JP3894039B2 - Operation method of electrodeionization equipment - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for operating an electrodeionization apparatus for producing deionized water used in various industries in the fields of semiconductors, liquid crystals, pharmaceuticals, foods, electric power, etc. The present invention relates to an operation method of an electrodeionization apparatus that can increase the removal rate of CO _{2 in} the apparatus and reliably obtain high-quality treated water.
[0002]
[Prior art]
Conventionally, for the production of deionized water used in various industries such as semiconductor manufacturing factory, liquid crystal manufacturing factory, pharmaceutical industry, food industry, electric power industry, etc. A plurality of anion exchange membranes and cation exchange membranes are alternately arranged to form a concentration chamber and a desalting chamber alternately, and the desalting chamber is made of an ion exchange resin, an ion exchange fiber or a graft exchanger. In addition, an electrodeionization apparatus in which a cation exchanger is mixed or packed in a multilayer is widely used.
[0003]
The electrodeionization device generates H ⁺ ions and OH ⁻ ions by water dissociation and continuously regenerates the ion exchanger filled in the desalting chamber, enabling efficient desalting treatment. Thus, it does not require a regeneration treatment using chemicals such as the ion exchange resin apparatus that has been widely used so far, and complete continuous water sampling is possible, and an excellent effect that high-purity water is obtained is exhibited.
[0004]
In such an electrodeionization apparatus, the specific resistance of the treated water may be significantly reduced due to a change in the conductivity of the feed water. According to the research of the present inventors, there are dissolved salts, silica, carbonic acid and the like as those affecting the performance of the electrodeionization apparatus, and among these, even a small amount of carbonic acid can greatly affect the performance. It was found.
[0005]
In order to remove CO ₂ with such an electrodeionization apparatus, it is necessary to generate an ion by causing the following ionization reaction to occur in the demineralization chamber.
CO ₂ + OH ⁻ → HCO ₃ ⁻ (pKa = 6.35)
In order to promote such an ionization reaction, it is effective to increase the current density of the electrodeionization apparatus, and it is known that the CO ₂ removal rate is improved by increasing the current density.
[0006]
[Problems to be solved by the invention]
However, when the current density of the electrodeionization device is increased, a calcium carbonate scale is generated in the concentration chamber, or power is consumed excessively, resulting in an increase in power cost.
[0007]
The present invention relates to a method for operating an electrodeionization apparatus capable of maintaining a stable treated water specific resistance value by detecting the CO ₂ concentration in the supply water and controlling to apply an appropriate applied voltage / current. The purpose is to provide.
[0008]
[Means for Solving the Problems]
In the operation method of the electrodeionization apparatus of the present invention, an anion exchange membrane and a cation exchange membrane are arranged between an anode and a cathode to form a concentration chamber and a desalting chamber, and an ion exchanger is placed in the desalting chamber. In the method of operating a charged electrodeionization apparatus, the current value of the electrodeionization apparatus is such that the current efficiency is always 25% or less, preferably 10 to 25% with respect to the change in the CO ₂ concentration in the feed water. It is characterized by controlling.
[0009]
In this way, by controlling the current value so that the current efficiency of the electrodeionization apparatus is 25% or less, preferably 10 to 25%, the specific resistance of the produced water (treated water) is, for example, 15 MΩ · cm or more. High value.
[0010]
In the present invention, the CO ₂ concentration and the conductivity of the feed water, the flow rate and conductivity of the product water is measured, it is preferable to control the current value so that the current efficiency is the range.
[0011]
Further, in the present invention, considering the strong influence of carbonic acid as described above, the pH of the feed water is reduced by 0.5 or more, the increase in conductivity is 5 μS / cm or less, and the specific resistance value of the produced water is The current value may be increased by 5% or more when the voltage drops by 3 MΩ · cm or more.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail.
[0013]
The operation method of the electrodeionization apparatus of the present invention is such that the current efficiency is always 25% or less, preferably 10 to 25% even if the CO ₂ concentration in the feed water varies, and other operating conditions May be the same as before.
[0014]
Therefore, a part of the supply water (this water is usually pretreated in order by an activated carbon tower and a reverse osmosis membrane separator, etc.) is supplied to the concentration chamber of the electrodeionization device, and the remainder is supplied to the demineralization chamber. Then, deionization treatment is performed, and the outflow water from the desalination chamber may be taken out as treated water (product water). In normal cases, part of the effluent from the concentrating chamber is discharged out of the system, and the remainder is circulated to the supply side of the concentrating chamber.
[0015]
The circulation of the effluent in the concentrating chamber is performed for the purpose of improving the water recovery rate. However, the amount of the circulated water is not particularly limited, and is usually about 50 to 95% of the effluent in the concentrating chamber. It is preferable to carry out the operation under conditions where the water recovery rate of the ion device is about 0.5 to 0.95.
[0016]
The removal mechanism of carbonic acid by this electrodeionization apparatus is as follows. That is, carbonic acid is converted to bicarbonate ion by an ionization reaction with hydroxide ions.
CO ₂ + OH ⁻ → HCO ₃ ⁻
The ionization reaction requires a supply of OH ⁻ ions, which is brought about by water dissociation.
H ₂ O → H ⁺ + OH ⁻
Therefore, when the amount of CO ₂ is increased, the amount of OH ⁻ is increased accordingly.
[0017]
In addition, since it is necessary to move the ionized bicarbonate ion quickly to the concentration chamber, it is necessary to increase the voltage as the amount of CO ₂ increases.
[0018]
The present inventor experimentally observed the relationship between the CO ₂ removal rate (specific resistance of the produced water) and the current efficiency by the electrodeionization apparatus. As shown in FIG. 1, the current efficiency was about 20%. The specific resistance of the production water (treated water) becomes the maximum when it is about, and the specific resistance of the production water is reduced below or above that; the specific resistance is 15 MΩ · cm or more when the current efficiency is 25% or less It was recognized that The present invention is based on such knowledge, and by setting the current efficiency to 25% or less, preferably 10 to 25%, more preferably 12 to 22%, high-quality product water with high specific resistance is obtained. It is a thing.
[0019]
As shown in FIG. 1, even if the current efficiency is 20% or less, the treated water specific resistance value does not increase any more. This suggests that the current efficiency may be about 20% even if the CO ₂ concentration in the feed water changes.
[0020]
The inventor varied the CO ₂ concentration in the feed water, controlled the current value so that the current efficiency was 20%, operated the electrodeionization apparatus, and measured the specific resistance of the produced water at that time. The result is shown in FIG. In FIG. 2, the electrode area is 5 dm ² .
[0021]
As shown in FIG. 2, even if the CO ₂ concentration in the feed water is increased, the specific resistance of the production water is increased to a high value of 15 MΩ · cm or more by increasing the current value so that the current efficiency is maintained at 20%. Become. In this way, the specific resistance value of the treated water is stabilized by increasing the current value by an amount equivalent to the increase in equivalent conductivity due to CO ₂ .
[0022]
The relationship between the current value and the current efficiency in the electrodeionization apparatus is expressed by the following equation.
I = a · Q (Cf−Cp) / η
I: Required current [A]
Q: Production water flow rate [L / min · cell]
Cf: Supply water equivalent conductivity [μS / cm]
Cp; treated water conductivity [μS / cm]
η: Current efficiency [%]
a; Constant Cf is the sum of silica equivalent conductivity, carbon dioxide equivalent conductivity and total equivalent conductivity other than these ionic species.
[0023]
This constant a is normally a = 1.31. “/ Cell” indicates “per desalination chamber”.
[0024]
Therefore, the current efficiency is 25% or less, preferably 10 to 25%, most preferably the current efficiency η = 20%, and the conductivity of the feed water, the flow rate and the conductivity of the production water are substituted into the above formula, and the calculation is performed. What is necessary is just to energize the electrodeionization apparatus with the current value.
[0025]
In addition, since CO ₂ in supply water is most likely to fluctuate and the equivalent conductivity is large, it is preferable to continuously measure the CO ₂ concentration in supply water using a CO ₂ meter. Instead of the CO ₂ meter, the CO ₂ concentration may be obtained from pH and IC measurement values using a pH meter and an inorganic carbon (IC) meter.
[0026]
However, in the present invention, instead of measuring the CO ₂ concentration in the feed water and controlling the current value accordingly, the variation in the measured value of the specific resistance of the production water is mainly due to the variation in the CO ₂ concentration in the feed water. Therefore, the necessary energization current value may be calculated based on the supply CO ₂ concentration fluctuation value estimated from the fluctuation of the specific resistance of the production water. Specifically, when the pH of the feed water is reduced by 0.5 or more, the increase in conductivity is 5 μS / cm or less, and the specific resistance value of the produced water is reduced by 3 MΩ · cm or more, the energization current value is 5 % May be increased.
[0027]
【The invention's effect】
As described above, according to the present invention, stable treated water quality can be obtained by operating the electrodeionization apparatus with an appropriate current value corresponding to the change in the CO ₂ concentration in the feed water. In addition, excessive power consumption can be prevented, and economic efficiency is increased.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between current efficiency and treated water resistivity.
FIG. 2 is a diagram showing the relationship between the supply water CO ₂ concentration and the treated water specific resistance value.

