Membrane distillation device and operating method thereof

Abstract

The treated liquid in the water supply tank (1) of the present invention is supplied to the hollow fiber (11) of the membrane module (10) through the water supply pump (2), the piping (3), the heat exchanger (4), and the piping (5), and then returns to the water supply tank (1). The secondary side (17) is decompressed by the pressure reducing device (25). The water in the treated liquid flowing through the hollow fiber (11) evaporates and is sucked out from the secondary side (17) to the piping (20). The gas phase part (1a) of the water supply tank is connected to the secondary side (17) via the piping (7) and the valve (8), and the pressure difference P1-P2 between the two is 20 to 90 kPa.

Description

Technical Field

[0001] The invention relates to a membrane distillation device and an operating method thereof. Background Art

[0002] A membrane distillation apparatus is configured to concentrate the liquid by passing a heated liquid through the primary side of a hydrophobic flat membrane or a hydrophobic hollow fiber membrane, lowering the vapor pressure on the secondary side of the membrane to below that on the primary side. This allows only vapor to be recovered from the liquid. Unlike conventional distillation apparatuses, membrane distillation increases the evaporation surface area through the use of a membrane, offering the advantage of miniaturization.

[0003] An example of a conventional membrane distillation apparatus is shown in FIG. Figure 2 The treated liquid stored in the feed water tank 1 is sent to the heat exchanger (heater) 4 via the pipe 3 having the feed water pump 2 , and after being heated by heat exchange with the high-temperature fluid, is sent to the membrane module 10 via the pipe 5 .

[0004] In this conventional example, the membrane module 10 is a hollow fiber membrane module. In the membrane module 10, a plurality of hollow fibers 11 are arranged in a hollow fiber bundle and arranged vertically. The upper and lower ends of the hollow fiber bundle are bundled with potting materials 12 and 13.

[0005] The hollow fiber bundle is arranged in the housing 14 of the membrane module 10. An inlet chamber 15 is formed above the upper potting material 12, and an outlet chamber 16 is formed below the lower potting material 13. The interior of each hollow fiber 11 is connected to the inlet chamber 15 and the outlet chamber 16, respectively.

[0006] The treated liquid from the pipe 5 flows from the inlet chamber 15 into each hollow fiber 11 (primary side), passes through the hollow fiber 11 and flows to the outlet chamber 16 , and returns from the outlet chamber 16 to the feed water tank 1 via the pipe 6 .

[0007] Outside the hollow fibers 11 within the housing 14 and between the potting materials 12 , 13 is the secondary side 17 .

[0008] The secondary side 17 communicates with a decompression device 25 via a pipe (steam pipe) 20 , a heat exchanger (cooler) 21 , a pipe 22 , a condensed water tank 23 , and a pipe 24 . The decompression device 25 reduces the pressure on the secondary side 17 .

[0009] When the treated liquid flows through the hollow fibers 17 (primary side), water in the treated liquid permeates the hollow fibers 11, becomes vapor, and is drawn out from the secondary side 17 to the pipe 20. It is then cooled by the heat exchanger 21 to form condensed water. The condensed water is stored in the condensed water tank 23.

[0010] In this conventional example, the secondary side 17 in the casing 14 communicates with the upper portion (gas phase portion 1 a ) in the feed water tank 1 via the pipe 7 .

[0011] Pressure sensors P1 , P2 , and P3 are provided to detect the pressures in the gas phase portion 1 a of the feed water tank 1 , the pipe 20 , and the pipe 5 .

[0012] Prior art literature

[0013] Patent Literature

[0014] Patent Document 1: Japanese Patent Application Laid-Open No. 3-52627.

[0015] In the membrane distillation apparatus described above, the negative pressure from the pressure reducing device 25 is transmitted from the secondary side 17 through the piping 7 to the feed water tank 1. This reduces the pressure in the piping 3 on the suction side of the feed water pump 2, creating a risk of cavitation. To prevent this cavitation, the water head (water level) H within the feed water tank 1 must be increased, and this requires a design that increases the height of the feed water tank 1. Summary of the Invention

[0016] Problems to be solved by the invention

[0017] An object of the present invention is to provide a membrane distillation apparatus and an operating method thereof that can prevent cavitation of a feed water pump without increasing the height of a feed water tank.

[0018] Technical solutions to problems

[0019] The membrane distillation device of the present invention is characterized in that the membrane distillation device comprises: a membrane module, which is divided into a primary side and a secondary side by a hydrophobic membrane in the shell; a treated liquid circulation supply mechanism, which has a water supply tank and a water supply pump, and is used to circulate the treated liquid to the primary side of the membrane module; a pressure reducing device, which reduces the pressure on the secondary side of the membrane module; and a connecting mechanism, which connects the secondary side of the membrane module with the gas phase part of the water supply tank, and the pressure difference between the gas phase part of the water supply tank and the secondary side of the membrane module is 20 to 90 kPa.

[0020] In one aspect of the present invention, the communication portion includes a pressure difference adjustment mechanism for adjusting the pressure difference.

[0021] In one aspect of the present invention, the communication portion having the pressure difference adjustment mechanism is a pipe equipped with a valve.

