JP7768282B2 - Method for operating an electrodeionization apparatus - Google Patents

Description

The present invention relates to a method for operating an electrodeionization apparatus that increases the removal rate of weak electrolytes such as boron, enabling the production of high-quality treated water.

Electrodeionization devices are widely used to produce deionized water for use in various industries, including semiconductor and liquid crystal manufacturing plants, pharmaceutical, food, and power plants, as well as consumer and research facilities. As shown in Figure 1, this electrodeionization device 1 has multiple anion and cation exchange membranes 4 and 5 arranged between electrode plates 2 and 3 connected to electrodes (anode 2A and cathode 3A), forming a concentration chamber C and a deionization chamber D. Deionization chamber D is filled with a mixture, multilayer structure, or single layer of anion and cation exchangers, such as ion exchange resins. E+ denotes the anode chamber, and E- denotes the cathode chamber. Water to be treated (e.g., water treated using a reverse osmosis membrane) W1 is supplied to deionization chamber D of the electrodeionization device 1 to obtain treated water W2. Concentrated water W3 is supplied to concentration chamber C in the opposite direction to the deionization chambers, and concentrated wastewater W5 is discharged. Electrode water W4 is supplied to anode chamber E+ and cathode chamber E-, and electrode wastewater W6 is discharged.

In recent years, the quality of ultrapure water required in semiconductor factories and the like has been increasing, and the required concentration of boron, a weak electrolyte in ultrapure water, has been reduced to a level of 1 ppt or less. However, in order to remove boron to a high degree, it is necessary to promote ionization according to the following formula:
H ₃ BO ₃ +OH ⁻ → B(OH) ⁴⁻ (pKa=9.24)

Conventionally, it has been believed that operating an electrodeionization device at a higher current density is effective in promoting this ionization reaction, and that increasing the current density can increase the boron removal rate.

However, the boron removal rate of an electrodeionization device is significantly affected by fluctuations in other factors, such as the boron concentration in the concentrated water and the boron concentration in the feedwater (water to be treated). Therefore, simply setting a high current density as one of the operating conditions for an electrodeionization device is not enough to reliably remove weak electrolytes such as boron. Therefore, there is currently a strong demand for indicators for setting operating conditions that can reliably remove weak electrolytes.

The present invention was made in consideration of the above-mentioned problems, and aims to provide a method for operating an electrodeionization apparatus that increases the removal rate of weak electrolytes such as boron and enables the production of high-quality treated water.

To achieve the above object, the present invention provides a method for operating an electrodeionization apparatus comprising an anode, a cathode, and a plurality of anion-exchange membranes and cation-exchange membranes arranged between them to form concentration compartments and deionization compartments, and the deionization compartments are filled with ion exchangers, the method comprising operating the electrodeionization apparatus under conditions that satisfy the following formulas (1) and (2 ) (Invention 1):
50<A<200...(1)
(In formula (1), A: operating current density of the electrodeionization device [A/m ² ])
AA/CDout<0.2...(2)
(In formula (2), AA: the concentration of boron that moves from the concentration compartment to the deionization compartment by diffusion [ng/L] calculated by the following formula (3),
CDout: boron concentration of treated water from electrodeionization device [ng/L]
AA=D×( _CC ×A/dx)×Q×10 ³ [ng/L] ...(3)
(In the formula (3), D: diffusion coefficient [m ² /sec],
C _C : boron concentration at the inlet or outlet of the concentration compartment adjacent to the treated water outlet of the desalination compartment [ng/L];
A: operating current value of the electrodeionization device [A/m ² ],
dx: thickness of ion exchange membrane [m],
Q: Flow rate of the cell in the deionization compartment of the electrodeionization device [L/sec]

In particular, in the above invention (Invention 1),
Formula (2) is AA/CDout<0.1 (2)
It is preferable that (Invention 2).

