JP2024087529A - Ultrapure water production apparatus and method for operating same - Google Patents

Abstract

To minimize wasteful consumption of water and reduce the cost of producing ultrapure water.
[Solution] The ultrapure water manufacturing system 10 has an ultrafiltration membrane device 15 having an ultrafiltration membrane that separates the treated water into permeate water and concentrated water, a concentrated water line L13 through which the concentrated water from the ultrafiltration membrane device 15 flows, a drainage line L14 that branches off from the concentrated water line L13 and discharges the concentrated water flowing through the concentrated water line L13 to the outside, a return water line L14 that branches off from the concentrated water line L13 and returns the concentrated water flowing through the concentrated water line L13 to the upstream side of the ultrafiltration membrane device 15, and flow path switching means V1, V2 that are provided near the branch point of the concentrated water line L13, the drainage line L14, and the return water line L15 and switch whether the concentrated water line L13 is connected to the drainage line L14 or the return water line L15.
[Selected Figure] Figure 1

Description

The present invention relates to an ultrapure water production apparatus and its operating method.

In the manufacturing process of semiconductor devices and liquid crystal devices, ultrapure water, from which impurities have been highly removed, is used for various purposes such as cleaning processes. Since the fine particles contained in ultrapure water are a direct cause of reduced device yields, their size (particle size) and number (concentration) must be strictly controlled. Ultrapure water is generally produced by sequentially treating raw water (tap water, river water, groundwater, industrial water, etc.) in a pretreatment system, a primary pure water system, and a secondary pure water system (subsystems). In order to reduce the fine particles in the ultrapure water, many subsystems are equipped with an ultrafiltration (UF) membrane device at the most downstream side.

In the UF membrane device installed in the subsystem, a cross-flow method is usually adopted in which the water to be treated is supplied parallel to the membrane surface of the UF membrane, and the part of the water that does not pass through the membrane is discharged as concentrated water. In this case, it is preferable that all of the concentrated water from the UF membrane device is discharged to the outside, considering the risk of fine particles leaking into the permeate water (ultrapure water) supplied to the use point, or of clogging due to the accumulation of fine particles on the membrane surface (see, for example, Patent Document 1).

JP 2020-171892 A

However, constantly discharging concentrated water from the UF membrane device to the outside is not desirable from the perspective of effective water utilization (water conservation). In addition, regardless of whether ultrapure water is used at the point of use, a certain amount of water is always wasted, which has a significant impact on the amount of primary pure water produced by the primary pure water system and is also undesirable in terms of the production costs of ultrapure water.

The object of the present invention is to provide an ultrapure water production apparatus and an operating method thereof that minimizes wasteful consumption of water and reduces the cost of producing ultrapure water.

In order to achieve the above-mentioned object, the ultrapure water production apparatus of the present invention is an ultrapure water production apparatus that produces ultrapure water by sequentially treating water to be treated, and includes an ultrafiltration membrane device having an ultrafiltration membrane that separates the water to be treated into permeate water and concentrated water, a concentrated water line through which the concentrated water from the ultrafiltration membrane device flows, a drainage line that branches off from the concentrated water line and discharges the concentrated water flowing through the concentrated water line to the outside, a reflux water line that branches off from the concentrated water line and returns the concentrated water flowing through the concentrated water line to the upstream side of the ultrafiltration membrane device, and a flow path switching means that is provided at or near the branch point of the concentrated water line, the drainage line, and the reflux water line, and that switches whether the concentrated water line is connected to the drainage line or the reflux water line.

The method of operating the ultrapure water production apparatus of the present invention includes a process for producing ultrapure water by sequentially treating water to be treated, and the process for producing ultrapure water includes a process for supplying the water to be treated to an ultrafiltration membrane device having an ultrafiltration membrane and separating the water into permeate and concentrated water, a process for discharging the separated concentrated water to the outside, and a process for returning the separated concentrated water to the upstream side of the ultrafiltration membrane device instead of discharging the separated concentrated water to the outside.

With this type of ultrapure water production system and its operating method, it is possible to switch between discharging concentrated water from the ultrafiltration membrane device to the outside or returning it to the upstream side of the ultrafiltration membrane device as needed. This makes it possible to reduce the amount of concentrated water discharged from the ultrafiltration membrane device to the outside without increasing the risk of fine particles leaking into the permeate from the ultrafiltration membrane device or clogging due to the accumulation of fine particles on the membrane surface.

As described above, the present invention can minimize wasteful consumption of water and reduce the cost of producing ultrapure water.

1 is a schematic configuration diagram of an ultrapure water production apparatus according to one embodiment of the present invention.

The following describes an embodiment of the present invention with reference to the drawings.

Figure 1 is a schematic diagram showing the configuration of an ultrapure water production apparatus according to one embodiment of the present invention. Note that the configuration of the ultrapure water production apparatus shown in the figure is merely an example and does not limit the present invention.

