System and method for extracting salinity difference energy of saline water

Abstract

The invention relates to the field of utilization of marine renewable energy sources, in particular to a salt water salinity difference energy extraction system and method. The system comprises a heat exchange unit, an osmotic membrane distillation unit, a pressure delay osmotic membrane unit, a reverse osmosis unit, a PX pressure exchange device and a booster pump. The low-salt wastewater enters an OMD hot side chamber after heat exchange and temperature rise, a sodium chloride solution enters an OMD cold side chamber, water molecules are migrated to a cold side in a steam form under the driving of temperature difference and concentration difference, an obtained sodium chloride dilute solution enters a PRO low-salt side chamber, a high-salt solution pressurized by a pressure exchange device enters a PRO high-salt side chamber, and the low-salt wastewater enters an OMD cold side chamber. Water is produced under the driving of osmotic pressure difference, and salinity difference energy is converted into mechanical energy; the low-salt solution is pressurized and then enters an RO unit for deep desalination, and fresh water is prepared. According to the invention, through OMD-PRO-RO multi-process coupling, efficient and stable extraction of salinity gradient energy, wastewater reduction and resource cooperative utilization are realized.

Description

Technical Field

[0001] This invention relates to the field of marine renewable energy utilization, and in particular to a system and method for extracting salinity gradient energy from brine. Background Technology

[0002] Pressure-delayed osmosis (PRO) membrane technology is a promising new membrane technology that has attracted much attention in recent years. It consists of a pressure-delayed osmosis membrane and low-salt and high-salt side chambers on either side of the membrane. Typically, low-salt solutions such as river water are used as the feed solution, while high-salt solutions such as seawater (brine) are used as the draw solution, which are respectively introduced into the low-salt and high-salt side chambers of the PRO device. Under osmotic pressure, water from the low-salt side chamber enters the high-salt side chamber, causing an increase in the volume of the solution in the high-salt side chamber. This converts salinity gradient energy into mechanical or electrical energy for utilization, making it one of the most promising technologies for extracting salinity gradient energy from seawater (brine). However, when low-salt solutions such as river water or wastewater are used as the feed solution in the PRO process, the river water (wastewater) comes into direct contact with the porous support layer of the pressure-delayed osmosis membrane, which has a larger pore size. Large organic molecules and small particles in the water can easily cause irreversible membrane fouling, such as pore blockage, thus compromising the stable extraction of salinity gradient energy. Although the forward osmosis (FO)-PRO coupling process can ensure the stable extraction of salinity gradient energy to a certain extent, since both the FO and PRO processes are concentration-driven membrane separation processes, if the concentration of the intermediate circulating solution increases, although the pure water extraction efficiency of the FO process increases, the salinity gradient energy extraction efficiency of the PRO process will decrease. Therefore, there is a strong competitive and balancing relationship between the FO unit and the PRO unit in this coupling process, which makes it impossible to achieve efficient and stable extraction of salinity gradient energy.

[0003] The bottlenecks of forward osmosis-pressure delayed osmosis (FO-PRO) coupling technology are mainly reflected in the following aspects: (1) Contradictory driving force: Both FO and PRO are concentration-driven processes, and there is an irreconcilable contradiction in the demand for intermediate draw solution concentration—FO requires high-concentration draw solution to increase water flux, while PRO requires low-concentration draw solution to maintain high osmotic pressure difference. This makes it impossible for the system to optimize the performance of the two units at the same time, and the overall energy conversion efficiency is limited; (2) Pollutant penetration problem: Although the FO membrane can retain most macromolecular pollutants, small molecule organic matter, inorganic ions and trace surfactants will pass through solute reverse osmosis (RSF). (3) Irreversible membrane fouling: The PRO membrane support layer has a complex structure and large pore size. Once pollutants enter, they are difficult to remove completely. Standard chemical cleaning can only restore 60-70% of the initial flux, and the membrane life is significantly shortened. (4) System complexity and energy consumption: The FO-PRO system needs to be equipped with a complex intermediate circulation solution regeneration unit, and the regeneration process consumes a lot of energy, which offsets part of the salinity gradient energy gain. (5) Poor water quality adaptability: When treating oily, high organic matter or high hardness wastewater, the FO membrane is prone to serious fouling, and the system stability drops sharply.

[0004] Furthermore, traditional FO-PRO technology places stringent requirements on membrane materials, necessitating a balance between high water flux, high salt rejection rate, and fouling resistance, thus limiting membrane performance. While high-performance TFC and TFN membrane materials possess excellent water flux and selectivity, their fouling resistance is often poor, leading to rapid performance degradation upon direct contact with wastewater. Studies have shown that unprotected TFC-PRO membranes can experience a water flux degradation rate exceeding 35% within 15 days when treating municipal wastewater, requiring membrane cleaning every 7-10 days, significantly increasing operating costs and maintenance complexity. More critically, FO-PRO systems cannot effectively utilize low-quality thermal energy, resulting in a single energy utilization method and low overall energy efficiency. Summary of the Invention

