JPS62279808A - Membrane distiller - Google Patents

Abstract

PURPOSE:To enhance thermal efficiency and to reduce distillation cost by using a hydrophobic porous membrane produced by a biaxial stretching method. CONSTITUTION:A hydrophobic porous membrane 2 produced by a biaxial stretching method wherein the membrane is formed by stretching and extending tetrafluoroethylene or the like i.e. in the vertical direction together with the right and left direction is fitted to the inside of a housing 1 made of acrylic resin. Liquid to be treated which is incorporated in a conical beaker 5A set in a heating tank 4 heated by a heater 3 is circulated to one side comparted by the hydrophobic porous membrane 2 incorporated in the housing 1 and pure water which is absorbing liquid incorporated in a beaker 5B set in a cooling tank 8 cooled by a cooler 7 is circulated to the other side comparted by the membrane 2 and water content incorporated in the liquid to be treated is evaporated via the membrane 2 and the vapor is directly absorbed by the absorbing liquid.

Description

[Detailed Description of the Invention] 3. Detailed Description of the Invention <Industrial Application Field> The present invention is a membrane distillation system used for the concentration of alcohol, desalination of seawater, production of distilled water, concentration of non-volatile substances, etc. It is related to the device.

<Prior art> So-called hydrophobic porous membranes, which are made of hydrophobic materials such as tetrafluoroethylene, polypropylene, polyvinylidene fluoride, and polyethylene, and have micropores in them, do not allow liquid to pass through. Furthermore, because it has the property of allowing gases such as steam to pass through, it is used as a material for raincoats, sportswear such as skiing, golf, etc., and hats, and the fine pores of the film can be used to remove particulates present in the liquid. It is currently used as a precision filtration membrane for this purpose.

Furthermore, in recent years, it has been proposed to use the hydrophobic porous membrane in a membrane distillation apparatus.

In other words, a liquid to be treated is brought into contact with one side of a hydrophobic porous membrane, and a different type of liquid or the same type of liquid present in the liquid to be treated is evaporated through the membrane, and the liquid is brought into contact with the other side of the membrane. It cools the transferred vapor to obtain a liquid, which is used for concentrating alcohol, desalinating seawater, producing distilled water, etc.

Membrane distillation apparatuses that have been proposed so far can be roughly divided into direct methods and indirect methods.

In other words, in the direct method, the liquid to be treated is passed through one side partitioned by the hydrophobic porous membrane while being in contact with the membrane, and the vapor pressure lower than the vapor pressure of the liquid to be evaporated from the liquid to be treated is passed through the other side. In this method, an absorption liquid is passed through the membrane while being in contact with the membrane, and vapor evaporated from the contact surface between the membrane and the liquid to be treated is passed through the fine pores of the membrane, and is condensed and absorbed on the surface of the absorption liquid.

In addition, the indirect method means that the liquid to be treated passes through one side partitioned by the hydrophobic porous membrane while being in contact with the membrane, and a cooling surface is placed on the other side through a space, and the liquid permeates through the membrane. The vapor is diffused in the space, and then condensed and liquefied on the cooling surface.

<Problems to be Solved by the Invention> The outline of the membrane distillation apparatus is as described above. In order to create a difference in the vapor pressure of the membrane, it is generally necessary to make the temperature of the liquid to be treated passing through one side of the membrane higher than that of the fluid passing through the other side. Therefore, when membrane distillation is performed while creating such a temperature difference, not only vapor movement occurs due to the vapor pressure difference, but also heat movement occurs due to the temperature difference. Here, the former heat transfer due to the movement of vapor is called evaporation latent heat, and the latter heat transfer based on the temperature difference is called conduction heat, and the sum of the evaporation latent heat and conduction heat is the heat transfer through the membrane.

The purpose of a membrane distillation apparatus is the movement of steam, and of the total heat transfer, the latent heat of vaporization is the amount of heat required for the movement of vapor. Therefore, the larger the proportion of the latent heat of vaporization is, , it can be said that the thermal efficiency in the membrane distillation apparatus is high.

In other words, in a membrane distillation apparatus, it is advantageous to use a hydrophobic porous membrane that can reduce conductive heat and increase latent heat of vaporization.

In view of this, the present inventor conducted trial-and-error experiments using various membranes in order to find the most suitable hydrophobic porous membrane for use in a membrane distillation apparatus, and as a result, the material and thickness of the membrane were found to be Even if they are the same, the thermal efficiency varies considerably depending on the manufacturing method, and it was found that the thermal efficiency of the membrane manufactured by the biaxial stretching method is better than that of the hydrophobic porous membrane manufactured by the axle stretching method, without exception. did.

<Means for Solving the Problems> The present invention has been made based on the above-mentioned knowledge, and therefore, a hydrophobic porous membrane manufactured by a biaxial stretching method that allows gas to pass through but not liquid to pass therethrough is used. The present invention relates to a membrane distillation apparatus characterized by the following.

Effect〉 The present invention will be explained in detail below.

