Device for preparing deuterium-depleted water by utilizing multi-stage cascade membrane distillation

Abstract

The utility model discloses a device for preparing deuterium-depleted water by utilizing multi-stage cascade membrane distillation, and mainly solves the problems that the existing hydrogen isotope water separation device is complicated in structure and process flow and higher in early-stage equipment arrangement cost. The device comprises a raw material supply device, a plurality of M * N cascade membrane rectification modules, a cooling water device, a heating device and a condensation collection device, wherein M represents the number of rows and is greater than or equal to 1; n represents the column number, Ngt; 1. Compared with a traditional water rectification scheme, the membrane rectification device is small, low in manufacturing cost and convenient to operate; through the reasonable cascade design, the next stage can utilize the waste heat of the raw material water of the previous stage, the heat loss of the raw material water in the cold and hot circulation process is reduced, and compared with a single stage, the energy consumption can be reduced; meanwhile, through multi-stage cascade connection, the defect that the single-stage membrane distillation separation factor is low is overcome, the deuterium abundance of the deuterium-depleted aquatic product is reduced, and the overall yield is increased.

Description

Technical Field

[0001] The utility model relates to the technical field of hydrogen isotope water separation, in particular to a device for preparing deuterium-depleted water by using multi-stage cascade membrane distillation. Background Art

[0002] The main production process of deuterium-depleted water is the isotope separation process. The isotopes of hydrogen mainly include hydrogen (H), deuterium (D), and tritium (T). Deuterium mainly exists in water in the form of HDO, with an element abundance of about 156.25ppm; tritium has almost zero stock in nature and needs to be produced by neutron bombardment of Li6 target plates or using heavy water reactors. T easily reacts with water to produce HTO. On the other hand, isotopes have the same electronic configuration and almost the same physical and chemical properties, so achieving efficient isotope separation is still a huge challenge. The traditional methods for separating hydrogen isotopes from water mainly include exchange and distillation, and the corresponding typical processes are dual-temperature exchange (Girdler-sulfide process) and water distillation (Water distillation). Dual-temperature exchange is the separation and enrichment of isotopes by transferring hydrogen isotopes between two different components of raw materials when they come into contact. Specifically, this method mainly uses the fact that the equilibrium constant of the reaction between H2S and HDO varies with temperature. At 303K, the equilibrium constant (separation factor) is 2.33, while at 403K, the equilibrium constant is 1.82. Water distillation uses the difference in vapor pressure of the components to be separated to allow the heavy components to enter the liquid phase and the light components to be enriched in the gas phase. However, the factor of single-stage water separation can only reach 1.026 (temperature 100°C). The Glace Bay heavy water production plant in Canada uses the GS+water distillation cascade to produce heavy water. Among them, GS adopts a three-stage cascade method. The first stage uses 6 large dual-temperature exchange columns with a diameter of 7m and a height of 60m to extract deuterium from seawater. GS produces deuterium-enriched water with a concentration of 20%, and then the deuterium-enriched water is concentrated to pure deuterium water (concentration: 99.75%) through water distillation. The annual output of heavy water in the plant is about 400 tons, and the daily power consumption is about 300MW.

[0003] The above two methods have the following problems: ① The device structure is complex, the process flow is complex, and the initial equipment investment cost is high; ② The whole process has high energy consumption. Since both the GS and distillation processes require water to be continuously heated to 100°C and then cooled through a heat exchanger for re-concentration by reflux, the energy consumption is high and it is a typical energy-intensive process; ③ The relevant raw materials are toxic and corrosive, such as H2S gas, which will cause harm to equipment and human body.

[0004] Membrane distillation (MD) is a heat-driven separation process that combines single-stage distillation with membrane separation technology.Figure 1 As shown, the two sides of the membrane are the high-temperature side and the low-temperature side respectively. The raw material water on the high-temperature side is in direct contact with the hydrophobic porous membrane. Due to the surface tension, the raw material water cannot directly penetrate through the hydrophobic membrane. The volatile components on the high-temperature side evaporate and diffuse into the low-temperature side and condense under the action of the temperature gradient on both sides. Other non-volatile components or hardly volatile components are completely or partially retained on the raw material water side, thus realizing the separation of non-volatile components from volatile components, or easily volatile components from hardly volatile components. The membrane separation process is essentially a non-isothermal process combining mass transfer and heat transfer. Its separation ability not only comes from the saturated vapor pressure difference of the mixture, but also from the diffusion rate difference of different components in the membrane. Summary of the Invention

