A method and apparatus for treating a solution by percolative vacuum membrane distillation

Abstract

The application discloses a method and device for treating solution by seepage flow type vacuum membrane distillation. The method uses a hydrophobic tubular microporous membrane with an average pore diameter of 10-100 nm to assemble a membrane element, and a feed liquid is forced to flow in the inner side of the membrane tube, and a vacuum is drawn on the outer side of the membrane tube to form a negative pressure cavity. The steam generated by the hot feed liquid penetrates the membrane pores to enter the negative pressure cavity, and the gas phase components are condensed to obtain distillate. The liquid phase components form a seepage flow in the membrane pores, flow to the outer side of the membrane tube, and are separately discharged and collected. The seepage flow is subjected to secondary evaporation and concentration under the negative pressure environment. By adjusting the concentration of the feed liquid, the operation temperature, the operation vacuum degree and the circulation flow rate of the feed liquid, the transmembrane steam flux is adjusted, and the reversible regulation and control of the seepage flow and the water production flux are realized. The method and device can be used for concentrating inorganic alkali and inorganic acid solutions and preparing pure water from salt solutions, and the membrane separation performance can be restored on line without backwashing or chemical cleaning, so that the operation cost is significantly reduced.

Description

Technical Field

[0001] This invention relates to the field of membrane distillation technology, and more particularly to a method and apparatus for treating solutions by percolation vacuum membrane distillation. Background Technology

[0002] Membrane distillation (MD) technology is a membrane separation process that uses a hydrophobic microporous membrane to block the permeation of liquid solutions while allowing gaseous components to pass through. Based on the condensation method of the gaseous components after passing through the membrane pores, MD processes can be classified into air-gap membrane distillation (AGMD), direct contact membrane distillation (DCMD), gas-swept membrane distillation (SGMD), and vacuum membrane distillation (VMD), and are widely used in seawater / brackish water desalination, ultrapure water preparation, high-concentration chemical solution concentration, and the separation of heat-sensitive substances.

[0003] The core of membrane distillation technology lies in maintaining the hydrophobicity of the membrane pores. Polypropylene (PP), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), or other superhydrophobic materials are typically used as membrane materials. Traditional membrane distillation processes consider hydrophobic membranes to be in an ideal state of "only allowing air to pass through but not water to pass through," and it is essential to strictly prevent the membrane pores from being wetted.

[0004] There is a competitive relationship between the permeation states of the gas and liquid phases within the membrane pores. When the amount of gas across the membrane is small and the amount of liquid is dominant, large droplets generated inside the membrane pores easily connect to form continuous liquid channels, ultimately causing the solution to permeate through the membrane pores into the product water side, resulting in membrane distillation failure. This phenomenon is particularly pronounced when using conventional hydrophobic membranes with pore sizes larger than 0.2 μm, which is currently the mainstream pore size range for membrane distillation. Conversely, when the amount of gas across the membrane is dominant, the high-speed gas flow can carry away the tiny droplets condensed inside the membrane pores, preventing the droplets from forming a continuous state and interrupting the liquid's "conduction." In this case, only a small amount of liquid permeates through the membrane pores in the form of "percolation."

[0005] Currently, the conventional technical approach to address membrane pore wetting failure is to shut down the system and backwash or chemically clean the membrane pores. This maintenance method not only increases operating costs but also prevents the system from operating continuously and stably, limiting the large-scale industrial application of membrane distillation technology in complex conditions such as high-concentration solution concentration. Summary of the Invention

[0006] In one aspect, the present invention provides a method for treating a solution by percolation vacuum membrane distillation. The method includes: assembling a membrane element using a hydrophobic tubular microporous membrane; forcing a feed liquid to circulate inside the hydrophobic tubular microporous membrane; forming a sealed cavity by the outer side of the hydrophobic tubular microporous membrane and a membrane shell; maintaining the operating vacuum level within the sealed cavity by evacuation; the vapor generated by the feed liquid inside the hydrophobic tubular microporous membrane passing through the membrane pores into the negative pressure cavity; the gas phase component being drawn out from the negative pressure cavity and condensed to obtain a distillate; and the percolation formed within the membrane pores flowing to the outer side of the hydrophobic tubular microporous membrane for secondary evaporation and concentration under negative pressure, wherein the average pore size of the membrane pores is 10 nm to 100 nm.

[0007] Furthermore, the method adjusts the transmembrane steam flux by controlling at least one of the concentration, temperature, circulation flow rate, and operating vacuum, thereby achieving reversible control of the permeate flow rate and the permeate flux.

[0008] Furthermore, when the method experiences an increase in percolation flow and a decrease in permeate flux during operation, the transmembrane steam flux is increased by at least one of the following methods: increasing the operating vacuum, increasing the feed liquid temperature, increasing the feed liquid circulation rate, and decreasing the feed liquid concentration, thereby suppressing liquid phase percolation and restoring permeate performance.

