RU2206376C2 - Method of manufacturing membrane-type tubular filter elements - Google Patents

Abstract

FIELD: chemical engineering. SUBSTANCE: manufacture of filter elements for ultra- and microfiltering of liquid mixtures for their concentration, separation, and purification of components consists in formation on open porous surface of liquid film from homogenous solution of 8-25 wt % fluoropolymer, 20-40 wt % nonsolvent, and, the rest, solvent, after which fluoropolymer is hardened to form selectively permeable membrane. Nonsolvent is selected from group including lower aliphatic alcohol, mixture of lower aliphatic alcohol with acetic acid, mixture of lower aliphatic alcohol with polyvinylpyrrolidone, and ternary mixture of lower aliphatic alcohol with acetic acid and polyvinylpyrrolidone. Substrate for membrane may be open-pore tubes from glass, carbon and organic plastic, ceramics or graphite, abc- plastic, and polyvinylchloride. EFFECT: enabled hydrophilization of membranes, enhanced their water permeability, and prevented pollution of filter elements. 2 cl, 1 tbl, 16 ex

Description

 The invention relates to a method for producing membrane tubular filtering elements for ultrafiltration and microfiltration of liquid mixtures for the purpose of concentration, separation and purification of their components.

 A known method of producing membrane filter elements (US patent 4810384, CL 210/636, 1989) by forming on a substrate a liquid film from a homogeneous 12.6-16 wt.% Solution of polyvinylidene fluoride, dimethylformamide and a non-solvent (1.5-3.5 wt. % lithium chloride and water) and curing of polyvinylidene fluoride to form a selectively permeable membrane film. The substrate on which the polyvinylidene fluoride solution is watered must be resistant to dimethylformamide. The membranes obtained by the specified method are stable when used in acidic and alkaline environments. However, their disadvantage is high hydrophobicity. Hydrophilization of such membranes is carried out in an oxygen plasma field. The process is complex, requires special expensive equipment and highly qualified service.

 A known method of producing membrane filter elements (ed. St. USSR 883100, class C 08 L 27/18, 1981) by forming on an inert substrate a liquid film from a homogeneous solution (5.8-8.1 wt.%) Of a tetrafluoroethylene-vinylidene fluoride copolymer , dimethylacetamide and a non-solvent (9.8-10.7 wt.% glycerol and 0.7-6.7 wt.% formic acid) and curing the copolymer to form a membrane. However, the copolymer solutions used have a low dynamic viscosity. It is difficult to obtain a tubular membrane uniform in thickness due to the self-draining of the solution. In addition, such solutions are unstable and easily turn into jelly. By the specified method receive only large-pore microfiltration membranes.

Closest to the claimed technical solution is a method of obtaining a membrane filter element (RF patent 2119817, CL 01 D 71/32, 27/06, 1998) by dissolving a copolymer of tetrafluoroethylene with vinylidene fluoride at a temperature of 30-50 ^o C in a volatile solvent (acetone) by mixing the resulting solution with a mixture of isopropyl alcohol and water at a temperature of 18-50 ^o With obtaining a working solution, impregnating it in a heated state at a speed of 1-10 m / min flat porous substrate with a pore size of 5 to 500 microns from non-woven material on poly based propylene or polyethylene terephthalate fibers, short-term (0.5-1.0 min) exposure to evaporate part of the solvent and partially cure the working solution and subsequent step drying at a temperature of 45-53, 55-65 and 90-100 ^o C. Content in the working solution , wt.%: copolymer of 7.5-11.5; acetone 69.1-65.3; water 8.5-6.5; isopropyl alcohol 16.4-15.0. The porous support must be inert with respect to the copolymer solvent.

 However, by the known method receive only microfilters with a pore size of 0.10-0.65 microns. The fluoropolymer working solutions used have a very low dynamic viscosity, seep through the substrate, due to the rapid self-draining, they are unsuitable for the manufacture of tubular filter elements. The presence of water in the working fluoropolymer solutions makes them unstable, with a decrease in temperature and a slight evaporation of the volatile solvent, they easily spontaneously turn into jelly and become unsuitable for the manufacture of the membrane. The process of forming the membrane structure is multistage, complex, energy-intensive.