[0005] The purpose of this invention is to provide a highly efficient and stable system and method for extracting salinity gradient energy from brine. By using a permeate membrane distillation-pressure delayed permeation coupling process, membrane fouling is reduced, and the strong competitive and balancing relationship between the FO unit and the PRO unit in the coupling process is avoided, thereby achieving highly efficient and stable extraction of salinity gradient energy from brine.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: On one hand, the present invention provides a brine salinity gradient energy extraction system, comprising: a heat exchange unit, a permeate membrane distillation unit, a pressure-delayed permeate membrane unit, a reverse osmosis unit, a PX pressure exchange device, and a booster pump; The permeate membrane distillation unit includes a hot-side chamber and a cold-side chamber. The hot-side chamber has an inlet at the top and a concentrated wastewater outlet at the bottom; the cold-side chamber has an inlet at the bottom and an outlet at the top. The pressure-delayed permeate membrane unit includes a low-salt side chamber and a high-salt side chamber. The low-salt side chamber has an inlet at the top and an outlet at the bottom; the high-salt side chamber has an inlet at the bottom and an outlet at the top. The heat exchange unit is connected via a pipeline supplying heated low-salt wastewater. The hot-side chamber inlet is connected to the cold-side chamber outlet via a sodium chloride dilute solution pipeline, and the low-salt side chamber outlet is also connected to the cold-side chamber inlet via a sodium chloride solution pipeline. The high-salt side chamber outlet is connected sequentially via pipelines to the booster pump, reverse osmosis unit, PX pressure exchange device, and high-salt side chamber inlet. A porous hydrophobic membrane is installed between the hot-side and cold-side chambers. A pressure-delayed osmosis membrane is installed between the low-salt and high-salt side chambers. The reverse osmosis unit is equipped with a freshwater outlet; the PX pressure exchange device is equipped with a concentrated brine outlet and a salt solution inlet.

[0007] Preferably, the porous hydrophobic membrane is any one of polytetrafluoroethylene, polyvinylidene fluoride, or polypropylene membrane; the average pore size of the hydrophobic membrane is 0.10~0.80 μm, and the thickness is 50~100 μm. If the pore size of the hydrophobic membrane is too small, the mass transfer resistance increases and the permeation flux decreases; however, if the pore size of the hydrophobic membrane is too large, the membrane wetting tendency is aggravated. If the membrane thickness is too small, the heat insulation effect is poor, causing the heat loss problem to be aggravated; however, if the membrane thickness is too thick, the mass transfer resistance increases, affecting the permeation flux. Specifically, Gore's GORE® Membrane (PTFE, pore size 0.10~0.20 μm, thickness 50~80 μm), Solvay's Kynar® PVDF membrane (pore size 0.45 μm, thickness 70 μm), or Asahi Kasei's Accurel® PP membrane (pore size 0.80 μm, thickness 100 μm) can be used.

[0008] Preferably, the pressure-delayed permeation membrane assembled in the PRO unit is any one of the following: cellulose acetate membrane (such as the HTI-CTA-ES model from HTI Corporation, USA), thin-walled composite membrane (such as the RE2540-PRO model from CSM Corporation, South Korea), and thin-walled composite nanomembrane (TFN, self-made in the laboratory, using PES ultrafiltration membrane as a substrate, with nanomaterials introduced through interfacial polymerization).

[0009] On the other hand, the present invention provides a method for salinity gradient extraction using the aforementioned salinity gradient extraction system, specifically comprising the following steps: (1) The low-salt wastewater is heated to 55~70℃ by heat exchange and sent into the hot side chamber of the permeate membrane distillation (OMD) unit. The sodium chloride solution is sent into the cold side chamber of the permeate membrane distillation unit at the same time. Driven by the temperature difference and concentration difference on both sides of the porous hydrophobic membrane, the water molecules in the low-salt wastewater migrate into the sodium chloride solution in the form of vapor to obtain a dilute sodium chloride solution. (2) A dilute sodium chloride solution is sent into the low-salt side chamber of the pressure delayed osmosis membrane (PRO) unit, and a high-pressure high-salt solution is sent into the high-salt side chamber of the pressure delayed osmosis membrane unit simultaneously. Driven by the osmotic pressure difference across the PRO membrane, water molecules in the dilute sodium chloride solution permeate into the high-salt solution, resulting in a low-salt solution and a sodium chloride solution. The sodium chloride solution is returned to the cold side chamber of the osmosis membrane distillation unit for recycling. (3) The low-salt solution is pressurized to 50~80 bar by a booster pump and sent to the reverse osmosis (RO) unit for deep desalination to obtain concentrated brine and fresh water; (4) The high-pressure high-salt solution in step (2) is obtained by sending the brine and the high-salt solution into the PX pressure exchange device for pressure exchange, thereby increasing the pressure of the high-salt solution to 13~20 bar, which is the high-pressure high-salt solution.

[0010] Preferably, in step (1), the low-salt wastewater has a salt content of <2000 mg / L, a hardness of <500 mg / L, an oil content of <1 mg / L, and a surfactant content of <0.01 mmol / L. These water quality indicators ensure that the low-salt wastewater will not cause scaling, oil stains, or wetting damage to the OMD hydrophobic membrane, thus ensuring the long-term stable operation of the system.