[0009] Furthermore, when the feed solution is an inorganic alkaline solution or an inorganic acid solution, the solution is concentrated through the percolation liquid; when the feed solution is a salt solution, pure water is produced through the distillate liquid.

[0010] Furthermore, the average pore size of the membrane is 20nm to 50nm, for example, 20nm, 30nm, or 40nm, and the material is selected from one or more of polypropylene, polyvinylidene fluoride, and polytetrafluoroethylene.

[0011] Furthermore, the air flux of the hydrophobic tubular microporous membrane at 25°C is greater than 3000 L / (m²). 2 (·h·bar), the breakthrough pressure of pure water is greater than 2.6 bar.

[0012] Furthermore, the membrane elements are arranged vertically, and a permeate flow gap is reserved between the hydrophobic tubular microporous membranes; the gas phase component is drawn out from the upper end of the negative pressure cavity, and the permeate is collected from the lower end, and the permeate is discharged and collected by gravity.

[0013] Furthermore, when the feed solution is a sodium hydroxide solution, the feed solution circulation flow rate is 0.5 m / s to 0.8 m / s, the operating temperature is 65℃ to 85℃, the mass concentration of the feed solution is 9.91% to 48.54%, and the operating vacuum degree is -0.096 MPa to -0.051 MPa.

[0014] Furthermore, when the feed solution is a sodium hydroxide solution, the conductivity of the distillate obtained by separation is less than 4 μS / cm; the percolate is enriched by solute after being concentrated twice under vacuum, and solute crystals are precipitated when cooled to room temperature.

[0015] Furthermore, when the feed solution is a phosphoric acid solution, the feed solution circulation flow rate is 0.5 m / s to 0.8 m / s, the operating temperature is 65℃ to 85℃, the mass concentration of the feed solution is 10.44% to 41.88%, and the operating vacuum degree is -0.096 MPa to -0.074 MPa.

[0016] Furthermore, when the feed solution is a phosphoric acid solution, the conductivity of the distillate obtained by separation is less than 4 μS / cm; the pH value of the percolate after vacuum secondary concentration is higher than that of the feed solution.

[0017] Further, the feed solution is an acetic acid solution, the feed solution circulation rate is 0.8 m / s, the operating temperature is 50℃~85℃, the mass concentration of the feed solution is 2.0%~44.5%, and the operating vacuum degree is -0.096MPa~-0.085MPa; preferably, by lowering the operating temperature, the transmembrane gas phase diffusion of acetic acid molecules is suppressed, thereby reducing the acetic acid concentration in the product water.

[0018] Furthermore, the concentrated liquid is a sodium chloride solution with a mass concentration of 5% to 15%; the operating temperature is 50℃ to 90℃; and the operating vacuum is -0.095MPa.

[0019] Furthermore, the membrane material of the hydrophobic tubular microporous membrane is polypropylene with an average pore size of 20 nm; the permeate is concentrated by secondary evaporation on the outside of the membrane tube, and then discharged by gravity and cooled for crystallization.

[0020] The present invention also provides a percolation-type vacuum membrane distillation apparatus, comprising a membrane element, a heat exchanger, a distillate collection tank, a vacuum pump, and a percolation collection tank, wherein: The membrane element is vertically arranged and includes a shell and a plurality of parallel hydrophobic tubular microporous membranes disposed therein, with a sealed cavity formed between the shell and the hydrophobic tubular microporous membranes; The membrane element has a pipe at its upper part that communicates with the heat exchanger, and a pipe at its lower part that communicates with the permeate collection tank; The heat exchanger is connected to the distillate collection tank via a pipe, and the distillate collection tank is connected to the vacuum pump via a pipe. The average pore size of the hydrophobic tubular microporous membrane is 20 nm to 50 nm, and the material is selected from one or more of polypropylene, polyvinylidene fluoride, and polytetrafluoroethylene.

[0021] The advantages and positive effects of this invention are as follows: 1. This invention uses a small-pore hydrophobic tubular microporous membrane with an average pore size of 10nm to 100nm, combined with a high-flux steam flow generated by negative pressure suction, so that the tiny droplets condensed in the membrane pores can be carried out by the steam in time, avoiding the droplets from agglomerating and growing into a continuous liquid column in the membrane pores, thus delaying the process of membrane pore wetting failure from a physical mechanism.

[0022] 2. In this invention, the permeate is collected separately on the outer wall of the membrane. The permeate undergoes secondary evaporation and concentration under negative pressure, and its solute concentration is higher than that of the hot feed liquid. This not only avoids the permeate from contaminating the distillate, but also achieves additional enrichment of solute, thereby improving the overall concentration efficiency.

[0023] 3. For volatile organic acid solutions such as acetic acid, this invention utilizes the difference in gas phase diffusion resistance of small-pore hydrophobic membranes, combined with low-temperature operation to reduce the evaporation rate of acetic acid, effectively reducing the permeation of acetic acid molecules with vapor through the membrane pores, and significantly improving the membrane's retention efficiency for volatile solutes.

