JP6359431B2 - Porous hollow fiber membrane, method for producing porous hollow fiber membrane, and water purification method - Google Patents

Description

  The present invention relates to a porous hollow fiber membrane, a method for producing a porous hollow fiber membrane, and a water purification method.

Clear water that can be reclaimed miscellaneous water by treating drinking water and industrial water from natural water source water such as river water, lake water, and groundwater that is suspended water, and domestic wastewater such as sewage In the sewage treatment to obtain water, it is essential to remove the suspension (turbidity) by performing a solid-liquid separation operation (turbidity removal operation). The removal is removal of suspensions (clays, colloids, bacteria, etc.) derived from natural water source water, which is suspension water for water treatment, and suspensions of activated water and activated sludge for sewage treatment. It is removal of the suspended matter (sludge etc.) in the treated water which carried out biological treatment (secondary treatment) by etc. Conventionally, these turbidity removal operations have been mainly performed by a precipitation method, a sand filtration method, or an agglomeration precipitation sand filtration method, but in recent years, a membrane filtration method has become widespread. Examples of the advantages of the membrane filtration method include the following.
(1) The turbidity level of the obtained water is high and stable (the safety of the obtained water is high).
(2) The installation space for the filtration device is small.
(3) Automatic operation is easy.

  For example, in clean water treatment, membrane filtration as an alternative to coagulation sedimentation sand filtration or as a means to further improve the quality of treated water that has been installed after the aggregation sedimentation sand filtration and filtered. Is used. Regarding sewage treatment, studies are underway to use a membrane filtration method to separate sludge from sewage secondary treated water.

  For the turbidity operation by these membrane filtration methods, a hollow-fiber ultrafiltration membrane or a microfiltration membrane (with a pore diameter of several nm to several hundred nm) is mainly used. As a filtration method using a hollow fiber filtration membrane, there are two methods, an internal pressure filtration method for filtering from the inner surface side to the outer surface side of the membrane and an external pressure filtration method for filtering from the outer surface side to the inner surface side. There is. Among these, since it is possible to increase the membrane surface area on the side in contact with the raw water, an external pressure filtration system that can reduce the turbidity load per unit membrane surface area is advantageous. Patent Documents 1 to 3 disclose a hollow fiber and a manufacturing method thereof.

  Patent Document 4 discloses a porous multilayer hollow fiber membrane made of a thermoplastic resin that has both fine pores and high water permeability and is excellent in strength, suitable for filtration, and a stable production method thereof. For this purpose, a hollow fiber molding nozzle having an annular discharge port is used, and a melt-kneaded product containing a thermoplastic resin and an organic liquid is discharged from the annular discharge port. In a method for producing a porous multilayer hollow fiber membrane by extracting and removing an organic liquid, the hollow fiber molding nozzle has two or more annular discharge ports arranged concentrically, and is different from adjacent discharge ports A melt-kneaded material having a composition is discharged, the melt-kneaded material discharged from at least one annular discharge port also contains inorganic fine powder, and the inorganic fine powder is also extracted and removed from the obtained multilayer hollow fiber shape. Manufacturing method disclosed It has been. Further, Patent Document 5 includes a braid and a membrane material, and the membrane material includes a first porous layer having a dense layer adjacent to the braid outer surface and a dense layer adjacent to the first porous layer. For the purpose of providing a composite porous membrane comprising a second porous layer having a braid and a membrane material, the membrane material having a dense layer adjacent to the outer surface of the braid A composite porous membrane comprising a first porous layer and a second porous layer having a dense layer adjacent to the first porous layer is disclosed.

  As described above, turbidity removal by membrane filtration has many advantages over conventional precipitation methods and sand filtration methods, and as a result, it has become popular in water treatment and sewage treatment as an alternative and complementary technology to conventional methods. It's getting on.

Incidentally, a thermally induced phase separation method is known as a method for producing a porous membrane. In this manufacturing method, a thermoplastic resin and an organic liquid are used. As the organic liquid, a solvent that does not dissolve the thermoplastic resin at room temperature but dissolves at a high temperature, that is, a latent solvent, is used. More specifically, in the thermally induced phase separation method, a thermoplastic resin and an organic liquid are kneaded at a high temperature, the thermoplastic resin is dissolved in the organic liquid, and then cooled to room temperature to induce phase separation. This is a method for producing a porous body by removing a liquid. The thermally induced phase separation method has the following advantages.
(A) Films can be formed even with polymers such as polyethylene that do not dissolve at room temperature in commonly used solvents.
(B) Since the film is formed by cooling and solidifying after melting at a high temperature, particularly when the thermoplastic resin is a crystalline resin, crystallization is promoted during film formation, and a high-strength film is likely to be obtained.

  From the above advantages, the thermally induced phase separation method is frequently used as a method for producing a porous membrane (for example, see Non-Patent Documents 1 to 4).

JP 60-139815 A JP-A-3-215535 JP-A-4-065055 International Publication No. 2007/043553 International Publication No. 2004/043579

Editorial Committee for Encyclopedia of Plastics and Functional Polymer Materials, “Encyclopedia of Plastics and Functional Polymer Materials”, Industrial Research Committee, February 2004, pages 672-679 Hideto Matsuyama, “Preparation of Polymeric Porous Membrane by Thermally Induced Phase Separation (TIPS)”, Chemical Engineering, Chemical Industries, June 1998, pages 45-56 Akira Takizawa, “Membrane”, IPC, January 1992, pages 404-406 D. R. Lloyd, et. al. , "Journal of Membrane Science", 64, 1991, pp. 1-11. Y. Watanabe, R.A. Bian, Membrane, 24 (6), 1999, 310-318.

  However, a technique for performing stable membrane filtration operation over a long period of time has not been established, and this hinders widespread use of membrane filtration methods (see Non-Patent Document 5 above). The cause of hindering the stable operation of the membrane filtration method is mainly the deterioration of the water permeability of the membrane due to abrasion of the membrane surface. The deterioration of the water permeation performance is caused by clogging (fouling) of the membrane due to turbid substances or the like (see Non-Patent Document 5 above) and the membrane surface being rubbed by the suspension.

