JPS5848202B2 - Method for manufacturing tubular semipermeable membrane - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a tubular semipermeable membrane used in a tubular reverse osmosis separation device.

Reverse osmosis membranes, or semipermeable membranes, which can separate substances or molecules with relatively small particle sizes such as ions and low-molecular substances, are used in the treatment of industrial wastewater, sewage treatment, desalination of brine and seawater, and even It is used in purification, concentration, and desalination processes in the food, pharmaceutical, brewing, and fermentation fields.

As one of the purification, concentration, or desalination methods using this semipermeable membrane, a semipermeable membrane is provided on the inner surface of a strong porous support tube, and the solution, that is, the liquid to be treated, is flowed through the tube under pressure higher than its osmotic pressure. A method is known in which the solvent in the liquid to be treated is caused to flow out into a chamber surrounding the porous support tube.

The device for implementing this method is a so-called tubular reverse osmosis separation device.

The tubular semipermeable membrane used in such reverse osmosis separation equipment is
While it is desired that the separation performance ranges from low to high separation rates, it is desirable to have a separation performance that is suitable for each application. In terms of physical strength and pH resistance, it is desired that the material has excellent durability such as chlorine resistance and microbial decomposition resistance.

By the way, cellulose acetate membranes, which are well-known as semipermeable membranes, are generally produced by dissolving cellulose acetate in a suitable solvent and adding certain additives to improve water permeability and other functions to prepare a membrane-forming solution, which is then flowed. It is made by spreading it, drying it appropriately, solidifying it by immersing it in water, and then subjecting it to further heat treatment.

This cellulose acetate membrane is a so-called asymmetric membrane having a membrane structure consisting of a surface skin layer and a porous support layer, but its membrane performance depends on the type of each component of the membrane forming solution used.
It varies widely depending on the amount and film forming conditions such as drying after casting, immersion in water, and heat treatment.

For this reason, many specific membrane forming liquids and membrane forming conditions have been proposed for the manufacturing method of cellulose acetate membranes, but most of these are aimed at flat membranes and are not suitable for tubular reverse osmosis separation devices. There are few membranes targeting tubular membranes used in this field, and therefore, not many tubular membranes have been found that can meet the above-mentioned demands.

On the other hand, the inventors used a specific membrane-forming solution for manufacturing cellulose acetate-based tubular membranes, and after casting the membrane on the inner surface of the tube, forced ventilation inside the tube during drying. We devised a method in which the solvent in the membrane-forming solution was evaporated, and by this method we were able to obtain relatively good results in terms of membrane performance such as separation characteristics, water permeability, and durability.

However, in the proposed method, the homogeneity of the membrane performance over the entire length of the membrane is somewhat inferior, the water permeability or separation characteristics tend to vary in the longitudinal direction of the tubular membrane, and there is still room for improvement in the tube, such as the amount of water permeation. There was room left.

This invention avoids this drawback and achieves uniform performance over the entire length of the membrane, since membranes exhibiting the above-mentioned inhomogeneous characteristics are inferior in commercial value or usability when used in tubular reverse osmosis separation devices. The present invention aims to provide a novel and useful method for manufacturing a tubular semipermeable membrane which can further satisfy the above-mentioned requirements regarding the membrane properties of the tubular semipermeable membrane.

That is, this invention contains 5 to 30% by weight of hydroxy acid and 5 to 15% by weight of lower alcohol, and the combined ratio (weight ratio) of hydroxy acid to lower alcohol is 0.5 to 3. After applying a cellulose acetate-based film-forming solution to the inside of the tube, aeration treatment is performed from one end of the tube toward the other end, and vice versa. 5 to 2 of the solvent in the film forming solution is
0% by weight is evaporated, and then the tubular body is immersed in water in a direction opposite to the final ventilation direction to solidify the membrane-forming liquid, and after this solidification, the tubular semipermeable membrane is further heat-treated. This relates to a manufacturing method.

