Method for operating an electrodeionization apparatus

Abstract

To provide an operation method for an electrolytic deionization apparatus that enhances the removal rate of weakly electrolyzed substances such as boron, enabling the production of treated water with high water quality.SOLUTION: There is provided an operation method for an electrolytic deionization apparatus which includes multiple anion exchange membranes and cation exchange membranes arranged between an anode and a cathode to form a concentration chamber and a desalination chamber, in which the desalination chamber is filled with an ion exchange material, and which is operated under conditions satisfying the following equations (1) and (2): 50<A<200...(1) (In equation (1), A: operating current density of the electrolytic deionization apparatus [A / m2]) AA / CDout<0.2...(2) (In equation (2), AA: concentration [ng / L] of boron moving from the concentration chamber to the desalination chamber by diffusion; CDout: boron concentration [ng / L] in treated water of the electrolytic deionization apparatus.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[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. [Background technology]

[0002] Electrodeionization apparatuses are widely used to produce deionized water for use in various industries, including semiconductor and liquid crystal manufacturing plants, pharmaceutical and food industries, and power plants, as well as in consumer and research facilities. As shown in Figure 1, this electrodeionization apparatus 1 has a structure in which multiple anion and cation exchange membranes 4 and 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+ is the anode compartment, and E- is 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. Electrodewater 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 other facilities has been increasing, and the required concentration of boron, a weak electrolyte in ultrapure water, has been reduced to below 1 ppt. However, to remove boron to a high degree, it is necessary to promote ionization according to the following formula. H3BO3+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. Summary of the Invention [Problem to be solved by the invention]

[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. [Means for solving the problem]

[0007] In order to achieve the above object, the present invention provides a method for operating an electrodeionization apparatus 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, and the deionization compartments are filled with ion exchangers, the method comprising the steps of: and (2 )of The present invention provides a method for operating an electrodeionization apparatus under conditions that satisfy the above requirements (Invention 1). 50 <A<200 ···(1) (In equation (1), A: operating current density of the electrodeionization device [A / m 2 ) AA / CDout<0.2 (2) (In formula (2), AA is calculated by the following formula (3) Boron concentration transferred from the concentration compartment to the deionization compartment by diffusion [ng / L] , CDout: Boron concentration in the treated water from the electrodeionization device (ng / L) AA=D×(C C ×A / dx)×Q×10 3 [ng / L] (3) (In equation (3), D: diffusion coefficient [m 2 / second], C C : Concentration chamber adjacent to the treated water outlet of the desalination chamber room Boron concentration at the inlet or outlet (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 desalting compartment of the electrodeionization device (L / sec)

[0008] In particular, in the above invention (Invention 1), Equation (2) is AA / CDout<0.1 (2) It is preferable that (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 discovered that, in an electrodeionization apparatus, the quality of treated water from the deionization compartment must be determined by considering 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. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a schematic diagram showing an electrodeionization apparatus to which the present invention can be applied. [Figure 2] FIG. 1 is a schematic diagram showing an electrodeionization apparatus used in Example 1. [Figure 3] FIG. 1 is a schematic diagram showing an electrodeionization apparatus used in Comparative Example 1. [Figure 4] 1 is a graph showing the relationship between the boron concentration in concentrated water and the boron concentration in treated water, which are actually measured values ​​and values ​​calculated using formula AA. [Figure 5] FIG. 1 is a schematic diagram showing an electrodeionization apparatus used in Example 6. [Figure 6] 1 is a graph showing the relationship between the calculated value of the formula AA / CDout and the boron removal rate. [Figure 7] 1 is a graph showing the relationship between current density and boron removal rate. [Figure 8] 1 is a graph showing the relationship between current density and boron concentration in treated water. [Figure 9] 1 is a graph showing the relationship between the calculated value of AA / CDout and the boron concentration of treated water. [Figure 10] FIG. 1 is a schematic diagram showing the electrodeionization apparatus used in Example 15. DETAILED DESCRIPTION OF THE INVENTION

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

[0012] <Electrodeionization device and its operating method> The electrodeionization apparatus to which the present invention can be applied is not particularly limited, and for example, an apparatus as shown in FIG. and (2 )of The system may be operated according to conventional methods, except that it is operated under conditions that satisfy the above requirements. 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 device 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. During this process, 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 effluent waters 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 expressed by the following formula (1): and (2 )of It is prescribed to satisfy. 50 <A<200 ···(1) (Wherein, A: operating current density of the electrodeionization device [A / m 2 ) AA / CDout<0.2 (2) (wherein AA is calculated by the following formula (3) Boron concentration transferred from the concentration compartment to the deionization compartment by diffusion [ng / L] , CDout: Boron concentration in treated water W2 (ng / L) AA=D×(C C ×A / dx)×Q×10 3 [ng / L] (3) (In the formula, C C : Concentration chamber adjacent to the treated water outlet of the desalination chamber room Boron concentration at the inlet or outlet (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 diffusion coefficient (permeability) varies depending on the ion exchange membrane used. The smaller the membrane pore size, the smaller the diffusion coefficient. On the other hand, the larger the membrane pore size, the larger the diffusion coefficient. Specifically, it can be calculated as follows:

[0015] As mentioned above, equation (1) and (2) By satisfying the above conditions, boron in the water to be treated (feed water) can be removed reliably and efficiently to obtain high-quality deionized water (treated water).