These inventions (Inventions 1 and 2) enable reliable and efficient removal of boron, resulting in high-quality treated water. This is due to the following reasons: The inventors discovered that increasing the current (current density) during operation of an electrodeionization apparatus makes it difficult to increase the removal rate of weak electrolytes such as boron. After investigating the causes, they discovered that, in an electrodeionization apparatus, the quality of treated water in the deionization compartment must take into account factors such as increasing the current (current density) in the deionization compartment to promote the removal of weak electrolytes such as boron, and the diffusion factor of permeation from the concentration compartment to the deionization compartment through the ion exchange membrane. They found that if the influence of diffusion becomes too great, increasing the current (current density) will not improve the removal of weak electrolytes such as boron. Based on these findings, the inventors conducted various experiments and discovered that operating an electrodeionization apparatus so as to satisfy a specific relationship can reliably and efficiently remove boron, resulting in high-quality treated water. This led to the invention of the present invention.

1 is a schematic diagram showing an electrodeionization apparatus to which the present invention can be applied. FIG. 1 is a schematic diagram showing an electrodeionization apparatus used in Example 1. FIG. 1 is a schematic diagram showing an electrodeionization apparatus used in Comparative Example 1. 1 is a graph showing the relationship between the boron concentration in concentrated water and the boron concentration in treated water, and between the actual measured value and the calculated value using formula AA. FIG. 1 is a schematic diagram showing an electrodeionization apparatus used in Example 6. 1 is a graph showing the relationship between the calculated value of the formula AA/CDout and the boron removal rate. 1 is a graph showing the relationship between current density and boron removal rate. 1 is a graph showing the relationship between current density and boron concentration in treated water. 1 is a graph showing the relationship between the calculated value of AA/CDout and the boron concentration of treated water. FIG. 1 is a schematic diagram showing an electrodeionization apparatus used in Example 15.

The operating method of the electrodeionization apparatus of the present invention is described in detail below.

<Electrodeionization Apparatus and Operation Method Thereof>
The electrodeionization apparatus to which the present invention can be applied is not particularly limited, and for example, an apparatus as schematically shown in FIG. 1 can be used. The apparatus may be operated in a conventional manner, except that the following formulas (1) and (2 ) are satisfied. Specifically, water to be treated (e.g., water treated by a reverse osmosis membrane) W1 is supplied to the deionization chamber D of the electrodeionization apparatus to obtain treated water W2. Concentrated water W3 is supplied to the concentration chamber C to discharge concentrated wastewater W5, and electrode water W4 is supplied to the anode chamber E+ and cathode chamber E- to discharge electrode wastewater W6. Deionization is performed by applying an electric current to the cathode and anode, and the effluent from the deionization chamber is extracted as treated water (deionized water) W2. The wastewaters W5 and W6 from the concentration chamber and electrode chamber are either discharged outside the system or circulated to the supply side of the raw water to be treated.

In this embodiment, the operating conditions of the electrodeionization apparatus are specified so as to satisfy the following formulas (1) and (2 ) .
50<A<200...(1)
(wherein A is the operating current density of the electrodeionization device [A/m ² ])
AA/CDout<0.2...(2)
(Wherein, AA: concentration of boron that moves from the concentrating compartment to the deionizing compartment by diffusion [ng/L] calculated by the following formula (3),
CDout: boron concentration of treated water W2 [ng/L]
AA=D×( _CC ×A/dx)×Q×10 ³ [ng/L] ...(3)
(Wherein, C _C : boron concentration [ng/L] at the inlet or outlet of the concentration compartment adjacent to the treated water outlet of the desalting compartment;
A: operating current value of the electrodeionization device [A/m ² ],
dx: thickness of ion exchange membrane [m],
Q: Cell flow rate in the deionization compartment of the electrodeionization device [L/sec]
D: Diffusion coefficient [m ² sec/sec] is a value that differs for each electrodeionization device, and the permeability coefficient (diffusion coefficient) of a substance varies depending on the ion exchange membrane used. If the membrane pore size is small, the diffusion coefficient will be small. On the other hand, if the membrane pore size is large, the diffusion coefficient will be large. Specifically, it can be calculated as follows:

By satisfying the above-mentioned formulas (1) and (2) , boron in the water to be treated (feed water) can be reliably and efficiently removed, and high-quality deionized water (treated water) can be obtained.

