JP2890469B2 - Method for producing porous separation membrane - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a separation membrane used in a membrane separation method for concentrating and separating a volatile organic liquid component from an aqueous solution thereof.

(Prior Art) Generally, there is a great need to separate and extract an organic liquid from an aqueous solution of an organic liquid generated or accumulated in a reaction system or various processes while concentrating the organic liquid. Technologies related to membrane separation include reverse osmosis, ultrafiltration,
In recent years, technologies such as diffusion dialysis, hemodialysis, electrodialysis, gas separation, and pervaporation have been remarkably developed.However, in conventional separation technologies, aqueous solutions of low-concentration organic liquids have been developed. It has been difficult to efficiently and efficiently separate organic liquid components from water. In the pervaporation method that is currently attracting attention,
When attempting to separate organic liquids from aqueous solutions, almost all membrane materials selectively allow water to permeate, and require large-capacity equipment to maintain a high degree of vacuum, resulting in an energy-intensive process. There was a problem.

Therefore, the uphill transport type membrane separation method using a hydrophobic polymer porous membrane, which concentrates the liquid-liquid type volatile organic liquid component, which uses temperature as the main driving force, or the secondary side of the membrane Japanese Patent Application No. 60-38 discloses a separation method in which an inert gas is flown and the permeate is collected.
810 and Japanese Patent Application No. 62-27218.

(Problems to be Solved by the Invention) The uphill transport type membrane separation method proposed in Japanese Patent Application No. 60-38810 discloses a liquid having a temperature difference as a main driving force.
This is a liquid direct contact type separation method. This separation method requires the use of a hydrophobic polymer porous membrane having an average micropore diameter of 20 to 1000 °. When the surface of the membrane gets wet with the volatile organic liquid aqueous solution of the object to be separated, the object to be separated penetrates into the micropores of the membrane in a liquid state, and the phenomenon of separation does not occur. By inferring the mechanism of this separation, it is considered that the object to be separated turns into vapor at the primary liquid film interface, enters the micropores of the membrane, and permeates through the micropores of the membrane in a vapor state. . Furthermore, since a separation coefficient exceeding the vapor-liquid equilibrium has been found depending on the membrane material and the state of the membrane, during the permeation process, specific components in the separation target that have a high affinity for the polymer of the membrane material are removed. It is considered that the specific component is concentrated on the secondary side because it selectively adsorbs on the primary membrane surface and preferentially causes surface diffusion accompanied by adsorption and desorption on the micropore surface in the membrane. Japanese Patent Application No. 62-4035, Japanese Patent Application No. 62-4036, and Japanese Patent Application No. 62-149087, wherein a silicone polymer, a ketone resin or poly (1-trimethylsilyl-1-propyne) is coated on the film surface. The fact that the composite membrane in which the polymer was coated on the membrane surface exhibited high separation performance is considered to support the idea of this separation mechanism.

Considering the mechanism of selective permeation, the uphill transport type membrane separation method, as in the case of dialysis and ultrafiltration, allows the substance to be separated to permeate through the membrane in a liquid state, as in dialysis and ultrafiltration. It can be said that this is a completely different separation method from the liquid-liquid separation method in which separation occurs due to the difference in the molecular size of the product. In addition, a separation method in which the component to be separated is permeated by dissolution and diffusion, such as a pervaporation method using a so-called dense membrane, or a membrane distillation method using a membrane having an average micropore diameter of 0.1 μm to 5 μm. It is considered that such a separation method is different from a separation method in which such a component to be separated permeates through a membrane by a simple viscous flow.

We have proposed an uphill transport type membrane separation method which is considered to be effective for the concentration of this volatile organic liquid aqueous solution.
It has been proposed in Japanese Patent Publication No. 38810, and has been further studied. However, in the case of the membrane used in the proposal of Japanese Patent Application No. 60-38810, the separation performance was not sufficiently satisfactory. Therefore, as a method for obtaining high separation performance, as described above, for example, Japanese Patent Application No. Sho 62
No.-4035, Japanese Patent Application No. 62-4036, Japanese Patent Application No. 62-1490
No. 87 proposes a composite film having a film surface coated with a polymer such as a silicone polymer, a ketone resin, or poly (1-trimethylsilyl-1-propyne). However, when these methods for improving the separation performance are attempted, as a general tendency, as the separation performance becomes higher, the amount of the substance permeating through the membrane per unit time and per unit membrane area, that is, the permeation rate, conversely increases as the separation performance becomes higher. There was a drawback of lowering.