 The technical problem to which the present invention is directed is to develop a simpler method for producing a wide range of tubular filter elements with a chemically resistant fluoropolymer, selectively permeable membrane by using stable, more viscous fluoropolymer working solutions suitable for forming membranes on long open-porous tubes, including those obtained based on polymers that are unstable to solvents, which are usually used for the manufacture of working solutions f orpolimerov.

 This problem is solved due to the fact that in the known method for producing membrane filter elements with a fluoropolymer membrane, a liquid film is formed on an open-porous substrate from a homogeneous solution of a fluoropolymer, a solvent and a non-solvent, and cured by a fluoropolymer to form a selectively permeable membrane, according to the invention, a solution is applied to the surface of an open-porous tube a fluoropolymer in which a lower aliphatic (isopropyl, ethyl or methyl) s is used as a non-solvent irt or its mixture (by weight) 1: 0.4-1.5 with acetic acid or 1: 0.016-0.180 with polyvinylpyrrolidone, or 1: 0.5-1.5: 0.015-0.035 with acetic acid and polyvinylpyrrolidone at the same time the following ratio of the components of the solution, wt.%: fluoropolymer 8-25, non-solvent 20-40, solvent - the rest.

 The solution to the problem was found in the use of a large (20-40 wt.%) Concentration of a mildly acting non-solvent. It turned out that for this lower aliphatic alcohols or mixtures thereof with acetic acid, or with polyvinylpyrrolidone, or with acetic acid and polyvinylpyrrolidone at the same time can be used. It was experimentally established that the concentrations and ratios of the components of the non-solvent indicated above are acceptable, and that additives to the lower aliphatic alcohol of acetic acid or polyvinylpyrrolidone, or their mixture, can simultaneously significantly change the operational characteristics of the obtained tubular filter elements with a fluoropolymer membrane, for example, water permeability.

 As membrane-forming fluoropolymers according to the invention, for example, soluble copolymers of vinylidene fluoride with tetrafluoroethylene or polyvinylidene fluoride are used, their solvent is acetone, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, tetrahydrofuran, etc., or mixtures thereof.

 An open-porous tube can be an open-porous tube made of any inert material (ceramic, graphite, stainless steel, glass, carbon or organoplastics, polypropylene, as well as polymers, for example, ABS plastic based on acrylonitrile butadiene styrene), which swell or dissolve in ordinary solvents for membrane-forming fluoropolymer.

 The open-porous tubes used in the proposed method have a length of up to 3 m, porosity of about 30%, average pore size of 5-20 μm and can be saturated with water, alcohol and other liquids that are soluble in water.

Preparation of a working solution of fluoropolymer in a solvent is carried out with stirring and heating to 40-80 ^o C until complete dissolution, then after lowering the temperature to 30-50 ^o C, a non-solvent is gradually added to the resulting solution.

 Filtering the finished working solution of the fluoropolymer is carried out through a material that traps foreign particles and gels.

 Dehydrated working solution is stored in a hermetically sealed container, usually at room temperature.

 Watering the working solution to the surface of open-porous tubes is carried out at speeds from 1 to 8 cm / s, while lower viscosity working solutions are applied at a higher speed.

 The curing of the fluoropolymer in the cast layer of the molding solution is carried out by dry or wet method. The air exposure of the liquid film of the working solution cast on the surface of an open-porous tube is from several seconds to 5 minutes.

 As a precipitation bath for curing the fluoropolymer membrane, water or its mixtures with a solvent and a non-solvent (up to 10 wt.% Of the latter) are used, which accumulate in water as a result of precipitation of the fluoropolymer from the working solution.

 Comparative analysis shows that the invention is distinguished by the novelty of the technical solution.

 The proposed method for producing membrane tubular filtering elements is characterized by the use of fluoropolymer solutions containing 20-40 wt. % non-solvent, which is used as the lower aliphatic alcohol or its mixture (by weight) with acetic acid 1: 0.4-1.5, or with polyvinylpyrrolidone 1: 0.016-0.180, or with acetic acid and polyvinylpyrrolidone simultaneously 1: 0, 5-1.5: 0.015-0.035. Despite the popularity of the use of isopropyl alcohol as a non-solvent in working fluoropolymer solutions to obtain selectively permeable membranes, its use in such large (20-40 wt.%) Concentrations, as well as in the above combinations with acetic acid or polyvinylpyrrolidone, or another is new at the same time.