When manufacturing the hydrophobic porous membrane, for example, when manufacturing a flat membrane, a hydrophobic synthetic resin material such as tetrafluoroethylene, polypropylene, polyvinylidene fluoride, polyethylene, etc. is made into a plastic state and stretched only in the left and right direction. an axle stretching method in which a film is formed by spreading the material, and a biaxial stretching method in which a film is formed by stretching the material in a plastic state not only in the horizontal direction but also in the vertical direction, for example. be.

In addition, when producing a tubular hydrophobic porous membrane, there is an axle stretching method in which the material is extruded from a ring-shaped nozzle and formed while being pulled only from the outflow direction of the nozzle. Arrange a conical spacer whose bottom circumference is larger than the diameter of the nozzle so that it widens toward the end, and pull it from the direction of flow while pushing the inner surface of the tube wider with the spacer from inside the tube flowing out from the nozzle. There is a biaxial stretching method.

Initially, the inventors of the present invention had no connection whatsoever with the differences in these manufacturing methods, and considered the differences in the materials such as tetrafluoroethylene, polypropylene, polyvinylidene fluoride, and polyethylene, the differences in film thickness, and the film shapes such as flat films and tubes. I was researching the differences in thermal efficiency in membrane distillation of various hydrophobic porous membranes due to differences in membrane distillation, but I found that there are two groups with superior thermal efficiency and one with inferior thermal efficiency, despite having exactly the same material, membrane thickness, and membrane shape. It turns out that there are two groups:

When we investigated the differences in detail, we found that the group with superior thermal efficiency was without exception hydrophobic porous membranes produced by biaxial stretching, and the group with poor thermal efficiency was the exception (axle stretching). It was discovered that the membrane was a hydrophobic porous membrane manufactured by.

It is currently unclear why the hydrophobic porous membrane produced by the biaxial stretching method has better thermal efficiency than the membrane produced by the axle stretching method, but The micropores formed on the membrane surface of the hydrophobic porous membrane are not called pores because they extend only from the left and right sides.
For example, the micropores of the hydrophobic porous membrane produced by the biaxial stretching method have an elongated slid shape with a short axis of about 1 μm and a long axis of 15 μm or more, extending in the left and right stretching directions.
Since the hole extends from the left and right direction and also from the top and bottom directions alternating at right angles to this direction, the hole has a circular shape in which the left and right diameters and the top and bottom diameters are approximately equal in diameter, for example, from 0.02 μm to 10 μm. It is thought that the underlying cause may be the difference in the shape of the micropores.

In any case, the reason is not clear, but when comparing the thermal efficiency of a hydrophobic porous membrane manufactured by the axle stretching method and a hydrophobic porous membrane manufactured by the biaxial stretching method, the latter is superior to the former. Since the thermal efficiency is clearly high, it is much more advantageous to use a hydrophobic porous membrane manufactured by a biaxial stretching method for use in a membrane distillation apparatus.

Regarding membrane distillation equipment that conventionally uses hydrophobic porous membranes,
Although various papers have been published, there is no mention that the thermal efficiency of the membrane varies depending on the method of manufacturing the membrane, as in the present invention, and the present invention is based on completely new knowledge.

As mentioned above, the hydrophobic porous membrane used in the present invention has different axial directions and angles in the left-right direction and the left-right direction.
For example, if the film is stretched in multiple directions, such as orthogonal up and down directions, at least in the direction of two wheels, it can be in the form of a flat film, furthermore, it can be in the form of a tube stretched through the aforementioned spacer, or a flat film produced by a biaxial stretching method. Any shape can be used, such as a spiral shape formed into a bag shape and further formed into a spiral shape, or a tube shape or tube shape obtained by joining both ends of the flat membrane. In addition, the fine pores on the membrane surface have a diameter of 0.02 μm to 1
It is desirable to use one with a diameter of 0 μm.

Any material can be used as long as it is a hydrophobic substance and can be stretched in a plastic state, but ethylene tetrafluoride, polypropylene, and polyvinylidene fluoride are highly hydrophobic and easy to stretch. Preferably, one selected from polyethylene and polyethylene is preferred, and tetrafluoroethylene is one of the four types having a large hydrophobicity.

Furthermore, the membrane distillation apparatus of the present invention is equipped with a hydrophobic porous membrane manufactured by the above-mentioned biaxial stretching method, and the liquid to be treated is passed through one side partitioned by the membrane while being in contact with the membrane. In addition, there is a direct method in which an absorbing liquid with a lower vapor pressure than the liquid to be evaporated from the liquid to be evaporated is passed through the membrane on the other side while in contact with the membrane, or an indirect method in which a cooling surface is placed on the other side through a space. It can be used for various purposes such as desalination of seawater, production of demineralized water (distilled water), concentration of non-volatile liquids, concentration of volatile liquids, and removal of water from blood.

Effects> As explained above, the membrane distillation apparatus of the present invention uses a hydrophobic porous membrane with high thermal efficiency manufactured by a specific method among hydrophobic porous membranes, so the distillation cost can be significantly reduced. This is a great benefit to industry.

