KR20200085385A - Method of desalinating seawater adopting photocatalyst and system for the same - Google Patents

Abstract

Provided is a seawater desalination method that can replace a conventional desalination post-treatment process to meet the water quality standards required by the industry in the seawater desalination process. In the seawater desalination method of the present invention, the treated water passes through a photocatalytic device after a main desalination process, and thus the concentration of chlorine ions in final product water can be reduced at low cost.

Description

Method of desalinating seawater adopting photocatalyst and system for the same}

The present invention relates to a seawater desalination method and system using a photocatalyst. More specifically, the present invention relates to a system employing a method and a device capable of meeting the water quality conditions of chlorine ions that are relatively high in seawater desalination processes.

Current seawater desalination methods include a pre-treatment process to remove impurities from the seawater that has been taken, then heat the seawater using a heat source and condense the steam generated to obtain fresh water, and reverse osmosis by applying high pressure to seawater. There are reverse osmosis (RO), which produces fresh water by passing seawater through a semi-permeable membrane, and multistage evaporation, which is similar to a multistage evaporation method, but suitable for medium-sized facilities.

Of these, the reverse osmosis method is the most widely used. In the case of treated water that has passed through the reverse osmosis membrane according to the reverse osmosis method, additional treatment may be required to meet the water quality standards.

That is, industrial water, industrial pure water facilities, and seawater desalination plant facilities require processes such as ion exchange resin, electric desalination, and capacitive desalination after the reverse osmosis process to meet the water quality standards of the final production water.

Since the ion exchange resin (IX) process is a non-continuous process, it is inevitable to stop the ultrapure water production process during the period of regeneration or replacement of the ion exchange resin. do. In addition, there is a disadvantage of low efficiency at low concentrations, and when the inflow amount is unstable, the life of the recycled resin may be shortened and maintenance cost may increase.

EDI (electro desalination) requires strict inflow water quality compared to the process using ion exchange resin, and when the water quality (CaCO ₃ ) of raw water is 1ppm or more, there is a problem in that the removal efficiency drops rapidly. In addition, if the pressure in the production compartment is not maintained higher than the pressure of the concentration compartment by more than 5 to 10 psig due to the unstable flow rate, ions transferred to the concentrate can move back to the production compartment, making it difficult to maintain a constant flow rate. have.

CDI (Capacitor Desalination) is a kind of electrosorption technology that removes ionic substances by using adsorption reactions in electrode double insects formed at the electrode interface when a potential is applied to the electrode. In this method, when an ionic substance is saturated adsorbed on the electrode surface, ion adsorption is no longer possible. Electrodes having a large surface area capable of adsorbing ions and having low electrical resistance have been studied, but they are difficult to apply in actual fields due to cost problems. In addition, although the energy cost is theoretically low, the process of adsorbing ions to the electrode is an energy storage step and the process of desorption is an energy release step, so energy must be used. In fact, energy costs are incurring more than necessary.

The present invention provides a seawater desalination method that can satisfy the water quality conditions of relatively high chlorine ions that may be required in the water treatment process.

Another object of the present invention is to provide a seawater desalination system that adopts a device capable of meeting the water quality conditions of relatively high chlorine ions that may be required in a water treatment process.

1. The first embodiment of the present invention

i) a pretreatment process for removing suspended matter from the seawater, ii) a first desalination process of the treated water after the pretreatment process, and iii) a second desalination process of the treated water after the first desalination process, and the second desalination process The process can present a method of desalination in seawater, characterized in that it is performed using a photocatalytic reaction.

2. In the first embodiment, the photocatalytic reaction may be made by irradiating TiO ₂ with UV.

3. In the first to second embodiments, the pretreatment process may be performed using precipitation, media filtration, pressurized flotation (DAF), or ultrafiltration.

4. In the first to third embodiments, the first desalination process may be performed using a reverse osmosis method or a multi-step evaporation method.

5. In the first to fourth embodiments, the second desalination process may be performed using a reverse osmosis method for brackish water (BWRO), capacitive desalination (CDI), electric desalination (EDI) or ion exchange resin (IX). .

6. In order to implement the first to fifth embodiments, a seawater desalination system including a photocatalytic reaction device for reducing the concentration of chlorine ions can be proposed.

