JP2013052349A - Water making method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a water making method, which assures a certain quantity of water to be treated without depending on the temperature and salt concentration of raw water in a desalinizing apparatus for salt water which obtains freshwater from salt water such as sea water and brine using a reverse osmosis membrane module.SOLUTION: The water making method is carried out by using the desalinizing apparatus for salt water which comprises a supply pump 1 pressurizing salt water, a first stage reverse osmosis membrane module 2 separating fresh water and concentrated water from the pressurized water by a reverse osmosis membrane, a second stage reverse osmosis membrane module 3 separating fresh water, which is treated water, and concentrated water by a reverse osmosis membrane from the fresh water obtained by the first stage reverse osmosis membrane module 2, and a bypass line to take out the fresh water obtained by the first stage reverse osmosis membrane module 2 bypassing the second stage reverse osmosis membrane module 3, as treated water directly. In the water making method, the appropriateness decision about the use of the bypass line is carried out depending on the temperature of the salt water and the salt concentration.

Description

  The present invention relates to a fresh water generation method using a salt water desalination apparatus that obtains fresh water from salt water using a reverse osmosis membrane module, and more specifically, an optimum operation method is determined according to the temperature and salt concentration of salt water. The present invention relates to a fresh water generation method using a salt water desalination apparatus.

  Seawater desalination and brine desalination using the reverse osmosis membrane method can separate and remove salinity and harmful substances without phase change, and are easy to manage and energy efficient. It is used in the field. To prevent reverse osmosis membrane permeability and separability from decreasing, sand filtration, coagulation sedimentation, pressurized flotation, microfiltration membrane and ultrafiltration membrane filtration are usually performed before supplying seawater or brine to the reverse osmosis membrane. Pre-processing is performed using a method such as

  Based on the permeation principle of reverse osmosis membranes, it is necessary to make the supply water pressure higher than the osmotic pressure using a high-pressure pump or the like so that the supply water containing a certain amount of salt, such as seawater or brine, can permeate the reverse osmosis membrane. . The osmotic pressure is related to the salinity concentration. For example, when seawater is separated by a reverse osmosis membrane, a pressure of at least about 3 MPa or more and at least about 5 MPa is required in consideration of practicality. Even in the case of irrigation, a pressure of at least about 1 MPa is required.

  Also, if the required treated water quality cannot be obtained with only the first-stage reverse osmosis membrane module because of the quality of the treated water, the treated water from the first-stage reverse osmosis membrane module is supplied to the second-stage reverse osmosis membrane module. In some cases, two-stage processing is performed. By performing the two-stage treatment, it is possible to obtain a good treated water quality that cannot be obtained only by the one-stage reverse osmosis membrane module. On the other hand, as a demerit in performing the two-stage process, there is a decrease in the total recovery rate. The recovery rate indicates the ratio of treated water amount / feed water amount, and the closer to 1, the greater the amount of treated water obtained from the same amount of feed water. The total recovery rate indicates a ratio of the second-stage treated water amount / first-stage supplied water amount in a salt water desalination apparatus equipped with a two-stage reverse osmosis membrane module. When processing seawater with a general TDS (total dissolved salt concentration) of 35000 mg / L, the recovery rate of the first-stage reverse osmosis membrane module is about 30% to 60%, and the recovery of the second-stage reverse osmosis membrane module The rate is often about 70% to 90%. In other words, the treated water could be produced at a recovery rate of about 30% to 60% in the first stage treatment, but the total recovery rate is reduced to 21% to 54% by using the second stage treatment.

  If the supply water is a water whose salt concentration is clear in advance, the necessary quality and quantity of treated water can be obtained by designing the equipment according to the supply water salt concentration. In equipment that must use various raw water as feed water, the supply salt concentration is not clear from the time of design, and low salt concentration water that can be treated in one stage is treated in two stages. If this happens, the amount of treated water will be reduced.

  Various proposals have been made to solve this problem. Patent Document 1 proposes to provide a line that bypasses the second-stage reverse osmosis membrane module so that two-stage treatment is performed when the raw water is seawater and one-stage treatment is possible when brine is used. In Patent Document 2, when raw water is seawater, two-stage treatment is performed, and when brine is supplied, the first-stage feed water is directly supplied to the second-stage reverse osmosis membrane module, that is, the second-stage reverse osmosis. It has been proposed to use the membrane module as a first-stage reverse osmosis membrane module. In either case, the effect of increasing the amount of treated water can be obtained when the supplied water is brine. However, when the raw water is seawater, a two-stage treatment is assumed, and the total recovery rate must be low.