[0011] Preferably, in step (1), the sodium chloride solution has a salt content of 2000~4000 mg / L. This concentration range ensures that: on the one hand, the sodium chloride solution has a salt content higher than that of low-salt wastewater (<2000 mg / L), forming a positive vapor pressure difference that drives water molecules to migrate; on the other hand, the concentration of the sodium chloride solution returned to OMD after PRO concentration is still lower than that of sea brine (30000~47000 mg / L), ensuring that the PRO process has a sufficient osmotic pressure difference.

[0012] The temperature of the low-salt wastewater on the hot side of the OMD unit is set at 55-70℃. This is to ensure sufficient vapor pressure difference to drive water molecules to cross the membrane efficiently, while avoiding excessive temperature that could cause the hydrophobic membrane to wet, scale, or age. This temperature range also allows for the utilization of low-grade heat sources such as solar energy and industrial waste heat, thereby improving the system's energy efficiency.

[0013] The operating pressure on the high-salt solution side of the PRO unit concentration chamber is set at 13-20 bar because within this range (approximately half the effective osmotic pressure difference between concentrated and dilute water), the power density of salt gradient energy converted into mechanical energy reaches its maximum. At the same time, this pressure is well matched with the PX pressure exchange device, which can efficiently extract energy without aggravating concentration polarization or damaging the membrane module due to excessive pressure.

[0014] Preferably, in step (1), the ratio of the influent flow rate of the low-salt wastewater to the influent flow rate of the sodium chloride solution is 4-5:1. This flow rate ratio ensures that the concentration of the dilute sodium chloride solution is close to the salinity of the low-salt wastewater, maximizing the driving force of the vapor pressure difference, while avoiding excessive concentration of the low-salt wastewater.

[0015] Preferably, in step (2), the salt content of the high-salt solution is 30,000~47,000 mg / L; the ratio of the influent volume of the dilute sodium chloride solution to the high-salt solution is 1:0.8~1; this flow ratio can effectively control the concentration polarization of the membrane surface and maintain a high osmotic pressure difference.

[0016] Compared with the prior art, the technical solution of the present invention has the following significant advantages: (1) In the traditional FO-PRO coupling technology, although the FO membrane can retain some pollutants, the presence of solute reverse osmosis makes it difficult to prevent pollutants from penetrating into the support layer during long-term operation, resulting in flux decay and irreversible performance degradation. In order to solve the above technical problems, the technical solution of this invention uses a permeation membrane distillation unit to replace the FO and pressure delayed osmosis unit for coupling. The OMD unit uses a hydrophobic microporous membrane, which only allows water to pass through in the form of steam, completely blocking pollutants such as oil, surfactants, macromolecular organic matter and colloids in low-salt wastewater from entering the intermediate circulating liquid sodium chloride solution. This fundamentally avoids pollutants from contacting and contaminating the porous support layer of the PRO membrane, thus solving the technical problems of PRO flux decay and irreversible performance degradation.

[0017] (2) The OMD process of this invention is mainly driven by temperature difference and vapor pressure difference, while the PRO process is driven by concentration difference (osmotic pressure difference). The two have different types of driving forces and do not interfere with each other. Therefore, compared with FO, OMD can independently optimize the concentration of intermediate circulating liquid. The concentration of sodium chloride solution can be controlled to be slightly higher than that of low-salt wastewater, avoiding the inherent competitive contradiction in FO-PRO where the efficiency of FO increases but the efficiency of PRO decreases due to the increase of intermediate liquid concentration, thus maximizing the overall energy efficiency of the system.

[0018] (3) OMD can use low-grade thermal energy such as solar energy, geothermal energy, and industrial waste heat as driving force to convert waste heat into a prerequisite for salinity gradient energy extraction, thereby achieving multi-energy complementarity and cascade utilization. In contrast, FO-PRO relies entirely on chemical potential difference and cannot integrate thermal energy resources.

[0019] (4) The OMD unit can reduce the flow rate of low-salt wastewater by 75-80%, which greatly reduces the load of subsequent sewage treatment; the sea brine diluted by PRO enters the RO unit, which significantly reduces the salinity of its influent and reduces the energy consumption per ton of RO water by >10%; the RO permeate is used for steam production in thermal power plants, and the concentrated brine enters the salt chemical process, realizing the synergy of multi-dimensional resources of "energy-water-chemical", and the comprehensive benefits far exceed the single energy recovery mode of FO-PRO.

[0020] (5) The OMD-PRO system has significant advantages over the FO-PRO system in key performance indicators. According to the experimental data of Comparative Example 1, under the same conditions of treating municipal wastewater reuse concentrate with high pollution risk, the average power generation density of the PRO unit of the OMD-PRO system reaches 3.80 W / m², while that of the FO-PRO system is only 3.20 W / m², with an energy conversion efficiency improvement of about 18.8%. In terms of long-term operational stability, the PRO membrane flux decay rate of the OMD-PRO system is only 8.3% after 180 days of continuous operation, while the flux decay rate of the FO-PRO system is as high as 28.7% within 30 days, with a decay rate more than 3 times that of the former. In terms of operation and maintenance costs, the membrane cleaning frequency of the OMD-PRO system is 4.9 times / year (about once every 75 days), while that of the FO-PRO system is as high as 12.2 times / year (about once every 30 days), reducing the cleaning frequency by nearly 60%. Furthermore, the OMD-PRO system exhibits a more stable water flux (average OMD water flux of 13.50 μm / s) due to its lower intermediate circulating liquid concentration and effective suppression of internal concentration polarization (ICP). In contrast, the FO-PRO system suffers from reverse solute infiltration and superimposed pollution, resulting in a water flux of only 2.78 μm / s for the FO unit. This comparison of performance parameters clearly demonstrates that the OMD-PRO system comprehensively outperforms the FO-PRO system in core indicators such as power generation efficiency, pollution resistance, operational stability, and maintenance economy.