[0005] The purpose of the present invention is to provide a device for preparing low-deuterium water by using multi-stage cascaded membrane distillation, mainly solving the problems of complex structure of the existing hydrogen isotope water separation device, complex technological process, and large initial equipment investment cost.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0007] A device for preparing low-deuterium water by using multi-stage cascaded membrane distillation includes a raw material feeding device, multiple membrane rectification modules cascaded in M×N levels, a cooling water device, a heating device, and a condensation collection device; wherein, M represents the number of rows, M≥1; N represents the number of columns, N>1;

[0008] When M = 1, the raw material feeding device is connected to the feed inlet of the membrane rectification module in the first row and the first column. The condensation discharge port of the membrane rectification module in the first row and the first column is connected to the feed inlet of the membrane rectification module in the next column until the condensation discharge port of the membrane rectification module in the last column is connected to the condensation collection device; the raw material feeding device and between every two membrane rectification modules, the raw material is heated through the heating device; wherein, the raw material discharge port of each membrane rectification module is connected to the raw material feeding device; the cooling water device is connected to each membrane rectification module;

[0009] When N>1, the raw material feeding device is connected to the feed inlet of the membrane rectification module in the first row and the first column. The condensation discharge port of each membrane rectification module in each row is correspondingly connected to the feed inlet of the membrane rectification module in the next column in the same row until the condensation discharge port of each membrane rectification module in the last column of each row is connected to the condensation collection device; the raw material discharge port of each membrane rectification module in each column is correspondingly connected to the feed inlet of the membrane rectification module in the next row until the raw material discharge port of each membrane rectification module in the last row is connected to the raw material feeding device; the raw material feeding device and between every two membrane rectification modules, the raw material is heated through the heating device; the cooling water device is connected to each membrane rectification module.

[0010] Furthermore, in the present utility model, the membrane distillation module includes two left and right clamping plates, two sealing gaskets disposed closely to the inner sides of the two clamping plates, a cooling plate disposed closely to the sealing gasket near the cooling water inlet side and facing the raw material feed side, a support plate disposed closely to the cooling plate and facing the raw material feed side, a support plate disposed closely to the cooling plate and facing the raw material feed side, a hydrophobic separation film disposed on the support plate facing the raw material feed side, a raw material water inlet and outlet disposed on the clamping plate near the raw material feed side, and a cooling water inlet and outlet disposed on the clamping plate near the cooling water device side; wherein, the condensate discharge port of the membrane distillation module is located between the cooling plate and the hydrophobic separation film.

[0011] Furthermore, in the present utility model, thermometers are disposed on both of the two left and right clamping plates.

[0012] Furthermore, in the present utility model, the raw material device or between every two membrane distillation modules is connected to the inlet of the raw material water inlet and outlet on the clamping plate through a raw material inlet pipeline, and a raw material delivery pump and a raw material flowmeter are disposed on the raw material inlet pipeline.

[0013] Furthermore, in the present utility model, the cooling water device is connected to the inlet of the cooling water inlet and outlet on the clamping plate through a cooling water inlet pipeline and is connected to the outlet of the cooling water inlet and outlet on the clamping plate through a cooling water return pipeline; a cooling water peristaltic pump and a cooling water flowmeter are disposed on the cooling water inlet pipeline.

[0014] Furthermore, in the present utility model, the sealing gasket is made of a silica gel gasket.

[0015] Furthermore, in the present utility model, the hydrophobic separation film is a film having hydrophobicity and heavy and light water separation functions.

[0016] Compared with the prior art, the present utility model has the following beneficial effects:

[0017] (1) The present utility model uses a cascaded membrane distillation module to prepare low-deuterium water by the method of membrane distillation. The hydrophobic separation film has hydrophobicity. When water flows on the surface of the film, only water vapor can pass through the film, and liquid water will not permeate through the film. The water vapor enters the pores of the hydrophilic polypropylene and condenses into liquid water after encountering the stainless steel cooling plate. When the water vapor passes through the film, isotope separation occurs due to different diffusion rates caused by different zero-point energies and adsorption energies at the chemical affinity sites in the channel, thereby obtaining low-deuterium water.