Claims (5)

A method for operating an electrodeionization apparatus in which an anion exchange membrane and a cation exchange membrane are arranged between an anode and a cathode to form a concentration chamber and a desalting chamber, and the desalting chamber is filled with an ion exchanger. In
A method for operating an electrodeionization apparatus, wherein the current value of the electrodeionization apparatus is controlled so that the current efficiency is always 25% or less with respect to a change in the CO ₂ concentration in the supply water. _{2. The} electrodeionization method according to claim 1, wherein the CO2 concentration and conductivity of the feed water, the flow rate and conductivity of the production water are measured, and the current value is controlled so that the current efficiency falls within the range. How to operate the device. The operation method of the electrodeionization apparatus according to claim 2, wherein a current value to be energized is calculated according to the following equation.
I = a · Q (Cf−Cp) / η
I: Required current [A]
Q: Production water flow rate [L / min · cell]
Cf: Supply water equivalent conductivity [μS / cm]
Cp; treated water conductivity [μS / cm]
η: Current efficiency [%]
a: Constant According to claim 2 or 3, the method operation of electrodeionization apparatus characterized by measuring the CO ₂ concentration in the feed water by CO ₂ meter. In claim 1, when the pH of the feed water is reduced by 0.5 or more, the increase in conductivity is 5 μS / cm or less, and the specific resistance value of the produced water is reduced by 3 MΩ · cm or more, the current value is 5%. The operating method of the electrodeionization apparatus characterized by raising above.

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