A hydrogel, membrane assembly and distillation method for membrane distillation

Abstract

The application provides a hydrogel, a membrane assembly and a distillation method for membrane distillation, wherein the hydrogel takes polyurethane as a support material, polyvinyl alcohol is loaded on the polyurethane, or a reaction product of polyvinyl alcohol and sodium alginate, or a reaction product of polyvinyl alcohol, sodium alginate and carboxylated carbon nanotubes. The application utilizes the interaction between water and polymer in the hydrogel to change the hydrogen bond network and the state of water, so as to reduce the water evaporation enthalpy. The permeation flux of the system is improved without increasing the energy consumption, thereby reducing the specific energy consumption of water production, optimizing the operation condition, and providing a new strategy for solving the problem of low heat utilization rate of membrane distillation.

Description

Technical Field

[0001] This invention belongs to the field of membrane distillation technology, and in particular relates to a hydrogel, membrane module and distillation method for membrane distillation. Background Technology

[0002] Thermally conductive vacuum membrane distillation (CH-VMD) can effectively reduce the negative impact of temperature polarization, resulting in a higher permeate flux than traditional membrane distillation. However, the system's thermal efficiency and permeate flux remain key factors limiting its development. In existing technologies, some have proposed using hydrogels to improve the evaporation rate at the solar interface. Highly hydrateable light-absorbing hydrogels (h-LAH), using polyvinyl alcohol and chitosan as the hydrateable framework and polypyrrole as the light absorber, can reduce the energy required for water evaporation (<50%). Furthermore, it has been demonstrated that improving the hydratability of h-LAH ​​can alter the state of water and partially activate it, thereby promoting evaporation and increasing the solar steam generation rate to 3.6 kg / m³. 2 •h. Some researchers have proposed preparing hydrogels resistant to high salinity by introducing polyelectrolyte materials. An anionic polyelectrolyte-based hydrogel (APH) with polyvinyl alcohol as the backbone and poly(3,4-ethylenedioxythiophene):poly(sodium p-benzenesulfonate) as the light absorber was prepared using a freeze-thaw method. This APH serves as an evaporator, combining photothermal properties (evaporating seawater) with electrostatic repulsion (preventing solid salt crystallization). It is evident that hydrogel materials, due to their unique three-dimensional network structure and internal water state, have been applied in research to improve water evaporation. However, most current research focuses only on solar interface evaporation. Summary of the Invention

[0003] In view of this, the present invention aims to propose a hydrogel, membrane module, and distillation method for membrane distillation. It utilizes the water-polymer interactions within the hydrogel to alter the hydrogen bond network and the state of water, thereby reducing the enthalpy of water vaporization. This increases the system's permeate flux without increasing energy consumption, thereby reducing the specific energy consumption of produced water, while simultaneously optimizing operating conditions, providing a new strategy for addressing the problem of low heat utilization in membrane distillation.

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

[0005] A hydrogel for membrane distillation, using polyurethane as a support material, with polyvinyl alcohol loaded on the polyurethane, or a product of the reaction of polyvinyl alcohol and sodium alginate, or a product of the reaction of polyvinyl alcohol, sodium alginate and carboxylated carbon nanotubes.

[0006] Preferably, the thickness of the polyurethane is 2 to 4 mm.

[0007] This invention also provides a method for preparing a hydrogel for membrane distillation as described above, comprising the following steps:

[0008] 1) Wash polyurethane with a length of 8-9cm, a width of 2-3cm, and a thickness of 2-4mm repeatedly with anhydrous ethanol and deionized water, and then dry it at 40-60℃ for 1-2 hours.

[0009] 2) Mix 10-20g of 10.0-15.0wt% polyvinyl alcohol solution and 120-150μL of 40-50wt% glutaraldehyde solution mechanically to form solution a. Glutaraldehyde acts as a crosslinking agent, reacting with the hydroxyl groups in polyvinyl alcohol and sodium alginate to form covalent bonds, thereby enhancing the three-dimensional network structure of the hydrogel and improving its mechanical strength and stability.