[0021] Therefore, the OMD average water flux of the technical solution of this invention is 13.10~14.40 μm / s, the low-salinity wastewater flow is reduced by 75-80%, the average power generation density of PRO is 3.48~3.78 W / m², the energy consumption per ton of RO water is reduced by 12.0~14.1%, the 180-day flux decay rate of PRO membrane is 7.5~8.6%, and the membrane cleaning frequency is 4.2~4.8 times / year. Compared with the prior art, the technical solution of this invention, through physical phase change to isolate pollutants, decoupling of driving force mechanism, and multi-process synergistic optimization, is comprehensively superior to FO-PRO technology in terms of anti-fouling, energy efficiency stability, system adaptability, and comprehensive resource utilization, providing a reliable path for the engineering and efficient extraction of salinity gradient energy from marine brine. Attached Figure Description

[0022] Figure 1 This is a process flow diagram of a specific embodiment of the brine-salt energy differential extraction system of the present invention.

[0023] In the diagram: 1. Heat exchange unit; 2. Permeate membrane distillation unit; 3. Pressure delayed permeate membrane unit; 4. Reverse osmosis unit; 5. PX pressure exchange device; 6. Booster pump; 2-1 Hot side chamber; 2-2 Cold side chamber; 2-3 Porous hydrophobic membrane; 3-1 Low-salt side chamber; 3-2 High-salt side chamber; 3-3 Pressure delayed permeate membrane. Detailed Implementation

[0024] The embodiments of the present invention will now be described in conjunction with specific examples.

[0025] Figure 1 This is a process flow diagram of a specific embodiment of the brine salt energy differential extraction system of the present invention, as shown below. Figure 1 As shown, low-salt wastewater is fed into heat exchange unit 1 and heated to 55-70°C, then fed into the hot-side chamber 2-1 of the permeate membrane distillation unit 2. Simultaneously, sodium chloride solution is fed into the cold-side chamber 2-2 of the permeate membrane distillation unit 2. Driven by the temperature and concentration differences across the porous hydrophobic membrane, water molecules in the low-salt wastewater migrate to the sodium chloride solution in the form of vapor, resulting in a dilute sodium chloride solution and concentrated wastewater. The dilute sodium chloride solution is then fed into the low-salt side chamber 3-1 of the pressure-delayed permeate membrane unit 3, while the high-salt solution, pressurized by the PX pressure exchange device 5, is simultaneously fed into the high-salt side chamber 3-1 of the pressure-delayed permeate membrane unit 3. In the salt-side chamber 3-2, driven by the osmotic pressure difference across the PRO membrane, water molecules from the dilute sodium chloride solution permeate into the high-salt solution, resulting in a low-salt solution and a sodium chloride solution. The sodium chloride solution is then recycled back to the cold-side chamber 2-2 of the osmotic membrane distillation unit. The low-salt solution is sent to the booster pump 6 to be pressurized to 50-80 bar, and then sent to the reverse osmosis unit 4 for deep desalination, yielding concentrated brine and fresh water. The concentrated brine and the high-salt solution are then sent to the PX pressure exchange device 5 for pressure exchange, increasing the pressure of the high-salt solution to 13-20 bar before it is sent to the high-salt side chamber 3-2 of the pressure-delayed osmotic membrane unit. The heat source for the heat exchange unit 1 includes low-grade heat energy such as solar energy, geothermal energy, and industrial waste heat.

[0026] The heat exchange unit 1, the permeate membrane distillation unit 2, the pressure delayed permeate membrane unit 3, the reverse osmosis unit 4, the PX pressure exchange device 5, and the booster pump 6 can all be manufactured or purchased using existing technologies.

[0027] The pharmaceuticals and materials used in the following examples and comparative examples can all be purchased through commercial channels, and the low-salinity wastewater and sea brine were obtained locally. The parameters for water flux, power generation density, and energy consumption per ton of water in the experimental method are referenced in the following papers (paper title: Investigation of the reduced specific energy consumption of the RO-PRO hybrid system based on temperature-enhanced pressure retarded osmosis, https: / / doi.org / 10.1016 / j.memsci.2019.03.079, paper title: Temperature-enhanced Pressure Retarded Osmosis Powered by Solar Energy: Experimental Validation, Economic Consideration, and Potential Implication, https: / / doi.org / 10.1016 / j.cherd.2021.04.024). Additionally, the FO-PRO coupling system is based on data analysis from literature reports (paper title: The forward osmosis-pressure retarded osmosis (FO-PRO) hybrid system: A new process to mitigate membrane fouling for sustainable osmotic powergeneration). (https: / / doi.org / 10.1016 / j.memsci.2018.04.036) Example 1