[0018] (2) Compared with the traditional water rectification scheme, the membrane distillation device of the present utility model is small, has low manufacturing cost and is convenient to operate; and through reasonable cascade design, the next stage can utilize the waste heat of the raw water of the previous stage, reducing the heat loss of the raw water during the cold and hot circulation process, saving energy consumption compared with a single stage; at the same time, through multi-stage cascading, the disadvantage of low separation factor of single-stage membrane distillation is made up for, the deuterium abundance of the low-deuterium water product decreases, and the overall yield increases. Brief Description of the Drawings

[0019] Figure 1 It is a schematic diagram of the overall structure of the present utility model.

[0020] Figure 2 It is an exploded structure schematic diagram of the membrane distillation module in the present utility model.

[0021] Figure 3 It is a simplified diagram of the membrane distillation module with single-stage cascading in Embodiment 1 of the present utility model.

[0022] Figure 4 It is a simplified diagram of the membrane distillation module with 2×5-stage cascading in Embodiment 2 of the present utility model.

[0023] Figure 5 It is a simplified diagram of the membrane distillation module with 5×5-stage cascading in Embodiment 3 of the present utility model.

[0024] Figure 6 It is a schematic diagram of the material balance of a single membrane distillation structure in the embodiments of the present utility model.

[0025] Among them, the names corresponding to the reference numerals are:

[0026] 1 - Raw material feeding device, 2 - Membrane distillation module, 3 - Cooling water device, 4 - Heating device, 5 - Condensation collection device, 20 - Clamping plate, 21 - Sealing gasket, 22 - Cooling plate, 23 - Support plate, 24 - Hydrophobic separation film, 25 - Thermometer, 26 - Raw material inlet pipeline, 27 - Raw material transfer pump, 28 - Raw material flowmeter, 29 - Cooling water return pipeline, 210 - Cooling water peristaltic pump, 211 - Cooling water flowmeter. Detailed Embodiments

[0027] The present utility model will be further described below in conjunction with the drawings and embodiments. The implementation manners of the present utility model include but are not limited to the following embodiments.

[0028] Embodiment

[0029] As Figure 1 、 2As shown in the figure, a device for preparing low-deuterium water by using multi-stage cascaded membrane distillation disclosed by the present utility model includes a raw material feeding device 1, multiple membrane distillation modules 2 in an M×N-stage cascade, a cooling water device 3, a heating device 4, and a condensation collection device 5; wherein, M represents the number of rows, M≥1; N represents the number of columns, N>1.

[0030] In the present utility model, the membrane distillation module 2 includes two left and right clamping plates 20, two sealing gaskets 21 disposed closely inside the two clamping plates 20, a cooling plate 22 disposed closely on the side of the sealing gasket 21 close to the cooling water inlet towards the raw material inlet side, a support plate 23 disposed closely on the side of the cooling plate 22 towards the raw material inlet side, a support plate 23 disposed closely on the side of the cooling plate 22 towards the raw material inlet side, a hydrophobic separation thin film 24 disposed on the side of the support plate 23 towards the raw material inlet side, a raw material water inlet and outlet disposed on the clamping plate 20 close to the raw material inlet side, and a cooling water inlet and outlet disposed on the clamping plate 20 close to the cooling water device 3 side; wherein, the condensation discharge port of the membrane distillation module 2 is located between the cooling plate 22 and the hydrophobic separation thin film 24. Among them, thermometers 25 are disposed on both of the left and right clamping plates 20. The sealing gasket 21 adopts a silica gel gasket.

[0031] The hydrophobic separation thin film 24 in the membrane distillation module 2 in this embodiment adopts a thin film with hydrophobicity and heavy and light water separation functions, preferably a graphene oxide thin film. The present utility model is not limited to the graphene oxide thin film, and any that has a separation effect on hydrogen isotopes can be used as the hydrophobic separation thin film.

[0032] In the present utility model, the raw material device or between every two membrane distillation modules 2 is connected to the inlet of the raw material water inlet and outlet on the clamping plate 20 through a raw material water inlet pipeline 26, and a raw material delivery pump 27 and a raw material flowmeter 28 are disposed on the raw material water inlet pipeline 26.

