SUPERHYDROPHOBIC MICROFILTRATION MEMBRANE FOR MEMBRANE DISTILLATION, FILTRATION MODULE FOR MEMBRANE DISTILLATION COMPRISING THE SAME, AND METHOD FOR MANUFACTURING THE SAME

Abstract

Disclosed are a superhydrophobic microfiltration membrane capable of facilitating higher permeate flux without separation performance deterioration when performing a water treatment based on a membrane distillation method, a filtration module for membrane distillation comprising the same, and a method for manufacturing the same. The superhydrophobic microfiltration membrane of the present invention comprises a porous member having a plurality of fine pores having an average pore size of 1 m to 100 m and has a pure water contact angle of 130 or more.

Claims

1. A superhydrophobic microfiltration membrane for membrane distillation, wherein the superhydrophobic microfiltration membrane comprises a porous member having a plurality of fine pores having an average pore size of 1 m to 100 m and has a pure water contact angle of 130 or more.

2. The superhydrophobic microfiltration membrane of claim 1, wherein: the average pore size of the plurality of fine pores is 10 m to 100 m; and a 99% nominal pore size of the plurality of fine pores is 110 m or less.

3. The superhydrophobic microfiltration membrane of claim 1, wherein: the average pore size of the plurality of fine pores is 20 m to 90 m; and a 99% nominal pore size of the plurality of fine pores is 95 m or less.

4. The superhydrophobic microfiltration membrane of claim 1, wherein: the average pore size of the plurality of fine pores is 35 m to 80 m; and a 99% nominal pore size of the plurality of fine pores is 85 m or less.

5. The superhydrophobic microfiltration membrane of claim 1, wherein the pure water contact angle is 150 or more.

6. The superhydrophobic microfiltration membrane of claim 1, wherein the porous member includes at least one selected from the group consisting of polytetrafluoroethylene, polyethylene, and polyvinylidene fluoride.

7. The superhydrophobic microfiltration membrane of claim 1, wherein the porous member is surface-treated by a plasma sputtering.

8. The superhydrophobic microfiltration membrane of claim 1, wherein a surface of the porous member is modified with at least one selected from the group consisting of CF.sub.3, CF.sub.2H, CF.sub.2, and CH.sub.2CF.sub.3.

9. The superhydrophobic microfiltration membrane of claim 1, wherein: the superhydrophobic microfiltration membrane further comprises a hydrophobic layer on the porous member; the hydrophobic layer comprises a mixture of nanoparticles and polymer base material; the nanoparticles includes at least one selected from the group consisting of (i) silica particle, (ii) CaCO.sub.3 particle, and (iii) Boehmite particle; and the polymer base material includes at least one selected from the group consisting of (i) a copolymer of fluoroalkyl and methyl methacryl, (ii) a fluorine-containing polymer, and (iii) Anatase.

10. A filtration module for membrane distillation comprising: a housing; and a filtration membrane dividing an inner space of the housing into a first flow path constituting a part of a feed water circulation path and a second flow path constituting a part of a permeate circulation path, wherein the filtration membrane is the hydrophobic microfiltration membrane of claim 1.

11. A method for manufacturing a hydrophobic microfiltration membrane for membrane distillation, the method comprising: forming a porous member having a plurality of fine pores having an average pore size of 1 m to 100 m; and making a surface of the porous member superhydrophobic to such a degree that the superhydrophobic microfiltration membrane has a pure water contact angle of 130 or more.

12. The method of claim 11, wherein: the average pore size of the plurality of fine pores is 10 m to 100 m; and a 99% nominal pore size of the plurality of fine pores is 110 m or less.

13. The method of claim 11, wherein: the average pore size of the plurality of fine pores is 20 m to 90 m; and a 99% nominal pore size of the plurality of fine pores is 95 m or less.

14. The method of claim 11, wherein: the average pore size of the plurality of fine pores is 35 m to 80 m; and a 99% nominal pore size of the plurality of fine pores is 85 m or less.

15. The method of claim 11, wherein the pure water contact angle is 150 or more.

16. The method of claim 11, wherein the porous member is formed of at least one selected from the group consisting of polytetrafluoroethylene, polyethylene, and polyvinylidene fluoride by means of a 3D printer.

17. The method of claim 11, wherein the making the surface of the porous member superhydrophobic comprises performing a surface treatment of the porous member by means of a plasma sputtering.

