JPH067644A - Film distillation apparatus - Google Patents

Abstract

PURPOSE:To improve the permeation performance of a film distillation method using a hydrophobic porous film and to decrease heat loss. CONSTITUTION:This apparatus is constituted of a film distillation cell 100 consisting of a raw liquid chamber 103 in which raw liquid 102 flows and a cooling water chamber 107 in which cooling water 104 flows and a hydrophobic porous film 101 interposed between both (103 and 105), a heat exchanger 108 for performing heat exchange between an outlet cooling water 106 of higher temp. and an inlet raw liquid 107 of lower temp., a heater 109 for further heating the raw liquid at the outlet of the heat exchanger 108, a cooler 110 for cooling the cooling water of higher temp. at the outlet of the heat exchanger, pumps 111, 112, raw liquid and cooling tanks 115, 116 and a pressure reducing device 117.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a membrane distillation method using a hydrophobic porous membrane, and more particularly to a system capable of significantly reducing heat loss due to sensible heat transfer by improving a permeation rate.

[0002]

2. Description of the Related Art In a membrane distillation method using a hydrophobic porous membrane, due to the nature of the membrane that gas is permeable but liquid is impermeable,
An indirect contact method in which a fluid is brought into contact with a membrane to flow, vapor generated from this fluid and condensed through the membrane is cooled and condensed on a cooling surface located on the opposite side of the membrane, and vapor passing through the membrane is opposite to the membrane. There is a direct contact method in which another fluid is directly absorbed into the fluid. In the former method, since it is composed of the region where the undiluted solution flows, the region where the condensed water flows, the region where the water vapor moves, the film and the cooling surface, the structure of the device is complicated, and not only is it difficult to realize the device, Since the generated steam moves not only in the film but also in the moving layer (diffusion layer) of the steam, its movement resistance lowers the amount of generated steam, and the amount of generated steam per unit membrane area becomes smaller. It has the advantage that it can be taken out directly. The latter method is a simple device in which the undiluted solution and the cooling water flow through the membrane, and the vapor travels only through the membrane, so it is extremely short and the resistance to movement is small, so the membrane should be used effectively. However, the produced water must be taken out of the cooling water. A system diagram of the direct contact membrane distillation method is shown in FIG. This system uses a hydrophobic porous membrane 201
The stock solution chamber 203 and the cooling water 20 through which the stock solution 202 flows
4, a membrane distillation cell 200 including a cooling water chamber 205
Cooling water outlet 206 with high temperature and undiluted solution inlet 20 with low temperature
7, a heat exchanger 208 for further heating the stock solution at the outlet of the heat exchanger 208, a cooler 210 for cooling water having a high temperature at the exit of the heat exchanger, a stock solution, and a pump 21 for the cooling water.
1 and 212. The stock solution 213 is sent to the heat exchanger 208 by the stock solution pump 211 and exchanges heat with the cooling water 206 having a high temperature. Further, this stock solution is heated to a predetermined temperature in the heater 209 and sent to the stock solution chamber 203 of the membrane distillation cell 200. Here, undiluted solution 20
The steam generated through the hydrophobic porous film 201 from No. 2 passes through the hydrophobic porous film 201, enters the cooling water chamber 205 side, and is condensed (absorbed) into the cooling water 204 flowing at a low temperature. The cooling water, which has received steam from the stock solution 202 and has a high temperature due to the latent heat of the steam at the same time, is subjected to heat by the stock solution in the heat exchanger 208, and then the cooling water passes through the cooler 210 by the cooling water pump 212. A part of the water is taken out of the system as produced water 214 and then sent to the membrane distillation cell 200 again.

Devices related to this type of device are disclosed in, for example, Japanese Patent Application Laid-Open Nos. 50-3753 and 60-118205.
Publication and Proceedings of the 15th I
STS (1986) p.1355-1359).

[0004]

The above-mentioned prior art has a problem in that a huge amount of heat energy is required because heat recovery, especially transfer of sensible heat through the membrane, is not taken into consideration. . That is, in the present technology, the high-temperature stock solution and the low-temperature cooling water are in contact with each other through the membrane or the thin air layer, so that the flow forms a kind of heat exchanger and heat transfer occurs due to the temperature difference. Here, the movement of water vapor occurs as a driving force due to the difference between the vapor pressure of the stock solution and the vapor pressure of the cooling water. Therefore, on the stock solution side, the vapor pressure is as high as possible. Side has low vapor pressure, lower temperature,
The amount of vapor permeation per unit membrane area becomes large. However, as described above, since the structure of the heat exchanger uses the membrane as the heat transfer surface, if the temperature difference between the stock solution and the cooling water is large, the amount of transfer of sensible heat also increases and the stock solution has Not only is the temperature not available effectively due to steam generation, but also an excessive amount of heat is transferred, so a considerable amount of heat is required for the amount of water produced. In the system shown in FIG. 2, a heat exchanger for heat recovery is installed between the raw water inlet and the cooling water outlet, but the heat loss is still large. Furthermore, both a heater and a cooler are required, and the required energy is large.

