Method for operating electric deionization apparatus

Abstract

Provided is a method for operating an electric deionization apparatus in which a concentration chamber and a desalination chamber are formed by arranging a plurality of anion exchange membranes and cation exchange membranes between a positive electrode and a negative electrode, and the desalination chamber is filled with an ion exchanger. The electric deionization apparatus is operated under the conditions that satisfy the following formulae (1), (2), and (3).　(1): 50 < A < 200, (2): AA / CDout < 0.2, (3): AA = D × (Cc × A) / dx) × Q × 103 (ng / L) According to this method for operating an electric deionization apparatus, the removal rate of weak electrolytes such as boron is increased, and treated water of high water quality can be obtained.

Description

Method for operating an electrodeionization apparatus

[0001] The present invention relates to a method for operating an electrodeionization apparatus that increases the removal rate of weak electrolytes such as boron and enables the production of treated water of high quality.

[0002] Electrodeionization apparatuses are widely used to produce deionized water for use in various industries, such as semiconductor manufacturing plants, liquid crystal manufacturing plants, pharmaceutical manufacturing, food manufacturing, and power generation, as well as consumer and research facilities. As shown in Figure 1, this electrodeionization apparatus 1 has a structure in which multiple anion exchange membranes 4 and cation exchange membranes 5 are arranged between electrode plates 2 and 3 connected to electrodes (anode 2A and cathode 3A), forming a concentration compartment C and a deionization compartment D. The deionization compartment D is filled with a mixture, multilayer structure, or a single layer of anion and cation exchangers, such as ion exchange resins. E+ denotes the anode compartment, and E- denotes the cathode compartment. Water to be treated (e.g., water treated using a reverse osmosis membrane) W1 is supplied to the deionization compartment D of the electrodeionization apparatus 1 to obtain treated water W2. Concentrated water W3 is supplied to the concentration compartment C in the opposite direction to the deionization compartments, and concentrated wastewater W5 is discharged. Furthermore, electrode water W4 is supplied to the anode compartment E+ and the cathode compartment E-, and electrode wastewater W6 is discharged.

[0003] 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 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)

[0004] In order to promote this ionization reaction, it has been considered effective to operate the electrodeionization device at a high current density, and it has been shown that increasing the current density can increase the boron removal rate.

[0005] 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 feed water (water to be treated). Therefore, simply setting a high current density as one of the operating conditions of an electrodeionization device is not enough to reliably remove weak electrolytes such as boron. Therefore, there is a strong demand for indicators for setting operating conditions that can reliably remove weak electrolytes.

[0006] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is 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.

[0007] In order 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 being operated under conditions that satisfy the following formulas (1), (2), and (3) (Invention 1): 50<A<200 (1) (where A is the operating current density of the electrodeionization apparatus [A / m ２ ) AA / CDout<0.2 (2) (where AA: operating conditions of the electrodeionization apparatus calculated by the following formula (3), CDout: boron concentration of treated water from the electrodeionization apparatus [ng / L]) AA=D×(C Ｃ ×A / dx)×Q×10 ３ [ng / L] (3) (where D: diffusion coefficient [m ２ / second], C Ｃ : boron concentration at the inlet or outlet of the concentrated water adjacent to the treated water outlet of the deionization chamber [ng / L], A: operating current value of the electrodeionization device [A / m ２ dx: thickness of ion exchange membrane [m], Q: flow rate of cell in deionization compartment of electrodeionization apparatus [L / sec])

[0008] In particular, in the above invention (invention 1), it is preferable that the formula (2) is AA / CDout<0.1 (2) (invention 2).

[0009] 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 did not significantly increase the removal rate of weak electrolytes such as boron. After investigating the cause, they found that, in determining the quality of treated water from the deionization compartments of an electrodeionization apparatus, it is necessary to consider two factors: the factor that promotes the removal of weak electrolytes such as boron by increasing the current (current density) in the deionization compartment; and the factor that contributes to diffusion through the ion exchange membrane from the concentration compartment to the deionization compartment. When the influence of diffusion becomes too great, the removal of weak electrolytes such as boron does not progress even when the current (current density) is increased. Based on these findings, the inventors conducted various experiments and found that operating an electrodeionization apparatus so as to satisfy a predetermined relationship can reliably and efficiently remove boron, resulting in high-quality treated water. This led to the invention of the present invention.