18. The method of claim 11, wherein the making the surface of the porous member superhydrophobic comprises modifying the surface of the porous member with at least one selected from the group consisting of CF.sub.3, CF.sub.2H, CF.sub.2, and CH.sub.2CF.sub.3.

19. The method of claim 11, wherein: the making the surface of the porous member superhydrophobic comprises forming a hydrophobic layer on the porous member; the hydrophobic layer is formed of a mixture of nanoparticles and polymer base material; the nanoparticles includes at least one selected from the group consisting of (i) silica particle, (ii) CaCO.sub.3 particle, and (iii) Boehmite particle; and the polymer base material includes at least one selected from the group consisting of (i) a copolymer of fluoroalkyl and methyl methacryl, (ii) a fluorine-containing polymer, and (iii) Anatase.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0036] The accompanying drawing, which is included to provide a further understanding of the invention and is incorporated in and constitute a part of this application, illustrate an embodiment of the invention and together with the description serves to explain the principle of the invention.

[0037] FIG. 1 schematically shows a membrane distillation system according to an embodiment of the present invention.

MODE FOR INVENTION

[0038] Hereinafter, the embodiments of the present invention will be described in detail with reference to the annexed drawing. The embodiments of the present invention are described only for illustrative purposes to provide better understanding of the invention and are not intended to limit the invention thereto.

[0039] It will be apparent to those having ordinary skill in the art that various modifications and variations are possible, without departing from the scope and spirit of the invention. Therefore, the present invention encompasses the inventions as defined by the appended claims and the modifications and variations equivalent thereto as well.

[0040] Hereinafter, the membrane distillation system of the present invention will be described in detail. FIG. 1 illustrates a direct contact membrane distillation system.

[0041] The membrane distillation system 100 of the present invention comprises a filtration module 110 performing water treatment, a feed water storage tank 120 where a feed water to be treated is stored, and a permeate storage tank 130 where a permeate produced by the filtration module 110 is stored.

[0042] As illustrated in FIG. 1, the filtration module 110 according to an embodiment of the present invention comprises a housing 111 and a filtration membrane 112. The filtration membrane 112 is installed in the housing 111 and divides the inner space of the housing 111 into the first flow path FP1 and the second flow path FP2. The first flow path FP1 constitutes a part of the feed water circulation path, and the second flow path FP2 constitutes a part of the permeate circulation path.

[0043] Although the filtration module 110 illustrated in FIG. 1 includes a flat sheet membrane as the filtration membrane 112, the filtration membrane 112 of the present invention is not limited to a flat sheet membrane and may be filtration membranes of various shapes, e.g., a hollow fiber membrane. If the filtration membrane is a hollow fiber membrane, the space between the housing and the hollow fiber membrane will serve as the first flow path for the feed water and the lumen of the hollow fiber membrane will serve as the second flow path for the permeate.

[0044] The feed water stored in the feed water storage tank 120 is supplied to the filtration module 110 by the first pump P1. If the feed water is seawater, the seawater may be directly supplied from a sea to the filtration module 110 by the first pump P1 without passing through the feed water storage tank 120.

[0045] As shown in FIG. 1, for the phase change at the surface of the filtration membrane 112, the feed water may be heated by the heating unit 140 just before supplied to the filtration module 110. If the temperature of the feed water is sufficiently high just like the seawater around the Middle East area, the seawater-heating process by the heating unit 140 may be omitted.

[0046] In order to minimized the energy consumption, the heating unit 140 may be a heat exchanger for transferring the waste heat of a power plant to the feed water (i.e., a heat exchanger where the heat is exchanged between the feed water and the steam of high temperature discharged after rotating a turbine of the power plant).

[0047] When the feed water supplied to the filtration module 110 passes through the first flow path FP1, a portion thereof transformed into a steam penetrates the filtration membrane 112 and enters the second flow path FP2, and the rest returns back to the feed water storage tank 120.

[0048] If the feed water is seawater, after passing through the first flow path FP1, the feed water may be directly discharged to the sea instead of returning back to the feed water storage tank 120.

[0049] Although a clean water is stored in the permeate storage tank 130 before the filtration starts, it is gradually replaced with the permeate as the filtration proceeds. Hereinafter, for the convenience of explanation, the clean water will also be called permeate.

[0050] The permeate stored in the permeate storage tank 130 is supplied to the filtration module 110 by the second pump P2.