[0028] (OMD-PRO-RO system, treating real municipal wastewater) (1) OMD unit operations: The low-salinity wastewater is taken from the effluent of the secondary biological sedimentation tank of a municipal wastewater treatment plant. This water, without further treatment, is directly taken from the effluent channel and contains 640 mg / L of salt (approximately 0.011 M as NaCl), 420 mg / L of hardness (as CaCO3), 0.9 mg / L of oil, and 0.009 mmol / L of surfactant. This water is usually discharged directly into urban waterways, but it is now used in this system to achieve synergistic energy recovery and pollution reduction. After being heated to 60°C by a heat exchange unit, the wastewater is sent to the hot-side chamber of the permeate membrane distillation unit. The OMD is equipped with Gore® Membrane (PTFE material, 0.15 μm pore size, 60 μm thickness). Simultaneously, a 3000 mg / L sodium chloride solution is pumped into the cold-side chamber. The hot and cold side feed solutions circulate in reverse at a volumetric flow ratio of 4.5:1 to match the low influent salinity and maximize the vapor pressure difference. After OMD treatment, the volume of low-salt wastewater was reduced from 100 m³ / h to 23 m³ / h, a reduction rate of 77%. The concentrate was returned to the equalization tank of the wastewater treatment plant for co-treatment.

[0029] (2) PRO unit operation: A dilute sodium chloride solution (measured salinity of approximately 620 mg / L) flowing from the top of the OMD cold-side chamber is piped into the low-salt side chamber of the pressure-delayed osmosis membrane unit. This chamber is equipped with a CSM RE2540-PRO thin-walled composite membrane from South Korea, which features high flux and good antifouling performance. Simultaneously, the high-salt solution uses natural seawater from the Bohai Bay nearshore area, with a salinity of 35,000 mg / L. After recovering the pressure energy of the RO concentrate using a PX pressure exchange device, it is pressurized to 16 bar and sent into the PRO high-salt side chamber. The feed volume ratio of the dilute sodium chloride solution to the high-salt solution is set at 1:0.9 to suppress concentration polarization. Driven by osmotic pressure difference, water molecules migrate from the freshwater side to the concentrated water side, causing the high-salt solution to expand in volume for power generation, and generating a low-salt solution (about 22,000 mg / L) for the RO unit to process; the concentrated sodium chloride solution (about 3,050 mg / L) is returned to the OMD cold side chamber for recycling through the return pipeline, forming a closed intermediate liquid system.

[0030] (3) RO unit operation: The low-salt solution from the PRO unit outlet is pressurized to 60 bar by a booster pump and then sent to the reverse osmosis unit for deep desalination. The RO permeate is high-purity freshwater with a conductivity of less than 500 μS / cm, which can be used for municipal reuse or industrial cooling. The discharged concentrated brine has a salt content of approximately 70,000 mg / L. Because it does not contain organic pollutants, it is transported to a nearby salt chemical plant as a raw material for salt production, thus realizing resource utilization.

[0031] (4) Performance indicators: The system operated continuously and stably for 180 days without membrane wetting or irreversible fouling. The average water flux of the OMD reached 13.50 μm / s; the average power density of the PRO unit was 3.80 W / m², indicating that the low intermediate solution concentration effectively suppressed internal concentration polarization (ICP); due to the dilution effect of the PRO on seawater, the salinity of the RO feed water decreased from 35,000 mg / L to 22,000 mg / L, and the energy consumption per ton of RO water decreased from 3.45 kWh / m³ in the traditional seawater desalination process to 2.99 kWh / m³, a reduction rate of 12.8%; the PRO membrane flux decay rate was only 8.3%, and the membrane cleaning frequency was 4.9 times / year (approximately once every 75 days). Example 2

[0032] (OMD-PRO-RO system, lower limit boundary condition) (1) OMD unit operations: The low-salinity wastewater is concentrated water from a municipal reclaimed water plant after MBR + ozone advanced treatment. Originally used for industrial cooling, its characteristics are: salt content 2000 mg / L, hardness 500 mg / L, oil content 1.0 mg / L, and surfactant 0.010 mmol / L. After being heated to 55°C, it enters the hot-side chamber of the OMD (Optical Deposition Method) and is fitted with a Solvay Kynar® PVDF membrane (0.45 μm pore size, 70 μm thickness). A sodium chloride solution (2000 mg / L) enters the cold-side chamber at a volumetric flow rate ratio of 4:1. The wastewater reduction rate is 76%.

[0033] (2) PRO unit operation: A dilute sodium chloride solution (≈1800 mg / L) was introduced into the low-salt side chamber of the PRO membrane (CSM RE2540-PRO membrane). The high-salt solution was seawater from Bohai Bay (30000 mg / L), pressurized to 13 bar by PX, with a volumetric flow rate ratio of 1:1.

[0034] (3) RO unit operation: Low-salt solution is desalinated at 50 bar, fresh water is used in thermal power plants, and concentrated brine (≈72000 mg / L) is used to produce salt.