[0033] In the present utility model, the cooling water device 3 is connected to the inlet of the cooling water inlet and outlet on the clamping plate 20 through a cooling water inlet pipeline 29, and is connected to the outlet of the cooling water inlet and outlet on the clamping plate 20 through a cooling water return pipeline; a cooling water peristaltic pump 210 and a cooling water flowmeter 211 are disposed on the cooling water inlet pipeline 29.

[0034] In membrane distillation, based on material balance, the permeation rate and hydrogen isotope concentration at the raw material inlet end, outlet end, and transmembrane permeation end of each membrane distillation module are calculated. As Figure 6 shown, raw material water with a certain concentration enters the module at a certain rate. A part of the raw material water diffuses through the GO thin film in the form of vapor and condenses in the gap in the module. Heavy hydrogen (deuterium, tritium) water has a slower diffusion rate in the thin film, so more of it remains in the raw material water and flows out through the bottom raw material water outlet and enters the next stage or returns to the raw material pool.

[0035] In the production of low-deuterium water, the concentration of raw water is the natural abundance of 156.25 ppm. The rate at which the low-deuterium water enters the membrane distillation module is related to the size of the module and can be adjusted. The film area in the current experiment is 10 cm 2 , and the rate of raw water entering the module is 10 mL / min, and the cooling water flow rate is 15 mL / min. The transmembrane flux is related to the physical properties of the film. If the film is a polymer hydrophilic film, the flux can reach 67.8 kg·m -2 .h -1 , for a polymer hydrophobic membrane (pore size 1 μm), the flux can reach 1 kg·m -2 .h -1 , when using a film with molecular sieve properties such as graphene oxide film, the flux can only reach 0.1 - 0.6 kg·m -2 .h -1 . After a reasonable design of the membrane module size, it can ensure that the transmembrane flux meets the design requirements.

[0036] Assume that the rate and concentration of the raw water entering the module are V i and C i , the transmembrane permeation rate and concentration are V p and C p , and the rate and concentration of the water flowing out of the module are V o and C o , then the material balance of the entire module can be described by the following formula:

[0037] V i =V p +V o (Formula 1)

[0038] V i C i =V p C p +V o C o (Formula 2)

[0039] C o =αC p (Formula 3)

[0040] V p =xV i (Formula 4)

[0041] Among them, α is the H2O / HDO separation factor of the GO film. Here, the H2O / HDO separation factor is taken as 1.15, and x is the ratio of the raw water entering the module to the transmembrane mass. Although the transmembrane rate is small (0.05 kg·m -2 .h -1), but the transmembrane mass can be increased by expanding the single-stage membrane area. Here, we assume x = 0.5.

[0042] Then, by combining Equation (2) and Equation (3), we can obtain:

[0043]

[0044] Let We can obtain:

[0045]

[0046] By further combining Equation (1), Equation (4), and Equation (6), we can obtain:

[0047]

[0048] For a single-column cascade design with n-stage trays in series, the raw water for each stage comes from the transmembrane distilled water of the previous stage, and its outlet concentration is:

[0049]

[0050] Then, the concentration C p,n of the transmembrane distilled water corresponding to the nth-stage tray, the outflow module water velocity V o,n , and the transmembrane permeation rate V p,n can be expressed by the following equations respectively:

[0051]

[0052] V o,n = (1 - x) n V i (Equation 10)

[0053] V p,n = x n V i (Equation 11)

[0054] According to Equations (8) to (11), the material parameters of each stage of the cascade can be balanced.

[0055] Example 1

[0056] In this example, a membrane distillation module with a single-column cascade of 5 stages is taken as an example. As Figure 3 shown, in this example, the distilled water of each stage is used as the raw water for the next stage for distillation, and the outlet wastewater is directly collected and discharged uniformly. Through 5-stage cascading, the concentration of the distilled water drops from 100% of the initial concentration to 70%. However, under this operating condition, the output after 5-stage membrane distillation filtration is only 3.125% of the raw water input rate, which is much lower than 100% of the waste liquid input rate.

[0057] Example 2

[0058] This example takes a double-column cascaded 5-stage membrane distillation module as an example. As Figure 4 shown, in this example, in order to make full use of the heat of the raw water and reduce the heat loss of the raw water during the cold and hot circulation process, similar to the single-column design, the distilled water at each stage is preheated to a specified temperature again and then enters the next stage to act on the raw water for further distillation. Among them, the re-preheating temperature is related to the process. Generally speaking, the cooling water temperature at each stage is generally about 10-20 °C, the raw water temperature is set between 30-80 °C, and the re-preheating temperature can be any temperature set between 30-80 °C.