[0022] In one embodiment of the present invention, the communication portion is a pipe having an inner diameter that is 1 / 4 or less of the inner diameter of the steam pipe, and the steam pipe is connected to the secondary side so as to decompress the secondary side and suck out steam.

[0023] The operating method of the membrane distillation apparatus of the present invention is the operating method of the membrane distillation apparatus of the present invention, characterized in that the opening of the valve is adjusted so that the pressure difference is 20 to 90 kPa.

[0024] Effects of the Invention

[0025] According to the present invention, by adjusting the pressure difference between the secondary side and the gas phase in the water supply tank to 20 to 90 kPa, the pressure on the suction side of the water supply pump can be ensured, thereby preventing cavitation. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a block diagram of a membrane distillation apparatus according to an embodiment.

[0027] Figure 2 This is a diagram showing the configuration of a conventional membrane distillation apparatus.

[0028] Figure 3 This is a graph showing the relationship between boiling point and gauge pressure. DETAILED DESCRIPTION

[0029] Figure 1 The configuration of a membrane distillation apparatus according to an embodiment is shown.

[0030] In this embodiment, a valve 8 for adjusting the pressure difference between the secondary side 17 and the gas phase 1a of the feed water tank 1 is provided in the piping 7 connecting the secondary side 17 of the membrane module 10 and the gas phase 1a. It should be noted that the pressure difference between the secondary side 17 and the gas phase 1a can be adjusted by reducing the diameter of the piping 7 instead of installing the valve 8. Alternatively, the diameter of the piping 7 can be reduced and the valve 8 can be installed. Alternatively, the diameter of a portion of the piping 7 can be reduced.

[0031] When reducing the diameter of piping 7, the inner diameter of piping 7 is preferably less than 1 / 4 (25%) of the inner diameter of steam piping 20, preferably 0.5 to 25%, and particularly preferably 1 to 20%. It should be noted that the purpose of piping 7 is to remove condensate or precipitate remaining on the secondary side of membrane module 10 before restarting the membrane distillation apparatus after a shutdown or during operation. Even a small inner diameter of piping 7 has little impact on the operational management of the membrane distillation apparatus. On the other hand, a large inner diameter of piping 7 requires frequent opening and closing of valve 8 to maintain a stable pressure differential at a specified value, making operational management difficult. Furthermore, if valve 8 is closed, there is a possibility that condensate or precipitate will remain on the secondary side of membrane module 10, complicating the stable operation of the membrane distillation apparatus.

[0032] The length of the pipe 7 is preferably 0.2 to 4 m, and particularly preferably 0.5 to 2 m.

[0033] In the present invention, the opening of the valve 8 (or the diameter of the pipe 7) is set so that the difference P1-P2 between the pressure P1 of the gas phase portion 1a of the water supply tank detected by the pressure sensor P1 and the pressure P2 of the secondary side 17 in the shell 14 detected by the pressure sensor P2 is 20 to 90 kPa, preferably 20 to 70 kPa.

[0034] This prevents the pressure of the gas phase portion 1 a from being excessively reduced, and prevents cavitation on the suction side of the feed water pump 2 without increasing the water head H of the feed water tank 1 .

[0035] The gauge pressure of P2 (difference from atmospheric pressure) is preferably -100 to -40 kPa, particularly preferably about -100 to -60 kPa.

[0036] It should be noted that Figure 1 Other components and Figure 2 The same symbol indicates the same part.

[0037] As the above-mentioned hollow fiber 11, a hydrophobic hollow fiber membrane is preferred. The inner diameter (diameter) of the hollow fiber is preferably 0.1 to 3.0 mm, particularly preferably 0.5 to 2.0 mm, and the thickness, i.e., the difference between the outer diameter (radius) and the inner diameter (radius) of the hollow fiber, is preferably 0.01 to 1.0 mm, particularly preferably 0.1 to 0.5 mm. The length of the hollow fiber is 200 to 2000 mm, particularly preferably 300 to 1000 mm. The filling rate of the hollow fiber (the sum of the cross-sectional area of ​​the hollow fiber / the cross-sectional area of ​​the shell) is preferably 5% to 50%, particularly preferably 15% to 35%. As materials for the hollow fiber, polysulfone, polyethersulfone, polyethylene, polypropylene, polyvinylidene fluoride and polytetrafluoroethylene can be cited, but are not limited to these.

[0038] In the above description, hollow fibers are used, but hydrophobic flat membranes may also be used.

[0039] Preferred treatment liquids include acidic solutions such as salt, phosphoric acid, hydrochloric acid, hydrofluoric acid, and sulfuric acid; alkaline wastewater such as ammonia; and aqueous solutions such as plating wastewater. The flow rate (linear velocity) of the treatment liquid within the hollow fiber is preferably 400 to 2000 mm / sec, particularly preferably 700 to 1500 mm / sec. The temperature of the treatment liquid upon entering the membrane module 10 is preferably 45 to 150°C, particularly preferably 50 to 80°C.

[0040] It should be noted that the above-mentioned preferred conditions are merely preferred examples, and the present invention is not limited to the above-mentioned conditions.

