JPH07100133B2 - Membrane distillation device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Field of Industrial Application> The present invention relates to a membrane distillation apparatus used for such purposes as concentration of alcohol, desalination of seawater, production of distilled water, concentration of nonvolatile substances, and the like.

<Prior Art> A so-called hydrophobic porous membrane in which a hydrophobic material such as ethylene tetrafluoride, polypropylene, polyvinylidene fluoride, or polyethylene is formed into a membrane and the membrane has fine pores does not allow liquid to pass through. In addition, because it has the property of passing vapors and other gases, it can be used as a material for raincoats, sportswear such as skis, golf, and hats, and the fine pores of the film can be used to remove fine particles present in the liquid. It is currently used as a microfiltration membrane for this purpose.

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

That is, the liquid to be treated is brought into contact with one side of the hydrophobic porous membrane, different types of liquids or liquids of the same type existing in the liquid to be treated are evaporated through the membrane, and the other side of the membrane is evaporated. It cools the transferred vapor to obtain a liquid, which is used for concentration of alcohol, desalination of seawater, production of distilled water, etc.

Membrane distillation apparatuses proposed hitherto are roughly classified into a direct method and an indirect method.

That is, the direct method allows a liquid to be treated to pass through one side partitioned by the hydrophobic porous membrane while being in contact with the membrane, and has a vapor pressure lower than that of the liquid to be evaporated from the liquid to be treated on the other side. This is a method in which the absorbing liquid is passed while being in contact with the film, and the vapor evaporated from the contact surface between the film and the liquid to be treated is transmitted through the fine pores of the film to be condensed and absorbed on the surface of the absorbing liquid.

The indirect method is to pass the liquid to be treated on one side partitioned by the hydrophobic porous membrane while making contact with the membrane, and to arrange a cooling surface on the other side through a space to permeate from the membrane. The vapor is diffused in the space and then condensed on the cooling surface to be liquefied.

<Problems to be Solved by the Invention> The outline of the membrane distillation apparatus is as described above, and is between the liquid on one side partitioned by the hydrophobic porous membrane and the fluid on the other side such as the absorbing liquid or the gas. In order to make the vapor pressures of the liquids different, it is generally necessary to make the temperature of the liquid to be treated which is passed through one side partitioned by the membrane higher than that of the liquid which is passed through the other side. Therefore, when membrane distillation is carried out while causing such a temperature difference, not only the vapor movement due to the vapor pressure difference but also the heat movement due to the temperature difference occurs. 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. The sum of the evaporation latent heat and conduction heat is the heat transfer moving through the film.

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

In other words, in the membrane distillation apparatus, it is advantageous to use a hydrophobic porous membrane that can have smaller conduction heat and larger evaporation latent heat.

In view of this point, the present inventor conducted trial and error experiments using various membranes in order to find an optimal hydrophobic porous membrane to be used in a membrane distillation apparatus, and as a result, the material or the thickness of the membrane was changed. Even if they are equal, the thermal efficiency is considerably different depending on the manufacturing method, and the membrane produced by the biaxial stretching method has better thermal efficiency than the hydrophobic porous membrane produced by the uniaxial stretching method. I found out.

<Means for Solving Problems> The present invention has been made based on the above-mentioned findings, and uses a hydrophobic porous membrane having a property of passing gas but not liquid, which is produced by a biaxial stretching method. The present invention relates to a membrane distillation apparatus characterized in that

<Operation> The present invention will be described in detail below.

When forming the hydrophobic porous film, for example, when manufacturing a flat film, a hydrophobic synthetic resin material such as ethylene tetrafluoride, polypropylene, polyvinylidene fluoride, and polyethylene is stretched in the lateral direction only in a plastic state. A uniaxial stretching method of forming a film by spreading the material and a biaxial stretching method of forming a film by stretching the material in a plastic state in the horizontal direction as well as in the vertical direction, for example, and spreading. is there.

Also when producing a tubular hydrophobic porous membrane, the material is extruded from a ring-shaped nozzle, a uniaxial stretching method of forming while pulling only from the outflow direction of the nozzle, and the outlet of the nozzle, It is formed by arranging a conical spacer whose bottom circumference is larger than the diameter of the nozzle so that it spreads out toward the end, and pulling it from the outflow direction while pushing the inner surface of the tube out of the inside of the tube flowing out from the nozzle. There is a biaxial stretching method.

