Method for controlling electrodeionization device

Abstract

This method for controlling an electrodeionization device is for an operation for changing the amount of water supplied to an electrodeionization device 1. The passing water flow rate is changed such that a flow rate of water supply W0 to a desalting chamber 36 of the electrodeionization device 1 is 50％ or less per minute with respect to a maximum water supply flow rate in the desalting chamber 36 of the electrodeionization device 1 while a flow rate of concentrated water W5 discharged from the electrodeionization device 1 is kept constant. According to such an electrodeionization device operation method, it is possible to prevent a reduction in quality of treated water when an operation in which the passing water flow rate is changed is performed.

Description

Control method for an electrodeionizer

[0001] The present invention relates to an operating method for an electrodeionizer, and more particularly to an operating method for an electrodeionizer that prevents a deterioration in treated water quality when operating with varying water flow rates.

[0002] Traditionally, ultrapure water used in the electronics industry, such as semiconductors, is produced by treating raw water in an ultrapure water production system consisting of a pretreatment device, a primary pure water production device, and a secondary pure water production device (subsystem) that processes the primary pure water.

[0003] The primary water production equipment that constitutes such ultrapure water production systems is a highly versatile system that is used in various fields other than ultrapure water production, such as pharmaceuticals and food products. The primary water production equipment typically consists of a two-stage reverse osmosis (RO) membrane system and an electrodeionizer. The reverse osmosis (RO) membrane system removes silica and salts, as well as ionic and colloidal TOC.

[0004] In general, an electrodeionizer has a configuration in which cation exchange membranes and anion exchange membranes are alternately arranged between the cathode and anode to alternately form a desalination chamber and a concentration chamber, and the desalination chamber is filled with an ion exchange resin, thereby removing various inorganic or organic anions and cations.

[0005] When water is supplied to the desalination chamber of this electrodeionizer, ions in the water move towards either the positive or negative ion exchange resin in the chamber due to their charge. The moved ions pass through the ion exchange resin and enter the concentration chamber, thus producing highly desalinized pure water in the desalination chamber. On the other hand, the ions that move to the concentration chamber are discharged as concentrated water.

[0006] From the perspective of stably producing primary pure water of a predetermined quality, the electrodeionizer was operated under constant water supply conditions. Therefore, the primary pure water produced by the primary pure water production system, including the electrodeionizer, was supplied in the required amount to the sub-tank of the secondary pure water production system, while any surplus primary pure water produced was circulated and reused within the primary pure water production system.

[0007] However, as mentioned above, conventional operating methods for primary pure water production systems involve supplying more water than necessary to electrodeionizers and other equipment for processing, leaving room for improvement in terms of energy efficiency. Therefore, Patent Document 1 discloses a method for controlling the amount of water supplied to an electrodeionizer so as to maintain a constant water level in the tank downstream of the electrodeionizer.

[0008] Patent No. 6368510

[0009] However, according to our investigation, as described in Patent Document 1, changing the flow rate of the electrodeionizer can cause a decrease in the quality of the treated water, and this tendency is particularly pronounced when the feedwater to the desalination chamber of the electrodeionizer contains a relatively large amount of carbonate ions.

[0010] The present invention has been made in view of the above problems, and aims to provide an operating method for an electrodeionizer that prevents a decrease in treated water quality when operating with varying water flow rates.

[0011] In view of the above objectives, the present invention provides a control method for an electrodeionizer that operates by changing the amount of water supplied to the electrodeionizer, wherein the flow rate of the water supplied to the desalination chamber of the electrodeionizer is changed by 50% / min or less relative to the maximum flow rate of the water supplied to the desalination chamber of the electrodeionizer (Invention 1).

[0012] According to the present invention (Invention 1), if the change in the water flow rate in the desalination chamber of the electrodeionizer is 50% / min or less relative to the maximum water flow rate, the deterioration of the treated water quality can be suppressed even if the water supply to the desalination chamber of the electrodeionizer is changed. This is because when the flow rate in the desalination chamber of the electrodeionizer is changed, the state of the adsorption zone of the ion exchange resin inside the electrodeionizer also changes. When the flow rate in the desalination chamber of the electrodeionizer is reduced, ions adsorbed on the ion exchange resin at the bottom of the desalination chamber move to the concentration chamber, and the proportion of regenerative ion exchange resin in the bottom of the desalination chamber increases, which is assumed to improve the treated water quality. On the other hand, the concentration difference between the bottom of the desalination chamber and the bottom of the concentration chamber increases, and the movement of ions from the concentration chamber to the desalination chamber due to concentration diffusion increases, which deteriorates the treated water quality. When the rate of change in flow rate is large, the effect of concentration diffusion becomes large, and it is thought that the treated water quality deteriorates. It is estimated that the threshold for this rate of change in flow rate is 50% / min or less, and especially 25% / min or less.