(Means for Solving the Problems) The present invention is characterized in that a film is formed from a mixed organic solvent-based polymer solution containing a polyvinylidene fluoride-based polymer, a solvent thereof, and an organic solvent having higher volatility than the solvent. A method for producing a porous separation membrane for concentrating a volatile component-containing aqueous solution, wherein a volatile organic liquid aqueous solution used in a liquid-liquid type membrane separation method for concentrating a volatile organic liquid component using temperature as a driving force for separation It relates to a concentration membrane.

As the polyvinylidene fluoride polymer used in the present invention, for example, polyvinylidene fluoride homopolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-propylene hexafluoride copolymer, or
These mixtures and the like can be mentioned, and it is preferable that the mixture contains at least 50% by weight of polyvinylidene fluoride. As the average molecular weight of the polyvinylidene fluoride-based polymer, the molecular weight of a generally available polymer is sufficient.

Examples of the solvent for the polyvinylidene fluoride polymer include N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, diethylacetamide, diethylformamide, hexamethylphosphoramide, tetramethylurea, and dimethylsulfoxide. In addition, a highly volatile organic solvent may be any solvent that has a lower boiling point and a higher vapor pressure than a polyvinylidene fluoride-based polymer solvent at the temperature of the polymer solution during film formation. Alternatively, it need not swell. Such organic solvents include
For example, n-hexane, cyclohexanone, hydrocarbons such as toluene, tetrachloroethylene, 1,2-dichloroethylene, halogenated hydrocarbons such as trichloroethylene, diethyl ether, ethers such as 1,4-dioxane, Esters such as methyl acetate, methanol, isopropyl alcohol, alcohols such as 1-butanol and the like, but ease of handling, considering the safety, relatively mild volatility of toluene, tetrachloroethylene, Preferred are 1,4-dioxane, isopropyl alcohol and the like.

As the composition of such a polymer solution comprising a polyvinylidene fluoride polymer, its solvent and an organic solvent having a higher volatility than the solvent, the polymer concentration is 10% by weight to 50% by weight.
Is preferred. Further, while the solvent of the polyvinylidene fluoride-based polymer has a strong polarity, a solvent having a higher volatility than the solvent generally has a weaker polarity and is often a non-solvent for the polymer. Therefore, unlike a solvent that swells or dissolves the polymer, if a highly volatile solvent is added in a large amount to the solvent of the polyvinylidene fluoride-based polymer, the polymer gels. Therefore, there is an upper limit to the proportion of the highly volatile organic solvent. Although it cannot be said uniformly depending on the type of the polymer and the solvent, empirically, it is 0.
The content is preferably from 05% by weight to 20% by weight, and even if added at a higher ratio, the performance is hardly remarkably improved.

The micropore diameter of the membrane used in the present invention is 20% or more in average pore diameter,
Preferably not more than 1000 、, especially 300 Å or more, 100
It is preferably 0 ° or less. When the average pore size is smaller than 20 mm, the volatile organic liquid component does not pass through preferentially even in a gaseous state, and when the average pore size is larger than 1000 mm, the pore size distribution naturally exists. The supplied volatile organic liquid easily permeates through the membrane even in a liquid state, so that membrane separation cannot be performed. However, the pore size of the porous membrane is relatively small at the pores on the surface of the membrane, and the pore size is large on the inside. It is considered that the impermeability to an actual aqueous solution is greater than the impermeability to water. The membrane of the present invention is preferably used for separating a substance having a Stokes radius of 1/10 or less with respect to the average pore diameter of the membrane and substantially impermeable to the membrane. If the ratio of the Stokes radius to the average micropore radius is greater than 1/10, the volatile organic liquid component, which is the substance to be separated, does not readily permeate water. In the present invention,
“Substantially impermeable” means that the material is impermeable to the membrane in a liquid state and permeates in a gaseous state. In the present invention, since the substance to be separated is used in a method of permeating the membrane in a vaporized state, the substance to be separated is separated without wetting the membrane.

In order to advantageously exhibit the separation performance of the membrane, it is necessary to have a larger volume porosity in addition to the average pore diameter and to have relatively large pores inside the membrane. Volume porosity is typically 20
% Or more, preferably 40% or more, is advantageous as long as the mechanical properties of the film are not impaired. In addition, the amount of permeated water is 50-50
00mlh ^-1 mmHg ^-1 m ² , Nitrogen permeability is 0.005 to 1.0cm ³ (ST
P) It is preferably in the range of cm ⁻² s ⁻¹ cmHg ⁻¹ .