The ultrapure water production system 10 has a primary pure water tank 11, a pump 12, an ultraviolet oxidation device 13, an ion exchange device 14, and an ultrafiltration (UF) membrane device 15. These are connected in series to a circulation line L1 to form a secondary pure water system (subsystem), which sequentially processes the primary pure water produced in the primary pure water system (not shown) to produce ultrapure water, and supplies the ultrapure water to the point of use 1.

The water to be treated (primary pure water) stored in the primary pure water tank 11 is pumped out by the pump 12 and supplied to the ultraviolet oxidation device 13 where it is irradiated with ultraviolet light, whereby the total organic carbon (TOC) in the water to be treated is decomposed. The water to be treated is then subjected to ion exchange processing in the ion exchange device 14 to remove metals and the like, and fine particles are removed in the UF membrane device 15. The ultrapure water thus obtained is returned to the primary pure water tank 11 through the circulation line L1, but a portion of it is supplied to the use point 1 through the water supply line L2 branched off from the circulation line L1 in response to a water collection request from the use point 1. A primary pure water supply line L3 is connected to the primary pure water tank 11, and primary pure water is supplied from the primary pure water system as necessary.

The primary pure water tank 11, pump 12, ultraviolet oxidation device 13, and ion exchange device 14 may be those commonly used in subsystems of ultrapure water production equipment. For example, the ion exchange device 14 may be a non-regenerative mixed-bed ion exchange device (cartridge polisher) filled with a mixed bed of cation exchange resin and anion exchange resin. A detailed description of the configuration and functions of these devices will be omitted, and the following description will focus on the configuration related to the UF membrane device 15.

The UF membrane device 15 is composed of a cross-flow type UF membrane module, and has a UF membrane that separates the water to be treated (treated water from the ion exchange device 14) supplied parallel to the membrane surface into concentrated water containing impurities and permeate water from which the impurities have been removed. The UF membrane device 15 is connected to a supply line L11 that supplies the water to be treated to the UF membrane device 15, an ultrapure water line L12 that circulates the permeate water (ultrapure water) from the UF membrane device 15, and a concentrated water line L13 that circulates the concentrated water from the UF membrane device 15. The upstream side of the supply line L11 and the downstream side of the ultrapure water line L12 are each connected to the primary pure water tank 11, and the supply line L11 and the ultrapure water line L12 form the circulation line L1. The concentrated water line L13 branches into a drainage line L14 that discharges a portion of the concentrated water to the outside, and a return water line L15 that returns the remainder to the primary pure water tank 11. In addition, the return water line L15 may be connected to the supply line L11 upstream of the pump 12 instead of being connected to the primary pure water tank 11.

The ultrapure water production system 10 also includes a flowmeter 16, a constant flow valve 17, on-off valves V1 and V2, and a control unit (not shown) as components for controlling the membrane separation process performed by the UF membrane device 15.

The flowmeter 16 is provided in the supply line L11 between the pump 12 and the ultraviolet oxidation device 13, and has the function of detecting the flow rate of the water to be treated (primary pure water) flowing through the supply line L11. The constant flow valve 17 is provided in the concentrated water line L13, and functions as a flow rate maintaining means for maintaining the flow rate of the concentrated water flowing through the concentrated water line L13 at a constant value. The on-off valves V1 and V2 are provided in the drain line L14 and the reflux water line L15, respectively, and function as a flow path switching means for switching whether the concentrated water line L13 is connected to the drain line L14 or the reflux water line L15. Note that instead of the on-off valves V1 and V2, a three-way valve may be provided at the branch point between the concentrated water line L13, the drain line L14, and the reflux water line L15.

The control unit (control means) controls the inverter of the pump 12 to perform flow control to adjust the flow rate of the ultrapure water flowing through the ultrapure water line L12 to a set flow rate. Specifically, the control unit controls the rotation speed of the pump 12 so that the flow rate of the primary pure water detected by the flow meter 16 is constant (predetermined set flow rate). For example, when the water temperature decreases, the viscosity of the water increases, and as a result, the flow rate of the permeate separated by the UF membrane device 15 decreases. The control unit increases the rotation speed of the pump 12 to compensate for this decrease, thereby increasing the supply pressure of the water to be treated to the UF membrane device 15. Also, when the water temperature increases, the viscosity of the water decreases, and as a result, the flow rate of the permeate separated by the UF membrane device 15 increases, but the control unit reduces the rotation speed of the pump 12 to counteract this increase, thereby reducing the supply pressure of the water to be treated to the UF membrane device 15. In this way, the rotation speed of the pump 12 is adjusted, and the supply pressure of the water to be treated to the UF membrane device 15 is adjusted, thereby maintaining a constant flow rate of the ultrapure water flowing through the ultrapure water line L2. Note that the flow meter 16 may be provided in the ultrapure water line L12, but considering the possibility that elution from the flow meter 16 may affect the quality of the ultrapure water, it is preferable to provide it in the supply line L11 as shown in the figure.