As described above, in this invention method, a cellulose acetate-based film-forming liquid is cast onto the inner surface of the tube and then dried by forced ventilation inside the tube, similar to the devised method described above.
At this time, the film-forming solution contains specific amounts of hydroxy acids and lower alcohols in specific proportions, and the ventilation process is performed from one end to the other without making the ventilation direction one direction, and then the ventilation is performed in the opposite direction. The main idea is that the treatment is not necessarily carried out, and that the aeration rates in both of the above directions are specified, and that after drying by the aeration treatment, the material is immersed in water in the opposite direction to the final aeration direction.

According to this method, the drawbacks of the previously proposed methods can be avoided, and a homogenized tubular semipermeable membrane with little variation in water permeability and separation characteristics over the entire length of the membrane can be obtained.

Although the reason for this is not necessarily clear at present, the coagulation effect due to the evaporation of the solvent in the membrane forming solution and the replacement of the remaining solvent with water is made uniform in the longitudinal direction of the pipe by the above-mentioned specific means. This seems to be because the thickness of the surface skin layer and other membrane structures of the resulting asymmetric membrane are homogenized as a whole.

Further, according to the method of the present invention, the film properties can be further improved.

That is, it is possible to impart predetermined separation characteristics depending on the purpose of use and maintain high water permeability even when achieving a high separation rate.

This is mainly based on the fact that a specific amount of lower alcohol and a specific amount of hydroxy acid are used together in a specific ratio as additives in the film forming solution.

Furthermore, mechanical strength and other durability can be improved by using such specific additive components, and preferably by using cellulose acetate having a specific degree of oxidation.

In this invention, a film forming solution is first prepared, and this film forming solution generally consists of cellulose acetate, an organic solvent for dissolving this acetate, and additives used to improve film performance.

Average degree of acetylation for cellulose acetate is 57-60
It is preferred to use % by weight.

If the degree of acetylation is too low, mechanical strength, pH resistance,
In addition to impairing durability such as resistance to microbial decomposition, there is a tendency for separation characteristics to worsen, and conversely, if the temperature is too high, good results in terms of separation characteristics cannot be obtained.

Such cellulose acetate usually has an acetylation degree of 55.5.
~55.8% by weight of cellulose diacetate and degree of acetylation 6
0.4 to 60.7% by weight of cellulose triacetate is mixed and used so that the average degree of acetylation is within the above range.

The amount of mixed acetate used is usually 10 to 20% of the total film forming solution.
It is desirable to set it as weight%.

If this amount is too small, the viscosity of the membrane-forming liquid will become too low, making it difficult to uniformly cast it onto the collapsed tube, and the membrane strength will decrease, resulting in poor pressure resistance.

Furthermore, if the above amount is too large, dissolution in an organic solvent will be hindered and the viscosity of the film-forming solution will become too high, making it difficult to cast onto the inner surface of the tube.

In this specification, the degree of acetylation of cellulose acetate is
It is expressed by converting the acetyl group content in the acetate molecule into acetic acid content.

Therefore, for the above acetylation degree (CH3CO group molecular weight/CH3CO
When multiplied by OH molecular weight)-=0.717, it becomes the so-called degree of acetylation.

As the organic solvent, a wide variety of known organic solvents capable of dissolving cellulose acetate as described above can be used, but a particularly suitable solvent is a mixed solvent of acetone and 1,4-dioxane.

Here, 1,4-dioxane has the ability to dissolve both cellulose diacetate and cellulose triacetate, while acetone only shows the ability to dissolve cellulose diacetate, but it can be used in combination with 1,4-dioxane. This brings about good results in terms of adjusting the viscosity of the film-forming solution and controlling the amount of solvent evaporation during drying.

The combined ratio (weight ratio) is acetone = 1,4-dioxane =
The ratio is preferably 1:1 to 3, particularly preferably 1:1.5 to 20.

The amount of the organic solvent used is determined depending on the amount of cellulose acetate, but is generally 50 to 80% by weight, preferably 65 to 75% by weight of the entire film forming solution.

In addition, when acetone and 1,4-dioxane are used together in the above proportions, the amounts used for each of these components are 15 to 40% by weight of acetone and 30 to 40% by weight of 14-dioxane in the entire membrane forming solution.
It is desirable that the content be 50% by weight.

