System for producing ultra-pure water

Details System for producing ultra-pure water System for producing ultra-pure water

1996-11-04

2001-02-13

Fortuna, Ana (Department: 1723)

Liquid purification or separation

Processes

Liquid/liquid solvent or colloidal extraction or diffusing...

C210S257200, C210S638000, C210S900000, C204S529000, C204S634000

Reexamination Certificate

active

06187201

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a system for producing ultra-pure water widely used in semiconductor plants, nuclear power plants and pharmaceutical plants.
BACKGROUND ART
Ultra-pure water containing only trace amounts of electrolytes, minute particles, organic substances (to be referred to as TOC hereinafter) has been used in semiconductor plants, nuclear power plants and pharmaceutical plants. As a system for producing such ultra-pure water, a combination of an ion-exchange unit packed with an ion-exchange resin and a reverse osmosis unit has been used widely. With such system incorporating an ion-exchange resin, however, TOC elating from the ion-exchange resin becomes so big in amount that in many cases it can not be eliminated completely by the subsequent unit on reverse osmosis.
As a means to meet such problem, the present inventors proposed a combination of an electrodialysis unit packed with ion-exchange membranes selectively permeable to monovalent cations and a reverse osmosis unit (Japanese Patent Application No. Hei-4(1992)-25180).
This system practically arrests elution of TOC: the unit on electro-dialysis removes monovalent cations such as Na+ or K+ even at extremely low concentrations, and the unit on reverse osmosis is excellent in eliminating polyvalent ions. Therefore, this system allows stable production of ultra-pure water of an extremely high purity.
However, when a variety of crude waters were treated with the system described above, it was found that elimination of TOC by the electrodialysis unit varies greatly according to the quality of the water treated, and does not proceed as expected for certain waters. More precisely, while a water sample is treated, water molecules are cleaved in association with desaltination. Because this cleavage of water molecules might affect the ion-exchange membrane, divalent cations are allowed intentionally to remain in the water, to prevent their damaging effects to the ion-exchange membrane. Contrary to expectation, however, this maneuver allows anions to remain in the water, and the amount of the anions corresponds in electric charges to those of cations to be kept intentionally in the water, especially of divalent cations such as Ca
2
+ and Mg
2
+. How much and what kind of anions remain in the water during such treatment vary according to the ionic balance or pH of the original water sample, but roughly speaking, inorganic carbonate ions tend to remain in the water. Among such inorganic carbonate ions, HCO
3
— (bicarbonate ion) more tends to remain in the water because it can not be readily captured by the subsequent reverse osmosis unit. Thus, with this system the quality of water falls rather near the outlet.
To solve this problem, the present inventors have tried hard to develop a ultra-pure water producing system that allows production of water practically free not only from TOC but also from cations and anions by combining an electrodialysis unit capable of thoroughly removing monovalent cations and anions, and a reverse osmosis unit capable of removing polyvalent cations and anions, and further to confer the system a property which dispenses with such a maintenance work as the renewal treatment of ion-exchange resins with HCl or NaOH, that is, a necessary maintenance work indispensable to conventional systems.
As a result, the inventors advanced a step beyond the previous system which depended on the use of an electrodialysis unit only with membranes selectively permeable to monovalent cations, developed a system wherein another set of membranes selectively permeable to monovalent anions is added to above-described membranes, and found that this system could solve the problem. This finding led to the present proposal of this invention.
The present invention, being based on the finding described above, aims at providing a system for producing ultra-pure water which will allow thorough elimination of not only electrolytes, minute particles and organic substances from the crude water submitted, but also monovalent cations and anions, and divalent and polyvalent cations and anions therefrom.
DISCLOSURE OF THE INVENTION
The ultra-pure water producing system of this invention is characterized by comprising an electrodialysis unit with membranes selectively permeable to monovalent cations and another set of membranes selectively permeable to monovalent anions, and a reverse osmosis unit connected in series at the subsequent stage. This system is further characterized by having a decarbonating means together with an acidifying means at a stage prior to the electrodialysis unit.
The electrodialysis unit of the system of this invention depends on the combinational use of membranes selectively permeable to monovalent cations and another set of membranes selectively permeable to monovalent anions. The membrane selectively permeable to monovalent cations and applicable to this system includes practically all appropriate membranes publicly known, but membranes whose selective permeability constants to Na+ or a representative monovalent cation and to Ca
2
+ or a representative divalent cation are both less than 0.5 (P
ca
Na
) or membranes whose permeability constant to monovalent cations is less than 0.2 are preferably used.
As such preferred membrane selectively permeable to monovalent cations, for example, the membrane described in the Japanese Patent Laid-Open No.SHO-62(1987)-205135 can be mentioned: a modified cation-exchange membrane that is produced by having a polymer comprising quaternary ammonium base and a vinyl compound composed of 3 or more vinylbenzyl groups coated at least on one of its surfaces. The vinyl compound composed of 3 or more vinylbenzyl groups includes a compound which results from the reaction of a primary amine such as methyl amine or ethyl amine with three or more vinylbenzyl halides. This membrane selectively permeable to monovalent cations, having the selective permeability constant of less than 0.2, is most adequate for this invention.
The membrane selectively permeable to monovalent anions and applicable to this system includes practically all appropriate membranes publicly known, but membranes whose selective permeability constants to Cl— or a representative monovalent anion and to SO42- or a representative divalent anion are both less than 0.5 (P
SO4
/
Cl
) or membranes whose permeability constant to monovalent anions is less than 0.2 are preferably used.
Such preferable membrane selectively permeable to monovalent anions includes:
an anion-exchange resin membrane made of an organic polymer having anion-exchange groups, possessed of selective permeabilities to different negative ionic species and having a thin layer coated on its surface which results from the condensation of a compound with aldehyde, that is, a compound with one or more amino groups about an aromatic nucleus which, when condensed with aldehyde group or methylol group, will produce a resin with a well-developed cross-linking structure (Japanese Patent Publication No.SHO-36(1961)-15258); an anion-exchange membrane having a thin layer made of organic low-molecule electrolytes having cation exchange groups or made of linear polymeric electrolytes having no cross-linking structure applied thereupon (Japanese Patent Laid-Open No.SHO-45(1970)-19980); and
a membrane produced by a process wherein an ion-exchange membrane precursor in which a part of functional groups convertible to highly basic anion-exchange groups has been inactivated against the quaternary amine treatment is treated so as to give a quaternary amine compound which is then exposed to a peroxide solution, or reversely, the precursor is exposed to a peroxide solution, then to be treated so as to give a quaternary amine compound (Japanese Patent Publication No.SHO-56(1981)-47213).
The electrodialysis unit of this invention comprises an electrodialysis tank in which the membranes selectively permeable to monovalent cations and those selectively permeable to monovalent anions are arranged alternately. Supplying a crude wat

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