Initially, the inventors of the present invention were completely irrelevant with respect to the difference in these manufacturing methods, and were different in materials such as tetrafluoroethylene, polypropylene, polyvinylidene fluoride, polyethylene, etc., in film thickness, or in flat film, tubular film, etc. We were studying the difference in the thermal efficiency in the membrane distillation of various hydrophobic porous membranes due to the difference in the thermal efficiency. It turned out that it can be divided into two groups of the existing group.

And when the differences were investigated in detail, the group with excellent thermal efficiency was the hydrophobic porous membrane produced by the biaxial stretching method without exception, and the group with poor thermal efficiency was the uniaxial stretching method without exception. It was found that it was a hydrophobic porous membrane manufactured by.

It is not clear at this time why the hydrophobic porous membrane produced by the biaxial stretching method has better thermal efficiency than the membrane produced by the uniaxial stretching method, but it is not clear by the uniaxial stretching method. The micropores formed on the film surface of the manufactured hydrophobic porous film are stretched from only the left and right, so rather than called pores,
For example, it has a long and narrow slit shape with a short diameter of about 1 μm and a long diameter of 15 μm or more extending in the left and right stretching directions, while the fine pores of the hydrophobic porous membrane produced by the biaxial stretching method are For example, since it is also stretched in the vertical direction alternating with this at right angles, the horizontal diameter of the hole and the vertical diameter have a substantially equal circular shape with a diameter of 0.02 μm to 10 μm, for example.
Alternatively, it is considered that there is a distant cause in such a difference in the shape of the fine holes.

In any case, the reason is not clear, but comparing the thermal efficiencies of the hydrophobic porous membrane produced by the uniaxial stretching method and the hydrophobic porous membrane produced by the biaxial stretching method, the latter is better than the former. Since the thermal efficiency is obviously higher, it is more advantageous to use the hydrophobic porous membrane used in the membrane distillation apparatus manufactured by the biaxial stretching method.

Conventionally, regarding the membrane distillation apparatus using a hydrophobic porous membrane,
Although various papers have been published, there is no description that the thermal efficiency is different due to the difference in the manufacturing method of the membrane as in the present invention, and the present invention is based on a completely new finding.

As the hydrophobic porous membrane used in the present invention, as described above, the angle is different from the left-right direction and the axial direction of the left-right direction.
For example, in the case of a film stretched from at least two or more biaxial directions, such as a vertical direction, which is orthogonal, a flat film shape, a tube shape stretched through the above-mentioned spacer, and a flat film manufactured by a biaxial stretching method. It is possible to use any shape such as a spiral shape in which the membrane is formed into a bag shape and further formed in a spiral shape, or a tubular shape or a tubular shape obtained by joining both ends of the flat membrane. Further, it is desirable to use micropores having a diameter of 0.02 μm to 10 μm in which the right and left diameters are substantially equal to the upper and lower diameters as the fine holes on the film surface.

Any material can be used as long as it is a hydrophobic substance and can be stretched in a plastic state, but as a material having strong hydrophobicity and easy stretching, tetrafluoroethylene, polypropylene, polyvinylidene fluoride , Polyethylene is preferred, and tetrafluoroethylene is the most hydrophobic of the four.

Further, the membrane distillation apparatus in the present invention is equipped with the hydrophobic porous membrane produced by the above-mentioned biaxial stretching method, and the liquid to be treated is passed through on one side partitioned by the membrane while being brought into contact with the membrane. At the same time, a direct method of passing an absorbing liquid having a vapor pressure lower than that of the liquid to be evaporated from the liquid to be treated on the other side while contacting the membrane, or an indirect method in which a cooling surface is arranged on the other side via a space Can be used as a method, desalination of seawater, production of demineralized water (distilled water), concentration of non-volatile liquids,
It can be used for various purposes such as concentration of volatile liquid and removal of water from blood.

<Effect> As described above, the membrane distillation apparatus of the present invention uses the hydrophobic porous membrane having a high thermal efficiency, which is manufactured by a specific method, among the hydrophobic porous membranes, so that the distillation cost is significantly reduced. It can be done, and it has a great advantage to the industry.

Examples will be described below in order to clarify the effects of the present invention.