In the above-mentioned flow control, the flow rate of the concentrated water separated by the UF membrane device 15 also changes according to the change in the supply pressure of the water to be treated to the UF membrane device 15, but since the constant flow valve 17 is provided in the concentrated water line L13, the flow rate of the concentrated water flowing through the concentrated water line L13 can be kept constant. As a result, as described later, by specifying the time period during which the concentrated water from the UF membrane device 15 is discharged to the outside, the amount of wastewater discharged per day can be easily grasped. Here, the specified flow rate of the constant flow valve 17 may be, on the one hand, sufficient to prevent clogging of the membrane due to fouling or scaling, and, on the other hand, sufficient to prevent damage to the membrane due to an increase in pressure loss. However, making the specified flow rate of the constant flow valve 17 larger than necessary is not preferable in terms of energy consumption, since the flow rate required for the pump 12 becomes larger than necessary, resulting in a larger size of the pump 12. Therefore, the specified flow rate of the constant flow valve 17 is set taking into consideration the permeation flux of the UF membrane of the UF membrane device 15 and the minimum flow rate of the concentrated water required for the UF membrane device 15. The minimum flow rate of concentrated water required for the UF membrane device 15 means the minimum flow rate of concentrated water that must be passed through the concentrated water line L13 to prevent clogging of the membrane due to fouling or scaling.

In addition to the flow rate control described above, the control unit also performs drainage volume control to adjust the drainage volume of concentrated water discharged from the UF membrane device 15 to the outside per day to a target drainage volume. Specifically, the control unit has a timer function that opens and closes the on-off valves V1 and V2 according to a predetermined schedule and switches whether the concentrated water line L13 is connected to the drainage line L14 or the reflux water line L15. More specifically, during a specified time period, the on-off valve V1 of the drainage line L14 is opened and the on-off valve V2 of the reflux water line L15 is closed, thereby connecting the concentrated water line L13 and the drainage line L15. As a result, the concentrated water from the UF membrane device 15 is discharged to the outside from the concentrated water line L13 through the drainage line L14. On the other hand, during a time period other than the specified time period, the on-off valve V1 of the drainage line L14 is closed and the on-off valve V2 of the reflux water line L15 is opened, thereby connecting the concentrated water line L13 and the reflux water line L15. As a result, the concentrated water from the UF membrane device 15 is returned to the primary pure water tank 11 via the concentrated water line L13 and the return water line L15.

Here, the length of the specified time period (concentrated water drainage time) is determined based on the target amount of concentrated water drained per day, which is determined as described below, and the flow rate of concentrated water flowing through concentrated water line L13 (set flow rate by constant flow valve 17). For example, if the set flow rate of concentrated water flowing through concentrated water line L13 is 120 L/h, and the target amount of concentrated water drained per day is set to 480 L, the concentrated water drainage time is set to 4 hours. In this way, by specifying the concentrated water drainage time, the amount of concentrated water drained per day can be adjusted to the target drainage amount.

By controlling the discharge amount in this manner, the amount of concentrated water discharged per day can be reduced compared to when concentrated water from the UF membrane device 15 is constantly discharged to the outside, suppressing wasteful consumption of water and realizing water conservation. In addition, by reducing the amount of concentrated water discharged to the outside from the UF membrane device 15, the supply amount of primary pure water supplied to the primary pure water tank 11 through the primary pure water supply line L3 can also be reduced in a similar manner, i.e., the amount of primary pure water produced by the primary pure water system can also be reduced. As a result, a reduction in the production cost of ultrapure water can be achieved for the entire system.

If we only consider the effective use of water (water saving), it is possible to always return the concentrated water from the UF membrane device 15 to the primary pure water tank 11, but in that case, there is a risk of increasing the risk of leakage of fine particles into the permeate water and clogging due to the accumulation of fine particles on the membrane surface. In contrast, in this embodiment, as described above, the concentrated water from the UF membrane device 15 is switched between being discharged to the outside and being returned to the primary pure water tank 11 as necessary. Therefore, it is possible to increase the amount of concentrated water returned to the primary pure water tank 11 to an extent that does not increase the above-mentioned risk, and it is also possible to reduce the amount of concentrated water discharged per day. Therefore, it is preferable that the amount of concentrated water discharged per day (target amount of discharged water) is determined taking into consideration the above-mentioned risks, and specifically, it is preferable that it is determined based on the maximum amount of ultrapure water used per day at the use point 1, for example, 0.01 to 10% of the maximum amount of use. In addition, in the ultrapure water production system 10, a constant amount of the ultrapure water produced is always circulated through the circulation line L1, so the maximum amount of ultrapure water used per day is calculated based on the difference between the set flow rate in the flow rate control of the permeate water and the predetermined circulation flow rate of the ultrapure water.