  This abrasion on the membrane surface does not occur during the operation of the membrane filtration method, but air-washing suspensions deposited on the outer surface of the membrane (hereinafter sometimes abbreviated as “membrane outer surface”) by external pressure filtration, etc. This is mainly caused when peeling from the outer surface of the film. However, since this phenomenon itself has not been well known, development of a technique for dealing with deterioration of water permeability performance due to film surface abrasion has not been made. Currently, as a technique for dealing with film surface abrasion, Japanese Patent Application Laid-Open No. 11-138164 merely discloses the use of a film having a high breaking strength as a means for suppressing film performance change due to air bubbling cleaning.

  Then, an object of this invention is to provide the porous hollow fiber membrane which can maintain high filtration performance over a long period of time.

  As a result of diligent efforts to solve the above-mentioned problems of the prior art, the present inventors include a porous membrane layer prepared by heating, melting and kneading polyethylene and a solvent having a viscosity average molecular weight of a specific value or more. The present inventors have found that the porous hollow fiber membrane has excellent scratch resistance and can suppress deterioration of water permeability, thereby completing the present invention.

That is, the present invention is as follows.
[1] A porous membrane layer containing polyethylene having a viscosity average molecular weight of 1,000,000 or more,
The porous membrane layer has a pore diameter of 0.3 μm or more and 1.0 μm or less, a pore aspect ratio of 10 or less, and a porosity of 70% or more,
Pure water flux is 5000 LMH or more,
A porous hollow fiber membrane having a membrane scratch resistance of 80% or more.
[2] The porous hollow fiber membrane according to [1], comprising the porous membrane layer located on the outer surface side and a hollow support located on the inner surface side.
[3] The porous hollow fiber membrane according to [2], wherein the support is a knitted string.
[4] The porous hollow fiber membrane according to [3], wherein the knitted string includes polyethylene terephthalate.
[5] A method for producing a porous hollow fiber membrane according to any one of [1] to [4],
Polyethylene having a viscosity average molecular weight of 1 million or more and a solvent are heated to 220 ° C. or more, and melt-kneaded using an extruder having a cylinder length L to cylinder diameter D of 10 or more (L / D). A melt-kneading step for producing a membrane stock solution;
A cooling and solidification step in which the film-forming stock solution is cooled and solidified to form a porous membrane layer by heat-induced phase separation; and
A method for producing a porous hollow fiber membrane having
[6] The method for producing a porous hollow fiber membrane according to [5], comprising 90% by mass or more of the polyethylene and the solvent with respect to 100% by mass of the membrane-forming stock solution.
[7] The method for producing a porous hollow fiber membrane according to [5] or [6], wherein the polyethylene is contained in an amount of 5% by mass to 20% by mass with respect to 100% by mass of the membrane-forming stock solution.
[8] The production of the porous hollow fiber membrane according to any one of [5] to [7], wherein the pore size of the porous hollow fiber membrane is controlled by the viscosity average molecular weight of the polyethylene. Method.
[9] The film-forming stock solution is further discharged from a flow path between an inner pipe and an outer pipe of a double-tube nozzle, and further includes a discharge step of discharging the solvent from the flow path in the inner pipe.
The method for producing a porous hollow fiber membrane according to any one of [5] to [8], wherein in the cooling and solidifying step, the membrane-forming stock solution that has passed through the discharging step is formed.
[10] The porous hollow fiber membrane includes the porous membrane layer located on the outer surface side, and a hollow support located on the inner surface side,
A stacking step of discharging the film-forming stock solution from a flow path between an inner tube and an outer tube of a double-tube nozzle, discharging the support from the inner tube, and laminating the film-forming stock solution on the support; Have
The method for producing a porous hollow fiber membrane according to any one of [5] to [8], wherein in the cooling and solidifying step, the membrane-forming stock solution that has undergone the laminating step is formed.
[11] A water purification method for filtration using the porous hollow fiber membrane according to any one of [1] to [4].

  According to the porous hollow fiber membrane of the present invention, high filtration performance can be maintained over a long period of time.

It is sectional drawing which shows typically the porous hollow fiber membrane which is one Embodiment of the porous hollow fiber membrane which concerns on this invention. It is the figure which expanded a part of sectional drawing of FIG.

  Hereinafter, a mode for carrying out the present invention (hereinafter referred to as “the present embodiment”) will be described in detail. The following embodiments are examples for explaining the present invention, and are not intended to limit the present invention to the following contents. The present invention can be implemented with appropriate modifications within the scope of the gist.

[Porous hollow fiber membrane]
The porous hollow fiber membrane according to the present embodiment includes a porous membrane layer containing polyethylene having a viscosity average molecular weight of 1,000,000 or more, a pure water flux is 5,000 LMH or more, and a membrane scratch resistance is 80% or more. . The porous membrane layer has a pore diameter of 0.3 μm or more and 1.0 μm or less, a pore aspect ratio of 10 or less, and a porosity of 70% or more.

  FIG. 1 schematically shows a cross section of a porous hollow fiber membrane which is an example of a porous hollow fiber membrane according to the present embodiment. The porous hollow fiber membrane shown in FIG. 1 has a two-layer structure. One porous membrane layer is a porous membrane layer 1a having an outer surface FA, and the other hollow support is a hollow portion 1c. And a braided string 1b having an inner surface FB. The porous membrane layer 1a is made of a thermoplastic resin, and the knitted string 1b is made of a hollow knitted string. In addition, the porous membrane layer 1a may contain up to about 5% by mass of components (impurities and the like) other than the thermoplastic resin.

  In addition, although the porous hollow fiber membrane of the two-layer structure was illustrated here, the porous hollow fiber membrane may be a single layer structure of the porous membrane layer. Further, the porous hollow fiber membrane shown in FIG. 1 is premised on the adoption of an external pressure filtration method, but in the present embodiment, an internal pressure filtration method may be adopted. When the internal pressure filtration method is adopted, A single layer structure of the porous membrane layer is preferable from the viewpoint of production.