[0059] The difference is that the raw water at each stage needs to flow through the two module raw material chambers in two columns of the same stage and be further distilled to maximize the use of the heat of the raw water. It should be noted that after the raw water passes through the first chamber, there will be a certain degree of decrease according to the different flow rates. At this time, the raw water needs to be supplemented with heat once to make the raw water reach the set working conditions. In addition, in the second-stage connection column, the raw water at each stage comes from the product water of the previous stage of the same system and also from the raw water of the first column of the same stage. When the raw water with the same concentration of 100% enters the double-column cascaded design again, the concentration of the relatively concentrated product water at the tail end is 103%, accounting for 89% of the total flow; while the concentration of the diluted product water is 73%, accounting for 11% of the total flow. Compared with 3% of the single column, the total flow at the dilution end has been greatly improved.

[0060] Example 3

[0061] The multi-column cascade mode is carried out by using more than 3 membrane distillation modules. In particular, when the number of columns is the same as the number of modules in each column, an N×N multi-stage cascade mode is formed, as Figure 5 shown. Through the gradually increasing cascade, it can be found that as the initial raw water flows column by column on the first-stage tray of each stage, the isotope concentration in the raw water gradually increases. After flowing on the first-stage tray, the raw water concentration is 107%, and when it reaches the outlet of the first-stage tray in the fifth column, its concentration reaches 140%. Therefore, when flowing out from the permeation end of the fifth-stage tray in the fifth column, its concentration is still as high as 91%. Therefore, generally speaking, due to the too high concentration of the initial raw water in the fifth-column cascade, increasing the cascade design is not conducive to reducing the total concentration at the depletion end, and can only increase the flow at the depletion end. On the other hand, by observing the concentration at the permeation end of the fifth stage of each column, it can also be found that the permeation end concentration has increased, rising from 70% in the first column to 91% in the fifth column in turn. It is precisely this negative impact that the concentration at the depletion end of the 5×5 multi-column cascade design is higher than that of the 2×5 double-column cascade design, rising from 73% to 83%. At the same time, due to the addition of the multi-column cascade design, the flow at the depletion end has also increased significantly, rising from 11% to 50%.

[0062] By introducing multi-stage cascading, we assume that when the separation factors are 1.15 and 2.0 respectively, the concentration and production of low-deuterium water at the depleted end of the multi-column cascade are shown in Table 1 (when the raw water is natural water, the deuterium abundance is 156.25 ppm). It can be seen that through reasonable cascade design, the deuterium abundance of low-deuterium water increases, the overall yield increases, and the energy consumption decreases accordingly.

[0063] Table 1 Output of low-deuterium water in the case of multi-column cascade (5×5)

[0064]

[0065] In Table 1, assuming that the inlet flow rate of the raw water is 100% and the concentration is 100%. In terms of the overall yield, the corresponding flow rate yield of the 1×5 cascade is 3.125% and the concentration yield is 70%; the corresponding yields of the 2×5 cascade are 11% and 73%; the corresponding yields of the 1×5 cascade are 50% and 83%; assuming that the power consumption of a single module is equal, which is X / block·hour, then the power required for the 1×5, 2×5, and 5×5 cascade designs to produce 1 kg of low-deuterium water with a concentration of 85% is:

[0066] Y1 = [(70% / 85%) / 0.03125] * 5X = 131.8X

[0067] Y2 = [(73% / 85%) / 0.11] * 10X = 78X

[0068] Y1 = [(83% / 85%) / 0.5] * 25X = 48.8X

[0069] It can be seen that recycling the filtered product water as the raw water for the next echelon can effectively reduce energy consumption.

[0070] Therefore, through the above design, compared with the traditional water rectification scheme, the membrane distillation device of the present utility model is small, has a low manufacturing cost, and is convenient to operate; and through reasonable cascade design, the deuterium abundance of low-deuterium water increases, the overall yield increases, and the energy consumption decreases accordingly.