[0051] As shown in FIG. 1, for the phase change of the feed water at the surface of the filtration membrane 112, the permeate may be cooled by the cooling unit 150 just before supplied to the filtration module 110.

[0052] When the permeate of relatively low temperature supplied to the filtration module 110 passes through the second flow path FP2, a portion of the feed water of relatively high temperature passing through the first flow path FP1, i.e., a portion of the feed water contacting the filtration membrane 112, undergoes phase change due to the temperature difference and changes into a steam. The steam penetrates the filtration membrane 112, moves to the permeate of low temperature, condenses into water, and flows into the permeate storage tank 130 along with the original permeate.

[0053] Hereinafter, the filtration membrane 112 of the present invention will be described in more detail.

[0054] The filtration membrane 112 of the present invention is a superhydrophobic microfiltration membrane which comprises a porous member having a plurality of fine pores desirably having an average pore size of 1 m to 100 m, more desirably 10 m to 100 m, further more desirably 20 m to 90 m, and still further more desirably 35 m to 80 m, and desirably has a pure water contact angle of 130 or more, more desirably 150 or more.

[0055] The average pore size of the filtration membrane 112 refers to a statistical mean value of the pore size and can be determined by using a pore size distribution graph obtained by LLDP (Liquid-Liquid Displacement Porosimetry) conducted on a sample taken from the central part of the filtration membrane 112.

[0056] The pure water contact angle of the filtration membrane 112 refers to a static contact angle and can be determined by dropping a pure water droplet on the surface of the filtration membrane 112 and measuring the angle between the surfaces of the filtration membrane 112 and the droplet.

[0057] Since a membrane distillation method uses the temperature difference between feed water and permeate, which are on opposite sides of a membrane, the temperature difference needs to be maintained above a predetermined level in order to continuously perform the filtration using membrane distillation and guarantee a permeate flux of a certain amount or more. In other words, the filtration membrane for membrane distillation must be able to inhibit or prevent the heat transfer from the feed water of relatively high temperature to the permeate of relatively low temperature.

[0058] Therefore, the porous member may include at least one selected from the group consisting of polytetrafluoroethylene (PTFE), polyethylene (PE), and polyvinylidene fluoride (PVDF) in order to make the filtration membrane 112 of the present invention have both high hydrophobicity and low thermal conductivity.

[0059] The filtration membrane 112 of the present invention has an average pore size of 1 m or more, thereby enabling the permeate flux as high as required for commercialization of the membrane distillation method, e.g., permeate flux of 20 LMH or higher under the standard condition of temperature difference of 40 C. between feed water and permeate.

[0060] Since the filtration membrane 112 of the present invention has superhydrophobicity so that the pure water contact angle thereof is 130 or more, although the fine pores have relatively large average pore size of 1 m or more, the wetting of the filtration membrane 112 can be inhibited and only the steam can penetrate the filtration membrane 112. In spite of the superhydrophobicity of the filtration membrane 112 of the present invention, however, if the fine pores have an average pore size more than 100 m, there would be a risk that the liquid containing the impurities (e.g., salts such as NaCl) will also penetrates the membrane and the separation performance (i.e., salt rejection) will deteriorate.

[0061] A surface treatment of the porous member by a plasma sputtering may be performed to increase the surface roughness of the porous member, thereby making the filtration membrane 112 superhydrophobic.

[0062] Alternatively, the filtration membrane 112 may be made superhydrophobic by modifying the surface of the porous member with at least one selected from the group consisting of CF.sub.3, CF.sub.2H, CF.sub.2, and CH.sub.2CF.sub.3.

[0063] According to another embodiment of the present invention, the surface of the porous member which has been surface-treated by a plasma sputtering may be modified with a fluorinated functional group.

[0064] According to further another embodiment of the present invention, the filtration membrane 112 may further comprise a hydrophobic layer on the porous member. The hydrophobic layer may comprise nanoparticles and a polymer base material.

[0065] The nanoparticles may include at least one selected from the group consisting of (i) silica particle, (ii) CaCO.sub.3 particle, and (iii) Boehmite particle, and the polymer base material may include at least one selected from the group consisting of (i) a copolymer of fluoroalkyl and methyl methacryl, (ii) a fluorine-containing polymer, and (iii) Anatase.