 The authors experimentally established that the fluoropolymer working solutions obtained and used in the proposed method have a higher dynamic viscosity, less leakage and self-draining ability, are thermally stable, do not cause deformation of open-porous tubes based on polymers that are not resistant to solvents (acetone, dimethylacetamide, dimethylformamide , methylpyrrolidone, etc.), commonly used to obtain selectively permeable fluoropolymer membranes. As a result, it is possible to obtain a wide assortment of high-quality membrane tubular filtering elements based on open-porous tubes both from inert materials and from polymers that are unstable to solvents, usually used for the manufacture of working solutions of fluoropolymers, for example, from ABS plastic. Achieving the result became possible not from the prior art, but from the materials of this application for invention. It turned out that the use of a fluoropolymer working solution as a non-solvent for the production of a tubular membrane filter element of lower aliphatic alcohol or its mixtures with acetic acid, or polyvinylpyrrolidone, or with acetic acid and polyvinylpyrrolidone at the same time in the indicated ratios and concentrations allows to obtain the most suitable working solutions for manufacturing open tubular filter elements with the necessary viscosity, thermal stability, non-deforming openly porous tubes, as well as to regulate and improve in a wide range the operational properties of the resulting membrane tubular filter elements.

 No technical solutions have been identified that have features that match the distinctive features of the proposed solution. The mismatch of technical properties in terms of the positive effect of the claimed and known objects indicates that as a result there is a new set of features of the solution, leading to the emergence of a new property that ensures the achievement of a positive effect, which allows us to recognize the proposed method meets the criterion of "significant differences" and the inventive condition level.

 The invention is illustrated by the following examples.

 Example 1

150 g of acetone are poured into a flask with a stirrer and a hydraulic shutter placed in a thermostat and 51 g of a copolymer of tetrafluoroethylene (23-25 wt.%) With vinylidene fluoride (GOST 25428-82) of the F-42L brand are added to it. They include a stirrer and thermostat heating. The temperature in the flask was raised to 40 ^o C. The contents of the flask were stirred until the fluoropolymer was completely dissolved and cooled to 30 ^o C, then 99 g of isopropyl alcohol was gradually poured from the dropping funnel to the contents of the flask with stirring. The contents of the flask are further stirred for 60 minutes. The resulting fluoropolymer solution (wt.%: Fluoropolymer 17, solvent 50, non-solvent 33) is poured into the filter, filtered and dehydrated for a day. Ready working solution of fluoropolymer has a viscosity of 4.6 Pa • s, is stable when stored at room temperature. Using a special device, it is watered at a speed of 2 cm / s on the surface of an open-porous fiberglass tube with a layer thickness of 450 μm. The open-porous tube with the applied solution is lowered into a precipitation bath containing water at a temperature of 12 ^° C. After 30 minutes, the resulting membrane filter element is transferred to a washing bath, where it is washed three times with a fresh portion of water three times every 20 minutes. The water permeability test of the obtained ultrafiltration element for demineralized water at a pressure of 0.2 MPa and a temperature of 25 ^o C is 25 dm ³ / (m ² • h • at), after 5 hours from the start of testing 20 dm ³ / (m ² • h • at). The specified fluoropolymer solution was also deposited on an open-pore ABS plastic tube (ABS TU 2211-015-00203521-96 brand), and no deformation of the tube was noted (the ABS plastic tube placed in acetone completely decomposes after a few minutes and turns in gel).

 Example 2

A method of obtaining a membrane tubular filter element, as in example 1, but 186 g of acetone are loaded into the flask, then 54 g of copolymer of grade Ф42Л and then 60 g of isopropyl alcohol are added. The finished working solution of the copolymer (wt.%: Fluoropolymer 18, solvent 62, non-solvent 20) has a viscosity of 10 Pa • s and is stable during storage. Watering the solution is carried out at a speed of 3 cm / s on the surface of the carbon-fiber open-porous tube with a layer thickness of 450 microns. The water permeability of the obtained ultrafiltration element was 150 dm ³ / (m ² • h • at), after 5 hours from the start of the test 90 dm ³ / (m ² • h • at). When applying the solution to an open-pore ABS plastic tube, its deformation is not noted.