The time period during which the concentrated water from the UF membrane device 15 is discharged to the outside is not particularly limited, but is preferably a time period during which there is no demand for ultrapure water at the point of use 1, i.e., a time period during which the permeate water from the UF membrane device 15 is not supplied to the point of use 1 as ultrapure water and is entirely returned to the primary pure water tank 11, such as a holiday or nighttime time period. In other words, during a time period during which there is a demand for ultrapure water at the point of use 1, it is preferable to return the concentrated water from the UF membrane device 15 to the primary pure water tank 11 without discharging it to the outside. This makes it possible to reduce the amount of primary pure water supplied to the primary pure water tank 11 compared to when the concentrated water from the UF membrane device 15 is discharged to the outside during such a time period, i.e., to reduce the amount of primary pure water produced by the primary pure water system.

In this embodiment, the concentrated water line L13 is provided with a constant flow valve 17, but other flow rate maintaining means, such as a combination of a flow meter and a flow rate adjustment valve, may be used as long as the flow rate of the concentrated water flowing through the concentrated water line L13 can be kept constant. However, for example, during times when there is no demand for ultrapure water at the use point 1, the circulating flow rate of ultrapure water along the circulation line L1 (the set flow rate in the flow rate control of the permeate water) may be changed, and the rotation speed of the pump 12 may be changed accordingly. Even in such cases, it is preferable to use the constant flow valve 17, since it is easier to ensure the minimum flow rate of concentrated water required for the UF membrane device 15.

1 Point of use 10 Ultrapure water production device 11 Primary pure water tank 12 Pump 13 Ultraviolet oxidation device 14 Ion exchange device 15 Ultrafiltration membrane device 16 Flow meter 17 Constant flow valve L1 Circulation line L2 Water supply line L3 Primary pure water supply line L11 Supply line L12 Ultrapure water line L13 Concentrated water line L14 Drain line L15 Circulation water line V1, V2 Opening and closing valve

Claims (8)

An ultrapure water producing apparatus for producing ultrapure water by sequentially treating water to be treated,
An ultrafiltration membrane device having an ultrafiltration membrane that separates the water to be treated into a permeate and a concentrated water;
A concentrated water line for circulating concentrated water from the ultrafiltration membrane device;
A drain line that branches off from the concentrated water line and discharges the concentrated water flowing through the concentrated water line to the outside;
A reflux water line that branches off from the concentrated water line and refluxes the concentrated water flowing through the concentrated water line to the upstream side of the ultrafiltration membrane device;
an ultrapure water producing apparatus having a flow path switching means provided at or near the branch point of the concentrated water line, the drain line, and the reflux water line, for switching whether the concentrated water line is connected to the drain line or the reflux water line. The ultrapure water production apparatus according to claim 1, further comprising a control means for controlling the flow path switching means and switching whether the concentrated water line is connected to the drain line or the reflux water line according to a predetermined schedule. The ultrapure water production apparatus according to claim 2, wherein the control means connects the concentrated water line to the drain line during a predetermined time period of a day, and connects the concentrated water line to the reflux water line during a time period other than the predetermined time period. A flow rate maintaining means for maintaining a constant flow rate of the concentrated water flowing through the concentrated water line,
4. The ultrapure water producing apparatus according to claim 3, wherein the length of the specified time period is determined based on a target drainage amount per day of concentrated water discharged from the concentrated water line through the drainage line to the outside and the flow rate of the concentrated water maintained constant. The control means performs flow rate control to adjust the flow rate of the permeate from the ultrafiltration membrane device to a set flow rate,
5. The ultrapure water producing apparatus according to claim 4, wherein the target discharge amount is determined based on the set flow rate of permeated water from the ultrafiltration membrane device. The ultrapure water production apparatus according to any one of claims 3 to 5, wherein the specified time period is a time period during which the permeate from the ultrafiltration membrane device is not supplied to the point of use as the ultrapure water, and all of the permeate is returned to the upstream side of the ultrafiltration membrane device. a subsystem for treating primary pure water as the water to be treated to produce the ultrapure water,
6. The ultrapure water producing system according to claim 1, wherein the ultrafiltration membrane device is provided at the most downstream side of the subsystem. A method for operating an ultrapure water production apparatus, comprising the steps of sequentially treating water to be treated to produce ultrapure water,
The step of producing ultrapure water comprises:
A step of supplying the water to be treated to an ultrafiltration membrane device having an ultrafiltration membrane and separating the water into permeate and concentrated water;
Discharging the separated concentrated water to the outside;
and returning the separated concentrated water to the upstream side of the ultrafiltration membrane device instead of discharging it to the outside.

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