[0013] In the above invention (Invention 1), the inorganic carbon concentration of the feedwater supplied to the desalination chamber is 10 to 3000 μg / L as CO2 ２ It may also be acceptable (Invention 2).

[0014] According to this invention (Invention 2), a higher inorganic carbon concentration in the feedwater supplied to the desalination chamber makes water quality deterioration more likely, but an inorganic carbon concentration of 10 to 3000 μg / L as CO2 ２ Even if the cost is high, it can suppress the deterioration of the water quality of the treated water.

[0015] In the above invention (Invention 2), it is preferable to pass the water supply from the desalination chamber of the electrodeionizer or the treated water of the electrodeionizer through the concentration chamber of the electrodeionizer in the opposite direction to the water flow direction through the desalination chamber (Invention 3).

[0016] According to this invention (Invention 3), even when the inorganic carbon dioxide concentration of the feedwater supplied to the desalination chamber is high, the deterioration of the water quality of the treated water can be further suppressed.

[0017] In the above inventions (Inventions 1 to 3), it is preferable that the minimum water flow rate in the desalination chamber of the electrodeionizer is 30% or more of the maximum water flow rate in the desalination chamber of the electrodeionizer (Invention 4).

[0018] According to this invention (Invention 4), if the water flow rate in the desalination chamber of the electrodeionizer is fluctuated too much, it will lead to a decrease in the water quality of the treated water of the electrodeionizer. However, by setting it to 30% or more, a decrease in water quality due to a reduction in the water supply to the desalination chamber is avoided.

[0019] According to the control method for the electrodeionizer of the present invention, the limit value of the flow rate fluctuation rate relative to the maximum flow rate of the feedwater in the desalination chamber of the electrodeionizer, which can suppress the deterioration of the water quality of the treated water, is set to 50% / min or less. Therefore, even if the flow rate of the feedwater in the desalination chamber of the electrodeionizer is changed, the deterioration of the water quality of the treated water can be suppressed.

[0020] This is a flow chart showing an ultrapure water production system to which the control method for an electrodeionizer according to the present invention can be applied. This is a schematic diagram showing the control structure of an electrodeionizer in the control method for an electrodeionizer according to the present invention. This is a schematic diagram showing an electrodeionizer used in the control method for an electrodeionizer according to the present invention. This is a schematic diagram showing the water flow state of an electrodeionizer used in the control method for an electrodeionizer according to the present invention. This is a graph showing the flow rate of feedwater supplied to the desalination chamber of the electrodeionizer in Example 1 and the change in resistivity (MΩ・cm). This is a graph showing the flow rate of feedwater supplied to the desalination chamber of the electrodeionizer in Comparative Example 1 and the change in resistivity (MΩ・cm).

[0021] The control method of the electrodeionizer of the present invention will be described below with reference to the attached drawings. For the purpose of explanation, a diagram in which the electrodeionizer is installed in an ultrapure water production system will be used in part, but the control method of the electrodeionizer of the present invention is not limited to this ultrapure water production system and can be used in various fields such as pharmaceuticals and food.

[0022] (Urpure Water Production System) Figure 1 is a flow diagram showing an ultrapure water production system capable of implementing the control method of the electrodeionizer 1 according to one embodiment of the present invention. As shown in Figure 1, the ultrapure water production system 100 consists of three stages of equipment: a pretreatment device 2, a primary pure water production device 3 including the electrodeionizer 1 (indicated as CDI in Figure 1), and a secondary pure water production device (subsystem) 4. In the pretreatment device 2 of such an ultrapure water production system 100, the raw water W is pretreated by filtration, coagulation and sedimentation, microfiltration membranes, etc., and suspended solids are mainly removed.