  FIG. 2 is an enlarged view of a part of the cross section shown in FIG. 1, and shows a part of the separation membrane portion 1a and the braid 1b shown in FIG. The separation membrane shown in FIG. 2 includes a skin layer and a porous membrane layer as the porous membrane layer 1a, and a braided filament that is assembled as a vertical cord and a braided filament as a braided cord 1b. It consists of.

  The inner diameter of the porous hollow fiber membrane is preferably 0.4 mm or more and 5 mm or less. If the inner diameter is 0.4 mm or more, the pressure loss of the liquid flowing in the porous hollow fiber membrane can be prevented, and if it is 5 mm or less, sufficient compressive strength and bursting strength are expressed with a relatively thin film thickness. It's easy to do. This inner diameter is more preferably 0.5 mm or more and 3.0 mm or less, and further preferably 0.6 mm or more and 1.0 mm or less.

  The thickness of the porous hollow fiber membrane of this embodiment is preferably 100 μm or more and 500 μm or less, more preferably 200 μm or more and 300 μm or less. When the thickness is 100 μm or more, the strength against compression becomes higher, and when the thickness is 500 μm or less, the pressure loss due to the membrane resistance can be further reduced.

  The pure water flux (water permeability), which is an index for the water permeability of the porous hollow fiber membrane, is to inject pure water into the hollow portion of the porous hollow fiber membrane and measure the amount of pure water that permeates from the hollow portion to the outer surface. Can be determined. In detail, it can measure according to the method described in the following Example.

  The pure water flux of the porous hollow fiber membrane of this embodiment is 5000 LMH or more, preferably 5,000 LMH or more and 30,000 LMH or less, more preferably 5,000 LMH or more and 20,000 LMH or less. When the pure water flux is 5000 LMH or more, water permeability during actual filtration can be easily maintained, and when it is 30,000 LMH or less, bacterial groups such as Escherichia coli can be blocked. In order to obtain a porous hollow fiber membrane having a pure water flux in such a range, for example, the spinning conditions such as adjusting the polymer concentration and adjusting the spinning temperature may be adjusted.

  The membrane surface scratch resistance, which is an index for the scratch resistance of the porous hollow fiber membrane, is measured by placing the porous hollow fiber membrane in sandblast and measuring the amount of pure water permeation after 10 minutes. Can be measured by. In detail, it can measure according to the method described in the following Example.

  The abrasion resistance surface abrasion rate of the porous hollow fiber membrane of this embodiment is 80% or more, preferably 80% or more and 100% or less, more preferably 90% or more and 100% or less. When the film scratch resistance is 80% or more, the scratch resistance is sufficiently good.

  The main cause of the decrease in water permeability due to rubbing is that the pores on the membrane surface are crushed by rubbing between adjacent porous hollow fiber membranes. By using polyethylene having a viscosity average molecular weight of 1 million or more as the main component of the porous hollow fiber membrane, the pores are crushed even if the adjacent membranes rub against each other due to the slidability and wear resistance of the material. Can be suppressed and the pore shape of the membrane surface can be maintained.

<Porous membrane layer>
The porous membrane layer of this embodiment contains polyethylene having a viscosity average molecular weight of 1,000,000 or more, and has a large number of pores on the surface. The porous membrane layer preferably has a network structure. Usually, the structure of the porous layer includes a network structure and a spherical structure, and the network structure having a larger volume of the connected portion is generally more than the spherical structure having a smaller volume of the connected thermoplastic resin. Has high elongation. Therefore, even if the braid portion changes in size and the porous structure is elongated, a network structure in which micro breakage or the like of the polymer connecting portion hardly occurs is preferable.

<Polyethylene>
The porous membrane layer of this embodiment includes polyethylene having a viscosity average molecular weight of 1 million or more. The polyethylene having a viscosity average molecular weight of 1 million or more is not particularly limited, but can be prepared by, for example, suspension polymerization using a metallocene catalyst.

  The polyethylene of this embodiment has a viscosity average molecular weight of 1 million or more, preferably 1.5 to 7 million, and more preferably 2 to 4 million. When the viscosity average molecular weight is 1,000,000 or more, the wear resistance is improved. On the other hand, when the viscosity average molecular weight is 7 million or less, the viscosity is low and spinning is easy.

<Viscosity average molecular weight (Mv)>
About the viscosity average molecular weight of the polyethylene of this embodiment, it calculated | required by the method shown below according to ISO1628-3 (2010). First, 20 mg of powdered polyethylene was weighed in a melting tube, and the melting tube was replaced with nitrogen, and then 20 mL of decahydronaphthalene (with 1 g / L of 2,6-di-t-butyl-4-methylphenol added) And stirred at 150 ° C. for 2 hours to dissolve the powdered polyethylene. The solution was measured in a thermostatic bath at 135 ° C. using a Canon-Fenske viscometer (manufactured by Shibata Kagaku Kikai Kogyo Co., Ltd .: product number-100) for the drop time (ts) between the marked lines. Similarly, the drop time (ts) between the marked lines was measured in the same manner for samples in which the amount of polyethylene powder was changed to 10 mg, 5 mg, and 2.5 mg. The dropping time (tb) of only decahydronaphthalene without powdered polyethylene as a blank was measured. The reduced viscosity (ηsp / C) of the powdered polyethylene determined according to the following formula (1) is plotted, and the concentration (C) (unit: g / dL) and the reduced viscosity of the ultrahigh molecular weight ethylene copolymer powder are plotted. A linear equation of (ηsp / C) was derived, and the intrinsic viscosity ([η]) extrapolated to a concentration of 0 was determined.
ηsp / C = (ts / tb−1) /0.1 (unit: dL / g) (1)
Next, the viscosity average molecular weight (Mv) was calculated using the value of the intrinsic viscosity [η] using the following formula (2).
Mv = (5.34 × 104) × [η] 1.49 (2)

  The pore diameter, pore aspect ratio, and porosity of the porous membrane layer of the present embodiment can be obtained by, for example, taking an SEM image of the porous membrane layer using an electron microscope and analyzing the image. it can. In detail, it can measure according to the method described in the following Example.

