed charged membranes is positively charged. Any structure may be used as long as it is coated with a three-dimensionally crosslinked organic polymer having a group, and the structure of the organic polymer is not particularly limited.

However, in the present invention, workability,
From the viewpoint of processability and the like, it is desirable that the organic polymer itself is soluble in a solvent. Therefore, it is preferable that the organic polymer is three-dimensionally crosslinked after coating on the composite semipermeable membrane. As such an organic polymer, a polymer having a functional group capable of causing a cross-linking reaction with a positively charged group in the molecule, which is itself soluble in a solvent is used. For example, as an example, a polymer having at least two hydroxyl groups and / or amino groups in a molecule together with a positively charged group (hereinafter referred to as polymer A), or at least two hydroxyl groups and / or amino groups together with a positively charged group in a molecule And a polymer having a group and two protected isocyanate groups (hereinafter referred to as polymer B).

Here, the positively charged group includes an ammonium group, a phosphonium group, a sulfonium group and the like. In addition, the protected isocyanate group,
An isocyanate group blocked with a blocking agent or an isocyanate group protected in the form of an amine imide group.

Various blocking agents for blocking the isocyanate group are known, for example, phenols such as phenol and cresol, alcohols such as methanol, ethanol and methyl cellosolve, methyl ethyl ketoxime and acetaldehyde oxime. And the like.

Examples of the polymer A include a homopolymer of hydroxypropyltrimethylammonium methacrylate and a copolymer with other polymerizable monomers, and a copolymer of ethyltrimethylammonium methacrylate with hydroxyethyl methacrylate. Copolymers and quaternized copolymers of 4-vinylpyridine and hydroxyethyl methacrylate can be exemplified.

The polymer B is, for example, 2
A copolymer of an isocyanate monomer obtained by blocking methacryloyloxyethylene isocyanate with an appropriate blocking agent and hydroxypropyltrimethylammonium methacrylate chloride, a copolymer of the above blocked isocyanate with 4-vinylpyridine and hydroxyethyl methacrylate Quaternized copolymer, copolymer of vinyl monomer having an amine imide group such as 1,1-dimethyl-1- (2-hydroxypropyl) amine methacrylimide and hydroxypropyltrimethylammonium methacrylate chloride, etc. Can be mentioned.

The above polymers A and B are both soluble in water and alcohol. Accordingly, in the present invention, the crosslinked polymer layer can be formed on the semipermeable ultrathin film of the conventional composite semipermeable membrane by, for example, the following various methods.

In order to form a crosslinked polymer layer formed by crosslinking the polymer A, an aqueous solution or alcohol solution of the polymer A is applied to the composite semipermeable membrane, dried, and then a polyisocyanate as a polyfunctional crosslinking agent is applied. The solution in which the compound is dissolved is brought into contact with the solution, and if necessary, the polymer A is crosslinked between the molecules by heating.

As another method, a polyfunctional polyisocyanate compound blocked with a blocking agent as described above is added to an aqueous solution or alcohol solution of polymer A,
After applying the obtained solution to the composite semipermeable membrane, heated to a temperature equal to or higher than the dissociation temperature of this blocked polyisocyanate,
The polyisocyanate compound may be liberated to cause a crosslinking reaction with the polymer A.

The polyisocyanate compound used here is not particularly limited, and examples thereof include tolylene diisocyanate and diphenylmethane diisocyanate, polymers thereof, isophorone diisocyanate hexamethylene diisocyanate, triphenylmethane triisocyanate, tris (p- Isocyanate phenyl) thiophosphite; adducts of trimethylolpropane with tolylene diisocyanate; adducts of trimethylolpropane with xylylene diisocyanate;

In order to form the crosslinked polymer layer of the polymer B on the composite semipermeable membrane, for example, a water or alcohol solution of the polymer B is applied on the composite semipermeable membrane, and the blocked isocyanate is applied. What is necessary is just to heat to a temperature higher than the dissociation temperature to release the isocyanate group and to crosslink between molecules or / and intramolecularly.

In order to promote the above-mentioned cross-linking reaction between isocyanate groups and hydroxyl groups,
If necessary, a catalyst such as a tertiary amine or an organotin compound can be used.

Even if the organic polymer does not have a crosslinkable functional group as described above, the composite semipermeable membrane may be irradiated with an electron beam after being coated with the organic polymer having a positive charge, or may be irradiated with an organic polymer. By mixing a peroxide in the coalescing solution, coating the composite semipermeable membrane, and then heating, a radical is generated on the organic polymer skeleton to perform three-dimensional crosslinking.

In the present invention, the thickness of the crosslinked polymer layer thus formed is usually preferably in the range of 10 Å to 10 μm. When the thickness is less than 10 angstroms, salt removal performance is hardly improved even when the obtained composite semipermeable membrane is used in a two-stage RO system,
On the other hand, when it exceeds 10 μm, the water permeability of the obtained membrane is remarkably reduced, which is not preferable.