[0024] 4. This invention can be used not only for concentrating high-concentration solutions (such as sodium hydroxide, phosphoric acid, and sodium chloride solutions), but also for producing pure water from these and other solutions (such as seawater). In water production mode, steam condenses after passing through the membrane pores to obtain high-purity freshwater with a conductivity of less than 7 μS / cm. Simultaneously, the permeate is concentrated on the outer wall of the membrane and collected separately, achieving effective separation of freshwater and concentrated water. The entire process can be continuously and stably operated for more than 2 weeks without backwashing, and is suitable for seawater desalination, brackish water desalination, and industrial pure water preparation. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the process flow of a percolation vacuum membrane distillation device. Detailed Implementation

[0026] Unless otherwise stated, all technical and scientific terms used herein have the meanings commonly understood by one of ordinary skill in the art.

[0027] The term "or" refers to a single element among the listed selectable elements, unless the context explicitly indicates otherwise.

[0028] The terms “including,” “having,” and similar expressions used herein do not exclude elements not listed. These terms also cover situations where the text consists only of the listed elements.

[0029] We discovered that during vacuum membrane distillation, when the hot solution inside the membrane evaporates on the membrane surface, the gas / liquid interface is not perfectly clear; instead, a "gas-liquid hybrid layer" exists. This gas-liquid mixture, upon entering the irregular membrane pores, undergoes collisions, coalescence, and capillary condensation, forming tiny droplets that exhibit liquid wettability. Based on the gas / liquid interface properties during solution evaporation and the membrane pore structure, capillary condensation within the membrane pores is difficult to completely avoid. Therefore, if we can use hydrophobic membranes with smaller pore sizes to reduce the probability of droplet coalescence and rely on high-flux gas flow to promptly transfer the condensed droplets, allowing them to drain as permeate on the other side of the membrane and be concentrated again through evaporation, we can potentially fundamentally prevent the formation of a continuous liquid column within the membrane pores, thereby eliminating the need for backwashing and achieving longer operating cycles.

[0030] Figure 1 This invention illustrates a membrane element and apparatus for realizing percolation-type vacuum membrane distillation. It includes a membrane element, a feed circulation loop, a condensation recovery unit, and a vacuum pumping unit. The membrane element 1 includes a hydrophobic tubular microporous membrane 2 (also referred to herein as a "membrane tube"), a membrane shell 3, an end plate 4, and a sealing head 5. The hydrophobic tubular microporous membrane 2 within the membrane element 1 is welded to the end plate 4, and the end plate 4 is welded to the membrane shell 3 and the sealing head 5 to prevent leakage outside the membrane pores and ensure that the collected percolate originates entirely from the membrane pores.

[0031] The hydrophobic tubular microporous membrane 4 has an average pore size of 10 nm to 100 nm, preferably 20 nm to 50 nm, and the membrane material is preferably polypropylene (PP). At 25°C, the air flux through the membrane tube is greater than 3000 L / (m²). 2 The breakthrough pressure of pure water is greater than 2.6 bar (·h·bar). The membrane element 3 is arranged vertically, and a permeate flow gap is reserved between the hydrophobic tubular microporous membranes 2. The inner side of the membrane shell 3 and the end plate 4 enclose a closed negative pressure cavity 6. A gas phase outlet 7 is provided at the upper end of the negative pressure cavity 6, and a permeate outlet 8 is provided at the lower end.

[0032] The feed liquid in the feed tank (not shown in the figure) is pumped into the hydrophobic tubular microporous membrane 4 of the membrane element 1 by a circulation pump for forced circulation. The gas phase outlet of the negative pressure chamber 6 is connected in sequence to the heat exchanger 9 and the distillate collection tank 10. The distillate collection tank 10 is connected to a vacuum pump (not shown in the figure) and is equipped with a regulating valve 11. The permeate outlet 8 is connected to the permeate collection tank 12. An exhaust valve 13 is provided on the top of the permeate collection tank 12. One end of the exhaust valve 13 is connected to the inside of the permeate collection tank 12, and the other end is connected to the atmosphere. The exhaust valve 13 is used to keep closed during the collection process to maintain negative pressure, and to open during discharge to introduce atmosphere to break the vacuum and allow the permeate to be discharged smoothly.

[0033] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0034] Example 1: Percolation vacuum membrane distillation concentration of sodium hydroxide solution 1. Membrane elements and devices Membrane elements were assembled using polypropylene (PP) membrane tubes. The outer diameter of the membrane tubes was 2.5 mm, the inner diameter was 1.5 mm, the average pore size was 40 nm, and the air flux at 25℃ was >3000 L / (m²). 2 The breakthrough pressure for pure water is >2.6 bar (·h·bar). The effective length of the membrane element is 240 mm, with 85 built-in membrane tubes, and the membrane area calculated based on the outer diameter is 0.16 m². 2 Inside membrane element 3, the membrane tube is welded to the end plate, the end plate is welded to the membrane shell, and the end cap to prevent leakage outside the membrane pores.