[0010] FIG. 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. FIG. 2 is a graph showing the relationship between the boron concentration of concentrated water and the boron concentration of treated water, which are actual measured values ​​and values ​​calculated using formula AA. FIG. 2 is a schematic diagram showing an electrodeionization apparatus used in Example 6. FIG. 3 is a graph showing the relationship between the calculated value of formula AA / CDout and the boron removal rate. FIG. 4 is a graph showing the relationship between current density and boron removal rate. FIG. 5 is a graph showing the relationship between current density and boron concentration of treated water. FIG. 6 is a graph showing the relationship between the calculated value of AA / CDout and the boron concentration of treated water. FIG. 7 is a schematic diagram showing an electrodeionization apparatus used in Example 15.

[0011] The method of operating the electrodeionization apparatus of the present invention will now be described in detail.

[0012] <Electrodeionization Apparatus and Operation Method> 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), (2), and (3) 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 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.

[0013] In this embodiment, the operating conditions of the electrodeionization apparatus are specified to satisfy the following formulas (1), (2), and (3): 50<A<200 (1) (where A is the operating current density of the electrodeionization apparatus [A / m ２ ) AA / CDout<0.2 (2) (where AA: operating condition of the electrodeionization apparatus calculated by the following formula (3), CDout: boron concentration of treated water W2 [ng / L]) AA=D×(C Ｃ ×A / dx)×Q×10 ３ [ng / L] ...(3) (in the formula, C Ｃ : boron concentration at the inlet or outlet of the concentrated water adjacent to the treated water outlet of the deionization chamber [ng / L], A: operating current value of the electrodeionization device [A / m ２ ], dx: thickness of ion exchange membrane [m], Q: cell flow rate in the desalting compartment of the electrodeionization device [L / sec]), D: diffusion coefficient [m ２ The permeability coefficient (diffusion coefficient) of a substance varies depending on the ion exchange membrane used. The smaller the pore size of the membrane, the smaller the diffusion coefficient. On the other hand, the larger the pore size of the membrane, the larger the diffusion coefficient. Specifically, it can be calculated as follows:

[0014] D (diffusion coefficient) is expressed as C in Equation (3).Ｃ , A, dx, and Q are known or configurable values, so these values ​​are substituted into equation (3). Also, AA is the boron concentration increased by diffusion, so in the counterflow method, at an arbitrary current value A, supply water (C Ｃ ) is passed through the desalination compartment, and ultrapure water with boron reduced as much as possible (boron concentration less than 1 ppt, Cdin) is passed through the desalination compartment. AA is calculated by subtracting the boron concentration of the water to be treated in the desalination compartment (Cdin) from the boron concentration of the treated water in the desalination compartment (Cdout). This AA value is then substituted into equation (3) to calculate the AA.

[0015] By satisfying the above-mentioned formulas (1) to (3), 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.

[0016] In this embodiment, formula (3) was derived using the following procedure. 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 on the treated water, the larger AA / CDout becomes. 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 / m ２ / sec] and is defined as the amount of a certain property passing through a unit area per unit time. D is the diffusion coefficient [m ２ / sec], and C is the concentration [mol / m ３ ] and x is the position [m].)

[0017] 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 Boron concentration near the membrane surface on the concentration compartment side (C Ｃ，ｍ ) is concentrated by applying a current (A) according to the following formula iii:Ｃ，ｍ =C Ｃ ×βA iii (wherein β is a constant term for convenience.)

[0018] 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 as D = D' × β, the flux (J) is expressed as the following formula vi. J = -D(C Ｃ × 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 of the deionization compartment, to obtain the following equations vii and viiii. AA = D(C Ｃ ×A / dx)×Q [μg / L] ...vii AA=D×(C Ｃ ×A / dx)×Q×10 ３ [ng / L] ...viii As a result, the formula (3) of the present invention was derived.

[0019] Although the present invention has been described above based on the above-mentioned embodiment, various modifications of the present invention 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 the anion exchange membranes or cation exchange membranes are partially connected to form a continuous deionization compartment.