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

The experimental apparatus is a conical beaker 5A mounted in a heating tank 4 in which a hydrophobic porous membrane 2 is mounted in a housing 1 made of acrylic resin having a thickness of about 20 mm and heated to a constant temperature by a heater 3. The liquid to be treated is circulated to one side partitioned by the hydrophobic porous membrane 2 in the housing 1 using the pump 6A, and is cooled to a constant temperature by the cooler 7
Pure water, which is the absorbing liquid in the conical beaker 5B placed inside the pump, is pumped by the pump 6B and the hydrophobic porous membrane 2 in the housing 1
It is circulated to the other side partitioned by, the water in the liquid to be treated is evaporated through the hydrophobic porous membrane 2 due to the temperature difference, and the vapor is directly absorbed by the absorbing liquid.

In addition, both conical beakers 5A and 5B are hermetically sealed, an air vent pipe 9A is attached to the conical beaker 5A side, and the conical beaker 5B is filled with pure water as an absorbing liquid, and the water increased by membrane distillation is bypassed by a bypass pipe 10 The conical beaker 5C placed on the electronic balance 11 is caused to overflow more, and the overflow water is received by the microcomputer 12 so that the weight gained at regular intervals can be recorded by the microcomputer 12. The conical beaker 5C is also tightly plugged, but an air vent pipe 9B is attached. Reference numerals 13, 14, 15, and 16 are thermometers.

A hydrophobic porous membrane having slit-like fine pores having a short diameter of 1 μm and a long diameter of 15 μm, which is manufactured by a uniaxial stretching method in a housing 1 of an experimental apparatus as shown in the figure, and a biaxial stretching method (stretching in a lateral direction). Along with the biaxially extending in the vertical direction intersecting at right angles with the axial direction in the left-right direction)
Exactly the same hydrophobic porous material was installed except that it had circular micropores with a diameter of 0.8 μm, and the thermal efficiencies due to the difference between the two manufacturing methods were compared, and the results are shown in Tables 1 and 2. . The hydrophobic porous membrane used in the experiment was made of ethylene tetrafluoride and had an effective area of 75 cm ^{2 and two} thicknesses of 150 μm and 100 μm, respectively.
The table shows the results for the film thickness of 150 μm, and the second table shows the results for the film thickness of 100 μm. The flow rates of the liquid to be treated and the absorption liquid are both
It was set to 1.5 / min.

Although a 5000 mg / salt solution was used as the liquid to be treated, there was no increase in salinity in the distilled water received by the conical beaker 5C.

The thermal efficiency is the temperature at each point and the flow rate of distilled water received by the conical beaker 5C is a constant value, the temperature at each point when it is in a so-called steady state and the flow rate of distilled water received by the conical beaker 5C (membrane permeation Flow velocity) can be used to calculate from both the treated liquid side and the absorbing liquid side.

For example, the inlet temperature of the treated liquid is 42.1 ° C and the outlet temperature of the treated liquid is 42.1 ° C for the hydrophobic porous membrane produced by the uniaxial stretching method in Table 1.
The thermal efficiency calculated from the liquid to be treated using the values of 0.2 ° C and the membrane permeation flow rate of 0.19 m ³ / m ² · day is as follows.

Thermal efficiency = (42.1-40.2) x 1.5 / min x 1 Kcal / ° C = 28.5 Kcal / min The membrane permeation flow rate is 0.19 cm ³ / m ² · day, which is converted to an effective membrane area of 75 cm ² and flow rate per minute. , 0.99 ml / min. Therefore, using the constant evaporation latent heat of 570 kcal / ml, the latent heat of evaporation is calculated as Table 1 shows the thermal efficiencies (the average values of the calculated values from the treated liquid side and the absorbed liquid side) at the respective temperatures of the respective thin films of the hydrophobic porous membranes calculated by the above calculation method, As shown in Table 2, the hydrophobic porous membrane produced by the biaxial stretching method has a thermal efficiency of about 40 to 60% higher than that produced by the uniaxial stretching method. It is shown.

[Brief description of drawings]

FIG. 1 is an explanatory view 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 tube , 10 ... Bypass pipe 11 ... Electronic balance 12 ... Microcomputer 13-16 ... Thermometer

Claims (2)

[Claims] 1. A membrane distillation apparatus characterized by using a hydrophobic porous membrane produced by a biaxial stretching method and having a property of allowing gas to pass therethrough but not allowing liquid to pass therethrough. 2. The material of the hydrophobic porous membrane is ethylene tetrafluoride,
The membrane distillation apparatus according to claim 1, wherein the membrane distillation apparatus is any one selected from polypropylene, polyvinylidene fluoride, and polyethylene.