Next, the additive component used to improve membrane performance is a combination system of hydroxy acid and lower alcohol.
The combined ratio (weight ratio) of hydroxy acid:lower alcohol is 1:0.5-3, particularly preferably 1:1-1.5.

Hydroxy acids include glycolic acid, lactic acid, α-oxy-n-butyric acid, α-oxy-isobutyric acid, etc., and lower alcohols include methanol, ethanol, n-butyric acid, etc.
-Propanol, inpropanol and the like.

Among the lower alcohols, methanol and ethanol are preferred, and methanol is particularly preferred.

By using such a hydroxy acid and a lower alcohol in combination, the amount of evaporation can be reduced during drying of the membrane forming solution, and a high separation rate can be obtained even when the heat treatment temperature is relatively low.
As a result, a semipermeable membrane with high water permeability can be obtained.

The amount of the additive used is 10 to 35% by weight, preferably 15 to 25% by weight of the entire film forming solution.

In addition, when hydroxy acid and lower alcohol are used together in the above proportions, the amount of each of these components used is 5 to 30% by weight of hydroxy acid and 5 to 15% by weight of lower alcohol in the entire film forming solution.
It is desirable to set it as weight%.

If the amount of hydroxy acid is too small, the water permeability will be poor, and if the amount is too large, the separation properties will be likely to be impaired.

Furthermore, if the amount of lower alcohol is too small, both separation properties and water permeability will be low, and if it is too large, complete dissolution of cellulose acetate may be impaired.

The typical liquid composition of the film-forming solution consisting of each of the above components is as follows: 10-20% by weight of cellulose acetate with an average degree of acetylation of 57-60% by weight, 15-40% by weight of acetone,
- 30-50% by weight of dioxane, 5-30% of hydroxy acid
% by weight and lower alcohols from 5 to 15% by weight.

Although the membrane-forming liquid of this liquid combination gives good membrane performance even in the production of flat membranes, it can be particularly advantageously applied to the method of the present invention aimed at producing tubular membranes.

In the method of this invention, the above-mentioned film-forming liquid is used, and the film-forming liquid is cast onto the inner cylinder of the tube.

During this casting application, generally the tube is supported vertically,
While supplying the film-forming liquid to the lower end of the tube, a core with an outer diameter slightly smaller than the inner diameter of the tube is introduced into the tube, and the above-mentioned supply liquid is cast onto the inner surface of the tube through this core. .

Note that a tow cord or compressed air is used to thread the core.

As the tube to be cast coated, a metal tube such as a glass tube or a stainless steel tube with an extremely smooth inner surface is used.

Further, the porous support tube in the reverse osmosis separation device may be applied as is.

This support tube is required to have sufficient mechanical strength and water permeability as its original characteristics, and also to have a smooth inner surface and good leakage with the membrane forming solution.

In addition to porous fiber-reinforced plastic tubes with smooth inner surfaces, such support tubes can also be made of woven or non-woven fabrics formed into a tube shape, particularly by arranging a high-density cloth on the inside and a low-density cloth on the outside. Examples include integrated lamination.

After the coating is cast on the inner surface of the tube in this way, the inside of the tube is aerated.In this process, in particular, the aeration treatment is performed from one end of the tube toward the other end, and vice versa. 5 to 15 m alternately and together with ventilation treatment toward one end.
This should be done at least twice in both directions at an air flow rate of /sec.

In aeration treatment in only one direction, the amount of solvent evaporated tends to vary between the inlet side and the outlet side due to the influence of the solvent evaporated on the gas inlet side.

However, the alternating aeration process avoids this drawback and allows evaporation to occur in a substantially uniform amount in the longitudinal direction of the tube.

The reason for setting the ventilation speed in each direction to 5 to 15 m/s is as follows.
If the ventilation speed is slower than 5 m/sec, the above effects will be poor, and while a higher ventilation speed is more desirable for the above effects, if it exceeds 15 m/sec, the film forming solution may scatter or the liquid surface may be disturbed. This is because there is a risk of causing serious defects in film properties.

Means for such aeration treatment include a method of pressurizing an inert gas such as air, nitrogen, or carbon dioxide gas with a compressor, and a method of sucking the above-mentioned gas with an aspirator or the like.