JP 2006-263542 A Japanese Patent No. 3957080

  On the other hand, in the two-stage treatment by the reverse osmosis membrane module, it has been found that there is a problem of a decrease in the amount of treated water due to an increase in the viscosity of water at low water temperature. Even with seawater with the same salt concentration, the water permeability of the reverse osmosis membrane decreased due to an increase in viscosity when the water temperature decreased, and supply water had to be supplied at a higher pressure in order to obtain the same amount of treated water. For example, without installing a pump on the supply line of the second-stage reverse osmosis membrane module, the treated water line of the first-stage reverse osmosis membrane module is directly connected to the supply line of the second-stage reverse osmosis membrane module, and the first-stage reverse osmosis membrane module When trying to supply continuously to the second-stage reverse osmosis membrane module by the pump of the membrane module supply line, in the case of seawater with a general salt concentration of 35000mg / L, if the water temperature falls below about 15 ° C, the general reverse osmosis It has been found that a pressure exceeding the design pressure of the membrane, 8.4 MPa, is required. In that case, since it is not normally allowed to operate at a pressure exceeding the design pressure, measures such as lowering the total recovery rate must be taken to operate with a reduced amount of treated water. That is, there is a problem that the rated water volume cannot be obtained at a low water temperature.

  An object of the present invention is to provide a fresh water generation method that ensures a constant amount of treated water regardless of the temperature and salt concentration of raw water in a salt water desalination apparatus that obtains fresh water from salt water such as seawater and brine using a reverse osmosis membrane module. It is to be.

The present invention for solving the above-described problems has the following characteristics.
(1) Supply pump for boosting salt water, reverse osmosis of fresh water obtained by the first-stage reverse osmosis membrane module and the first-stage reverse osmosis membrane module for separating the pressurized salt water into fresh water and concentrated water by a reverse osmosis membrane The second-stage reverse osmosis membrane module that separates the treated water into fresh water and concentrated water, and the fresh water obtained from the first-stage reverse osmosis membrane module by bypassing the second-stage reverse osmosis membrane module is treated as it is. A fresh water generation method using a salt water desalination apparatus provided with a bypass line for taking out as a method for determining whether or not to use a bypass line according to the temperature and salt concentration of salt water.
(2) The bypass line is used when the inequality C <15T ² -2000T + 72800 holds when the salt water temperature is T [° C.] and the salt concentration is C [mg / L]. Fresh water generation method.

  According to the present invention, when seawater with relatively high temperature and low salt concentration is raw water, two-stage treatment is performed, and the required amount of water and water quality can be ensured. When seawater with relatively low temperature and high salt concentration is raw water, By bypassing without using the second-stage reverse osmosis membrane module, both the required amount of water and water quality can be ensured.

It is a flowchart which shows the salt water desalination apparatus which made it possible to measure the temperature and salt concentration of the salt water which concern on the water preparation method of this invention. It is a flowchart which shows the conventional salt water desalination apparatus which has a 1st step | paragraph reverse osmosis membrane module and a 2nd step | paragraph reverse osmosis membrane module.

  In order to describe an embodiment of the present invention, first, a conventional salt water desalination apparatus having a first-stage reverse osmosis membrane module and a second-stage reverse osmosis membrane module will be described as a comparative example.

  As shown in FIG. 2, the conventional salt water desalination apparatus mainly includes a feed pump 1, a first-stage reverse osmosis membrane module 2 composed of a reverse osmosis membrane (RO membrane), and a second-stage reverse osmosis membrane composed of a reverse osmosis membrane. Osmosis membrane module 3, second stage reverse osmosis membrane module supply valve 4, second stage reverse osmosis membrane module bypass valve 5, second stage for switching water flow to bypass line and second stage reverse osmosis membrane module 3 It consists of a check valve 6 that prevents backflow to the reverse osmosis membrane module.

  The flow of salt water desalination of a conventional salt water desalination apparatus is typically as described below. The salt water introduced from the pretreatment device is pressurized by the supply pump 1 and supplied to the first-stage reverse osmosis membrane module 2. In the osmosis membrane module 2, fresh water and concentrated water are separated by the reverse osmosis membrane method, and the fresh water is supplied to the second-stage reverse osmosis membrane module 3. In the second-stage reverse osmosis membrane module 3, it is separated into fresh water and concentrated water by the reverse osmosis membrane method, and the fresh water is taken out as treated water. The concentrated water discharged from the first-stage reverse osmosis membrane module 2 and the concentrated water discharged from the second-stage reverse osmosis membrane module 3 are both discharged out of the system as concentrated water.

  A water production method using a conventional salt water desalination apparatus is typically as follows. When the salt water introduced into the pretreatment device is sea water, the second stage reverse osmosis membrane module 3 is used to obtain treated water. That is, the second-stage reverse osmosis membrane module supply valve 4 is opened, the second-stage reverse osmosis membrane module bypass valve 5 is closed, and the total amount of fresh water obtained by the first-stage reverse osmosis membrane module 2 is Supply to the reverse osmosis membrane module 3 to obtain treated water. When the brine supplied for pretreatment is brine, treated water is obtained by bypassing the second-stage reverse osmosis membrane module 3. That is, the second-stage reverse osmosis membrane module supply valve 4 is closed, the second-stage reverse osmosis membrane module bypass valve 5 is opened, and the fresh water obtained from the first-stage reverse osmosis membrane module 2 is treated directly as treated water. To discharge.