Further, since it is difficult to dissipate heat especially in a station or spacecraft in outer space, a system with less waste heat is desired.

An object of the present invention is to provide a new system for reducing heat loss due to the transfer of sensible heat.

[0007]

In order to solve such a problem, a means for reducing the air concentration in the membrane in the case of direct contact and for the indirect contact, that is, maintaining a reduced pressure state is installed. You can solve it.

[0008]

That is, in the membrane distillation method, it is always necessary to provide a temperature difference between the raw liquid side and the cooling water side in order to make a vapor pressure difference. Therefore, the stock solution side must be heated and the cooling water side must be cooled by the amount of heat. Therefore, in order to perform the distillation operation, in addition to the heat energy required for the phase change, the heat energy corresponding to the heat loss due to the sensible heat transfer is required, and the heat energy about twice the latent heat is required. Are known.
However, if a vacuum is maintained in the film or in the air layer, that is, the region where the permeating water vapor diffuses and moves, the diffusion speed of the water vapor in the film increases significantly, and the amount of transfer by latent heat is the same as the amount of sensible heat transfer. Can be increased. Therefore, the transfer amount of sensible heat can be increased to a negligible amount with respect to the transfer amount of latent heat, and the heat loss is significantly reduced.

[0009]

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 shows an example of the direct contact membrane distillation method. This system comprises a membrane distillation cell 100 including a stock solution chamber 103 in which a stock solution 102 flows through a hydrophobic porous membrane 101 and a cooling water chamber 105 in which a cooling water 104 flows, a high temperature cooling water outlet 106 and a low temperature stock solution inlet. Heat exchanger 108 between the heat exchanger 107 and the heater 107, a heater 109 for further heating the stock solution at the outlet of the heat exchanger 108, a cooler 110 for cooling water having a high temperature at the exit of the heat exchanger, pumps 111, 1 for the stock solution and the cooling water.
12, a stock solution, cooling water tanks 115 and 116, and a pressure reducing device 117. The stock solution 113 contained in the stock solution tank 115 is transferred to the heat exchanger 1 by the stock solution pump 111.
It is sent to 08 and is heat-exchanged with the cooling water 106 with high temperature.
Further, this stock solution is heated to a predetermined temperature in the heater 109 and sent to the stock solution chamber 103 of the membrane distillation cell 100. Here, the steam generated from the undiluted solution 102 through the hydrophobic porous film 101 passes through the hydrophobic porous film 101, enters the cooling water chamber 105 side, and flows at a low temperature.
4 is condensed (absorbed). The cooling water, which has received steam from the undiluted solution 102 and has a high temperature due to the latent heat of the steam, is heated by the undiluted solution in the heat exchanger 108,
After the water is once stored in the cooling water tank 116, it is passed through the cooler 110 by the cooling water pump 112, a part of the water is taken out of the system as the produced water 114, and then sent to the membrane distillation cell 100 again. Here, the inside of the system of the present system is a closed system, and is adjusted by a decompression device 117 to be in a vacuum state or a pressure lower than atmospheric pressure. Therefore, the air in the film is removed, and the movement of water vapor in the film approaches from convection to diffusion. Diffusion rate F in normal film
Is known to follow the one-sided diffusion equation shown in Eq.

[0011]

[Equation 1]

Where D _M is the diffusion coefficient of water vapor in the film,
π is the total pressure, m is the molecular weight of water, R is the gas constant, Pa is the partial pressure of air, and δ is the film thickness. In this equation, the value of F becomes large as the air partial pressure becomes small, and becomes infinite as the vacuum becomes perfect, and the evaporation rate in the membrane distillation method is governed by the heat supply amount. The result calculated by Equation 1 is shown in FIG. The calculation condition in this result is that the raw water temperature is 6
Although the temperature is 0 ° C and the cooling water temperature is 25 ° C, the permeability coefficient (F / ΔP
The value of () is defined by
When it goes down to 0 mmHg, the transmission coefficient becomes about 1.5 times. (Note that 138 mmHg is the saturated vapor pressure of water at 60 ° C.)
Further, the ratio of the total amount of heat transferred through the film to the amount of heat transferred by latent heat is shown in the upper part of FIG. 3, and the ratio of latent heat also increases as the total pressure decreases. Therefore, by lowering the pressure in the system, it is possible to increase the permeation rate and reduce heat loss due to sensible heat transfer.