Forcibly evaporating the solvent in the membrane-forming solution through the above aeration treatment brings about good results in improving the separation properties and other membrane properties, but the amount of evaporation should be kept within the range of 5 to 20% by weight.

The reason for this is that if the amount is less than 5% by weight, the shape of the skin layer on the surface side of the membrane will be insufficient, and in this case, the skin layer will break due to shrinkage due to heat treatment in the final process or elongation due to pressure during use. This is because there is a risk of causing
This is because when the amount exceeds % by weight, the skin layer becomes too thick, resulting in a sharp drop in water permeability.

In addition, in this invention, a hydroxy acid and a lower alcohol are used together as additives, but in this case, in the range of 5 to 20% by weight of evaporation, the smaller the evaporation amount, the better the separation characteristics, and the better the water permeability. Sexuality improves.

This trend is completely opposite to the general tendency that the higher the amount of evaporation, the better the separation properties and the lower the amount of water permeation.

The reason for this is not necessarily clear, but it is believed that when lower alcohols are used together with hydroxycarboxylic acids, they have the effect of increasing both separation properties and water permeability. This is thought to be due to the fact that while it varies depending on the composition ratio with the acid, this composition ratio changes due to the volatilization of the lower alcohol accompanying the evaporation of the solvent.

In this invention, after a specific amount of the solvent in the film forming solution is evaporated by the above method, the film is immersed in water in a direction opposite to the final aeration direction to solidify.

Only by employing such a coagulation means and the above-mentioned aeration treatment can the film properties be made homogeneous over the entire length of the film.

The detailed reason for this effect produced by specifying the direction of immersion in water is unknown, but it appears that the non-uniformity of performance caused by the evaporation process and the non-uniformity of performance caused by the coagulation (immersion) process are offset. This seems to be for the purpose of

In other words, even if the ventilation direction is alternately changed during evaporation, the effect of the last ventilation direction will still remain, so the unevenness of the evaporation amount caused by this will be offset by the unevenness that occurs during the solidification process, resulting in a homogeneous overall result. It seems that improved performance can be obtained.

The water temperature during immersion in water may generally be about 0 to 25°C, and within this temperature range, the higher the water temperature, the higher the salt separation property.

After this solidification, the tubular semipermeable membrane of the present invention can be obtained by subjecting it to heat treatment according to a conventional method.

The purpose of the above heat treatment is to shrink the coagulated membrane and promote crystallization to strengthen its mechanical strength and improve separation performance, and generally a temperature of up to 100°C is applied, but in this invention, the membrane forming solution By using a hydroxy acid and a lower alcohol together as additives, it is possible to obtain high separation performance or mechanical strength even if the heat treatment temperature is set to a relatively low temperature of 60 to 70°C. This will bring about better results.

In the above heat treatment, if the tube is a glass tube or a metal tube, the coagulated membrane is first drawn out from inside the tube, and then subjected to the heat treatment while being inserted into a porous support tube in a reverse osmosis separation device.

When the rectangular tube is the above-mentioned porous support tube, it is heat-treated as it is.

The tubular semipermeable membrane of the present invention thus obtained is highly homogenized with little variation in water permeability and separation properties over the entire membrane length, and membrane properties such as separation properties, water permeability, and durability are uniform throughout the membrane. It is highly improved and has high practical value.

Examples of the present invention will be described below to explain it more specifically.

Note that in the following, parts and % mean parts by weight and % by weight, respectively.

Example 1 Cellulose triacetate (A-435-858 manufactured by Eastman Chemical Company; degree of acetylation 60.7%) and cellulose diacetate (E-400- manufactured by Eastman Chemical Company)
25; acetylation degree 55.8%) and the average acetylation degree after mixing is 5.
A film-forming solution having the following composition was prepared, which was used in combination at a ratio of 8.2%.

Cellulose triacetate 7 parts Cellulose diacetate 7 parts 1 4-dioxane
43 parts acetone 27
Part lactic acid 7 parts Methanol 9 parts The above film forming solution was cast onto the inner surface of a glass tube with a length of 3000 mm and an inner diameter of 12.5 mm, and then poured from one end of the glass tube to the other end for 10 minutes. By ventilation for 15 seconds at a speed of 5 m/sec, and then for 15 seconds at the same speed in the opposite direction to the above, 13% of the solvent in the film-forming solution was evaporated.