[0016] In this embodiment, the formula (3) is derived in the following manner. The AA defined in equation (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 equation (3) is based on Fick's law, the fundamental law regarding the diffusion of substances (equation 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 C,m =C C ×βA iii (In the formula, 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 equation i) J=-D´(C C,m / dx)=-D´(C C ×βA / dx) (v: Substitute iii into equation iv) Here, if the diffusion coefficient D is set to D = D' × β, the flux (J) is given by the following equation vi. J=-D(C C ×A / dx) ···vi Since J is the flux, the permeation amount can be calculated by multiplying it by Q (L / sec), which is the cell flow rate in the desalting compartment, to obtain the following equations vii and viii. AA=D(C C ×A / dx)×Q [μg / L] ···vii AA=D×(C C ×A / dx)×Q×103 [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. [Example]

[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+ is the anode compartment, and E- is the cathode compartment. An ultrapure water supply pipe is provided in communication with ultrapure water (UPW) supply source 6, and this ultrapure water supply pipe branches into supply pipes 7 and 8. A tank 11 containing boron solution B of a known concentration as a chemical component is provided, and supply pipe 12 equipped with a pump 13 connects this tank 11 to supply pipe 7. Thus, ultrapure water containing boron solution B is supplied to 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 to discharge concentrated wastewater W5. Furthermore, electrode water W4 is supplied to the anode chamber E+ and the 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, particles of 50 nm or more in diameter and 1000 particles / L or less, viable bacteria of 1 particle / L or less, total organic carbon 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 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 (ion exchange membrane thickness): 5.0 x 10 -4 [m](500 [μm]) D (diffusion coefficient): (2.5 x 10 -19 [m 2 / sec〕

[0026] 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 (at the inlet of chamber C), the current density, and the value of A 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] 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.

[0028] 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 (at the inlet of chamber C), the current density, and the value of A in equation (3).

[0029] [Table 1]

[0030] From Table 1 and Figure 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 acidic. For this reason, most of the boron is in the form of molecular boron (H3BO3), and the following relationship holds: H3BO3+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 caused by 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 almost the same. 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 pipe is provided in communication with an ultrapure water (UPW) supply source 6. This ultrapure water pipe branches into pipes 7 and 8, allowing the supply water for deionization chamber D to flow in the opposite direction to the supply water for concentration chamber C, anode chamber E+, and cathode chamber E−. A tank 11 containing boron solution B of a known concentration as a chemical component is provided. A supply pipe 12 equipped with a pump 13 runs from this tank 11 to pipe 7. A tank 15 containing boron solution B of a known concentration as a chemical component is provided. A supply pipe 16 equipped with a pump 17 runs from this tank 15 to pipe 8. As a result, ultrapure water with boron solution B added is supplied from tank 11 to deionization compartment D, while ultrapure water with boron solution B 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 with boron added at different concentrations to deionization compartment D and concentration compartment C. Reference numerals 14A and 14B denote boron concentration measuring means, respectively.

[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 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.

[0035] [Table 2]

[0036] 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 effect of diffusion becomes greater, and the boron removal rate drops sharply when AA / CDout exceeds 0.2. 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.

[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 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. 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] [Table 3]

[0040] 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 AA / CDout value also increases, and the boron concentration in treated water W2 gradually approaches a constant value. In particular, when the AA / CDout value 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 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 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.

[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 pipe is provided in communication with ultrapure water (UPW) supply source 6, and this ultrapure water pipe branches into water pipe 7 and water pipe 8 along the way, so that the supply water for deionization chamber D flows in the same direction as the concentrating chamber C, anode chamber E+, and cathode chamber E-. A tank 11 containing boron solution B of a known concentration as a chemical component is prepared, and a supply pipe 12 equipped with a pump 13 connects this tank 11 to water pipe 7. A tank 15 containing boron solution B of a known concentration as a chemical component is prepared, and a supply pipe 16 equipped with a pump 17 connects this tank 15 to water pipe 8. As a result, ultrapure water with boron solution B added is supplied from tank 11 to deionization compartment D, anode compartment E+, and cathode compartment E-, while ultrapure water with boron solution B added is supplied from tank 15 to concentration compartment C. This results in a structure in which ultrapure water with boron 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 was measured under these operating conditions. The results are shown in Table 4, along with the boron concentrations of concentrated water W3, concentrated wastewater W5, and 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.

[0045] [Table 4]

[0046] As is clear from Table 4, the concentration of the treated water adjacent to the outlet of the desalination chamber room When we focus on the inlet or outlet concentration of the desalinated water D, the formula AA is the same whether the desalinated water D and the concentration chamber C are passed in the same direction (parallel flow) or the desalinated water D and the concentration chamber C are passed in the same direction (counter flow). roomBy summarizing the effect of diffusion in terms of "inlet or outlet concentration of" we can see that the same formula can be applied to both counterflow and parallel flow. [Explanation of symbols]

[0047] 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) Source 7 Water pipe 8 Water 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 chamber D Desalination room E+ Anode chamber E- Cathode chamber W1 Treated water W2 treated water W3 Concentrated water W4 electrode water W5 Concentrated wastewater W6 electrode drainage

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 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 the formula (1), A is the 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] A=D×(C Ｃ ×A / dx)×Q×10 ３ 〔n / L〕 ・・・(3) (In formula (3), D: diffusion coefficient [m ２ / second], 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]

2. Formula (2) is AA / CDout<0.1 (2) 2. The method of claim 1, wherein:

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