[0010] 3) Add 500-550 μL of 1-1.2 mol / L HCl solution to solution a, and mix thoroughly by mechanical stirring.

[0011] 4) Place the dried polyurethane into the mixture from step 3), spread it evenly, and let it stand for 2-4 hours;

[0012] 5) Soak the polyurethane treated in step 4) in water for 24 to 48 hours to obtain hydrogel SH-1.

[0013] Preferably, in step 3), 0.1–0.2 g of sodium alginate is added to solution a to prepare hydrogel SH-2; the remaining steps are the same.

[0014] Preferably, in step 3), 0.1–0.2 g of sodium alginate and 0.1–0.3 g of hydroxylated carbon nanotubes are added to solution a to prepare hydrogel SH-3; the remaining operation steps are the same.

[0015] The present invention also provides a membrane assembly comprising a hydrogel for membrane distillation as described above.

[0016] Preferably, it consists of two parts: a feed side and a permeation side, with two sealing rings in between to prevent liquid leakage. An external heat source transfers heat to the hydrogel and feed liquid via a heat carrier. The hydrogel, 8–9 cm long, 2–3 cm wide, and 2–4 mm thick, is placed in a feed channel with a depth of 3–6 mm and is in close contact with the heat carrier. The effective area is 16–20 cm². 2 The polytetrafluoroethylene hydrophobic membrane is placed on the other side of the heat carrier.

[0017] Preferably, the membrane is a polytetrafluoroethylene (PTFE) membrane with a thickness of 20–80 μm, a pore size of 0.1–0.2 μm, a porosity of 80%–85%, a contact angle of 130°–145°, and a liquid osmotic pressure of 130–135 kPa.

[0018] This invention also provides a method for thermally conductive vacuum membrane distillation, using a membrane assembly comprising a hydrogel as described above for membrane distillation. The thermally conductive vacuum membrane distillation system 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 carrier. A peristaltic pump circulates the feed liquid. A vacuum pump is used to generate a vacuum on the permeate side of the membrane. Water vapor is condensed on the permeate side to form permeate. The distillation is carried out under the following conditions: feed flow rate of 5–10 cm / s, heat source temperature of 100–200 °C, and vacuum degree of 0.08–0.1 MPa.

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] This invention targets the CH-VMD system. First, a polyurethane (PU) / polyvinyl alcohol (PVA) hydrogel with polyurethane as the support material was prepared. Then, sodium alginate (SA), a polyelectrolyte, was introduced to prepare a negatively charged PU / PVA / SA hydrogel. Based on this, carboxylated carbon nanotubes (MWCNT-COOH), a thermally conductive material, were added to prepare a PU / PVA / SA / MWCNT-COOH hydrogel. The three hydrogels were characterized and analyzed, and their effects on the temperature, permeate flux, thermal efficiency, and specific energy consumption within the CH-VMD system were compared. Then, the optimization effect on the system was verified from the perspective of the water-state changes within the hydrogels. This further promotes the development of membrane distillation technology.

[0021] The theoretical enthalpy of vaporization of pure water is 2357.6 kJ / kg, while the measured and calculated enthalpy of vaporization of SH-1 is 1195 kJ / kg, that of SH-2 is 1136 kJ / kg, and that of SH-3 is 1082 kJ / kg. Compared with pure water, the enthalpy of vaporization of all hydrogels is reduced.

[0022] The flux of a conventional CH-VMD system is 11.16 L / m³. 2 The system flux reached 17.94 L / m³ after adding SH-1 hydrogel as the evaporation material. 2 The flux of SH-1 was 20.38 L / m³, a 60.75% improvement compared to the conventional CH-VMD system (SH-0). This is because the PVA in SH-1 is a hydrophilic polymer, and its hydrophilic polymer chains cross-link in a large amount of water to form a unique three-dimensional network structure. The hydrogen bonding between water molecules and polymer chains weakens the hydrogen bonding between water molecules, thereby reducing the energy requirement for evaporation and thus improving the flux. 2Compared to the pure PVA gel of SH-1, the addition of sodium alginate increased the number of hydrophilic groups in the hydrogel, resulting in a higher proportion of intermediate water and thus a further increase in the water evaporation rate by 13.60%. SH-3 achieved a flux of up to 22.35 L / m³. 2 Compared to SH-2, SH-3 improves heat transfer efficiency and reduces heat loss to the circulating feed liquid by introducing the high thermal conductivity of carboxylated carbon nanotubes, thus increasing the evaporation rate by 9.67%.