  The pore diameter of the porous membrane layer of this embodiment is 0.3 μm or more and 1.0 μm or less, preferably 0.1 μm or more and 2.0 μm or less, more preferably 0.3 μm or more and 1.0 μm or less. When this pore diameter is 0.3 μm or more, more excellent water permeability can be obtained, and when it is 1.0 μm or less, turbid components contained in the suspended water are not easily removed. In order to obtain a porous membrane layer having a pore diameter in such a range, for example, the spinning conditions such as adjusting the polymer concentration and adjusting the spinning temperature may be adjusted.

  The aspect ratio of the pores of the porous membrane layer of this embodiment is 10 or less, preferably 2 or more and 20 or less, more preferably 4 or more and 10 or less. When the aspect ratio of the pores is 10 or less, the pore diameter distribution tends to be narrowed, and when the aspect ratio is 2 or more, the compression resistance strength tends to increase. The aspect ratio of the pore is the ratio of the major axis to the minor axis of the pore (major axis / minor axis), so the lower limit is 1.0. In order to obtain a porous hollow fiber membrane having an aspect ratio in such a range, for example, the spinning conditions such as adjusting the polymer concentration and adjusting the spinning temperature may be adjusted.

  The porosity of the porous membrane layer of this embodiment is 70% or more, preferably 70% or more and 90% or less, more preferably 70% or more and 80% or less. When the porosity is 70% or more, water permeability is easily obtained, and when it is 90% or less, mechanical strength is easily obtained. In order to obtain a porous hollow fiber membrane having a porosity in such a range, for example, the spinning conditions such as adjusting the polymer concentration and adjusting the spinning temperature may be adjusted.

<Support>
As the porous hollow fiber membrane according to this embodiment, from the viewpoint of external pressure filtration, it is preferable to include the porous membrane layer located on the outer surface side and the hollow support located on the inner surface side. .

  Although it does not specifically limit as a hollow support body, For example, what comprised the braided string (braid), the nonwoven fabric, etc. in the tubular shape is mentioned. The support made of knitted string is formed by assembling a plurality of yarns (multifilament yarns) formed by bundling one fiber or two or more fibers into a tubular shape. It is preferable that the support is a braided string because the pressure resistance is high.

  Specific examples of materials contained in the hollow support include nylons, nylon 66, polyamides such as aromatic polyamide, polyesters such as polyethylene terephthalate, polylactic acid, and polyglycol, and polyolefins such as polyethylene and polypropylene. , Polyvinyl chloride such as polyvinyl chloride and polyvinylidene chloride, polyfluorine such as polytetrafluoroethylene and polyvinylidene fluoride, polyvinyl alcohol, polyacrylonitrile, polyuric acid, polyalkylene paraoxybenzoate, and polyurethane Synthetic polymer materials such as cellulose, protein, natural polymer materials such as seed wool fibers and asbestos; inorganic materials such as metal fibers, carbon fibers and silicate fibers; and combinations of the above materials It is done. These can be selected appropriately depending on the application. In applications such as water treatment, synthetic polymer materials are preferred, and polyethylene terephthalate is more preferred because of its high cost and flexibility in fiber shape.

  The thickness of the fiber is not particularly limited, but is preferably 1 μm or more and 100 μm or less in diameter. If the diameter is 1 μm or more, the surface fluffing can be further suppressed and higher adhesiveness with the porous layer can be exhibited. If the diameter is 100 μm or less, the resulting braid is firmly assembled and exhibits higher compressive strength. it can. In the case of a multifilament yarn, the number of fibers in one yarn is preferably 10 or more and 1000 or less. If the number is 10 or more, the flexibility of the multifilament yarn and the braid made of the yarn becomes higher, and as a result, a porous hollow fiber membrane having a high cleaning effect that is easily shaken by air scrubbing or the like is obtained. On the other hand, if it is 1000 or less, the multifilament yarn does not become too thick, and a braid having higher compressive strength can be obtained. The number of braids is preferably 5 or more and 100 or less. If it is 5 or more, the resulting braid can exhibit higher compressive strength, and if it is 100 or less, the structural change due to shrinkage can be suppressed to a more preferable range.

  The cross-sectional shape of the support in the present embodiment is not limited to the circular shape as shown in FIG. 1, and the effect can be exhibited in various shapes such as a triangular shape, a quadrangular shape, and other irregular shapes. From the viewpoint of productivity and durability against crushing of the hollow portion, a circular shape is preferable.

[Method for producing porous hollow fiber membrane]
The manufacturing method of the porous hollow fiber membrane of this embodiment is demonstrated. The method for producing a porous hollow fiber membrane according to the present embodiment includes a melt-kneading step in which polyethylene and a solvent are melt-kneaded to produce a membrane-forming stock solution, and the membrane-forming stock solution is cooled and solidified to form a porous membrane layer. Cooling and solidifying step. As a specific example, using an extruder equipped with a cylinder having a viscosity average molecular weight of 1,000,000 or more and a solvent heated to 220 ° C. or more and a ratio of length L to diameter D (L / D) of 10 or more. Manufacturing comprising a melt-kneading step in which a film-forming stock solution is prepared by melt-kneading, and a cooling-solidifying step in which the film-forming stock solution is cooled and solidified to form a film by a heat-induced phase separation method to form a porous membrane layer By the method, a porous hollow fiber membrane can be produced.

[Melting and kneading process]
In the melt-kneading step of this embodiment, polyethylene and a solvent are melt-kneaded by an extruder equipped with a cylinder having a length L to diameter D ratio (L / D) of 10 or more to produce a film-forming stock solution. . The extruder is not particularly limited as long as the ratio of the cylinder length to the cylinder diameter D (L / D) is 10 or more, but L / D is preferably 10 or more and 100 or less, and preferably 10 or more and 50. The following is more preferable. Moreover, since an extruder strengthens kneading | mixing, the extruder whose cylinder is biaxial is preferable.