  In addition, it is preferable that the salt water introduce | transduced from the pre-processing equipment is normally pre-processed before processing with the reverse osmosis membrane module 2, and can also be preferably employ | adopted also in the salt water desalination apparatus of this invention. The position where the pretreatment facility is introduced is usually upstream of the supply pump 1, and the pretreatment facility is also introduced upstream of the supply pump in FIG. Here, precision membrane filtration or ultramembrane filtration, activated carbon filtration, a safety filter, etc. are used as pretreatment equipment. Moreover, chemical | medical solution additions, such as a disinfectant, a flocculant, a reducing agent, pH adjuster, a scale inhibitor, can be performed as needed.

  Here, the supply pump 1 has various types. However, in the present invention, the type is not particularly limited as long as the desired pressure and flow rate can be obtained. For example, a piston type such as a plunger pump is used. These pumps, centrifugal pumps, centrifugal pumps, multistage centrifugal pumps, and the like can be appropriately used according to the purpose.

  The salt water referred to in the present invention is a general term for salt-containing water, and a relatively low concentration salt water generally referred to as brine having a chloride ion concentration of about 300 to 15,000 mg / l, or a chloride ion concentration. This refers to relatively high-concentration salt water, generally referred to as seawater, of about 15,000 to 40,000 mg / l, but there is no clear distinction between seawater and brine, and relatively low-concentration seawater may be treated as brine. . In addition, seawater that is close to 0 ° C, such as the Arctic Circle, and seawater that exceeds 40 ° C in the Middle East are also subject to saltwater. If the installation location is a salt water desalination system, the annual minimum and maximum water temperatures of the water area will be investigated, and the head of the high-pressure pump will be adjusted so that the required amount of treated water can be obtained in the minimum and maximum temperature ranges. Although the number of reverse osmosis membrane modules can be selected, it is desirable that a necessary amount of treated water be obtained in a temperature range as wide as possible in a saltwater desalination apparatus for disaster countermeasures.

  That is, it is desirable to change the operation method according to the salt concentration and temperature of salt water, instead of changing the operation method depending on the type of raw water being seawater or brine. That is, a threshold value is set for each of the salt concentration and the temperature, and when the salt water exceeds the threshold value, the second-stage reverse osmosis membrane module is used. An operation method such as bypassing the membrane module can be employed. As a means for measuring the salt concentration of salt water in that case, the indication value of the electric conductivity meter incorporated in the apparatus may be converted into the salt concentration, or a threshold value is provided for the indication value itself of the electric conductivity meter. Also good. Further, sampling of salt water may be performed to perform TDS concentration analysis, or a threshold value may be set by performing partial or plural ion concentration analysis in salt water. The temperature measurement method is not limited as long as the water temperature can be measured, and a thermocouple, a resistance thermometer, a radiation thermometer, a liquid column thermometer, a bimetal thermometer, and the like can be used.

  Here, the reverse osmosis membrane used in the first-stage reverse osmosis membrane module 2 and the second-stage reverse osmosis membrane module 3 according to the present invention refers to other components that permeate some components of the supply liquid, for example, salt. It is a semipermeable membrane that does not allow permeation. The material can be a polymer material such as cellulose acetate polymer, polyamide, polyester, polyimide, vinyl polymer. Examples of membrane forms include hollow fiber membranes and flat membranes. In this invention, it can utilize regardless of the raw material and membrane form of a reverse osmosis membrane.

  The reverse osmosis membrane element is a form for practical use of the above reverse osmosis membrane. It is incorporated into a flat membrane, spiral, tubular, plate and frame element, and the hollow fiber membrane is bundled. However, in the present invention, it does not depend on the form of these reverse osmosis membrane elements.

  A reverse osmosis membrane module is a module in which one to several of the above-mentioned reverse osmosis membrane elements are placed in parallel in a pressure vessel, and the combination, number, and arrangement are arbitrarily determined according to the purpose. Can do.