7. In order to implement the first to sixth embodiments, a pretreatment process apparatus for removing suspended matter from seawater, ii) a first desalination process apparatus and iii) a second desalination process apparatus, and the second desalination process apparatus Can present a seawater desalination system characterized in that it comprises a photocatalytic reaction device.

8. In order to implement the first to seventh embodiments, the photocatalytic reaction device may include a TiO ₂ and UV irradiation device, which may present a seawater desalination system.

9. In order to implement the first to eighth embodiments, the photocatalytic reaction device may propose a seawater desalination system that is installed in a bed type or a reactor type.

Seawater desalination method according to the present invention, by applying a photocatalyst and a method for reducing chlorine ions using UV and a device therefor, after the desalination process of the seawater desalination process, compared to the existing ion exchange resin process or a process such as CDI It is very useful because it can dramatically reduce operating costs. The method of the present invention uses a photocatalyst that can be repeatedly used, and does not incur an additional cost for treatment of the cleaning waste liquid generated after the use of chemicals involved in the existing process. In addition, the oxidizing material generated from the photocatalytic reaction can prevent and disinfect the final production water.

1A to 1C are graphs showing changes in the concentration of Cl ^- ions according to reaction time when treated water is injected into a UV-TiO ₂ reaction apparatus according to an embodiment of the present invention.
2 is a schematic diagram of a seawater desalination system employing a Cl ^- ion concentration reduction device according to an embodiment of the present invention.
3A to 3C are UV-TiO ₂ photocatalyst devices for performing Cl ⁻ ion concentration reduction in treated water according to an exemplary embodiment of the present invention.

Hereinafter, the present invention will be described in detail.

The following phrases and terms are used to help understand the description, and should not be interpreted limitedly.

The seawater desalination method of the present invention comprises: i) a pretreatment process for removing suspended matter from seawater, ii) a first desalination process of treated water after the pretreatment process, and iii) a second desalination process of treated water after the first desalination process. Included, the second desalination process is characterized in that it is carried out using a photocatalyst.

In general, pretreatment processes for treating suspended matter in treated water include precipitation, media filtration, DAF (Dissolved Air Flotation), and ultrafiltration. Coagulation, a method of sedimentation, is a method in which colloidal substances and fine floating substances become destabilized and destabilized particles first adhere to each other when rapidly mixed by adding a coagulant. Flocculation is a method in which particles destabilized due to agglomeration are mixed with each other slowly to form a floc quickly.

Light contaminants such as algae, oil, and grease among floating materials are difficult to remove by media filtration and can be removed using a pressurized flotation device. This DAF technology is treated by supplying a coagulant to the supply of dissolved air and the influent line, and as a result, the concentrated sludge is expressed as floating in water. This material can be recovered over the surface of the pressure flotation device.

Ultrafiltration is a process of removing particulate matter, such as sand, silt, algae, microorganisms, and organic pollutants in treated water using an ultrafiltration membrane, and prevents reverse osmosis membranes from being blocked by subsequent substances due to these substances. It is used for stable operation of the process.

In the pretreatment step of the present invention, one or more of these steps can be used in combination as necessary.

The ii) first desalination process is a main process of a seawater desalination method, and may be performed using a reverse osmosis method or a multistage evaporation method.

The Seawater Reverse Osmosis (SWRO) used in the reverse osmosis method has a salt removal rate of about 99% or more, and the treated water after this SWRO process usually has a total dissolved solids (TDS) concentration of 250 to 900 mg. /L, which exceeds the water quality standards for drinking water or industrial water.

Therefore, in order to meet the water quality standards that are more strictly required as desired, a second desalination process is additionally required.

To this end, for example, there is a method of using a reverse osmosis membrane (Brackish Water Reverse Osmosis, BWRO) with a lower efficiency than the reverse osmosis membrane.

This BWRO process is widely used because it is easy to connect with the main process, SWRO, and is relatively easy to operate.

In general, major demand sources of industrial water, such as petrochemical companies and power plants, provide stable water quality conditions to the supply source so that they can be used immediately without their own water treatment process. In particular, the water quality conditions of chlorine ions are higher than other items. Doing. In such a case, by adopting a method capable of reducing the chlorine ion concentration of the present invention, it is possible to more effectively satisfy the water quality condition of the feed water.

The table below shows the water quality requirements when SWRO and BWRO processes have been adopted in the recent seawater desalination process (Source: KDI Public Investment Management Center, 2018 Preliminary Feasibility Study Report, Daesan Forestry Area Industrial Waterworks (Seawater Desalination) Project).