In this embodiment, the formula (3) is derived in the following manner.
The AA defined in formula (3) represents the approximate concentration of boron that moves from the concentration compartment to the deionization compartment due to diffusion. Therefore, the greater the influence of diffusion in the treated water, the greater the AA/CDout. The AA defined in formula (3) is based on Fick's law, which is the fundamental law regarding the diffusion of substances (formula i).
J=-D'(dC/dx)...i
(Where J is the diffusion flux or flux [mol/ ^m2 /sec], defined as the amount of a property passing through a unit area per unit time, D is the diffusion coefficient [ ^m2 /sec], C is the concentration [mol/ ^m3 ], and x is the position [m].)

Here, the boron concentration near the membrane surface is expressed by the following formula ii, since the concentration compartment side is greater than the deionization compartment side.
dC=C _C,m -C _D,m ≒ C _C,m ...ii
∵C _C,m ≫-C _D,m
The boron concentration (C _C,m ) near the membrane surface on the concentrating compartment side is concentrated by applying a current (A) according to the following formula iii.
C _C,m =C _C ×βA...iii
(In the formula, β is a constant term for convenience.)

Then, by substituting formulas ii and iii into formula i above, the following formulas iv and v are obtained.
J = -D'(dC/dx) = -D'(C _C,m /dx) ... (iv: Substitute ii into formula i)
J = -D'(C _C,m /dx) = -D'(C _C × βA/dx) ... (v: Substitute iii into formula iv)
Here, if the diffusion coefficient D is set to D=D'×β, the flux (J) is given by the following formula vi.
J=-D( _CC ×A/dx)...vi
Since J is the flux, the permeation amount is calculated by multiplying it by Q [L/sec], which is the cell flow rate in the deionization compartment, to obtain the following equations vii and viii.
AA=D( _CC ×A/dx)×Q [μg/L] ・・・vii
AA=D×( _CC ×A/dx)×Q×10 ³ [ng/L] ...viii
As a result, the formula (3) of the present invention was derived.

The present invention has been described above based on the above-mentioned embodiment, but various modifications are possible. For example, the electrodeionization device may be one in which a plurality of anion exchange membranes and cation exchange membranes are arranged between an anode and a cathode to form concentration compartments and deionization compartments, or one in which anion exchange membranes or cation exchange membranes are partially connected to form a continuous deionization compartment.

The process for calculating operating conditions for the electrodeionization apparatus operating method of the present invention will be explained using the following specific example.

[Examples 1 to 5]
A test electrodeionization apparatus as shown in FIG. 2 was prepared. In FIG. 2, electrodeionization apparatus 1 has a structure in which an anion exchange membrane 4 and a cation exchange membrane 5 are arranged between electrode plates 2 and 3 connected to electrodes (anode 2A and cathode 3A), forming two concentration compartments C and one deionization compartment D. In the figure, E+ is the anode compartment, and E- is the cathode compartment. An ultrapure water supply pipe is provided in communication with an ultrapure water (UPW) supply source 6, and this ultrapure water supply pipe branches into water supply pipes 7 and 8. A tank 11 containing a boron solution B of a known concentration as a chemical component is provided, and a supply pipe 12 equipped with a pump 13 is connected to this tank 11 and to the supply pipe 7. Thus, ultrapure water containing boron solution B is supplied to the concentration compartment C on the cathode compartment E- side, and ultrapure water is supplied to the other concentration compartment C, deionization compartment D, anode compartment E+, and cathode compartment E-. The reference numerals 14A and 14B denote boron concentration measuring means. The water to be treated W1 is supplied to the deionization chamber D of the electrodeionization apparatus 1 to obtain treated water W2, while concentrated water W3 is supplied to the concentration chamber C and concentrated wastewater W5 is discharged. Furthermore, electrode water W4 is supplied to the anode chamber E+ and the cathode chamber E- and electrode wastewater W6 is discharged.