FIG. 4 shows an embodiment of the indirect contact type membrane distillation method. In this system, a stock solution chamber 403 in which a stock solution 402 flows through a hydrophobic porous membrane 401 and a cooling plate 414, a cooling water chamber 405 in which a cooling water 404 flows, and water vapor generated from the stock solution are diffused and condensed on the cooling plate 414. Chamber 406
Membrane distillation cell 400 consisting of, stock solution, cooling water pump 40
7, 408, stock solution tank 409 including a heater, cooling water tank 410, stock solution tank 411, stock solution supply pump 41
2 and a pressure reducing device 413. The stock solution 402 contained in the stock solution tank 409 is sent to the stock solution chamber 403 of the membrane distillation cell 400 by a stock solution pump 407. Here, the vapor generated from the undiluted solution 402 through the hydrophobic porous film 401 passes through the hydrophobic porous film 401, and passes through the air chamber.
It enters the 406 side and condenses on the cooling plate 414 cooled by the cooling water 404 flowing at a low temperature. The cooling water is the cooling water pump 40.
8 is sent to the membrane distillation cell 400 to form a circulation system.
Here, the inside of the system of the present system is a closed system, and is adjusted by the decompression device 413 to be in a vacuum state or a pressure lower than atmospheric pressure. Therefore, the air in the membrane and the air in the air layer are removed, and the movement of the water vapor in the membrane approaches from convection to the convection, resulting in a significant increase in the permeation rate.
Therefore, in practical use, the film area can be reduced, which leads to downsizing of the device.

FIG. 5 shows an example of a depressurizable membrane module structure used in the direct contact membrane distillation method. This module 500 is intended for tubular membranes or hollow fiber membranes. The module is provided with an inlet 501 and an outlet 502 of the undiluted solution, a cooling water inlet 503 and an outlet 504, and five inlets and outlets of a vacuum exhaust system 505 for reducing the pressure, and a cooling water flowing region and a vacuum exhaust system. Are completely distinguished.
Here, when the stock solution and the cooling water are caused to flow from the respective inlets and outlets, ordinary membrane distillation can be performed. However, the vacuum exhaust system 50
When 5 is activated, the pressure in the decompression unit 508 is reduced, and at the same time, the air inside the membrane can be removed through the tubular or hollow fiber membrane, and membrane distillation under reduced pressure conditions can be performed.

[0015]

According to the present invention, the problem of heat loss due to the transfer of sensible heat in a membrane distillation system can be solved, and the permeation rate per unit membrane area is increased, so that the membrane area in the system can be reduced, and the apparatus size can be reduced. Can be realized. With this system, the amount of heat radiation can be greatly reduced, and it can be used sufficiently in a closed system.

[Brief description of drawings]

FIG. 1 is a system diagram of an example of a direct contact membrane distillation method.

FIG. 2 is a system diagram of a conventional membrane distillation system.

FIG. 3 is a characteristic diagram of a trial calculation example of the effect of decompression.

FIG. 4 is a system diagram of an example of an indirect contact membrane distillation method.

FIG. 5 is an explanatory diagram of an example of a membrane module structure.

[Explanation of symbols]

100 ... Membrane distillation cell, 101 ... Hydrophobic porous membrane, 108
... Heat exchanger, 109 ... Undiluted solution heater, 110 ... Cooling water cooler, 117 ... Decompression device.

Claims (7)

[Claims] 1. A first region in which a high temperature liquid flows, a second region in which a liquid of a temperature lower than the fluid flows in the first region, and a hydrophobic porous membrane separating the two regions. A heater for heating the fluid flowing through the first region and a cooler for cooling the fluid flowing through the second region, wherein vapor generated from the fluid flowing through the first region is hydrophobic. In the hydrophobic membrane in a membrane distillation method in which the fluid flowing in the first region is distilled and concentrated by being condensed and absorbed by the fluid flowing in the second region after passing through the porous membrane. Membrane distillation apparatus having at least one means for lowering the pressure of 2. A first region in which a liquid having a high temperature flows, a second region in which a liquid having a temperature lower than that of the fluid in the first region flows, and the region between the first and second regions. A third structure sandwiched between a hydrophobic porous membrane separating the first region side and a cooling plate separating the second region side between the first and second regions. An area and a heater for heating the fluid flowing through the first area, and a cooler for cooling the fluid flowing through the second area.
Vapor generated from the fluid flowing in the first region passes through the hydrophobic porous membrane and is condensed on the cooling plate cooled by the fluid flowing in the second region, whereby the first region In the membrane distillation method in which the fluid flowing through the column is distilled and recovered from the third region, at least one means for lowering the pressure in the third region is provided. 3. The membrane distillation apparatus according to claim 1, wherein the high temperature fluid is an aqueous solution, and the vapor passing through the hydrophobic porous membrane is water vapor. 4. A human being in a closed system space such as a submarine or a spacecraft according to claim 1 or 2,
Membrane distillation equipment that is wastewater from animals and plants. 5. The membrane distillation apparatus according to claim 1, wherein the membrane is a membrane made of polytetrafluoroethylene, polypropylene or polyethylene. 6. A membrane module comprising a first region in which a liquid flows, a second region in which another liquid flows, and a hydrophobic porous membrane separating the two regions, wherein A membrane module comprising at least one means for reducing the pressure inside the membrane. 7. The membrane module according to claim 6, wherein the membrane is a membrane made of polytetrafluoroethylene, polypropylene or polyethylene.