Item Chlorine ion (mg/L) Electrical conductivity (μS/L) TDS (mg/L) Total hardness (mg/L) pH Water quality below 10 100 or less 100 or less below 10 6.5~7.5

In this regard, the basic design simulation data for SWRO and BWRO processes are as follows.

[Table 2]

The RO projection program used to derive this is Nitto Group IMS Design-2015, using the Seawater Reverse Osmosis (SWRO) considering the salt concentration range of TDS of 35,000 mg/L, which is the standard seawater standard. The existing implementation design conditions were simulated. When designing all reverse osmosis membrane plants, the RO projection data provided by the reverse osmosis membrane production company to be applied as a basic design data can be used to determine the operating conditions of the membrane, and the basic material balance and RO projection program distributed by the reverse osmosis membrane manufacturer can be used. You can check the reverse osmosis membrane arrangement conditions according to the operation recovery rate, the required drug selection and injection amount according to the raw water quality.

As shown in Table 1, the water quality criteria for desalination of seawater and applied as industrial water is 10 mg/L or less for chlorine ions, and TDS is 100 mg/L or less.

If you look at Table 2, employing the SWRO and BWRO process, when reviewing by applying the above water quality standards TDS condition is met one (reference is 100 mg / L or treated water 20.73 mg / L Im), Cl ^- ions in the It can be seen that the concentration conditions were exceeded (the standard is 10 mg/L, but the Cl ^- ion concentration in the treated water is 12 mg/L, which is about 2 mg/L).

Therefore, if the purpose is to use industrial water, there is a need to reduce the Cl ^- concentration of treated water that has undergone SWRO and BWRO processes again.

To this end, in theory, the CDI, EDI, or IX process may be further processed, but these processes are impractical in that the equipment cost and operation cost are very large for the purpose of meeting the water quality requirements desired by the customer, as described above. .

In this situation, chlorine ions using the photocatalyst of the present invention Removal techniques can be a very suitable solution. That is, in order to further reduce the concentration of chlorine ions in the second desalination step, the desired purpose can be achieved by introducing treated water that has undergone the first desalination process into the photocatalytic reaction process using TiO ₂ and UV.

In the method of the present invention, TiO ₂ is irradiated with ultraviolet light to pass the treated water through a photocatalyst device that causes catalysis to remove chlorine ions by a photocatalytic reaction.

This photocatalytic reaction may be performed by irradiating ultraviolet wavelengths such as UV-C (200-280 nm), UV-B (280-315 nm), and UV-A (315-400 nm) for a few minutes to 2 hours. The photocatalytic reaction may vary in efficiency depending on the concentration and operating conditions of the photocatalyst, the wavelength range of UV irradiation, and the like, and the conditions of the photocatalytic reaction may be adjusted by a person skilled in the art according to the purpose and need.

According to one embodiment of the present invention, the concentration of chlorine ions in the treated water that has been subjected to the photocatalytic reaction process using TiO ₂ and UV is reduced by about 18% to 25% compared to before the process (see FIGS. 1A to 1C ). ). Therefore, it can be seen that the chlorine ion reduction process using TiO ₂ and UV can be sufficiently reduced to a level required for use in industrial water reactors, for example, by being introduced after the first desalination process. .

In the second desalination process using the photocatalyst of the present invention, the photocatalytic reaction process using TiO ₂ and UV may be performed alone, or may be used in combination with one or more of the BWRO, CDI, EDI, or IX processes.

According to another aspect of the present invention, a device for photocatalytic reaction using TiO ₂ and UV used in the second desalination process to reduce the concentration of chlorine ions (Cl ⁻ ) (hereinafter referred to as a UV-TiO ₂ device) A seawater desalination system is provided.

For example, the seawater desalination system of the present invention comprises i) a pretreatment process device for removing suspended matter from seawater, ii) a first desalination process device for the treated water and iii) a second desalination process device for the treated water, The second desalination process device includes a photocatalyst and a UV irradiation device.

The pretreatment process apparatus is not particularly limited, and a known precipitation, media filtration, DAF (Dissolved Air Flotation), or ultrafiltration apparatus may be used. The same device may be used in multiple connections, or multiple types of devices may be used in combination.

The apparatus for the first desalination process of the treated water is also not particularly limited, and includes a device for a reverse osmosis process known in the art or a device for a multi-step evaporation method.