  For example, Patent Document 4 discloses a method for producing a flat film using ultrahigh molecular weight polyethylene as a raw material. In order to be suitably used as a separation membrane, a porous hollow fiber membrane is preferable to a flat membrane because the membrane area can be increased even with the same footprint. In order to form a porous hollow fiber membrane, the hollow fiber membrane is obtained by using a medium shear as described in Patent Document 4 (L (cylinder length) / D (cylinder diameter) of the twin screw extruder is about 6). For use in molding, the compatibility between the solvent and the ultra high molecular weight polyethylene is not sufficient. Therefore, it is preferable to discharge the film forming stock solution from a double tube nozzle provided in the extruder through shearing in a cylinder having an L / D of 10 or more.

<Solvent>
The three-dimensional solubility parameter P (determined according to the following formula (3)) of the solvent of this embodiment is preferably less than 3.0, more preferably less than 2.0, and even more preferably less than 1.8. And particularly preferably less than 1.5. When this value is less than 3.0, a film-forming stock solution in which polyethylene is sufficiently dissolved or dispersed in a solvent can be obtained.
P = ((σ _dm −σ _dp ) ² + (σ _pm −σ _pp ) ² + (σ _hm −σ _hp ) ² ) ^1/2 (3)
(In formula (3), σ _dm and σ _dp represent the dispersion force terms of the solvent and polyethylene, σ _pm and σ _pp represent the dipole binding force terms of the solvent and polyethylene, respectively, and σ _hm and σ _hp represent the solvent. And hydrogen bond terms of polyethylene respectively.)

  For example, a solvent having a three-dimensional solubility parameter P represented by the above formula of less than 3.0, such as di-2-ethylhexyl phthalate (DOP), diisononyl phthalate (DINP), and diisodecyl phthalate (DIDP) is suitable. Can be used.

  The total content of polyethylene and the solvent contained in the film-forming stock solution is preferably 90% by weight or more, more preferably 80% by weight or more and 100% by weight or less, more preferably 100% by weight of the film-forming stock solution. Is 90% by mass or more and 100% by mass or less. When the content is 90% by mass or more, the original properties of the polymer are easily obtained.

  The content of polyethylene contained in the film-forming stock solution is preferably 5% by mass or more and 20% by mass or less, more preferably 10% by mass or more and 20% by mass or less, and still more preferably, with respect to 100% by mass of the film-forming stock solution. Is 10 mass% or more and 18 mass% or less. When this value is 5% by mass or more, higher mechanical strength is easily obtained. On the other hand, when it is 20% by mass or less, the viscosity of the melt-kneaded product becomes lower, so that the porous membrane layer can be easily formed.

  The size of the pore diameter of the porous hollow fiber membrane can be controlled by the size of the viscosity average molecular weight of polyethylene. For example, by increasing the viscosity average molecular weight of polyethylene, the pore diameter can be increased without greatly changing the porosity.

  The film-forming stock solution is composed of two components of polyethylene and a solvent, and may be composed of three components of polyethylene, a solvent and an inorganic fine powder. Ingredients may be included. When inorganic fine powder is used, the primary particle size of the inorganic fine powder contained in the film-forming stock solution is preferably 50 nm or less, more preferably 5 nm or more and 20 nm or less. In addition, when manufacturing a porous membrane layer using the membrane forming stock solution containing inorganic fine powder, the manufacturing method of a porous hollow fiber membrane further has the process of extracting and removing inorganic fine powder after a cooling solidification process. Is preferred.

  Specific examples of the inorganic fine powder include silica fine powder, titanium oxide, and lithium chloride. Among these, silica fine powder is preferable from the viewpoint of cost. The above-mentioned “primary particle size of inorganic fine powder” means a value obtained from analysis of an electron micrograph. That is, a group of inorganic fine powder is first pretreated by the method of ASTM D3849. Then, the particle diameter of 3000-5000 inorganic fine powders copied to the transmission electron micrograph is measured, and the primary particle diameter of the inorganic fine powders is calculated by arithmetically averaging these values.

  In the melt-kneading step, the polyethylene and the solvent are heated to 220 ° C. or higher and melt-kneaded. In that case, it is preferably 200 ° C. or higher and 300 ° C. or lower, more preferably 200 ° C. or higher and 250 ° C. or lower. By heating to 220 ° C. or higher, kneading is further strengthened. On the other hand, heating to 300 ° C. or lower can suppress decomposition of the polymer. Heating can be performed by using an electric heater for the extruder.

[Discharge process]
In the method for producing a porous hollow fiber membrane of the present embodiment, for example, the porous membrane layer can be formed in a hollow shape by a discharge step of discharging a film-forming stock solution from a nozzle. Moreover, it is preferable from a viewpoint of quality improvement that said nozzle is a double tube nozzle. As a discharge process using a double tube nozzle, for example, a film forming stock solution is discharged from a flow channel between an inner tube and an outer tube of a double tube nozzle, and a solvent is discharged from a flow channel in the inner tube, And those that discharge gas from the flow path in the inner pipe, but those that discharge the solvent from the flow path in the inner pipe are more preferable from the viewpoint of reducing coating film defects. The film-forming stock solution that has undergone this discharge step is formed by a cooling and solidification step that will be described later.

[Lamination process]
In the method for producing a porous hollow fiber membrane of the present embodiment, for example, the porous membrane layer can be formed on the support by a laminating process in which a membrane-forming stock solution is discharged and laminated on the support. Moreover, it is preferable from a viewpoint of a quality improvement that it is the lamination process which discharges | stacks a film forming undiluted solution and a support body from a double tube nozzle, and laminates | stacks. When the porous hollow fiber membrane includes a porous membrane layer located on the outer surface side and a hollow support located on the inner surface side, for example, as a laminating step using a double tube nozzle, for example, a membrane-forming stock solution Is discharged from the flow path between the inner tube and the outer tube of the double tube nozzle, the support is discharged from the inner tube, and the film-forming stock solution is laminated on the support, resulting in a decrease in coating film defects. preferable. The film-forming stock solution that has undergone this laminating process is formed into a film by a cooling and solidification process that will be described later.