Hereinafter, the multistage RO system of the present invention will be described with reference to the drawings. FIG. 1 is an explanatory diagram showing a two-stage RO system as an example of a multi-stage RO system.
In FIG. 1, reference numeral 1 denotes a first-stage membrane module;
Reference numeral 11 denotes a stock solution chamber of the module 1 and reference numeral 12 denotes a permeate solution chamber. Reference numeral 2 denotes a second-stage membrane module, reference numeral 21 denotes a stock solution chamber of the module 2, and reference numeral 22 denotes a permeation chamber. The first
The permeated liquid chamber 12 of the first-stage module 1 communicates with the stock solution chamber 21 of the second-stage module 2 so that both modules 1 and 2 are connected in series. 31 and 32 are each module 1,
2 is a pressure regulating valve provided at the outlet of the stock solution chamber.
The pressure of the stock solution chamber of each module 1 and 2 is adjusted by 1 and 32. Reference numeral 4 denotes a liquid sending pump, and reference numeral 5 denotes a stock solution tank.

In order to perform the membrane treatment of the undiluted solution according to the present invention, the pressure of the undiluted solution chamber of each of the modules 1 and 2 is set to a predetermined pressure by operating the pressure regulating valves 31 and 32, and the liquid sending pump 4 is driven. In this case, if the undiluted solution contains a metal salt, for example, NaCl, and the first-stage membrane module 1 is in a negatively charged state, an electrostatic attraction in the permeation direction acts on Na ⁺ ions. Therefore, the rejection of Na ⁺ is lower than the rejection of Cl ⁻ , and the permeate in the first-stage module 1 is N
The a ⁺ concentration is higher than the Cl ^- concentration. The permeate reaches the second stage module 2 and is treated by the membrane of the module. However, since the charge state of the membrane of the second-stage module 2 is opposite to that of the first-stage module 1 and is in a positive charge state, the rejection of Na ⁺ is larger than that of Cl ^−. In the permeated liquid of the second-stage membrane module 2, the difference between the Na ⁺ concentration and the Cl ⁻ concentration is reduced (the imbalance between both ion concentrations is eliminated), and the permeated liquid quality can be improved.

In the above, NaOH is injected into the permeate of the first-stage module 1 and the carbon dioxide in the permeate is converted into HCO ₃
^{When the} carbon dioxide gas is finally removed from the permeated liquid by converting these ions into CO ² or CO ₃ ^2- and removing these ions in the second-stage module 2, the permeated liquid in the first-stage membrane module 1 N
be included in a large amount is aOH of Na ^+, it is effective because the number can be removed by the second stage membrane module 2.

Further, NaOH is injected into the stock solution tank 5, the carbon dioxide gas of the stock solution is converted into HCO ₃ ^- or CO ₃ ^2- , and these ions are removed by the first-stage module 1 to remove the carbon dioxide gas. When finally removing the permeate from the permeate, anions such as HCO ₃ ^- and CO ₃ ^2- can be efficiently removed by the first-stage module 1 in the negatively charged state. Even if a large amount of Na ⁺ of NaOH is contained in the first permeate, most of it can be removed by the second-stage membrane module 2, which is more effective.

The composite semipermeable membrane used for the membrane modules 1 and 2 generally has a permeation flow of about 1.0 m ³ / m ² · day at an operating pressure of 20 kgf / cm ² or less, preferably 15 kgf / cm ² or less. A bundle and at least 80% or more, preferably 9%
Those having a salt rejection of 0% or more are used. The form of the membrane module to be used is not limited at all, for example,
Tubular, hollow fiber, plate-and-frame, and spiral shapes are included.

In the two-stage RO system, the permeated liquid storage tank of the first-stage module 1 and the permeated liquid in the storage tank are supplied to the stock solution chamber of the second-stage module 2 at a predetermined pressure. If the liquid supply pump is provided for the first stage module 1 as well as the second stage module 2, the permeated liquid chamber 1
An ordinary membrane module that can be designed to have a constant pressure can be used.
In addition, a membrane module group connected in parallel can be used for the modules 1 and 2 in each stage.

[0031]

The multi-stage RO system of the present invention combines membrane modules having different charges, and is particularly insolubilized not only by a specific composite semipermeable membrane, that is, by a conventional adsorption mechanism but also by three-dimensional crosslinking. The use of a composite semi-permeable membrane made of an organic polymer having a fixed positive charge to form a coating with excellent durability enables efficient salt rejection performance, improved permeate quality, and further repetition. Even when used, there is an advantage that a stable permeated liquid quality can be obtained for a long period of time without dropping the organic polymer having a positive fixed charge and without deteriorating the performance of the whole system. Therefore, the present invention is particularly effective in the production of ultrapure water, for example, a primary pure water system in semiconductor production.

[0032]

EXAMPLES The present invention will be described below with reference to examples.
The present invention is not limited to these examples.