[0071] The above embodiments are only one of the preferred embodiments of the present utility model and should not be used to limit the protection scope of the present utility model. Any modifications or polishings made without substantial significance in the main design concept and spirit of the present utility model, as long as the technical problems solved are still the same as those of the present utility model, should be included in the protection scope of the present utility model.

Claims

1. An apparatus for preparing low-deuterium water by using multi-stage cascade membrane distillation, characterized in that, It includes a raw material feeding device (1), multiple membrane distillation modules (2) in an M×N - stage cascade, a cooling water device (3), a heating device (4), and a condensation collection device (5); where M represents the number of rows, M≥1; N represents the number of columns, N>1; When M = 1, the raw material feeding device (1) is connected to the feed inlet of the membrane distillation module (2) in the first row and first column. The condensation outlet of the membrane distillation module (2) in the first row and first column is connected to the feed inlet of the membrane distillation module (2) in the next column until the condensation outlet of the membrane distillation module (2) in the last column is connected to the condensation collection device (5); between the raw material feeding device (1) and every two membrane distillation modules (2), the raw material is heated through the heating device (4); where the raw material outlet of each membrane distillation module (2) is connected to the raw material feeding device (1); the cooling water device (3) is connected to each membrane distillation module (2); When N>1, the raw material feeding device (1) is connected to the feed inlet of the membrane distillation module (2) in the first row and first column. The condensation outlet of each membrane distillation module (2) in each row is correspondingly connected to the feed inlet of the membrane distillation module (2) in the next column in the same row until the condensation outlets of the membrane distillation modules (2) in the last column of each row are all connected to the condensation collection device (5); the raw material outlet of each membrane distillation module (2) in each column is correspondingly connected to the feed inlet of the membrane distillation module (2) in the next row until the raw material outlets of each membrane distillation module (2) in the last row are connected to the raw material feeding device (1); between the raw material feeding device (1) and every two membrane distillation modules (2), the raw material is heated through the heating device (4); the cooling water device (3) is connected to each membrane distillation module (2).

2. The device for preparing low-deuterium water by using multi-stage cascade membrane distillation according to claim 1, wherein The membrane distillation module (2) includes two left - and - right clamping plates (20), two sealing gaskets (21) arranged closely on the inner sides of the two clamping plates (20), a cooling plate (22) arranged closely on the sealing gasket (21) close to the cooling water inlet side and facing the raw material feed side, a support plate (23) arranged closely on the cooling plate (22) and facing the raw material feed side, a support plate (23) arranged closely on the cooling plate (22) and facing the raw material feed side, a hydrophobic separation film (24) arranged on the support plate (23) facing the raw material feed side, a raw material water inlet and outlet arranged on the clamping plate (20) close to the raw material feed side, and a cooling water inlet and outlet arranged on the clamping plate (20) close to the cooling water device (3); where the condensation outlet of the membrane distillation module (2) is located between the cooling plate (22) and the hydrophobic separation film (24).

3. The device for preparing low-deuterium water by using multi-stage cascade membrane distillation according to claim 2, characterized in that, Thermometers (25) are arranged on both of the left - and - right clamping plates (20).

4. The device for preparing low-deuterium water by using multi-stage cascade membrane distillation according to claim 3, characterized in that, The raw material device or between every two membrane distillation modules (2) is connected to the inlet of the raw material water inlet and outlet on the clamping plate (20) through a raw material inlet pipeline (26). A raw material transfer pump (27) and a raw material flowmeter (28) are arranged on the raw material inlet pipeline (26).

5. The device for preparing low-deuterium water by using multi-stage cascade membrane distillation according to claim 4, characterized in that, The cooling water device (3) is connected to the inlet of the cooling water inlet and outlet on the clamping plate (20) through a cooling water inlet pipeline (29), and is connected to the outlet of the cooling water inlet and outlet on the clamping plate (20) through a cooling water return pipeline; a cooling water peristaltic pump (210) and a cooling water flowmeter (211) are arranged on the cooling water inlet pipeline (29).

6. The device for preparing low-deuterium water by using multi-stage cascade membrane distillation according to claim 5, characterized in that, The sealing gasket (21) is made of a silicone gasket.

7. The device for preparing low-deuterium water by using multi-stage cascade membrane distillation according to claim 6, characterized in that, The hydrophobic separation film (24) is a film with hydrophobicity and heavy-light water separation function.

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