[0023] The primary pure water production apparatus 3 includes a reverse osmosis membrane apparatus 5 for treating the pre-treated water W1, a degassing membrane apparatus 6, an ultraviolet oxidation apparatus 7, an electrodeionizer 1, and a water supply pump 8 for supplying the pre-treated water W1 to the electrodeionizer 1. The primary pure water production apparatus 3 removes most of the electrolytes, fine particles, live bacteria, etc. from the pre-treated water W1 and decomposes organic matter.

[0024] Subsystem 4 consists of a sub-tank 11, which is a storage tank located downstream of the electrodeionizer 1, for storing primary pure water W2 produced by the primary pure water production device 3 (in this embodiment, since the electrodeionizer 1 is located at the end of the primary pure water production device 3, this corresponds to the treated water from the electrodeionizer 1; the same applies hereinafter), an ultraviolet oxidation device 12, a non-regenerative mixed-bed ion exchange device 13, and an ultrafiltration (UF) membrane 14 as a membrane filtration device, which treat the primary pure water W2 supplied from the sub-tank 11 via a pump (not shown). In addition, an RO membrane separation device may be provided as needed. In this subsystem 4, the ultraviolet oxidation device 12 oxidizes and decomposes trace amounts of organic matter (TOC components) contained in the primary pure water W2, and then the non-regenerative mixed-bed ion exchange device 13 processes it to remove residual carbonate ions, organic acids, anionic substances, and even metal ions and cationic substances by ion exchange. Then, the ultrafiltration (UF) membrane 14 removes the fine particles to produce ultrapure water W3, which is supplied to the use point 15, and the unused ultrapure water W3 is returned to the sub-tank 11.

[0025] In this embodiment, as shown in Figure 2, the primary pure water production apparatus 3 is equipped with a water supply pump 8 for controlling the flow rate of water W0 supplied to the electrodeionizer 1. The electrodeionizer 1, which is in communication with the water supply pump 8, is equipped with a DC power supply 9, and the treated water W2 from the electrodeionizer 1 can be supplied to a sub-tank 11, which is a water storage tank located downstream of the electrodeionizer 1.

[0026] Furthermore, the flow path 25 of the concentrated water W5 in the electrodeionizer 1 is equipped with a control valve 26 and a flow meter 27 for arbitrarily controlling the flow rate of the concentrated water W5. In addition, the flow path 22 of the treated water (desalinated water) W2 in the electrodeionizer 1 is also equipped with a control valve 23 and a flow meter 24.

[0027] The control device 28, equipped with a personal computer and the like, can increase or decrease the flow rate of water W0 supplied to the electrodeionizer 1 by controlling the water supply pump 8, and can arbitrarily increase or decrease the flow rate of the flow path 22 and / or flow path 25 by controlling the control valves 23 and 26. The control device 28 can also transmit measurement data from the flow meter 24 and the flow meter 27. In addition, the sub-tank 11 may be provided with a level switch 21 for measuring the amount of water stored, and the amount of treated water W2 produced may be controlled according to the measurement data of the amount of water stored in the sub-tank 11.

[0028] (Electrodeionizer) Here, the electrodeionizer 1 can preferably be one having the configuration shown in Figures 3 and 4.

[0029] In Figure 3, the electrodeionizer 1 alternately forms a concentration chamber 35 and a desalination chamber 36 by arranging multiple anion exchange membranes 33 and cation exchange membranes 34 alternately between electrodes (anode 31, cathode 32). The desalination chamber 36 is filled with a mixture or multi-layered ion exchange material (anion exchange material and cation exchange material) consisting of an ion exchange resin, ion exchange fiber, or graft exchange material. The concentration chamber 35, as well as the anode chamber 37 and cathode chamber 38, are also filled with ion exchange material.

[0030] This electrodeionizer 1 is provided with a water supply means (not shown) for passing water W0 through the desalination chamber 36 to extract treated water (desalination water) W2, and a concentrated water supply means (not shown) for passing water to be concentrated W4 through the concentration chamber 35. In this embodiment, the water to be concentrated W4 is introduced into the concentration chamber 35 from the side of the desalination chamber 36 closest to the outlet for the treated water W2, and flows out from the side of the desalination chamber 36 closest to the inlet for the water supply W0. In other words, the water to be concentrated W4 is introduced into the concentration chamber 35 from the opposite direction to the flow direction of the water supply W0 in the desalination chamber 36, and concentrated water W5 is discharged.