[0041] Example

[0042] [Comparative Example 1]

[0043] <Experimental equipment and conditions>

[0044] The assay was run under the following conditions Figure 2 The values ​​of the feed water tank pressure P1 and the membrane secondary side pressure P2 for the device shown are as follows.

[0045] Hollow fiber 11: made of polytetrafluoroethylene.

[0046] Inner diameter 1.0mm, thickness 0.3mm, length 600mm, 1 piece.

[0047] The housing 14 of the membrane module 10 has an inner diameter of 10 mm and a height of 690 mm.

[0048] Feed water: NaCl 1% aqueous solution.

[0049] Water supply temperature: 60℃.

[0050] Membrane module inlet pressure P3: -80kPa (gauge pressure).

[0051] Water supply: 40 mL / min (linear velocity in the hollow fiber 850 mm / sec).

[0052] Secondary side pressure P2: -90kPa (gauge pressure).

[0053] The inner diameter of the steam pipe 20 is 6 mm.

[0054] The inner diameter of the pipe 7 is 6 mm.

[0055] Length of the pipe 7: 1 m.

[0056] Liquid level height H: 50cm.

[0057] Results and Investigation

[0058] As the pressure on the secondary side of the membrane is reduced, the pressure in the feed water tank 1 is also rapidly reduced. When the feed water tank pressure P1 reaches -70 kPa, cavitation occurs, and boiling begins when it reaches -80 kPa.

[0059] Figure 3 The relationship between the pressure and boiling point of the aqueous solution is shown in FIG. Based on this relationship, it is assumed that boiling occurs at -80 kPa when the feed water temperature is 60°C.

[0060] In Comparative Example 1, cavitation occurred at the stage of -70 kPa. Therefore, it is considered that a liquid level height of at least 3 m is required in order to prevent cavitation from occurring even at the stage of -90 kPa.

[0061] [Example 1]

[0062] <Experimental equipment and conditions>

[0063] The same structure as in Comparative Example 1 was used except that the inner diameter of the pipe 7 was 0.75 mm and a valve 8 capable of adjusting the opening was provided in the pipe 7. Figure 1 The membrane distillation apparatus shown (the inner diameter of the pipe 7 is 12.5% ​​of the inner diameter of the steam pipe 20) was used for the distillation test under the same conditions as in Comparative Example 1 except that the opening of the valve 8 was adjusted so that the membrane module inlet pressure P3 was -40 kPa (gauge pressure).

[0064] Results and Investigations

[0065] In this embodiment, since the opening of the valve 8 is adjusted so that the membrane module inlet pressure P3 is -40 kPa, P1-P2 is 50 kPa, and the pump 2 can be operated without cavitation.

[0066] Furthermore, since the inner diameter of the pipe 7 in Example 1 is sufficiently smaller than that in Comparative Example 1, the speed of pressure reduction in the feed water tank 1 is slower than that on the secondary side 17 , making pressure regulation easier.

[0067] As described above, it was confirmed that by adjusting the feed water tank pressure to 20 to 90 kPa, operation without causing cavitation was possible even at a liquid level of 50 cm.

[0068] While the present invention has been described in detail with reference to specific embodiments thereof, it will be apparent to those skilled in the art that various modifications can be made therein without departing from the spirit and scope of the invention.

[0069] This application is based on Japanese Patent Application No. 2020-134727 filed on August 7, 2020, the entire contents of which are incorporated herein by reference.

[0070] Description of Reference Signs

[0071] 1 water tank

[0072] 2 water pump

[0073] 4.21 Heat exchanger

[0074] 8 valves

[0075] 10 membrane modules

[0076] 11 Hollow Fiber

[0077] 12, 13 potting materials

[0078] 14 Housing

[0079] 15 Inflow chamber

[0080] 16 Outflow Chamber

[0081] 17 Secondary side

[0082] 25 Pressure relief device.

Claims

1. A membrane distillation apparatus, characterized in that: The membrane distillation apparatus comprises: Membrane module, the shell is separated into primary side and secondary side by a hydrophobic membrane; a treated liquid circulation supply mechanism comprising a water supply tank and a water supply pump, for circulating the treated liquid to the primary side of the membrane module; a decompression device for decompressing the secondary side of the membrane module; and A communication mechanism connects the secondary side of the membrane module with the gas phase of the water supply tank, The communication mechanism includes a pressure difference adjustment mechanism for adjusting the pressure difference between the gas phase portion of the water supply tank and the secondary side of the membrane module to 20 to 90 kPa.

2. The membrane distillation apparatus according to claim 1, wherein The communication mechanism having the pressure difference adjustment mechanism is a pipe equipped with a valve.

3. The membrane distillation apparatus according to claim 2, wherein The inner diameter of the pipe is 1 / 4 or less of the inner diameter of the steam pipe connected to the secondary side so as to reduce the pressure on the secondary side and suck and discharge steam.

4. A method for operating a membrane distillation apparatus, which is the method for operating the membrane distillation apparatus according to claim 2, characterized in that: The opening of the valve is adjusted so that the pressure difference is 20 to 90 kPa.

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