The second desalination process device must essentially include a UV-TiO ₂ device for the photocatalytic reaction, and may include the UV-TiO ₂ device alone, but perform the above-described BWRO, CDI, EDI or IX process. In order to be configured in combination with devices known in the industry.

The UV-TiO ₂ device is not particularly limited, and a known photocatalytic device or a modified product can be used. For example, It may be configured in various forms including a UV light source unit, a reaction tank, and a filtration device. In particular, the filtration device includes a membrane or a filter, and may be provided to prevent leakage of particulate matter such as a catalyst in treated water.

UV-TiO ₂ device according to an embodiment of the present invention is a reactor type or a filter type bed (depending on the reaction time (contact time) and the concentration of the photocatalyst, the efficiency according to the UV wavelength, etc. in accordance with the water quality conditions of the incoming raw water and treated water ( bed) type.

2 is an example of a seawater desalination system including a UV-TiO ₂ device according to the present invention.

Specifically, FIG. 2 is a pre-treatment process apparatus (1), a first desalination apparatus (2), a UV-TiO ₂ apparatus (3) in the form of a bed as a second desalination apparatus, or a UV-TiO ₂ apparatus (4) in the form of a reactor It shows a seawater desalination system comprising a. The pretreatment process apparatus includes a media filtration apparatus, and in the filter process, particulate matter and solid substances such as sand, sand, etc., which greatly affect the life and performance of the MF/UF (microfiltration/ultrafiltration) membrane, are previously used. In the MF/UF process equipment, particulate matter such as sand, silt, algae, microorganisms and organic contaminants are removed.

The first desalination device 2 includes a reverse osmosis membrane device. The reverse osmosis membrane device is not particularly limited, and any one known in the art may be used. The treated water that has passed through the reverse osmosis membrane device flows into the second desalination device (3 or 4) to further reduce the chlorine ion concentration.

The second desalination device includes a bed type UV-TiO ₂ device (3) or a reaction tank type UV-TiO ₂ device (4), and either one or both are selected according to needs and purposes. It is also possible to construct a seawater desalination system.

The treated water that has undergone the reverse osmosis membrane process passes through a photocatalytic reaction device equipped with a UV light source (32 or 42) and a TiO ₂ (31 or 41) to produce a water quality with a reduced chlorine ion concentration. In the figure, 33 is a mesh that serves to confine the catalyst, and 43 is an immersion type separation membrane for separating the catalyst in the treated water.

The second desalination apparatus from the UV-TiO by placing a water bath (44 or 5) to the sequencing of the _second device, to average the flow rate of the raw water to flow into the UV-TiO ₂ system or, UV-TiO ₂ system It is possible to recover water from which chlorine ions have been reduced.

On the other hand, concentrated water that has been concentrated to one side of the reverse osmosis membrane device may be introduced into the energy recovery device. The energy recovery device is a method of using the pressure of the concentrated water discharged at high pressure as an energy source and transmitting it to the seawater source water side flowing at a low pressure.

TiO ₂ used in the method of the present invention is a catalyst and can be reused because there is no change in physical properties before and after the reaction. Can be used in conjunction. That is, the photocatalytic reaction device using TiO ₂ and UV of the present invention has an advantage that can simplify the equipment and also facilitate the coupling or linkage with other processes.

By those skilled in the art to which the present invention pertains, all simple modifications or changes of the present invention that can be easily implemented can be considered to be included in the scope of the present invention.

Example 1

The chlorine ion reduction experiment is a batch-type experiment to perform a photocatalytic reaction using TiO ₂ and UV, Four UV lamps and a 10 L capacity reactor were used (see Figures 3A and 3B). In order to block external light, the surface of the reaction tank was coated and used (refer to FIG. 3C), although not shown in the drawing, a temperature control device and a stirrer were installed to keep the raw water condition constant at all times.

The specifications of the photocatalytic reaction device used in the examples are shown in the table below.

Reactor size 10L (0.3m(H)*0.2m(w)*0.3m(L)) UV lamp length 0.13m UV lamp diameter 0.025m UV lamp number 4 Power consumption 4W wavelength 253.7nm

Raw water was prepared by dissolving NaCl (SAMCHUN PURE CHEMICAL CO., LTD.NaCl Extra pure, Assay ≥99.0%) in ultrapure water produced using a HIQ ultrapure water production system and producing it at 200 mg/L based on TDS.