[0035] (4) Performance indicators: After 180 days of operation, the OMD flux was 13.10 μm / s, the PRO power was 3.48 W / m², the energy consumption decreased by 12.0%, the decay rate was 8.6%, and the cleaning frequency was 4.8 times / year. Example 3

[0036] (OMD-PRO-RO system, high limit boundary condition) (1) OMD unit operations: The low-salinity wastewater, consisting of biochemical and sand filtration tailwater from the industrial park (1500 mg / L, hardness 480 mg / L, oil content 0.95 mg / L, surfactant 0.0095 mmol / L), was heated to 70°C and then entered the hot side of the OMD system. GORE® Membrane (PTFE, 0.20 μm, 50 μm) was installed. A sodium chloride solution of 4000 mg / L was used at a volumetric flow rate ratio of 5:1. The wastewater reduction rate was 79%.

[0037] (2) PRO unit operation: A dilute sodium chloride solution (≈1750 mg / L) was introduced into the low-salt side chamber of the CSM PRO membrane. The high-salt solution was underground brine (47000 mg / L), pressurized to 20 bar with PX at a volumetric flow rate ratio of 1:0.8.

[0038] (3) RO unit operation: Desalination is carried out at 80 bar. The fresh water is used for thermal power plants, and the concentrated brine (≈83000 mg / L) is used for salt field crystallization.

[0039] (4) Performance indicators: The OMD flux is 14.40 μm / s, the PRO power is 3.72 W / m², the energy consumption is reduced by 13.9%, the attenuation rate is 7.5%, and the cleaning is performed 4.2 times per year. Example 4

[0040] (OMD-PRO-RO system, upper limit boundary of OMD hydrophobic membrane parameters) (1) OMD unit operations: The low-salinity wastewater was RO concentrate from a reclaimed water plant (1800 mg / L, hardness 490 mg / L, oil content 0.98 mg / L, surfactant 0.0098 mmol / L), entering the hot side of the OMD at 60°C. An Asahi Kasei Accurel® PP membrane (0.80 μm pore size, 100 μm thickness) was used. A sodium chloride solution of 4000 mg / L was used at a volumetric flow rate ratio of 5:1. The wastewater reduction rate was 78%.

[0041] (2) PRO unit operation: A dilute sodium chloride solution (≈1780 mg / L) was introduced into the low-salt side chamber of a laboratory-made TFN membrane. The high-salt solution was underground brine (47000 mg / L), at 20 bar, with a volumetric flow rate ratio of 1:0.8.

[0042] (3) RO unit operation: Desalination at 80 bar: fresh water is used in thermal power plants, and concentrated brine (≈85000 mg / L) is used in the chlor-alkali industry.

[0043] (4) Performance indicators: The OMD flux is 14.12 μm / s, the PRO power is 3.78 W / m², the energy consumption is reduced by 14.1%, the degradation is 7.8%, and the cleaning is performed 4.4 times per year. Example 5

[0044] (OMD-PRO-RO system, PRO operating pressure lower limit boundary) (1) OMD unit operations: Same as Example 2: Concentrate from reclaimed water plant (2000 mg / L), 55°C, Kynar® PVDF membrane, sodium chloride solution 2000 mg / L, volumetric flow ratio 4:1.

[0045] (2) PRO unit operation: A dilute sodium chloride solution enters the low-salt side chamber of the PRO, but instead uses an HTI-CTA-ES cellulose acetate membrane from HTI Corporation (as permitted in claim 3). The high-salt solution is seawater from the Bohai Bay (30,000 mg / L), pressurized to 13 bar with PX at a volumetric flow rate ratio of 1:1.

[0046] (3) RO unit operation: 50 bar desalination, fresh water is used in thermal power plants, and concentrated brine is used for salt production.

[0047] (4) Performance indicators: The OMD flux was 13.10 μm / s, the PRO power was 3.48 W / m², the energy consumption decreased by 12.0%, the degradation was 8.6%, and the cleaning was required 4.8 times / year. The system's compatibility with different PRO membranes was verified. Example 6

[0048] (OMD-PRO-RO system, lower limit boundary of OMD hydrophobic membrane parameters) (1) OMD unit operations: The low-salinity wastewater is reclaimed water from a municipal wastewater treatment plant (1000 mg / L, hardness 480 mg / L, oil content 0.92 mg / L, surfactant 0.0092 mmol / L), entering the OMD hot side at 55°C. It is equipped with GORE® Membrane (PTFE, pore size 0.10 μm, thickness 50 μm, lower limit of claim 2). A sodium chloride solution of 2000 mg / L is used at a volumetric flow rate ratio of 4:1. The volume reduction rate is 75%.

[0049] (2) PRO unit operation: A dilute sodium chloride solution (≈980 mg / L) was introduced into the low-salt side chamber of the HTI-CTA-ES membrane. The high-salt solution was seawater from Bohai Bay (30000 mg / L), at 13 bar, with a volumetric flow rate ratio of 1:1.

[0050] (3) RO unit operation: 50 bar desalination, fresh water is used in thermal power plants, and concentrated brine is used for salt production.