[0066] The wetting of the filtration membrane 112 is caused mainly by the pores of relatively large pore size. The smaller the number of the pores of large pore size is, the higher the anti-wetting property of the filtration membrane 112 is so that satisfactory medium and long term filtration performance can be secured. Thus, according to an embodiment of the present invention, 99% of the pores of the porous member desirably has pore size of 100 m or less, more desirably 95 m or less, and further more desirably 85 m or less. In other words, the pore size corresponding to the pore cumulative number of 99% in the cumulative distribution of pore size in ascending order (hereinafter, 99% nominal pore size) is desirably 100 m or less, more desirably 95 m or less, and further more desirably 85 m or less. The 99% nominal pore size of the filtration membrane 112 can be obtained by means of LLDP (Liquid-Liquid Displacement Porosimetry).

[0067] Hereinafter, a method for manufacturing the filtration membrane 112 of the present invention will be described in detail.

[0068] The method of the present invention comprises forming a porous member having a plurality of fine pores having an average pore size of 1 m to 100 m, more desirably 10 m to 100 m, and making a surface of the porous member superhydrophobic.

[0069] As explained above, the porous member may be formed of at least one selected from the group consisting of polytetrafluoroethylene (PTFE), polyethylene (PE), and polyvinylidene fluoride (PVDF) by means of any conventional membrane-manufacturing method.

[0070] If the porous member is formed using a conventional membrane-manufacturing method, however, there would be a risk of pore size deviation of such degree that a lot of pores having diameters larger than the average pore size (e.g., diameters larger than 100 m) might exist. Such big pores are likely to induce the membrane wetting, thereby degrading the separation performance (i.e., salt rejection). Accordingly, in order to make the pore sizes of the plurality of fine pores uniform (i.e., in order to minimize the pore size deviation), the porous member may be formed by means of a 3D printer.

[0071] By the step of making the surface of the porous member superhydrophobic, the filtration membrane 112 of the present invention can gain high hydrophobicity of such degree that the pure water contact angle thereof is 130 or more, more desirably 150 or more.

[0072] The step of making the surface of the porous member superhydrophobic may comprise performing a surface treatment of the porous member by means of a plasma sputtering. By the surface treatment, the surface roughness of the porous member increases and the filtration membrane 112 can gain the superhydrophobicity so that the pure water contact angle thereof is 130 or more.

[0073] The plasma sputtering may be performed using a RF power source in a vacuum. For example, it may be performed using a bias voltage of 700 V in the mixture gas of oxygen and argon (molar ratio=2:1) for 2 hours.

[0074] Alternatively, the step of making the surface of the porous member superhydrophobic may comprise modifying the surface of the porous member with a fluorinated functional group. The fluorinated function group may be at least one selected from the group consisting of CF.sub.3, CF.sub.2H, CF.sub.2, and CH.sub.2CF.sub.3. For example, after a plasma etching of the surface of the porous member is performed to roughen the surface, the surface of the porous member may be modified by generating a plasma in a fluorinated gas environment.

[0075] According to another embodiment of the present invention, the step of making the surface of the porous member superhydrophobic may comprise forming a hydrophobic layer on the porous member. The hydrophobic layer may be formed of a mixture of nanoparticles and a polymer base material by using a conventional coating method (e.g., spray coating, dip coating, and etc.).

[0076] The nanoparticles may include at least one selected from the group consisting of (i) silica particle, (ii) CaCO.sub.3 particle, and (iii) Boehmite particle, and the polymer base material may include at least one selected from the group consisting of (i) a copolymer of fluoroalkyl and methyl methacryl, (ii) a fluorine-containing polymer, and (iii) Anatase.

[0077] Hereinafter, the present invention will be described in more detail with reference to the following Examples and Comparative Examples. The following Examples are only given for better understanding of the present invention and should not be construed as limiting the scope of the present invention.

Example 1

[0078] A PTFE porous member having an average pore size of 1 m and a 99% nominal pore size of 1.2 m was formed by using a 3D printer. Subsequently, a plasma etching (1.3 kV, 50 mA) was performed on the surface of the porous member in an air atmosphere of 2 Torr for 20 minutes to roughen the surface, and then the surface of the porous member was modified by filling the chamber with CHF.sub.3 gas and generating plasma (2.2 kV, 80 mA) for 5 minutes while maintaining the pressure at 4 Torr, thereby completing a filtration membrane.