  In this invention, like embodiment (Example) shown in FIG. 1, the flow of a salt water desalination apparatus is the same as a comparative example (FIG. 2). However, as shown in FIG. 1, it is desirable to have a water temperature meter 7 as a means for measuring the water temperature and a salt concentration meter 8 as a means for measuring the salt concentration on the upstream side of the supply pump 1. The water temperature meter and / or the salt concentration meter are not necessarily incorporated in the apparatus, and the salt water sampling may be performed to measure the water temperature and / or the salt concentration. The water temperature meter is not limited as long as it can measure the water temperature, and a thermocouple, a resistance thermometer, a radiation thermometer, a liquid column thermometer, a bimetal thermometer, or the like can be used. The salt concentration meter is not limited as long as it can measure the salt concentration, or can measure a physical quantity that can be converted into the salt concentration, or can measure a physical quantity that can be used as an alternative to the salt concentration. The indicated value may be converted into a salinity concentration or a TDS concentration analysis may be performed. Alternatively, partial or multiple ion concentration analysis in salt water may be performed and the concentration may be used.

  The fresh water generation method of the present invention is characterized in that it is determined whether to use or bypass the second-stage reverse osmosis membrane module 3 according to the temperature and salt concentration of salt water. By providing this feature, the fresh water generation method of the present invention makes it possible to increase the amount of treated water at a low temperature, as compared to an operation method that bypasses only by salt concentration.

  Specific examples of the present invention are shown below. In this example, the target salt concentration of the treated water is set to 200 mg / L or less in terms of TDS concentration. At this time, when only the first-stage reverse osmosis membrane module 2 is used and the operation is performed with the second-stage reverse osmosis membrane module 3 being bypassed, the treated water salt concentration (TDS) is 200 mg / L. The correlation between water temperature and salt concentration was as follows.

  From Table 1, for example, when the salt water salt concentration is 25000 mg / L, if the water temperature is 30.7 ° C. or less, even if the operation bypassing the second-stage reverse osmosis membrane module 3 is performed, the treated water salt concentration is 200 mg / L or less. Treated water can be obtained. Conversely, if the water temperature exceeds 30.7 ° C, treated water having a treated water salt concentration of 200 mg / L or less cannot be obtained unless the second stage reverse osmosis membrane module 3 is operated. I understand that.

  Here, when the temperature of the salt water is T [° C.] and the salt concentration of the salt water is C [mg / L], an approximate curve showing the relational expression of T and C is obtained from the results of Table 1, and the following formula 1 is obtained. .

C = 15T ² −2000T + 72800 (Formula 1)
That is, when the following formula 2 is satisfied, the treated water salt concentration of 200 mg / L can be satisfied even if the operation is performed by bypassing the second-stage reverse osmosis membrane module 3, and when the formula 2 is not satisfied, the second-stage reverse osmosis membrane module 3 is bypassed. It can be seen that the use of the reverse osmosis membrane module 3 can satisfy the treated water salt concentration of 200 mg / L. Thus, in the fresh water generation method of the present invention, it is preferable to use Formula 2 to determine whether to use or bypass the second-stage reverse osmosis membrane module 3.

C <15T ² −2000T + 72800 (Formula 2)
(However, the temperature of salt water is T [° C.] and the salt concentration of salt water is C [mg / L].)
By judging the operation method using Equation 2, it can be seen that even if it is seawater, if the seawater temperature is high enough to satisfy Equation 2, it will be possible to obtain treated water that satisfies treated water quality only by one-stage treatment. The total recovery rate can be increased, and a reduction in the amount of treated water at low temperatures can be prevented.

  In the examples, it is assumed that the treated water salt concentration is 200 mg / L or less, but this threshold value can be freely changed and applied when applied to other plants. Moreover, since Formula 2 can also take various formulas with various plants, it is possible to derive Formula 2 according to the applied plant by performing simulation.

  Thus, based on the fresh water generation method of the present invention, by determining whether to use or bypass the second-stage reverse osmosis membrane module 3 according to the temperature and salt concentration of salt water, the temperature and salt concentration of raw water are determined. Regardless of this, a certain amount of treated water can be secured.

1: Supply pump 2: 1st stage reverse osmosis membrane module 3: 2nd stage reverse osmosis membrane module 4: 2nd stage reverse osmosis membrane supply valve 5: 2nd stage reverse osmosis membrane bypass valve 6: 2nd stage reverse osmosis membrane Module fresh water outlet check valve 7: Water temperature gauge 8: Salt concentration meter

Claims (2)

Supply pump that boosts salt water, first-stage reverse osmosis membrane module that separates pressurized salt water into fresh water and concentrated water by reverse osmosis membrane, and treats fresh water obtained by first-stage reverse osmosis membrane module by reverse osmosis membrane To bypass the second-stage reverse osmosis membrane module that separates the fresh water and concentrated water, and the second-stage reverse osmosis membrane module, and extract fresh water obtained from the first-stage reverse osmosis membrane module as treated water as it is. A fresh water generation method using a salt water desalination apparatus having a bypass line, wherein the use of the bypass line is determined according to the temperature and salt concentration of the salt water. The fresh water generation method according to claim 1, wherein a bypass line is used when the inequality C <15T ² -2000T + 72800 holds when the temperature of the salt water is T [° C.] and the salt concentration of the salt water is C [mg / L]. .

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