[0023] The effects of hydrogels SH-1, SH-2, and SH-3 on the desalination flux of CH-VMD were investigated. SH-1 can increase the permeation flux of the system by more than 48% under different salt concentrations. SH-2 and SH-3, due to the presence of sodium alginate polyelectrolyte, enable the hydrogels to generate negatively charged groups, thereby repelling ions in seawater and further increasing the permeation flux to more than 52%, with salt rejection rates exceeding 99%.

[0024] The TE values ​​of the CH-VMD systems with hydrogels were all higher than those without hydrogels. Among them, the TE (thermal efficiency) value of SH-1 was as high as 82.8%; the TE (thermal efficiency) value of SH-2 was 77.1%; and the TE (thermal efficiency) value of SH-3 was 75.0%. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of a membrane module;

[0026] Figure 2 A schematic diagram of the experimental setup for the CH-VMD system;

[0027] Figure 3 For the preparation of hydrogels;

[0028] Figure 4 Cross-sectional electron micrographs of different hydrogels: (a) SH-1; (b) SH-2; (c) SH-3

[0029] Figure 5 Fourier transform infrared spectra of polyurethane and hydrogel SH-1, SH-2, and SH-3;

[0030] Figure 6 (a) DSC test plots for hydrogels SH-1, SH-2, and SH-3; (b) integral plot of enthalpy of evaporation calculation.

[0031] Figure 7 Figures showing the evaporation performance of hydrogels in the CH-VMD system: (a) Temperature changes at the same location on the upper surface of SH-0, SH-1, SH-2, and SH-3; (b) Comparison of system flux.

[0032] Figure 8 The following graphs show the desalination performance of hydrogels in the CH-VMD system: (a) System flux when the feed solution is 0.8 wt% NaCl solution; (b) System flux when the feed solution is 3.5 wt% NaCl solution; (c) System flux when the feed solution is 10 wt% NaCl solution; (d) Comparison of system flux and salt rejection rate for different hydrogels under different feed water concentrations.

[0033] Figure 9 This is a comparison chart of the thermal efficiency and specific energy consumption of the system when using different hydrogels. Detailed Implementation

[0034] 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.

[0035] The present invention will be described in detail below with reference to the embodiments.

[0036] The experimental materials and reagents used in this invention are shown in Table 1.

[0037] Table 1

[0038]

[0039] The experimental instruments used in this invention are shown in Table 2.

[0040] Table 2

[0041]

[0042] This invention uses a commercially available hydrophobic polytetrafluoroethylene (PTFE) membrane with a thickness of 50 μm, a pore diameter of 0.1 μm, a porosity of 81%, a contact angle of 143°, and a liquid osmotic pressure of 132 kPa.

[0043] The membrane module measures 80mm × 20mm × 4mm and can accommodate a hydrophobic membrane with an effective area of ​​16cm². Its structure is as follows: Figure 1 As shown. The membrane module consists of two parts: a feed side and a permeate side, with two sealing rings in between to prevent liquid leakage. An external heat source transfers heat to the hydrogel and feed liquid via a heat carrier. The hydrogel, 8cm long, 2cm wide, and 3mm thick, is placed in a 4mm deep feed channel, tightly against the heat carrier. The effective area is 16cm². 2 The PTFE hydrophobic membrane is placed on the other side of the heat carrier.

[0044] This invention employs a laboratory-scale CH-VMD system, such as Figure 2As 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 carrier. A peristaltic pump circulates the feed liquid. A vacuum pump is used to create a vacuum on the permeate side of the membrane, where water vapor is condensed to form permeate.