[0035] The membrane element is placed vertically, and the hot feed liquid undergoes forced circulation within the membrane tube. The outer side of the membrane, the inner wall of the membrane shell, and the inner wall of the end plate form a closed cavity. A steam outlet is located at the upper end of this cavity, and a permeate outlet is located at the lower end. The permeate outlet is connected to a permeate collection tank, forming a blind end. The steam outlet is connected to one end of the heat exchanger. The distillate collection tank has three ports. The other end of the heat exchanger is connected to the first port of the distillate collection tank. The second port of the distillate collection tank is connected to a vacuum pump, and the third port is connected to one end of a regulating valve. The other end of the regulating valve is open to the atmosphere, and the operating vacuum of the membrane distillation is adjusted through the regulating valve.

[0036] 2. Performance Testing Prepare a sodium hydroxide (NaOH) solution as the feed solution to be concentrated. The feed solution is forced to circulate in the membrane tube, and the circulation flow rate is controlled at 0.5 m / s to 0.8 m / s.

[0037] (1) Variable concentration test: Under the conditions of circulation flow rate of 0.8 m / s and operating vacuum of -0.096 MPa, the operating temperature was set to 85℃, 75℃ and 65℃ respectively, and the permeate flux and seepage flow rate under different NaOH mass concentrations were tested.

[0038] (2) Variable parameter control and in-situ recovery test: At a NaOH concentration of 9.91% and a temperature of 85℃, the operating vacuum was reduced from -0.084MPa to -0.051MPa; at a temperature of 75℃ and a concentration of 9.91%, the circulation flow rate was reduced from 0.8m / s to 0.5m / s. The flux change before and after the parameter change was tested, and after the flux decayed (the permeate flow increased), in-situ recovery tests were conducted by increasing the operating vacuum, increasing the feed temperature, increasing the feed circulation flow rate, or decreasing the feed concentration.

[0039] 3. Results (1) Under the conditions of a circulation flow rate of 0.8 m / s and an operating vacuum of -0.096 MPa, the results of the variable concentration test show that: At 85℃, the NaOH concentration increased from 9.91% to 48.54%, and the water production rate increased from 14.44 L / (m³) 2 The concentration of ·h) decreased to 1.79 L / (m 2 The seepage flow rate first increased and then decreased, from 0.06 L / (m³). 2 The concentration of ·h) increased to a maximum of 0.73 L / (m 2 (At this point, the NaOH concentration is 33.32%). When the concentration is further increased to 48.54%, there is almost no seepage.

[0040] At 75℃, the concentration increased from 9.91% to 44.24%, and the water production increased from 11.25 L / (m³) 2 The concentration of ·h) decreased to 2.14 L / (m 2 •h); During the process of NaOH concentration increasing from 9.91% to 33.32%, the seepage flow rate (calculated by the change in liquid volume in the seepage collection tank) increased from 0.02 L / (m³) 2 The concentration of 1.73 L / (m³) increased to a maximum value of 1.73 L / (m³). 2 (·h) (concentration 20.2%), then reduced to 0.29 L / (m 2 ·h).

[0041] At 65℃, the concentration increased from 9.91% to 37.59%, and the water production increased from 8.9 L / (m³) 2 The concentration of ·h) decreased to 1.78 L / (m 2 •h); During the process of NaOH concentration increasing from 9.91% to 33.32%, the seepage flow rate increased from 0.26 L / (m³) 2 The concentration of ·h) increased to a maximum of 0.68 L / (m 2 (·h) (concentration 20.2%), then reduced to 0.38 L / (m 2 ·h).

[0042] The conductivity of the distillate at all concentrations was less than 4 μS / cm.

[0043] (2) The test results of variable parameter control show that: When the operating vacuum at 85℃ decreased from -0.084MPa to -0.051MPa, the water production rate increased from 13.14L / (m³). 2 The concentration of ·h) decreased to 0.32 L / (m 2 The seepage flow rate is 0.33 L / (m³). 2 •h) increased to 2.07L / (m 2 ·h).

[0044] When the operating vacuum at 75℃ decreased from -0.096MPa to -0.066MPa, the water production rate increased from 11.25L / (m³). 2The concentration of ·h) decreased to 0.14 L / (m 2 The seepage flow rate is 0.02 L / (m³). 2 •h) increased to 3.0L / (m 2 ·h).

[0045] When the operating vacuum at 65℃ decreased from -0.096MPa to -0.078MPa, the water production rate increased from 8.9L / (m³). 2 The concentration of ·h) decreased to 0.08 L / (m 2 The seepage flow rate is 0.26 L / (m³). 2 •h) increased to 3.3L / (m 2 ·h).

[0046] When the flow velocity decreased from 0.8 m / s to 0.5 m / s at 75℃, the water production rate decreased from 11.25 L / (m³) 2 The concentration of ·h) decreased to 9.93 L / (m 2 ·h), the seepage flow rate is 0.015L / (m 2 The concentration of ·h) increased to 0.08 L / (m 2 ·h).