 Example 3

A method of obtaining a membrane tubular filter element, as in example 1, but 100 g of acetone, 24 g of copolymer of brand Ф42Л and then a mixture of 120 g of isopropyl alcohol and 56 g of acetone are loaded into the flask. The finished solution (wt.%: Fluoropolymer 8, solvent 52, non-solvent 40) has a viscosity of 2 Pa • s, is stable during storage, and the copolymer solution is watered at a speed of 6 cm / s on the surface of an open-porous fiberglass tube with a thickness of 450 μm. The water permeability of the obtained microfiltration element is 1700 dm ³ / (m ² • h • at), after 5 hours from the start of the test 810 dm ³ / (m ² • h • at). When applying the solution to an open-pore tube made of ABS plastic, no deformation is observed.

If the tube with the applied solution is left to rotate for 8 min in air, then the surface of the liquid film is blown with air for 1 min, an ultrafiltration element with a water permeability of 340 dm ³ / (m ² • h • at) is obtained, after 5 h from the start of the test, 80 dm ³ / (m ² • h • at).

 Example 4

A tubular filter element is obtained, as in example 1, by dissolving 90 g of a copolymer of brand Ф42Л in 240 g of dimethylacetamide, then add a mixture consisting of 125 g of ethyl alcohol and 45 g of demythylacetamide. The solution (wt.%: Fluoropolymer 18, solvent 57, non-solvent 25) has a viscosity of 35 Pa • s, is stable during storage, pouring it onto an open-pore ABS plastic tube is carried out at a speed of 2 cm / s. The water permeability of the obtained ultrafiltration element is 250 dm ³ / (m ² • h • at), after 5 hours from the start of the test 120 dm ³ / (m ² • h • at).

 Example 5

A tubular filter element is obtained, as in example 1, by dissolving 51.9 g of the copolymer in 143.1 g of acetone; add a mixture consisting of 75 g of isopropyl alcohol and 30 g of acetic acid. The solution (wt.%: Fluoropolymer 17.3, solvent 47.7, non-solvent 35; 1: 0.4 ratio of isopropyl alcohol to acetic acid) has a viscosity of 23 Pa • s, is stable during storage. The water permeability of the obtained ultrafiltration element based on ABS plastic is 520 dm ³ / (m ² • h • at), after 5 hours from the start of testing 190 dm ³ / (m ² • h • at). The open-pore ABS plastic tube did not deform when the solution was applied to it.

 Example 6

A tubular filter element is obtained, as in example 1, by dissolving 85 g of a copolymer of brand Ф42Л in 270 g of dimethylacetamide, then add a mixture consisting of 95 g of methyl alcohol and 50 g of acetic acid. The solution (wt.%: Fluoropolymer 17, dimethylacetamide 54, methyl alcohol 19, acetic acid 10; the ratio of alcohol to acetic acid 1: 0.53), has a viscosity of 150 Pa • s, is stable when stored by pouring it on an open-pored ABS pipe -plastic is carried out at a speed of 5 cm / s. The water permeability of the obtained ultrafiltration element is 350 dm ³ / (m ² • h • at), after 5 hours from the start of the test 200 dm ³ / (m ² • h • at).

 Example 7

A tubular filter element is obtained, as in Example 1, by dissolving 47.5 g of a copolymer of brand Ф42Л in 200 g of acetone, then add a mixture consisting of 50 g of isopropyl alcohol, 75 g of acetic acid and 127.5 g of acetone. The finished solution (wt.%: Fluoropolymer 9.5, solvent 65.5, non-solvent 25; alcohol: acetic acid ratio 1: 1.5) has a viscosity of 0.25 Pa • s, is stable during storage. The water permeability of the obtained microfiltration element is 1800 dm ³ / (m ² • h • at), after 5 hours from the start of testing 580 dm ³ / (m ² • h • at). When this solution is applied to an open-pore ABS plastic tube, its deformation is not noted.