Regarding the shape of the membrane, considering the process of membrane formation, in the case of a sheet-like membrane, the conditions such as the viscosity of the polymer solution can be selected from a relatively wide range because the support is used, whereas the hollow fiber Since the support cannot be used with the membrane of
The conditions for film formation are considerably limited as compared with sheet-like films. Further, even when the film is formed from the same polymer solution, the strength and separation performance of the obtained film may be significantly different due to the difference in conditions based on the difference in shape. In the uphill transport type membrane separation method, any shape of membrane such as a sheet-like membrane and a hollow fiber membrane can be used, but from a practical viewpoint, a hollow fiber membrane is advantageous. It is considered to be.

The sheet-like film is formed by casting a solution comprising the above-mentioned polyvinylidene fluoride polymer and its solvent and an organic solvent having a higher volatility than the solvent at a constant thickness on a solid surface, a support or a porous support film. It is obtained by discharging or coating, evaporating the solvent for a certain period of time, and replacing the solvent with a coagulating solvent. Here, the solid surface is a smooth surface of a solid that does not dissolve in the solvent in the polymer solution or the solvent to be replaced, and a glass plate, a plate made of polytetrafluoroethylene, a metal plate, or the like can be used. Further, the support is a membrane strength reinforcing material which does not dissolve in the solvent of the polymer mixed solution or the solvent to be replaced and has substantially no separation performance, and nonwoven fabric, cloth, metal mesh and the like can be used. Furthermore, a porous support membrane is a porous membrane that does not dissolve in a solvent in the polymer solution or a solvent to be replaced, and as an inorganic porous membrane, an organic porous membrane such as porous glass and porous ceramics. For example, porous films of various polymers can be used. Here, as the polymer of the organic porous support membrane, polyethylene, polypropylene, polytetrafluoroethylene and a copolymer of the above polymers, polyacrylonitrile, polyacrylic acid, polyacrylate, polymethacrylic acid, polymethacrylic acid Ester, polyacrylamide, vinyl polymers such as polyvinyl alcohol and their copolymers and their blended polymers, polyesters, polyamides,
Examples include polysiloxanes, polyphosphazenes, and cellulose polymers. On the other hand, in the case of a hollow fiber membrane, a method comprising discharging a solution composed of the above-mentioned polyvinylidene fluoride polymer and its solvent and a highly volatile solvent into a hollow shape while flowing a fluid from an annular base to a central portion, There is a method of coating the surface of the hollow fiber support membrane, evaporating the solvent for a certain period of time, and then replacing the solvent with a coagulating solvent. When an annular base is used, the fluid flowing in the center is a liquid,
As a gas such as a coagulating liquid such as water or alcohol, an incompatible liquid, and a mixed liquid thereof, air, nitrogen, argon, or the like can be used. Hollow fiber support membranes are porous membranes that do not dissolve in the solvent in the polymer solution or the solvent to be replaced, and as inorganic porous membranes, porous glass, porous ceramics, etc. Polymeric porous membranes can be used. Here, as the polymer of the porous support membrane, polyethylene, polypropylene, a copolymer of polytetrafluoroethylene and the above polymer, polyacrylonitrile, polyacrylic acid, polyacrylate, polymethacrylic acid, polymethacrylic ester, Polyacrylamide, vinyl polymers such as polyvinyl alcohol and their copolymers and their blended polymers, polyesters, polyamides,
Examples include polysiloxanes, polyphosphazenes, and cellulose polymers.

The coagulation solvent is a solvent that is miscible with the solvent in the polymer solution at the time of the above-described film formation, does not dissolve the polyvinylidene fluoride-based polymer, and precipitates the solid component of the polymer solution. Such solvents include methanol, ethanol,
Examples thereof include alcohols such as propanol, water, and the like, and mixtures of these with other solvents.

In order to prepare a porous membrane in a dry state while maintaining the porous structure in a suitable state, solvent replacement drying in which the organic solvent miscible with water is replaced with a non-solvent of a polyvinylidene fluoride-based polymer and then dried is preferable. It may be prepared by a method of drying the water-containing film under mild conditions.

A volatile organic liquid aqueous solution to which the present invention can be applied can be basically applied to a substance in which the composition of the organic liquid substance in the gas phase in the gas-liquid equilibrium of the aqueous solution is larger than the composition in the liquid phase. . Examples of such substances include methanol, ethanol, n-propanol, iso-propanol, n-butanol, t-butanol, acetone, tetrahydrofuran, 1,4-dioxane, methylamine,
Ethylamine, acetonitrile, methyl ethyl ketone,
Examples include methyl acetate and ethyl acetate.