[0031] The water to be concentrated W4 introduced into the concentration chamber 35 can be the same as the feedwater W0 supplied to the desalination chamber 36, but as shown in Figure 4, the treated water W2 obtained from the desalination chamber 36 can also be used as the water to be concentrated W4.

[0032] (Control method for the electrodeionizer) The control method for the electrodeionizer 1 according to this embodiment will be described below.

[0033] The control method for the electrodeionizer 1 according to this embodiment involves changing the flow rate of the feedwater W0 supplied to the desalination chamber 36 of the electrodeionizer 1 while maintaining a constant flow rate of the concentrated water W5 discharged from the electrodeionizer 1. Normally, when the flow rate of the feedwater W0 of the electrodeionizer 1 is reduced, the concentration difference between the lower part of the desalination chamber 36 and the lower part of the concentration chamber 35 increases, and the movement of ions from the concentration chamber 35 to the desalination chamber 36 due to concentration diffusion tends to increase. Therefore, in this embodiment, the flow rate of the feedwater W0 in the desalination chamber 36 of the electrodeionizer 1 is changed to 50% / min or less, particularly 25% / min or less, relative to its maximum flow rate. This makes it possible to maintain the water quality (e.g., resistivity) of the treated water (desalination water) W2.

[0034] In one embodiment of the control method, it is preferable that the minimum flow rate of the feedwater W0 of the electrodeionizer 1 during flow rate fluctuations is 30% or more of the maximum flow rate of the feedwater in the desalination chamber of the electrodeionizer. If the flow rate of the feedwater W0 of the electrodeionizer 1 falls below the minimum value, the feedwater in the desalination chamber 36 may decrease too much, potentially degrading the water quality of the treated water W2, which is undesirable.

[0035] In the control method according to this embodiment, the flow rate of the concentrated water W5 discharged from the electro-deionization device 1 is controlled to be kept constant. For example, as shown in FIG. 2, in response to the change in the amount of the feed water W0, the control valves 23 and 26 are controlled by the control device 28 to control the flow rates of the treated water W2 and the concentrated water W5 of the electro-deionization device 1, which makes it possible. That is, the amount of the treated water W2 may be adjusted so that the recovery rate varies while the amount of the concentrated water W5 is kept constant. Here, being kept constant means that the change in the flow rate of the concentrated water W5 discharged from the electro-deionization device 1 is in the range of 90% to 110%.

[0036] In addition, in the control method according to another embodiment, the water recovery amount of the electro-deionization device 1 is not particularly limited, but is preferably 50 to 99%.

[0037] In the control method according to an embodiment, the conductivity of the feed water W0 supplied to the desalination chamber 36 of the electro-deionization device 1 is not particularly limited, but is preferably 0.1 to 5 mS / m. Further, in this embodiment, the inorganic carbon concentration of the feed water W0 supplied to the desalination chamber may be at a relatively high level of 10 to 3000 μg / L as CO2. The higher the inorganic carbon concentration is, if the reduction rate of the flow rate is large, the water quality is likely to deteriorate. However, in this embodiment, the effect of suppressing the deterioration of the water quality is great. The current efficiency of the feed water W0 to the electro-deionization device 1 is preferably 1 to 30%.

[0038] As described above, the present invention has been described based on the above embodiments. However, the present invention is not limited to the above embodiments, and may be configured by devices such as a pretreatment device, a primary pure water production device, and a secondary pure water production device (subsystem). As long as the primary pure water production device is an ultrapure water production system including an electro-deionization device, the configuration of each device is not limited and can be applied to various configurations.

[0039] Hereinafter, the present invention will be described more specifically based on examples. However, the present invention is not limited to the following examples.

[0040] 〔Example 1〕 VN - 55EX - 2 (manufactured by Evora Water Technologies) was used as the electro - deionization device. Water with a conductivity of 1 mS / m and an inorganic carbon concentration of 400 μg / L as CO ２ was used as the feed water W0 to the desalination chamber and the water to be concentrated W4, and water was passed through at a recovery rate of 94 - 95% such that the concentrated water flowed in the opposite direction (counter - flow) to the treated water as shown in Fig. 3.

[0041] This electro - deionization device was operated at a desalination chamber flow rate of 12.4 m ３ / h and a concentration chamber flow rate of 0.66 m ３ / h. While operating, the flow rate of the desalination chamber was decreased at a rate of 2.7 m ３ / h per minute (flow rate change speed: 22% / min), and the flow rate of the desalination chamber was set to 9.75 m ３ / h. The change in the resistivity of the treated water and the change in the flow rate of the desalination chamber during this period were measured. The results are shown in Fig. 5.