The cell size of this electrodeionization device 1 was 48.5 mm wide x 230 mm high x 5.0 mm thick, and deionization compartment D and concentration compartment C were each filled with ion exchange resin (a mixed resin of anion exchange resin and cation exchange resin).

In this example, ultrapure water has the following characteristics: resistivity: 18.1 MΩ·cm or more; fine particles: 1000 particles/L or less with a particle size of 50 nm or more; viable bacteria: 1 particle/L or less; TOC: 1 μg/L or less; total silicon: 0.1 μg/L or less; metals: 1 ng/L or less; ions: 10 ng/L or less; hydrogen peroxide: 30 μg/L or less; and a water temperature of 25±2°C.

The electrodeionization apparatus 1 described above was operated at an operating current density of 100 [A/m ² ], while varying the boron concentration of the concentrated water supplied from the boron solution B to one of the concentration compartments C from 0 to 1000 ppb.

In this case, the calculation parameters were set as follows:
Q (cell flow rate in the deionization compartment of the electrodeionization apparatus): 0.005 [L/sec]
dx (thickness of ion exchange membrane): 5.0 × 10 ⁻⁴ [m] (500 [μm])
D (diffusion coefficient): (2.5×10 ⁻¹⁹ [m ² /sec]

The boron concentration of treated water W2 was measured under these operating conditions. The results are shown in Table 1, along with the boron (B) concentration of the water to be treated (feedwater) W1, the boron (B) concentration of concentrated water W3 (inlet to chamber C), the current density, and the value of AA in equation (3). Figure 4 also shows the relationship between the boron concentration of treated water W2 and the boron concentration of concentrated water W3.

[Comparative Example 1]
The electrodeionization apparatus was operated in the same manner as in Example 3, except that for testing purposes, the boron solution B was added to the concentrating compartment C on the anode compartment E+ side as shown in FIG.

The boron concentration of treated water W2 was measured under these operating conditions. The results are shown in Table 1, along with the boron (B) concentration of the water to be treated (feedwater) W1, the boron (B) concentration of concentrated water W3 (inlet to chamber C), the current density, and the value of AA in formula (3).

4, it can be seen that boron is mixed into the treated water W2 via the cation exchange membrane 5. However, because the cation exchange membrane 5 is a solid superacid, the pH inside the cation exchange membrane 5 is extremely low and is an acidic condition. For this reason, most of the boron is in the form of molecular boron (H ₃ BO ₃ ), and the following relational formula is satisfied:
H ₃ BO ₃ +OH ⁻ → B(OH) ₄ ⁻ (pKa=9.24)

Therefore, it can be said that passage through the cation exchange membrane 5 is due to concentration diffusion, rather than charge transfer caused by voltage application. Furthermore, Figure 4 shows that the boron concentration in treated water W2 (◯) and the calculated value AA (●) of the concentration increase due to diffusion are nearly identical. From these facts, it can be said that the effect of diffusion through the cation exchange membrane can be calculated using formula AA.

[Examples 6 to 9 and Comparative Example 2]
A test electrodeionization apparatus as shown in FIG. 5 was prepared. In FIG. 5, electrodeionization apparatus 1 has basically the same configuration as that shown in FIG. 2. An ultrapure water supply pipe is provided in communication with an ultrapure water (UPW) supply source 6. This ultrapure water supply pipe branches into water supply pipe 7 and water supply pipe 8, and the supply water for deionization chamber D flows in the opposite direction to the supply water for concentration chamber C, anode chamber E+, and cathode chamber E−. A tank 11 containing a boron solution B of a known concentration as a chemical component is prepared. A supply pipe 12 equipped with a pump 13 runs from this tank 11 to water supply pipe 7. A tank 15 containing a boron solution B of a known concentration as a chemical component is prepared. A supply pipe 16 equipped with a pump 17 runs from this tank 15 to water supply pipe 8. As a result, ultrapure water to which boron solution B has been added is supplied from tank 11 to deionization compartment D, and ultrapure water to which boron solution B has been added is supplied from tank 15 to concentration compartment C, anode compartment E+, and cathode compartment E-. This structure makes it possible to supply ultrapure water to which boron has been added at different concentrations to deionization compartment D and concentration compartment C. Reference numerals 14A and 14B each represent boron concentration measuring means.