[0051] (4) Performance indicators: OMD flux: 13.23 μm / s; PRO power: 3.48 W / m²; energy consumption decreased by 12.4% (due to dilute RO feed water); degradation: 8.5%; cleaning frequency: 4.8 times / year. Comparative Example 1

[0052] (FO-PRO system, treating real municipal wastewater) (1) FO unit operation: The effluent from the secondary biological treatment plant (NaCl 640 mg / L), identical to that in Example 1, was used as the FO feed solution. The draw solution was 0.2 M NaCl (≈11700 mg / L). A typical TFC hollow fiber forward osmosis membrane (AL-FS mode, 22°C) was used. Although the FO membrane retains large molecules, small molecule organic acids, Cl⁻, and surfactants continuously enter the intermediate solution through solute reverse osmosis (RSF).

[0053] (2) PRO unit operation: The FO outlet dilution extract (≈0.18 M) is used as the PRO freshwater feed, and 1.2 M NaCl (≈70000 mg / L) is used as the concentrate feed. The PRO unit is equipped with a CSM RE2540-PRO membrane (AL-DS mode).

[0054] (3) RO unit operation: No RO unit was set up, no freshwater was produced, and there was no resource utilization pathway for concentrated brine.

[0055] (4) Performance indicators: The average water flux of FO is only 2.78 μm / s; due to the high concentration of intermediate liquid and severe ICP, the average power generation density of PRO is only 3.20 W / m²; the flux decay rate of PRO membrane reaches 28.7% within 30 days, and the cleaning frequency is 12.2 times / year; the system cannot operate continuously for 180 days and is not feasible for engineering. Comparative Example 2

[0056] (OMD-PRO-RO system, hot side temperature exceeds limit) (1) OMD unit operations: The low-salinity wastewater is MBR + ozone concentrate (2000 mg / L) from the reclaimed water plant, which is incorrectly heated to 85°C (exceeding the 55–70°C range of claim 4) and enters the hot side of the OMD, where a GORE® PTFE membrane (0.10 μm, 50 μm) is installed.

[0057] (2) PRO unit operation: A dilute sodium chloride solution was introduced into the CSM PRO membrane at a concentration of 30,000 mg / L seawater at 13 bar.

[0058] (3) RO unit operation: 50 bar desalination.

[0059] (4) Performance indicators: The initial flux of the OMD membrane was 14.23 μm / s. After 72 hours, the membrane became wetted, and the flux dropped sharply to 0. The PRO membrane degraded by more than 40% within a week. Cleaning was ineffective, and the system crashed. Comparative Example 3

[0060] (OMD-PRO-RO system, hydrophobic membrane pore size exceeds limit) (1) OMD unit operations: The low-salinity wastewater is industrial park tailwater (2000 mg / L), which enters the OMD at 60°C, but uses a PTFE membrane with a pore size of 1.20 μm (exceeding the 0.10–0.80 μm range of claim 2).

[0061] (2) PRO unit operation: A dilute sodium chloride solution was introduced into the CSM PRO membrane at 13 bar of seawater.

[0062] (3) RO unit operation: 50 bar desalination.

[0063] (4) Performance indicators: The initial flux of OMD was 13.65 μm / s, with local wetting after 48 hours and complete failure within 3 days; the flux of PRO decreased by more than 35%; the system crashed and cleaning was ineffective.

[0064] The key performance parameters of the embodiments and comparative examples are summarized in Table 1.

[0065]

[0066] Note: "—" indicates that no RO unit is set or the data is meaningless; " / " indicates that the system has crashed and cannot obtain valid data.

[0067] Comparative Example 1 compares the OMD-PRO-RO system of this invention with the conventional FO-PRO system under the same conditions of treating municipal wastewater with high pollution risk (NaCl 640 mg / L), highlighting the decisive role of the OMD unit in pollutant isolation. The results show that the FO-PRO system suffers from severe internal concentration polarization (ICP) due to a high intermediate draw solution concentration (≈0.18 M), and small-molecule organic acids, Cl⁻, and trace surfactants continuously invade the porous support layer of the PRO membrane through solute reverse osmosis (RSF), resulting in a PRO membrane flux decline of up to 28.7% within 30 days and a cleaning frequency exceeding 12 times per year (approximately once every 30 days), making long-term stable operation difficult. In contrast, the OMD-PRO-RO system in Example 1, through a physical phase change mechanism (only water vapor permeates), completely blocks pollutant migration, keeping the PRO membrane flux decline rate below 8.3%, reducing the cleaning frequency to 4.9 times / year, and simultaneously increasing the power generation density to 3.80 W / m² (18.8% higher than FO-PRO). This comparative example fully verifies the necessity and technical superiority of replacing FO with OMD as the pre-unit in this invention, fundamentally solving the dual problems of driving force competition and contaminant penetration in the FO-PRO system.