The concentrations of aqueous solutions of these substances to which the present invention can be applied are:
From the viewpoint of taking advantage of the features of the method of the present invention, a relatively low concentration region is preferable, and 0.5 to 20% by weight is appropriate. The upper limit of the aqueous solution concentration is determined mainly by the concentration at which the aqueous solution to be separated does not wet the membrane. This is generally difficult to specify because the physicochemical properties of the polymer of the membrane material, the micropore diameter of the membrane, the surface tension of the separation target, and the like are involved.

(Example) Next, an Example demonstrates this invention.

The experiment of the concentration method of the liquid-liquid type volatile organic liquid aqueous solution was performed by a method schematically shown in FIG. That is, a 5% aqueous ethanol solution is adjusted to 50 ° C. from the supply liquid tank 1 and supplied to the membrane module 4 for circulation. On the other hand, on the secondary side of the membrane
A 5% aqueous ethanol solution adjusted to 15 ° C. is circulated through the permeate tank 5. After the start of the experiment, sample liquid is taken from the supply liquid tank (high temperature side) and the permeate liquid tank (low temperature side) at predetermined time intervals, and the concentration thereof is measured with a differential refractometer.

After completion of the experiment, the liquid weights of the supply liquid tank and the permeate liquid tank are measured. The concentration on the high temperature side decreases and the concentration on the low temperature side increases. The permeation rate of ethanol in the initial stage of the experiment was calculated from the time-dependent changes in the concentration on the high temperature side and the low temperature side and the amount of liquid permeation. ^{That, C H (t) = {} E H (O) -Qe (t) -Σ C H (n) S H (n)} / E HT (t)
^{(1) C L (t)} = {E L (O) -Qe (t) -Σ C L (n) S L (n)} / E LT (t)
(2) Here, C (t) is the concentration of ethanol t hours after the start of the experiment, and the suffix H indicates the high temperature side and L indicates the low temperature side. C
(N) indicates the density of the n-th sampling. S (n)
Indicates the amount of the n-th sampling. E (O), E
(T) shows the amount of ethanol before the start of the experiment and t time after the start, respectively. E _HT (t) and E _LT (t) are the total liquid volume on the high temperature side and the low temperature side at time t after the start of the experiment, respectively. Using the above equations, the ethanol permeation rate (Je) and the water permeation rate (Jw) were calculated by the curve fitting method in consideration of the relations of the equations (3) and (4). ^{Determine the} separation coefficient α ^EtOH . Qe (t) is the amount of permeation of ethanol after t hours.

^{Qe (t) = A ∫ t} Je dt (3) Qw (t) = A ∫ t Jw dt (4) α EtOH = (Je / Jw) / (C H (t) / {1-C H (t) ｝ (5) The average pore diameter of the membrane was measured by the method described below, that is, the water permeability (Lp) of the membrane and the diffusive separation property (Pm) of the solute were measured using methanol, ethanol,
It was measured with propanol, butanol, acetone, etc., and calculated using the relationship of the following equation.

Pm = (D / L) (H / ts ² ) (6) Lp = (H / L) ｛(Rp / (8η)｝ (7) where D is the diffusion coefficient of the solute, L is the film thickness, and H Is the water content, ts is the solute curvature, Rp is the average micropore radius, η is the viscosity of water, and ts was determined from the following equation.

fsw ⁰ = RT / D (8) fsw = (RT / Pm−Vs / Lp) (H / L) (9) ts = fsw / fsw ⁰ (10) where R is a gas constant, and T is a value at the time of measurement. Absolute temperature, Vs, is the partial molar volume of the solute.

Example 1 Pennwold polyvinylidene fluoride KYNAR460 and 74
0 to give a polymer concentration of 24.5% and a solution viscosity of 10 at 110 ° C.
50 g of tetrachloroethylene was added to 1000 g of a polymer solution using dimethyl sulfoxide (DMSO) as a solvent and prepared to have a poise of 100 poise, to prepare a spinning solution. At this time, the amount of tetrachloroethylene based on the total solvent in the polymer solution is
6.6% by weight. This spinning stock solution is spun from an annular hollow fiber spinneret while pouring an aqueous solution of DMSO 80% into the hollow portion, coagulated in water at 45 ° C., and then washed with water.
A water-containing polyvinylidene fluoride-based hollow fiber membrane was obtained. This membrane was immersed in methanol and n-hexane sequentially, replaced,
Air dried. Outer diameter of hollow fiber membrane is 1011μm, inner diameter is 750μ
m, the volume porosity was 68%.

After replacing the dried hollow fiber with water via methanol, the water permeability and the diffusion permeability of ethanol were measured, and the average pore diameter (Rp) and the curvature (ts) were determined. Rp is 497A, ts is 2.
21, The ratio of Rp to Stokes radius was 256. The amount of permeated water is 857 mlh ^-1 mmHg ^-1 m ^-2 , and the amount of permeated nitrogen is 0.054 cm ³ (S
TP) cm ^-2 s ^-1 cmHg ^-1 .