The electrodeionization apparatus 1 described above was operated at an operating current density of 100 [A/m ² ], with the boron concentrations of the concentrated water W3 and electrode water W4 supplied to the concentration chamber C, anode chamber E+, and cathode chamber E- varying from 0 to 1000 ppb, and water to be treated (feedwater) with a boron concentration of 10 ppb was supplied to the deionization chamber D. The calculation parameters were the same as in Example 1.

The boron (B) concentration of treated water W2 was measured under these operating conditions. The results are shown in Table 2, along with the boron (B) concentration of the water to be treated (feedwater), the boron (B) concentration of concentrated water W3 (inlet to chamber C), the current density, and the boron removal rate. The formula AA/CDout (CDout) was calculated based on these data, and the results are also shown in Table 2. Furthermore, the relationship between this calculation result and the boron removal rate is shown in Figure 6.

In Figure 6, the index AA/CDout was used as the basis. AA indicates the increase in concentration due to diffusion, and CDout indicates the boron concentration in the treated water. Therefore, AA/CDout indicates the estimated proportion of the boron concentration in the treated water that has increased due to the effects of diffusion. As is clear from Table 2 and Figure 6, the smaller the AA/CDout, the higher the boron removal rate; as AA/CDout increases, the impact of diffusion becomes greater, and when AA/CDout exceeds 0.2, the boron removal rate drops sharply. In particular, it is clear that in order to maintain a high boron removal rate, it is preferable to keep AA/CDout at 0.1 or less.

[Examples 10 to 12, and Comparative Examples 3 and 4]
In Example 6, the boron concentrations of the concentrated water W3 and electrode water W4 supplied to the concentrating chamber C, anode chamber E+, and cathode chamber E- were set to 100 ppb, and the boron concentration of the water to be treated W1 supplied to the deionization chamber D was set to 10 ppb. The electrodeionization apparatus 1 was operated with the operating current density varied from 25 to 300 [A/m ² ]. The calculation parameters were the same as those in Example 1.

The boron concentration of treated water W2 was measured under these operating conditions. The results are shown in Table 3, along with the current density, the value of the formula AA/CDout, and the boron (B) removal rate. Figure 7 shows the relationship between current density and boron removal rate, Figure 8 shows the relationship between current density and boron concentration removal rate in treated water W2, and Figure 9 shows the relationship between the formula AA/CDout and boron concentration removal rate.

As is clear from Table 3 and Figures 7 to 9, generally, the higher the current density, the better the boron removal rate, but the value of AA/CDout also increases, and the boron concentration in treated water W2 gradually approaches a constant value. In particular, it was found that when the value of AA/CDout exceeds 0.1, the boron concentration in treated water W2 significantly converges to a constant value.

[Examples 13 and 14]
In Example 6, the system was operated at an operating current density of 100 [A/m ² ], with the boron concentrations of the concentrated water W3 and electrode water W4 supplied to the concentration chamber C, anode chamber E+, and cathode chamber E- set to 100 ppb and 250 ppb, respectively, and water to be treated with a boron concentration of 10 ppb was supplied to the deionization chamber D. The calculation parameters were the same as in Example 1.

The boron concentration of treated water W2 was measured under these operating conditions. The results are shown in Table 4, along with the uron concentration of concentrated water W3, the boron concentration of concentrated wastewater W5, the boron concentration of concentrated water W3 at the inlet of the concentration chamber adjacent to the outlet of treated water W2, the value of the formula AA/CDout, and the boron (B) removal rate.