[0047] When the permeate flux decreased, it was quickly restored by increasing the operating vacuum, raising the temperature, increasing the flow rate (e.g., reducing the flow rate from 0.5 m / s to 0.8 m / s), or decreasing the feed concentration. The entire testing period lasted 4 weeks, during which repeated tests were conducted under various concentration conditions. The permeate flux and permeate quality remained stable, and no backwashing or chemical cleaning was required throughout the process.

[0048] (3) In all NaOH solution membrane distillation concentration tests, the turbidity of the liquid in the permeate collection tank was significantly higher than that of the NaOH feed solution. In particular, when the NaOH feed solution was concentrated to 48.54%, the feed solution remained clear and transparent after cooling to room temperature; while the permeate in the permeate collection tank underwent secondary evaporation and concentration under negative pressure, resulting in a high concentration of solute. After cooling to room temperature, it reached a supersaturated state, and white precipitate and wall-attached substances appeared (a large amount of white solute crystals were precipitated and accompanied by wall-attached phenomena).

[0049] 4. Conclusion The permeate production of NaOH solution decreases with increasing concentration, while the permeate flow initially increases and then decreases. By adjusting the operating vacuum, temperature, flow rate, or concentration to increase the transmembrane vapor flux, the vapor phase can occupy the membrane pores to suppress liquid phase permeate flow, achieving in-situ reversible recovery of permeate performance. Simultaneously, the secondary evaporation and concentration of the permeate outside the membrane allows for the precipitation and crystallization of high-concentration solutes.

[0050] Example 2: Percolation vacuum membrane distillation concentration of phosphoric acid solution 1. Membrane elements and devices The membrane element is assembled using the same polypropylene membrane tube as in Example 1. The membrane element is placed vertically, and the hot feed liquid undergoes forced circulation within the membrane tube. The outer side of the membrane and the inner wall of the membrane shell form a closed cavity. A steam outlet is provided at the upper end of this cavity, and a permeate outlet is provided at the lower end (connected to the permeate collection tank to form a blind end).

[0051] 2. Performance Testing Prepare a phosphoric acid (H3PO4) solution as the concentrate, and control the circulation flow rate at 0.5 m / s to 0.8 m / s.

[0052] (1) Variable concentration test: Under the conditions of circulation flow rate of 0.8 m / s and operating vacuum of -0.096 MPa, the operating temperature was set to 85℃, 75℃ and 65℃ respectively, and the permeate flux and seepage flow rate were tested under different phosphoric acid mass concentrations (10.44%~41.88%).

[0053] (2) Variable parameter control test: At a phosphoric acid concentration of 10.44% and a temperature of 85℃, the operating vacuum was reduced from -0.096MPa to -0.074MPa; the circulation flow rate was reduced from 0.8m / s to 0.5m / s. At a phosphoric acid concentration of 21.57% and a temperature of 75℃, the circulation flow rate was reduced from 0.8m / s to 0.5m / s. The flux change before and after the parameter changes was tested, and after the flux decayed, in-situ recovery tests were conducted by increasing the operating vacuum or increasing the feed liquid circulation flow rate.

[0054] 3. Results (1) Under the conditions of a circulation flow rate of 0.8 m / s and an operating vacuum of -0.096 MPa, the results of the variable concentration test show that: At 85℃, the phosphoric acid concentration increased from 10.44% to 41.88%, and the water production rate increased from 20.3 L / (m³) 2 The concentration of ·h) decreased to 16.78 L / (m 2 ·h); seepage flow rate from 0.02L / (m 2 The concentration of ·h) increased to a maximum of 0.19 L / (m 2 (At this point, the feed concentration is 23.31%), and when the concentration is further increased to 41.88%, the permeation flow rate decreases to 0.05 L / (m³). 2 ·h).

[0055] At 75℃, the concentration increased from 10.44% to 41.88%, and the water production increased from 13.84 L / (m³) 2 The concentration of ·h) decreased to 10.32 L / (m 2 •h); During the process of increasing the concentration of phosphoric acid solution from 10.44% to 27.79%, the permeation flow rate increased from 0.06 L / (m³) 2 The concentration of ·h) increased to a maximum of 0.11 L / (m 2(·h) (concentration 23.31%), then reduced to 0.05 L / (m 2 ·h).

[0056] At 65℃, the concentration increased from 10.44% to 41.88%, and the water production increased from 8.94 L / (m³) 2 The concentration of ·h) decreased to 4.96 L / (m 2 •h); During the process of increasing the concentration of phosphoric acid solution from 10.44% to 27.79%, the permeation flow rate increased from 0.015L / (m³) 2 The concentration of ·h) increased to a maximum of 0.14 L / (m 2 (·h) (concentration 21.57%), then reduced to 0.06 L / (m 2 ·h).

