JPS6157205A - Treatment of aqueous solution by thermo-pervaporation method - Google Patents

Abstract

PURPOSE:To perform thermo-pervaporation effectively even with respect to an aqueous solution, of which the surface tension is low because a surfactant or org. substance is contained, without performing the membrane permeation of the aqueous solution, by using a microporous membrane having minute pores with an average pore size of a specific value or less. CONSTITUTION:A membrane tube 2 comprising a microporous membrane made of a polytetrafluoroethylene having minute pores with an average pore size of 0.4mum or less is coaxially arranged in an outer pipe 1. An aqueous solution containing a surfactant or org. substance and having low surface tension of 65- 25dyn/cm is heated and recirculated to an aqueous solution passage 3 through an introducing pipe 4 and a discharge pipe 5. Steam generated from the aqueous solution is transmitted through the membrane tube 2 to reach a steam diffusion space 10 and cooled by a heat transfer pipe 9 to be converted to condensed water which is, in turn, guided out of the system from a discharge pipe 13. By this method, thermo-pervaporation can be effectively adapted without preliminarily removing the surfactant or org. substance in the aqueous solution.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating an aqueous solution using a thermopervaporation method, and more specifically, the present invention relates to a method for treating an aqueous solution using a thermopervaporation method. Concerning a method of obtaining condensed water.

For example, as described in Japanese Patent Publication No. 49-45461, water is separated from an aqueous solution, or the aqueous solution is
As a method for condensation, an aqueous solution heated to a predetermined temperature, such as hot sea water, is passed through one side of a hydrophobic microporous membrane that allows water vapor to pass through but not the aqueous solution itself. Water vapor is passed through the microporous membrane, cooled and condensed by a low-temperature heat transfer wall placed opposite to the other side of the membrane, and in this way an aqueous solution is formed on one side of the microporous membrane. A liquid separation method using thermopervaporation, in which S condenses and condensed water is obtained on the other side, is already known.

For concentrating and separating liquids using the thermo-vaporage condensation method, a hydrophobic heat-resistant microporous membrane made of a fluororesin such as polytetrafluoroethylene is typically used as a separation means. The method is t'f of aqueous solution.
It is suitable for separating water from iaM and its aqueous solution. For example, a typical aqueous solution whose main component is water usually has a surface tension of about 72 to 6 Q dyn/cm, and the hydrophobicity of the fluororesin is maintained in such an aqueous solution, so that the membrane permeation of the aqueous solution is difficult. The thermopervaporation method can concentrate the aqueous solution and also obtain condensed water. However, when the aqueous solution contains a surfactant or an organic substance and has a small surface tension of 65 to 25 dyn/cm, the critical surface tension may be lowered, such as with a microporous membrane made of polytetrafluoroethylene. Even a microporous membrane made of the smallest resin loses its hydrophobic effect in such an aqueous solution, and as a result, the microporous membrane allows the aqueous solution to pass through, and the removal rate for solutes decreases. At the same time, an aqueous solution may be mixed into the condensed water, reducing its purity.

Therefore, when the aqueous solution contains a surfactant as mentioned above, it is preferable to apply the thermopervaporation method after removing the surfactant. Not only when it contains organic substances, but also when it contains trace amounts of organic substances such as seawater, the surface tension of the aqueous solution is small due to its surfactant action, so it is necessary to apply the thermopervaporation method in advance. It is preferable to remove these organic substances and increase their surface tension.However, it is not easy to remove surfactants and trace amounts of organic substances from aqueous solutions, and rather they can be removed from aqueous solutions with high efficiency. What is required is a method to do so.

Therefore, as a result of intensive research in order to meet the above-mentioned demands, the present inventors unexpectedly found that by using a microporous membrane having micropores of a predetermined pore size, it is possible to contain surfactants and organic substances. The inventors have discovered that the thermopervaporation method can be effectively applied to an aqueous solution having a low surface tension without substantially permeating the aqueous solution through a membrane, leading to the present invention.

In the present invention, an aqueous solution is brought into contact with one side of a hydrophobic microporous membrane that does not allow an aqueous solution to pass through, but allows a water vapor to pass through, generates water vapor from this aqueous solution, and transfers this to the other side of the microporous membrane. In a method for treating an aqueous solution using a thermopervaporation method in which it is permeated, cooled, and condensed, an aqueous solution having a surface tension in the range of 65 to 25 dyn/cm is passed through a microporous membrane having micropores with an average pore size of 0.4 μm or less. It is characterized by being brought into contact.