[0042] 〔Comparative Example 1〕 The same electro - deionization device as in Example 1 was used. Water with a conductivity of 1 mS / m and an inorganic carbon concentration of 600 μg / L as CO ２ was used as the feed water W0 to the desalination chamber and the water to be concentrated W4, and water was passed through at a recovery rate of 90 - 95% such that the concentrated water flowed in the same direction (parallel flow) as the treated water.

[0043] This electro - deionization device was operated at a desalination chamber flow rate of 12.4 m ３ / h and a concentration chamber flow rate of 0.66 m3 / h. While operating, the flow rate of the desalination chamber was decreased at a rate of 6.4 m ３ / h per minute (flow rate change speed: 52% / min), and the flow rate of the desalination chamber was set to 6.00 m ３ / h. The change in the resistivity of the treated water and the change in the flow rate of the desalination chamber during this period were measured. The results are shown in Fig. 6.

[0044] As is clear from Figures 5 and 6, in Example 1, where the flow rate change rate was 22% / min, the resistivity of the treated water was maintained at 18.0 MΩ·cm or higher, whereas in Comparative Example 1, where the flow rate change rate was 52% / min, the resistivity decreased to approximately 17.5 MΩ·cm. This is because changing the flow rate in the desalination chamber of the electrodeionizer also changes the state of the adsorption zone of the ion exchange resin inside the electrodeionizer. When the flow rate in the desalination chamber of the electrodeionizer is reduced, ions adsorbed on the ion exchange resin at the bottom of the desalination chamber move to the concentration chamber, and it is assumed that the proportion of regenerative ion exchange resin in the bottom of the desalination chamber increases, improving the quality of the treated water. On the other hand, the concentration difference between the bottom of the desalination chamber and the bottom of the concentration chamber increases, and the movement of ions from the concentration chamber to the desalination chamber due to concentration diffusion increases, degrading the quality of the treated water. When the flow rate change rate is high, the effect of concentration diffusion becomes greater, and it is thought that the quality of the treated water deteriorates. It is estimated that the threshold for this flow rate change rate is 50% / min.

[0045] 100 Ultrapure Water Production System 1 Electrodeionizer 2 Pretreatment device 3 Primary pure water production device 4 Secondary pure water production device (subsystem) 5 Reverse osmosis membrane device 6 Degassing membrane device 7 Ultraviolet oxidation device 8 Water supply pump 9 DC power supply 11 Subtank 12 Ultraviolet oxidation device 13 Non-regenerative mixed-bed ion exchange device 14 Ultrafiltration (UF) membrane 15 Use point 21 Level switch (water level measuring means) 22 Flow path for desalinated water (treated water) 23, 26 Control valves 24, 27 Flow meter 25 Flow path for concentrated water 28 Control device 31 Anode (electrode) 32 Cathode (electrode) 33 Anion exchange membrane 34 Cation exchange membrane 35 Concentration chamber 36 Desalinated chamber 37 Anode chamber 38 Cathode chamber W Raw water W0 Water supply for electrodeionizer W1 Pre-treated water (feedwater) W2 Treated water from electrodeionizer (primary pure water) W3 Ultrapure water (secondary pure water) W4 Water to be concentrated W5 Concentrated water

Claims

1. A control method for an electrodeionizer that operates by changing the amount of water supplied to the electrodeionizer, wherein the flow rate of the water supplied to the desalination chamber of the electrodeionizer is changed by 50% / min or less relative to the maximum flow rate of the water supplied to the desalination chamber of the electrodeionizer.

2. The inorganic carbon concentration of the feedwater supplied to the desalination chamber is 10 to 3000 μg / L as CO2 ２ The control method for an electrodeionizer according to claim 1.

3. A control method for an electrodeionizer according to claim 2, wherein the water supply from the desalination chamber of the electrodeionizer or the treated water of the electrodeionizer is passed through the concentration chamber of the electrodeionizer in the opposite direction to the water flow direction of the desalination chamber.

4. A control method for an electrodeionizer according to any one of claims 1 to 3, wherein the minimum water flow rate in the desalination chamber of the electrodeionizer is 30% or more of the maximum water supply flow rate in the desalination chamber of the electrodeionizer.

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