[0057] At the same concentration, the water production of phosphoric acid solution is greater than that of NaOH solution, while the percolation rate is less. The conductivity of the distillate is less than 4 μS / cm.

[0058] (2) The test results of variable parameter control show that: When the operating vacuum at 85℃ decreased from -0.096MPa to -0.074MPa, the water production rate increased from 20.3L / (m³) 2 The concentration of ·h) decreased to 15.99 L / (m 2 The seepage flow rate is 0.02 L / (m³). 2 •h) increased to 0.15L / (m 2 ·h).

[0059] When the circulation velocity at 85℃ decreased from 0.8 m / s to 0.5 m / s, the water production rate decreased from 20.3 L / (m³) 2 The concentration of ·h) decreased to 17.53 L / (m 2 The seepage flow rate is 0.02 L / (m³). 2 ·h) increased to 0.08L / (m 2 ·h).

[0060] When the operating vacuum at 75℃ decreased from -0.096MPa to -0.082MPa, the water production rate increased from 13.84L / (m³). 2 The concentration of ·h) decreased to 9.42 L / (m 2 ·h), the seepage flow rate is 0.06L / (m 2 •h) increased to 0.16L / (m 2 ·h).

[0061] At 75℃ and a phosphoric acid concentration of 21.57%, when the circulation flow rate decreased from 0.8 m / s to 0.5 m / s, the permeate flow rate decreased from 13.6 L / (m³) 2 The concentration of ·h) decreased to 12.7 L / (m 2·h), the seepage flow rate is 0.04 L / (m³). 2 The concentration of ·h) increased to 0.13 L / (m 2 ·h).

[0062] When the operating vacuum at 65℃ decreased from -0.096MPa to -0.085MPa, the water production rate increased from 8.92L / (m³) 2 The concentration of ·h) decreased to 3.82 L / (m 2 ·h), the seepage flow rate is 0.015L / (m 2 •h) increased to 0.24L / (m 2 ·h).

[0063] When the permeate flux decreased, it was quickly restored by increasing the operating vacuum or increasing the feed circulation rate. During four weeks of continuous testing, the permeate flux remained stable, and no backwashing or chemical cleaning was required throughout the process.

[0064] (3) In all test phases, the pH value of the phosphoric acid solution in the permeate collection tank was higher than that of the corresponding feed liquid. This phenomenon indicates that the permeate underwent intense secondary evaporation and concentration under the negative pressure environment outside the membrane, resulting in a high enrichment of solute.

[0065] 4. Conclusion The permeate and seepage patterns of phosphoric acid solution are consistent with those of NaOH solution. By adjusting the operating vacuum and circulation rate, seepage suppression and in-situ reversible recovery of permeate performance can also be achieved. The pH value of the seepage liquid is higher than that of the feed liquid, confirming the effect of negative pressure secondary evaporation concentration on high-concentration inorganic acid systems, indicating that this process has significant advantages in the concentration and recovery of inorganic acid systems.

[0066] Example 3: Percolation vacuum membrane distillation concentration of acetic acid solution 1. Membrane elements and devices The membrane element is assembled using the same polypropylene membrane tube as in Example 1. The membrane element is placed vertically, and the hot feed liquid undergoes forced circulation within the membrane tube. The outer side of the membrane and the inner wall of the membrane shell form a closed cavity. A steam outlet is provided at the upper end of this cavity, and a permeate outlet is provided at the lower end (connected to the permeate collection tank to form a blind end).

[0067] An acetic acid (CH3COOH) solution was prepared as the feed liquid to be concentrated, and the circulation flow rate was set to 0.8 m / s. Given the volatility of acetic acid, this embodiment focuses on investigating the effect of lowering the operating temperature on controlling the transmembrane gas-phase diffusion rate of acetic acid molecules.

[0068] Acetic acid is a volatile organic acid, and during membrane distillation, it evaporates through the membrane pores and enters the product water. Because a hydrophobic membrane with a 40nm pore size is used, the membrane pores effectively trap acetic acid molecules during vacuum membrane distillation, thus separating and concentrating the acetic acid solution. For acetic acid, a volatile organic acid, temperature is the most significant factor affecting the concentration of acetic acid in the product water, followed by the concentration of acetic acid in the feed solution, and then the degree of vacuum. Furthermore, the product water flow rate of the acetic acid solution is greater than that of a NaOH solution of the same concentration, while the permeate flow rate is slightly lower.

[0069] 2. Performance Testing and Results (1) High-temperature and high-concentration test: The operating temperature was set at 85℃, and acetic acid solutions with a mass concentration of 11.5% to 44.5% were treated. The operating vacuum was matched within the range of -0.096MPa to -0.085MPa. The results showed that when the concentration was 11.5% and the vacuum was -0.096MPa, the water production rate was 22.26L / (m³). 2 The permeate water concentration was 9.5% when the vacuum was adjusted to -0.085 MPa; however, when the vacuum was adjusted to -0.085 MPa, the permeate water concentration remained almost unchanged, but the permeate flow rate decreased to 19.04 L / (m³). 2 (·h). When the concentration increased to 44.5%, the permeate flow rate decreased to 20.27 L / (m³). 2 (·h), the acetic acid concentration in the product water increased to 12.5%. At high temperatures, the water vapor flux was large, and the gas phase effectively occupied the membrane pores, forming a carrier and competitive inhibition of acetic acid molecules.