[0020] The process for calculating the operating conditions in the method for operating an electrodeionization apparatus of the present invention will be described below based on specific examples.

[0021] Examples 1 to 5 A test electrodeionization apparatus as shown in Figure 2 was prepared. In Figure 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+ indicates the anode compartment, and E- indicates the cathode compartment. An ultrapure water supply pipe is connected to an ultrapure water (UPW) supply source 6 and 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 prepared, and a supply pipe 12 equipped with a pump 13 is connected from this tank 11 to water supply pipe 7. As a result, ultrapure water containing boron solution B is supplied to the concentrating chamber C on the cathode chamber E- side, while ultrapure water is supplied to the other concentrating chamber C, deionization chamber D, anode chamber E+, and cathode chamber E-. Reference numerals 14A and 14B denote boron concentration measuring means. Water to be treated W1 is supplied to the deionization chamber D of the electrodeionization device 1 to obtain treated water W2, while concentrated water W3 is supplied to the concentrating chamber C and concentrated wastewater W5 is discharged. Furthermore, electrode water W4 is supplied to the anode chamber E+ and cathode chamber E-, and electrode wastewater W6 is discharged.

[0022] The cell size of this electrodeionization device 1 was 48.5 mm wide x 230 mm high x 5.0 mm thick, and the 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).

[0023] In this example, ultrapure water has a resistivity of 18.1 MΩ cm or more, fine particles of 50 nm or more in diameter and 1,000 particles / L or less, viable bacteria of 1 particle / L or less, TOC of 1 μg / L or less, total silicon of 0.1 μg / L or less, metals of 1 ng / L or less, ions of 10 ng / L or less, hydrogen peroxide of 30 μg / L or less, and a water temperature of 25±2° C.

[0024] In the electrodeionization apparatus 1 described above, the operating current density is 100 [A / m ２ The boron concentration of the concentrated water supplied from the boron solution B to one of the concentration compartments C was varied from 0 to 1000 ppb.

[0025] In this case, the calculation parameters were set as follows: Q (cell flow rate in the desalting compartment of the electrodeionization device): 0.005 [L / sec]; dx (thickness of the ion exchange membrane): 5.0 × 10 －４ [m] (500 [μm]) D (diffusion coefficient): (2.5 × 10 －１９ [m ２ / second]

[0026] The boron concentration of treated water W2 under these operating conditions was measured. 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 (at the inlet of chamber C), the current density, and the value of AA in equation (3). The relationship between the boron concentration of treated water W2 and the boron concentration of concentrated water W3 is shown in Figure 4.

[0027] Comparative Example 1 An 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 chamber C on the anode chamber E+ side as shown in FIG.

[0028] The boron concentration of treated water W2 under these operating conditions was measured. The results are shown in Table 1 together with the boron (B) concentration of the water to be treated (feedwater) W1, the boron (B) concentration of concentrated water W3 (at the inlet of chamber C), the current density, and the value of AA in formula (3).

[0029]

[0030] From Table 1 and FIG. 4, it can be seen that boron is mixed into the treated water W2 via the cation exchange membrane 5. However, since the cation exchange membrane 5 is a solid superacid, the pH in the cation exchange membrane 5 is extremely low and is an acidic condition. Therefore, most of the boron is in the form of molecular boron (H ３ BO ３ ) and the following relational expression is obtained: H ３ BO ３ +OH － → B(OH) ４ － (pKa=9.24)

[0031] Therefore, it can be said that passage through the cation exchange membrane 5 is due to concentration diffusion, not charge transfer due to voltage application. Furthermore, it can be seen from Figure 4 that the boron concentration in the 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.

[0032] Examples 6 to 9 and Comparative Example 2 A test electrodeionization apparatus as shown in Figure 5 was prepared. In Figure 5, electrodeionization apparatus 1 has basically the same configuration as that shown in Figure 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 supply pipes 7 and 8, allowing the supply water to deionization chamber D to flow in the opposite direction to the supply water to 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 was prepared. Supply pipe 12 equipped with a pump 13 connected from this tank 11 to supply pipe 7. A tank 15 containing a boron solution B of a known concentration as a chemical component was prepared. Supply pipe 16 equipped with a pump 17 connected from 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, 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.