[0045] The effective area is 16cm. 2 The PTFE membrane (2cm×8cm) is placed in a feed channel with a depth of 4mm. The feed channel is connected to an inlet and an outlet for the circulation of the feed liquid.

[0046] The heat transfer medium is made of aluminum sheet with high thermal conductivity. The aluminum sheet is perforated to avoid blocking the transmembrane process of water vapor.

[0047] 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.

[0048] The external heat source was set to a temperature of 200℃, a vacuum of 0.1MPa, and a feed flow rate of 6.08cm / s.

[0049] All temperature data in the experiment were measured using a multi-channel temperature recorder. The internal temperature change of the heat transfer fluid was measured from the start of heating to 2 hours of operation, with measurement points set on the upper surface of the hydrogel. The conductivity of the feed solution and product water was measured using a Rayleigh magnetic conductivity meter. The experiment was repeated three times, and the results are reported as averages.

[0050] Example 1

[0051] Loading hydrogels onto polyurethane, such as Figure 3 As shown, the specific implementation steps are as follows:

[0052] ① Wash the polyurethane material with a length of 8cm, a width of 2cm, and a thickness of 3mm repeatedly with anhydrous ethanol and deionized water, and then dry it at 45℃ for 1 hour.

[0053] ② Mix 10g of 10.0wt% PVA solution and 120μL of 50wt% GA solution mechanically until homogeneous; this mixture is called solution a.

[0054] ③ Add 500 μL of 1.2 mol / L HCl solution to solution a, and mix thoroughly by mechanical stirring.

[0055] ④ Apply the above mixture evenly onto the dried polyurethane and let it stand for 2 hours to obtain the prepared polyurethane-based hydrogel sample.

[0056] ⑤ Soak the prepared sample in 500 mL of deionized water for 24 hours to obtain the final hydrogel sample, which is named SH-1.

[0057] ⑥ Using the same method, 0.1g of sodium alginate was added to solution a to prepare hydrogel SH-2.

[0058] ⑦ Using the same method, 0.1g of sodium alginate and 0.2g of hydroxylated carbon nanotubes were added to solution a to prepare hydrogel SH-3.

[0059] The obtained hydrogel was analyzed:

[0060] ① The morphology and structure of SH-1, SH-2, and SH-3 were characterized by scanning electron microscopy (SEM), such as... Figure 4 As shown, the hydrogel has a porous network structure with an average pore size of approximately 10 μm. The porosity of the polyurethane itself and the porous structure within the hydrogel provide the physical conditions for rapid water transport and evaporation, as well as vapor diffusion. Even after the addition of SA, the porous structure of the hydrogel remains, with pores of 1–2 μm on the pore walls, exhibiting the typical structure of a homogeneous hydrogel. SH-3 consists of a porous network structure with pore sizes of 50–200 μm, and pores of 1–2 μm are also distributed on the pore walls, indicating that the addition of MWCNT-COOH has no significant impact on the internal morphology of the hydrogel.

[0061] ②The chemical composition of the hydrogel was analyzed using Fourier transform infrared spectroscopy (FTIR). For example... Figure 5 As shown, SH-1 has three main peaks located at 3298 cm⁻¹. -1 2916.4cm -1 and 1090.3cm -1 At these locations, the vibrations correspond to the OH stretching vibration, CH symmetric stretching vibration, and CO stretching vibration of the hydroxyl group, respectively. Among them, 1090.3 cm⁻¹ -1 This is a characteristic peak of PVA. In the infrared spectrum of SH-2, the OH stretching absorption peak shifts to lower wavenumbers, indicating that the presence of SA weakens the hydrogen bonding interactions between PVA molecules. Its carboxyl and hydroxyl groups form hydrogen bonds with the hydroxyl groups in PVA. This hydrogen bond formation alters the stretching vibrations of the OH bonds in the PVA hydroxyl groups, leading to a red shift of the OH characteristic peak. Polyurethane at 1537.3 cm⁻¹... -1 The characteristic peaks at 1091 cm⁻¹ are in-plane bending vibrations of NH and tensile vibrations of CN, while at 1091 cm⁻¹... -1The characteristic peaks at the specified locations are a result of CC stretching vibrations. The spectrum of SH-3 exhibits characteristic peaks of both SH-2 and polyurethane, demonstrating the feasibility of in-situ synthesis of composite hydrogels within polyurethane. Compared to SH-1, the -OH peak of SH-3 shifts towards higher wavenumbers, exhibiting a blue shift. This blue shift is likely due to the opening of the π bond in MWCNT-COOH, leading to hydrogen bonds between the abundant hydroxyl and carboxyl groups in MWCNT-COOH and the hydroxyl groups of the PVA macromolecule.

[0062] ③ Using a differential scanning calorimeter (DSC), the heat change of the material under a temperature / time program was detected to verify that the enthalpy of vaporization of water in the hydrogel can be reduced. Figure 6 As shown in (a), the heat flux signal first rises to its highest value with increasing temperature, then suddenly drops until it smooths out; this process represents water evaporation. The figure shows that composite hydrogels SH-2 and SH-3 have broader peaks that gradually decay, thus exhibiting different evaporation behaviors compared to SH-1. The integral of the heat flux curve over time represents the enthalpy of water evaporation, as shown... Figure 6 As shown in (b), the area of ​​the peaks in the figure represents the magnitude of the enthalpy change, i.e., the total amount of heat absorbed during the process. The theoretical enthalpy of evaporation for pure water is 2357.6 kJ / kg, while the measured and calculated enthalpies of evaporation for SH-1, SH-2, and SH-3 are 1195 kJ / kg, 1136 kJ / kg, and 1082 kJ / kg, respectively. Compared to pure water, all hydrogels have lower enthalpies of evaporation. Therefore, less energy is required when using hydrogels for evaporation, and the permeation flux of the system can be increased while keeping the total heat input to the system constant.

[0063] The effects of hydrogels SH-1, SH-2, and SH-3 on the evaporation of the CH-VMD system:

[0064] Experimental procedure:

[0065] CH-VMD experiments were conducted using different prepared hydrogels.

[0066] The feed solution was pure water, the feed flow rate was 6.08 cm / s, the heat carrier temperature was 60℃, and the vacuum degree was 100 kPa. The throughput of the CH-VMD system was compared when using hydrogels SH-1, SH-2, SH-3, and without hydrogel (SH-0). The temperature of the hydrogel surface was measured using a temperature recorder from the start of heating until 2 hours of operation.

[0067] Experimental results:

[0068] like Figure 7As shown in (a), without hydrogel, the temperature at the same point in the feed channel rises slowly over time, reaching a peak after 100 minutes, with a maximum temperature of only 35.6°C. Conversely, with the addition of different hydrogels to the system, the surface temperatures are all significantly higher than SH-0, reaching over 40°C. The surface temperature of SH-3 rises particularly rapidly, reaching a maximum of 44°C. This is because SH-3 contains carboxylated carbon nanotubes, which possess excellent thermal conductivity. The presence of carboxyl and hydroxyl groups increases the interaction between carbon nanotubes and PVA, water, etc., generating hydrogen bonds. This increases the heat conduction path and heat transfer efficiency, thereby improving the thermal conductivity of carboxylated carbon nanotubes. Furthermore, the presence of carboxyl groups enhances the hydrophilicity of the carbon nanotube surface, making it easier for them to form closer contact with water or other liquid media, accelerating heat transfer. Simultaneously, the enhanced hydrophilicity reduces thermal resistance and increases the heat transfer rate. Therefore, when SH-3 is used, its upper surface can receive more heat.

[0069] Depend on Figure 7 (b) It can be seen that the flux of the conventional CH-VMD system is 11.16 L / m. 2 With the addition of SH-1, the flux can reach 17.94 L / m³. 2 The flux increased by 60.75%. This is because the PVA in SH-1 is a hydrophilic polymer, and its hydrophilic polymer chains cross-link in a large amount of water to form a unique three-dimensional network structure. The hydrogen bonding between water molecules and polymer chains weakens the hydrogen bonding between water molecules, thereby reducing the energy requirement for evaporation, thus improving the flux. When SH-2 is used, the system flux is 20.38 L / m³. 2 The increase in flux (·h) is due to the addition of sodium alginate, which increases the number of hydrophilic groups in the hydrogel, leading to a higher proportion of intermediate water and thus a 13.6% increase in the water evaporation rate. Using SH-3, the flux reached a high of 22.35 L / m³. 2 Compared to SH-2, SH-3 improves heat transfer efficiency and reduces heat loss of circulating feed liquid by introducing the high thermal conductivity of carboxylated carbon nanotubes, thereby increasing the evaporation rate by 9.67%.

[0070] Furthermore, using the same energy input U in By comparing the enthalpy of vaporization of the hydrogel with the known theoretical value of 2357.6 kJ / kg, the equivalent enthalpy of vaporization of the hydrogel was estimated:

[0071] U in =E equ ·m g =E0·m0

[0072] Among them, U inE0 is the energy used for water evaporation; E0 is the known enthalpy of vaporization of water, 2357.6 kJ / kg; m0 is the mass change of pure water when tested using only pure water; m g E represents the change in mass of pure water after the addition of the hydrogel. equ It is the equivalent enthalpy of vaporization of water in the hydrogel.

[0073] The calculation results are shown in Table 3. Compared with the known enthalpy of vaporization of liquid water, the equivalent enthalpy of vaporization of water in other hydrogels is reduced to varying degrees. This result shows the same trend as the result obtained from DSC testing, which verifies that hydrogels can reduce the enthalpy of vaporization of water, thereby increasing the permeation flux of the system.

[0074] Table 3

[0075]

[0076] Effects of hydrogels SH-1, SH-2, and SH-3 on the desalination flux of CH-VMD

[0077] Experimental procedure:

[0078] CH-VMD experiments were conducted using different prepared hydrogels.

[0079] The feed flow rate is 6.08 cm / s, the heat carrier temperature is 60℃, and the vacuum degree is 100 kPa.

[0080] The experiment investigated the effect of different hydrogels on the system flux when the feed solution was set to a solution with different salinities.

[0081] Experimental results:

[0082] like Figure 8 As shown in Figure ac, the osmotic flux of the system significantly increased after the addition of hydrogels at different concentrations. SH-1 increased the osmotic flux by over 48% at different salt concentrations, while SH-2 and SH-3, due to the presence of sodium alginate polyelectrolyte, generated negatively charged groups in the hydrogels, thereby repelling ions from seawater and further increasing the osmotic flux to over 52%. Figure 8 (d) It can be concluded that the hydrogel is less affected by concentration. Regardless of the type of hydrogel, the osmotic flux of the system fluctuates slightly in salt water of different concentrations, but remains stable within a certain range, and the salt rejection rate is above 99%.

[0083] Energy consumption is a key factor restricting the development of MD systems, so it is necessary to analyze the thermal efficiency and specific energy consumption of the system.

[0084] CH-VMD experiments were conducted using three types of hydrogels prepared by the method in Example 1. The CH-VMD system using hydrogels showed a significant advantage in throughput compared to systems without hydrogels. The thermal efficiency and specific energy consumption of the three prepared hydrogels were analyzed and compared.

[0085] Experimental results:

[0086] like Figure 9 The values ​​represent the TE and SEC of the CH-VMD system using different types of hydrogels. The results show that the TE values ​​of the CH-VMD system with hydrogel are all higher than those without hydrogel, with SH-1 reaching as high as 82.8%, 19.6% higher than SH-0. This is because the presence of hydrogel reduces the heat required for water evaporation, resulting in a higher permeation flux and thus generating more heat for water evaporation at the same heat carrier temperature, i.e., Q. v The enthalpy change of water in SH-2 and SH-3 is higher than that of SH-1, resulting in a higher TE value. Although the permeation flux of SH-2 and SH-3 is greater than that of SH-1, their TE values ​​are actually lower. This is because the enthalpy of water evaporation in SH-1, SH-2, and SH-3 is reduced by 710.9 kJ / kg, 907.7 kJ / kg, and 1036 kJ / kg respectively compared to the enthalpy of pure water evaporation. The reduction in enthalpy change of SH-2 and SH-3 is 27.7% and 45.7% greater than that of SH-1, respectively. Meanwhile, the permeation flux of the three hydrogels increases by 7.23 L / m³ compared to SH-0. 2 ·h、8.23L / m 2 ·h、9.47L / m 2 The increases in flux for SH-2 and SH-3 were 13.7% and 30.9% greater than that for SH-1, respectively. This indicates that the increase in flux for SH-2 and SH-3 was not as significant as the decrease in enthalpy of vaporization, resulting in calculated TE values ​​smaller than SH-1 but greater than SH-0. This demonstrates that using hydrogels to increase the water evaporation rate is an effective method to improve the system's TE. SEC represents the energy consumption per unit of water produced; it is positively correlated with heat input and negatively correlated with TE. A lower SEC value indicates less heat required to produce a unit mass of pure water. As shown in the figure, the use of hydrogels significantly reduced the system's SEC, with SH-3 reaching as low as 0.70 kWh / L, a 48.2% reduction compared to SH-0's 1.36 kWh / L. The SEC values ​​for SH-1 and SH-2 were 0.80 kWh / L and 0.75 kWh / L, respectively, also showing significant reductions compared to SH-0.

[0087] 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 method for preparing a hydrogel for membrane distillation, characterized in that: Using polyurethane as the support material, with a thickness of 2–4 mm, polyvinyl alcohol, sodium alginate, and carboxylated carbon nanotubes are loaded onto the polyurethane; the process includes the following steps. 1) Wash polyurethane with a length of 8-9cm, a width of 2-3cm, and a thickness of 2-4mm repeatedly with anhydrous ethanol and deionized water, and then dry it at 40-60℃ for 1-2 hours. 2) Mechanically stir 10-20g of 10.0-15.0 wt% polyvinyl alcohol solution and 120-150μL of 40-50 wt% glutaraldehyde solution until homogeneous; this mixture is called solution a. 3) Add 500-550 μL of 1-1.2 mol / L HCl solution, 0.1-0.2 g of sodium alginate and 0.1-0.3 g of carboxylated carbon nanotubes to solution a, and mix them evenly by mechanical stirring. 4) Place the dried polyurethane into the mixture from step 3), spread it evenly, and let it stand for 2-4 hours; 5) Soak the polyurethane treated in step 4) in water for 24 to 48 hours to obtain a hydrogel.

2. A membrane module, characterized in that: The hydrogel for membrane distillation prepared by the method described in claim 1 comprises a feed side and a permeate side, with two sealing rings in between to prevent liquid leakage; an external heat source transfers heat to the hydrogel and feed liquid via a heat carrier; the hydrogel, 8-9 cm long, 2-3 cm wide, and 2-4 mm thick, is placed in a feed channel with a depth of 3-6 mm and is in close contact with the heat carrier; the effective area is 16-20 cm². 2 The polytetrafluoroethylene hydrophobic membrane is placed on the other side of the heat carrier.

3. The membrane module according to claim 2, characterized in that: The polytetrafluoroethylene membrane has a thickness of 20–80 μm, a pore size of 0.1–0.2 μm, a porosity of 80%–85%, a contact angle of 130°–145°, and a liquid osmotic pressure of 130–135 kPa.

4. A method for thermally conductive vacuum membrane distillation, characterized in that: Using the membrane module as described in claim 2 or 3, the thermally conductive vacuum membrane distillation system 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 through an aluminum heat carrier; a peristaltic pump circulates the feed liquid; a vacuum pump is used to generate a vacuum on the permeate side of the membrane; water vapor is condensed on the permeate side to form permeate; the process is carried out under the following conditions: feed flow rate of 5–10 cm / s, heat source temperature of 100–200 °C, and vacuum degree of 0.08–0.1 MPa.

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