According to the thermopervaporation method of the present invention, the microporous membrane used substantially does not permeate an aqueous solution with a small surface tension in the range of 65 to 25 dyrr/cm, but allows water vapor to permeate. Therefore, it is necessary that the average pore diameter of the micropores is 0.4 μm or less, preferably 0.3 μm or less.

When the micropore diameter is, for example, 0.5 μm or more, the aqueous solution itself having a low surface tension as described above permeates through the membrane at least partially, and the aqueous solution is mixed into the condensed water. Although the film thickness is not particularly limited, it is usually 1 to 300 μm, preferably 5 to 50 μm.

In the present invention, the average pore diameter of the micropores of the microporous membrane is
8 STM Designation: , F31640
means the average pore diameter measured by the method described in .

That is, a microporous membrane is completely wetted and maintained with an appropriately selected organic solvent (n-hexyl ether having a surface tension of about 22 dyn/cm is used in the present invention), and nitrogen is applied to one side of the membrane. The gas is brought into contact with the membrane under continuous pressure, and the flow rate of nitrogen gas permeated through the membrane against the pressure of nitrogen gas is measured to determine the nitrogen gas permeation flow rate line (al) through the wet membrane.

Separately, the nitrogen gas permeation flow rate line fb) is determined in the same manner for the dried microporous membrane. This line (bl is substantially a linear function of nitrogen gas pressure and forms a straight line. Next, the straight line 'Ki (
Find the intersection point I between c) and the above line (a), and find the nitrogen gas pressure P (anllg) corresponding to that point. Then, the surface tension of the existing solvent is γ (mN/m), and the contact angle is θ. The average pore diameter d (μm) of the micropores of the microporous membrane can be obtained as follows.

゛In the present invention, the above-mentioned microporous membrane is specifically a microporous membrane made of a fluororesin such as polytetrafluoroethylene resin, vinylidene fluoride resin, ethylene-tetrafluoroethylene copolymer resin, etc. A high quality membrane is particularly preferably used because it has excellent heat resistance and hydrophobicity. However, even for microporous membranes made of hydrophilic resins such as polysulfone or cellulose resin, the surface can be coated with an agricultural water resin such as fluororesin or silicone resin to provide a hydrophobic microporous surface. In some cases, these resin films can also be used.

Note that since microporous membranes generally have low strength, they may be supported on an appropriate support. Such a support only needs to be able to reinforce the microporous membrane and allow water vapor to pass therethrough, and for example, a woven or nonwoven fabric made of polyamide or a porous tube made of ceramic is preferably used.

1 and 2 show an example of a thermopervaporation device according to the present invention.

That is, a membrane tube 2 made of a microporous membrane is disposed coaxially within the outer tube 1, and a tube for an aqueous solution heated to a predetermined temperature is further disposed between the membrane tube and the outer tube. An aqueous solution passage 3 is formed.

An aqueous solution inlet pipe 4 and an aqueous outlet pipe 5 are connected to the aqueous solution passage 3, and the aqueous solution is appropriately replenished into the aqueous solution circuit from an aqueous solution supply pipe 7 equipped with a valve 6, and circulated through the aqueous solution circuit. If necessary, the aqueous solution is heated to a predetermined temperature by heaters 8 provided in these pipes. Further, although not shown, a portion of the aqueous solution is discharged from the aqueous solution circuit through a discharge pipe as necessary.

A heat transfer tube 9 having a heat transfer wall is further disposed coaxially inside the membrane tube 2, and a vapor diffusion space 1 is formed between the membrane tube 2 and the membrane tube 2.
Appropriate intervals are provided so that 0 exists. The heat exchanger tube is made of a material with high heat conductivity, such as a thin-walled metal tube. These heat transfer tubes include an inlet pipe 11 and an outlet pipe 12 for the cooling medium.
are connected, and a cooling medium such as cooling water is passed through the heat exchanger tube in a VA ring.

Connected to the lower end of the vapor diffusion space is an outlet pipe 13 for taking out the condensed water that has been cooled and condensed by the heat transfer tube 9 in the vapor space.

In the above-mentioned apparatus, an aqueous solution heated to a predetermined temperature is introduced into the aqueous solution passage 3, and water vapor generated from the aqueous solution passes through the membrane tube 2, reaches the vapor diffusion space 10, and diffuses through the vapor diffusion space. It is cooled in the heat exchanger tube 9 to produce condensed water, and this condensed water flows down the surface of the heat exchanger tube to the condensed water thinning tube 13.
4 outside the device.