[0033] In the electrodeionization apparatus 1 described above, the operating current density is 100 [A / m ２ 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- were varied 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.

[0034] The boron (B) concentration of treated water W2 under these operating conditions was measured. 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 (at the inlet of 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.

[0035]

[0036] In Figure 6, the index AA / CDout was used as the standard. 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 influence of diffusion. As is clear from Table 2 and Figure 6, the smaller the AA / CDout, the higher the boron removal rate. As the AA / CDout increases, the influence of diffusion becomes greater, and when the AA / CDout exceeds 0.2, the boron removal rate drops sharply. In particular, it can be seen that in order to maintain a high boron removal rate, it is preferable to keep the AA / CDout at 0.1 or less.

[0037] [Examples 10 to 12, and Comparative Examples 3 and 4] In Example 6, the boron concentration of the concentrated water W3 and electrode water W4 supplied to the concentration chamber C, anode chamber E+, and cathode chamber E- was 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 operating current density was set to 25 to 300 [A / m ２ The electrodeionization apparatus 1 was operated by varying the temperature from 0.01 to 0.01. The parameters for calculation were the same as those in Example 1.

[0038] The boron concentration of treated water W2 under these operating conditions was measured. The results are shown in Table 3 together with the current density, the value of the formula AA / CDout, and the boron (B) removal rate. The relationship between current density and boron removal rate is shown in Figure 7, the relationship between current density and boron concentration removal rate in treated water W2 is shown in Figure 8, and the relationship between the formula AA / CDout and boron concentration removal rate is shown in Figure 9.

[0039]

[0040] 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, when the value of AA / CDout exceeds 0.1, the boron concentration in treated water W2 significantly converges to a constant value.

[0041] [Examples 13 and 14] In Example 6, the operating current density was 100 [A / m ２ 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- were 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.

[0042] The boron concentration of treated water W2 under these operating conditions was measured. The results are shown in Table 4 together 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 concentrating chamber adjacent to the outlet of treated water W2, the value of the formula AA / CDout, and the boron (B) removal rate.

[0043] 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, 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. 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 provided. 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.

[0044] The boron concentration of treated water W2 under these operating conditions was measured. The results are shown in Table 4 together with the boron concentrations of concentrated water W3, concentrated wastewater W5, and concentrated wastewater W5 at the outlet of the concentrating chamber adjacent to the outlet of treated water W2, the value of the mathematical formula AA / CDout, and the boron (B) removal rate.

[0045]

[0046] As is clear from Table 4, when focusing on the inlet or outlet concentration of the concentrated water adjacent to the treated water outlet of the desalination compartment, formula AA is the same whether the desalinated water D and the concentrating compartment C are passed in the same direction (parallel flow) or the desalinated water D and the 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 concentrated water 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.

[0047] REFERENCE SIGNS LIST 1 Electrodeionization apparatus 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 Pump 14A, 14B, 14C, 14D Boron concentration measuring means 15 Tank 16 Supply pipe 17 Pump C Concentration compartment D Deionization compartment 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

1. 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, with the deionization compartments filled with ion exchangers, the method being performed under conditions that satisfy the following formulas (1), (2), and (3): 50<A<200 (1) (where A is the operating current density of the electrodeionization apparatus [A / m ２ ) AA / CDout<0.2 (2) (where AA: operating conditions of the electrodeionization apparatus calculated by the following formula (3), CDout: boron concentration of treated water from the electrodeionization apparatus [ng / L]) AA=D×(C Ｃ ×A / dx)×Q×10 ３ [ng / L] (3) (where D: diffusion coefficient [m ２ / second], C Ｃ : boron concentration at the inlet or outlet of the concentrated water adjacent to the treated water outlet of the deionization chamber [ng / L], A: operating current value of the electrodeionization device [A / m ２ dx: thickness of ion exchange membrane [m], Q: flow rate of cell in deionization compartment of electrodeionization apparatus [L / sec]) 2. The method for operating an electrodeionization apparatus according to claim 1, wherein the formula (2) is: AA / CDout<0.1 (2).

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