 Example 8

A tubular filter element is obtained, as in Example 1, by dissolving 85 g of polyvinylidene fluoride (F22 brand fluoropolymer) in 260 g of dimethylacetamide, then pour a mixture consisting of 50 g of isopropyl alcohol, 50 g of acetic acid and 55 g of dimethylacetamide. The solution (wt.%: Fluoropolymer 17, solvent 63, non-solvent 20; alcohol to acetic acid ratio 1: 1), has a viscosity of 15.0 Pa • s, is stable during storage. The water permeability of the obtained ultrafiltration element using an open-porous ceramic tube 590 dm ³ / (m ² • h • at), after 5 hours from the start of the test 500 dm ³ / (m ² • h • at). When applying the solution to an open-pore ABS plastic tube, its deformation is not noted.

 Example 9

A tubular filter element is obtained, as in Example 1, by dissolving 1.8 g of polyvinylpyrrolidone with a molecular weight of 10,000 and 100 g of a copolymer of brand Ф42Л in 270 g of dimethylformamide, then a mixture consisting of 98.2 g of isopropyl alcohol and 30 g of dimethylformamide is added. The solution (wt.%: Fluoropolymer 20, solvent 60, non-solvent 20; alcohol to polyvinylpyrrolidone ratio 1: 0.018) has a viscosity of 72 Pa • s and is stable during storage. The rate of application of the solution to the open-porous graphite tube is 1 cm / s. The water permeability of the obtained ultrafiltration element is 290 dm ³ / (m ² • h • at), after 5 hours from the start of testing 120 dm ³ / (m ² • h • at). When applying the solution to an open-pore tube made of ABS plastic, its deformation is not observed.

 Example 10

A tubular filter element is obtained, as in Example 1, by dissolving 95 g of a copolymer of grade Ф42Л in 200 g of acetone, then add a mixture of 2.5 g of polyvinylpyrrolidone, 153 g of isopropyl alcohol and 49.5 g of acetone. The solution (wt.%: Fluoropolymer 19, solvent 49.9, non-solvent 31.1; ratio of alcohol to polyvinylpyrrolidone 1: 0.016) has a viscosity of 25 Pa • s, is stable during storage. The water permeability of the obtained ultrafiltration element is 190 dm ³ / (m ² • h • at), after 5 hours from the start of testing 70 dm ³ / (m ² • h • at). When applying the solution to an open-porous tube made of ABS plastic, no deformation is observed.

 Example 11

A tubular filter element is obtained, as in Example 1, by dissolving 75 g of a copolymer of brand Ф42Л and 22 g of polyvinylpyrrolidone in 200 g of dimethylformamide, then a mixture consisting of 129 g of isopropyl alcohol and 74 g of dimethylformamide is added. The solution (wt.%: Fluoropolymer 15, solvent 54.8, non-solvent 30.2; alcohol to polyvinylpyrrolidone ratio 1: 0.017) has a viscosity of 20 Pa • s, is stable during storage. The water permeability of the obtained microfiltration element is 910 dm ³ / (m ² • h • at), after 5 hours from the start of testing 740 dm ³ / (m ² • h • at). When applying the solution to an open-pore tube made of ABS plastic, no deformation is observed.

 Example 12

A tubular filter element is obtained, as in example 1, by dissolving 2 g of polyvinylpyrrolidone and 77.5 g of brand copolymer Ф42Л in 200 g of acetone, then a mixture of 85 g of isopropyl alcohol, 90 g of acetic acid and 45.5 g of acetone is added. The solution (wt.%: Fluoropolymer 15.5, solvent 49.1, non-solvent 35.4; ratio of alcohol, acetic acid and polyvinylpyrrolidone 1: 0.06: 0.024) has a viscosity of 3.7 Pa • s, is stable during storage. The rate of irrigation of the solution on an open-porous tube was 6 cm / s. The water permeability of the obtained ultrafiltration element is 650 dm ³ / (m ² • h • at), after 5 hours from the start of testing 530 dm ³ / (m ² • h • at). When applying the solution to an open-porous polyvinyl chloride tube, it is not deformed.

 Example 13

A tubular filter element is obtained, as in example 1, by dissolving 125 g of a copolymer of grade F42L and 1.0 g of polyvinylpyrrolidone in a mixture of 122.5 g of dimethylformamide and 120 g of acetone, then add a mixture of 65 g of isopropyl alcohol, 36.5 g of acetic acid acid and 30 g of acetone. The solution (wt.%: Fluoropolymer 25, solvent 54.5, non-solvent 20.5; the ratio of alcohol, acetic acid and polyvinylpyrrolidone 1: 0.56: 0.015) has a viscosity of 160 Pa • s, is thermostable during storage. Watering the working solution on an open-porous organoplastic tube is carried out at a speed of 1 cm / s. The water permeability of the obtained ultrafiltration element is 230 dm ³ / (m ² • h • at), after 5 hours from the start of testing 80 dm ³ / (m ² • h • at).