[0068] Comparative Example 2 focuses on the system failure caused by the OMD hot-side operating temperature exceeding the range defined in claim 4 (85°C vs. 55~70°C), revealing the critical impact of temperature parameters on the functional integrity of the hydrophobic membrane. Despite a high initial water flux (14.23 μm / s), the excessively high hot-side temperature disrupted the gas-liquid interface stability of the PTFE hydrophobic membrane, leading to irreversible membrane wetting within 72 hours. Oil and humic acid contaminants penetrated into the sodium chloride circulating solution, subsequently contaminating the downstream PRO membrane module. Within a week, the PRO membrane flux decreased by more than 40%, and conventional chemical cleaning could not restore performance, necessitating membrane module replacement and complete system failure. These results clearly demonstrate that strictly controlling the hot-side temperature within 55~70°C is not only necessary to maintain a sufficient vapor pressure differential to drive efficient mass transfer, but also a critical boundary condition to prevent thermal aging and wetting failure of the hydrophobic membrane, reflecting the scientific and engineering necessity of the present invention in setting process parameters.

[0069] Comparative Example 3 analyzes the case where the pore size of the OMD hydrophobic membrane exceeds the range defined in claim 2 (1.20 μm vs. 0.10~0.80 μm), clarifying the decisive role of membrane structure parameters in the long-term reliable operation of the system. Although the water flux was acceptable in the initial stage (13.65 μm / s), the excessively large pore size significantly weakened the capillary resistance (Laplace pressure), making it difficult to maintain a stable non-wetting state. Local wetting appeared after 48 hours, and spread to the entire membrane surface within 72 hours, causing contaminant penetration and intermediate liquid contamination. Within 3 days, the OMD function was completely lost, the PRO membrane flux dropped sharply by more than 35%, the system could not continue to operate, and cleaning was ineffective. This result proves that limiting the pore size of the hydrophobic membrane to the range of 0.10~0.80 μm is the technical balance point that balances high mass transfer flux and anti-wetting stability. It is an indispensable structural basis for achieving long-term, efficient, and stable extraction with salinity gradient energy and cannot be arbitrarily replaced or relaxed.

[0070] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that cannot be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. In the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A salinity gradient energy extraction system, characterized in that, include: Heat exchange unit (1), permeate membrane distillation unit (2), pressure delayed permeate membrane unit (3), reverse osmosis unit (4), PX pressure exchange device (5) and booster pump (6); The permeate membrane distillation unit (2) includes a hot-side chamber (2-1) and a cold-side chamber (2-2). The hot-side chamber (2-1) has an inlet at the top and a concentrated wastewater outlet at the bottom. The cold-side chamber (2-2) has an inlet at the bottom and an outlet at the top. The pressure-delayed permeate membrane unit (3) includes a low-salt side chamber (3-1) and a high-salt side chamber (3-2). The low-salt side chamber (3-1) has an inlet at the top and an outlet at the bottom. The high-salt side chamber (3-2) has an inlet at the bottom and an outlet at the top. The heat exchange unit (1) is connected to the inlet of the hot-side chamber (2-1) via a heated low-salt wastewater pipeline. The outlet of the cold side chamber (2-2) is connected to the inlet of the low-salt side chamber (3-1) via a sodium chloride dilute solution pipeline. The outlet of the low-salt side chamber (3-1) is connected to the inlet of the cold side chamber (2-2) via a sodium chloride solution pipeline. The outlet of the high-salt side chamber (3-2) is connected in sequence via pipeline to the booster pump (6), the reverse osmosis unit (4), the PX pressure exchange device (5), and the inlet of the high-salt side chamber (3-2). A porous hydrophobic membrane (2-3) is provided between the hot side chamber (2-1) and the cold side chamber (2-2). A pressure-delayed osmosis membrane (3-3) is provided between the low-salt side chamber (3-1) and the high-salt side chamber (3-2).

2. The salinity gradient energy extraction system according to claim 1, characterized in that, The porous hydrophobic membrane (2-3) is any one of polytetrafluoroethylene, polyvinylidene fluoride or polypropylene membrane; the average pore size of the hydrophobic membrane is 0.10~0.80 μm and the thickness is 50~100 μm.

3. The salinity gradient energy extraction system according to claim 1, characterized in that, The pressure-delayed permeation membrane (3-3) is any one of cellulose acetate membrane, thin-walled composite membrane, or thin-walled composite nanomembrane.

4. A method for extracting salt water using salinity gradient energy, characterized in that, Using the salinity gradient energy extraction system according to any one of claims 1 to 3, the process includes the following steps: (1) Heat the low-salt wastewater to 55~70℃ and send it into the hot side chamber (2-1) of the permeate membrane distillation unit. Simultaneously send the sodium chloride solution into the cold side chamber (2-2) of the permeate membrane distillation unit to obtain a dilute sodium chloride solution. (2) A dilute sodium chloride solution is sent into the low-salt side chamber (3-1) of the pressure delayed osmosis membrane unit, and a high-pressure high-salt solution is sent into the high-salt side chamber (3-2) of the pressure delayed osmosis membrane unit simultaneously to obtain a low-salt solution and a sodium chloride solution. The sodium chloride solution is returned to the cold side chamber (2-2) of the osmosis membrane distillation unit for recycling. (3) Pressurize the low-salt solution to 50~80 bar and send it into the reverse osmosis unit (4) for deep desalination to obtain concentrated brine and fresh water; (4) The high-pressure high-salt solution in step (2) is obtained by sending concentrated brine and high-salt solution into the PX pressure exchange device (5) for pressure exchange, thereby increasing the pressure of the high-salt solution to 13~20 bar, which is the high-pressure high-salt solution.

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