  For example, Patent Document 5 discloses a method of coating a porous membrane layer on a braid. When this method is adopted, the film-forming stock solution is discharged from the flow path between the inner tube and the outer tube of the double tube nozzle, and the braid is discharged from the inner tube to perform spinning. If the braid is taken out in accordance with the discharge of the membrane-forming stock solution, a porous hollow fiber membrane in which the porous membrane layer is coated on the braid can be obtained. The porous hollow fiber membrane thus obtained can achieve both higher opening ratio and higher strength.

[Cooling and solidification process]
In the cooling and solidification step of the present embodiment, the porous membrane layer can be formed by cooling and solidifying the film-forming stock solution to form a film by a thermally induced phase separation method. As a method for forming a porous membrane layer, a dry / wet method (non-solvent phase separation method) in which a phase separation is caused by contact with a non-solvent to form a porous layer, and a phase separation is caused by cooling to produce a porous layer. There is a heat-induced phase separation method for forming a porous layer. Among these, a thermally induced phase separation method is preferable because a homogeneous membrane can be obtained. In the thermally induced phase separation method, for example, phase separation can be caused by immersing the membrane forming stock solution in water at about 30 ° C.

  In this way, the porous hollow fiber membrane of this embodiment provided with a porous membrane layer can be obtained. In addition, the manufacturing method of this embodiment may have the other process which the manufacturing method of a porous hollow fiber membrane can have besides the above, unless the effect of this invention is inhibited.

[Water purification method]
The water purification method of the present embodiment performs filtration using the porous hollow fiber membrane described above. Examples of the suspended water to be filtered include natural water, domestic wastewater, and treated water thereof. Examples of natural water include river water, lake water, groundwater, and seawater. Treated water obtained by subjecting these natural waters to sedimentation treatment, sand filtration treatment, coagulation sedimentation sand filtration treatment, ozone treatment, activated carbon treatment, and the like is also included in the suspension water to be treated. An example of domestic wastewater is sewage, for example. Sewage primary treated water that has been subjected to screen filtration and sedimentation treatment, sewage secondary treated water that has been subjected to biological treatment, and tertiary that has undergone treatment such as coagulation sedimentation sand filtration, activated carbon treatment and ozone treatment. Treated (highly treated) water is also included in the suspension water to be treated. Suspended water to be treated can include turbidity (humic colloid, organic colloid, clay, bacteria, etc.) composed of fine organic substances, inorganic substances, and organic-inorganic mixtures of the order of μm or less. It can also be used to concentrate and purify raw water containing relatively hard particles such as polishing wastewater.

The water quality of suspended water (such as the above-mentioned natural water, domestic wastewater, and treated water thereof) can be generally expressed by turbidity and organic substance concentration, which are representative water quality indicators, alone or in combination. When the water quality is classified by turbidity (average turbidity, not instantaneous turbidity), it is roughly divided into low turbid water with turbidity less than 1, turbid water with turbidity of 1 to less than 10, high turbidity with turbidity of 10 to less than 50, It can be classified into ultra-turbid water with turbidity of 50 or more. Moreover, when water quality is classified by organic substance concentration (total organic carbon (TOC): mg / L) (average value, not instantaneous value), it is roughly less than 1 low TOC water, 1 or more 4 It can be classified into less than medium TOC water, 4 or more and less than 8 high TOC water, and 8 or more ultra-high TOC water. Basically, the higher the turbidity or TOC, the easier it is to clog the filtration membrane. Therefore, the higher the turbidity or TOC, the higher the turbidity or TOC, the porous hollow fiber membrane of this embodiment (for example, the porous hollow fiber shown in FIG. 1). The effect of using the membrane 10, 20) is increased.
More specifically, the present invention provides a method for removing turbidity of natural water, domestic wastewater, and suspension water, which is treated water, by membrane filtration, and has little deterioration in water permeability due to membrane clogging. An object of the present invention is to provide a porous hollow fiber membrane with little deterioration in water permeability due to rubbing.

  Hereinafter, although an Example is given and this embodiment is described in detail, this embodiment is not limited at all unless it exceeds the summary. Each physical property value in Examples and Comparative Examples was measured and evaluated by the following methods.

(1) Aperture ratio, porosity, hole diameter and aspect ratio Using an electron microscope (product name “SU8000 series”) manufactured by HITACHI, scanning electron microscope (SEM) images of the surface and cross section of the film at an acceleration voltage of 3 kV The photo was taken at 5000x. A cross-sectional sample for an electron microscope was obtained by cutting a membrane sample frozen in ethanol into round slices. Next, using the image analysis software Winroof 6.1.3 (product name, manufactured by Mitani), “noise removal” of the SEM image in the area of 100 mm × 100 mm is performed by the numerical value “6”, and further by a single threshold value. By binarization, binarization was performed using “threshold value: 105”. The aperture ratio was determined by determining the occupied area ratio of the binarized image on the surface of the film thus obtained, and the porosity was determined by determining the occupied area ratio of the binarized image of the cross section of the film. The above measurement was performed on 10 SEM images, and the average value was used.
For the hole diameter, in the SEM image in the 100 mm × 100 mm region of the surface, the hole area of each hole is added in order from the smallest hole diameter to each hole existing on the surface, and the sum is the hole area of each hole. Was determined by the hole diameter at which the hole reached 50% of the total sum.
As for the aspect ratio of the pore, when measuring the pore diameter, the major axis and minor axis of the pore are measured, and the aspect ratio is calculated by (major axis) / (minor axis). About 100 outer surface holes were measured, an arithmetic arithmetic average was obtained, and the value was taken as the aspect ratio of the outer surface holes.