REFERENCE EXAMPLE 1 29 g of methyl ethyl ketoxime was dissolved in 50 g of benzene, and 51.6 g of 2-methacryloyloxyethylene isocyanate was added dropwise at a temperature of 25 ° C. over about 40 minutes. Stirred for hours. The obtained reaction product was analyzed by proton NMR to confirm that it was a blocked product in which methyl ethyl ketoxime was almost quantitatively added to 2-methacryloyloxyethylene isocyanate.

Reference Example 2 16 g of hydroxypropyltrimethylammonium methacrylate chloride and 8 g of the blocked isocyanate compound obtained in Reference Example 1 were dissolved in 60 g of methanol.
0.4 g of azobisisobutyronitrile was added thereto, and the mixture was stirred at 60 ° C. for 6 hours under a nitrogen gas atmosphere to obtain a copolymer having a quaternary ammonium group.

Reference Example 3 6 g of hydroxypropyltrimethylammonium methacrylate chloride and 18 g of butyl methacrylate were dissolved in 60 g of methanol, and 0.7 g of azobisisobutyronitrile was added thereto.
After stirring for an hour, a copolymer having a quaternary ammonium group was obtained. When this copolymer was dried by heating, it was non-crosslinkable but insoluble in water.

Reference Example 4 According to the method described in JP-A-2-187135, an aromatic polyamide-based composite semipermeable membrane having negative charge was obtained.

Reference Example 5 1 g of the copolymer obtained in Reference Example 2 was dissolved in water, and 1% by weight
An aqueous solution was prepared. To this, 0.005 g of 1,4-azabicyclo (2,2,2) octane was added as a crosslinking catalyst. The solution obtained in this manner was applied on the composite semipermeable membrane of Reference Example 4 so as to have a dry film thickness of 0.05 μm.
Heating was performed for 0 minutes to crosslink the copolymer to obtain a positively chargeable composite semipermeable membrane.

Reference Example 6 1 g of the copolymer obtained in Reference Example 3 was dissolved in ethanol.
A 1% by weight ethanol solution was prepared. The solution thus obtained was applied on the composite semipermeable membrane of Reference Example 4 so as to have a dry film thickness of 0.05 μm, and heated at 150 ° C. for 10 minutes to obtain a positively charged composite semipermeable membrane. I got

Example 1 Using the composite semipermeable membranes obtained in Reference Examples 4 and 5, spiral modules were respectively manufactured. A module prepared using the negatively chargeable composite semipermeable membrane obtained from Reference Example 4 was used in the first stage, and a module was prepared using the positively chargeable composite semipermeable membrane obtained from Reference Example 5 The two-stage RO system was configured by using the module thus completed in the second stage, and this test was performed as follows.

The undiluted solution had an electric conductivity of 100 μS / cm and a pH =
Using 6.5 well water, the supply pressure to the first stage module was adjusted to 30 kg / cm ² and the supply pressure to the second stage module was adjusted to 15 kg / cm ² . NaOH is added to the stock solution tank.
And the pH was adjusted so that the specific resistance of the permeate of the second stage module was the highest. Evaluation was performed continuously under these conditions, and the change with time in the maximum specific resistance value of the permeate of the second stage module was examined. The results are shown in FIG.

Comparative Example 1 In Example 1, a module manufactured using the composite semipermeable membrane obtained in Reference Example 6 was used instead of the module manufactured using the composite semipermeable membrane obtained in Reference Example 5. The same two-stage RO system test as in Example 1 was performed, and the results are shown in FIG.

As is apparent from the results shown in FIG. 2, the multi-stage RO system of the present invention is a conventional multi-stage RO system using a composite semipermeable membrane having an uncrosslinked type polymer having a positive charge of a comparative example. It can be seen that a stable permeate quality can be obtained over a long period of time as compared with

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an example of the multi-stage RO system of the present invention. FIG. 2 is a graph showing the change over time in the specific resistance of the permeated water in the multi-stage RO system.

[Explanation of symbols]

 1 First-stage membrane module 2 Second-stage membrane module

Claims (3)

(57) [Claims] 1. A method for membrane-treating a liquid with a plurality of membrane modules connected in series, wherein at least one membrane module is formed by cross-linking a negatively chargeable composite semipermeable membrane with a polymer having a positive charge. Covered with a crosslinked polymer layer consisting of
Multistage reverse osmosis system characterized by using a composite semipermeable membrane. 2. A negatively chargeable composite semipermeable membrane as the composite semipermeable membrane.
But polymers having a quaternary ammonium group and a hydroxyl group
Is converted into an isocyanate group intramolecularly and / or intermolecularly.
Multistage reverse osmosis system of claim 1, wherein the use of coated composite semipermeable membrane with a more crosslinked formed by cross-linked polymer layer. 3. A method for producing ultrapure water by subjecting raw water to membrane treatment using a plurality of membrane modules connected in series,
At least one membrane module has a composite semi-transmissive
A method for producing ultrapure water, comprising using a composite semipermeable membrane whose membrane is covered with a crosslinked polymer layer formed by crosslinking a polymer having a positive charge.