[Examples 15 and 16]
A test electrodeionization apparatus as shown in Figure 10 was prepared. In Figure 10, electrodeionization apparatus 1 has basically the same configuration as that shown in Figure 1. An ultrapure water supply pipe is provided in communication with an ultrapure water (UPW) supply source 6. This ultrapure water supply pipe branches into supply pipes 7 and 8 along the way, so that the supply water for deionization chamber D flows in the same direction as the water for concentration chamber C, anode chamber E+, and cathode chamber E-. A tank 11 containing a boron solution B of a known concentration as a chemical component is provided, and a supply pipe 12 equipped with a pump 13 connects this tank 11 to supply pipe 7. A tank 15 containing a boron solution B of a known concentration as a chemical component is also provided, and a supply pipe 16 equipped with a pump 17 connects this tank 15 to supply pipe 8. As a result, ultrapure water to which boron solution B has been added is supplied from tank 11 to deionization compartment D, anode compartment E+, and cathode compartment E-, and ultrapure water to which boron solution B has been added is supplied from tank 15 to concentration compartment C. This results in a structure in which ultrapure water to which boron has been added at different concentrations is supplied to deionization compartment D and concentration compartment C. Note that 14C and 14D are boron concentration measuring means, respectively.

The boron concentration of treated water W2 was measured under these operating conditions. The results are shown in Table 4, along with the boron concentration of concentrated water W3, the boron concentration of concentrated wastewater W5, the boron concentration of concentrated wastewater W5 at the outlet of the concentration chamber adjacent to the outlet of treated water W2, the value of the formula AA/CDout, and the boron (B) removal rate.

As is clear from Table 4, when focusing on the inlet or outlet concentration of the concentrating compartment adjacent to the treated water outlet of the desalination compartment, formula AA is the same whether desalinated water D and concentrating compartment C are passed in the same direction (parallel flow) or whether desalinated water D and concentrating compartment C are passed in the same direction (counter flow). Therefore, by summarizing the effect of diffusion in terms of the "inlet or outlet concentration of the concentrating compartment adjacent to the treated water outlet of the desalting compartment," it can be seen that the same formula can be applied to both counter flow and parallel flow.

1 Electrodeionization device 2, 3 Electrode plate 2A Anode (electrode)
3A cathode (electrode)
4. Anion exchange membrane (AM)
5. Cation exchange membrane (CM)
6 Ultrapure water (UPW) supply source 7 Water supply pipe 8 Water supply pipe 11 Tank 12 Supply pipe 13 Pumps 14A, 14B, 14C, 14D Boron concentration measuring means 15 Tank 16 Supply pipe 17 Pump C Concentration chamber D Deionization chamber E+ Anode chamber E- Cathode chamber W1 Water to be treated W2 Treated water W3 Concentrated water W4 Electrode water W5 Concentrated wastewater W6 Electrode wastewater

Claims (2)

A method for operating an electrodeionization apparatus comprising an anode, a cathode, and a plurality of anion exchange membranes and cation exchange membranes arranged between the anode and cathode to form concentration compartments and deionization compartments, and the deionization compartments are filled with ion exchangers, the method comprising operating the electrodeionization apparatus under conditions that satisfy the following formulas (1) and (2 ) :
50<A<200...(1)
(In formula (1), A: operating current density of the electrodeionization device [A/m ² ])
AA/CDout<0.2...(2)
(In formula (2), AA: the concentration of boron that moves from the concentration compartment to the deionization compartment by diffusion [ng/L] calculated by the following formula (3),
CDout: boron concentration of treated water from electrodeionization device [ng/L]
AA=D×( _CC ×A/dx)×Q×10 ³ [ng/L] ...(3)
(In the formula (3), D: diffusion coefficient [m ² /sec],
C _C : boron concentration at the inlet or outlet of the concentration compartment adjacent to the treated water outlet of the desalination compartment [ng/L];
A: operating current value of the electrodeionization device [A/m ² ],
dx: thickness of ion exchange membrane [m],
Q: Flow rate of the cell in the deionization compartment of the electrodeionization device [L/sec] Formula (2) is AA/CDout<0.1 (2)
2. The method of claim 1, wherein: