A modified heat carrier, a membrane assembly and a distillation method for thermally conductive vacuum membrane distillation

Abstract

The application provides a modified heat carrier, a film assembly and a distillation method for heat-conducting vacuum film distillation. The surface of an aluminum heat carrier is modified by a graphene heat dissipation coating and / or a super-hydrophobic silicon dioxide coating to prepare a novel composite heat carrier which can effectively improve the heat and mass transfer process of the system and improve the performance of the heat-conducting vacuum film distillation.

Description

Technical Field

[0001] This invention belongs to the field of membrane distillation, and in particular relates to a modified heat carrier, membrane module, and distillation method for thermally conductive vacuum membrane distillation. Background Technology

[0002] Thermally conductive vacuum membrane distillation (CH-VMD) can effectively reduce the negative impact of temperature polarization, resulting in higher permeate flux than traditional membrane distillation. However, the system's thermal efficiency and permeate flux still need improvement, which is a key factor restricting the development of this technology. In existing technologies, a simple nanoparticle-water system has been proposed, where a layer of hydrophobic silica nanoparticles is deposited on the water surface, effectively improving the water vapor generation efficiency. However, this method is only suitable for static liquids; in flowing liquids, the coverage of nanoparticles cannot be guaranteed. Another approach involves preparing multifunctional three-dimensional porous graphene for steam generation through nitrogen doping, effectively achieving energy conversion from sunlight to high-energy steam through thermal localization. However, both methods have certain drawbacks. How to increase the system's permeate flux without increasing energy consumption, thereby reducing the specific energy consumption of produced water, while simultaneously optimizing operating conditions and improving the heat utilization rate of membrane distillation, has become an urgent problem to be solved. Summary of the Invention

[0003] In view of this, the present invention aims to propose a modified heat transfer medium, membrane module and distillation method for thermally conductive vacuum membrane distillation, to prepare a novel composite heat transfer medium that can effectively improve the heat and mass transfer process of the system, increase the permeate flux of the system without increasing energy consumption, thereby reducing the specific energy consumption of the produced water, and optimize operating conditions, providing a new strategy for solving the problems of low permeate flux and heat utilization in membrane distillation technology.

[0004] To achieve the above objectives, the technical solution of the present invention is implemented as follows:

[0005] A modified heat transfer medium for thermally conductive vacuum membrane distillation, wherein the surface of the aluminum heat transfer medium is modified by a graphene heat dissipation coating and / or a superhydrophobic silica coating.

[0006] Preferably, the resin-based waterborne graphene heat dissipation coating used in the graphene heat dissipation coating is prepared through the following steps:

[0007] 1) Add graphene to distilled water and sonicate for 5-30 minutes to disperse it evenly, thereby preparing a graphene suspension with a graphene mass concentration of 0.5-1.5%; preferably, the graphene mass concentration is 1%.

[0008] 2) At a rotation speed of 600-1000 r / min, the graphene suspension is slowly added to the uniformly dispersed epoxy resin emulsion and allowed to disperse uniformly for 30-60 min, wherein the mass ratio of graphene to epoxy resin is (0.8-1.2):(1.8-2.2); preferably, the mass ratio of graphene to epoxy resin is 1:2.

[0009] 3) Add silicon carbide powder and stir electromagnetically for 2 hours to obtain resin-type waterborne graphene heat dissipation coating, wherein the mass ratio of graphene suspension to silicon carbide is (7.5~8.5):1; preferably, the mass ratio of graphene suspension to silicon carbide is 8:1.

[0010] Preferably, the graphene heat dissipation coating is obtained through the following steps: the prepared resin-based water-based graphene heat dissipation coating is loaded into a spray gun, an aluminum heat carrier is fixed on a glass plate, and the coating is sprayed every 40cm. 2 The amount of coating applied is 6 mL to 15 mL. After drying, an aluminum carrier modified with a graphene heat dissipation coating, namely GPE-Al heat carrier, is obtained.

[0011] Preferably, the hydrophobic nano-silica gel solution used in the superhydrophobic silica coating is prepared through the following steps:

[0012] 1) Tetraethyl orthosilicate (TEOS) is hydrolyzed and condensed in ethanol (EtOH) to form silica sol;

[0013] 2) Add ammonia and stir for 1 hour. Let the mixture age at room temperature for 3-7 days to form a silica sol suitable for coating.

[0014] 3) Add 8-12 mL of hexamethyldisilazane (HMDS) to the sol and stir for 1 hour. After standing for 1 day, a usable hydrophobic nano silica gel solution is obtained.

[0015] The molar ratio of TEOS:EtOH:NH3:H2O is (0.9~1.1):(34.2~41.8):(0.9~1.2):(2.7~3.3); preferably, it is 1:38:1:3.

[0016] Preferably, the superhydrophobic silica coating is obtained through the following steps: a prepared hydrophobic nano-silica gel solution is sprayed onto an aluminum heat transfer medium using a spray gun, causing the nanoparticles to adhere to the aluminum heat transfer medium at intervals of 40 cm. 2 The spraying amount is 6mL to 15mL; after drying for 1 to 10 minutes, a superhydrophobic surface can be obtained, and the aluminum heat transfer medium modified with superhydrophobic silica coating is used.

[0017] Preferably, the aluminum heat transfer medium is etched before being coated with a hydrophobic nano-silica gel solution. The etching process includes the following steps.

[0018] 1) Use 600-grit sandpaper to sand the aluminum heat carrier in a cross shape, then rinse the aluminum heat carrier with distilled water.

[0019] 2) Soak it in hydrochloric acid with a volume concentration of 1:(2.5~3.5) for 10~14h to make its surface uniform, then remove it with pliers and rinse off the residual acid on its surface with water; preferably, the volume concentration of hydrochloric acid is 1:3.

[0020] 3) Place it in a constant temperature water bath at 90-100℃ for 20-40 minutes, then remove it from the water bath with pliers and dry it in nitrogen at 70-90℃ for 1.5-2.5 hours to obtain the etched aluminum heat transfer medium.

[0021] Etching is used to improve the adhesion of nanoparticles to the heat carrier. When silicon dioxide is used for modification, the heat carrier needs to be etched.

[0022] Preferably, the aluminum heat carrier is perforated with a hole diameter of 3mm, a horizontal spacing of 10cm, a vertical spacing of 2cm, and the holes are centered.

[0023] The present invention also provides a membrane assembly comprising the modified heat transfer fluid as described above.

[0024] This invention also provides a method for thermally conductive vacuum membrane distillation, using a membrane module assembled with the modified heat carrier as described above, wherein the method is carried out under the following conditions: a feed liquid layer thickness of 1–4 mm, a feed flow rate of 22–26 cm / s, a heat source temperature of 180–220 °C, and a vacuum degree of 85–95 kPa; preferably, the method is carried out under the following conditions: a feed liquid layer thickness of 1 mm, a feed flow rate of 24 cm / s, a heat source temperature of 200 °C, and a vacuum degree of 90 kPa.

[0025] Compared with the prior art, the present invention has the following advantages:

[0026] When GPE-Al heat transfer fluid was applied to the CH-VMD system, the permeation flux increased from 9.28 L / (m³) 2 The concentration of ·h) increased to 10.55 L / (m 2 (·h), the increase was 13.7%.

[0027] When the SiO2-Al heat transfer fluid was applied to the CH-VMD system, the system's permeation flux increased from 9.28 L / (m²). 2 The concentration of ·h) was increased to 10.81 L / (m 2 (·h), the increase was 16.5%.

[0028] A GPE / SiO2-Al heat transfer medium was prepared by combining a graphene heat dissipation coating with hydrophobic silica nanoparticle modification technology. It was compared with Al heat transfer medium (liquid layer thicknesses of 4 mm and 1 mm), GPE-Al heat transfer medium, and SiO2-Al heat transfer medium in a CH-VMD system to examine the differences in permeation flux. The temperature distribution and changes within the system were recorded using a temperature recorder when different heat transfer medium types were used. The results show that the GPE / SiO2-Al heat transfer medium can increase the permeation flux by 71.6%, and the overall temperature increase within the system is approximately 0.5–1.7 °C, demonstrating significant advantages. A comparison of the thermal efficiency and specific energy consumption of the system using different types of heat transfer medium in this patent shows that the GPE / SiO2-Al heat transfer medium achieves the highest thermal efficiency and the lowest specific energy consumption.

[0029] The CH-VMD system can achieve a salt rejection rate of over 99.99% when using the above-mentioned heat transfer fluid types, resulting in excellent product water quality. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of a membrane module;

[0031] Figure 2 A schematic diagram of the experimental setup for a laboratory-scale CH-VMD system;

[0032] Figure 3 A physical image of the CH-VMD system;

[0033] Figure 4 Preparation of GPE-Al heat transfer medium;

[0034] Figure 5 (a) Al heat transfer fluid; (b) GPE-Al heat transfer fluid; Water contact angle.

[0035] Figure 6 (a) SEM image of the Al heat transfer medium surface; (c) total energy spectrum; (e) energy spectrum layering diagram; (b) SEM image of the GPE-Al heat transfer medium surface; (d) total energy spectrum; (f) energy spectrum layering diagram;

[0036] Figure 7 Infrared emissivity of Al and GPE-Al heat transfer fluid;

[0037] Figure 8 The changes in (a) flux and (b) water temperature of GPE-Al heat transfer fluid and Al heat transfer fluid in CH-VMD system;

[0038] Figure 9 A schematic diagram and a physical image of a hydrophobic silica nanoparticle heat carrier being sprayed.

[0039] Figure 10 (a) SEM image of the SiO2-Al heat transfer medium surface; (b) total spectrum distribution map of the energy dispersive spectroscopy (EDS) surface scan; (c) layered image of the EDS surface scan; (d) contact angle test image.

[0040] Figure 11 This is a diagram showing the effect of the SiO2-Al heat transfer fluid on the system's permeation flux.

[0041] Figure 12 Schematic diagram of enhanced heat and mass transfer using GPE / SiO2-Al heat transfer medium;

[0042] Figure 13 This is a diagram showing the impact of different types of heat transfer fluids on the system's permeation flux.

[0043] Figure 14 The following graphs show the temperature changes at different points inside the CH-VMD system with heating time under different heat carrier conditions: (a) Overall temperature change; (b) Water temperature change; (c) Membrane side temperature change; (d) Feed side temperature change.

[0044] Figure 15 (a) Thermal efficiency and specific energy consumption of the CH-VMD system when using different heat transfer fluids; (b) Flux and salt rejection rate. Detailed Implementation

[0045] Unless otherwise defined, the technical terms used in the following embodiments have the same meanings as commonly understood by those skilled in the art. Unless otherwise specified, the experimental reagents used in the following embodiments are conventional biochemical reagents; and the experimental methods described are conventional methods.

[0046] The present invention will be described in detail below with reference to embodiments.

[0047] Table 1. Experimental materials and reagents used in this invention

[0048]

[0049]

[0050] Table 2 Experimental instruments used in this invention

[0051]

[0052] The present invention uses a hydrophobic polytetrafluoroethylene (PTFE) membrane with a thickness of 50 μm, a pore size of 0.1 μm, a porosity of 81%, a contact angle of 143°, and a liquid osmotic pressure of 132 kPa.

[0053] The membrane module has internal dimensions of 75mm × 30mm × 4mm and can accommodate a hydrophobic membrane with an effective area of ​​22.5cm². Its structure is as follows: Figure 1 As shown, the membrane module consists of a feed side and a permeate side, with two sealing rings in between to prevent feed leakage. The heat carrier transfers heat from an external heat source to the membrane-water interface for water evaporation, and the resulting water vapor can be collected by passing through the hydrophobic membrane.

[0054] This invention employs a laboratory-scale CH-VMD system, such as Figure 2 As shown, the experimental setup consists of a cold water circulation system, a heating system, and a vacuum condensation and collection system. An external heat source transfers heat to the feed liquid via an aluminum heat transfer medium.

[0055] The effective area is 40cm² 2 A PTFE membrane (4cm×10cm) is placed on a feed channel with a depth of 1mm, which is used for the circulation of the feed liquid.

[0056] The heat transfer medium is made of aluminum with high thermal conductivity. It is perforated to avoid blocking the water vapor transmembrane process. The perforation diameter is 3mm, the horizontal spacing of each perforation is 10cm, the vertical spacing is 2cm, and the perforation is centered.

[0057] A peristaltic pump is used to circulate the feed solution, and water vapor is condensed on the permeate side. A vacuum pump is used to create a vacuum (0–100 kPa) on the permeate side of the membrane.

[0058] A 35 g / L NaCl solution was used as the feed liquid, and the external heat source was set to a temperature of 200 °C, a vacuum of 95 kPa, and a feed flow rate of 24 cm / s.

[0059] All temperature data in the experiment were measured using a multi-channel temperature recorder. The temperature recorder was used to measure the internal temperature change of the system from the start of heating of the heat carrier to 2 hours of operation. Three measurement points were set up: measurement point 1 was the temperature of the feed liquid, with a thermocouple placed in the feed channel; measurement point 2 was the temperature at the heat carrier / water interface, with a thermocouple attached to the lower surface of the heat carrier; and measurement point 3 was the temperature at the heat carrier / membrane interface, with a thermocouple placed on the upper surface of the heat carrier. The initial feed liquid temperature was 26℃. The circulating feed liquid was first heated using the heat carrier while temperature measurement was performed. After 1 hour, the vacuum pump was turned on, and the CH-VMD system officially started operation, with continued temperature measurement. The conductivity of the feed liquid and permeate was measured using a conductivity meter. Since the thickness of the feed liquid layer inside the membrane module affects the system flux, a control experiment was conducted with a feed liquid layer thickness of 1 mm and 4 mm.

[0060] The experiment was repeated three times, and the results were reported as the average.

[0061] Actual photos of the experimental setup are shown below. Figure 3 As shown.

[0062] Example 1

[0063] The Al heat transfer medium was modified by coating both sides with graphene (GPE) heat dissipation coatings, and the new heat transfer medium was named GPE-Al heat transfer medium. The graphene heat dissipation coating uses resin-based waterborne graphene, such as... Figure 4 As shown, the specific implementation steps are as follows:

[0064] 1) Add 10g of graphene to appropriate distilled water and then disperse it evenly with ultrasound for 5 minutes to obtain a graphene suspension with a graphene mass concentration of 1%.

[0065] 2) At a rotation speed of 800 r / min, the graphene suspension was slowly added to the previously uniformly dispersed epoxy resin emulsion and allowed to disperse uniformly for 1 hour, wherein the mass ratio of graphene to epoxy resin was 1:2.

[0066] 3) Add silicon carbide powder and stir electromagnetically for 2 hours to obtain resin-type waterborne graphene heat dissipation coating, wherein the mass ratio of graphene suspension to silicon carbide is 8:1.

[0067] 4) Load the prepared resin-based waterborne graphene heat dissipation coating into the spray gun, fix the heat carrier with a size of 4cm×10cm on the glass plate and spray it. The spraying amount is 6mL. After drying, the GPE-Al heat carrier is obtained.

[0068] The obtained GPE-Al heat transfer fluid was analyzed:

[0069] ① The size of the contact angle can directly reflect the hydrophilicity or hydrophobicity of an object's surface. Analysis of the contact angles of Al heat transfer fluids and GPE-Al heat transfer fluids shows that, for example... Figure 5 As shown, both have hydrophilic surfaces, but the GPE-Al heat transfer medium has a lower contact angle and stronger hydrophilicity.

[0070] ② Surface scanning electron microscopy and energy dispersive spectroscopy analysis were performed on the heat transfer fluid before and after modification, such as... Figure 6 As shown, the graphene coating exhibits a loose, layered stacked distribution with uniform sheet size. This well-distributed layered structure is beneficial for realizing its unique properties. A comparison of the energy dissipation spectra of the two heat carriers also reveals that carbon elements constitute the majority of the surface of the GPE-Al heat carrier, indicating that the graphene heat dissipation coating is uniformly distributed on the surface of the heat carrier and achieves near-complete coverage, consistent with the modification results.

[0071] ③ Since this invention primarily utilizes the radiative heat dissipation of graphene, the infrared emissivity of the GPE-Al heat transfer medium was characterized. From Figure 7As can be seen, the infrared emissivity of the GPE-Al heat transfer medium is above 0.7, while the infrared emissivity of the ordinary Al heat transfer medium is always below 0.1. This indicates that after modification by coating with graphene heat dissipation coating, the infrared radiation performance of the heat transfer medium is significantly improved, which can greatly enhance the radiation heat dissipation of the heat transfer medium.

[0072] Experiment 1

[0073] The Influence of GPE-Al Heat Transfer Fluid on CH-VMD System

[0074] Experimental procedure:

[0075] CH-VMD experiments were conducted using a graphene-modified heat transfer medium.

[0076] The permeation flux of Al heat transfer fluid and GPE-Al heat transfer fluid was compared within 30 min under the following conditions: feed liquid layer thickness of 1 mm, feed flow rate of 24 cm / s, heat source temperature of 200 ℃, and vacuum degree of 90 kPa.

[0077] Experimental results:

[0078] like Figure 8 As shown, the GPE-Al heat transfer fluid can effectively improve the system's permeation flux. Compared to the Al heat transfer fluid, the system flux is increased from 9.6 L / (m²). 2 The concentration of ·h) increased to 10.8 L / (m 2 •h), the increase was 11.5% ( Figure 8 a).

[0079] The temperature of the feed liquid circulation was measured. The feed liquid gains heat during circulation, thus its temperature rises slowly. Figure 8 As can be seen from b, before the vacuum was turned on, the GPE-Al heat transfer medium showed a significantly better heating effect on the feed liquid than the Al heat transfer medium, with a temperature difference of approximately 2°C after 60 minutes of heating. This demonstrates that the graphene coating can effectively improve the heat exchange between the heat transfer medium and the feed liquid, allowing more heat to be transferred to the feed liquid and concentrated at the evaporation surface. Because a phase change occurs at the evaporation surface, heat transferred from the external heat source is continuously consumed, resulting in a temperature difference between the heat transfer medium and the liquid layer, leading to heat localization. This result indicates that using the GPE-Al heat transfer medium can enhance heat transfer, thereby increasing the system throughput.

[0080] from Figure 8As shown in Figure b, the temperature inside the system is significantly affected before and after vacuuming. After the vacuum pump is turned on, the system vacuum level is 90 kPa, and the water temperature in both the GPE-Al heat transfer fluid system and the Al heat transfer fluid system drops by about 5°C. This is because water vapor is generated under vacuum after the system starts operating, and water evaporation absorbs a large amount of latent heat. At the same time, the originally warmer air in the system is also extracted, resulting in a rapid drop in the water temperature. After the water temperature in the system drops, it gradually stabilizes. The figure also shows that after the temperature drops, the water temperature in the system using GPE-Al heat transfer fluid is still higher than that in the system using Al heat transfer fluid. After the system stabilizes, the temperature difference between the two is about 1.5°C, further demonstrating that GPE-Al heat transfer fluid has an enhanced heat transfer effect.

[0081] Example 2

[0082] A hydrophobic nano-silica gel solution was prepared using the sol-gel method to obtain a superhydrophobic silica coating adhered to a heat transfer medium. For example... Figure 9 As shown, the specific implementation steps are as follows:

[0083] 1) A superhydrophobic silica coating was prepared by treating a silica sol obtained by hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in ethanol (EtOH) with hexamethyldisilazane (HMDS). The molar ratio was adjustable within a certain range, TEOS:EtOH:NH3:H2O = 1:38:1:3 (all water was obtained from a 28% ammonia solution, with a volume ratio of TEOS:EtOH:NH3·H2O = 5:50:1.8).

[0084] 2) Add ammonia and stir for 1 hour. Let the mixture age at room temperature for 5 days to form a silica sol suitable for coating.

[0085] 3) Add 10 mL of HMDS to the sol and stir for 1 hour. It can be used after standing for 1 day.

[0086] 4) Use a spray gun to spray the prepared modified sol onto one side of the heat carrier, so that the nanoparticles adhere to the heat carrier. After drying for 1 minute, a superhydrophobic surface can be obtained. The amount of spraying on one side is 6 mL, and the size of the heat carrier is 4 cm × 10 cm.

[0087] To improve the adhesion of nanoparticles to the aluminothermic heat transfer medium, the aluminum heat transfer medium needs to be etched before spraying the hydrophobic nano-silica gel solution, including the following steps:

[0088] 1) Use 600-grit sandpaper to sand the heat carrier in a cross shape, then rinse with distilled water.

[0089] 2) Soak it in hydrochloric acid with a volume concentration of 1:3 for 1 hour to make its surface uniform, then remove it with pliers and rinse it with distilled water to remove any remaining acid.

[0090] 3) Place it in a 100℃ constant temperature water bath, take it out after 1 hour, and dry it at 80℃ for 2 hours to obtain the etched heat carrier.

[0091] The obtained SiO2-Al heat transfer fluid was analyzed:

[0092] Analysis of the SiO2-Al heat transfer medium using surface scanning electron microscopy and energy dispersive spectroscopy clearly revealed that silica nanoparticles were uniformly attached to the surface of the heat transfer medium. Figure 10 a) Furthermore, the silica nanoparticles on the surface cross-condense and interconnect to form a three-dimensional rough network structure with randomly distributed micro- and nano-pores. The prepared SiO2-Al heat transfer medium has a good hierarchical rough structure and a hydrophobic angle of 152°. Figure 10 d). Energy dispersive spectroscopy (EDS) analysis of the SiO2-Al heat transfer fluid surface. Figure 10 b, Figure 10 c) The surface has the largest proportion of Si and O elements and the smallest proportion of Al elements, indicating that the coating has a high coverage on the heat carrier surface.

[0093] Experiment 2

[0094] Effect of SiO2-Al heat transfer fluid on CH-VMD flux

[0095] Experimental procedure:

[0096] CH-VMD experiments were conducted using a heat transfer medium coated with SiO2 on one side.

[0097] The feed liquid thickness is 1 mm, the feed flow rate is 24 cm / s, the heat source temperature is 200℃, and the vacuum degree is 90 kPa.

[0098] The experiment investigated the effect of placing the modified side of the SiO2-Al heat transfer medium towards the feed liquid side and the membrane side on the system flux, and compared it with the unmodified Al heat transfer medium.

[0099] Experimental results:

[0100] from Figure 11 As can be seen, the experimental results obtained by placing the SiO2-Al heat transfer medium towards the feed liquid side and the membrane side are completely different. When SiO2 is placed towards the feed liquid side, the system flux increases significantly, from 9.28 L / (m²). 2 The concentration of ·h) was increased to 10.81 L / (m 2The flux was increased by approximately 16.5% when the silica nanoparticles on the SiO2-Al heat carrier were oriented towards the feed side, but the system flux was lower when the nanoparticles were oriented towards the membrane side compared to when using the Al heat carrier. Experimental results indicate that oriented the hydrophobic silica nanoparticles on the SiO2-Al heat carrier towards the feed side can effectively improve the system flux.

[0101] Example 3

[0102] Preparation of GPE / SiO2-Al heat transfer medium:

[0103] Following the methods of Example 1 and Example 2, two modification treatments were performed on the GPE / SiO2-Al heat transfer medium, such as... Figure 12 As shown, graphene modification is applied to one side of the heat carrier to enhance its radiative heat dissipation, allowing the water layer to absorb more heat and thus increasing the system flux. Simultaneously, silica nanoparticles are used to modify the other side of the heat carrier (feed / liquid side) to alter the liquid's saturated vapor pressure and enhance the evaporation rate. The prepared graphene heat dissipation coating and the hydrophobic nano-silica gel solution are sprayed onto both sides of the aluminum heat carrier using a spray gun and then dried for later use.

[0104] Experiment 3

[0105] CH-VMD experiments were conducted using the GPE / SiO2-Al heat transfer medium prepared by the method in Example 3. The permeation flux of the CH-VMD system was compared using four heat transfer mediums under five conditions: GPE-Al, SiO2-Al, and GPE / SiO2-Al with a feed layer thickness of 1 mm, and Al heat transfer medium with feed layer thicknesses of 1 mm and 4 mm, respectively.

[0106] Experimental procedure:

[0107] The feed flow rate is 24 cm / s, the heat source temperature is 200℃, and the vacuum degree is 90 kPa.

[0108] Experimental results:

[0109] like Figure 13 As shown in the figure, Al(4mm) represents Al heat carrier with a liquid layer thickness of 4mm, meaning that no experimental conditions optimization was performed.

[0110] Al(1mm) indicates that the Al heat transfer medium has a liquid layer thickness of 1mm.

[0111] GPE-Al indicates that the heat transfer medium has been modified with graphene heat dissipation coatings on both sides of Al.

[0112] SiO2-Al (liquid sample measurement) indicates that the side of the heat carrier in contact with the liquid has undergone hydrophobic silica modification treatment.

[0113] GPE / SiO2-Al indicates that the side of the heat carrier in contact with the liquid is modified with hydrophobic silica, and the side in contact with the film is modified with a graphene heat dissipation coating.

[0114] from Figure 13 It is evident that the Al heat transfer medium exhibits the lowest permeation flux without experimental optimization. After optimizing the liquid layer thickness, the permeation fluxes obtained using heat transfer mediums modified with graphene and silica are quite similar, while the GPE / SiO2-Al heat transfer medium combining the two modification methods shows a significant advantage in flux.

[0115] After a series of optimizations, the system flux was increased by 71.6% compared to the Al (4mm) heat carrier studied in previous experiments, and also increased by 7% and 9.6% respectively compared to the graphene and silica coating modifications alone.

[0116] Experiment 4

[0117] CH-VMD experiments were conducted using the GPE / SiO2-Al heat transfer medium prepared by the method in Example 3. A temperature recorder was used to measure the feed liquid temperature, the surface temperature of the heat transfer medium in contact with water, and the membrane surface temperature when using GPE / SiO2-Al, GPE-Al, and Al heat transfer mediums, respectively. The temperature changes of the feed liquid, the surface of the heat transfer medium in contact with water, and the membrane surface were measured from the start of heating to 2 hours of operation.

[0118] Experimental results:

[0119] like Figure 14 As shown, Figure 14 'a' represents the temperature change at different locations within the system over heating time when using different types of heat transfer fluids. The initial water temperature was 26℃. All data were measured in the first half, where the heat transfer fluid was used to heat the circulating feed liquid, and the second half, where the CH-VMD system operated after the vacuum pump was turned on. The sudden drop in temperature indicates the activation of the vacuum pump. At this point, heat is carried out of the system along with water vapor and condensed, thus showing a trend of temperature decrease over time, eventually approaching equilibrium. The graph shows that the temperature rises fastest in all parts of the system when using GPE-Al heat transfer fluid. The overall temperature is relatively close when using Al heat transfer fluid and GPE / SiO2-Al heat transfer fluid, with only a few locations showing significant temperature differences. Figure 14 b、 Figure 14 c. Figure 14 d represents the water temperature, membrane surface temperature, and feed temperature of different types of heat transfer fluids, respectively. A detailed analysis follows:

[0120] ① Comparison Figure 14In the three sets of data in b, the water temperature rose the fastest and remained relatively high after equilibrium was reached when using the GPE-Al heat transfer medium. In contrast, the water temperatures heated by the Al and GPE / SiO2-Al heat transfer mediums were significantly lower, and their temperature curves almost overlapped, indicating that there was virtually no difference between the two in heating the feed liquid. During the temperature drop phase after the vacuum pump was turned on, the water temperature obtained using the GPE / SiO2-Al heat transfer medium was slightly higher than that heated by the Al heat transfer medium. This is because the GPE / SiO2-Al heat transfer medium has a graphene heat dissipation coating on the side near the membrane, allowing it to transfer more heat to the water.

[0121] ② Figure 14 c represents the temperature change of the membrane surface under the three heat transfer mediums. The figure shows that the GPE-Al heat transfer medium still has the best heat transfer performance, with the membrane surface temperature 1.2℃ higher than when using the Al heat transfer medium. The membrane surface temperature is higher when using the GPE / SiO2-Al heat transfer medium than when using the Al heat transfer medium, and the surface temperature is also higher during the rising phase.

[0122] ③ Figure 14 d represents the temperature change at the interface between the three heat transfer fluids and the feed liquid. The temperature of the GPE / SiO2-Al heat transfer fluid after heating is still the highest under the action of the graphene heat dissipation layer.

[0123] Comparing the temperature changes within the system under the three heat transfer mediums, the system temperature increased by 0.5–1.7°C when using GPE-Al heat transfer medium compared to the other mediums, demonstrating the significant advantage of GPE-Al heat transfer in heat transfer. In contrast, the GPE / SiO2-Al heat transfer medium showed no significant advantage in temperature increase compared to the other two. Its single-sided graphene heat dissipation coating structure only resulted in slightly higher temperatures at the film surface and in the water compared to the Al heat transfer medium, which is merely one factor contributing to its flux improvement. The key to its enhanced CH-VMD performance lies in the combined effect of increased evaporation rate and enhanced heat transfer.

[0124] In summary, the water layer above the heat carrier can enhance radiative heat dissipation through graphene, thereby strengthening the heat transfer process. In the lower part of the heat carrier, nanoparticles enhance the evaporation effect, thereby strengthening the mass transfer process of the system. The internal temperature and permeation flux of the system are significantly improved.

[0125] Experiment 5

[0126] Energy consumption is a key factor restricting the development of MD systems; therefore, it is necessary to analyze the system's thermal efficiency and specific energy consumption. CH-VMD experiments were conducted using the GPE / SiO2-Al heat transfer medium prepared by the method in Example 3. When using the GPE / SiO2-Al heat transfer medium, the CH-VMD system exhibits advantages in flux and membrane surface temperature distribution compared to Al, GPE-Al, and SiO2-Al heat transfer mediums. The thermal efficiency and specific energy consumption of the prepared heat transfer mediums were analyzed and compared.

[0127] Experimental results:

[0128] ①For example Figure 15 As shown in Figure a, a lower specific energy consumption (SEC) means that the system requires less heat to produce a unit mass of pure water, thus reducing the system's operating costs. From Figure 15 As shown in Figure a, when the system uses Al heat transfer fluid (liquid layer thickness 4 mm), the specific energy consumption is as high as 1.22 kWh / L. However, when using GPE / SiO2-Al heat transfer fluid, the specific energy consumption can be reduced to as low as 0.712 kWh / L, a reduction of 41%. It can be seen that after adjusting the liquid layer thickness and modifying the heat transfer fluid, the system's energy consumption is significantly reduced.

[0129] ②For example Figure 15 As shown in Figure a, comparing the thermal efficiency (TE) of the systems, the system using GPE / SiO2-Al heat transfer medium achieved the highest TE, reaching 91%. For the Al heat transfer medium, when the liquid layer thickness decreased from 4 mm to 1 mm, the system TE increased from 53.6% to 73.9%, an increase of 37.9%. This demonstrates that reducing the liquid layer thickness significantly improves the system TE. The GPE / SiO2-Al heat transfer medium yielded the highest flux and had the highest TE, indicating the most efficient utilization of heat.

[0130] ③ Figure 15 b represents the salt rejection rate and flux under different types of heat transfer fluids. When using different types of modified heat transfer fluids, the system flux gradually increases, while the salt rejection rate of the system is above 99.99%, demonstrating the excellent performance of CH-VMD in desalination.

[0131] 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, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A modified heat transfer fluid for thermally conductive vacuum membrane distillation, characterized in that: The aluminum heat carrier has a graphene heat dissipation coating on the side that contacts the membrane, and a superhydrophobic silica coating on the side that contacts the liquid. The resin-based water-based graphene heat dissipation coating used in the graphene heat dissipation coating is prepared through the following steps. 1) Add graphene to distilled water and sonicate for 5-30 minutes to disperse it evenly, thus preparing a graphene suspension with a graphene mass concentration of 0.5-1.5%; 2) At a rotation speed of 600~1000r / min, the graphene suspension was slowly added to the uniformly dispersed epoxy resin emulsion and allowed to disperse evenly for 30~60min, wherein the mass ratio of graphene to epoxy resin was (0.8~1.2):(1.8~2.2). 3) Add silicon carbide powder and stir for 2 hours to obtain resin-type waterborne graphene heat dissipation coating, wherein the mass ratio of graphene suspension to silicon carbide is (7.5~8.5):1; The hydrophobic nano-silica gel solution used in the superhydrophobic silica coating is prepared through the following steps. 1) Tetraethyl orthosilicate (TEOS) is hydrolyzed and condensed in ethanol (EtOH) to form silica sol; 2) Add ammonia and stir for 1 hour. Let the mixture age at room temperature for 3-7 days to form a silica sol suitable for coating. 3) Add 8-12 mL of hexamethyldisilazane (HMDS) to the sol and stir for 1 hour. After standing for 1 day, a usable hydrophobic nano silica gel solution is obtained. The molar ratio of TEOS:EtOH:NH3:H2O is (0.9~1.1):(34.2~41.8):(0.9~1.2):(2.7~3.3).

2. The modified heat transfer fluid for thermally conductive vacuum membrane distillation according to claim 1, characterized in that: In the preparation of the resin-based waterborne graphene heat dissipation coating used in the graphene heat dissipation coating, step 1) the mass concentration of graphene is 1%; step 2) the mass ratio of graphene to epoxy resin is 1:2; and step 3) the mass ratio of graphene suspension to silicon carbide is 8:

1.

3. The modified heat transfer fluid for thermally conductive vacuum membrane distillation according to claim 1, characterized in that: The graphene heat dissipation coating is obtained through the following steps: The prepared resin-based water-based graphene heat dissipation coating is loaded into a spray gun, an aluminum heat carrier is fixed to a glass plate, and the coating is sprayed every 40cm. 2 The amount of coating applied is 6mL~15mL. After drying, the heat transfer medium modified by the graphene heat dissipation coating is obtained, namely GPE-Al heat transfer medium.

4. The modified heat transfer fluid for thermally conductive vacuum membrane distillation according to claim 1, characterized in that: In the preparation of the hydrophobic nano silica gel solution used in the superhydrophobic silica coating, the molar ratio of TEOS:EtOH:NH3:H2O in step 3) is 1:38:1:

3.

5. The modified heat transfer fluid for thermally conductive vacuum membrane distillation according to claim 1, characterized in that: The superhydrophobic silica coating is obtained through the following steps: A prepared hydrophobic nano-silica gel solution is sprayed onto the surface of an aluminum heat carrier using a spray gun, with a spacing of 40 cm². 2 The spraying amount is 6mL~15mL; after drying for 1~10min, a heat transfer medium modified with a superhydrophobic silica coating can be obtained.

6. The modified heat transfer fluid for thermally conductive vacuum membrane distillation according to claim 5, characterized in that: The aluminum heat transfer medium undergoes etching treatment before being coated with a hydrophobic nano-silica gel solution. The etching treatment includes the following steps. 1) Use 600-grit sandpaper to sand the aluminum heat carrier in a cross shape, then rinse with distilled water; 2) Soak it in hydrochloric acid with a volume concentration of 1:(2.5~3.5) for 10~14 hours to make its surface uniform, then remove it with pliers and rinse the surface with distilled water to remove any residual acid. 3) Place it in a constant temperature water bath at 90~100℃. After 20~40 minutes, take it out with pliers and dry it at 70~90℃ for 1.5~2.5 hours to obtain the etched aluminum heat transfer medium.

7. The modified heat transfer fluid for thermally conductive vacuum membrane distillation according to claim 1, characterized in that... The aluminum heat carrier is perforated with a diameter of 3mm, and the horizontal spacing between each hole is 10cm and the vertical spacing is 2cm. The holes are centered.

8. A membrane module, characterized in that... It includes the modified heat transfer fluid as described in any one of claims 1 to 7.

9. A method for thermally conductive vacuum membrane distillation, characterized in that... The membrane module assembled using the modified heat carrier as described in any one of claims 1 to 7 is carried out under the following conditions: the feed liquid layer thickness is 1 to 4 mm, the feed flow rate is 22 to 26 cm / s, the heat source temperature is 180 to 220 °C, and the vacuum degree is 85 to 95 kPa.

10. The method for thermally conductive vacuum membrane distillation according to claim 9, characterized in that... The experiment was conducted under the following conditions: feed liquid layer thickness of 1 mm, feed flow rate of 24 cm / s, heat source temperature of 200 ℃, and vacuum degree of 90 kPa.

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