(2) Pure water flux After being immersed in ethanol, one end of a wet hollow fiber membrane having a length of about 10 cm, which was repeatedly immersed in pure water several times, was sealed, and an injection needle was inserted into the hollow portion from the other end. Pure water at 25 ° C. is injected into the hollow portion from the injection needle at a pressure of 0.1 MPa in an environment of ° C., the amount of pure water permeating through the outer surface is measured, and the pure water flux is expressed by the following equation (4). Were determined.
Pure water flux [L / m ² / h (LMH)] = 60 × (permeated water amount [L]) / {π × (membrane outer diameter [m]) × (membrane effective length [m]) × (measurement time [ min])} (4)
Here, the membrane effective length refers to the net membrane length excluding the portion where the injection needle is inserted.

(3) Film Surface Abrasion Rate The film surface abrasion rate is an index for judging the degree of water permeability performance deterioration due to film surface abrasion. After dipping in ethanol, wet hollow fiber membranes (approximately 10 cm long) that have been dipped in pure water several times are arranged on a metal plate and fine sand (particle size 130 μm, product name “Fuji Brown FRR # 120”). ) Suspended water in which 20% by mass was suspended in water was sprayed at a pressure of 0.07 MPa from a nozzle set 70 cm above the membrane and sprayed on the outer surface of the membrane. After spraying for 10 minutes, the membrane was turned over and suspended water was sprayed for 10 minutes under the same conditions. Before and after spraying, the pure water flux was measured by performing the same operation as that after sealing one end of the wet hollow fiber membrane in “(2) Pure water flux”. The rate was determined.
Scratch resistance [%] = 100 × (pure water flux after spraying [LMH]) / (pure water flux before spraying [LMH]) (5)

(4) Permeability performance retention rate during suspension water filtration The permeation performance retention rate during suspension water filtration is an index for judging the degree of water permeation performance degradation due to clogging (fouling). After dipping in ethanol, filtration was performed by an external pressure method with a membrane effective length of 11 cm using a wet hollow fiber membrane that was repeatedly dipped in pure water several times. First, pure water was filtered at a filtration pressure of 10 m ³ per day per 1 m ² of the outer membrane surface area, the permeated water was collected for 2 minutes, and the amount of the collected water was defined as the initial pure water permeability. Next, the river surface water (Fuji River surface water: turbidity 2.2, TOC concentration 0.8 ppm), which is a natural suspension water, is filtered for 10 minutes at the same filtration pressure as when the initial pure water permeability was measured, The permeated water was collected for 2 minutes from 8 minutes to 10 minutes after the start of filtration, and the amount of the collected water was taken as the water permeability during suspension water filtration. From the water permeability, the water permeability retention rate during suspension water filtration was calculated by the following formula (6). All operations were performed at 25 ° C. and a film surface linear velocity of 0.5 m / sec.
Permeability retention ratio during suspension water filtration [%] = 100 × (water permeability during suspension water filtration [g]) / (initial pure water permeability [g]) (6)
In addition, each parameter in Formula (6) is computed from following formula (7), (8), (9).
Filtration pressure = {(input pressure [MPa]) + (output pressure [MPa])} / 2 (7)
Surface area outside membrane [m ² ] = π × (outer diameter of hollow fiber membrane [m]) × (effective length of hollow fiber membrane [m]) (8)
Membrane surface linear velocity [m / s] = 4 × (filtered water amount [m ³ / s]) / {π × (hollow fiber membrane inner diameter [m]) ² −π × (membrane outer diameter [m]) ² } (9)

In this measurement, the filtration pressure of the suspended water is not the same for each membrane, but the initial pure water permeability (also the permeability at the time of suspension water filtration start) is the filtration pressure that permeates 10 m ³ per day per 1 m ^{2 of the} membrane surface area. Set. This is because in actual water treatment and sewage treatment, the membrane is usually used in quantitative filtration operation (a method in which the filtration pressure is adjusted so that a constant amount of filtered water is obtained within a certain period of time). Therefore, in this measurement, the deterioration of water permeability performance can be compared under conditions as close as possible to the conditions of quantitative filtration operation within the range of measurement using one hollow fiber membrane.

(5) HSP distance (three-dimensional solubility parameter)
The HSP distance [dPVDF-d solvent] is “Hansen, Charles (2007) Hansen Solubility Parameters: A user's handbook, Second Edition. It was determined by the method described.

[Example 1]
A porous membrane of Example 1 was obtained using a double-structure spinning nozzle (double tube nozzle). Specifically, first, as a thermoplastic resin, 12.5% by mass of ultra high molecular weight polyethylene (manufactured by Asahi Kasei Chemicals, product name “UH-900”, viscosity average molecular weight: 3.3 × 10 ⁶ ), and bisphthalate (2-ethylhexyl) (DEHP) 87.5 mass% was prepared.

These were melt-kneaded at 240 ° C. with a twin-screw kneading extruder (TEM-37, Toshiba Machine, L / D: 32) to obtain a film forming stock solution in the extruder. Next, at 240 ° C., the above-described flow path between the inner tube and the outer tube of the double tube nozzle (outer layer outermost diameter 2.0 mm, hollow portion forming layer outermost diameter 0.9 mm) installed at the exit of the extruder A hollow fiber-shaped molded product was obtained by discharging the film-forming stock solution and discharging air from the flow path in the inner tube. At this time, the shear rate obtained by dividing the discharge flow rate of the film forming solution by the film forming solution passing cross-sectional area of the double tube nozzle was set to 500 to 5,000 s ⁻¹ .

  The discharged (extruded) hollow fiber-shaped molded article was idled at a distance of 50 mm, and then thermally induced phase separation was allowed to proceed in water at 30 ° C. The hollow fiber-like molded product after the heat-induced phase separation was taken up at a speed of 30 m / min and wound up in a skein. The hollow fiber-shaped molding after winding was immersed in isopropyl alcohol to extract and remove bis (2-ethylhexyl) phthalate to obtain a porous hollow fiber membrane.

  Table 1 shows the blending composition, production conditions, and various performances of the obtained porous hollow fiber membrane.