Example 2

[0079] A filtration membrane was obtained in the same manner as in Example 1 except that the PTFE porous member had an average pore size of 10 m and a 99% nominal pore size of 11.8 m.

Example 3

[0080] A filtration membrane was obtained in the same manner as in Example 1 except that the PTFE porous member had an average pore size of 20 m and a 99% nominal pore size of 23.3 m.

Example 4

[0081] A filtration membrane was obtained in the same manner as in Example 1 except that the PTFE porous member had an average pore size of 35 m and a 99% nominal pore size of 40.5 m.

Example 5

[0082] A filtration membrane was obtained in the same manner as in Example 1 except that the PTFE porous member had an average pore size of 100 m and a 99% nominal pore size of 109.5 m.

Example 6

[0083] A filtration membrane was obtained in the same manner as in Example 1 except that the PTFE porous member was prepared by using a Melt Spinning Cold Stretching (MSCS) method and the PTFE porous member had an average pore size of 25 m and a 99% nominal pore size of 85.2 m.

Comparative Example 1

[0084] A commonly used PTFE filtration membrane having an average pore size of 0.1 m and a 99% nominal pore size of 7.2 m was prepared.

Comparative Example 2

[0085] A filtration membrane was obtained in the same manner as in Example 1 except that the PTFE porous member had an average pore size of 101.5 m and a 99% nominal pore size of 118.7 m.

Comparative Example 3

[0086] A filtration membrane was obtained in the same manner as in Example 1 except that the surface-modifying process was omitted.

[0087] Direct contact membrane distillation processes were carried out using the filtration membranes of the aforementioned Examples and Comparative Examples under the following Standard Temperature Difference Condition and Low Temperature Difference Condition, respectively. A feed water containing 50 S/cm of NaCl was used, the circulation flow rate was 80 mL/min, and the pressure of the circulated water was 0.01 bar. The permeate fluxes and salt rejections were measured respectively and the results thereof are shown in the following Table 1.

[0088] Standard Temperature Difference Condition

[0089] This is the condition corresponding to a case where the seawater heated with a waste heat generated in volume at a power plant having a cooling tower operated on the coast is used as the feed water. Feed water of 60 C. and permeate of 20 C. were used.

[0090] Low Temperature Difference Condition

[0091] This is the condition corresponding to a case where the seawater of Middle East area and the underground water are used as the feed water and the permeate, respectively. Feed water of 40 C. and permeate of 20 C. were used.

TABLE-US-00001 TABLE 1 Standard Low Temp. Difference Temp. Difference Porous Member Condition Condition 99% (60 C./20 C.) (40 C./20 C.) Average Nominal Permeate Salt Permeate Salt Pore size Pore Size Surface Flux Rejection Flux Rejection (m) (m) Modification (LMH) (%) (LMH) (%) Ex. 1 1 1.2 yes 84 >99 15 >99 Ex. 2 10 11.8 yes 550 >99 41 >99 Ex. 3 20 23.3 yes 825 >99 62 >99 Ex. 4 35 40.5 yes 960 >99 75 >99 Ex. 5 100 109.5 yes 1620 95 96 94 Ex. 6 25 85.2 yes 880 97 68 96 Comp. 0.1 7.2 yes 15 >99 2 >99 Ex. 1 Comp. 101.5 118.7 yes 1770 82 108 81 Ex. 2 Comp. 1 1.2 no 95 85 17 84 Ex. 3

[0092] As can be seen in Table 1, all the filtration membranes of Examples 1 to 6 showed excellent salt rejections higher than 95% (on the other hand, the filtration membrane of Comparative Example 2 the pore sizes of the porous member of which were larger than 100 m and the filtration membrane of Comparative Example 3 prepared without surface modification respectively showed salt rejections lower than 85%) and, at the same time, showed permeate fluxes 5.6 times or more higher than and 7.5 times or more higher than those of the filtration membrane of Comparative Example 1, the porous member of which had an average pore size of 0.1 m, under the standard temperature difference condition and low temperature difference condition, respectively. As explained above, such a high permeate flux enables the commercialization of membrane distillation method.

[0093] Particularly, the filtration membranes of Examples 1 to 3 whose porous members have 99% nominal pore sizes smaller than 85 m showed more excellent salt rejections (i.e., salt rejections more than 99%) than those of Examples 5 and 6 having 99% nominal pore sizes larger than 85 m.