 Example 14

A tubular filter element is obtained, as in Example 1, by dissolving 55 g of an F42L copolymer in 200 g of methylpyrrolidone, then add a mixture of 65 g of isopropyl alcohol, 81.5 g of methylpyrrolidone, 97.5 g of acetic acid and 1 g of polyvinylpyrrolidone. The solution (wt.%: Fluoropolymer 11, solvent 56.3, non-solvent 32.7; the ratio of alcohol, acetic acid and polyvinylpyrrolidone 1: 1.5: 0.015) has a viscosity of 8.0 Pa • s, is thermostable during storage. Watering the solution on an open-porous tube is carried out at a speed of 8 cm / s. The water permeability of the obtained microfiltration element is 2100 dm ³ / (m ² • h • at), after 5 hours from the start of testing 680 dm ³ / (m ² • h • at). When applying the solution to an open-porous tube made of ABS plastic, no deformation is observed.

 Example 15

A tubular filter element is obtained, as in Example 1, by dissolving 100 g of a copolymer of brand Ф42Л in 196.5 g of dimethylcetamide, then add a mixture of 100 g of isopropyl alcohol, 50 g of acetic acid, 3.5 g of polyvinylpyrrolidone and 50 g of dimethylacetamide. The solution (wt.%: Fluoropolymer 20, solvent 49.3, non-solvent 30.7; ratio of alcohol, acetic acid and polyvinylpyrrolidone 1: 0.5: 0.035) has a viscosity of 89 Pa • s, is stable during storage. The water permeability of the obtained ultrafiltration element is 400 dm ³ / (m ² • h • at), after 5 hours from the start of testing 270 dm ³ / (m ² • h • at). When applying the solution to an open-pore ABS plastic tube, no deformation of the tube is observed.

 Example 16 (according to the prototype).

The tubular filter element is obtained, as in example 3, from a working solution, which is prepared by dissolving 40 g of copolymer brand Ф42Л in 200 g of acetone, then add a mixture consisting of 82.5 g of isopropyl alcohol, 145.5 g of acetone and 32.5 g water. The solution (wt.%: Fluoropolymer 8, solvent 69, non-solvent 22.9; alcohol to water ratio 1: 0.40) has a viscosity of 0.15 Pa • s. During storage and lowering the temperature, it easily turns into jelly and becomes unsuitable for obtaining a membrane on the surface of an open-porous tube. This solution immediately after preparation at a temperature of 40 ^o With a speed of 8 cm / s is applied to the surface of an open-porous tube. It seeps heavily through the wall of an open-pore tube. The water permeability of the obtained microfiltration element is 21750 dm ³ / (m ² • h • at), after 5 hours from the start of testing 10500 dm ³ / (m ² • h • at). In the obtained filter element, the membrane is nonuniform in thickness along the length of the tube due to the rapid self-draining of the working solution, although the tube is quickly moved to a horizontal position and rotated at a speed of 30 rpm.

 The data are tabulated.

 The table shows that upon the preparation of membrane tubular filtering elements by forming a liquid film on an open-porous tube from a homogeneous solution of a fluoropolymer, a solvent and a non-solvent, and curing the fluoropolymer in a precipitation bath when high concentrations of aliphatic alcohol are used as a non-solvent (examples 1-4), or a mixture thereof (by weight) 1.0: 0.4-1.5 with acetic acid (examples 5-8), or 1.0: 0.016-0.18 with polyvinylpyrrolidone (examples 9-11 ), or 1: 0.5-1.5: 0.015-0.035 with acetic acid and polyvinylpyrrolidone one belt (examples 12-15) with the following ratios of the components of the solution (wt.%): fluoropolymer 8-25, non-solvent 20-40, solvent (acetone, dimethylformamide, dimethylacetamide, methylpyrrolidone or mixtures thereof (example 13)) - the rest is provided by obtaining more highly viscous storage stable solutions even with a very high concentration of fluoropolymer (up to 25 wt.%), suitable for producing high-quality filter elements with a fluoropolymer membrane in a wide range (ultra- and microfilters), both by dry and wet methods (example 3) .