Thereafter, the glass tube was immersed in water at 10° C. in a direction opposite to the direction of the subsequent ventilation to solidify the film-forming liquid on the inner surface of the glass tube.

The tubular semipermeable membrane of the present invention was produced by pulling out the obtained coagulated membrane from the inside of the glass tube, inserting it into a fiber-reinforced plastic pipe having a large number of perforations, and heat-treating it with hot water at 65°C for 10 minutes. Ta.

In order to investigate the membrane performance of this semipermeable membrane, 0.5% sodium chloride I
- Pressure 42 kg/i, temperature 2 using IJum aqueous solution
A desalting test was conducted under the condition of 5℃, and the entire length of the membrane was divided into 5 equal parts from one end to the other, and divided into 5 parts A, B, C, D, and E.
The area was divided into five sections, and the water permeability and salt separation rate in each section were determined using the following method.

The above test results were as shown in Tables 1 and 2 below.

In the same table, Comparative Examples 1 to 4 indicate the test results of tubular semipermeable membranes made by the following method.

Comparative Example 1 A tubular semipermeable membrane made in exactly the same manner as in Example 1 except that the direction of immersion in water was reversed.

Comparative Example 2 By setting the ventilation direction only in one direction from one end side to the other end side and making the ventilation time 30 seconds, the amount of solvent in the film forming solution on the inner surface of the glass tube was reduced to almost the same amount as in Example 1. A tubular semipermeable membrane made in the same manner as in Example 1, except that the direction of immersion in water was the same as the above-mentioned ventilation direction.

Comparative Example 3 Comparative Example 2 except that the direction of immersion in water was reversed to the ventilation direction.
A tubular semipermeable membrane made in exactly the same manner as .

Comparative Example 4 The ventilation speed was 2 ml seconds, and the ventilation time in each direction was 8
A tubular semipermeable membrane made in exactly the same manner as in Example 1, except that approximately the same amount of solvent in the membrane forming solution on the inner surface of the glass tube was evaporated as in Example 1 by setting the time to 0 seconds.

As is clear from Tables 1 and 2 above, according to the method of this invention, a nearly uniform salt separation rate is obtained over the entire length of the membrane, and it is also extremely uniform over the entire length. It can be seen that high water permeability is obtained.

Example 2 Using the film-forming solution prepared in Example 1, it was cast onto the inner surface of the same glass tube as in Example 1, and then 10.577+ was applied from one end of the glass tube to the other end. ．． /second for 10 seconds, then in the opposite direction to the above for 10 seconds at the same speed.
By blowing air for seconds, 10% of the solvent in the film forming solution was evaporated.

Thereafter, the glass tube was immersed in water at 20° C. in a direction opposite to the direction of the subsequent ventilation to solidify the film-forming liquid on the inner surface of the glass tube.

The tubular semipermeable membrane of the present invention was produced by pulling the obtained coagulated membrane out of the glass tube, inserting it into a fiber-reinforced plastic pipe with a large number of perforations, and heat-treating it with hot water at 70°C for 10 minutes. Ta.

When this semipermeable membrane was subjected to a desalination test similar to that described in Example 1, almost uniform water permeation and salt separation rate were obtained over the entire length of the membrane, and the overall average water permeation was 0. 8
5 m/yn' f3, overall average salt separation rate is 97.
It was 5%.

Example 3 A tubular semipermeable membrane was produced in exactly the same manner as in Example 2, except that 5 parts of glycolic acid was used instead of 7 parts of lactic acid in the membrane forming solution composition described in Example 1.

The water permeation rate and salt separation rate of this **semipermeable membrane are almost uniform over the entire length of the membrane, and the average water permeation rate is 0. 9 4 m7'
m day, the average salt separation rate was 98%.

Example 4 Four types of tubular tubes of the present invention were prepared in exactly the same manner as in Example 2, except that the ventilation time was changed to change the amount of evaporation of the solvent in the film-forming solution, and the water temperature during immersion in water was set to 0°C. Created a semi-permeable membrane.