[0070] (2) Medium-temperature and low-concentration test: The operating temperature was set at 60℃, and acetic acid solutions with a mass concentration of 2.0% to 11.5% were treated. The operating vacuum was matched within the range of -0.096MPa to -0.085MPa. The results showed that when the concentration was 2.0%, the permeate flow rate was 7.89 L / (m³). 2 The permeate concentration was only 0.18% at a concentration of 11.5% and a vacuum of -0.096 MPa, with a permeate flow rate of 8.59 L / (m³). 2 The permeate contains 3.65% acetic acid; when the vacuum is adjusted to -0.085 MPa, the acetic acid concentration in the permeate remains almost unchanged, but the permeate flow rate decreases to 6.81 L / (m³). 2 (·h). Lowering the temperature significantly inhibited the volatilization and transmembrane diffusion of acetic acid molecules.

[0071] (3) Low-temperature specific concentration test: The operating temperature was set at 50℃, and an acetic acid solution with a mass concentration of 11.5% was treated. The operating vacuum was maintained at -0.096MPa. The results showed that the permeate flow rate was maintained at 6.2L / (m³). 2 The acetic acid concentration in the produced water was further reduced to 3.5% (·h).

[0072] During four weeks of continuous testing, the permeate flow and retention performance remained stable by adjusting parameters such as operating temperature and vacuum, without requiring backwashing or chemical cleaning throughout the process.

[0073] 3. Conclusion The permeate flow behavior of the acetic acid solution is consistent with that of the NaOH and phosphoric acid systems, with permeate flux decreasing as feed concentration increases. For acetic acid, a volatile organic acid, operating temperature is the most significant factor affecting the acetic acid concentration in the permeate, followed by the concentration of acetic acid in the feed solution. Within the test range of this embodiment, the operating vacuum level significantly affects the permeate flux, but has a relatively small impact on the acetic acid concentration in the permeate. By lowering the operating temperature, the permeation of acetic acid molecules with water vapor through the membrane pores can be significantly reduced, thereby improving the retention efficiency of the small-pore hydrophobic membrane for volatile solutes.

[0074] Example 4: Water production by percolation vacuum membrane distillation of sodium chloride solution 1. Membrane elements and devices The membrane element was assembled using polypropylene (PP) membrane tubes with a wall thickness of 2.5 mm, an inner diameter of 1.5 mm, and an average pore size of 20 nm. The effective length of the membrane element was 240 mm, containing 85 membrane tubes, with an effective membrane area of ​​0.16 m². 2 The membrane element is a shell-and-tube heat exchanger structure, placed vertically.

[0075] 2. Process Flow A heated sodium chloride solution from the feed tank is injected into the membrane tube side and flows out from the other end, returning to the feed tank for recirculation. A negative pressure (vacuum) is applied to the shell side of the membrane element, generating a large amount of vapor on the membrane surface. At this time, some solution inside the membrane pores also permeates to the outer wall of the membrane tube, evaporating again under high temperature and negative pressure. This vapor flows into a heat exchanger for cooling and is stored in a distillate collection tank (in this embodiment, a freshwater collection tank). The liquid concentrated on the outer membrane wall flows under gravity into a permeate collection tank (in this embodiment, a concentrate collection tank) connected to the shell side.

[0076] 3. Performance Testing and Results (1) High-temperature water production test: The hot feed liquid was a 10% sodium chloride solution, the temperature was 90℃, and the vacuum side pressure was -0.095MPa. The freshwater production rate was 20.3L / (m³). 2 The permeate conductivity was 4.2 μS / cm; the concentrate flow rate was 15 ml / h (referring to the total concentrate collection volume of the entire membrane element); precipitate was obtained upon standing. This process was run continuously for 2 weeks without backwashing.

[0077] (2) Medium-temperature water production test: The hot feed liquid was a 10% sodium chloride solution, the temperature was 50℃, and the vacuum side pressure was -0.095MPa. The freshwater production rate was 6.2L / (m³). 2The permeate conductivity was 6.5 μS / cm; the concentrate flow rate was 96 ml / h (referring to the total concentrate collection volume of the entire membrane element); precipitate was obtained upon standing. This process was run continuously for 2 weeks without backwashing.

[0078] 4. Conclusion Percolation vacuum membrane distillation using PP hydrophobic membrane tubes with a pore size of 20 nm can effectively produce pure water from sodium chloride solution. At 90℃, the freshwater yield can reach 20.3 L / (m³). 2 The permeate has a conductivity as low as 4.2 μS / cm, resulting in excellent permeate quality. The permeate undergoes secondary evaporation and concentration under high temperature and negative pressure on the outer wall of the membrane, producing solid precipitates in the concentrated water. No backwashing is required throughout the process, ensuring stable continuous operation.