The dried hollow fiber membrane was cut into a length of about 30 cm, bundled into 14 pieces, and inserted into an acrylic case. This case is about 20cm
In this structure, two inlets and outlets for fluid are provided on the side surface of the acrylic pipe. Both ends of the pipe serve as inlets and outlets for low-temperature side fluid and the side portions serve as inlets and outlets for high-temperature side fluid. After inserting the hollow fiber, both ends of the case were potted with an epoxy adhesive, and after curing, both ends were cut to form an opening of the hollow fiber membrane.

An experiment of a method for concentrating a volatile organic liquid aqueous solution in a liquid-liquid system is as follows.
It carried out by the above-mentioned method using ethanol 5 wt% aqueous solution.
In the present embodiment, the temperatures of the high-temperature side fluid and the low-temperature side fluid at the module inlet were 51.0 ° C. and 15.0 ° C., respectively. At this time, the permeation rates of ethanol and water at the beginning of the experiment were 0.45 respectively.
6, 0.994 kgm ^-2 h ^-1 , and the separation factor α ^EtOH for ethanol was 8.75. Comparing this result with the separation performance of the ordinary polyvinylidene fluoride hollow fiber membrane without adding a volatile solvent shown in Comparative Example 1, the permeation rates of ethanol and water were 2.7 times and 1.7 times, respectively, and the separation factor was 1.6 times. Has improved.

Example 2 Using 1,4-dioxane instead of tetrachloroethylene of Example 1 and using the same other polymer solution,
Under the same conditions, a polyvinylidene fluoride hollow fiber membrane was prepared,
When a similar concentration experiment was performed, the permeation rate for ethanol and water was greatly improved.

Example 3 A polyvinylidene fluoride-based hollow fiber membrane was prepared under the same conditions as in Example 1 except that the amount of tetrachloroethylene was 100 g, and a similar concentration experiment was performed. Significantly improved.

Comparative Example 1 A polyvinylidene fluoride-based hollow fiber membrane was prepared using a polymer solution in which a solvent having a higher volatility than that of a polyvinylidene fluoride-based polymer such as tetrachloroethylene was added in Example 1, and a concentration method was similarly performed. Was conducted. The concentration of the aqueous ethanol solution was 5 wt%, and the temperatures of the high-temperature side fluid and the low-temperature side fluid were 49.6 ° C. and 14.0 ° C., respectively. At this time, the permeation rates of ethanol and water were 0.17 and 0.59 kg, respectively.
m ^-2 h ^-1 , and the separation coefficient α ^EtOH was 5.42.

(Effects of the Invention) According to the present invention, improved permeability and separation selectivity can be used in a novel liquid-liquid system membrane separation method for selectively concentrating and separating an organic liquid from a volatile organic liquid aqueous solution. And a method for producing the same.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a membrane separation experiment apparatus used in an example of the present invention. 1 is a supply (or primary) liquid tank, 2 is a supply liquid circulation pump, 3 is a supply liquid side heat exchanger,
Is a membrane module, and 9 and 10 are the inlet and outlet of the feed liquid side module, respectively. 5 is a permeate (or secondary) liquid tank,
6 is a permeate-side heat exchanger, 7 is a pressure regulating valve, 8 is a permeate-side circulation pump, and 11 and 12 are an inlet and an outlet of a permeate-side membrane module, respectively.

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI // C08L 27:12 (56) References JP-A-49-126572 (JP, A) JP-A-60-216804 (JP, A JP-A-64-38103 (JP, A) JP-A-61-21702 (JP, A) JP-A-60-97001 (JP, A) JP-A-62-14905 (JP, A) 503323 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) B01D 71/34 B01D 61/36 D01F 6/12 C08J 9/28

Claims (3)

(57) [Claims] 1. A method for concentrating an aqueous solution containing volatile components, comprising forming a film from a polymer solution of a mixed organic solvent system containing a polyvinylidene fluoride polymer, a solvent thereof and an organic solvent having a higher volatility than the solvent. A method for producing a porous separation membrane. 2. The porous separation according to claim 1, wherein the proportion of the highly volatile organic solvent in the polymer solution is 0.05% by weight or more and 20% by weight or less based on the total amount of the solvent. Manufacturing method of membrane. 3. The method according to claim 1, wherein the polyvinylidene fluoride-based polymer contains at least 50% by weight of polyvinylidene fluoride.