Looking at Table 2, the total amount of ions present in water is 20.73 mg/L based on TDS, and the concentrations of individual Cl ions and individual Na ions are 12.009 mg/L and 7.647 mg/L, respectively. It can be seen that L becomes almost the value of TDS. Therefore, since it is confirmed that almost all of the ions of the treated water that has undergone the SWRO and BWRO processes remain as Na or Cl ions, experimental water (raw water) in which NaCl is dissolved in pure water can be replaced therewith. In most RO studies related to seawater, artificial raw water is generally used as NaCl.

The experiment was conducted by mixing powder particles to a concentration of 1 g/L of TiO ₂ (manufacturer: Daejung Chemical Co., Ltd.) as a photocatalyst in a 200 mg/L NaCl solution based on TDS. In addition, TiO ₂ , a photocatalyst, can be adsorbed with ionic substances by changing the potential on the particle surface, so it reacts for 30 minutes without UV irradiation to evaluate the efficiency of chlorine ion removal by adsorption by the photocatalyst itself. Ion removal efficiency was checked. After that, a UV lamp having a wavelength band of 253.7 nm was operated to react for 2 hours to examine the efficiency of removing chlorine ions in raw water according to UV irradiation.

As a result, as shown in FIG. 1A, it was found that when reacted for 30 minutes without UV irradiation, there was little efficiency in removing chlorine ions, but after UV irradiation, about 25% of chlorine ions were removed for about 2 hours.

Therefore, it is confirmed that the Cl ion concentration of the treated water can be further reduced by applying the chlorine ion reduction method using this apparatus after the main desalination process of the seawater desalination process.

Example 2

As the number of experiments, the experiment was conducted under the same conditions as in Example 1, except that TiO ₂ (manufacturer: Daejung Gold Co., Ltd.), which is a photocatalyst, was adjusted to a concentration of 2 g/L in a 200 mg/L NaCl solution based on TDS. .

As a result, as shown in FIG. 1B, it was found that when reacted for 30 minutes without UV irradiation, chlorine ion removal efficiency was little, but after UV irradiation, about 18% of chlorine ions were removed for about 2 hours.

Example 3

As the number of experiments, the experiment was conducted under the same conditions as in Example 1, except that TiO ₂ (manufacturer: Daejung Gold Co., Ltd.), which is a photocatalyst, was adjusted to a concentration of 1 g/L in a 100 mg/L NaCl solution based on TDS. .

As a result, as shown in FIG. 1C, when reacted for 30 minutes without UV irradiation, it was found that chlorine ion removal efficiency was little, but after UV irradiation, about 22% of chlorine ions were removed for about 2 hours.

1: Pretreatment process equipment
2: First desalination device
3: Bed type photocatalytic reaction Device
4: Reactor type photocatalytic reaction device
5: Production water storage tank
31, 41: photocatalyst (TiO ₂ )
32, 42: UV light source
33: mans
43: membrane or filter
44: Raw water tank
45: raw water pump
46: suction pump

Claims (9)

i) a pretreatment process for removing suspended matter from the seawater, ii) a first desalination process of the treated water after the pretreatment process, and iii) a second desalination process of the treated water after the first desalination process, and the second desalination process The seawater desalination method, characterized in that the process is carried out using a photocatalytic reaction. The method of claim 1, wherein the photocatalytic reaction is achieved by irradiating TiO ₂ with UV. The method of claim 1, wherein the pre-treatment process is carried out using precipitation, media filtration, DAF (pressurized flotation) or ultrafiltration. The method of claim 1, wherein the first desalination process is performed using a reverse osmosis method or a multi-step evaporation method. The method according to claim 1, wherein the second desalination process is performed using a reverse osmosis method for brackish water (BWRO), capacitive desalination (CDI), electrodesalination (EDI) or ion exchange resin (IX). Seawater desalination system comprising a photocatalytic reaction device for reducing the concentration of chlorine ions. A seawater desalination system comprising a pretreatment process device for removing suspended matter from seawater, ii) a first desalination process device, and iii) a second desalination process device, wherein the second desalination process device comprises a photocatalytic reaction device. . The seawater desalination system according to claim 6 or 7, wherein the photocatalytic reaction device comprises a TiO ₂ and UV irradiation device. The seawater desalination system according to claim 6 or 7, wherein the photocatalytic reaction device is installed in a bed type or a reactor type.

Images