[0079] In addition, by interchangeing the tube side and shell side of the membrane element, the same effect of percolation membrane distillation for water production and concentrate collection can be achieved; or by connecting the membrane shell to the atmosphere to form gas-swept membrane distillation, the result is similar.

[0080] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for treating a solution using a percolation vacuum membrane distillation process, comprising: A membrane element is formed by assembling a hydrophobic tubular microporous membrane. The feed liquid is forced to circulate inside the hydrophobic tubular microporous membrane. The hydrophobic tubular microporous membrane and the membrane shell enclose the outer side to form a sealed cavity. The operating vacuum in the sealed cavity is maintained by evacuation. The vapor generated inside the hydrophobic tubular microporous membrane by the feed liquid permeates through the membrane pores into the negative pressure chamber. The gaseous components are drawn out from the negative pressure chamber and condensed to obtain distillate. The permeate formed inside the membrane pores flows to the outside of the hydrophobic tubular microporous membrane and undergoes secondary evaporation and concentration under negative pressure. The average pore size of the membrane is 10 nm to 100 nm.

2. The method according to claim 1, wherein the transmembrane steam flux is adjusted by controlling at least one of the concentration, temperature, circulation flow rate, and operating vacuum, thereby achieving reversible control of the permeate flow rate and the permeate flux.

3. The method according to claim 1 or 2, wherein when the percolation flow rate increases and the permeate flux decreases during operation, the transmembrane steam flux is increased by at least one of the following methods: increasing the operating vacuum, increasing the feed liquid temperature, increasing the circulation flow rate of the feed liquid, and decreasing the concentration of the feed liquid, thereby suppressing liquid phase percolation to restore permeate performance.

4. The method according to any one of claims 1-3, wherein when the feed solution is an inorganic alkaline solution or an inorganic acid solution, the solution is concentrated through the percolate; and when the feed solution is a salt solution, pure water is produced through the distillate.

5. The method according to any one of claims 1-4, wherein the average pore size of the membrane pores is 20 nm to 50 nm, and the material is selected from one or more of polypropylene, polyvinylidene fluoride, and polytetrafluoroethylene.

6. The method according to any one of claims 1-5, wherein the hydrophobic tubular microporous membrane has an air flux greater than 3000 L / (m²) at 25°C. 2 (·h·bar), the breakthrough pressure of pure water is greater than 2.6 bar.

7. The method according to any one of claims 1-6, wherein the membrane element is arranged vertically, and a permeate flow gap is reserved between the hydrophobic tubular microporous membranes; the gas phase component is drawn out from the upper end of the negative pressure cavity, and the permeate is collected from the lower end, wherein the permeate is discharged and collected by gravity.

8. The method according to any one of claims 1-7, wherein, The feed solution is a sodium hydroxide solution, with a circulation flow rate of 0.5 m / s to 0.8 m / s, an operating temperature of 65℃ to 85℃, a mass concentration of 9.91% to 48.54%, and an operating vacuum of -0.096 MPa to -0.051 MPa. The feed solution is a phosphoric acid solution, with a circulation flow rate of 0.5 m / s to 0.8 m / s, an operating temperature of 65℃ to 85℃, a mass concentration of 10.44% to 41.88%, and an operating vacuum of -0.096 MPa to -0.074 MPa. The feed solution is an acetic acid solution, with a circulation flow rate of 0.8 m / s, an operating temperature of 50℃~85℃, a mass concentration of 2.0%~44.5%, and an operating vacuum degree of -0.096MPa~-0.085MPa; or The feed solution is a sodium chloride solution with a mass concentration of 5% to 15%; the operating temperature is 50℃ to 90℃; and the operating vacuum is -0.095MPa.

9. The method according to any one of claims 1-8, wherein the membrane material of the hydrophobic tubular microporous membrane is polypropylene with an average pore size of 20 nm.

10. A percolation-type vacuum membrane distillation apparatus, comprising a membrane element, a heat exchanger, a distillate collection tank, a vacuum pump, and a percolation collection tank, wherein: The membrane element is vertically arranged and includes a shell and a plurality of parallel hydrophobic tubular microporous membranes disposed therein, with a sealed cavity formed between the shell and the hydrophobic tubular microporous membranes; The membrane element has a pipe at its upper part that communicates with the heat exchanger, and a pipe at its lower part that communicates with the permeate collection tank; The heat exchanger is connected to the distillate collection tank via a pipe, and the distillate collection tank is connected to the vacuum pump via a pipe. The average pore size of the hydrophobic tubular microporous membrane is 20 nm to 50 nm, and the material is selected from one or more of polypropylene, polyvinylidene fluoride, and polytetrafluoroethylene.

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