As described above, according to the method of the present invention, the average pore size is 0.4 μm or less, preferably 0.3 μm or less, even for an aqueous solution whose surface tension is low because it contains a surfactant or an organic substance.
Microporous membranes with micropores of micrometers or less do not allow the aqueous solution itself to pass through the membrane, but allow only water vapor to pass through the membrane. Therefore, thermopervapor can be effectively used without removing surfactants and organic substances from the aqueous solution in advance. It is possible to concentrate aqueous solutions and obtain condensed water using the ration method.

Therefore, the method according to the present invention is suitable, for example, for the desalination of seawater, as well as the concentration of various aqueous solutions containing organic substances, and the separation of water from aqueous solutions, and is useful, for example, in the food and pharmaceutical industries. Suitable for concentration separation of components and wastewater treatment, specifically, concentration of fish and shellfish extracts, fruit juice concentration of mandarin oranges, treatment of pectin and gelatin aqueous solutions, wastewater treatment of potato wastewater, dyeing, pulp wastewater, etc. obtain.

Examples of the present invention are listed below.

Example 1 As shown in Figure 1, polytetrafluoroethylene micropores with a thickness of 60 μm and having micropores with various average pore diameters are placed inside a synthetic resin outer tube with a diameter of 40 μm. The membranes were arranged coaxially to form a 11-way tube with a diameter of about 28 mm, and a stainless steel heat exchanger tube was further arranged within this membrane tube so that the width of the vapor diffusion space was 2.3 mm. , a thermopervaporation device was constructed. The effective membrane area in the device is 240cn!
Met.

In this device, a 3.5% by weight aqueous sodium chloride solution A with a surface tension cuff of 6 dyn/cm and 3.5% by weight
Sodium chloride water? Aqueous solution B prepared by adding sodium dodecylbenzenesulfonate to gt& to a concentration equal to or higher than the critical micelle concentration to adjust the surface tension to 31 dyn/cm.
Each of the aqueous solutions was heated to a temperature of 60°C and passed through an aqueous solution passage, and cooling water at a temperature of 4°C was passed through a heat transfer tube to concentrate each aqueous solution and obtain condensed water.

As the results are shown in the table, surface tension cuff 6 dyn/cm
For aqueous solution A with a surface tension of 31 dyn/cm, the same results were obtained for both condensed water acquisition rate and salt removal rate regardless of the micropore diameter, but for aqueous solution B with a surface tension of 31 dyn/cm, the condensed water acquisition rate and salt removal rate Both salt rates have a remarkable dependence on the micropore diameter, and according to Examples 1 and 2 of the present invention, the average pore diameter was 0.3.
According to an apparatus equipped with a microporous membrane having micropores of micrometers or less, condensed water can be obtained without an aqueous solution passing through the membrane. However, when the micropore diameter is larger than this, it is clear that membrane permeation of the aqueous solution occurs from the rapid decrease in the salt removal rate and the significant increase in the condensed water acquisition rate.

In addition, in the case of Comparative Example 1, although the membrane was not wetted by the aqueous solution B, the aqueous solution permeated through the membrane. In the case of Comparative Examples 2 and 3, the membrane was completely wetted by the aqueous solution B, especially in the latter case.
Even when the pump that circulated the aqueous solution to the aqueous solution passage was stopped, the aqueous solution permeated through the membrane due to its static pressure.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view showing an example of thermopervaporation according to the present invention, FIG. 2 is a sectional view taken along line A-A in FIG. The average pore diameter is determined by ASTM Designation: F316-
80 is a graph showing an example obtained according to the method described in No. 80. 1... Outer tube, 2... Membrane tube, 3... Aqueous solution passage, 4
...Aqueous solution inlet pipe, 5...Aqueous solution outlet pipe, 7...
Aqueous solution supply pipe, 9... Heat transfer tube, 10... Vapor diffusion space, 11... Cooling medium introduction pipe, 12... Cooling medium outlet pipe, 13... Condensed water extraction pipe.

Claims (1)

[Claims] (1) An aqueous solution is brought into contact with one side of the hydrophobic microporous membrane that does not allow the aqueous solution to pass through, but allows water vapor to pass through, and water vapor is generated from the aqueous solution and is allowed to pass through the other side of the microporous membrane. , in a method for treating an aqueous solution using the thermopervaporation method, which involves cooling and condensing, the surface tension is 6.
An aqueous solution in the range of 5 to 25 dyn/cm with an average pore size of 0
．． A method for treating an aqueous solution by a thermopervaporation method, which comprises bringing the solution into contact with a microporous membrane having micropores of 4 μm or less.