[Example 2]
A porous hollow fiber membrane of Example 2 was obtained in the same manner as in Example 1 except that diisodecyl phthalate (DIDP) was used in place of bis (2-ethylhexyl) phthalate as a solvent. Table 1 shows the blending composition, production conditions, and various performances of the obtained porous hollow fiber membrane.

[Example 3]
A film was formed in the same manner as in Example 1 except that a polyester braided string (BO00B manufactured by Bonsing, inner diameter: 1.2 mm, outer diameter: 1.9 mm) was used for the inner pipe of the double pipe nozzle. 3 porous hollow fiber membranes were obtained. Table 1 shows the composition, production conditions, and various performances of the porous hollow fiber membrane of Example 3 obtained.

[Example 4]
The porous hollow of Example 4 was formed in the same manner as in Example 1 except that a stainless steel mesh pipe (40 mesh wire mesh formed into an inner diameter of 1.0 mm and an outer diameter of 2.0 mm) was used for the inner layer. A yarn membrane was obtained. Table 1 shows the composition, production conditions, and various performances of the porous hollow fiber membrane of Example 4 obtained.

[Examples 5 to 8]
As ultra-high molecular weight polyethylene, instead of the one having a viscosity average molecular weight of 3.3 × 10 ⁶ , one having a viscosity average molecular weight of 1.0 × 10 ⁶ (manufactured by Asahi Kasei Chemicals Co., Ltd., product name “UH650”), viscosity average molecular weight 0 × 10 ⁶ (Asahi Kasei Chemicals, product name “UH850”), viscosity average molecular weight 4.3 × 10 ⁶ (Asahi Kasei Chemicals, product name “UH950”), or viscosity average molecular weight 6.0 A porous hollow fiber membrane of Examples 5, 6 and 7 was obtained in the same manner as in Example 1 except that the x10 ⁶ product (product name “UH970” manufactured by Asahi Kasei Chemicals Corporation) was used. . In Table 1, the compounding composition of the porous hollow fiber membrane of obtained Examples 5-7, manufacturing conditions, and various performance are shown. Thus, by adjusting the viscosity average molecular weight of the ultra-high molecular weight polyethylene used as the raw material, the pore size could be controlled without changing the scratch resistance.

[Comparative Example 1]
Example 1 except that high density polyethylene (manufactured by Asahi Kasei Chemicals Corporation, product name “SH-800”, viscosity average molecular weight: 2.4 × 10 ⁵ ) was used as the thermoplastic resin instead of ultrahigh molecular weight polyethylene. In the same manner, a porous hollow fiber membrane of Comparative Example 1 was obtained. Table 1 shows the blending composition, production conditions, and various performances of the obtained porous hollow fiber membrane.

  As described above, it can be seen that the porous hollow fiber membrane having a membrane scratch resistance of 80% or more has excellent scratch resistance and high water permeability retention.

  According to the porous hollow fiber membrane of the present invention, a porous hollow fiber membrane that can maintain high filtration performance over a long period of time, a method for producing the same, and a water purification method using the porous hollow fiber membrane can be obtained. The present invention has industrial applicability in fields such as water treatment.

  DESCRIPTION OF SYMBOLS 1 ... Porous hollow fiber membrane, 1a ... Porous membrane layer, 1b ... Knitted string, 1c ... Hollow part, FA ... Outer surface, FB ... Inner surface

Claims (10)

A porous membrane layer located on the outer surface side and containing polyethylene having a viscosity average molecular weight of 1,000,000 or more, and a hollow support located on the inner surface side ,
The porous membrane layer has a pore diameter of 0.3 μm or more and 1.0 μm or less, a pore aspect ratio of 10 or less, and a porosity of 70% or more,
Pure water flux is 5000 LMH or more,
A porous hollow fiber membrane having a membrane scratch resistance of 80% or more. The porous hollow fiber membrane according to claim 1 , wherein the support is a knitted string. The porous hollow fiber membrane according to claim 2 , wherein the knitted string includes polyethylene terephthalate. It is a manufacturing method of the porous hollow fiber membrane according to any one of claims 1 to 3 ,
Polyethylene having a viscosity average molecular weight of 1 million or more and a solvent are heated to 220 ° C. or more, and melt-kneaded using an extruder having a cylinder length L to cylinder diameter D of 10 or more (L / D). A melt-kneading step for producing a membrane stock solution;
A cooling and solidification step in which the film-forming stock solution is cooled and solidified to form a porous membrane layer by heat-induced phase separation; and
A method for producing a porous hollow fiber membrane having The manufacturing method of the porous hollow fiber membrane of Claim 4 which contains the said polyethylene and the said solvent 90 mass% or more with respect to 100 mass% of said membrane forming stock solutions. The manufacturing method of the porous hollow fiber membrane of Claim 4 or 5 which contains 5 to 20 mass% of said polyethylene with respect to 100 mass% of said membrane-forming stock solutions. The manufacturing method of the porous hollow fiber membrane as described in any one of Claims 4-6 which controls the magnitude | size of the hole diameter of the said porous hollow fiber membrane with the magnitude | size of the viscosity average molecular weight of the said polyethylene. Discharging the film-forming stock solution from the flow path between the inner pipe and the outer pipe of the double-tube nozzle, and further discharging the solvent from the flow path in the inner pipe,
In the said cooling solidification process, the manufacturing method of the porous hollow fiber membrane as described in any one of Claims 4-7 which forms the said film forming undiluted solution which passed through the said discharge process. The porous hollow fiber membrane comprises the porous membrane layer located on the outer surface side, and a hollow support located on the inner surface side,
A stacking step of discharging the film-forming stock solution from a flow path between an inner tube and an outer tube of a double-tube nozzle, discharging the support from the inner tube, and laminating the film-forming stock solution on the support; Have
In the said cooling solidification process, the manufacturing method of the porous hollow fiber membrane as described in any one of Claims 4-7 which forms the said film forming undiluted solution which passed through the said lamination process. The water purification method which filters using the porous hollow fiber membrane as described in any one of Claims 1-3 .