 Very high concentrations (20-40 wt.%) Of the non-solvent in the solutions for the production of fluoropolymer selectively permeable membranes allow the use of open-porous tubes made of polymers that are unstable to the action of commonly used solvents for fluoropolymers to obtain high-quality membrane tubular filtering elements (examples 4-6, 12). Such open-porous tubes can be cheaper and more affordable, for example, from ABS plastic (examples 4-6).

 Additives to the composition of the non-solvent polyvinylpyrrolidone increase the permeability of the membrane tubular filter elements with a fluoropolymer membrane (see examples 1 and 10, 6 and 11).

 Obtained according to the prototype (example 16) fluoropolymer solutions are low viscosity, thermodynamically unstable, easily gelled and become unsuitable for processing. The shelf life is very limited. Only volatile solvents are used. During the preparation, storage and processing of these solutions, it is necessary to strictly maintain the elevated temperature in order to prevent gelation in them. This greatly complicates the hardware design of the process (heated pipelines, etc.).

 Obtained according to the prototype (example 16), the membranes are microfiltration, have only large pores with a size of 0.1-0.7 microns. Such membranes are not suitable for separation and deep purification of components of process liquids or wastewater.

 In the claimed method, the non-solvent of the fluoropolymer in the working solution for the preparation of membranes is either a lower aliphatic alcohol or non-aqueous mixtures thereof with acetic acid at a ratio of 1: 0.4-1.5 by weight, or with polyvinylpyrrolidone at a ratio of 1: 0.016-0.18 by weight, or with acetic acid and polyvinylpyrrolidone in a ratio of 1: 0.5-1.5: 0.015-0.035 by weight.

 This fundamental difference allows introducing into the working solution to obtain membranes an exceptionally large (20-40 wt.% Based on the weight of the solution) amount of fluoropolymer non-solvent even at high (15-25 wt.%) Concentrations of the latter.

 Using the proposed method for producing tubular filter elements provides the following advantages compared to known methods.

 1. The possibility of obtaining a wide assortment of pore size and, accordingly, water permeability of chemically resistant fluoropolymer membranes on the surface of open-porous tubes from more viscous thermodynamically stable solutions containing readily and volatile solvents.

 2. To reduce the cost of tubular filter elements with a fluoroplastic membrane by using cheaper components (aliphatic alcohol, acetic acid) of the working solution, as well as by the possibility of using open-porous tubes from cheaper and more affordable materials, such as ABS plastic, polyvinyl chloride.

 3. To ensure hydrophilization of fluoropolymer membranes due to polyvinylpyrrolidone, which improves water permeability, prevents contamination of tubular filter elements and the need for frequent washing.

 4. More simple and convenient instrumentation of the process.

Claims (3)

 1. A method of producing membrane tubular filtering elements by forming on an open-porous substrate a liquid film from a homogeneous solution of a fluoropolymer, a solvent and a non-solvent, and curing the fluoropolymer with the formation of a selectively permeable membrane, characterized in that a fluoropolymer solution is applied to the surface of the open-ended tube in the following ratio of the solution components, wt. %: fluoropolymer 8-25, non-solvent 20-40, selected from the group consisting of lower aliphatic alcohol, a mixture of lower aliphatic alcohol with acetic acid in a ratio of 1: 0.4-1.5 by weight, a mixture of lower aliphatic alcohol with polyvinylpyrrolidone in a ratio of 1: 0.016-0.180 by weight or a mixture of lower aliphatic alcohol, acetic acid and polyvinylpyrrolidone in a ratio of 1: 0.5- 1.5: 0.015-0.035 by weight, solvent - the rest.  2. The method of producing membrane filter elements according to claim 1, characterized in that open-porous tubes of glass, carbon, organoplastics, ceramics or graphite are used as a substrate for the membrane.  3. The method of producing membrane filter elements according to claim 1, characterized in that open-porous tubes made of polymers — ABS plastic, polyvinyl chloride, swellable or soluble in ordinary solvents for a membrane-forming fluoropolymer — are used as a substrate for the membrane.

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