The relationship between the performance of these semipermeable membranes and the amount of solvent evaporated was investigated, and the results were as shown in Table 3 below.

The water permeability and salt separation rate in the table are average values over the entire length of the membrane.

As is clear from the above table, each tubular semipermeable membrane has various water permeability and salt separation rate depending on the amount of solvent evaporation. Since it contains carboxylic acid and lower alcohol, it exhibits a unique phenomenon in which the salt separation rate increases as the evaporation amount decreases in the solvent evaporation amount range (5 to 20%) as shown in the above examples.

If the amount of solvent evaporation exceeds the above range and exceeds 20%, the water permeability will be extremely reduced, and if it is less than 5%, it must be strong enough to withstand heat and pressure during heat treatment or use. It was confirmed that both methods were impractical.

Example 5 Four types of film forming liquids having different average degrees of acetylation were prepared by changing the ratio of cellulose triacetate and cellulose diacetate to the film forming liquid composition described in Example 1, and using these film forming liquids, Four types of tubular semipermeable membranes of the present invention were prepared in exactly the same manner as in Example 2.

The relationship between the performance of these semipermeable membranes and the degree of acetylation was investigated, and the results were as shown in Table 4 below.

For reference, the results of Example 2, which consisted of a membrane forming solution with an average degree of acetylation of 58.2%, are also listed in the same table.

In addition, the water permeability and salt separation rate in the table are average values over the entire length of the membrane.

As is clear from the above table, in the method of this invention, if cellulose acetate with an average degree of acetylation is used is 57 to 60%, water permeation can be maintained at a relatively high level and the salt separation rate can be increased. It can be seen that good results are obtained in terms of membrane performance.

Comparative Example 5 A tubular semipermeable membrane was produced in exactly the same manner as in Example 2, except that 9 parts of methanol in the membrane forming solution composition described in Example 1 was not used.

The average water permeability over the entire length of this semipermeable membrane is 6. 4 r
tf:/tri'·day, the average salt separation rate was 5%.

On the other hand, since the heat treatment temperature in the above example was as low as 70°C, when this temperature was changed to 95°C, the average water permeation rate of the obtained tubular semipermeable membrane was 0. The average salt separation rate was 95% at 32 m/rrr'·day.

As can be understood by comparing the above results with the results of Example 2, in the method for manufacturing a tubular semipermeable membrane, it is better to use a lower alcohol together with a hydroxycarboxylic acid as an additive component of the membrane forming solution. It can be seen that a high average salt separation rate and a sufficiently satisfactory average water permeation rate can be obtained by applying a low heat treatment temperature, which is more desirable with the same membrane properties.

Claims (1)

[Scope of Claims] 1 Contains 5 to 30% by weight of hydroxy acid and 5 to 15% by weight of lower alcohol, and the combined ratio (weight ratio) of hydroxy acid and lower alcohol is hydroxy acid:lower alcohol-1:0. After applying the cellulose acetate-based film-forming solution described in 5 to 3 on the inner surface of the tube, aeration treatment is performed from one end of the tube to the other end, and vice versa. Alternately and together with ventilation treatment for 5 to 1
5 to 20% by weight of the solvent in the film-forming solution is evaporated by carrying out at least two passes in both directions at an aeration rate of 5 m/sec, and then the tube is immersed in water in the opposite direction to the final aeration direction. A method for manufacturing a tubular semipermeable membrane, which comprises immersing the membrane to solidify the membrane-forming solution, and further heat-treating the membrane after solidification. 2. The method for producing a tubular semipermeable membrane according to claim 1, which uses a membrane forming solution having an average degree of acetylation of cellulose acetate of 57 to 60% by weight. 3 Cellulose acetate 10-20% by weight with an average degree of acetylation of 57-60% by weight, acetone 15-40% by weight, ■,
4-dioxane 30-50% by weight, hydroxy acid 5-3
2. The method for producing a tubular semipermeable membrane according to claim 1, which uses a membrane forming solution containing 0% by weight and 5 to 15% by weight of lower alcohol.