Examples will be described below to make the effects of the present invention more clear.

<Example> Using the experimental apparatus of a membrane distillation apparatus as shown in Fig. 1,
The following experiment was conducted.

The experimental device has a hydrophobic porous membrane 2 installed in a housing 1 made of acrylic resin with a thickness of approximately 20 m, and a heater 3.
Using the pump 6A, the EI liquid in the conical beak 5A placed in the heating tank 4 heated to a constant temperature is
Al-pure water is circulated through one side of the housing 1 partitioned by the hydrophobic porous membrane 2, and the absorption liquid in the conical beaker 5B placed in the cooling tank 8 cooled to a constant temperature by the cooler 7 is used. is circulated to the other side of the housing 1 partitioned by the hydrophobic porous membrane 2 using the pump 6B, and the water in the liquid to be treated is evaporated via the hydrophobic porous membrane 2 due to the temperature difference, The vapor is directly absorbed by the absorption liquid.

In addition, both conical beakers 5A and 5B are tightly plugged, and an air vent pipe 9A is attached to the conical beaker 5A side.
Conical Ruby Force 5B is filled with pure water which is Absorption 1,
The water increased by membrane distillation overflows from the bypass pipe 10, and the overflow water is transferred to the electronic balance 11.
The structure is such that a microcomputer 12 can record the increase in weight at regular intervals due to the conical ruby force 5C placed above. Note that the conical ruby 5C is also sealed, but an air vent pipe 9B is attached. Also 13.14
．． 15.16 is a thermometer.

A hydrophobic porous membrane having slit-like micropores with a short diameter of 1 μm and a long diameter of 15 μm manufactured by the axle stretching method is housed in the housing 1 of the experimental apparatus as shown in the figure, and a hydrophobic porous membrane having slit-like micropores with a short diameter of 1 μm and a long diameter of 15 μm manufactured by the axle stretching method and a biaxial stretching method (stretched in the left-right direction The same hydrophobic porous membrane except that it has circular micropores with a diameter of 0.8 μm manufactured by stretching with two axes in the vertical direction perpendicular to the left-right axis direction is attached. The thermal efficiency was compared between the two manufacturing methods, and the results are shown in Tables 1 and 2. The hydrophobic porous membrane used in the experiment was made of tetrafluoroethylene, with an effective area of 75 cI+1, and two types of membrane thickness, 150 μm and 100 μm, respectively. Table 2 shows the results when the film thickness was 100 μm. Further, the flow rates of both the liquid to be treated and the absorption liquid were 1.51/min.

Although a 5000 g/6 salt solution was used as the liquid to be treated, there was no increase in salt content at all in the distilled water subjected to the conical Ruby force of 5C.

Table 2 (film thickness 100 μm) Thermal efficiency is the temperature at each point and the conical Ruby force when the flow rate of distilled water received by the temperature at each point and the conical Ruby force of 5C becomes a constant value, a so-called steady state. It can be calculated from both the treated liquid side and the absorption liquid side using the flow rate (membrane permeation flow rate) of distilled water received at 5C.

For example, using the treated liquid inlet temperature of 42.1°C and the treated liquid outlet temperature of 40.2°C and 2 m permeation flow rate of 0.19 i/day O value of the hydrophobic porous membrane manufactured by the axle stretching method shown in Table 1. The thermal efficiency calculated from the treated liquid side is as follows.

Thermal efficiency'-(42,1-40,2)Xl, 51/minXI
Kcaj! /'C = 2.85 Kcal/minute The membrane permeation flow rate is 0.19 n(/rrlday, and when converted to an effective membrane area of 75 cd and a flow rate per spectral spectrum, it is 0.99 mj!/min, and therefore the evaporation rate is a constant. When the latent heat of vaporization is calculated using the latent heat of 570 Kcaj2/mJ, it is: =0.55 Kcal/min - 19.3% At each temperature for each film thickness of both hydrophobic porous membranes calculated by the above calculation method The thermal efficiency (the average value of the calculated value from the treated liquid side and the calculated value from the absorption liquid side) is shown in Tables 1 and 2, but the hydrophobic porous membrane manufactured by the biaxial stretching method However, it has been shown that the thermal efficiency of these materials is approximately 40 to 60% higher than that produced by the axle stretching method.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the flow of the experimental apparatus of the membrane distillation apparatus used in the examples. 1...Housing 2...Hydrophobic porous membrane 3
... Heater 4 ... Heating tank 5 ... Conical beaker 6 ... Pump 7 ... Cooler
8...Cooling tank 9...Air vent pipe 1o...
・Bypass pipe 11...Electronic balance 12...Microcomputer 13-16...Thermometer

Claims (1)

[Scope of Claims] 1. A membrane distillation device characterized by using a hydrophobic porous membrane manufactured by a biaxial stretching method that allows gas to pass through but not liquid. 2. The membrane distillation apparatus according to claim 1, wherein the hydrophobic porous membrane is made of one selected from tetrafluoroethylene, polypropylene, polyvinylidene fluoride, and polyethylene.